A Study to Learn More About How Risankizumab Works in Young Participants With Ulcerative Colitis

Ulcerative colitis (UC) is a type of inflammatory bowel disease that causes inflammation and bleeding from the lining of the rectum and colon (large intestine). This study will assess how Risankizumab moves through the body as well as how safe and effective it is in treating pediatric participants with moderate to severely active UC. Adverse events and change in disease activity will be assessed. Risankizumab is an approved medication for moderate to severe UC in multiple countries and is being developed for the treatment of UC in pediatrics. This study is comprised of 3 cohorts that may participate in 3 substudies (SS). Cohort 1 will enroll participants with ages from 6 to less than 18 years. Cohort 2 will enroll participants with ages from 2 to less than 6 years. Cohort 3 will enroll participants with ages from 2 to less than 18 years. SS1 is an open-label induction period where participants will receive a weight-based induction regimen of risankizumab. SS2 is a double-blind maintenance period where participants will be randomized to receive 1 of 2 doses of weight-based maintenance regimen of risankizumab. SS3 is an open-label extension period where participants will receive risankizumab based off of their response in SS2. Around 120 pediatric participants with UC will be enrolled at around 80 sites worldwide. Participants in SS1 will receive risankizumab intravenously during the 12-week induction period. Participants in SS2 will receive risankizumab subcutaneously during the 52-week randomized maintenance period. Participants in SS3 will receive risankizumab subcutaneously during the 208-week open label period. Participants will be followed-up for approximately 140 days. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.