Understanding the placebo response is critical to interpreting treatment efficacy, particularly for agents with a ceiling to their therapeutic effect, where an increasing placebo response makes it harder to detect potential benefit. The objective of this study is to assess the change in placebo responses over time in rheumatoid arthritis (RA) randomized placebo-controlled trials (RCT) for drug licensing authorization.
The Cochrane Controlled Trials Register database was searched to identify RCT of biological or targeted synthetic disease-modifying antirheumatic drugs (DMARD) in RA. Studies were excluded if patients were conventional synthetic DMARD (csDMARD)-naive, not receiving background csDMARD therapy, or were biologic experienced. Metaregression model was used to evaluate changes in American College of Rheumatology (ACR) 20, ACR50, and ACR70 treatment response over time.
There were 32 trials in total: anti-tumor necrosis factor therapy (n = 15), tocilizumab (n = 4), abatacept (n = 2), rituximab (n = 2), and Janus kinase inhibitors (n = 9). From 1999 to 2018, there was no significant trend in the age or sex of patients in the placebo arm. Disease duration, swollen joint count, and 28-joint count Disease Activity Score using erythrocyte sedimentation rate at baseline all significantly declined over time. There was a statistically significant increase in placebo ACR50 and ACR70 responses (ACR50 β = 0.41, 95% CI 0.09-0.74, p = 0.01; ACR70 β = 0.18, 95% CI 0.04-0.31, p = 0.01) that remained significant after controlling for potential confounders.
There has been a rise in the placebo response in RA clinical trials over the last 2 decades. Shifting RA phenotype, changes in trial design, and expectation bias are possible explanations for this phenomenon. This observation has important implications when evaluating newer novel agents against established therapies.