A 12-week, placebo-controlled study (6002-US-006) of istradefylline in Parkinson disease

Moving to a non-dopaminergic approach for the treatment of OFF fluctuations in Parkinson's disease

In levodopa treated patients with Parkinson's disease (PD), the standard approach to managing motor fluctuations is to adjust dopaminergic therapy. However, despite the availability of a wide armamentarium of dopaminergic medications, most patients treated with levodopa will still experience significant OFF time, and it is increasingly clear that motor fluctuations have a significant non-dopaminergic component. In this narrative review, we compare and contrast the therapeutic profiles of the only two non-dopaminergic medications approved in the US for the management of OFF time, namely amantadine and istradefylline. When compared against each other the two agents exemplify two different pharmacological approaches to treatment. Whereas amantadine has a multimodal pharmacology, istradefylline has highly specific actions at A2A receptors which are highly expressed in the indirect pathway of the basal ganglia. We discuss how both offer an important alternative approach to treatment, without increasing total dopaminergic load. Clinicians can also consider that amantadine and istradefylline each have overlapping indications with classic dopaminergic medications, but with distinct mechanisms of action that can complement each other to reduce motor complications in patients already being treated with other dopaminergic agents.

Comparative Safety of Istradefylline Among Parkinson Disease Adjunctive Therapies: A Systematic Review and Meta-analysis of Randomized Controlled Studies

INTRODUCTION

Adjunctive therapies to treat OFF episodes resulting from long-term levodopa treatment in Parkinson disease (PD) are hampered by safety and tolerability issues. Istradefylline offers an alternative mechanism (adenosine A2A receptor antagonist) and therefore potentially improved tolerability.

METHODS

A systematic review of PD adjuncts published in 2011 was updated to include randomized controlled trials published from January 1, 2010-April 15, 2019. Pairwise meta-analyses were updated, and Bucher indirect comparisons were used to generate estimates of relative safety, presented as odds ratio (OR) and 95% confidence interval (CI) for comparators versus istradefylline.

RESULTS

Fifty-seven randomized controlled trials involving 11,517 patients were included in the meta-analysis. Relative to istradefylline, dopamine agonists and catechol-O-methyl transferase (COMT) inhibitors had statistically significant higher odds of dyskinesia and somnolence. Monoamine oxidase-B inhibitors had significantly higher odds of hypotension. Amantadine extended-release (ER) had statistically significant higher odds of hallucination, orthostatic hypotension, insomnia, and withdrawals due to adverse events. All interventions combined had significantly higher odds of dyskinesia versus istradefylline 20 mg and somnolence versus istradefylline 40 mg. Considering overall incidence of adverse events, COMT inhibitors and amantadine ER had statistically significant higher odds versus both istradefylline doses (COMT versus istradefylline 40 mg, OR: 1.33; 95% CI: 1.03, 1.75; versus istradefylline 20 mg, OR: 1.32; 95% CI: 1.01, 1.72; amantadine ER versus istradefylline 40 mg, OR: 3.45; 95% CI: 1.85, 6.25; versus istradefylline 20 mg, OR: 3.33; 95% CI: 1.82, 6.25).

CONCLUSION

Istradefylline was associated with a generally favorable safety profile relative to other adjunct medications in this study.

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.

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.

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.

Anti-parkinsonian activity of the adenosine A2A receptor antagonist/inverse agonist KW-6356 as monotherapy in MPTP-treated common marmosets

KW-6356 is a novel adenosine A_2A receptor antagonist/inverse agonist that not only blocks binding of adenosine to adenosine A_2A receptor but also inhibits the constitutive activity of adenosine A_2A receptor. The efficacy of KW-6356 as both monotherapy and an adjunct therapy to L-3,4-dihydroxyphenylalanine (L-DOPA)/decarboxylase inhibitor in Parkinson's disease (PD) patients has been reported. However, the first-generation A_2A antagonist istradefylline, which is approved for use as an adjunct treatment to L-DOPA/decarboxylase inhibitor in adult PD patients experiencing OFF episodes, has not shown statistically significant efficacy as monotherapy. In vitro pharmacological studies have shown that the pharmacological properties of KW-6356 and istradefylline at adenosine A_2A receptor are markedly different. However, the anti-parkinsonian activity and effects on dyskinesia of KW-6356 in PD animal models and the differences in the efficacy between KW-6356 and istradefylline are unknown. The present study investigated the anti-parkinsonian activity of KW-6356 as monotherapy in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated common marmosets, and its efficacy was directly compared with that of istradefylline. In addition, we investigated whether or not repeated administration of KW-6356 induced dyskinesia. Oral administration of KW-6356 reversed motor disability in a dose-dependent manner up to 1 mg/kg in MPTP-treated common marmosets. The magnitude of anti-parkinsonian activity induced by KW-6356 was significantly greater than that of istradefylline. Repeated administration of KW-6356 induced little dyskinesia in MPTP-treated common marmosets primed to exhibit dyskinesia by prior exposure to L-DOPA. These results indicate that KW-6356 can be a novel non-dopaminergic therapy as monotherapy without inducing dyskinesia in PD patients.

