Abstract
Differences in efficacy between antipsychotics and placebo in schizophrenia trials have decreased over the past decades. Previous studies have tried to identify potential explanatory factors focusing on response to placebo; however, it is still not clear which factors, if any, specifically moderate drug-response, as they may be different from those moderating placebo-response. Therefore, in this meta-regression analysis we explore whether there is an interaction between drug-response and placebo-response in terms of effect size. We systematically searched multiple electronic databases, ClinicalTrials.gov, and the US Food and Drug Administration website for randomized, placebo-controlled trials investigating the efficacy of antipsychotics in patients with acute schizophrenia (last update: October 2016). The main outcome was the change on the Brief Psychiatric Rating Scale or the Positive and Negative Syndrome Scale score from baseline to endpoint after at least 3 weeks of treatment. Multiple patient-, design-, and drug-related potential predictors of response were analyzed by meta-regressions, as predefined in the study protocol. Overall, 167 trials with 28,102 participants were included. Publication year, the number of participants and sites, mean dose, minimum severity threshold as an inclusion criterion, chronicity, industry sponsorship, type of rating scale, diagnostic criteria, and number of medications had a different impact on drug and placebo response. By contrast, baseline severity, duration of wash-out, study duration, and study region affected drug and placebo response in a similar way without a net effect on effect sizes. No other factors had a significant effect on either drug-response or placebo-response. In conclusion, as individual moderators may have different impact on placebo-response and drug-response, it is important to consider also the specific factors influencing drug-response in order to fully understand the difference between antipsychotics and placebo. These results shed further light on the phenomenon of decreasing effect size of antipsychotics for schizophrenia over time and should help design future randomized controlled trials in the field (Prospero registration number CRD42013003342).