A Study to Evaluate the Safety, Pharmacokinetics, Pharmacodynamics, and Efficacy of Escalating Doses of SAN-300 in Patients with Active Rheumatoid Arthritis with Inadequate Response to Disease-Modifying Anti-rheumatic Drug(s).

Inclusion Criteria:

1. Diagnosed with RA for ≥ 6 months according to American College of Rheumatology (ACR)/European League Against Rheumatism (EULAR) Classification Criteria 2010
2. 18 to 75 years of age, inclusive, at the time of informed consent
3. Swollen joint count of ≥ 6 (66-joint count) and tender joint count of ≥ 6 (68-joint count) at Screening and randomization
4. Inadequate response to therapy or discontinuation of therapy because of unacceptable toxicity from at least one prior traditional or biologic disease-modifying anti-rheumatic drug (DMARD)
5. Stable dose of methotrexate (≥ 15 mg/week and ≤ 25 mg/week) for ≥ 6 weeks before randomization

Exclusion Criteria:

1. Functional Class IV as defined by ACR classification of functional status in RA
2. History of significant systemic involvement secondary to RA (e.g., vasculitis, pulmonary fibrosis, or Felty's syndrome)
3. History of malignancy or carcinoma in situ within the 5 years before Screening or any history of melanoma. Patients with history of excised or adequately treated non-melanoma skin cancer are eligible
4. Evidence of clinically significant uncontrolled concurrent diseases such as cardiovascular, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, renal, and/or other major diseases
5. History of recurrent clinically significant infections
6. Current active infection or serious local infection (e.g., cellulitis, abscess) or systemic infection (e.g., pneumonia, septicemia) within 3 months before randomization
7. History of severe allergic or anaphylactic reactions to other biologic agents
8. History of allergies to murine protein
9. Surgery within 3 months before randomization (other than minor cosmetic surgery or minor dental procedures) or plans for a surgical procedure during the Treatment Period or Follow-up Period
10. History of tuberculosis or latent infection currently undergoing treatment
11. History of malaria
12. Treatment regimen with prednisone that is either over 10 mg/day (or equivalent dose of another corticosteroid) or is not taken at a stable dose of ≤ 10 mg/day for at least 4 weeks before randomization
13. Intra-articular corticosteroid injection(s) within 4 weeks before randomization
14. Any live immunization/vaccination, including against Herpes zoster, within 4 weeks before randomization. Live vaccinations must also be avoided throughout the study
15. Abnormal laboratory value at Screening or Day -1 considered clinically significant
16. Positive for hepatitis C virus (HCV) antibody or hepatitis B surface antigen (HBsAg)
17. Positive for human immunodeficiency virus (HIV) antibody
18. History of tuberculosis or positive QuantiFERON®-TB Gold test (QFT)