P09.06.A INVESTIGATING NOVEL RADIOTHERAPY-BH3 MIMETIC COMBINATIONS IN GLIOBLASTOMA

Abstract BACKGROUND Glioblastoma (GBM) is a highly aggressive primary brain tumour. Treatment consists of maximal safe tumour resection, chemotherapy and radiotherapy; however, this aggressive treatment offers only modest improvement in outcomes, with average life expectancy of approximately 12 months. Consequently, novel, effective and easily translatable therapies are urgently required. While radiotherapy is an effective treatment for many solid tumours, GBM displays an innate radio-resistance that contributes to treatment failure. Therefore, there is a need to identify chemotherapeutics that can be used as radiosensitizers to increase efficacy. Notably, a key pathway involved in therapeutic resistance is apoptotic evasion, driven by upregulation of pro-survival BCL-2 proteins. Recent advances in drug development have produced a range of BH3 mimetics - novel chemotherapeutics with potential to reinstate apoptosis by targeting specific pro-survival molecules. We hypothesized that the radiation sensitivity of GBM cells would be enhanced by using BH3 mimetics to inhibit pro-survival molecules and release the apoptotic block. MATERIAL AND METHODS A panel of BH3 mimetics was tested in combination with radiotherapy across six patient-derived GBM cell lines, using 3D clonogenic culture systems and 3D cell viability assays that recapitulate in vivo growth patterns and responses. Cleavage of PARP1 was also investigated as a marker of apoptosis via Western blotting. MCL-1 and BCL-XL protein levels were measured by IHC in a G7 GBM orthotopic xenograft model pre- and post- radiotherapy. A pharmacokinetic study was performed using the same orthotopic model, enabling drug delivery evaluation and informing treatment scheduling. Building on these data, an in vivo survival study investigating A1331852 as a radio-sensitising agent was carried out. RESULTS The BCL-XL inhibitor A1331852 significantly radio-sensitised multiple GBM lines and induced apoptosis, detected by PARP cleavage. The pharmacokinetic study found A1331852 to be blood brain barrier penetrant at 25 mg/kg, with increased drug detected in tumour vs normal brain tissue. Kaplan-Meier analysis of the survival study revealed a statistically significant difference in survival between mice treated with radiotherapy alone compared to mice treated with radiotherapy plus A133852 (χ² = 4.661, df = 1, p = 0.0309), with median survival of 36 days vs 78 days. Importantly, a lack of toxicity was observed throughout the study. CONCLUSION Overall, we have demonstrated that dependence on BCL-2 family proteins is a vulnerability that can be targeted to improve radiotherapy response in GBM. Furthermore, we have identified a promising BH3 mimetic, A1331852, to use in combination with radiotherapy and have completed a proof-of-concept in vivo study that confirms the potential of BH3 mimetics to improve outcomes for patients with GBM.