The novel adenosine A2a receptor antagonist ST1535 potentiates the effects of a threshold dose of l-DOPA in MPTP treated common marmosets

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.

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.

Molecular Mechanisms and Therapeutic Strategies for Levodopa-Induced Dyskinesia in Parkinson’s Disease: A Perspective Through Preclinical and Clinical Evidence

Parkinson’s disease (PD) is the second leading neurodegenerative disease that is characterized by severe locomotor abnormalities. Levodopa (L-DOPA) treatment has been considered a mainstay for the management of PD; however, its prolonged treatment is often associated with abnormal involuntary movements and results in L-DOPA-induced dyskinesia (LID). Although LID is encountered after chronic administration of L-DOPA, the appearance of dyskinesia after weeks or months of the L-DOPA treatment has complicated our understanding of its pathogenesis. Pathophysiology of LID is mainly associated with alteration of direct and indirect pathways of the cortico-basal ganglia-thalamic loop, which regulates normal fine motor movements. Hypersensitivity of dopamine receptors has been involved in the development of LID; moreover, these symptoms are worsened by concurrent non-dopaminergic innervations including glutamatergic, serotonergic, and peptidergic neurotransmission. The present study is focused on discussing the recent updates in molecular mechanisms and therapeutic approaches for the effective management of LID in PD patients.

Emerging Therapeutic Strategies for Parkinson’s Disease and Future Prospects: A 2021 Update

Parkinson’s disease (PD) is a neurodegenerative disorder pathologically distinguished by degeneration of dopaminergic neurons in the substantia nigra pars compacta. Muscle rigidity, tremor, and bradykinesia are all clinical motor hallmarks of PD. Several pathways have been implicated in PD etiology, including mitochondrial dysfunction, impaired protein clearance, and neuroinflammation, but how these factors interact remains incompletely understood. Although many breakthroughs in PD therapy have been accomplished, there is currently no cure for PD, only trials to alleviate the related motor symptoms. To reduce or stop the clinical progression and mobility impairment, a disease-modifying approach that can directly target the etiology rather than offering symptomatic alleviation remains a major unmet clinical need in the management of PD. In this review, we briefly introduce current treatments and pathophysiology of PD. In addition, we address the novel innovative therapeutic targets for PD therapy, including α-synuclein, autophagy, neurodegeneration, neuroinflammation, and others. Several immunomodulatory approaches and stem cell research currently in clinical trials with PD patients are also discussed. Moreover, preclinical studies and clinical trials evaluating the efficacy of novel and repurposed therapeutic agents and their pragmatic applications with encouraging outcomes are summarized. Finally, molecular biomarkers under active investigation are presented as potentially valuable tools for early PD diagnosis.

The regulation of circadian entrainment in mice by the adenosine the A 2A /A 1 receptor antagonist CT1500

Circadian entrainment in mice relies primarily on photic cues that trigger the transcription of the core clock genes Period1/2 in the suprachiasmatic nucleus (SCN), thus aligning the phase of the clock with the dawn/dusk cycle. It has been shown previously that this pathway is directly regulated by adenosine signalling and that adenosine A_2A/A₁ receptor antagonists can both enhance photic entrainment and phase shift circadian rhythms of wheel-running behaviour in mice. In this study, we tested the ability of CT1500, a clinically safe adenosine A_2A/A₁ receptor antagonist to effect circadian entrainment. We show that CT1500 lengthens circadian period in SCN ex vivo preparations. Furthermore, we show in vivo that a single dose of CT1500 enhances re-entrainment to a shifted light dark cycle in a dose-dependent manner in mice and also phase shifts the circadian clock under constant dark with a clear time-of-day related pattern. The phase response curve shows CT1500 causes phase advances during the day and phase delays at dusk. Finally, we show that daily timed administration of CT1500 can entrain the circadian clock to a 24 h rhythm in free-running mice. Collectively, these data support the use of CT1500 in the treatment of disorders of circadian entrainment.

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.

Can adenosine A2A receptor antagonists modify motor behavior and dyskinesia in experimental models of Parkinson's disease?

Current treatment of the motor symptoms of Parkinson's disease (PD) focuses on dopamine replacement therapies. While these treatments are initially highly effective, with long-term use and disease progression, the therapeutic response is often limited by the development of motor complications, dopaminergic side effects, and residual unresponsive motor and non-motor symptoms. An alternative or additive treatment approach may be to target non-dopaminergic receptors within the motor control pathways, which function to modulate basal ganglia output. Adenosine A_2A receptors are one potential non-dopaminergic target as they are selectively localized to the basal ganglia and to the indirect output pathway known to modulate the striato-thalamo-cortical loops critical to the expression of the motor symptoms of PD. This paper reviews the preclinical evidence base for the ability of adenosine A_2A receptor blockade to influence motor function and modulate dyskinesia expression. There is consensus that adenosine A_2A receptor antagonists - administered either as a monotherapy or in combination with l-DOPA or dopamine agonists - improve motor function in both rodent and primate models of PD, and should be effective for treating the motor symptoms of PD in humans. Importantly, the improvements in motor function were seen in the absence of dyskinesia. The introduction of a non-dopaminergic approach to modifying basal ganglia function provides a useful addition to the range of available therapies for treating PD, and there is a rational basis for a drug that focuses on modifying basal ganglia output.

