PLX3397 is a selective inhibitor of Fms and Kit activity. The objective of this study is to evaluate the safety, pharmacokinetics (PK), pharmacodynamics (PD), and drug-drug interaction (DDI) of orally administered PLX3397 during 2 weeks of dosing in patients with rheumatoid arthritis (RA) who are on maintenance methotrexate. This study is planned to provide data to inform dose selection for a subsequent 12 week dose ranging study in RA.