ABSTRACT
[reaction: see text] This work describes the synthesis of two novel macrocyclic taxoid constructs by ring-closing olefin metathesis (RCM) and their biological evaluation. Computational studies examine conformational profiles of 1 and 2 for their fit to the beta-tubulin binding site determined by electron crystallography. The results support the hypothesis that paclitaxel binds to microtubules in a "T" conformation.
Subject(s)
Antineoplastic Agents, Phytogenic/chemical synthesis , Antineoplastic Agents, Phytogenic/pharmacology , Paclitaxel/chemical synthesis , Paclitaxel/pharmacology , Binding Sites , Crystallography, X-Ray , Cyclization , Humans , Indicators and Reagents , Models, Molecular , Molecular Conformation , Paclitaxel/analogs & derivatives , Tubulin/chemistry , Tumor Cells, CulturedABSTRACT
The chemotherapeutic drug Taxol is known to interact within a specific site on beta-tubulin. Although the general location of the site has been defined by photoaffinity labeling and electron crystallography, the original data were insufficient to make an absolute determination of the bound conformation. We have now correlated the crystallographic density with analysis of Taxol conformations and have found the unique solution to be a T-shaped Taxol structure. This T-shaped or butterfly structure is optimized within the beta-tubulin site and exhibits functional similarity to a portion of the B9-B10 loop in the alpha-tubulin subunit. The model provides structural rationalization for a sizeable body of Taxol structure-activity relationship data, including binding affinity, photoaffinity labeling, and acquired mutation in human cancer cells.
Subject(s)
Paclitaxel/analogs & derivatives , Paclitaxel/chemistry , Paclitaxel/metabolism , Taxoids , Tubulin/metabolism , Binding Sites , Crystallography , Docetaxel , Drug Resistance, Neoplasm , Microscopy, Electron , Models, Molecular , Molecular Conformation , Photoaffinity Labels , Structure-Activity Relationship , Tubulin/chemistry , Tubulin/geneticsABSTRACT
Numerous structure-activity studies combining synthesis and bioassay have been performed for the anti-cancer drug Taxol. The four-membered D-ring, an oxetane, is one of four structural features regarded to be essential for biological activity. This proposition is examined by application of a Taxol-epothilone minireceptor, K(i) estimation for microtubule binding and docking of Taxol analogues into a model of the Taxol-tubulin complex. In this way, we evaluate the two characteristics considered responsible for oxetane function: (1) rigidification of the tetracyclic Taxol core to provide an appropriate framework for presenting the C-2, C-4, C-13 side chains to the microtubule protein and (2) service as a hydrogen-bond acceptor. An energy decomposition analysis for a series of Taxol analogues demonstrates that the oxetane ring clearly operates by both mechanisms. However, a broader analysis of four-membered ring containing compounds, C- and D-seco derivatives, and structures with no oxetane equivalent underscores that the four-membered ring is not necessary for Taxol analogue bioactivity. Other functional groups and ligand-protein binding characteristics are fully capable of delivering Taxol biobehavior as effectively as the oxetane D-ring. This insight may contribute to the design and development of novel anticancer drugs.
