P53 status plays no role in radiosensitizing effects of SN-38, a camptothecin derivative

Purpose: Topoisomerase inhibitors including camptothecin are being studied as potential radiosensitizers. CPT-11 is a derivative of camptothecin and i s clinically available. In this study, we investigated the effects of SN-38 (an active metabolite of CPT-11) on Four nonirradiated and irradiated muri ne fibroblast cell lines with different p53 statuses to clarify the role of p53 in the radiosensitizing activity of SN-38. Materials and methods: Four fibroblast cell lines, MT158, MT158/neo, MT158/wtp53 and MT158/mp53 with t he same genetic background but with different p53 statuses, were used. Expo nentially growing cells were treated with SN-38 (200 nM) and incubated with the drug for 30 min. Cells were then irradiated (0 to 12 Gy) and further i ncubated with the drug for 2 h. The cell survival rate was determined using a conventional clonogenic assay. The effects of the treatments on the cell cycle were analyzed with a how cytometric assay. Apoptosis after these tre atments was also detected by an annexin V assay. Results: There were no sig nificant differences in sensitivity to radiation or SN-38 treatment among t hese cell lines. The combined treatment of irradiation and SN-38 showed sup raadditive effects in all four cell lines independent of their p53 status. Transient arrest in G(2) with a decreased percentage of cells in both the S and G(1) phases was observed 8 h after treatment with either SN-38 alone, radiation or their combination, regardless of the p53 status. No significan t differences in frequency of apoptosis were observed between treatment and control groups in two cell lines with or without wild-type p53. Conclusion : The combination of irradiation and SN-38 treatment showed supraadditive e ffects in all four cell lines tested, and the p53 status did not play a rol e in the combination effect.