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

New and emerging medical therapies in Parkinson's disease

INTRODUCTION

Parkinson's disease (PD) is one of the most challenging neurodegenerative disorders to treat as it manifests with a large variety of troublesome, and often disabling, motor and non-motor symptoms. Despite limitations, such as motor and other complications, levodopa remains the most effective drug in the treatment of PD.

AREAS COVERED

In this review, we focus on phase 2 and 3 studies describing new and emerging medical therapies in PD. We discuss new formulations of levodopa, medications that prolong levodopa response and ameliorate levodopa-induced dyskinesias, and innovative delivery methods that are currently being evaluated in clinical trials or are in development with the promise of better efficacy and tolerability. We also describe novel non-dopaminergic drugs that have been identified for treatment of motor and non-motor symptoms. A specific section is designated for potential disease modifying therapies.

EXPERT OPINION

Alternative formulations of levodopa appear to be promising especially to help with the motor fluctuations either by providing sustained benefits with controlled released formulations or ameliorate sudden OFF by formulations such as inhaled levodopa. Several different medications affecting non-dopaminergic pathways are being evaluated which may aide levodopa. As the understanding of the disease grows further, numerous novel neuroprotective or disease modifying therapies have been suggested. This along with development of medications to treat various non-motor symptoms will help improve quality of life of patients with PD.

Clinical efficacy of istradefylline versus rTMS on Parkinson's disease in a randomized clinical trial

OBJECTIVE

To compare the efficacy of istradefylline (20 mg/day, 40 mg/day) and repetitive transcranial magnetic stimulation (rTMS) (1 Hz, 10 Hz) as an adjunct therapy to levodopa in the treatment of Parkinson's disease (PD).

METHODS

A total of 132 PD patients from China were randomly assigned to receive 20 mg/day istradefylline plus sham-rTMS (Group I), 40 mg/day istradefylline plus sham-rTMS (Group II), placebo plus 1 Hz rTMS (Group III) and placebo plus 10 Hz rTMS (Group IV) for 12 weeks. Unified Parkinson's Disease Rating Scale (UPDRS) part III score was the primary outcome. Clinical Global Impression-Global Improvement (CGI-I) was the secondary outcome. The change in daily off time in Groups I and II was also recorded.

RESULTS

After 12 weeks of treatment, the changes in UPDRS part III score were -6.05, -6.39, -5.91 and -6.46 for Groups I, II, III and IV, respectively, and the difference was not significant. The difference in CGI-I among the four groups was not significant. The daily off time was reduced by -1.43 hours in Group I and -1.62 hours in Group II. No severe adverse events occurred among the four groups.

CONCLUSION

These results indicate that, as augmentation agents to levodopa in the treatment of PD, istradefylline and rTMS had comparable efficacy and tolerability.

Beneficial and detrimental role of adenosine signaling in diseases and therapy

Adenosine is a major signaling nucleoside that orchestrates cellular and tissue adaptation under energy depletion and ischemic/hypoxic conditions by activation of four G protein-coupled receptors (GPCR). The regulation and generation of extracellular adenosine in response to stress are critical in tissue protection. Both mouse and human studies reported that extracellular adenosine signaling plays a beneficial role during acute states. However, prolonged excess extracellular adenosine is detrimental and contributes to the development and progression of various chronic diseases. In recent years, substantial progress has been made to understand the role of adenosine signaling in different conditions and to clarify its significance during the course of disease progression in various organs. These efforts have and will identify potential therapeutic possibilities for protection of tissue injury at acute stage by upregulation of adenosine signaling or attenuation of chronic disease progression by downregulation of adenosine signaling. This review is to summarize current progress and the importance of adenosine signaling in different disease stages and its potential therapeutic effects.

Gastroretentive carbidopa/levodopa, DM-1992, for the treatment of advanced Parkinson's disease

OBJECTIVES

This study was undertaken to compare efficacy, tolerability, and pharmacokinetics of DM-1992, an extended-release formulation of carbidopa/levodopa (CD/L-dopa) with immediate-release (IR) CD/L-dopa in patients with advanced Parkinson's disease.

METHODS

This randomized, open-label, crossover study included a 3-d baseline and two 10-d treatment periods. Patients with daily OFF time of 2.5 h or more taking 400 mg or more L-dopa/d in four or more divided doses were titrated to stable regimens of DM-1992 2 times per day or CD/L-dopa IR 3 times to 8 times per day. Patients were allowed to take rescue CD/L-dopa as needed. Using home diaries, patients recorded OFF time and ON time with or without troublesome dyskinesia during baseline and treatment days 7 through 9. During 12-h clinic visits on day 10, plasma samples were collected for pharmacokinetics, and motor performance was assessed hourly.

