A phase I single and repeated dose pharmacokinetic study of oral V81444, a selective non-xanthine adenosine A2A receptor antagonist

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.

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.

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.

Adenosine 2A Receptor Antagonists for the Treatment of Motor Symptoms in Parkinson's Disease

Background

Treatment of motor fluctuations in Parkinson's disease (PD) remains an unmet challenge. Adenosine 2A (A_2A) receptors are located along the indirect pathway and represent a potential target to enhance l-3,4-dihydroxyphenylalanine (l-DOPA) antiparkinsonian action.

Methods

This article summarizes the preclinical and clinical literature on A_2A antagonists in PD, with a specific focus on their effect on off time, on time, and dyskinesia.

Findings

Several A_2A receptor antagonists have been tested in preclinical studies and clinical trials. In preclinical studies, A_2A antagonists enhanced l-DOPA antiparkinsonian action without exacerbating dyskinesia, but A_2A antagonists were generally administered in combination with a subthreshold dose of l-DOPA, which is different to the paradigms used in clinical trials, where A_2A antagonists were usually added to an optimal antiparkinsonian regimen. In clinical settings, A_2A antagonists generally reduced duration of off time, by as much as 25% in some studies. The effect of on time duration is less clear, and in a few studies an exacerbation of dyskinesia was reported. Two A_2A antagonists have been tested in phase III settings: istradefylline and preladenant. Istradefylline was effective in two phase III trials, but ineffective in another; the drug has been commercially available in Japan since 2013. In contrast, preladenant was ineffective in a phase III trial and the drug was discontinued. A phase III study with tozadenant will begin in 2015; the drug was effective at reducing off time in a phase IIb study. Other A_2A antagonists are in development at the preclinical and early clinical levels.