Disease progression trajectory curves to estimate saved time in Alzheimer's disease trials

With the recent successful disease-modifying therapies against Alzheimer's disease (AD), there have been discussions on easily interpretable measures for treatment effects. Among them, saved time for patients treated with a new drug as compared to patients randomized to the placebo group offers easier interpretation than the reduced percentage in outcome decline at last visit which were commonly used in AD trials. The existing method to calculate saved time utilized the disease progression trajectory of the placebo group and the treatment effect at the last visit. We propose to develop two new methods that use disease progression trajectories of both groups: (1) slope adjusted method; and (2) area under the curve method. We used data from the two donanemab trials and the donepezil trial to illustrate the application of the proposed methods and conducted simulation studies to compare these methods. When a drug has a constant treatment effect over time or early and middle difference in the disease progression, the area under the curve method often has the saved time being longer than the existing method. When the treatment effect is an increasing function of time before the last visit as observed in disease-modifying therapy trials, the slope adjusted method could have a larger saved time as compared to the existing method. In many cases, the area under the curve method often has the smallest standard deviation of saved time.