RESULTS

Thirty-four patients were enrolled; mean baseline L-dopa dosage was 968 mg/d. After titration, CD/L-dopa IR was dosed 4.8 times per day and DM-1992, 2 times per day. Rescue CD/L-dopa IR was given 1.3 times during the DM-1992 arm and 0.2 times during the CD/L-dopa IR arm. The reduction from baseline in % OFF time was greater for DM-1992 compared with CD/L-dopa IR (-5.52% vs. +1.33%; P = 0.0471). At steady-state, compared with CD/L-dopa IR, DM-1992 exhibited a smoother plasma L-dopa concentration profile mostly because of a significantly higher (day 10) predose L-dopa concentration, associated with enhanced motor performance. Although more patients taking DM-1992 had one or more adverse events (AEs) than CD/L-dopa IR patients (35% vs. 15%), no pattern to the AEs was seen, nor any resulting discontinuations.

CONCLUSIONS

DM-1992 was associated with a reduction in %OFF time compared with CD/L-dopa IR despite a reduced dosing frequency. Although the open-label study design and the greater number of rescue doses during the DM-1992 arm call for caution in interpreting the results, the elevated predose plasma L-dopa concentration (12 h after DM-1992 administration) lends objective support to our findings, suggesting that phase 3 studies are warranted.

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.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

The safety of istradefylline for the treatment of Parkinson's disease

INTRODUCTION

Antagonism of the A2A receptor improves motor behavior in patients with Parkinson's disease (PD), according to results of clinical studies which confirm findings of previous experimental research. The xanthine derivative, istradefylline , has the longest half-life out of the available A2A receptor antagonists. Istradefylline easily crosses the blood-brain barrier and shows a high affinity to the human A2A receptor.

AREAS COVERED

This narrative review aims to discuss the safety and tolerability of istradefylline against the background of the currently available drug portfolio for the treatment of PD patients.

EXPERT OPINION

Istradefylline was safe and well tolerated in clinical trials, which have focused on l-DOPA-treated PD patients. The future of istradefylline as a complementary drug for modulation of the dopaminergic neurotransmission also relies on its potential to act like an l-DOPA plus dopamine agonist sparing future treatment alternative and to reduce the risk of predominant l-DOPA-related onset of motor complications in addition to its direct ameliorating effect on motor symptoms. Dopamine-substituting drugs may dose-dependently produce systemic side effects, particularly onset of hypotension and nausea by peripheral dopamine receptor stimulation. Istradefylline does not interfere with these peripheral receptors and therefore shows a good safety and tolerability profile.

Treatment of advanced Parkinson's disease

PURPOSE OF REVIEW

Later stage Parkinson's disease, sometimes referred to as advanced disease, has been characterized by motor complication, as well as by the potential emergence of nonlevodopa responsive motor and nonmotor symptoms. The management of advanced stage Parkinson's disease can be complex. This review summarizes the currently available treatment strategies for addressing advanced Parkinson's disease.

RECENT FINDINGS

We will discuss the latest pharmacological strategies (e.g., inhibitors of dopamine-metabolizing enzymes, dopamine agonists, and extended release dopamine formulations) for addressing motor dysfunction. We will summarize the risks and benefits of current invasive treatments. Finally, we will address the current evidence supporting the treatment of nonmotor symptoms in the advanced Parkinson's disease patient. We will conclude by detailing the potential nonpharmacological and multidisciplinary approaches for advanced stage Parkinson's disease.

SUMMARY

The optimization of levodopa is, in most cases, the most powerful therapeutic option available; however, medication optimization requires an advanced understanding of Parkinson's disease. Failure of conventional pharmacotherapy should precipitate a discussion of the potential risks and benefits of more invasive treatments. Currently, there are no comparative studies of invasive treatment. Among the invasive treatments, deep brain stimulation has the largest amount of existing evidence, but also has the highest individual per patient risk. Nonmotor symptoms will affect quality of life more than the motor Parkinson's disease symptoms, and these nonmotor symptoms should be aggressively treated. Many advanced Parkinson's disease patients will likely benefit from multi and interdisciplinary Parkinson's disease teams with multiple professionals collaborating to develop a collective and tailored strategy for an individual patient.

