An interaction of cisplatin and radiation in two rat yolk sac tumour cell lines with different radiosensitivities in vitro.

We investigated the interaction of cisplatin and radiation in two rat yolk sac tumour cell lines with different radiosensitivities in vitro. The effects were assessed by clonogenic assay. D0 for the radiosensitivity of the radioresistant variant cell line, NMT-1R was 139 cGy, which was 1.3 times larger than that for the parent radiosensitive cell line, NMT-1 (D0 = 107 cGy). The concentration of cisplatin required to reduce colony formation by 50% at 1 h treatment (ID50 of cisplatin) was 0.25 microgram/ml for NMT-1, whereas that for NMT-1R was 1.0 microgram/ml. Cisplatin potentiation of radiation sensitivity was manifested by the decrease in the slope of the radiation dose-response curve. D0's for NMT-1 and NMT-1R were 83 and 100 cGy in combined treatment with ID50 of cisplatin immediately before radiation. The enhancement ratios of cisplatin were therefore 1.30 for NMT-1 and 1.39 for NMT-1R respectively in D0. No different enhancement ratio for cisplatin was observed in the time course of combination treatment with cisplatin and radiation within the interval of 6 h. There was no clear change in cell cycle distribution within 6 h after treatment with ID50 of cisplatin for both cell lines. In conclusion, cisplatin had a synergistic effect on both cell lines which was independent of the time course and sequence in combination with radiation within 6 h.