Iodine-Hoechst Enhances X-ray Sensitivity of Cancer Cells Under Normal and Low Oxygen Conditions

Iodine-Hoechst (IH) is a derivative of Hoechst dye that binds minor grooves of DNA. This chemical has been used for cell labeling studies such as nuclear staining; however, its use in radiation therapy has not been explored fully. We investigated enhancement of X-ray sensitivity of cancer cells by IH.

Cell survival was examined over a range of X-ray doses, survival curves were generated, and the enhancement ratio (ER) was calculated. Double-strand DNA breaks were examined by a γH2AX assay. We then investigated whether the radiosensitivity-enhancing effect of IH occurs under low oxygen conditions. Additionally, reactive oxygen species (ROS) production was measured using dichlorodihydrofluorescein diacetate. As a first step to investigate the effect of IH on X-ray inhibition of tumor growth, we employed the chorioallantoic membrane (CAM) assay, a simple, versatile cancer model.

IH enhanced X-ray sensitivity of cancer cells, with an ER of 1.29 based on 10% survival values. X-ray-induced DNA double-strand breaks were increased by IH. Upon exposing cells to low oxygen conditions, X-ray sensitivity decreased, consistent with the idea that X-ray sensitivity decreases under low oxygen. Even under this condition, IH enhancement persisted, revealing that the radiosensitivity-enhancing effect was not significantly affected by oxygen. This enhancement does not involve ROS, as X-ray-induced ROS production was unchanged by IH. In the CAM assay, transplantation of cancer cells led to tumor formation within the egg. Intravenous injection of IH resulted in the delivery of IH to the tumor. X-ray irradiation of the tumor inhibited tumor growth, and this was enhanced by IH.

Our results reveal that IH enhances radiation sensitivity of cancer cells, and this effect is observed even under low oxygen conditions.