Inhibitors of tubulin polymerization

Combretastatins are a class of compounds originally derived from the African Willow tree (Combretum caffrum) and are powerful reversible inhibitors of tubulin polymerization. This class of molecules has been shown to bind to the colchicine binding site of tubulin, by the same mode of action as mentioned above (Sect. 1.2). Combretastatins consist of a ris-slilbcnc core structure. To date, there have been several compounds that have shown promise as potential anticancer drugs. However, development of these compounds as anticancer agents is limited by issues of chemical stability, bioavailibilty, toxicity, and solubility. 

The most famous of these compounds is combretastatin A-4 (CA-4,7), isolated by Pettit et al. in 1989 [30]. Pettit s research led to the isolation and structural determination of a series of phenanthrenes, dihydrophenanthrene, stilbene, and bibenzyl compounds [31]. CA-4 (7), alongside CA-1 (8), was found to be an extremely active inhibitor of tubulin polymerization [30,32]. The major problems associated with these compounds were poor bioavailability and low aqueous solubility [33,34], and hence, research in the field was turned to designing better alternatives with the hope of eradicating the negative properties of these potent compounds. 

The synthesis of biologically important heterocyclic stilbene and chalcone derivatives of combretastatins has been discussed. Combretastatins have been shown to be inhibitors of tubulin polymerization. In many cases the compounds described in this chapter were included because of an interesting synthesis or structure, although limited biological data were found. It is the author s opinion that a great number of the compounds contained within this review are worthy of further investigation as potential tubulin binders. 

Another structurally related series is the 2-ary 1-1,8-naphthyridin-4-ones (37 to 48, see Table 6.7), which contain a second nitrogen in the aromatic A ring. Compounds with meta-substituted phenyls (methoxy-, chloro-, or fluoro-) or a-naphthyl groups at the C-2 position showed potent cytotoxicity in the NCI 60 human tumor cell line panel with GI50 values in the low micromolar to nanomolar range (Tables 6.7 and 6.8).51 The tumor cell line selectivity varies with the various substituents. 2-(3 -Methoxyphenyl)-naphthyridinone (37) was significantly more cytotoxic in several cancer cell lines than the corresponding 2-(3 -meth-oxyphenyl)-quinolone (36). Both compound classes were potent inhibitors of tubulin polymerization the 2-ary 1-1,8-naphthyridin-4-ones had activity nearly comparable with those... 

Although carbamates of deacetylcolchicine were prepared earlier and found to be potent inhibitors of tubulin polymerization, and of binding of radiolabeled colchicine to tubulin (<5), a natural representative of this class of compounds only recently was found in extracts of Colchicum cicilium (9). The structure of carbamate 4 was established on the basis of spectral data and was proved to be correct by synthesis from demecolcine and ethylchloroformate. 

Although 2-methoxy-5-aryltropones lacking the B ring of colchicine are not true colchicinoids, they are discussed since some were found to be potent inhibitors of tubulin polymerization in vitro. (70). This has stimulated much interest in this type of compound as well as research to achieve their practical synthesis. 

Phenolic colchicines found as plant consituents and as metabolites (1) are less potent antitubulin compounds than their fully methylated analogs (Fig. 26). It can be seen that 10-SCH3 analogs of colchicines (thiocol-chicines) were always more potent inhibitors of tubulin polymerization... 

Radiolabeled 3-demethyl-3-chloroacetylthiocolchicine with a l4C label in the chloroacetyl moiety (DCTC) was found to be a potent inhibitor of tubulin polymerization and of colchicine binding to tubulin. The reaction was 80-90% inhibited in the presence of saturating, amounts of known antitubulin compounds such as podophyllotoxin, combretastatin A-4, and colchicine itself. The tubulin /3 subunit was labeled 5-6 times faster than the a subunit. Cyanogen bromide digestion of the /3 subunit which had reacted covalently with DCTC indicated that at least three positions in /3-tubulin had reacted with DCTC. Purification and amino acid sequencing of these peptides are in progress (138). 