Adenosinergic Pathway in Parkinson's Disease: Recent Advances and Therapeutic Perspective

Parkinson's disease (PD) is a neurodegenerative disease characterized pathologically by α-synuclein (α-syn) aggregation. In PD, the current mainstay of symptomatic treatment is levodopa (L-DOPA)-based dopamine (DA) replacement therapy. However, the development of dyskinesia and/or motor fluctuations which is relevant to levodopa is restricting its long-term utility. Given that the ability of which is to modulate the striato-thalamo-cortical loops and function to modulate basal ganglia output, the adenosinergic pathway (AP) is qualified as a potential promising non-DA target. As an indispensable component of energy production pathways, AP modulates cellular metabolism and gene regulation in both neurons and neuroglia cells through the recognition and degradation of extracellular adenosine. In addition, AP is geared to the initiation, evolution, and resolution of inflammation as well. Besides the above-mentioned crosstalk between the adenosine and dopamine signaling pathways, the functions of adenosine receptors (A₁R, A_2AR, A_2BR, and A₃R) and metabolism enzymes in modulating PD pathological process have been extensively investigated in recent decades. Here we reviewed the emerging findings focused on the function of adenosine receptors, adenosine formation, and metabolism in the brain and discussed its potential roles in PD pathological process. We also recapitulated clinical studies and the preclinical evidence for the medical strategies targeting the Ado signaling pathway to improve motor dysfunction and alleviate pathogenic process in PD. We hope that further clinical studies should consider this pathway in their monotherapy and combination therapy, which would open new vistas to more targeted therapeutic approaches.

Targeting G Protein-Coupled Receptors in the Treatment of Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disease characterized in part by the deterioration of dopaminergic neurons which leads to motor impairment. Although there is no cure for PD, the motor symptoms can be treated using dopamine replacement therapies including the dopamine precursor L-DOPA, which has been in use since the 1960s. However, neurodegeneration in PD is not limited to dopaminergic neurons, and many patients experience non-motor symptoms including cognitive impairment or neuropsychiatric disturbances, for which there are limited treatment options. Moreover, there are currently no treatments able to alter the progression of neurodegeneration. There are many therapeutic strategies being investigated for PD, including alternatives to L-DOPA for the treatment of motor impairment, symptomatic treatments for non-motor symptoms, and neuroprotective or disease-modifying agents. G protein-coupled receptors (GPCRs), which include the dopamine receptors, are highly druggable cell surface proteins which can regulate numerous intracellular signaling pathways and thereby modulate the function of neuronal circuits affected by PD. This review will describe the treatment strategies being investigated for PD that target GPCRs and their downstream signaling mechanisms. First, we discuss new developments in dopaminergic agents for alleviating PD motor impairment, the role of dopamine receptors in L-DOPA induced dyskinesia, as well as agents targeting non-dopamine GPCRs which could augment or replace traditional dopaminergic treatments. We then discuss GPCRs as prospective treatments for neuropsychiatric and cognitive symptoms in PD. Finally, we discuss the evidence pertaining to ghrelin receptors, β-adrenergic receptors, angiotensin receptors and glucagon-like peptide 1 receptors, which have been proposed as disease modifying targets with potential neuroprotective effects in PD.

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.

