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

Novel and experimental therapeutics for the management of motor and non-motor Parkinsonian symptoms

BACKGROUND  : Both motor and non-motor symptoms of Parkinson's disease (PD) have a substantial detrimental influence on the patient's quality of life. The most effective treatment remains oral levodopa. All currently known treatments just address the symptoms; they do not completely reverse the condition.

METHODOLOGY

In order to find literature on the creation of novel treatment agents and their efficacy for PD patients, we searched PubMed, Google Scholar, and other online libraries.

RESULTS

According to the most recent study on Parkinson's disease (PD), a great deal of work has been done in both the clinical and laboratory domains, and some current scientists have even been successful in developing novel therapies for PD patients.

CONCLUSION

The quality of life for PD patients has increased as a result of recent research, and numerous innovative medications are being developed for PD therapy. In the near future, we will see positive outcomes regarding PD treatment.

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.

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.

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

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

An Update on Nondopaminergic Treatments for Motor and Non-motor Symptoms of Parkinson's Disease

Nondopaminergic neurotransmitters such as adenosine, norepinephrine, serotonin, glutamate, and acetylcholine are all involved in Parkinson's disease (PD) and promote its symptoms. Therefore, nondopaminergic receptors are key targets for developing novel preparations for the management of motor and non-motor symptoms in PD, without the potential adverse events of dopamine replacement therapy. We reviewed English-written articles and ongoing clinical trials of nondopaminergic treatments for PD patients till 2014 to summarize the recent findings on nondopaminergic preparations for the treatment of PD patients. The most promising research area of nondopaminergic targets is to reduce motor complications caused by traditional dopamine replacement therapy, including motor fluctuations and levodopa-induced dyskinesia. Istradefylline, Safinamide, and Zonisamide were licensed for the management of motor fluctuations in PD patients, while novel serotonergic and glutamatergic agents to improve motor fluctuations are still under research. Sustained- release agents of Amantadine were approved for treating levodopa induced dyskinesia (LID), and serotonin 5HT1B receptor agonist also showed clinical benefits to LID. Nondopaminergic targets were also being explored for the treatment of non-motor symptoms of PD. Pimavanserin was approved globally for the management of hallucinations and delusions related to PD psychosis. Istradefylline revealed beneficial effect on daytime sleepiness, apathy, depression, and lower urinary tract symptoms in PD subjects. Droxidopa may benefit orthostatic hypotension in PD patients. Safinamide and Zonisamide also showed clinical efficacy on certain non-motor symptoms of PD patients. Nondopaminergic drugs are not expected to replace dopaminergic strategies, but further development of these drugs may lead to new approaches with positive clinical implications.

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.

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.

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.

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.

Piperazine- and Piperidine-Containing Thiazolo[5,4-d]pyrimidine Derivatives as New Potent and Selective Adenosine A2A Receptor Inverse Agonists

The therapeutic use of A_2A adenosine receptor (AR) antagonists for the treatment of neurodegenerative disorders, such as Parkinson and Alzheimer diseases, is a very promising approach. Moreover, the potential therapeutic role of A_2A AR antagonists to avoid both immunoescaping of tumor cells and tumor development is well documented. Herein, we report on the synthesis and biological evaluation of a new set of piperazine- and piperidine- containing 7-amino-2-(furan-2-yl)thiazolo[5,4-d]pyrimidine derivatives designed as human A_2A AR antagonists/inverse agonists. Binding and potency data indicated that a good number of potent and selective hA_2A AR inverse agonists were found. Amongst them, the 2-(furan-2-yl)-N⁵-(2-(4-phenylpiperazin-1-yl)ethyl)thiazolo[5,4-d]pyrimidine-5,7-diamine 11 exhibited the highest A_2A AR binding affinity (K_i = 8.62 nM) as well as inverse agonist potency (IC₅₀ = 7.42 nM). In addition, bioinformatics prediction using the web tool SwissADME revealed that 8, 11, and 19 possessed good drug-likeness profiles.

