A Study to Evaluate Efficacy and Safety of ASP015K in Patients With Rheumatoid Arthritis (RA) Who Had an Inadequate Response to Methotrexate (MTX) Treatment

Inclusion Criteria:

• Subject has RA of < 10 years duration at baseline that was diagnosed according tothe 1987 American College of Rheumatology (ACR) criteria or the 2010 AmericanCollege of Rheumatology/European League against Rheumatism (ACR/EULAR) criteria

• Subject who did not receive the following drugs, or received the drugs with stabledosage for at least 28 days prior to the baseline (start of treatment) for RAtreatment:

• Non-steroidal anti-inflammatory drugs (NSAIDs; excluding topical formulationswith a local action), oral morphine or equivalent opioid analgesics (≤ 30mg/day), acetaminophen, or oral corticosteroids (≤ 10 mg/day in prednisoloneequivalent)

• At screening subject has active RA as evidenced by both of the following:

• ≥ 6 tender/painful joints (using 68-joint assessment)

• ≥ 6 swollen joints (using 66-joint assessment)

• CRP (latex agglutination test) of ≥ 1.00 mg/dL at screening.

• Subject meets the ACR 1991 Revised Criteria for the Classification of GlobalFunctional Status in RA Class I, II or, III at screening

• Inadequate responders to MTX which was continuously administered for at least 90days prior to screening and MTX ≥ 8 mg/week for at least 28 days prior to baseline.However, inadequate responder to MTX < 8 mg/week is eligible if intoleranceprecludes dose increase and defined as MTX-IR

• Subject is able to continue stable dose of MTX (a maximum of 16 mg/week) from atleast 28 days prior to screening until the end of treatment

• Subject has bone erosion at the joint (as evidenced by x-rays of hands and feet)assessed in mTSS and any of the following apply at screening. Bone erosion may beevidenced by x-rays within 90 days prior to baseline.

• Positive anti-CCP antibody: ≥ 4.5 U/mL

• Positive rheumatoid factor: > 15 IU/mL

Exclusion Criteria:

• Subject has received a biologic DMARD within the specified period

• Inadequate responders to biologic DMARD as determined byinvestigator/sub-investigator

• Subject has received intra-articular, intravenous, intramuscular or endorectal (excluding suppositories for anal diseases) corticosteroid within 28 days prior tobaseline

• Subject has participated in any study of ASP015K and has received ASP015K or placebo

• Subject has received other investigational drugs within 90 days or within 5half-lives, whichever is longer, prior to baseline

• Subject has received plasma exchange therapy within 60 days prior to baseline

• Subject has undergone joint drainage, has received local anesthesia and nerve block,or has received articular cartilage protectant at the assessed joint within 28 daysprior to baseline

• Subject has undergone surgery and has residual effects in the assessed joints at thediscretion of investigator/sub-investigator, or is scheduled to undergo surgery thatmay affect the study evaluation of the assessed joints at the discretion ofinvestigator/sub-investigator

• A diagnosis of inflammatory arthritis (psoriatic arthritis, ankylosing spondylitis,SLE, sarcoidosis, etc.) other than RA

• Any of the following laboratory values at screening:

• Hemoglobin < 9.0 g/dL

• Absolute neutrophil count < 1000/μL

• Absolute lymphocyte count < 800/μL

• Platelet count < 75000/μL

• ALT ≥ 2 ×ULN

• AST ≥ 2 × ULN

• Total bilirubin (TBL) ≥ 1.5 × ULN

• Estimated GFR ≤ 40 mL/min as measured by the MDRD method

• β-D-glucan ≥ 11 pg/mL

• Positive HBs antigen, HBc antibody, HBs antibody or HBV-DNA quantitation (However, subject with negative HBs antigen and HBV-DNA quantitation, andpositive HBc antibody and/or HBs antibody is eligible if HBV-DNA is monitoredby HBV-DNA quantitation at every scheduled visit after initiation of study drugadministration.)

• Positive HCV antibody

• Subject has a history of or concurrent active tuberculosis (TB)

• Subject has a history of or concurrent interstitial pneumonia andinvestigator/sub-investigator judges that it is inappropriate for the subject toparticipate in this study

• Subject has a history of or concurrent malignant tumor (except for successfullytreated basal cell carcinoma)

• Subject has received live or live attenuated virus vaccination within 56 days priorto baseline. (Inactivated vaccines including influenza and pneumococcal vaccines areallowed.)

• Subject has any ongoing severe, progressive, or uncontrolled renal, hepatic,hematological, gastrointestinal, metabolic, endocrine, pulmonary, cardiac,neurological, infectious, or autoimmune disease except for RA (excluding Sjogren'ssyndrome and chronic thyroiditis), or any ongoing illness which would make thesubject unsuitable for the study as determined by the investigator/sub-investigator

• Subject has a history of clinically significant allergy. (Clinically significantallergy includes allergies such as systemic urticaria induced by specific antigensand drugs, anaphylaxis, and allergy associated with shock necessitating hospitalizedtreatment.)

• Subject has concurrent cardiac failure, defined as NYHA classification Class III orhigher, or a history of it

• Subject has concurrent prolonged QT syndrome or a history of it. Subject hasprolonged QT interval (defined as QTc ≥ 500 msec. Subject has QTc ≥ 500 msec atretest will be excluded) at screening

• Subject has a history of positive HIV infection

• Subject has congenital short QT syndrome or a history of it. Subject has shortenedQT interval (defined as QTc < 330 msec. Subject has QTc < 330 msec at retest will beexcluded) at screening.