Disease activity improvements with optimal discriminatory ability between treatment arms: applicability in early and established rheumatoid arthritis clinical trials

BACKGROUND: The ACR20 has been validated as the best discriminator of efficacy in placebo-controlled trials, but not in head-to-head trials comparing effective therapies in patients with rheumatoid arthritis (RA). We assessed the most discriminatory ACR response and most discriminatory percent improvement in disease activity measures for Simplified Disease Activity index (SDAI), Clinical Disease Activity index (CDAI), and 28-joint Disease Activity Score based on C-reactive protein (DAS28(CRP)) using different patient populations and trial designs. METHODS: Data from two placebo-controlled studies in established RA and two head-to-head studies in early RA were analyzed. The numeric ACR response for each treatment and P value for the difference between treatments were calculated at multiple time points to determine the ACR response associated with the lowest P value. Similarly, values for percent improvement from baseline in SDAI, CDAI, and DAS28(CRP) with the most discrimination between treatments were examined. RESULTS: In the head-to-head early RA trials, the minimum P value and greatest treatment difference between the active comparator arms at 6 months was achieved at higher ACR rates and greater percent improvements in other disease activity measures. In established RA, lower responses (minimum P value and maximum treatment difference) and smaller improvements in disease activity scores had better discriminatory ability at 6 months. CONCLUSIONS: The most discriminatory ACR response rate and percent improvement in disease activity measures were higher in head-to-head active comparator trials in early RA versus placebo-controlled trials in established RA. This difference should be considered in future clinical trial designs. TRIAL REGISTRATION: NCT00195663, NCT00420927, NCT00195702.