Design, synthesis and cytotoxic activities of naphthyl analogues of combretastatin A-4

The 3,4,5-trimethoxyphenyl and 3-hydroxy-4-methoxyphenyl rings of combretastatin A-4 are deemed optimal for its activity as antimitotic agent. The replacement of either one by a naphthalene ring results in compounds with a potency comparable to that of the parent compound. These results show that the naphthalene ring is a good surrogate for the 3,4,5-trimethoxyphenyl or the 3-hydroxy-4-methoxyphenyl rings of combretastatin A-4 and that neither of them is essential for the antitumor activity.     

Synthesis,cytotoxic evaluation and molecular docking study of 2-alkylthio-4-(2,3,4-trimethoxyphenyl)-5-aryl-thiazoles as tubulin polymerization inhibitors

A series of cis-restricted 2-alkylthio-4-(2,3,4-trimethoxyphenyl)-5-aryl-thiazole analogues of combretastatin A-4 were synthesized and investigated for inhibition of cell proliferation against three cancer cell lines, HT-29, MCF-7, and AGS, and a normal mouse fibroblastic cell line, NIH-3T3, using an MTT assay. The biological study showed that 2-(methylthio) substituted compounds showed little cytotoxic activity against the four cell lines. In contrast, the presence of the 2-(benzylthio) group on the thiazole ring resulted in a significant improvement in cytotoxic activity relative to the 2-(methylthio) substituted derivatives. Furthermore, the inhibition of tubulin polymerization by some potent compounds was evaluated. All the compounds studied were moderate tubulin polymerization inhibitors. The flow cytometry analysis confirmed that the synthesized compounds led to cell cycle arrest at the G₂/M phase. Docking simulation was performed to insert these compounds into the crystal structure of tubulin at the colchicine binding site to determine a probable binding model.     

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.     

Synthesis, biological evaluation and molecular modeling of 1,2,3-triazole analogs of combretastatin A-1

The synthesis, cytotoxicity, inhibition of tubulin polymerization data and anti-angiogenetic effects of seven 1,5-disubstituted 1,2,3-triazole analogs and two 1,4-disubstituted 1,2,3-triazole analogs of combretastatin A-1 (1) are reported herein. The biological studies revealed that the 1,5-disubstituted 1,2,3-triazoles 3-methoxy-6-(1-(3,4,5-trimethoxyphenyl)-1H-1,2,3-triazol-5-yl)benzene-1,2-diol (6), 3-methoxy-6-(1-(3,4,5-trimethoxyphenyl)-1H-1,2,3-triazol-5-yl)benzene-1,2-diamine (8) and 5-(2,3-difluoro-4-methoxyphenyl)-1-(3,4,5-trimethoxyphenyl)-1H-1,2,3-triazole (9) were the three most active compounds regarding inhibition of both tubulin polymerization and angiogenesis. Molecular modeling studies revealed that combretastatins 1 and 2 and analogs 5-11 could be successfully docked into the colchicine binding site of α,β-tubulin.     

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.     

Design, synthesis and biological evaluation of dihydronaphthalene and benzosuberene analogs of the combretastatins as inhibitors of tubulin polymerization in cancer chemotherapy

A novel series of dihydronaphthalene and benzosuberene analogs bearing structural similarity to the combretastatins in terms of 1,2-diarylethene, trimethoxyphenyl, and biaryl functionality has been synthesized. The compounds have been evaluated in regard to their ability to inhibit tubulin assembly and for their cytotoxicity against selected human cancer cell lines. From this series of compounds, benzosuberene analogs 2 and 4 inhibited tubulin assembly at concentrations comparable to that of combretastatin A-4 (CA4) and combretastatin A-1 (CA1). Furthermore, analog 4 demonstrated remarkable cytotoxicity against the three human cancer cell lines evaluated (for example GI(50)=0.0000032 microM against DU-145 prostate carcinoma).     

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.     

