Epothilones, a new class of microtubule-stabilizing agents with a taxol-like mechanism of action.

Tubulin polymerization into microtubules is a dynamic process, with the equilibrium between growth and shrinkage being essential for many cellular processes. The antineoplastic agent taxol hyperstabilizes polymerized microtubules, leading to mitotic arrest and cytotoxicity in proliferating cells. Using a sensitive filtration-calorimetric assay to detect microtubule nucleating activity, we have identified epothilones A and B as compounds that possess all the biological effects of taxol both in vitro and in cultured cells. The epothilones are equipotent and exhibit kinetics similar to taxol in inducing tubulin polymerization into microtubules in vitro (filtration, light scattering, sedimentation, and electron microscopy) and in producing enhanced microtubule stability and bundling in cultured cells. Furthermore, these 16-membered macrolides are competitive inhibitors of [3H]taxol binding, exhibiting a 50% inhibitory concentration almost identical to that of taxol in displacement competition assays. Epothilones also cause cell cycle arrest at the G2-M transition leading to cytotoxicity, similar to taxol. In contrast to taxol, epothilones retain a much greater toxicity against P-glycoprotein-expressing multiple drug resistant cells. Epothilones, therefore, represent a novel structural class of compounds, the first to be described since the original discovery of taxol, which not only mimic the biological effects of taxol but also appear to bind to the same microtubule-binding site as taxol.