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.

PBF509, an Adenosine A2A Receptor Antagonist With Efficacy in Rodent Models of Movement Disorders

Adenosine A_2A receptor (A_2AR) antagonists have emerged as complementary non-dopaminergic drugs to alleviate Parkinson's disease (PD) symptomatology. Here, we characterize a novel non-xhantine non-furan A_2AR antagonist, PBF509, as a potential pro-dopaminergic drug for PD management. First, PBF509 was shown to be a highly potent ligand at the human A_2AR, since it antagonized A_2AR agonist-mediated cAMP accumulation and impedance responses with K_B values of 72.8 ± 17.4 and 8.2 ± 4.2 nM, respectively. Notably, these results validated our new A_2AR-based label-free assay as a robust and sensitive approach to characterize A_2AR ligands. Next, we evaluated the efficacy of PBF509 reversing motor impairments in several rat models of movement disorders, including catalepsy, tremor, and hemiparkinsonism. Thus, PBF509 (orally) antagonized haloperidol-mediated catalepsy, reduced pilocarpine-induced tremulous jaw movements and potentiated the number of contralateral rotations induced by L-3,4-dihydroxyphenylalanine (L-DOPA) in unilaterally 6-OHDA-lesioned rats. Moreover, PBF509 (3 mg/kg) inhibited L-DOPA-induced dyskinesia (LID), showing not only its efficacy on reversing parkinsonian motor impairments but also acting as antidyskinetic agent. Overall, here we describe a new orally selective A_2AR antagonist with potential utility for PD treatment, and for some of the side effects associated to the current pharmacotherapy (i.e., dyskinesia).

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.

A Meta-Analysis of Adenosine A2A Receptor Antagonists on Levodopa-Induced Dyskinesia In Vivo

Background

Long-term use of levodopa (l-dopa) is inevitably complicated with highly disabling fluctuations and drug-induced dyskinesias, which pose major challenges to the existing drug therapy of Parkinson's disease.

Methods

In this study, we conducted a systematic review and meta-analysis to assess the efficacy of A2A receptor antagonists on reducing l-dopa-induced dyskinesias (LID).

Results

Nine studies with a total of 152 animals were included in this meta-analysis. Total abnormal involuntary movements (AIM) score, locomotor activity, and motor disability were reported as outcome measures in 5, 5, and 3 studies, respectively. Combined standardized mean difference (SMD) estimates were calculated using a random-effects model. We pooled the whole data and found that, when compared to l-dopa alone, A2A receptor antagonists plus l-dopa treatment showed no effect on locomotor activity (SMD -0.00, 95% confidence interval (CI): -2.52 to 2.52, p = 1.0), superiority in improvement of motor disability (SMD -5.06, 95% CI: -9.25 to -0.87, p = 0.02) and more effective in control of AIM (SMD -1.82, 95% CI: -3.38 to -0.25, p = 0.02).

Conclusion

To sum up, these results demonstrated that A2A receptor antagonists appear to have efficacy in animal models of LID. However, large randomized clinical trials testing the effects of A2A receptor antagonists in LID patients are always warranted.

Dyskinesias and impulse control disorders in Parkinson's disease: From pathogenesis to potential therapeutic approaches

Dopaminergic treatment in Parkinson's disease (PD) reduces the severity of motor symptoms of the disease. However, its chronic use is associated with disabling motor and behavioral side effects, among which levodopa-induced dyskinesias (LID) and impulse control disorders (ICD) are the most common. The underlying mechanisms and pathological substrate of these dopaminergic complications are not fully understood. Recently, the refinement of imaging techniques and the study of the genetics and molecular bases of LID and ICD indicate that, although different, they could share some features. In addition, animal models of parkinsonism with LID have provided important knowledge about mechanisms underlying such complications. In contrast, animal models of parkinsonism and abnormal impulsivity, although useful regarding some aspects of human ICD, do not fully resemble the clinical phenotype of ICD in patients with PD, and until now have provided limited information. Studies on animal models of addiction could complement the previous models and provide some insights into the background of these behavioral complications given that ICD are regarded as behavioral addictions. Here we review the most relevant advances in relation to imaging, genetics, biochemistry and pharmacological interventions to treat LID and ICD in patients with PD and in animal models with a view to better understand the overlapping and unique maladaptations to dopaminergic therapy that are associated with LID and ICD.

