RecruitingPhase 2Drug trialA Study to Evaluate Etrasimod Treatment in Adolescents With Ulcerative ColitisThe purpose of this study is to determine the safety, efficacy, and pharmacokinetics (PK) of etrasimod for the treatment of moderately to severely active ulcerative colitis in adolescents (≥ 12 years up to \< 18 years of age). Participants who will complete the total 52-week treatment period will have the opportunity to continue in a Long-Term Extension (LTE) Period of up to 4 years (5 years after study enrollment).