A predictive probability interim design for phase II clinical trials with continuous endpoints

Molecular targeted therapies come often with lower toxicity profiles than traditional cytotoxic treatments, thus shifting drug development paradigm into establishing evidence of biological activity, target modulation, and pharmacodynamics effects of these therapies in early phase trials. Therefore, these trials need to address simultaneous evaluation of safety, proof-of-concept biological marker activity, or changes in continuous tumor size instead of binary response rate. Interim analyses are typically incorporated in the trial due to concerns regarding excessive toxicity and ineffective new treatment. There is a lack of interim strategies developed to monitor futility and/or efficacy for these types of continuous outcomes, especially in single-arm phase II trials. We propose a 2-stage design based on predictive probability to accommodate continuous endpoints, assuming a normal distribution with known variance. Simulation results and case study demonstrated that the proposed design can incorporate an interim stop for futility as well as for efficacy while maintaining desirable design properties. As expected, using continuous tumor size resulted in reduced sample sizes for both optimal and minimax designs. A limited exploration of various priors was performed and shown to be robust. As research rapidly moves to incorporate more molecular targeted therapies, it will accommodate new types of outcomes while allowing for flexible stopping rules to continue optimizing trial resources and prioritize agents with compelling early phase data.