Tubulin-assembly activity

The route of taxanes was finally unveiled with the use of a covalent ligand of the taxane binding site [21], Cyclostreptin, abacterial natural product [46,47] with weak, but irreversible tubulin assembly activity and strong apparent binding affinity for the paclit-axel site, covalently labels both a residue placed in the lumenal site of paclitaxel Asn228 and a residue previously proposed to be in the external binding site Thr220 (Fig. 3a). 

Walker et al prepared lOa-spiro epoxide (42a) and its 7-methoxymethyl (MOM) ether (42b), which exhibited comparable cytotoxicity and tubulin assembly activity with paclitaxel. 

Several taxol analogs have been prepared in which taxol is linked to another molecule as a dimeric derivative. Thus the dimeric taxanes 13.2.1 and 13.2.2 were prepared by Appendino and Danieli and their colleagues from taxol and 2 -deacetoxyaustrospicatine (423). Although neither compound was cytotoxic, 13.2.2 retained approximately a third of taxol s tubulin-assembly activity. 

Samaranayake G, Magri NF, Jitrangsri C, Kingston DGI (1991) Modified Taxols. 5. Reaction of Taxol with Electrophilic Reagents, and Preparation of a Rearranged Taxol Derivative with Tubulin Assembly Activity. J Org Chem 56 5114... 

FIGURE 8.11 Multiple signal-transduction pathways initiated by calmodulin. Calmodulin bound to Ca2+ interacts and activates many enzymes, opening up a wide range of possible cellular responses. Abbreviations MAP-2, microtubule-associated protein 2 NO, nitric oxide Tau, tubulin assembly unit. 

The role of microtubules in secretion is more clearly defined. Colchicine and vinblastine inhibit secretion, even in cytochalasin-B-treated cells, and D2O (which promotes tubulin assembly) enhances secretion in cytochalasin-treated cells. Microtubules may also be necessary for the translocation of phagocytic vesicles from the neutrophil periphery into the central region of the cytoplasm. Drugs affecting microtubule assembly may inhibit particle-induced oxidase activation or else increase oxidase activation in response to soluble agents such as fMet-Leu-Phe. 

David-Pfeuty, T., Simon, C., and Pantaloni, D. (1979). Effect of antimitotic drugs on tubulin GTPase activity and self-assembly./ Biol. Chem. 254, 11696-11702. 

More recently, additional 2H "I J REDOR measurements on new PTX derivatives bound to MT provided three new interatomic distances that helped to define more precisely the bound conformation of PTX [87], The two PTX analogues labeled with fluorine and deuterium were designed to determine the three key distances based on the measurement of 2II— "I distances, as indicated in Fig. 9 (compounds 4 and 5). These interatomic distances rule out the polar and nonpolar conformers and further support the T-taxol conformation (Tablel). The polar conformation is rejected on the basis of distance IV, as the 4.5 A is much shorter than the > 8 A determined by REDOR. The nonpolar conformation can be ruled out on the basis of distances I and II, that are too short in the nonpolar conformer compared to the experimental ones (1.8 and 3.6 A short). Besides, bridged taxanes resembling the nonpolar conformation displayed no activity in the tubulin assembly assay [88], PTX-NY closely matches the REDOR distances. Nevertheless, it is discarded as the bioactive form of PTX due to its poor fit to the EC density [87],... 

In addition to the inhibition of MT polymerization, more properties are shared by vinca domain compounds. As a hallmark, they inhibit the tubulin GTPase activity linked to MT assembly [42] as well as that induced by colchicine (see [33] and... 

The extension of the C-13 side chain to its homologated one furnished poorly active paclitaxel and docetaxel analogs in tubulin assembly assay. The authors assumed that the poor activity of the analogs may have originated from their different conformations from that of paclitaxel in water. 

In 2001, two D-thia analogs, 5(20)-thia-docetaxel 97a and 7-deoxy-5(20)-thia-docetaxel 97b, were synthesized. Compound 97a is two orders of magnitude less cytotoxic than paclitaxel, but it retained tubulin assembly ability stronger than in a previous report. Meanwhile, 97b is inactive in tubulin assay and 3 times more cytotoxic than 97a against KB cells, but it is still much less active than both 7-deoxy-lO-acetyl-docetaxel and docetaxel. 

The new compounds have been evaluated for their effects on tubulin assembly and for cytotoxic and apoptotic activities. 

T.media possesses antitumour activity (e.g. IL against L1210 murine leukemia cells of 3.6 0.3 nM) higher than that of taxol (e.g. 57.0 3.0 nM) and is also active against adriamycin resistant cells (32). Taxol C possesses selective cytotoxicity in the NCI human cell line screen (33). 10-Deacetyltaxol C, A -methyltaxol C (130) and taxol D (133) also showed activity close to that of taxol in the tubulin assembly assay (32,33). Table 1 lists the different taxanes isolated thus far from different Taxus species. 

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