From a variety of 10-substituted analogs of colchicine it was found that 10-demethoxy-10-ethylcolchicine was a good inhibitor of tubulin polymerization, suggesting that steric, rather than electronic, effects of the 10-substituent are of importance for binding to tubulin (140). 

As reported by Apers et al. [89], podophyllotoxin derivatives can be divided into two groups in terms of their mechanism of action 1 Inhibitors of tubulin polymerization (such as podophyllotoxin), 2 Inhibitors of DNA topoisomerase II (such as etoposide and teniposide). Combining both pharmacophores leads to compounds with a dual mechanism of action, such as azatoxin. 

These simple semi-synthetic procedures have been extensively used in the preparation of new allocolchicinoids for biological screening purposes [11], Recently, these molecules found a new interest when it was shown by scientists at Angiogene Pharmaceuticals that A/-acetylcolchinol phosphate 35, termed ZD6126, is a prodrug of jV-acetylcolchinol 31 which causes the selective destruction of tumour vasculature [74], This compound, obtained semi-synthetically from colchicine, is currently under phase I clinical trials. Another patent from Brandeis University described methyl ketone 36 as a reversible inhibitor of tubulin polymerization (in contrast to colchicine), with 100-fold cytotoxicity compared to colchicine toward a number of cell lines [75]. 

In the course of their synthesis of colchicine [102], Banwell and coworkers found that the tricyclic intermediate 74, Fig. (15), aromatized in the presence of //-toluenesulfonic acid to give the colchinol analogue 75 [103]. The methyl ester 76 and the previously described methyl ether 72 obtained from 75 were found to be potent inhibitors of tubulin polymerization. 

Figure 9.3 Structures of inhibitors of tubulin polymerization (9.1-9.5) and of monastrol 9.6, an inhibitor of mitotic spindle assembly, found through FCG.

Figure 13.15. Inhibition of mitosis by vincristine, a The mitotic spindle consists of tubulin fibers, which are helical polymers of a,P tubulin dimers, b Structure of vincristine, an inhibitor of tubulin polymerization, c Mode of action of vincristine. The drug binds to an a,P dimer. These dimers retain the ability to associate with growing filaments however, the further attachment of dimers is inhibited. At high concentrations, vincristin-associated tubulin is also sequestrated within mis-assembled polymers.

Romagnoli R, Baraldi PG, Carrion MD et al (2007) Synthesis and biological evaluation of 2-and 3-aminobenzo[b]thiophene derivatives as antimitotic agents and inhibitors of tubulin polymerization. J Med Chem 50 2273-2277... 

Conformationally restricted analogues of lavendustin A were prepared by M. Cushman and co-wokers as cyctotoxic inhibitors of tubulin polymerization. ... 

Among the modifications of ring A, fluorcombstatin and related 3-halostilbenes have been recently reported [7, 14]. The fluoro, chloro, and bromo halocombstatins were nearly equivalent to Combretastatin A-4 as inhibitors of tubulin polymerization and of the binding of colchicine to tubulin and retained the powerful human cancer cell line inhibitory activity of combretastatin A-4. 

These compounds have been evaluated as cytotoxic agents and inhibitors of tubulin polymerization. 

The investigation of a variety of benzophenones analogues related to Phenstatin [(3,4,5-trimethoxyphenyl),(3 -hydroxy-4 -methoxyphenyl) ketone], evidenced some lead compounds in this new type of inhibitor of tubulin polymerization [(3,4,5-trimethoxy phenyl, (2 -amino-4... 

Benzo[b]furan and indole analogues of some recently identified benzo[b]thiophene inhibitors of tubulin polymerization have been synthesized, and their biological activity has been investigated. Among them, the compound 2-(3-hydroxy,4-methoxyphenyl),3-(3,4,5-trimethoxybenzoyl)-6-methoxyfuran-2-one 81 was the most active, exhibiting 5 times the level of potency of Combretastatin A-4. However, it is less potent than Combretastatin A-4 as inhibitor of [3H]colchicine... 

Table 1 The isolated yields of spongiostatins 1-9 and their cytoxicity in the ISCI s 60 cell line screen and as an inhibitor of tubulin polymerization [43].

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