LAUSR

Glioblastoma (GBM), the most common primary malignant brain tumor in adults, is an incurable disease that has an inevitable tendency to recur following standard of care surgery, radiotherapy, and temozolomide chemotherapy. There is an urgent need for improved medical therapies. The topoisomerase I inhibitor irinotecan has been investigated and used in the treatment of GBM for over 20 years. Clinical trials investigating irinotecan as monotherapy and in combination with temozolomide and bevacizumab have failed to show an overall survival benefit for newly diagnosed and recurrent GBM. Consequently, irinotecan is not commonly recommended for GBM treatment. However, these trials were conducted prior to the advent of molecularly-guided GBM subtyping, which is now a cornerstone of glioma diagnostics according to the updated WHO 2021 classification. To understand the extent to which molecularly-defined glioma subgroups differentially respond to clinically relevant therapies, we investigated the sensitivity of GBM patient-derived xenografts (PDXs) against a panel of chemotherapeutic agents, including temozolomide, bevacizumab, and irinotecan. All GBM PDXs (n=9) were responsive to irinotecan when grown as subcutaneous tumors. Two PDX lines exhibited hypersensitivity to irinotecan, which was marked by treatment-induced regression of subcutaneous tumors and increased median survival of 61 and 90+ days. Molecular analyses revealed that these hypersensitive lines were both ATRX-deficient tumors that displayed the alternative lengthening of telomeres (ALT) phenotype, while the less sensitive lines were all ATRX-intact and demonstrated TERT activation. One of the hypersensitive lines was IDH1R132H/WT while the other was IDH1R132H/− (historically termed secondary GBM; grade 4 astrocytoma per 2021 WHO classification). Cell-based studies showed that irinotecan sensitivity did not correlate with D-2HG producing capacity of the cells and was not impacted by inhibition of the mutant IDH1 enzyme. Irinotecan treatment significantly prolonged survival of orthotopic ALT+ PDXs relative to a vehicle control (64 days versus 41.5 days median survival). These results suggest that ALT+ gliomas may be particularly sensitive to topoisomerase inhibitor therapy. Our study highlights the need to interpret glioma clinical trial results in light of key molecular biomarkers, including indicators of ALT activity (e.g. ATRX deficiency, ultrabright telomeric foci, and C-circles). In addition, our results suggest that ALT-related biomarkers may be useful for identifying glioma patients who may benefit from topoisomerase inhibitor therapy. Citation Format: Stephen T. Keir, Heng Liu, Justin T. Low, Christopher J. Pirozzi, Nathan Reynolds, Eric S. Lipp, Annick Desjardins, Henry S. Friedman, Mustafa Khasraw, Darell D. Bigner, David M. Ashley, Matthew S. Waitkus. Alternative lengthening of telomeres is associated with hypersensitivity to topoisomerase 1 inhibition in glioblastoma xenograft models. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4924.