Emerging therapies in Parkinson disease - repurposed drugs and new approaches

Parkinson disease (PD) treatment options have conventionally focused on dopamine replacement and provision of symptomatic relief. Current treatments cause undesirable adverse effects, and a large unmet clinical need remains for treatments that offer disease modification and that address symptoms resistant to levodopa. Advances in high-throughput drug screening methods for small molecules, developments in disease modelling and improvements in analytical technologies have collectively contributed to the emergence of novel compounds, repurposed drugs and new technologies. In this Review, we focus on disease-modifying and symptomatic therapies under development for PD. We review cellular therapies and repurposed drugs, such as nilotinib, inosine, isradipine, iron chelators and anti-inflammatories, and discuss how their success in preclinical models has paved the way for clinical trials. We provide an update on immunotherapies and vaccines. In addition, we review non-pharmacological interventions targeting motor symptoms, including gene therapy, adaptive deep brain stimulation (DBS) and optogenetically inspired DBS. Given the many clinical phenotypes of PD, individualization of therapy and precision of treatment are likely to become important in the future.

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.

Therapeutic strategies for Parkinson disease: beyond dopaminergic drugs

Existing therapeutic strategies for managing Parkinson disease (PD), which focus on addressing the loss of dopamine and dopaminergic function linked with degeneration of dopaminergic neurons, are limited by side effects and lack of long-term efficacy. In recent decades, research into PD pathophysiology and pharmacology has focused on understanding and tackling the neurodegenerative processes and symptomology of PD. In this Review, we discuss the challenges associated with the development of novel therapies for PD, highlighting emerging agents that aim to target cell death, as well as new targets offering a symptomatic approach to managing features and progression of the disease.

An mGlu4-Positive Allosteric Modulator Alleviates Parkinsonism in Primates

BACKGROUND

Levodopa remains the gold-standard treatment for PD. However, it becomes less effective as the disease progresses and produces debilitating side effects, such as motor fluctuations and l-dopa-induced dyskinesia. Modulation of metabotropic glutamate receptor 4 represents a promising antiparkinsonian approach in combination with l-dopa, but it has not been demonstrated in primates.

OBJECTIVE

We studied whether a novel positive allosteric modulator of the metabotropic glutamate receptor 4, PXT002331 (foliglurax), could reduce parkinsonism in primate models.

METHODS

We assessed the therapeutic potential of PXT002331 in three models of MPTP-induced parkinsonism in macaques. These models represent three different stages of disease evolution: early stage and advanced stage with and without l-dopa-induced dyskinesia.

RESULTS

As an adjunct to l-dopa, PXT002331 induced a robust and dose-dependent reversal of parkinsonian motor symptoms in macaques, including bradykinesia, tremor, posture, and mobility. Moreover, PXT002331 strongly decreased dyskinesia severity, thus having therapeutic efficacy on both parkinsonian motor impairment and l-dopa-induced dyskinesia. PXT002331 brain penetration was also assessed using PET imaging in macaques, and pharmacodynamic analyses support target engagement in the therapeutic effects of PXT002331.

CONCLUSIONS

This work provides a demonstration that a positive allosteric modulator of metabotropic glutamate receptor 4 can alleviate the motor symptoms of PD and the motor complications induced by l-dopa in primates. PXT002331 is the first compound of its class to enter phase IIa clinical trials. © 2018 International Parkinson and Movement Disorder Society.

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.

Repurposing drugs to treat l-DOPA-induced dyskinesia in Parkinson's disease

In this review, we discuss the opportunity for repurposing drugs for use in l-DOPA-induced dyskinesia (LID) in Parkinson's disease. LID is a particularly suitable indication for drug repurposing given its pharmacological diversity, translatability of animal-models, availability of Phase II proof-of-concept (PoC) methodologies and the indication-specific regulatory environment. A compound fit for repurposing is defined as one with appropriate human safety-data as well as animal safety, toxicology and pharmacokinetic data as found in an Investigational New Drug (IND) package for another indication. We first focus on how such repurposing candidates can be identified and then discuss development strategies that might progress such a candidate towards a Phase II clinical PoC. We discuss traditional means for identifying repurposing candidates and contrast these with newer approaches, especially focussing on the use of computational and artificial intelligence (AI) platforms. We discuss strategies that can be categorised broadly as: in vivo phenotypic screening in a hypothesis-free manner; in vivo phenotypic screening based on analogy to a related disorder; hypothesis-driven evaluation of candidates in vivo and in silico screening with a hypothesis-agnostic component to the selection. To highlight the power of AI approaches, we describe a case study using IBM Watson where a training set of compounds, with demonstrated ability to reduce LID, were employed to identify novel repurposing candidates. Using the approaches discussed, many diverse candidates for repurposing in LID, originally envisaged for other indications, will be described that have already been evaluated for efficacy in non-human primate models of LID and/or clinically. This article is part of the Special Issue entitled 'Drug Repurposing: old molecules, new ways to fast track drug discovery and development for CNS disorders'.

