A Study to Evaluate the Pharmacokinetics, Safety and Efficacy of Afimkibart (RO7790121) in Children With Moderately to Severely Active Ulcerative Colitis

Who May Qualify: - Bodyweight \>= 10 kilogram (kg) - Confirmed diagnosis of UC - Demonstrated intolerance or inadequate response (IR) to one or more of the following categories of drugs: systemic corticosteroids, immunomodulators, and/or biologic therapies as outlined in the protocol Who Should NOT Join This Trial: - Monogenic disorder pertaining to infant onset inflammatory bowel disease (IBD) - Current diagnosis of Crohn's disease (CD), abdominal/intrabdominal/perianal fistula and/or abscess, indeterminant colitis, IBD-unclassified, microscopic colitis, ischemic colitis, infectious colitis, radiation colitis, or active diverticular disease - Presence of an ostomy or ileoanal pouch - Current diagnosis or suspicion of primary sclerosing cholangitis - Any major surgery within 6 weeks prior to screening or a major planned surgery during the study - Active tuberculosis (TB) infection suggested by positive TB testing, clinical symptoms, and/or chest imaging (X-ray or CT) Always talk to your doctor about whether this trial is right for you.