Treatment of Parkinson's disease : what's on the horizon?

Few neurological diseases have received as much attention and investment in research as Parkinson's disease. Although great strides have been made in the development of agents to treat this neurodegenerative disease, none yet address the underlying problem associated with it, the progressive loss of dopaminergic neurons. Current therapeutic strategies for Parkinson's disease focus primarily on reducing the severity of its symptoms using dopaminergic medications. Although providing substantial benefit, these agents are burdened by adverse effects and long-term complications. This review highlights new and emerging therapies for Parkinson's disease, categorised as symptomatic, neuroprotective and neurorestorative, although at times, this distinction is not easily made. Novel symptomatic treatments target nondopaminergic areas in the hope of avoiding the motor complications seen with dopaminergic therapies. Two emerging treatment approaches under investigation are adenosine A(2A) receptor antagonists (such as istradefylline [KW-6002]) and glutamate AMPA receptor antagonists (such as talampanel [LY-300164]). In 2003, the results from two studies using istradefylline in patients with Parkinson's disease were published, with both showing a positive benefit of the study drug when used as adjunctive therapy to levodopa. In non-human primate models of Parkinson's disease, talampanel has been found to have antiparkinsonian effects when administered as high-dose monotherapy and antidyskinetic effects on levodopa-induced dyskinesias. NS-2330, another drug currently undergoing clinical trials, is a triple monoamine reuptake inhibitor that has therapeutic potential in both Parkinson's and Alzheimer's disease. A phase II proof-of-concept study is currently underway in early Parkinson's disease. However, a recently published study in advanced Parkinson's disease showed no therapeutic benefit of NS-2330 in this patient population. Even more exciting are agents that have a neuroprotective or neurorestorative role. These therapies aim to prevent disease progression by targeting the mechanisms involved in the pathogenesis of Parkinson's disease. Several lines of investigation for neuroprotective therapies have been taken, including the antioxidant coenzyme Q10 (ubidecarenone) and anti-apoptotic agents such as CEP-1347. Studies in patients with Parkinson's disease with coenzyme Q10 have suggested that it slows down functional decline. The PRECEPT study is currently in progress to assess the neuroprotective role of CEP-1347 in the early phase of the disease. Gene therapy is another exciting arena and includes both potentially neuroprotective and neurorestorative agents. Novel methods include subthalamic glutamic acid decarboxylase gene therapy and the use of glial cell line-derived neurotrophic factor (GDNF). Eleven of 12 patients have been enrolled in the first FDA-approved phase I subthalamic glutamic acid decarboxylase gene therapy trial for Parkinson's disease, with currently no evidence of adverse events. GDNF delivered intracerebroventricularly was studied in a small population of patients with Parkinson's disease, but unfortunately did not reveal positive results. Other methods of administering GDNF include direct delivery via infusions into the basal ganglia and the use of viral vectors; thus far, these approaches have shown promising results. This is an exciting and rewarding time for research into Parkinson's disease. With so many therapies currently under investigation, the time is ripe for the beginning of a new phase of treatment strategies.