High-grade gliomas, including glioblastomas, are aggressive cancers of the brain that usually recur despite standard treatment, including radiation therapy. These mechanisms conferring persistence to tumor cells may be explained by DNA damage response pathways, especially those relying on ataxia-telangiectasia mutated (ATM) and DNA-dependent protein kinase (DNA-PK), which induce the repair of radiation-induced DNA damage. The effects of the ATM/DNA-PK inhibitor XRD-0394 were evaluated in in vitro models of glioma.