Evaluation of the genotoxic activity of paclitaxel by the in vitro micronucleus test in combination with fluorescent in situ hybridization of a DNA centromeric probe and the alkaline single cell gel electrophoresis technique (comet assay) in human T-lymphocytes.

Paclitaxel is a recent chemotherapeutic agent that inhibits tubulin depolymerization in tumoral cells. Despite its increasing use against various human cancers, the genotoxicity of paclitaxel has never been studied on normal human cells. The in vitro genotoxic effects of the drug were evaluated with two complementary mutagenesis tests on human T-lymphocytes: (1) the cytokinesis-blocked micronuclei assay (CBMN) in combination with fluorescent in situ hybridization (FISH) of nonspecific centromeric probes and (2) the comet assay performed in three ways: on stimulated lymphocytes as in the CBMN, and on freshly isolated lymphocytes at both 4 and 37 degrees C. A slight cytotoxicity of 2.5 to 10 nM paclitaxel was found in the CBMN and a significant increase in the binucleated micronucleated cell rates was observed, with a concentration-dependent manner. In the FISH analysis, more than 85% of the micronuclei (MN) were centromere positive, and a ratio of 72. 2 to 78.6% of these MN contained more than one centromere. Moreover, at 10 nM of paclitaxel, 35.6% of the cells are multimicronucleated lymphocytes. Unexpectedly, paclitaxel induced single-strand breaks on proliferating lymphocytes at 5 and 7.5 nM but not in resting cells, even at 5 to 15 microM. These in vitro results showed that (1) paclitaxel does not present any direct DNA action in resting cells, (2) DNA damage detected in stimulated lymphocytes may be linked either to a high frequency of cells in the S-phase cell cycle or to a direct DNA damaging effect on replicating cells, and (3) paclitaxel is a strong in vitro aneugenic drug on human normal cells, at clinically relevant concentrations. Copyright 1999. Wiley-Liss, Inc.