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. We have found 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 in preclinical models by mycophenolate mofetil (MMF), an FDA-approved and CNS-penetrant inhibitor of purine synthesis. In this research proposal we will determine how the RT response and purine synthesis regulate one another in GBM. We will also determine if the GBMs with the greatest activity of purine synthesis derive the greatest benefit from MMF treatment. Finally, we will perform a clinical trial to determine the maximum tolerated dose of MMF given in combination with RT for patients with GBM and confirm that this dose reaches active concentrations in GBM tissue. Together, these studies will (1) Determine mechanistic links between the RT response and purine metabolism in GBM that will facilitate the rational combination of metabolic inhibitors with DNA damage inducing therapeutics, (2) Determine whether measuring purine synthesis rates could predict GBM response to MMF treatment, and (3) Determine whether combined RT and MMF should be evaluated in randomized trials for patients with GBM.