This study is testing a new drug called vixarelimab for people with ulcerative colitis (UC). UC is a disease that causes swelling and sores in the large intestine. The study will compare vixarelimab to a placebo (a fake treatment) to see how well it works, how safe it is, and how the body processes it. The study has two parts: an induction period to see if the drug starts working, and an optional extension period to see if the drug keeps working. To join, you must have had UC for at least 3 months and have not responded well to other treatments. You cannot join if you have other bowel diseases like Crohn's disease or have had a surgery called colectomy.

• The study has two parts; the second part is optional.
• All participants in the later part will receive vixarelimab.
• Eligibility depends on previous response to UC treatments.