A Study in Participants With Rheumatoid Arthritis on Background Methotrexate Therapy

Inclusion Criteria

• Must have active RA
• Must regularly use methotrexate (MTX) for at least 12 weeks before your participation in this study
• Must have American College of Rheumatology (ACR) functional class I, II, or III
• Must have C-reactive protein (CRP) measurement > 1.2 times upper limit of normal (ULN) or Erythrocyte Sedimentation Rate (ESR) > ULN [28 millimeters/hour (mm/hr)]
• Have laboratory values that in the opinion of the investigator do not pose an unacceptable risk to the participants if study drug would be administered
• Must have venous access sufficient to allow blood sampling as per the protocol
• Must be reliable and willing to be available for the duration of the study and are willing to follow study procedures
• Must be able to read, understand, and give written informed consent approved by Lilly or its designee and the ethical review board (ERB) governing the site
• Male participants: agree to use 2 forms of highly effective methods of birth control with female partners of childbearing potential during the study
• If you are a woman and you could become pregnant during this study, you must talk to the study doctor about birth control. You are required to use 2 forms of highly effective methods of birth control to avoid getting pregnant during the study
• If you are a post-menopausal woman, you must be at least 45 years of age and have not menstruated for the last 12 months
• If you are a woman between 40 and 45 years of age, test negative for pregnancy, and have not menstruated during the last 12 months only, you must have an additional blood test
• For participants receiving corticosteroids, you must be on a dose not to exceed 10 mg of prednisone daily (or equivalent) and have been on the same dosing regimen for at least 6 weeks prior to randomization
• Continue to meet inclusion criteria for Parts A and B as applicable
• Part D only: have completed the 52 weeks (Week 24 to Week 76) of participation in Part C of the study without permanent study drug discontinuation and have not completed the Follow-Up Visit (approximately 28 days after the last dose of study drug)

Exclusion Criteria

• Must not have received any parenteral corticosteroid administered by intra-articular, intramuscular (IM), or intravenous (IV) injection within 6 weeks prior to baseline
• Must not be concomitantly using non-steroidal anti-inflammatory drugs (NSAIDS), unless you are on a stable dose within the last 4 weeks
• Must not have received any prior biologic disease modifying anti-rheumatic drug (DMARD) therapy [such as Tumor necrosis factor-alpha (TNFα), interleukin (IL)-1, IL-6, T-cell or B-cell target therapies)
• Must not have used DMARDs other than methotrexate (MTX), hydroxychloroquine, or sulfasalazine within the last 8 weeks
• Must not have used leflunomide within the last 12 weeks and have not received cholestyramine to speed up the elimination of leflunomide from your body
• Must not have previously been randomized, completed or withdrawn from this study or any other study investigating LY3009104
• Must not have received prior treatment with an oral JAK inhibitor
• Must not have a current or recent (within the last 30 days) viral, bacterial, fungal, or parasitic infection
• Must not have had a serious infection (for example, pneumonia, cellulitis, or bone or joint infections) or atypical mycobacterial infection within the last 6 months
• Must not have had symptomatic herpes zoster or herpes simplex infection within the last 90 days or have a history of disseminated/complicated herpes zoster
• Must not have evidence of human immunodeficiency virus (HIV) and/or positive human HIV antibodies
• Must not have evidence of hepatitis C virus (HCV) or active hepatitis B
• Must not have evidence or suspicion of active or latent tuberculosis (TB)
• Must not have another serious disorder or illness
• Must not be exposed to a live vaccine within the last 12 weeks
• Must not have donated more than 500 milliliters (mL) of blood