A Magnetic Resonance Imaging Study and Arthroscopic Biopsy Substudy in Subjects With Active Rheumatoid Arthritis Receiving VX-509, an Oral JAK3 Inhibitor

Inclusion Criteria: * Male and female subjects 18 to 65 years of age (inclusive) * Diagnosis of RA * Swollen joint count of ≥6 out of 66 joints and tender joint count of ≥6 out of 68 joints * Seropositivity based on either a positive rheumatoid factor or anti cyclic citrullinated peptide antibody at screening -OR- known erosive disease based on previous X-ray report or erosions detected on screening hand and foot X-ray * Baseline CRP level or Westergren erythrocyte sedimentation rate ≥1.2 × upper limit of normal * Receiving stable therapy with 1 of the following DMARDs: methotrexate, sulfasalazine, leflunomide, anti-malarial drug, or penicillamine * Palpable 2+ synovitis of the wrist or ≥2 MCPs in the MRI-designated hand Exclusion Criteria: * History or presence of a clinically significant medical disorder other than RA that, in the opinion of the investigator and medical monitor, would pose a risk to subject safety or interfere with the study evaluation, procedures, or completion. * Inflammatory, rheumatological disorders other than RA, where arthritis may be a prominent feature * Planned surgery during the study * History of alcohol or drug abuse, or excessive alcohol consumption * History of tuberculosis (TB) infection of any kind (pulmonary or extrapulmonary, active or latent), regardless of history of anti-TB treatment. * Pregnant or nursing an infant or with a life partner who is pregnant, nursing, or planning to become pregnant