Safety, Tolerability, PK and PD of Biosimilar Drug Ritumax® Compared to Original Drug MabThera®

Inclusion Criteria: 1. Signed Patient Information Sheet and Informed Consent Form to participate in the study. 2. Men and women aged 18 years and older. 3. Patient diagnosed with rheumatoid arthritis of at least 6-month duration, determined according to classification criteria of the American College of Rheumatology (ACR) and the European League Against Rheumatism (EULAR) (ACR-EULAR 2010) or the American College of Rheumatology (ACR1987) (in case if the diagnosis of rheumatoid arthritis was determined till 2010). 4. Active phase of the disease confirmed due to the combination of the following parameters: * DAS28 \> 3,2 * 5 (of 28) of tender and 5 (of 28) of swollen joints * CRP level ≥1,5 mg/dL and/or ESR \> 28 mm/h * positive test for rheumatoid factor and/or CCP antibodies 5. Patients with negative response to or intolerability to the DMARD therapy. 6. Current outpatient therapy of rheumatoid arthritis: * continuous therapy with Methotrexate for at least 12 weeks prior to screening * stable doses of Methotrexate (10-25 mg per week) within 4 weeks prior to screening * when currently under the corticosteroids therapy, the dose should be stable within the last 4 weeks prior to screening and be less than 10 mg of Prednisolone or its equivalent * when currently under the NSAID therapy, the dose should be stable within the last 4 weeks prior to screening Exclusion Criteria: The patient will be deemed ineligible for the study meeting any of the following criteria: 1. Other inflammatory arthropathies apart from rheumatoid arthritis (e.g. gout, reactive arthritis, psoriatic arthritis, seronegative spondyloarthropathy, Lyme desease) or other system autoimmune diseases (e.g. systemic lupus erythematosus, inflammatory bowel disease, pneumosclerosis or Felly's syndrome, sclerodermia, inflammatory myopathy, mixed collagenosis or other crossed syndrome). Patients with secondary Sjorgen's syndrome or secondary limited cutaneous vasculitis with a rheumatoid arthritis background may participate in the study. 2. Chronic heart failure Class III or IV in New York Heart Association (NYHA) classification and clinically significant ventricular arrhythmias (ventricular tachycardia, ventricular fibrillation). 3. Serious chronic pulmonary diseases (COPD, bronchial asthma); functional dyspnea of severity level III and higher (due to the Medical Science Research Council scale for dyspnea). 4. Signs of significant uncontrolled concomitant disease, e.g. renal, hepatic, gastrointestinal, endocrine system and nervous system disorders, which, according to the Investigator's opinion, could prevent the patient's participation in the study. 5. Any surgical procedure, including bone and joint surgeries, or synovectomy (including arthrodesis or endoprosthesis replacement), performed within 12 weeks prior to screening or planned to be performed within 24 weeks after (except for small surgical procedures, requiring a local anesthesia or no anesthesia). 6. Infectious diseases.