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Parkinson's Disease

2026-01-03T00:50:32Z

Techsensus: /* Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder associated with the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficit in the basal ganglia, which are critical for initiating and smoothing movement. The predominant motor symptoms advancing from this deficit include tremor, muscle stiffness (rigidity) and slowness of movement (bradykinesia)<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa, a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed into the bloodstream in the gastrointestinal tract, chiefly in the duodenum and proximal jejunum, and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, an adequate concentration of levodopa in the blood serum is a prerequisite for the exertion of the therapeutic effect. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes complicated, manifesting as unpredictable motor fluctuations and dyskinesias. There is no cure for Parkinson's disease, although medications and non-drug therapies can alleviate the symptoms and improve daily functioning. In advanced cases, patients may undergo surgical treatments such as deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation of Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficit as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor) and non-motor features (such as loss of smell, sleep disorder, gastrointestinal issues, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is clinical, based on a neurological examination and response to dopaminergic therapy, with imaging (such as MRI and DaT-SPECT) mainly used to exclude other causes of the symptoms; research into biomarkers is ongoing and their application is not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although during the course of the disease levodopa-resistant symptoms develop, and advanced therapies (such as deep brain stimulation, DBS) are an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis. An example of a wearable device accessible to patients is the Apple Watch with app StrivePD of Runelabs, which allows for the continuous monitoring of dyskinesia and the specific PD tremor frequency in the affected arm on the iPhone.<ref>Powers, R.; Etezadi-Amoli, M.; Arnold, E. M.; Kianian, S.; Mance, I.; Gibiansky, M.; Trietsch, D.; Alvarado, A. S.; Kretlow, J. D.; Herrington, T. M.; Brillman, S.; Huang, N.; Lin, P. T.; Pham, H. A.; Ullal, A. V. Smartwatch Inertial Sensors Continuously Monitor Real-World Motor Fluctuations in Parkinson’s Disease. ''Science Translational Medicine'' '''2021''', ''13'' (579), eabd7865. <nowiki>https://doi.org/10.1126/scitranslmed.abd7865</nowiki></ref><ref>'''StrivePD''': A digital health platform and companion tool empowering people with Parkinson’s disease by providing real-time symptom tracking, personalized health insights, medication adherence support, and access to clinical trials through wearable data and mobile applications. Available at <nowiki>https://www.strive.group/</nowiki></ref> Under development is the smartwatch app PD-Trends which in addition to tremor duration and intensity also monitors aspects of movement and sleep and shows longer trends in symptom development. It also aims to support Android platforms once the Apple app is online. <ref>'''PD-Trends.com'''. ''Parkinson’s Disease Trends and Data''. <nowiki>https://www.pd-trends.com</nowiki></ref>

Broadly speaking, Parkinson’s disease can be subdivided into three disease stages:

<nowiki>*</nowiki> The early stage, in which response to dopaminergic medication is generally stable throughout the day, without response fluctuations. The person with PD (PwPD) can, under drug treatment, function normally in many respects.

<nowiki>*</nowiki> Mid-stage, in which motor fluctuations (the ON-OFF phenomenon) come to the fore. Even with extensive dosing schedules (levodopa doses up to eight or more times a day), some people may still not reach a stable response, limiting daily functioning. In this phase, advanced therapies such as continuous dopaminergic drug administration via subcutaneous/intestinal pump and/or deep brain stimulation surgery may be considered.

<nowiki>*</nowiki> Late-stage disease. Dopamine-resistant and non-motor symptoms such as postural instability/falling, cognitive deficits/dementia, hallucinations and autonomic failure dominate the clinical picture. The patient’s functioning is determined mostly by these symptoms, which are poorly susceptible to drug treatment.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction in 1961<ref>Birkmayer W, Hornykiewicz O. The effect of l-3,4-dihydroxyphenylalanine (=DOPA) on akinesia in parkinsonism (originally published in German in the journal: Wiener Klinische Wochenschrift, volume 73, no. 45, pp. 787–788 (1961) Parkinsonism and Related Disorders 4 (1998) 59–60</ref>and demonstrated their impact on the patients with PD<ref>Cotzias GC, Van Woert MH, Schiffer LM. Aromatic amino acids and modification of parkinsonism. N Engl J Med. 1967;276(7):374-379.</ref><ref>Cotzias GC, Papavasiliou, PS, Gellene R. Modification of parkinsonism — chronic treatment with L-dopa. N Engl J Med. 1969;280(7):337-345

</ref>. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma. It has a short half-life of 30-90 minutes, so it is typically administered in combination with a peripheral decarboxylase inhibitor (PDI), such as carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of levodopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of levodopa to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>. Advances in infusion therapies include foslevodopa/foscarbidopa. <ref>Rigon, L.; Fogliano, C.; Odin, P.; Antonini, A. Infusion Therapies for Parkinson’s Disease: Where Are We in 2025? ''Expert Opinion on Drug Delivery'' '''2025''', ''22'' (???), 1–4. <nowiki>https://doi.org/10.1080/17425247.2025.2577710</nowiki></ref>

Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in the therapeutic window of levodopa becoming narrower and the development of the ‘ON-OFF phenomenon’<ref name=":0" />which is characterized on one end of the spectrum by hypodopaminergic symptoms (stiffness, slowness, freezing) and on the other end by hyperdopaminergic excessive movement (dyskinesia). This highlights the need for precise levodopa dosing. The narrowing of the levodopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and OFF-time (return of PD symptoms).

== (Lack of) correlation between peripheral levodopa concentration and clinical symptoms ==

=== Pathophysiological complexity of the disease ===
Parkinson’s disease is a complex condition. Not only do the symptoms encompass a wide range of motor and non-motor complaints, the underlying pathophysiological mechanisms are also manifold. This limits dopaminergic therapy in important ways:

* Not all dopaminergic systems in the brain are affected at the same pace<ref>Rodríguez-Oroz, M. C.; Jahanshahi, M.; Krack, P.; Litvan, I.; Macías, R.; Bezard, E.; Obeso, J. A. Initial Clinical Manifestations of Parkinson’s Disease: Features and Pathophysiological Mechanisms. ''The Lancet Neurology'' '''2009''', ''8'' (12), 1128–1139. <nowiki>https://doi.org/10.1016/S1474-4422(09)70293-5</nowiki>.</ref> , with as a consequence that a certain levodopa dose can result in an eudopaminergic state in the motor circuit but a hyperdopaminergic state in the limbic circuit. The clinical correlate of this is a patient whose motor symptoms may be well-controlled, but cumbersome psychotic symptoms occur as an unintended effect.

* During the disease course, in addition to the dopaminergic deficit, deficits of other neurotransmitter systems<ref>Devos, D.; Defebvre, L.; Bordet, R. Dopaminergic and Non-Dopaminergic Pharmacological Hypotheses for Gait Disorders in Parkinson’s Disease. ''Fundamental & Clinical Pharmacology'' '''2010''', ''24'' (4), 407–421. <nowiki>https://doi.org/10.1111/j.1472-8206.2009.00798.x</nowiki>.</ref> (such as cholinergic, (nor)adrenergic and serotonergic systems) develop in addition to neurotransmitter-independent neurodegeneration. Several debilitating symptoms, such as autonomic and cognitive symptoms and disturbed balance/falling, are therefore levodopa-resistant.

* Varying extents of postsynaptic dopaminergic degeneration occur. This means that not only is the amount of (pre-synaptic) dopamine limited, but there is also damage to the (postsynaptic) neurons that have to make use of the dopamine. Even an optimal concentration of dopamine in the synaptic cleft may therefore be unable to adequately attenuate hypodopaminergic symptoms.

=== Correlation between peripheral levodopa concentration and dopamine available to cerebral neurons ===

A multitude of steps stands between (oral) levodopa intake and concentration of dopamine available to dopaminergic neurons in the brain.

1. After oral ingestion of levodopa, the tablets have to dissolve in the stomach, and then be transported to the small intestine for absorption. Many people with Parkinson’s disease have a lower acidity<ref>Fasano, A.; Visanji, N. P.; Liu, L. W. C.; Lang, A. E.; Pfeiffer, R. F. Gastrointestinal Dysfunction in Parkinson’s Disease. ''The Lancet Neurology'' '''2015''', ''14'' (6), 625–639. <nowiki>https://doi.org/10.1016/S1474-4422(15)00007-1</nowiki>.</ref> of the stomach (e.g. due to medication use or ''Helicobacter pylori'' infection), and an estimated 70-100% of people with Parkinson’s disease have gastroparesis<ref>Marrinan, S.; Emmanuel, A. V.; Burn, D. J. Delayed Gastric Emptying in Parkinson’s Disease. ''Movement Disorders'' '''2014''', ''29'' (1), 23–32. <nowiki>https://doi.org/10.1002/mds.25708</nowiki>.</ref> (delayed stomach emptying). This leads to impaired dissolution of levodopa and delayed transfer of levodopa to the small intestine, respectively.

2. Once levodopa has arrived in the duodenum and proximal jejunum for absorption, it has to be actively transported over the bowel mucosa by saturable LNAA transporters (transporters for Large Neutral Amino Acids). These transporters are both sensitive to pH, working optimally at a pH between 6.2 – 7.4,<ref>Pedrosa de Menezes, A. L.; Bloem, B. R.; Beckers, M.; Piat, C.; Benarroch, E. E.; Savica, R. Molecular Variability in Levodopa Absorption and Clinical Implications for the Management of Parkinson’s Disease. ''Journal of Parkinson’s Disease'' '''2024''', ''14'' (7), 1353–1368. <nowiki>https://doi.org/10.3233/JPD-240036</nowiki></ref> as well as sensitive to so-named ''protein competition''. That is: after a protein-rich meal, dietary proteins/amino acids compete with levodopa<ref>Contin, M.; Martinelli, P. Pharmacokinetics of Levodopa. ''Journal of Neurology'' '''2010''', ''257'' (Suppl. 2), S253–S261. <nowiki>https://doi.org/10.1007/s00415-010-5728-8</nowiki>.</ref> for the transport system, reducing levodopa’s bioavailability. As noted, stomach acidity is often less pronounced in people with Parkinson’s disease, and due to delayed gastric emptying, dietary proteins may be present in the small intestine for extended periods after a meal. This limits predictability of the speed and magnitude with which levodopa is absorbed into the bloodstream.

3. Both in the intestinal lumen as well as in peripheral blood, levodopa is metabolized to dopamine and other metabolites. This is unwanted, as dopamine cannot cross the blood-brain barrier; for pharmacotherapeutic effect, levodopa has to cross the blood-brain barrier in unmetabolized form. While peripheral decarboxylase inhibitors are able to partially inhibit peripheral decarboxylation, there is interindividual variability in the activity of levodopa-metabolizing enzymes such as AADC and catechol-O-aminotransferase (COMT). Part of this variability is genetic<ref>Devos D, Lejeune S, Cormier-Dequaire F, Tahiri K, Charbonnier-Beaupel F, Rouaix N, Duhamel A, Sablonniere B, Bonnet AM, Bonnet C, et al. Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in parkinson’s disease. PARKINSONISM RELAT D. 2014;20(2):170–5.</ref><ref>Sampaio TF, Dos SE, de Lima G, Dos AR, Da SR, Asano A, Asano N, Crovella S, de Souza P. MAO-B and COMT genetic variations associated with Levodopa treatment response in patients with parkinson’s disease. J CLIN PHARMACOL. 2018;58(7):920–6.</ref> , but AADC activity also varies by sex, disease duration and use of dopaminergic medication. The amount of levodopa that succeeds in being absorbed into the blood stream is therefore not necessarily the amount available at the blood-brain barrier.

4. At the blood-brain barrier, protein competition exists as well. <ref>Lees, A. J. The On-Off Phenomenon. ''Journal of Neurology, Neurosurgery & Psychiatry'' '''1989''', ''52'' (Suppl. 29), 29–37. <nowiki>https://doi.org/10.1136/jnnp.52.Suppl.29</nowiki>.</ref> Speed and magnitude of levodopa transport over the blood-brain barrier thus depends on the concentration of (dietary) amino acids in the blood.

5. Once in the brain, enzymes such as AADC and COMT metabolize levodopa and, together with monoamine oxidase B (MAO-B), determine the resultant amount of dopamine available in the synaptic cleft of dopaminergic neurons. Here, as well, there is interindividual variation in the activities of the enzymes.

Given these multiple steps of which the magnitude is – to a large extent – poorly predictable, levodopa plasma concentration correlates poorly with dopamine available to neurons in the brain.

Furthermore, the above-described steps apply in individuals without other levodopa-bioavailability-altering conditions. There are multiple circumstances <ref>Beckers, M.; Bloem, B. R.; Verbeek, M. M. Mechanisms of Peripheral Levodopa Resistance in Parkinson’s Disease. ''npj Parkinson’s Disease'' '''2022''', ''8'' (1), 56. <nowiki>https://doi.org/10.1038/s41531-022-00321-y</nowiki>.</ref> under which the correlation between intake of levodopa and clinical effect becomes even more unpredictable, e.g. if intestinal absorption is limited (e.g. due to inflammation of the intestinal lining), if levodopa is prematurely metabolized by gut bacteria, or if there is an overactivity of the AADC enzyme.

=== Correlation between bioavailable dopamine and control of PD symptoms ===
As described earlier in the text, not all PD symptoms respond to levodopa to the same extent. In addition to the presence of non-dopaminergic symptoms (which by definition do not respond to levodopa), even symptoms who are normally levodopa-responsive can display variation in their response. Factors inducing these variation include, amongst others, (psychological and physical) stress<ref>Helmich, R. C. G. and the Systems Neurology Group at the Donders Centre for Cognitive Neuroimaging investigate the cerebral mechanisms of movement disorders, especially the pathophysiology of Parkinson’s disease tremor and compensatory network changes, using neuroimaging and neurophysiological methods. Recent works include neuroimaging studies on tremor circuitry, longitudinal brain compensation in Parkinson’s progression, and clinical trials examining propranolol’s effects on tremor.</ref> as well as energy level/fatigue.

For the reasons outlined in the above paragraphs, peripheral levodopa pharmacokinetics correlate poorly to the actual level of symptom control. A 2013 study <ref>Fasano, A.; Bove, F.; Gabrielli, M.; Petracca, M.; Zocco, M. A.; Ragazzoni, E.; Barbaro, F.; Piano, C.; Fortuna, S.; Tortora, A.; Di Giacopo, R.; Campanale, M.; Gigante, G.; Lauritano, E. C.; Navarra, P.; Marconi, S.; Gasbarrini, A.; Bentivoglio, A. R. The Role of Small Intestinal Bacterial Overgrowth in Parkinson’s Disease. ''Movement Disorders'' '''2013''', ''28'' (9), 1241–1249. <nowiki>https://doi.org/10.1002/mds.25522</nowiki></ref> demonstrated that the presence of motor fluctuations did not correlate to pharmacokinetic data.

Thus, peripheral levodopa concentration monitoring would need to be integrated with measurement of clinical symptoms to be useful in clinical practice.
== State of the Art ==

==== Current Methods to Measure levodopa ====
The current gold standards to measure levodopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly levodopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref><ref>Batchu K, Probst D, Satomura T, Younce J, Sode K, The Development and Application of An Engineered Direct Electron Transfer Enzyme for Continuous Levodopa Monitoring,

''npj Biosensing'', 2(1) ('''2025'''). <nowiki>https://www.nature.com/articles/s44328-024-00020-z</nowiki> </ref>

==== Matrix ====
Several matrices are available in which levodopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure levodopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, levodopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the levodopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable levodopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce levodopa concentration fluctuations in blood. Continuous sensing would also help researchers better understand the pharmacokinetics of levodopa across different compartments, including intestinal lumen, blood, ISF and cerebrospinal fluid (CSF). This could improve treatment strategies for mid-stage PD patients and also help make a step towards a long-term vision of closed-loop levodopa therapy for PD with integrated levodopa concentration & motor symptom sensing in which continuous (e.g. subcutaneous, intestinal) levodopa administration is automatically adjusted based upon the sensor data, potentially reducing fluctuations and improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2026-01-02T01:16:00Z

Techsensus: /* General information */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder associated with the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficit in the basal ganglia, which are critical for initiating and smoothing movement. The predominant motor symptoms advancing from this deficit include tremor, muscle stiffness (rigidity) and slowness of movement (bradykinesia)<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa, a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed into the bloodstream in the gastrointestinal tract, chiefly in the duodenum and proximal jejunum, and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, an adequate concentration of levodopa in the blood serum is a prerequisite for the exertion of the therapeutic effect. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes complicated, manifesting as unpredictable motor fluctuations and dyskinesias. There is no cure for Parkinson's disease, although medications and non-drug therapies can alleviate the symptoms and improve daily functioning. In advanced cases, patients may undergo surgical treatments such as deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation of Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficit as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor) and non-motor features (such as loss of smell, sleep disorder, gastrointestinal issues, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is clinical, based on a neurological examination and response to dopaminergic therapy, with imaging (such as MRI and DaT-SPECT) mainly used to exclude other causes of the symptoms; research into biomarkers is ongoing and their application is not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although during the course of the disease levodopa-resistant symptoms develop, and advanced therapies (such as deep brain stimulation, DBS) are an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis. An example of a wearable device accessible to patients is the Apple Watch with app StrivePD of Runelabs, which allows for the continuous monitoring of dyskinesia and the specific PD tremor frequency in the affected arm on the iPhone.<ref>Powers, R.; Etezadi-Amoli, M.; Arnold, E. M.; Kianian, S.; Mance, I.; Gibiansky, M.; Trietsch, D.; Alvarado, A. S.; Kretlow, J. D.; Herrington, T. M.; Brillman, S.; Huang, N.; Lin, P. T.; Pham, H. A.; Ullal, A. V. Smartwatch Inertial Sensors Continuously Monitor Real-World Motor Fluctuations in Parkinson’s Disease. ''Science Translational Medicine'' '''2021''', ''13'' (579), eabd7865. <nowiki>https://doi.org/10.1126/scitranslmed.abd7865</nowiki></ref><ref>'''StrivePD''': A digital health platform and companion tool empowering people with Parkinson’s disease by providing real-time symptom tracking, personalized health insights, medication adherence support, and access to clinical trials through wearable data and mobile applications. Available at <nowiki>https://www.strive.group/</nowiki></ref> Under development is the smartwatch app PD-Trends which in addition to tremor duration and intensity also monitors aspects of movement and sleep and shows longer trends in symptom development. It also aims to support Android platforms once the Apple app is online. <ref>'''PD-Trends.com'''. ''Parkinson’s Disease Trends and Data''. <nowiki>https://www.pd-trends.com</nowiki></ref>

Broadly speaking, Parkinson’s disease can be subdivided into three disease stages:

<nowiki>*</nowiki> The early stage, in which response to dopaminergic medication is generally stable throughout the day, without response fluctuations. The person with PD (PwPD) can, under drug treatment, function normally in many respects.

<nowiki>*</nowiki> Mid-stage, in which motor fluctuations (the ON-OFF phenomenon) come to the fore. Even with extensive dosing schedules (levodopa doses up to eight or more times a day), some people may still not reach a stable response, limiting daily functioning. In this phase, advanced therapies such as continuous dopaminergic drug administration via subcutaneous/intestinal pump and/or deep brain stimulation surgery may be considered.

<nowiki>*</nowiki> Late-stage disease. Dopamine-resistant and non-motor symptoms such as postural instability/falling, cognitive deficits/dementia, hallucinations and autonomic failure dominate the clinical picture. The patient’s functioning is determined mostly by these symptoms, which are poorly susceptible to drug treatment.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction in 1961<ref>Birkmayer W, Hornykiewicz O. The effect of l-3,4-dihydroxyphenylalanine (=DOPA) on akinesia in parkinsonism (originally published in German in the journal: Wiener Klinische Wochenschrift, volume 73, no. 45, pp. 787–788 (1961) Parkinsonism and Related Disorders 4 (1998) 59–60</ref>and demonstrated their impact on the patients with PD<ref>Cotzias GC, Van Woert MH, Schiffer LM. Aromatic amino acids and modification of parkinsonism. N Engl J Med. 1967;276(7):374-379.</ref><ref>Cotzias GC, Papavasiliou, PS, Gellene R. Modification of parkinsonism — chronic treatment with L-dopa. N Engl J Med. 1969;280(7):337-345

</ref>. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma. It has a short half-life of 30-90 minutes, so it is typically administered in combination with a peripheral decarboxylase inhibitor (PDI), such as carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of levodopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of levodopa to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>. Advances in infusion therapies include foslevodopa/foscarbidopa for example [Rigon et al, Expert Opinion Drug Delivery, 2025].

Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in the therapeutic window of levodopa becoming narrower and the development of the ‘ON-OFF phenomenon’<ref name=":0" />which is characterized on one end of the spectrum by hypodopaminergic symptoms (stiffness, slowness, freezing) and on the other end by hyperdopaminergic excessive movement (dyskinesia). This highlights the need for precise levodopa dosing. The narrowing of the levodopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and OFF-time (return of PD symptoms).

== (Lack of) correlation between peripheral levodopa concentration and clinical symptoms ==

=== Pathophysiological complexity of the disease ===
Parkinson’s disease is a complex condition. Not only do the symptoms encompass a wide range of motor and non-motor complaints, the underlying pathophysiological mechanisms are also manifold. This limits dopaminergic therapy in important ways:

* Not all dopaminergic systems in the brain are affected at the same pace<ref>Rodríguez-Oroz, M. C.; Jahanshahi, M.; Krack, P.; Litvan, I.; Macías, R.; Bezard, E.; Obeso, J. A. Initial Clinical Manifestations of Parkinson’s Disease: Features and Pathophysiological Mechanisms. ''The Lancet Neurology'' '''2009''', ''8'' (12), 1128–1139. <nowiki>https://doi.org/10.1016/S1474-4422(09)70293-5</nowiki>.</ref> , with as a consequence that a certain levodopa dose can result in an eudopaminergic state in the motor circuit but a hyperdopaminergic state in the limbic circuit. The clinical correlate of this is a patient whose motor symptoms may be well-controlled, but cumbersome psychotic symptoms occur as an unintended effect.

* During the disease course, in addition to the dopaminergic deficit, deficits of other neurotransmitter systems<ref>Devos, D.; Defebvre, L.; Bordet, R. Dopaminergic and Non-Dopaminergic Pharmacological Hypotheses for Gait Disorders in Parkinson’s Disease. ''Fundamental & Clinical Pharmacology'' '''2010''', ''24'' (4), 407–421. <nowiki>https://doi.org/10.1111/j.1472-8206.2009.00798.x</nowiki>.</ref> (such as cholinergic, (nor)adrenergic and serotonergic systems) develop in addition to neurotransmitter-independent neurodegeneration. Several debilitating symptoms, such as autonomic and cognitive symptoms and disturbed balance/falling, are therefore levodopa-resistant.

* Varying extents of postsynaptic dopaminergic degeneration occur. This means that not only is the amount of (pre-synaptic) dopamine limited, but there is also damage to the (postsynaptic) neurons that have to make use of the dopamine. Even an optimal concentration of dopamine in the synaptic cleft may therefore be unable to adequately attenuate hypodopaminergic symptoms.

=== Correlation between peripheral levodopa concentration and dopamine available to cerebral neurons ===

A multitude of steps stands between (oral) levodopa intake and concentration of dopamine available to dopaminergic neurons in the brain.

1. After oral ingestion of levodopa, the tablets have to dissolve in the stomach, and then be transported to the small intestine for absorption. Many people with Parkinson’s disease have a lower acidity<ref>Fasano, A.; Visanji, N. P.; Liu, L. W. C.; Lang, A. E.; Pfeiffer, R. F. Gastrointestinal Dysfunction in Parkinson’s Disease. ''The Lancet Neurology'' '''2015''', ''14'' (6), 625–639. <nowiki>https://doi.org/10.1016/S1474-4422(15)00007-1</nowiki>.</ref> of the stomach (e.g. due to medication use or ''Helicobacter pylori'' infection), and an estimated 70-100% of people with Parkinson’s disease have gastroparesis<ref>Marrinan, S.; Emmanuel, A. V.; Burn, D. J. Delayed Gastric Emptying in Parkinson’s Disease. ''Movement Disorders'' '''2014''', ''29'' (1), 23–32. <nowiki>https://doi.org/10.1002/mds.25708</nowiki>.</ref> (delayed stomach emptying). This leads to impaired dissolution of levodopa and delayed transfer of levodopa to the small intestine, respectively.

2. Once levodopa has arrived in the duodenum and proximal jejunum for absorption, it has to be actively transported over the bowel mucosa by saturable LNAA transporters (transporters for Large Neutral Amino Acids). These transporters are both sensitive to pH, working optimally at a pH between 6.2 – 7.4,<ref>Pedrosa de Menezes, A. L.; Bloem, B. R.; Beckers, M.; Piat, C.; Benarroch, E. E.; Savica, R. Molecular Variability in Levodopa Absorption and Clinical Implications for the Management of Parkinson’s Disease. ''Journal of Parkinson’s Disease'' '''2024''', ''14'' (7), 1353–1368. <nowiki>https://doi.org/10.3233/JPD-240036</nowiki></ref> as well as sensitive to so-named ''protein competition''. That is: after a protein-rich meal, dietary proteins/amino acids compete with levodopa<ref>Contin, M.; Martinelli, P. Pharmacokinetics of Levodopa. ''Journal of Neurology'' '''2010''', ''257'' (Suppl. 2), S253–S261. <nowiki>https://doi.org/10.1007/s00415-010-5728-8</nowiki>.</ref> for the transport system, reducing levodopa’s bioavailability. As noted, stomach acidity is often less pronounced in people with Parkinson’s disease, and due to delayed gastric emptying, dietary proteins may be present in the small intestine for extended periods after a meal. This limits predictability of the speed and magnitude with which levodopa is absorbed into the bloodstream.

3. Both in the intestinal lumen as well as in peripheral blood, levodopa is metabolized to dopamine and other metabolites. This is unwanted, as dopamine cannot cross the blood-brain barrier; for pharmacotherapeutic effect, levodopa has to cross the blood-brain barrier in unmetabolized form. While peripheral decarboxylase inhibitors are able to partially inhibit peripheral decarboxylation, there is interindividual variability in the activity of levodopa-metabolizing enzymes such as AADC and catechol-O-aminotransferase (COMT). Part of this variability is genetic<ref>Devos D, Lejeune S, Cormier-Dequaire F, Tahiri K, Charbonnier-Beaupel F, Rouaix N, Duhamel A, Sablonniere B, Bonnet AM, Bonnet C, et al. Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in parkinson’s disease. PARKINSONISM RELAT D. 2014;20(2):170–5.</ref><ref>Sampaio TF, Dos SE, de Lima G, Dos AR, Da SR, Asano A, Asano N, Crovella S, de Souza P. MAO-B and COMT genetic variations associated with Levodopa treatment response in patients with parkinson’s disease. J CLIN PHARMACOL. 2018;58(7):920–6.</ref> , but AADC activity also varies by sex, disease duration and use of dopaminergic medication. The amount of levodopa that succeeds in being absorbed into the blood stream is therefore not necessarily the amount available at the blood-brain barrier.

4. At the blood-brain barrier, protein competition exists as well. <ref>Lees, A. J. The On-Off Phenomenon. ''Journal of Neurology, Neurosurgery & Psychiatry'' '''1989''', ''52'' (Suppl. 29), 29–37. <nowiki>https://doi.org/10.1136/jnnp.52.Suppl.29</nowiki>.</ref> Speed and magnitude of levodopa transport over the blood-brain barrier thus depends on the concentration of (dietary) amino acids in the blood.

5. Once in the brain, enzymes such as AADC and COMT metabolize levodopa and, together with monoamine oxidase B (MAO-B), determine the resultant amount of dopamine available in the synaptic cleft of dopaminergic neurons. Here, as well, there is interindividual variation in the activities of the enzymes.

Given these multiple steps of which the magnitude is – to a large extent – poorly predictable, levodopa plasma concentration correlates poorly with dopamine available to neurons in the brain.

Furthermore, the above-described steps apply in individuals without other levodopa-bioavailability-altering conditions. There are multiple circumstances <ref>Beckers, M.; Bloem, B. R.; Verbeek, M. M. Mechanisms of Peripheral Levodopa Resistance in Parkinson’s Disease. ''npj Parkinson’s Disease'' '''2022''', ''8'' (1), 56. <nowiki>https://doi.org/10.1038/s41531-022-00321-y</nowiki>.</ref> under which the correlation between intake of levodopa and clinical effect becomes even more unpredictable, e.g. if intestinal absorption is limited (e.g. due to inflammation of the intestinal lining), if levodopa is prematurely metabolized by gut bacteria, or if there is an overactivity of the AADC enzyme.

=== Correlation between bioavailable dopamine and control of PD symptoms ===
As described earlier in the text, not all PD symptoms respond to levodopa to the same extent. In addition to the presence of non-dopaminergic symptoms (which by definition do not respond to levodopa), even symptoms who are normally levodopa-responsive can display variation in their response. Factors inducing these variation include, amongst others, (psychological and physical) stress<ref>Helmich, R. C. G. and the Systems Neurology Group at the Donders Centre for Cognitive Neuroimaging investigate the cerebral mechanisms of movement disorders, especially the pathophysiology of Parkinson’s disease tremor and compensatory network changes, using neuroimaging and neurophysiological methods. Recent works include neuroimaging studies on tremor circuitry, longitudinal brain compensation in Parkinson’s progression, and clinical trials examining propranolol’s effects on tremor.</ref> as well as energy level/fatigue.

For the reasons outlined in the above paragraphs, peripheral levodopa pharmacokinetics correlate poorly to the actual level of symptom control. A 2013 study <ref>Fasano, A.; Bove, F.; Gabrielli, M.; Petracca, M.; Zocco, M. A.; Ragazzoni, E.; Barbaro, F.; Piano, C.; Fortuna, S.; Tortora, A.; Di Giacopo, R.; Campanale, M.; Gigante, G.; Lauritano, E. C.; Navarra, P.; Marconi, S.; Gasbarrini, A.; Bentivoglio, A. R. The Role of Small Intestinal Bacterial Overgrowth in Parkinson’s Disease. ''Movement Disorders'' '''2013''', ''28'' (9), 1241–1249. <nowiki>https://doi.org/10.1002/mds.25522</nowiki></ref> demonstrated that the presence of motor fluctuations did not correlate to pharmacokinetic data.

Thus, peripheral levodopa concentration monitoring would need to be integrated with measurement of clinical symptoms to be useful in clinical practice.
== State of the Art ==

==== Current Methods to Measure levodopa ====
The current gold standards to measure levodopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly levodopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref><ref>Batchu K, Probst D, Satomura T, Younce J, Sode K, The Development and Application of An Engineered Direct Electron Transfer Enzyme for Continuous Levodopa Monitoring,

''npj Biosensing'', 2(1) ('''2025'''). <nowiki>https://www.nature.com/articles/s44328-024-00020-z</nowiki> </ref>

==== Matrix ====
Several matrices are available in which levodopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure levodopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, levodopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the levodopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable levodopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce levodopa concentration fluctuations in blood. Continuous sensing would also help researchers better understand the pharmacokinetics of levodopa across different compartments, including intestinal lumen, blood, ISF and cerebrospinal fluid (CSF). This could improve treatment strategies for mid-stage PD patients and also help make a step towards a long-term vision of closed-loop levodopa therapy for PD with integrated levodopa concentration & motor symptom sensing in which continuous (e.g. subcutaneous, intestinal) levodopa administration is automatically adjusted based upon the sensor data, potentially reducing fluctuations and improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2025-12-26T22:58:57Z

Techsensus: /* General information */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder associated with the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficit in the basal ganglia, which are critical for initiating and smoothing movement. The predominant motor symptoms advancing from this deficit include tremor, muscle stiffness (rigidity) and slowness of movement (bradykinesia)<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed into the bloodstream in the gastrointestinal tract, chiefly in the duodenum and proximal jejunum, and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, an adequate concentration of levodopa in the blood serum is a prerequisite for the exertion of the therapeutic effect. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes complicated, manifesting as unpredictable motor fluctuations and dyskinesias. There is no cure for Parkinson's disease, although medications and non-drug therapies can alleviate the symptoms and improve daily functioning. In advanced cases, patients may undergo surgical treatments such as deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation of Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficit as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor) and non-motor features (such as loss of smell, sleep disorder, gastrointestinal issues, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is clinical, based on a neurological examination and response to dopaminergic therapy, with imaging (such as MRI and DaT-SPECT) mainly used to exclude other causes of the symptoms; research into biomarkers is ongoing and their application is not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although during the course of the disease levodopa-resistant symptoms develop, and advanced therapies (such as deep brain stimulation, DBS) are an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis. An example of a wearable device accessible to patients is the Apple Watch with app StrivePD of Runelabs, which allows for the continuous monitoring of dyskinesia and the specific PD tremor frequency in the affected arm on the iPhone.<ref>Powers, R.; Etezadi-Amoli, M.; Arnold, E. M.; Kianian, S.; Mance, I.; Gibiansky, M.; Trietsch, D.; Alvarado, A. S.; Kretlow, J. D.; Herrington, T. M.; Brillman, S.; Huang, N.; Lin, P. T.; Pham, H. A.; Ullal, A. V. Smartwatch Inertial Sensors Continuously Monitor Real-World Motor Fluctuations in Parkinson’s Disease. ''Science Translational Medicine'' '''2021''', ''13'' (579), eabd7865. <nowiki>https://doi.org/10.1126/scitranslmed.abd7865</nowiki></ref><ref>'''StrivePD''': A digital health platform and companion tool empowering people with Parkinson’s disease by providing real-time symptom tracking, personalized health insights, medication adherence support, and access to clinical trials through wearable data and mobile applications. Available at <nowiki>https://www.strive.group/</nowiki></ref> Under development is the smartwatch app PD-Trends which in addition to tremor duration and intensity also monitors aspects of movement and sleep and shows longer trends in symptom development. It also aims to support Android platforms once the Apple app is online. <ref>'''PD-Trends.com'''. ''Parkinson’s Disease Trends and Data''. <nowiki>https://www.pd-trends.com</nowiki></ref>

Broadly speaking, Parkinson’s disease can be subdivided into three disease stages:

<nowiki>*</nowiki> The early stage, in which response to dopaminergic medication is generally stable throughout the day, without response fluctuations. The person with PD (PwPD) can, under drug treatment, function normally in many respects.

<nowiki>*</nowiki> Mid-stage, in which motor fluctuations (the ON-OFF phenomenon) come to the fore. Even with extensive dosing schedules (levodopa doses up to eight or more times a day), some people may still not reach a stable response, limiting daily functioning. In this phase, advanced therapies such as continuous dopaminergic drug administration via subcutaneous/intestinal pump and/or deep brain stimulation surgery may be considered.

<nowiki>*</nowiki> Late-stage disease. Dopamine-resistant and non-motor symptoms such as postural instability/falling, cognitive deficits/dementia, hallucinations and autonomic failure dominate the clinical picture. The patient’s functioning is determined mostly by these symptoms, which are poorly susceptible to drug treatment.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction in 1961. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with a peripheral decarboxylase inhibitor (PDI), such as carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in the therapeutic window of levodopa becoming narrower and the development of the ‘ON-OFF phenomenon’<ref name=":0" />which is characterized on one end of the spectrum by hypodopaminergic symptoms (stiffness, slowness, freezing) and on the other end by hyperdopaminergic excessive movement (dyskinesia). This highlights the need for precise L-Dopa dosing. The narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and OFF-time (return of PD symptoms).

== (Lack of) correlation between peripheral levodopa concentration and clinical symptoms ==

=== Pathophysiological complexity of the disease ===
Parkinson’s disease is a complex condition. Not only do the symptoms encompass a wide range of motor and non-motor complaints, the underlying pathophysiological mechanisms are also manifold. This limits dopaminergic therapy in important ways:

* Not all dopaminergic systems in the brain are affected at the same pace<ref>Rodríguez-Oroz, M. C.; Jahanshahi, M.; Krack, P.; Litvan, I.; Macías, R.; Bezard, E.; Obeso, J. A. Initial Clinical Manifestations of Parkinson’s Disease: Features and Pathophysiological Mechanisms. ''The Lancet Neurology'' '''2009''', ''8'' (12), 1128–1139. <nowiki>https://doi.org/10.1016/S1474-4422(09)70293-5</nowiki>.</ref> , with as a consequence that a certain levodopa dose can result in an eudopaminergic state in the motor circuit but a hyperdopaminergic state in the limbic circuit. The clinical correlate of this is a patient whose motor symptoms may be well-controlled, but cumbersome psychotic symptoms occur as an unintended effect.

* During the disease course, in addition to the dopaminergic deficit, deficits of other neurotransmitter systems<ref>Devos, D.; Defebvre, L.; Bordet, R. Dopaminergic and Non-Dopaminergic Pharmacological Hypotheses for Gait Disorders in Parkinson’s Disease. ''Fundamental & Clinical Pharmacology'' '''2010''', ''24'' (4), 407–421. <nowiki>https://doi.org/10.1111/j.1472-8206.2009.00798.x</nowiki>.</ref> (such as cholinergic, (nor)adrenergic and serotonergic systems) develop in addition to neurotransmitter-independent neurodegeneration. Several debilitating symptoms, such as autonomic and cognitive symptoms and disturbed balance/falling, are therefore levodopa-resistant.

* Varying extents of postsynaptic dopaminergic degeneration occur. This means that not only is the amount of (pre-synaptic) dopamine limited, but there is also damage to the (postsynaptic) neurons that have to make use of the dopamine. Even an optimal concentration of dopamine in the synaptic cleft may therefore be unable to adequately attenuate hypodopaminergic symptoms.

=== Correlation between peripheral levodopa concentration and dopamine available to cerebral neurons ===

A multitude of steps stands between (oral) levodopa intake and concentration of dopamine available to dopaminergic neurons in the brain.

1. After oral ingestion of levodopa, the tablets have to dissolve in the stomach, and then be transported to the small intestine for absorption. Many people with Parkinson’s disease have a lower acidity<ref>Fasano, A.; Visanji, N. P.; Liu, L. W. C.; Lang, A. E.; Pfeiffer, R. F. Gastrointestinal Dysfunction in Parkinson’s Disease. ''The Lancet Neurology'' '''2015''', ''14'' (6), 625–639. <nowiki>https://doi.org/10.1016/S1474-4422(15)00007-1</nowiki>.</ref> of the stomach (e.g. due to medication use or ''Helicobacter pylori'' infection), and an estimated 70-100% of people with Parkinson’s disease have gastroparesis<ref>Marrinan, S.; Emmanuel, A. V.; Burn, D. J. Delayed Gastric Emptying in Parkinson’s Disease. ''Movement Disorders'' '''2014''', ''29'' (1), 23–32. <nowiki>https://doi.org/10.1002/mds.25708</nowiki>.</ref> (delayed stomach emptying). This leads to impaired dissolution of levodopa and delayed transfer of levodopa to the small intestine, respectively.

2. Once levodopa has arrived in the duodenum and proximal jejunum for absorption, it has to be actively transported over the bowel mucosa by saturable LNAA transporters (transporters for Large Neutral Amino Acids). These transporters are both sensitive to pH, working optimally at a pH between 6.2 – 7.4,<ref>Pedrosa de Menezes, A. L.; Bloem, B. R.; Beckers, M.; Piat, C.; Benarroch, E. E.; Savica, R. Molecular Variability in Levodopa Absorption and Clinical Implications for the Management of Parkinson’s Disease. ''Journal of Parkinson’s Disease'' '''2024''', ''14'' (7), 1353–1368. <nowiki>https://doi.org/10.3233/JPD-240036</nowiki></ref> as well as sensitive to so-named ''protein competition''. That is: after a protein-rich meal, dietary proteins/amino acids compete with levodopa<ref>Contin, M.; Martinelli, P. Pharmacokinetics of Levodopa. ''Journal of Neurology'' '''2010''', ''257'' (Suppl. 2), S253–S261. <nowiki>https://doi.org/10.1007/s00415-010-5728-8</nowiki>.</ref> for the transport system, reducing levodopa’s bioavailability. As noted, stomach acidity is often less pronounced in people with Parkinson’s disease, and due to delayed gastric emptying, dietary proteins may be present in the small intestine for extended periods after a meal. This limits predictability of the speed and magnitude with which levodopa is absorbed into the bloodstream.

3. Both in the intestinal lumen as well as in peripheral blood, levodopa is metabolized to dopamine and other metabolites. This is unwanted, as dopamine cannot cross the blood-brain barrier; for pharmacotherapeutic effect, levodopa has to cross the blood-brain barrier in unmetabolized form. While peripheral decarboxylase inhibitors are able to partially inhibit peripheral decarboxylation, there is interindividual variability in the activity of levodopa-metabolizing enzymes such as AADC and catechol-O-aminotransferase (COMT). Part of this variability is genetic<ref>Devos D, Lejeune S, Cormier-Dequaire F, Tahiri K, Charbonnier-Beaupel F, Rouaix N, Duhamel A, Sablonniere B, Bonnet AM, Bonnet C, et al. Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in parkinson’s disease. PARKINSONISM RELAT D. 2014;20(2):170–5.</ref><ref>Sampaio TF, Dos SE, de Lima G, Dos AR, Da SR, Asano A, Asano N, Crovella S, de Souza P. MAO-B and COMT genetic variations associated with Levodopa treatment response in patients with parkinson’s disease. J CLIN PHARMACOL. 2018;58(7):920–6.</ref> , but AADC activity also varies by sex, disease duration and use of dopaminergic medication. The amount of levodopa that succeeds in being absorbed into the blood stream is therefore not necessarily the amount available at the blood-brain barrier.

4. At the blood-brain barrier, protein competition exists as well. <ref>Lees, A. J. The On-Off Phenomenon. ''Journal of Neurology, Neurosurgery & Psychiatry'' '''1989''', ''52'' (Suppl. 29), 29–37. <nowiki>https://doi.org/10.1136/jnnp.52.Suppl.29</nowiki>.</ref> Speed and magnitude of levodopa transport over the blood-brain barrier thus depends on the concentration of (dietary) amino acids in the blood.

5. Once in the brain, enzymes such as AADC and COMT metabolize levodopa and, together with monoamine oxidase B (MAO-B), determine the resultant amount of dopamine available in the synaptic cleft of dopaminergic neurons. Here, as well, there is interindividual variation in the activities of the enzymes.

Given these multiple steps of which the magnitude is – to a large extent – poorly predictable, levodopa plasma concentration correlates poorly with dopamine available to neurons in the brain.

Furthermore, the above-described steps apply in individuals without other levodopa-bioavailability-altering conditions. There are multiple circumstances <ref>Beckers, M.; Bloem, B. R.; Verbeek, M. M. Mechanisms of Peripheral Levodopa Resistance in Parkinson’s Disease. ''npj Parkinson’s Disease'' '''2022''', ''8'' (1), 56. <nowiki>https://doi.org/10.1038/s41531-022-00321-y</nowiki>.</ref> under which the correlation between intake of levodopa and clinical effect becomes even more unpredictable, e.g. if intestinal absorption is limited (e.g. due to inflammation of the intestinal lining), if levodopa is prematurely metabolized by gut bacteria, or if there is an overactivity of the AADC enzyme.