Design and synthesis of five-membered heterocyclic derivatives of istradefylline with comparable pharmacological activity

Parkinson's disease (PD) is a common degenerative disease of the central nervous system among the elderly. Istradefylline, an FDA-approved adenosine A_2A receptor antagonist (anti-PD drug), has good efficacy. However, it has been reported that the double bond of istradefylline is easily converted into cis-configuration when exposed to an indoor environment or direct light in a dilute solution. In order to find more stable adenosine A_2A receptor antagonists with similar pharmacological efficacy to istradefylline, the compounds series I-1 (12 compounds) was designed by maintaining the xanthine skeleton of istradefylline unchanged and replacing the trans-double bond with thiazole or benzothiazole and other biologically active heterocyclic compounds. These compounds were synthesized via multi-step experiment and successfully confirmed through different characterization techniques for their ability to inhibit cAMP formation in A_2A AR overexpressing cells. The thiazole derivative of istradefylline (Compound I-1-11, I-1-12) exhibited significant activity (IC₅₀  = 16.74 ± 4.11 μM, 10.36 ± 3.09 μM), as compared to istradefylline (IC₅₀  = 5.05 ± 1.32 μM). In addition, the molecular docking of benzothiazole derivatives I-1-11 and thiazole derivatives I-1-12 with higher inhibition rate were carried out and compared with istradefylline. The molecular docking results showed that I-1-11 and I-1-12 anchored in the same site as that of XAC (3REY) with predicted affinity binding energy -6.63 kcal/mol and - 6.75 kcal/mol, respectively. Validation through dynamics simulation also showed stable interactions, with fluctuations <3 Å and MM/GBSA energy <-20 kcal/mol. Hence, this study could provide a basis for the rational design of adenosine A_2A receptor antagonists with better potency.

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

The adenosine A2A receptor subtype is recognized as a non-dopaminergic pharmacological target for the treatment of neurodegenerative disorders, notably Parkinson's disease (PD). The selective A2A 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 A2A 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.

Emerging Nondopaminergic Medications for Parkinson's Disease: Focusing on A2A Receptor Antagonists and GLP1 Receptor Agonists

Parkinson's disease (PD) is a severe neurodegenerative disease characterized by classic motor features associated with the loss of dopaminergic neurons and appearance of Lewy bodies in the substantia nigra. Due to the complexity of PD, a definitive diagnosis in the early stages and effective management of symptoms in later stages are difficult to achieve in clinical practice. Previous research has shown that colocalization of A2A receptors (A2AR) and dopamine D2 receptors (D2R) may induce an antagonistic interaction between adenosine and dopamine. Clinical trials have found that the A2AR antagonist istradefylline decreases dyskinesia in PD and could be used as an adjuvant to levodopa treatment. Meanwhile, the incretin hormone glucagon-like peptide 1 (GLP1) mainly facilitates glucose homeostasis and insulin signaling. Preclinical experiments and clinical trials of GLP1 receptor (GLP1R) agonists show that they may be effective in alleviating neuroinflammation and sustaining cellular functions in the central nervous system of patients with PD. In this review, we summarize up-to-date findings on the usefulness of A2AR antagonists and GLP1R agonists in PD management. We explain the molecular mechanisms of these medications and their interactions with other neurotransmitter receptors. Furthermore, we discuss the efficacy and limitations of A2AR antagonists and GLP1R agonists in clinical practice.

Targeting Adenosine Receptors in Neurological Diseases

Adenosine plays a significant role in neurotransmission process by controlling the blood pressure, while adenosine triphosphate (ATP) acts as a neuromodulator and neurotransmitter and by activation of P2 receptors, regulates the contractility of the heart. Adenosine signaling is essential in the process of regeneration by regulating proliferation, differentiation, and apoptosis of stem cells. In this review, we have selected neurological disorders (Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, multiple sclerosis, and epilepsy) with clinical trials using antagonists and epigenetic tools targeting adenosine receptor as a therapeutic approach in the treatment of these disorders. Promising results have been reported from many clinical trials. It has been found that higher expression levels of A2A and P2X7 receptors in neurological disorders further complicate the disease condition. Therefore, modulations of these receptors by using antagonists of these receptors or SAM (S-adenosylmethionine) therapy as an epigenetic tool could be useful in reversing the complications of these disorders. Finally, we suggest that modulation of adenosine receptors in neurological disorders can increase the regenerative phase by increasing the rate of proliferation and differentiation in the damaged tissues.

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.

Update on Parkinson's Disease Therapy

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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.

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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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.

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.

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.

