This study characterizes the pharmacokinetic effects of ASP015K on Tacrolimus in healthy volunteers.

The subjects will be confined to the unit for 13 days and have a brief follow-up visit to obtain hematology blood samples. Numerous blood and urine samples will be taken to determine the pharmacokinetics of the drugs.

Inclusion Criteria:

• If female, the subject is at least 2 years post menopausal or is surgically sterile per documentation provided by a third party medical professional and the subject is not pregnant as documented by a negative serum pregnancy test
• If male, the subject agrees to sexual abstinence and/or to use a highly effective method of birth control during the study period
• Subject is medically healthy, with no clinically significant medical history or abnormalities observed upon physical examination or 12-lead electrocardiogram (ECG)
• Subjects must weigh at least 45 kg and have a body mass index (BMI) of 18-32 kg/m2

Exclusion Criteria:

• Subject has a history of chronic diarrhea
• Subject has been vaccinated within the last 60 days prior to study drug administration
• The subject has a previous history of any clinically significant neurological, gastro-intestinal, renal, hepatic, pulmonary, metabolic, cardiovascular, psychiatric, endocrine, hematological disorder or disease
• Subject has a positive test for hepatitis C antibody, or positive for hepatitis B surface antigen (HBsAg)
• Subject has a history of the human immunodeficiency virus (HIV) antibody
• The subject has an absolute neutrophil count (ANC) < 2500 cells/mm3 Subject has had clinically significant illness within 1 month prior to study drug administration
• Subject has a recent history (within the last 6 months) of drug or alcohol abuse or a positive urine screen for alcohol or drugs of abuse/illegal drugs