A common pharmacophore for epothilone and taxanes: Molecular basis for drug resistance conferred by tubulin mutations in human cancer cells

The epothilones are naturally occurring antimitotic drugs that share with the taxanes a similar mechanism of action without apparent structural similarity. Although photoaffinity labeling and electron crystallographic studies have identified the taxane-binding site on beta-tubulin, similar data are not available for epothilones. To identify tubulin residues important for epothilone binding, we have isolated two epothilone-resistant human ovarian carcinoma sublines derived in a single-step selection with epothilone A or B. These epothilone-resistant sublines exhibit impaired epothilone- and taxane-driven tubulin polymerization caused by acquired beta-tubulin mutations (beta274(Thr-->Ile) and beta282(Arg-->Gln)) located in the atomic model of alphabeta-tubulin near the taxane-binding site. Using molecular modeling, we investigated the conformational behavior of epothilone, which led to the identification of a common pharmacophore shared by taxanes and epothilones. Although two binding modes for the epothilones were predicted, one mode was identified as the preferred epothilone conformation as indicated by the activity of a potent pyridine-epothilone analogue. In addition, the structure-activity relationships of multiple taxanes and epothilones in the tubulin mutant cells can be fully explained by the model presented here, verifying its predictive value. Finally, these pharmacophore and activity data from mutant cells were used to model the tubulin binding of sarcodictyins, a distinct class of microtubule stabilizers, which in contrast to taxanes and the epothilones interact preferentially with the mutant tubulins. The unification of taxane, epothilone, and sarcodictyin chemistries in a single pharmacophore provides a framework to study drug-tubulin interactions that should assist in the rational design of agents targeting tubulin.