Rasagiline in neurodegeneration

Treatment of Parkinson's Disease by MAO-B Inhibitors, New Therapies and Future Challenges - A Mini-Review

BACKGROUND

One of the most prevalent neurodegenerative diseases with increasing age is Parkinson's disease (PD). Its pathogenesis is unclear and mainly confined to glutamate toxicity and oxidative stress. The dyskinesia and motor fluctuations and neuroprotective potential are the major concerns which are still unmet in PD therapy.

OBJECTIVE

This article is a capsulization of the role of MAO-B in the treatment of PD, pharmacological properties, safety and efficiency, clinical evidence through random trials, future therapies and challenges.

CONCLUSION

MAO-B inhibitors are well tolerated for the treatment of PD because of their pharmacokinetic properties and neuroprotective action. Rasagiline and selegiline were recommended molecules for early PD and proven safe and provide a modest to significant rise in motor function, delay the use of levodopa and used in early PD. Moreover, safinamide is antiglutamatergic in action. When added to Levodopa, these molecules significantly reduce the offtime with a considerable improvement of non-motor symptoms. This review also discusses the new approaches in therapy like the use of biomarkers, neurorestorative growth factors, gene therapy, neuroimaging, neural transplantation, and nanotechnology. Clinical evidence illustrated that MAOB inhibitors are recommended as monotherapy and added on therapy to levodopa. A large study and further evidence are required in the field of future therapies to unwind the complexity of the disease.

Early versus delayed initiation of pharmacotherapy in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease and pathologically is characterised by a progressive loss of dopaminergic cells of the nigrostriatal pathway. Clinically, PD is mainly defined by the presence of the motor symptoms of bradykinesia, rigidity, rest tremor and postural instability, but non-motor symptoms such as depression, dementia and autonomic disturbances are recognised as integral parts of the disease. Although pharmacotherapy for PD was introduced almost 50 years ago, and has improved significantly over the intervening period, the timing of initiation of treatment in newly diagnosed PD remains controversial. While some physicians favour an early start of pharmacotherapy at or soon after diagnosis, others prefer to delay pharmacological treatment until a certain degree of disability has developed. This article aims to discuss the advantages and disadvantages of both strategies by exploring their effects on symptoms, disease progression and quality of life. Although the data on putative disease-modifying effects of early pharmacological intervention in PD are still inconclusive, we believe that the most important indication for an early initiation of anti-parkinsonian treatment should be to maintain the quality of life of PD patients and to secure their socioeconomic status as long as possible.

Defining the Role of the Monoamine Oxidase-B Inhibitors for Parkinson's Disease

Inhibitors of monoamine oxidase-B (MAO-B) occupy an important place in the treatment of Parkinson's disease. Selegiline was the first MAO-B to be used therapeutically, while rasagiline is a second-generation drug with higher potency and selectivity. Safinamide is an investigational MAO-B inhibitor with non-dopaminergic properties that may provide advantages over its predecessors. As a class, MAO-B inhibitors are safe and well tolerated and provide symptomatic benefit both as monotherapy and in combination with other antiparkinsonian medications from early to late stages of disease. In combination with levodopa, MAO-B inhibitors may improve motor fluctuations and allow for lower total doses of levodopa. Patient characteristics and preferences can be important factors in deciding between agents. As a class, MAO-B inhibitors have shown promise as disease-modifying agents, but the clinical trial evidence to date has not been strong enough to afford them such a label. Future research may help further elucidate their relative merits and clarify their role in altering disease progression.

Targeting mitochondria for neuroprotection in Parkinson's disease

SIGNIFICANCE

Several genetic causes of familial Parkinson's disease (PD) have now been identified and include mutations of genes encoding mitochondrial proteins. Mitochondrial complex I toxins can induce dopaminergic cell death and produce a parkinsonian state. Importantly, defects of mitochondrial function have been identified in postmortem substantia nigra from pathologically proven cases of PD.

RECENT ADVANCES

These observations provide compelling evidence to support the notion that mitochondria play an important role in the pathogenesis of PD. Thus, targeting mitochondrial function to delay or prevent neuronal cell death would represent a logical means to modify the course of this disease. Several attempts have already been made in this respect, and have been tested in clinical trial.

CRITICAL ISSUES

To date, there is no unequivocal evidence for an effective intervention to slow the disease. However, several novel mitochondrial targets are now emerging, including the potential to manipulate the mitochondrial pool to maintain function via biogenesis and mitophagy.

FUTURE DIRECTIONS

This development in drug targets needs to be supported by a parallel improvement in clinical trial design to be able to detect a neuroprotective or disease-modifying effect over a reasonable time scale.