=== Correlation between bioavailable dopamine and control of PD symptoms ===
As described earlier in the text, not all PD symptoms respond to levodopa to the same extent. In addition to the presence of non-dopaminergic symptoms (which by definition do not respond to levodopa), even symptoms who are normally levodopa-responsive can display variation in their response. Factors inducing these variation include, amongst others, (psychological and physical) stress<ref>Helmich, R. C. G. and the Systems Neurology Group at the Donders Centre for Cognitive Neuroimaging investigate the cerebral mechanisms of movement disorders, especially the pathophysiology of Parkinson’s disease tremor and compensatory network changes, using neuroimaging and neurophysiological methods. Recent works include neuroimaging studies on tremor circuitry, longitudinal brain compensation in Parkinson’s progression, and clinical trials examining propranolol’s effects on tremor.</ref> as well as energy level/fatigue.

For the reasons outlined in the above paragraphs, peripheral levodopa pharmacokinetics correlate poorly to the actual level of symptom control. A 2013 study <ref>Fasano, A.; Bove, F.; Gabrielli, M.; Petracca, M.; Zocco, M. A.; Ragazzoni, E.; Barbaro, F.; Piano, C.; Fortuna, S.; Tortora, A.; Di Giacopo, R.; Campanale, M.; Gigante, G.; Lauritano, E. C.; Navarra, P.; Marconi, S.; Gasbarrini, A.; Bentivoglio, A. R. The Role of Small Intestinal Bacterial Overgrowth in Parkinson’s Disease. ''Movement Disorders'' '''2013''', ''28'' (9), 1241–1249. <nowiki>https://doi.org/10.1002/mds.25522</nowiki></ref> demonstrated that the presence of motor fluctuations did not correlate to pharmacokinetic data.

Thus, peripheral levodopa concentration monitoring would need to be integrated with measurement of clinical symptoms to be useful in clinical practice.
== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure L-Dopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce levodopa concentration fluctuations in blood. Continuous sensing would also help researchers better understand the pharmacokinetics of levodopa across different compartments, including intestinal lumen, blood, ISF and cerebrospinal fluid (CSF). This could improve treatment strategies for mid-stage PD patients and also help make a step towards a long-term vision of closed-loop levodopa therapy for PD with integrated levodopa concentration & motor symptom sensing in which continuous (e.g. subcutaneous, intestinal) levodopa administration is automatically adjusted based upon the sensor data, potentially reducing fluctuations and improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2025-12-26T22:39:53Z

Techsensus: /* General information */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder associated with the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficit in the basal ganglia, which are critical for initiating and smoothing movement. The predominant motor symptoms advancing from this deficit include tremor, muscle stiffness (rigidity) and slowness of movement (bradykinesia)<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed into the bloodstream in the gastrointestinal tract, chiefly in the duodenum and proximal jejunum, and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, an adequate concentration of levodopa in the blood serum is a prerequisite for the exertion of the therapeutic effect. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes complicated, manifesting as unpredictable motor fluctuations and dyskinesias. There is no cure for Parkinson's disease, although medications and non-drug therapies can alleviate the symptoms and improve daily functioning. In advanced cases, patients may undergo surgical treatments such as deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation of Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficit as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor) and non-motor features (such as loss of smell, sleep disorder, gastrointestinal issues, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is clinical, based on a neurological examination and response to dopaminergic therapy, with imaging (such as MRI and DaT-SPECT) mainly used to exclude other causes of the symptoms; research into biomarkers is ongoing and their application is not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although during the course of the disease levodopa-resistant symptoms develop, and advanced therapies (such as deep brain stimulation, DBS) are an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis. An example of a wearable device accessible to patients is the Apple Watch with app StrivePD of Runelabs, which allows for the continuous monitoring of dyskinesia and the specific PD tremor frequency in the affected arm on the iPhone.<ref>Powers, R.; Etezadi-Amoli, M.; Arnold, E. M.; Kianian, S.; Mance, I.; Gibiansky, M.; Trietsch, D.; Alvarado, A. S.; Kretlow, J. D.; Herrington, T. M.; Brillman, S.; Huang, N.; Lin, P. T.; Pham, H. A.; Ullal, A. V. Smartwatch Inertial Sensors Continuously Monitor Real-World Motor Fluctuations in Parkinson’s Disease. ''Science Translational Medicine'' '''2021''', ''13'' (579), eabd7865. <nowiki>https://doi.org/10.1126/scitranslmed.abd7865</nowiki></ref><ref>'''StrivePD''': A digital health platform and companion tool empowering people with Parkinson’s disease by providing real-time symptom tracking, personalized health insights, medication adherence support, and access to clinical trials through wearable data and mobile applications. Available at <nowiki>https://www.strive.group/</nowiki></ref> Under development is the smartwatch app PD-Trends which in addition to tremor duration and intensity also monitors aspects of movement and sleep and shows longer trends in symptom development. It also aims to support Android platforms once the Apple app is online. <ref>'''PD-Trends.com'''. ''Parkinson’s Disease Trends and Data''. <nowiki>https://www.pd-trends.com</nowiki></ref>

Broadly speaking, Parkinson’s disease can be subdivided into three disease stages:

<nowiki>*</nowiki> The early stage, in which response to dopaminergic medication is generally stable throughout the day, without response fluctuations. The person with PD(PwPD) can, under drug treatment, function normally in many respects.

<nowiki>*</nowiki> Mid-stage, in which motor fluctuations (the ON-OFF phenomenon) come to the fore. Even with extensive dosing schedules (levodopa doses up to eight or more times a day), some people may still not reach a stable response, limiting daily functioning. In this phase, advanced therapies such as continuous dopaminergic drug administration via subcutaneous/intestinal pump and/or deep brain stimulation surgery may be considered.

<nowiki>*</nowiki> Late-stage disease. Dopamine-resistant and non-motor symptoms such as postural instability/falling, cognitive deficits/dementia, hallucinations and autonomic failure dominate the clinical picture. The patient’s functioning is determined mostly by these symptoms, which are poorly susceptible to drug treatment.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction in 1961. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with a peripheral decarboxylase inhibitor (PDI), such as carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in the therapeutic window of levodopa becoming narrower and the development of the ‘ON-OFF phenomenon’<ref name=":0" />which is characterized on one end of the spectrum by hypodopaminergic symptoms (stiffness, slowness, feezing) and on the other end by hyperdopaminergic excessive movement (dyskinesia). This highlights the need for precise L-Dopa dosing. The narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and OFF-time (return of PD symptoms).

== (Lack of) correlation between peripheral levodopa concentration and clinical symptoms ==

=== Pathophysiological complexity of the disease ===
Parkinson’s disease is a complex condition. Not only do the symptoms encompass a wide range of motor and non-motor complaints, the underlying pathophysiological mechanisms are also manifold. This limits dopaminergic therapy in important ways:

* Not all dopaminergic systems in the brain are affected at the same pace<ref>Rodríguez-Oroz, M. C.; Jahanshahi, M.; Krack, P.; Litvan, I.; Macías, R.; Bezard, E.; Obeso, J. A. Initial Clinical Manifestations of Parkinson’s Disease: Features and Pathophysiological Mechanisms. ''The Lancet Neurology'' '''2009''', ''8'' (12), 1128–1139. <nowiki>https://doi.org/10.1016/S1474-4422(09)70293-5</nowiki>.</ref> , with as a consequence that a certain levodopa dose can result in an eudopaminergic state in the motor circuit but a hyperdopaminergic state in the limbic circuit. The clinical correlate of this is a patient whose motor symptoms may be well-controlled, but cumbersome psychotic symptoms occur as an unintended effect.

* During the disease course, in addition to the dopaminergic deficit, deficits of other neurotransmitter systems<ref>Devos, D.; Defebvre, L.; Bordet, R. Dopaminergic and Non-Dopaminergic Pharmacological Hypotheses for Gait Disorders in Parkinson’s Disease. ''Fundamental & Clinical Pharmacology'' '''2010''', ''24'' (4), 407–421. <nowiki>https://doi.org/10.1111/j.1472-8206.2009.00798.x</nowiki>.</ref> (such as cholinergic, (nor)adrenergic and serotonergic systems) develop in addition to neurotransmitter-independent neurodegeneration. Several debilitating symptoms, such as autonomic and cognitive symptoms and disturbed balance/falling, are therefore levodopa-resistant.

* Varying extents of postsynaptic dopaminergic degeneration occur. This means that not only is the amount of (pre-synaptic) dopamine limited, but there is also damage to the (postsynaptic) neurons that have to make use of the dopamine. Even an optimal concentration of dopamine in the synaptic cleft may therefore be unable to adequately attenuate hypodopaminergic symptoms.

=== Correlation between peripheral levodopa concentration and dopamine available to cerebral neurons ===

A multitude of steps stands between (oral) levodopa intake and concentration of dopamine available to dopaminergic neurons in the brain.

1. After oral ingestion of levodopa, the tablets have to dissolve in the stomach, and then be transported to the small intestine for absorption. Many people with Parkinson’s disease have a lower acidity<ref>Fasano, A.; Visanji, N. P.; Liu, L. W. C.; Lang, A. E.; Pfeiffer, R. F. Gastrointestinal Dysfunction in Parkinson’s Disease. ''The Lancet Neurology'' '''2015''', ''14'' (6), 625–639. <nowiki>https://doi.org/10.1016/S1474-4422(15)00007-1</nowiki>.</ref> of the stomach (e.g. due to medication use or ''Helicobacter pylori'' infection), and an estimated 70-100% of people with Parkinson’s disease have gastroparesis<ref>Marrinan, S.; Emmanuel, A. V.; Burn, D. J. Delayed Gastric Emptying in Parkinson’s Disease. ''Movement Disorders'' '''2014''', ''29'' (1), 23–32. <nowiki>https://doi.org/10.1002/mds.25708</nowiki>.</ref> (delayed stomach emptying). This leads to impaired dissolution of levodopa and delayed transfer of levodopa to the small intestine, respectively.

2. Once levodopa has arrived in the duodenum and proximal jejunum for absorption, it has to be actively transported over the bowel mucosa by saturable LNAA transporters (transporters for Large Neutral Amino Acids). These transporters are both sensitive to pH, working optimally at a pH between 6.2 – 7.4,<ref>Pedrosa de Menezes, A. L.; Bloem, B. R.; Beckers, M.; Piat, C.; Benarroch, E. E.; Savica, R. Molecular Variability in Levodopa Absorption and Clinical Implications for the Management of Parkinson’s Disease. ''Journal of Parkinson’s Disease'' '''2024''', ''14'' (7), 1353–1368. <nowiki>https://doi.org/10.3233/JPD-240036</nowiki></ref> as well as sensitive to so-named ''protein competition''. That is: after a protein-rich meal, dietary proteins/amino acids compete with levodopa<ref>Contin, M.; Martinelli, P. Pharmacokinetics of Levodopa. ''Journal of Neurology'' '''2010''', ''257'' (Suppl. 2), S253–S261. <nowiki>https://doi.org/10.1007/s00415-010-5728-8</nowiki>.</ref> for the transport system, reducing levodopa’s bioavailability. As noted, stomach acidity is often less pronounced in people with Parkinson’s disease, and due to delayed gastric emptying, dietary proteins may be present in the small intestine for extended periods after a meal. This limits predictability of the speed and magnitude with which levodopa is absorbed into the bloodstream.

3. Both in the intestinal lumen as well as in peripheral blood, levodopa is metabolized to dopamine and other metabolites. This is unwanted, as dopamine cannot cross the blood-brain barrier; for pharmacotherapeutic effect, levodopa has to cross the blood-brain barrier in unmetabolized form. While peripheral decarboxylase inhibitors are able to partially inhibit peripheral decarboxylation, there is interindividual variability in the activity of levodopa-metabolizing enzymes such as AADC and catechol-O-aminotransferase (COMT). Part of this variability is genetic<ref>Devos D, Lejeune S, Cormier-Dequaire F, Tahiri K, Charbonnier-Beaupel F, Rouaix N, Duhamel A, Sablonniere B, Bonnet AM, Bonnet C, et al. Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in parkinson’s disease. PARKINSONISM RELAT D. 2014;20(2):170–5.</ref><ref>Sampaio TF, Dos SE, de Lima G, Dos AR, Da SR, Asano A, Asano N, Crovella S, de Souza P. MAO-B and COMT genetic variations associated with Levodopa treatment response in patients with parkinson’s disease. J CLIN PHARMACOL. 2018;58(7):920–6.</ref> , but AADC activity also varies by sex, disease duration and use of dopaminergic medication. The amount of levodopa that succeeds in being absorbed into the blood stream is therefore not necessarily the amount available at the blood-brain barrier.

4. At the blood-brain barrier, protein competition exists as well. <ref>Lees, A. J. The On-Off Phenomenon. ''Journal of Neurology, Neurosurgery & Psychiatry'' '''1989''', ''52'' (Suppl. 29), 29–37. <nowiki>https://doi.org/10.1136/jnnp.52.Suppl.29</nowiki>.</ref> Speed and magnitude of levodopa transport over the blood-brain barrier thus depends on the concentration of (dietary) amino acids in the blood.

5. Once in the brain, enzymes such as AADC and COMT metabolize levodopa and, together with monoamine oxidase B (MAO-B), determine the resultant amount of dopamine available in the synaptic cleft of dopaminergic neurons. Here, as well, there is interindividual variation in the activities of the enzymes.

Given these multiple steps of which the magnitude is – to a large extent – poorly predictable, levodopa plasma concentration correlates poorly with dopamine available to neurons in the brain.

Furthermore, the above-described steps apply in individuals without other levodopa-bioavailability-altering conditions. There are multiple circumstances <ref>Beckers, M.; Bloem, B. R.; Verbeek, M. M. Mechanisms of Peripheral Levodopa Resistance in Parkinson’s Disease. ''npj Parkinson’s Disease'' '''2022''', ''8'' (1), 56. <nowiki>https://doi.org/10.1038/s41531-022-00321-y</nowiki>.</ref> under which the correlation between intake of levodopa and clinical effect becomes even more unpredictable, e.g. if intestinal absorption is limited (e.g. due to inflammation of the intestinal lining), if levodopa is prematurely metabolized by gut bacteria, or if there is an overactivity of the AADC enzyme.

=== Correlation between bioavailable dopamine and control of PD symptoms ===
As described earlier in the text, not all PD symptoms respond to levodopa to the same extent. In addition to the presence of non-dopaminergic symptoms (which by definition do not respond to levodopa), even symptoms who are normally levodopa-responsive can display variation in their response. Factors inducing these variation include, amongst others, (psychological and physical) stress<ref>Helmich, R. C. G. and the Systems Neurology Group at the Donders Centre for Cognitive Neuroimaging investigate the cerebral mechanisms of movement disorders, especially the pathophysiology of Parkinson’s disease tremor and compensatory network changes, using neuroimaging and neurophysiological methods. Recent works include neuroimaging studies on tremor circuitry, longitudinal brain compensation in Parkinson’s progression, and clinical trials examining propranolol’s effects on tremor.</ref> as well as energy level/fatigue.

For the reasons outlined in the above paragraphs, peripheral levodopa pharmacokinetics correlate poorly to the actual level of symptom control. A 2013 study <ref>Fasano, A.; Bove, F.; Gabrielli, M.; Petracca, M.; Zocco, M. A.; Ragazzoni, E.; Barbaro, F.; Piano, C.; Fortuna, S.; Tortora, A.; Di Giacopo, R.; Campanale, M.; Gigante, G.; Lauritano, E. C.; Navarra, P.; Marconi, S.; Gasbarrini, A.; Bentivoglio, A. R. The Role of Small Intestinal Bacterial Overgrowth in Parkinson’s Disease. ''Movement Disorders'' '''2013''', ''28'' (9), 1241–1249. <nowiki>https://doi.org/10.1002/mds.25522</nowiki></ref> demonstrated that the presence of motor fluctuations did not correlate to pharmacokinetic data.

Thus, peripheral levodopa concentration monitoring would need to be integrated with measurement of clinical symptoms to be useful in clinical practice.
== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure L-Dopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce levodopa concentration fluctuations in blood. Continuous sensing would also help researchers better understand the pharmacokinetics of levodopa across different compartments, including intestinal lumen, blood, ISF and cerebrospinal fluid (CSF). This could improve treatment strategies for mid-stage PD patients and also help make a step towards a long-term vision of closed-loop levodopa therapy for PD with integrated levodopa concentration & motor symptom sensing in which continuous (e.g. subcutaneous, intestinal) levodopa administration is automatically adjusted based upon the sensor data, potentially reducing fluctuations and improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2025-12-26T21:10:48Z

Techsensus: /* History and Current Situation 0f Parkinson's Disease */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder associated with the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficit in the basal ganglia, which are critical for initiating and smoothing movement. The predominant motor symptoms advancing from this deficit include tremor, muscle stiffness (rigidity) and slowness of movement (bradykinesia)<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed into the bloodstream in the gastrointestinal tract, chiefly in the duodenum and proximal jejunum, and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, an adequate concentration of levodopa in the blood serum is a prerequisite for the exertion of the therapeutic effect. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes unpredictable, manifesting as unpredictable motor fluctuations and dyskinesias. Therefore, monitoring levodopa levels serves as a crucial tool for optimizing dosing regimens. There is no cure for Parkinson's disease, although medications and non-drug therapies can alleviate the symptoms and improve daily functioning. In advanced cases, patients may undergo surgical treatments such as deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation 0f Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficit as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor) and non-motor features (such as loss of smell, sleep disorder, gastrointestinal issues, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is clinical, based on a neurological examination and response to dopaminergic therapy, with imaging (such as MRI and DaT-SPECT) mainly used to exclude other causes of the symptoms; research into biomarkers is ongoing and their application is not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although during the course of the disease levodopa-resistant symptoms develop, and advanced therapies (such as deep brain stimulation, DBS) are an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis. An example of a wearable device accessible to patients is the Apple Watch with app StrivePD of Runelabs, which allows for the continuous monitoring of dyskinesia and the specific PD tremor frequency in the affected arm on the iPhone.<ref>Powers, R.; Etezadi-Amoli, M.; Arnold, E. M.; Kianian, S.; Mance, I.; Gibiansky, M.; Trietsch, D.; Alvarado, A. S.; Kretlow, J. D.; Herrington, T. M.; Brillman, S.; Huang, N.; Lin, P. T.; Pham, H. A.; Ullal, A. V. Smartwatch Inertial Sensors Continuously Monitor Real-World Motor Fluctuations in Parkinson’s Disease. ''Science Translational Medicine'' '''2021''', ''13'' (579), eabd7865. <nowiki>https://doi.org/10.1126/scitranslmed.abd7865</nowiki></ref><ref>'''StrivePD''': A digital health platform and companion tool empowering people with Parkinson’s disease by providing real-time symptom tracking, personalized health insights, medication adherence support, and access to clinical trials through wearable data and mobile applications. Available at <nowiki>https://www.strive.group/</nowiki></ref> Under development is the smartwatch app PD-Trends which in addition to tremor duration and intensity also monitors aspects of movement and sleep and shows longer trends in symptom development. It also aims to support Android platforms once the Apple app is online. <ref>'''PD-Trends.com'''. ''Parkinson’s Disease Trends and Data''. <nowiki>https://www.pd-trends.com</nowiki></ref>

Broadly speaking, Parkinson’s disease can be subdivided into three disease stages:

<nowiki>*</nowiki> The early stage, in which response to dopaminergic medication is generally stable throughout the day, without response fluctuations. The person with PD can (under drug treatment) function normally in many respects.

<nowiki>*</nowiki> Mid-stage, in which motor fluctuations (the ON-OFF phenomenon) come to the fore. Even with extensive dosing schedules (levodopa doses up to eight or more times a day), some people may still not reach a stable response, limiting daily functioning. In this phase, advanced therapies such as continuous dopaminergic drug administration via subcutaneous/intestinal pump and/or deep brain stimulation surgery may be considered.

<nowiki>*</nowiki> Late-stage disease. Dopamine-resistant and non-motor symptoms such as postural instability/falling, cognitive deficits/dementia, hallucinations and autonomic failure dominate the clinical picture. The patient’s functioning is determined mostly by these symptoms, which are poorly susceptible to drug treatment.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction in 1961. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with a peripheral decarboxylase inhibitor (PDI), such as carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in the therapeutic window of levodopa becoming narrower and the development of the ‘ON-OFF phenomenon’<ref name=":0" />which is characterized on one end of the spectrum by hypodopaminergic symptoms (stiffness, slowness, feezing) and on the other end by hyperdopaminergic excessive movement (dyskinesia). This highlights the need for precise L-Dopa dosing. The narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and OFF-time (return of PD symptoms).

== (Lack of) correlation between peripheral levodopa concentration and clinical symptoms ==

=== Pathophysiological complexity of the disease ===
Parkinson’s disease is a complex condition. Not only do the symptoms encompass a wide range of motor and non-motor complaints, the underlying pathophysiological mechanisms are also manifold. This limits dopaminergic therapy in important ways:

* Not all dopaminergic systems in the brain are affected at the same pace<ref>Rodríguez-Oroz, M. C.; Jahanshahi, M.; Krack, P.; Litvan, I.; Macías, R.; Bezard, E.; Obeso, J. A. Initial Clinical Manifestations of Parkinson’s Disease: Features and Pathophysiological Mechanisms. ''The Lancet Neurology'' '''2009''', ''8'' (12), 1128–1139. <nowiki>https://doi.org/10.1016/S1474-4422(09)70293-5</nowiki>.</ref> , with as a consequence that a certain levodopa dose can result in an eudopaminergic state in the motor circuit but a hyperdopaminergic state in the limbic circuit. The clinical correlate of this is a patient whose motor symptoms may be well-controlled, but cumbersome psychotic symptoms occur as an unintended effect.

* During the disease course, in addition to the dopaminergic deficit, deficits of other neurotransmitter systems<ref>Devos, D.; Defebvre, L.; Bordet, R. Dopaminergic and Non-Dopaminergic Pharmacological Hypotheses for Gait Disorders in Parkinson’s Disease. ''Fundamental & Clinical Pharmacology'' '''2010''', ''24'' (4), 407–421. <nowiki>https://doi.org/10.1111/j.1472-8206.2009.00798.x</nowiki>.</ref> (such as cholinergic, (nor)adrenergic and serotonergic systems) develop in addition to neurotransmitter-independent neurodegeneration. Several debilitating symptoms, such as autonomic and cognitive symptoms and disturbed balance/falling, are therefore levodopa-resistant.

* Varying extents of postsynaptic dopaminergic degeneration occur. This means that not only is the amount of (pre-synaptic) dopamine limited, but there is also damage to the (postsynaptic) neurons that have to make use of the dopamine. Even an optimal concentration of dopamine in the synaptic cleft may therefore be unable to adequately attenuate hypodopaminergic symptoms.

=== Correlation between peripheral levodopa concentration and dopamine available to cerebral neurons ===

A multitude of steps stands between (oral) levodopa intake and concentration of dopamine available to dopaminergic neurons in the brain.

1. After oral ingestion of levodopa, the tablets have to dissolve in the stomach, and then be transported to the small intestine for absorption. Many people with Parkinson’s disease have a lower acidity<ref>Fasano, A.; Visanji, N. P.; Liu, L. W. C.; Lang, A. E.; Pfeiffer, R. F. Gastrointestinal Dysfunction in Parkinson’s Disease. ''The Lancet Neurology'' '''2015''', ''14'' (6), 625–639. <nowiki>https://doi.org/10.1016/S1474-4422(15)00007-1</nowiki>.</ref> of the stomach (e.g. due to medication use or ''Helicobacter pylori'' infection), and an estimated 70-100% of people with Parkinson’s disease have gastroparesis<ref>Marrinan, S.; Emmanuel, A. V.; Burn, D. J. Delayed Gastric Emptying in Parkinson’s Disease. ''Movement Disorders'' '''2014''', ''29'' (1), 23–32. <nowiki>https://doi.org/10.1002/mds.25708</nowiki>.</ref> (delayed stomach emptying). This leads to impaired dissolution of levodopa and delayed transfer of levodopa to the small intestine, respectively.

2. Once levodopa has arrived in the duodenum and proximal jejunum for absorption, it has to be actively transported over the bowel mucosa by saturable LNAA transporters (transporters for Large Neutral Amino Acids). These transporters are both sensitive to pH, working optimally at a pH between 6.2 – 7.4,<ref>Pedrosa de Menezes, A. L.; Bloem, B. R.; Beckers, M.; Piat, C.; Benarroch, E. E.; Savica, R. Molecular Variability in Levodopa Absorption and Clinical Implications for the Management of Parkinson’s Disease. ''Journal of Parkinson’s Disease'' '''2024''', ''14'' (7), 1353–1368. <nowiki>https://doi.org/10.3233/JPD-240036</nowiki></ref> as well as sensitive to so-named ''protein competition''. That is: after a protein-rich meal, dietary proteins/amino acids compete with levodopa<ref>Contin, M.; Martinelli, P. Pharmacokinetics of Levodopa. ''Journal of Neurology'' '''2010''', ''257'' (Suppl. 2), S253–S261. <nowiki>https://doi.org/10.1007/s00415-010-5728-8</nowiki>.</ref> for the transport system, reducing levodopa’s bioavailability. As noted, stomach acidity is often less pronounced in people with Parkinson’s disease, and due to delayed gastric emptying, dietary proteins may be present in the small intestine for extended periods after a meal. This limits predictability of the speed and magnitude with which levodopa is absorbed into the bloodstream.

3. Both in the intestinal lumen as well as in peripheral blood, levodopa is metabolized to dopamine and other metabolites. This is unwanted, as dopamine cannot cross the blood-brain barrier; for pharmacotherapeutic effect, levodopa has to cross the blood-brain barrier in unmetabolized form. While peripheral decarboxylase inhibitors are able to partially inhibit peripheral decarboxylation, there is interindividual variability in the activity of levodopa-metabolizing enzymes such as AADC and catechol-O-aminotransferase (COMT). Part of this variability is genetic<ref>Devos D, Lejeune S, Cormier-Dequaire F, Tahiri K, Charbonnier-Beaupel F, Rouaix N, Duhamel A, Sablonniere B, Bonnet AM, Bonnet C, et al. Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in parkinson’s disease. PARKINSONISM RELAT D. 2014;20(2):170–5.</ref><ref>Sampaio TF, Dos SE, de Lima G, Dos AR, Da SR, Asano A, Asano N, Crovella S, de Souza P. MAO-B and COMT genetic variations associated with Levodopa treatment response in patients with parkinson’s disease. J CLIN PHARMACOL. 2018;58(7):920–6.</ref> , but AADC activity also varies by sex, disease duration and use of dopaminergic medication. The amount of levodopa that succeeds in being absorbed into the blood stream is therefore not necessarily the amount available at the blood-brain barrier.

4. At the blood-brain barrier, protein competition exists as well. <ref>Lees, A. J. The On-Off Phenomenon. ''Journal of Neurology, Neurosurgery & Psychiatry'' '''1989''', ''52'' (Suppl. 29), 29–37. <nowiki>https://doi.org/10.1136/jnnp.52.Suppl.29</nowiki>.</ref> Speed and magnitude of levodopa transport over the blood-brain barrier thus depends on the concentration of (dietary) amino acids in the blood.

5. Once in the brain, enzymes such as AADC and COMT metabolize levodopa and, together with monoamine oxidase B (MAO-B), determine the resultant amount of dopamine available in the synaptic cleft of dopaminergic neurons. Here, as well, there is interindividual variation in the activities of the enzymes.

Given these multiple steps of which the magnitude is – to a large extent – poorly predictable, levodopa plasma concentration correlates poorly with dopamine available to neurons in the brain.

Furthermore, the above-described steps apply in individuals without other levodopa-bioavailability-altering conditions. There are multiple circumstances <ref>Beckers, M.; Bloem, B. R.; Verbeek, M. M. Mechanisms of Peripheral Levodopa Resistance in Parkinson’s Disease. ''npj Parkinson’s Disease'' '''2022''', ''8'' (1), 56. <nowiki>https://doi.org/10.1038/s41531-022-00321-y</nowiki>.</ref> under which the correlation between intake of levodopa and clinical effect becomes even more unpredictable, e.g. if intestinal absorption is limited (e.g. due to inflammation of the intestinal lining), if levodopa is prematurely metabolized by gut bacteria, or if there is an overactivity of the AADC enzyme.

=== Correlation between bioavailable dopamine and control of PD symptoms ===
As described earlier in the text, not all PD symptoms respond to levodopa to the same extent. In addition to the presence of non-dopaminergic symptoms (which by definition do not respond to levodopa), even symptoms who are normally levodopa-responsive can display variation in their response. Factors inducing these variation include, amongst others, (psychological and physical) stress<ref>Helmich, R. C. G. and the Systems Neurology Group at the Donders Centre for Cognitive Neuroimaging investigate the cerebral mechanisms of movement disorders, especially the pathophysiology of Parkinson’s disease tremor and compensatory network changes, using neuroimaging and neurophysiological methods. Recent works include neuroimaging studies on tremor circuitry, longitudinal brain compensation in Parkinson’s progression, and clinical trials examining propranolol’s effects on tremor.</ref> as well as energy level/fatigue.

For the reasons outlined in the above paragraphs, peripheral levodopa pharmacokinetics correlate poorly to the actual level of symptom control. A 2013 study <ref>Fasano, A.; Bove, F.; Gabrielli, M.; Petracca, M.; Zocco, M. A.; Ragazzoni, E.; Barbaro, F.; Piano, C.; Fortuna, S.; Tortora, A.; Di Giacopo, R.; Campanale, M.; Gigante, G.; Lauritano, E. C.; Navarra, P.; Marconi, S.; Gasbarrini, A.; Bentivoglio, A. R. The Role of Small Intestinal Bacterial Overgrowth in Parkinson’s Disease. ''Movement Disorders'' '''2013''', ''28'' (9), 1241–1249. <nowiki>https://doi.org/10.1002/mds.25522</nowiki></ref> demonstrated that the presence of motor fluctuations did not correlate to pharmacokinetic data.

Thus, peripheral levodopa concentration monitoring would need to be integrated with measurement of clinical symptoms to be useful in clinical practice.
== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure L-Dopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce levodopa concentration fluctuations in blood. Continuous sensing would also help researchers better understand the pharmacokinetics of levodopa across different compartments, including intestinal lumen, blood, ISF and cerebrospinal fluid (CSF). This could improve treatment strategies for mid-stage PD patients and also help make a step towards a long-term vision of closed-loop levodopa therapy for PD with integrated levodopa concentration & motor symptom sensing in which continuous (e.g. subcutaneous, intestinal) levodopa administration is automatically adjusted based upon the sensor data, potentially reducing fluctuations and improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2025-12-24T21:42:21Z

Techsensus: /* Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder associated with the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficit in the basal ganglia, which are critical for initiating and smoothing movement. The predominant motor symptoms advancing from this deficit include tremor, muscle stiffness (rigidity) and slowness of movement (bradykinesia)<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed into the bloodstream in the gastrointestinal tract, chiefly in the duodenum and proximal jejunum, and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, an adequate concentration of levodopa in the blood serum is a prerequisite for the exertion of the therapeutic effect. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes unpredictable, manifesting as unpredictable motor fluctuations and dyskinesias. Therefore, monitoring levodopa levels serves as a crucial tool for optimizing dosing regimens. There is no cure for Parkinson's disease, although medications and non-drug therapies can alleviate the symptoms and improve daily functioning. In advanced cases, patients may undergo surgical treatments such as deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation 0f Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficit as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor) and non-motor features (such as loss of smell, sleep disorder, gastrointestinal issues, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is clinical, based on a neurological examination and response to dopaminergic therapy, with imaging (such as MRI and DaT-SPECT) mainly used to exclude other causes of the symptoms; research into biomarkers is ongoing and their application is not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although during the course of the disease levodopa-resistant symptoms develop, and advanced therapies (such as deep brain stimulation, DBS) are an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis.

Broadly speaking, Parkinson’s disease can be subdivided into three disease stages:

<nowiki>*</nowiki> The early stage, in which response to dopaminergic medication is generally stable throughout the day, without response fluctuations. The person with PD can (under drug treatment) function normally in many respects.

<nowiki>*</nowiki> Mid-stage, in which motor fluctuations (the ON-OFF phenomenon) come to the fore. Even with extensive dosing schedules (levodopa doses up to eight or more times a day), some people may still not reach a stable response, limiting daily functioning. In this phase, advanced therapies such as continuous dopaminergic drug administration via subcutaneous/intestinal pump and/or deep brain stimulation surgery may be considered.

<nowiki>*</nowiki> Late-stage disease. Dopamine-resistant and non-motor symptoms such as postural instability/falling, cognitive deficits/dementia, hallucinations and autonomic failure dominate the clinical picture. The patient’s functioning is determined mostly by these symptoms, which are poorly susceptible to drug treatment.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction in 1961. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with a peripheral decarboxylase inhibitor (PDI), such as carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in the therapeutic window of levodopa becoming narrower and the development of the ‘ON-OFF phenomenon’<ref name=":0" />which is characterized on one end of the spectrum by hypodopaminergic symptoms (stiffness, slowness, feezing) and on the other end by hyperdopaminergic excessive movement (dyskinesia). This highlights the need for precise L-Dopa dosing. The narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and OFF-time (return of PD symptoms).

== (Lack of) correlation between peripheral levodopa concentration and clinical symptoms ==

=== Pathophysiological complexity of the disease ===
Parkinson’s disease is a complex condition. Not only do the symptoms encompass a wide range of motor and non-motor complaints, the underlying pathophysiological mechanisms are also manifold. This limits dopaminergic therapy in important ways:

* Not all dopaminergic systems in the brain are affected at the same pace<ref>Rodríguez-Oroz, M. C.; Jahanshahi, M.; Krack, P.; Litvan, I.; Macías, R.; Bezard, E.; Obeso, J. A. Initial Clinical Manifestations of Parkinson’s Disease: Features and Pathophysiological Mechanisms. ''The Lancet Neurology'' '''2009''', ''8'' (12), 1128–1139. <nowiki>https://doi.org/10.1016/S1474-4422(09)70293-5</nowiki>.</ref> , with as a consequence that a certain levodopa dose can result in an eudopaminergic state in the motor circuit but a hyperdopaminergic state in the limbic circuit. The clinical correlate of this is a patient whose motor symptoms may be well-controlled, but cumbersome psychotic symptoms occur as an unintended effect.

* During the disease course, in addition to the dopaminergic deficit, deficits of other neurotransmitter systems<ref>Devos, D.; Defebvre, L.; Bordet, R. Dopaminergic and Non-Dopaminergic Pharmacological Hypotheses for Gait Disorders in Parkinson’s Disease. ''Fundamental & Clinical Pharmacology'' '''2010''', ''24'' (4), 407–421. <nowiki>https://doi.org/10.1111/j.1472-8206.2009.00798.x</nowiki>.</ref> (such as cholinergic, (nor)adrenergic and serotonergic systems) develop in addition to neurotransmitter-independent neurodegeneration. Several debilitating symptoms, such as autonomic and cognitive symptoms and disturbed balance/falling, are therefore levodopa-resistant.

* Varying extents of postsynaptic dopaminergic degeneration occur. This means that not only is the amount of (pre-synaptic) dopamine limited, but there is also damage to the (postsynaptic) neurons that have to make use of the dopamine. Even an optimal concentration of dopamine in the synaptic cleft may therefore be unable to adequately attenuate hypodopaminergic symptoms.

=== Correlation between peripheral levodopa concentration and dopamine available to cerebral neurons ===

A multitude of steps stands between (oral) levodopa intake and concentration of dopamine available to dopaminergic neurons in the brain.

1. After oral ingestion of levodopa, the tablets have to dissolve in the stomach, and then be transported to the small intestine for absorption. Many people with Parkinson’s disease have a lower acidity<ref>Fasano, A.; Visanji, N. P.; Liu, L. W. C.; Lang, A. E.; Pfeiffer, R. F. Gastrointestinal Dysfunction in Parkinson’s Disease. ''The Lancet Neurology'' '''2015''', ''14'' (6), 625–639. <nowiki>https://doi.org/10.1016/S1474-4422(15)00007-1</nowiki>.</ref> of the stomach (e.g. due to medication use or ''Helicobacter pylori'' infection), and an estimated 70-100% of people with Parkinson’s disease have gastroparesis<ref>Marrinan, S.; Emmanuel, A. V.; Burn, D. J. Delayed Gastric Emptying in Parkinson’s Disease. ''Movement Disorders'' '''2014''', ''29'' (1), 23–32. <nowiki>https://doi.org/10.1002/mds.25708</nowiki>.</ref> (delayed stomach emptying). This leads to impaired dissolution of levodopa and delayed transfer of levodopa to the small intestine, respectively.

2. Once levodopa has arrived in the duodenum and proximal jejunum for absorption, it has to be actively transported over the bowel mucosa by saturable LNAA transporters (transporters for Large Neutral Amino Acids). These transporters are both sensitive to pH, working optimally at a pH between 6.2 – 7.4,<ref>Pedrosa de Menezes, A. L.; Bloem, B. R.; Beckers, M.; Piat, C.; Benarroch, E. E.; Savica, R. Molecular Variability in Levodopa Absorption and Clinical Implications for the Management of Parkinson’s Disease. ''Journal of Parkinson’s Disease'' '''2024''', ''14'' (7), 1353–1368. <nowiki>https://doi.org/10.3233/JPD-240036</nowiki></ref> as well as sensitive to so-named ''protein competition''. That is: after a protein-rich meal, dietary proteins/amino acids compete with levodopa<ref>Contin, M.; Martinelli, P. Pharmacokinetics of Levodopa. ''Journal of Neurology'' '''2010''', ''257'' (Suppl. 2), S253–S261. <nowiki>https://doi.org/10.1007/s00415-010-5728-8</nowiki>.</ref> for the transport system, reducing levodopa’s bioavailability. As noted, stomach acidity is often less pronounced in people with Parkinson’s disease, and due to delayed gastric emptying, dietary proteins may be present in the small intestine for extended periods after a meal. This limits predictability of the speed and magnitude with which levodopa is absorbed into the bloodstream.

3. Both in the intestinal lumen as well as in peripheral blood, levodopa is metabolized to dopamine and other metabolites. This is unwanted, as dopamine cannot cross the blood-brain barrier; for pharmacotherapeutic effect, levodopa has to cross the blood-brain barrier in unmetabolized form. While peripheral decarboxylase inhibitors are able to partially inhibit peripheral decarboxylation, there is interindividual variability in the activity of levodopa-metabolizing enzymes such as AADC and catechol-O-aminotransferase (COMT). Part of this variability is genetic<ref>Devos D, Lejeune S, Cormier-Dequaire F, Tahiri K, Charbonnier-Beaupel F, Rouaix N, Duhamel A, Sablonniere B, Bonnet AM, Bonnet C, et al. Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in parkinson’s disease. PARKINSONISM RELAT D. 2014;20(2):170–5.</ref><ref>Sampaio TF, Dos SE, de Lima G, Dos AR, Da SR, Asano A, Asano N, Crovella S, de Souza P. MAO-B and COMT genetic variations associated with Levodopa treatment response in patients with parkinson’s disease. J CLIN PHARMACOL. 2018;58(7):920–6.</ref> , but AADC activity also varies by sex, disease duration and use of dopaminergic medication. The amount of levodopa that succeeds in being absorbed into the blood stream is therefore not necessarily the amount available at the blood-brain barrier.

4. At the blood-brain barrier, protein competition exists as well. <ref>Lees, A. J. The On-Off Phenomenon. ''Journal of Neurology, Neurosurgery & Psychiatry'' '''1989''', ''52'' (Suppl. 29), 29–37. <nowiki>https://doi.org/10.1136/jnnp.52.Suppl.29</nowiki>.</ref> Speed and magnitude of levodopa transport over the blood-brain barrier thus depends on the concentration of (dietary) amino acids in the blood.

5. Once in the brain, enzymes such as AADC and COMT metabolize levodopa and, together with monoamine oxidase B (MAO-B), determine the resultant amount of dopamine available in the synaptic cleft of dopaminergic neurons. Here, as well, there is interindividual variation in the activities of the enzymes.

Given these multiple steps of which the magnitude is – to a large extent – poorly predictable, levodopa plasma concentration correlates poorly with dopamine available to neurons in the brain.

Furthermore, the above-described steps apply in individuals without other levodopa-bioavailability-altering conditions. There are multiple circumstances <ref>Beckers, M.; Bloem, B. R.; Verbeek, M. M. Mechanisms of Peripheral Levodopa Resistance in Parkinson’s Disease. ''npj Parkinson’s Disease'' '''2022''', ''8'' (1), 56. <nowiki>https://doi.org/10.1038/s41531-022-00321-y</nowiki>.</ref> under which the correlation between intake of levodopa and clinical effect becomes even more unpredictable, e.g. if intestinal absorption is limited (e.g. due to inflammation of the intestinal lining), if levodopa is prematurely metabolized by gut bacteria, or if there is an overactivity of the AADC enzyme.

=== Correlation between bioavailable dopamine and control of PD symptoms ===
As described earlier in the text, not all PD symptoms respond to levodopa to the same extent. In addition to the presence of non-dopaminergic symptoms (which by definition do not respond to levodopa), even symptoms who are normally levodopa-responsive can display variation in their response. Factors inducing these variation include, amongst others, (psychological and physical) stress<ref>Helmich, R. C. G. and the Systems Neurology Group at the Donders Centre for Cognitive Neuroimaging investigate the cerebral mechanisms of movement disorders, especially the pathophysiology of Parkinson’s disease tremor and compensatory network changes, using neuroimaging and neurophysiological methods. Recent works include neuroimaging studies on tremor circuitry, longitudinal brain compensation in Parkinson’s progression, and clinical trials examining propranolol’s effects on tremor.</ref> as well as energy level/fatigue.

For the reasons outlined in the above paragraphs, peripheral levodopa pharmacokinetics correlate poorly to the actual level of symptom control. A 2013 study <ref>Fasano, A.; Bove, F.; Gabrielli, M.; Petracca, M.; Zocco, M. A.; Ragazzoni, E.; Barbaro, F.; Piano, C.; Fortuna, S.; Tortora, A.; Di Giacopo, R.; Campanale, M.; Gigante, G.; Lauritano, E. C.; Navarra, P.; Marconi, S.; Gasbarrini, A.; Bentivoglio, A. R. The Role of Small Intestinal Bacterial Overgrowth in Parkinson’s Disease. ''Movement Disorders'' '''2013''', ''28'' (9), 1241–1249. <nowiki>https://doi.org/10.1002/mds.25522</nowiki></ref> demonstrated that the presence of motor fluctuations did not correlate to pharmacokinetic data.

