Neuroprotection in Parkinson's disease: clinical trials

Advances in our understanding of the cause and pathogenesis of Parkinson's disease (PD) have permitted the rational selection of putative neuroprotective agents for study in PD. However, the list of agents that might provide neuroprotective effects derived from laboratory studies is daunting, and we face the challenge of determining which agents to bring to the clinic and how to find the resources (patients and funds) to properly study so many promising therapeutic opportunities.1 Appropriate outcome variables that are not confounded by any symptomatic effect of the drug and are acceptable to clinicians and regulatory authorities also remain to be defined. The first clinical trials designed to test the capacity of putative neuroprotective agents to alter the natural history of PD have now been performed and illustrate some of these problems. The DATATOP (Deprenyl and Tocopherol Antioxidant Therapy of PD) study used the time to reach a disease milestone in untreated PD patients (ie, need for levodopa) as the primary end point. However, interpretation of results was confounded by the drug's symptomatic effect. The SINDEPAR (Sinemet-Deprenyl-Parlodel) study used the change in motor score between initial visit and final visit after washout of all study medications as the primary end point. However, here too there were concerns about confounding symptomatic effects, because antiparkinsonian medications have now been shown to have a long duration response that can persist for weeks and perhaps even months after withdrawal. More recent studies have used surrogate markers of the integrity of nigrostriatal function such as striatal uptake of fluorodopa on positron emission tomography (PET) or beta-CIT-on single-photon emission computerized tomography (SPECT) as primary outcome measures. However, it has not yet been confirmed that striatal uptake of these isotopes does in fact correlate with the remaining number of dopamine neurons or terminals, and the possibility of a confounding pharmacological effect has not yet been completely excluded. To date, no drug has been established to have a neuroprotective effect in PD, and none has been approved for a neuroprotective indication. Furthermore, regulatory agencies have not yet agreed that any of the outcome measures currently used will be acceptable for approval of a new drug. Resolution of these issues is of critical importance to convince pharmaceutical companies to expend the hundreds of millions of dollars necessary to bring a new drug to market. Drugs that already have been approved in PD for their symptomatic effects, such as dopamine agonists or propargylamines (eg, selegiline), offer the best opportunity for establishing that a drug is neuroprotective in PD in the immediate future, but herein also lies the difficulty of establishing that any benefits observed are not solely because of the drug's symptomatic properties. Currently, this will most likely entail demonstrating that the drug provides benefit for PD patients for both imaging and clinical markers of disease progression.