Combination Therapy for the Treatment of Diffuse Midline Gliomas

--COHORTS 1, 2, AND 3 CLOSED--- INCLUSION CRITERIA: COHORT 1A AND 1B: * New diagnosis of DMG with imaging and/or pathology consistent with a DMG, including spinal cord tumors. In cohort 1B, previous tumor tissue confirmation of DMG is mandatory and pathology must be consistent with a DMG including diffuse midline glioma Histone 3 lysine 27 - mutant (H3K27M); World Health Organization (WHO) grade III and IV H3 wildtype gliomas. * Must be within 6 weeks of diagnosis to begin standard of care radiation therapy on study. COHORT 2A AND 2B: * Diagnosis of DMG with imaging and/or pathology consistent with a DMG, including spinal cord tumors, who have complete standard-of-care radiation therapy. In Cohort 2B, previous tumor tissue confirmation of DMG is mandatory and pathology must be consistent with a DMG including diffuse midline glioma H3K27M mutant; WHO grade III and IV H3 wildtype gliomas. * Participants must be within 4-14 weeks of completion of radiation. Radiation should have started within 6 weeks of diagnosis. COHORT 3A AND 3B: * Diagnosis of recurrent DMG with imaging and/or pathology consistent with a DMG, including spinal cord tumors, who have complete standard-of-care radiation therapy. In cohort 3B, previous tumor tissue confirmation of DMG is mandatory and pathology must be consistent with a DMG including diffuse midline glioma H3K27M mutant; WHO grade III and IV H3 wildtype gliomas. * Participants must have evidence of progression and not have received any tre…