Glioblastoma (GBM) is a disease of the whole brain, with infiltrative tumor cells protected by an intact BBB. GBM has a poor prognosis despite aggressive treatment, in part due to lack of adequate drug permeability at the BBB. Standard of care GBM therapies include radiation and cytotoxic chemotherapy that lead to DNA damage. Subsequent activation of DNA damage response (DDR) pathways can induce resistance. Various DDR inhibitors, targeting the key regulators of these pathways such as ataxia telangiectasia mutated and Rad3-related (ATR), are being explored as radio- and chemo-sensitizers. Elimusertib, a novel ATR kinase inhibitor, can prevent repair of damaged DNA, increasing efficacy of DNA damaging cytotoxic therapies. Robust synergy was observed