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Objectives: This study evaluated the efficacy and safety of baricitinib, an oral Janus kinase (JAK)1/JAK2 inhibitor, in patients with moderately to severely active rheumatoid arthritis (RA) and inadequate response to methotrexate (MTX) therapy., Methods: In this phase 3, double-blind, 52-week, placebo-controlled study, 290 patients with moderately to severely active RA and inadequate response to MTX were randomly assigned 1:1 to placebo or baricitinib 4-mg once daily, stratified by country (China, Brazil, Argentina) and presence of joint erosions. Primary endpoint measures included American College of Rheumatology 20% response (ACR20) at week 12. Secondary endpoints included changes in Health Assessment Questionnaire-Disability Index (HAQ-DI) and Disease Activity Score for 28-joint counts (DAS28)-high-sensitivity C-reactive protein (hsCRP), Simplified Disease Activity Index (SDAI) score ≤3.3, mean duration of morning joint stiffness, severity of morning joint stiffness numeric rating scale (NRS 0-10), worst tiredness NRS, and worst joint pain NRS at week 12., Results: Most patients (approximately 80%) were from China. More patients achieved ACR20 response at week 12 with baricitinib than with placebo (58.6% vs. 28.3%; p<0.001). Statistically significant improvements were also seen in HAQ-DI, DAS28-hsCRP, morning joint stiffness, worst tiredness, and worst joint pain in the baricitinib group compared to placebo at week 12. Through week 24, rates of treatment-emergent adverse events, including infections, were higher for baricitinib compared to placebo, while serious adverse event rates were similar between baricitinib and placebo., Conclusions: In patients with RA who had an inadequate response to MTX, baricitinib was associated with significant clinical improvements as compared with placebo.