Predictors of Placebo Response in the Study of Oxytocin in Autism to Improve Reciprocal Social Behaviors

Background: Although randomized clinical trials (RCTs) have investigated several treatments for social communication difficulties and repetitive behavior in autism, none has yet shown consistent superiority over placebo. Placebo response in autism RCTs may impede the ability to detect meaningful treatment effects. Objective: We sought to identify individual-level predictors of placebo response in Study of Oxytocin in Autism to improve Reciprocal Social Behaviors (SOARS-B), a 24-week RCT of intranasal oxytocin for social impairment in autistic youth. In our primary analysis, we examined predictors of change in the Aberrant Behavior Checklist-modified Social Withdrawal (ABC-mSW) score at 24 weeks in SOARS-B participants taking placebo. Secondary analyses examined predictors of ABC-mSW change at 12 weeks and of Clinical Global Impressions-Improvement at 24 and 12 weeks. We also examined predictors of response among SOARS-B participants taking oxytocin. Methods: For each analysis, we first used lasso (least absolute shrinkage and selection operator) regression to identify potentially influential predictors from a large group that included demographic factors, rating scale data, and prescribed medications. We then estimated an unpenalized linear regression model for the outcome of interest that included only variables retained by the optimal lasso. We considered variables with statistically significant coefficients to be influential predictors. Results: Higher baseline ABC-mSW score was the only significant predictor of greater ABC-mSW change in the placebo group at 24 and 12 weeks. Conclusions: In SOARS-B, higher baseline severity on a measure of reciprocal social communication predicted greater placebo response. This is consistent with the finding that lower social communication adaptive functioning was associated with greater placebo response in recent RCTs of balovaptan for social impairment in autism. However, it contrasts with findings from a trial of citalopram for repetitive behavior in autism, in which lower baseline severity of a composite of autistic and mood symptoms predicted greater placebo response. This may indicate that different factors contribute to placebo response in different symptom domains.