Smaller Oxygen Nanobubbles More Effectively Suppress Colitis and Colon Carcinogenesis than Larger Oxygen Nanobubbles

Oxygen nanobubble (NBO₂) water is reportedly a promising therapeutic and radiosensitizing agent against solid cancers. However, the significance of nanobubble size in inflammation-associated colorectal carcinogenesis in vivo remains elusive. We investigated whether small NBO₂ water exerts stronger preventive effects against colitis and colorectal carcinogenesis in an azoxymethane/dextran sulfate sodium-induced mouse model of colitis-associated cancer. Differences in particle size between the small and large NBO₂ water samples were confirmed by atomic force microscopy. The mice received drinking water containing either small or large sized NBO₂ throughout the experiment. Small NBO₂ water significantly reduced disease activity index scores, histopathological colitis scores, colonic shortening, CD68-positive inflammatory macrophage density, and tumor numbers. However, body weight, water intake, food intake, and spleen weight were unaffected. Immunohistochemistry revealed that small NBO₂ water reduced the percentage of Ki-67-positive tumor cells and the proportions of hypoxia-inducible factor-1α-positive epithelial and stromal cells, whereas no significant differences were observed in CD8- or forkhead box P3-positive cells. We conclude that nanometer-sized oxygen bubbles prevent inflammation-associated colorectal carcinogenesis, and that particle size is a critical determinant of biological effects. Small amounts of NBO₂ water may help control colitis and tumor development by alleviating hypoxia in the tumor microenvironment.