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.

Animal models of Parkinson׳s disease: Effects of two adenosine A2A receptor antagonists ST4206 and ST3932, metabolites of 2-n-Butyl-9-methyl-8-[1,2,3]triazol-2-yl-9H-purin-6-ylamine (ST1535)

Antagonism of the adenosine A2A receptor represents a promising strategy for non-dopaminergic treatment of Parkinson׳s disease (PD). Previously, the adenosine A2A receptor antagonist ST1535 was shown to possess potential beneficial effects in animal models of PD. Two metabolites of ST1535, namely ST3932 and ST4206, were tested in vitro to assess their affinity and activity on cloned human A2A adenosine receptors, and their metabolic profile. Additionally, ST3932 and ST4206 were investigated in vivo in animal models of PD following oral/intraperitoneal administration of 10, 20 and 40mg/kg using ST1535 as a reference compound. ST3932 and ST4206 displayed high affinity and antagonist behaviour for cloned human adenosine A2A receptors. The Ki values for ST1535, ST3932 and ST4206 were 8, 8 and 12nM, respectively, and their IC50 values on cyclic AMP were 427, 450 and 990nM, respectively. ST1535, ST3932 and ST4206 antagonized (orally) haloperidol-induced catalepsy in mice, potentiated (intraperitoneally) the number of contralateral rotations induced by l-3,4-dihydroxyphenylalanine (l-DOPA) (3mg/kg) plus benserazide (6mg/kg) in 6-Hydroxydopamine hydrobromide (6-OHDA)-lesioned rats, and increased mouse motor activity by oral route. Thus, ST3932 and ST4206, two ST1535 metabolites, show a pharmacological activity similar to ST1535, both in vitro and in vivo, and may be regarded as an interesting pharmacological alternative to ST1535.

History and perspectives of A2A adenosine receptor antagonists as potential therapeutic agents

Growing evidence emphasizes that the purine nucleoside adenosine plays an active role as a local regulator in different pathologies. Adenosine is a ubiquitous nucleoside involved in various physiological and pathological functions by stimulating A1 , A2A , A2B , and A3 adenosine receptors (ARs). At the present time, the role of A2A ARs is well known in physiological conditions and in a variety of pathologies, including inflammatory tissue damage and neurodegenerative disorders. In particular, the use of selective A2A antagonists has been reported to be potentially useful in the treatment of Parkinson's disease (PD). In this review, A2A AR signal transduction pathways, together with an analysis of the structure-activity relationships of A2A antagonists, and their corresponding pharmacological roles and therapeutic potential have been presented. The initial results from an emerging polypharmacological approach are also analyzed. This approach is based on the optimization of the affinity and/or functional activity of the examined compounds toward multiple targets, such as A1 /A2A ARs and monoamine oxidase-B (MAO-B), both closely implicated in the pathogenesis of PD.

Adenosine A2A receptor antagonists in Parkinson's disease: progress in clinical trials from the newly approved istradefylline to drugs in early development and those already discontinued

Neurotransmitters other than dopamine, such as norepinephrine, 5-hydroxytryptamine, glutamate, adenosine and acetylcholine, are involved in Parkinson's disease (PD) and contribute to its symptomatology. Thus, the progress of non-dopaminergic therapies for PD has attracted much interest in recent years. Among new classes of drugs, adenosine A2A antagonists have emerged as promising candidates. The development of new highly selective adenosine A2A receptor antagonists, and their encouraging anti-parkinsonian responses in animal models of PD, has provided a rationale for clinical trials to evaluate the therapeutic potential and the safety of these agents in patients with PD. To date, the clinical research regarding A2A antagonists and their potential utilization in PD therapy continues to evolve between drugs just or previously discontinued (preladenant and vipadenant), new derivatives in development (tozadenant, PBF-509, ST1535, ST4206 and V81444) and the relatively old drug istradefylline, which has finally been licensed as an anti-parkinsonian drug in Japan. All these compounds have been shown to have a good safety profile and be well tolerated. Moreover, results from phase II and III trials also demonstrate that A2A antagonists are effective in reducing off-time, without worsening troublesome dyskinesia, and in increasing on-time with a mild increase of non-troublesome dyskinesia, in patients at an advanced stage of PD treated with L-DOPA. In addition, early findings suggest that A2A antagonists might also be efficacious as monotherapy in patients at an early stage of PD. This review summarizes pharmacological and clinical data available on istradefylline, tozadenant, PBF-509, ST1535, ST4206, V81444, preladenant and vipadenant.

