AN INTERACTION OF CISPLATIN AND RADIATION IN 2 RAT YOLK-SAC TUMOR-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. D-0 for the radiosensi tivity of the radioresistant variant cell line, NMT-1R was 139 cGy, wh ich was 1 . 3 times larger than that for the parent radiosensitive cel l line, NMT-1 (D-0 = 107 cGy). The concentration of cisplatin required to reduce colony formation by 50% at 1 h treatment (ID50 of cisplatin ) was 0 . 25 mu g/ml for NMT-1, whereas that for NMT-1R was 1 . 0 mu g /ml. Cisplatin potentiation of radiation sensitivity was manifested by the decrease in the slope of the radiation dose-response curve. D-0's for NMT-1 and NMT-1R were 83 and 100 cGy in combined treatment with I D50 Of cisplatin immediately before radiation. The enhancement ratios of cisplatin were therefore 1 . 30 for NMT-1 and 1 . 39 for NMT-1R res pectively in D-0. No different enhancement ratio for cisplatin was obs erved in the time course of combination treatment with cisplatin and r adiation 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 seque nce in combination with radiation within 6 h.