Adenosine A2A receptor antagonist istradefylline (KW-6002) reduces "off" time in Parkinson's disease: a double-blind, randomized, multicenter clinical trial (6002-US-005)

Factors associated with a placebo effect in Parkinson's disease in clinical trials: a meta-analysis

OBJECTIVES

Outcomes of clinical trials of treatment in patients with Parkinson's disease (PD) may be influenced by placebo effects. The aim of this study was to determine the factors associated with placebo effects in Parkinson's disease (PD) for guidance with design of future clinical trials.

METHODS

Factors associated with placebo effects in PD were examined in a meta-analysis using a random effects model with pooling of placebo effects on the Unified Parkinson's Disease Rating Scale part III (UPDRS III) or Movement Disorder Society sponsored revision of UPDRS III (MDS-UPDRS III). The following prespecified variables were included in the analyses: with or without drug at baseline, with or without a placebo run-in phase, with or without motor fluctuation, published year, number of study sites, placebo administration period, age, sex, disease duration, and daily levodopa dose. Publication bias was assessed by visual inspection of funnel plots and adjusted using the trim-and-fill method.

RESULTS

Thirty-eight articles with a total of 4828 subjects satisfied the inclusion criteria. There was a significant placebo effect using UPDRS III or MDS-UPDRS III (SMD = - 0.25; 95% CI - 0.35 to - 0.14; p < 0.001, I2 = 92%). Subgroup and/or multivariate meta-regression analyses revealed that placebo effects were associated with advanced PD (p = 0.04), drug exposure at baseline (p < 0.001), placebo administration period (p < 0.001), and disease duration (p < 0.01).

CONCLUSIONS

The results of this study are important as guidance in design of future clinical trials in which the influence of placebo effects is minimized.

Modulatory effects of acupuncture on raphe nucleus-related brain circuits in patients with chronic neck pain: A randomized neuroimaging trial

OBJECTIVE

Acupuncture has shown promise in treating neck pain. Clinical trials have shown mixed results, possibly due to heterogeneous methodologies and the lack of knowledge regarding underlying brain circuit mechanism of action. In this study, we investigated the specific contribution of the serotonergic system in treating neck pain, and the specific brain circuits involved.

METHODS

A total of 99 patients with chronic neck pain (CNP) were randomized to receive true acupuncture (TA) or sham acupuncture (SA) 3 times weekly for 4 weeks. Patients with CNP in each group were assessed for primary outcomes by measuring the Visual Analog Scale (VAS) and the duration of each attack; secondary outcomes were measured using the Neck Disability Index (NDI), Northwick Park Neck Pain Questionnaire (NPQ), McGill Pain Questionnaire (MPQ), Self-rating Anxiety Scale (SAS), Self-rating Depression Scale (SDS) and the 12-item Short Form Quality Life Scale (SF-12); levels of functional circuits connectivity were assessed using resting-state functional magnetic resonance imaging in the dorsal (DR) and median (MR) raphe nucleus, before and after undergoing acupuncture.

RESULTS

Patients receiving TA showed more extensive symptom improvement compared with SA. Regarding the primary outcomes, changes observed in the TA group were as follows: VAS = 16.9 mm (p < 0.001) and the duration of each attack = 4.30 h (p < 0.001); changes in the SA group: VAS = 5.41 mm (p = 0.138) and the duration of each attack = 2.06 h (p = 0.058). Regarding the secondary outcomes, changes in the TA group: NDI = 7.99 (p < 0.001), NPQ = 10.82 (p < 0.001), MPQ = 4.23 (p < 0.001), SAS = 5.82 (p < 0.001), SDS = 3.67 (p = 0.003), and SF-12 = 3.04 (p < 0.001); changes in the SA group: NDI = 2.97 (p = 0.138), NPQ = 5.24 (p = 0.035) and MPQ = 2.90 (p = 0.039), SAS = 1.48 (p = 0.433), SDS = 2.39 (p = 0.244), and SF-12 = 2.19 (p = 0.038). The modulatory effect of TA exhibited increased functional connectivity (FC) between the DR and thalamus, between the MR and parahippocampal gyrus, amygdala, and insula, with decreased FC between the DR and lingual gyrus and middle frontal gyrus, between the MR and middle frontal gyrus. Furthermore, changes in the DR-related circuit were specifically associated with the intensity and duration of pain, and the MR-related circuit was correlated with the quality of life with CNP.

CONCLUSION

These results demonstrated the effectiveness of TA in treating neck pain and suggested that it regulates CNP by reconfiguring the function of the raphe nucleus-related serotonergic system.

Management of OFF condition in Parkinson disease

Parkinson disease (PD) impacts nearly 1 million individuals in the United States. Nearly every patient with PD will require therapy with dopamine in the form of levodopa as the disease progresses. In more advanced stages of the disease, patients will experience motor fluctuations and require adjustment to their medication regimens to maintain good control of their symptoms. During the last 10 years, several new therapeutic treatment options have come to the market to treat motor fluctuations and improve patient quality of life. Some of these agents represent additional options to previously available drug classes, such as the catechol-O-methyl transferase (COMT) inhibitor, opicapone, and monoamine-oxidase B-inhibitor (MAO-B inhibitor), safinamide, as well as new dosage forms for available therapeutics. One new agent, istradefylline, has a novel mechanism in the treatment of PD. The place in therapy for these newer therapeutic options will be explored through a series of patient cases. This article focuses on evidence-based recommendations for the use of these newer options in the management of patients experiencing OFF episodes.

Comparative efficacy and safety of adjunctive drugs to levodopa for fluctuating Parkinson's disease - network meta-analysis

It remains unclear which adjunctive drug for Parkinson's disease (PD) in combination with levodopa is more effective, tolerable, and safe. We aimed to compare the efficacy, tolerability, and safety among anti-PD drugs from several classes in patients with fluctuating PD who received levodopa through network meta-analysis (NMA). Twelve anti-PD drugs belonging to 4 different drug classes (dopamine agonists, monoamine oxidase type B inhibitors, catechol-O-methyl transferase inhibitors, and an adenosine A2A receptor antagonist) were selected. We systematically searched PubMed, Embase, and the Cochrane Library for eligible randomized controlled trials (RCTs) comparing placebo with anti-PD drug or among anti-PD drugs in patients with PD who experienced motor fluctuations or wearing-off and received levodopa. We included 54 RCTs in the analysis. The NMA was performed under a frequentist framework using a random-effects model. The efficacy outcome was change in daily off-time, and the tolerability outcome was discontinuation due to all causes. Safety outcomes included discontinuation due to adverse events (AEs) and the incidence of AEs, dyskinesia, hallucination, and orthostatic hypotension. According to the surface under the cumulative ranking curve (SUCRA) in the NMA, ropinirole transdermal patch (SUCRA, 0.861) ranked the highest in efficacy, followed by pramipexole (0.762), ropinirole extended release (ER) (0.750), and safinamide (0.691). In terms of tolerability, ropinirole (0.954) ranked the highest, followed by pramipexole (0.857), safinamide (0.717), and ropinirole ER (0.708). Each anti-PD drug had different SUCRA ranking profiles for the safety outcomes. These findings suggest that ropinirole, pramipexole, and safinamide are well-balanced anti-PD drugs that satisfy both efficacy and tolerability outcomes.

Molecular and Structural Insight into Adenosine A2A Receptor in Neurodegenerative Disorders: A Significant Target for Efficient Treatment Approach

All biological tissues and bodily fluids include the autacoid adenosine. The P1 class of purinergic receptors includes adenosine receptors. Four distinct G-protein-coupled receptors on the cellular membrane mediate the effects of adenosine, whose cytoplasmic content is regulated by producing/degrading enzymes and nucleoside transporters. A2A receptor has received a great deal of attention in recent years because it has a wide range of potential therapeutic uses. A2B and, more significantly, A2A receptors regulate numerous physiological mechanisms in the central nervous system (CNS). The inferior targetability of A2B receptors towards adenosine points that they might portray a promising medicinal target since they are triggered only under pharmacological circumstances (when adenosine levels rise up to micromolar concentrations). The accessibility of specific ligands for A2B receptors would permit the exploration of such a theory. A2A receptors mediate both potentially neurotoxic and neuroprotective actions. Hence, it is debatable to what extent they play a role in neurodegenerative illnesses. However, A2A receptor blockers have demonstrated clear antiparkinsonian consequences, and a significant attraction exists in the role of A2A receptors in other neurodegenerative disorders. Amyloid peptide extracellular accumulation and tau hyperphosphorylation are the pathogenic components of AD that lead to neuronal cell death, cognitive impairment, and memory loss. Interestingly, in vitro and in vivo research has shown that A2A adenosine receptor antagonists may block each of these clinical symptoms, offering a crucial new approach to combat a condition for which, regrettably, only symptomatic medications are currently available. At least two requirements must be met to determine whether such receptors are a target for diseases of the CNS: a complete understanding of the mechanisms governing A2A-dependent processes and the availability of ligands that can distinguish between the various receptor populations. This review concisely summarises the biological effects mediated by A2A adenosine receptors in neurodegenerative disorders and discusses the chemical characteristics of A2A adenosine receptor antagonists undergoing clinical trials. Selective A2A receptor blocker against neurodegenerative disorders.

