Epothilone A is a competitive inhibitor of the binding of [³H] paclitaxel to tubulin polymers, with a K_i of 0.6-1.4 μM. In Vitro: Epothilone A is a competitive inhibitor of the binding of [³H] paclitaxel to tubulin polymers. The apparent K_i value for Epothilone A is 1.4 μM by Hanes analysis and 0.6 μM by Dixon analysis^[1]. Epothilone A, is noted to be highly cytotoxic (IC₅₀=0.05 μM) in vitro when applied to the human T-24 bladder carcinoma cell line. The binding affinity of Epothilone A to tubulin is of the same order of magnitude as the binding affinity of paclitaxel to tubulin based on competition assays. The IC₅₀ for displacement of 100 nM of (³H) paclitaxel from the tubulin binding site is 3.6 μM for paclitaxel, 2.3 μM for Epothilone A, and 3.3 μM for patupilone^[2].