The purpose of this study is to evaluate the effects of JNJ-38518168 on the pharmacokinetics (how the body handles a drug) of methotrexate in participants with rheumatoid arthritis.

This is an open-label (physician and participant will know the name of the assigned drug), multi-center, non-randomized, drug/drug interaction study of JNJ-38518168 in participants with rheumatoid arthritis (RA) on a stable oral methotrexate (MTX) dose (7.5-25 mg). The treatment phase will last 16 days. Participants will be housed in the clinic for dosing and study procedures from Day -1 to Day 2 and again from Day 14 to Day 16. Participants completing treatment will have a follow-up visit between Days 22 and 25. Participants who discontinue the study prematurely will complete the Day 22 to 25 follow-up procedures at the time of discontinuation or as soon as possible thereafter. The duration of participation in the study for an individual participant will be up to 7 weeks (including screening). Participant safety will be monitored. Day 1: A single oral dose of the participant's weekly methotrexate dose. Days 2-15: Weekly MTX co-administered with two 50-mg capsules of JNJ-38518168, administered daily.

Inclusion Criteria:

• Have a body mass index (BMI) between 18 and 38 kg/m2 inclusive, and a body weight between 50-110 kg, inclusive
• Have a diagnosis of Rheumatoid Arthritis functional class I-III for at least 3 months prior to Screening
• Have been treated with and tolerated oral MTX treatment for a minimum of 3 months prior to screening and must have been on a once-weekly stable MTX dose (taken as a single dose) (between 7.5-25 mg/week) for a minimum of 1 month
• Participants may continue on stable non-steroid anti-inflammatory drugs and corticosteroid background therapy for RA or stable therapies for other conditions as prescribed by physician, provided that the doses of such therapy are stable for at least 1 month prior to screening and will remain unchanged for the entire study duration.

Exclusion Criteria:

• Have current signs or symptoms of liver or renal insufficiency or cardiac, vascular, pulmonary, gastrointestinal, endocrine, neurologic, hematologic, psychiatric, or metabolic disturbances that are severe, progressive or uncontrolled in the study doctor's opinion
• Have been treated with a monoclonal antibody for RA or have had a serious infection with 2 weeks of the study treatment period.