Importance of time to ON versus wearing OFF in total daily OFF time experienced by patients with Parkinson's disease

Most patients with Parkinson's disease (PD) receiving levodopa (LD)/DOPA decarboxylase inhibitors develop motor fluctuations with an increasing amount of OFF time, negatively impacting patient quality of life. Herein, we review the evidence supporting the substantial, yet underappreciated contribution of delays in time to ON (including delayed ON and no ON) to total daily OFF time. Most clinical studies use patient diaries that do not capture time to ON and wearing OFF separately as related to LD dosing, and consequently, most OFF time has generally been attributed to wearing OFF. Hence, most treatment regimens focus on reducing wearing OFF by changing LD dosing/formulations and/or using "ON-extenders" (eg, catechol-o-methyltransferase inhibitors, monoamine oxidase-B inhibitors, extended-release amantadine, and adenosine A2A receptor antagonists). However, the literature describing approved treatments for PD that has focused on delays in time to ON is sparse and suggests this type of OFF may comprise more than twice the amount of total daily OFF time as wearing OFF. Here, we advocate for the importance of measuring and adequately addressing delays in time to ON and build support for the consistent inclusion of the time to ON measurement in future clinical trials.

Clinical Use of On-Demand Therapies for Patients with Parkinson's Disease and OFF Periods

On-demand therapies for Parkinson's disease (PD) provide rapid, reliable relief for patients experiencing OFF periods; however, practical guidelines on the use of these therapies are not generally available. This paper reviews the use of on-demand treatments. Motor fluctuations occur in nearly all patients with PD after long-term use of levodopa. As the goal of PD treatment is to provide good ON time, on-demand treatments that have a more rapid reliable onset than the slower-acting oral medications provide rapid relief for OFF periods. All current on-demand treatments bypass the gastrointestinal tract, providing dopaminergic therapy directly into the blood stream by subcutaneous injection, through the buccal mucosa, or by inhalation into the pulmonary circulation. On-demand treatments are fast acting (10- to 20-min onset), with maximum, reliable, and significant responses reached within 30 min after administration. Oral medications pass through the gastrointestinal tract and thus have slower absorption owing to gastroparesis and competition with food. On-demand therapies, by providing fast-acting relief, can have a positive impact on a patient's quality of life when patients are experiencing OFF periods.

Adenosine A2A antagonists and Parkinson's disease

Although there is no cure for Parkinson's disease (PD), there are several classes of medications with various mechanisms of action that can help improve the functionality of someone with PD. Dopamine derivatives are first line therapies for PD, hence dopamine receptor agonists (DAs) have been shown to improve functionality of symptoms in PD patients. The two main formulations of dopamine agonist medications in PD therapy are ergoline and non-ergoline derivatives. Additionally, it has been shown that PD can involve irregularities in other neurotransmitters, such as acetylcholine, norepinephrine, and serotonin, hence why non-dopaminergic medications are also vital in PD management. Examples include NMDA receptor antagonists, dopamine antagonists (i.e. neuroleptics), acetylcholine receptor antagonists, serotonin receptor 2A agonists, and adenosine A2 antagonists. In general, dopaminergic medications are the most effective in improving motor involvement with PD, whereas non-dopaminergic medications tend to focus on the non-motor involvement of PD. In this chapter, we will focus on the chemistry and medication background on dopaminergic vs non-dopaminergic therapy, with a focus of adenosine A2 antagonists at the end.

The adenosine A2A receptor antagonist/inverse agonist, KW-6356 enhances the anti-parkinsonian activity of L-DOPA with a low risk of dyskinesia in MPTP-treated common marmosets

The adenosine A_2A receptor antagonist/inverse agonist, KW-6356 has been shown to be effective in Parkinson's disease (PD) patients as monotherapy and as an adjunct therapy to L-3,4-dihydroxyphenylalanine (L-DOPA)/decarboxylase inhibitor. However, the effects of KW-6356 combined with L-DOPA on anti-parkinsonian activity and established dyskinesia has not been investigated in preclinical experiments. We examined the effects of combination of KW-6356 with L-DOPA in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated common marmosets. Oral administration of KW-6356 (1 mg/kg) enhanced the anti-parkinsonian activities of various doses of L-DOPA (2.5-10 mg/kg). In MPTP-treated common marmosets primed with L-DOPA to show dyskinesia, KW-6356 (1 mg/kg) also enhanced the anti-parkinsonian activities of various doses of L-DOPA (1.25-10 mg/kg) but not dyskinesia. Chronic co-administration of KW-6356 (1 mg/kg) with a low dose of L-DOPA (2.5 mg/kg) for 21 days increased the degree of dyskinesia induced by the low dose of L-DOPA, but the amplitude of dyskinesia induced by combined administration of KW-6356 (1 mg/kg) with L-DOPA (2.5 mg/kg) was lower than that induced by an optimal dose of L-DOPA (10 mg/kg). These results suggest that KW-6356 can be used to potentiate the effects of a wide range of L-DOPA doses with a low risk of dyskinesia for the treatment of PD.

Safety review of current pharmacotherapies for levodopa-treated patients with Parkinson's disease

INTRODUCTION

Levodopa remains the gold standard for treatment of Parkinson's disease (PD). Patients develop complications with disease progression, necessitating adjunctive therapy to control fluctuations in motor and non-motor symptoms and dyskinesia. Knowledge of medication safety and tolerability is critical to ascertain the benefit-risk ratio and select an adjunctive therapy that provides the highest chance for medication adherence. Posing a challenge are the sheer abundance of options, stemming from the development of several new drugs in recent years, as well as differences in commercial drug availability worldwide.

AREAS COVERED

This review evaluates the efficacy, safety, and tolerability of current US FDA-approved pharmacotherapies for levodopa-treated PD patients, including dopamine agonists, monoamine oxidase type-B inhibitors, catechol-O-methyltransferase inhibitors, the N-methyl-D-aspartate receptor antagonist amantadine, and the adenosine receptor antagonist istradefylline. Data were taken from pivotal phase III randomized controlled and post-surveillance studies, when available, that directly led to FDA-approval.

EXPERT OPINION

No strong evidence exists to support use of a specific adjunctive treatment for improving Off time. Only one medication has demonstrated improvement in dyskinesia in levodopa-treated PD patients; however, every patient cannot tolerate it and therefore adjunctive therapy should be tailored to an individual's symptoms and risk for specific adverse effects.

Levodopa-Induced Dyskinesias in Parkinson's Disease: An Overview on Pathophysiology, Clinical Manifestations, Therapy Management Strategies and Future Directions

Since its first introduction, levodopa has become the cornerstone for the treatment of Parkinson's disease and remains the leading therapeutic choice for motor control therapy so far. Unfortunately, the subsequent appearance of abnormal involuntary movements, known as dyskinesias, is a frequent drawback. Despite the deep knowledge of this complication, in terms of clinical phenomenology and the temporal relationship during a levodopa regimen, less is clear about the pathophysiological mechanisms underpinning it. As the disease progresses, specific oscillatory activities of both motor cortical and basal ganglia neurons and variation in levodopa metabolism, in terms of the dopamine receptor stimulation pattern and turnover rate, underlie dyskinesia onset. This review aims to provide a global overview on levodopa-induced dyskinesias, focusing on pathophysiology, clinical manifestations, therapy management strategies and future directions.

