As recruitment for oncology clinical trials has become more difficult, there appears to have been an increase in the number of studies that allow patients in the control arm to “crossover” and receive the experimental therapy after disease progression. Although some researchers worry that allowing such crossover may abolish gains in progression-free survival in the experimental arm, the possibility that crossover might inadvertently benefit the experimental arm has not been addressed. In clinical trials in which the experimental agent has little or no intrinsic activity and is used to modulate an active combination, such crossover might negatively affect the overall survival of the control arm. Because resistance to the active combination—manifested as disease progression—has occurred, the likelihood of benefit from adding the experimental drug is reduced. Consequently, patients who were randomly assigned to the control arm continue to receive the now inactive combination after crossover, whereas patients in the experimental arm who discontinue study participation may seek out potentially effective salvage regimens. This difference in subsequent therapies may confer an advantage to the experimental arm that is manifested as gains beyond those achieved in progression-free survival, gains that occur not because the experimental therapy induced a change in tumor biology that persists beyond treatment discontinuation but because the control arm suffers by continuing to receive a therapy on which their tumor is progressing. Such an outcome may explain the recently reported trial results for iniparib in triple-negative breast cancer. Given that allowing patients in the control arm to receive the experimental agent may confound interpretation of overall survival, such crossover should not be used indiscriminately, especially if the experimental agent has little or no intrinsic activity.
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