OptIMMize-1

Psoriasis is a chronic, systemic, inflammatory disease in which skin cells build up and develop thick, red and white scaly patches on the skin. There is an unmet medical need for effective treatment in pediatric patients and this study is being done to evaluate risankizumab in pediatric participants with moderate to severe plaque psoriasis. This study will assess the change in disease symptoms. Risankizumab is a drug being studied for the treatment for plaque psoriasis in pediatric participants. This study has 4 parts. Part 1: Participants aged 12 < 18 will receive a fixed dose of risankizumab. Part 2: Participants aged 12 < 18 will receive; - Period A: Risankizumab or ustekinumab based on body weight followed by; - Period B: Risankizumab or no treatment. - Period C: Re-treatment with risankizumab (if needed). Part 3: Participants aged 6 < 12 will receive risankizumab based on body weight. Part 4: Participants aged 6 < 12 will receive risankizumab based on body weight (Japan only: Participants aged 12 > 18 will receive risankizumab based on body weight). Around 132 participants will be enrolled in approximately 50 sites worldwide. Risankizumab and ustekinumab are given as a subcutaneous (under the skin) injection. Parts 1, 3, and 4: Risankizumab for 40 weeks with a follow-up call 20 weeks later for a study duration of approximately 65 weeks. Part 2: - Period A: Risankizumab or ustekinumab for 16 weeks. - Period B: Risankizumab or no treatment for 36 weeks. - Period C: Re-treatment with risankizumab for 16 weeks. Follow-up call 20 weeks later for a study duration of approximately 81 weeks. Participants from each Part who meet eligibility criteria for an open-label extension (OLE) study may continue on risankizumab for 216 additional weeks. There may be a higher burden for study participants compared to standard treatment. Participants will attend monthly visits and medical assessments will check the effect of treatment through blood tests, questionnaires, and checking for side effects.