A Study of LY4213663 in Healthy Participants and Participants With Rheumatoid Arthritis

This study is testing a new investigational drug called LY4213663 — first in healthy volunteers to check safety, then in people with rheumatoid arthritis (RA) to see if it helps with joint inflammation. **You may be eligible if...** - You are a healthy adult with a BMI between 18 and 32 (healthy volunteer arm) - OR you have been diagnosed with rheumatoid arthritis for at least 3 months, weigh at least 50 kg, and have a BMI between 18 and 40 (RA arm) **You may NOT be eligible if...** - You have severe RA (Class 4 by medical guidelines) - You have had a recent active infection - You have an abnormal heart tracing (ECG) at screening - You have significant heart, lung, liver, kidney, or nerve problems Talk to your doctor to see if this trial is right for you.