The purpose of the study is to evaluate the effect of LX3305 on methotrexate (MTX) pharmacokinetics and to evaluate the safety and tolerability of LX3305 given over 14 days in subjects with stable rheumatoid arthritis that are receiving stable doses of MTX.

Inclusion Criteria:

• Males and females ≥ 18 years old
• Must be willing to practice 2 adequate methods of contraception for the duration of the study
• Rheumatoid arthritis present for at least 3 months; functional class I, II, or III as defined by ACR criteria
• Treatment with methotrexate (7.5 to 25 mg per week) for at least the last 3 months, and currently receiving stable dose methotrexate for at least one month prior to the start of the study
• Ability to provide written informed consent

Exclusion Criteria:

• Women who are pregnant or nursing
• History of other current inflammatory arthritis
• History of opportunistic infection
• History of recurrent infections or current infection 2 weeks prior to start of study
• Presence of significant, uncontrolled medical problems
• Treatment with any disease-modifying anti-rheumatoid drugs other than methotrexate within 4 weeks prior to start of study
• Use of chondroitin sulfate, glucosamine sulfate, minocycline, or matrix metalloproteinase inhibitors, H-2 blockers, proton pump inhibitors, or misoprostol within 4 weeks prior to study start
• Receipt of live vaccine within 8 weeks prior to study start
• Rheumatoid arthritis, functional class IV as defined by ACR criteria

22 days

An adverse event is defined as any noxious, pathological, or unintended change in anatomical, physiological, or metabolic functions as indicated by physical signs or symptoms occurring in any phase of the clinical study whether or not associated with the study medication and whether or not considered related to the study medication.