A drug interaction study to evaluate the effect of ASP015K (twice daily) on the pharmacokinetics (PK) of once weekly oral methotrexate (MTX).

Patients check in on day -1 and remain confined until all the exit procedures are performed on the morning of day 10. Patients to return for 1 post-treatment follow-up visit on day 13.

Patients receive a single dose of methotrexate on day 1 and day 8 and ASP015K (twice daily) on days 3 through 8 plus the morning of day 9.

Inclusion Criteria:

1. If female, subject must be at least 2 years post menopausal or is surgically sterile per documentation provided by a medical professional and the subject is not pregnant as documented by a negative serum pregnancy test at Screening and negative urine pregnancy test at check-in
2. If male, subject must agree to sexual abstinence and/or to use a highly effective method of birth control during the study period and for 60 days after the last dose of study drug
3. Subject must have a clinical diagnosis of Rheumatoid Arthritis (RA) at least 6 months prior to Screening
4. Subject must be on a stable 15 - 25 mg dose of methotrexate > 28 days prior to Screening
5. If subject is on a non-biologic disease modifying anti-rheumatic drug (DMARDs) therapy, the dose must be stable > 28 days prior to Screening
6. Subject must be willing and able to comply with the study requirements

Exclusion Criteria:

1. Subject has a previous history of any clinically significant neurological, gastrointestinal, renal, hepatic, pulmonary, metabolic, cardiovascular, psychiatric, endocrine, hematological disorder or disease, or any other medical condition that would preclude participation in the study
2. Subject has a known history of positive test for hepatitis B surface antigen (HbsAg) or hepatitis C antibody or history of a positive test for human immunodeficiency virus (HIV) infection
3. Subject has received live virus vaccination within the last 30 days prior to study drug administration
4. Subject has a Body Mass Index (BMI) > 35 (kg/m2)
5. Subject is on biologic disease modifying anti-rheumatic drug (DMARDs) therapy
6. Subject has a current history of anemia as defined by hemoglobin of less than 12 g/dL
7. 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 at Screening or check-in
8. Subject has received any investigational agent within 30 days of screening

Access to anonymized individual participant level data will not be provided for this trial. Further details on Astellas' data sharing policy can be found at https://www.clinicaltrials.astellas.com/transparency/.