Synthesis and biological evaluation of metabolites of 2-n-butyl-9-methyl-8-[1,2,3]triazol-2-yl-9H-purin-6-ylamine (ST1535), a potent antagonist of the A2A adenosine receptor for the treatment of Parkinson's disease

The synthesis and preliminary in vitro evaluation of five metabolites of the A2A antagonist ST1535 (1) are reported. The metabolites, originating in vivo from enzymatic oxidation of the 2-butyl group of the parent compound, were synthesized from 6-chloro-2-iodo-9-methyl-9H-purine (2) by selective C-C bond formation via halogen/magnesium exchange reaction and/or palladium-catalyzed reactions. The metabolites behaved in vitro as antagonist ligands of cloned human A2A receptor with affinities (Ki 7.5-53 nM) comparable to that of compound 1 (Ki 10.7 nM), thus showing that the long duration of action of 1 could be in part due to its metabolites. General behavior after oral administration in mice was also analyzed.

New pharmacological avenues for the treatment of L-DOPA-induced dyskinesias in Parkinson's disease: targeting glutamate and adenosine receptors

INTRODUCTION

Parkinson's disease (PD) therapy is still centered on the use of L-3,4-dihydroxyphenylalanine (L-DOPA), which is hampered by numerous side effects, including abnormal involuntary movements known as L-DOPA-induced dyskinesias (LIDs). LIDs are the result of pre- and postsynaptic changes at the corticostriatal level, induced by chronic and pulsatile stimulation of striatal dopaminergic receptors. These changes impact on synaptic plasticity and involve also selected, nondopaminergic receptors expressed by striatal projection neurons.

AREAS COVERED

Among nondopaminergic receptors, glutamate receptors - NMDA and mGluR5 subtypes in particular - and adenosine A(2A) receptors are those most likely involved in LIDs. The aim of the present review is to summarize results of studies undertaken with specific antagonists of these receptors, first conducted in animal models of LIDs, which in selected cases have been translated into clinical trials.

EXPERT OPINION

Selected antagonists of glutamate and adenosine receptors have been proposed as anti-dyskinetic agents. Promising results have been obtained in preclinical investigations and in initial clinical trials, but long-term safety, tolerability and efficacy studies in patients are still required. The current development of novel antagonists, including tools able to act on receptor mosaics, may provide innovative tools for LIDs management in the next future.

Inflammation in Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disease that is characterized by the degeneration of dopaminergic neurons in the substantia nigra pars compacta. Inflammatory responses manifested by glial reactions, T cell infiltration, and increased expression of inflammatory cytokines, as well as other toxic mediators derived from activated glial cells, are currently recognized as prominent features of PD. The consistent findings obtained by various animal models of PD suggest that neuroinflammation is an important contributor to the pathogenesis of the disease and may further propel the progressive loss of nigral dopaminergic neurons. Furthermore, although it may not be the primary cause of PD, additional epidemiological, genetic, pharmacological, and imaging evidence support the proposal that inflammatory processes in this specific brain region are crucial for disease progression. Recent in vitro studies, however, have suggested that activation of microglia and subsequently astrocytes via mediators released by injured dopaminergic neurons is involved. However, additional in vivo experiments are needed for a deeper understanding of the mechanisms involved in PD pathogenesis. Further insight on the mechanisms of inflammation in PD will help to further develop alternative therapeutic strategies that will specifically and temporally target inflammatory processes without abrogating the potential benefits derived by neuroinflammation, such as tissue restoration.

Adenosine A(2A) Receptor Antagonists and Parkinson's Disease

This Review summarizes and updates the work on adenosine A(2A) receptor antagonists for Parkinson's disease from 2006 to the present. There have been numerous publications, patent applications, and press releases within this time frame that highlight new medicinal chemistry approaches to this attractive and promising target to treat Parkinson's disease. The Review is broken down by scaffold type and will discuss the efforts to optimize particular scaffolds for activity, pharmacokinetics, and other drug discovery parameters. The majority of approaches focus on preparing selective A(2A) antagonists, but a few approaches to dual A(2A)/A(1) antagonists will also be highlighted. The in vivo profiles of compounds will be highlighted and discussed to compare activities across different chemical series. A clinical report and update will be given on compounds that have entered clinical trials.

Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A(2A) receptors (A(2A)R) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D₂ and adenosine A(2A) receptors in the basal ganglia. At present it is believed that A(2A)R antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A(2A)R antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D₂ receptors (D₂Rs) expressed in the striatum are known to form heteromers with A(2A) adenosine receptors. Thus, the development of heteromer-specific A(2A) receptor antagonists represents a promising strategy for the identification of more selective and safer drugs.