Unprecedented therapeutic potential with a combination of A2A/NR2B receptor antagonists as observed in the 6-OHDA lesioned rat model of Parkinson's disease

In Parkinson's disease, the long-term use of dopamine replacing agents is associated with the development of motor complications; therefore, there is a need for non-dopaminergic drugs. This study evaluated the potential therapeutic impact of six different NR2B and A2A receptor antagonists given either alone or in combination in unilateral 6-OHDA-lesioned rats without (monotherapy) or with (add-on therapy) the co-administration of L-Dopa: Sch-58261+ Merck 22; Sch-58261+Co-101244; Preladenant + Merck 22; Preladenant + Radiprodil; Tozadenant + Radiprodil; Istradefylline + Co-101244. Animals given monotherapy were assessed on distance traveled and rearing, whereas those given add-on therapy were assessed on contralateral rotations. Three-way mixed ANOVA were conducted to assess the main effect of each drug separately and to determine whether any interaction between two drugs was additive or synergistic. Additional post hoc analyses were conducted to compare the effect of the combination with the effect of the drugs alone. Motor activity improved significantly and was sustained for longer when the drugs were given in combination than when administered separately at the same dose. Similarly, when tested as add-on treatment to L-Dopa, the combinations resulted in higher levels of contralateral rotation in comparison to the single drugs. Of special interest, the activity observed with some combinations could not be described by a simplistic additive effect and involved more subtle synergistic pharmacological interactions. The combined administration of A2A/NR2B-receptor antagonists improved motor behaviour in 6-OHDA rats. Given the proven translatability of this model such a combination may be expected to be effective in improving motor symptoms in patients.

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.

Tozadenant (SYN115) in patients with Parkinson's disease who have motor fluctuations on levodopa: a phase 2b, double-blind, randomised trial

BACKGROUND

Many patients with Parkinson's disease have motor fluctuations despite treatment with available drugs. Tozadenant (SYN115) is an oral, selective adenosine A2A receptor antagonist that improves motor function in animal models of Parkinson's disease. We aimed to assess the safety and efficacy of tozadenant as an adjunct to levodopa in patients with Parkinson's disease who have motor fluctuations on levodopa.

METHODS

We did an international, multicentre, phase 2b, randomised, double-blind, placebo-controlled, parallel-group, dose-finding clinical trial of tozadenant in levodopa-treated patients with Parkinson's disease who had motor fluctuations (at least 2·5 h off-time per day). Eligible patients were randomly assigned via a computer-generated randomisation schedule to receive tozadenant 60, 120, 180, or 240 mg or matching placebo twice daily for 12 weeks. All study management, site personnel, and patients were masked to treatment assignment. The primary outcome was change from baseline to week 12 in hours per day spent in the off-state (assessed from Parkinson's disease diaries completed by patients). This study is registered at ClinicalTrials.gov, number NCT01283594.

FINDINGS

Of 420 randomised patients (mean age 63·3 [SD 8·3] years; mean duration of Parkinson's disease 8·7 [4·7] years), 403 provided post-baseline diary data and 337 completed study treatment. Compared with placebo, mean daily off-time was significantly reduced in the combined tozadenant 120 mg twice-daily and 180 mg twice-daily group (-1·1 h, 95% CI -1·8 to -0·5; p=0·0006), the tozadenant 120 mg twice-daily group (-1·1 h, -1·8 to -0·4; p=0.0039), and the tozadenant 180 mg twice-daily group (-1·2 h, -1·9 to -0·4; p=0·0039). The most common adverse events in these groups were dyskinesia (seven [8%] of 84 patients in the placebo group, 13 [16%] of 82 in the 120 mg twice-daily group, and 17 [20%] of 85 in the 180 mg twice-daily group), nausea (three [4%], 9 [11%], and ten [12%]), and dizziness (one [1%], four [5%], and 11 [13%]). Tozadenant 60 mg twice daily was not associated with a significant reduction in off-time, and tozadenant 240 mg twice daily was associated with an increased rate of discontinuation because of adverse events (17 [20%] of 84 patients).

INTERPRETATION

Tozadenant at 120 or 180 mg twice daily was generally well tolerated and was effective at reducing off-time. Further investigation of tozadenant treatment in phase 3 trials is warranted.

FUNDING

Biotie Therapies.

Antidepressant activity of the adenosine A2A receptor antagonist, istradefylline (KW-6002) on learned helplessness in rats

RATIONALE

Istradefylline, an adenosine A2A receptor antagonist, improves motor function in animal models of Parkinson's disease (PD) and in patients with PD. In addition, some A2A antagonists exert antidepressant-like activity in rodent models of depression, such as the forced swim and the tail suspension tests.

OBJECTIVE

We have investigated the effect of istradefylline on depression-like behaviors using the rat learned helplessness (LH) model.

