Project Summary/Abstract Glioblastoma (GBM) is the most common aggressive primary brain tumor and is uniformly fatal with a median survival of around 1.5 years. Like surgery and chemotherapy, radiation (RT) is a critical treatment for nearly every patient with GBM and has repeatedly improved patient survival in multiple randomized trials. Still, 80% of GBMs recur within the high dose RT field. Thus, there is a critical need to develop strategies to overcome GBM RT resistance to further improve patient outcomes. GBM cells exhibit profound cancer-specific metabolic abnormalities, including elevated purine synthesis, to fuel proliferation, invasion and survival. Using mice bearing intracranial orthotopic patient-derived brain tumors, my research has established that the metabolic phenotype of elevated purine synthesis also mediates resistance to RT in GBM by promoting the repair of RT-induced DNA damage. This purine-mediated RT resistance can be overcome by treatment with mycophenolate mofetil (MMF), an FDA-approved and CNS-penetrant inhibitor of purine biosynthesis. In this research proposal I will determine how the RT response and purine synthesis regulate one another in GBM. By employing a variety of cutting-edge metabolomic techniques and patient-derived GBM models, I will 1) define the biosynthetic pathway GBMs use to generate purines, and 2) determine the RT response mechanism by which GBMs increase purine levels to resist RT-induced DNA damage. Findings from the experiments proposed here will expand our understanding of how tumors modulate metabolism to promote therapeutic resistance, inform how to combine metabolic inhibitors with standard therapies, and lay the mechanistic groundwork for clinical trials at the University of Michigan that targeting purine biosynthesis to augment RT in GBM patients.