This is a Phase 1 study, involving a 29-day treatment period, designed to evaluate the pharmacokinetics and the effectiveness of investigational study drug ARRY-371797 in treating rheumatoid arthritis in patients receiving stable doses of methotrexate, and to further evaluate the drug's safety. Approximately 30 patients from the US will be enrolled in this study.

Key Inclusion Criteria:

• Diagnosed with rheumatoid arthritis (RA) at least 3 months prior to study start based upon the American College of Rheumatology (ACR) 1987 Revised Criteria.
• On a stable weekly dose of both methotrexate (MTX) (10 - 25 mg/week inclusively; either oral or parenteral) and folate (≥ 5 mg weekly ) for ≥ 6 weeks prior to study start and is willing to continue on these regimens for the duration of the study.
• Meets the ACR 1991 Revised Criteria for Global Functional Status in RA, Class I, II, or III.
• Completed an appropriate washout period if treated with specified therapies.
• Patients may continue on specified stable background therapy for RA (doses should be stable for at least 6 weeks prior to the first dose of study drug, unless the patient stops due to documented disease improvement, with Sponsor approval).
• Additional criteria exist.

Key Exclusion Criteria:

• Diagnosis of any other inflammatory or non-inflammatory arthritis (e.g. spondyloarthritis; fibromyalgia, psoriatic arthritis, crystal-proven gout) that may interfere with disease activity assessments and/or clinically apparent osteoarthritis which would affect subsequent efficacy measures.

• Has received any of the following prior treatments:

  1. Within 4 weeks of first dose of study drug: anakinra (Kineret®), etanercept (Enbrel®);
  2. Within 8 weeks of first dose of study drug: infliximab (Remicade®), adalimumab (Humira®), leflunomide (Arava®);
  3. At any time: rituximab (Rituxan®), alemtuzumab (CamPath®), any experimental B cell targeting agents.

• Treatment with potent CYP3A inhibitors or inducers, disease-modifying antirheumatic drugs (DMARDs) and/or biologic response modifiers (BRMs) during the study.

• Additional criteria exist.