Estimand Endpoints for Longitudinal Measures of Continuous Disease Progression with an Alzheimer's Disease Example

The ICH E9 (R1) Addendum provides a framework to define an estimand and perform sensitivity analysis. The clinical endpoint (i.e., variable, response, outcome) is one of the important estimand attributes. In our opinion, the selection of the endpoint in Alzheimer's disease requires more exploration beyond what is used currently. The change in a cognitive and functional assessment scale from baseline to a specific time point of interest is often used as a primary or key secondary endpoint in clinical trials. However, such a change from baseline to the time point of interest may not reflect the benefit of the treatment over the course of treatment duration and may be difficult to intuitively understand by patients and clinicians. For two patients with the same change from baseline, the patient with rapid disease progression in the beginning is considered to have overall worse quality of life compared to the other patient with slow disease progression in the beginning but rapid progression toward the end. We explore time-averaged measurement (TAM) as a new endpoint and propose using the relative change to quantify the treatment difference. Estimands under the ICH E9 (R1) Addendum were considered by using various strategies in handling intercurrent events and used corresponding methods for handling missing data. We illustrate the use of TAM and compare the results with other commonly used estimand endpoints (the change from baseline, the relative disease progression model, and the slope of disease progression) for different estimands and imputation methods from retrospective analyses of a historical study.