Are adenosine antagonists, such as istradefylline, caffeine, and chocolate, useful in the treatment of Parkinson's disease?

Strategies for the Treatment of Parkinson's Disease: Beyond Dopamine

Parkinson’s disease (PD) is the second-leading cause of dementia and is characterized by a progressive loss of dopaminergic neurons in the substantia nigra alongside the presence of intraneuronal α-synuclein-positive inclusions. Therapies to date have been directed to the restoration of the dopaminergic system, and the prevention of dopaminergic neuronal cell death in the midbrain. This review discusses the physiological mechanisms involved in PD as well as new and prospective therapies for the disease. The current data suggest that prevention or early treatment of PD may be the most effective therapeutic strategy. New advances in the understanding of the underlying mechanisms of PD predict the development of more personalized and integral therapies in the years to come. Thus, the development of more reliable biomarkers at asymptomatic stages of the disease, and the use of genetic profiling of patients will surely permit a more effective treatment of PD.

Tribolium castaneum as a whole-animal screening system for the detection and characterization of neuroprotective substances

Parkinson's disease (PD) is a movement disorder caused by the progressive loss of dopaminergic neurons. Natural antioxidants and plant extracts with neuroprotective properties offer a promising new therapeutic approach for PD patients, but a suitable large-scale screening system is required for their discovery and preclinical analysis. Here we used the red flour beetle (Tribolium castaneum ) as a whole-animal screening system for the detection and characterization of neuroprotective substances. Paraquat was added to the diet of adult beetles to induce PD-like symptoms, which were quantified using a novel positive geotaxis behavioral assay. These paraquat-induced behavioral changes were reduced in beetles fed on diets supplemented with l-dihydroxyphenylalanine, ascorbic acid, curcumin, hempseed flour, or the Chinese herb gou-teng. T. castaneum is, therefore, a valuable model for the screening of neuroprotective substances in chemical libraries and plant extracts and could be developed as a model for the preclinical testing of therapeutic candidates for the treatment of neurodegenerative diseases, such as PD.

A(2A) adenosine receptors and Parkinson's disease severity

OBJECTIVES

In the last decade, increasing evidence suggests a key role of adenosine in Parkinson's disease (PD) and A2A adenosine receptors (A2A ARs) as an important pharmacological target in PD. An overexpression of A2A ARs has been found in putamen and in peripheral blood cells of PD patients. The primary aim of this study was to verify whether the alterations in A2A ARs in lymphocytes of PD subjects correlate with disease severity.

MATERIAL AND METHODS

A consecutive sample of PD patients was enrolled. A clinical examination and a face-to-face interview were carried out. A2A ARs were investigated to verify the affinity and receptor density in lymphocyte membranes. The data were compared with those found in healthy controls. Moreover, the correlation between A2A AR density and affinity and clinical variables was evaluated in PD patients.

RESULTS

In human lymphocyte membranes from PD patients, an increase in A2A AR density and a decrease in A2A AR affinity were found if compared with healthy subjects. A statistically significant correlation between the A2A AR density or affinity and specific clinical parameters as motor and cognitive impairment was detected. Patients with higher A2A AR density and lower affinity were more likely to exhibit motor complications.

CONCLUSIONS

Parkinson's disease patients show an A2A AR upregulation in lymphocyte membranes if compared with healthy subjects. The correlation found between A2A AR density or affinity and clinical parameters highlights the central role of A2A AR modulation in the pharmacological treatment for PD and could suggest the putative role of A2A AR as a candidate biomarker of PD severity.

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.

New small molecules for the treatment of Parkinson's disease

IMPORTANCE OF THE FIELD

Parkinson's disease (PD) is characterized by a slowly ongoing neuronal death, which affects neurotransmitter metabolism and causes a wide variety of motor and non-motor features. Until now, therapy approaches have predominantly focused on motor behavior associated with dopamine substitution.

AREAS COVERED IN THIS REVIEW

This review aims to discuss putative reasons for recent failures of investigated treatment approaches, and to introduce currently tested and future compounds.

WHAT THE READER WILL GAIN

We will describe how development programs of novel molecules now additionally consider non-motor features of PD as promising targets in order to obtain regulatory approval. Regulatory authorities increasingly exert influence on trial designs, demanding therapeutic effects that are not always clinically feasible given the variety of manifestations of the disease entity known as PD.

