A Study to Evaluate the Long-term Safety and Efficacy of ASP015K in Subjects Previously Enrolled in a Phase 2 ASP015K Rheumatoid Arthritis Study

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