A2AR antagonist treatment for multiple sclerosis: Current progress and future prospects

Emerging evidence suggests that both selective and non-selective Adenosine A2A receptor (A2AR) antagonists could effectively protect mice from experimental autoimmune encephalomyelitis (EAE), which is the most commonly used animal model for multiple sclerosis (MS) research. Meanwhile, the recent FDA approval of Nourianz® (istradefylline) in 2019 as an add-on treatment to levodopa in Parkinson's disease (PD) with "OFF" episodes, along with its proven clinical safety, has prompted us to explore the potential of A2AR antagonists in treating multiple sclerosis (MS) through clinical trials. However, despite promising findings in experimental autoimmune encephalomyelitis (EAE), the complex and contradictory role of A2AR signaling in EAE pathology has raised concerns about the feasibility of using A2AR antagonists as a therapeutic approach for MS. This review addresses the potential effect of A2AR antagonists on EAE/MS in both the peripheral immune system (PIS) and the central nervous system (CNS). In brief, A2AR antagonists had a moderate effect on the proliferation and inflammatory response, while exhibiting a potent anti-inflammatory effect in the CNS through their impact on microglia, astrocytes, and the endothelial cells/epithelium of the blood-brain barrier. Consequently, A2AR signaling remains an essential immunomodulator in EAE/MS, suggesting that A2AR antagonists hold promise as a drug class for treating MS.

How and why the adenosine A2A receptor became a target for Parkinson's disease therapy

Dopaminergic therapy for Parkinson's disease has revolutionised the treatment of the motor symptoms of the illness. However, it does not alleviate all components of the motor deficits and has only limited effects on non-motor symptoms. For this reason, alternative non-dopaminergic approaches to treatment have been sought and the adenosine A2A receptor provided a novel target for symptomatic therapy both within the basal ganglia and elsewhere in the brain. Despite an impressive preclinical profile that would indicate a clear role for adenosine A2A antagonists in the treatment of Parkinson's disease, the road to clinical use has been long and full of difficulties. Some aspects of the drugs preclinical profile have not translated into clinical effectiveness and not all the clinical studies undertaken have had a positive outcome. The reasons for this will be explored and suggestions made for the further development of this drug class in the treatment of Parkinson's disease. However, one adenosine A2A antagonist, namely istradefylline has been introduced successfully for the treatment of late-stage Parkinson's disease in two major areas of the world and has become a commercial success through offering the first non-dopaminergic approach to the treatment of unmet need to be introduced in several decades.

The role of adenosine A2A receptors in Alzheimer's disease and tauopathies

Adenosine signals through four distinct G protein-coupled receptors that are located at various synapses, cell types and brain areas. Through them, adenosine regulates neuromodulation, neuronal signaling, learning and cognition as well as the sleep-wake cycle, all strongly impacted in neurogenerative disorders, among which Alzheimer's Disease (AD). AD is a complex form of cognitive deficits characterized by two pathological hallmarks: extracellular deposits of aggregated β-amyloid peptides and intraneuronal fibrillar aggregates of hyper- and abnormally phosphorylated Tau proteins. Both lesions contribute to the early dysfunction and loss of synapses which are strongly associated to the development of cognitive decline in AD patients. The present review focuses on the pathophysiological impact of the A_2ARs dysregulation observed in cognitive area from AD patients. We are reviewing not only evidence of the cellular changes in A_2AR levels in pathological conditions but also describe what is currently known about their consequences in term of synaptic plasticity, neuro-glial miscommunication and memory abilities. We finally summarize the proof-of-concept studies that support A_2AR as credible targets and the clinical interest to repurpose adenosine drugs for the treatment of AD and related disorders. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Serotonin 5-HT1A receptors and their interactions with adenosine A2A receptors in Parkinson's disease and dyskinesia

The dopamine neuronal loss that characterizes Parkinson's Disease (PD) is associated to changes in neurotransmitters, such as serotonin and adenosine, which contribute to the symptomatology of PD and to the onset of dyskinetic movements associated to levodopa treatment. The present review describes the role played by serotonin 5-HT_1A receptors and the adenosine A_2A receptors on dyskinetic movements induced by chronic levodopa in PD. The focus is on preclinical and clinical results showing the interaction between serotonin 5-HT_1A receptors and other receptors such as 5-HT_1B receptors and adenosine A_2A receptors. 5-HT_1A/1B receptor agonists and A_2A receptor antagonists, administered in combination, contrast dyskinetic movements induced by chronic levodopa without impairing motor behaviour, suggesting that this drug combination might be a useful therapeutic approach for counteracting the PD motor deficits and dyskinesia associated with chronic levodopa treatment. This article is part of the Special Issue on "The receptor-receptor interaction as a new target for therapy".

Apomorphine Sublingual Film Compared with Subcutaneous Apomorphine for OFF Episodes in Parkinson's Disease: An Open-Label, Randomized, Crossover Study

BACKGROUND

Apomorphine sublingual film (SL-APO) and subcutaneous apomorphine (SC-APO) have been used for the treatment of OFF episodes in Parkinson's disease (PD). No study has prospectively compared efficacy and safety of these formulations.

OBJECTIVE

To compare SL-APO with SC-APO for treatment of OFF episodes in PD.

METHODS

An open-label, randomized, crossover study assessed SL-APO versus SC-APO in patients with PD and OFF episodes (N = 113). Doses were optimized in randomly assigned order. SL-APO dose initiation (10 mg) occurred in clinic; further dose optimization (15-30 mg; 5-mg increments) occurred primarily at home. SC-APO dosing (2-6 mg; 1-mg increments) occurred entirely in clinic. After a 3-7-day washout, patients were randomized 1 : 1 to 4 weeks of treatment with their optimized dose of SL-APO or SC-APO, followed by washout and 4 weeks of crossover treatment.

RESULTS

Propensity score matching applied on 159 patients (STN-DBS n = 75, MED n = 84) resulted in 40 patients in each treatment group. At 36-month follow-up, STN-DBS led to significantly better PDSS and PDQ-8 change scores, which were significantly correlated. We observed no significant effects for HADS and no significant correlations between change scores in PDSS, HADS, and LEDD.

CONCLUSIONS

We report Class IIb evidence of beneficial effects of STN-DBS on quality of sleep at 36-month follow-up, which were associated with QoL improvement independent of depression and dopaminergic medication. Our study highlights the importance of sleep for assessments of DBS outcomes.

RESULTS

No difference was observed between SL-APO and SC-APO for change from predose to 90 minutes postdose in Movement Disorder Society Unified Parkinson's Disease Rating Scale Part III score at week 4 (primary endpoint), assessed by a blinded rater (-13.6 vs. -13.8, respectively; p = NS). Overall, 72.2% of patients preferred SL-APO compared with SC-APO/no preference (p = 0.0002) per the Treatment Preference Questionnaire (secondary endpoint). Patients reported greater satisfaction with SL-APO compared with SC-APO, per mean scores of convenience (73.7 vs. 53.5) and global satisfaction (63.9 vs. 57.6) on the Treatment Satisfaction Questionnaire for Medication (other endpoint). The safety profiles of both treatments were generally comparable and were well-tolerated.

CONCLUSIONS

Patients reported overall preference for and greater satisfaction with SL-APO over SC-APO.

Motor response with apomorphine sublingual film and levodopa in patients with OFF episodes

Aim: Evaluate timing of motor improvement with carbidopa/levodopa (CD/LD) and apomorphine sublingual film (SL-APO) in patients with Parkinson's disease and OFF episodes. Methods: A post hoc pooled analysis from two studies assessed Movement Disorder Society Unified Parkinson's Disease Rating Scale Part III (MDS-UPDRS-III) scores and investigator-rated FULL ON. Results: At 15 and 30 min following the prescribed first daily CD/LD dose, mean improvements in MDS-UPDRS-III scores were -6.7 and -16.3, respectively, and FULL ON was achieved by 6.5 and 41.8% of patients. Following an optimized SL-APO dose, mean improvements in MDS-UPDRS-III scores were -13.9 and -22.9, and FULL ON was achieved by 34.7 and 81.0% of patients. Conclusion: Concomitant administration of SL-APO with carbidopa/levodopa may be useful for delayed ON.

