An Adaptive Phase 2a/2b Study of LY3871801 in Adult Participants With Rheumatoid Arthritis

The main purpose of this study is to evaluate the efficacy and safety of LY3871801 in adult participants with active moderately-to-severe rheumatoid arthritis (RA).

Inclusion Criteria: * Have a diagnosis of adult-onset RA for at least 3 months prior to screening as defined by the 2010 ACR/European League Against Rheumatism (EULAR) classification criteria. * Have moderately-to-severely active RA at screening and baseline, defined by the presence of * ≥6 swollen joints based on 66 joint count, and * ≥6 tender joints based on 68 joint count. * Have had a history of failure (an inadequate response, intolerance, or loss of response) to at least 1 conventional synthetic disease modifying anti rheumatic drug (csDMARD) and either 1 biologic (bDMARD), or targeted synthetic (tsDMARD) treatment. Exclusion Criteria: * Have Class IV RA according to ACR revised criteria * Have presence of 1 or more significant concurrent medical conditions per investigator judgment, including but not limited to * poorly controlled diabetes or hypertension * chronic kidney disease stage IIIa or IIIb, IV, or V * symptomatic heart failure according to New York Heart Association class II, III, or IV * myocardial infarction,unstable angina pectoris, stroke, or transient ischemic attack within the past 12 months before randomization * severe chronic pulmonary disease, for example, requiring oxygen therapy * major chronic inflammatory disease or connective tissue disease other than RA, including but not limited to * systemic lupus erythematosus * psoriatic arthritis * axial spondyloarthritis,including ankylosing spondylitis and non-radiographic axial spondyloarthritis * reactive arthritis * gout * scleroderma * polymyositis * dermatomyositis * active fibromyalgia, or * multiple sclerosis