Molecular docking studies of 1-(substituted phenyl)-3-(naphtha [1, 2-d] thiazol-2-yl) urea/thiourea derivatives with human adenosine A(2A) receptor

Computational assessment of the binding interactions of drugs is an important component of computer-aided drug design paradigms. In this perspective, a set of 30 1-(substituted phenyl)-3-(naphtha[1, 2-d] thiazol-2-yl) urea/thiourea derivatives showing antiparkinsonian activity were docked into inhibitor binding cavity of human adenosine A(2A) receptor (AA2AR) to understand their mode of binding interactions in silico. Lamarckian genetic algorithm methodology was employed for docking simulations using AutoDock 4.2 program. The results signify that the molecular docking approach is reliable and produces a good correlation coefficient (r(2) = 0.483) between docking score and antiparkinsonian activity (in terms of % reduction in catalepsy score). Potent antiparkinsonian agents carried methoxy group in the phenyl ring, exhibited both hydrophilic and lipophilic interactions with lower energy of binding at the AA(2A)R. These molecular docking analyses should, in our view, contribute for further development of selective AA(2A)R antagonists for the treatment of Parkinson's disease.

Preladenant, a selective A(2A) receptor antagonist, is active in primate models of movement disorders

Parkinson's Disease (PD) and Extrapyramidal Syndrome (EPS) are movement disorders that result from degeneration of the dopaminergic input to the striatum and chronic inhibition of striatal dopamine D(2) receptors by antipsychotics, respectively. Adenosine A(2A) receptors are selectively localized in the basal ganglia, primarily in the striatopallidal ("indirect") pathway, where they appear to operate in concert with D(2) receptors and have been suggested to drive striatopallidal output balance. In cases of dopaminergic hypofunction, A(2A) receptor activation contributes to the overdrive of the indirect pathway. A(2A) receptor antagonists, therefore, have the potential to restore this inhibitor imbalance. Consequently, A(2A) receptor antagonists have therapeutic potential in diseases of dopaminergic hypofunction such as PD and EPS. Targeting the A(2A) receptor may also be a way to avoid the issues associated with direct dopamine agonists. Recently, preladenant was identified as a potent and highly selective A(2A) receptor antagonist, and has produced a significant improvement in motor function in rodent models of PD. Here we investigate the effects of preladenant in two primate movement disorder models. In MPTP-treated cynomolgus monkeys, preladenant (1 or 3 mg/kg; PO) improved motor ability and did not evoke any dopaminergic-mediated dyskinetic or motor complications. In Cebus apella monkeys with a history of chronic haloperidol treatment, preladenant (0.3-3.0 mg/kg; PO) delayed the onset of EPS symptoms evoked by an acute haloperidol challenge. Collectively, these data support the use of preladenant for the treatment of PD and antipsychotic-induced movement disorders.

Caffeine and a selective adenosine A2A receptor antagonist induce sensitization and cross-sensitization behavior associated with increased striatal dopamine in mice

Background

Caffeine, a nonselective adenosine A₁ and A_2A receptor antagonist, is the most widely used psychoactive substance in the world. Evidence demonstrates that caffeine and selective adenosine A_2A antagonists interact with the neuronal systems involved in drug reinforcement, locomotor sensitization, and therapeutic effect in Parkinson's disease (PD). Evidence also indicates that low doses of caffeine and a selective adenosine A_2A antagonist SCH58261 elicit locomotor stimulation whereas high doses of these drugs exert locomotor inhibition. Since these behavioral and therapeutic effects are mediated by the mesolimbic and nigrostriatal dopaminergic pathways which project to the striatum, we hypothesize that low doses of caffeine and SCH58261 may modulate the functions of dopaminergic neurons in the striatum.

Methods

In this study, we evaluated the neuroadaptations in the striatum by using reverse-phase high performance liquid chromatography (HPLC) to quantitate the concentrations of striatal dopamine and its metabolites, dihydroxylphenylacetic acid (DOPAC) and homovanilic acid (HVA), and using immunoblotting to measure the level of phosphorylation of tyrosine hydroxylase (TH) at Ser31, following chronic caffeine and SCH58261 sensitization in mice. Moreover, to validate further that the behavior sensitization of caffeine is through antagonism at the adenosine A_2A receptor, we also evaluate whether chronic pretreatment with a selective adenosine A_2A antagonist SCH58261 or a selective adenosine A₁ antagonist DPCPX can sensitize the locomotor stimulating effects of caffeine.

Results

Chronic treatments with low dose caffeine (10 mg/kg) or SCH58261 (2 mg/kg) increased the concentrations of dopamine, DOPAC and HVA, concomitant with increased TH phosphorylation at Ser31 and consequently enhanced TH activity in the striatal tissues in both caffeine- and SCH58261-sensitized mice. In addition, chronic caffeine or SCH58261 administration induced locomotor sensitization, and locomotor cross-sensitization to caffeine was observed following chronic treatment of mice with SCH58261 but not with DPCPX.