A2A Adenosine Receptor Gene Deletion or Synthetic A2A Antagonist Liberate Tumor-Reactive CD8+ T Cells from Tumor-Induced Immunosuppression

Tumor hypoxia-driven accumulation of extracellular adenosine was shown to facilitate tumor evasion by engaging the immunosuppressive, intracellular cAMP-elevating A₂ adenosine receptors (A₂R) on tumor-reactive effector T cells, but there remains a need for careful evaluation of the limiting factors and properties of A₂R blockade-enabled antitumor immunity. In studies of A_2AR and/or A_2BR gene-deficient mice, we found that A_2AR deletion-but not A_2BR deletion-liberates endogenous CD8⁺ T cell antitumor immunity against weakly immunogenic MCA205 sarcomas. Studies of adoptively transferred A_2AR^-/-, A_2BR^-/-, or A_2AR^-/-/A_2BR^-/- tumor-reactive T cells confirmed that immunosuppression in the tumor microenvironment was mediated by A_2AR on CD8⁺ T cells. Treatment with A_2AR antagonist mimicked A_2AR gene deletion in adoptive T cell immunotherapy. This therapeutic benefit of targeting A_2AR was independent of the anatomical location of tumor growth. The enhanced antitumor reactivity also led to the eradication of established intracranial tumors, which was associated with mouse survival and the maintenance of long-lasting, tumor-specific immunological memory. The blockade of the A_2AR on adoptively transferred T cells by synthetic A_2AR antagonist led to higher levels of IFN-γ secretion by tumor-infiltrating CD8⁺ T cells. These data clarify the mechanism of hypoxia-driven immunosuppression in the tumor microenvironment by A_2AR on tumor-reactive CD8⁺ T cells and show that selective A_2AR antagonists can be effective in improving the outcomes of T cell-based immunotherapies. Demonstration of the T cell dose dependency of tumor rejection points to a major limitation of current cancer immunotherapies, in which the presence of sufficient numbers of tumor-reactive T cells in a patient is not known.

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.

Neurotoxins as Preclinical Models for Parkinson's Disease

Translational medicine is one of the major concerns in this century. While significant advances have been made with scientific knowledge, the translation of their promising results has not led to any new therapies. In Parkinson's disease, a long list of clinical studies, based on preclinical models with exogenous neurotoxins, has failed. Therefore, the aim of this opinion paper is to open discussion about preclinical models for Parkinson's disease based on neurotoxins.

Serotonergic Approaches in Parkinson's Disease: Translational Perspectives, an Update

Parkinson's disease (PD) has long been seen as a disorder caused by degeneration of the dopaminergic system, leading to the classic motor manifestations of the disease. However, there is now overwhelming evidence that PD is more than a disease merely caused by dopamine depletion. It is well-known that a myriad of other neurotransmitters are affected by the disease process. One such neurotransmitter is serotonin (5-HT). 5-HT has been shown to play a role in several motor and nonmotor manifestations of PD, including tremor, cognition, depression and psychosis. 5-HT also seems to play a critical role in L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia. A breadth of preclinical studies and clinical trials have been conducted that aimed at modulating the 5-HT system in order to alleviate depression, cognitive deficits, psychosis, and dyskinesia. In this Review, we summarize recent advances in the 5-HT field in PD, but with a translational emphasis. We start by presenting a novel nonhuman primate model of PD that presents with dual dopamine and 5-HT lesions. We then present preclinical and clinical data that introduce new concepts, such as the use of biased and partial agonists, as well as molecules recently introduced to the field of PD, such as eltoprazine, pimavanserin, nelotanserin, and SYN-120, to enhance therapeutic benefit while minimizing adverse events, notably on parkinsonian disability.

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).

