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

Topoisomerase inhibitors including camptothecin are being studied as potential radiosensitizers. CPT-11 is a derivative of camptothecin and is clinically available. In this study, we investigated the effects of SN-38 (an active metabolite of CPT-11) on four nonirradiated and irradiated murine fibroblast cell lines with different p53 statuses to clarify the role of p53 in the radiosensitizing activity of SN-38.

Four fibroblast cell lines, MT158, MT158/neo, MT158/wtp53 and MT158/mp53 with the same genetic background but with different p53 statuses, were used. Exponentially 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 incubated 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 flow cytometric assay. Apoptosis after these treatments was also detected by an annexin V assay.

There were no significant differences in sensitivity to radiation or SN-38 treatment among these cell lines. The combined treatment of irradiation and SN-38 showed supraadditive effects in all four cell lines independent of their p53 status. Transient arrest in G2 with a decreased percentage of cells in both the S and G1 phases was observed 8 h after treatment with either SN-38 alone, radiation or their combination, regardless of the p53 status. No significant differences in frequency of apoptosis were observed between treatment and control groups in two cell lines with or without wild-type p53.

The combination of irradiation and SN-38 treatment showed supraadditive effects in all four cell lines tested, and the p53 status did not play a role in the combination effect.