Photoaffinity Labeled Taxols

The analog 11.2.6 was prepared by Ojima and his colleagues using tritiation of the corresponding a/ -unsaturated amide derivative to incorporate the tritium label (389). 

Taxol analogs 11.2.6 and 11.2.7 with photoaffinity labels at the 7-and 10-positions were prepared by similar methods to those used for the analog 11.2.6 (390). Analog 11.2.6 had the better cytotoxicity, with IC50 values approximately twice those of taxol in several cell lines. 

Finally, the interesting analog 11.2.8 was also prepared by Ojima the biotin moiety was included to serve to as an affinity label so as to assist in the isolation of labeled fragments from photoaffinity studies (391). 

The related analog 11,2.9 was also prepared in both deuterium and tritium labeled forms, but its bioactivity was such that it was not the ideal derivative for photoaffinity labeling studies (392). 

Taxol analogs labeled at the 2-position have been prepared by Kingston and by Georg. The tritiated 2-(m-azidobenzoyl) analog 11.2.10 was prepared by reductive debromination of the corresponding 3-nitro-4-bromobenzoyl derivative with sodium [ H]-borohydride (393), and the corresponding non-radioactive analogs 11.2.11 and 11.2.12 were prepared by standard methods (388). 

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Recently, we proposed a new bioactive conformation of paclitaxel, RKDOR-Taxol [50], based on (i) the 19F-13C distances obtained by the REDOR experiment [49], (ii) the photoaffinity labeling of microtubules [51], (iii) the crystal structure (PDB code 1TUB) of the Zn2+-stabilized aP-tubulin dimer model determined by cryo-electron microscopy (cryo-EM) [52], and (iv) molecular modeling (Monte Carlo Macromodel) [50], In this computational biology analysis, we first docked a paclitaxel-photoaffinity label molecule to the position identified by our photoaffinity labeling study and then optimized the... 

The synthesis of labeled analogs of taxol has been an important aspect of research on the drug. Radioactively labeled analogs are needed for metabolism and related studies, while photoaffinity-labeled and fluorescently labeled analogues have been used to study the interaction of taxol with tubulin. This section will focus on the synthetic aspects of this work the biochemical studies of the taxol-tubulin interaction will be discussed in a later section. 

The location of the binding site of taxol on tubulin has been determined by photoaffinity labeling using several differently labeled taxol analogs. An early study showed that taxol labeled -tubulin as opposed to a-tubulin 466), and this result was confirmed with the azidophenyl taxol analog 15.1.1 467). 

Georg GI, Harriman GCB (1992) Taxol Photoaffinity Label 7-(p-Azidobenzoyl)-taxol Synthesis and Biological Evaluation. Bioorg Med Chem Lett 2 735... 

Georg GI, Harriman GCB, Park H, Himes RH (1994) Taxol Photoaffinity Labels 2. Synthesis and Biological Evaluation of N-(4-Azidobenzoyl)-N-debenzoyltaxol, N-(4-Azido-2,3,5,6-tetrafluorobenzoyl)-N-debenzoyltaxol, and 7-(4-Azido-2,3,5,6-tet-rafluorobenzoyl)taxol. Bioorg Med Chem Lett 4 487... 

Rao S, Horwitz SB, Ringel I (1992) Direct photoaffinity labeling of tubulin with taxol. J Natl Cancer Inst 84 785... 

More detailed studies showed that 3 -(p-azidobenzamido)taxol (11.2.1) photolabeled -tubulin preferentially 387) and that the site of labeling was in the N-terminal 31 amino acid unit 386). On the other hand, 2- m-azidobenzoyl)taxol (11.2.10) labeled a peptide containing amino acids 217-231 of -tubulin 393). The photoaffinity probe 11.2.5 was also found to bind (most probably) to /3-tubulin 389), while the probe 11.2.6 was shown to bind to Arg in -tubulin 468). 

Dasgupta D, Park H, Harriman GCB, Georg GI, Himes RH (1994) Synthesis of a Photoaffinity Taxol Analogue and Its Use in Labeling Tubulin. J Med Chem 37 2976... 

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