The belated US FDA approval of the adenosine A2A receptor antagonist istradefylline for treatment of Parkinson's disease

After more than two decades of preclinical and clinical studies, on August 27, 2019, the US Food and Drug Administration (FDA) approved the adenosine A_2A receptor antagonist Nourianz® (istradefylline) developed by Kyowa Hakko Kirin Inc., Japan, as an add-on treatment to levodopa in Parkinson's disease (PD) with "OFF" episodes. This milestone achievement is the culmination of the decade-long clinical studies of the effects of istradefylline in more than 4000 PD patients. Istradefylline is the first non-dopaminergic drug approved by FDA for PD in the last two decades. This approval also provides some important lessons to be remembered, namely, concerning disease-specific adenosine signaling and targeting subpopulation of PD patients. Importantly, this approval paves the way to foster entirely novel therapeutic opportunities for adenosine A_2A receptor antagonists, such as neuroprotection or reversal of mood and cognitive deficits in PD and other neuropsychiatric diseases.

Targeting Purinergic Signaling and Cell Therapy in Cardiovascular and Neurodegenerative Diseases

Extracellular purines exert several functions in physiological and pathophysiological mechanisms. ATP acts through P2 receptors as a neurotransmitter and neuromodulator and modulates heart contractility, while adenosine participates in neurotransmission, blood pressure, and many other mechanisms. Because of their capability to differentiate into mature cell types, they provide a unique therapeutic strategy for regenerating damaged tissue, such as in cardiovascular and neurodegenerative diseases. Purinergic signaling is pivotal for controlling stem cell differentiation and phenotype determination. Proliferation, differentiation, and apoptosis of stem cells of various origins are regulated by purinergic receptors. In this chapter, we selected neurodegenerative and cardiovascular diseases with clinical trials using cell therapy and purinergic receptor targeting. We discuss these approaches as therapeutic alternatives to neurodegenerative and cardiovascular diseases. For instance, promising results were demonstrated in the utilization of mesenchymal stem cells and bone marrow mononuclear cells in vascular regeneration. Regarding neurodegenerative diseases, in general, P2X7 and A_2A receptors mostly worsen the degenerative state. Stem cell-based therapy, mainly through mesenchymal and hematopoietic stem cells, showed promising results in improving symptoms caused by neurodegeneration. We propose that purinergic receptor activity regulation combined with stem cells could enhance proliferative and differentiation rates as well as cell engraftment.

Receptor Ligands as Helping Hands to L-DOPA in the Treatment of Parkinson's Disease

Levodopa (LD) is the most effective drug in the treatment of Parkinson’s disease (PD). However, although it represents the “gold standard” of PD therapy, LD can cause side effects, including gastrointestinal and cardiovascular symptoms as well as transient elevated liver enzyme levels. Moreover, LD therapy leads to LD-induced dyskinesia (LID), a disabling motor complication that represents a major challenge for the clinical neurologist. Due to the many limitations associated with LD therapeutic use, other dopaminergic and non-dopaminergic drugs are being developed to optimize the treatment response. This review focuses on recent investigations about non-dopaminergic central nervous system (CNS) receptor ligands that have been identified to have therapeutic potential for the treatment of motor and non-motor symptoms of PD. In a different way, such agents may contribute to extending LD response and/or ameliorate LD-induced side effects.

Development of Adenosine A2A Receptor Antagonists for the Treatment of Parkinson's Disease: A Recent Update and Challenge

Parkinson's disease (PD) is a neurodegenerative disease with significant unmet medical needs. The current dopamine-centered treatments aim to restore motor functions of patients without slowing the disease progression. Long-term usage of these drugs is associated with diminished efficacy, motor fluctuation, and dyskinesia. Furthermore, the nonmotor features associated with PD such as sleep disorder, pain, and psychiatric symptoms are poorly addressed by the dopaminergic treatments. Adenosine receptor A_2A antagonists have emerged as potential treatment for PD in the past decade. Here we summarize the recent work (2015-2018) on adenosine receptor A_2A antagonists and discuss the challenge and opportunity for the treatment of PD.

Chemical management of levodopa-induced dyskinesia in Parkinson's disease patients

INTRODUCTION

Levodopa-induced dyskinesias (LID) appears in more than 50% of Parkinson's disease patients after 5 years of treatment and clinicians always have to ensure that there is a balance between the beneficial effect of the treatment and the potential complications.

AREAS COVERED

In this review, the authors discuss the treatment of LID. Treatment can be divided into strategies for preventing their occurrence, modification of dopaminergic therapy, and providing more continuous dopaminergic stimulation as well as the use of nondopaminergic drugs.