New Combretastatin Analogs as Anticancer Agents: Design,Synthesis, Microtubules Polymerization Inhibition,and Molecular Docking Studies

A new series of 4-(4-methoxyphenyl)-5-(3,4,5-trimethoxyphenyl)-4H-1,2,4-triazole-3-thiol derivatives were synthesized as analogs for the anticancer drug combretastatin A-4 ( CA-4 ) and characterized using FT-IR, ¹H-NMR, ¹³CNMR, and HR-MS techniques. The new CA-4 analogs were designed to meet the structural requirements of the highest expected anticancer activity of CA-4 analogs by maintaining ring A 3,4,5-trimethoxyphenyl moiety, and at the same time varying the substituents effect of the triazole moiety (ring B ). In silico analysis indicated that compound 3 has higher total energy and dipole moment than colchicine and the other analogs, and it has excellent distribution of electron density and is more stable, resulting in an increased binding affinity during tubulin inhibition. Additionally, compound 3 was found to interact with three apoptotic markers, namely p53, Bcl-2, and caspase 3. Compound 3 showed strong similarity to colchicine , and it has excellent pharmacokinetics properties and a good dynamic profile. The in vitro anti-proliferation studies showed that compound 3 is the most cytotoxic CA-4 analog against cancer cells (IC₅₀ of 6.35 μM against Hep G2 hepatocarcinoma cells), and based on its selectivity index (4.7), compound 3 is a cancer cytotoxic-selective agent. As expected and similar to colchicine , compound 3 -treated Hep G2 hepatocarcinoma cells were arrested at the G2/M phase resulting in induction of apoptosis. Compound 3 tubulin polymerization IC₅₀ (9.50 μM) and effect on V_max of tubulin polymerization was comparable to that of colchicine (5.49 μM). Taken together, the findings of the current study suggest that compound 3 , through its binding to the colchicine-binding site at β-tubulin, is a promising microtubule-disrupting agent with excellent potential to be used as cancer therapeutic agent.     

Synthesis and biological evaluation of 2-substituted-4-(3′,4′,5′-trimethoxyphenyl)-5-aryl thiazoles as anticancer agents

Antitumor agents that bind to tubulin and disrupt microtubule dynamics have attracted considerable attention in the last few years. To extend our knowledge of the thiazole ring as a suitable mimic for the cis-olefin present in combretastatin A-4, we fixed the 3,4,5-trimethoxyphenyl at the C4-position of the thiazole core. We found that the substituents at the C2- and C5-positions had a profound effect on antiproliferative activity. Comparing compounds with the same substituents at the C5-position of the thiazole ring, the moiety at the C2-position influenced antiproliferative activities, with the order of potency being NHCH₃ > Me ? N(CH₃)₂. The N-methylamino substituent significantly improved antiproliferative activity on MCF-7 cells with respect to C2-amino counterparts. Increasing steric bulk at the C2-position from N-methylamino to N,N-dimethylamino caused a 1–2 log decrease in activity. The 2-N-methylamino thiazole derivatives 3b, 3d and 3e were the most active compounds as antiproliferative agents, with IC₅₀ values from low micromolar to single digit nanomolar, and, in addition, they are also active on multidrug-resistant cell lines over-expressing P-glycoprotein. Antiproliferative activity was probably caused by the compounds binding to the colchicines site of tubulin polymerization and disrupting microtubule dynamics. Moreover, the most active compound 3e induced apoptosis through the activation of caspase-2, -3 and -8, but 3e did not cause mitochondrial depolarization.     

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.     

Design,synthesis, in vitro antiproliferative activity and apoptosis-inducing studies of 1-(3′,4′,5′-trimethoxyphenyl)-3-(2′-alkoxycarbonylindolyl)-2-propen-1-one derivatives obtained by a molecular hybridisation approach

