DDDR-34. Bcl-xL inhibition enhances the efficacy of Temozolomide by targeting senescent glioblastoma cells

INTRODUCTION Alkylating agent temozolomide (TMZ) is a standard chemotherapeutic agent in the current treatment of glioma. However, its clinical effects are insufficient, especially for glioblastomas (GBM) that nearly inevitably recur. The primary cytotoxic mechanism of TMZ involves the direct modification of DNA and resulting futile cycle of DNA mismatch repair that induces cell death via multiple mechanisms including apoptosis, necrosis, and autophagy as well as cellular senescence. In this study, we investigated the degree to which TMZ induces cellular senescence and whether senolytic drugs, which selectively target senescent cells, could enhance the therapeutic effect of TMZ. METHODS Using GBM cell lines and patient-derived GBM cell lines, we first investigated whether TMZ induces senescence by testing β-galactosidase activity and expression of senescence-associated secretory phenotype (SASP) factors. We screened various senolytic drugs for their abilities to enhance the anti-tumor effects of TMZ. RESULTS In GBM cell lines and patient-derived GBM cell lines, TMZ treatment inhibited cell proliferation and consequently induced cellular senescence. However, apoptosis induction was hardly detectable after treatment with TMZ alone. Our subsequent screening with senolytic small molecule agents revealed that Navitoclax (ABT-263), a pan-Bcl-2 inhibitor, showed a combination effect with TMZ. Unlike TMZ alone, the combination treatment of TMZ with Navitoclax induced apoptosis. Furthermore, Bcl-xL specific inhibitors exhibited the combination efficacy with TMZ, whereas Bcl-2 specific inhibitors did not, indicating that targeting Bcl-xL mediated the senolytic effect. CONCLUSION Our results suggest cellular senescence may represent a critical mechanism of GBM evasion of TMZ treatment. Bcl-xL inhibition enhances anti-GBM efficacy of TMZ by further driving senescent GBM cells to apoptosis.