Levodopa-Induced Dyskinesia in Parkinson's Disease: Pathogenesis and Emerging Treatment Strategies

The most commonly used treatment for Parkinson's disease (PD) is levodopa, prescribed in conjunction with carbidopa. Virtually all patients with PD undergo dopamine replacement therapy using levodopa during the course of the disease's progression. However, despite the fact that levodopa is the "gold standard" in PD treatments and has the ability to significantly alleviate PD symptoms, it comes with side effects in advanced PD. Levodopa replacement therapy remains the current clinical treatment of choice for Parkinson's patients, but approximately 80% of the treated PD patients develop levodopa-induced dyskinesia (LID) in the advanced stages of the disease. A better understanding of the pathological mechanisms of LID and possible means of improvement would significantly improve the outcome of PD patients, reduce the complexity of medication use, and lower adverse effects, thus, improving the quality of life of patients and prolonging their life cycle. This review assesses the recent advancements in understanding the underlying mechanisms of LID and the therapeutic management options available after the emergence of LID in patients. We summarized the pathogenesis and the new treatments for LID-related PD and concluded that targeting pathways other than the dopaminergic pathway to treat LID has become a new possibility, and, currently, amantadine, drugs targeting 5-hydroxytryptamine receptors, and surgery for PD can target the Parkinson's symptoms caused by LID.

Istradefylline for OFF Episodes in Parkinson’s Disease: A US Perspective of Common Clinical Scenarios

The effective management of OFF episodes remains an important unmet need for patients with Parkinson’s disease (PD) who develop motor complications with long-term levodopa therapy. Istradefylline is a selective adenosine A_2A receptor antagonist for the treatment of patients with PD experiencing OFF episodes while on levodopa/decarboxylase inhibitor. Originally approved in Japan, istradefylline was recently approved in the USA. In this article, we provide a specific review of the four clinical studies that the FDA included in the approval of istradefylline in the USA, and discuss common clinical scenarios, based on our experience, where treatment with istradefylline may benefit patients experiencing motor fluctuations.

The Pharmacological Potential of Adenosine A2A Receptor Antagonists for Treating Parkinson's Disease

The adenosine A_2A receptor subtype is recognized as a non-dopaminergic pharmacological target for the treatment of neurodegenerative disorders, notably Parkinson’s disease (PD). The selective A_2A receptor antagonist istradefylline is approved in the US and Japan as an adjunctive treatment to levodopa/decarboxylase inhibitors in adults with PD experiencing OFF episodes or a wearing-off phenomenon; however, the full potential of this drug class remains to be explored. In this article, we review the pharmacology of adenosine A_2A receptor antagonists from the perspective of the treatment of both motor and non-motor symptoms of PD and their potential for disease modification.

Evaluating the impact of adjunctive istradefylline on the cumulative dose of levodopa-containing medications in Parkinson's disease: study protocol for the ISTRA ADJUST PD randomized, controlled study

Background

Levodopa remains the most effective symptomatic treatment for Parkinson’s disease (PD) more than 50 years after its clinical introduction. However, the onset of motor complications can limit pharmacological intervention with levodopa, which can be a challenge when treating PD patients. Clinical data suggest using the lowest possible levodopa dose to balance the risk/benefit. Istradefylline, an adenosine A_2A receptor antagonist indicated as an adjunctive treatment to levodopa-containing preparations in PD patients experiencing wearing off, is currently available in Japan and the US. Preclinical and preliminary clinical data suggested that adjunctive istradefylline may provide sustained antiparkinsonian benefits without a levodopa dose increase; however, available data on the impact of istradefylline on levodopa dose titration are limited. The ISTRA ADJUST PD study will evaluate the effect of adjunctive istradefylline on levodopa dosage titration in PD patients.

Methods

This 37-week, multicenter, randomized, open-label, parallel-group controlled study in PD patients aged 30–84 years who are experiencing the wearing-off phenomenon despite receiving levodopa-containing medications ≥ 3 times daily (daily dose 300–400 mg) began in February 2019 and will continue until February 2022. Enrollment is planned to attain 100 evaluable patients for the efficacy analyses. Patients will receive adjunctive istradefylline (20 mg/day, increasing to 40 mg/day) or the control in a 1:1 ratio, stratified by age, levodopa equivalent dose, and presence/absence of dyskinesia. During the study, the levodopa dose will be increased according to symptom severity. The primary study endpoint is the comparison of the cumulative additional dose of levodopa-containing medications during the treatment period between the adjunctive istradefylline and control groups. Secondary endpoints include changes in efficacy rating scales and safety outcomes.

Discussion

This study aims to clarify whether adjunctive istradefylline can reduce the cumulative additional dose of levodopa-containing medications in PD patients experiencing the wearing-off phenomenon, and lower the risk of levodopa-associated complications. It is anticipated that data from ISTRA ADJUST PD will help inform future clinical decision-making for patients with PD in the real-world setting.

Trial registration

Japan Registry of Clinical Trials, jRCTs031180248; registered 12 March 2019.

[Adenosine A2A Receptor Antagonists as a Treatment Option for Parkinson's Disease?]

In Parkinson's disease, the focus has long been on motor symptoms and therapy with dopaminergic substances. In recent years, the importance of non-motor symptoms has been increasingly recognized, as they occur early in the course of the disease and restrict considerably the quality of life. However, this also made the need for treatment of non-dopaminergic deficits obvious. Adenosine A_2A receptor antagonists were identified as an additional therapy, since the adenosine A_2A receptors are non-dopaminergic and selectively localized in the basal ganglia. This means that the striato-thalamo-cortical loops can be modulated. An adenosine A_2A receptor antagonist was already approved in Japan in 2013 and in the USA in 2019 as an add-on to L-DOPA. Approval for this drug in Europe is expected in the near future. In this overview, we present the theoretical basis and current data on its efficacy and therapeutic use.

nnEfficacy and safety of istradefylline for Parkinson's disease: A systematic review and meta-analysis

As an adenosine receptor A2A antagonist, istradefylline is used as an adjunctive agent of levodopa to improve motor symptoms in advanced Parkinson's disease (PD) patients. In this study, we re-evaluated the effects of istradefylline on treating the motor symptoms of PD patients. We performed a literature search up to November 2021 from electronic databases. Eligible studies were synthesized for efficacy, tolerability, OFF time, Unified Parkinson's Disease Rating Scale part III score, ON state with dyskinesia, and the incidence of treatment-emergent adverse events. As a result, nine clinical studies with 2727 subjects on istradefylline treatment for PD patients were included. Our results showed that compared to placebo, istradefylline exhibited a statically significant difference in efficacy (1.39 [1.15 to 1.69]; p = 0.001), decreasing OFF time (-0.58 [-1.01 to -0.16]; p = 0.007), and improving ON state with dyskinesia (0.69 [0.02 to 1.37]; p = 0.043). For tolerability, UPDRS III, and adverse effects, there was no significant difference between istradefylline and placebo. In conclusion, the results suggest that istradefylline exhibits an efficient and well-tolerated role in treating PD patients. Randomized controlled trials and long-term studies are still required to investigate the effects of istradefylline on motor and non-motor symptoms of PD in future research.

Istradefylline: A novel agent in the treatment of “off” episodes associated with levodopa/carbidopa use in Parkinson disease

The current gold standard for treatment of Parkinson disease (PD) is levodopa/carbidopa (L/C), but long-term treatment frequently results in motor complications, such as wearing-off and motor fluctuations (eg, dyskinesia, “on-off” phenomenon). Istradefylline is a new drug with a unique pharmacologic profile that was approved by the FDA for use as adjunctive treatment to L/C in adult patients with PD experiencing “off” episodes. The drug was shown to reduce “off” time in 4 randomized, double-blind, placebo-controlled studies. The most common adverse effects are dyskinesia, dizziness, constipation, nausea, hallucinations, and insomnia. Unlike many drugs that treat PD, istradefylline is a nondopaminergic drug that exerts its effects via adenosine A2A receptor antagonism. The major drug interactions involve inhibitors or inducers of CYP3A4 as well as tobacco smoking via induction of CYP1A1. Istradefylline is taken once daily as a 20- or 40-mg dose, except in cases involving drug interactions or hepatic impairment. The cost of the drug is relatively expensive, which has implications for Medicare and private insurance coverage. Istradefylline is an alternative option to dopaminergic drugs such as dopamine agonists, monoamine oxidase B inhibitors, and catechol-O-methyltransferase inhibitors as an adjunct to L/C in patients with motor fluctuations, but clinical use will further define its role in treatment of PD.

Should “on-demand” treatments for Parkinson’s disease OFF episodes be used earlier?

