Description_x000D_ Biochem/physiol Actions_x000D_ (-)-Epothilone A is a microtubule (MT) stabilizing drug and natural macrolide antitumor from myxobacteria Sorangium cellulosum. EpoA exhibits kinetics similar to paclitaxel by inducing tubulin polymerization in vitro and producing enhanced microtubule stability and bundling in cultured cells. In contrast to paclitaxel, Epothilone A exhibits a greater cytotoxicity against P-glycoprotein-expressing multidrug resistant (MDR) cells (IC50 = 20 nM for MDR CCRF-CEM/VBL100 cells). Epothilone A is a competitve inhibitor of 3H-paclitaxel binding with comparable IC50 to paclitaxel in displacement competition assays. EpoA causes cell cycle arrest at the G2/M transition leading to cytotoxicity.