Thus, peripheral levodopa concentration monitoring would need to be integrated with measurement of clinical symptoms to be useful in clinical practice.
== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure L-Dopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce levodopa concentration fluctuations in blood. Continuous sensing would also help researchers better understand the pharmacokinetics of levodopa across different compartments, including intestinal lumen, blood, ISF and cerebrospinal fluid (CSF). This could improve treatment strategies for mid-stage PD patients and also help make a step towards a long-term vision of closed-loop levodopa therapy for PD with integrated levodopa concentration & motor symptom sensing in which continuous (e.g. subcutaneous, intestinal) levodopa administration is automatically adjusted based upon the sensor data, potentially reducing fluctuations and improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2025-12-24T21:39:54Z

Techsensus: /* Levodopa Biosensors */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder associated with the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficit in the basal ganglia, which are critical for initiating and smoothing movement. The predominant motor symptoms advancing from this deficit include tremor, muscle stiffness (rigidity) and slowness of movement (bradykinesia)<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed into the bloodstream in the gastrointestinal tract, chiefly in the duodenum and proximal jejunum, and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, an adequate concentration of levodopa in the blood serum is a prerequisite for the exertion of the therapeutic effect. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes unpredictable, manifesting as unpredictable motor fluctuations and dyskinesias. Therefore, monitoring levodopa levels serves as a crucial tool for optimizing dosing regimens. There is no cure for Parkinson's disease, although medications and non-drug therapies can alleviate the symptoms and improve daily functioning. In advanced cases, patients may undergo surgical treatments such as deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation 0f Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficit as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor) and non-motor features (such as loss of smell, sleep disorder, gastrointestinal issues, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is clinical, based on a neurological examination and response to dopaminergic therapy, with imaging (such as MRI and DaT-SPECT) mainly used to exclude other causes of the symptoms; research into biomarkers is ongoing and their application is not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although during the course of the disease levodopa-resistant symptoms develop, and advanced therapies (such as deep brain stimulation, DBS) are an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis.

Broadly speaking, Parkinson’s disease can be subdivided into three disease stages:

<nowiki>*</nowiki> The early stage, in which response to dopaminergic medication is generally stable throughout the day, without response fluctuations. The person with PD can (under drug treatment) function normally in many respects.

<nowiki>*</nowiki> Mid-stage, in which motor fluctuations (the ON-OFF phenomenon) come to the fore. Even with extensive dosing schedules (levodopa doses up to eight or more times a day), some people may still not reach a stable response, limiting daily functioning. In this phase, advanced therapies such as continuous dopaminergic drug administration via subcutaneous/intestinal pump and/or deep brain stimulation surgery may be considered.

<nowiki>*</nowiki> Late-stage disease. Dopamine-resistant and non-motor symptoms such as postural instability/falling, cognitive deficits/dementia, hallucinations and autonomic failure dominate the clinical picture. The patient’s functioning is determined mostly by these symptoms, which are poorly susceptible to drug treatment.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction in 1961. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with a peripheral decarboxylase inhibitor (PDI), such as carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in the therapeutic window of levodopa becoming narrower and the development of the ‘ON-OFF phenomenon’<ref name=":0" />which is characterized on one end of the spectrum by hypodopaminergic symptoms (stiffness, slowness, feezing) and on the other end by hyperdopaminergic excessive movement (dyskinesia). This highlights the need for precise L-Dopa dosing. ''Figure 1'' illustrates how the narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and OFF-time (return of PD symptoms).

== (Lack of) correlation between peripheral levodopa concentration and clinical symptoms ==

=== Pathophysiological complexity of the disease ===
Parkinson’s disease is a complex condition. Not only do the symptoms encompass a wide range of motor and non-motor complaints, the underlying pathophysiological mechanisms are also manifold. This limits dopaminergic therapy in important ways:

* Not all dopaminergic systems in the brain are affected at the same pace<ref>Rodríguez-Oroz, M. C.; Jahanshahi, M.; Krack, P.; Litvan, I.; Macías, R.; Bezard, E.; Obeso, J. A. Initial Clinical Manifestations of Parkinson’s Disease: Features and Pathophysiological Mechanisms. ''The Lancet Neurology'' '''2009''', ''8'' (12), 1128–1139. <nowiki>https://doi.org/10.1016/S1474-4422(09)70293-5</nowiki>.</ref> , with as a consequence that a certain levodopa dose can result in an eudopaminergic state in the motor circuit but a hyperdopaminergic state in the limbic circuit. The clinical correlate of this is a patient whose motor symptoms may be well-controlled, but cumbersome psychotic symptoms occur as an unintended effect.

* During the disease course, in addition to the dopaminergic deficit, deficits of other neurotransmitter systems<ref>Devos, D.; Defebvre, L.; Bordet, R. Dopaminergic and Non-Dopaminergic Pharmacological Hypotheses for Gait Disorders in Parkinson’s Disease. ''Fundamental & Clinical Pharmacology'' '''2010''', ''24'' (4), 407–421. <nowiki>https://doi.org/10.1111/j.1472-8206.2009.00798.x</nowiki>.</ref> (such as cholinergic, (nor)adrenergic and serotonergic systems) develop in addition to neurotransmitter-independent neurodegeneration. Several debilitating symptoms, such as autonomic and cognitive symptoms and disturbed balance/falling, are therefore levodopa-resistant.

* Varying extents of postsynaptic dopaminergic degeneration occur. This means that not only is the amount of (pre-synaptic) dopamine limited, but there is also damage to the (postsynaptic) neurons that have to make use of the dopamine. Even an optimal concentration of dopamine in the synaptic cleft may therefore be unable to adequately attenuate hypodopaminergic symptoms.

=== Correlation between peripheral levodopa concentration and dopamine available to cerebral neurons ===

A multitude of steps stands between (oral) levodopa intake and concentration of dopamine available to dopaminergic neurons in the brain.

1. After oral ingestion of levodopa, the tablets have to dissolve in the stomach, and then be transported to the small intestine for absorption. Many people with Parkinson’s disease have a lower acidity<ref>Fasano, A.; Visanji, N. P.; Liu, L. W. C.; Lang, A. E.; Pfeiffer, R. F. Gastrointestinal Dysfunction in Parkinson’s Disease. ''The Lancet Neurology'' '''2015''', ''14'' (6), 625–639. <nowiki>https://doi.org/10.1016/S1474-4422(15)00007-1</nowiki>.</ref> of the stomach (e.g. due to medication use or ''Helicobacter pylori'' infection), and an estimated 70-100% of people with Parkinson’s disease have gastroparesis<ref>Marrinan, S.; Emmanuel, A. V.; Burn, D. J. Delayed Gastric Emptying in Parkinson’s Disease. ''Movement Disorders'' '''2014''', ''29'' (1), 23–32. <nowiki>https://doi.org/10.1002/mds.25708</nowiki>.</ref> (delayed stomach emptying). This leads to impaired dissolution of levodopa and delayed transfer of levodopa to the small intestine, respectively.

2. Once levodopa has arrived in the duodenum and proximal jejunum for absorption, it has to be actively transported over the bowel mucosa by saturable LNAA transporters (transporters for Large Neutral Amino Acids). These transporters are both sensitive to pH, working optimally at a pH between 6.2 – 7.4,<ref>Pedrosa de Menezes, A. L.; Bloem, B. R.; Beckers, M.; Piat, C.; Benarroch, E. E.; Savica, R. Molecular Variability in Levodopa Absorption and Clinical Implications for the Management of Parkinson’s Disease. ''Journal of Parkinson’s Disease'' '''2024''', ''14'' (7), 1353–1368. <nowiki>https://doi.org/10.3233/JPD-240036</nowiki></ref> as well as sensitive to so-named ''protein competition''. That is: after a protein-rich meal, dietary proteins/amino acids compete with levodopa<ref>Contin, M.; Martinelli, P. Pharmacokinetics of Levodopa. ''Journal of Neurology'' '''2010''', ''257'' (Suppl. 2), S253–S261. <nowiki>https://doi.org/10.1007/s00415-010-5728-8</nowiki>.</ref> for the transport system, reducing levodopa’s bioavailability. As noted, stomach acidity is often less pronounced in people with Parkinson’s disease, and due to delayed gastric emptying, dietary proteins may be present in the small intestine for extended periods after a meal. This limits predictability of the speed and magnitude with which levodopa is absorbed into the bloodstream.

3. Both in the intestinal lumen as well as in peripheral blood, levodopa is metabolized to dopamine and other metabolites. This is unwanted, as dopamine cannot cross the blood-brain barrier; for pharmacotherapeutic effect, levodopa has to cross the blood-brain barrier in unmetabolized form. While peripheral decarboxylase inhibitors are able to partially inhibit peripheral decarboxylation, there is interindividual variability in the activity of levodopa-metabolizing enzymes such as AADC and catechol-O-aminotransferase (COMT). Part of this variability is genetic<ref>Devos D, Lejeune S, Cormier-Dequaire F, Tahiri K, Charbonnier-Beaupel F, Rouaix N, Duhamel A, Sablonniere B, Bonnet AM, Bonnet C, et al. Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in parkinson’s disease. PARKINSONISM RELAT D. 2014;20(2):170–5.</ref><ref>Sampaio TF, Dos SE, de Lima G, Dos AR, Da SR, Asano A, Asano N, Crovella S, de Souza P. MAO-B and COMT genetic variations associated with Levodopa treatment response in patients with parkinson’s disease. J CLIN PHARMACOL. 2018;58(7):920–6.</ref> , but AADC activity also varies by sex, disease duration and use of dopaminergic medication. The amount of levodopa that succeeds in being absorbed into the blood stream is therefore not necessarily the amount available at the blood-brain barrier.

4. At the blood-brain barrier, protein competition exists as well. <ref>Lees, A. J. The On-Off Phenomenon. ''Journal of Neurology, Neurosurgery & Psychiatry'' '''1989''', ''52'' (Suppl. 29), 29–37. <nowiki>https://doi.org/10.1136/jnnp.52.Suppl.29</nowiki>.</ref> Speed and magnitude of levodopa transport over the blood-brain barrier thus depends on the concentration of (dietary) amino acids in the blood.

5. Once in the brain, enzymes such as AADC and COMT metabolize levodopa and, together with monoamine oxidase B (MAO-B), determine the resultant amount of dopamine available in the synaptic cleft of dopaminergic neurons. Here, as well, there is interindividual variation in the activities of the enzymes.

Given these multiple steps of which the magnitude is – to a large extent – poorly predictable, levodopa plasma concentration correlates poorly with dopamine available to neurons in the brain.

Furthermore, the above-described steps apply in individuals without other levodopa-bioavailability-altering conditions. There are multiple circumstances <ref>Beckers, M.; Bloem, B. R.; Verbeek, M. M. Mechanisms of Peripheral Levodopa Resistance in Parkinson’s Disease. ''npj Parkinson’s Disease'' '''2022''', ''8'' (1), 56. <nowiki>https://doi.org/10.1038/s41531-022-00321-y</nowiki>.</ref> under which the correlation between intake of levodopa and clinical effect becomes even more unpredictable, e.g. if intestinal absorption is limited (e.g. due to inflammation of the intestinal lining), if levodopa is prematurely metabolized by gut bacteria, or if there is an overactivity of the AADC enzyme.

=== Correlation between bioavailable dopamine and control of PD symptoms ===
As described earlier in the text, not all PD symptoms respond to levodopa to the same extent. In addition to the presence of non-dopaminergic symptoms (which by definition do not respond to levodopa), even symptoms who are normally levodopa-responsive can display variation in their response. Factors inducing these variation include, amongst others, (psychological and physical) stress<ref>Helmich, R. C. G. and the Systems Neurology Group at the Donders Centre for Cognitive Neuroimaging investigate the cerebral mechanisms of movement disorders, especially the pathophysiology of Parkinson’s disease tremor and compensatory network changes, using neuroimaging and neurophysiological methods. Recent works include neuroimaging studies on tremor circuitry, longitudinal brain compensation in Parkinson’s progression, and clinical trials examining propranolol’s effects on tremor.</ref> as well as energy level/fatigue.

For the reasons outlined in the above paragraphs, peripheral levodopa pharmacokinetics correlate poorly to the actual level of symptom control. A 2013 study <ref>Fasano, A.; Bove, F.; Gabrielli, M.; Petracca, M.; Zocco, M. A.; Ragazzoni, E.; Barbaro, F.; Piano, C.; Fortuna, S.; Tortora, A.; Di Giacopo, R.; Campanale, M.; Gigante, G.; Lauritano, E. C.; Navarra, P.; Marconi, S.; Gasbarrini, A.; Bentivoglio, A. R. The Role of Small Intestinal Bacterial Overgrowth in Parkinson’s Disease. ''Movement Disorders'' '''2013''', ''28'' (9), 1241–1249. <nowiki>https://doi.org/10.1002/mds.25522</nowiki></ref> demonstrated that the presence of motor fluctuations did not correlate to pharmacokinetic data.

Thus, peripheral levodopa concentration monitoring would need to be integrated with measurement of clinical symptoms to be useful in clinical practice.
== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure L-Dopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce levodopa concentration fluctuations in blood. Continuous sensing would also help researchers better understand the pharmacokinetics of levodopa across different compartments, including intestinal lumen, blood, ISF and cerebrospinal fluid (CSF). This could improve treatment strategies for mid-stage PD patients and also help make a step towards a long-term vision of closed-loop levodopa therapy for PD with integrated levodopa concentration & motor symptom sensing in which continuous (e.g. subcutaneous, intestinal) levodopa administration is automatically adjusted based upon the sensor data, potentially reducing fluctuations and improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2025-12-24T21:38:45Z

Techsensus: /* (Lack of) correlation between peripheral levodopa concentration and clinical symptoms */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder associated with the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficit in the basal ganglia, which are critical for initiating and smoothing movement. The predominant motor symptoms advancing from this deficit include tremor, muscle stiffness (rigidity) and slowness of movement (bradykinesia)<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed into the bloodstream in the gastrointestinal tract, chiefly in the duodenum and proximal jejunum, and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, an adequate concentration of levodopa in the blood serum is a prerequisite for the exertion of the therapeutic effect. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes unpredictable, manifesting as unpredictable motor fluctuations and dyskinesias. Therefore, monitoring levodopa levels serves as a crucial tool for optimizing dosing regimens. There is no cure for Parkinson's disease, although medications and non-drug therapies can alleviate the symptoms and improve daily functioning. In advanced cases, patients may undergo surgical treatments such as deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation 0f Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficit as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor) and non-motor features (such as loss of smell, sleep disorder, gastrointestinal issues, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is clinical, based on a neurological examination and response to dopaminergic therapy, with imaging (such as MRI and DaT-SPECT) mainly used to exclude other causes of the symptoms; research into biomarkers is ongoing and their application is not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although during the course of the disease levodopa-resistant symptoms develop, and advanced therapies (such as deep brain stimulation, DBS) are an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis.

Broadly speaking, Parkinson’s disease can be subdivided into three disease stages:

<nowiki>*</nowiki> The early stage, in which response to dopaminergic medication is generally stable throughout the day, without response fluctuations. The person with PD can (under drug treatment) function normally in many respects.

<nowiki>*</nowiki> Mid-stage, in which motor fluctuations (the ON-OFF phenomenon) come to the fore. Even with extensive dosing schedules (levodopa doses up to eight or more times a day), some people may still not reach a stable response, limiting daily functioning. In this phase, advanced therapies such as continuous dopaminergic drug administration via subcutaneous/intestinal pump and/or deep brain stimulation surgery may be considered.

<nowiki>*</nowiki> Late-stage disease. Dopamine-resistant and non-motor symptoms such as postural instability/falling, cognitive deficits/dementia, hallucinations and autonomic failure dominate the clinical picture. The patient’s functioning is determined mostly by these symptoms, which are poorly susceptible to drug treatment.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction in 1961. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with a peripheral decarboxylase inhibitor (PDI), such as carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in the therapeutic window of levodopa becoming narrower and the development of the ‘ON-OFF phenomenon’<ref name=":0" />which is characterized on one end of the spectrum by hypodopaminergic symptoms (stiffness, slowness, feezing) and on the other end by hyperdopaminergic excessive movement (dyskinesia). This highlights the need for precise L-Dopa dosing. ''Figure 1'' illustrates how the narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and OFF-time (return of PD symptoms).

== (Lack of) correlation between peripheral levodopa concentration and clinical symptoms ==

=== Pathophysiological complexity of the disease ===
Parkinson’s disease is a complex condition. Not only do the symptoms encompass a wide range of motor and non-motor complaints, the underlying pathophysiological mechanisms are also manifold. This limits dopaminergic therapy in important ways:

* Not all dopaminergic systems in the brain are affected at the same pace<ref>Rodríguez-Oroz, M. C.; Jahanshahi, M.; Krack, P.; Litvan, I.; Macías, R.; Bezard, E.; Obeso, J. A. Initial Clinical Manifestations of Parkinson’s Disease: Features and Pathophysiological Mechanisms. ''The Lancet Neurology'' '''2009''', ''8'' (12), 1128–1139. <nowiki>https://doi.org/10.1016/S1474-4422(09)70293-5</nowiki>.</ref> , with as a consequence that a certain levodopa dose can result in an eudopaminergic state in the motor circuit but a hyperdopaminergic state in the limbic circuit. The clinical correlate of this is a patient whose motor symptoms may be well-controlled, but cumbersome psychotic symptoms occur as an unintended effect.

* During the disease course, in addition to the dopaminergic deficit, deficits of other neurotransmitter systems<ref>Devos, D.; Defebvre, L.; Bordet, R. Dopaminergic and Non-Dopaminergic Pharmacological Hypotheses for Gait Disorders in Parkinson’s Disease. ''Fundamental & Clinical Pharmacology'' '''2010''', ''24'' (4), 407–421. <nowiki>https://doi.org/10.1111/j.1472-8206.2009.00798.x</nowiki>.</ref> (such as cholinergic, (nor)adrenergic and serotonergic systems) develop in addition to neurotransmitter-independent neurodegeneration. Several debilitating symptoms, such as autonomic and cognitive symptoms and disturbed balance/falling, are therefore levodopa-resistant.

* Varying extents of postsynaptic dopaminergic degeneration occur. This means that not only is the amount of (pre-synaptic) dopamine limited, but there is also damage to the (postsynaptic) neurons that have to make use of the dopamine. Even an optimal concentration of dopamine in the synaptic cleft may therefore be unable to adequately attenuate hypodopaminergic symptoms.

=== Correlation between peripheral levodopa concentration and dopamine available to cerebral neurons ===

A multitude of steps stands between (oral) levodopa intake and concentration of dopamine available to dopaminergic neurons in the brain.

1. After oral ingestion of levodopa, the tablets have to dissolve in the stomach, and then be transported to the small intestine for absorption. Many people with Parkinson’s disease have a lower acidity<ref>Fasano, A.; Visanji, N. P.; Liu, L. W. C.; Lang, A. E.; Pfeiffer, R. F. Gastrointestinal Dysfunction in Parkinson’s Disease. ''The Lancet Neurology'' '''2015''', ''14'' (6), 625–639. <nowiki>https://doi.org/10.1016/S1474-4422(15)00007-1</nowiki>.</ref> of the stomach (e.g. due to medication use or ''Helicobacter pylori'' infection), and an estimated 70-100% of people with Parkinson’s disease have gastroparesis<ref>Marrinan, S.; Emmanuel, A. V.; Burn, D. J. Delayed Gastric Emptying in Parkinson’s Disease. ''Movement Disorders'' '''2014''', ''29'' (1), 23–32. <nowiki>https://doi.org/10.1002/mds.25708</nowiki>.</ref> (delayed stomach emptying). This leads to impaired dissolution of levodopa and delayed transfer of levodopa to the small intestine, respectively.

2. Once levodopa has arrived in the duodenum and proximal jejunum for absorption, it has to be actively transported over the bowel mucosa by saturable LNAA transporters (transporters for Large Neutral Amino Acids). These transporters are both sensitive to pH, working optimally at a pH between 6.2 – 7.4,<ref>Pedrosa de Menezes, A. L.; Bloem, B. R.; Beckers, M.; Piat, C.; Benarroch, E. E.; Savica, R. Molecular Variability in Levodopa Absorption and Clinical Implications for the Management of Parkinson’s Disease. ''Journal of Parkinson’s Disease'' '''2024''', ''14'' (7), 1353–1368. <nowiki>https://doi.org/10.3233/JPD-240036</nowiki></ref> as well as sensitive to so-named ''protein competition''. That is: after a protein-rich meal, dietary proteins/amino acids compete with levodopa<ref>Contin, M.; Martinelli, P. Pharmacokinetics of Levodopa. ''Journal of Neurology'' '''2010''', ''257'' (Suppl. 2), S253–S261. <nowiki>https://doi.org/10.1007/s00415-010-5728-8</nowiki>.</ref> for the transport system, reducing levodopa’s bioavailability. As noted, stomach acidity is often less pronounced in people with Parkinson’s disease, and due to delayed gastric emptying, dietary proteins may be present in the small intestine for extended periods after a meal. This limits predictability of the speed and magnitude with which levodopa is absorbed into the bloodstream.

3. Both in the intestinal lumen as well as in peripheral blood, levodopa is metabolized to dopamine and other metabolites. This is unwanted, as dopamine cannot cross the blood-brain barrier; for pharmacotherapeutic effect, levodopa has to cross the blood-brain barrier in unmetabolized form. While peripheral decarboxylase inhibitors are able to partially inhibit peripheral decarboxylation, there is interindividual variability in the activity of levodopa-metabolizing enzymes such as AADC and catechol-O-aminotransferase (COMT). Part of this variability is genetic<ref>Devos D, Lejeune S, Cormier-Dequaire F, Tahiri K, Charbonnier-Beaupel F, Rouaix N, Duhamel A, Sablonniere B, Bonnet AM, Bonnet C, et al. Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in parkinson’s disease. PARKINSONISM RELAT D. 2014;20(2):170–5.</ref><ref>Sampaio TF, Dos SE, de Lima G, Dos AR, Da SR, Asano A, Asano N, Crovella S, de Souza P. MAO-B and COMT genetic variations associated with Levodopa treatment response in patients with parkinson’s disease. J CLIN PHARMACOL. 2018;58(7):920–6.</ref> , but AADC activity also varies by sex, disease duration and use of dopaminergic medication. The amount of levodopa that succeeds in being absorbed into the blood stream is therefore not necessarily the amount available at the blood-brain barrier.

4. At the blood-brain barrier, protein competition exists as well. <ref>Lees, A. J. The On-Off Phenomenon. ''Journal of Neurology, Neurosurgery & Psychiatry'' '''1989''', ''52'' (Suppl. 29), 29–37. <nowiki>https://doi.org/10.1136/jnnp.52.Suppl.29</nowiki>.</ref> Speed and magnitude of levodopa transport over the blood-brain barrier thus depends on the concentration of (dietary) amino acids in the blood.

5. Once in the brain, enzymes such as AADC and COMT metabolize levodopa and, together with monoamine oxidase B (MAO-B), determine the resultant amount of dopamine available in the synaptic cleft of dopaminergic neurons. Here, as well, there is interindividual variation in the activities of the enzymes.

Given these multiple steps of which the magnitude is – to a large extent – poorly predictable, levodopa plasma concentration correlates poorly with dopamine available to neurons in the brain.

Furthermore, the above-described steps apply in individuals without other levodopa-bioavailability-altering conditions. There are multiple circumstances <ref>Beckers, M.; Bloem, B. R.; Verbeek, M. M. Mechanisms of Peripheral Levodopa Resistance in Parkinson’s Disease. ''npj Parkinson’s Disease'' '''2022''', ''8'' (1), 56. <nowiki>https://doi.org/10.1038/s41531-022-00321-y</nowiki>.</ref> under which the correlation between intake of levodopa and clinical effect becomes even more unpredictable, e.g. if intestinal absorption is limited (e.g. due to inflammation of the intestinal lining), if levodopa is prematurely metabolized by gut bacteria, or if there is an overactivity of the AADC enzyme.

=== Correlation between bioavailable dopamine and control of PD symptoms ===
As described earlier in the text, not all PD symptoms respond to levodopa to the same extent. In addition to the presence of non-dopaminergic symptoms (which by definition do not respond to levodopa), even symptoms who are normally levodopa-responsive can display variation in their response. Factors inducing these variation include, amongst others, (psychological and physical) stress<ref>Helmich, R. C. G. and the Systems Neurology Group at the Donders Centre for Cognitive Neuroimaging investigate the cerebral mechanisms of movement disorders, especially the pathophysiology of Parkinson’s disease tremor and compensatory network changes, using neuroimaging and neurophysiological methods. Recent works include neuroimaging studies on tremor circuitry, longitudinal brain compensation in Parkinson’s progression, and clinical trials examining propranolol’s effects on tremor.</ref> as well as energy level/fatigue.

For the reasons outlined in the above paragraphs, peripheral levodopa pharmacokinetics correlate poorly to the actual level of symptom control. A 2013 study <ref>Fasano, A.; Bove, F.; Gabrielli, M.; Petracca, M.; Zocco, M. A.; Ragazzoni, E.; Barbaro, F.; Piano, C.; Fortuna, S.; Tortora, A.; Di Giacopo, R.; Campanale, M.; Gigante, G.; Lauritano, E. C.; Navarra, P.; Marconi, S.; Gasbarrini, A.; Bentivoglio, A. R. The Role of Small Intestinal Bacterial Overgrowth in Parkinson’s Disease. ''Movement Disorders'' '''2013''', ''28'' (9), 1241–1249. <nowiki>https://doi.org/10.1002/mds.25522</nowiki></ref> demonstrated that the presence of motor fluctuations did not correlate to pharmacokinetic data.

Thus, peripheral levodopa concentration monitoring would need to be integrated with measurement of clinical symptoms to be useful in clinical practice.
== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure L-Dopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce Levodopa fluctuations. Continuous sensing would also help researchers better understand the pharmacokinetics of Levodopa across different bodily fluids, namely ISF, blood, and cerebrospinal fluid (CSF). This could improve treatment strategies for late-stage PD patients and also help make a step towards the long-term vision of closed-loop Levodopa therapy for PD wherein continual oral administration would no longer be required, improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2025-12-24T21:29:07Z

Techsensus: /* (Lack of) correlation between peripheral levodopa concentration and clinical symptoms */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder associated with the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficit in the basal ganglia, which are critical for initiating and smoothing movement. The predominant motor symptoms advancing from this deficit include tremor, muscle stiffness (rigidity) and slowness of movement (bradykinesia)<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed into the bloodstream in the gastrointestinal tract, chiefly in the duodenum and proximal jejunum, and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, an adequate concentration of levodopa in the blood serum is a prerequisite for the exertion of the therapeutic effect. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes unpredictable, manifesting as unpredictable motor fluctuations and dyskinesias. Therefore, monitoring levodopa levels serves as a crucial tool for optimizing dosing regimens. There is no cure for Parkinson's disease, although medications and non-drug therapies can alleviate the symptoms and improve daily functioning. In advanced cases, patients may undergo surgical treatments such as deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation 0f Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficit as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor) and non-motor features (such as loss of smell, sleep disorder, gastrointestinal issues, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is clinical, based on a neurological examination and response to dopaminergic therapy, with imaging (such as MRI and DaT-SPECT) mainly used to exclude other causes of the symptoms; research into biomarkers is ongoing and their application is not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although during the course of the disease levodopa-resistant symptoms develop, and advanced therapies (such as deep brain stimulation, DBS) are an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis.

Broadly speaking, Parkinson’s disease can be subdivided into three disease stages:

<nowiki>*</nowiki> The early stage, in which response to dopaminergic medication is generally stable throughout the day, without response fluctuations. The person with PD can (under drug treatment) function normally in many respects.

<nowiki>*</nowiki> Mid-stage, in which motor fluctuations (the ON-OFF phenomenon) come to the fore. Even with extensive dosing schedules (levodopa doses up to eight or more times a day), some people may still not reach a stable response, limiting daily functioning. In this phase, advanced therapies such as continuous dopaminergic drug administration via subcutaneous/intestinal pump and/or deep brain stimulation surgery may be considered.

<nowiki>*</nowiki> Late-stage disease. Dopamine-resistant and non-motor symptoms such as postural instability/falling, cognitive deficits/dementia, hallucinations and autonomic failure dominate the clinical picture. The patient’s functioning is determined mostly by these symptoms, which are poorly susceptible to drug treatment.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction in 1961. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with a peripheral decarboxylase inhibitor (PDI), such as carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in the therapeutic window of levodopa becoming narrower and the development of the ‘ON-OFF phenomenon’<ref name=":0" />which is characterized on one end of the spectrum by hypodopaminergic symptoms (stiffness, slowness, feezing) and on the other end by hyperdopaminergic excessive movement (dyskinesia). This highlights the need for precise L-Dopa dosing. ''Figure 1'' illustrates how the narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and OFF-time (return of PD symptoms).

== (Lack of) correlation between peripheral levodopa concentration and clinical symptoms ==

=== Pathophysiological complexity of the disease ===
Parkinson’s disease is a complex condition. Not only do the symptoms encompass a wide range of motor and non-motor complaints, the underlying pathophysiological mechanisms are also manifold. This limits dopaminergic therapy in important ways:

* Not all dopaminergic systems in the brain are affected at the same pace<ref>Rodríguez-Oroz, M. C.; Jahanshahi, M.; Krack, P.; Litvan, I.; Macías, R.; Bezard, E.; Obeso, J. A. Initial Clinical Manifestations of Parkinson’s Disease: Features and Pathophysiological Mechanisms. ''The Lancet Neurology'' '''2009''', ''8'' (12), 1128–1139. <nowiki>https://doi.org/10.1016/S1474-4422(09)70293-5</nowiki>.</ref> , with as a consequence that a certain levodopa dose can result in an eudopaminergic state in the motor circuit but a hyperdopaminergic state in the limbic circuit. The clinical correlate of this is a patient whose motor symptoms may be well-controlled, but cumbersome psychotic symptoms occur as an unintended effect.

* During the disease course, in addition to the dopaminergic deficit, deficits of other neurotransmitter systems<ref>Devos, D.; Defebvre, L.; Bordet, R. Dopaminergic and Non-Dopaminergic Pharmacological Hypotheses for Gait Disorders in Parkinson’s Disease. ''Fundamental & Clinical Pharmacology'' '''2010''', ''24'' (4), 407–421. <nowiki>https://doi.org/10.1111/j.1472-8206.2009.00798.x</nowiki>.</ref> (such as cholinergic, (nor)adrenergic and serotonergic systems) develop in addition to neurotransmitter-independent neurodegeneration. Several debilitating symptoms, such as autonomic and cognitive symptoms and disturbed balance/falling, are therefore levodopa-resistant.

* Varying extents of postsynaptic dopaminergic degeneration occur. This means that not only is the amount of (pre-synaptic) dopamine limited, but there is also damage to the (postsynaptic) neurons that have to make use of the dopamine. Even an optimal concentration of dopamine in the synaptic cleft may therefore be unable to adequately attenuate hypodopaminergic symptoms.

=== Correlation between peripheral levodopa concentration and dopamine available to cerebral neurons ===

A multitude of steps stands between (oral) levodopa intake and concentration of dopamine available to dopaminergic neurons in the brain.

1. After oral ingestion of levodopa, the tablets have to dissolve in the stomach, and then be transported to the small intestine for absorption. Many people with Parkinson’s disease have a lower acidity [MB1] of the stomach (e.g. due to medication use or ''Helicobacter pylori'' infection), and an estimated 70-100% of people with Parkinson’s disease have gastroparesis [MB2] (delayed stomach emptying). This leads to impaired dissolution of levodopa and delayed transfer of levodopa to the small intestine, respectively.

2. Once levodopa has arrived in the duodenum and proximal jejunum for absorption, it has to be actively transported over the bowel mucosa by saturable LNAA transporters (transporters for Large Neutral Amino Acids). These transporters are both sensitive to pH, working optimally at a pH between 6.2 – 7.4,[MB3] as well as sensitive to so-named ''protein competition''. That is: after a protein-rich meal, dietary proteins/amino acids compete with levodopa [MB4] for the transport system, reducing levodopa’s bioavailability. As noted, stomach acidity is often less pronounced in people with Parkinson’s disease, and due to delayed gastric emptying, dietary proteins may be present in the small intestine for extended periods after a meal. This limits predictability of the speed and magnitude with which levodopa is absorbed into the bloodstream.

3. Both in the intestinal lumen as well as in peripheral blood, levodopa is metabolized to dopamine and other metabolites. This is unwanted, as dopamine cannot cross the blood-brain barrier; for pharmacotherapeutic effect, levodopa has to cross the blood-brain barrier in unmetabolized form. While peripheral decarboxylase inhibitors are able to partially inhibit peripheral decarboxylation, there is interindividual variability in the activity of levodopa-metabolizing enzymes such as AADC and catechol-O-aminotransferase (COMT). Part of this variability is genetic[MB5] , but AADC activity also varies by sex, disease duration and use of dopaminergic medication. The amount of levodopa that succeeds in being absorbed into the blood stream is therefore not necessarily the amount available at the blood-brain barrier.

4. At the blood-brain barrier, protein competition exists as well. [MB6] Speed and magnitude of levodopa transport over the blood-brain barrier thus depends on the concentration of (dietary) amino acids in the blood.

5. Once in the brain, enzymes such as AADC and COMT metabolize levodopa and, together with monoamine oxidase B (MAO-B), determine the resultant amount of dopamine available in the synaptic cleft of dopaminergic neurons. Here, as well, there is interindividual variation in the activities of the enzymes.

Given these multiple steps of which the magnitude is – to a large extent – poorly predictable, levodopa plasma concentration correlates poorly with dopamine available to neurons in the brain.

Furthermore, the above-described steps apply in individuals without other levodopa-bioavailability-altering conditions. There are multiple circumstances [MB7] under which the correlation between intake of levodopa and clinical effect becomes even more unpredictable, e.g. if intestinal absorption is limited (e.g. due to inflammation of the intestinal lining), if levodopa is prematurely metabolized by gut bacteria, or if there is an overactivity of the AADC enzyme.
----[MB1]Ref 10.1016/S1474-4422(15)00007-1

[MB2]Ref 10.1002/mds.25708

[MB3]Ref: 10.3233/JPD-240036

[MB4]Ref doi 10.1007/s00415-010-5728-8

[MB5]Refs:

· Devos D, Lejeune S, Cormier-Dequaire F, Tahiri K, Charbonnier-Beaupel F, Rouaix N, Duhamel A, Sablonniere B, Bonnet AM, Bonnet C, et al. Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in parkinson’s disease. PARKINSONISM RELAT D. 2014;20(2):170–5.

· Sampaio TF, Dos SE, de Lima G, Dos AR, Da SR, Asano A, Asano N, Crovella S, de Souza P. MAO-B and COMT genetic variations associated with Levodopa treatment response in patients with parkinson’s disease. J CLIN PHARMACOL. 2018;58(7):920–6.

[MB6]Ref: 10.1136/jnnp.52.Suppl.29

[MB7]Ref 10.1038/s41531-022-00321-y

=== Correlation between bioavailable dopamine and control of PD symptoms ===
As described earlier in the text, not all PD symptoms respond to levodopa to the same extent. In addition to the presence of non-dopaminergic symptoms (which by definition do not respond to levodopa), even symptoms who are normally levodopa-responsive can display variation in their response. Factors inducing these variation include, amongst others, (psychological and physical) stress[MB1] as well as energy level/fatigue.

For the reasons outlined in the above paragraphs, peripheral levodopa pharmacokinetics correlate poorly to the actual level of symptom control. A 2013 study [MB2] demonstrated that the presence of motor fluctuations did not correlate to pharmacokinetic data.

Thus, peripheral levodopa concentration monitoring would need to be integrated with measurement of clinical symptoms to be useful in clinical practice.
[MB1]See e.g. research works by the group of Rick Helmich

[MB2]Ref 10.1002/mds.25522

== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure L-Dopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce Levodopa fluctuations. Continuous sensing would also help researchers better understand the pharmacokinetics of Levodopa across different bodily fluids, namely ISF, blood, and cerebrospinal fluid (CSF). This could improve treatment strategies for late-stage PD patients and also help make a step towards the long-term vision of closed-loop Levodopa therapy for PD wherein continual oral administration would no longer be required, improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2025-12-24T21:28:10Z

Techsensus: /* Pathophysiological complexity of the disease */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder associated with the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficit in the basal ganglia, which are critical for initiating and smoothing movement. The predominant motor symptoms advancing from this deficit include tremor, muscle stiffness (rigidity) and slowness of movement (bradykinesia)<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed into the bloodstream in the gastrointestinal tract, chiefly in the duodenum and proximal jejunum, and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, an adequate concentration of levodopa in the blood serum is a prerequisite for the exertion of the therapeutic effect. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes unpredictable, manifesting as unpredictable motor fluctuations and dyskinesias. Therefore, monitoring levodopa levels serves as a crucial tool for optimizing dosing regimens. There is no cure for Parkinson's disease, although medications and non-drug therapies can alleviate the symptoms and improve daily functioning. In advanced cases, patients may undergo surgical treatments such as deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation 0f Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficit as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor) and non-motor features (such as loss of smell, sleep disorder, gastrointestinal issues, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is clinical, based on a neurological examination and response to dopaminergic therapy, with imaging (such as MRI and DaT-SPECT) mainly used to exclude other causes of the symptoms; research into biomarkers is ongoing and their application is not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although during the course of the disease levodopa-resistant symptoms develop, and advanced therapies (such as deep brain stimulation, DBS) are an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis.

Broadly speaking, Parkinson’s disease can be subdivided into three disease stages:

<nowiki>*</nowiki> The early stage, in which response to dopaminergic medication is generally stable throughout the day, without response fluctuations. The person with PD can (under drug treatment) function normally in many respects.

<nowiki>*</nowiki> Mid-stage, in which motor fluctuations (the ON-OFF phenomenon) come to the fore. Even with extensive dosing schedules (levodopa doses up to eight or more times a day), some people may still not reach a stable response, limiting daily functioning. In this phase, advanced therapies such as continuous dopaminergic drug administration via subcutaneous/intestinal pump and/or deep brain stimulation surgery may be considered.

<nowiki>*</nowiki> Late-stage disease. Dopamine-resistant and non-motor symptoms such as postural instability/falling, cognitive deficits/dementia, hallucinations and autonomic failure dominate the clinical picture. The patient’s functioning is determined mostly by these symptoms, which are poorly susceptible to drug treatment.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction in 1961. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with a peripheral decarboxylase inhibitor (PDI), such as carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in the therapeutic window of levodopa becoming narrower and the development of the ‘ON-OFF phenomenon’<ref name=":0" />which is characterized on one end of the spectrum by hypodopaminergic symptoms (stiffness, slowness, feezing) and on the other end by hyperdopaminergic excessive movement (dyskinesia). This highlights the need for precise L-Dopa dosing. ''Figure 1'' illustrates how the narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and OFF-time (return of PD symptoms).

== (Lack of) correlation between peripheral levodopa concentration and clinical symptoms ==

=== Pathophysiological complexity of the disease ===
Parkinson’s disease is a complex condition. Not only do the symptoms encompass a wide range of motor and non-motor complaints, the underlying pathophysiological mechanisms are also manifold. This limits dopaminergic therapy in important ways:

* Not all dopaminergic systems in the brain are affected at the same pace<ref>Rodríguez-Oroz, M. C.; Jahanshahi, M.; Krack, P.; Litvan, I.; Macías, R.; Bezard, E.; Obeso, J. A. Initial Clinical Manifestations of Parkinson’s Disease: Features and Pathophysiological Mechanisms. ''The Lancet Neurology'' '''2009''', ''8'' (12), 1128–1139. <nowiki>https://doi.org/10.1016/S1474-4422(09)70293-5</nowiki>.</ref> , with as a consequence that a certain levodopa dose can result in an eudopaminergic state in the motor circuit but a hyperdopaminergic state in the limbic circuit. The clinical correlate of this is a patient whose motor symptoms may be well-controlled, but cumbersome psychotic symptoms occur as an unintended effect.

* During the disease course, in addition to the dopaminergic deficit, deficits of other neurotransmitter systems<ref>Devos, D.; Defebvre, L.; Bordet, R. Dopaminergic and Non-Dopaminergic Pharmacological Hypotheses for Gait Disorders in Parkinson’s Disease. ''Fundamental & Clinical Pharmacology'' '''2010''', ''24'' (4), 407–421. <nowiki>https://doi.org/10.1111/j.1472-8206.2009.00798.x</nowiki>.</ref> (such as cholinergic, (nor)adrenergic and serotonergic systems) develop in addition to neurotransmitter-independent neurodegeneration. Several debilitating symptoms, such as autonomic and cognitive symptoms and disturbed balance/falling, are therefore levodopa-resistant.

* Varying extents of postsynaptic dopaminergic degeneration occur. This means that not only is the amount of (pre-synaptic) dopamine limited, but there is also damage to the (postsynaptic) neurons that have to make use of the dopamine. Even an optimal concentration of dopamine in the synaptic cleft may therefore be unable to adequately attenuate hypodopaminergic symptoms.

Correlation between peripheral levodopa concentration and dopamine available to cerebral neurons

A multitude of steps stands between (oral) levodopa intake and concentration of dopamine available to dopaminergic neurons in the brain.

1. After oral ingestion of levodopa, the tablets have to dissolve in the stomach, and then be transported to the small intestine for absorption. Many people with Parkinson’s disease have a lower acidity [MB1] of the stomach (e.g. due to medication use or ''Helicobacter pylori'' infection), and an estimated 70-100% of people with Parkinson’s disease have gastroparesis [MB2] (delayed stomach emptying). This leads to impaired dissolution of levodopa and delayed transfer of levodopa to the small intestine, respectively.

2. Once levodopa has arrived in the duodenum and proximal jejunum for absorption, it has to be actively transported over the bowel mucosa by saturable LNAA transporters (transporters for Large Neutral Amino Acids). These transporters are both sensitive to pH, working optimally at a pH between 6.2 – 7.4,[MB3] as well as sensitive to so-named ''protein competition''. That is: after a protein-rich meal, dietary proteins/amino acids compete with levodopa [MB4] for the transport system, reducing levodopa’s bioavailability. As noted, stomach acidity is often less pronounced in people with Parkinson’s disease, and due to delayed gastric emptying, dietary proteins may be present in the small intestine for extended periods after a meal. This limits predictability of the speed and magnitude with which levodopa is absorbed into the bloodstream.

3. Both in the intestinal lumen as well as in peripheral blood, levodopa is metabolized to dopamine and other metabolites. This is unwanted, as dopamine cannot cross the blood-brain barrier; for pharmacotherapeutic effect, levodopa has to cross the blood-brain barrier in unmetabolized form. While peripheral decarboxylase inhibitors are able to partially inhibit peripheral decarboxylation, there is interindividual variability in the activity of levodopa-metabolizing enzymes such as AADC and catechol-O-aminotransferase (COMT). Part of this variability is genetic[MB5] , but AADC activity also varies by sex, disease duration and use of dopaminergic medication. The amount of levodopa that succeeds in being absorbed into the blood stream is therefore not necessarily the amount available at the blood-brain barrier.

4. At the blood-brain barrier, protein competition exists as well. [MB6] Speed and magnitude of levodopa transport over the blood-brain barrier thus depends on the concentration of (dietary) amino acids in the blood.

5. Once in the brain, enzymes such as AADC and COMT metabolize levodopa and, together with monoamine oxidase B (MAO-B), determine the resultant amount of dopamine available in the synaptic cleft of dopaminergic neurons. Here, as well, there is interindividual variation in the activities of the enzymes.

Given these multiple steps of which the magnitude is – to a large extent – poorly predictable, levodopa plasma concentration correlates poorly with dopamine available to neurons in the brain.

