Abstract BACKGROUND High-grade gliomas (HGGs) are a leading cause of cancer-associated death in children. Radiation therapy (RT) continues to be the only standard-of-care treatment across HGG subtypes, but disease frequently recurs following only a transient response. We have recently identified that HGGs utilize the pro-survival BCL-2 family of proteins to evade cell death after radiation, and we demonstrated that effective inhibition of BCL-2 with the CNS-penetrant drug venetoclax could be utilized to disrupt this resistance and augment radiation-induced cell killing. While this treatment significantly prolonged survival in orthotopic mouse models of pediatric HGG, tumors invariably developed resistance to single-agent venetoclax and recurred. METHODS We profiled tumors from venetoclax-resistant animals for alternate BCL-2 family binding. We then assessed susceptibility to MCL1 inhibition in venetoclax-treated or treatment naïve culture models. Synergy was tested in vitro, and mitochondrial ROS production and induction of apoptosis was assessed using the MCL1-specific agent S63845. Finally, venetoclax and S63845 were tested in combination following fractionated radiotherapy in orthotopic models of DIPG. RESULTS MCL1 is an acquired dependency following treatment with RT + venetoclax. S63845 synergizes with venetoclax following RT to drive mitochondrial ROS production and apoptosis. The combination of venetoclax and S63845 significantly prolongs survival in xenograft models of DIPG. CONCLUSIONS Combinatorial targeting with BH3 mimetics augments radiation-induced cell killing and may open new therapeutic avenues for pediatric HGG.

HGG-41. COMBINATORIAL BH3-MIMETIC STRATEGY FOR OVERCOMING RADIORESISTANCE IN PEDIATRIC HIGH-GRADE GLIOMA