Conclusions

Our study demonstrated that low dosages of caffeine and a selective adenosine A_2A antagonist SCH58261 elicited locomotor sensitization and cross-sensitization, which were associated with elevated dopamine concentration and TH phosphorylation at Ser31 in the striatum. Blockade of adenosine A_2A receptor may play an important role in the striatal neuroadaptations observed in the caffeine-sensitized and SCH58261-sensitized mice.

A new ethyladenine antagonist of adenosine A(2A) receptors: behavioral and biochemical characterization as an antiparkinsonian drug

Adenosine A(2A) receptor antagonists have emerged as an attractive non-dopaminergic target in clinical trials aimed at evaluating improvement in motor deficits in Parkinson's disease (PD). Moreover, preclinical studies suggest that A(2A) receptor antagonists may slow the course of the underlying neurodegeneration of dopaminergic neurons. In this study, we evaluated the efficacy of the new adenosine A(2A) receptor antagonist 8-ethoxy-9-ethyladenine (ANR 94) in parkinsonian models of akinesia and tremor. In addition, induction of the immediate early gene zif-268, and neuroprotective and anti-inflammatory effects of ANR 94 were evaluated. ANR 94 was effective in reversing parkinsonian tremor induced by the administration of tacrine. ANR 94 also counteracted akinesia (stepping test) and sensorimotor deficits (vibrissae-elicited forelimb-placing test), as well as potentiating l-dopa-induced contralateral turning behavior in 6-hydroxydopamine (6-OHDA) lesion model of PD. Potentiation of motor behavior in 6-OHDA-lesioned rats was not associated with increased induction of the immediate early gene zif-268 in the striatum, suggesting that ANR 94 does not induce long-term plastic changes in this structure. Finally, in a subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD, ANR 94 protected nigrostriatal dopaminergic neurons from degeneration and counteracted neuroinflammatory processes by contrasting astroglial (glial fibrillary acidic protein, GFAP) and microglial (CD11b) activation. A(2A) receptor antagonism represents a uniquely realistic opportunity for improving PD treatment, since A(2A) receptor antagonists offer substantial symptomatic benefits and possibly disease-modifying activity. The characterization of ANR 94 may represent a further therapeutic opportunity for the treatment of PD with this new class of drugs.

Dual-target-directed drugs that block monoamine oxidase B and adenosine A(2A) receptors for Parkinson's disease

Inadequacies of the current pharmacotherapies to treat Parkinson's disease (PD) have prompted efforts to identify novel drug targets. The adenosine A(2A) receptor is one such target. Antagonists of this receptor (A(2A) antagonists) are considered promising agents for the symptomatic treatment of PD. Evidence suggests that A(2A) antagonists may also have neuroprotective properties that may prevent the development of the dyskinesia that often complicates levodopa treatment. Because the therapeutic benefits of A(2A) antagonists are additive to that of dopamine replacement therapy, it may be possible to reduce the dose of the dopaminergic drugs and therefore the occurrence of side effects. Inhibitors of monoamine oxidase (MAO)-B also are considered useful tools for the treatment of PD. When used in combination with levodopa, inhibitors of MAO-B may enhance the elevation of dopamine levels after levodopa treatment, particularly when used in early stages of the disease when dopamine production may not be so severely compromised. Furthermore, MAO-B inhibitors may also possess neuroprotective properties in part by reducing the damaging effect of dopamine turnover in the brain. These effects of MAO-B inhibitors are especially relevant when considering that the brain shows an age-related increase in MAO-B activity. Based on these observations, dual-target-directed drugs, compounds that inhibit MAO-B and antagonize A(2A) receptors, may have value in the management of PD. This review summarizes recent efforts to develop such dual-acting drugs using caffeine as the lead compound.

The effects of systemic, intrastriatal, and intrapallidal injections of caffeine and systemic injections of A2A and A1 antagonists on forepaw stepping in the unilateral 6-OHDA-lesioned rat

RATIONALE AND OBJECTIVES

Given that adenosine A2A antagonists appear to be therapeutic in several animal models of Parkinson's disease (PD), we examined the extent to which caffeine and selective A2A and A1 antagonists could enhance contralateral forepaw stepping in the unilateral 6-OHDA-lesioned rat.

MATERIALS AND METHODS

Following unilateral injections of 12 microg 6-OHDA into the medial forebrain bundle (MFB), frequency of stepping with both front paws was counted separately as the paws were dragged anteriorally and laterally by a treadmill.