Furthermore, the above-described steps apply in individuals without other levodopa-bioavailability-altering conditions. There are multiple circumstances [MB7] under which the correlation between intake of levodopa and clinical effect becomes even more unpredictable, e.g. if intestinal absorption is limited (e.g. due to inflammation of the intestinal lining), if levodopa is prematurely metabolized by gut bacteria, or if there is an overactivity of the AADC enzyme.
----[MB1]Ref 10.1016/S1474-4422(15)00007-1

[MB2]Ref 10.1002/mds.25708

[MB3]Ref: 10.3233/JPD-240036

[MB4]Ref doi 10.1007/s00415-010-5728-8

[MB5]Refs:

· Devos D, Lejeune S, Cormier-Dequaire F, Tahiri K, Charbonnier-Beaupel F, Rouaix N, Duhamel A, Sablonniere B, Bonnet AM, Bonnet C, et al. Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in parkinson’s disease. PARKINSONISM RELAT D. 2014;20(2):170–5.

· Sampaio TF, Dos SE, de Lima G, Dos AR, Da SR, Asano A, Asano N, Crovella S, de Souza P. MAO-B and COMT genetic variations associated with Levodopa treatment response in patients with parkinson’s disease. J CLIN PHARMACOL. 2018;58(7):920–6.

[MB6]Ref: 10.1136/jnnp.52.Suppl.29

[MB7]Ref 10.1038/s41531-022-00321-y

=== Correlation between bioavailable dopamine and control of PD symptoms ===
As described earlier in the text, not all PD symptoms respond to levodopa to the same extent. In addition to the presence of non-dopaminergic symptoms (which by definition do not respond to levodopa), even symptoms who are normally levodopa-responsive can display variation in their response. Factors inducing these variation include, amongst others, (psychological and physical) stress[MB1] as well as energy level/fatigue.

For the reasons outlined in the above paragraphs, peripheral levodopa pharmacokinetics correlate poorly to the actual level of symptom control. A 2013 study [MB2] demonstrated that the presence of motor fluctuations did not correlate to pharmacokinetic data.

Thus, peripheral levodopa concentration monitoring would need to be integrated with measurement of clinical symptoms to be useful in clinical practice.
[MB1]See e.g. research works by the group of Rick Helmich

[MB2]Ref 10.1002/mds.25522

== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure L-Dopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce Levodopa fluctuations. Continuous sensing would also help researchers better understand the pharmacokinetics of Levodopa across different bodily fluids, namely ISF, blood, and cerebrospinal fluid (CSF). This could improve treatment strategies for late-stage PD patients and also help make a step towards the long-term vision of closed-loop Levodopa therapy for PD wherein continual oral administration would no longer be required, improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2025-12-24T21:25:24Z

Techsensus: /* Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder associated with the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficit in the basal ganglia, which are critical for initiating and smoothing movement. The predominant motor symptoms advancing from this deficit include tremor, muscle stiffness (rigidity) and slowness of movement (bradykinesia)<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed into the bloodstream in the gastrointestinal tract, chiefly in the duodenum and proximal jejunum, and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, an adequate concentration of levodopa in the blood serum is a prerequisite for the exertion of the therapeutic effect. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes unpredictable, manifesting as unpredictable motor fluctuations and dyskinesias. Therefore, monitoring levodopa levels serves as a crucial tool for optimizing dosing regimens. There is no cure for Parkinson's disease, although medications and non-drug therapies can alleviate the symptoms and improve daily functioning. In advanced cases, patients may undergo surgical treatments such as deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation 0f Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficit as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor) and non-motor features (such as loss of smell, sleep disorder, gastrointestinal issues, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is clinical, based on a neurological examination and response to dopaminergic therapy, with imaging (such as MRI and DaT-SPECT) mainly used to exclude other causes of the symptoms; research into biomarkers is ongoing and their application is not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although during the course of the disease levodopa-resistant symptoms develop, and advanced therapies (such as deep brain stimulation, DBS) are an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis.

Broadly speaking, Parkinson’s disease can be subdivided into three disease stages:

<nowiki>*</nowiki> The early stage, in which response to dopaminergic medication is generally stable throughout the day, without response fluctuations. The person with PD can (under drug treatment) function normally in many respects.

<nowiki>*</nowiki> Mid-stage, in which motor fluctuations (the ON-OFF phenomenon) come to the fore. Even with extensive dosing schedules (levodopa doses up to eight or more times a day), some people may still not reach a stable response, limiting daily functioning. In this phase, advanced therapies such as continuous dopaminergic drug administration via subcutaneous/intestinal pump and/or deep brain stimulation surgery may be considered.

<nowiki>*</nowiki> Late-stage disease. Dopamine-resistant and non-motor symptoms such as postural instability/falling, cognitive deficits/dementia, hallucinations and autonomic failure dominate the clinical picture. The patient’s functioning is determined mostly by these symptoms, which are poorly susceptible to drug treatment.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction in 1961. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with a peripheral decarboxylase inhibitor (PDI), such as carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine by the enzyme aromatic L-amino acid decarboxylase (AADC) <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in the therapeutic window of levodopa becoming narrower and the development of the ‘ON-OFF phenomenon’<ref name=":0" />which is characterized on one end of the spectrum by hypodopaminergic symptoms (stiffness, slowness, feezing) and on the other end by hyperdopaminergic excessive movement (dyskinesia). This highlights the need for precise L-Dopa dosing. ''Figure 1'' illustrates how the narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and OFF-time (return of PD symptoms).

== (Lack of) correlation between peripheral levodopa concentration and clinical symptoms ==

=== Pathophysiological complexity of the disease ===
Parkinson’s disease is a complex condition. Not only do the symptoms encompass a wide range of motor and non-motor complaints, the underlying pathophysiological mechanisms are also manifold. This limits dopaminergic therapy in important ways:

* Not all dopaminergic systems in the brain are affected at the same pace[MB1] , with as a consequence that a certain levodopa dose can result in an eudopaminergic state in the motor circuit but a hyperdopaminergic state in the limbic circuit. The clinical correlate of this is a patient whose motor symptoms may be well-controlled, but cumbersome psychotic symptoms occur as an unintended effect.

* During the disease course, in addition to the dopaminergic deficit, deficits of other neurotransmitter systems [MB2] (such as cholinergic, (nor)adrenergic and serotonergic systems) develop in addition to neurotransmitter-independent neurodegeneration. Several debilitating symptoms, such as autonomic and cognitive symptoms and disturbed balance/falling, are therefore levodopa-resistant.

* Varying extents of postsynaptic dopaminergic degeneration occur. This means that not only is the amount of (pre-synaptic) dopamine limited, but there is also damage to the (postsynaptic) neurons that have to make use of the dopamine. Even an optimal concentration of dopamine in the synaptic cleft may therefore be unable to adequately attenuate hypodopaminergic symptoms.

[MB1]Ref doi 10.1016/S1474-4422(09)70293-5

[MB2]Ref 10.1111/j.1472-8206.2009.00798.x
<nowiki>=== Correlation between peripheral levodopa concentration and dopamine available to cerebral neurons ===</nowiki>

A multitude of steps stands between (oral) levodopa intake and concentration of dopamine available to dopaminergic neurons in the brain.

1. After oral ingestion of levodopa, the tablets have to dissolve in the stomach, and then be transported to the small intestine for absorption. Many people with Parkinson’s disease have a lower acidity [MB1] of the stomach (e.g. due to medication use or ''Helicobacter pylori'' infection), and an estimated 70-100% of people with Parkinson’s disease have gastroparesis [MB2] (delayed stomach emptying). This leads to impaired dissolution of levodopa and delayed transfer of levodopa to the small intestine, respectively.

2. Once levodopa has arrived in the duodenum and proximal jejunum for absorption, it has to be actively transported over the bowel mucosa by saturable LNAA transporters (transporters for Large Neutral Amino Acids). These transporters are both sensitive to pH, working optimally at a pH between 6.2 – 7.4,[MB3] as well as sensitive to so-named ''protein competition''. That is: after a protein-rich meal, dietary proteins/amino acids compete with levodopa [MB4] for the transport system, reducing levodopa’s bioavailability. As noted, stomach acidity is often less pronounced in people with Parkinson’s disease, and due to delayed gastric emptying, dietary proteins may be present in the small intestine for extended periods after a meal. This limits predictability of the speed and magnitude with which levodopa is absorbed into the bloodstream.

3. Both in the intestinal lumen as well as in peripheral blood, levodopa is metabolized to dopamine and other metabolites. This is unwanted, as dopamine cannot cross the blood-brain barrier; for pharmacotherapeutic effect, levodopa has to cross the blood-brain barrier in unmetabolized form. While peripheral decarboxylase inhibitors are able to partially inhibit peripheral decarboxylation, there is interindividual variability in the activity of levodopa-metabolizing enzymes such as AADC and catechol-O-aminotransferase (COMT). Part of this variability is genetic[MB5] , but AADC activity also varies by sex, disease duration and use of dopaminergic medication. The amount of levodopa that succeeds in being absorbed into the blood stream is therefore not necessarily the amount available at the blood-brain barrier.

4. At the blood-brain barrier, protein competition exists as well. [MB6] Speed and magnitude of levodopa transport over the blood-brain barrier thus depends on the concentration of (dietary) amino acids in the blood.

5. Once in the brain, enzymes such as AADC and COMT metabolize levodopa and, together with monoamine oxidase B (MAO-B), determine the resultant amount of dopamine available in the synaptic cleft of dopaminergic neurons. Here, as well, there is interindividual variation in the activities of the enzymes.

Given these multiple steps of which the magnitude is – to a large extent – poorly predictable, levodopa plasma concentration correlates poorly with dopamine available to neurons in the brain.

Furthermore, the above-described steps apply in individuals without other levodopa-bioavailability-altering conditions. There are multiple circumstances [MB7] under which the correlation between intake of levodopa and clinical effect becomes even more unpredictable, e.g. if intestinal absorption is limited (e.g. due to inflammation of the intestinal lining), if levodopa is prematurely metabolized by gut bacteria, or if there is an overactivity of the AADC enzyme.
----[MB1]Ref 10.1016/S1474-4422(15)00007-1

[MB2]Ref 10.1002/mds.25708

[MB3]Ref: 10.3233/JPD-240036

[MB4]Ref doi 10.1007/s00415-010-5728-8

[MB5]Refs:

· Devos D, Lejeune S, Cormier-Dequaire F, Tahiri K, Charbonnier-Beaupel F, Rouaix N, Duhamel A, Sablonniere B, Bonnet AM, Bonnet C, et al. Dopa-decarboxylase gene polymorphisms affect the motor response to L-dopa in parkinson’s disease. PARKINSONISM RELAT D. 2014;20(2):170–5.

· Sampaio TF, Dos SE, de Lima G, Dos AR, Da SR, Asano A, Asano N, Crovella S, de Souza P. MAO-B and COMT genetic variations associated with Levodopa treatment response in patients with parkinson’s disease. J CLIN PHARMACOL. 2018;58(7):920–6.

[MB6]Ref: 10.1136/jnnp.52.Suppl.29

[MB7]Ref 10.1038/s41531-022-00321-y

=== Correlation between bioavailable dopamine and control of PD symptoms ===
As described earlier in the text, not all PD symptoms respond to levodopa to the same extent. In addition to the presence of non-dopaminergic symptoms (which by definition do not respond to levodopa), even symptoms who are normally levodopa-responsive can display variation in their response. Factors inducing these variation include, amongst others, (psychological and physical) stress[MB1] as well as energy level/fatigue.

For the reasons outlined in the above paragraphs, peripheral levodopa pharmacokinetics correlate poorly to the actual level of symptom control. A 2013 study [MB2] demonstrated that the presence of motor fluctuations did not correlate to pharmacokinetic data.

Thus, peripheral levodopa concentration monitoring would need to be integrated with measurement of clinical symptoms to be useful in clinical practice.
[MB1]See e.g. research works by the group of Rick Helmich

[MB2]Ref 10.1002/mds.25522

== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure L-Dopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce Levodopa fluctuations. Continuous sensing would also help researchers better understand the pharmacokinetics of Levodopa across different bodily fluids, namely ISF, blood, and cerebrospinal fluid (CSF). This could improve treatment strategies for late-stage PD patients and also help make a step towards the long-term vision of closed-loop Levodopa therapy for PD wherein continual oral administration would no longer be required, improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2025-12-24T21:12:32Z

Techsensus: /* History and Current Situation 0f Parkinson's Disease */

Parkinson's Disease

2025-12-24T20:44:35Z

Techsensus: /* General information */

Parkinson's Disease

2025-12-02T16:19:47Z

Techsensus: /* General information */

== General information ==

The theme of SensUs 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder caused by the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficiency in the basal ganglia, which are critical for initiating and smoothing movement. The primary motor symptoms advancing from this deficit include tremor, muscle rigidity, slowness of movement, and postural instability.<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed from the gastrointestinal tract and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, the concentration of levodopa in the blood serum directly influences the therapeutic effect and the onset of motor complications. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes unpredictable, leading to motor fluctuations and dyskinesias. Therefore, monitoring levodopa levels serves as a crucial tool for optimizing dosing regimens. There is no cure for Parkinson's disease, although medications and therapies can manage the symptoms. In advanced cases, patients may undergo surgical treatments like deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation 0f Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficiency as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor, postural instability) and non-motor features (such as loss of smell, sleep disorder, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is primarily clinical, based on a neurological examination and response to dopaminergic therapy; research into biomarkers and imaging (e.g. DAT scans) is ongoing but not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although long-term use often leads to motor fluctuations and dyskinesias, and DBS is an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction around 1970. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with an AAADI ('''a'''romatic L-'''a'''mino '''a'''cid '''d'''ecarboxylase '''i'''nhibitor), like carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Long-term usage of L-Dopa can lead to complications. Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in greater fluctuations in L-Dopa levels <ref name=":0" />. This highlights the need for precise L-Dopa dosing. ''Figure 1'' illustrates how the narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and off-time (return of PD symptoms).

== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

SensUs 2026 proposes to measure L-Dopa in simulated ISF in the range 3-50 μM, covering therapeutic concentrations as well as concentrations related to the management of levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce Levodopa fluctuations. Continuous sensing would also help researchers better understand the pharmacokinetics of Levodopa across different bodily fluids, namely ISF, blood, and cerebrospinal fluid (CSF). This could improve treatment strategies for late-stage PD patients and also help make a step towards the long-term vision of closed-loop Levodopa therapy for PD wherein continual oral administration would no longer be required, improving the patients’ quality of life.<ref name=":0" />

MediaWiki:Sidebar

2025-11-28T13:29:29Z

Techsensus:

MediaWiki:Sidebar

2025-11-28T13:28:02Z

Techsensus:

Main Page

2025-11-28T13:14:32Z

Techsensus: /* SensUs 2024/2025 */

=== SensUs 2025/2026 ===
The theme of SensUs 2025/2026 is [[Parkinson's Disease]].

Parkinson's Disease

2025-11-19T11:06:39Z

Techsensus: /* Levodopa Biosensors */

== General information ==

The theme of 2026 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder caused by the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficiency in the basal ganglia, which are critical for initiating and smoothing movement. The primary motor symptoms advancing from this deficit include tremor, muscle rigidity, slowness of movement, and postural instability.<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed from the gastrointestinal tract and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, the concentration of levodopa in the blood serum directly influences the therapeutic effect and the onset of motor complications. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes unpredictable, leading to motor fluctuations and dyskinesias. Therefore, monitoring levodopa levels serves as a crucial tool for optimizing dosing regimens. There is no cure for Parkinson's disease, although medications and therapies can manage the symptoms. In advanced cases, patients may undergo surgical treatments like deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation 0f Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficiency as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor, postural instability) and non-motor features (such as loss of smell, sleep disorder, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is primarily clinical, based on a neurological examination and response to dopaminergic therapy; research into biomarkers and imaging (e.g. DAT scans) is ongoing but not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although long-term use often leads to motor fluctuations and dyskinesias, and DBS is an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction around 1970. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with an AAADI ('''a'''romatic L-'''a'''mino '''a'''cid '''d'''ecarboxylase '''i'''nhibitor), like carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Long-term usage of L-Dopa can lead to complications. Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in greater fluctuations in L-Dopa levels <ref name=":0" />. This highlights the need for precise L-Dopa dosing. ''Figure 1'' illustrates how the narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and off-time (return of PD symptoms).

== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

The approximate concentration range of L-Dopa in ISF is 3-50 μM. SensUs 2026 proposes to focus on the upper ranges of the concentration, in order to manage levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce Levodopa fluctuations. Continuous sensing would also help researchers better understand the pharmacokinetics of Levodopa across different bodily fluids, namely ISF, blood, and cerebrospinal fluid (CSF). This could improve treatment strategies for late-stage PD patients and also help make a step towards the long-term vision of closed-loop Levodopa therapy for PD wherein continual oral administration would no longer be required, improving the patients’ quality of life.<ref name=":0" />

Parkinson's Disease

2025-11-18T17:03:45Z

Techsensus: /* General information */

Parkinson's Disease

2025-11-15T19:23:28Z

Techsensus: First complete draft

== General information ==

The theme of 2025/26 is Parkinson's disease and levodopa monitoring. Parkinson's disease is a progressive neurodegenerative disorder caused by the loss of dopamine-producing neurons in the substantia nigra region of the brain. This results in a severe dopamine deficiency in the basal ganglia, which are critical for initiating and smoothing movement. The primary motor symptoms advancing from this deficit include tremor, muscle rigidity, slowness of movement, and postural instability.<ref>AANS. (2024, April 30). ''Parkinson’s Disease''. American Association of Neurological Surgeons. <nowiki>https://www.aans.org/patients/conditions-treatments/parkinsons-disease</nowiki></ref> Generally, the management of motor symptoms relies on the administration of levodopa (L-Dopa), a dopamine precursor that can cross the blood-brain barrier, unlike dopamine itself.<ref>StatPearls. (n.d.). ''Levodopa''. In NCBI Bookshelf. <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK482140/</nowiki> — “Unlike dopamine, levodopa can cross the blood-brain barrier (BBB).”</ref> <ref>Chen, Y., et al. (2025). ''Translation-Neurodegeneration'', 14:10. “Unlike dopamine, levodopa crosses the blood–brain barrier …” <nowiki>https://doi.org/10.1186/s40035-025-00467-8</nowiki></ref>Levodopa is absorbed from the gastrointestinal tract and transported via the bloodstream to the brain, where it is decarboxylated into dopamine to restore motor function. Consequently, the concentration of levodopa in the blood serum directly influences the therapeutic effect and the onset of motor complications. A standard therapeutic range for plasma levodopa is typically considered to be between 2 - 7.6 µM following a dose.<ref name=":0">Probst, D.; Batchu, K.; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ACS Sensors 2024, 9 (8), 3828–3839.\href{<nowiki>https://doi.org/10.1021/acssensors.4c00602}{https://doi.org/10.1021/acssensors.4c00602}</nowiki> </ref> As the disease progresses, the relationship between dose and clinical response becomes unpredictable, leading to motor fluctuations and dyskinesias. Therefore, monitoring levodopa levels serves as a crucial tool for optimizing dosing regimens. There is no cure for Parkinson's disease, although medications and therapies can manage the symptoms. In advanced cases, patients may undergo surgical treatments like deep brain stimulation to help control motor symptoms.<ref>Perestelo-Pérez, L., et al. (2019). ''Efficacy and Safety of Deep Brain Stimulation in the Treatment of Parkinson’s Disease: A Systematic Review and Meta-analysis of Randomized Controlled Trials.'' ''Frontiers in Neurology'', 10, 857. <nowiki>https://doi.org/10.3389/fneur.2019.00857</nowiki></ref><ref>Odekerken, V. J. J., et al. (2015). ''Deep brain stimulation in Parkinson's disease: meta-analysis of randomized controlled trials.'' ''Movement Disorders'', 30(10), 1501–1510. <nowiki>https://doi.org/10.1002/mds.26237</nowiki></ref>

== History and Current Situation 0f Parkinson's Disease ==

Although tremor-like syndromes appeared in historical accounts, Parkinson’s disease was first clearly defined in 1817, when James Parkinson published ''An Essay on the Shaking Palsy'', describing six cases with trembling at rest, slowness, and a peculiar posture. <ref>Parkinson, J. (1817). ''An Essay on the Shaking Palsy''. London: Whittingham & Rowland.</ref> In the late 19th century, Jean-Martin Charcot refined this clinical picture, distinguishing rigidity and bradykinesia, and popularized the eponym “Parkinson’s disease.” <ref>LaFia, D. J. (1967). ''The Shaking Palsy 1817–1967''. ''JAMA''. </ref> Pathological and biochemical studies in the 20th century uncovered degeneration of the substantia nigra and the presence of Lewy bodies, leading to the understanding of dopamine deficiency as a core mechanism. <ref name=":1">Lees, A. J. (2017). The history of Parkinson’s disease: early clinical descriptions and neurological therapies. ''Brain'', 140(3), 843–848. <nowiki>https://doi.org/10.1093/brain/awx035</nowiki></ref> Treatment evolved from early anticholinergic drugs to the introduction of levodopa in the 1960s, and later to surgical and neuromodulation techniques such as deep brain stimulation (DBS). <ref name=":1" />

Parkinson’s disease (PD) is now a rapidly growing global health challenge. In 2021, about 11.77 million people worldwide were estimated to be living with PD, with age-standardized prevalence at ~ 138.6 per 100,000 and incidence at ~ 15.63 per 100,000.<ref name=":2">GBD 2021 Parkinson’s Disease Collaborators. (2024). Global burden of Parkinson’s disease from 1990 to 2021: a population-based study. ''Lancet Neurology / GBD Data''.</ref> Over the past three decades, rates of PD incidence, prevalence, and disability (DALYs) have all increased, particularly in men. <ref name=":2" /> Projections suggest that by 2050, the number of people with PD could more than double to ~ 25.2 million, largely driven by aging populations. <ref>Xie, Y., et al. (2024). Projections for prevalence of Parkinson’s disease by 2050: modeling study based on Global Burden of Disease 2021. ''BMJ / PubMed''. </ref>

On a biological level, PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and accumulation of α-synuclein aggregates (Lewy bodies). <ref name=":1" /> Both genetic factors (e.g. LRRK2, GBA) and environmental exposures (e.g. pesticides) contribute to disease risk. <ref name=":1" /> Clinically, PD manifests with motor symptoms (bradykinesia, rigidity, resting tremor, postural instability) and non-motor features (such as loss of smell, sleep disorder, autonomic problems, depression), many of which may begin years before motor onset. <ref name=":1" /> Diagnosis is primarily clinical, based on a neurological examination and response to dopaminergic therapy; research into biomarkers and imaging (e.g. DAT scans) is ongoing but not yet standard. <ref name=":1" /> Treatment remains symptomatic: levodopa is the cornerstone, although long-term use often leads to motor fluctuations and dyskinesias, and DBS is an option for selected patients. <ref name=":1" /> Major challenges include disease heterogeneity, the lack of reliable biomarkers for early detection or progression, and unequal access to advanced therapies — but wearable and biosensor technologies are promising tools for monitoring and early diagnosis.

== Role of Levodopa(L-Dopa) in the Treatment of Parkinson's Disease ==
Levodopa has remained the benchmark treatment for PD since its introduction around 1970. It has a therapeutic reference range of 0.76-1.25 μM in cerebrospinal fluid (CSF) and 2-7.6 μM in blood plasma <ref name=":0" />. It has a short half-life of 30-90 minutes, so it is typically administered in combination with an AAADI ('''a'''romatic L-'''a'''mino '''a'''cid '''d'''ecarboxylase '''i'''nhibitor), like carbidopa (exclusively an L- isomer) or benserazide (a racemic mixture), to increase the efficacy of L-Dopa entering into the CNS ('''c'''entral '''n'''ervous '''s'''ystem) by minimizing the peripheral conversion of L-Dopa to dopamine <ref>Carvey, P. M. Dopa-Decarboxylase Inhibitors. ''Encyclopedia of Movement Disorders'' '''2010''', 313–316. <nowiki>https://doi.org/10.1016/b978-0-12-374105-9.00318-x</nowiki></ref>.

Long-term usage of L-Dopa can lead to complications. Early in PD, the brain can store and regulate dopamine well, providing stable symptom relief. However, as the disease progresses to later stages, this ability of the brain weakens, resulting in greater fluctuations in L-Dopa levels <ref name=":0" />. This highlights the need for precise L-Dopa dosing. ''Figure 1'' illustrates how the narrowing of the L-Dopa therapeutic window over time leads to '''l'''evodopa-'''i'''nduced '''d'''yskinesia (LID; involuntary, uncontrolled movements) and off-time (return of PD symptoms).

== State of the Art ==

==== Current Methods to Measure L-Dopa ====
The current gold standards to measure L-Dopa are Liquid-Chromatography-Mass Spectrometry (LC-MS) and High Performance Liquid Chromatography (HPLC). However, these methods are time-consuming, costly, and are performed in centralized laboratories, making them impractical for timely adjustment of L-Dopa does for PD patients. This causes a need for reliable, affordable, quick, and more user-friendly L-Dopa testing.<ref>Kuldeep Mahato; Moon, J.-M.; Chochanon Moonla; Longardner, K.; Ghodsi, H.; Litvan, I.; Wang, J. Biosensor Strip for Rapid On‐Site Assessment of Levodopa Pharmacokinetics along with Motor Performance in Parkinson’s Disease. ''Angewandte Chemie International Edition'' '''2024'''. <nowiki>https://doi.org/10.1002/anie.202403583</nowiki></ref>

==== Matrix ====
Several matrices are available in which L-Dopa can be measured by a biosensor, namely blood (plasma), sweat, or interstitial fluid (ISF). ISF has been selected as the matrix for SensUs 2026, due to the ease of accessibility compared to blood plasma and the more stable composition compared to sweat <ref>Peterson, K. L.; Shukla, R. P.; Daniele, M. A. Percutaneous Wearable Biosensors: A Brief History and Systems Perspective. ''Advanced Sensor Research'' '''2024'''. <nowiki>https://doi.org/10.1002/adsr.202400068</nowiki></ref>.

Currently, ISF is used for continuous glucose monitoring. Interstitial skin fluid (ISF) makes up 75% of extracellular fluid and 15-25% of body weight <ref name=":3">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I.,

Felner, E. I., Jones, D. P., Miller, G. W., Prausnitz, M. R. (2020). Sampling interstitial fluid

from human skin using a microneedle patch. Science Translational Medicine, 12(571).

<nowiki>https://doi.org/10.1126/scitranslmed.aaw0285</nowiki></ref>. It surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum <ref name=":3" />.

The approximate concentration range of L-Dopa in ISF is 3-50 μM. SensUs 2026 proposes to focus on the upper ranges of the concentration, in order to manage levodopa-induced dyskinesia.

==== Test Stability ====
In solution, L-Dopa is chemically unstable and naturally degrades over time due to interactions with proteins and oxidative processes, leading to its conversion to dopamine or other byproducts in the L-Dopa metabolic pathway. The degradation rate can be slowed down by <ref>Pappert, E. J.; Buhrfiend, C.; Lipton, J. W.; Carvey, P. M.; Stebbins, G. T.; Goetz, C. G. Levodopa Stability in Solution: Time Course, Environmental Effects, and Practical Recommendations for Clinical Use. ''Movement Disorders'' '''1996''', ''11'' (1), 24–26. <nowiki>https://doi.org/10.1002/mds.870110106</nowiki></ref>:

* refrigeration or freezing: stable for approximately one week;
* ascorbate addition: stable for approximately three days.

== Levodopa Biosensors ==
A continuous wearable L-Dopa sensor would help doctors to make informed decisions about personalized patient treatment and how to adjust medication in real-time to reduce Levodopa fluctuations. Continuous sensing would also help researchers better understand the pharmacokinetics of Levodopa across different bodily fluids, namely ISF, blood, and cerebrospinal fluid (CSF). This could improve treatment strategies for late-stage PD patients and also help make a step towards the long-term vision of closed-loop Levodopa therapy for PD wherein continual oral administration would no longer be required, improving the patients’ quality of life.<ref>Probst, D.; Kartheek Batchu; Younce, J. R.; Sode, K. Levodopa: From Biological Significance to Continuous Monitoring. ''ACS Sensors'' '''2024''', ''9'' (8), 3828–3839. <nowiki>https://doi.org/10.1021/acssensors.4c00602</nowiki></ref>

Parkinson's Disease

2025-10-13T13:04:41Z

Techsensus: History added

Parkinson's Disease

2025-10-02T09:11:44Z

Techsensus: /* Levodopa biosensors */

Parkinson's Disease

2025-09-29T20:27:07Z

Techsensus: Added general structure

Parkinson's Disease

2025-09-29T20:22:26Z

Techsensus: /* General information */

Parkinson's Disease

2025-09-29T16:11:47Z

Techsensus: Created page for SensUs 2025/2026

== General information ==

== History 0f Parkinson's Disease ==

Acute Kidney Injury

2025-04-24T14:29:28Z

Techsensus: Techsensus moved page Acute Kidney Injury to Kidney Failure: Name change from Acute Kidney Injury to Kidney Failure decided in 2025

#REDIRECT [[Kidney Failure]]

Kidney Failure

2025-04-24T14:29:28Z

Techsensus: Techsensus moved page Acute Kidney Injury to Kidney Failure: Name change from Acute Kidney Injury to Kidney Failure decided in 2025

== General information ==

The theme of SensUs 2024/2025 is kidney failure. Kidney failure is characterized by one or both kidneys losing their renal function, namely, the ability to filter waste matter from the blood. This results in an accumulation of waste in the bloodstream, altering the ionic homeostasis of the blood. There are 5 stages of kidney failure depending on glomerular filtration rate (GFR) which measures the blood filtration rates of the kidneys (in (mL/min)), with the preliminary signs advancing to kidney failure including fatigue, nausea, swelling, etc.<ref name="Ref2> End-stage renal disease - Diagnosis and treatment - Mayo Clinic. (2023, October 10). https://www.mayoclinic.org/diseases-conditions/end-stage-renal-disease/diagnosis-treatment/drc-20354538 </ref> Generally, the clearance of substances that are freely filtered but not secreted or reabsorbed by the kidneys is used to estimate the GFR in clinical settings, with creatinine meeting the criteria.<ref name="Ref4"> López-Giacoman, S., & Madero, M. (2015). Biomarkers in chronic kidney disease, from kidney function to kidney damage. World Journal of Nephrology, 4(1), 57. https://doi.org/10.5527/wjn.v4.i1.57 </ref>
Creatinine is a product of the metabolism of creatine, which is produced in the liver from three amino acids, methionine, arginine, and glycine, and stored in muscle to be used as a source of energy once phosphorylated. Creatinine is normally excreted through the kidneys. Healthy kidneys are responsible for filtering creatinine out of the bloodstream, as it is a freely filtered metabolite that is not secreted or reabsorbed. Consequently, during kidney failure when the GFR reduces, there is a buildup of high levels of creatinine in the blood. A standard range of serum creatinine levels (SCr) for healthy men is 0.7 - 1.3 mg/dL (61.9 - 114.9 µmol/L), and for healthy women is 0.6 – 1.1 mg/dL (53 – 97.2 µmol/L).<ref name="Ref5"> Creatinine blood test. (n.d.-b). Mount Sinai Health System. https://www.mountsinai.org/health-library/tests/creatinine-blood-test#:~:text=Normal%20Results,less%20muscle%20mass%20than%20men </ref> As diet and hydration has a negligible impact on serum creatinine levels, it serves as a reliable indicator of renal function. There is no cure for kidney failure, although maintaining a proper diet and medications can slow the progression of the disease. A person with kidney failure needs to undergo dialysis treatment or kidney transplantation. These two treatments allow the normal, healthy functioning of the kidneys.<ref name="Ref6">World Kidney Day. (2019, June 7). Chronic Kidney Disease - World Kidney Day. World Kidney Day -. https://www.worldkidneyday.org/facts/chronic-kidney-disease/ </ref>

== History of Kidney Failure ==
Some of the earliest knowledge about kidney and urinary diseases dates all the way back to 9000BC. It comes from the cradle of Western civilization, Mesopotamia, from the cuneiform clay tablets of Acadia, Assyria, and Babylon that contain references to urinary obstruction, stone, cysts, urethritis, stricture, and urethral discharge. In ancient Babylon physicians made diagnoses depending on the appearance of the urine. They treated symptoms with remedies derived from plants or minerals. Drugs were administered by blowing them through a tube into the urethra, most likely also to relieve urinary obstruction, and using alcohol as an anaesthetic. Much of the medical information generated in Mesopotamia was later transported to the Mediterranean, especially to Greece.<ref name = "Ref7"> Geller, M. J., and Cohen, S. L. Kidney and urinary tract disease in ancient Babylonia, with translations of the cuneiform sources. Kidney International 1995; 47: 1811–1815.</ref> <ref name = "Ref8"> Mujais, S. The future of the realm: medicine and divination in ancient Syro-Mesopotamia. Am J Nephrol 1999; 19: 133–139.</ref>
Except for infections which cause symptoms such as pyuria, pain and fever, at the time (about 450BC), most diseases of the renal parenchyma were unknown in Greek and Roman antiquity. Treatments for uraemia included the use of hot baths, sweating therapies, bloodletting and enemas. First records of urinary diseases are found in the Hippocratic Corpus, a collection of some 60 studies that are believed to represent the work of several medical writers. How much was written by Hippocrates himself remains uncertain. Nevertheless, Hippocrates of Cos (460–377 BCE) is regarded as the father of medicine, and many of the aphorisms attributed to him refer to diseases of the kidney.<ref name = "Ref7"/> <ref name = "Ref8"/> <ref name = "Ref9">Diamandopoulos, A. A. Twelve centuries of nephrological writings in the Graeco-Roman world of the Eastern Mediterranean (from Hippocrates to Aetius Amidanus). Nephrol Dial Transplant 1999; 14[Suppl 2]: 2–9.
</ref> <ref name = "Ref10">Marandola, P., Musitelli, S., Jallous, H., Speroni, A., de Bastiani, T. The Aristotelian kidney. Am J Nephrol 1994; 14: 302–306.
</ref> <ref name = "Ref11">Marketos, S. G., Eftychiadis, A. G., Diamandopoulos, A. Acute renal failure according to ancient Greek and Byzantine medical writers. J R Soc Med 1993; 86: 290–293.</ref> <ref name = "Ref12">F Reubi, [On the history of kidney disease], Schweiz Med Wochenschr 1987; 7;117(10): 369-76
</ref>
Even in the Renaissance renal diseases were still not being properly identified and oedema was generally thought to be related to liver disease. In 1827, Richard Bright provided the first, almost complete clinical description of the various forms of acute and chronic glomerulonephritis and showed that they were accompanied by macroscopic changes in the kidneys. Between 1850 and 1885, Frerichs, Klebs and Langhans described the primary glomerular lesions. Scottish chemist Thomas Graham first described dialysis in 1854. He used osmosis to separate dissolved substances and remove water through semi-permeable membranes, although he did not apply the method to medicine.<ref name = "Ref12"/>
The first human dialysis machine was constructed in 1943 by Dr Willem Kolff. His work to create an artificial kidney began in the late 1930s when he was working in a small ward at the University of Groningen Hospital in the Netherlands. Kolff’s machine is considered the first modern drum dialyzer. The first patient in the world to be treated by repeated haemodialysis was Clyde Shields in 1960 in Seattle. After the early successes in Seattle, haemodialysis established itself as the treatment of choice worldwide for chronic and acute kidney failure. Membranes, dialyzers and dialysis machines were continuously improved and manufactured industrially in ever-increasing numbers. A major step forward was the development of the first hollow-fibre dialyzer in 1964. This technology replaced the until-then traditional membranous tubes and flat membranes with a number of capillary-sized hollow membranes. This procedure allowed for the production of dialyzers with a surface area large enough to fulfil the demands of efficient dialysis treatment.<ref name = "Ref13">History of the kidney disease treatment, https://www.sgkpa.org.uk/main/history-of-the-kidney-disease-treatment
</ref> <ref name = "Ref14">The History of Dialysis, https://www.fresenius.com/history-of-dialysis
</ref>
Over the years that followed, thanks to the development of appropriate industrial manufacturing technologies, it became possible to produce large numbers of disposable dialyzers at a reasonable price. Today, dialyzers are made from entirely synthetic polysulfone, a plastic that exhibits exceptionally good filtering efficiency and tolerability for patients.<ref name = "Ref14"/>

== Mechanism of Kidney Failure ==
Kidney failure is characterized by an abrupt decline in renal function, leading to a reduction in the glomerular filtration rate (GFR) and the subsequent accumulation of nitrogenous waste products in the body.<ref name = "Ref15"> Lote, C.J., Harper, L. and Savage, C.O. (1996) ‘Mechanisms of acute renal failure’, British Journal of Anaesthesia, 77(1), pp. 82–89. doi:10.1093/bja/77.1.82.</ref> The clinical signs of kidney failure are characterized by either an elevation in serum creatinine levels, a decrease in urine output, or both.<ref name = "Ref16">Ronco, C., Bellomo, R. and Kellum, J.A. (2019) ‘Acute kidney injury’, The Lancet, 394(10212), pp. 1949–1964. doi:10.1016/s0140-6736(19)32563-2
</ref> The causes of this disorder can then be classified into three categories, namely, pre-renal, intrinsic renal or post renal.<ref name = "Ref15"/>

Pre-renal kidney failure is a term used to describe the condition in which there is a systemic circulation disorder leading to a reduction in renal perfusion and subsequently a reduction in GFR.<ref name = "Ref17"> Kellum, J.A. and Lameire, N. (2013) ‘Diagnosis, evaluation, and management of Acute Kidney Injury: A KDIGO summary (part 1)’, Critical Care, 17(1), p. 204. doi:10.1186/cc11454. </ref> Notable causes that can contribute to pre-renal kidney failure include reduced blood volume, peripheral vasodilation, reduced arterial pressure or impaired cardiac function, leading to a reduced cardiac output.<ref name = "Ref17"/> Characterising a condition as pre-renal implies that addressing the root cause of the circulatory disorder, by improving cardiac function or replenishing lost volume, may lead to the restoration of GFR.<ref name = "Ref16"/> However, in most cases, pre-renal failure is often followed by intrinsic renal failure where the GFR of a patient may not be restored, despite addressing pre-renal causes.

Intrinsic renal failure refers to direct damage to the kidney itself and is categorised by the location of the injury, most commonly occuring to the glomerulus or the tubule, and include the interstitial or vascular portions of the kidney.<ref name = "Ref18"> Sharfuddin, A.A. et al. (2012) ‘Acute kidney injury’, Brenner and Rector’s The Kidney, pp. 1044–1099. doi:10.1016/b978-1-4160-6193-9.10030-2.
</ref> The typical causes for each include the inflammation and structural damage of the glomerular cells (glomerulonephritis), interstitial cells (acute interstitial nephritis) or the tubular epithelial cells (acute tubular necrosis).<ref name = "Ref16"/> These conditions themselves can be a result of immune complexes from systemic illnesses, ischemic causes such as prolonged periods of severe hypovolemia or hypotension, nephrotoxic causes such as exposure to exogenous or endogenous toxins <ref name = "Ref18"/>, or hypersensitivity reactions to medications such as antibiotics.<ref name = "Ref19">Praga, M. and González, E. (2010) ‘Acute interstitial nephritis’, Kidney International, 77(11), pp. 956–961. doi:10.1038/ki.2010.89. </ref>

Post-renal failure or obstructive renal failure are caused by disease states downstream of the kidneys resulting in extrarenal obstruction of urinary flow.<ref name = "Ref20">Raup, V.T., Chang, S.L. and Eswara, J.R. (2018) ‘Post-renal acute kidney injury: Epidemiology, presentation, pathophysiology, diagnosis, and management’, Core Concepts in Acute Kidney Injury, pp. 247–256. doi:10.1007/978-1-4939-8628-6_16.
</ref> These can be related to neurogenic bladder conditions, obstructed urinary catheters, bladder stones, or cancers of the bladder, prostate or ureter.<ref name = "Ref20"/>

The GFR in mL/min can be calculated with the following formula: '''GFR''' = ( UX · V̇ ) / PX. Here, UX and PX are the concentrations of substance X in urine and plasma in mg/mL respectively, with V̇ being the urine flow in mL/min. Ideally X is a substance that is freely filtered but not secreted or reabsorbed by the kidneys, subsequently having the same concentration in the plasma and glomerular filtrate.<ref name = "Ref21">Pocock, G., Richards, C.D. and Richards, D.A. (2013) Human physiology. Oxford: Oxford University Press.</ref>These criteria are largely met by creatinine, and the creatinine clearance (CCr) obtained from this formula is generally used to measure GFR in clinical practice.<ref name = "Ref22">Delgado, C. et al. (2022) ‘A unifying approach for GFR estimation: Recommendations of the NKF-ASN task force on reassessing the inclusion of race in diagnosing kidney disease’, American Journal of Kidney Diseases, 79(2). doi:10.1053/j.ajkd.2021.08.003.</ref> Other diagnostic tools also include serum creatinine levels (SCr) as in the case of renal dysfunction, the creatinine clearance by the kidneys is reduced and therefore the creatinine concentration in the blood rises.<ref name = "Ref21"/>

== State of the Art ==
=== Matrix ===
The fluid matrix for biosensor measurements in SensUs 2024/2025 is Interstitial Skin Fluid (ISF). ISF is the most prevalent fluid in the body, making up 75% of extracellular fluid and 15-25% of body weight.<ref name = "Ref24">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I., Felner, E. I., Jones, D. P., Miller, G. W., & Prausnitz, M. R. (2020). Sampling interstitial fluid from human skin using a microneedle patch. Science Translational Medicine, 12(571). https://doi.org/10.1126/scitranslmed.aaw0285
</ref> ISF surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum.<ref name = "Ref25">Samant, Pradnya P, and Mark R Prausnitz. “Mechanisms of Sampling Interstitial Fluid from Skin Using a Microneedle Patch.” Proceedings of the National Academy of Sciences of the United States of America, 2018, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5939066/
</ref> Due to its accessibility and similarity in composition to serum, ISF is a suitable candidate for continuous monitoring <ref name = "Ref23">Friedel, M., Thompson, I. a. P., Kasting, G. B., Polsky, R., Cunningham, D., Soh, H. T., & Heikenfeld, J. (2023b). Opportunities and challenges in the diagnostic utility of dermal interstitial fluid. Nature Biomedical Engineering. https://doi.org/10.1038/s41551-022-00998-9</ref> and is currently used in clinical settings for continuous glucose monitoring (CGM).