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OFF episodes are common in patients as Parkinson’s disease progresses.

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OFF episodes are typically managed with “ON-extenders” and changes in levodopa dosing.

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OFF episodes persist despite conventional treatment.

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“On-demand” therapies can rapidly improve OFF symptoms.

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A shift to earlier complementary use of “on-demand” therapies should be considered.

We discuss a shift in the treatment paradigm for OFF episode management in patients with Parkinson’s disease, based on clinical experience in the United States (US). Three “on-demand” treatments are currently available in the US as follows: subcutaneous apomorphine, levodopa inhalation powder, and sublingual apomorphine. We empirically propose that “on-demand” treatments can be utilized as a complementary treatment when OFF episodes emerge and can be utilized when needed rather than reserving these treatments only until other treatment approaches (adjustment of baseline treatment and/or addition of adjunctive treatment with “ON-extenders”) have failed. Current treatment approaches combine “ON-extenders” with increasing levodopa dosing and/or frequency to treat OFF episodes. Yet, OFF episodes often persist, with a substantial amount of daily OFF time. OFF episode treatment is hindered by variable gastrointestinal (GI) absorption of oral levodopa, reflecting GI dysmotility and protein competition. Novel “on-demand” treatments bypass the gut and can improve OFF symptoms more rapidly and reliably than oral levodopa. With the emergence of novel “on-demand” treatments, we conclude that a shift in treatment paradigm to the earlier, complementary use of these medications be considered.

Istradefylline Versus Opicapone for "Off" Episodes in Parkinson's Disease: A Systematic Review and Meta-Analysis

Background:

In recent times, the US-FDA approved istradefylline and opicapone as an adjunct to levodopa/carbidopa for managing the "off" episodes in Parkinson’s disease.

Purpose:

Current meta-analysis was performed to determine the safety and efficacy of these drugs in the management of “off” episodes and to recognize which among them would provide therapeutic benefits clinically.

Methods:

A thorough literature search was performed through the Cochrane Library, PubMed, and clinicaltrials.gov for a period from January 2003 to October 2020, with the following keywords: Istradefylline, KW-6002, opicapone, BIA 9-1067, and Parkinson’s disease. Those randomized, double-blind placebo/active comparator-controlled trials that analyzed the efficacy and safety of istradefylline and opicapone and that were published in the English language were included. In this analysis, the outcomes focused on the least square mean change in “off” time and Unified Parkinson’s Disability Rating Scale (UPDRS) III score from baseline to the end of the study, and the incidence of treatment-emergent adverse events (TEAEs) and dyskinesia.

Results:

Both drugs have shown significant reduction in “off” time duration (mean difference [MD] = –0.70; 95% CI [–1.11, –0.30]; P < 0.001 for istradefylline and MD = –0.85; 95% CI [–1.09, –0.61]; P < .001 for opicapone). Istradefylline showed significant improvement in UPDRS III (MD = –1.56; 95% CI [–2.71, –0.40]; P < .008), but the same was not observed with opicapone (MD = –0.63; 95% CI [–1.42, –0.15]; P < .12). The incidence of TEAEs and dyskinesia reportedly were higher in the intervention group rather than with the placebo, (risk ratio RR =1.11, 95% CI [1.02,1.20] for istradefylline and RR =1.12, 95% CI [1.00,1.25] for opicapone, and for dyskinesia particularly, the incidence was higher with opicapone as compared to istradefylline (RR = 3.47, 95% CI [2.17, 5.57], and RR = 1.77, 95% CI [1.29, 2.44], respectively).

Conclusions:

Both drugs were comparable in efficacy; however, istradefylline seemed to be better in reducing the UPDRS III score. Although the incidence of TEAEs and dyskinesia were higher with both the drugs, the incidence of dyskinesia was more in the opicapone group.

Efficacy of Istradefylline, an Adenosine A2A Receptor Antagonist, as Adjunctive Therapy to Levodopa in Parkinson's Disease: A Pooled Analysis of 8 Phase 2b/3 Trials

BACKGROUND

Istradefylline is a selective adenosine A2A receptor antagonist for the treatment of patients with Parkinson's disease (PD) experiencing OFF episodes while on levodopa/decarboxylase inhibitor.

OBJECTIVE

This pooled analysis of eight randomized, placebo-controlled, double-blind phase 2b/3 studies evaluated the efficacy and safety of istradefylline.

METHODS

Istradefylline was evaluated in PD patients receiving levodopa with carbidopa/benserazide and experiencing motor fluctuations. Eight 12- or 16-week trials were conducted (n = 3,245); four of these studies were the basis for istradefylline's FDA approval. Change in OFF time as assessed in patient-completed 24-h PD diaries at Week 12 was the primary endpoint. All studies were designed with common methodology, thereby permitting pooling of data. Pooled analysis results from once-daily oral istradefylline (20 and 40 mg/day) and placebo were evaluated using a mixed-model repeated-measures approach including study as a factor.

RESULTS

Among 2,719 patients (placebo, n = 992; 20 mg/day, n = 848; 40 mg/day, n = 879), OFF hours/day were reduced at Week 12 at istradefylline dosages of 20 mg/day (least-squares mean difference [LSMD] from placebo in reduction from baseline [95%CI], -0.38 h [-0.61, -0.15]) and 40 mg/day (-0.45 h [-0.68, -0.22], p < 0.0001); ON time without troublesome dyskinesia (ON-WoTD) significantly increased. Similar results were found in the four-study pool (OFF hours/day, 20 mg/day, -0.75 h [-1.10, -0.40]; 40 mg/day, -0.82 h [-1.17, -0.47]). Istradefylline was generally well-tolerated; the average study completion rate among istradefylline-treated patients across all studies was 89.2%. Dyskinesia was the most frequent adverse event (placebo, 9.6%; 20 mg/day, 16.1%; 40 mg/day, 17.7%).

CONCLUSION

In this pooled analysis, istradefylline significantly improved OFF time and ON-WoTD relative to placebo and was well-tolerated.

Neuroinflammation and L-dopa-induced abnormal involuntary movements in 6-hydroxydopamine-lesioned rat model of Parkinson's disease are counteracted by combined administration of a 5-HT1A/1B receptor agonist and A2A receptor antagonist

Several lines of evidence have strongly implicated neuroinflammation in Parkinson's disease (PD) progression and l-dopa-induced dyskinesia. The present study investigated whether early subchronic pretreatment with the serotonin 5-HT_1A/1B receptor agonist eltoprazine plus the adenosine A_2A receptor antagonist preladenant counteracted l-dopa-induced abnormal involuntary movements (AIMs, index of dyskinesia), and neuroinflammation, in unilateral 6-hydroxydopamine(6-OHDA)-lesioned rat model of PD. The immunoreactivity of glial fibrillary acidic protein (GFAP), and the colocalization of ionized calcium binding adaptor molecule-1 (IBA-1), with interleukin (IL)-1β, tumor-necrosis-factor-α (TNF-α) and IL-10 were evaluated in the denervated caudate-putamen (CPu) and substantia nigra pars-compacta (SNc). The combined subchronic pretreatment with l-dopa plus eltoprazine and preladenant reduced AIMs induced by acute l-dopa challenge in these rats and decreased GFAP and IBA-1 immunoreactivity induced by the drug in both CPu and SNc, with reduction in IL-1β in IBA-1-positive cells in both CPu and SNc, and in TNF-α in IBA-1-positive cells in SNc. Moreover, a significant increase in IL-10 in IBA-1-positive cells was observed in SNc. Evaluation of immediate early-gene zif-268 (index of neuronal activation) after l-dopa challenge, showed an increase in its expression in denervated CPu of rats pretreated with l-dopa or l-dopa plus preladenant compared with vehicle, whereas rats pretreated with eltoprazine, with or without preladenant, had lower zif-268 expression. Finally, tyrosine hydroxylase and dopamine transporter examined to evaluate neurodegeneration, showed a significant equal decrease in all experimental groups. The present findings suggest that combination of l-dopa with eltoprazine and preladenant may be promising therapeutic strategy for delaying the onset of dyskinesia, preserving l-dopa efficacy and reducing neuroinflammation markers in nigrostriatal system of 6-OHDA-lesioned rats.

