Epothilone A (Epo A) is a biologically active macrolide produced from the fermentation of the soil bacteria S. cellulosum that was discovered as a potent antimicrotubule agent.^1,2,3 In competition assays, epo A binds to β-tubulin with a similar order of magnitude as the binding of paclitaxel to β-tubulin (IC50s = 2.3 and 3.6 µM, respectively).^1,2,3,4 Epo A is cytotoxic to human T-24 bladder carcinoma cells (IC50 = 0.05 µM in vitro) but has poor pharmacological properties and is 2-fold less potent in stabilizing microtubules compared to epothilone B .¹