A Study of LY3541860 in Adult Participants With Moderately to Severely Active Rheumatoid Arthritis

The main purpose of this study is to evaluate the effects of LY3541860 in adult participants with moderately to severely active Rheumatoid Arthritis with inadequate response to a least one biologic disease-modifying antirheumatic drug (bDMARD)or targeted synthetic disease-modifying antirheumatic drug (tsDMARD). Study participation is approximately 50 weeks, including a 6-week screening period, 24-week treatment period, and a 20-week safety follow up period.

Inclusion Criteria: * Have a diagnosis of adult-onset Rheumatoid Arthritis (RA) for at least 3 months prior to screening, as defined by the 2010 ACR/EULAR classification criteria. * Have moderately to severely active RA, at screening (Visit 1) and baseline (Visit 2), defined by the presence of * ≥6 swollen joints based on 66 joint count (and ≥4 swollen joints on 28 joint count), and * ≥6 tender joints based on 68 joint count (and ≥4 tender joints on 28 joint count). * Have active synovitis in ≥1 joint, in hands or wrists, of the investigator choosing at screening, having an MRI synovitis RAMRIS score of ≥2 as determined from the central reading of the images. Exclusion Criteria: * Have a diagnosis or history of malignant disease within 5 years prior to baseline, with the exceptions of * basal cell or squamous epithelial carcinomas of the skin that have been resected with no evidence of metastatic disease for 3 years, or * cervical carcinoma in situ, with no evidence of recurrence within the 5 years prior to baseline. * Have estimated glomerular filtration rate (eGFR) of \<45 milliliter per minute (mL/minute) from serum creatinine using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation. * Have a current or recent active infection. * 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 3a or b, 4, or 5 * symptomatic heart failure according to New York Heart Association Class 2, 3, or 4 * myocardial infarction, unstable angina pectoris, stroke, or transient ischemic attack within the past 12 months before baseline * 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 * Have received any prior B-cell targeted therapy (for example, anti-CD19 antibodies or anti-CD20 antibodies, such as rituximab). * Received corticosteroids, opioids, conventional disease-modifying antirheumatic drug (cDMARD), with changes in dose or have received Janus kinase (JAK) inhibitors within 28 days prior to the screening MRI or plan on receiving these drugs during the study.