Early termination of a phase II clinical trial

This is a report of an ad hoc application of "predictive probability" to support the decision to terminate a traditional fixed sample phase II clinical trial of endocrine therapy in breast cancer after accrual of only 75% of the planned sample (18 of 25). The analysis was motivated by concerns over low accrual rates and a lower than expected response rate. Statistical analyses included the computation of several sets of probability estimates. The accumulated data were sufficient to reject H0: pi = 0.6 (the desired response rate) in favor of H1: pi less than 0.6 (p less than or equal to 0.02), and at study completion the 95% confidence interval for pi was expected to fail to include 0.6 with probability 0.92. Under no circumstances could the final observed response rate exceed 0.52. These and other computations provided assurance that early termination of the trial was unlikely to miss a treatment as good or better than the standard.