Review ArticleNaphthalene Combretastatin Analogues: Synthesis,Cytotoxicity and Antitubulin Activity

Synthesis and evaluation of new combretastatin analogues with varied modifications on the bridge and the aromatic rings, have shown that the 2-naphthyl moiety is a good surrogate for the 3-hydroxy-4-methoxyphenyl (B-ring) of combretastatin A-4. Other bicyclic systems, such as 6(7)-quinolyl and 5-indolyl, can replace the B-ring, but they produce less potent analogues in the cytotoxicity and tubulin polymerization inhibition assays. Other modifications are detrimentral for the potency of the studied analogues. The 2-naphthyl combretastatin 53 and the related 6-quinolyl combretastatin 106 analogues are the most potent among the derivatives of this type, whereas 92 and 95 are the most potent among the naphthalene derivatives with a heterocycle in the bridge. Previous and new results in this family of combretastatin analogues are discussed.     

A new family of quinoline and quinoxaline analogues of combretastatins

The 3-hydroxy-4-methoxyphenyl ring of combretastatin A-4 can be replaced by a 2-naphthyl moiety without significant loss of cytotoxicity and inhibition of tubulin polymerization potency. In this paper we show that the 6- or 7-quinolyl systems can in turn replace both cyclic moieties, keeping in the first case most of the potency as cytotoxic agent and in the second case as inhibitor of tubulin polymerization, related to the activities displayed by model compounds.     

Isocombretastatins A: 1,1-Diarylethenes as potent inhibitors of tubulin polymerization and cytotoxic compounds

Isocombretastatins A are 1,1-diarylethene isomers of combretastatins A. We have synthesized the isomers of combretastatin A-4, deoxycombretastatin A-4, 3-amino-deoxycombretastatin A-4 (AVE-8063), naphthylcombretastatin and the N-methyl- and N-ethyl-5-indolyl analogues of combretastatin A-4. Analogues with a 2,3,4-trimethoxyphenyl ring instead of the 3,4,5-trimethoxyphenyl ring have also been prepared. The isocombretastatins A strongly inhibit tubulin polymerization and are potent cytotoxic compounds, some of them with IC₅₀s in the nanomolar range. This new family of tubulin inhibitors shows higher or comparable potency when compared to phenstatin or combretastatin analogues. These results suggest that one carbon bridges with a geminal diaryl substitution can successfully replace the two carbon bridge of combretastatins and that the carbonyl group of phenstatins is not essential for high potency.     

Synthesis and biological evaluation of new disubstituted analogues of 6-methoxy-3-(3',4',5'-trimethoxybenzoyl)-1H-indole (BPR0L075), as potential antivascular agents

6-Methoxy-3-(3',4',5'-trimethoxybenzoyl)-1H-indole (BPR0L075) (1) is a potent inhibitor of tubulin polymerization which exhibits both in vitro and in vivo activities against a broad spectrum of solid tumors. This compound was designed as a heterocyclic analogue of combretastatin A4 (CA-4), a natural stilbene derivative that disrupts the tumor vasculature and causes tumor regression. In the present work, we describe the design and synthesis of several new disubstituted analogues of 1, along with their biological evaluation as potential antivascular agents. Compound 13, bearing a hydroxyl group at the 7-position of the indole nucleus that mimics the hydroxyl group at the 3-position of the B-ring of CA-4, was identified as a potent inhibitor of tubulin polymerization and also as a cytotoxic agent against B16 melanoma cells at sub-micromolar concentration. In addition, compound 13 displayed marked morphological activity (rounding up) at nanomolar concentrations on endothelial cells (EA.hy 926 cells), which is indicative of potential antivascular activity. Interestingly, the corresponding 7-O-mesylate derivative 28 (an intermediate in the synthesis of 13), was also found active in cellular assays, although it was moderately active in the tubulin polymerization inhibition test. Finally, in order to better understand the SAR of disubstituted analogues of 1, two other position isomers (10 and 14), were synthesized and evaluated for their biological activities. It was noted that the 7-hydroxysubstituted analogue 13 was more potent than the 5-hydroxysubstituted analogue 10. In conclusion, this work has allowed the identification of biologically potent CA-4 analogues (13 and 28) and also contributes to a better understanding of the SAR of 1.     

Increased endothelial cell selectivity of triazole-bridged dihalogenated A-ring analogues of combretastatin A-1

The antiproliferative activity on ovarian cancer (SK-OV-3) cells of a series of triazole-bridged combretastatin analogues (37, 38, 40-43) containing dihalogenation of the A-ring is reported, and compared with their trimethoxy analogues (5, 15, 39). It was found that dihalogenation with either bromine or iodine was a tolerated modification when compared to the parent compound combretastatin (CA-4, 1) and had less effect than B-ring modification on potency. These compounds exhibited G(2)/M arrest, and maintained antitubulin activity. Further assays on human umbilical vein endothelial cells (HUVECs) demonstrated the potential antivascular effects of these triazoles. Of particular note was a 3,5-diiodo-4-methoxyaryl triazole (43) which had promising 7-fold selectivity for HUVECs over ovarian cancer cells.     

