This clinical trial is focused on studying a type of brain cancer called high-grade glioma in children and young adults. The study will explore the effects of a treatment that combines two medications: Abemaciclib and Temozolomide. Abemaciclib, also known by its code name LY2835219, is a medication that comes in the form of tablets or granules and is taken by mouth. Temozolomide is another medication used in this study, also taken orally.

The purpose of the study is to determine if adding Abemaciclib to the standard chemotherapy treatment with Temozolomide can provide additional benefits for patients who have been newly diagnosed with high-grade glioma and have already undergone radiotherapy. Radiotherapy is a treatment that uses high-energy rays to target and kill cancer cells. The study will compare the combination of Abemaciclib and Temozolomide to the use of Temozolomide alone.

Participants in the study will receive either the combination treatment or Temozolomide alone. The study will monitor the participants over a period to assess the effectiveness of the treatments. The goal is to see if the combination treatment can improve outcomes for patients with high-grade glioma. The study will involve regular check-ups, including lab tests and imaging of the tumor, to track the progress and effects of the treatment.