New photoaffinity analogs of paclitaxel

Tritiation of azido-labeled diiodo cabazitaxel (Jevtana) and docetaxel (Taxotere) derivatives to generate 3 H-photoaffinity probes

Radiolabelled azidophenyl analogues can make powerful photoaffinity probes for the identification of molecular targets. We describe our efforts to prepare tritiated azidophenyl analogues of the taxols cabazitaxel and docetaxel. Late-stage tritiation by isotope exchange with diiodo precursors resulted in reduction of the azide moiety, which could only be overcome by addition of high excess of a sacrificial azide. Iodine-deuterium exchange experiments on a model system established that deiodination with concomitant azide reduction is a general problem when performing such isotope-exchange reactions on azide-containing aryl iodides.

Efficient purification of paclitaxel from yews using high-performance displacement chromatography technique

Paclitaxel was purified using high-performance displacement chromatography (HPDC) technique, but not by the mechanism of HPDC. On small scale, paclitaxel was extracted with methanol from dry needles of Taxus canadensis and was enriched by extracting with chloroform after removing water-soluble hydrophilic components and hexane-soluble hydrophobic components. Then, 93-99% purity of paclitaxel was obtained using the HPDC technique. On large scale, taxanes were enriched by solvent partitioning between acetic acid/MeOH/H(2)O and hexane and extracted with CH(2)Cl(2). Taxanes except paclitaxel were further removed by extracting with methanol-water-trifluoroacetic acid (1.0:98.9:0.1, v/v/v). Applying HPDC technique to water-insoluble substances is problematic as this method requires a highly aqueous solvent system. In order to overcome this incompatibility, a system was set up where paclitaxel, although in low concentration, was extracted by methanol-water-trifluoroacetic acid (10.0:89.9:0.1, v/v/v). Recycling the extracting solvent to ensure minimal volume, the extracted paclitaxel was adsorbed on a C(18) trap column. A C(18) column of 4.6mm internal diameter was then connected to the trap column. The HPDC technique was thus carried out using an isocratic acetonitrile-water-trifluoroacetic acid (30.0:69.9:0.1, v/v/v) mobile phase consisting of a displacer cetylpyridinium trifluoroacetate (3mg/mL). Paclitaxel was co-eluted with the displacer and spontaneously crystallized. The crystal (114mg) showed 99.4% purity and only 10% of paclitaxel in the starting crude extract was lost during the enrichment/purification processes. This large scale purification method was successfully applied to purify paclitaxel from Chinese yew in small scale, suggesting general applicability of the method. This is the first report of purifying a water-insoluble natural product using HPDC technique.

Stereoselective synthesis of photoreactive peptidomimetic gamma-secretase inhibitors

The first asymmetric synthesis of novel, potent photoreactive gamma-secretase inhibitors 2 and 3 has been accomplished. Two stereoselective methods for the preparation of lactone 9 are described. Protected benzophenone intermediate 19 is prepared via an aldol-elimination reaction followed by a PtO(2)-catalyzed asymmetric hydrogenation. Two routes leading from 19 to compounds 2 and 3 are evaluated. The application of 3 as an activity-based probe has been demonstrated by localizing gamma-secretase activity in the plasma membrane of intact cells.

Characterization of the Taxol binding site on the microtubule. Identification of Arg(282) in beta-tubulin as the site of photoincorporation of a 7-benzophenone analogue of Taxol

Photoaffinity labeling methods have allowed a definition of the sites of interaction between Taxol and its cellular target, the microtubule, specifically beta-tubulin. Our previous studies have indicated that [(3)H]3'-(p-azidobenzamido)Taxol photolabels the N-terminal 31 amino acids of beta-tubulin (Rao, S., Krauss, N. E., Heerding, J. M., Swindell, C. S., Ringel, I., Orr, G. A., and Horwitz, S. B. (1994) J. Biol. Chem. 269, 3132-3134) and [(3)H]2-(m-azidobenzoyl)Taxol photolabels a peptide containing amino acid residues 217-233 of beta-tubulin (Rao, S., Orr, G. A., Chaudhary, A. G., Kingston, D. G. I., and Horwitz, S. B. (1995) J. Biol. Chem. 270, 20235-20238). The site of photoincorporation of a third photoaffinity analogue of Taxol, [(3)H]7-(benzoyldihydrocinnamoyl) Taxol, has been determined. This analogue stabilizes microtubules polymerized in the presence of GTP, but in contrast to Taxol, does not by itself enhance the polymerization of tubulin to its polymer form. CNBr digestion of [(3)H]7-(benzoyldihydrocinnamoyl)Taxol-labeled tubulin, with further arginine-specific cleavage by clostripain resulted in the isolation of a peptide containing amino acid residues 277-293. Amino acid sequence analysis indicated that the photoaffinity analogue cross-links to Arg(282) in beta-tubulin. Advances made by electron crystallography in understanding the structure of the tubulin dimer have allowed us to visualize the three sites of photoincorporation by molecular modeling. There is good agreement between the binding site of Taxol in beta-tubulin as determined by photoaffinity labeling and electron crystallography.

Photoaffinity analogs for multidrug resistance-related transporters and their use in identifying chemosensitizers

A major obstacle in cancer treatment is the development of resistance to multiple chemotherapeutic agents in tumor cells. The hallmark of this multidrug resistance (MDR) is overexpression of the MDR 1 P-glycoprotein or the multidrug resistance protein MRP1. It is well documented that these proteins confer MDR in cancer cells. Much evidence indicates that control of intracellular drug levels in MDR cells is determined by P-glycoprotein or MRP, and therefore these proteins are suitable targets for identifying MDR-reversing agents (MDR modulators). We originally explored the drug-binding ability of P-glycoprotein by synthesizing and using radioactive photoaffinity analogs of vinblastine. Since our initial discovery that P-glycoprotein binds to vinblastine photoaffinity analogs, many P-glycoprotein- and MRP-specific photoaffinity analogs have been developed. In this review, photoaffinity analogs which specifically bind to P-glycoprotein or MRP are discussed. Moreover, utilizing these photoprobes to identify, characterize and localize the drug binding sites of P-glycoprotein and MRP is described. Using P-glycoprotein-specific photoaffinity analogs in combination with site-directed antibodies to several domains of this protein has allowed the localization of the general binding domains of some of the cytotoxic agents an MDR modulators on P-glycoprotein. However, the molecular architecture of the drug binding sites, their exact location on the P-glycoprotein molecule, and the total number of the drug binding sites remain to be determined. This review discusses recent advances in delineating the structure of the drug-binding sites of P-glycoprotein. Moreover, novel MRP1 photoaffinity analogs are reviewed. Copyright 1999 Harcourt Publishers Ltd.