The purpose of this study is to determine the relative bioavailability and pharmacokinetic profiles from three strengths of a new tablet formulation of ASP015K and to evaluate the safety and tolerability after a single dose of three different strengths of a new tablet formulation.

Eligible subjects will be admitted on Day-1 and remain confined on the unit for 16 days. Each subject will receive a single dose of study drug per the defined treatment periods on the mornings of Day 1, Day 6 and Day11. There will be a minimum of 5 days between each consecutive dose group. Any subject discontinuing the study prior to completion should have all end of study evaluations completed.

Inclusion Criteria:

• Subject weighs at least 45 kg and has a body mass index (BMI) of 18-32 kg/m2
• Male subject agrees to sexual abstinence, is surgically sterile or is using a medically acceptable method to prevent pregnancy and agrees to continue using this method until 90 days after the last dose of study drug administration
• Male subject agrees to not donate sperm until 90 days after the dose of study drug administration
• Female subject is surgically sterile or is post-menopausal and is not pregnant and is not lactating
• Subject's 12-lead electrocardiogram (ECG) is normal
• Subject must be capable of swallowing multiple tablets

Exclusion Criteria:

• Subject has a previous history of any clinically significant gastro-intestinal, neurological, renal, hepatic, pulmonary, metabolic, cardiovascular, psychiatric, endocrine, hematological disorder or disease, malignancy excluding non-melanoma skin cancer or any other medical condition
• Subject has had major GI surgery (such as colectomy, cholecystectomy, etc)
• Subject has a history of consuming more than 14 units of alcoholic beverages per week or has a history of alcoholism or drug/chemical/ substance abuse within past 2 years
• Subject has/had a symptomatic, viral, bacterial (including upper respiratory infection), or fungal (non-cutaneous) infection within 1 week
• Subject has a positive test for hepatitis C antibody, or positive test for hepatitis B surface antigen (HBsAg), or positive hepatitis B core antibody
• Subject has a history of the human immunodeficiency virus (HIV) antibody
• Subject has a positive tuberculosis (TB) skin test, Quantiferon Gold test or T-SPOT® test
• Subject received any vaccine within 60 days
• Subject received an experimental agent within 30 days
• Subject has an absolute neutrophil count (ANC) < 2500 cells/mm3
• 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
• Subject has a history of smoking or has used tobacco-containing products and nicotine or nicotine-containing products in the past six months