Exploration of chalcones and related heterocycle compounds as ligands of adenosine receptors: therapeutics development

Adenosine receptors (ARs) are ubiquitously distributed throughout the mammalian body where they are involved in an extensive list of physiological and pathological processes that scientists have only begun to decipher. Resultantly, AR agonists and antagonists have been the focus of multiple drug design and development programmes within the past few decades. Considered to be a privileged scaffold in medicinal chemistry, the chalcone framework has attracted a substantial amount of interest in this regard. Due to the potential liabilities associated with its structure, however, it has become necessary to explore other potentially promising compounds, such as heterocycles, which have successfully been obtained from chalcone precursors in the past. This review aims to summarise the emerging therapeutic importance of adenosine receptors and their ligands, especially in the central nervous system (CNS), while highlighting chalcone and heterocyclic derivatives as promising AR ligand lead compounds.

Effect of dopaminergic medication on adenosine 2A receptor availability in patients with Parkinson's disease

OBJECTIVE

To assess the necessity of withdrawing dopaminergic medication in Parkinson's disease (PD) patients for accurate estimation of adenosine 2A receptor (A_2AR) availability using [¹¹C]TMSX PET imaging. This was accomplished by studying the short-term effect of the cessation of dopaminergic medication on A_2AR availability in non-dyskinetic patients with PD treated with dopaminergic medication.

METHODS

Eight PD patients (age 67.9 ± 5.6 years; 6 men, 2 women) without dyskinesia were enrolled in this study. A_2AR availability was measured using PET imaging with a [7-methyl-¹¹C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthine ([¹¹C]TMSX) radioligand after a short term cessation of dopaminergic medication (12hrs for levodopa, 24hrs for dopamine agonists and MAO-B inhibitors). Repeated PET imaging was performed while the patients were back 'on' their regular dopaminergic medication (median 13 days after first imaging). Conventional MRI was acquired for anatomical reference. Specific binding of [¹¹C]TMSX was quantified as distribution volume ratios (DVR) for caudate, pallidum and putamen using Logan graphical method with clustered gray matter reference region.

RESULTS

No significant differences were observed for the DVRs in all three striatal regions between 'on' and 'off' medication states. Strong correlations were also observed between the two states. Statistical equivalence was found in pallidum (TOST equivalence test, p = 0.045) and putamen (TOST equivalence test, p = 0.022), but not in caudate DVR (TOST equivalence test, p = 0.201) between the two medication states.

CONCLUSIONS

Our results show that dopaminergic medication has no significant short-term effect on the availability of A_2A receptors in putamen and pallidum of patients with PD. However, relatively poor repeatability was demonstrated in the caudate.

[Separation and identification of impurities from intermediates of istradefylline]

Istradefylline is a novel selective adenosine A2A receptor antagonist that is used to treat Parkinson's disease and improve motor dysfunction in the early stage of this disease. During the synthesis of intermediate A1 (6-amino-1,3-diethyl-2,4-(1H,3H)-pyrimidinedione), at least two by-products were formed under alkaline or high-temperature conditions. In a previous study, one of the by-products in the synthesis of the intermediate was studied, and its structure was identified as (E)-N-ethyl-2-cyano-3-ethylamino-2-butene amide. In this study, we used high performance liquid chromatography (HPLC) to analyze another impurity formed during the synthesis of A1, and the following steps were executed: 0.4 g of intermediate was weighed and added to a 50 mL beaker, followed by the sequential addition of 8 mL water and 8 mL acetonitrile, and then, ultrasonic dissolution was performed. Finally, the solution was filtered through a 0.45-μm organic membrane and the test sample solution was obtained. We used the Agilent zorbax C18 chromatography column, with acetonitrile (A)/water(B) as the mobile phase under gradient elution ((tmin/A∶B)=t0/20∶80, t15/60∶40, t20-t50/90∶10). The detector wavelength is 268 nm. In order to separate the impurity from A1, we used a Ceres B preparative column, with acetonitrile-water (30/70, v/v) as the mobile phase. The flow rate was set at 30 mL/min, and the detection wavelength was 268 nm. The structure of the impurity was confirmed by high-resolution mass spectrometry (HRMS), one-dimensional nuclear magnetic resonance (NMR), and two-dimensional nuclear magnetic resonance (2D NMR), and characterized by single-crystal X-ray diffraction (XRD). In MS experiments, an electrospray ionization (ESI) source was used with positive ion scanning. In the NMR experiments, we used tetramethylsilane (TMS) as the internal standard and deuterated dimethyl sulfoxide (DMSO-d6) as the solvent to obtain the spectra. The results of preparative high performance liquid chromatography (Prep-HPLC) showed that good separation effect could be achieved by isocratic elution, and the impurity was perfectly separated. The1H-NMR spectral data are as follows:1H-NMR (600 MHz, DMSO): δ 1.01 (q, J=6.9 Hz, 3H), 1.02 (q, J=6.9 Hz, 3H), 1.07 (t, J=6.9 Hz, 3H), 3.04 (p, J=6.8 Hz, 2H), 3.74 (q, J=7.0 Hz, 2H), 3.94 (q, J=7.1 Hz, 2H), 5.85 (s, 1H). The 13C-NMR spectral data are summarized as follows: 13C-NMR (150 MHz, DMSO): δ13.9, 14.1, 15.9, 34.6, 34.9, 36.9, 81.9, 152.2, 153.3, 159.3, 162.0. The impurity was characterized by single-crystal XRD, and its spatial structure was further verified and determined as 1-(1,3-diethyl-2,6-dioxo-1,2,3,6-tetrahydropyrimidin-4-yl)-3-ethylurea. Based on the chemical structure of the impurity, we propose the following mechanism for the impurity: when A1 is synthesized under alkaline conditions or at high temperature, excessive diethylurea continues to undergo amidation with A1 to obtain this by-product. Although the formation mechanism of the impurity studied in this paper is different from that of the intermediate A1 impurity (E)-N-ethyl-2-cyano-3-ethylamino-2-butene amide, both the impurities are formed at high temperatures. Both will be accompanied by A1 in the subsequent reaction of istradefylline synthesis. The relationship between drug impurities and drug safety is a complex relationship that is affected by many factors. Generally, most impurities in drugs have potential biological activities, and some even interact with the drugs, thus affecting their efficacy and safety and inducing toxic effects. Therefore, to ensure the quality of istradefylline, it is necessary to control the impurity content during the production. The findings of this paper may provide guidelines for controlling the impurity content in istradefylline.

Istradefylline - a first generation adenosine A2A antagonist for the treatment of Parkinson's disease

Introduction It is now accepted that Parkinson's disease (PD) is not simply due to dopaminergic dysfunction, and there is interest in developing non-dopaminergic approaches to disease management. Adenosine A_2A receptor antagonists represent a new way forward in the symptomatic treatment of PD.Areas covered In this narrative review, we summarize the literature supporting the utility of adenosine A_2A antagonists in PD with a specific focus on istradefylline, the most studied and only adenosine A_2A antagonist currently in clinical use.Expert opinion: At this time, the use of istradefylline in the treatment of PD is limited to the management of motor fluctuations as supported by the results of randomized clinical trials and evaluation by Japanese and USA regulatory authorities. The relatively complicated clinical development of istradefylline was based on classically designed studies conducted in PD patients with motor fluctuations on an optimized regimen of levodopa plus adjunctive dopaminergic medications. In animal models, there is consensus that a more robust effect of istradefylline in improving motor function is produced when combined with low or threshold doses of levodopa rather than with high doses that produce maximal dopaminergic improvement. Exploration of istradefylline as a 'levodopa sparing' strategy in earlier PD would seem warranted.

Allosteric Interactions between Adenosine A2A and Dopamine D2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A_2A and dopamine D₂ receptors (R). Stimulation of A_2AR inhibits and blockade of A_2AR enhances D₂R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D₂R via its interaction with A_2AR. Reciprocal A_2AR/D₂R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A_2AR and D₂R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor–receptor interactions within A_2AR/D₂R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson’s disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A_2AR/D₂R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

Update on Parkinson's Disease Therapy

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Development, Efficacy and Safety of Once-daily, Bedtime, Extended-release Amantadine (Gocovri®) to Treat Dyskinesia and OFF Time in Parkinson’s Disease

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Patient Preferences for Treating "OFF" Episodes in Parkinson's Disease: A Discrete Choice Experiment

INTRODUCTION

Several on-demand treatments are available for management of "OFF" episodes in patients with Parkinson's disease (PD). We evaluated patients' preferences for features of theoretical on-demand treatment options.

