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We consider the design of phase II trials evaluating combinations of experimental therapies. In the modern era, many immunotherapy and targeted therapy regimens are being developed as combination regimens, including combinations consisting only of experimental agents. In some clinical or drug development scenarios, it may be difficult to isolate the effect of the individual agents composing a combination of this type, which makes the evaluation of the combination challenging. One such scenario arises when none of the agents making up the experimental combination have demonstrated single-agent activity in the clinical setting of interest. One solution to this problem is to use a randomized comparative trial in which the combination of interest is compared with 1 or both of its component agents, but some modifications to more traditional randomized comparative phase II trials must be made because all arms in such a trial would be experimental. In this manuscript, we present sensible modifications to randomized phase II trial designs that can be used in 2 common drug development scenarios of this type and provide a detailed discussion of the practical aspects of designing these trials. We also include 2 worked examples to further illustrate how to design such a trial.

Designing and interpreting single-arm phase II trials of combinations of agents is challenging because it can be difficult, based on historical data, to identify levels of activity for which the combination would be worth pursuing. We identified Cancer Therapy Evaluation Program single-arm combination trials that were activated in 2008–2017 and tabulated their design characteristics and results. Positive trials were evaluated as to whether they provided credible evidence that the combination was better than its constituents. A total of 125 trials were identified, and 120 trials had results available. Twelve had designs where eligible patients were required to be resistant or refractory to all but one element of the combination. Only 17.8% of the 45 positive trials were deemed to provide credible evidence that the combination was better than its constituents. Of the 10 positive trials with observed rates 10 percentage points higher than their upper (alternative hypothesis) targets, only five were deemed to provide such credible evidence. Many trials were definitively negative, with observed clinical activity at or below their lower (null hypothesis) targets. Ideally, use of single-arm combination trials should be restricted to settings where each agent is known to have minimal monotherapy activity (and a randomized trial is infeasible). In these settings, an observed signal is attributable to synergy and thus could be used to decide whether the combination is worth pursuing. In other settings, credible evidence can still be obtained if the observed activity is much higher than expected, but experience suggests that this is a rare occurrence.