EVALUATION OF THE CYTOTOXIC MECHANISM MEDIATED BY BACCATIN-III, THE SYNTHETIC PRECURSOR OF TAXOL

Baccatin III, which is used as a precursor for the semisynthesis of ta xol, showed cytotoxic activity against a variety of cancer cell lines in culture, with ED(50) values ranging from approximately 8 to 50 mu M . Although the potency of this response is much lower than that mediat ed by taxol, it was interesting to note that any significant cytotoxic response could be mediated by this compound. Thus, it was considered of potential value to investigate the mechanism of cytotoxic action, C onsistent with an antimitotic mode of action, baccatin III induced cul tured cells to accumulate in the G(2) + M phases of the cell cycle. Ho wever, unlike taxol, which potentiates the polymerization of tubulin, baccatin III mediated an antimitotic response through inhibition of th e polymerization reaction, similar to colchicine, podophyllotoxin, or vinblastine. Accordingly, baccatin III was unable to reduce the extent of Ca2+-induced depolymerization, a hallmark of the biological respon se mediated by taxol. To further explore the mode of antimitotic activ ity facilitated by baccatin III, competitive interactions with the col chicine, podophyllotoxin, and vinblastine binding sites of tubulin wer e investigated. Baccatin III displaced the binding of radiolabeled col chicine or radiolabeled podophyllotoxin, but did not displace the bind ing of radiolabeled vinblastine, Greater affinity with the colchicine binding site was observed and the kinetics of inhibition were shown to be mixed. The side chain of taxol, which differentiates the molecule from baccatin III and is known to be of requisite importance for the u nique activity mediated by taxol, is not by itself active in any of th ese processes. Thus, the baccatin III nucleus of taxol may lead to an interaction with tubulin through traditional binding sites. Facilitate d by this interaction, the intact molecule of taxol may thereby be per mitted to potentiate tubulin polymerization and block cells in the mit otic phase of the cell cycle.