A Study to Assess the Safety, Tolerability and Pharmacokinetics of ASP015K in Healthy Subjects

Inclusion Criteria: * Female subject must be either: * Of non-childbearing potential: 1. postmenopausal (defined as at least 1 year without any menses) prior to screening, 2. or documented surgically sterile or status post-hysterectomy (at least 1 month prior to screening). * Or, if of childbearing potential: 1. must have a negative pregnancy test at screening and day -1. 2. must use highly effective contraception consisting of 2 forms of birth control (1 of which must be a barrier method) starting at screening and throughout the study period and for 90 days after final study drug administration. * 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 2 forms of birth control (1 of which must be a barrier method) starting at screening and continuing throughout the study period, and for 90 days after final study drug administration. * Male subject must not donate sperm starting at screening and continuing throughout the study period, and for 90 days after final study drug administration. * Subject has a Body Mass Index (BMI) range of 18.5 to 32.0 kg/m2, inclusive, and must weigh at least 50 kg at screening. * Subject must be capable of swallowing multiple (up to 20) tablets. * Subject agrees not to participate in another investigational study while on treatment. 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 formulations used. * Subject has any of the liver function tests (aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase, gamma-glutamyl transferase or total bilirubin) above the ULN at screening or day -1. If the result is outside the limits, the assessment may be repeated once at screening and day -1. * Subject has any clinically significant history of allergic conditions. * Subject has any history or evidence of any clinically significant cardiovascular, gastrointestinal (GI), endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, renal and/or other major disease or malignancy, as judged by the investigator or designee. * Subject has/had febrile illness or symptomatic, viral, bacterial (including upper respiratory infection) or fungal (non-cutaneous) infection within 1 week prior to day -1. * Subject has any clinically significant abnormality following the investigator's review of the physical examination, electrocardiogram (ECG) and protocol-defined clinical laboratory tests at screening or day -1. * Subject has a mean pulse \< 40 or \> 90 beats per minute, mean systolic blood pressure (BP) \> 140 mmHg or mean diastolic BP \> 90 mmHg (measurements taken in triplicate after subject has been resting in sitting position for 5 minutes) at screening or day -1. * Subject has a mean QTcF interval of \> 430 msec (for males) and \> 450 msec (for females) at screening or day -1. If the mean QTcF exceeds the limits above, 1 additional triplicate ECG can be taken. If this triplicate also gives an abnormal result, the subject should be excluded. * 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), hormonal contraceptives and intermittent acetaminophen (no more than 2g per day). * Subject has smoked or has used tobacco-containing products and nicotine or nicotine-containing products in the past 6 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 the past 2 years prior to screening (Note: 1 unit = 12 ounces of beer, 4 ounces of wine or 1 ounce of spirits). * Subject has a positive test for alcohol, drugs of abuse or cotinine at screening or day -1. * Subject anticipates an inability to abstain from xanthine (e.g., caffeine), grapefruit, Seville oranges (including marmalade), star fruit or any products containing these items from 72 hours prior to day -1 and throughout the duration of the study. * Subject has used any inducer of metabolism (e.g., barbiturates, rifampin) in the 3 months prior to day -1. * Subject has had any significant blood loss, donated 1 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 day -1. * Subject has a positive test for hepatitis B surface antigen, anti-hepatitis A virus (Immunoglobulin M) antibody, anti-hepatitis C virus antibody, hepatitis B core antibody or anti-human immunodeficiency virus Type 1 or Type 2 at screening. * Subject has a positive tuberculosis skin test, Quantiferon Gold® or T-SPOT® test at screening. * Subject has 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. If the result is outside the limits, the assessment may be repeated once at screening and day -1. * Subject has had major GI surgery or has a medical condition that may inhibit the absorption and/or metabolism of study drug. * Subject has participated in any interventional clinical study or has been treated with any investigational drugs within 30 days or 5 half-lives of the drug, whichever is longer, prior to screening. * Subject has any other condition, which in the opinion of the investigator, precludes the subject's participation in the study. * Subject is an employee of the Astellas Group, Janssen Pharmaceuticals or vendors involved in the study. * Subject has participated in a prior study with ASP015K.