DISRUPTING REDOX BALANCE VIA THE TETRAHYDROBIOPTERIN PATHWAY IN GLIOBLASTOMA

Abstract Aberrant redox statuses are observed in glioblastoma (GBM), and we previously identified GTP cyclohydrolase I (GCH1) to be a redox regulator upregulated in brain tumor initiating cells (BTICs). GCH1 is a rate-limiting enzyme in the de novo synthesis of tetrahydrobiopterin (BH4), a cofactor that produces catecholamine precursors and nitric oxide (NO) and, once used, becomes 7,8-dihydrobiopterin (BH2). Regeneration of BH2 into BH4 by dihydrofolate reductase (DHFR) helps to maintain proper BH4/BH2 ratios for redox balance. Although the BH4 pathway has traditionally been studied in the vasculature system for its regulation of NO, our previous work and that of others suggests the GCH1/BH4 pathway plays a critical redox role including in neoplastic cells. In silico analysis of primary and recurrent gliomas indicate high expression of BH4 related enzymes that correlated with worse patient survival in both primary and recurrent gliomas. The observed elevation of the BH4 pathway not only emphasizes its importance, but a therapeutic opportunity for improving survival in glioma patients. By repurposing FDA approved drugs known to cross the blood brain barrier and previously suggested as anti-glioma therapies, combining inhibitors for the de novo synthesis (sulfasalazine) and regeneration (pyrimethamine) of BH4 could prove to be an effective strategy for targeting the GCH1/BH4 through redox disruption. Preliminary data BTICs isolated from patient derived xenografts (PDXs) indicated reduced viability when treated with sulfasalazine (SASP) and pyrimethamine (PYR). Furthermore, we observed lower/depleted levels of BH4 relative to BH2 when BTICs were treated with SASP and PYR. Lastly, there is an increase in mitochondrial ROS upon SASP and PYR treatment, suggesting dysregulated redox states. Importantly, temozolomide resistant GBM cells remained sensitive to SASP and PYR. Taken together, our preliminary data suggests the plausibility of targeting the GCH1/BH4 pathway with SASP and PYR to disrupt redox balance in glioma through the depletion of BH4.