Worldwide research is ongoing on the development of continuous ISF biosensors for analytes such as glucose, urea, and cortisol, with urea being the most relevant to kidney failure.<ref name = "Ref26">Chen, Q., Zhao, Y., & Liu, Y. (2021b). Current development in wearable glucose meters. Chinese Chemical Letters, 32(12), 3705–3717. https://doi.org/10.1016/j.cclet.2021.05.043</ref> <ref name = "Ref27">Venugopal, M., Arya, S. K., Chornokur, G., & Bhansali, S. (2011b). A realtime and continuous assessment of cortisol in ISF using electrochemical impedance spectroscopy. Sensors and Actuators A: Physical, 172(1), 154–160. https://doi.org/10.1016/j.sna.2011.04.028
</ref>

=== Continuous glucose monitoring ===
Glucose sensors are commercially available for continuous monitoring, primarily used in diabetes management.<ref name = "Ref28">Johnston, L. et al. (2021) ‘Advances in biosensors for continuous glucose monitoring towards wearables’, Frontiers in Bioengineering and Biotechnology, 9. doi:10.3389/fbioe.2021.733810.
</ref> Most of the CGM biosensors with ISF as a matrix are catalytic biosensors, using glucose oxidase (GOD) as the recognition molecule to bind with glucose.<ref name = "Ref28"/> Microneedle array electrodes have been used for CGM, e.g. by functionalizing them through entrapment of GOD in an electropolymerized film <ref name = "Ref30">Sharma, S. et al. (2016) ‘Evaluation of a minimally invasive glucose biosensor for continuous tissue monitoring’, Analytical and Bioanalytical Chemistry, 408(29), pp. 8427–8435. doi:10.1007/s00216-016-9961-6.
</ref>, or by non-enzymatic amperometric readout.<ref name = "Ref31">Lee, S.J. et al. (2016) ‘A patch type non-enzymatic biosensor based on 3D sus micro-needle electrode array for minimally invasive continuous glucose monitoring’, Sensors and Actuators B: Chemical, 222, pp. 1144–1151. doi:10.1016/j.snb.2015.08.013.
</ref> Other examples of CGM biosensors in ISF include an enzymatic open circuit potential biosensor using GOD <ref name = "Ref32">Song, Y. et al. (2016) ‘Design and preparation of open circuit potential biosensor for in vitro and in vivo glucose monitoring’, Analytical and Bioanalytical Chemistry, 409(1), pp. 161–168. doi:10.1007/s00216-016-9982-1.
</ref> and an electrochemical glucose sensor composed of electroplated nanoporous platinum.<ref name = "Ref33">Yoon, H. et al. (2018) ‘Wearable, robust, non-enzymatic continuous glucose monitoring system and its in vivo investigation’, Biosensors and Bioelectronics, 117, pp. 267–275. doi:10.1016/j.bios.2018.06.008.
</ref>

=== Continuous sensing of cortisol ===
Continuous measurement of cortisol in ISF has been done using the electrochemical impedance (EIS) technique.<ref name = "Ref27"/> This technique involves gold microelectrode arrays functionalized with a self-assembled monolayer (SAM) to fabricate a disposable, electrochemical cortisol immunosensor. <ref name = "Ref27"/>

=== Continuous sensing of urea ===
Urea is an analyte that is relevant for kidney failure. Gold microneedle arrays have been studied for electrochemical sensing of urea.<ref name = "Ref35">Şenel, M., Dervisevic, M., & Voelcker, N. H. (2019). Gold microneedles fabricated by casting of gold ink used for urea sensing. Materials Letters, 243, 50–53. https://doi.org/10.1016/j.matlet.2019.02.014
</ref> Furthermore, wearable potentiometric biosensors have been studied for on-body and on-site monitoring of urea in sweat.<ref name = "Ref36">Ibáñez-Redín, G., Cagnani, G. R., Gomes, N. O., Raymundo‐Pereira, P. A., Machado, S. a. S., Gutierrez, M. A., Krieger, J. E., & Oliveira, O. N. (2023). Wearable potentiometric biosensor for analysis of urea in sweat. Biosensors and Bioelectronics, 223, 114994. https://doi.org/10.1016/j.bios.2022.114994
</ref>

== Creatinine Biosensors ==
Creatinine is a key indicator of renal function and is measured using various methods. The Jaffe reaction involves creatinine reacting with alkaline picrate to form a measurable orange-red complex, but its drawback lies in low specificity, due to interference from substances like glucose and bilirubin.<ref name = "Ref38">Creatinine - SensUS Wiki. (n.d.). https://wiki.sensus.org/index.php?title=Creatinine
</ref> Also enzymatic techniques are used for creatinine detection, e.g. creatininase amidohydrolase or creatinine deaminase in conjunction with other enzymes to convert creatinine to creatine and subsequently produce measurable hydrogen peroxide.<ref name = "Ref38"/> While enzymatic sensors are specific and sensitive, they have their drawbacks in terms of lack of stability and sensitivity to changes in pH, temperature and humidity.<ref name = "Ref38"/>

Commercially available analytical systems, such as Abbott's i-STAT system and Nova Biomedical's StatSensor CREAT, leverage enzymes and electrochemistry to provide creatinine measurements, offering a linear correlation between current and creatinine concentration.<ref name = "Ref38"/>

Potentiometric creatinine biosensors have been developed using different immobilization techniques and enzyme combinations. Potentiometric biosensors for creatinine detection rely on creatinine iminohydrolase (CIH) and subsequent ammonia detection. The sensors exhibit a linear range of 0.02 – 20.0 mM and a minimum detection limit of 10 µM, with 30 – 60 s response time.<ref name = "Ref37">Pundir, C., Kumar, P., & Jaiwal, R. (2019b). Biosensing methods for determination of creatinine: A review. Biosensors and Bioelectronics, 126, 707–724. https://doi.org/10.1016/j.bios.2018.11.031
</ref>

Nanomaterials are also being studied for creatinine detection.<ref name = "Ref40">Narimani, R., Esmaeili, M., Rasta, S. H., Khosroshahi, H. T., & Mobed, A. (2020). Trend in creatinine determining methods: Conventional methods to molecular‐based methods. Analytical Science Advances, 2(5–6), 308–325. https://doi.org/10.1002/ansa.202000074
</ref>
Sensors have been demonstrated with sensitivity in the range of 0.2 – 1.4 µM.<ref name = "Ref40"/>

Lastly, a biosensor based on particle motion (BPM) has been studied for continuous creatinine sensing.<ref name = "Ref42">Yan, J. et al. (2020) ‘Continuous small-molecule monitoring with a digital single-particle switch’, ACS Sensors, 5(4), pp. 1168–1176. doi:10.1021/acssensors.0c00220.
</ref> The sensor has a competitive format, with anti-creatinine antibodies and creatinine-analogues. The measurement range was 10–1000 μM.<ref name = "Ref42"/>

== References ==

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Techsensus: Created page with "== General information == The theme of SensUs 2024 is Kidney failure also referred to as acute kidney injury (AKI). Kidney failure is characterized by one or both kidneys losing their renal function, namely, the ability to filter waste matter from the blood. This results in an accumulation of waste in the bloodstream, altering the ionic homeostasis of the blood. There are 5 stages of kidney failure depending on glomerular filtration rate (GFR) which measures the blood f..."

== General information ==

The theme of SensUs 2024 is Kidney failure also referred to as acute kidney injury (AKI). Kidney failure is characterized by one or both kidneys losing their renal function, namely, the ability to filter waste matter from the blood. This results in an accumulation of waste in the bloodstream, altering the ionic homeostasis of the blood. There are 5 stages of kidney failure depending on glomerular filtration rate (GFR) which measures the blood filtration rates of the kidneys (in (mL/min)), with the preliminary signs advancing to kidney failure including fatigue, nausea, swelling, etc.<ref name="Ref2> End-stage renal disease - Diagnosis and treatment - Mayo Clinic. (2023, October 10). https://www.mayoclinic.org/diseases-conditions/end-stage-renal-disease/diagnosis-treatment/drc-20354538 </ref> Generally, the clearance of substances that are freely filtered but not secreted or reabsorbed by the kidneys is used to estimate the GFR in clinical settings, with creatinine meeting the criteria.<ref name="Ref4"> López-Giacoman, S., & Madero, M. (2015). Biomarkers in chronic kidney disease, from kidney function to kidney damage. World Journal of Nephrology, 4(1), 57. https://doi.org/10.5527/wjn.v4.i1.57 </ref>
Creatinine is a product of the metabolism of creatine, which is produced in the liver from three amino acids, methionine, arginine, and glycine, and stored in muscle to be used as a source of energy once phosphorylated. Creatinine is normally excreted through the kidneys. Healthy kidneys are responsible for filtering creatinine out of the bloodstream, as it is a freely filtered metabolite that is not secreted or reabsorbed. Consequently, during kidney failure when the GFR reduces, there is a buildup of high levels of creatinine in the blood. A standard range of serum creatinine levels (SCr) for healthy men is 0.7 - 1.3 mg/dL (61.9 - 114.9 µmol/L), and for healthy women is 0.6 – 1.1 mg/dL (53 – 97.2 µmol/L).<ref name="Ref5"> Creatinine blood test. (n.d.-b). Mount Sinai Health System. https://www.mountsinai.org/health-library/tests/creatinine-blood-test#:~:text=Normal%20Results,less%20muscle%20mass%20than%20men </ref> As diet and hydration has a negligible impact on serum creatinine levels, it serves as a reliable indicator of renal function. There is no cure for chronic kidney disease (CKD), although maintaining a proper diet and medications can slow the progression of the disease. A person with kidney failure needs to undergo dialysis treatment or kidney transplantation. These two treatments allow the normal, healthy functioning of the kidneys.<ref name="Ref6">World Kidney Day. (2019, June 7). Chronic Kidney Disease - World Kidney Day. World Kidney Day -. https://www.worldkidneyday.org/facts/chronic-kidney-disease/ </ref>

== History of Acute Kidney Injury ==
Some of the earliest knowledge about kidney and urinary diseases dates all the way back to 9000BC. It comes from the cradle of Western civilization, Mesopotamia, from the cuneiform clay tablets of Acadia, Assyria, and Babylon that contain references to urinary obstruction, stone, cysts, urethritis, stricture, and urethral discharge. In ancient Babylon physicians made diagnoses depending on the appearance of the urine. They treated symptoms with remedies derived from plants or minerals. Drugs were administered by blowing them through a tube into the urethra, most likely also to relieve urinary obstruction, and using alcohol as an anaesthetic. Much of the medical information generated in Mesopotamia was later transported to the Mediterranean, especially to Greece.<ref name = "Ref7"> Geller, M. J., and Cohen, S. L. Kidney and urinary tract disease in ancient Babylonia, with translations of the cuneiform sources. Kidney International 1995; 47: 1811–1815.</ref> <ref name = "Ref8"> Mujais, S. The future of the realm: medicine and divination in ancient Syro-Mesopotamia. Am J Nephrol 1999; 19: 133–139.</ref>
Except for infections which cause symptoms such as pyuria, pain and fever, at the time (about 450BC), most diseases of the renal parenchyma were unknown in Greek and Roman antiquity. Treatments for uraemia included the use of hot baths, sweating therapies, bloodletting and enemas. First records of urinary diseases are found in the Hippocratic Corpus, a collection of some 60 studies that are believed to represent the work of several medical writers. How much was written by Hippocrates himself remains uncertain. Nevertheless, Hippocrates of Cos (460–377 BCE) is regarded as the father of medicine, and many of the aphorisms attributed to him refer to diseases of the kidney.<ref name = "Ref7"/> <ref name = "Ref8"/> <ref name = "Ref9">Diamandopoulos, A. A. Twelve centuries of nephrological writings in the Graeco-Roman world of the Eastern Mediterranean (from Hippocrates to Aetius Amidanus). Nephrol Dial Transplant 1999; 14[Suppl 2]: 2–9.
</ref> <ref name = "Ref10">Marandola, P., Musitelli, S., Jallous, H., Speroni, A., de Bastiani, T. The Aristotelian kidney. Am J Nephrol 1994; 14: 302–306.
</ref> <ref name = "Ref11">Marketos, S. G., Eftychiadis, A. G., Diamandopoulos, A. Acute renal failure according to ancient Greek and Byzantine medical writers. J R Soc Med 1993; 86: 290–293.</ref> <ref name = "Ref12">F Reubi, [On the history of kidney disease], Schweiz Med Wochenschr 1987; 7;117(10): 369-76
</ref>
Even in the Renaissance renal diseases were still not being properly identified and oedema was generally thought to be related to liver disease. In 1827, Richard Bright provided the first, almost complete clinical description of the various forms of acute and chronic glomerulonephritis and showed that they were accompanied by macroscopic changes in the kidneys. Between 1850 and 1885, Frerichs, Klebs and Langhans described the primary glomerular lesions. Scottish chemist Thomas Graham first described dialysis in 1854. He used osmosis to separate dissolved substances and remove water through semi-permeable membranes, although he did not apply the method to medicine.<ref name = "Ref12"/>
The first human dialysis machine was constructed in 1943 by Dr Willem Kolff. His work to create an artificial kidney began in the late 1930s when he was working in a small ward at the University of Groningen Hospital in the Netherlands. Kolff’s machine is considered the first modern drum dialyzer. The first patient in the world to be treated by repeated haemodialysis was Clyde Shields in 1960 in Seattle. After the early successes in Seattle, haemodialysis established itself as the treatment of choice worldwide for chronic and acute kidney failure. Membranes, dialyzers and dialysis machines were continuously improved and manufactured industrially in ever-increasing numbers. A major step forward was the development of the first hollow-fibre dialyzer in 1964. This technology replaced the until-then traditional membranous tubes and flat membranes with a number of capillary-sized hollow membranes. This procedure allowed for the production of dialyzers with a surface area large enough to fulfil the demands of efficient dialysis treatment.<ref name = "Ref13">History of the kidney disease treatment, https://www.sgkpa.org.uk/main/history-of-the-kidney-disease-treatment
</ref> <ref name = "Ref14">The History of Dialysis, https://www.fresenius.com/history-of-dialysis
</ref>
Over the years that followed, thanks to the development of appropriate industrial manufacturing technologies, it became possible to produce large numbers of disposable dialyzers at a reasonable price. Today, dialyzers are made from entirely synthetic polysulfone, a plastic that exhibits exceptionally good filtering efficiency and tolerability for patients.<ref name = "Ref14"/>

== Mechanism of Acute Kidney Injury ==
Acute kidney injury (AKI), also known as acute renal failure (ARF), is characterized by an abrupt decline in renal function, leading to a reduction in the glomerular filtration rate (GFR) and the subsequent accumulation of nitrogenous waste products in the body.<ref name = "Ref15"> Lote, C.J., Harper, L. and Savage, C.O. (1996) ‘Mechanisms of acute renal failure’, British Journal of Anaesthesia, 77(1), pp. 82–89. doi:10.1093/bja/77.1.82.</ref> The clinical signs of acute kidney injury (AKI) are characterized by either an elevation in serum creatinine levels, a decrease in urine output, or both.<ref name = "Ref16">Ronco, C., Bellomo, R. and Kellum, J.A. (2019) ‘Acute kidney injury’, The Lancet, 394(10212), pp. 1949–1964. doi:10.1016/s0140-6736(19)32563-2
</ref> The causes of this disorder can then be classified into three categories, namely, pre-renal, intrinsic renal or post renal.<ref name = "Ref15"/>

Pre-renal kidney failure is a term used to describe the condition in which there is a systemic circulation disorder leading to a reduction in renal perfusion and subsequently a reduction in GFR.<ref name = "Ref17"> Kellum, J.A. and Lameire, N. (2013) ‘Diagnosis, evaluation, and management of Acute Kidney Injury: A KDIGO summary (part 1)’, Critical Care, 17(1), p. 204. doi:10.1186/cc11454. </ref> Notable causes that can contribute to pre-renal kidney failure include reduced blood volume, peripheral vasodilation, reduced arterial pressure or impaired cardiac function, leading to a reduced cardiac output.<ref name = "Ref17"/> Characterising a condition as pre-renal implies that addressing the root cause of the circulatory disorder, by improving cardiac function or replenishing lost volume, may lead to the restoration of GFR.<ref name = "Ref16"/> However, in most cases, pre-renal failure is often followed by intrinsic renal failure where the GFR of a patient may not be restored, despite addressing pre-renal causes.

Intrinsic renal failure refers to direct damage to the kidney itself and is categorised by the location of the injury, most commonly occuring to the glomerulus or the tubule, and include the interstitial or vascular portions of the kidney.<ref name = "Ref18"> Sharfuddin, A.A. et al. (2012) ‘Acute kidney injury’, Brenner and Rector’s The Kidney, pp. 1044–1099. doi:10.1016/b978-1-4160-6193-9.10030-2.
</ref> The typical causes for each include the inflammation and structural damage of the glomerular cells (glomerulonephritis), interstitial cells (acute interstitial nephritis) or the tubular epithelial cells (acute tubular necrosis).<ref name = "Ref16"/> These conditions themselves can be a result of immune complexes from systemic illnesses, ischemic causes such as prolonged periods of severe hypovolemia or hypotension, nephrotoxic causes such as exposure to exogenous or endogenous toxins <ref name = "Ref18"/>, or hypersensitivity reactions to medications such as antibiotics.<ref name = "Ref19">Praga, M. and González, E. (2010) ‘Acute interstitial nephritis’, Kidney International, 77(11), pp. 956–961. doi:10.1038/ki.2010.89. </ref>

Post-renal failure or obstructive renal failure are caused by disease states downstream of the kidneys resulting in extrarenal obstruction of urinary flow.<ref name = "Ref20">Raup, V.T., Chang, S.L. and Eswara, J.R. (2018) ‘Post-renal acute kidney injury: Epidemiology, presentation, pathophysiology, diagnosis, and management’, Core Concepts in Acute Kidney Injury, pp. 247–256. doi:10.1007/978-1-4939-8628-6_16.
</ref> These can be related to neurogenic bladder conditions, obstructed urinary catheters, bladder stones, or cancers of the bladder, prostate or ureter.<ref name = "Ref20"/>

The GFR in mL/min can be calculated with the following formula: '''GFR''' = ( UX · V̇ ) / PX. Here, UX and PX are the concentrations of substance X in urine and plasma in mg/mL respectively, with V̇ being the urine flow in mL/min. Ideally X is a substance that is freely filtered but not secreted or reabsorbed by the kidneys, subsequently having the same concentration in the plasma and glomerular filtrate.<ref name = "Ref21">Pocock, G., Richards, C.D. and Richards, D.A. (2013) Human physiology. Oxford: Oxford University Press.</ref>These criteria are largely met by creatinine, and the creatinine clearance (CCr) obtained from this formula is generally used to measure GFR in clinical practice.<ref name = "Ref22">Delgado, C. et al. (2022) ‘A unifying approach for GFR estimation: Recommendations of the NKF-ASN task force on reassessing the inclusion of race in diagnosing kidney disease’, American Journal of Kidney Diseases, 79(2). doi:10.1053/j.ajkd.2021.08.003.</ref> Other diagnostic tools also include serum creatinine levels (SCr) as in the case of renal dysfunction, the creatinine clearance by the kidneys is reduced and therefore the creatinine concentration in the blood rises.<ref name = "Ref21"/>

== State of the Art ==
=== Matrix ===
The fluid matrix for biosensor measurements in SensUs 2024 is Interstitial skin fluid (ISF). ISF is the most prevalent fluid in the body, making up 75% of extracellular fluid and 15-25% of body weight.<ref name = "Ref24">Samant, P. P., Niedzwiecki, M. M., Raviele, N., Tran, V., Mena-Lapaix, J., Walker, D. I., Felner, E. I., Jones, D. P., Miller, G. W., & Prausnitz, M. R. (2020). Sampling interstitial fluid from human skin using a microneedle patch. Science Translational Medicine, 12(571). https://doi.org/10.1126/scitranslmed.aaw0285
</ref> ISF surrounds cells and tissues, serving as an interface between blood and cells. It may be a source of biomarkers in addition to blood biomarkers, as research shows that 83% of proteins found in blood serum are also present in ISF, but 50% of proteins in ISF are not found in serum.<ref name = "Ref25">Samant, Pradnya P, and Mark R Prausnitz. “Mechanisms of Sampling Interstitial Fluid from Skin Using a Microneedle Patch.” Proceedings of the National Academy of Sciences of the United States of America, 2018, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5939066/
</ref> Due to its accessibility and similarity in composition to serum, ISF is a suitable candidate for continuous monitoring <ref name = "Ref23">Friedel, M., Thompson, I. a. P., Kasting, G. B., Polsky, R., Cunningham, D., Soh, H. T., & Heikenfeld, J. (2023b). Opportunities and challenges in the diagnostic utility of dermal interstitial fluid. Nature Biomedical Engineering. https://doi.org/10.1038/s41551-022-00998-9</ref> and is currently used in clinical settings for continuous glucose monitoring (CGM).

Worldwide research is ongoing on the development of continuous ISF biosensors for analytes such as glucose, urea, and cortisol, with urea being the most relevant to kidney failure.<ref name = "Ref26">Chen, Q., Zhao, Y., & Liu, Y. (2021b). Current development in wearable glucose meters. Chinese Chemical Letters, 32(12), 3705–3717. https://doi.org/10.1016/j.cclet.2021.05.043</ref> <ref name = "Ref27">Venugopal, M., Arya, S. K., Chornokur, G., & Bhansali, S. (2011b). A realtime and continuous assessment of cortisol in ISF using electrochemical impedance spectroscopy. Sensors and Actuators A: Physical, 172(1), 154–160. https://doi.org/10.1016/j.sna.2011.04.028
</ref>

=== Continuous glucose monitoring ===
Glucose sensors are commercially available for continuous monitoring, primarily used in diabetes management.<ref name = "Ref28">Johnston, L. et al. (2021) ‘Advances in biosensors for continuous glucose monitoring towards wearables’, Frontiers in Bioengineering and Biotechnology, 9. doi:10.3389/fbioe.2021.733810.
</ref> Most of the CGM biosensors with ISF as a matrix are catalytic biosensors, using glucose oxidase (GOD) as the recognition molecule to bind with glucose.<ref name = "Ref28"/> Microneedle array electrodes have been used for CGM, e.g. by functionalizing them through entrapment of GOD in an electropolymerized film <ref name = "Ref30">Sharma, S. et al. (2016) ‘Evaluation of a minimally invasive glucose biosensor for continuous tissue monitoring’, Analytical and Bioanalytical Chemistry, 408(29), pp. 8427–8435. doi:10.1007/s00216-016-9961-6.
</ref>, or by non-enzymatic amperometric readout.<ref name = "Ref31">Lee, S.J. et al. (2016) ‘A patch type non-enzymatic biosensor based on 3D sus micro-needle electrode array for minimally invasive continuous glucose monitoring’, Sensors and Actuators B: Chemical, 222, pp. 1144–1151. doi:10.1016/j.snb.2015.08.013.
</ref> Other examples of CGM biosensors in ISF include an enzymatic open circuit potential biosensor using GOD <ref name = "Ref32">Song, Y. et al. (2016) ‘Design and preparation of open circuit potential biosensor for in vitro and in vivo glucose monitoring’, Analytical and Bioanalytical Chemistry, 409(1), pp. 161–168. doi:10.1007/s00216-016-9982-1.
</ref> and an electrochemical glucose sensor composed of electroplated nanoporous platinum.<ref name = "Ref33">Yoon, H. et al. (2018) ‘Wearable, robust, non-enzymatic continuous glucose monitoring system and its in vivo investigation’, Biosensors and Bioelectronics, 117, pp. 267–275. doi:10.1016/j.bios.2018.06.008.
</ref>

=== Continuous sensing of cortisol ===
Continuous measurement of cortisol in ISF has been done using the electrochemical impedance (EIS) technique.<ref name = "Ref27"/> This technique involves gold microelectrode arrays functionalized with a self-assembled monolayer (SAM) to fabricate a disposable, electrochemical cortisol immunosensor. <ref name = "Ref27"/>

=== Continuous sensing of urea ===
Urea is an analyte that is relevant for AKI. Gold microneedle arrays have been studied for electrochemical sensing of urea.<ref name = "Ref35">Şenel, M., Dervisevic, M., & Voelcker, N. H. (2019). Gold microneedles fabricated by casting of gold ink used for urea sensing. Materials Letters, 243, 50–53. https://doi.org/10.1016/j.matlet.2019.02.014
</ref> Furthermore, wearable potentiometric biosensors have been studied for on-body and on-site monitoring of urea in sweat.<ref name = "Ref36">Ibáñez-Redín, G., Cagnani, G. R., Gomes, N. O., Raymundo‐Pereira, P. A., Machado, S. a. S., Gutierrez, M. A., Krieger, J. E., & Oliveira, O. N. (2023). Wearable potentiometric biosensor for analysis of urea in sweat. Biosensors and Bioelectronics, 223, 114994. https://doi.org/10.1016/j.bios.2022.114994
</ref>

== Creatinine Biosensors ==
Creatinine is a key indicator of renal function and is measured using various methods. The Jaffe reaction involves creatinine reacting with alkaline picrate to form a measurable orange-red complex, but its drawback lies in low specificity, due to interference from substances like glucose and bilirubin.<ref name = "Ref38">Creatinine - SensUS Wiki. (n.d.). https://wiki.sensus.org/index.php?title=Creatinine
</ref> Also enzymatic techniques are used for creatinine detection, e.g. creatininase amidohydrolase or creatinine deaminase in conjunction with other enzymes to convert creatinine to creatine and subsequently produce measurable hydrogen peroxide.<ref name = "Ref38"/> While enzymatic sensors are specific and sensitive, they have their drawbacks in terms of lack of stability and sensitivity to changes in pH, temperature and humidity.<ref name = "Ref38"/>

Commercially available analytical systems, such as Abbott's i-STAT system and Nova Biomedical's StatSensor CREAT, leverage enzymes and electrochemistry to provide creatinine measurements, offering a linear correlation between current and creatinine concentration.<ref name = "Ref38"/>

Potentiometric creatinine biosensors have been developed using different immobilization techniques and enzyme combinations. Potentiometric biosensors for creatinine detection rely on creatinine iminohydrolase (CIH) and subsequent ammonia detection. The sensors exhibit a linear range of 0.02 – 20.0 mM and a minimum detection limit of 10 µM, with 30 – 60 s response time.<ref name = "Ref37">Pundir, C., Kumar, P., & Jaiwal, R. (2019b). Biosensing methods for determination of creatinine: A review. Biosensors and Bioelectronics, 126, 707–724. https://doi.org/10.1016/j.bios.2018.11.031
</ref>

Nanomaterials are also being studied for creatinine detection.<ref name = "Ref40">Narimani, R., Esmaeili, M., Rasta, S. H., Khosroshahi, H. T., & Mobed, A. (2020). Trend in creatinine determining methods: Conventional methods to molecular‐based methods. Analytical Science Advances, 2(5–6), 308–325. https://doi.org/10.1002/ansa.202000074
</ref>
Sensors have been demonstrated with sensitivity in the range of 0.2 – 1.4 µM.<ref name = "Ref40"/>

Lastly, a biosensor based on particle motion (BPM) has been studied for continuous creatinine sensing.<ref name = "Ref42">Yan, J. et al. (2020) ‘Continuous small-molecule monitoring with a digital single-particle switch’, ACS Sensors, 5(4), pp. 1168–1176. doi:10.1021/acssensors.0c00220.
</ref> The sensor has a competitive format, with anti-creatinine antibodies and creatinine-analogues. The measurement range was 10–1000 μM.<ref name = "Ref42"/>

== References ==

MediaWiki:Sidebar

2024-12-04T14:59:14Z

Traumatic Brain Injury

2024-12-04T14:58:32Z

Techsensus: Created page with "Release of GFAP after astrocyte injury. <ref name="Arti1">Glial Fibrillary Acidic Protein in Blood as a Disease Biomarker of Neuromyelitis Optica Spectrum Disorders. Front. Neurol., 17 March 2022, https://www.frontiersin.org/articles/10.3389/fneur.2022.865730/full#F1</ref> == General information == The theme of SensUs 2023 is Traumatic Brain Injury (TBI). It is often stated in the literature that TBI is a..."

[[File: Release_of_GFAP_after_astrocyte_injury.png|thumb|360px|Release of GFAP after astrocyte injury. <ref name="Arti1">Glial Fibrillary Acidic Protein in Blood as a Disease Biomarker of Neuromyelitis Optica Spectrum Disorders. Front. Neurol., 17 March 2022, https://www.frontiersin.org/articles/10.3389/fneur.2022.865730/full#F1</ref>]]

== General information ==
The theme of SensUs 2023 is Traumatic Brain Injury (TBI). It is often stated in the literature that TBI is a silent epidemic with an estimated 64–74 million new cases presenting each year.<ref name="Arti2">Rattani, A., Gupta, S., Baticulon, R. E., Hung, Y. C., Punchak, M., Agrawal, A., Adeleye, A. O., Shrime, M. G., Rubiano, A. M., Rosenfeld, J. V., & Park, K. B. (2019). Estimating the global incidence of traumatic brain injury. Journal of Neurosurgery, 130(4), 1080–1097. https://doi.org/10.3171/2017.10.jns17352 </ref> The impairments suffered by many TBI patients, such as memory loss, cognitive dysfunction, or behavioural disturbance, are often not visible. The economic and social impact is considerable, with an estimate of direct medical expenditures and indirect costs (e.g., loss of productivity) attributed to TBI exceeding $60 billion in 2000 in the USA. TBI is defined as an alteration in brain function, or other evidence of brain pathology, caused by an external force.<ref name="Arti3">Menon, D. K., Schwab, K., Wright, D. W., & Maas, A. I. (2010). Position Statement: Definition of Traumatic Brain Injury. Archives of Physical Medicine and Rehabilitation, 91(11), 1637–1640. https://doi.org/10.1016/j.apmr.2010.05.017 </ref> The severity of injury in TBI is classified as mild, moderate, or severe with mild TBIs being the most common.<ref name="Arti4">Iaccarino, C., Gerosa, A., & Viaroli, E. (2021). Epidemiology of Traumatic Brain Injury. In S. Honeybul & A. G. Kolias (Eds.), Traumatic Brain Injury. Springer. https://link.springer.com/chapter/10.1007/978-3-030-78075-3_1 </ref> Alteration in brain function can be manifest by loss or decreased level of consciousness, alteration in mental state, incomplete memory for the event, or neurological deficits. Examples of external forces include the head striking or being struck by an object, rapid acceleration or deceleration of the brain, penetration of the brain by a foreign object, and exposure to forces associated with blasts.

GFAP is a protein found in the glial cells of neural tissue. Glial cells are non-neuronal cells that provide physical and metabolic support to neurons. Blood biomarker levels of GFAP reflect acute injury to brain tissue since the biomarker levels increase strongly and fast within hours after the trauma occurred. Therefore, it has potential use in the emergency unit and intensive care unit, directly after the TBI. Furthermore, the biomarker persists for an extended period with a half-life of 48 hours, making it a favourable biomarker to use in both the acute and subacute phases of injury. The concentration of GFAP peaks at about 20 hours after the injury. Reference serum levels of GFAP range from 0.02 — 0.35 𝜇g/L and the cut-off value of a negative CT scan for GFAP is 0.35 𝜇g/L.<ref name="Arti5">Papa, L., Brophy, G. M., Welch, R. D., Lewis, L. M., Braga, C. F., Tan, C. N., Ameli, N. J., Lopez, M. A., Haeussler, C. A., Mendez Giordano, D. I., Silvestri, S., Giordano, P., Weber, K. D., Hill-Pryor, C., & Hack, D. C. (2016). Time Course and Diagnostic Accuracy of Glial and Neuronal Blood Biomarkers GFAP and UCH-L1 in a Large Cohort of Trauma Patients With and Without Mild Traumatic Brain Injury. JAMA Neurology, 73(5), 551. https://doi.org/10.1001/jamaneurol.2016.0039 </ref>

== History of TBI ==
=== Pre-historic period ===
TBIs play an important role in the evolution of humans. Studies of skeletons from the mesolithic period (c. 7000 BC) suggest an inverse relationship between the risk of skull fracture and the progress of civilization. The palaeolithic period is a period in prehistory that occurred over a million years ago. In those times, man's predecessor was a semi-erect hominid now named Australopithecus africanus; a damaged skull from this species found in South Africa reveals the first evidence of brain injury.<ref name="Arti6">Rose, F. C. (1997). The history of head injuries: An overview*. Journal of the History of the Neurosciences, 6(2), 154–180. https://doi.org/10.1080/09647049709525700 </ref>

One of the earliest methods used for treating TBI is called Trepanation. Trepanning, also known as trepanation, is a surgical intervention in which a hole is drilled or scraped into the human skull. The earliest evidence of trepanation performed by the man himself appears in the Neolithic period. The primary theories for the practice of trepanation in ancient times include spiritual purposes and treatment for epilepsy, headache, head wound, and mental disorders.

=== Ancient History ===
The first written evidence of TBI is documented in an ancient Egyptian text known as the Edwin Smith Papyrus which was written around 1650-1550 BC and believed to be a copy of an even older manuscript written around 3000 BC. It describes various head injuries and symptoms and classifies them based on their presentation and tractability.<ref name="Arti7">Sanchez, G. M., & Burridge, A. L. (2007). Decision making in head injury management in the Edwin Smith Papyrus. Neurosurgical Focus, 23(1), 1–9. https://doi.org/10.3171/foc-07/07/e5 </ref> This text is so old that it goes as far as mentioning magic as a last resort in terminal cases. To get an idea of what treatment for a brain injury was like then, bandaging the head wound with meat and applying a honey and oil type of dressing until healed were common methods.<ref name="Arti8">The History of Brain Injuries. (2018, March 29). Intrepid Fallen Heroes Fund. Retrieved October 17, 2022, from https://www.fallenheroesfund.org/the-history-of-brain-injuries </ref> There are several biblical references to head injuries which are reported to have occurred in the 12-10th century BC. These references to head injuries include: Sisera’s death at the hands of Jael, skull fractures on Abimelech, and the most famous death of Goliath by David.<ref name="Arti9">Feinsod, M. (1997). Three head injuries: The biblical account of the deaths of Sisera, Abimelech and Goliath. Journal of the History of the Neurosciences, 6(3), 320–324. https://doi.org/10.1080/09647049709525717 </ref>

Hippocrates of Kos, also known as Hippocrates II, was a Greek physician of the classical period who is considered one of the most outstanding figures in the history of medicine and is referred to as the "Father of Medicine”. The first systematic approach to head injuries was by Hippocrates. The Hippocratic Corpus consisted of 76 Treatises, one of which was "On Injuries of the Head".<ref name="Arti10">Hippocrates. (n.d.). On Injuries of the Head by Hippocrates (F. Adams, Trans.). The Internet Classics Archive. Retrieved October 17, 2022, from http://classics.mit.edu/Hippocrates/headinjur.13.13.html </ref>

=== Medieval History ===
In the Middle Ages, physicians further described head injury symptoms and the term concussion became more widespread. Berengario da Carpi, an Italian physician, published one of the first books on head injuries. He categorized head injuries into lacerations, contusions and perforations, each of which could be associated with a fracture. A prime example from the 16th century is also one of history’s most well-known figures. A recent study argues that King Henry VIII of England’s erratic behavior was a result of possible repeated traumatic brain injuries. Researchers have made a compelling case citing notes that describe changes including memory loss, irritability, impulsive nature, and insomnia. All known today as common symptoms of a traumatic brain injury.<ref name="Arti11">Ikram, M. Q., Sajjad, F. H., & Salardini, A. (2016). The head that wears the crown: Henry VIII and traumatic brain injury. Journal of Clinical Neuroscience, 28, 16–19. https://doi.org/10.1016/j.jocn.2015.10.035 </ref>

=== Modern Era ===
It was first suggested in the 18th century that symptoms arising from a head injury are not due to a fractured skull, but to injury of the brain. Percival Pott (1713-1788) was one of the first to emphasize that it was the neurological status and not the skull fracture that determined whether surgical intervention was indicated. One of the most notable instances of brain injury took place in the 1800s. Phineas Gage survived an accident where an iron rod penetrated his head and destroyed a good portion of his left frontal lobe. Prior to his accident, Gage was described as “even-tempered”, but his demeanor shifted significantly afterwards. Due to the definitive nature of his injury and the personality changes that followed, many cite this as the first case illustrating mood and personality shifts directly resulting from a brain injury.
The 20th century saw the advancement of technologies that improved treatment and diagnosis such as the development of imaging tools including CT and MRI, and, in the 21st century, diffusion tensor imaging (DTI). The introduction of intracranial pressure monitoring in the 1950s has been credited with beginning the "modern era" of head injury.<ref name="Arti12">Marshall, L. F. (2000). Head Injury: Recent Past, Present, and Future. Neurosurgery, 47(3), 546–561. https://doi.org/10.1097/00006123-200009000-00002</ref> In the 1970s, awareness of TBI as a public health problem grew, and a great deal of progress has been made since then in brain trauma research, such as the discovery of primary and secondary brain injury. Prevention of TBI has also become immensely important with several efforts made in that direction such as: introduction of helmets in the army and motorcyclists, airbags in motor vehicles etc. The 1990s saw the development and dissemination of standardized guidelines for the treatment of TBI, with protocols for a range of issues such as drugs and management of intracranial pressure.

== Mechanism of Traumatic Brain Injury ==
The mechanical force applied to the head displaces the brain. Depending on the direction and magnitude of the force, certain neurological functions can be disrupted. The damage to the brain within the skull can present as strain, tissue distortion and shearing of axons. The processes for damage control are activated immediately, while the damage continues to evolve. The damage occurs as a result of the cellular and molecular processes taking place in response to mechanical forces. First, the mechanical force disrupts the neuronal membrane, which rapidly increases the extracellular potassium concentration. This activates a positive feedback loop where more potassium ions are released, which leads to a cascade of responses that eventually lead to a period of hyperglycolysis in the brain followed by higher permeability of the Blood-Brain-Barrier (BBB) and the induction of cytotoxic edema in the brain.<ref name="Arti13">Giordano, K. R., & Lifshitz, J. (2021). Pathophysiology of Traumatic Brain Injury. In S. Honeybul & A. G. Kolias (Eds.), Traumatic Brain Injury. Springer. https://doi.org/10.1007/978-3-030-78075-3_2 </ref> The latter has a detrimental effect on all cells in the brain tissue.

Very important cell types in the brain, aside from neurons, are the neuroglia. These cell types are broadly recognized as having a supportive role. There are 6 types of neuroglia, each with their own function [[File:Types-of-neuroglia_brain-physiology-cells-QBI.png|thumb|250px|Types of glia. <ref name="Arti16">https://qbi.uq.edu.au/brain-basics/brain/brain-physiology/types-glia</ref>]]. In the central nervous system (CNS), the astrocytes are a main type of glial cells. They contribute to the maintenance of homeostasis in the CNS through reactive astrogliosis.<ref name="Arti14">Sofroniew, M. V., & Vinters, H. V. (2009). Astrocytes: biology and pathology. Acta Neuropathologica, 119(1), 7–35. https://doi.org/10.1007/s00401-009-0619-8 </ref> This process is triggered by all types of brain insults and has features that aid recovery, but simultaneously have a potential to inflict damage on the brain. For example, the astrocytes break down their glycogen to supply adjacent neurons with lactate, which the neurons use as fuel to recover. However, the astrocytes also play a critical role in water movements through the brain and in pathological conditions, this can mediate oedema. In all cases of reactive astrogliosis, GFAP is up-regulated. The degree to which the GFAP expression changes, is only dependent on the severity of the trauma to the CNS and not on the morphological appearance of the reactive astrogliosis. <ref name="Arti15">Verkhratsky, A., & Butt, A. (2013). General Pathophysiology of Neuroglia. In Glial Physiology and Pathophysiology (1st ed.). John Wiley & Sons, Ltd. https://doi.org/10.1002/9781118402061 </ref>

When the cause of the increase in GFAP production is brain injury, one can imagine there are several ways for the molecule to end up in the extracellular space instead of remaining in the intracellular space where it was produced [[File:Expression of GFAP in reactive astrogliosis.png|thumb|270px|Expression of GFAP in reactive astrogliosis.<ref name="Arti18">Verkhratsky, A., & Butt, A. (2013). General Pathophysiology of Neuroglia. In Glial Physiology and Pathophysiology (1st ed.). John Wiley & Sons, Ltd. https://doi.org/10.1002/9781118402061</ref>]]. To give an example, necrosis leads to leakage of cellular molecules.<ref name="Arti19">Messing, A., & Brenner, M. (2020). GFAP at 50. ASN Neuro, 12, 175909142094968. https://doi.org/10.1177/1759091420949680</ref> Due to the greater permeability of the BBB, GFAP concentrations increase in the blood as well. Within the first hour of injury, elevated levels of serum GFAP can be detected. At twenty hours of injury, the serum level of GFAP reaches its peak. During the next 52 hours, the GFAP level will decrease slowly. Moreover, there is no significant increase in serum levels of GFAP in patients without TBI.<ref name="Arti17">Abdelhak, A., Foschi, M., Abu-Rumeileh, S., Yue, J. K., D’Anna, L., Huss, A., Oeckl, P., Ludolph, A. C., Kuhle, J., Petzold, A., Manley, G. T., Green, A. J., Otto, M., & Tumani, H. (2022). Blood GFAP as an emerging biomarker in brain and spinal cord disorders. Nature Reviews Neurology, 18(3), 158–172. https://doi.org/10.1038/s41582-021-00616-3</ref> Therefore, a biosensor that detects the levels of GFAP can be used to determine the presence of traumatic brain injury in patients who have received a significant force to their head.

== Medical application and relevance ==
The global incidence of TBI is estimated to be 27 to 69 million a year <ref name="Arti20">Dewan, M. C., Rattani, A., Gupta, S., Baticulon, R. E., Hung, Y. C., Punchak, M., Agrawal, A., Adeleye, A. O., Shrime, M. G., Rubiano, A. M., Rosenfeld, J. V., & Park, K. B. (2019). Estimating the global incidence of traumatic brain injury. Journal of Neurosurgery, 130(4), 1080–1097. https://doi.org/10.3171/2017.10.jns17352</ref> <ref name="Arti21">James, S. L., Theadom, A., Ellenbogen, R. G., Bannick, M. S., Montjoy-Venning, W., Lucchesi, L. R., Abbasi, N., Abdulkader, R., Abraha, H. N., Adsuar, J. C., Afarideh, M., Agrawal, S., Ahmadi, A., Ahmed, M. B., Aichour, A. N., Aichour, I., Aichour, M. T. E., Akinyemi, R. O., Akseer, N., . . . Murray, C. J. L. (2019). Global, regional, and national burden of traumatic brain injury and spinal cord injury, 1990–2016: a systematic analysis for the Global Burden of Disease Study 2016. The Lancet Neurology, 18(1), 56–87. https://doi.org/10.1016/s1474-4422(18)30415-0</ref>. Across all severities of TBI, mortality is quite low at 3% <ref name="Arti22">Georges, A., & Das, J. M. (2022). Traumatic Brain Injury [Internet]. In StatPearls. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK459300/</ref> this was determined in the United States of America, a country where the healthcare system is quite developed. While mortality is low, the long term effects of TBI can be detrimental to a person’s quality of life. Some of the acute symptoms lessen or resolve over time, such as dizziness or nausea. Other consequences do not become apparent until a long period of time has passed, for instance psychiatric conditions.<ref name="Arti23">Seel, R. T., Macciocchi, S., & Kreutzer, J. S. (2010). Clinical Considerations for the Diagnosis of Major Depression After Moderate to Severe TBI. Journal of Head Trauma Rehabilitation, 25(2), 99–112. https://doi.org/10.1097/htr.0b013e3181ce3966</ref>
Currently, the Glasgow coma scale (GCS) is the only standardized way to assess patients with a suspected TBI. The GCS provides a practical method for assessing impairment of conscious level in response to defined stimuli.[[File:The glasgow coma scale.png|thumb|290px|The Glasgow coma scale.<ref name="Arti24">https://smhs.gwu.edu/urgentmatters/news/keep-it-simple-acute-gcs-score-binary-decision</ref>]] Depending on the final score, the TBI can be classified as minor, moderate or severe.

