Study to Investigate the Safety and Efficacy of Tregalizumab in Subjects (MTX-IR) With Active Rheumatoid Arthritis

Inclusion Criteria: 1. Subject demonstrates active RA according to the 1987 American College of Rheumatology (ACR) or 2010 ACR/European League Against Rheumatism (EULAR) classification criteria for RA with functional class I-III for ≥6 months. 2. Subject receives oral or parenteral MTX treatment for ≥12 weeks (overall), with an unchanged mode of application and stable MTX dose of ≥15 mg per week (or ≥12.5 mg per week in case of MTX intolerance), but no more than the highest locally approved dose for RA, for ≥8 weeks prior to baseline. The dose of MTX is expected to remain stable throughout the study and may be adjusted only for safety reasons. If applicable, the dose of folic acid must be unchanged for ≥8 weeks prior to baseline. 3. Subject meets the following two criteria at both screening and baseline: - At least 6 swollen joints at 28-joint assessment. - At least 6 tender joints at 28-joint assessment. 4. Subject has an erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP) above the upper limit of normal (ULN) at screening. These tests may be repeated once during the screening period at the discretion of the investigator. 5. Subject is ≥18 and ≤75 years of age. 6. Subject has a body mass index ≥18 and ≤35 kg/m². 7. Subject receives treatment with corticosteroids ≤10 mg prednisone equivalent, stable for at least 4 weeks prior to baseline and during the study, if applicable. 8. Subject receives treatment with non-steroidal anti-inflammatory drugs (NSAIDs), stable for at least 2 weeks prior to baseline and during the study, if applicable. 9. Female subjects of childbearing potential: has both a negative serum pregnancy test at screening and a negative urine pregnancy test at baseline. 10. Subject is judged to be in good general health as determined by the investigator based upon the results of medical history, laboratory profile, physical examination, chest X-ray (within 3 months before screening date is acceptable), and 12-lead electrocardiogram (ECG). 11. Subject has a cluster of differentiation 4 (CD4) cell count of \> 400/µl at screening. Exclusion Criteria: 1. Subject has previous exposure to any systemic biologic therapy (e.g., etanercept, adalimumab, rituximab, abatacept, tocilizumab), to Janus kinase (JAK) or spleen tyrosine kinase (SYK) inhibitors, or to Tregalizumab. Previous treatment with an anti-TNF agent is allowed only, if all of the following criteria apply: - treatment was stopped for reasons other than lack of efficacy or adverse events (AEs) - treatment was stopped at least 12 weeks or five half-lives of the compound prior to baseline (whichever is longer), and - the treatment period did not exceed 6 weeks. 2. Subject received treatment with conventional disease-modifying anti-rheumatic drugs (DMARDs) apart from MTX in the 12 weeks prior to baseline, and for DMARD leflunomide in the 24 weeks prior to baseline (except where specific leflunomide wash-out procedures were completed, following applicable guidelines). 3. Subject has been treated with intra-articular or parenteral administration of corticosteroids in the 4 weeks prior to baseline. Inhaled corticosteroids for stable medical conditions are allowed. 4. Subject has undergone joint surgery in the 12 weeks prior to baseline (at joints to be assessed within the study) or has undergone major surgery (e.g., abdominal surgery) in the 8 weeks prior to baseline. 5. Subject has a history of acute inflammatory joint disease of an origin other than RA or subject has any other rheumatic disease other than RA (e.g., mixed connective tissue disease, seronegative spondylarthropathy, psoriatic arthritis, Reiter's syndrome, fibromyalgia, systemic lupus erythematosus or any arthritis with onset prior to age 17 years). However, subjects may have secondary Sjögren's syndrome.