Taxol is converted to 7-epitaxol, a biologically active isomer, in cell culture medium

The hydrolysis products of taxol have been isolated by high-performance liquid chromatography and identified by nuclear magnetic resonance and mass spectroscopy. In contrast to taxol, the major hydrolysis product, baccatin III, has little cytotoxic activity and does not promote in vitro microtubule assembly. In cell culture medium, the concentration of taxol decreases with time and 7-epitaxol, which exhibits properties comparable to those of taxol both on cells and on in vitro microtubule polymerization, is formed. Baccatin III is found in small quantities in the cell medium, although it is barely detectable within the cells. It is concluded that 7-epitaxol is the major derivative of taxol found in cells and that its presence does not alter, in a major way, the overall biological activity of taxol.