EXPERT OPINION

Amantadine is currently considered the most effective drug for the treatment of LID. Several compounds developed to target adenosine, adrenergic, glutamatergic, and serotonergic receptors have shown to significantly decrease dyskinesias in animal models. However, despite promising preclinical results, translation to clinical practice remains challenging and majority of these compounds failed to decrease LID in randomized controlled trials with moderate-to-advanced parkinsonian patients. Despite promising results with nondopaminergic drugs, treatment of dyskinesias is still challenging and largely due to their side effects. Future research should focus on developing treatments that can provide continuous dopaminergic delivery throughout the day in a noninvasive manner. Studies on the impact of the early administration of long-acting formulations of levo-3,4-dihydroxy-phenylalanine on dyskinesias are also necessary.

Parkinson's Disease: Basic Pathomechanisms and a Clinical Overview

PD is a common and a debilitating degenerative movement disorder. The number of patients is increasing worldwide and as yet there is no cure for the disease. The majority of existing treatments target motor symptom control. Over the last two decades the impact of the genetic contribution to PD has been appreciated. Significant discoveries have been made, which have advanced our understanding of the pathophysiological and molecular basis of PD. In this chapter we outline current knowledge of the clinical aspects of PD and the basic mechanistic understanding.

Levodopa-induced dyskinesia: clinical features, incidence, and risk factors

Symptoms of Parkinson's disease have been controlled with levodopa for many years; however, motor complications consisting of wearing off of medication effect and dyskinesias tend to occur within a few years of starting levodopa. Motor complications can begin a few months after taking levodopa, with the average time to onset estimated to be 6.5 years. Dyskinesias can be troublesome and require intervention. Levodopa-induced dyskinesia can be composed of a variety of movement disorders including chorea, dystonia, ballism, myoclonus, and akathisia. Based on the clinical pattern, the most common dyskinesia is chorea and choreoathetosis. The clinical manifestations can be divided into three main categories based on their clinical movement patterns and the temporal correlation between the occurrence of dyskinesia and the levodopa dosing: on or peak-dose dyskinesias, biphasic dyskinesias, and Off dyskinesias. Severe cases of dyskinesia have been reported, with the extreme being dyskinesia-hyperpyrexia syndrome. The prevalence of LID has been reported in many studies, but the reported incidence varies. The rate of LID development is from 3 to 94%. The prevalence of LID mainly depends on age at onset, disease duration, and severity, and duration of levodopa therapy. Some of the risk factors for the development of dyskinesia are modifiable. Modifiable risk factors include levodopa dose and body weight. Non-modifiable risk factors include age, gender, duration of disease, clinical subtype, disease progression, disease severity, and genetic factors.

Current Nondopaminergic Therapeutic Options for Motor Symptoms of Parkinson's Disease

Objective:

The aim of this study was to summarize recent studies on nondopaminergic options for the treatment of motor symptoms in Parkinson's disease (PD).

Data Sources:

Papers in English published in PubMed, Cochrane, and Ovid Nursing databases between January 1988 and November 2016 were searched using the following keywords: PD, nondopaminergic therapy, adenosine, glutamatergic, adrenergic, serotoninergic, histaminic, and iron chelator. We also reviewed the ongoing clinical trials in the website of clinicaltrials.gov.

Study Selection:

Articles related to the nondopaminergic treatment of motor symptoms in PD were selected for this review.

Results:

PD is conventionally treated with dopamine replacement strategies, which are effective in the early stages of PD. Long-term use of levodopa could result in motor complications. Recent studies revealed that nondopaminergic systems such as adenosine, glutamatergic, adrenergic, serotoninergic, histaminic, and iron chelator pathways could include potential therapeutic targets for motor symptoms, including motor fluctuations, levodopa-induced dyskinesia, and gait disorders. Some nondopaminergic drugs, such as istradefylline and amantadine, are currently used clinically, while most such drugs are in preclinical testing stages. Transitioning of these agents into clinically beneficial strategies requires reliable evaluation since several agents have failed to show consistent results despite positive findings at the preclinical level.

Conclusions:

Targeting nondopaminergic transmission could improve some motor symptoms in PD, especially the discomfort of dyskinesia. Although nondopaminergic treatments show great potential in PD treatment as an adjunct therapy to levodopa, further investigation is required to ensure their success.

