Taxoid site

Hamel, E., Sackett, D. L., Vourloumis, D., and Nicolaou, K. C. (1999). The coral-derived natural products eleutherobin and sarcodictyins A and B Effects on the assembly of purified tubulin with and without microtubule-associated proteins and binding at the polymer taxoid site. Biochemistry 38, 3490-3498. 

It has been noted that the global shape of other taxoid site ligands, such as PTX, DTX or epothilones, can be described as consisting of a cyclic core and a side-chain tail [124], Although DDM structure is not cyclic and hence its conformation is less restricted, it seems that DDM folds in such a way to resemble the shape of the other drugs. 

Fig. 30 Taxoid site on (3-tubulin and predicted peloruside site on a-tubulin. a Surface representation (view from the inner side of the microtubule) of a tubulin dimer with FIX (red) bound to P-tubulin (green) and peloruside A (orange) bound to the predicted site in a-tubulin (blue), b View of the peloruside binding site. Hydrogen bonds are represented as yellow dashed lines, and the residues involved in these bonds are labeled. Some secondary structure elements are also labeled, c View of the taxol binding site. Some secondary structure elements are labeled. In panels b and c, H7 is colored in orange, and the N-terminal and intermediate domains are colored in green and blue, respectively. (Reprinted with permission from [17]. Copyright 2006 American Chemical Society)...

It was soon established that fijianolide B does not bind to the taxoid site of tubulin.223 This prompted an investigation into the similarities and synergistic effects of fijianolide B and other taxoid site agents. Unlike epothilone A, fijianolide B was found to act synergistically with taxol at lower temperatures. 

Peloruside A 14 (Scheme 6.1 Part 2) was isolated from a New Zealand Mycale hentschei marine sponge and initially showed activity against P388 murine leukaemia cells at 10 ng/mL.98 Peloruside s cytotoxicity profile and structural similarity to bryostatin led to the examination of protein kinase C (PKC) as a possible mode of action.242 This was determined to be incorrect and it was soon established that the remarkable activity of peloruside was through the stabilisation of microtubules at a site distinct from the taxoid site.243... 

Fig. 12 Natural compounds of diverse structure affecting the taxoid site.

Table 3 Status of several natural / semisynthetic compounds of diverse structure, affecting the taxoid site currently in clinical development...

Gaitanos, T.N., Buey, R.M., Diaz, J.F., Northcote, P.T., Teesdale-Spitde, P., Andreu, J.M., and Miller, J.H. (2004) Peloruside A does not bind to the taxoid site on P-tubulin and retains its activity in multidrug-resistant cell lines. Cancer Res., 64, 5063-5067. 

The metabolism of taxol by Eucalyptus perriniana cell suspension cultures has been recently reported to induce hydrolyses of ester bonds at C-13, C-10 and C-2 [222]. At this moment only very few data have been published about the microbial metabolism of taxoid compounds only site specific hydrolyses of acyl side-chains at C-13 or C-10 by extracellular and intracellular esterases of Nocar-dioides albus SC13,911 and N. luteus SC13,912, respectively, have been reported [223]. On the other hand, Hu et al. [224-226] have recently described some fungal biotransformations of related natural taxane diterpenes extracted from Chinese yews or their cell cultures, in order to obtain new active substances or precursors for hemisynthesis. The taxadiene 145, a 14 -acetylated derivative... 

Thus, the fluorine probe approach has proved useful for the conformational analysis of paclitaxel and taxoids in connection with the determination of possible bioactive conformations.77 The previously unrecognized conformer C might be the molecular structure first recognized by the P-tubulin binding site on microtubules. 

The problem was overcome with the use of mildly fixed microtubules [10] in which the paclitaxel binding site is unaltered, while protected from cold and dilution depolymerisation and a paclitaxel molecule bound to a fluorescent probe (either fluorescein or difluorofluorescein) [8], The fluorescent derivatives of paclitaxel were then tested to check that they compete with taxoids for the same site, with the same 1 1 stoichiometry producing the same celular effects as paclitaxel and docetaxel, thus it can be assumed that they bind for the same site [9], Using these stabilized microtubules, it is possible to determine precisely the binding constant of the fluorescent taxoid which was found to be of the order of 108 M at 25°C. 

Fig. 9 Insight of the outer (a) and (b) inner surface of a high resolution microtubule model, showing two different types of pores, I and II (see text). Green beads, polar residues yellow beads, hydrophobic residues red beads, acid residues blue beads, basic residues white beads, paclitaxel bound at its site grey beads, nucleotide. Detail of a pore type I viewed from above (c). Ribbon representation of two neighbour P-tubulin subunits as seen from the plus end of the microtubule, paclitaxel, GDP and the four residues forming a putative taxoid binding site are shown in Van der Waals representation. Taken from [22]...

Buey RM, Calvo E, Barasoain I, Pineda O, Edler MC, Matesanz R, Cerezo G, Vanderwal CD, Day BW, Sorensen EJ, Lopez JA, Andreu JM, Hamel E, Diaz JF (2007) Cyclostreptin binds covalently to microtubule pores and lumenal taxoid binding sites. Nat Chem Biol 3 117-125... 

Most research has focused on the development of paclitaxel analogs or prodrugs with enhanced specificity MDR reversal and orally effective taxoids have also been developed recently. Meanwhile, scientists have gained insights into the mechanism of action of taxoids at molecular level, that is, binding sites on tubulin and dynamics of tubulin polymerization. It is worth pointing out that the SAR results derived from traditional medicinal chemistry have shown the essential... 

Andreu, J. M. Barasoain, I. The interaction of baccatin III with Taxol binding site of microtubules determined by a homogeneous assay with fluorescent taxoid. Biochemistry, 2001, 40 11975-11984. 

Figure 5.2 Primary sites of hydroxylation on the second-generation taxoids by P450 family enzymes.

Andreu JM, Barasoain I (2001) The Interaction of Baccatin III with the Taxol Binding Site of Microtubules Determined by a Homogeneous Assay with Fluorescent Taxoid. Biochemistry 40 11975... 

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