RESULTS

Acute, as well as chronic, oral administration of istradefylline significantly improved the inescapable shock (IES)-induced escape deficit with a degree of efficacy comparable to chronic treatment with the tricyclic antidepressant desipramine and the selective serotonin (5-HT) reuptake inhibitor, fluoxetine. Both the A1/A2A receptor nonspecific antagonist theophylline and the moderately selective antagonist CGS15943, but not the A1 selective antagonist DPCPX, ameliorated the IES-induced escape deficit. The enhancement of escape response by istradefylline was reversed by a local injection of the A2A specific agonist CGS21680 either into the nucleus accumbens, the caudate-putamen, or the paraventricular nucleus of the hypothalamus, but not by the A1 specific agonist R-PIA into the nucleus accumbens. Moreover, neither the 5-HT2A/2C receptor antagonist methysergide or the adrenergic α 2 antagonist yohimbine, nor the β-adrenergic antagonist propranolol, affected the improvement of escape response induced by istradefylline.

CONCLUSIONS

Istradefylline exerts antidepressant-like effects via modulation of A2A receptor activity which is independent of monoaminergic transmission in the brain. Istradefylline may represent a novel treatment option for depression in PD as well as for the motor symptoms.

Striatal adenosine A2A receptor expression is controlled by S-adenosyl-L-methionine-mediated methylation

Adenosine A2A receptor (A2AR) is a G protein-coupled receptor enriched in the striatum for which an increased expression has been demonstrated in certain neurological diseases. Interestingly, previous in vitro studies demonstrated that A2AR expression levels are reduced after treatment with S-adenosyl-L-methionine (SAM), a methyl donor molecule involved in the methylation of important biological structures such as DNA, proteins, and lipids. However, the in vivo effects of SAM treatment on A2AR expression are still obscure. Here, we demonstrated that 2 weeks of SAM treatment produced a significant reduction in the rat striatal A2AR messenger RNA (mRNA) and protein content as well as A2AR-mediated signaling. Furthermore, when the content of 5-methylcytosine levels in the 5'UTR region of ADORA2A was analyzed, this was significantly increased in the striatum of SAM-treated animals; thus, an unambiguous correlation between SAM-mediated methylation and striatal A2AR expression could be established. Overall, we concluded that striatal A2AR functionality can be controlled by SAM treatment, an issue that might be relevant for the management of these neurological conditions that course with increased A2AR expression.

Increased striatal adenosine A2A receptor levels is an early event in Parkinson's disease-related pathology and it is potentially regulated by miR-34b

Adenosine A2A receptor (A2AR) is a G-protein coupled receptor that stimulates adenylyl cyclase activity. In the brain, A2ARs are found highly enriched in striatal GABAergic medium spiny neurons, related to the control of voluntary movement. Pharmacological modulation of A2ARs is particularly useful in Parkinson's disease (PD) due to their property of antagonizing dopamine D2 receptor activity. Increases in A2AR levels have been described in PD patients showing an important loss of dopaminergic denervation markers, but no data have been reported about A2AR levels in incidental PD brains. In the present report, we show that increased A2ARs protein levels were also detected in the putamen of incidental PD cases (Braak PD stages 1-2) with respect to age-matched controls. By contrast, A2ARs mRNA levels remained unchanged, suggesting that posttranslational mechanisms could be involved in the regulation of A2ARs. It has been described how miR-34b/c downregulation is an early event in PD cases. We found that miR-34b levels are also significantly reduced in the putamen of incidental PD cases and along disease progression. Given that 3'UTR of A2AR contains a predicted target site for miR-34b, the potential role of this miRNA in protein A2AR levels was assessed. In vitro studies revealed that endogenous A2AR protein levels increased when miR-34b function was blocked using a specific anti-miR-34b. Moreover, using a luciferase reporter assay with point mutations in a miR-34b predicted binding site within the 3'UTR region of A2AR mRNA abolished the effect of the miRNA using a miR-34b mimic. In addition, we showed a reduced percentage of DNA methylation in the 5'UTR region of ADORA2A in advanced PD cases. Overall, these findings reveal that increased A2AR protein levels occur in asymptomatic PD patients and provide new insights into the molecular mechanisms underlying A2AR expression levels along the progression of this neurodegenerative disease.

