[Antimitotic agents].

Microtubules are one of the major filament of the cytoskelton and play a role in various biological functions such as mitosis, cell motility and intracellular transport. Therefore, microtubules are considered one of the most important molecular targets for cancer chemotherapy. Tubulin is one of the major microtubular components, and its polymerization and depolymerization regulate microtubular dynamics. Other microtubular components such as microtubule-associated protein (MAPs), actin, and intermediate and microfilaments have also been demonstrated to be involved in microtubular dynamics. Recent studies provide evidence that the functions of MAPs and filaments in microtubule assembly are regulated by phosphorylation, which is catalyzed by mitogenactivated protein kinase (MAP kinase) and cdc2 kinase. Antimitotic agents that disrupt microtubules can be classified in two categories according to the mechanism of action, vinca alkaloids and taxanes. Vinca alkoloids, estramustine, rhizoxin, and E7010 inhibit microtubule polymerization. In contrast, taxanes such as paclitaxel and docetaxel promote polymerization of microtubules and enhance microtubule stability. We have demonstrated that paclitaxel inhibits the catalytic activity of MAP kinase and cdc2 kinase in lung cancer cell lines. This biological effect may be responsible for the increased affinity between MAP2 and tubulins, resulting in promotion of microtubule assembly. Factors that contribute to the resistance to antimitotic agents include intracellular accumulation of the drugs, genetic or functional alternations in tubulin, and alternations in MAP kinase cascade. Antimitotic agents showed a broad spectrum of preclinical antitumor activity. Clinical trials of taxanes revealed that they were effective for several cancers which were advanced or resistant against other anticancer drugs, especially for breast cancers, ovarian cancers and non-small cell lung cancers.