Modification of potentially lethal damage in irradiated Chinese hamster V79 cells after incorporation of halogenated pyrimidines.

Radiosensitization of exponentially growing and plateau phase Chinese hamster V79 cells by incorporation of halogenated pyrimidines (HP) was investigated for different culture conditions that influenced repair. For this purpose cells were grown for 72 h with 0, 1, 2 and 4 microM of chloro-(CldUrd), bromo- (BrdUrd) or iodo-deoxyuridine (IdUrd) and were subsequently irradiated with gamma-rays from a 197Cs source, either in exponential growth or in plateau-phase. Cell survival after irradiation was determined by clonogenic assay. In exponentially growing cultures thymidine-replacement in the DNA of the cells after incubation with 4 microM of CldUrd, BrdUrd and IdUrd was 22.3, 32.7 and 12.7%, respectively. In plateau-phase cultures the percentage thymidine replacement in the DNA of the cells after incubation during growth with 4 microM CldUrd, BrdUrd and IdUrd was 27.5, 33.8 and 10.7%, respectively. Linear-quadratic analyses of the radiation survival curves were performed. In exponentially growing cells a marked increase by a factor 2-3 of the value of alpha was obtained. The beta term significantly increased only in cells which were grown in the presence of BrdUrd and which were trypsinized and replated immediately after irradiation. In plateau-phase cells which were trypsinized and plated immediately after irradiation both alpha and beta increased up to a factor 2-3 with increasing incorporation of halogenated pyrimidines. In plateau phase cells which were allowed to repair potentially lethal damage (PLD) for 6 h and subsequently trypsinized and plated, alpha increased by a factor 3-4. In these latter conditions changes in beta were smaller. In exponentially growing cells in which repair was allowed after irradiation by plating prior to the treatment, the alpha values decreased for all the HP drugs tested as compared to the alpha of cells plated immediately after irradiation. In contrast, delay of plating for plateau phase cells yielded increased alpha values not only when compared with the alpha of plateau phase cells plated immediately after treatment but also when compared with the alpha value of radiosensitized exponentially growing cells. The increase of alpha might be interpreted as an enhancement in the expression of PLD. The larger contribution of fixation of PLD might be due to initial DNA damage and/or to inhibition of PLD repair resulting from incorporation of HP. The increase of beta might be attributed to enhanced interaction or to fixation of sublethal damage (SLD). In view of clinical applications of HP it is of interest that sensitization is not abolished in plateau-phase cells.