A comparative study of the radiation and/or doxorubicin (DOX) survival response for synchronous populations of Chinese hamster V79 cells and two DOX-resistant variants (77A and LZ-8) was performed. The greatest cellular radiation sensitivity was observed in mitosis, while the greatest resistance was observed during late S phase for the three cell lines. The variation in radiation response throughout the cell cycle was expressed as a change in the width of the shoulder of the survival curves$(D_{q})$ with little change in D0. This suggests that each phase of the cell cycle has a different capacity for accumulation of radiation injury. The radiation age-response function for the three cell lines revealed that 77A and LZ-8 cells were more radiosensitive than V79 cells throughout the cell cycle. Exposure of synchronous populations to DOX (1.84 μM for V79, 9.21 μM for 77A, and 921 μM for LZ-8) for 1 h as a function of cell cycle phase revealed that V79, 77A, and LZ-8 cells exhibited the greatest sensitivity to DOX in mitosis and the most resistance to DOX during S phase, as indicated by the differences in the slope of the initial component of the survival curve. Levels of P-glycoprotein (P-gp) are probably not a factor contributing to DOX age-response function since P-gp levels remain constant throughout the cell cycle in all three cell lines. Synchronous populations of V79, 77A, and LZ-8 cells sequentially treated with DOX and radiation at various cell cycle phases were also analyzed. The results showed that the interaction between radiation and DOX damage resulted in a reduced cellular capacity for the accumulation of radiation damage throughout the cell cycle, as indicated by a decrease in the width of the shoulder of the survival curve. Overall, both DOX-sensitive V79 cells and DOX-resistant 77A and LZ-8 cells exhibited (1) a similar age-response function for radiation or DOX, and (2) no differences in the effects of DOX on radiation-induced damage throughout the cell cycle. These results indicate that acquired resistance to DOX associated with increased levels of P-gp in the cell membrane did not appear to affect the age-response function for radiation or DOX, and the nature of the interaction between damage caused by radiation and DOX was also not affected.

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