Title: Activities of the Microtubule-stabilizing Agents Epothilones A and B with Purified Tubulin and in Cells Resistant to Paclitaxel (Taxol®)
Abstract: Epothilones A and B, natural products with minimal structural analogy to taxoids, have effects similar to those of paclitaxel (Taxol®) in cultured cells and on microtubule protein, but differ from paclitaxel in retaining activity in multidrug-resistant cells. We examined interactions of the epothilones with purified tubulin and additional cell lines, including a paclitaxel-resistant ovarian carcinoma line with an altered β-tubulin. The epothilones, like paclitaxel, induced tubulin to form microtubules at low temperatures and without GTP and/or microtubule-associated proteins. The epothilones are competitive inhibitors of the binding of [<sup>3</sup>H]paclitaxel to tubulin polymers. The apparent <i>K</i><sub>i</sub> values for epothilones A and B were 1.4 and 0.7 μM by Hanes analysis and 0.6 and 0.4 μM by Dixon analysis. In the paclitaxel-sensitive human cell lines we examined, epothilone B had greater antiproliferative activity than epothilone A or paclitaxel, while epothilone A was usually less active than paclitaxel. A multidrug-resistant colon carcinoma line and the paclitaxel-resistant ovarian line retained sensitivity to the epothilones. With <i>Potorous tridactylis</i> kidney epithelial (PtK<sub>2</sub>) cells examined by indirect immunofluorescence, microtubule bundles appeared more rapidly following epothilone B treatment, and there were different proportions of various mitotic aberrations following treatment with different drugs.