A Study to Assess the Relative Bioavailability Between Two ASP015K Tablets and the Food Effect of a New Tablet in Healthy Adult Subjects

Inclusion Criteria: * Female subject must be of non-childbearing potential (i.e., post-menopausal \[defined as at least 1 year without menses prior to screening\], or documented surgically sterile or status post hysterectomy \[at least 1 month prior to screening\]). * Female subject must have a negative pregnancy test at screening and day -1 of treatment period 1. * Female subject must not donate ova starting at screening and throughout the study period, and for 90 days after the final study drug administration. * Male subject and his female spouse/partner who is of childbearing potential must be using highly effective contraception consisting of two forms of birth control (one of which must be a barrier method) starting at screening and continue throughout the study period and for 90 days after final study drug administration. * Male subject must not donate sperm starting at screening and throughout the study period and for 90 days after final study drug administration. * Subject has a Body Mass Index (BMI) range of 18.5-32.0 kg/m2, inclusive, and must weigh at least 50 kg at screening. Exclusion Criteria: * Female subject who has been pregnant within 6 months before screening assessment or breast feeding within 3 months before screening. * Subject has a known or suspected hypersensitivity to ASP015K, or any components of the formulation used. * Subject has any clinically significant history of allergic conditions (including drug allergies, asthma, eczema, or anaphylactic reactions, but excluding untreated, asymptomatic, seasonal allergies at time of dosing). * Subject has/had febrile illness or symptomatic, viral, bacterial (including upper respiratory infection), or fungal (non-cutaneous) infection within 1 week prior to first clinic check-in. * Subject has used any prescribed or non-prescribed drugs (including vitamins, natural and herbal remedies, e.g., St. John's Wort) in the 2 weeks prior to study drug administration, with the exception of hormone replacement therapy (HRT), intermittent acetaminophen (to a maximum of 2 g/day). * Subject has smoked or has used tobacco-containing products and nicotine or nicotine-containing products in the past six months prior to screening. * Subject has a history of consuming more than 14 units of alcoholic beverages per week within 6 months prior to screening or has a history of alcoholism or drug/chemical substance abuse within past 2 years prior to screening (Note: one unit = 12 ounces of beer, 4 ounces of wine or 1 ounce of spirits) prior to day -1 of treatment period 1. * Subject has had any significant blood loss, donated one unit (450 mL) of blood or more, or received a transfusion of any blood or blood products within 60 days or donated plasma within 7 days prior to clinic check-in on day -1 of treatment period 1. * Subject has a positive test for hepatitis B surface antigen (HBsAg), anti-hepatitis A virus (HAV) (Immunoglobulin \[Ig\] M), anti-hepatitis C virus (HCV), hepatitis B core antibody, or anti-human immunodeficiency virus (HIV) type 1 or type 2 at screening. * Subject has a positive tuberculosis (TB) skin test, Quantiferon Gold® test or T-SPOT® test at screening. * Subject received any vaccine within 60 days prior to study drug administration. * Subject has an absolute neutrophil count (ANC) \< 2000 cells/mm3 or a creatine phosphokinase (CPK) \> 1.5 x ULN at screening or day -1 of treatment period 1. * Subject has had major gastrointestinal (GI) surgery or has a medical condition, which may inhibit the absorption and/or metabolism of study drug.