A Study to Assess a Medicine Called Tovorafenib in Japanese Children and Young Adults With Brain Tumours

The purpose of this study is to evaluate safety and the way the body absorbs, distributes and gets rid of the study drug tovorafenib in the body in Japanese children, adolescents and young adults with specific brain tumours. This includes how the drug is absorbed, distributed and eliminated from the body (called pharmacokinetics). The study will also test how well the drug works to shrink brain tumours. In this study, all participants will receive tovorafenib orally once weekly. There will be four periods in this study: 1. Screening period (up to 4 weeks): Participants will be evaluated to determine if they can take part in the study, requiring at least one visit to the study centre. 2. Treatment period (up to 24 months): All eligible participants will receive tovorafenib. This requires five visits for the first 2 months (Cycle 1 and Cycle 2) followed by one visit every month (at the start of each treatment cycle). Participants will receive the first oral dose of tovorafenib on Day 1 of Cycle 1 at the study clinic. After Day 1, participants will need to take tovorafenib once weekly on Day 8, Day 15 and Day 22. Participants will be required to come to the study clinic in person at least five times for Cycle 1 and Cycle 2. In addition, participants will have one remote visit (telephone call) during Cycle 1. After Cycle 2, only one in-person clinic visit is required at the start of each treatment cycle. A participant will stop treatment if their disease gets worse, if treatment has a harmful effect, or if they do not want to take part in the study anymore. 3. End-of-Treatment Safety Follow-Up (30 days): Participants will have a clinic visit 30 days after stopping treatment to check their health. 4. Long-Term Follow-Up (up to 2 years): Participants will be monitored every 3 months unless they start a new anti-cancer treatment or leave the study. During the study, participants will undergo various health measurements and observations, including blood sampling and urine collections. Each participant will be in this study for up to approximately 4 years. Tovorafenib will be provided to participants who tolerate it for as long as their disease does not progress. Once tovorafenib becomes approved and commercially available in Japan, participants may transition to the commercial drug for continued treatment. A participant may withdraw consent to participate at any time.