This clinical trial studies the use of risankizumab (also known as ABBV-066) in young patients aged 2 to 18 years who have ulcerative colitis, a condition where the inner lining of the large intestine becomes inflamed and develops small sores or ulcers. The study focuses on patients whose disease is moderately to severely active, meaning their symptoms significantly affect their daily life and have not improved enough with other treatments.

The medication will be given in two different ways: as an intravenous infusion (through a vein) and as a subcutaneous injection (under the skin) using a pre-filled syringe. Some participants will receive the actual medication while others will receive a placebo. The study consists of three main parts: an initial open-label treatment period where all participants receive the medication, followed by a period where participants are randomly assigned to either continue the medication or receive placebo, and finally a long-term extension period.

The purpose of this study is to understand how the medication moves through the body, how well it works, and how safe it is for young patients with ulcerative colitis. The study will measure various aspects of the disease improvement, including the healing of the intestinal lining and the reduction of symptoms such as stomach pain and frequent bowel movements. The total duration of treatment may continue for up to 248 weeks.