Combination Therapy with Pharmacological Ascorbate and Rucosopasem Selectively Radiosensitizes Colorectal Tumors to Clinically Relevant Radiation Doses

Pentaazamacrocyclic Mn (II)-containing (MnPAM) superoxide dismutase mimetics can selectively modulate the effects of radiotherapy in tumors and normal tissues by enhancing oxidative stress in tumors and mitigating it in normal tissues. However, radiosensitizing effects in tumors are only achieved with stereotactic body radiotherapy-type dosing (≥7 Gy per fraction). These effects are limited at lower fractional doses that are clinically relevant for many tumors, including locally advanced rectal cancer. Here, we show that combining the MnPAM dismutase mimetic, rucosopasem manganese (GC4711), with pharmacological ascorbate significantly enhances radiosensitization to fractionated radiation doses relevant in colorectal cancer treatment (2-5 Gy) compared to either agent alone in multiple tumor models in vitro and significantly prolongs tumor growth delay in vivo. Mechanistically, enhanced radiosensitization with combination therapy is mediated by increased oxygen consumption and hydrogen peroxide generation. Despite having no effect on oxygen consumption in tumor cell media on its own, GC4711 significantly increases pharmacological ascorbate-induced oxygen consumption. Inducible catalase expression, or administration of exogenous catalase, attenuates the anti-cancer activity when combined. As predicted, increased peroxide fluxes with combination therapy enhanced postirradiation DNA damage and G2/M cell cycle accumulation. Importantly, treatment with pharmacological ascorbate and/or GC4711 had no significant effect on postirradiation clonogenic survival or DNA damage in non-malignant FHs74 enterocytes. These results support the hypothesis that adding pharmacological ascorbate to MnPAM dismutase mimetics selectively induces more peroxide-mediated radiosensitization than either agent alone in tumors without increasing radiosensitivity in cells treated with clinically relevant radiation fraction sizes used for radiotherapy of colorectal cancers.