A Study to Assess the Efficacy and Safety of Induction Therapy With Afimkibart (Also Known as RO7790121) in Participants With Moderately to Severely Active Ulcerative Colitis

Inclusion Criteria:

• Confirmed diagnosis of UC

• Moderately to severely active UC assessed by mMS

• Bodyweight >= 40 kilogram (kg)

• Up to date with colorectal cancer (CRC) screening performed according to localstandards

• Demonstrated inadequate response, loss of response and/or intolerance to at leastone protocol-specified conventional or advanced UC therapy

• Males and females of childbearing potential must meet protocol criteria forcontraception requirements

Exclusion Criteria:

• Currently known complications of UC (e.g. fulminant colitis, toxic megacolon)

• Current diagnosis of Crohn's disease (CD) or indeterminate colitis, microscopiccolitis, ischemic colitis, infectious colitis, radiation colitis

• Presence of an ostomy or ileoanal pouch

• Current diagnosis or suspicion of primary sclerosing cholangitis

• Pregnancy or breastfeeding, or intention of becoming pregnant during the study

• Past or current evidence of definite low-grade or high-grade colonic dysplasia oradenomas or neoplasia not completely removed

• History of malignancy within 5 years, with the exception of malignancies adequatelytreated with resection for non-metastatic basal cell or squamous cell cancer or insitu cervical cancer

• Evidence of infection with Clostridioides difficile (C. difficile; formerly known asClostridium difficile), cytomegalovirus (CMV), human immunodeficiency virus (HIV),Hepatitis B (HBV), Hepatitis C (HCV)

• Has evidence of active tuberculosis (TB), latent TB not successfully treated (perlocal guidance) or inadequately treated TB

• Has received protocol-specified prohibited medicines, including known exposure toany type of anti-TL1A therapy