TAKE HOME MESSAGE

In the past, research pitfalls have resulted in the failure of promising new compounds. Among the many reasons for this are massive placebo responses; the participation of too many investigators, with consequent wide variations of efficacy assessments; and a misconception of preclinical drug development, with models of PD that do not mimic its clinical nature. A few compounds are now being tested that have modes of action indirectly modulating the dopamine system; however, critical analysis of the preclinical and clinical research concept and drug approval is warranted to prevent further frustration in this field.

Impacts of methylxanthines and adenosine receptors on neurodegeneration: human and experimental studies

Neurodegenerative disorders are some of the most feared illnesses in modern society, with no effective treatments to slow or halt this neurodegeneration. Several decades after the earliest attempt to treat Parkinson's disease using caffeine, tremendous amounts of information regarding the potential beneficial effect of caffeine as well as adenosine drugs on major neurodegenerative disorders have accumulated. In the first part of this review, we provide general background on the adenosine receptor signaling systems by which caffeine and methylxanthine modulate brain activity and their role in relationship to the development and treatment of neurodegenerative disorders. The demonstration of close interaction between adenosine receptor and other G protein coupled receptors and accessory proteins might offer distinct pharmacological properties from adenosine receptor monomers. This is followed by an outline of the major mechanism underlying neuroprotection against neurodegeneration offered by caffeine and adenosine receptor agents. In the second part, we discuss the current understanding of caffeine/methylxantheine and its major target adenosine receptors in development of individual neurodegenerative disorders, including stroke, traumatic brain injury Alzheimer's disease, Parkinson's disease, Huntington's disease and multiple sclerosis. The exciting findings to date include the specific in vivo functions of adenosine receptors revealed by genetic mouse models, the demonstration of a broad spectrum of neuroprotection by chronic treatment of caffeine and adenosine receptor ligands in animal models of neurodegenerative disorders, the encouraging development of several A(2A) receptor selective antagonists which are now in advanced clinical phase III trials for Parkinson's disease. Importantly, increasing body of the human and experimental studies reveals encouraging evidence that regular human consumption of caffeine in fact may have several beneficial effects on neurodegenerative disorders, from motor stimulation to cognitive enhancement to potential neuroprotection. Thus, with regard to neurodegenerative disorders, these potential benefits of methylxanthines, caffeine in particular, strongly argue against the common practice by clinicians to discourage regular human consumption of caffeine in aging populations.

Caffeine protects against disruptions of the blood-brain barrier in animal models of Alzheimer's and Parkinson's diseases

Sporadic Alzheimer's disease (AD) and Parkinson's disease (PD) are two of the most common neurodegenerative diseases and as such they represent major public health problems. Finding effective treatments for AD and PD represents an unmet and elusive goal largely because these diseases are chronic and progressive, and have a complicated and ill-understood pathogenesis. Although the underlying mechanisms are not fully understood, caffeine, the most commonly ingested psychoactive drug in the world, has been shown in human and animal studies to be protective against AD and PD. One mechanism implicated in the pathogenesis of AD and PD is blood-brain barrier (BBB) dysfunction and we reported recently that caffeine exerts protective effects against AD and PD at least in part by keeping the BBB intact. The present review focuses on the role of BBB dysfunction in the pathogenesis of AD and PD, caffeine's protective effects against AD and PD, and potential mechanisms whereby caffeine protects against BBB leakage.

A2A adenosine receptor overexpression and functionality, as well as TNF-alpha levels, correlate with motor symptoms in Parkinson's disease

The antagonistic interaction between adenosine and dopamine receptors could have important pathophysiological and therapeutic implications in Parkinson's disease (PD). The primary aim of this study was to investigate the expression, affinity, and density of A(1), A(2A), A(2B), and A(3) adenosine receptors (ARs) and D(2) dopamine receptors (D(2)Rs) in PD. An increase in A(2A)AR density in putamen was found. The presence and functionality of ARs in human lymphocyte and neutrophil membranes from patients with PD revealed a specific A(2A)AR alteration compared with healthy subjects. A statistically significant linear correlation among the A(2A)AR density, functionality, or tumor necrosis factor-alpha (TNF-alpha) levels and Unified Parkinson's Disease Rating Scale (UPDRS) motor score was reported. Adenosine concentration and TNF-alpha levels were increased in plasma of patients with PD. In rat adrenal pheochromocytoma (PC12) cells, a widely useful model, adenosine antagonists decreased dopamine uptake, and an opposite effect was mediated by A(2A) agonists. This is the first report showing the presence of an A(2A)AR alteration in putamen in PD that mirrors a similar up-regulation in human peripheral blood cells. Moreover, the correlation found between A(2A)AR density or A(2A) agonist potency and UPDRS motor score highlights the central role of A(2A)ARs in the pharmacological treatment of PD.

