The impact of opioid withdrawal symptoms on pain outcomes in enriched enrollment randomized withdrawal trials: a mediation meta-analysis of trials submitted to the US Food and Drug Administration

The validity of enriched enrollment randomized withdrawal (EERW) trials to evaluate the efficacy of opioids for the treatment of chronic pain has been questioned. Enriched enrollment randomized withdrawal trials include an open-label titration phase to identify treatment responders who tolerate the drug, followed by a double-blind randomized phase in which responders either continue the drug or switch to placebo. A key concern is that the apparent efficacy of opioids in EERW trials may be attributable to induction of withdrawal symptoms among participants switched to placebo. We used individual participant data from 13 EERW trials (N = 5070) submitted to the US Food and Drug Administration (FDA) to estimate the extent to which withdrawal symptoms mediated the treatment effect of opioids on pain. The primary mediator was the maximum Subjective Opioid Withdrawal Scale score during the randomized phase. The primary outcome was the change in pain intensity (numeric rating scale) from randomization baseline to week 12. The pooled average treatment effect was -0.71 (95% confidence interval [CI], -0.87 to -0.55) on the numeric rating scale. Withdrawal symptoms did not significantly mediate the effect of opioids on pain overall, accounting for 2% of the pooled treatment effect (95% CI, -1% to 4%). However, significant mediation was observed in 3 individual trials (range, 8% to 28% of treatment effect mediated). Although withdrawal symptoms did not systematically bias efficacy findings in EERW trials of opioids submitted to the FDA, they contributed to overestimation in some cases. These findings support incorporating mediation analyses in future EERW trials to ensure accurate interpretation of study results.