Tubulin dimers polymerize to form microtubules. They have been used for many years in treating lymphomas and acute childhood leukemia, respectively. Vincristine and vinblastine inhibit cancer cell reproduction by promoting microtubule disassembly. They bind to the tubulin dimers. When the tubulin-alkaloid complex attaches to the microtubule, polymerization is terminated and depolymerization begins.
Mitosis is arrested at metaphase.
The taxoids, paclitaxel and the related semisynthetic docetaxel, are examples of novel new anticancer agents provided by plants. In contrast to the Vinca alkaloids, paclitaxel and docetaxel induce assembly of microtubules and stabilize microtubule networks. Cells treated in vitro with paclitaxel form disorganized bundles of microtubules in all phases of the cell cycle. During cell division, paclitaxel induces the formation of many abnormal spindle asters. Cells are either arrested in mitosis or in G or S phases.