Adaptive enrichment designs with a continuous biomarker

A popular design for clinical trials assessing targeted therapies is the two‐stage adaptive enrichment design with recruitment in stage 2 limited to a biomarker‐defined subgroup chosen based on data from stage 1. The data‐dependent selection leads to statistical challenges if data from both stages are used to draw inference on treatment effects in the selected subgroup. If subgroups considered are nested, as when defined by a continuous biomarker, treatment effect estimates in different subgroups follow the same distribution as estimates in a group‐sequential trial. This result is used to obtain tests controlling the familywise type I error rate (FWER) for six simple subgroup selection rules, one of which also controls the FWER for any selection rule. Two approaches are proposed: one based on multivariate normal distributions suitable if the number of possible subgroups, k , is small, and one based on Brownian motion approximations suitable for large k . The methods, applicable in the wide range of settings with asymptotically normal test statistics, are illustrated using survival data from a breast cancer trial.