RESULTS

The MFB lesions decreased contralateral stepping by 74-83%, and 8 mg/kg 3,4-dihydroxy-L-phenylalanine (L-DOPA) increased contralateral stepping by 25-26%. Caffeine given systemically (15 mg/kg) or into the dorsal striatum or external globus pallidus (GPE; 20-40 microg) increased contralateral forepaw stepping by 14%, 27%, and 26%, respectively, and enhanced the effect of 8 mg/kg L-DOPA on stepping. The selective A(2A) antagonist SCH-58261 (2 mg/kg) also increased stepping by 13% and enhanced the therapeutic effect of L-DOPA, whereas the selective A(1) [corrected] antagonist 8-cyclopentyltheophylline (3-7 mg/kg) and A(1) agonist N(6)-cyclopentyladenosine (0.03-0.2 mg/kg) had no effect. None of these drugs appeared to produce dyskinesic effects.

CONCLUSIONS

In this well-validated animal model of the akinesic effects of PD, caffeine and a selective A2A, but not an A1, antagonist were able to provide both monotherapeutic and adjunctive therapeutic effects. These data are consistent with the hypothesis that A2A antagonists may be therapeutic in human PD patients and indicate that the dorsal striatum and GPE are critical sites of therapeutic action.

Adenosine A(2A) receptors in Parkinson's disease treatment

Latest results on the action of adenosine A(2A) receptor antagonists indicate their potential therapeutic usefulness in the treatment of Parkinson's disease. Basal ganglia possess high levels of adenosine A(2A) receptors, mainly on the external surfaces of neurons located at the indirect tracts between the striatum, globus pallidus, and substantia nigra. Experiments with animal models of Parkinson's disease indicate that adenosine A(2A) receptors are strongly involved in the regulation of the central nervous system. Co-localization of adenosine A(2A) and dopaminergic D2 receptors in striatum creates a milieu for antagonistic interaction between adenosine and dopamine. The experimental data prove that the best improvement of mobility in patients with Parkinson's disease could be achieved with simultaneous activation of dopaminergic D2 receptors and inhibition of adenosine A(2A) receptors. In animal models of Parkinson's disease, the use of selective antagonists of adenosine A(2A) receptors, such as istradefylline, led to the reversibility of movement dysfunction. These compounds might improve mobility during both monotherapy and co-administration with L-DOPA and dopamine receptor agonists. The use of adenosine A(2A) receptor antagonists in combination therapy enables the reduction of the L-DOPA doses, as well as a reduction of side effects. In combination therapy, the adenosine A(2A) receptor antagonists might be used in both moderate and advanced stages of Parkinson's disease. The long-lasting administration of adenosine A(2A) receptor antagonists does not decrease the patient response and does not cause side effects typical of L-DOPA therapy. It was demonstrated in various animal models that inhibition of adenosine A(2A) receptors not only decreases the movement disturbance, but also reveals a neuroprotective activity, which might impede or stop the progression of the disease. Recently, clinical trials were completed on the use of istradefylline (KW-6002), an inhibitor of adenosine A(2A) receptors, as an anti-Parkinson drug.

A novel adenosine A(1) and A(2A) receptor antagonist ASP5854 ameliorates motor impairment in MPTP-treated marmosets: comparison with existing anti-Parkinson's disease drugs

Recent evidence indicates that adenosine A(2A) receptor antagonists hold therapeutic potential for the treatment of Parkinson's disease (PD). A study on the novel adenosine A(1) and A(2A) receptor dual antagonist 5-[5-amino-3-(4-fluorophenyl)pyrazin-2-yl]-1-isopropylpyridine-2(1H)-one (ASP5854) showed it to be effective in various rodents models of PD and cognition. In the present study, we further investigated the potential of ASP5854 as an anti-PD drug using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated common marmosets, which is a highly predictive model of clinical efficacy in PD, and compared its effect with those of existing anti-PD drugs. ASP5854 significantly and dose-dependently improved the total motor disability score for 7h at doses higher than 1mg/kg, and significantly increased total locomotor activity at doses higher than 0.1mg/kg without adverse effects. l-3,4-Dihydroxyphenylalanine+benserazide and bromocriptine also significantly improved the motor disability score and the hypolocomotion caused by MPTP treatment in a dose-dependent fashion. This amelioration was significant at 32+8 and 10-32 mg/kg, respectively, although bromocriptine induced severe emesis. Trihexiphenidyl also significantly improved the total motor disability score at doses of 10-32 mg/kg; however, while a significant increase in the total locomotor activity was observed at 10mg/kg, the drug induced ataxia-like behavior at 32 mg/kg. On the other hand, neither selegiline nor amantadine improved the total motor disability and hypolocomotion. These data substantiate the evidence that the novel adenosine antagonist ASP5854 exerts comparable anti-PD activity with existing anti-PD drugs, which indicates that ASP5854 might have potential to ameliorate motor deficits in PD.

