Manipulation of Redox Metabolism Using Pharmacologic Ascorbate Opens a Therapeutic Window for Radio-Sensitization by ATM Inhibitors in Colorectal Cancer

Purpose: Ataxia telangiectasia mutated kinase (ATM) inhibitors are potent radiosensitizers that influence DNA damage responses and redox metabolism. However, their clinical application has been limited due to the risk of excessive toxicity to normal tissues. Pharmacologic ascorbate (P-AscH-, administered intravenously to achieve millimolar plasma concentrations) selectively enhances hydrogen peroxide (H2O2)-induced oxidative stress and radiosensitization in tumors, while acting as an antioxidant to mitigate radiation-induced damage in normal tissues, including the bowel. We hypothesized that P-AscH- could improve the therapeutic index of ATM inhibitor-based chemoradiation by simultaneously enhancing the tumor-specific effects of ATM inhibitors and reducing off-target toxicity in adjacent normal tissues.
Methods and Materials: Clonogenic survival was evaluated in human colorectal cancer cell lines (HCT116, SW480, HT29), murine colorectal cancer cell lines (CT26, MC38), and normal cells (human umbilical vein endothelial cells, FHs74) after radiation with or without DNA repair inhibitors and P-AscH-. Tumor growth delay was studied in mice bearing HCT116 or MC38 tumors treated with fractionated radiation (5 Gy × 3) ± the ATM inhibitor KU60019 ± P-AscH-. Intestinal injury, oxidative damage, and transforming growth factor β (TGF-β) immunoreactivity were assessed via immunohistochemistry following whole abdominal radiation (10 Gy) ± KU60019 ± P-AscH-. Cell cycle distribution and ATM subcellular localization were analyzed using flow cytometry and immunohistochemistry. The role of intracellular H2O2 fluxes was examined with a doxycycline-inducible catalase transgene.
Results: The combination of KU60019 and P-AscH- significantly KU-60019 enhanced radiosensitization in colorectal cancer models both in vitro and in vivo by promoting H2O2-mediated oxidative damage to proteins, increasing DNA damage, disrupting the post-radiation G2 cell cycle checkpoint, and inhibiting ATM nuclear localization. Notably, P-AscH- concurrently reduced intestinal toxicity and oxidative damage associated with KU60019 treatment.
Conclusions: Our findings suggest that redox-modulating agents such as P-AscH- can enhance the clinical translation of ATM inhibitors by maximizing tumor radiosensitization while protecting normal tissues from radiation-induced damage.