Participants will be randomly assigned to receive either standard treatment or standard treatment plus the study drug vorasidenib. All participants will receive radiation therapy followed by temozolomide. After the first year, participants will continue taking either vorasidenib or a placebo until their tumor worsens or their doctor recommends stopping treatment. The medications are taken by mouth once a day.

People that have newly diagnosed advanced brain cancer.

Participants may be able to join if they are age 12 or older with a grade 3 IDH-mutant astrocytoma and have tumor tissue available for testing. They must have recently had surgery or a biopsy, be planning to receive radiation therapy and chemotherapy, and be able to undergo MRI scans. Participants cannot have received prior cancer treatments for this tumor, have cancer that has spread to the spinal cord or brain lining, be pregnant, or have certain serious infections, heart conditions, or tumor bleeding.

This study is comparing standard treatment with standard treatment plus the study drug vorasidenib in people with this type of brain tumor. Researchers want to learn if adding vorasidenib can help shrink the tumor or keep it from growing compared with the usual treatment alone.