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Higher BMI Progressively Reduces JAK Inhibitor Efficacy in Rheumatoid Arthritis
In the largest individual patient data meta-analysis of JAK inhibitor trials to date, patients with class 3 obesity showed approximately 22% lower odds of achieving a meaningful clinical response compared with healthy-weight patients, with an adjusted relative risk for ACR20 of 0.78 (95% CI 0.71–0.85). This effect followed a clear dose-response gradient across all BMI categories and was absent in the placebo arm, indicating that obesity specifically diminishes the therapeutic benefit of JAK inhibitors rather than reflecting general disease severity differences.
What Was Studied
This study investigated whether body mass index modifies the clinical response to JAK inhibitors — tofacitinib, baricitinib, and upadacitinib — in adults with rheumatoid arthritis. The primary outcomes were ACR20 response rate and DAS28-CRP score at each trial’s predefined primary efficacy timepoint, assessed across the full spectrum of BMI categories from healthy weight through class 3 obesity.
How It Was Studied
Researchers conducted an individual patient data (IPD) meta-analysis, pooling participant-level data from 16 phase 3 randomised controlled trials identified through a systematic search of ClinicalTrials.gov up to January 2025. Data were accessed via the Vivli data-sharing platform, covering 11,883 adult patients with rheumatoid arthritis. The analytic approach used one-stage mixed-effects models treating BMI as both a continuous and categorical variable, with healthy-weight participants serving as the reference group and placebo recipients as an internal control. A two-stage IPD meta-analysis was used for validation, and risk of bias was assessed with the Cochrane Risk of Bias 2 tool, yielding a low overall rating across contributing trials.
What Was Observed
- Class 3 obesity was associated with the greatest reduction in treatment response: patients with a BMI of 40 kg/m² or above had about 22% lower likelihood of achieving ACR20 response compared with healthy-weight patients (adjusted RR 0.78, 95% CI 0.71–0.85). Even overweight patients showed a modest but statistically meaningful reduction of about 6% (adjusted RR 0.94, 95% CI 0.91–0.98).
- Disease activity scores followed the same gradient: compared with healthy-weight patients, DAS28-CRP scores were on average 0.21 units higher in class 1 obesity (95% CI 0.12–0.31) and 0.54 units higher in class 3 obesity (95% CI 0.37–0.70), indicating progressively worse residual disease activity at higher BMI levels.
- The BMI effect was specific to active treatment: no equivalent BMI-related gradient in clinical outcomes was observed among placebo recipients, strongly suggesting that obesity acts as an effect modifier of JAK inhibitor pharmacological benefit rather than simply a marker of more refractory disease.
- Results were consistent across trials: between-study heterogeneity was low to moderate (I² = 0–40%), and the findings were robust across both analytical approaches, supporting the reliability of the pooled estimates.
Why This Matters
Obesity affects nearly one-third of rheumatoid arthritis patients in this dataset, yet its impact on treatment response has not previously been quantified at this scale using individual patient data. The finding that obesity functions as an effect modifier — not merely a prognostic factor — has direct implications for how clinicians interpret treatment non-response in higher-BMI patients. The authors conclude that these findings support incorporating weight management into routine rheumatoid arthritis care and call for greater BMI representation in future clinical trials, which have historically underrepresented patients with severe obesity.
How to Read This Result
While the large sample size, low risk of bias, and internal placebo control strengthen confidence in these findings, the exclusion of filgotinib trials due to missing variables means the results may not extend to all JAK inhibitors, and some residual confounding inherent in BMI-stratified analyses of trial data cannot be fully excluded.
Limitations
Filgotinib trials could not be included because of insufficient variable data, which limits the scope of conclusions across the full JAK inhibitor class. The study did not incorporate input from people with lived experience of rheumatoid arthritis at any stage of design or conduct. Additionally, the underlying trial datasets may themselves have underrepresented patients with severe obesity, which could affect the precision of estimates at the highest BMI categories.