This is a randomized study of the safety and tolerability of multiple doses of REGN88 in rheumatoid arthritis patients who are receiving treatment with methotrexate.

This is a multi-centered, randomized, double-blind, placebo-controlled, ascending parallel-group study of the safety and tolerability of REGN88 in patients with rheumatoid arthritis who are receiving concomitant methotrexate.

This study will be conducted in 3 parts, and will include a total of 6 dose cohorts. Part B will not begin until the safety of Part A has been assessed. Part C will not begin until enrollment in Part B is complete.

Inclusion Criteria:

• Patients with an established diagnosis of Rheumatoid Arthritis
• Patients currently treated with concomitant methotrexate for at least 12 weeks, with a stable dose for at least 6 weeks

Exclusion Criteria:

• Persistent chronic or current active infections
• Patients who have taken anakinra within 2 weeks
• Patients who have taken etanercept, cyclosporine, mycophenolate, tacrolimus, gold, penicillamine, sulfasalazine or hydroxychloroquine within 4 weeks
• Patients who have taken adalimumab within 6 weeks
• Patients who have taken abatacept, azathioprine, cyclophosphamide or infliximab within 12 weeks
• Patients who have taken leflunomide or rituximab within 6 months
• Patients who have had prior treatment with tocilizumab or any other anti-IL-6 medication
• Significant arthritis or other medical condition that could interfere with study evaluations
• Participation in any clinical research study evaluating another investigational drug within 30 days