Combined gamma‐irradiation and subsequent cisplatin treatment in human squamous carcinoma cell lines sensitive and resistant to cisplatin

PURPOSE: To investigate the effects of combined radiation and subsequent cisplatin treatment on the human squamous carcinoma cell line SCC-25 and its cisplatin-resistant derivative SCC-25/CP. MATERIALS AND METHODS: SCC-25 and SCC-25/CP cells were treated with various gamma-ray doses (5 cGy-7 Gy) followed 60 min later by cisplatin treatment and subsequently assayed for survival using a conventional colony assay. For SCC-25, the subsequent cisplatin treatment was 0.1, 1, 10 and 20 microM for 1 h. For the more cisplatin-resistant SCC-25/CP cells, the subsequent cisplatin treatment was 10 and 50 microM for 1 h. RESULTS: The cisplatin-resistant SCC-25/CP cells were not cross-resistant to gamma-irradiation. Subsequent treatment with an LD50 concentration of cisplatin (10 and 50 microM for SCC-25 and SCC-25/CP, respectively) resulted in radiosensitization for SCC-25/CP but not for SCC-25 cells. Gamma-irradiation of SCC-25/CP cells followed by treatment with 10 and 50 microM cisplatin for 1 h resulted in radiation survival curves displaying a significant low-dose hypersensitive region followed by increased radioresistance at higher doses. A total of 10 microM cisplatin resulted in radiosensitization confined to the low-dose region (0.05 and 0.25 Gy), whereas the higher cisplatin treatment of 50 microM resulted in the appearance of a hypersensitive region together with a reduction of the increased radioresistance region. In contrast, cisplatin treatment (0.1, 1, 10 and 20 microM for 1 h) of SCC-25 cells had no significant effect on survival following 2.5 or 7.0 Gy and actually resulted in an increased low-dose radiation survival (0.05, 0.25 and 1 Gy) when survival was corrected for cisplatin treatment (p<0.01 for all cisplatin concentrations tested). CONCLUSIONS: The significant radiosensitization for SCC-25/CP given subsequent treatment with 50 microM cisplatin indicates cisplatin can inhibit the increased radioresistance response in SCC-25/CP cells. In contrast, the subsequent cisplatin treatment of SCC-25 cells can enhance their survival following low radiation doses.