Abstract 690: Investigation of BH-3 mimetics as radiosensitizing agents in glioblastoma

Abstract Glioblastoma (GBM) is a highly aggressive primary brain tumor. Treatment consists of maximal safe tumor resection, chemotherapy (temozolomide) and radiotherapy; however, this aggressive treatment offers only a 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 tumors, 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, which is driven by upregulation of pro-survival BCL-2 proteins that inhibit the pro-apoptotic proteins necessary for the apoptotic response. 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. A panel of BH3 mimetics was tested in combination with radiotherapy across three patient-derived GBM cell lines (G7, E2 and R15), using a 3D clonogenic culture system that recapitulates in vivo growth patterns and responses. We found that the BCL-XL inhibitor, A1331852, significantly radiosensitized all 3 GBM lines. In addition, Western blot analysis revealed A1331852 to induce PARP1 cleavage in G7 cells when used as a single agent. Subsequent in vivo studies were conducted in our well characterized orthotopic G7 xenograft murine model of GBM. Immunohistochemical studies showed that the pro-survival proteins MCL-1 and BCL-XL were upregulated in irradiated tumors. Furthermore, a pharmacokinetic study in the same orthotopic murine model, allowing for drug delivery evaluation, and optimized treatment scheduling. Importantly, A1331852 was found to be blood brain barrier penetrant at 25 mg/kg, with increased drug detected in tumor vs normal brain tissue. From these data, a survival study investigating A1331852 as a radiosensitizing agent was carried out in the same G7 GBM orthotopic xenograft model. Kaplan-Meier analysis showed an increase in survival of mice treated with A133852 plus radiotherapy compared with the radiotherapy only group (p = 0. 0309). 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 RT and have completed a proof-of-concept in vivo study that confirms the potential of BH3 mimetics to improve outcomes for patients with GBM. Citation Format: Louise R. Dutton, Clara Mullen, Anna L. Koessinger, Mark Jackson, Katrina Stevenson, Anthony J. Chalmers, Stephen Tait, Kirsteen J. Campbell, Joanna L. Birch. Investigation of BH-3 mimetics as radiosensitizing agents in glioblastoma abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 690.