We used MHC-I immunopeptidomics to study how MTX-241F, a selective EGFR/PI3K inhibitor, alters antigen presentation in diffuse midline glioma (H3K27M) and glioblastoma xenografts. MHC-I–bound peptides from vehicle- and drug-treated tumors were profiled by LC–MS/MS to identify tumor- and treatment-specific peptides, including candidate brain-enriched biomarkers and immunotherapy targets.