The primary objective of this study is to evaluate the effects of CKD-506 on signs and symptoms of RA in subjects with moderate-to-severe RA who are inadequate responders to methotrexate.

Inclusion Criteria:

• Diagnosis of RA for at least 6 months prior to Screening, currently meet the 2010 American College of Rheumatology (ACR)/European League Against Rheumatism (EULAR) criteria for RA, and are ACR functional class I-III.

• Have active RA

• Ongoing treatment with a stable dose of MTX as described below:

  1. Use of oral or injectable MTX on a continuous basis for at least 12 weeks prior to Baseline and on a stable dose and route of administration between 15 mg and 25 mg/weekly for at least 8 weeks prior to Baseline and planned during the study.
  2. Subjects should be on an adequate and stable dose of folic acid for at least 4 weeks prior to first administration of study treatment and planned during the study.

• Women of childbearing potential must use a medically acceptable means of birth control and agree to continue its use during the study and for at least 12 weeks after the last dose of study treatment.

• Women of childbearing potential must have a negative serum pregnancy test at Screening and urine pregnancy test at Baseline

• Sexually active men, if not surgically sterile, must agree to use a medically acceptable form of contraception during the study and continue its use for at least 12 weeks after the last dose of study treatment.

Exclusion Criteria:

• Treatments for RA as follows: JAK inhibitors at any time; use of any currently licensed biologics with DMARD properties at any time.
• Use of oral steroids at a dose >10 mg/day of prednisone or prednisone equivalent or at a dose that has not been stable for at least 4 weeks prior to Screening.
• Use of nonsteroidal anti-inflammatory drugs (NSAIDs) which have not been at a stable dose or route of administration for at least 2 weeks prior to Baseline and planned during the study.
• History of tuberculosis (TB) infection.
• Positive serology for human immunodeficiency virus 1 or 2, hepatitis B virus or hepatitis C virus.
• Currently active infection or history of infection within the last 2 weeks of Screening or Baseline