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Washout and delayed start designs for identifying disease modifying effects in slowly progressive diseases using disease progression analysis

Treatment of a slowly progressive disease, such as Parkinson's and Alzheimer's disease should slow down or even reverse progression of the disease. For estimating the actual treatment effect a clinical trial should allow the time course of treatment effects to be distinguished from those due to natural disease progression. A symptomatic treatment effect has a rapid onset, which will disappear after treatment cessation, whereas disease modifying effects are slow and persistent. Our objective was to compare the power of washout and delayed start designs for distinguishing symptomatic and disease modifying effects using disease progression modelling. The effect of dropout due to worsening disease status on the power was also considered.