Study Overview: This research is testing a treatment called etrasimod for teenagers (12 to 18 years old) with a condition called ulcerative colitis (UC). UC is a disease that causes swelling and sores in the colon or large intestine. The study will check if etrasimod is safe and works well. Also, it will study how the body processes the drug, known as pharmacokinetics (PK).

Study Duration and Participation: The main part of the study lasts for 52 weeks (about one year). If the treatment seems helpful, participants can choose to stay in the study for up to four more years, or until the drug is approved in their country.

Eligibility: To join, participants must have moderate to severe UC. They can continue some other UC treatments. However, they cannot join if they have severe colitis, Crohn’s disease, or certain other types of colitis.

• 52-week main study, with a possible 4-year extension.
• Must have moderately to severely active UC.
• Cannot have certain types of colitis or Crohn’s disease.