Abstract: AIMTo investigate the radiosensitizing effect and mechanism of action of staurosporine (STP) in human colon carcinoma HT-29 and breast cancer MCF-7/ADR cells. METHODSThe effect of STP on the cytotoxicity of X-ray was determined by clonogenic assay. The effect of STP on cell cycle arrest induced by X irradiation was studied in two cell lines by using flow cytometry, Western Blotting was preformed to indicate the changes of cyclin B1 and cdc2 protein levels. RESULTSSTP sensitized the two cell lines to X-ray by clonogenic assay. STP potentiated the cytotoxicity of X-ray by 2.10- and 2.09-fold in HT-29 and MCF-7/ADR cells. Flow cytometry assay showed that exposure of HT-29 and MCF-7/ADR cells to X-ray caused cells arrest in G₂ phase. The percentage of arrest G₂ phase cells were 56% and 52.7%, respectively. The addition of STP after irradiation resulted in a dose-dependent reduction of G₂ phase arrest induced by X-ray. Furthermore, the results showed that STP blocked decrease of cyclin B1 expression induced by X-ray, while mitotic index measurement indicated that X-ray-irradiated cells treated with STP entered mitosis. The data suggested that the potentiation of cytotoxicity of X-ray by STP is associated with the suppression of cyclin B1 expression, which result in the abrogation of G₂ arrest, before the cells entered into M phase, they had not enough time to repair. CONCLUSIONSTP is a potent G₂ checkpoint abrogator and markedly enhanced the cytotoxicity of X irradiation in the p53 mutant cancer cells.