Antioxidant Nanomedicine Significantly Enhances the Survival Benefit of Radiation Cancer Therapy by Mitigating Oxidative Stress‐Induced Side Effects

Oxidative stress‐induced off‐target effects limit the therapeutic window of radiation therapy. Although many antioxidants have been evaluated as radioprotective agents, none of them are in widespread clinical use, owing to the side effects of the antioxidants themselves and the lack of apparent benefit. Aiming for a truly effective radioprotective agent in radiation cancer therapy, the performance of a self‐assembling antioxidant nanoparticle (herein denoted as redox nanoparticle; RNP) is evaluated in the local irradiation of a subcutaneous tumor‐bearing mouse model. Since RNP is covered with a biocompatible shell layer and possesses a core–shell type structure of several tens of nanometers in size, its lifetime in the systemic circulation is prolonged. Moreover, since 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO), one of the most potent antioxidants, is covalently encapsulated in the core of RNP, it exerts intense antioxidant activity and induces fewer adverse effects by avoiding leakage of the TEMPO molecules. Preadministration of RNP to the mouse model effectively mitigates side effects in normal tissues and significantly extends the survival benefit of radiation cancer therapy. Moreover, RNP pretreatment noticeably increases the apoptosis/necrosis ratio of radiation‐induced cell death, a highly desirable property to reduce the chronic side effects of ionizing irradiation. Redox nanoparticle (RNP) is a self‐assembling antioxidant nanomedicine that exerts significantly prolonged systemic circulation and highly potent reactive oxygen species (ROS) scavenging activity. In contrast to previous predictions, ROS scavenging in tumors does not sacrifice cancer cell killing efficacy of ionizing radiation, results in significantly enhanced survival benefit of radiation cancer therapy.