The main purpose of this study is to investigate efficacy, pharmacokinetics and safety of the drug in pediatric participants with moderately to severely active ulcerative colitis (UC).

Participants who complete the study will have the option to enter the long-term extension Study I6T-MC-AMAZ (AMAZ).

Inclusion Criteria:

• Males or females weighing ≥10 kg and ≥2 and <18 years old at the time of consent for screening.
• Have moderate to severe UC.
• Have failed corticosteroids, biologics, (for example, anti-tumor necrosis factor (TNF) antibodies or anti-integrin antibodies), immunomodulators (for example, azathioprine, thiopurines or methotrexate) or Janus Kinase (JAK)-Inhibitor treatment.
• Have UC at least 3 months in duration before baseline, which includes endoscopic evidence of UC corroborated by a histopathology report.

Exclusion Criteria:

• Have Crohn's disease, Inflammatory Bowel Disease Unclassified, ulcerative proctitis or primary sclerosing cholangitis.
• Have immune deficiency syndrome.
• Previous bowel resection or intestinal surgery.
• Evidence of toxic megacolon.
• History or current evidence of cancer of the gastrointestinal tract.

Anonymized individual patient level data will be provided in a secure access environment upon approval of a research proposal and a signed data sharing agreement.

Data are available 6 months after the primary publication and approval of the indication studied in the US and European Union (EU), whichever is later. Data will be indefinitely available for requesting.

A research proposal must be approved by an independent review panel and researchers must sign a data sharing agreement.