Abstract LB-359: Inhibition of DNA repair by the dual PI3K/mTOR inhibitor NVP-BEZ235 as a novel radiosensitization strategy for glioblastoma.

Abstract

Abstract Glioblastomas (GBM) are deadly brain tumors with extremely poor prognosis. The standard of care for GBM is tumor resection followed by treatment with ionizing radiation (IR) and the DNA alkylating agent temozolomide. As GBMs are highly radioresistant, it is crucial to identify effective radiosensitizers for improving therapeutic outcome. We find that NVP-BEZ235, a dual PI3K/mTOR inhibitor that is in clinical trials, potently inhibits both ATM and DNA-PKcs, the two major kinases that respond to IR-induced DNA double-strand breaks (DSBs). Consequently, this drug blocks non-homologous end joining (NHEJ) as well as homologous recombination (HR), the two major pathways of DSB repair. We find that low concentrations of NVP-BEZ235 (100 nM) can block DSB repair in a panel of human GBM lines resulting in significant radiosensitization. This radiosensitizing effect is more potent than that seen with much higher concentrations (10 μm) of current inhibitors of DNA-PKcs or ATM that are being optimized for clinical testing. In order to examine the effects of NVP-BEZ235 in vivo, we sub-cutaneously injected mice with radioresistant U87 glioma cells ectopically expressing EGFRvIII and treated with clinically-relevant doses of the drug. We find that NVP-BEZ235 blocks the activation of both DNA-PKcs and ATM in irradiated tumors resulting in significant attenuation of DSB repair. Tumors treated with a combination of IR and NVP-BEZ235 exhibit elevated DSBs and reduced Ki67 positivity compared to tumors treated with only IR or NVP-BEZ235. Consequently, a combination of IR and NVP-BEZ235 results in dramatic inhibition of tumor growth, in contrast to almost no inhibition with either treatment modality alone. Finally, we established that NVP-BEZ235 can cross the blood-brain barrier and therefore examined its effectiveness in an orthotopic brain tumor model. We find that NVP-BEZ235 given with IR can attenuate the growth of intracranial tumors generated from U87-EGFRvIII cells, thereby extending survival of tumor-bearing mice. Importantly, NVP-BEZ235 also sensitizes subcutaneous and intracranial tumors to temozolomide by attenuating the repair of secondary (replication-associated) DSBs that are generated upon temozolomide treatment. In sum, this study provides support for the potential use of NVP-BEZ235 as a radio- and chemo-sensitizer for improving the outcome of GBM therapy. Citation Format: Carlos Gil Del Alcazar, Molly Gillam, Nozomi Tomimatsu, Xiaohuan Gao, Bipasha Mukherjee, Sandeep Burma. Inhibition of DNA repair by the dual PI3K/mTOR inhibitor NVP-BEZ235 as a novel radiosensitization strategy for glioblastoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-359. doi:10.1158/1538-7445.AM2013-LB-359