While the GCS score describes the current condition of the patient, its usage has many shortcomings, one of which is the fact that physicians seem to struggle with remembering the exact levels of the scale.<ref name="Arti25">Riechers, R. G., Ramage, A., Brown, W., Kalehua, A., Rhee, P., Ecklund, J. M., & Ling, G. S. (2005). Physician Knowledge of the Glasgow Coma Scale. Journal of Neurotrauma, 22(11), 1327–1334. https://doi.org/10.1089/neu.2005.22.1327</ref> Although there is a dose-response relationship with regard to the severity of the TBI and the severity of the outcome, the GCS does not effectively and uniformly predict mortality rate. <ref name="Arti26">Institute of Medicine (US) Committee on Gulf War and Health: Brain Injury in Veterans and Long-Term Health Outcomes. (2008). Gulf War and Health: Volume 7 (Vol. 7) [Internet]. National Academies Press. https://doi.org/10.17226/12436</ref> <ref name="Arti27">Cho, D. Y., & Wang, Y. C. (1997). Comparison of the APACHE III, APACHE II and Glasgow Coma Scale in acute head injury for prediction of mortality and functional outcome. Intensive Care Medicine, 23(1), 77–84. https://doi.org/10.1007/s001340050294</ref>

Other ways for diagnosing TBI can include various imaging tests like a CT scan or an MRI scan. However, these imaging techniques are often not sensitive/specific enough for milder TBI. According to experts, only 5-10% of mild TBI patients have abnormal CT/MRI scans. The problem with these scans is that they can only detect damage on a macroscopic level, whereas mild TBI manifests primarily at a microscopic level. Therefore, doctors can mistakenly believe that patients with a standard CT or MRI scan have not suffered a TBI.<ref name="Arti28">McKinlay, A., Lin, A., & Than, M. (2018). A comparison of emergency department medical records to parental self-reporting of traumatic brain injury symptoms. Concussion, 3(1), CNC52. https://doi.org/10.2217/cnc-2017-0017</ref> All in all, improvements are needed to objectively and effectively determine whether or not a patient is suffering from a TBI.

== State of the Art ==
Currently, the concentration of GFAP is typically measured in blood plasma (liquid component of blood in which blood cells are absent) or serum (plasma from which the clotting proteins have been removed). Classic methods for the detection of GFAP include ELISA (Enzyme-Linked Immunosorbent Assay).

{| class="wikitable" style="margin-bottom:0"
!Test Name
!Sample Volume (μL)
!Sample matrix
!Range (pg/mL)
!Precision (CV%)
!Incubation time (min)
!Measuring Technique
!Assay type
!Links
|-
|GFAP CLIA Assay, from Eagle Biosciences
|100
|serum, CSF
|10-640
|Intra-Assay: CV=1%, Inter-Assay: CV=6%
|150
|ELISA
|solid phase enzyme-linked sandwich immunosorbent
|https://eaglebio.com/product/gfap-clia-assay-kit/ https://eaglebio.com/wp-content/uploads/2022/01/gfp31-L01-gfap-clia-assay-kit-package-insert-2022.pdf
|-
|Human GFAP ELISA Kit, from FineTest
|
|serum, plasma, tissue homogenates, other biological fluids
|313-20000
|Intra-Assay: CV<8%, Inter-Assay: CV<10%
|
|ELISA
|sandwich
|https://www.fn-test.com/product/eh0410/ https://www.fn-test.com/content/uploads/product/manuals/elisa/EH0410.pdf
|-
|Human GFAP ELISA Kit, Producer not known, distributed by Mybiosource
|>10
|serum, plasma, tissue homogenates
|625-40000
|Intra-Assay: CV<8%, Inter-Assay: CV<10%
|≈300
|ELISA
|quantitative sandwich enzyme immunoassay
|https://www.mybiosource.com/gfap-human-elisa-kits/glial-fibrillary-acidic-protein/704044 https://cdn.mybiosource.com/tds/protocol_manuals/000000-799999/MBS704044.pdf
|-
|Human GFAP ELISA Kit, from LSBio
|>10
|plasma, serum
|156 - 10000
|Intra-Assay: CV<10%, Inter-Assay: CV<12%
|≈300
|ELISA
|sandwich
|https://www.lsbio.com/elisakits/human-gfap-sandwich-elisa-elisa-kit-ls-f4258/4258?trid=247 https://www.lsbio.com/elisakits/manualpdf/ls-f4258.pdf
|-
|GFAP Human ProcartaPlex™ Simplex Kit, from Thermo
|>25
|plasma, serum, CSF
|8 - 32900
|
|
|ELISA
|sandwich
|https://www.thermofisher.com/order/catalog/product/EPX010-12336-901
|-
|GFAP ELISA Kit, from Aviva Systems Biology
|>2
|Serum, plasma, other biological fluids
|31.25-2000
|Intra-Assay: CV<10%, Inter-Assay: CV<12%
|300
|ELISA
|sandwich enzyme-linked immuno-sorbent
|https://www.avivasysbio.com/gfap-elisa-kit-bovine-okcd02567.html https://www.avivasysbio.com/pub/media/pdf/products/OKCD02567.pdf
|-
|}<div style="margin-bottom:1em">''Table 1: Commercially available GFAP ELISA test assays. 

As is clear from the above table, the classic ELISA technique is widely available as commercial kits, which makes it a convenient method for biomarker detection in patient samples. Nevertheless, a relatively long detection time (of at least a few hours) and the requirement of a laboratory environment precludes its use in GP surgeries or patients’ homes.<ref name="Arti29">Xu, L., Ramadan, S., Akingbade, O. E., Zhang, Y., Alodan, S., Graham, N., Zimmerman, K. A., Torres, E., Heslegrave, A., Petrov, P. K., Zetterberg, H., Sharp, D. J., Klein, N., & Li, B. (2021). Detection of Glial Fibrillary Acidic Protein in Patient Plasma Using On-Chip Graphene Field-Effect Biosensors, in Comparison with ELISA and Single-Molecule Array. ACS Sensors, 7(1), 253–262. https://doi.org/10.1021/acssensors.1c02232</ref> Classic ELISAs are laboratory-based assay, which are not suited for rapid testing as targeted in the SensUs competition.The same applies to more advanced laboratory assays, such as mass spectrometry and single molecular array (Simoa) technology.
Recently a commercial GFAP test has been developed by Abbott on its i-STAT Alinity biosensor platform. The specifications are listed in the Table below. The i-STAT platform is used in hospitals. However, the i-STAT TBI Plasma test is not intended to be used in point-of-care settings.

{| class="wikitable" style="margin-bottom:0"
!Test Name
!Sample Volume (μL)
!Sample matrix
!Range (pg/mL)
!Precision (CV%)
!Incubation time (min)
!Measuring Technique
!Assay type
!Links
|-
|i-STAT TBI Plasma
|>20
|plasma
|30-10000
|
|15
|cartridge for analyzer
|
|https://www.globalpointofcare.abbott/en/product-details/apoc/istat-tbi-plasma.html https://www.globalpointofcare.abbott/en/product-details/apoc/istat-alinity.html
|-
|}<div style="margin-bottom:1em">''Table 2: Commercially available GFAP biosensor. 

== Lab protocols ==
During the preparation of the blood samples, one should avoid physical contact with blood by wearing gloves, glasses, and lab coats. Needles and lancets should be used only once and disposed of in a sharps container for decontamination. Furthermore, cuts already present on hands or arms should be covered with plasters to avoid blood-on-blood contact. After completing the samples, gloves should be removed and hands should be washed thoroughly. The waste should be disposed of in specific biohazard waste bins or bags.

== References ==
<references />

Acute Inflammation with a Focus on Sepsis

2024-12-04T14:58:20Z

Techsensus: Created page with "Crystal structure of IL-6. <ref name="Arti1">Crystal structure of IL-6 as published in the Protein Data Bank rendered in Pymol (PDB: 1ALU), 2006, Ramin Herati</ref> == General information == The theme of SensUs 2022 is acute inflammation in intensive care with a focus on sepsis. According to the World Health Organization, sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to..."

[[File: IL6_Crystal_Structure.rsh.png|thumb|360px|Crystal structure of IL-6. <ref name="Arti1">Crystal structure of IL-6 as published in the Protein Data Bank rendered in Pymol (PDB: 1ALU), 2006, Ramin Herati</ref>]]

== General information ==
The theme of SensUs 2022 is acute inflammation in intensive care with a focus on sepsis. According to the World Health Organization, sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. It causes 11 million deaths each year and affects an estimated 49 million people worldwide.<ref name="Arti2">WHO - Health topics - Sepsis, WHO, 2021, https://www.who.int/health-topics/sepsis#tab=tab_1 </ref> According to experts, the diagnosis of sepsis is often set too late as the symptoms arise when organ injury has already occurred, hence the millions of deaths. Sepsis is one of several acute inflammatory responses; others can be caused by injurious agents such as allergens, toxins, burns, and frostbite.<ref name="Arti3">Acute Inflammatory Response, StatPearls Publishing LLC, 2020, Hannoodee, Sally Nasuruddin, D. N. </ref> Clinically, acute inflammation is characterized by five cardinal signs: redness, increased heat, swelling, pain, and functio laesa (i.e. loss of function). It may be regarded as the first line of defense against injury, releasing signaling molecules such as cytokines and chemokines to assist in healing the body and returning to homeostasis. <ref name="Arti4">Concise Pathology (3rd ed.), Appleton & Lange, 1997, Chandrasoma, P., & Taylor, C. R. </ref>

Interleukin (IL)-6 is a pleiotropic cytokine with a wide range of biological activities. It induces pro- and anti-inflammatory reactions and is rapidly induced in the course of acute inflammatory reactions. IL-6 is produced by lymphoid and nonlymphoid cells and helps regulate immune reactivity, the acute phase response, inflammation, oncogenesis, and hematopoiesis. Studies have shown that IL-6 appears to be both a marker and mediator of sepsis and persists in the plasma much longer than other proinflammatory cytokines.<ref name="Arti5">Interleukin-6. Critical Care Medicine, 33(12 Suppl), S463-5, 2005, Song, M., & Kellum, J. A. https://doi.org/10.1097/01.ccm.0000186784.62662.a1 </ref> Therefore IL-6 serves as a valuable biomarker for the early detection of sepsis.

== History of Sepsis ==

The oldest report that associates sepsis with wounds goes all the way back to a discovery by Edwin Smith. In 1826 he found a papyrus in Luxor, Egypt, which was written around 1600 BC. This papyrus seemed to be a copy of an even older manuscript written around 3000 BC. In this manuscript, 48 cases of traumatic lesions between wounds, fractures, and dislocations are mentioned. Clear references to fever as a secondary phenomenon in the wound – with emphasis on the fever as a part of the monitoring of the patients’ evolution - are found in five out of the forty-eight references. Thus, without being familiar with the concept of infection or inflammation, these Egyptian physicians were able to identify some clear signs of what we know nowadays as local suppuration and systemic infection.<ref name="Arti6">The History of Sepsis from Ancient Egypt to the XIX Century, (M. C. F. P. E.-L. Azevedo (Ed.); p. Ch. 1). IntechOpen, 2012, Botero, J. S. H., https://doi.org/10.5772/51484 </ref><ref name="Arti7"> The last 100 years of sepsis. American Journal of Respiratory and Critical Care Medicine, 173(3), 256–263, 2006, Vincent, J.-L., & Abraham, E., https://doi.org/10.1164/rccm.200510-1604OE </ref><ref name="Arti8"> Sepsis History, https://www.news-medical.net/health/Sepsis-History.aspx, 2018, Ryding, S. </ref>

The first reported use of the word sepsis (σηψις) is in a poem of Homer in the Iliad, where it is a derivative of the word sepo (σηπω), which translates to “I rot”. Yet, the first use of sepsis in a medical context can be found in the Hippocratic corpus, written around 400 BC. The use of this word was related to the phenomenon discovered by the Egyptians. Hippocrates described sepsis as a dangerous odiferous biological decay that could occur in the body. Furthermore, it was believed that this decay took place in the colon and from there “dangerous principles” were released, which could cause “auto-intoxication”. Hippocrates was the first one to try and find antisepsis properties and potential medicinal compounds.<ref name="Arti6">The History of Sepsis from Ancient Egypt to the XIX Century, (M. C. F. P. E.-L. Azevedo (Ed.); p. Ch. 1). IntechOpen, 2012, Botero, J. S. H., https://doi.org/10.5772/51484 </ref><ref name="Arti9">Sepsis and septic shock: a history. Critical Care Clinics, 25(1), 83–101, viii, 2009, Funk, D. J., Parrillo, J. E., & Kumar, A., https://doi.org/10.1016/j.ccc.2008.12.003</ref><ref name="Arti8"> Sepsis History, https://www.news-medical.net/health/Sepsis-History.aspx, 2018, Ryding, S. </ref>

In the nineteenth century, large growth in the knowledge on the origin and transmission of infectious diseases occurred. One of the physicians who contributed significantly to this development was Ignaz Semmelweiss (1818-1865). He was a physician in Vienna, Austria. In 1841, while working on a maternity ward in a hospital he noticed that there was a high rate of death from childbed fever. Nowadays this is also known as puerperal sepsis. He made the observation that women whose deliveries were assisted by midwives had a significantly lower percentage of infection (2%) than deliveries assisted by medical students (16%). Back then, the medical students practiced both autopsies and childbirth deliveries on the same day without washing their hands. When one of Semmelweis’ colleagues died of an infection, Semmelweis made the connection between the medical students, the deliveries, the autopsies, and puerperal sepsis. Semmelweis’ comment on this situation was “The fingers and hands of students and doctors, soiled by recent dissections, carry those death-dealing cadaver’s poisons into the genital organs of women in childbirth”. When a handwashing policy was implemented, the rates of puerperal sepsis dropped to under 3%.<ref name="Arti9">Sepsis and septic shock: a history. Critical Care Clinics, 25(1), 83–101, viii, 2009, Funk, D. J., Parrillo, J. E., & Kumar, A., https://doi.org/10.1016/j.ccc.2008.12.003</ref><ref name="Arti7"> The last 100 years of sepsis. American Journal of Respiratory and Critical Care Medicine, 173(3), 256–263, 2006, Vincent, J.-L., & Abraham, E., https://doi.org/10.1164/rccm.200510-1604OE </ref><ref name="Arti8"> Sepsis History, https://www.news-medical.net/health/Sepsis-History.aspx, 2018, Ryding, S. </ref>

In 1964 Dr. Edward Frank from Boston published a management strategy for septic shock. This strategy consisted of continuous monitoring of systemic arterial pressure, central venous pressure, cardiac output, urinary output, blood volume, blood chemistries, gases, pH and electrolytes. Some of these are still used nowadays, such as blood monitoring and urinary output. Aided by the discovery of antibiotics by Alexander Fleming, it was also recommended to find the cause of the infection.<ref name="Arti10">The History of Sepsis Management Over the Last 30 Years. Elsevier, 15(2), 116–117, 2014, Zehava L., N., https://daneshyari.com/article/preview/3235901.pdf </ref>

== Mechanism of Acute Inflammation ==

When the human body is subjected to harmful stimuli, an inflammatory response is activated to remove these stimuli and, if necessary, initiate a healing process. Cellular and molecular events take place to minimize injury and infection. Common characteristics of inflammation on tissue level are redness, heat, pain and loss of tissue function, which all result from local immune, vascular and inflammatory cell responses to infection or injury.<ref name="Arti11">Inflammatory responses and inflammation-associated diseases in organs, Oncotarget, 9(6), 7204–7218, 2018, Chen, L., Deng, H., Cui, H., Fang, J., Zuo, Z., Deng, J., Li, Y., Wang, X., & Zhao, L., https://doi.org/10.18632/oncotarget.23208 </ref> The mentioned events are a consequence of specific complex molecular pathways involving different types of receptors, transcription factors, leukocytes and eventually cytokines that induce inflammatory responses. Even though different stimuli may evoke different inflammation pathways in the human body, in general, a common mechanism is applied which can be summarized in four steps as stated in research done by Chen et al., 2018 <ref name="Arti11">Inflammatory responses and inflammation-associated diseases in organs, Oncotarget, 9(6), 7204–7218, 2018, Chen, L., Deng, H., Cui, H., Fang, J., Zuo, Z., Deng, J., Li, Y., Wang, X., & Zhao, L., https://doi.org/10.18632/oncotarget.23208 </ref>:

1. Cell surface pattern receptors recognize harmful stimuli

2. Inflammatory pathways are activated

3. Inflammatory markers are released

4. Inflammatory cells are recruited

Pathogen-associated molecular patterns (PAMPs) trigger inflammatory responses through activation of specific pattern recognition receptors. As a result, the production of proinflammatory cytokines is induced. Proinflammatory cytokines are produced predominantly by activated macrophages and are involved in the upregulation of inflammatory reactions.<ref name="Arti12">Cytokines, inflammation, and pain. International Anesthesiology Clinics, 45(2), 27–37, 2007, Zhang, J.-M., & An, J., https://doi.org/10.1097/AIA.0b013e318034194e </ref> Interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) are cytokines that mediate receptor activation in order to trigger crucial intracellular signaling pathways that may start the healing process.

However, in the case of acute inflammation, the response to an infection is dysregulated and often disproportional to the severity of the infection. The response gets overheated, overactivated, and can damage the body from within. Potential consequences of this overly strong reaction include infections, organ dysfunction (severe sepsis), or septic shock which is a state of circulatory failure where circulatory, cellular and metabolic abnormalities are associated with an increased risk of death. These reactions are often caused by coagulation (i.e. formation of blood clots) dysregulation. The hypercoagulability of sepsis is thought to be driven by the release of tissue factor from disrupted endothelial cells.<ref name="Arti13">Sepsis: The evolution in definition, pathophysiology, and management. SAGE Open Medicine, 7, 2050312119835043–2050312119835043, 2019, Gyawali, B., Ramakrishna, K., & Dhamoon, A. S., https://doi.org/10.1177/2050312119835043 </ref> When the human body suffers from severe sepsis, activated monocytes and endothelial cells, along with circulating microvesicles, become sources of tissue factor<ref name="Arti14">Role of extracellular vesicles in the development of sepsis-induced coagulopathy. Journal of Intensive Care, 6, 68, 2018, Iba, T., & Ogura, H., https://doi.org/10.1186/s40560-018-0340-6 </ref>.

This factor then causes the systemic activation of the coagulation cascade resulting in the production of thrombin, activation of platelets, and formation of platelet–fibrin clots. These structures can result in local perfusion defects leading to tissue hypoxia and organ dysfunction <ref name="Arti13">Sepsis: The evolution in definition, pathophysiology, and management. SAGE Open Medicine, 7, 2050312119835043–2050312119835043, 2019, Gyawali, B., Ramakrishna, K., & Dhamoon, A. S., https://doi.org/10.1177/2050312119835043 </ref>. Moreover, research has shown that dysregulated apoptotic immune cell-death plays a crucial part in immune dysfunction and mortality of sepsis. Apoptosis is a “programmed cell death” to limit damage of surrounding tissue during the immune response<ref name="Arti15">Apoptosis: a review of programmed cell death. Toxicologic Pathology, 35(4), 495–516, 2007, Elmore, S., https://doi.org/10.1080/01926230701320337 </ref>. It is a vital component of many processes in the human body such as cell turnover, proper development and functioning of the immune system <ref name="Arti15">Apoptosis: a review of programmed cell death. Toxicologic Pathology, 35(4), 495–516, 2007, Elmore, S., https://doi.org/10.1080/01926230701320337 </ref> Most cells that undergo enhanced apoptosis in sepsis are of lymphoid origin, hence less immune cells are left to fight off the infection itself <ref name="Arti16">Host–pathogen interactions in sepsis. The Lancet Infectious Diseases, 8(1), 32–43, 2008, van der Poll, T., & Opal, S. M., https://doi.org/https://doi.org/10.1016/S1473-3099(07)70265-7 </ref>. Since no effective treatment for sepsis exists yet, early diagnosis and recognition is crucial.<ref name="Arti17">Sepsis and septic shock: current approaches to management. Internal Medicine Journal, 49(2), 160–170., 2019, Thompson, K., Venkatesh, B., & Finfer, S., https://doi.org/10.1111/imj.14199 </ref>

This is where IL-6 plays an important part. As mentioned, IL-6 is a cytokine that functions as a crucial mediator during the acute phase of response to inflammation in sepsis.<ref name="Arti18">Diagnostic and prognostic value of interleukin-6, pentraxin 3, and procalcitonin levels among sepsis and septic shock patients: a prospective controlled study according to the Sepsis-3 definitions. BMC Infectious Diseases, 19(1), 968, 2019, Song, J., Park, D. W., Moon, S., Cho, H.-J., Park, J. H., Seok, H., & Choi, W. S., https://doi.org/10.1186/s12879-019-4618-7 </ref> Research on the clinical value of IL-6 in patients with sepsis and septic shock describes that IL-6 is considered controversial regarding its diagnostic and prognostic values, where meta-analysis of diagnostic value of IL-6 has shown that IL-6 only offers moderate success in differentiating sepsis from non-infectious systemic inflammatory response syndrome (SIRS).<ref name="Arti19">Role of interleukin-6 to differentiate sepsis from non-infectious systemic inflammatory response syndrome. Cytokine, 88, 126–135, 2016, Ma, L., Zhang, H., Yin, Y.-L., Guo, W.-Z., Ma, Y.-Q., Wang, Y.-B., Shu, C., & Dong, L.-Q., https://doi.org/10.1016/j.cyto.2016.08.033 </ref> Hence it is recommended that IL-6 is used as a biomarker to confirm infection rather than differentiate between sepsis and SIRS<ref name="Arti19">Role of interleukin-6 to differentiate sepsis from non-infectious systemic inflammatory response syndrome. Cytokine, 88, 126–135, 2016, Ma, L., Zhang, H., Yin, Y.-L., Guo, W.-Z., Ma, Y.-Q., Wang, Y.-B., Shu, C., & Dong, L.-Q., https://doi.org/10.1016/j.cyto.2016.08.033 </ref>.

== Medical Application & Relevance ==

As mentioned, sepsis causes each year more than 11 million deaths while affecting approximately 50 million people worldwide.<ref name="Arti2">WHO - Health topics - Sepsis, WHO, 2021, https://www.who.int/health-topics/sepsis#tab=tab_1 </ref> Mortality rates from sepsis, according to data from the Surviving Sepsis Campaign 2012, were approximately 41% in Europe versus approximately 28.3% in the United States.<ref name="Arti20">Surviving Sepsis Campaign: association between performance metrics and outcomes in a 7.5-year study. Critical Care Medicine, 43(1), 3–12, 2015, Levy, M. M., Rhodes, A., Phillips, G. S., Townsend, S. R., Schorr, C. A., Beale, R., Osborn, T., Lemeshow, S., Chiche, J.-D., Artigas, A., & Dellinger, R. P., https://doi.org/10.1097/CCM.0000000000000723 </ref> Severe sepsis causes dysfunction of multiple organs with as a consequence a state of chronic critical illness involving severe immune dysfunction and catabolism.<ref name="Arti21">Sepsis: pathophysiology and clinical management. BMJ, 353, i1585, 2016, Gotts, J. E., & Matthay, M. A., https://doi.org/10.1136/bmj.i1585 </ref> Despite efforts of experimental and clinical research during the last thirty years, the ability to positively influence the course and outcome of the disease continues to be inadequate. Options of successful and specific interventions are limited and often unsuccessful. If sepsis is detected early and has not affected vital organs yet, administration of different antibiotics have shown to increase chances of survival.<ref name="Arti22">Sepsis—Pathophysiology and Therapeutic Concepts. Frontiers in Medicine, 8, 609, 2021, Jarczak, D., Kluge, S., & Nierhaus, A., https://doi.org/10.3389/fmed.2021.628302 </ref><ref name="Arti20">Surviving Sepsis Campaign: association between performance metrics and outcomes in a 7.5-year study. Critical Care Medicine, 43(1), 3–12, 2015, Levy, M. M., Rhodes, A., Phillips, G. S., Townsend, S. R., Schorr, C. A., Beale, R., Osborn, T., Lemeshow, S., Chiche, J.-D., Artigas, A., & Dellinger, R. P., https://doi.org/10.1097/CCM.0000000000000723 </ref>

Furthermore, early symptoms of sepsis are rather generic, e.g. fever, chills, feeling dizzy or nausea, and thus are not often attributed to sepsis in the early stages.<ref name="Arti23">NHS inform Scotland - Sepsis, https://www.nhsinform.scot/illnesses-and-conditions/blood-and-lymph/sepsis, 2020 </ref> When sepsis goes undetected for too long, or if effective treatment is withheld, sepsis can rapidly progress to septic shock where risk of mortality increases by 7.6% each hour . Hence, a patient suffering from sepsis must be diagnosed correctly in time to prevent worsening of the disease with death as a potential consequence.<ref name="Arti22">Sepsis—Pathophysiology and Therapeutic Concepts. Frontiers in Medicine, 8, 609, 2021, Jarczak, D., Kluge, S., & Nierhaus, A., https://doi.org/10.3389/fmed.2021.628302 </ref>

Currently, sepsis is diagnosed with standard laboratory techniques, which takes 12-72 hours before results are obtained. These tests are based on blood cultures looking for bacterial or viral infections.<ref name="Arti24">How is sepsis diagnosed and treated?, Centers for Disease Control and Prevention. (n.d.). Retrieved September 28, 2021, from https://www.cdc.gov/sepsis/diagnosis/ </ref> However, research shows that sepsis cannot always be attributed to only the infectious agent and the immune response that follows, but also to more complex and indirect factors such as significant alterations in coagulation, immunosuppression and organ dysfunction.<ref name="Arti13">Sepsis: The evolution in definition, pathophysiology, and management. SAGE Open Medicine, 7, 2050312119835043–2050312119835043, 2019, Gyawali, B., Ramakrishna, K., & Dhamoon, A. S., https://doi.org/10.1177/2050312119835043 </ref> Hence, it is often difficult to diagnose sepsis in a patient on time since multiple causes might be at hand. Early diagnosis is therefore key to battle sepsis. With a biosensor that detects IL-6 in the early stages, symptoms can be classified as belonging to sepsis and consequently, the mortality and severity of sepsis can be decreased. Sepsis is treatable if it is identified and treated quickly which in most cases leads to full recovery with no lasting problems<ref name="Arti23">NHS inform Scotland - Sepsis, https://www.nhsinform.scot/illnesses-and-conditions/blood-and-lymph/sepsis, 2020 </ref>.

== State of the Art ==

Currently, the concentration of unbound IL-6 is typically measured in serum, i.e. blood samples are clotted and refrigerated-centrifuged within 25-40 minutes after collection. The serum is then suited for use in several immunoassays, e.g., ELISA, CLIA, ECLIA, and RIA. Alternatively, the serum can be frozen and stored at -20 ℃ or lower for later use.<ref name="Arti25">Plasma and Serum Preparation, ThermoFisher Scientific, 2007, https://www.thermofisher.com/nl/en/home/references/protocols/cell-and-tissue-analysis/elisa-protocol/elisa-sample-preparation-protocols/plasma-and-serum-preparation.html </ref><ref name="Arti26">Measuring cytokine levels in blood. Importance of anticoagulants, processing, and storage conditions. Journal of Immunological Methods, 153(1–2), 115–124, 1992, Thavasu, P. W., Longhurst, S., Joel, S. P., Slevin, M. L., & Balkwill, F. R., https://doi.org/10.1016/0022-1759(92)90313-i </ref> In addition, commercial assays also allow for measurement in plasma. Whole blood is collected into anticoagulant-treated tubes e.g., EDTA-treated or citrate-treated. Cells are removed from plasma by centrifugation for 10 minutes at 1000-2000 x g using a refrigerated centrifuge. Centrifugation for 15 minutes at 2000 x g depletes platelets in the plasma sample.

{| class="wikitable" style="margin-bottom:0"
!Company
!Product
!Test name
!Sample volume (μL)
!Sample matrix
!Range (pg/mL)
!Sensitivity (pg/mL)
!Precision (CV%)
!Incubation time (min)
!Measuring Technique
!Assay type
|-
|Beckman Coulter
|A16369
|Access IL-6 Assay
|110
|Serum or plasma
|0.5 - 1500
|0.5
|N/A
|35
|CLIA
|One-step immunoenzymatic sandwich assay
|-
|Roche COBAS
|5109442190
|Elecsys IL-6
|30
|Serum or plasma
|1.5 - 5000
|1.5
|N/A
|18
|ECLIA
|Sandwich
|-
|ThermoFisher Scientific
|MAN0014630
|Human IL-6 ELISA Kit
|100
|Serum, plasma, buffered solution, or cell culture supernatants
|7.8 - 2500
|< 2.0
|Intra-assay: CV = 6.2%, Inter-assay CV = 7.9%
|180
|ELISA
|Solid-phase sandwich
|-
|R&D Systems Inc.
|D6050
|Quantikine® Human IL-6 Immunoassay
|100
|Serum, plasma, or cell culture supernatants
|3.1 - 300
|0.7
|Intra-assay: CV = 2.0% - 4.2%, Inter-assay: CV = 3.8% - 6.4%
|270
|ELISA
|Sandwich
|-
|abcam
|ab178013
|Human IL-6 ELISA Kit
|N/A
|Serum, EDTA plasma, cit plasma, cell culture supernatants
|7.8 - 500
|1.6
|Intra-assay CV = 2.1%, Inter-assay CV = 2.4%
|90
|ELISA
|Sandwich
|-
|RayBio
|ELH-IL6
|RayBio® Human IL-6 ELISA Kit
|100
|Serum, plasma, cell culture supernatants
|3.0 - 1000
|3.0
|Intra-assay CV =< 10%, Inter-assay CV =< 12%
|150
|ELISA
|Sandwich

|}<div style="margin-bottom:1em">''Table 1: Commercially available IL-6 well-plate assays. 

Immunoassays, in general, provide a relatively slow method for measuring specific biomarkers like IL-6. Therefore, faster methods are the subject of intensive research. A commercially available rapid test is the Milenia® QuickLine IL-6 test: Milenia Biotec.<ref name="Arti27">Milenia Biotec, 2016, https://www.milenia-biotec.com/en/product/il6/#nav-overview </ref> This lateral flow immunoassay is designed for the semi-quantitative evaluation of human Interleukin-6 in serum, plasma, cell culture supernatant, amniotic fluid or cerebrospinal fluid.<ref name="Arti28">Rapid and sensitive detection of interleukin-6 in serum via time-resolved lateral flow immunoassay, Analytical Biochemistry, 588, 113468, 2020, Huang, D., Ying, H., Jiang, D., Liu, F., Tian, Y., Du, C., Zhang, L., & Pu, X., https://doi.org/10.1016/j.ab.2019.113468</ref><ref name="Arti29">Development of quantum dot-based fluorescence lateral flow immunoassay strip for rapid and quantitative detection of serum interleukin-6, Journal of Clinical Laboratory Analysis, 35(5), e23752, 2021, Tang, J., Wu, L., Lin, J., Zhang, E., & Luo, Y., https://doi.org/10.1002/jcla.23752 </ref> In the Milenia® QuickLine IL-6 test, the sample is pipetted in the sample application port. IL-6 of the patient's sample binds to a first monoclonal anti-IL-6 antibody conjugated to gold particles. The IL-6-loaded gold particles diffuse through the membrane and overflow the test line (T). There, a second monoclonal antibody specific for IL-6 is coated on the membrane; so the gold particles are bound specifically and become visible as a colored line. The color intensity is proportional to the concentration of IL-6 in the sample and intensifies during the incubation time. The surplus of gold particles continues to diffuse over the test strip. The conjugate specific antibodies printed as a control line on the membrane (control line, C) capture the gold conjugate and therefore a visible line develops during the incubation time.<ref name="Arti30">A Point of Care Lateral Flow Assay for Rapid and Colorimetric Detection of Interleukin 6 and Perspectives in Bedside Diagnostics, J Clin Med Res, 2(2), 1–16, 2020, de Souza Sene, I., Costa, V., Bras, D. C., de Oliveira Farias, E. A., Nunes, G. E., & Bechtold, I. H., https://doi.org/https://doi.org/10.37191/Mapsci-2582-4333-2(2)-032 </ref><ref name="Arti31">Duplex Shiny app quantification of the sepsis biomarkers C-reactive protein and interleukin-6 in a fast quantum dot labeled lateral flow assay, Journal of Nanobiotechnology, 18(1), 130, 2020, Ruppert, C., Kaiser, L., Jacob, L. J., Laufer, S., Kohl, M., & Deigner, H.-P., https://doi.org/10.1186/s12951-020-00688-1 </ref>

{| class="wikitable" style="margin-bottom:0"
!Company
!Product
!Test name
!Sample volume (μL)
!Sample matrix
!Range (pg/mL)
!Sensitivity (pg/mL)
!Incubation time (min)
!Measuring Technique
|-
|Milenia Biotec
|MQL6 1
|Milenia® QuickLine IL-6 test
|100
|Serum, plasma, cell culture supernatants, amniotic fluid, cerebrospinal fluid
|50 - 10000
|50
|20
|LFIA
|-

|}<div style="margin-bottom:1em">''Table 2: Commercially available rapid test, Milenia® Quickline IL-6 test. 

== Safety & Lab protocols ==

Safety

Interleukin-6 is present in blood plasma, thus the sample preparation in the lab will also take place with human blood plasma as matrix. It is of crucial importance to handle human blood plasma with care due to potential pathogens that can be transmitted, such as Hepatitis viruses and HIV.<ref name="Arti32">Recommendations for the production, control and regulation of human plasma for fractionation, World Health Organization, 2007, https://www.who.int/bloodproducts/publications/TRS941Annex4blood.pdf </ref> Sigma Aldrich has screened the blood plasma in advance on HIV, Hepatitis B and Hepatitis C, however caution is recommended all the same.<ref name="Arti33">Plasma from human, Sigma Aldrich, Retrieved November 12, 2021 from https://www.sigmaaldrich.com/NL/en/product/sigma/p9523, n.d. </ref> Specific lab protocols and rules will be listed below.

Lab protocols

During the preparation of the blood samples, one should avoid physical contact with blood by wearing gloves, glasses, and lab coats. Needles and lancets should be used only once and disposed of in a sharps container for decontamination.<ref name="Arti34">Serum / Plasma Specimens - Safety, Centers for Disease Control and Prevention, 2016, https://www.cdc.gov/dpdx/diagnosticprocedures/serum/safety.html </ref> Furthermore, cuts already present on hands or arms should be covered with plasters to avoid blood-on-blood contact. After completing the samples, gloves should be removed and hands should be washed thoroughly.<ref name="Arti34">Serum / Plasma Specimens - Safety, Centers for Disease Control and Prevention, 2016, https://www.cdc.gov/dpdx/diagnosticprocedures/serum/safety.html </ref> The waste should be disposed of in specific biohazard waste bins or bags.

Il-6 will be provided by HyTest. Instructions for storage will also be provided by HyTest. In general, human interleukin-6 is not a dangerous substance according to GHS ((Globally Harmonized System of Classification and Labeling of Chemicals).<ref name="Arti35">Safety Data Sheet - Recombinant Human Interleukin-6 (IL-6), BioVision Inc., 2015, https://www.biovision.com/documentation/sds/4143_SDS.pdf</ref> Potential health effects include:

* harm if inhaled
* respiratory tract irritation
* harmful if absorbed through skin
* skin irritation
* eye irritation
* harmful if swallowed<ref name="Arti35">Safety Data Sheet - Recombinant Human Interleukin-6 (IL-6), BioVision Inc., 2015, https://www.biovision.com/documentation/sds/4143_SDS.pdf</ref>

== References ==
<references />

Influenza A

2024-12-04T14:58:08Z

Techsensus: Created page with " Structure of the influenza virus.<ref name="Arti1">Host Protective Immune Responses against Influenza A Virus Infection, MDPI, 2020, Hi Eun Jung, Heung Kyu Lee, https://www.mdpi.com/1999-4915/12/5/504/htm</ref> == General information== The theme of SensUs 2021 is acute respiratory viruses. The current Covid-19 pandemic has made it apparent that large virus outbreaks can cause immense harm to human health and can disrupt soci..."

[[File:Influenzastructure.png|thumb|360px|Structure of the influenza virus.<ref name="Arti1">Host Protective Immune Responses against Influenza A Virus Infection, MDPI, 2020, Hi Eun Jung, Heung Kyu Lee, https://www.mdpi.com/1999-4915/12/5/504/htm</ref>]]

== General information==

The theme of SensUs 2021 is acute respiratory viruses. The current Covid-19 pandemic has made it apparent that large virus outbreaks can cause immense harm to human health and can disrupt society as a whole. One of the most common respiratory viruses is influenza A. Therefore, the influenza virus serves as an interesting research topic for this year’s Competition.

The influenza A virus is known to cause influenza in birds and some mammals, like humans. Several subtypes of the influenza A virus have been isolated from wild birds. Occasionally, viruses can be transmitted from wild birds to domestic animals, like chickens or pigs. This may give rise to human influenza.<ref name="Arti2">Transmission of Avian Influenza A Viruses Between Animals and People, CDC, 2015, https://www.cdc.gov/flu/avianflu/virus-transmission.htm</ref>

Influenza A viruses are negative-sense, single-stranded RNA viruses. Subtypes of influenza A are characterized by proteins on the outermost membrane of the virus, called hemagglutinin (H or HA) and neuraminidase (N or NA). H and N are the antigens of the virus, and play an important role in the interaction between the host’s immunological response and the virus. Recently, researchers have reported the discovery of an antibody that is generally effective against all subtypes of the influenza A virus.<ref name="Arti3">Super antibody' fights off flu, BBC, 2011, James Gallagher, https://www.bbc.com/news/health-14324901</ref> Hemagglutinin will serve as this year's biomarker. In infected patients, bound hemagglutinin indicates the presence of the influenza virus. In the Testing Event, unbound hemagglutinin proteins will be used to mimic the immunologic response of the virus.

The subtype which will be used in SensUs 2021 is H1N1. Historically, H1N1 has been responsible for most deaths due to influenza. It is a popular influenza strain for research purposes.<ref name="Arti4">Prevalent Eurasian avian-like H1N1 swine influenza virus with 2009 pandemic viral genes facilitating human infection, Research Gate, 2020, Sun, Honglei & Xiao, Yihong & Liu, Jiyu & Wang, Dayan & Li, Fangtao & Wang, Chenxi & Li, Chong & Zhu, Junda & Song, Jingwei & Sun, Haoran & Zhimin, Jiang & Liu, Litao & Zhang, Xin & Wei, Kai & Dongjun, Hou & Pu, Juan & Sun, Yipeng & Tong, Qi & Bi, Yuhai & Liu, Jinhua https://www.researchgate.net/publication/342555087_Prevalent_Eurasian_avian-like_H1N1_swine_influenza_virus_with_2009_pandemic_viral_genes_facilitating_human_infection</ref><ref name="Arti5">Comparison of Hospitalized Patients With ARDS Caused by COVID-19 and H1N1, CHEST, Xiao Tang, Rong-Hui Du, Rui Wang, Tan-Ze Cao, Lu-Lu Guan, Cheng-Qing Yang, Qi Zhu, Ming Hu, Xu-Yan Li, Ying Li, Li-Rong Liang, Zhao-Hui Tong, Bing Sun, Peng Peng, Huan-Zhong Shi, 2020, https://www.sciencedirect.com/science/article/pii/S0012369220305584</ref><ref name="Arti6">Landscape of coordinated immune responses to H1N1 challenge in humans, Journal of Clinical Investigation, 2020, Zainab Rahil, Rebecca Leylek, Christian M. Schürch, Han Chen, Zach Bjornson-Hooper, Shannon R. Christensen, Pier Federico Gherardini, Salil S. Bhate, Matthew H. Spitzer, Gabriela K. Fragiadakis, Nilanjan Mukherjee, Nelson Kim, Sizun Jiang, Jennifer Yo, Brice Gaudilliere, Melton Affrime, Bonnie Bock, Scott E. Hensley, Juliana Idoyaga, Nima Aghaeepour, Kenneth Kim, Garry P. Nolan, David R. McIlwain https://www.sciencedirect.com/science/article/pii/S0012369220305584</ref> Due to its popularity among researchers, antigens and antibodies are commercially available, making H1N1 suitable as a target for the SensUs Competition.
Influenza A vaccines for humans have been developed. New versions of the vaccines are developed twice per year for use all over the world, which is necessary due to rapid mutations of the influenza virus. Every year during flu season, a large part of the population is vaccinated in order to protect individuals against the virus. However, due to unforeseen mutations of the virus, it might be possible that in a certain year a vaccine will prove ineffective. In that case large portions of the population would be at risk and a pandemic could occur. The probability of a major influenza A pandemic is estimated to be around 0.5-1% each year.<ref name="Arti7">Pandemic risk: how large are the expected losses?, WHO, 2017, Victoria Y Fan, Dean T Jamisonb & Lawrence H Summers, https://www.who.int/bulletin/volumes/96/2/17-199588.pdf</ref>

==History of influenza (A)==

A lack of data up until 1500 AC complicates the research on influenza before that period.<ref name="Arti8">Internet‐Based Intelligence in Public Health Emergencies, NATO Science for Peace and Security Series - E: Human and Societal Dynamics, 2013 Mordini E., Green M., https://www.iospress.nl/book/internet%E2%80%90based-intelligence-in-public-health-emergencies/</ref> Possibly the first influenza pandemic occurred around 6000 BC in China. The symptoms of human influenza seem to have been clearly described by Hippocrates, roughly 2,400 years ago<ref name="Arti9">2,500-year Evolution of the Term Epidemic, Emerging infectious diseases, 2006, Martin PM, Martin-Granel E, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3373038/</ref> Although the virus seems to have caused epidemics throughout human history, historical data on influenza is difficult to interpret, due to the fact that symptoms of influenza are similar to those found in other respiratory diseases, like respiratory syncytial virus (RSV).

