Ulcerative Colitis (UC) is a condition where the large intestine (colon) becomes swollen and sore. This study is testing a drug called vixarelimab to see if it helps people with moderate to severe UC. The study wants to see if vixarelimab is safe, works well, and understand how it moves through the body (pharmacokinetics). A placebo is a fake treatment that looks like the real thing but doesn't have any medicine in it. Some people in the study will get vixarelimab and some will get a placebo.

The study has two parts: an induction period to see if vixarelimab can help people feel better, and an optional active treatment extension (ATE) period where everyone gets the real drug to see how long it works. If you have had UC for at least 3 months and haven't responded well to other treatments, you might qualify. You can't be in the study if you have Crohn's disease or have had certain surgeries.

• Study has two parts: initial treatment and optional extended treatment.
• Participants may receive either the study drug or a placebo to start.
• Eligibility requires a history of UC and issues with previous treatments.