Synthesis, evaluation and structural studies of antiproliferative tubulin-targeting azetidin-2-ones

A series of azetidin-2-ones substituted at positions 1, 3 and 4 of the azetidinone ring scaffold were synthesised and evaluated for antiproliferative, cytotoxic and tubulin-binding activity. In these compounds, the cis double bond of the vascular targeting agent combretastatin A-4 is replaced with the azetidinone ring in order to enhance the antiproliferative effects displayed by combretastatin A-4 and prevent the cis/trans isomerisation that is associated with inactivation of combretastatin A-4. The series of azetidinones was synthetically accessible via the Staudinger and Reformatsky reactions. Of a diverse range of heterocyclic derivatives, 3-(2-thienyl) analogue 28 and 3-(3-thienyl) analogue 29 displayed the highest potency in human MCF-7 breast cancer cells with IC(50) values of 7 nM and 10nM, respectively, comparable to combretastatin A-4. Compounds from this series also exhibited potent activity in MDA-MB-231 breast cancer cells and in the NCI60 cell line panel. No significant toxicity was observed in normal murine breast epithelial cells. The presence of larger, bulkier groups at the 3-position, for example, 3-naphthyl derivative 21 and 3-benzothienyl derivative 26, resulted in relatively lower antiproliferative activity in the micromolar range. Tubulin-binding studies of 28 (IC(50)=1.37 μM) confirmed that the molecular target of this series of compounds is tubulin. These novel 3-(thienyl) β-lactam antiproliferative agents are useful scaffolds for the development of tubulin-targeting drugs.     

Diarylmethyloxime and hydrazone derivatives with 5-indolyl moieties as potent inhibitors of tubulin polymerization

We describe the synthesis and biological evaluation of a series of diarylmethyloxime and diarylmethylhydrazone analogues that contain an indole ring and different modifications on the nitrogen of the bridge. Several compounds showed potent tubulin polymerization inhibitory action as well as cytotoxic activity against cancer cell lines. The N-methyl-5-indolyl substituted analogues are more potent than ethyl substituted ones. The most potent inhibitors of tubulin polymerization are the diarylketones and the diaryloximes. The cytotoxicity against several cancer cell lines is lower for the oximes than for the ketones. Other substitutions on the imine nitrogen greatly reduce the tubulin inhibitory and/or cytotoxic potencies.     

Synthesis and biological activity of naphthalene analogues of phenstatins: naphthylphenstatins

Novel phenstatin analogues with a 2-naphthyl moiety combined with either a 2,3,4- or a 3,4,5-trimethoxyphenyl ring have been synthesized, and their tubulin polymerization inhibiting and cytotoxic activities have been evaluated. The 2-naphthyl ring is a better replacement for the 3-hydroxy-4-methoxyphenyl ring in the phenstatin series than in the combretastatin series. For the naphthylphenstatins, the carbonyl is required, and the preferred orientation of the trimethoxyphenyl ring is the one found in combretastatins.     

SAR studies of capsazepinoid bronchodilators. Part 1: The importance of the catechol moiety and aspects of the B-ring structure

Capsazepine as well as its derivatives and analogues are general inhibitors of constriction of human small airways. From a systematic variation of the capsazepine structure, divided into four regions, SARs were established. This part concerns the catechol moiety of the A-ring as well as the 2,3,4,5-tetrahydro-1H-2-azepine moiety (the B-ring) of capsazepine. It is revealed that a conformational constrain (as a fused ring) is important and that compounds with a six-membered B-ring (as a 1,2,3,4-tetrahydroisoquinoline) in general are more potent than the corresponding isoindoline, 2,3,4,5-tetrahydro-1H-2-benzazepine and 2,3,4,5-tetrahydro-1H-3-benzazepine derivatives.     

