Evading Pgp activity in drug-resistant cancer cells : a structural and functional study of antitubulin furan metotica compounds

Tumor resistance to antitubulin drugs resulting from P-glycoprotein (Pgp) drug-efflux activity, increased expression of the bIII tubulin isotype, and alterations in the drug-binding sites are major obstacles in cancer therapy. Consequently, novel antitubulin drugs that overcome these challenges are of substantial interest. Here, we study a novel chemotype named furan metotica that localizes to the colchicine-binding site in b-tubulin, inhibits tubulin polymerization, and is not antagonized by Pgp. To elucidate the structure–activity properties of this chiral chemotype, the enantiomers of its most potent member were separated and their absolute configurations determined by X-ray crystallography. Both isomers were active and inhibited all 60 primary cancer cell lines tested at the U.S. National Cancer Institute. They also efficiently killed drug-resistant cancer cells that overexpressed the Pgp drug-efflux pump 106-fold. In vitro, the R-isomer inhibited tubulin polymerization at least 4-fold more potently than the S-isomer, whereas in human cells the difference was 30-fold. Molecular modeling showed that the two isomers bind to b-tubulin in distinct manners: the R-isomer binds in a colchicine-like mode and the S-isomer in a podophyllotoxin-like fashion. In addition, the dynamic binding trajectory and occupancy state of the R-isomer were energetically more favorable then those of the S isomer,explaining the observed differences in biologic activities. The ability of a racemic drug to assume the binding modes of two prototypical colchicine-site binders represents a novel mechanistic basis for antitubulin activity and paves the way toward a comprehensive design of novel anticancer agents.