Effect of 2'-OH acetylation on the bioactivity and conformation of 7-O-[N-(4'-fluoresceincarbonyl)-L-alanyl]taxol. A NMR-fluorescence microscopy study.

The relationship between conformation, 2'-OH acetylation, and bioactivity of two fluorescent taxoids has been investigated by a combination of NMR and fluorescence microscopy techniques. These taxoids present the structure of taxol with the 7-OH group esterified with the N-(4'-fluoresceincarbonyl)-L-alanine group and with the 2'-OH group free (taxoid 2) or acetylated (taxoid 3). The larger water solubility of 2 and 3 compared with taxol allowed a detailed NMR study in DMSO-d6/D2O (3/7), showing that both taxoids adopt a similar collapsed conformation in which the hydrophobic groups 2-O-benzoyl, 3'-phenyl and 4-O-acetyl are in close proximity, with the fluorescein group displaying unrestricted motion. On the other hand, while taxoid 2 retains essentially the ability of taxol to induce in vitro microtubule assembly and to bind to cell microtubules, the 2'-acetylated derivative 3 does not show immediate activity. However, when taxoid 3 is left in the cell culture, the slow hydrolysis of the 2'-acetate group in the medium liberates the cytotoxic, microtubule-specific taxoid 2. The intense emission of this active derivative (2) allows the accurate recording of the drug-cell interaction from the very initial steps using fluorescence microscopy. These experiments show conclusively, for the first time in cell cultures, that a free 2'-OH group in taxol is essential for the recognition of the drug by the binding site of cellular microtubules.