The most infamous and lethal outbreak was the 1918 flu pandemic, which lasted until 1920. The number of deaths is unknown, but estimates range from 17 to 100 million people.<ref name="Arti10">Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic, American journal of epidemiology, 2018, Spreeuwenberg P, Kroneman M, Paget J, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7314216/</ref>. This pandemic has been described as "the greatest medical holocaust in history"<ref name="Arti11"> Reviewing the History of Pandemic Influenza: Understanding Patterns of Emergence and Transmission
, Pathogens, 2016, Saunders-Hastings PR, Krewski D, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5198166/</ref> and may have killed as many people as the plague (Black Death). This huge death toll was caused by an extremely high infection rate of up to 50% and the severity of the symptoms, suspected to be caused by cytokine storms in which the innate immune system causes an uncontrolled and excessive release of pro-inflammatory signaling molecules called cytokines.
One of the most recent outbreaks of influenza was the 2009 Swine Flu. Similar to the Spanish Flu, it was also of the subtype H1N1. The death toll of the 2009 pandemic is estimated to be around 150,000 to 575,000.<ref name="Arti12">2009 H1N1 Pandemic (H1N1pdm09 virus), centers for disease control and prevention, 2010, https://www.cdc.gov/flu/pandemic-resources/2009-h1n1-pandemic.html</ref>

==Structure of the influenza virus==

The virus particle (virion) is 80–120 nanometers in diameter.<ref name="Arti13">Native Morphology of Influenza Virions, Frontiers in microbiology, 2011, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249889/</ref> The virion shape can be spherical, elliptical, or even filamentous with a length of tens of micrometers<ref name="Arti13">Native Morphology of Influenza Virions, Frontiers in microbiology, 2011, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249889/</ref>. The virion is made up of a viral envelope containing two main types of proteins, wrapped around a central core. The two large proteins found on the outside of viral particles are hemagglutinin (HA) and neuraminidase (NA). HA is a protein that mediates binding of the virion to target cells and entry of the viral genome into the target cell, and therefore plays an important role in infecting healthy cells. NA is involved in releasing the progeny viruses once a cell has been infected and has started producing the virus itself.<ref name="Arti14">Influenza A penetrates host mucus by cleaving sialic acids with neuraminidase, Virology journal , 2013, Cohen M, Zhang XQ, Senaati HP, Chen HW, Varki NM, Schooley RT, Gagneux P, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3842836/</ref>

These two proteins are a target of interest for antiviral drugs.<ref name="Arti15">Recent Strategies in the Search for New Anti-Influenza Therapies, Current Drug Targets , 2003, J.C. Wilson, M. von Itzstein, https://www.eurekaselect.com/63785/article</ref> Furthermore, they are also the antigen proteins to which a host antibodies can bind and trigger an immune response. Influenza type A viruses are categorized into different subtypes, or strains, based on which type of these two proteins is present on the surface of the virion. Currently, there are 16 subtypes of HA and 9 subtypes of NA known to exist. The most prevalent form of the different subtypes is H1N1. Single hemagglutinin-neuraminidase proteins, in which both HA and NA are found in a single protein, also exist. However, these will not be used in SensUs 2021.

==Mechanism of infection==
[[File:Flu-infect.jpg |right|thumb|200px|An influenza virus infecting a cell.<ref name="Arti31">Sugars on Cell Surface Are Key to Flu Infections, National Institute of Health, 2008, https://www.nih.gov/news-events/nih-research-matters/sugars-cell-surface-are-key-flu-infections</ref>]]

The involvement of the hemagglutinin and neuraminidase proteins in the infection of H1N1 is essential for the virus reproduction, and a more specific mechanism is discussed below.
The hemagglutinin protein (HA) has the role of searching for the sialic acid receptors in respiratory-lining cell membranes. Upon binding of this protein and the receptor, fusion of the virus and the cell membrane is facilitated with the help of glycan proteins. The virus then enters the cell where it sheds its shell and approaches the cell’s nucleus. Using the host replication mechanisms, copies of the virus are made. At this step of the infection process, important viral proteins are synthesized. These newly replicated viral elements subsequently leave through the cell membrane and infect other cells. To inhibit the exit of the viral components, sialic acid receptors on the cell can bind the HA glycoproteins. This is where viral evolution/mutability can play a role in the expansion of the capabilities of the virus.
The neuraminidase glycoprotein (N) has the role of cleaving the sialic acid receptors, allowing the exit of the viral components which then go in search of a new host.
After infection is complete, the H1N1 virus triggers cell apoptosis, leading to the death of the cell and spread of the virions.

==Medical application==
The influenza virus spreads when people with the flu cough, sneeze or talk. Infected people transfer tiny droplets to people close to them and infect them by the handover of droplets. During an outbreak, it is important to be able to control and prevent the virus from spreading further, for example, by implementing control measures. Those control measures can be determined based on the reproduction number. The reproduction number is defined as: “the expected number of secondary cases produced by a single infection in a completely susceptible population”.<ref name="Arti16">Notes on R0, 2007, James Holland Jones, https://web.stanford.edu/~jhj1/teachingdocs/Jones-on-R0.pdfe</ref> A clear overview of the number of infected people is needed to calculate the reproduction number.

For disease control, it is crucial to know the infection rate in a specific region. The results may influence critical decisions such as whether to perform other diagnostic testing or to implement infection prevention and control measures for influenza. Furthermore, manpower can be in short supply during a pandemic and the speed at which someone can be tested and receive the result is of vital importance.

Biosensors that will be developed in SensUs 2021 are envisioned to be used outside the hospital in a point-of-care (POC) setting. Inside the hospital there is not a big advantage, as there are already very specific and accurate tests available for that setting.<ref name="Arti28">Diagnosing Flu, CDC, 2020, https://www.cdc.gov/flu/symptoms/testing.htm</ref><ref name="Arti29">Information on Rapid Molecular Assays, RT-PCR, and other Molecular Assays for Diagnosis of Influenza Virus Infection, CDC, 2020, https://www.cdc.gov/flu/professionals/diagnosis/molecular-assays.htm</ref> Important applications are fast testing at the general practioner and at home. Therefore the test should be easy to use. The biosensors will be designed to enable a fast yes/no answer, based on measuring the concentration of hemagglutinin particles in the sample. The biosensors will not distinguish between different virus subtypes, as the subtype causing the pandemic is assumed to be known.

==State of the Art==
There are various influenza tests available on the market. The most common tests are ‘rapid influenza diagnostic tests’, also called RIDTs. RIDTs provide results in a qualitative way within approximately 10-15 minutes and work by detecting the parts of the virus that stimulate an immune response.<ref name="Arti17">Rapid Influenza Diagnostic Tests, 2016, Centers for Disease Control and Prevention, https://www.cdc.gov/flu/professionals/diagnosis/clinician_guidance_ridt.htm</ref>
The table contains examples of available rapid influenza diagnostic tests that provide results in 10-15 minutes. In the SensUs Competition, a saliva based test will be developed. Moreover, SensUs strives to stimulate developement of sensors that provide results within 5-10 minutes and that are as sensitive as possible. The SensUs Competition aims to innovate the field of influenza biosensing by using saliva as a matrix, by improving the speed of the test, and by targeting a high sensitivity.

{| class="wikitable" style="margin-bottom:0"
!Manufacturer
!Product
!CLIA Waived
!Platform
!Sensitivity
(PPA*)
!Specificity
(NPA**)
!Sample type
!LOD*** (H1N1)
TCID50/mL****
|-
|Abbott<ref name="Arti18">Abbott RIDT Product Information, Abbott, https://www.globalpointofcare.abbott/nl/product-details/binaxnow-influenza-a-and-b.html</ref>
|Binax Now Influenza A & B Card 2
|Yes
|DIGIVAL
|70-89%
|90-99%
|NPS, NS direct
|N/A
|-
|Becton Dickinson & Co.<ref name="Arti19">Becton Dickinson & Co. RIDT Product Information, Becton Dickinson & Co., https://www.bd.com/en-us/offerings/capabilities/microbiology-solutions/point-of-care-testing/veritor-system</ref>
|BD Veritor™ Flu A + B
|Yes
|BD veritor Reader
|82%
|98%
|NPS, NS direct
|3.3*102-5.0*103
|-
|Becton Dickinson & Co.<ref name="Arti19">Becton Dickinson & Co. RIDT Product Information, Becton Dickinson & Co., https://www.bd.com/en-us/offerings/capabilities/microbiology-solutions/point-of-care-testing/veritor-system</ref>
|BD Veritor™ Flu A + B
|Yes
|BD veritor plus Analyzer
|83,6%
|97,5%
|NPS, NS direct
|3.3*102-5.0*103
|-
|Quidel Corp.<ref name="Arti20">Quidel RIDT Product Information, Quidel, https://www.quidel.com/immunoassays/rapid-influenza-tests/sofia-influenza-fia</ref>
|Sofia® Influenza A + B FIA
|Yes
|Sofia(2) FIA Analyzer
|90-99%
|95-96%
|NS, NPS, NPA, NPW direct, NP, NPA, NPW in VTM
|N/A
|-
|Quidel Corp<ref name="Arti21">Quidel QuickVue RIDT Product Information, Quidel, https://www.quidel.com/sites/default/files/product/documents/EF1350313EN00_1.pdf</ref>
|QuickVue® Influenza A + B
|Yes
|N/A
|81,5%
|97,8%
|NS, NPS, NPA, NPW direct, NP, NPA, NPW in VTM
|1.63*103-4.4*103
|-
|Princeton BioMeditech Corp.<ref name="Arti22">BioSing RIDT Product Information, Princeton BioMeditech Corp., http://punchout.medline.com/product/BioSign-Rapid-Flu-AB-Antigen-Panel-Test-by-Princeton-BioMeditech/Influenza-Testing/Z05-PF176440?question=&index=P11&indexCount=11#mrkSpec</ref>
|BioSign® Flu A & B
LABSCO
|Yes
|N/A
|89,2%
|99,4%
|NS, NPS direct, NPA(waived), NPW(not waived)
|N/A
|-
|Remel/Thermo Fisher.<ref name="Arti23">XPECT™ RIDT Product Information, Remel/Thermo Fisher, https://www.thermofisher.com/document-connect/document-connect.html?url=https%3A%2F%2Fassets.thermofisher.com%2FTFS-Assets%2FMBD%2FInstructions%2FIFU24600%2520.pdf&title=WHBlY3QgRmx1IEEgYW5kIEI=</ref>
|XPECT™ Flu A & B
LABSCO
|No
|N/A
|89-100%
|100%
|NW, NS
|1.63*103-4.41*103
|-
|Sekisui Diagnostics, <ref name="Arti24">Acucy Influenza A&B Test Product Information, Sekisui Diagnostics, https://www.sekisuidiagnostics.com/wp-content/uploads/2019/08/Acucy-Influenza-AB-Package-Insert.pdf</ref>
|Acucy Influenza A&B Test
|Yes
|The Acucy System
|96,4%
|96,0%
|NPS, NS direct
|1.4*101
|-
|Sekisui Diagnostics, <ref name="Arti25">OSOM Ultra Plus Flu A&B Test Product Information, Sekisui Diagnostics, https://www.sekisuidiagnostics.com/wp-content/uploads/2019/04/OSOM_P-52631-D__web.pdf</ref>
|OSOM Ultra Plus Flu A&B Test
|Yes
|N/A
|90,2-92,2%
|75,1-85,7%
|NPS, NPA, NPW, NS
|1.05*102
|-
|Access Bio, Inc., <ref name="Arti26">CareStart Flu A&B Plus Product Information, Access Bio, Inc., https://www.accessdata.fda.gov/cdrh_docs/pdf19/K191514.pdf</ref>
|CareStart Flu A&B Plus
|No
|N/A
|79,9%
|98,4%
|NPS
|5.0*103-9.6*103

|}<div style="margin-bottom:1em">''Table 1: Different rapid diagnostic tests for infuenza A'' 
'''*'''PPA: positive percent agreement (True positive / total true positive)

'''**'''NPA: negative percent agreement (True negative / total true negative)

'''***'''LOD: Limit of detection
'''****'''TCID50/mL represents the viral load at which 50% of cells are infected when a solution containing the virus is added to the cell culture.<ref name="Arti32">Limits of detection for FDA-authorized COVID-19 diagnostics, BioCentury, 2020, https://www.biocentury.com/article/304801/limits-of-detection-for-fda-authorized-covid-19-diagnostics</ref> Detection limits are often expressed in TCID50/mL. Copies/TCID50 for H1N1 is 2381 copies/TCID50 so 1 TCID50/mL represents 2381 copies per mL. It is a very rough method to determine concentration and it has a standard deviation of about 1048 copies/TCID50.<ref name="Arti33">Analytical Sensitivity Comparison between Singleplex Real-Time PCR and a Multiplex PCR Platform for Detecting Respiratory Viruses, PLoS One, 2015,Parker J, Fowler N, Walmsley ML, Schmidt T, Scharrer J, Kowaleski J, Grimes T, Hoyos S, Chen J., https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4646456/</ref></div>

CLIA waived tests are tests cleared for POC setting. These are simple tests with a relatively low risk for an incorrect result. Tests are scored by seven criteria on a scale of one to three, where a score of 1 indicates the lowest level of complexity. The tests may be called CLIA waived, if the score is low enough. The seven criteria are:<ref name="Arti27">Categorization Criteria CLIA test, FDA, 2020, Inc., https://www.fda.gov/node/365445#scorecard</ref>

1. Knowledge: scientific and technical knowledge required to use the test.

2. Training and experience: training needed to perform the test.

3. Reagents and materials preparation: stability of materials and material preparation.

4. Characteristics of operational steps: complexity of operational steps.

5. Calibration, quality control and proficiently testing materials: can be stable or labile.

6. Test system troubleshooting and equipment maintenance: the easiness and frequency of the troubleshooting and equipment maintenance.

7. Interpretation and judgement: required to perform analytic processes and resolution of problems.

An alternative type of testing is with a ‘molecular assay’, which is more accurate than RITDs, because it detects viral RNA or nucleic acids in respiratory specimens. This type of testing includes rapid molecular assays, RT-PCR and other nucleic acid amplification tests. Molecular tests are mostly used in hospitals, some are able to detect both influenza A and B. Others can identify different subtypes. The ‘molecular assays’ are more accurate and sensitive than RIDTs, but the time-to-result may be several hours.<ref name="Arti28">Diagnosing Flu, CDC, 2020, https://www.cdc.gov/flu/symptoms/testing.htm</ref><ref name="Arti29">Information on Rapid Molecular Assays, RT-PCR, and other Molecular Assays for Diagnosis of Influenza Virus Infection, CDC, 2020, https://www.cdc.gov/flu/professionals/diagnosis/molecular-assays.htm</ref>

==Safety==
When working with the influenza virus, influenza infection in humans can occur following a laboratory accident. For safety reasons, unbound hemagglutinin particles will be used in SensUs 2021, as a substitute for infectious influenza virus particles.

===Lab protocols===
The use of safety equipment combined with good practices is fundamental to laboratory safety and in helping to reduce the risks involved in dealing with biosafety hazards. Therefore, it is important that you consult with your local biosafety officer and comply to the safety rules of your own organization.

According to Article 4.84 of the Working Conditions Decree in The Netherlands, inactivated virus particles can be classified as category 1 of the biological agents<ref name="Arti30">1Short manual for the ML-I, ML-II laboratory(gmo-labs) and/or laboratory for working with biological agents / human materials, University of Twente, 2018,https://www.utwente.nl/.uc/f4fcab199010209618400ac90d20284e61d13c337a78900/short-manual-gmo.pdf</ref>, as they are unlikely to cause disease in humans. Therefore, the biosafety guidelines as defined in the ML-1 (minimum containment level) have to be followed. It is stipulated in the guidelines that general laboratory protocols such as the usage of barrier protection (lab coats, gloves, and face protection) when handling the samples are to be followed. The protection barriers are not always necessary from a microbiological perspective. However, it is compulsory when handling disinfectants or solvents.

== References ==

<references />

Valproate

2024-12-04T14:57:58Z

Techsensus: Created page with "<div style="float: right; clear: right; margin: -1em 0 0 1em; font-size: 85%"> {| class="wikitable" width="300px" |colspan="2"|300px<ref name="[46]">Pubchem (Last modified: 2019-12-08) Accessed on 12 December 2019, at [https://pubchem.ncbi.nlm.nih.gov/compound/Valproic-acid#section=3D-Conformer''https://pubchem.ncbi.nlm.nih.gov/compound/Valproic-acid#section=3D-Conformer''].</ref> |- !style="text-align:left;"|Drug Name |Valproate |- !style="text-align..."

<div style="float: right; clear: right; margin: -1em 0 0 1em; font-size: 85%">
{| class="wikitable" width="300px"
|colspan="2"|[[File:VPA_3D.png|300px]]<ref name="[46]">Pubchem (Last modified: 2019-12-08) Accessed on 12 December 2019, at [https://pubchem.ncbi.nlm.nih.gov/compound/Valproic-acid#section=3D-Conformer''https://pubchem.ncbi.nlm.nih.gov/compound/Valproic-acid#section=3D-Conformer''].</ref>
|-
!style="text-align:left;"|Drug Name
|Valproate
|-
!style="text-align:left;"|Systematic name
|2‐propyl‐pentanoic acid
|-
!style="text-align:left;"|Type
|Anti-epileptic drug (AED)
|-
!style="text-align:left;"|Molecular formula
|C8H16O2<ref name="[46]">Pubchem (Last modified: 2019-12-08) Accessed on 12 December 2019, at [https://pubchem.ncbi.nlm.nih.gov/compound/Valproic-acid#section=3D-Conformer''https://pubchem.ncbi.nlm.nih.gov/compound/Valproic-acid#section=3D-Conformer''].</ref>
|-
!style="text-align:left;"|Molecular weight
|144.21 g/mol<ref name="[46]">Pubchem (Last modified: 2019-12-08) Accessed on 12 December 2019, at [https://pubchem.ncbi.nlm.nih.gov/compound/Valproic-acid#section=3D-Conformer''https://pubchem.ncbi.nlm.nih.gov/compound/Valproic-acid#section=3D-Conformer''].</ref>
|}</div>

== General ==

Valproate (VPA) is a drug used in the treatment of epilepsy, bipolar disorder and spinal muscular atrophy <ref name=”[15]”> Sodium valproate (Epilim, Epival, Episenta). (2018, June 5) , at [https://www.netdoctor.co.uk/medicines/brain-nervous-system/a6665/epilim-sodium-valproate/ “https://www.netdoctor.co.uk/medicines/brain-nervous-system/a6665/epilim-sodium-valproate/”].</ref>
. VPA is used to control epileptic seizures, divided in two subcategories: focal and generalized seizures.

VPA was originally synthesized by Burton in 1882 and used as an organic solvent. Its antiepileptic effect was discovered by accident almost 100 years later by Meunier et al. (1963). It was registered as a drug in 1964 in France and later in many other countries. <ref name=”[35]”> , . Drugbank, Valproic Acid Retrieved from November 25, 2019, at [https://www.drugbank.ca/drugs/DB00313
“https://www.drugbank.ca/drugs/DB00313”].</ref><ref name=”[41]”> Perucca, E. (2012, August 29). Pharmacological and Therapeutic Properties of Valproate ,CNS Drugs
October 2002, Volume 16, Issue 10, pp 695–714. Retrieved November 28, 2019, at [https://link.springer.com/article/10.2165/00023210-200216100-00004 “https://link.springer.com/article/10.2165/00023210-200216100-00004.”]
</ref><ref name=”[45]”> Padmanabhan, R., Abdulrazzaq, Y. M., & Bastaki, S. M. A. (2000). Valproic acid-induced congenital malformations: Clinical and experimental observations , Congenital anomalies 40(4), 259-268. Retrieved December 9, 2019, at [https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1741-4520.2000.tb00923.x“https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1741-4520.2000.tb00923.x.”].</ref>

==Structure and interaction with albumin==

VPA (2‐propyl‐pentanoic acid) is a short-chain fatty acid administered intravenously or orally as sodium valproate, composed of sodium and VPA in a 1:1 molar ratio. VPA itself is highly hydrophobic, leading to a favourable entry to the central nervous system with good oral bioavailability. <ref name=”[1]”> Williams, J. H., Jayaraman, B., Swoboda, K. J., & Barrett, J. S. (2011, December 13). Population Pharmacokinetics of Valproic Acid in Pediatric Patients With Epilepsy: Considerations for Dosing Spinal Muscular Atrophy Patients. Retrieved October 17, 2019 , at [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3345311/ “https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3345311/”].</ref>
VPA in the blood exists in its free form as the valproate ion, but is highly bound to serum proteins. It mainly binds to albumin (90 - 95%), <ref name=”[2]”> Dasgupta, A. (2016). Chapter 4 - Monitoring Free Drug Concentration: Clinical Usefulness and Analytical Challenges. Retrieved October 17, 2019 , at [https://doi.org/10.1016/B978-0-12-802025-8.00004-0 “https://doi.org/10.1016/B978-0-12-802025-8.00004-0”].</ref>
with the unbound fraction increasing linearly from approximately 10% at 50 mg/L to approximately 30% at 200 mg/L total concentrations. <ref name=”[1]”> Williams, J. H., Jayaraman, B., Swoboda, K. J., & Barrett, J. S. (2011, December 13). Population Pharmacokinetics of Valproic Acid in Pediatric Patients With Epilepsy: Considerations for Dosing Spinal Muscular Atrophy Patients. Retrieved October 17, 2019 , at [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3345311/ “https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3345311/”].</ref>
Phenytoin, another well-known anti-epileptic drug (AED), is a competitor in this protein binding. This means that VPA binding to albumin decreases if phenytoin is taken, causing both free drug concentrations to increase. <ref name=”[3]”> Cramer, J. A., & Mattson, R. H. (1979). Valproic acid: in vitro plasma protein binding and interaction with phenytoin. Retrieved November 6, 2019 , at [https://www.ncbi.nlm.nih.gov/pubmed/121944 “https://www.ncbi.nlm.nih.gov/pubmed/121944”].</ref>

== Mechanism of action ==
===Pathophysiology of epilepsy===

Epilepsy is a disease of the central nervous system caused by disruptions in the electrical communication between neurons, more specifically, the imbalance in excitatory and inhibitory action of neurotransmitters at the synapses, which can lead to seizures, loss of awareness or unusual behavior. <ref name=”[13]”> Flyyn, S., & Babi, M. A. (2017). Pathophysiology of Epilepsy.Paragraph Anticonvulsants , at [https://www.sciencedirect.com/topics/medicine-and-dentistry/pathophysiology-of-epilepsy “https://www.sciencedirect.com/topics/medicine-and-dentistry/pathophysiology-of-epilepsy”].</ref> A seizure occurs when there is a large depolarization of many neurons that fire the action potential at the same time. This paroxysmal depolarizing shift can last for thirty seconds up to two minutes and leads the above mentioned effects.

Epileptic seizures appear in two types: focal and generalized. The focal seizure starts in one part of the brain, but due to the failure of inhibitory mechanisms it can spread further. The cause of it can be a trauma or cortex developmental disorder <ref name=”[43]”> (Nall, R. R. (2018, January 23). What are simple partial seizures? Retrieved December 7, 2019, at [ https://www.medicalnewstoday.com/articles/320696.php“ https://www.medicalnewstoday.com/articles/320696.php”].</ref> <ref name=”[44]”> (Leventer, R. J., Guerrini, R., & Dobyns, W. B. (2008, March 1). Malformations of cortical development and epilepsy. Retrieved December 7, 2019, at [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3181860/“https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3181860/”].</ref> The generalized seizure, on the other hand, is characterized by a sudden and large activity that occurs in more than one area of the brain at the same time, causing the hyperexcitability in the cortex and the neurons that connect the thalamus to the cortex. <ref name=”[38]”> (2016, May 22).2-Minute Neuroscience: Epilepsy. Retrieved November 26, 2019, at [https://www.youtube.com/watch?v=OGFQhLPaaOQ “https://www.youtube.com/watch?v=OGFQhLPaaOQ”].</ref>

The first type of focal seizures is the one without loss of consciousness, but with jerking of different body parts or a change in the perception of reality; the second type is focal seizures with impaired consciousness or change of awareness. Generalized seizures are absence seizures, tonic seizures, atonic seizures, clonic seizures and so on. <ref name=”[36]”> Epilepsy. (2019, August 10), at [https://www.mayoclinic.org/diseases-conditions/epilepsy/symptoms-causes/syc-20350093 “https://www.mayoclinic.org/diseases-conditions/epilepsy/symptoms-causes/syc-20350093”].</ref>
Focal seizures include automatisms, behavior arrest, hyperkinetic, autonomic, cognitive and emotional, while atonic, clonic, epileptic spasms, myoclonic and tonic seizures can be both focal or generalized. Generalized seizure types are absence with eyelid myoclonida, myoclonic absence, myoclonic - atonic, myoclonic - tonic - clonic. <ref name=”[42]”> Fisher, R. S., Cross, J. H., French, J. A., Higurashi, N., Hirsch, E., Jansen, F. E., … Zuberi, S. M. (2017, April). Operational classification of seizure types by the International League Against Epilepsy: Position Paper of the ILAE Commission for Classification and Terminology, at [ https://www.ncbi.nlm.nih.gov/pubmed/28276060
“h https://www.ncbi.nlm.nih.gov/pubmed/28276060
”].</ref>

One of the reasons for epilepsy development is the genetic background. While the exact background is still unknown, there are certain genes that were found to be involved in the progression of the epilepsy, for example SCN1A and SCN8A that are active in the production of sodium channels and can therefore contribute to the epilepsy succession. <ref name=”[14]”> Guo, W., Shang, D.-M., Cao, J.-H., Feng, K., He, Y.-C., Jiang, Y., … Gao, Y.-F. (2017). Identifying and Analyzing Novel Epilepsy-Related Genes Using Random Walk with Restart Algorithm. Introduction , at [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5309434/ “https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5309434/”].</ref>

===Valproate Mode of Action===
The exact mode of action of valproate on molecular level is unknown, as there are many different substances that participate simultaneously in the regulation of neuronal activity. <ref name=”[15]”> Sodium valproate (Epilim, Epival, Episenta). (2018, June 5) , at [https://www.netdoctor.co.uk/medicines/brain-nervous-system/a6665/epilim-sodium-valproate/ “https://www.netdoctor.co.uk/medicines/brain-nervous-system/a6665/epilim-sodium-valproate/”].</ref> VPA is connected to cortical inhibition in order to contribute to ‘neural synchrony’ and is known to provide protection from neural degradation and damage. <ref name=”[16]”> Williams, R. S. B., Cheng, L., Mudge, A. W., & Harwood, A. J. (2002). A common mechanism of action for three mood-stabilizing drugs , at [https://www.nature.com/articles/417292a “https://www.nature.com/articles/417292a”].</ref>
VPA inhibits histone deacetylase, which causes hyperacetylation of histones. The drug is furthermore connected to inositol depletion by preventing the gene prolyl oligopeptidase to be expressed through indirect inhibition of myo-inositoal-1-phophate-synthetase. <ref name=”[16]”> Williams, R. S. B., Cheng, L., Mudge, A. W., & Harwood, A. J. (2002). A common mechanism of action for three mood-stabilizing drugs , at [https://www.nature.com/articles/417292a “https://www.nature.com/articles/417292a”].</ref> In terms of epilepsy, it is believed that VPA is affecting the activity of GABA. By increasing the activity of GABA, the epileptic attacks are prevented. This is established through inhibition of succinic semialdehyde dehydrogenase which then increases the amount of succinic semialdehyde and subsequently increases GABA neurotransmission. <ref name=”[37]”> Flyyn, S., & Babi, M. A. (2017). Pathophysiology of Epilepsy , at [https://www.sciencedirect.com/topics/medicine-and-dentistry/pathophysiology-of-epilepsy “https://www.sciencedirect.com/topics/medicine-and-dentistry/pathophysiology-of-epilepsy”].</ref>

VPA affects the extracellular signal-regulated kinase pathway or ERK (Extracellular Receptor Kinase), which results in phosphorylation of ERK 1/2 . This has an effect on the expression of proteins that contribute to the plasticity of neurons and neuronal growth. The downside of this effect of VPA is the additional increase in GABA activity. VPA has an influence on fatty acids production, leading to lower membrane fluidity due to the presence of less sterols and glycerolipids. Subsequently, this increases the action potential threshold of the membrane and contributes to the antiepileptic effects of valproate . <ref name=”[17]”> , . Drugbank, Valproic Acid Retrieved from November 25, 2019, at [https://www.drugbank.ca/drugs/DB00313
“https://www.drugbank.ca/drugs/DB00313”].</ref>
===Pharmacokinetics of VPA===
When introduced orally, VPA is found to be absorbed in a period of 4 hours. However, introducing it in other ways results in the same Cmax, AUC and Cmin in a steady state, while all the differences in absorption are neglected. When VPA is taken as extended release tablet and combined with meals, the absorption time increases from 4 to 8 hours. On the other hand, the increase in absorption time in the same situation increases from 3.3 to 4.8 hours when taken as a sprinkle tablet. There is 90% bioavailability of VPA in all oral forms. On the other hand, there is 100% bioavailability in enteric-coated forms.
<ref name=”[18]”> Gugler, R., & Unruh, G. E. von. (2012, December 13). Clinical Pharmacokinetics of Valproic Acid , at [https://link.springer.com/article/10.2165/00003088-198005010-00002 “https://link.springer.com/article/10.2165/00003088-198005010-00002”].</ref>The average half-life of VPA is 13-19 hours, the volume of distribution is 11 L/1.73 m2 . The protein binding is linear at low concentrations, but non-linear and decreased at high concentrations, which may be due to the different affinity binding sites of albumin. Parts of the drug can be metabolized in different ways. The most relevant metabolism of VPA would be glucuronide conjugates and mitochodrial-ß oxidation. <ref name=”[17]”> , . Drugbank, Valproic Acid Retrieved from November 25, 2019, at [https://www.drugbank.ca/drugs/DB00313
“https://www.drugbank.ca/drugs/DB00313”].</ref>

Only 3% of the drug is eliminated through urine, about 30-50% is eliminated through hepatic metabolism, and about 40% being excreted through mitochondrial-beta oxidation. <ref name=”[18]”> Gugler, R., & Unruh, G. E. von. (2012, December 13). Clinical Pharmacokinetics of Valproic Acid , at [https://link.springer.com/article/10.2165/00003088-198005010-00002 “https://link.springer.com/article/10.2165/00003088-198005010-00002”].</ref>

===Efficacy and influence on hepatic function===
Optimal dose and effects are achieved with upper limit of 60 mg/kg/day. If the desired effects are not achieved (in other words, the seizures are not gone or the side effects are too strong), testing for blood levels of VPA needs to be executed in order to determine whether they fall within the optimal range of total VPA 50-100 µg/mL. Otherwise, the dose is altered in agreement with the doctor according to the individual state of the patient, his/her other conditions or other medication that they take. The toxicity level of valproate in blood, although not very conclusive, is taken as 150 µg/mL<ref name=”[19]”> valproic Acid Dosage Guide with Precautions. (2019, March 28), at [https://www.drugs.com/dosage/valproic-acid.html “https://www.drugs.com/dosage/valproic-acid.html”].</ref>
.

VPA impacts the hepatic drug metabolism by inhibiting it and displacing other strongly bound drugs from proteins, which implies that dosage changes need to be considered when using it in combination with other drugs. <ref name=”[20]”> Valproic Acid (2018, October 1) , at [https://labtestsonline.org/tests/valproic-acid“https://labtestsonline.org/tests/valproic-acid”].</ref> The normal limits of valproate in blood of 70-100 kg person is 1000-3000 mg/day <ref name=”[46]”> Farmacotherapeutisch Kompas. Retrieved December 19, 2019, at [https://www.farmacotherapeutischkompas.nl/bladeren/preparaatteksten/v/valproinezuur“https://www.farmacotherapeutischkompas.nl/bladeren/preparaatteksten/v/valproinezuur”].</ref>

==Number of patients==
Epilepsy affects around 50 million people worldwide. This includes people having less than one seizure per year. The prevalence of active epilepsy (i.e. patients who have frequent seizures or use medication) is between 0.4% and 1%. On a global scale, an estimated five million people are diagnosed with epilepsy each year.
Epileptic seizures can be controlled by using anti-epileptic drugs (AEDs). About 70% of epileptic patients becomes free of seizures by appropriate AEDs. Low-cost treatments are available, with daily medication that costs as little as US$ 5 per year.<ref name=”[21]”> WHO. (2019, June 20). Epilepsy. Retrieved October 30, 2019 , at [https://www.who.int/news-room/fact-sheets/detail/epilepsy “https://www.who.int/news-room/fact-sheets/detail/epilepsy”].</ref>

==Medical use and TDM==

Valproate (VPA) is usually taken as tablets of 250 mg or as a syrup with 250 mg per ml, but this may vary per product <ref name=”[39]”> Valproic Acid Capsules - FDA prescribing information, side effects and uses. (2018, June 1).Retrieved November 26, 2019, at [https://www.drugs.com/pro/valproic-acid-capsules.html“https://www.drugs.com/pro/valproic-acid-capsules.html”].</ref>.The dose of VPA is at first taken in small amounts and is then gradually increased until the satisfactory dose is reached.
Testing for valproate levels is important to check if the blood levels are within normal therapeutic range <ref name=”[20]”> Valproic Acid (2018, October 1) , at [https://labtestsonline.org/tests/valproic-acid“https://labtestsonline.org/tests/valproic-acid”].</ref>
. The test should measure the free concentration (i.e. the concentration of unbound valproate) as this fraction is pharmacologically active.
The recommended levels of unbound valproate are 6-22 µg/mL in blood <ref name=”[20]”> Valproic Acid (2018, October 1) , at [https://labtestsonline.org/tests/valproic-acid“https://labtestsonline.org/tests/valproic-acid”].</ref>. For epilepsy patients, the range for the treatment of total valproate should be 50-100 µg/mL. Overall, if a patient has a VPA concentration that falls within this range, does not have recurrent seizures and has minimal side effects, then the dose is said to be suitable<ref name=”[20]”> Valproic Acid (2018, October 1) , at [https://labtestsonline.org/tests/valproic-acid“https://labtestsonline.org/tests/valproic-acid”].</ref>
. To provide a patient their right amount of medication, therapeutic drug monitoring (TDM) is implemented in hospitals, because every patient responds differently to a certain dose of medication.
VPA is a conventional drug that is used as first line monotherapy for idiopathic generalized epilepsies. Its effectiveness is not clearly conclusive and varies between different patients. The doses are not generalized and depend on patient’s age and weight. Doses are taken every day, with some patients even two times a day. It is not advised, but sometimes necessary, to take VPA in combination with other seizure drugs (e.g., ethosuximide, lamotrigine, phenytoin, rufinamide, topira mate), some antidepressants or certain antibiotics<ref name=”[20]”> Valproic Acid (2018, October 1) , at [https://labtestsonline.org/tests/valproic-acid“https://labtestsonline.org/tests/valproic-acid”].</ref>. Medications based on VPA are harmful for the unborn child. If valproate is taken during pregnancy, research has shown that up to 4 in 10 babies are at risk of developmental disorders, and approximately 1 in 10 are at risk of birth defects. <ref name=”[22]”> gov.uk. (2018, March 23), at [https://www.gov.uk/guidance/valproate-use-by-women-and-girls“https://www.gov.uk/guidance/valproate-use-by-women-and-girls”].</ref>

==Safety & Lab protocols==
===Safety===
All anti-epileptic drugs have side-effects, including VPA. The risk of hyperammonemia is nearly 40% in patients ingesting intravenous VPA in the ICU setting <ref name=”[4]”> Lind, J., & Nordlund, P. (2019, July). Intravenous use of valproic acid in status epilepticus is associated with high risk of hyperammonemia, Seizure Vol. 69. Retrieved October 23, 2019, at [https://doi.org/10.1016/j.seizure.2019.03.020“https://doi.org/10.1016/j.seizure.2019.03.020”].</ref>
. Other adverse effects include thrombocytopenia and endocrine effects of women. Valproate is associated with a dose-related teratogenicity rate, with risk of major malformation higher than 30% at doses greater than 1100 mg/d. In utero exposure is also linked to dose-dependent reduced verbal IQ and autism<ref name=”[6]”> Abou-Khalil, & Bassel, W. (2019). Update on Antiepileptic Drugs 2019. Retrieved November 6, 2019, at [ https://insights.ovid.com/crossref?an=00132979-201904000-00014“ https://insights.ovid.com/crossref?an=00132979-201904000-00014”].</ref>
.
Children, who were exposed to VPA during birth, have a possible chance of a major congenital malformation (MCM) <ref name=”[7]”> Morrow , J., Russell, A., Guthrie, E., Parsons, L., Robertson, I., Waddell, R., … Craig, J. (2006, January 17). Malformation risks of antiepileptic drugs in pregnancy: a prospective study from the UK Epilepsy and Pregnancy Register. Retrieved October 23, 2019, at [https://jnnp.bmj.com/content/77/2/193“https://jnnp.bmj.com/content/77/2/193”].</ref>
. The risk of having severe consequences for these infants exposed to sodium valproate in utero has been estimated between 6% and 12%<ref name=”[8]”> Use of Sodium Valproate in Pregnancy. (2014, December 15). Retrieved October 23, 2019, at [https://www.medsafe.govt.nz/profs/PUArticles/December2014SodiumValproate.htm“https://www.medsafe.govt.nz/profs/PUArticles/December2014SodiumValproate.htm”].</ref>
. This can be prevented by reducing the dose of the drug. An example of endocrine effects might be idiosyncratic liver toxicity <ref name=”[9]”> Stewart, J. D., Horvath, R., Baruffini, E., Ferrero, I., Bulst, S., Watkins, P. B., … Chinnery, P. F. (2010, November). POLG determines the risk of sodium valproate induced liver toxicity. Retrieved October 23, 2019, at [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3841971/“https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3841971/”].</ref>

===Lab Protocols===

VPA is considered thermodynamically stable, which indicates that it is not reactive under normal environmental conditions. It should be stored in a metal can or drum and kept away from incompatible materials such as oxidizing agents, bases and strong reducing agents as ignition may result.

Working with VPA can be irritating when inhaled or when getting directly in contact with the yes. To prevent this, VPA needs to be handled in a fume hood, safety goggles, lab coats, and gloves need to worn. Direct skin contact or ingestion of VPA should be avoided. In case of contact, flush the specific body part and go to the doctor without delay <ref name=”[10]”> Chemwatch: 15242 Version No: 7.1.1.1 Safety Data Sheet (Conforms to Regulation (EU) No 2015/830)</ref>

==State of the art==

{| class="wikitable" style="margin-bottom:0"
!Company
!Product
!Test name
!Sample Volume (μL)
!Reportable range
!Dilution
!Precision
!Incubation time
!Measuring Technique
|-
|Beckman Coulter<ref name=”[23]”>Emit 2000 Valproic Acid Assay (2010, September) Retrieved from November 7, 2019''. Beckman.</ref> <ref name=”[24]”>Beckman Coulter system Reagent, AU400/AU400e (2012, February) Retrieved from November 7, 2019''. Beckman.</ref>
|AU2700/AU5400
|VALPROIC ACID EMIT® 2000
|3.5 μL
|< 150 μg/mL
|1:1
|Total CV < 4.3% Inter-assay: CV < 3.2%
|15-75 min.
|ELISA1
|-
|Roche COBAS<ref name=”[25]”>Cobas 8000 modular analyzer series (last update: 2019, November 8).</ref> <ref name=”[26]”>van Eckardstein, A et al. (2013). cobas 8000 Modular Analyzer Series Evaluated under Routine-like Conditions at 14 Sites in Australia, Europe, and the United States. Retrieved from October 23., at [https://pdfs.semanticscholar.org/beb9/eaea6d45ba8fec2f3049cc5e242d9d2cfb13.pdf
“https://pdfs.semanticscholar.org/beb9/eaea6d45ba8fec2f3049cc5e242d9d2cfb13.pdf
”].</ref>
|Cobas 8000
|Cobas 8000
|1.5–35
|3.15 – 150 mg/L <ref name=”[27]”>Therapeutic drug monitoring (2011). Retrieved from November 8, 2019.'' R-Biopharm AG.</ref>
|1:(3-121)
|Inter-assay: CV < 0.7% ~ 2.9% Total CV<3%
|9 min – 27 min
|ELISA1
|-
|ABBOTT LABORATORIES DIAGNOSTIC DIVISION<ref name=”[28]”>iValproic Acid B1P350 (2009,August).</ref>
<ref name=”[29]”> Free Valproic Acid Assay (2014, April). Retrieved from November 3, 2019, at [https://www.cadth.ca/sites/default/files/pdf/lab-tests/06_Free_Valproic_Acid_Assay_e.pdf/“https://www.cadth.ca/sites/default/files/pdf/lab-tests/06_Free_Valproic_Acid_Assay_e.pdf”].</ref>

|B1P350
|ARCHITECT iValproic Acid
|10
|2 - 150 μg/mL
|1:10
|Total CV≤ 7%
|29 min
|CMIA2
|-
|SIEMENS HEALTHCARE DIAGNOSTICS INC.<ref name=”[30]”> ADVIA 1900 Chemistry system</ref>
|67070
|ADVIA 1200 CHEMISTRYSYSTEM -VALPROIC ACID (VPA) ASSAY
|2-3
|NA
|1:5
|Inter-assay: CV = 0.2% - 5.3 % Total CV= 0.4%-5.3% <ref name=”[31]”> American Association for Clinical Chemistry, 70th AACC Annual Scientific Meeting (July, August, 2018). Retrieved from November 7, 2019</ref>

|3-21 min
|ELISA1
|-
|MICROGENICS CORPORATION<ref name=”[32]”>CEDIA Valproic Acid II Assay (2018, November). Retrieved from November 7, 2019</ref>
|62390
|CEDIA TDM ASSAY -VALPROIC ACID
|NA
|3.0-150 μg/mL
|NA
|Inter-assay: CV = 1.3% - 2.4 % Total CV= 1.8%-3.4%
|Reagent 1 : 2-5 min Reagent 2: 4-8 min
|ELISA1

|}<div style="margin-bottom:1em">''1 ELISA: Enzyme Linked Immunosorbent Assay'' Note: 2.CMIA: Chemiluminescent Microparticle Immunoassay
</div>

To measure unbound VPA, blood samples are treated by ultrafiltration, followed by an immunoassay, also referred to as ELISA. By performing the ultrafiltration, the protein-bound form of VPA is separated from its unbound form. After that, the level of the unbound fraction can be measured by an immunoassay, e.g. an ELISA kit. Alternatively, LC-MS can be used to measure VPA.<ref name=”[40]”> ao, S., Miao, H., Tao, X., Jiang, B., Xiao, Y., Cai, F., … Chen, W. (2011, July 1). LC–MS/MS method for simultaneous determination of valproic acid and major metabolites in human plasma, Journal of Chromatography B
Volume 879, Retrieved November 28, 2019, at [https://www.sciencedirect.com/science/article/pii/S1570023211003278?via=ihub“https://www.sciencedirect.com/science/article/pii/S1570023211003278?via=ihub”].</ref>
Steps such as solvent extraction or derivation must be executed prior to a HPLC assay, which takes significant time<ref name=”[29]”> Free Valproic Acid Assay (2014, April). Retrieved from November 3, 2019, at [https://www.cadth.ca/sites/default/files/pdf/lab-tests/06_Free_Valproic_Acid_Assay_e.pdf/“https://www.cadth.ca/sites/default/files/pdf/lab-tests/06_Free_Valproic_Acid_Assay_e.pdf”].</ref>
. CMIA is a special type of ELISA <ref name=”[34]”> Ilyas M., Ahmad. I (2014, July 12), Chemiluminescent microparticle immunoassay based detection and prevalence of HCV infection in district Peshawar Pakistan, third alinea Background, Retrieved from November 7, 2019, at [https://virologyj.biomedcentral.com/articles/10.1186/1743-422X-11-127“https://virologyj.biomedcentral.com/articles/10.1186/1743-422X-11-127”].</ref>
.
Several innovations are being investigated for VPA testing. For example, 2D-LC system (two-dimensional chromatography) was studied, allowing large volume injection, reducing interfering components, and reducing the analysis time and preventing most interference components by selecting useful sections in the “heart-cut” column(1D) from entering the analysis column (2D).<ref name=”[11]”> Liu, W., Shang, X., Yao, S., & Wang, F. (2019, August 20). A novel and nonderivatization method for the determination of valproic acid in human serum by two‐dimensional liquid chromatography. Retrieved October 17, 2019, at [https://doi.org/10.1002/bmc.4695.“https://doi.org/10.1002/bmc.4695.”].</ref>
. Another example is dried blood spot (DBS) followed by gas chromatography mass spectrometry (GC–MS), which does not require solvent extraction or elution. The limit of quantitation was 200 ng/mL. <ref name=”[12]”> Guo, M., Shao, L., Chen, X., Li, H., Wang, L., Pan, Y., & Tang, D. (2019, September 13). Assay of dried blood spot from finger prick for sodium valproate via ink auxiliary headspace gas chromatography mass spectrometry, Journal of Chromatography A Vol. 1601 p. 335-339. Retrieved October 24, 2019, at [ https://doi.org/10.1016/j.chroma.2019.05.039“ https://doi.org/10.1016/j.chroma.2019.05.039”].</ref>