Adenosine A2A Receptor Gene Knockout Prevents l-3,4-Dihydroxyphenylalanine-Induced Dyskinesia by Downregulation of Striatal GAD67 in 6-OHDA-Lesioned Parkinson's Mice

l-3,4-Dihydroxyphenylalanine (l-DOPA) remains the primary pharmacological agent for the symptomatic treatment of Parkinson’s disease (PD). However, the development of l-DOPA-induced dyskinesia (LID) limits the long-term use of l-DOPA for PD patients. Some data have reported that adenosine A_2A receptor (A_2AR) antagonists prevented LID in animal model of PD. However, the mechanism in which adenosine A_2AR blockade alleviates the symptoms of LID has not been fully clarified. Here, we determined to knock out (KO) the gene of A_2AR and explored the possible underlying mechanisms implicated in development of LID in a mouse model of PD. A_2AR gene KO mice were unilaterally injected into the striatum with 6-hydroxydopamine (6-OHDA) in order to damage dopamine neurons on one side of the brain. 6-OHDA-lesioned mice were then injected once daily for 21 days with l-DOPA. Abnormal involuntary movements (AIMs) were evaluated on days 3, 8, 13, and 18 after l-DOPA administration, and real-time polymerase chain reaction and immunohistochemistry for glutamic acid decarboxylase (GAD) 65 and GAD67 were performed. We found that A_2AR gene KO was effective in reducing AIM scores and accompanied with decrease of striatal GAD67, rather than GAD65. These results demonstrated that the possible mechanism involved in alleviation of AIM symptoms by A_2AR gene KO might be through reducing the expression of striatal GAD67.

mGlu5, Dopamine D2 and Adenosine A2A Receptors in L-DOPA-induced Dyskinesias

Patients with Parkinson's disease (PD) receiving L-3,4-dihydroxyphenylalanine (L-DOPA, the gold-standard treatment for this disease) frequently develop abnormal involuntary movements, termed L-DOPA-induced dyskinesias (LID). Glutamate overactivity is well documented in PD and LID. An approach to manage LID is to add to L-DOPA specific agents to reduce dyskinesias such as metabotropic glutamate receptor (mGlu receptor) drugs. This article reviews the contribution of mGlu type 5 (mGlu5) receptors in animal models of PD. Several mGlu5 negative allosteric modulators acutely attenuate LID in 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP) monkeys and 6-hydroxydopamine(6-OHDA)-lesioned rats. Chronic administration of mGlu5 negative allosteric modulators to MPTP monkeys and 6-OHDA rats also attenuates LID while maintaining the antiparkinsonian effect of L-DOPA. Radioligand autoradiography shows an elevation of striatal mGlu5 receptors of dyskinetic L-DOPA-treated MPTP monkeys but not in those without LID. The brain molecular correlates of the long-term effect of mGlu5 negative allosteric modulators treatments with L-DOPA attenuating development of LID was shown to extend beyond mGlu5 receptors with normalization of glutamate activity in the basal ganglia of L-DOPA-induced changes of NMDA, AMPA, mGlu2/3 receptors and VGlut2 transporter. In the basal ganglia, mGlu5 receptor negative allosteric modulators also normalize the L-DOPA-induced changes of dopamine D2receptors, their associated signaling proteins (ERK1/2 and Akt/GSK3β) and neuropeptides (preproenkephalin, preprodynorphin) as well as the adenosine A2A receptors expression. These results show in animal models of PD reduction of LID with mGlu5 negative allosteric modulation associated with normalization of glutamate, dopamine and adenosine receptors suggesting a functional link of these receptors in chronic treatment with L-DOPA.

Synthesis and in vivo Evaluation of Fluorine-18 and Iodine-123 Pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine Derivatives as PET and SPECT Radiotracers for Mapping A2A Receptors

Imaging agents that target adenosine type 2A (A2A ) receptors play an important role in evaluating new pharmaceuticals targeting these receptors, such as those currently being developed for the treatment of movement disorders like Parkinson's disease. They are also useful for monitoring progression and treatment efficacy by providing a noninvasive tool to map changes in A2A receptor density and function in neurodegenerative diseases. We previously described the successful evaluation of two A2A -specific radiotracers in both nonhuman primates and in subsequent human clinical trials: [(123) I]MNI-420 and [(18) F]MNI-444. Herein we describe the development of both of these radiotracers by selection from a series of A2A ligands, based on the pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine core of preladenant. Each of this series of 16 ligands was found to bind to recombinant human A2A receptor in the low nanomolar range, and of these 16, six were radiolabeled with either fluorine-18 or iodine-123 and evaluated in nonhuman primates. These initial in vivo results resulted in the identification of 7-(2-(4-(4-(2-[(18) F]fluoroethoxy)phenyl)piperazin-1-yl)ethyl)-2-(furan-2-yl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine ([(18) F]MNI-444) and 7-(2-(4-(2-fluoro-4-[(123) I]iodophenyl)piperazin-1-yl)ethyl)-2-(furan-2-yl)-7H-imidazo[1,2-c]pyrazolo[4,3-e]pyrimidin-5-amine ([(123) I]MNI-420) as PET and SPECT radiopharmaceuticals for mapping A2A receptors in brain.

