Adaptive nuclear GTP synthesis promotes glioblastoma treatment resistance and is a targetable vulnerability in patients.

Glioblastoma (GBM), the most lethal of all brain cancers, resists therapy by rewiring metabolism and relying on GTP signaling to promote DNA repair and radiation therapy (RT) resistance. How GBM modulates GTP levels for this signaling in response to RT-induced DNA damage, and the therapeutic tractability of this metabolic activity in the context of standard-of-care chemoradiation therapy, remain unaddressed. Here, we identify acute changes in glioma metabolism within hours of RT, including an acute post-RT rewiring of guanylate synthesis driven by nuclear translocation of the rate-limiting de novo guanylate synthesis enzyme IMPDH1. This subcellular IMPDH1 re-localization and nuclear GTP accumulation are dependent on the DNA damage signal kinase DNA-PK. Targeting intracranial GTP synthesis with the FDA-approved inhibitor mycophenolate mofetil (MMF) slows repair of DNA damage and extends survival of orthotopic murine models treated with combined RT and temozolomide. Extending our findings to humans, we performed a phase 0 clinical trial revealing that oral MMF administration leads to active intracranial drug concentrations, with target engagement indicated by reversal of IMPDH upstream and downstream metabolites in recurrent GBM tumors. Together, these findings implicate IMPDH as a potential metabolic target in GBM whose pharmacological inhibition is feasible and could complement standard-of-care chemoradiation therapy.