Abstract 2763: Tryptophan metabolism contributes to radiation-induced immune checkpoint reactivation in glioblastoma
Authors: Kesawani Pravin , Prabhu Antony , Kant Shiva , Kumar Praveen , Graham Stewart F. , Buelow Katie , Wilson George , Miller C Ryan , Chinnaiyan Prakash 
Published: August 2018

Abstract
Glioblastoma (GBM) is an aggressive brain tumor with limited treatment options. Immune checkpoint inhibitors designed to revert tumor-induced immune suppression have emerged as potent anti-cancer therapies. We performed cross-platform analyses coupling global metabolomic and gene-expression profiling in patient-derived gliomas. We identified aberrant tryptophan metabolism as a metabolic node in glioblastoma, which represents an emerging immune checkpoint. Specifically, GBM demonstrated an accumulation of tryptophan and kynurenine when compared to low-grade glioma (LGG), while kynurenate, a metabolite immediately downstream of kynurenine, was significantly lower in glioblastoma, resulting kynurenine/kynurenate ratio demonstrating &gt;90% accuracy in discriminating between GBM and LGG. These metabolic findings were corroborated by increased expression of indoleamine-2,3-dioxygenase-1 (IDO1) in GBM, a key enzyme involved in tryptophan metabolism and kynurenine production. Cross-platform analysis using gene expression arrays allowed for molecular subtyping of GBM, demonstrated that aberrant tryptophan metabolism was specific to classical and mesenchymal subtypes, while kynurenate accumulation, the metabolite elevated in LGG, was only evident in the proneural subtype. This metabolic phenotype was recapitulated in GBM preclinical models, which demonstrated robust IFNγ-induced IDO1 pathway activation and kynurenine production. The novel IDO1 inhibitor GDC-0919 demonstrated potent inhibition of tryptophan metabolism in our model and importantly, effectively crossed the blood-brain barrier. To explore the immune consequence of aberrant tryptophan metabolism in GBM, we extended investigations to an adult astrocytic, genetically engineered mouse (GEM) cell line to allow for in vivo studies using immune competent mice. Using this orthotopic mouse model, we demonstrated that although GDC-0919 as a single agent did not have anti-tumor activity, it had strong potential for enhancing radiation response in glioblastoma, which was further augmented when using a hypofractionated regimen. The immunological evaluation demonstrated that radiation response in glioblastoma involves immune stimulation, reflected by an increase in activated and cytotoxic T-cells, which is balanced by immune checkpoint reactivation, reflected by an increase in IDO1 expression and immunosuppressive regulatory T cells. GDC-0919 mitigated radiation-induced immune suppression and enhanced immune activation. These findings support clinical efforts designed to combine IDO1 inhibition with hypofractionated radiation in glioblastoma, offering the promise of harnessing a patient's immune system to attack these otherwise recalcitrant tumors.
Citation Format: Pravin Kesawani, Antony Prabhu, Shiva Kant, Praveen Kumar, Stewart F. Graham, Katie Buelow, George Wilson, C Ryan Miller, Prakash Chinnaiyan. Tryptophan metabolism contributes to radiation-induced immune checkpoint reactivation in glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2763.