The purpose of this study is to evaluate the long-term safety and efficacy of ASP015K in subjects with Rheumatoid Arthritis (RA) who have completed a preceding Phase 2 ASP015K RA study.

This study is comprised of a 104 week treatment period beginning on Day 1 of study drug administration and a 30-day follow-up period or permanent termination of clinical development of the product, whichever is earlier.

Inclusion Criteria:

• Subject completed the week 12 visit in one of the Phase 2, ASP015K Rheumatoid Arthritis studies within the previous 14 days.
• Male and Female subjects must be willing to comply with contraception requirements as well as restrictions regarding egg and sperm donation

Exclusion Criteria:

• Subject has any condition considered by the Principal Investigator or Medical Monitor to preclude adequate evaluation of drug safety
• Subject is scheduled to receive any investigational drug other than ASP015K during the course of the study
• Subject is scheduled to receive a prohibited medication
• Subject has a planned major surgery
• Subject discontinued study drug due to meeting study drug discontinuation criteria prior to completion of the Week 12 visit in the preceding study or fulfills study drug discontinuation criteria at the final study visit of the preceding study
• Subject has out of range laboratory values within 14 days of the Day 1 study dosing
• Absolute lymphocyte count (ALC) < 500/mm3
• Creatine Phosphokinase (CPK) > 1.5 x upper limit of normal unless the level has been stable for at least 2 consecutive blood draws (at least 7 days apart), and subject does not have symptoms of muscle aching, weakness, or severe unusual muscle cramps

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.