Combretastatin analogues in cancer biology: A prospective view

The stilbenoid combretastatin and its derivatives are potent inhibitors of angiogenesis and cell proliferation and induce apoptosis. They disrupt cytoskeletal dynamics and modulate cell morphology, motility, and invasion. Hence they have been viewed as potential as anticancer agents. The impediments of poor solubility and bioavailability and the spontaneous geometric isomerisation of combretastatin into an inactive form have led to intensive efforts towards evolving novel analogues to provide more efficacious biological outcome. Importantly, isomerically stable and biologically active cis-restricted analogues have been synthesised and tested. However, very few analogues have been tested in preclinical models to assess their effects on processes relevant to cancer development and progression. Hence the accent here is on the signalling systems operated by the new derivatives and their biological effects with reference to cancer progression. Combretastatins modulate an extensive network of signalling emphasising their varied versatility. Harnessing these systems and accentuating or counteracting aberrant signalling could open potential avenues of approach to the designing of novel derivatives with enhanced performance. The import of mammalian target of rapamycin pathway, which co-ordinates growth factor receptor signalling, epithelial-mesenchymal transition activation and angiogenic signalling, is emphasised. It may be viewed as a prime target for allosteric inhibition in combination with combretastatin analogues to ascertain their potential in cancer control.     

Synthesis and structure-activity relationships of N-{1-[(6-fluoro-2-naphthyl)methyl]piperidin-4-yl}benzamide derivatives as novel CCR3 antagonists

A novel class of potent CCR3 receptor antagonists were designed and synthesized starting from N-{1-[(6-fluoro-2-naphthyl)methyl]piperidin-4-yl}benzamide (1),which was found by subjecting our chemical library to high throughput screening (HTS). The CCR3 inhibitory activity of the synthesized compounds against eotaxin-induced Ca(2+) influx was evaluated using CCR3-expressing preB cells. Systematic chemical modifications of 1 revealed that the 6-fluoro-2-naphthylmethyl moiety was essential for CCR3 inhibitory activity in this new series of CCR3 antagonists. Further structural modifications of the benzamide and piperidine moieties of 1 led to the identification of exo-N-{8-[(6-fluoro-2-naphthyl)methyl]-8-azabicyclo[3.2.1]oct-3- yl}biphenyl-2-carboxamide [corrected] (31) as a potent CCR3 antagonist with an IC(50) value of 0.020 microM.     

Design, synthesis, and biological evaluation of combretastatin nitrogen-containing derivatives as inhibitors of tubulin assembly and vascular disrupting agents

A series of analogs with nitro or serinamide substituents at the C-2'-, C-5'-, or C-6'-position of the combretastatin A-4 (CA4) B-ring was synthesized and evaluated for cytotoxic effects against heart endothelioma cells, blood flow reduction to tumors in SCID mice, and as inhibitors of tubulin polymerization. The synthesis of these analogs typically featured a Wittig reaction between a suitably functionalized arylaldehyde and an arylphosphonium salt followed by separation of the resultant E- and Z-isomers. Several of these nitrogen-modified CA4 derivatives (both amino and nitro) demonstrate significant inhibition of tubulin assembly as well as cytotoxicity and in vivo blood flow reduction. 2'-Aminostilbenoid 7 and 2'-amino-3'-hydroxystilbenoid 29 proved to be the most active in this series. Both compounds, 7 and 29, have the potential for further pro-drug modification and development as vascular disrupting agents for treatment of solid tumor cancers and certain ophthalmological diseases.     

Synthesis and in vitro pharmacological studies of C(4) modified salvinorin A analogues

Salvinorin A is the most potent naturally occurring opioid agonist with a high selectivity and affinity for kappa-opioid receptor. To explore its structure-activity relationships, modifications at the C(4) position have been studied and a series of salvinorin A derivatives were prepared. These C(4)-modified salvinorin A analogues were screened for binding and functional activities at the human kappa-opioid receptor and several potent new agonists have been identified.     

SAR studies of capsazepinoid bronchodilators 3: The thiourea part (coupling region) and the 2-(4-chlorophenyl)ethyl moiety (C-region)

Certain derivatives and analogues of capsazepine are potent in vitro inhibitors of bronchoconstriction in human small airways. During an investigation of the dependency of the potency on the structural features of the capsazepinoids in the thiourea moiety (coupling region) and the 2-(4-chlorophenyl)ethyl moiety (C-region), it was revealed that capsazepinoids with a thiourea or an amide link between the B-ring and the C-region in general have a good bronchorelaxing activity, while urea is a less attractive choice. Further, it was shown that 1,2,3,4-tetrahydroisoquinolines with a 2-(phenyl)ethyl derivative as the C-region are considerably more potent than those with an octyl group, while 2,3,4,5-tetrahydro-1H-2-benzazepines were found to be more insensitive to the nature of the C-region.     

Novel sulfonate analogues of combretastatin A-4: potent antimitotic agents

Sulfonate analogues of combretastatin A-4 have been prepared. These compounds compete with colchicine and combretastatin A-4 for the colchicine binding site on tubulin and are potent inhibitors of tubulin polymerization and cell proliferation. Importantly, these compounds also inhibit the proliferation of P-glycoprotein positive (+) cancer cells, which are resistant to many other antitumor agents.     

