Effects of Ultrafine Single-Nanometer Oxygen Bubbles on Radiation Sensitivity in a Tumor-Bearing Mouse Model.

Radiation therapy against cancer cells often causes radiation resistance via accumulation of hypoxia-inducible factor 1 subunit alpha (HIF-1α) under hypoxic conditions and severe side effects. Radiation sensitizers without side effects are required to overcome hypoxia-induced radiation resistance and decrease radiation-related side effects in patients with refractory cancer. We previously developed oxygen nanobubble water (NBO 2 water) and demonstrated that it suppresses hypoxia-induced radiation resistance in cancer cell lines within the single-nanometer range. This study aimed to elucidate whether NBO 2 water could act as a radiosensitizer via regulation of HIF-1α in a tumor-bearing mouse model. Six-week-old female BALB/c mice subcutaneously injected with tumor cells received control water or NBO 2 water for 28 days, after which biochemical examinations and radiation treatment were performed. Hypoxic tumor regions were detected immunohistochemically. We found that NBO 2 water sensitized radiation reactivity in the xenografted tumors. Notably, NBO 2 water administration downregulated the accumulation of HIF-1α in xenografted tumors and did not affect the vital organs of healthy mice. The combination of radiation and single-nanometer NBO 2 water without severe side effects may be a promising therapeutic option to improve radiation sensitivity in cancer patients without tolerance to invasive treatments.