Development of a common pharmacophore model for taxol and the epothilones
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Taxol is an anticancer drug approved by the FDA for the treatment of ovarian, breast and lung carcinomas. The drug hyperstabilizes microtubules, disrupts normal microtubule organization and dynamics, blocks cells in mitosis, and induces apoptosis. In an in vitro system, Taxol can polymerize stable microtubules in the absence of GTP that is normally required for tubulin assembly. Recently, three new natural products, the epothilones, eleutherobin and discodermolide, whose chemical structures are highly diverse, have been reported to have mechanisms of action similar to that of Taxol, in that they all stabilize microtubules.;In vitro tubulin polymerization and sedimentation assays were used to define the structure activity relationships (SAR) for the epothilones, and to determine the structural requirements for their biological activity. With results from over 40 synthetic epothilone analogs, the SAR of the epothilones have been determined. The epoxide moiety can be replaced by a double bond at the C12, C13 positions. The acyl sector (C1-C8) is intolerant of modifications. The O-alkyl sector (C9-C15) and the aryl sector seemed quite tolerant of alterations.;Extensive SAR studies of Taxol had identified the C-13 side chain as an essential component for biological activity because baccatin III, that lacks the side chain at C-13, does not possess Taxol-like activities. It had also been reported that a meta azido substitution on the C-2 benzoyl ring could enhance the activity of the Taxol molecule. 2-m-azido baccatin III is a Taxol analog lacking the C-13 side chain but with a meta azido benzoyl group at the C-2 position. 2-m-azido baccatin III inhibited the growth of cultured cancer cells at nanomolar concentrations and exhibited Taxol-like activities.;Using the crystal structure of tubulin determined by Nogales et al., molecular modeling studies have been done to identify (1) the binding site of the 2-m-azido baccatin III molecule in beta-tubulin and (2) a common pharmacophore for Taxol, the epothilones and eleutherobin. The meta azido substituted C-2 benzoyl ring of 2-m-azido baccatin III fits into a binding pocket formed by His227 and Asp224 of beta-tubulin and the enhancing effect of the meta substitution is probably due to its strengthened interaction with Asp224. To confirm this model, the biological activity of 2-p-azido baccatin III was studied. As expected, it is inactive because the para substituent would not allow the binding of the C-2 benzoyl ring to beta-tubulin, due to steric hindrance. These studies point to the specificity and importance of the interaction of the meta substituent on the C-2 benzoyl ring with the Taxol binding site in beta-tubulin. (Abstract shortened by UMI.).
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