Parkinson's disease

Parkinson's disease is a common progressive bradykinetic disorder that can be accurately diagnosed. It is characterised by the presence of severe pars-compacta nigral-cell loss, and accumulation of aggregated alpha-synuclein in specific brain stem, spinal cord, and cortical regions. The main known risk factor is age. Susceptibility genes including alpha-synuclein, leucine rich repeat kinase 2 (LRRK-2), and glucocerebrosidase (GBA) have shown that genetic predisposition is another important causal factor. Dopamine replacement therapy considerably reduces motor handicap, and effective treatment of associated depression, pain, constipation, and nocturnal difficulties can improve quality of life. Embryonic stem cells and gene therapy are promising research therapeutic approaches.

Chocolate consumption is increased in Parkinson's disease. Results from a self-questionnaire study

Clinical observations in Parkinson's disease (PD) patients suggested an increased chocolate consumption. Chocolate contains high contents of various biogenic amines potentially influencing brain monoamine metabolism. 498 PD patients and their partners were evaluated by a structured self-questionnaire asking for consumption of chocolate and non-chocolate sweets, changes in chocolate consumption during the disease course, and depressive symptoms. Questionnaires from 274 patients (55 %) and 234 controls were eligible for further analysis. Consumption of chocolate was significantly higher in PD patients compared to controls, while consumption of non-chocolate sweets was similar in both groups. Our study suggests that chocolate consumption is increased in PD independent of concomitant depressive symptoms measured by BDI-1. Although reasons for increased chocolate consumption in PD remain elusive, it may hypothetically be a consequence of the high content of various biogenic amines and/or caffeine analogues with potential antiparkinsonian effects.

Adenosine receptor antagonists alter the stability of human epileptic GABAA receptors

We examined how the endogenous anticonvulsant adenosine might influence gamma-aminobutyric acid type A (GABA(A)) receptor stability and which adenosine receptors (ARs) were involved. Upon repetitive activation (GABA 500 microM), GABA(A) receptors, microtransplanted into Xenopus oocytes from neurosurgically resected epileptic human nervous tissues, exhibited an obvious GABA(A)-current (I(GABA)) run-down, which was consistently and significantly reduced by treatment with the nonselective adenosine receptor antagonist CGS15943 (100 nM) or with adenosine deaminase (ADA) (1 units/ml), that inactivates adenosine. It was also found that selective antagonists of A2B (MRS1706, 10 nM) or A3 (MRS1334, 30 nM) receptors reduced I(GABA) run-down, whereas treatment with the specific A1 receptor antagonist DPCPX (10 nM) was ineffective. The selective A2A receptor antagonist SCH58261 (10 nM) reduced or potentiated I(GABA) run-down in approximately 40% and approximately 20% of tested oocytes, respectively. The ADA-resistant, AR agonist 2-chloroadenosine (2-CA) (10 microM) potentiated I(GABA) run-down but only in approximately 20% of tested oocytes. CGS15943 administration again decreased I(GABA) run-down in patch-clamped neurons from either human or rat neocortex slices. I(GABA) run-down in pyramidal neurons was equivalent in A1 receptor-deficient and wt neurons but much larger in neurons from A2A receptor-deficient mice, indicating that, in mouse cortex, GABA(A)-receptor stability is tonically influenced by A2A but not by A1 receptors. I(GABA) run-down from wt mice was not affected by 2-CA, suggesting maximal ARs activity by endogenous adenosine. Our findings strongly suggest that cortical A2-A3 receptors alter the stability of GABA(A) receptors, which could offer therapeutic opportunities.

Current approaches to the treatment of Parkinson's disease

Enormous progress has been made in the treatment of Parkinson's disease (PD). As a result of advances in experimental therapeutics, many promising therapies for PD are emerging. Levodopa remains the most potent drug for controlling PD symptoms, yet is associated with significant complications such as the "wearing off" effect, levodopa-induced dyskinesias and other motor complications. Catechol-o-methyl-transferase inhibitors, dopamine agonists and nondopaminergic therapy are alternative modalities in the management of PD and may be used concomitantly with levodopa or one another. The neurosurgical treatment, focusing on deep brain stimulation, is reviewed briefly. Although this review has attempted to highlight the most recent advances in the treatment of PD, it is important to note that new treatments are not necessarily better than the established conventional therapy and that the treatment options must be individualized and tailored to the needs of each individual patient.