Abstract 811: Pharmacologic ascorbate opens a therapeutic window for ATM inhibition and radiotherapy in colorectal cancer

Purpose/Objective(s): The ATM protein is key to DNA double strand break (DSB) repair and ATM inhibitors are potent radiosensitizers. Clinical translation of these agents in combination with radiotherapy (RT) has been limited due to concerns for increased normal tissue toxicity. Pharmacologic Ascorbate (P-AscH-) radiosensitizes cancer cells via generation of a H2O2 flux while acting as a radioprotector in normal tissue via free radicle scavenging. We hypothesized that P-AscH- could open a therapeutic window for the combination of RT and ATM inhibition in colorectal cancer (CRC). Materials/Methods: Multiple human and murine CRC cell lines were used in vitro. Clonogenic survival was assessed after combinations of RT +/- P-AscH and DNA Repair Inhibitors (DRIs). Catalase expression was induced using HCT116 cells expressing a doxycycline-inducible catalase transgene. DSBs were quantified using neutral comet assays. Cell cycle distribution were assessed using flow cytometry. ATM localization/activation were assessed using IF. Normal tissue toxicity in vivo was assessed using IHC following whole-abdominal RT. Survival and tumor growth delay was assessed following 5Gyx3 +/- drug treatment to unilateral flank tumors in syngeneic/xenograft models. Results: DRIs were potent radiosensitizers in most CRC cell lines and the addition of P-AscH- further reduced clonogenic survival. In contrast, P-AscH- did not radiosensitize HUVEC or FHs-74int normal cell lines. P-AscH- significantly increased the number of DSBs in tumors after RT in vitro. P-AscH- simultaneously decreased nuclear localization and activation of pATM after RT and perturbed G2+M phase progression. In vivo, the addition of P-AscH- to RT + KU60019 significantly increased survival delayed tumor growth in syngeneic/xenograft models while ameliorating increased normal bowel toxicity as measured by jejunal crypt density, acute weight loss, rectal injury, and markers of oxidative stress following whole-abdominal RT. The effects of P-AscH were reversed by inducing the overexpression of catalase. Conclusion: P-AscH- improves both aspects of the therapeutic window of RT+ATM inhibition in CRC by simultaneously enhancing tumor efficacy while decreasing RT-mediated bowel toxicity. The effects of P-AscH- on clonogenic survival, initial and persistent DSBs, G2+M phase perturbations, ATM activation/localization, and in vivo survival and tumor growth delay were dependent on H202 flux. Normal tissue protection appears to be related to decreased oxidative stress. Citation Format: Cameron M. Callaghan, Ibrahim M. Abukhiran, Richard V. Van Rheeden, Amanda L. Kalen, Samuel N. Rodman, Michael S. Petronek, Kranti A. Mapuskar, Sarah L. Mott, Mitchell C. Coleman, Prabhat C. Goswami, John M. Buatti, Bryan G. Allen, Douglas R. Spitz, Joseph M. Caster. Pharmacologic ascorbate opens a therapeutic window for ATM inhibition and radiotherapy in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 811.