MITOTIC BLOCK INDUCED IN HELA-CELLS BY LOW CONCENTRATIONS OF PACLITAXEL (TAXOL) RESULTS IN ABNORMAL MITOTIC EXIT AND APOPTOTIC CELL-DEATH

Paclitaxel at low concentrations (10 nM for 20 h) induces similar to 9 0% mitotic block at the metaphase/anaphase transition in HeLa cells, a pparently by suppressing dynamics of spindle microtubules (M. A. Jorda n et al., Proc. Natl. Acad. Sci. USA, 90: 9552-9556, 1993). It is not known, however, whether inhibition of mitosis by such low paclitaxel c oncentrations results in cell death. In the present work, we found tha t after removal of paclitaxel (10 nM-1 mu M), blocked cells did not re sume proliferation. Instead, cells exited mitosis abnormally within 24 h. They did not progress through anaphase or cytokinesis but entered an interphase-like state (chromatin decondensed, and an interphase-lik e microtubule array and nuclear membranes reformed). Many cells (great er than or equal to 55%) contained multiple nuclei. Additional DNA syn thesis and polyploidy did not occur. DNA degradation into nucleosome-s ized fragments characteristic of apoptosis began during drug incubatio n and increased after drug removal. Cells died within 48-72 h, Incubat ion with paclitaxel (10 nM for 30 h) resulted in high intracellular dr ug accumulation (8.3 mu M) and little efflux after paclitaxel removal; intracellular retention of paclitaxel may contribute to its efficacy. The results support the hypothesis that the most potent chemotherapeu tic mechanism of paclitaxel is kinetic stabilization of spindle microt ubule dynamics.