Role of adenosine A2A receptors in motor control: relevance to Parkinson's disease and dyskinesia

Adenosine is an endogenous purine nucleoside that regulates several physiological functions, at the central and peripheral levels. Besides, adenosine has emerged as a major player in the regulation of motor behavior. In fact, adenosine receptors of the A_2A subtype are highly enriched in the caudate-putamen, which is richly innervated by dopamine. Moreover, several studies in experimental animals have consistently demonstrated that the pharmacological antagonism of A_2A receptors has a facilitatory influence on motor behavior. Taken together, these findings have envisaged A_2A receptors as a promising target for symptomatic therapies aimed at ameliorating motor deficits. Accordingly, A_2A receptor antagonists have been extensively studied as new agents for the treatment of Parkinson's disease (PD), the epitome of motor disorders. In this review, we provide an overview of the effects that adenosine A_2A receptor antagonists elicit in rodent and primate experimental models of PD, with regard to the counteraction of motor deficits as well as to manifestation of dyskinesia and motor fluctuations. Moreover, we briefly present the results of clinical trials of A_2A receptor antagonists in PD patients experiencing motor fluctuations, with particular regard to dyskinesia. Finally, we discuss the interaction between A_2A receptor antagonists and serotonin receptor agonists, since combined administration of these drugs has recently emerged as a new potential therapeutic strategy in the treatment of dyskinesia.

The effect of istradefylline for Parkinson's disease: A meta-analysis

Adenosine A_2A receptor antagonists are an alternative treatment strategy for Parkinson's disease. Several randomized placebo controlled studies have tested the effect of A_2A receptor antagonist istradefylline, and more robust evidence has been acquired. This meta-analysis aimed to provide evidence for its efficacy and safety on patients with Parkinson's disease. After a systematic literature search, we calculated the pooled standardized mean difference and risk ratio for continuous and dichotomous variables, respectively. Further, sensitivity analyses were performed to confirm the effect estimated by meta-analyses. Publication bias was assessed by funnel plot and deviation of intercept. Six studies satisfied our inclusion criteria. Istradefylline (40 mg/day) decreased off time and improved motor symptoms of Parkinson's disease in homogeneous studies. Istradefylline at 20 mg/day decreased off time and improved motor symptoms, but heterogeneity was found in the analysis of the former among studies. There was a significant effect of istradefylline on dyskinesia in homogeneous studies. Publication bias, however, was observed in the comparison of dyskinesia. Other adverse events showed no significant difference. The present meta-analysis suggests that istradefylline at 40 mg/day could alleviate off time and motor symptoms derived from Parkinson's disease. Dyskinesia might be worsened, but publication bias prevents this from being clear.

Targeting Adenosine Signaling in Parkinson's Disease: From Pharmacological to Non-pharmacological Approaches

Parkinson's disease (PD) is one of the most prevalent neurodegenerative disease displaying negative impacts on both the health and social ability of patients and considerable economical costs. The classical anti-parkinsonian drugs based in dopaminergic replacement are the standard treatment, but several motor side effects emerge during long-term use. This mini-review presents the rationale to several efforts from pre-clinical and clinical studies using adenosine receptor antagonists as a non-dopaminergic therapy. As several studies have indicated that the monotherapy with adenosine receptor antagonists reaches limited efficacy, the usage as a co-adjuvant appeared to be a promising strategy. The formulation of multi-targeted drugs, using adenosine receptor antagonists and other neurotransmitter systems than the dopaminergic one as targets, have been receiving attention since Parkinson's disease presents a complex biological impact. While pharmacological approaches to cure or ameliorate the conditions of PD are the leading strategy in this area, emerging positive aspects have arisen from non-pharmacological approaches and adenosine function inhibition appears to improve both strategies.

Effects of Rifampin on the Pharmacokinetics of a Single Dose of Istradefylline in Healthy Subjects

Istradefylline, a selective adenosine A_2A inhibitor, is under development for the treatment of Parkinson's disease. The effect of oral steady‐state rifampin 600 mg/day, a potent cytochrome P450 (CYP) 3A4 inducer, on the disposition of a single oral dose of istradefylline 40 mg was determined in a crossover study in 20 healthy subjects by measuring plasma concentrations of istradefylline and its M1 and M8 metabolites and their derived pharmacokinetic parameters. Based on the geometric mean ratio of log‐transformed data, rifampin reduced istradefylline exposure: C_max, 0.55 (90%CI, 0.49–0.62); AUC_last, 0.21 (90%CI, 0.19–0.22); and AUC_inf, 0.19 (90%CI, 0.18–0.20), indicating nonequivalence. These changes were primarily because of the effect of rifampin on the elimination parameters of istradefylline; mean CL/F was increased from 4.0 to 20.6 L/h, and mean t_1/2 was reduced from 94.8 to 31.5 hours. The effect of rifampin coadministration on the disposition of the istradefylline M1 and M8 metabolites was inconsistent and variable. Furthermore, as exposure of the istradefylline M1 and M8 metabolites in plasma was generally <9% of total drug exposure, it would be expected to have a negligible impact on the pharmacodynamic effect of istradefylline. Caution should be exercised when istradefylline is administered concurrently with strong CYP3A4 inducers and dose adjustment considered.

