This study looks at how a new medicine called Mirikizumab works for kids with moderate to severe ulcerative colitis (UC), a condition causing swelling and sores in the colon. The study will check how well the drug works, how it's processed in the body (also known as pharmacokinetics), and its safety. If you finish the study, you may join another longer-term study called AMAZ. To join, kids need to be between 2 and 18 years old and weigh at least 10 kg (about 22 pounds). They should have UC for at least 3 months and have tried other treatments like corticosteroids (medicines that help reduce swelling), biologics (like special antibodies), or other related medicines but did not get better. Kids with other conditions like Crohn's disease or certain past surgeries cannot join.

• Duration: Complete the study to join a longer-term study.
• Eligibility: Kids 2-18 years old with moderate to severe UC.
• Exclusions: Kids with certain diseases or past surgeries.