Advances in non-dopaminergic treatments for Parkinson's disease

Since the 1960's treatments for Parkinson's disease (PD) have traditionally been directed to restore or replace dopamine, with L-Dopa being the gold standard. However, chronic L-Dopa use is associated with debilitating dyskinesias, limiting its effectiveness. This has resulted in extensive efforts to develop new therapies that work in ways other than restoring or replacing dopamine. Here we describe newly emerging non-dopaminergic therapeutic strategies for PD, including drugs targeting adenosine, glutamate, adrenergic, and serotonin receptors, as well as GLP-1 agonists, calcium channel blockers, iron chelators, anti-inflammatories, neurotrophic factors, and gene therapies. We provide a detailed account of their success in animal models and their translation to human clinical trials. We then consider how advances in understanding the mechanisms of PD, genetics, the possibility that PD may consist of multiple disease states, understanding of the etiology of PD in non-dopaminergic regions as well as advances in clinical trial design will be essential for ongoing advances. We conclude that despite the challenges ahead, patients have much cause for optimism that novel therapeutics that offer better disease management and/or which slow disease progression are inevitable.

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.

Adenosine A₂A-receptor antagonist istradefylline enhances the motor response of L-DOPA without worsening dyskinesia in MPTP-treated common marmosets

The adenosine A₂A-receptor antagonist istradefylline decreases OFF time in patients with Parkinson's disease who are already treated with optimal doses of dopaminergic medication but can cause an increase in non-troublesome dyskinesia. Preclinical experiments have shown that A₂A antagonists are most effective in potentiating motor function when combined with sub-maximal doses of L-DOPA. However, the effects of combining istradefylline with sub-optimal L-DOPA treatment on established dyskinesia have not been studied. We now examine the effects of acute and repeated administration of istradefylline on dyskinesia in MPTP-treated common marmosets previously primed to exhibit involuntary movements by prior exposure to L-DOPA. In these animals, single dose acute oral administration of istradefylline (10 mg/kg) enhanced and prolonged the anti-parkinsonian effects of a sub-optimal dose of L-DOPA (2.5 mg/kg). The chronic co-administration of istradefylline (10 mg/kg) with L-DOPA (2.5 mg/kg) for 21 days did not worsen the severity of existing dyskinesia. Rather, the severity of dyskinesia tended to be reduced over the 21-day treatment period. These results suggest that istradefylline can be used to potentiate the effects of sub-optimal doses of L-DOPA in the treatment of Parkinson's disease without causing or worsening dyskinesia.

Two new adenosine receptor antagonists for the treatment of Parkinson's disease: istradefylline versus tozadenant

INTRODUCTION

Adenosine A2A receptors are localized in the brain, mainly within the caudate and putamen nuclei of the basal ganglia. Their activation leads to stimulation of the 'indirect' pathway. Conversely, administration of A2A receptor antagonists leads to inhibition of this pathway, which was translated into reduced hypomotility in several animal models of parkinsonism.

AREAS COVERED

In this review, the effects of two A2A receptor antagonists, istradefylline and tozadenant, on parkinsonian symptoms in animal and humans will be discussed.

EXPERT OPINION

Animal studies have shown potent antiparkinsonian effects for several A2A receptor antagonists, including istradefylline. In clinical trials, istradefylline reduced OFF time when administered with levodopa, but results are inconclusive. Results with tozadenant are scarce. Modification of thalamic blood flow compatible with reduced inhibition was noted in one small trial, followed by a significant reduction in OFF time in a larger one. Therefore, both drugs show promising efficacy for the reduction of OFF time in levodopa-treated Parkinson's disease patients, but further research is needed in order to obtain definitive conclusions.

Adenosine receptors and dyskinesia in pathophysiology

First, the recent progress in the pathogenesis of levodopa-induced dyskinesia was described. Serotonin neurons play an important role in conversion from levodopa to dopamine and in the release of converted dopamine into the striatum in the Parkinsonian state. Since serotonin neurons lack buffering effects on synaptic dopamine concentration, the synaptic dopamine markedly fluctuates depending on the fluctuating levodopa concentration in the serum after taking levodopa. The resultant pulsatile stimulation makes the striatal direct-pathway neurons get potential that releases excessive GABA into the output nuclei of the basal ganglia. When levodopa is administered, the stored GABA is released, the output nuclei become hypoactive, and then dyskinesias emerge. Second, effects of adenosine A2A receptor antagonists on dyskinesia were described. It has been demonstrated that the expression of adenosine A2A receptors is increased in Parkinson's disease (PD) patients with dyskinesias, suggesting that blockade of A2A receptors is beneficial for dyskinesias. Preclinical studies have shown that A2A receptor antagonists reduce liability of dyskinesias in PD models. Clinical trials have demonstrated that A2A antagonists increase functional ON-time (ON without troublesome dyskinesia) in PD patients suffering from wearing-off phenomenon, although they may increase dyskinesia in patients with advanced PD.

Treatment of Parkinson's disease: what's in the non-dopaminergic pipeline?