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.

Drug therapy for Parkinson’s disease: An update

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder, affecting about 1% of the population above the age of 65. PD is characterized by a selective degeneration of the dopaminergic neurons of the substantia nigra pars compacta. This results in a marked loss of striatal dopamine and the development of the characteristic features of the disease, i.e., bradykinesia, rest tremor, rigidity, gait abnormalities and postural instability. Other types of neurons/neurotransmitters are also involved in PD, including cholinergic, serotonergic, glutamatergic, adenosine, and GABAergic neurotransmission which might have relevance to the motor, non-motor, neuropsychiatric and cognitive disturbances that occur in the course of the disease. The treatment of PD relies on replacement therapy with levodopa (L-dopa), the precursor of dopamine, in combination with a peripheral decarboxylase inhibitor (carbidopa or benserazide). The effect of L-dopa, however, declines over time together with the development of motor complications especially dyskinesia in a significant proportion of patients within 5 years of therapy. Other drugs include dopamine-receptor-agonists, catechol-O-methyltransferase inhibitors, monoamine oxidase type B (MAO-B) inhibitors, anticholinergics and adjuvant therapy with the antiviral drug and the N-methyl-D-aspartate glutamate receptor antagonist amantadine. Although, these medications can result in substantial improvements in parkinsonian symptoms, especially during the early stages of the disease, they are often not successful in advanced disease. Moreover, dopaminergic cell death continues over time, emphasizing the need for neuroprotective or neuroregenerative therapies. In recent years, research has focused on non-dopaminergic approach such as the use of A_2A receptor antagonists: istradefylline and preladenant or the calcium channel antagonist isradipine. Safinamide is a selective and reversible inhibitor of MAO-B, a glutamate receptor inhibitor as well as sodium and calcium channel blocker. Minocycline and pioglitazone are other agents which have been shown to prevent dopaminergic nigral cell loss in animal models of PD. There is also an evidence to suggest a benefit from iron chelation therapy with deferiprone and from the use of antioxidants or mitochondrial function enhancers such as creatine, alpha-lipoic acid, l-carnitine, and coenzyme Q10.

Pharmacological strategies for the management of levodopa-induced dyskinesia in patients with Parkinson's disease

L-Dopa-induced dyskinesias (LID) are the most common adverse effects of long-term dopaminergic therapy in Parkinson's disease (PD). However, the exact mechanisms underlying dyskinesia are still unclear. For a long time, nigrostriatal degeneration and pulsatile stimulation of striatal postsynaptic receptors have been highlighted as the key factors for the development of LID. In recent years, PD models have revealed a wide range of non-dopaminergic neurotransmitter systems involved in pre- and postsynaptic changes and thereby contributing to the pathophysiology of LID. In the current review, we focus on therapeutic LID targets, mainly based on agents acting on dopaminergic, glutamatergic, serotoninergic, adrenergic, and cholinergic systems. Despite a large number of clinical trials, currently only amantadine and, to a lesser extent, clozapine are being used as effective strategies in the treatment of LID in clinical settings. Thus, in the second part of the article, we review the placebo-controlled trials on LID treatment in order to disentangle the changing scenario of drug development. Promising results include the extension of L-dopa action without inducing LID of the novel monoamine oxidase B- and glutamate-release inhibitor safinamide; however, this had no obvious effect on existing LID. Others, like the metabotropic glutamate-receptor antagonist AFQ056, showed promising results in some of the studies; however, confirmation is still lacking. Thus, to date, strategies of continuous dopaminergic stimulation seem the most promising to prevent or ameliorate LID. The success of future therapeutic strategies once moderate to severe LID occur will depend on the translation from preclinical experimental models into clinical practice in a bidirectional process.

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.

