This study is to evaluate the efficacy and safety of ASP015K monotherapy and to evaluate the dose-dependent response of ASP015K in moderate to severe Rheumatoid Arthritis (RA) subjects given ASP015K for 12 weeks.

This is a Phase 2b, randomized, double-blind, parallel-group, placebo-controlled, dose-finding, monotherapy, multi-center study with once daily oral ASP015K or matching placebo in subjects with moderate to severe RA, with or without prior antirheumatic medication, and regardless of responsiveness to the medication.

The study is comprised of up to a 4-week Screening period, a 12-week Treatment period and a 4-week Follow-up period.

Subjects in each treatment group will take ASP015K or matching placebo orally, once daily, after breakfast for 12 weeks after the screening period.

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

Access to anonymized individual participant level data collected during the study, in addition to study-related supporting documentation, is planned for studies conducted with approved product indications and formulations, as well as products terminated during development. Studies conducted with product indications or formulations that remain active in development are assessed after study completion to determine if Individual Participant Data can be shared. Further details on Astellas' data sharing policy can be found at https://www.clinicaltrials.astellas.com/transparency/.

Access to participant level data is offered to researchers after publication of the primary manuscript (if applicable) and is available as long as Astellas has legal authority to provide the data.

Researchers must submit a proposal to conduct a scientifically relevant analysis of the study data. The research proposal is reviewed by an Independent Research Panel. If the proposal is approved, access to the study data is provided in a secure data sharing environment after receipt of a signed Data Sharing Agreement.