The cytotoxicity of paclitaxel (taxol) is associated mainly with block in G2/ M phase, the most radiosensitive phase of the cell cycle. Nevertheless, taxol-induced modification of the effects of radiation may vary from clear sensitization to subadditivity. Therefore, this effect was studied in relation to drug-induced alterations in the distribution of cells in the phases of the cell cycle in tumor cells (EMT-6 and OV-1063) and normal skin fibroblasts. Cell survival was evaluated with two colorimetric assays. The cell cycle was evaluated by FACS analysis of doubly-labeled cells. The radiosensitivity of the different cells studied was similar, apart from the less radiosensitive human fibroblasts. However, their dose- and time-dependent sensitivity to taxol varied significantly. After 24 h exposure of EMT-6 cells to taxol ( IC50 ∼20 nM), the fraction of cells in G2/ M phase increased, the fraction in S phase decreased, and the proportion of possibly apoptotic cells with subdiploid and subtetraploid DNA content increased; this coincided with radiosensitization. In OV-1063 cells ( IC50 ∼3 nM), the drug-induced${\rm G}_{2}/{\rm M}\text{-phase}$ block was most pronounced, but the combined effect with radiation was merely additive. In human fibroblasts ( IC50 ∼35 nM), a minimal${\rm G}_{2}/{\rm M}\text{-phase}$ block with no change in the S phase and a massive elevation of apoptotic cells with subdiploid DNA content was accompanied by a subadditive combined effect with radiation. Six hours of exposure to taxol increased the fraction of cells in S phase in both nonsynchronized and S-phase-synchronized human fibroblasts ( G1 phase ∼65%, S phase ∼13%). This was accompanied by a pronounced subadditive effect of the combined treatment. However, in G1-phase synchronized human fibroblasts ( G1 phase ≥90%, S phase ∼3%), only the fraction of cells in G2/ M phase was slightly elevated, with a merely additive response to the combined treatment. The differences in the response to the combined treatment between slowly and rapidly proliferating cells in relation to modifications in the cell cycle are discussed.

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