Human abuse liability evaluation of CNS stimulant drugs

Psychoactive drugs that increase alertness, attention and concentration and energy, while also elevating mood, heart rate and blood pressure are referred to as stimulants. Despite some overlapping similarities, stimulants cannot be easily categorized by their chemical structure, mechanism of action, receptor binding profile, effects on monoamine uptake, behavioral pharmacology (e.g., effects on locomotion, temperature, and blood pressure), therapeutic indication or efficacy. Because of their abuse liability, a pre-market assessment of abuse potential is required for drugs that show stimulant properties; this review article focuses on the clinical aspects of this evaluation. This includes clinical trial adverse events, evidence of diversion or tampering, overdoses and the results of a human abuse potential study. While there are different types of human experimental studies that can be employed to evaluate stimulant abuse potential (e.g., drug discrimination, self-administration), only the human abuse potential study and clinical trial adverse event data are required for drug approval. The principal advances that have improved human abuse potential studies include using study enrichment strategies (pharmacologic qualification), larger sample sizes, better selection of endpoints and measurement strategies and more carefully considered interpretation of data. Because of the methodological advances, comparisons of newer studies with historical data is problematic and may contribute to a biased regulatory framework for the evaluation of newer stimulant-like drugs, such as A2 antagonists. This article is part of the Special Issue entitled 'CNS Stimulants'.

MitoPark mice, an animal model of Parkinson's disease, show enhanced prepulse inhibition of acoustic startle and no loss of gating in response to the adenosine A(2A) antagonist SCH 412348

RATIONALE

Psychoses are debilitating side effects associated with current dopaminergic treatments for Parkinson's disease (PD). Prepulse inhibition (PPI), in which a non-startling stimulus reduces startle response to a subsequent startle-eliciting stimulus, is important in filtering out extraneous sensory stimuli. PPI deficits induced by dopamine agonists can model symptoms of psychosis. Adenosine A(2A) receptor antagonists, being developed as novel PD treatments, indirectly modulate dopamine signaling in the basal ganglia and may have an improved psychosis profile which could be detected using the PPI model.

OBJECTIVES

The aims of this study is to characterize PPI in MitoPark mice, which exhibit progressive loss of dopamine signaling and develop a Parkinson-like motor phenotype, and assess standard and novel PD treatment effects on PPI in MitoPark mice, which more closely mimic the basal ganglia dopamine status of PD patients.

RESULTS

MitoPark mice displayed enhanced PPI as dopamine tone decreased with age, consistent with studies in intact mice that show enhanced PPI in response to dopamine antagonists. Paradoxically, older MitoParks were more sensitive to PPI disruption when challenged with dopamine agonists such as apomorphine or pramipexole. Alternatively, SCH 412348, an adenosine A(2A) antagonist, did not disrupt PPI in MitoPark mice at doses that normalized hypoactivity.

CONCLUSION

Use of MitoPark mice in the PPI assay to assess the potential for PD treatment to produce psychoses likely represents a more disease-relevant model. SCH 412348 does not differentially disrupt PPI as do dopamine agonists, perhaps indicative of an improved psychosis profile of adenosine A(2A) antagonists, even in PD patients with decreased dopamine tone in the basal ganglia.

Effects of the selective adenosine A2A receptor antagonist, SCH 412348, on the parkinsonian phenotype of MitoPark mice

Adenosine A2A receptors are predominantly localized on striatopallidal gamma-aminobutyric acid (GABA) neurons, where they are colocalized with dopamine D2 receptors and are involved in the regulation of movement. Adenosine A2A receptor antagonists have been evaluated as a novel treatment for Parkinson's disease and have demonstrated efficacy in a broad spectrum of pharmacological and toxicological rodent and primate models. Fewer studies have been performed to evaluate the efficacy of adenosine A2A receptor antagonists in genetic models of hypodopaminergic states. SCH 412348 is a potent and selective adenosine A2A receptor antagonist that shows efficacy in rodent and primate models of movement disorders. Here we evaluated the effects of SCH 412348 in the MitoPark mouse, a genetic model that displays a progressive loss of dopamine neurons. The dopamine cell loss is associated with a profound akinetic phenotype that is sensitive to levodopa (l-dopa). SCH 412348 (0.3-10mg/kg administered orally) dose dependently increased locomotor activity in the mice. Moreover, SCH 412348 retained its efficacy in the mice as motor impairment progressed (12-22 weeks of age), demonstrating that the compound was efficacious in mild to severe Parkinson's disease-like impairment in the mice. Additionally, SCH 412348 fully restored lost functionality in a measure of hind limb bradykinesia and partially restored functionality in a rotarod test. These findings provide further evidence of the anti-Parkinsonian effects of selective adenosine A2A receptor antagonists and predict that they will retain their efficacy in both mild and severe forms of motor impairment.

