Defining disease-modifying therapies for PD--a road map for moving forward.

A disease-modifying therapy that slows or stops disease progression is one of the major unmet needs in the management of Parkinson's disease. To date, no therapy has been approved for disease modification despite promising laboratory data and positive results in clinical trials. This is because confounding symptomatic or pharmacologic effects cannot be excluded. The delayed start study provides an opportunity to define therapies that provide benefit that cannot be explained by an early symptomatic effect alone. However, this trial design does not necessarily provide a meaningful measure of the effect of the intervention on cumulative disability. In contrast, the long-term simple study provides a measure of the effect of the drug on cumulative disability but does not address mechanism of action. Together these two trials provide a road map for defining a disease modifying drug and determining the long term cumulative effect of the drug on the disease.