Paclitaxel-tubulin binding

Islam MN, Song Y, Iskander MN. Investigation of structural requirements of anticancer activity at the paclitaxel/tubulin binding site using CoMFAand CoMSIA. J Mol Graph Model 2003 21 263-272. 

Paclitaxel (96) Diterpene taxoid Cabazitaxel Oncology Tubulin binding Phase III Sanofi-Aventis 658-662... 

DHA-paclitaxel Oncology Tubulin binding Phase III Luifr)oId 665, 666... 

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 first structural location of the taxane binding site [42] placed it in the interprotofilament space, thus supporting the biochemical results. However, this changed when the first high resolution 3D structure of the paclitaxel-tubulin complex was solved by electron-crystallography of a two-dimensional zinc-induced tubulin polymer [5]. The fitting of this structure into a three-dimensional reconstruction of microtubules from cryoelectron microscopy allowed a pseudo atomic resolution model of microtubules [43] in which the paclitaxel binding site was placed inside the lumen of the microtubules hidden from the outer solvent. 

From these results a structural binding pathway consistent with that observed kinetically was proposed [21], Paclitaxel binds fast to a site located in the surface of the microtubules, into the pore type I. Then it has to be transferred to the second luminal, final location. The transfer involves probably the switch of some of the elements of the first site, since only one molecule of paclitaxel can bind to each molecule of P-tubulin (Fig. 3b). 

Computational studies have been of paramount importance in order to integrate the results of the experimental studies indicated above with the electron crystallographic data known. According to Snyder and co-workers, a satisfactory and experimentally verifiable model of the tubulin-binding site and of the conformation of paclitaxel has been obtained by computational methods on the first electron crystallographic model. In this context, a new paclitaxel conformer, T-Taxol (Fig. 10c), was proposed [59, 81, 82],... 

Peluroside A was the first microtubule stabilizing agent whose conformation has been determined bound to microtubules (those of Paclitaxel and Epothilone were determined in non-microtubular tubulin [5, 12, 38, 91]). In the bound state, the NMR data, assisted by molecular mechanics calculations and docking experiments, indicated that only one (that present in water, B) of the two major conformations existing in water solution is bound to microtubules (a-tubulin). A model of the binding mode to tubulin has also been proposed [27], involving the a-tubulin monomer, in contrast with paclitaxel, which binds to the p-monomcr. 

The tubulin-binding properties of (-)-rhazinilam were discovered through screening of a number of Malaysian plant extracts [60]. Natural (-)-rhazinilam induces tubulin spiralization, inhibiting tubulin assembly in the same way as vinblastine-like alkaloids, and protects microtubules from cold disassembly such as with paclitaxel [67]. This effect has never been observed with other microtubule poisons. For this reason, and despite the in vivo inactivity of (-)-rhazinilam [67], a number of analogues have been prepared by semi-synthesis and total synthesis (see Sections 3.1.3. and 3.2.3.) in order to improve the pharmacological properties of this molecule. 

Taxine alkaloids are complex polycyclic compounds in which N is present but not as an integral part of a ring. The taxines are found in Taxus (yew) species (Taxaceae). Taxine A (C61 CIO ] C6-0-C0-CH(0H)-CH(N(CH3)2)-Phe) is substantially responsible for yew toxicity. The related polycyclic amide taxol (paclitaxel) and the closely related docetaxel are tubulin-binding, antimitotic cytotoxics that are used clinically as anticancer drugs. A variety of taxines have been isolated from Taxus species. 

Steric hindrance, that is, the introduction of (5)-Me to C-2, while 2 (/ )-OH is retained, makes a positive contribution to the antimmor activity as well as to the tubulin binding ability. This result may have come from the reduced rotation of the side chain, which thus enhances the ratios of bioactive conformers in all con-formers. The preparation of 2 -Me analog was usually undertaken by (3-lactam approach, in which 3-keto-(3-lactam was attacked by a nucleophile to yield stereo-selectively 3-methyl-3-OH (equivalent to the 2 position in paclitaxel) (3-lactam ready for attachment to baccatin core stmcmres. Battaglia et al. prepared a series of 2 (5)-Me of paclitaxel analogs from 10-DAB and 14(3-OH-10-DAB with different C-3 and 3 -A substitutents, and all compounds 6a-e are comparable with or more active than paclitaxel toward A2780 human lung carcinoma in vitro. °... 

EXPLORATION ON MECHANISM OF PACLITAXEL RELATED TO TUBULIN BINDING AND QUEST FOR ITS PHARMACOPHORE... 

It should be noted that in 2004, Ganesh et al. and in 2000, Snyder et claimed in their articles that only the T-conformation was the conformation to be adopted by paclitaxel when binding to tubulin, whereas collapsed polar and nonpolar conformations do not work. Their conclusion was supported by computer simulation and NMR experiments of semisynthetic taxoids, in which the C-4 alkyl terminal and the C3 -Ph ortfio-position were linked. ... 

Wrasidlo et al. demonstrated the impact of nature and position of different substitutents on the activity with the 2 - and 7-pyridinium and 2 -sulfonate paclitaxel derivatives. They found that the 2 -(A-methyl-pyridinium acetate) paclitaxel (118) behaved more likely as a prodmg, in comparison with 7-pyridinium and 2 -sulfo-nate derivatives, because it showed almost no cytotoxicity and tubulin binding ability in the absence of plasma. However, it exhibited higher in vivo activity and reduced system toxicitiy than those of paclitaxel, whereas 7-pyridinium and 2 -sul-fonate derivatives showed little activities in nude mice although they retained strong cytotoxicities. 

Vinca alkaloids (vincristine, vinblastine, vinorelbine) are derived from the periwinkle plant (Vinca rosea). These agents work by binding to tubulin at a site different than colchicine or paclitaxel. They block polymerization, which prevents the formation of the mitotic spindle, and are used as antineoplastic agents. Taxanes produce a stabilization of microtubules similar to colchicine, but by a different mechanism, and also halt cells in metaphase. Paclitaxel (taxol) is the taxane used clinically. It is derived from the bark of the pacific yew. Taxol disrupts several microtubule-based functions as completely as inhibitors of polymerization, emphasizing the importance of assembly/disassembly balance in microtubule function. Recently, it has been found that paclitaxel also binds to and inhibits the function of a protein called bcl-2, an inhibitor of one or more pathways involved in mediating apoptosis. PaclitaxeTs interference with this function promotes apoptosis in addition to its microtubule-related inhibition of cell division. 

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