Promise of pharmacogenomics for drug discovery, treatment and prevention of Parkinson's disease. A perspective

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by a heterogeneous array of motor and non-motor features. Anti-PD drugs that are in use target only the motor symptoms, may lose efficacy over time, and can cause serious adverse effects such as dyskinesia and psychosis. There are currently no preventative or disease modifying treatments. All attempts to develop disease modifying drugs have failed. Pharmacogenomics (PGx) has the potential to change the way new drugs are developed and the way drugs are prescribed. By using genetic markers that correlate with, and can therefore predict drug response, clinical trials can be designed to be enriched with individuals who are most likely to benefit from the drug, maximizing drug's efficacy, minimizing its adverse effects, and boosting the odds of successful drug discovery. Clinical application of PGx will help physicians to quickly and accurately determine the right drugs and the right doses for individuals, avoiding the lengthy trial and error approaches and adverse effects. In combination with known protective factors such as nicotine and caffeine, PGx may enable development of personalized methods for PD prevention and, by extension, care.

Moonlighting proteins and protein-protein interactions as neurotherapeutic targets in the G protein-coupled receptor field

There is serious interest in understanding the dynamics of the receptor-receptor and receptor-protein interactions in space and time and their integration in GPCR heteroreceptor complexes of the CNS. Moonlighting proteins are special multifunctional proteins because they perform multiple autonomous, often unrelated, functions without partitioning into different protein domains. Moonlighting through receptor oligomerization can be operationally defined as an allosteric receptor-receptor interaction, which leads to novel functions of at least one receptor protomer. GPCR-mediated signaling is a more complicated process than previously described as every GPCR and GPCR heteroreceptor complex requires a set of G protein interacting proteins, which interacts with the receptor in an orchestrated spatio-temporal fashion. GPCR heteroreceptor complexes with allosteric receptor-receptor interactions operating through the receptor interface have become major integrative centers at the molecular level and their receptor protomers act as moonlighting proteins. The GPCR heteroreceptor complexes in the CNS have become exciting new targets for neurotherapeutics in Parkinson's disease, schizophrenia, drug addiction, and anxiety and depression opening a new field in neuropsychopharmacology.

Interaction of adenosine receptors with other receptors from therapeutic perspective in Parkinson's disease

Altered dopaminergic neurotransmission in the basal ganglia is observed in Parkinson's disease (PD) and L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesias (LID). An attractive alternative for treating LID is to use adjunct drugs to modulate nondopaminergic neurotransmitter systems in the basal ganglia. For example, adenosine receptors have received attention over the past years for the treatment of PD and LID. Adenosine interacts closely with dopamine and plays an important role in the function of striatal GABAergic efferent neurons. Excitatory glutamatergic neurotransmission is also modulated by adenosine in the striatum. Hence, based on the unique cellular and regional distribution of this system, adenosine neurotransmission could have an important implication for the development of new therapeutic strategies targeting the basal ganglia disorders. Indeed, A2A adenosine receptor antagonists were shown to improve motor deficits in PD and to reduce the severity of LID. A2A receptor subtypes are selectively found on striatopallidal neurons and can couple with receptors of interest in PD, such as D2 dopamine and metabotropic glutamate receptor type 5 (mGlu5) receptors, and form functional heteromeric complexes. This chapter will review relevant studies investigating the role and contribution of adenosine receptor subtypes in pathophysiology of PD and LID. The interactions of adenosine receptors, especially A1 and A2A receptor subtypes, with other receptors implicated in the pathophysiology of PD and LID such as dopaminergic and glutamatergic receptors will be reviewed. The implication of these interactions in the development and expression of PD symptoms and LID needs further investigation to find novel drug targets.

Preladenant: an adenosine A2A receptor antagonist for Parkinson’s disease

Preladenant (SCH 420814) is a potent selective antagonist at the adenosine A2A receptor that is being studied for treatment in early Parkinson’s disease (PD) as a monotherapy, and in moderate-to-severe PD as an add on to levodopa therapy. Unlike other drugs used for this disease, preladenant modulates adenosine action at the striatal level in order to block the inhibitory action of the basal ganglia output nuclei. Animal models of PD suggested that preladenant could be an effective treatment, which was further supported in a Phase II study of subjects with idiopathic PD who demonstrated a benefit in reducing off-time with an increase in on-time. In this article, we review current perspectives concerning pharmacological approaches to PD, the pharmacological properties of preladenant, its efficiency and safety, as well as the results reported for parkinsonian subjects treated with this drug.

