24-week Study With Open Label Extension of VX-509, an Oral JAK3 Inhibitor, in Subjects Taking Methotrexate

Inclusion Criteria: * Male and female subjects, between 18 and 80 years of age (inclusive) * All subjects must have been diagnosed with RA * Must have a swollen joint count of ≥6 out of 66 joints and tender joint count of ≥6 out of 68 joints * Baseline CRP level must be above the upper limit of normal * All subjects must have been receiving stable MTX coadministered with folic or folinic acid (at least 5 mg/week) * Subjects may remain on 1 nonsteroidal anti-inflammatory medication during the study (aspirin ≤ 325 mg/day is allowed). * Subjects must not have received prior treatment with a JAK inhibitor * Subjects who are on an additional nonbiologic DMARD (e.g., sulfasalazine) must be willing to discontinue that DMARD after signing consent, except for hydroxychloroquine * Subjects may have received previous therapy with a single TNF inhibitor (e.g., etanercept, adalimumab, infliximab, golimumab, certolizumab pegol) * Females must have a negative pregnancy test prior to study dosing * Sexually active subjects and their partners must agree to contraceptive requirements 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. * Subjects with inflammatory, rheumatological disorders other than RA * Pregnant or nursing female subjects * Subjects who have a female partner who is pregnant, nursing, or planning to become pregnant * Subjects who have planned major surgery (e.g., joint replacement) or procedures during the study * History of drug abuse or positive drug screen * History of alcohol 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.