The purpose of this study is to evaluate the effect of verapamil, a P-glycoprotein (P-gp) inhibitor, on the pharmacokinetics of ASP015K. This study will also assess the safety and tolerability of ASP015K administered alone and also and in combination with verapamil.

Eligible subjects will be admitted to the clinical unit on day -1 and remain confined until day 15.

Inclusion Criteria:

• Subject has a Body Mass Index (BMI) range of 18.5-32.0 kg/m2, inclusive, and must weigh at least 50 kg
• Subject must be capable of swallowing multiple tablets
• Subject agrees not to participate in another investigational study while on treatment

Exclusion Criteria:

• Subject has a known or suspected hypersensitivity to verapamil, ASP015K, or any components of the formulations used.
• Subject has any of the liver function tests above the upper limit of normal (ULN)
• 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 any history or evidence of any clinically significant cardiovascular, GI, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, renal, and/or other major disease or malignancy
• Subject has/had febrile illness or symptomatic, viral, bacterial (including upper respiratory) infection, or fungal (noncutaneous) infection within 1 week prior to day -1.
• Subject has any clinically significant abnormality following physical examination, ECG, such as sick sinus syndrome, second- or third-degree atrioventricular block, or atrial flutter/atrial fibrillation, or clinical laboratory tests
• Subject has a mean pulse < 50 or > 90 beats per minute (bpm); mean systolic blood pressure (SBP) < 100 or > 140 mmHg; mean diastolic blood pressure (DBP) < 60 or > 90 mmHg (measurements taken in triplicate after subject has been resting in sitting position for 5 minutes)
• Subject has a mean QTcF interval of > 430 msec (for males) and > 450 msec (for females)
• Subject has used any prescribed or nonprescribed 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) and intermittent acetaminophen (no more than 2 g per day)
• Subject has smoked or has used tobacco-containing products and nicotine or nicotine-containing products in the past 6 months
• 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)
• Subject has a positive test for alcohol, drugs of abuse, or cotinine
• 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 past 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 past 7 days
• Subject has a positive test for hepatitis B surface antigen (HBsAg), anti-hepatitis A virus (Immunoglobulin [Ig] M), anti-hepatitis C virus (HCV), hepatitis B core antibody or anti-human immunodeficiency virus (HIV) type 1 or type 2
• Subject has a positive tuberculosis (TB) skin test, Quantiferon Gold® test or T-SPOT® test
• 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
• Subject has had major gastrointestinal (GI) surgery or has a medical condition, which 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