ESTRO 2023 - Abstract Book

S689

Monday 15 May 2023

ESTRO 2023

1 University of Oxford, Department of Oncology, Oxford, United Kingdom; 2 University of Manchester, School of Health Sciences, Manchester, United Kingdom; 3 University of Glasgow, School of Cancer Sciences, Glasgow, United Kingdom Purpose or Objective Radiotherapy is part of the standard of care in treating glioblastoma. However, irradiation of normal brain tissue is required to encompass this infiltrative disease. Radiation-induced neurotoxicity is mediated by neuroinflammation and blood-brain barrier disruption and causes irreversible cognitive impairment. Therefore, reducing neurotoxicity without costing tumor response would be beneficial. Recent preclinical studies have shown that whole-brain irradiation using FLASH radiotherapy can reduce radiation-induced neurotoxicity compared to conventional radiotherapy. PARP inhibitors have also been shown to enhance the radiosensitivity of multiple cancers and also reduce neuroinflammation in many brain diseases. Here we investigate if combining PARP inhibitors with hemibrain FLASH radiotherapy could further reduce radiation-induced neurotoxicity while maintaining tumor-killing efficacy. Materials and Methods Mice were randomly allocated to the following groups: 1) control; 2) FLASH irradiation (FLASH IR, mean dose rate= 2000 Gy/s); 3) Conventional dose rate irradiation (CONV IR, 0.1 Gy/s); 4) PARP inhibitor (PARPi, pamiparib, oral gavage, 12.5 mg/kg, twice daily for 7 days); 5) PARPi+CONV IR; and 6) PARPi+FLASH IR. Mice were irradiated once with 20 Gy from a 6 MeV electron beam to the right hemisphere. For combinatory therapy, the first dose of PARPi was administered 1 hour before irradiation. For normal tissue toxicity studies, normal C57BL6 mice were used. MRI, PET/CT, IHC and lightsheet microscopy were used to study structural and vascular changes in the brain. Systemic toxicity was assessed by weight change post-treatment. For tumor response, athymic nude mice with subcutaneous U87MG tumours were irradiated. Results Both PARPi+FLASH IR and PARPi+CONV IR showed around a 2-fold reduction in the tumor growth rate compared to control and PARPi groups. While a significant reduction in tumor growth rate was observed in mice treated with PARPi+CONV IR compared to CONV IR, a smaller, non-significant difference was observed between mice treated with PARPi+FLASH IR and FLASH IR alone. One week after irradiation, only mice treated with PARPi+CONV IR showed a significant weight reduction. A significant increase in brain perfusion in the irradiated hemisphere was observed in mice treated with CONV IR, while no significant differences in perfusion between hemispheres were seen in the other groups. Analysis of brain vasculature 1- month post-irradiation showed a reduction in vascular density for mice treated with CONV IR compared to FLASH IR. Conclusion The combinatory treatment of PARP inhibitor with radiotherapy prolonged tumor growth delay and reduced radiation- induced neurotoxicity. The synergistic tumor effect was more pronounced with CONV IR than FLASH IR. However, the combination of PARPi+CONV IR was also associated with increased acute systemic toxicity. Overall, this research demonstrates the potential of using FLASH radiotherapy together with PARP inhibitors for the treatment of glioblastoma. PD-0818 TRIB1 confers resistance to radiation treatment in GBM by inhibiting p53 function K. Singh 1 , C. Han 1 , J. Fleming 1 , A. Kumar 1 , C. Showalter 1 , Z. Tong 1 , V. Becker 1 , X. Meng 1 , A. Becker 1 , H. Manring 1 , S.J. Haque 1 , A. Chakravarti 1 1 The Ohio State University Comprehensive Cancer Center, Radiation Oncology, Columbus, USA Purpose or Objective Glioblastoma (GBM; WHO grade IV) is an aggressive disease with a low 5-year survival rate. Resistance to therapy is common which renders current modalities largely ineffective. We identified TRIB1, a Ser/Thr pseudokinase through a patient derived reverse translational approach as a potential driver of resistance mechanisms in GBM. TRIB1 functions as a scaffold to initiate Ubiquitin Proteasome System-mediated degradation of its substrates and has also been reported to activate the MAPK and Akt pathways in various cell types. Several studies show that TRIB1 contributes to chemotherapy resistance in various cancers. Therefore, we evaluated the role of TRIB1 in radiation therapy resistance routinely encountered in GBM. Materials and Methods Cells were irradiated using the X-Rad cabinet system. Site-directed mutagenesis was performed to substitute specific amino acid (W>A). Patient derived primary cell lines stably overexpressing wild type and mutant (W337A) TRIB1 transgenes were generated by antibiotic selection. Cells were injected intracranially in nude mice to develop tumors. Changes in tumor volume and overall survival (OS) were determined. Co-immunoprecipitation was performed to evaluate protein-protein interactions. Protein levels were detected by western blot. Results TRIB1 mRNA and protein levels increased after radiation treatment in GBM cell lines. We observed that TRIB1 overexpression in patient derived primary GBM (PDX) cell lines increased their viability after RT treatment. However, upon overexpression of mutant (W337A) TRIB1, this effect was reversed. We further observed a decrease in apoptosis of TRIB1 overexpressing PDX cells after RT treatment in vitro. Mice bearing TRIB1 transgene overexpressing tumors had the highest tumor volume and lowest OS whereas mice bearing mutant TRIB1 tumors had the highest OS and lower tumor volume at the end of experiment. Both p53 and HDAC1 formed complex(s) with TRIB1 in primary GBM cells lines, as revealed by co- immunoprecipitation. Further, western blot revealed that overexpression of TRIB1 decreased p21 and MDM2 levels in these cells. Conclusion Increased expression of TRIB1 in GBM cells contributes to radiation resistance by increasing the cell viability in vitro and promoting tumor growth in vivo. Mechanistic studies revealed that TRIB1 forms a complex with p53 and HDAC1 in primary GBM cell lines consequently inhibiting p53 function by facilitating its degradation through the E3-ligase COP1 or deacetylation through HDAC1, thereby decreasing radiation induced cell death. Taken together TRIB1 can be considered a potential therapeutic target in GBM.