Total synthesis and evaluation of C26-hydroxyepothilone D derivatives for photoaffinity labeling of beta-tubulin.

Three photoaffinity labeled derivatives of epothilone D were prepared by total synthesis, using efficient novel asymmetric synthesis methods for the preparation of two important synthetic building blocks. The key step for the asymmetric synthesis of (S,E)-3-(tert-butyldimethylsilyloxy)-4-methyl-5-(2-methylthiazol-4-yl)pent-4-enal involved a ketone reduction with (R)-Me-CBS-oxazaborolidine. For the synthesis of (5S)-5,7-di[(tert-butyldimethylsilyl)oxy]-4,4-dimethylheptan-3-one an asymmetric Noyori reduction of a beta-ketoester was employed. The C26 hydroxyepothilone D derivative was constructed following a well-established total synthesis strategy and the photoaffinity labels were attached to the C26 hydroxyl group. The photoaffinity analogues were tested in a tubulin assembly assay and for cytotoxicity against MCF-7 and HCT-116 cancer cell lines. The 3- and 4-azidobenzoic acid analogues were found to be as active as epothilone B in a tubulin assembly assay, but demonstrated significantly reduced cellular cytotoxicity compared to epothilone B. The benzophenone analogue was inactive in both assays. Docking and scoring studies were conducted that suggested that the azide analogues can bind to the epothilone binding site, but that the benzophenone analogue undergoes a sterically driven ligand rearrangement that interrupts all hydrogen bonding and therefore protein binding. Photoaffinity labeling studies with the 3-azidobenzoic acid derivative did not identify any covalently labeled peptide fragments, suggesting that the phenylazido side chain was predominantly solvent-exposed in the bound conformation.

(3R,5S,7as)-(3,5-Bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol, a novel neuroprotective agent.

Compounds that interact with microtubules, such as paclitaxel, have been shown to possess protective properties against beta-amyloid (Abeta) induced neurodegeneration associated with Alzheimer's disease. In this work, the novel agent (3R,5S,7as)-(3,5-bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol was investigated for effectiveness in protecting neurons against several toxic stimuli and its interaction with the microtubule network. Exposure of neuronal cultures to Abeta peptide in the presence of 5 nM (3R,5S,7as)-(3,5-bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol resulted in a 50% increase in survival. Neuronal cultures treated with other toxic stimuli such as staurosporine, thapsigargin, paraquat, and H(2)O(2) showed significantly enhanced survival in the presence of (3R,5S,7as)-(3,5-bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol. Microtubule binding and tubulin assembly studies revealed differences compared to paclitaxel but confirmed the interaction of (3R,5S,7as)-(3,5-bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol with microtubules. Furthermore, in vitro studies using bovine brain microvessel endothelial cells experiments suggest that (3R,5S,7as)-(3,5-bis(4-fluorophenyl)tetrahydro-1H-oxazolo[3,4-c]oxazol-7a-yl)methanol can readily cross the blood-brain barrier in a passive manner.

Total synthesis and evaluation of 22-(3-azidobenzoyloxy)methyl epothilone C for photoaffinity labeling of beta-tubulin.

The total synthesis of 22-(3-azidobenzoyloxy)methyl epothilone C is described as a potential photoaffinity probe to elucidate the beta-tubulin binding site. A sequential Suzuki-aldol-Yamaguchi macrolactonization strategy was utilized employing a novel derivatized C1-C6 fragment. The C22-functionalized analog exhibited good activity in microtubule assembly assays, but cytotoxicity was significantly reduced. Molecular modeling simulations indicated that excessive steric bulk in the C22 position is accommodated by the large hydrophobic pocket of the binding site. Photoaffinity labeling studies were inconclusive suggesting non-specific labeling.

Total synthesis and evaluation of C25-benzyloxyepothilone C for tubulin assembly and cytotoxicity against MCF-7 breast cancer cells.

The total synthesis of C25-benzyloxy epothilone C is described. A sequential Suzuki-Aldol-Yamaguchi macrolactonization strategy was utilized employing a novel derivatized C8-C12 fragment. The C25-benzyloxy analog exhibited significantly reduced biological activity in microtubule assembly and cytotoxicity assays. Molecular modeling simulations indicated that excessive steric bulk in the C25 position may reduce activity by disrupting key hydrogen bonds that are crucial for epothilone binding to beta-tubulin.

Epothilone and paclitaxel: unexpected differences in promoting the assembly and stabilization of yeast microtubules.

Paclitaxel (Taxol) and the epothilones are antimitotic agents that promote the assembly of mammalian tubulin and stabilization of microtubules. The epothilones competitively inhibit the binding of paclitaxel to mammalian brain tubulin, suggesting that the two types of compounds share a common binding site in tubulin, despite the lack of structural similarities. It is known that paclitaxel does not stabilize microtubules formed in vitro from Saccharomyces cerevisiae tubulin; thus, it would be expected that the epothilones would not affect yeast microtubules. However, we found that epothilone A and B do stimulate the formation of microtubules from purified yeast tubulin. In addition, epothilone B severely dampens the dynamics of yeast microtubules in vitro in a manner similar to the effect of paclitaxel on mammalian microtubules. We used current models describing paclitaxel and epothilone binding to mammalian beta-tubulin to explain why paclitaxel apparently fails to bind to yeast tubulin. We propose that three amino acid substitutions in the N-terminal region and at position 227 in yeast beta-tubulin weaken the interaction of the 3'-benzamido group of paclitaxel with the protein. These results also indicate that mutagenesis of yeast tubulin could help define the sites of interaction with paclitaxel and the epothilones.