The radiosensitizing effect of zinc oxide nanoparticles in sub-cytotoxic dosing is associated with oxidative stress in vitro

Radioresistance is an important cause of head and neck cancer therapy failure. Zinc oxide nanoparticles (ZnO-NP) mediate tumor-selective toxic effects. The aim of this study was to evaluate the potential for radiosensitization of ZnO-NP. The dose-dependent cytotoxicity of ZnO-NP$_{20 nm}$ and ZnO-NP$_{100 nm}$ was investigated in FaDu and primary fibroblasts (FB) by an MTT assay. The clonogenic survival assay was used to evaluate the effects of ZnO-NP alone and in combination with irradiation on FB and FaDu. A formamidopyrimidine-DNA glycosylase (FPG)-modified single-cell microgel electrophoresis (comet) assay was applied to detect oxidative DNA damage in FB as a function of ZnO-NP and irradiation exposure. A significantly increased cytotoxicity after FaDu exposure to ZnO-NP$_{20 nm}$ or ZnO-NP$_{100 nm}$ was observed in a concentration of 10 µg/mL or 1 µg/mL respectively in 30 µg/mL of ZnO-NP$_{20 nm}$ or 20 µg/mL of ZnO-NP$_{100 nm}$ in FB. The addition of 1, 5, or 10 µg/mL ZnO-NP$_{20 nm}$ or ZnO-NP$_{100 nm}$ significantly reduced the clonogenic survival of FaDu after irradiation. The sub-cytotoxic dosage of ZnO-NP$_{100 nm}$ increased the oxidative DNA damage compared to the irradiated control. This effect was not significant for ZnO-NP$_{20 nm}$. ZnO-NP showed radiosensitizing properties in the sub-cytotoxic dosage. At least for the ZnO-NP$_{100 nm}$, an increased level of oxidative stress is a possible mechanism of the radiosensitizing effect