METHODS

In a discrete choice experiment, US adults with self-reported PD of ≥5 years, or <5 years with "OFF" episodes, taking oral carbidopa/levodopa, selected between pairs of theoretical on-demand treatments that varied by mode of administration (with and without mode-specific adverse events [AEs]), time to FULL "ON," duration of "ON," and out-of-pocket cost for a 30-day supply. Data were analyzed with a random parameters logit model; results were used to calculate relative importance of treatment attributes, preference shares, and willingness to pay.

RESULTS

Among 300 respondents, 98% had "OFF" episodes. Across the range of attribute levels included in the survey, avoiding $90 cost was most important to respondents, followed by a preferable mode of administration with associated AEs and decreasing time to FULL "ON." Duration of "ON" was relatively less important. On average, respondents preferred a theoretical dissolvable sublingual film versus other theoretical treatments with alternative modes of administration. Respondents were willing to pay $28-$52 US dollars to switch from least- to more-preferred mode of administration with associated AEs, $58 to reach FULL "ON" in 15 versus 60 min, and $9 to increase duration of FULL "ON" from 1 to 2 h.

CONCLUSION

Respondents with PD valued lower out-of-pocket cost and a sublingual mode of administration with its associated AEs when choosing an on-demand treatment for "OFF" episodes.

Pharmacological interactions between adenosine A2A receptor antagonists and different neurotransmitter systems

While Parkinson's disease (PD) is traditionally characterized by dopaminergic neuron degeneration, several neurotransmitters and neuromodulators besides dopamine are also involved in the onset and progression of the disease and its symptoms. The other principal neurotransmitters/neuromodulators known to control basal ganglia functions and, in particular, motor functions, are GABA, glutamate, serotonin (5-HT), noradrenaline, acetylcholine, adenosine and endocannabinoids. Among these, adenosine is the most relevant, acting through its adenosine A_2A receptor. Work in experimental models of PD has established the effects of A_2A receptor antagonists, including the alleviation of disrupted dopamine functions and improved efficacy of dopamine replacement therapy. Moreover, positive interactions between A_2A receptor antagonists and both D₂ and D₁ receptor agonists have been described in vitro at the receptor-receptor level or in more complex in vivo models of PD, respectively. In addition, the interactions between A_2A receptor antagonists and glutamate ionotropic GluN_2B-containing N-Methyl-d-aspartic acid receptors, or metabotropic glutamate (mGlu) receptors, including both mGlu₅ receptor inhibitors and mGlu₄ receptor activators, have been reported in both in vitro and in vivo animal models of PD, as have positive interactions between A_2A and endocannabinoid CB₁ receptor antagonists. At the same time, a combination of A_2A receptor antagonists and 5-HT_1A-5-HT_1B receptor agonists have been described to modulate the expression of dyskinesia induced by chronic dopamine replacement therapy.

Do caffeine and more selective adenosine A2A receptor antagonists protect against dopaminergic neurodegeneration in Parkinson's disease?

The adenosine A_2A receptor is a major target of caffeine, the most widely used psychoactive substance worldwide. Large epidemiological studies have long shown caffeine consumption is a strong inverse predictor of Parkinson’s disease (PD). In this review, we first examine the epidemiology of caffeine use vis-à-vis PD and follow this by looking at the evidence for adenosine A_2A receptor antagonists as potential neuroprotective agents. There is a wealth of accumulating biological, epidemiological and clinical evidence to support the further investigation of selective adenosine A_2A antagonists, as well as caffeine, as promising candidate therapeutics to fill the unmet need for disease modification of PD.

The challenge of developing adenosine A2A antagonists for Parkinson disease: Istradefylline, preladenant, and tozadenant

Laboratory and clinical experience have pointed to the value of targeting motor pathways emerging from the striatum to treat problems arising in advanced Parkinson's disease (PD). These pathways are selectively populated with a subtype of adenosine binding sites (A_2A receptors) that offer a target for improving PD symptomatology. Several compounds were developed that possess high selectivity and potency for blocking this receptor. Three of these compounds - istradefylline, preladenant, and tozadenant - were chosen for clinical development programs that culminated in Phase 3 multicenter randomized clinical trials. Each of these drugs exert virtually no off-target neurochemical effects. Clinical trials with these drugs focused upon reducing OFF time when administered adjunctly to levodopa and other antiparkinsonian medications. Despite promising Phase 2 data, preladenant did not show efficacy when tested in a randomized placebo-controlled Phase 3 clinical trial. Reports of hematological toxicity necessitated ceasing an ongoing Phase 3 investigation of tozadenant. Following a challenging approval process, based on the results of randomized clinical trials carried out in the U.S. and Japan, istradefylline received approval in these countries for treatment of OFF episodes.

Istradefylline to Treat Patients with Parkinson's Disease Experiencing "Off" Episodes: A Comprehensive Review

Parkinson’s disease (PD) is a common neurodegenerative disorder that leads to significant morbidity and disability. PD is caused by a loss of dopaminergic, cholinergic, serotonergic, and noradrenergic neurons in the central nervous system (CNS), and peripherally; the syndromic parkinsonism symptoms of movement disorder, gait disorder, rigidity and tremor are mostly driven by the loss of these neurons in the basal ganglia. Unfortunately, a significant proportion of patients taking levodopa, the standard of care treatment for PD, will begin to experience a decrease in effectiveness at varying times. These periods, referred to as “off episodes”, are characterized by increased symptoms and have a detrimental effect on quality of life and disability. Istradefylline, a novel adenosine A2A receptor antagonist, is indicated as a treatment addition to levodopa/carbidopa in patients experiencing “off episodes”. It promotes dopaminergic activity by antagonizing adenosine in the basal ganglia. This review will discuss istradefylline as a treatment for PD patients with off episodes.

Pivotal Role of Fyn Kinase in Parkinson's Disease and Levodopa-Induced Dyskinesia: a Novel Therapeutic Target?

The exact etiology of Parkinson's disease (PD) remains obscure, although many cellular mechanisms including α-synuclein aggregation, oxidative damage, excessive neuroinflammation, and dopaminergic neuronal apoptosis are implicated in its pathogenesis. There is still no disease-modifying treatment for PD and the gold standard therapy, chronic use of levodopa is usually accompanied by severe side effects, mainly levodopa-induced dyskinesia (LID). Hence, the elucidation of the precise underlying molecular mechanisms is of paramount importance. Fyn is a tyrosine phospho-transferase of the Src family nonreceptor kinases that is highly implicated in immune regulation, cell proliferation and normal brain development. Accumulating preclinical evidence highlights the emerging role of Fyn in key aspects of PD and LID pathogenesis: it may regulate α-synuclein phosphorylation, oxidative stress-induced dopaminergic neuronal death, enhanced neuroinflammation and glutamate excitotoxicity by mediating key signaling pathways, such as BDNF/TrkB, PKCδ, MAPK, AMPK, NF-κB, Nrf2, and NMDAR axes. These findings suggest that therapeutic targeting of Fyn or Fyn-related pathways may represent a novel approach in PD treatment. Saracatinib, a nonselective Fyn inhibitor, has already been tested in clinical trials for Alzheimer's disease, and novel selective Fyn inhibitors are under investigation. In this comprehensive review, we discuss recent evidence on the role of Fyn in the pathogenesis of PD and LID and provide insights on additional Fyn-related molecular mechanisms to be explored in PD and LID pathology that could aid in the development of future Fyn-targeted therapeutic approaches.

A case study of foliglurax, the first clinical mGluR4 PAM for symptomatic treatment of Parkinson's disease: translational gaps or a failing industry innovation model?

INTRODUCTION

Approximately 40% of Parkinson's disease (PD) patients that take mostly dopamine receptor agonists for motor fluctuations, experience the return of symptoms between regular doses. This is a phenomenon known as 'OFF periods.' Positive allosteric modulators (PAMs) of metabotropic glutamate receptor 4 (mGluR4) are a promising non-dopaminergic mechanism with potential to address the unmet need of patients suffering from OFF periods. Foliglurax is the first mGluR4 PAM that has advanced into clinical testing in PD patients.

