A Study Evaluating the Efficacy and Safety of CKD-506 in Adult Subjects With Moderate-to-Severe Rheumatoid Arthritis and Inadequate Response 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