The objective of the study is to characterize the effect of multiple doses of ASP015K on the pharmacokinetics of a single dose of midazolam.

Inclusion Criteria:

• Subject weighing at least 45kg and BMI of 18-32 kg/m2, inclusive
• If female, 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, subject agrees to sexual abstinence, is surgically sterile, or is using a medically acceptable method to prevent pregnancy during the study period

Exclusion Criteria:

• History of any clinically significant disorder, disease or malignancy excluding non-melanoma skin cancer
• Recent history (within 6 months) of drug or alcohol abuse or positive urine screen for drugs of abuse/illegal drugs
• Treatment with prescription drugs or complementary and alternative medicines (CAM) within 14 days prior to study drug administration, or over the counter medication within 1 week prior to study drug administration
• A symptomatic, viral, bacterial or fungal infection within 1 week prior to clinic check-in
• Positive test for hepatitis C antibody, or positive test for hepatitis B surface antigen (HBsAg)
• History of the human immunodeficiency virus (HIV) antibody
• Positive tuberculosis (TB) skin test or Quantiferon Gold test
• Vaccinated within the last 30 days prior to study drug administration
• Received an experimental agent within 30 days or five half-lives, whichever is longer, prior to study drug administration
• 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 admission
• Absolute neutrophil count (ANC) <2500 cells/mm3