PACLITAXEL INHIBITS PROGRESSION OF MITOTIC CELLS TO G(1) PHASE BY INTERFERENCE WITH SPINDLE FORMATION WITHOUT AFFECTING OTHER MICROTUBULE FUNCTIONS DURING ANAPHASE AND TELEPHASE

Very low concentrations of paclitaxel, a clinically active anticancer agent isolated from the bark of the Pacific yew tree, were found to pr oduce micronuclei in human colon carcinoma cells, suggesting inhibitio n of mitotic spindle assembly or function. The possibility that paclit axel acts at the level of the mitotic spindle was investigated by eval uating its ability to inhibit the progression of mitotic cells to G(1) phase. Paclitaxel inhibited mitotic progression with a median inhibit ory concentration of 4 nM, a concentration equivalent to the median cy totoxic concentration, without arresting cells in mitosis. A direct co rrelation was shown to exist between the cytotoxic potency and ability to inhibit mitotic progression for analogues of paclitaxel and antimi crotubule agents but not for the topoisomerase II-active agents etopos ide and teniposide. After release from the nocodazole block, cells syn chronized in mitosis remained sensitive to very low concentrations of paclitaxel for <30 min, the time required for spindle formation, yet r emained sensitive to vinblastine for >90 min. This result indicates th at very low concentrations of paclitaxel inhibit formation of mitotic spindles in cells without affecting function of preformed spindles and without arresting cells in mitosis. Continuous exposure to low nanomo lar concentrations of paclitaxel for more than one cell cycle resulted in cells with DNA contents >4C and as much as 8C. These results suppo rt a hypothesis that, by not being capable of segregating sister chrom atids, paclitaxel-treated cells eventually reform nuclear membranes ar ound individual or clusters of chromosomes, revert to G(1) phase cells containing 4C DNA, and enter S phase, resulting in cells with as much as 8C DNA content. It is proposed that this is the primary cytotoxic mechanism of paclitaxel.