Brain tumors are the leading cause of cancer-related death in children. Tazemetostat is an FDA-approved enhancer of zeste homolog (EZH2) inhibitor. To determine its role in difficult-to-treat pediatric brain tumors, we examined EZH2 levels in a panel of 22 PDOX models and confirmed EZH2 mRNA over-expression in 9 GBM (34.6 ± 12.7-fold) and 11 medulloblastoma models (6.2 ± 1.7 in group 3, 6.0 ± 2.4 in group 4) accompanied by elevated H3K27me3 expression. Therapeutic efficacy was evaluated in 4 models (1 GBM, 2 medulloblastomas and 1 ATRT) via systematically administered tazemetostat (250 and 400 mg/kg, gavaged, twice daily) alone and in combination with cisplatin (5 mg/kg, i.p., twice) and/or radiation (2 Gy/day × 5 days). Compared with the untreated controls, tazemetostat significantly (Pcorrected < 0.05) prolonged survival times in ic-l1115atrt (101% at 400 mg kg) and ic-2305gbm (32% at 250 mg kg, 45% at 400 mg kg) in a dose-dependent manner. the addition of tazemetostat with radiation was evaluated in 3 models, with only one [ic-1078mb (group 4)] showing a substantial, though not statistically significant, prolongation in survival compared to radiation treatment alone. combining tazemetostat (250 mg kg) with cisplatin was not superior to cisplatin alone in any model. analysis of in vivo drug resistance detected predominance of ezh2-negative cells in the remnant pdox tumors accompanied by decreased h3k27me2 and h3k27me3 expressions. these data supported the use of tazemetostat in a subset of pediatric brain tumors and suggests that ezh2-negative tumor cells may have caused therapy resistance and should be prioritized for the search of new therapeutic targets.>

DOI 10.1038/s41374-021-00700-8