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Purpose: Ataxia telangiectasia mutated kinase (ATM) inhibitors are potent radiosensitizers that regulate DNA damage responses and redox metabolism, but they have not been translated clinically because of the potential for excess normal tissue toxicity. Pharmacologic ascorbate (P-AscH - ; intravenous administration achieving mM plasma concentrations) selectively enhances H 2 O 2 -induced oxidative stress and radiosensitization in tumors while acting as an antioxidant and mitigating radiation damage in normal tissues including the bowel. We hypothesized that P-AscH - could enhance the therapeutic index of ATM inhibitor-based chemoradiation by simultaneously enhancing the intended effects of ATM inhibitors in tumors and mitigating off-target effects in adjacent normal tissues., Methods and Materials: Clonogenic survival was assessed in human (human colon tumor [HCT]116, SW480, HT29) and murine (CT26, MC38) colorectal tumor lines and normal cells (human umbilical vein endothelial cell, FHs74) after radiation ± DNA repair inhibitors ± P-AscH - . Tumor growth delay was assessed in mice with HCT116 or MC38 tumors after fractionated radiation (5 Gy × 3) ± the ATM inhibitor KU60019 ± P-AscH - . Intestinal injury, oxidative damage, and transforming growth factor β immunoreactivity were quantified using immunohistochemistry after whole abdominal radiation (10 Gy) ± KU60019 ± P-AscH - . Cell cycle distribution and ATM subcellular localization were assessed using flow cytometry and immunohistochemistry. The role of intracellular H 2 O 2 fluxes was assessed using a stably expressed doxycycline-inducible catalase transgene., Results: KU60019 with P-AscH - enhanced radiosensitization in colorectal cancer models in vitro and in vivo by H 2 O 2 -dependent oxidative damage to proteins and enhanced DNA damage, abrogation of the postradiation G2 cell cycle checkpoint, and inhibition of ATM nuclear localization. In contrast, concurrent P-AscH - markedly reduced intestinal toxicity and oxidative damage with KU60019., Conclusions: We provide evidence that redox modulating drugs, such as P-AscH - , may facilitate the clinical translation of ATM inhibitors by enhancing tumor radiosensitization while simultaneously protecting normal tissues., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Purpose: The majority of colorectal cancers are resistant to cancer immune checkpoint inhibitors. Ionizing radiation (IR) and several radiosensitizers, including PARP inhibitors, can enhance responsiveness to immune checkpoint inhibitors by potentially complementary mechanisms of action. We assessed the ability of radiation and PARP inhibition to induce proimmunogenic changes in tumor cells and enhance their in vivo responsiveness to anti-PD-1 antibodies., Methods and Materials: We performed a candidate drug screen and used flow cytometry to assess effects of the PARP inhibitor veliparib on IR-mediated changes in MHC-1 antigen presentation and surface localization of immune-modulating proteins including PD-L1 and calreticulin in colorectal cancer tumor models. Reverse transcription polymerase chain reaction was used to assess the effects of veliparib and radiation on the expression of proinflammatory and immunosuppressive cytokines. The ability of concurrent PARP inhibition and subablative doses of radiation therapy to enhance in vivo responsiveness to anti-PD-1 antibodies was assessed using unilateral flank-tumor models with or without T-cell depletion., Results: Veliparib was a potent radiosensitizer in both cell lines. Radiation increased surface localization of MHC-1 and PD-L1 in a dose-dependent manner, and veliparib pretreatment significantly enhanced these effects with high (8 Gy) but not with lower radiation doses. Enhancement of MHC-1 and PD-L1 surface localization by IR and IR+ veliparib remained significant 1, 3, and 7 days after treatment. IR significantly increased delayed tumoral expression of proinflammatory cytokines interferon-Ƴ and CXCL10 but had no significant effect on the expression of IL-6 or TGF-β. Concurrent administration of veliparib and subablative radiation therapy (8 Gy × 2) significantly prolonged anti-PD-1-mediated in vivo tumor growth delay and survival in both tumor models. Moreover, these effects were more pronounced in the microsatellite instability-mutated MC38 tumor model. Enhancement of anti-PD-1 mediated tumor growth delay with veliparib and IR was attenuated by CD8+ T-cell depletion., Conclusions: We provide preclinical evidence for a novel therapeutic strategy to enhance responsiveness of colorectal tumors to immune checkpoint inhibitors., (Copyright © 2020 Elsevier Inc. All rights reserved.)