Dopamine depletion resulting from degeneration of nigrostriatal dopaminergic neurons is the primary neurochemical basis of the motor symptoms of Parkinson's disease (PD). While dopaminergic replacement strategies are effective in ameliorating these symptoms early in the disease process, more advanced stages of PD are associated with the development of treatment-related motor complications and dopamine-resistant symptoms. Other neurotransmitter and neuromodulator systems are expressed in the basal ganglia and contribute to the extrapyramidal refinement of motor function. Furthermore, neuropathological studies suggest that they are also affected by the neurodegenerative process. These non-dopaminergic systems provide potential targets for treatment of motor fluctuations, levodopa-induced dyskinesias, and difficulty with gait and balance. This review summarizes recent advances in the clinical development of novel pharmacological approaches for treatment of PD motor symptoms. Although the non-dopaminergic pipeline has been slow to yield new drugs, further development will likely result in improved treatments for PD symptoms that are induced by or resistant to dopamine replacement.

Antidepressant-like activity of the adenosine A(2A) receptor antagonist, istradefylline (KW-6002), in the forced swim test and the tail suspension test in rodents

RATIONALE

Depression is common in Parkinson's disease (PD) but its response to classical antidepressants is not clear. The adenosine A2A antagonist istradefylline is effective in the treatment of the motor symptoms of PD but inhibition of the adenosine A2A receptor may also induce antidepressant-like effects.

OBJECTIVE

We have investigated whether istradefylline might be effective in treating depression in PD using the forced swimming test (FST) and the tail suspension test (TST) in rodents.

RESULTS

Istradefylline significantly decreased immobility time in the FST in both rats and mice (0.16mg/kg and higher) with comparable efficacy to an equivalent dose of the tricyclic antidepressants, desipramine and imipramine. Both 8-OH-DPAT (5-HT1A agonist) and quinpirole (D2 agonist) also reduced the immobility time. The istradefylline-induced reduction of immobility time was attenuated by corticosterone. In addition, the combined use of a sub-threshold dose of istradefylline and the serotonin-noradrenaline reuptake inhibitor venlafaxine ameliorated depression-like behavior in the mouse FST. In the mouse TST, istradefylline (0.08mg/kg and higher) decreased immobility time. Moreover, co-administration of istradefylline with paroxetine or fluoxetine (selective serotonin reuptake inhibitors) or deprenyl (MAO-B inhibitor) at doses that did not show antidepressant-like effects when administered alone, resulted in a significant reduction in immobility time.

CONCLUSIONS

Istradefylline alone or co-administered with currently available antidepressants, may be useful for the treatment of depression as well as motor symptoms of PD. Its effects might be, at least in part, attributable to modulation of hypothalamic-pituitary-adrenal axis.

Advances in the pharmacological treatment of Parkinson's disease: targeting neurotransmitter systems

For several decades, the dopamine precursor levodopa has been the primary therapy for Parkinson's disease (PD). However, not all of the motor and non-motor features of PD can be attributed solely to dopaminergic dysfunction. Recent clinical and preclinical advances provide a basis for the identification of additional innovative therapeutic options to improve the management of the disease. Novel pharmacological strategies must be optimized for PD by: (i) targeting disturbances of the serotonergic, noradrenergic, glutamatergic, GABAergic, and cholinergic systems in addition to the dopaminergic system, and (ii) characterizing alterations in the levels of neurotransmitter receptors and transporters that are associated with the various manifestations of the disease.

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.

Long-term safety and efficacy of preladenant in subjects with fluctuating Parkinson's disease

BACKGROUND

Preladenant is a selective adenosine A₂A receptor antagonist under investigation for Parkinson's disease treatment.

METHODS

A phase 2 36-week open-label follow-up of a double-blind study using preladenant 5 mg twice a day as a levodopa adjunct in 140 subjects with fluctuating Parkinson's disease was conducted. The primary end point was adverse event (AE) assessment. Secondary (efficacy) analyses included hours/day spent in OFF and ON states and dyskinesia prevalence/severity.

RESULTS

The 36-week open-label phase was completed by 106 of 140 subjects (76%). AE-related treatment discontinuations occurred in 19 subjects (14%). Treatment-emergent AEs, reported by ≥15% of subjects, were dyskinesia (33%) and constipation (19%). Preladenant 5 mg twice a day provided OFF time reductions (1.4-1.9 hours/day) and ON time increases (1.2-1.5 hours/day) throughout the 36-week treatment relative to the baseline of the double-blind study.

CONCLUSIONS

Long-term preladenant treatment (5 mg twice a day) was generally well tolerated and provided sustained OFF time reductions and ON time increases.

