Long B H, Fairchild C R
Department of Experimental Therapeutics, Bristol-Myers Squibb Pharmaceutical Research Institute, Princeton, New Jersey 08543-4000.
Cancer Res. 1994 Aug 15;54(16):4355-61.
Very low concentrations of paclitaxel, a clinically active anticancer agent isolated from the bark of the Pacific yew tree, were found to produce micronuclei in human colon carcinoma cells, suggesting inhibition of mitotic spindle assembly or function. The possibility that paclitaxel acts at the level of the mitotic spindle was investigated by evaluating its ability to inhibit the progression of mitotic cells to G1 phase. Paclitaxel inhibited mitotic progression with a median inhibitory concentration of 4 nM, a concentration equivalent to the median cytotoxic concentration, without arresting cells in mitosis. A direct correlation was shown to exist between the cytotoxic potency and ability to inhibit mitotic progression for analogues of paclitaxel and antimicrotubule agents but not for the topoisomerase II-active agents etoposide and teniposide. After release from the nocodazole block, cells synchronized in mitosis remained sensitive to very low concentrations of paclitaxel for < 30 min, the time required for spindle formation, yet remained sensitive to vinblastine for > 90 min. This result indicates that 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 nanomolar concentrations of paclitaxel for more than one cell cycle resulted in cells with DNA contents > 4C and as much as 8C. These results support a hypothesis, that, by not being capable of segregating sister chromatids, paclitaxel-treated cells eventually reform nuclear membranes around individual or clusters of chromosomes, revert to G1 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.
紫杉醇是一种从太平洋紫杉树皮中分离出的具有临床活性的抗癌药物，人们发现极低浓度的紫杉醇会在人结肠癌细胞中产生微核，这表明其对有丝分裂纺锤体的组装或功能具有抑制作用。通过评估紫杉醇抑制有丝分裂细胞进入G1期的能力，研究了其作用于有丝分裂纺锤体水平的可能性。紫杉醇抑制有丝分裂进程，其半数抑制浓度为4 nM，该浓度与半数细胞毒性浓度相当，且不会使细胞停滞在有丝分裂期。结果表明，紫杉醇类似物和抗微管药物的细胞毒性效力与其抑制有丝分裂进程的能力之间存在直接相关性，但拓扑异构酶II活性药物依托泊苷和替尼泊苷则不存在这种相关性。从诺考达唑阻滞中释放后，处于有丝分裂同步化的细胞在<30分钟内（纺锤体形成所需时间）对极低浓度的紫杉醇仍保持敏感，但对长春碱的敏感时间>90分钟。这一结果表明，极低浓度的紫杉醇可抑制细胞中有丝分裂纺锤体的形成，而不影响已形成纺锤体的功能，也不会使细胞停滞在有丝分裂期。连续一个以上细胞周期暴露于低纳摩尔浓度的紫杉醇会导致细胞DNA含量>4C，甚至高达8C。这些结果支持了一种假说，即紫杉醇处理的细胞由于无法分离姐妹染色单体，最终会在单个或成簇的染色体周围重新形成核膜，恢复为含有4C DNA的G1期细胞，并进入S期，从而产生DNA含量高达8C的细胞。有人提出，这是紫杉醇的主要细胞毒性机制。