Camphorsulfonic acid catalysed facile tandem double Friedlander annulation protocol for the synthesis of phenoxy linked bisquinoline derivatives and discovery of antitubercular agents

A series of phenoxy linked bisquinoline derivatives were synthesised from the Friedlander annulation of 2-(4-acetylphenoxy)-1-aryl-1-ethanones with 2-aminobenzophenone in good yields using (±)-camphor-10-sulfonic acid (CSA) as the catalyst. These compounds were screened for their in vitro activity against Mycobacterium tuberculosis H37Rv (MTB) and among the 23 compounds screened, 2-(3-bromophenyl)-6-chloro-3-[4-(6-chloro-4-phenyl-2-quinolyl)phenoxy]-4-phenylquinoline (3q) and 2-(4-bromophenyl)-6-chloro-3-[4-(6-chloro-4-phenyl-2-quinolyl)phenoxy]-4-phenylquinoline (3o) were found to be the most active compounds with MIC of 1.1 and 2.2 μM against MTB. The cytotoxic effects against mouse fibroblasts (NIH 3T3) in vitro were evaluated for 3o and 3q, which displayed no toxic effects against mouse fibroblast cell line NIH 3T3.     

Development of combretastatins as potent tubulin polymerization inhibitors

The combretastatins are isolated from South African tree combretum caffrum kuntze. The lead compound combretastatin A-4 has displayed remarkable cytotoxic effect in a wide variety of preclinical tumor models and inhibits tubulin polymerization by interacting at colchicine binding site of microtubule. However, the structural simplicity of C A-4 is favorable for synthesis of various derivatives projected to induce rapid and selective vascular shutdown in tumors. Majority of the molecules have shown excellent antiproliferative activity and are able to inhibit tubulin polymerization as well as possible mechanisms of action have been investigated. In this review article, the synthesis and structure-activity relationships of C A-4 and immense number of its synthetic derivatives with various modifications on the A, B-rings, bridge carbons and their anti mitotic activities are discussed.     

Synthesis and antimycobacterial activities of ring-substituted quinolinecarboxylic acid/ester analogues. Part 1

Structural optimization of recently discovered new chemical entity, 2,8-dicyclopentyl-4-methylquinoline (DCMQ; MIC= 6.25 microg/mL, M. tuberculosis H37Rv) resulted in the synthesis of four new series of ring-substituted quinolinecarboxylic acids/esters constituting 45 analogues. All new derivatives were evaluated for in vitro antimycobacterial activities against M. tuberculosis H37Rv. Certain ring-substituted-2-quinolinecarboxylic acid ester and ring-substituted-2-quinoline acetic acid ester analogues described herein showed moderate to good inhibitory activity. In particular, three analogues methyl 4,5-dicyclopentyl-2-quinolinecarboxylate (3b), methyl 4,8-dicyclopentyl-2-quinolinecarboxylate (3c) and ethyl 2-(2,8-dicyclopentyl-4-quinolyl)acetate (14g) exhibited excellent MIC values of 1.00, 2.00 and 4.00microg/mL, respectively. Results obtained indicate that substitution of the quinoline ring with dicyclopentyl substituent presumably enhances the antimycobacterial activities in the quinoline analogues described herein.     

A 2-step synthesis of Combretastatin A-4 and derivatives as potent tubulin assembly inhibitors

A series of combretastatin derivatives were designed and synthesised by a two-step stereoselective synthesis by use of Wittig olefination followed by Suzuki cross-coupling. Interestingly, all new compounds (2a-2i) showed potent cell-based antiproliferative activities in nanomolar concentrations. Among the compounds, 2a, 2b and 2e were the most active across three cancer cell lines. In addition, these compounds inhibited the polymerisation of tubulin in vitro more efficiently than CA-4. They caused cell cycle arrest in G₂/M phase further confirming their ability to inhibit tubulin polymerisation.     

Lead identification of conformationally restricted benzoxepin type combretastatin analogs: synthesis,antiproliferative activity,and tubulin effects

We have synthesized a series of polymethoxylated rigid analogs of combretastatin A-4 which contain a benzoxepin ring in place of the usual ethylene bridge present in the natural combretastatin products. The compounds display antiproliferative activity when evaluated against the MCF-7 and MDA human breast carcinoma cell lines. 5-(3-Hydroxy-4-methoxyphenyl)-4-(3,4,5-trimethoxyphenyl)-2,3-dihydro-benzoxepine (11g) was found to be the most potent product when evaluated against the MCF-7 breast cancer cell line. A brief computational study of the structure–activity relationship for the synthesized compounds is presented. These 4,5-diarylbenzoxepins are identified as potentially useful scaffolds for the further development of antitumor agents which target tubulin.