Randomized trial of preladenant, given as monotherapy, in patients with early Parkinson disease

Objective:

To evaluate the adenosine 2a receptor antagonist preladenant as a nondopaminergic drug for the treatment of Parkinson disease (PD) when given as monotherapy.

Methods:

This was a randomized, 26-week, placebo- and active-controlled, parallel-group, multicenter, double-blind trial conducted in adults diagnosed with PD for <5 years who were not yet receiving l-dopa or dopamine agonists. Patients with a Unified Parkinson’s Disease Rating Scale (UPDRS) part 3 (motor function) score ≥10 and Hoehn & Yahr score ≤3 were randomized 1:1:1:1:1 to preladenant 2, 5, or 10 mg twice daily, rasagiline 1 mg (active-control) once daily, or placebo. The primary endpoint was the change from baseline at week 26 in the sum of UPDRS parts 2 (activities of daily living) and 3 scores (UPDRS2+3).

Results:

The number of patients treated was 1,007. Neither preladenant nor rasagiline was superior to placebo after 26 weeks. The differences vs placebo (95% confidence interval) in UPDRS2+3 scores (with a negative difference indicating improvement vs placebo) were preladenant 2 mg = 2.60 (0.86, 4.30), preladenant 5 mg = 1.30 (−0.41, 2.94), preladenant 10 mg = 0.40 (−1.29, 2.11), and rasagiline 1 mg = 0.30 (−1.35, 2.03). Post hoc analyses did not identify a single causal factor that could explain the finding of a failed trial. Preladenant was generally well-tolerated with few patients discontinuing due to adverse events (preladenant 7%, rasagiline 3%, placebo 4%).

Conclusions:

No evidence supporting the efficacy of preladenant as monotherapy was observed in this phase 3 trial. The lack of efficacy of the active control rasagiline makes it difficult to interpret the results.

Clinical trial registration:

Clinicaltrials.gov: NCT01155479.

Classification of evidence:

This study provides Class I evidence that for patients with early PD, preladenant is not effective as monotherapy at the doses studied (2, 5, 10 mg).

An evaluation of istradefylline treatment on Parkinsonian motor and cognitive deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated macaque models

Istradefylline (KW-6002), an adenosine A2A receptor antagonist, is used adjunct with optimal doses of L-3,4-dihydroxyphenylalanine (l-DOPA) to extend on-time in Parkinson's disease (PD) patients experiencing motor fluctuations. Clinical application of istradefylline for the management of other l-DOPA-induced complications, both motor and non-motor related (i.e. dyskinesia and cognitive impairments), remains to be determined. In this study, acute effects of istradefylline (60-100 mg/kg) alone, or with optimal and sub-optimal doses of l-DOPA, were evaluated in two monkey models of PD (i) the gold-standard 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated macaque model of parkinsonian and dyskinetic motor symptoms and (ii) the chronic low dose (CLD) MPTP-treated macaque model of cognitive (working memory and attentional) deficits. Behavioural analyses in l-DOPA-primed MPTP-treated macaques showed that istradefylline alone specifically alleviated postural deficits. When combined with an optimal l-DOPA treatment dose, istradefylline increased on-time, enhanced therapeutic effects on bradykinesia and locomotion, but exacerbated dyskinesia. Istradefylline treatment at specific doses with sub-optimal l-DOPA specifically alleviated bradykinesia. Cognitive assessments in CLD MPTP-treated macaques showed that the attentional and working memory deficits caused by l-DOPA were lowered after istradefylline administration. Taken together, these data support a broader clinical use of istradefylline as an adjunct treatment in PD, where specific treatment combinations can be utilised to manage various l-DOPA-induced complications, which importantly, maintain a desired anti-parkinsonian response.