Paclitaxel at low concentrations (10 nm for 20 h) induces ∼90% mitotic block at the metaphase/anaphase transition in HeLa cells, apparently by suppressing dynamics of spindle microtubules (M. A. Jordan et al., Proc. Natl. Acad. Sci. USA, 90: 9552–9556, 1993). It is not known, however, whether inhibition of mitosis by such low paclitaxel concentrations results in cell death. In the present work, we found that after removal of pacli-taxel (10 nm-1 µm), blocked cells did not resume 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-like microtubule array and nuclear membranes reformed). Many cells (≥55%) contained multiple nuclei. Additional DNA synthesis and polyploidy did not occur. DNA degradation into nucleosome-sized fragments characteristic of apoptosis began during drug incubation and increased after drug removal. Cells died within 48–72 h. Incubation with paclitaxel (10 nm for 20 h) resulted in high intracellular drug accumulation (8.3 µm) and little efflux after paclitaxel removal; intracellular retention of paclitaxel may contribute to its efficacy. The results support the hypothesis that the most potent chemotherapeutic mechanism of paclitaxel is kinetic stabilization of spindle microtubule dynamics.

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This work was supported by Grant CA 57291 from the National Cancer Institute.

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