Neuroimaging over the course of Parkinson's disease: from early detection of the at-risk patient to improving pharmacotherapy of later-stage disease

Brain imaging of striatal dopamine terminal degeneration serves an important role in the clinical management of Parkinson's disease (PD). Imaging biomarkers for interrogating dopaminergic systems are used for clarifying diagnosis when only subtle motor symptoms are present. However, motor dysfunction is not the earliest symptom of PD. There is increasing interest in identifying premotor PD patients, particularly because potential disease-modifying therapies are developed and the clinical imperative becomes early and accurate diagnosis. On the other end of the spectrum of the disease course, during later stages of PD, significant clinical challenges like levo-dopa-induced dyskinesias and medication on-off phenomenon become more prevalent. In this instance, better understanding of altered PD motor pathways suggests the potential utility of novel treatments targeting neuronal systems that are impacted by degenerating dopamine neurons and chronic dopamine replacement treatment. Molecular neuroimaging serves unique roles in both very early PD and later-stage disease, in the former, potentially pushing back the time of diagnosis, and in the latter, elucidating pathology relevant to new drug development.

Adenosine receptors as drug targets--what are the challenges?

Adenosine signalling has long been a target for drug development, with adenosine itself or its derivatives being used clinically since the 1940s. In addition, methylxanthines such as caffeine have profound biological effects as antagonists at adenosine receptors. Moreover, drugs such as dipyridamole and methotrexate act by enhancing the activation of adenosine receptors. There is strong evidence that adenosine has a functional role in many diseases, and several pharmacological compounds specifically targeting individual adenosine receptors--either directly or indirectly--have now entered the clinic. However, only one adenosine receptor-specific agent--the adenosine A2A receptor agonist regadenoson (Lexiscan; Astellas Pharma)--has so far gained approval from the US Food and Drug Administration (FDA). Here, we focus on the biology of adenosine signalling to identify hurdles in the development of additional pharmacological compounds targeting adenosine receptors and discuss strategies to overcome these challenges.

Adenosine A2A receptor antagonist istradefylline reduces daily OFF time in Parkinson's disease

BACKGROUND

We evaluated the efficacy and safety of istradefylline, a selective adenosine A2A receptor antagonist administered as adjunctive treatment to levodopa for 12 weeks in a double-blind manner in Parkinson's disease patients with motor complications in Japan.

METHODS

A total of 373 subjects were randomized to receive placebo (n=126), istradefylline 20 mg/day (n=123), or istradefylline 40 mg/day (n=124). The primary efficacy variable was the change in daily OFF time. Other secondary variables were also evaluated.

RESULTS

The change in daily OFF time was significantly reduced in the istradefylline 20 mg/day (-0.99 hours, P=.003) and istradefylline 40 mg/day (-0.96 hours, P=.003) groups compared with the placebo group (-0.23 hours). The most common adverse event was dyskinesia (placebo, 4.0%; istradefylline 20 mg/day, 13.0%; istradefylline 40 mg/day, 12.1%).

CONCLUSIONS

Istradefylline reduced daily OFF time and was well tolerated in Japanese PD patients with motor complications on levodopa treatment.

Adenosine A2A antagonists in Parkinson's disease: what's next?

Parkinson's disease (PD) is the second most prevalent neurodegenerative disorder, affecting up to 10 million people worldwide. Current treatment primarily involves symptom management with dopaminergic replacement therapy. Levodopa remains the most effective oral treatment, although long-term use is associated with complications such as wearing off, dyskinesias, and on-off fluctuations. Non-dopaminergic medications that improve PD symptoms and motor fluctuations are in demand. Adenosine A2A receptors are abundantly expressed within the basal ganglia and offer a unique target to modify abnormal striatal signaling associated with PD. Preclinical animal models have shown the ability of adenosine A2A receptor antagonists to improve PD motor symptoms, reduce motor fluctuations and dyskinesia, as well as protect against toxin-induced neuronal degeneration. Both istradefylline and preladenant have demonstrated moderate efficacy in reducing off time in PD patients with motor fluctuations. The safety and efficacy of this class of compounds continues to be defined and future studies should focus on non-motor symptoms, dyskinesias, and neuroprotection.

Istradefylline, an adenosine A₂A receptor antagonist, for patients with Parkinson's Disease: a meta-analysis

OBJECTIVES

To assess the efficacy and safety of istradefylline as an adjunct to levodopa in patients with Parkinson's Disease (PD).

METHODS

In this study, we searched the Cochrane Library, MEDLINE, Embase, China Academic Journal Full-text Database (CNKI), China Biomedical Literature Database (CBM), Chinese Scientific Journals Database (VIP), and Wanfang Database. The quality of included studies was strictly evaluated. Data analyses were performed by the Cochrane Collaboration's RevMan5.0 software.

