Purpose: Glioblastoma (GBM) is the most aggressive primary brain tumor in adults with a median survival of 15-20 months. Numerous approaches and novel therapeutics for treating GBM have been investigated in the setting of phase III clinical trials, including a recent analysis of the immune checkpoint inhibitor, Nivolumab (anti-PD-1), which failed to improve recurrent GBM patient survival. However, rather than abandoning immune checkpoint inhibitor treatment for GBM, which has shown promise in other types of cancer, ongoing studies are currently evaluating this therapeutic class when combined with other agents. Experimental Design: Here, we investigated immunocompetent orthotopic mouse models of GBM treated with the potent CNS-penetrating IDO1 enzyme inhibitor, BGB-5777, combined with anti-PD1 mAb, as well as radiation therapy (RT), based on our recent observation that tumor-infiltrating T cells directly increase immunosuppressive IDO1 levels in human GBM, the previously-described enhancement of immune cell functions after PD-1 blockade, as well as the pro-inflammatory effects of radiation. Results: Our results demonstrate a durable survival benefit from this novel three-agent treatment, but not for any single- or dual-agent combination. Unexpectedly, treatment efficacy required IDO1 enzyme inhibition in non-GBM cells, rather than tumor cells. Timing of effector T cell infiltration, animal subject age and usage of systemic chemotherapy, all directly impacted therapy-mediated survival benefit. Conclusions: These data highlight a novel and clinically-relevant immunotherapeutic approach with associated mechanistic considerations that have formed the basis of a newly-initiated phase I/II trial for GBM patients.
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