A Study to Evaluate the Efficacy and Safety of ASP015K in Moderate to Severe Rheumatoid Arthritis Subjects

Inclusion Criteria: * Subject has received a full explanation of the study drug and this study in advance, and written informed consent to participate in the study has been obtained from the subject himself/herself * Outpatient has RA that was diagnosed according to the 1987 revised criteria of the ACR at least 6 months prior to screening * At screening subject has active RA as evidenced by all of the following: * ≥ 6 tender/painful joints; * ≥ 6 swollen joints; * CRP of ≥ 0.5 mg/dL or ESR of ≥ 28 mm/h.C-Reactive Protein (CRP) of ≥ 0.8 mg/dL or Erythrocyte Sedimentation Rate (ESR) of ≥ 28 mm/hr * Subject meets the ACR 1991 Revised Criteria for the Classification of Global Functional Status in RA, Class I, II or, III at screening Exclusion Criteria: * Positive tuberculin (TB) test within 90 days of Screening * Abnormal chest x-ray indicative of an acute or chronic infectious process or malignancy * Receipt of live or live attenuated virus vaccination within 30 days prior to the first dose of study drug * Hepatitis B virus or hepatitis C virus carrier or has a history of a positive test for human immunodeficiency virus (HIV) infection * Any other autoimmune rheumatic disease, other than Sjogren's syndrome * Previous history of clinically significant infections or illness (requiring hospitalization or requiring parenteral therapy) within 90 days of the Screening visit, or a history of any illness that would preclude participation in the study * History of any malignancy, except for successfully treated basal or squamous cell carcinoma of the skin or in-situ carcinoma of the cervix * Does not meet specified washout criteria for the following RA medications: etanercept, certolizumab, adalimumab, golimumab, infliximab and tocilizumab, rituximab, abatacept, anakinra, methotrexate, sulfasalazine, hydroxychloroquine, gold, penicillamine or leflunomide * Previous intolerance to Janus kinase (JAK) inhibitors * Receipt of intra-articular or parenteral corticosteroid within 28 days prior to the first dose of study drug * Receipt of plasma exchange therapy within 60 days prior to the start of study drug * Receipt of any investigational agent within 30 days or 5 half-lives, whichever is longer, prior to first dose of study drug * Receipt of medications that are CYP3A substrates with narrow therapeutic range within 14 days prior to first dose of study drug * History of heart failure, defined as New York Heart Association (NYHA) grade 3 or greater