Levodopa-induced dyskinesias in Parkinson's disease: emerging treatments

Parkinson's disease therapy is still focused on the use of L-3,4-dihydroxyphenylalanine (levodopa or L-dopa) for the symptomatic treatment of the main clinical features of the disease, despite intensive pharmacological research in the last few decades. However, regardless of its effectiveness, the long-term use of levodopa causes, in combination with disease progression, the development of motor complications termed levodopa-induced dyskinesias (LIDs). LIDs are the result of profound modifications in the functional organization of the basal ganglia circuitry, possibly related to the chronic and pulsatile stimulation of striatal dopaminergic receptors by levodopa. Hence, for decades the key feature of a potentially effective agent against LIDs has been its ability to ensure more continuous dopaminergic stimulation in the brain. The growing knowledge regarding the pathophysiology of LIDs and the increasing evidence on involvement of nondopaminergic systems raises the possibility of more promising therapeutic approaches in the future. In the current review, we focus on novel therapies for LIDs in Parkinson's disease, based mainly on agents that interfere with glutamatergic, serotonergic, adenosine, adrenergic, and cholinergic neurotransmission that are currently in testing or clinical development.

A2A receptor antagonists do not induce dyskinesias in drug-naive or L-dopa sensitized rats

L-dopa, the precursor to dopamine, is currently the gold standard treatment for Parkinson's disease (PD). However, chronic exposure is associated with L-dopa-induced dyskinesias (LIDs), a serious side effect characterized by involuntary movements. Adenosine A2A receptor antagonists have been studied as a novel non-dopaminergic PD treatment. Because A2A receptor antagonists do not act on dopamine receptors, it has been hypothesized that they will not induce dyskinesias characteristic of L-dopa. To test this hypothesis in a rodent model, the A2A receptor antagonists SCH 412348 (3 mg/kg), vipadenant (10 mg/kg), caffeine (30 mg/kg), or istradefylline (3 mg/kg) were chronically (19-22 days) administered to Sprague Dawley rats, and dyskinetic behaviors were scored across this chronic dosing paradigm. Unlike L-dopa, there was no evidence of dyskinetic activity resulting from any of the four A2A receptor antagonists tested. When delivered to animals previously sensitized with L-dopa (6 mg/kg), SCH 412348, vipadenant, caffeine or istradefylline treatment produced no dyskinesias. When administered in combination with L-dopa (6 mg/kg), SCH 412348 (3 mg/kg) neither exacerbated nor prevented the induction of LIDs over the course of 19 days of treatment. Collectively, our data indicate that A2A receptor antagonists are likely to have a reduced dyskinetic liability relative to L-dopa but do not block dyskinesias when coadministered with L-dopa. Clinical studies are required to fully understand the dyskinesia profiles of A2A receptor antagonists.

In vitro pharmacological profile of the A2A receptor antagonist istradefylline

Adenosine A2A receptors are suggested to be a promising non-dopaminergic target for the treatment of Parkinson's disease (PD). Istradefylline is an adenosine A2A receptor antagonist that has been reported to exhibit antiparkinsonian activities in PD patients as well as both rodents and nonhuman primate models of PD. The aim of this study was to evaluate the in vitro pharmacological profile of istradefylline as an A2A receptor antagonist. Istradefylline exhibited high affinity for A2A receptors in humans, marmosets, dogs, rats, and mice. The affinities for the other subtypes of adenosine receptors (A1, A2B, and A3) were lower than that for A2A receptors in each species. Istradefylline demonstrated no significant affinity for other neurotransmitter receptors, including dopamine receptors (D1, D2, D3, D4, and D5). In addition, istradefylline hardly inhibited monoamine oxidase-A, monoamine oxidase-B, or catechol-O-methyl transferase. A kinetic analysis indicated that istradefylline reversibly binds to the human A2A receptors: The association reached equilibrium within 1 min, and the binding was also almost completely dissociated within 1 min. Istradefylline inhibited the A2A agonist CGS21680-induced accumulation of cAMP in the cultured cells and then shifted the concentration-response curve of CGS21680 to the right without affecting the maximal response of the agonist. These results indicate that istradefylline is a potent, selective, and competitive A2A receptor antagonist. The in vitro pharmacological profile of istradefylline helps to explain the in vivo profile of istradefylline and may be useful for clinical pharmacokinetic-pharmacodynamic considerations of efficacy and safety.