Interaction of ionizing radiation with the topoisomerase I poison camptothecin in growing V-79 and HeLa cells.

Interaction between gamma-rays and camptothecin (CPT) was investigated in vitro, using log phase HeLa-S3 cells and Chinese hamster V-79 fibroblasts. A plateau of toxicity was rapidly reached for both cell lines upon exposure to CPT alone, consistent with S-phase specificity of CPT. With synchronized HeLa cells, however, CPT proved cytotoxic after the middle of G1 phase. This effect was abolished by cotreatment with the DNA polymerase alpha inhibitor aphidicolin. CPT enhanced the initial slope of the radiation survival curves for asynchronous cells by a factor of 1.1 (HeLa) to 2.3 (V-79). This apparent radiation sensitization correlated with the intrinsic radiosensitivities of the drug-surviving fractions within the different compartments of the cell cycle. There was no evidence of mutual potentiation of CPT and radiation in terms of survival as well as with regard to the formation and rejoining of DNA double-strand breaks. V-79 cells exhibited pronounced postirradiation recovery. In contrast, the response of HeLa cells to drug did not vary appreciably for over 8-h following radiation. This difference proceeded from differential cell cycle redistribution. In both cell lines, acute irradiation produced depletion of the G1 compartment, accumulation at the S-G2 junction, and G2 arrest in proportion to the gamma-ray dose. However, while radiation brought about rapid depletion of the S-phase compartment in V-79 cells, it induced accumulation of HeLa cells into S phase. As a result, the CPT-sensitive HeLa cell population, consisting of the early-S, mid-S, and late G1 fractions, did not vary very much after irradiation. Exposure to CPT under conditions of low dose rate irradiation (1 Gy/h) selectively reversed the radiation effect on S-phase progression in V-79 cells; i.e., it induced accumulation of cells in S phase in the same way as found with HeLa cells. Isobolic analysis of survival data consistently showed supraadditivity of cell killing in both cell lines upon concomitant exposure to CPT and low dose rate irradiation. Cytokinetic cooperation appears to be the major determinant of cell survival in treatments associating CPT and radiation in growing cells. Attempts to predict the outcome of such a combined modality thus should take into consideration the response of the growing fraction of each cell line in terms of cell cycle-regulatory processes and redistribution.