== References ==

<references />

Adalimumab

2024-12-04T14:57:46Z

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<div style="float: right; clear: right; margin: -1em 0 0 1em; font-size: 85%">
{| class="wikitable" width="300px"
|colspan="2"|[[File:adalimumab.png|300px]]<ref name="[1]">Drugbank (2005). Adalimumab. Accessed on 6 November 2018, at [https://www.drugbank.ca/drugs/DB00051 ''https://www.drugbank.ca/drugs/DB00051''].</ref>
|-
!style="text-align:left;"|Drug Name
|Adalimumab
|-
!style="text-align:left;"|Systematic name
|Immunoglobulin G1, anti-(human tumor necrosis factor) (human monoclonal D2E7 heavy chain), disulfide with human monoclonal D2E7 light chain, dimer<ref name=”[2]”>U.S. National Library of Medicine (2018). Adalimumab. Accessed on 6 November 2018, at [https://chem.nlm.nih.gov/chemidplus/rn/331731-18-1 ''https://chem.nlm.nih.gov/chemidplus/rn/331731-18-1''].</ref>
|-
!style="text-align:left;"|Synonyms
|ADL, adalimumab-adaz, adalimumab-adbm, adalimumab-atto<ref name="[1]" />
|-
!style="text-align:left;"|Type
|Monoclonal antibody (mAb), Biologics, Disease-modifying antirheumatic drugs (DMARD)<ref name="[1]" />
|-
!style="text-align:left;"|Target
|Tumor necrosis factor alpha (TNF-⍺)
|-
!style="text-align:left;"|Molecular formula
|C6428H9912N1694O1987S46<ref name="[1]" />
|-
!style="text-align:left;"|Molecular weight
|148 kDa<ref name="[1]" />
|-
!style="text-align:left;"|Half life
|~20 days<ref name=”[3]”>Ternant, D. et al. (2014). Pharmacokinetics and concentration–effect relationship of adalimumab in rheumatoid arthritis. ''British Journal of Clinical Pharmacology, 79''(2), 286–297. [https://doi.org/10.1111/bcp.12509 ''doi:10.1111/bcp.12509'']</ref>
|}</div>

== Structure and History ==

=== Structure ===

Adalimumab is a fully human IgG1 monoclonal antibody (mAb) that binds specifically to TNF-α. The molecule consists of 1330 amino acids and its molecular weight is approximately 148 kDa.<ref name=”[4]”>Medsafe (2012). Humira solution for injection: Data Sheet. Accessed on 6 November 2018, at [http://www.medsafe.govt.nz/Medicines/SearchResult.asp ''http://www.medsafe.govt.nz/Medicines/SearchResult.asp''].</ref> It is composed of two H and two L polypeptide chains, with each containing three complementarity-determining regions in the heavy (VH) and light (VL) variable domains. Like the structure of IgG, adalimumab has two antigen-binding Fab domains linked to the Fc domain via a hinge. Six complementarity-determining regions of each H:L chain pair compose the antigen-binding site on the Fab domain of the mAb.<ref name=”[5]”>Tracey, D., Klareskog, L., Sasso, E.H., Salfeld, J.G., Tak, P.P. (2008). Tumor necrosis factor antagonist mechanisms of action: A comprehensive review. ''Pharmacology & Therapeutics, 117''(2), 244-279. [https://doi.org/10.1016/j.pharmthera.2007.10.001 ''doi:10.1016/j.pharmthera.2007.10.001'']</ref>

In addition to heavy and light variable regions, adalimumab consists of human IgG1:κ constant regions that are engineered by phage display technology. The phage display facilitates selection of a fully human antibody specific for a specific antigen, in this case tumor necrosis factor (TNF), from a large range of antibodies. If the desired antibody is rare in this range, a two-stage process is applied for a more rapid guided selection. For the generation of adalimumab, the first step is the usage of anti-human TNF murine antibody MAK195 for the isolation of a human antibody that can recognize the same neutralizing epitope as MAK195. This antibody has a low off-rate and high affinity for human TNF. VH and VL MAK195 are paired with human cognate repertoires. For these phage antibody libraries, recombinant human TNF serves as the antigen for the antigen binding selection. A fully human anti-TNF antibody is then generated by combining the selected human VH and VL genes. Early human anti-TNF antibodies were optimized in a second phase that mirrors the natural process for antibody optimization. It is produced in a Chinese hamster ovary host, transfected with a plasmid vector containing the expression cassettes for adalimumab light and heavy chains.<ref name=”[6]”>Boehncke, W.H., Radeke, H.H. (2007). Adalimumab. In Salfeld,J., Kupper, H., ''Biologics in General Medicine.'', 14-31, Springer-Verlag Berlin Heidelberg. [https://doi.org/10.1007/978-3-540-29018-6 ''doi:10.1007/978-3-540-29018-6'']</ref>

[[File:structure.png|thumb|Structure of adalimumab<ref name=”[7]”>Mitoma, H., Horiuchi, T., Tsukamoto, H., Ueda, N. (2018). Molecular mechanisms of action of anti-TNF-α agents – Comparison among therapeutic TNF-α antagonists. ''Cytokine, 101'', 56-63. [https://doi.org/10.1016/j.cyto.2016.08.014 ''doi:10.1016/j.cyto.2016.08.014'']</ref>|150px]]

=== History ===

The first version of adalimumab was engineered in the 90s and was named D2E7. BASF Pharma started the development of a TNF neutralizing human antibody with the use of phage display technology in 1993 in collaboration with Cambridge Antibody Technology. Phage display repertoires were used to guide the selection of human antibodies to a single epitope of antigen TNF-⍺. This technology was developed in 1991 by Cambridge Antibody Technology. A phage antibody library technology was developed that could be used to discover human antibodies of therapeutic value. To select the antibodies that bind to a desired antigen, enormous repertoires of human antibodies are displayed on the surfaces of millions of bacterial phages, i.e. phage antibody library.<ref name=”[8]”>Jespers, L.S., Roberts, A., Mahler, S.M., Winter, G., Hoogenboom, H.R. (1994). Guiding the selection of human antibodies from phage display repertoires to a single epitope of an antigen. ''Bio/Technology'', ''12''(9), 899–903. [https://doi.org/10.1038/nbt0994-899 ''doi:10.1038/nbt0994-899'']</ref><ref name=”[9]”>Den Broeder, A. et al. (2002). A single dose, placebo controlled study of the fully human anti-tumor necrosis factor-alpha antibody adalimumab (D2E7) in patients with rheumatoid arthritis. ''The Journal of Rheumatology, 29''(11), 2288-2298</ref>

The compound that later became adalimumab was identified within two years after the start of the development.<ref name=”[10]”>McCafferty, J. (2010). The long and winding road to antibody therapeutics,''mAbs, 2''(5), 459-460. [https://dx.doi.org/10.4161%2Fmabs.2.5.13088 ''doi:10.4161%2Fmabs.2.5.13088'']</ref> In 2002, Abbott received approvement of the Food and Drug Administration (FDA) for sales of Humira for the treatment of rheumatoid arthritis (RA).<ref name="[1]" /> It became to be the first fully human monoclonal antibody to be approved by the FDA. The patent on Humira belonged to AbbVie, a spin-off from Abbott, and is approved for the treatment of psoriatic arthritis, ankylosing spondylitis, Crohn’s disease, ulcerative colitis, hidradenitis suppurativa, juvenile idiopathic arthritis, uveitis and plaque psoriasis.<ref name=”[4]”>Medsafe (2012). Humira solution for injection: Data Sheet. Accessed on 6 November 2018, at [http://www.medsafe.govt.nz/Medicines/SearchResult.asp ''http://www.medsafe.govt.nz/Medicines/SearchResult.asp''].</ref> The patent expired in October 2018 in Europe, which led to the immediate launch of other biosimilars. In the US, AbbVie’s patent on Humira expired in 2016 but the company has managed to prolong its protection until 2023.<ref name=”[11]”>Loftus, P. (2017). AbbVie, Amgen Reach Settlement in Humira Patent Dispute, ''The Wall Street Journal''. Retrieved from [https://www.wsj.com/articles/abbvie-amgen-reach-settlement-in-humira-patent-dispute-1506635070 ''https://www.wsj.com/articles/abbvie-amgen-reach-settlement-in-humira-patent-dispute-1506635070''].</ref><ref name=”[12]”>Derbyshire, M. (2015). Patent expiry dates for best-selling biologicals, ''Generics and Biosimilars Initiative Journal, 4''(4), 178-179. [https://dx.doi.org/10.5639/gabij.2015.0404.040 ''doi:10.5639/gabij.2015.0404.040''].</ref>

== Mechanism of action ==

=== Pathophysiology of Rheumatoid Arthritis ===

TNF-⍺ is a proinflammatory cytokine and is part of the type II cytokine family. It is a ~26 kDa protein that is produced by activated macrophages, monocytes, and activated T-cells. Synthetization occurs as a transmembrane TNF (tmTNF). The ~15 kDa soluble form of TNF (sTNF) is released after proteolysis by a TNF-⍺ converting enzyme in the extracellular domain. It can bind to its receptors, TNF receptor 1 (TNFR1) and TNF receptor 2 (TNFR2). Both receptors are naturally monomeric and occur on cell surfaces and in soluble form.<ref name=”[13]”>Deora, A. et al. (2017). Transmembrane TNF-dependent uptake of anti-TNF antibodies. ''MAbs, 9''(4), 680-695. [https://doi.org/10.1080/19420862.2017.1304869 ''doi:10.1080/19420862.2017.1304869'']</ref><ref name=”[14]”>Haraoui, B., Bykerk, V. (2007). Etanercept in the treatment of rheumatoid arthritis. ''Therapeutics and clinical risk management'', ''3''(1), 99–105. [https://doi.org/10.2147/tcrm.2007.3.1.99 ''doi:10.2147/tcrm.2007.3.1.99'']</ref><ref name=”[15]”>Wang, J., Al-Lamki, R.S. (2013). Tumor Necrosis Factor Receptor 2: Its Contribution to Acute Cellular Rejection and Clear Cell Renal Carcinoma. ''BioMed Research International, 2013''(1), 1-11. [https://doi.org/10.1155/2013/821310 ''doi:10.1155/2013/821310'']</ref> Both sTNF and tmTNF bind to TNFR1 and TNFR2. The biological functions of TNF-⍺ are expressed by its binding to receptors. One of these biological functions is starting the inflammation process.

The pathophysiology of rheumatoid arthritis (RA) is generated by B- and T-cells, with a prominent role of the pro-inflammatory cytokines TNF-⍺ and IL-1. The permeation of CD4+ T cells into the synovium of the joint plays an important role in the inflammatory process. CD4+ T cells, activated by an antigen, stimulate the production of TNF-⍺, IL-1 and IL-6. This stimulation is the driving force behind ongoing inflammation in RA. It is thought that these CD4+ T cells also stimulate B cells in the production of immunoglobulins such as rheumatoid factor. Rheumatoid factor and IgG can form an immune complex, and this, in turn, contributes to the RA pathogenesis by activation of the complement system.<ref name=”[6]”>Boehncke, W.H., Radeke, H.H. (2007). Adalimumab. In Salfeld,J., Kupper, H., ''Biologics in General Medicine.'', 14-31, Springer-Verlag Berlin Heidelberg. [https://doi.org/10.1007/978-3-540-29018-6 ''doi:10.1007/978-3-540-29018-6'']</ref>

In patients with RA, elevated TNF-⍺ levels are found in the cartilage-pannus junction and the synovial tissue. It is produced locally in the synovium of joints. TNF-⍺ induces the production and secretion of cartilage-degrading matrix metalloproteinase enzymes (MMPs) from synovial fibroblasts. The MMPs inhibit tissue inhibitors of metalloproteinase. In total, this leads to the breakdown of collagen and joint destruction, due to matrix-degrading activities.<ref name=”[16]”>Alldred, A. (2001). Etanercept in rheumatoid arthritis. ''Expert Opinion on Pharmacotherapy, 2''(7), 1137-1148. [https://doi.org/10.1517/14656566.2.7.1137 ''doi:10.1517/14656566.2.7.1137'']</ref> Under the influence of pro-inflammatory cytokines, the macrophage colony stimulating factor (M-CSF) and the receptor activator of nuclear factor-kB ligand are activated. As a result, osteoclasts maturate and the osteoprotegerin ratio decreases, which leads to constant osteoclasts differentiation. Matured osteoclasts attached to the matrix, secrete hydrochloric acid and a proteolytic enzyme cathepsin K. These acids and enzymes destroy osteonectin and aggrecan, which result in chronic joint destruction.<ref name=”[17]”>Fazal, S.A. et al. (2018). A Clinical Update and Global Economic Burden of Rheumatoid Arthritis. ''Endocrine, Metabolic & Immune disorders - Drug Targets, 18''(2), 98-109. [https://doi.org/10.2174/1871530317666171114122417 ''doi:10.2174/1871530317666171114122417'']</ref>

=== Mode of action ===

Disease-modifying anti-rheumatic drugs (DMARDs) like adalimumab are used to reduce disease progression and to improve function by inhibiting the inflammation process. Adalimumab is a highly specific TNF-⍺ neutralizing antibody. Unlike other mAbs, adalimumab does not bind to other forms of TNF, like lymphotoxin-⍺. With its binding to soluble TNF-⍺, adalimumab inhibits the interaction of TNF-⍺ with TNFR1 and TNFR2 and prevents it from expression its biological function. With its Fab arms, it has the ability to crosslink two trimeric sTNF at the same time, which causes multimeric complexes to form.<ref name=”[5]”>Tracey, D., Klareskog, L., Sasso, E.H., Salfeld, J.G., Tak, P.P. (2008). Tumor necrosis factor antagonist mechanisms of action: A comprehensive review. ''Pharmacology & Therapeutics, 117''(2), 244-279. [https://doi.org/10.1016/j.pharmthera.2007.10.001 ''doi:10.1016/j.pharmthera.2007.10.001'']</ref><ref name=”[6]”>Boehncke, W.H., Radeke, H.H. (2007). Adalimumab. In Salfeld,J., Kupper, H., ''Biologics in General Medicine.'', 14-31, Springer-Verlag Berlin Heidelberg. [https://doi.org/10.1007/978-3-540-29018-6 ''doi:10.1007/978-3-540-29018-6'']</ref>

Adalimumab can also bind tmTNF. In a study that used a system with Jurkat T-cells to study the effect of adalimumab, the drug was found to induce complement-dependent cytotoxicity (CDC). The first component of complement activation (C1) is activated by the CH2 region of the Fc portion, which eventually leads to cell lysis. At the same time, adalimumab induces antibody-dependent cell-mediated cytotoxicity (ADCC). The CH2 and CH3 domains of the Fc domain of IgG1 play a role in the binding of adalimumab to the Fc receptors on a NK cell. This sets the lysis of the target cell in motion by granzyme B and perforin. Moreover, adalimumab is involved in reverse signaling. After the binding of adalimumab to tmTNF, cell apoptosis and cell cycle G0/G1 arrest are induced.<ref name=”[18]”>Horiuchi, T., Mitoma, H., Harashima, S., Tsukamoto, H., Shimoda, T. (2010). Transmembrane TNF-α: structure, function and interaction with anti-TNF agents. ''Rheumatology'', ''49''(7), 1215–1228. [https://doi.org/10.1093/rheumatology/keq031 ''doi:10.1093/rheumatology/keq031'']</ref>

=== Pharmacokinetics ===

Adalimumab is administered by the subcutaneous (SC) route. The absorption of adalimumab after SC administration is not fully understood. It is suggested that the absorption occurs like the diffusion of IgG across blood vessels and its relocation through lymphatic vessels. Flow through lymphatic vessels is slow, which causes the adsorption to last several days with a large interindividual variability.<ref name=”[19]”>Ternant, D., Bejan-Angoulvant, T., Passot, C., Mulleman, D., Paintaud, G. (2015). Clinical Pharmacokinetics and Pharmacodynamics of Monoclonal Antibodies Approved to Treat Rheumatoid Arthritis. ''Clinical Pharmacokinetics, 54''(11), 1107-1123. [https://doi.org/10.1007/s40262-015-0296-9 ''doi:10.1007/s40262-015-0296-9'']</ref> Peak serum concentrations of adalimumab are reached after around 5 days post administration. The absolute bioavailability after a single 40-mg dose was 64%. Concentrations of adalimumab in the synovial fluid of RA patients ranged from 31-96% of those in serum.<ref name=”[4]”>Medsafe (2012). Humira solution for injection: Data Sheet. Accessed on 6 November 2018, at [http://www.medsafe.govt.nz/Medicines/SearchResult.asp ''http://www.medsafe.govt.nz/Medicines/SearchResult.asp''].</ref>

As adalimumab is a large hydrophilic molecule, it should be confined to lymphatic vessels and blood vessels and report low tissue penetration. However, adalimumab penetrates cells through fluid phase endocytosis or through receptor-mediated endocytosis. Size prevents it from glomerular filtration and adalimumab is thus not eliminated via renal or biliary excretion. Elimination of adalimumab occurs non-specific (linear). The half-life is around twenty days.<ref name=”[3]”>Ternant, D. et al. (2014). Pharmacokinetics and concentration–effect relationship of adalimumab in rheumatoid arthritis. ''British Journal of Clinical Pharmacology, 79''(2), 286–297. [https://doi.org/10.1111/bcp.12509 ''doi:10.1111/bcp.12509'']</ref> This long serum half-life can be explained by the binding of neonatal Fc receptors on endothelial cells to the Fc domain of IgG at acidic pH. This protects IgG from catabolic activities and contributes to its long half-life. As adalimumab is much like IgG, it is thought that it goes through the same process as IgG.<ref name=”[7]”>Mitoma, H., Horiuchi, T., Tsukamoto, H., Ueda, N. (2018). Molecular mechanisms of action of anti-TNF-α agents – Comparison among therapeutic TNF-α antagonists. ''Cytokine, 101'', 56-63. [https://doi.org/10.1016/j.cyto.2016.08.014 ''doi:10.1016/j.cyto.2016.08.014'']</ref> The mechanisms by which antibodies are cleared from the circulation are not fully understood.<ref name=”[19]”>Ternant, D., Bejan-Angoulvant, T., Passot, C., Mulleman, D., Paintaud, G. (2015). Clinical Pharmacokinetics and Pharmacodynamics of Monoclonal Antibodies Approved to Treat Rheumatoid Arthritis. ''Clinical Pharmacokinetics, 54''(11), 1107-1123. [https://doi.org/10.1007/s40262-015-0296-9 ''doi:10.1007/s40262-015-0296-9'']</ref>

Interpatient variability of the pharmacokinetics can be explained by several factors, including antigenic burden, the presence of anti-drug antibodies (ADAbs) and body size. For most mAbs, the clearance and volume of distribution of mAbs increase with body size. These parameters are also reported to be higher in men than in women. For adalimumab, the clearance rate is around 40% higher in men. As the response to adalimumab is reported to increase with serum concentrations, a dosage adjustment by the influence of body weight and body surface area can be justified.<ref name=”[3]”>Ternant, D. et al. (2014). Pharmacokinetics and concentration–effect relationship of adalimumab in rheumatoid arthritis. ''British Journal of Clinical Pharmacology, 79''(2), 286–297. [https://doi.org/10.1111/bcp.12509 ''doi:10.1111/bcp.12509'']</ref><ref name=”[19]”>Ternant, D., Bejan-Angoulvant, T., Passot, C., Mulleman, D., Paintaud, G. (2015). Clinical Pharmacokinetics and Pharmacodynamics of Monoclonal Antibodies Approved to Treat Rheumatoid Arthritis. ''Clinical Pharmacokinetics, 54''(11), 1107-1123. [https://doi.org/10.1007/s40262-015-0296-9 ''doi:10.1007/s40262-015-0296-9'']</ref>

=== Efficacy ===

Adalimumab is an overall well-tolerated drug. However, some patients show an inadequate response to the drug. Around 10–30% of patients do not respond to the initial treatment and 23–46% of patients lose response over time.<ref name=”[20]”>Roda, G., Jharap, B., Neeraj, N., Colombel, J.F. (2016). Loss of Response to Anti-TNFs: Definition, Epidemiology, and Management. ''Clinical and translational gastroenterology'', ''7''(1), e135. [https://doi.org/10.1038/ctg.2015.63 ''doi:10.1038/ctg.2015.63'']</ref> Because adalimumab is an exogenous protein, it can induce an immune response. While the risk of the formation of anti-drug antibodies (ADAb) are very high for murine mAbs, the risk is also present for human antibodies like adalimumab. A decreased response in treatment of adalimumab can be associated with the formation of ADAb.<ref name=”[19]”>Ternant, D., Bejan-Angoulvant, T., Passot, C., Mulleman, D., Paintaud, G. (2015). Clinical Pharmacokinetics and Pharmacodynamics of Monoclonal Antibodies Approved to Treat Rheumatoid Arthritis. ''Clinical Pharmacokinetics, 54''(11), 1107-1123. [https://doi.org/10.1007/s40262-015-0296-9 ''doi:10.1007/s40262-015-0296-9'']</ref> For patients that show an inadequate response to adalimumab and by whom the presence of ADAb is registered, an increase in the dosage or dose frequency of adalimumab can lead to a decrease in detectable ADAb. By increasing the dosing frequency, it might overload the ability of the immune system to produce sufficient ADAb or it may induce immunotolerance to adalimumab.<ref name=”[21]”>Bartelds, G.M. et al. (2007). Clinical response to adalimumab: relationship to anti-adalimumab antibodies and serum adalimumab concentrations in rheumatoid arthritis. ''Annals of the Rheumatic Diseases'', ''66''(1), 921-926. [https://doi.org/10.1136/ard.2006.065615 ''doi:10.1136/ard.2006.065615'']</ref> This could potentially increase clinical outcome.

Not only the presence of ADAb is a predictor of a non-response. Some non-responders have no detectable ADAb or have adequate or high drug levels without clinical response. In patients without detectable drug levels, however, no clinical effect of adalimumab was registered.<ref name=”[19]”>Ternant, D., Bejan-Angoulvant, T., Passot, C., Mulleman, D., Paintaud, G. (2015). Clinical Pharmacokinetics and Pharmacodynamics of Monoclonal Antibodies Approved to Treat Rheumatoid Arthritis. ''Clinical Pharmacokinetics, 54''(11), 1107-1123. [https://doi.org/10.1007/s40262-015-0296-9 ''doi:10.1007/s40262-015-0296-9'']</ref><ref name=”[22]”>Pouw, M.F. et al. (2015). Key findings towards optimising adalimumab treatment: the concentration–effect curve. ''Annals of the rheumatic diseases, 74''(3), 513–518. [https://doi.org/10.1136/annrheumdis-2013-204172 ''doi:10.1136/annrheumdis-2013-204172'']</ref>
[[File:meandas28.png|left|thumb|Mean DAS28 improvement for adalimumab concentrations<ref name=”[22]”>Pouw, M.F. et al. (2015). Key findings towards optimising adalimumab treatment: the concentration–effect curve. ''Annals of the rheumatic diseases, 74''(3), 513–518. [https://doi.org/10.1136/annrheumdis-2013-204172 ''doi:10.1136/annrheumdis-2013-204172'']</ref>|500px]]
A good clinical response is characterized by a Disease Activity Score in 28 joints (DAS28) improvement of 1.2 and higher. Serum concentrations of around 3 µg/mL are sufficient to reach this threshold. Serum levels up to 8 µg/mL have a positive effect on the DAS28 score, therefore the probability of clinical response to adalimumab increases with the trough serum concentrations. It should be noted that serum levels surpassing 8 µg/mL do not contribute to further clinical improvement. The cut-off value to distinguish between good responders and non and moderate of responders was determined to be 5 µg/mL. Therefore, adalimumab serum trough concentrations in the range of 5-8 µg/mL were found to be predictive of good clinical response. A DAS28 improvement score below 0.6 is classified as a no response.<ref name=”[22]”>Pouw, M.F. et al. (2015). Key findings towards optimising adalimumab treatment: the concentration–effect curve. ''Annals of the rheumatic diseases, 74''(3), 513–518. [https://doi.org/10.1136/annrheumdis-2013-204172 ''doi:10.1136/annrheumdis-2013-204172'']</ref>

{| class="wikitable" style="border: none; background: none;"
! colspan="2" rowspan="2" style="border: none; background: none;"|
! colspan="3"| DAS28 improvement
|-
! >1.2 !! >0.6 and ≤ 1.2 !! ≤0.6
|-
! rowspan="3"| Present DAS28 score
! ≤3.2
| good response || moderate response || no response
|-
! >3.2 and ≤5.1
| moderate response || moderate response || no response
|-
! >5.1
| moderate response || no response || no response
|}<div style="margin-bottom:1em">''Table 1: EULAR response criteria DAS28.<ref name=”[23]”>Fransen, J., Van Riel, P.L.C.M. (2005) The Disease Activity Score and the EULAR response criteria, ''Clinical and Experimental Rheumatology, 23''(39), S93-S99.</ref>''</div>

The use of serum trough levels along with an assessment of disease activity, can serve as an early prediction of response to adalimumab in RA. The level reached in serum depends on factors like adsorption rate, distribution rate and clearance. Furthermore, these factors are influenced by physical states like gender, age and disease state.<ref name=”[22]”>Pouw, M.F. et al. (2015). Key findings towards optimising adalimumab treatment: the concentration–effect curve. ''Annals of the rheumatic diseases, 74''(3), 513–518. [https://doi.org/10.1136/annrheumdis-2013-204172 ''doi:10.1136/annrheumdis-2013-204172'']</ref>

For mAbs in general, patients with high disease activity have an increased amount of TNF-⍺. Due to target-mediated elimination, this increase leads to a increased target-mediated clearance which in turn leads to lower concentrations of TNF targeting mAbs. Consequently, patients that express a high disease activity have a lower exposure to anti-TNF-⍺ mAbs. RA patients should therefore receive higher mAb doses according to disease activity. To solidify this and to propose rational treatment strategies, more pharmacokinetic-pharmacodynamic studies are needed.<ref name=”[19]”>Ternant, D., Bejan-Angoulvant, T., Passot, C., Mulleman, D., Paintaud, G. (2015). Clinical Pharmacokinetics and Pharmacodynamics of Monoclonal Antibodies Approved to Treat Rheumatoid Arthritis. ''Clinical Pharmacokinetics, 54''(11), 1107-1123. [https://doi.org/10.1007/s40262-015-0296-9 ''doi:10.1007/s40262-015-0296-9'']</ref>

As the half-life of adalimumab is around twenty days, steady state is only achieved after 4 months after initiation of treatment. No loading dose is recommended for RA patients and therefore the time to reach steady-state is delayed. Instituting a higher loading dose to reach steady state faster could potentially decrease the risk of a non-response. This is seen in Crohn’s disease patients, in whom the use of loading doses was shown to increase treatment efficacy and decrease the risk of developing ADAb.<ref name=”[3]”>Ternant, D. et al. (2014). Pharmacokinetics and concentration–effect relationship of adalimumab in rheumatoid arthritis. ''British Journal of Clinical Pharmacology, 79''(2), 286–297. [https://doi.org/10.1111/bcp.12509 ''doi:10.1111/bcp.12509'']</ref>

== Safety ==

Biological DMARDs like adalimumab have a unique mechanism of action. Adalimumab blocks the overexpressed cytokine TNF-⍺ and with its binding, inhibits an important signaling protein in the normal immune response. TNF plays a critical role in the formation of post-infectious granuloma and its maintenance, which are important components for the host defenses against microbial pathogens, such as Mycobacterium Tuberculosis, histoplasmosis and other opportunistic infections. Deactivation of TNF can give way to granulomatous infectious diseases, like tuberculosis, histoplasmosis, and other less common conditions.<ref name=”[14]”>Haraoui, B., Bykerk, V. (2007). Etanercept in the treatment of rheumatoid arthritis. ''Therapeutics and clinical risk management'', ''3''(1), 99–105. [https://doi.org/10.2147/tcrm.2007.3.1.99 ''doi:10.2147/tcrm.2007.3.1.99'']</ref><ref name=”[24]”>Wallis, R.S., Broder, M.S., Wong, J.Y., Hanson, M.E., Beenhouwer, D.O. (2004). Granulomatous Infectious Diseases Associated with Tumor Necrosis Factor Antagonists. ''Clinical Infectious Diseases'', ''38''(9), 1261–1265. [https://doi.org/10.1086/383317 ''doi:10.1086/383317'']</ref>

The most common adverse effects of the usage of adalimumab are infections and immunological reactions, like hypersensitivity, injection-site and infusion-related reactions. More serious adverse events include the reactivation of tuberculosis and hepatitis B. Therefore, patients with a history of chronic infections and recurrent infections should avoid biologic therapies. Adalimumab is contra-indicated for patients with sepsis or who are at risk of sepsis, with active tuberculosis or other opportunistic infections.<ref name=”[4]”>Medsafe (2012). Humira solution for injection: Data Sheet. Accessed on 6 November 2018, at [http://www.medsafe.govt.nz/Medicines/SearchResult.asp ''http://www.medsafe.govt.nz/Medicines/SearchResult.asp''].</ref> Some studies report an increased risk of infections with higher doses of adalimumab. However, this correlation is not completely certain.<ref name=”[19]”>Ternant, D., Bejan-Angoulvant, T., Passot, C., Mulleman, D., Paintaud, G. (2015). Clinical Pharmacokinetics and Pharmacodynamics of Monoclonal Antibodies Approved to Treat Rheumatoid Arthritis. ''Clinical Pharmacokinetics, 54''(11), 1107-1123. [https://doi.org/10.1007/s40262-015-0296-9 ''doi:10.1007/s40262-015-0296-9'']</ref><ref name=”[25]”>Bongartz, T., Sutton, A.J., Sweeting, M.J,, Buchan, I., Matteson, E.L., Montori, V. (2016). Anti-TNF Antibody Therapy in Rheumatoid Arthritis and the Risk of Serious Infections and Malignancies Systematic Review and Meta-analysis of Rare Harmful Effects in Randomized Controlled Trials. ''JAMA, 295''(19), 2275-2285. [https://doi.org/10.1001/jama.295.19.2275 ''doi:10.1001/jama.295.19.2275'']</ref> Therefore, caution has to be taken with prescribing DMARDs and for prescribing the right dosage

== Lab protocols ==

Adalimumab in solution should be refrigerated between 2ºC to 8ºC and protected from light. It should not be frozen.<ref name=”[4]”>Medsafe (2012). Humira solution for injection: Data Sheet. Accessed on 6 November 2018, at [http://www.medsafe.govt.nz/Medicines/SearchResult.asp ''http://www.medsafe.govt.nz/Medicines/SearchResult.asp''].</ref> It can be harmful if inhaled and cause respiratory tract irritation. To prevent harmful absorption through the skin and prevent possible skin irritation, protective gloves are recommended. Moreover, swallowing the compound could be harmful.<ref name=”[26]”>BioVision (2018). Anti-TNF-a (Adalimumab): Datasheet. Accessed on 9 November 2018, at [https://www.biovision.com/anti-tnf-adalimumab-humanized-antibody.html ''https://www.biovision.com/anti-tnf-adalimumab-humanized-antibody.html''].</ref>

Samples containing adalimumab have to be analyzed within 72 hours. Otherwise the samples must be frozen at ≤ -18 °C for 12 months. Disposal of samples are to be performed according to your laboratory regulations.<ref name=”[27]”>MabTrack level adalimumab (2018). Leaflet ''MabTrack level adalimumab''. Sanquin.</ref>

== Medical Use and TDM ==

The drug is supplied as a solution for injection with a pH of 5.2. Current available dosage forms are 40 mg/0.8 mL, 40 mg/0.4 mL, 20 mg/0.4 mL and 10 mg/0.2 mL single-use prefilled syringe. There also exist prefilled pens of 40 mg/0.8 mL and 40 mg/0.4 mL.<ref name=”[4]”>Medsafe (2012). Humira solution for injection: Data Sheet. Accessed on 6 November 2018, at [http://www.medsafe.govt.nz/Medicines/SearchResult.asp ''http://www.medsafe.govt.nz/Medicines/SearchResult.asp''].</ref> ADL is administered once every week or every other week. The target steady state trough concentration is between 5 and 8 µg/mL.

In current practices, a patient can be classified as a non-reponder, when the concentration of adalimumab is at a normal range, but the patient has no decreased disease activity. A switch to a different medicine is often advised. When serum levels of adalimumab are too low, the underlying cause, either non-compliance or ADAb, is investigated. If the concentration is not extremely low, an increase of dosage can be considered.

Measurements of adalimumab levels are not routinely done. Therapeutic drug monitoring (TDM) is the practice of measuring the concentration of a specific drug in the bloodstream with the aim of using this data to optimize the individual dosing schemes of patients. TDM could provide a means to optimize the treatment with adalimumab. The introduction of a biosensor for adalimumab as a means for TDM would allow for better drug monitoring. It gives the possibility to detect non-responders in an early stage of treatment or to optimize dosing strategies. For patients with too high serum levels, dosage reduction to obtain serum levels between 5 and 8 µg/mL could be beneficial for the patient as the expensive drug is used more optimally. As ADAb can be the cause of the lower adalimumab serum levels, measurement of ADAb with a biosensor is also a possibility. However, as adalimumab can interfere with an assay that measures ADAb, measurement of adalimumab itself is to be prefered.<ref name=”[22]”>Pouw, M.F. et al. (2015). Key findings towards optimising adalimumab treatment: the concentration–effect curve. ''Annals of the rheumatic diseases, 74''(3), 513–518. [https://doi.org/10.1136/annrheumdis-2013-204172 ''doi:10.1136/annrheumdis-2013-204172'']</ref>

A treatment based on TDM has the potential to ensure a maximal clinical benefit with the lowest dose of the drug.<ref name=”[22]”>Pouw, M.F. et al. (2015). Key findings towards optimising adalimumab treatment: the concentration–effect curve. ''Annals of the rheumatic diseases, 74''(3), 513–518. [https://doi.org/10.1136/annrheumdis-2013-204172 ''doi:10.1136/annrheumdis-2013-204172'']</ref> Next to the health benefit, treatment could have an economic benefit, as it can be cost-saving in the long-term, especially in the case of dose reduction.

A possible difficulty in the implementation of personalized dosing schemes are the fixed dosing regimens. Drugs that are administered via SC route are provided in fixed dosing regimens, for example a 40-mg prefilled syringe. In case of intravenous dosing (IV), dosing adjustments for body size and disease activity are possible for anti-TNF-⍺ mAbs.<ref name=”[19]”>Ternant, D., Bejan-Angoulvant, T., Passot, C., Mulleman, D., Paintaud, G. (2015). Clinical Pharmacokinetics and Pharmacodynamics of Monoclonal Antibodies Approved to Treat Rheumatoid Arthritis. ''Clinical Pharmacokinetics, 54''(11), 1107-1123. [https://doi.org/10.1007/s40262-015-0296-9 ''doi:10.1007/s40262-015-0296-9'']</ref>

== State of the art of adalimumab assays ==

In the table below, a selection of the available adalimumab assays is listed.

{| class="wikitable" style="margin-bottom:0"
!Company
!Product
!Test name
!Sample Volume
!Reportable range
!Dilution ratio*
!Precision
!Incubation time
|-
|Sanquin<ref name=”[27]”>MabTrack level adalimumab (2018). Leaflet ''MabTrack level adalimumab''. Sanquin.</ref>
|M2910
|MabTrack level adalimumab
|5 μL
|1–30 µg/mL
|1:199 1:1499 1:1999
|Total CV < 15% Inter-assay: CV < 15.4%
|2 hours 10 min.
|-
|apDia<ref name=”[28]”>apDia Adalimumab ELISA (2016). Leaflet ''apDia Adalimumab ELISA.'' apDia.</ref>
|710201
|apDia Adalimumab ELISA
|10 μL
|0.5-12 µg/mL 2.0- 48 µg/mL
|1:99 1:399
|Intra-assay: CV < 10.1% Inter-assay: CV< 14.2%
|1 hour 40 min.
|-
|Theradiag<ref name=”[29]”>LTA002 LISA-TRACKER Adalimumab (2017). Leaflet ''LTA002 LISA-TRACKER Adalimumab''. Theradiag.</ref>
|LTA 002
|LISA-TRACKER Adalimumab
|5 μL
|0.3 - 16 µg/mL
|1:200
|Intra-assay: CV < 13.3% Inter-assay: CV< 9.7%
|2 hours
|-
|R-Biopharm AG<ref name=”[30]”>GN3043 RIDA®QUICK ADM Monitoring (2018). Leaflet ''GN3043 RIDA®QUICK ADM Monitoring''. R-Biopharm AG.</ref>
|GN3043
|RIDA®QUICK ADM Monitoring
|20 μL
|0.5 - 25 μg/ml
|1:499
|Intra-assay: CV < 16.8% Inter-assay: CV< 16.6%
|15 min.
|-
|BÜHLMANN<ref name=”[31]”>LF-TLAD25 Quantum Blue® Adalimumab (2018). Leaflet ''LF-TLAD25 Quantum Blue® Adalimumab''. BÜHLMANN.</ref>
|LF-TLAD25
|Quantum Blue® Adalimumab
|10 μL
|1.3 - 35 μg/mL
|1:19
|Intra-assay: CV < 28.6% Inter-assay: CV < 12.6%
|15 min

|}<div style="margin-bottom:1em">''Table 2: Selection of currently available systems for measuring adalimumab.'' *Note: the dilution ratio is defined as 1:x, with x the volume of added reagents relative to the volume of plasma sample</div>

To determine the trough levels of adalimumab, the samples must be taken within 24 hours prior to the drug administration.<ref name=”[27]”>MabTrack level adalimumab (2018). Leaflet ''MabTrack level adalimumab''. Sanquin.</ref>

These assays are sandwich-type assays with enzymatic labelling, except for the RIDA®QUICK ADM Monitoring and Quantum Blue® Adalimumab, which are based on lateral flow immunoassays. Mabtrack by Sanquin uses polystyrene microtiter wells with immobilized TNF-specific mouse monoclonal antibodies. These bind recombinant TNF. The adalimumab that is present in the sample binds to the bound TNF on the microtiter plate and an adalimumab/TNF/anti-TNF complex is formed. Next, a monoclonal ADAb labeled with horseradish peroxidase is added which binds to the complex. A substrate solution leads to the formation of a colored product, proportional to the amount of adalimumab present in the sample.<ref name=”[27]”>MabTrack level adalimumab (2018). Leaflet ''MabTrack level adalimumab''. Sanquin.</ref> The Adalimumab ELISA and LISA-TRACKER Adalimumab both have immobilized TNF-⍺ on the surface of the microwell plate. While the apDia Adalimumab ELISA makes use of ADAb conjugated with peroxidase to form a TNF-⍺/adalimumab/conjugated-ADAb complex, the LISA-TRACKER uses anti-human IgG biotinylated antibodies to form the complex, whereafter horseradish peroxydase labelled with streptavidin is added that binds to the complex.<ref name=”[28]”>apDia Adalimumab ELISA (2016). Leaflet ''apDia Adalimumab ELISA.'' apDia.</ref><ref name=”[29]”>LTA002 LISA-TRACKER Adalimumab (2017). Leaflet ''LTA002 LISA-TRACKER Adalimumab''. Theradiag.</ref>

== Numbers ==

Rheumatoid arthritis affects around 1% of the world population per year.<ref name=”[32]”>Marita Cross, M. et al. (2014). The global burden of rheumatoid arthritis: estimates from the Global Burden of Disease 2010 study. ''Annals of Rheumatic diseases, 73'', 1316-1322. [https://doi.org/10.1136/annrheumdis-2013-204627 ''doi:10.1136/annrheumdis-2013-204627'']</ref> Global estimates in 2010 reported a prevalence rate of 0.35% for women and 0.13% for men. The prevalence of RA is higher in more developed countries.<ref name=”[33]”>Fazal, S.A. et al. (2018). A Clinical Update and Global Economic Burden of Rheumatoid Arthritis. ''Endocrine, Metabolic & Immune disorders - Drug Targets, 18''(2), 98-109. [https://doi.org/10.2174/1871530317666171114122417 ''doi:10.2174/1871530317666171114122417'']</ref> A study in the US reported an overall lifetime risk for RA of 3.6% for women and 1.7% for men. This corresponds to around 1 in 28 women and 1 in 59 men that will develop RA in their lifetime.<ref name=”[34]”>Crowson, C.S. et al. (2011). The lifetime risk of adult-onset rheumatoid arthritis and other inflammatory autoimmune rheumatic diseases. ''Arthritis and Rheumatism, 63''(3), 633-639. [https://doi.org/10.1002/art.30155 ''doi:10.1002/art.30155'']</ref>

Globally in 2010, RA represented 0.49% of years lived with disability (YLD) and 0.19% of disability-adjusted-years (DALY). Across 31 nations in the period of 2009–2011, a total of 219,189 patients died, in whom RA was registered as the underlying cause of death. The YLDs for RA were 55/100000 population and the total DALYs were around 4.8 million.<ref name=”[33]”>Fazal, S.A. et al. (2018). A Clinical Update and Global Economic Burden of Rheumatoid Arthritis. ''Endocrine, Metabolic & Immune disorders - Drug Targets, 18''(2), 98-109. [https://doi.org/10.2174/1871530317666171114122417 ''doi:10.2174/1871530317666171114122417'']</ref>

The US reported having approximately $128 billion of direct and $47.0 billion of indirect costs billable to arthritis and related rheumatic conditions. In the UK, this number came down to £560 million a year in health care costs.<ref name=”[33]”>Fazal, S.A. et al. (2018). A Clinical Update and Global Economic Burden of Rheumatoid Arthritis. ''Endocrine, Metabolic & Immune disorders - Drug Targets, 18''(2), 98-109. [https://doi.org/10.2174/1871530317666171114122417 ''doi:10.2174/1871530317666171114122417'']</ref>
In 2017, adalimumab (Humira) was at the top of pharmaceutical products by sales worldwide. The drug generated more than 18.4 billion US dollars. Almost twice as much as Rituxan, who took second place with 9.2 billion dollars generated.<ref name=”[35]”>Statista (2018). Top 15 pharmaceutical products by sales worldwide in 2017. Accessed on 7 November 2018, at [https://www.statista.com/statistics/258022/top-10-pharmaceutical-products-by-global-sales-2011/ ''https://www.statista.com/statistics/258022/top-10-pharmaceutical-products-by-global-sales-2011/''].</ref> In 2015, Humira costs around $2669 per month in the US and $1362 in the UK.<ref name=”[36]”>Statista (2018). Average prices of Humira in selected countries in 2015. Accessed on 7 November 2018, at [https://www.statista.com/statistics/312014/average-price-of-humira-by-country/ ''https://www.statista.com/statistics/312014/average-price-of-humira-by-country/''].</ref>

== References ==

<references />