Role of adenosine A2A receptors in parkinsonian motor impairment and l-DOPA-induced motor complications

Adenosine A2A receptors have a unique cellular and regional distribution in the basal ganglia, being particularly concentrated in areas richly innervated by dopamine such as the caudate-putamen and the globus pallidus. Adenosine A2A receptors are selectively located on striatopallidal neurons and are capable of forming functional heteromeric complexes with dopamine D2 and metabotropic glutamate mGlu5 receptors. Based on the unique cellular and regional distribution of this receptor and in line with data showing that A2A receptor antagonists improve motor symptoms in animal models of Parkinson's disease (PD) and in initial clinical trials, A2A receptor antagonists have emerged as an attractive non-dopaminergic target to improve the motor deficits that characterize PD. Experimental data have also shown that A2A receptor antagonists do not induce neuroplasticity phenomena that complicate long-term dopaminergic treatments. The present review provides an updated summary of results reported in the literature concerning the biochemical characteristics and basal ganglia distribution of A2A receptors. We subsequently aim to examine the effects of adenosine A2A antagonists in rodent and primate models of PD and of l-DOPA-induced dyskinesia. Finally, concluding remarks are made on post-mortem human brains and on the translation of adenosine A2A receptor antagonists in the treatment of PD.

Characterization of the antiparkinsonian effects of the new adenosine A2A receptor antagonist ST1535: Acute and subchronic studies in rats

Antagonism of adenosine A2A receptor function has been proposed as an effective therapy in the treatment of Parkinson's disease. Thus, the study of new adenosine receptor antagonists is of great importance for the potential use of these drugs in clinical practice. The present study evaluated effects of the new preferential adenosine A2A receptor antagonist 2-butyl-9-methyl-8-(2H-1,2,3-triazol-2-yl)-9H-purin-6-ylamine (ST1535) in unilaterally 6-hydroxydopamine lesioned rats. Acute ST1535 dose-dependently potentiated contralateral turning behaviour induced by a threshold dose of l-3,4-dihydroxyphenylalanine (L-DOPA) (3 mg/kg i.p.), a classical test for antiparkinson drug screening. Subchronic (18 days, twice a day) ST1535 (20 mg/kg i.p.)+L-DOPA (3 mg/kg i.p.) did not induce sensitization to turning behaviour or abnormal involuntary movements during the course of treatment, indicating a low dyskinetic potential of the drug. Moreover, while subchronic administration of a fully effective dose of L-DOPA (6 mg/kg i.p.) significantly increased GABA synthesizing enzyme glutamic acid decardoxylase (GAD67), dynorphin and enkephalin mRNA levels in the lesioned striatum, subchronic ST1535 (20 mg/kg i.p.)+L-DOPA (3 mg/kg i.p.) did not modify any of these markers, although it induced a similar number of contralateral rotations at the beginning of treatment. Finally, acute administration of ST1535 (20 mg/kg i.p.) proved capable of reducing jaw tremors in tacrine model of Parkinson's disease tremor. Results showed that ST1535, in association with a low dose of L-DOPA, displayed antiparkinsonian activity similar to that produced by a full dose of L-DOPA without exacerbating abnormal motor side effects. Moreover, in agreement to other well characterized adenosine A2A receptor antagonists, ST1535 features antitremorigenic effects.

Adenosine A2A receptor antagonists improve deficits in initiation of movement and sensory motor integration in the unilateral 6-hydroxydopamine rat model of Parkinson's disease

Evidence obtained in rodent and primate models of Parkinson's disease (PD) and preliminary clinical trials, indicates that adenosine A(2A) receptor antagonists might represent a promising nondopaminergic therapeutic tool for the treatment of PD. Those studies demonstrated the ability of adenosine A(2A) receptor antagonists to potentiate l-dopa-mediated motor improvement, whereas very little is known about counteraction of specific motor deficits and on the effects of these compounds when administered alone. To this aim we evaluated the effects of different adenosine A(2A) receptor antagonists on initiation of movement deficits, gait impairment and sensory-motor deficits, induced in rats by a unilateral 6-hydroxydopamine lesion of dopaminergic nigrostriatal neurons. The following tests were used: (1) initiation time of stepping; (2) adjusting step (stepping with forelimb was measured as the forelimb was dragged laterally); (3) vibrissae-elicited forelimb placing (as index of sensory-motor integration deficits). Acute administration of the A(2A) receptor antagonists SCH 58261 (5 mg/kg i.p.) and ST 1535 (20 mg/kg i.p.) similarly to l-dopa (6 mg/kg i.p.) counteracted the impairments in the initiation time of stepping test, in the adjusting step and in the vibrissae-elicited forelimb placing induced by the lesion. The intensity of the effect was l-dopa > SCH 58261 > ST 1535. The results provide the first evidence that blockade of A(2A) receptors is effective in antagonizing specific motor deficit induced by DA neuron degeneration, such as initiation of movement and sensory-motor integration deficits, even without l-dopa combined administration.