AREAS COVERED

We summarize the chemistry, pharmacokinetics, and preclinical pharmacology of foliglurax. Translational PET imaging studies, clinical efficacy data, and a competitive landscape analysis of available therapies are presented to the readers. In this Perspective article, foliglurax is used as a case study to illustrate the inherent R&D challenges that companies face when developing drugs. These challenges include the delivery of drugs acting through novel mechanisms, long-term scientific investment, and commercial success and shorter-term positive financial returns.

EXPERT OPINION

Failure to meet the primary and secondary endpoints in a Phase 2 study led Lundbeck to discontinue the development of foliglurax. Understanding the evidence supporting compound progression into Phase 2 will enable the proper assessment of the therapeutic potential of mGluR4 PAMs.

Diet, Microbiota and Brain Health: Unraveling the Network Intersecting Metabolism and Neurodegeneration

Increasing evidence gives support for the idea that extra-neuronal factors may affect brain physiology and its predisposition to neurodegenerative diseases. Epidemiological and experimental studies show that nutrition and metabolic disorders such as obesity and type 2 diabetes increase the risk of Alzheimer's and Parkinson's diseases after midlife, while the relationship with amyotrophic lateral sclerosis is uncertain, but suggests a protective effect of features of metabolic syndrome. The microbiota has recently emerged as a novel factor engaging strong interactions with neurons and glia, deeply affecting their function and behavior in these diseases. In particular, recent evidence suggested that gut microbes are involved in the seeding of prion-like proteins and their spreading to the central nervous system. Here, we present a comprehensive review of the impact of metabolism, diet and microbiota in neurodegeneration, by affecting simultaneously several aspects of health regarding energy metabolism, immune system and neuronal function. Advancing technologies may allow researchers in the future to improve investigations in these fields, allowing the buildup of population-based preventive interventions and development of targeted therapeutics to halt progressive neurologic disability.

Medical Management and Prevention of Motor Complications in Parkinson's Disease

Levodopa is the most effective medication for the treatment of the motor symptoms of Parkinson's disease. However, over time, the clinical response to levodopa becomes complicated by a reduction in the duration and reliability of motor improvement (motor fluctuations) and the emergence of involuntary movements (levodopa-induced dyskinesia). Strategies that have been attempted in an effort to delay the development of these motor complications include levodopa sparing and continuous dopaminergic therapy. Once motor complications occur, a wide array of medical treatments is available to maximize motor function through the day while limiting dyskinesia. Here, we review the clinical features, epidemiology, and risk factors for the development of motor complications, as well as strategies for their prevention and medical management.

Soluble Epoxide Hydrolase Inhibition to Face Neuroinflammation in Parkinson's Disease: A New Therapeutic Strategy

Neuroinflammation is a crucial process associated with the pathogenesis of neurodegenerative diseases, including Parkinson's disease (PD). Several pieces of evidence suggest an active role of lipid mediators, especially epoxy-fatty acids (EpFAs), in the genesis and control of neuroinflammation; 14,15-epoxyeicosatrienoic acid (14,15-EET) is one of the most commonly studied EpFAs, with anti-inflammatory properties. Soluble epoxide hydrolase (sEH) is implicated in the hydrolysis of 14,15-EET to its corresponding diol, which lacks anti-inflammatory properties. Preventing EET degradation thus increases its concentration in the brain through sEH inhibition, which represents a novel pharmacological approach to foster the reduction of neuroinflammation and by end neurodegeneration. Recently, it has been shown that sEH levels increase in brains of PD patients. Moreover, the pharmacological inhibition of the hydrolase domain of the enzyme or the use of sEH knockout mice reduced the deleterious effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration. This paper overviews the knowledge of sEH and EETs in PD and the importance of blocking its hydrolytic activity, degrading EETs in PD physiopathology. We focus on imperative neuroinflammation participation in the neurodegenerative process in PD and the putative therapeutic role for sEH inhibitors. In this review, we also describe highlights in the general knowledge of the role of sEH in the central nervous system (CNS) and its participation in neurodegeneration. We conclude that sEH is one of the most promising therapeutic strategies for PD and other neurodegenerative diseases with chronic inflammation process, providing new insights into the crucial role of sEH in PD pathophysiology as well as a singular opportunity for drug development.

P1 Receptor Agonists/Antagonists in Clinical Trials - Potential Drug Candidates of the Future

BACKGROUND

Adenosine mediates various physiological and pathological conditions by acting on its four P1 receptors (A1, A2A, A2B and A3 receptors). Omnipresence of P1 receptors and their activation, exert a wide range of biological activities. Thus, its modulation is implicated in various disorders like Parkinson's disease, asthma, cardiovascular disorders, cancer etc. Hence these receptors have become an interesting target for the researchers to develop potential therapeutic agents. Number of molecules were designed and developed in the past few years and evaluated for their efficacy in various disease conditions.

OBJECTIVE

The main objective is to provide an overview of new chemical entities which have crossed preclinical studies and reached clinical trials stage following their current status and future prospective.

METHODS

In this review we discuss current status of the drug candidates which have undergone clinical trials and their prospects.

RESULTS

Many chemical entities targeting various subtypes of P1 receptors are patented; twenty of them have crossed preclinical studies and reached clinical trials stage. Two of them viz adenosine and regadenoson are approved by the Food and Drug Administration.

CONCLUSION

This review is an attempt to highlight the current status, progress and probable future of P1 receptor ligands which are under clinical trials as promising novel therapeutic agents and the direction in which research should proceed with a view to come out with novel therapeutic agents.

Freezing of gait in Parkinson's disease: pathophysiology, risk factors and treatments

Background

Freezing of gait (FOG) is a common, disabling symptom of Parkinson's disease (PD), but the mechanisms and treatments of FOG remain great challenges for clinicians and researchers. The main focus of this review is to summarize the possible mechanisms underlying FOG, the risk factors for screening and predicting the onset of FOG, and the clinical trials involving various therapeutic strategies. In addition, the limitations and recommendations for future research design are also discussed.

Main body

In the mechanism section, we briefly introduced the physiological process of gait control and hypotheses about the mechanism of FOG. In the risk factor section, gait disorders, PIGD phenotype, lower striatal DAT uptake were found to be independent risk factors of FOG with consistent evidence. In the treatment section, we summarized the clinical trials of pharmacological and non-pharmacological treatments. Despite the limited effectiveness of current medications for FOG, especially levodopa resistant FOG, there were some drugs that showed promise such as istradefylline and rasagiline. Non-pharmacological treatments encompass invasive brain and spinal cord stimulation, noninvasive repetitive transcranial magnetic stimulation (rTMS) or transcranial direct current stimulation (tDCS) and vagus nerve stimulation (VNS), and physiotherapeutic approaches including cues and other training strategies. Several novel therapeutic strategies seem to be effective, such as rTMS over supplementary motor area (SMA), dual-site DBS, spinal cord stimulation (SCS) and VNS. Of physiotherapy, wearable cueing devices seem to be generally effective and promising.

Conclusion

FOG model hypotheses are helpful for better understanding and characterizing FOG and they provide clues for further research exploration. Several risk factors of FOG have been identified, but need combinatorial optimization for predicting FOG more precisely. Although firm conclusions cannot be drawn on therapeutic efficacy, the literature suggested that some therapeutic strategies showed promise.

PET Imaging of the Adenosine A2A Receptor in the Rotenone-Based Mouse Model of Parkinson's Disease with [18F]FESCH Synthesized by a Simplified Two-Step One-Pot Radiolabeling Strategy

The adenosine A_2A receptor (A_2AR) is regarded as a particularly appropriate target for non-dopaminergic treatment of Parkinson’s disease (PD). An increased A_2AR availability has been found in the human striatum at early stages of PD and in patients with PD and dyskinesias. The aim of this small animal positron emission tomography/magnetic resonance (PET/MR) imaging study was to investigate whether rotenone-treated mice reflect the aspect of striatal A_2AR upregulation in PD. For that purpose, we selected the known A_2AR-specific radiotracer [¹⁸F]FESCH and developed a simplified two-step one-pot radiosynthesis. PET images showed a high uptake of [¹⁸F]FESCH in the mouse striatum. Concomitantly, metabolism studies with [¹⁸F]FESCH revealed the presence of a brain-penetrant radiometabolite. In rotenone-treated mice, a slightly higher striatal A_2AR binding of [¹⁸F]FESCH was found. Nonetheless, the correlation between the increased A_2AR levels within the proposed PD animal model remains to be further investigated.