EXTH-17. TARGETING CTPS1 AND NUCLEOTIDE BIOSYNTHESIS PATHWAYS AS A NOVEL THERAPEUTIC APPROACH IN MYC-AMPLIFIED MEDULLOBLASTOMA

Abstract MYC-amplified medulloblastoma (MB) tumors are the deadliest of all subgroups with a < 50% 5-year overall survival rate. High risk patients receive large doses of chemo- and radio- therapy (CT/RT) with most survivors experiencing debilitating neurological and cognitive sequelae making the identification of novel targets of the utmost importance. Recent whole-genome CRISPR screening identified the enzyme CTPS1 and other components of the de novo pyrimidine metabolism cascade as a possible cancer-specific vulnerability in MYC-amplified MB. Our study tested targeted inhibitors of this pathway to identify rational combinations for translation to pre-clinical models. A stable of treatment-naïve, patient-derived models of MYC-amplified MB were used and treated with various de novo pyrimidine synthesis inhibitors alone and in novel combinations. Analysis of proliferation, cell survival, and downstream signaling were performed both in vitro and in vivo. Inhibition of CTPS1 led to a significant decrease in cell proliferation and alterations in cell cycle with minimal increases in cell death, suggest a cytostatic mechanism. Previous work has shown de novo pathway inhibition results in reduction of the dNTP pool, leading to activation of the DNA damage response (DDR), seizing the cell cycle. Interestingly, the DDR inhibitor prexasertib (CHKi) robustly targets MYC-amplified MB tumors in pre-clinical models. Combining CHKi and CTPS1 inhibitors yields significant tumor cytotoxicity both in vitro and in vivo. We further demonstrated robust synergy of CHKi with other clinically relevant de novo pyrimidine metabolism inhibitors. The results of this investigation confirm CTPS1 and the de novo pyrimidine metabolism pathway as targets in MYC-amplified MB and identifies a novel rational combination for treatment in patients.