Efficacy and Safety Study of TAS5315 Compared With Placebo in Participants With Rheumatoid Arthritis.

Inclusion Criteria: * Have a diagnosis of RA according to 2010 ACR/ EULAR rheumatoid arthritis (RA) classification criteria * Have been treated with methotrexate (MTX) for at least 90 days prior to screening, and must be on a stable dose between 8 and 16 mg/week for at least 56 days prior to screening. * Have an inadequate response to MTX * Have a minimum of 6 swollen and 6 tender joints from 66/68 joint count * Have hsCRP of ≥ 0.6 mg/dL Exclusion Criteria: * Have been treated with conventional synthetic disease-modifying anti-rheumatic drug, except for MTX, within 28 days prior to randomization * Have an inadequate response to biologic disease-modifying anti-rheumatic drug or Have been treated with 2 biologic treatment * Have been treated with Janus Kinase inhibitors or other Bruton's Tyrosine Kinase inhibitors * Have a positive result for hepatitis B surface antigen/antibody, hepatitis B core antibody, hepatitis C virus antibody or human immunodeficiency virus antigen/antibody at screening * Have been treated with Oral steroids at dose above 10 mg/day of prednisone or prednisone equivalents * Have a diagnosis of Felty's syndrome * Have a positive result of the QuantiFERON®-tuberculosis (TB) Gold test or a T-spot ®-TB test at screening * Have a positive result of β-D-glucan at screening