RESULTS

Five randomized controlled trials (RCTs) were included. The result showed a significant reduction of the awake time per day spent in the OFF state and improvement of the Unified Parkinson's Disease Rating Scale (UPDRS) Part III in the ON state when receiving istradefylline compared with patients receiving placebo. There was no significant difference between the istradefylline 20mg and the istradefylline 40 mg groups in the UPDRS Part III in the ON state (WMD=1.27, 95% CI [-0.40, 2.95]). The results showed significant differences in dyskinesia (RR=1.63, 95% CI [1.16, 2.29]) compared to istradefylline 40 mg with placebo. There was no significant statistical difference with regard to other adverse events.

CONCLUSIONS

The present study showed that istradefylline is safe and effective as an adjunct to levodopa in patients with PD. Future large-scale, higher-quality, long-treatment, and placebo-controlled trials are needed.

Clinical features, pathophysiology, and treatment of levodopa-induced dyskinesias in Parkinson's disease

Dyskinetic disorders are characterized by excess of motor activity that may interfere with normal movement control. In patients with Parkinson's disease, the chronic levodopa treatment induces dyskinetic movements known as levodopa-induced dyskinesias (LID). This paper analyzed the pathophysiology, clinical manifestations, pharmacological treatments, and surgical procedures to treat hyperkinetic disorders. Surgery is currently the only treatment available for Parkinson's disease that may improve both parkinsonian motor syndrome and LID. However, this paper shows the different mechanisms involved are not well understood.

Adenosine A2A receptor in the monkey basal ganglia: ultrastructural localization and colocalization with the metabotropic glutamate receptor 5 in the striatum

The adenosine A(2A) receptor (A(2A) R) is a potential drug target for the treatment of Parkinson's disease and other neurological disorders. In rodents, the therapeutic efficacy of A(2A) R modulation is improved by concomitant modulation of the metabotropic glutamate receptor 5 (mGluR5). To elucidate the anatomical substrate(s) through which these therapeutic benefits could be mediated, pre-embedding electron microscopy immunohistochemistry was used to conduct a detailed, quantitative ultrastructural analysis of A(2A) R localization in the primate basal ganglia and to assess the degree of A(2A) R/mGluR5 colocalization in the striatum. A(2A) R immunoreactivity was found at the highest levels in the striatum and external globus pallidus (GPe). However, the monkey, but not the rat, substantia nigra pars reticulata (SNr) also harbored a significant level of neuropil A(2A) R immunoreactivity. At the electron microscopic level, striatal A(2A) R labeling was most commonly localized in postsynaptic elements (58% ± 3% of labeled elements), whereas, in the GPe and SNr, the labeling was mainly presynaptic (71% ± 5%) or glial (27% ± 6%). In both striatal and pallidal structures, putative inhibitory and excitatory terminals displayed A(2A) R immunoreactivity. Striatal A(2A) R/mGluR5 colocalization was commonly found; 60-70% of A(2A) R-immunoreactive dendrites or spines in the monkey striatum coexpress mGluR5. These findings provide the first detailed account of the ultrastructural localization of A(2A) R in the primate basal ganglia and demonstrate that A(2A) R and mGluR5 are located to interact functionally in dendrites and spines of striatal neurons. Together, these data foster a deeper understanding of the substrates through which A(2A) R could regulate primate basal ganglia function and potentially mediate its therapeutic effects in parkinsonism.

Future treatments for Parkinson's disease: surfing the PD pipeline

Our current wish list for the treatment of Parkinson's disease (PD) includes therapies that will provide robust and sustained antiparkinsonian benefit through the day, ameliorate or prevent dyskinesia, and slow or prevent the progression of the disease. In this article, I review selected new therapies in clinical development for motor features or treatment complications of PD, and some that may slow disease progression. These include adenosine 2a (A2a) antagonists (istradefylline, preladenant, and SYN115), levodopa/carbidopa intestinal gel (LCIG), IPX066--an extended-release formulation of carbidopa/levodopa, XP21279--a sustained-release levodopa prodrug, ND0611--a carbidopa subcutaneous patch, safinamide--a mixed mechanism of action medication that may provide both MAO-B and glutamate inhibition, PMY50028--an oral neurotrophic factor inducer, antidyskinesia medications (AFQ056 and fipamezole), and gene therapies (AAV2-neurturin and glutamic acid decarboxylase gene transfer). Some of these therapies will never be proven efficacious and will not come to market while others may play a key role in the future treatment of PD.