EXTH-41. IMPROVING THE RADIOSENSITIVITY OF DIFFUSE INTRINSIC PONTINE GLIOMAS (DIPG) BY TARGETING HYPOXIA AND MITOCHONDRIAL METABOLISM

Abstract

Abstract DIPG is the leading cause of brain tumor-related death in children. Radiotherapy is the only treatment that offers transient benefit and provides palliative care. DIPGs are hypo-perfused with tumor cells being exposed to hypoxia, a potent barrier to effective radiotherapy. Reducing the oxygen consumption rate (OCR) is therefore a potential strategy to reduce tumor hypoxia. We therefore performed a high-throughput screening with 1963 FDA-approved drugs that could reduce tumor hypoxia and OCR, thereby improving radiosensitivity of DIPGs. A subset of anti-diabetic drugs was identified, with phenformin amongst the most potent. It significantly reduced OCR in a panel of DIPG cultures and subsequently suppressed hypoxia in 3-dimensional DIPG neurosphere models that mimic the hypoxic microenvironment. In addition, phenformin demonstrated greater anti-tumor activity and radiosensitizing effect with much lower doses compared with metformin, a less potent biguanide that improved radiosensitivity of DIPG in a patient-derived xenograft (PDX) model in our previous study. The effect of phenformin was further enhanced by combining a second drug dichloroacetate that simultaneously attenuated phenformin-induced acidification rate. Specifically, the combination of phenformin and dichloroacetate induced higher levels of hypoxia inhibition, reactive oxygen species, DNA damage and apoptosis. RNA sequencing demonstrated significant changes induced by the combination treatment in cell-cycle, DNA repair, unfolded protein response and alternative energetic pathways. These changes were further validated by PCR array and western blotting at mRNA and protein levels. In addition, two master regulators that enhance the metabolic capacity of tumor cells through increased glycolysis thus contributing to radioresistance, HIF-1α and c-Myc, were also significantly suppressed by combination treatment. Ultimately, the triple combination of phenformin, DCA and irradiation demonstrated the most potent efficacy in inducing DNA damage, apoptosis and clonogenic inhibition in DIPG cultures. This promising triple combination therapy is currently being tested in our PDX cohort in vivo.