Next-Generation Atm Kinase Inhibitors Under Development Radiosensitize Glioblastoma With Conformal Radiation In A Mouse Orthotopic Model

Ataxia telangiectasia mutated (ATM) is a protein kinase which regulates the DNA damage response and is activated by radiation-induced DNA double-strand breaks. Glioblastoma multiforme is an inevitably fatal cancer in which surgery and chemo-radiation therapy continues to be the primary treatment modality. ATM kinase inhibitors increase radiosensitization of glioma cells and tumors. In particular, mutant p53 tumors seem to respond better to this treatment than those with normal p53. Thus, it might be possible to achieve cancer-specific targeting with an ATM kinase inhibitor (ATMi) as radiosensitizer and achieve tumor control and perhaps cure with lower doses of radiation. Conformal radiation therapy is now standard-of-care in order to spare normal healthy brain. Therefore, to precisely target the tumor and avoid excess toxicities to normal brain structures we utilized the small animal radiation research platform (SARRP) to deliver conformal radiation with cone beam CT imaging and treatment planning. Importantly, we aimed to test the efficacy of novel, orally bioavailable ATMi to radiosensitize GBM in an orthotopic mouse model using the SARRP. Several potent and selective next-generation ATMi developed by AstraZeneca were tested including AZ31 and AZ32, which has been shown to cross the blood brain barrier (BBB), as well as a more potent compound AZD1390, also shown to cross the BBB in preclinical species and currently in preclinical development. Mouse GL261 glioma (p53 mutant) cells were implanted intra-cranially into immunocompetent, syngeneic C57/bl6 mice followed by bioluminescent imaging (BLI) prior to randomization. ATM inhibitors were administered by oral gavage prior to delivering multiple fractions of 2-3 Gy of radiation on 2-4 consecutive days. Radiation was administered via SARRP irradiation to the site of the tumor with a 5x5 mm lateral field. In vitro radiosurvival and western blotting experiments were conducted in parallel with the mouse survival studies to assess in vivo efficacy (Kaplan-Meier). As expected, in vitro cell culture experiments demonstrated that ATM inhibitors blocked the DNA damage response and radiosensitized GL261 mouse and human glioma cells. Enhanced killing by mitotic catastrophe was demonstrated in cells with p53 knockdown. Median survival of mice treated with ATM inhibitor and radiation was significantly longer than radiation alone, ATMi alone, or untreated control mice (p = 0.001). No overt signs of treatment toxicity were observed with SARRP contrary to whole-head irradiated mice (137-Cs) that seemed to develop mucositis and difficulties drinking and eating at doses >10 Gy in combination with an ATMi. ATM inhibitors radiosensitize human and mouse glioma cells and significantly improve the survival of mice with orthotopic tumors also treated with conformal radiation.