Mitochondrial pathology in Parkinson's disease

The last 25 years have witnessed remarkable advances in our understanding of the etiology and pathogenesis of Parkinson's disease. The ability to undertake detailed biochemical analyses of the Parkinson's disease postmortem brain enabled the identification of defects of mitochondrial and free-radical metabolism. The discovery of the first gene mutation for Parkinson's disease, in alpha-synuclein, ushered in the genetic era for the disease and the subsequent finding of several gene mutations causing parkinsonism, 15 at the time of writing. Technological advances both in sequencing technology and software analysis have allowed association studies of sufficiently large size accurately to describe genes conferring an increased risk for Parkinson's disease. What has been so surprising is the convergence of these 2 separate disciplines (biochemistry and genetics) in terms of reinforcing the importance of the same pathways (ie, mitochondrial dysfunction and free-radical metabolism). Other pathways are also important in pathogenesis, including protein turnover, inflammation, and post-translational modification, particularly protein phosphorylation and ubiquitination. However, even these additional pathways overlap with each other and with those of mitochondrial dysfunction and oxidative stress. This review explores these concepts with particular relevance to mitochondrial involvement.

Polycyclic cage structures as lipophilic scaffolds for neuroactive drugs

Polycyclic cage scaffolds have been successfully used in the development of numerous lead compounds demonstrating activity in the central nervous system (CNS). Several neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, schizophrenia, and stroke, as well as drug abuse, can be modulated with polycyclic cage derivatives. These cage moieties, including adamantane and pentacycloundecane derivatives, improve the pharmacokinetic and pharmacodynamic properties of conjugated parent drugs and serve as an important scaffold in the design of therapeutically active agents for the treatment of neurological disorders. In this Minireview, we focus on the recent developments in the field of polycyclic cage compounds, as well as the relationship between the lipophilic character of these cage-derived drugs and the ability of such compounds to target and reach the CNS and improve the pharmacodynamic properties of compounds conjugated to it.

Rasagiline protects against alpha-synuclein induced sensitivity to oxidative stress in dopaminergic cells

Rasagiline is a propargylamine and irreversible monoamine oxidase (MAO) B inhibitor used for the treatment of Parkinson's disease (PD). It has demonstrated neuroprotective properties in laboratory studies. Current concepts of PD aetiopathogenesis include the role of alpha-synuclein, protein aggregation, free radical metabolism and mitochondrial dysfunction in contributing to cell death. We have used a combination of alpha-synuclein and free radical mediated toxicity in a dopaminergic cell line to provide a model of nigral toxicity in order to investigate the potential molecular mechanisms that mediate rasagiline protection. We demonstrate that rasagiline protects against cell death induced by the combination of free radicals generated by paraquat and either wild-type or A53T mutant alpha-synuclein over-expression. This protection was associated with a reduction in caspase 3 activation, a reduction in superoxide generation and a trend to ameliorate the fall in mitochondrial membrane potential. Rasagiline induced an increase in cellular glutathione levels. The results support a role for rasagiline in protecting dopaminergic cells against free radical mediated damage and apoptosis in the presence of alpha-synuclein over-expression. The data are of relevance to the interpretation of the potential mechanisms of action of rasagiline in explaining the results of disease modification trials in PD.

Targeted overexpression of human alpha-synuclein in oligodendroglia induces lesions linked to MSA-like progressive autonomic failure

Multiple system atrophy (MSA) is a rare neurodegenerative disease of undetermined cause manifesting with progressive autonomic failure (AF), cerebellar ataxia and parkinsonism due to neuronal loss in multiple brain areas associated with (oligodendro)glial cytoplasmic α-synuclein (αSYN) inclusions (GCIs). Using proteolipid protein (PLP)-α-synuclein (αSYN) transgenic mice we have previously reported parkinsonian motor deficits triggered by MSA-like αSYN inclusions. We now extend these observations by demonstrating degeneration of brain areas that are closely linked to progressive AF and other non-motor symptoms in MSA, in (PLP)-αSYN transgenic mice as compared to age-matched non-transgenic controls. We show delayed loss of cholinergic neurons in nucleus ambiguus at 12 months of age as well as early neuronal loss in laterodorsal tegmental nucleus, pedunculopontine tegmental nucleus and Onuf's nucleus at 2 months of age associated with αSYN oligodendroglial overexpression. We also report that neuronal loss triggered by MSA-like αSYN inclusions is absent up to 12 months of age in the thoracic intermediolateral cell column suggesting a differential dynamic modulation of αSYN toxicity within the murine autonomic nervous system. Although the spatial and temporal evolution of central autonomic pathology in MSA is unknown our findings corroborate the utility of the (PLP)-αSYN transgenic mouse model as a testbed for the study of oligodendroglial αSYN mediated neurodegeneration replicating both motor and non-motor aspects of MSA.