Clinical Trial Issues

Like AD, a major clinical trial issues for all CNS neurodegenerative diseases is in the design of studies to differentiate symptomatic from neuroprotective benefits of NCEs.

For PD, two different clinical study designs (drug wash-out and randomized delayed starts) have been used in attempts to distinguish this difference for NCEs.69 The first uses a drug wash-out period at the end of the treatment course. Patients are followed for different time periods following drug withdrawal to determine if a benefit persists in the absence of continued drug use. While this is a logical design, there are inherent problems. One issue is that the pharmacodynamic effects of treatment may outlast the duration of the physical presence of the drug. Another is that patients with more severe symptoms cannot tolerate discontinuation of a symptomatic treatment; thus patients with less severe symptoms may be preferentially retained in a study. The second design is a randomized delayed-start trial in which patients are randomized to treatment groups following receipt of placebo for various durations. This study design also has challenges: (1) treatment earlier in the course of disease may provide greater symptomatic benefit; and (2) patients may drop out of the study prior to receiving treatment.

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