Inhibition of microtubule function using tubulin targeting agents has received growing attention in the last several decades. The indole scaffold has been recognized as an important scaffold in the design of novel compounds acting as antimitotic agents. Indole-based chalcones, in which one of the aryl rings was replaced by an indole, have been explored in the last few years for their anticancer potential in different cancer cell lines. Eighteen novel (3′,4′,5′-trimethoxyphenyl)-indolyl-propenone derivatives with general structure 9 were synthesized and evaluated for their antiproliferative activity against a panel of four different human cancer cell lines. The highest IC₅₀ values were obtained against the human promyelocytic leukemia HL-60 cell line. This series of chalcone derivatives was characterized by the presence of a 2-alkoxycarbonyl indole ring as the second aryl system attached at the carbonyl of the 3-position of the 1-(3′,4′,5′-trimethoxyphenyl)-2-propen-1-one framework. The structure–activity relationship (SAR) of the indole-based chalcone derivatives was investigated by varying the position of the methoxy group, by the introduction of different substituents (hydrogen, methyl, ethyl or benzyl) at the N-1 position and by the activity differences between methoxycarbonyl and ethoxycarbonyl moieties at the 2-position of the indole nucleus. The antiproliferative activity data of the novel synthesized compounds revealed that generally N-substituted indole analogues exhibited considerably reduced potency as compared with their parent N-unsubstituted counterparts, demonstrating that the presence of a hydrogen on the indole nitrogen plays a decisive role in increasing antiproliferative activity. The results also revealed that the position of the methoxy group on the indole ring is a critical determinant of biological activity. Among the synthesized derivatives, compound 9e, containing the 2-methoxycarbonyl-6-methoxy-N-1H-indole moiety exhibited the highest antiproliferative activity, with IC₅₀ values of 0.37, 0.16 and 0.17?μM against HeLa, HT29 and MCF-7 cancer cell lines, respectively, and with considerably lower activity against HL-60 cells (IC₅₀: 18?μM). This derivative also displayed cytotoxic properties (IC₅₀ values ～1?μM) in the human myeloid leukemia U-937 cell line overexpressing human Bcl-2 (U-937/Bcl-2) via cell cycle progression arrest at the G₂-M phase and induction of apoptosis. The results obtained also demonstrated that the antiproliferative activity of this molecule is related to inhibition of tubulin polymerisation. The presence of a methoxy group at the C5- or C6-position of the indole nucleus, as well as the absence of substituents at the N-1-indole position, contributed to the optimal activity of the indole-propenone-3′,4′,5′-trimethoxyphenyl scaffold.     

Naphthalene 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 detrimental 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.     

Application of plant allylpolyalkoxybenzenes in synthesis of antimitotic phenstatin analogues

Phenstatin and its derivatives with the modified ring A have been synthesized, using plant allylpolyalkoxybenzenes as a starting material. The targeted molecules were evaluated in a phenotypic sea urchin embryo assay for antiproliferative activity. It was found that phenstatin ring A modifications yielded antimitotic compounds. The most effective myristicin derivative 7d (combretastatin A-2 analogue) was determined to be ca. 10 times more potent than phenstatin, displaying antimitotic tubulin-destabilizing activity at the same concentration range as combretastatins. In contrast to combretastatins, 7d featured the steric stability with potential for further design as anticancer agent.     

1,5-Disubstituted 1,2,3-triazoles as cis-restricted analogues of combretastatin A-4: Synthesis, molecular modeling and evaluation as cytotoxic agents and inhibitors of tubulin

A series of cis-restricted 1,5-disubstituted 1,2,3-triazole analogues of combretastatin A-4 (1) have been prepared. The triazole 12f, 2-methoxy-5-(1-(3,4,5-trimethoxyphenyl)-1H-1,2,3-triazol-5-yl)aniline, displayed potent cytotoxic activity against several cancer cell lines with IC₅₀ values in the nanomolar range. The ability of triazoles to inhibit tubulin polymerization has been evaluated, and 12f inhibited tubulin polymerization with IC₅₀ = 4.8 μM. Molecular modeling experiments involving 12f and the colchicine binding site of ,β-tubulin showed that the triazole moiety interacts with β-tubulin via hydrogen bonding with several amino acids.     

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.     

Application of the McMurry coupling reaction in the synthesis of tri- and tetra-arylethylene analogues as potential cancer chemotherapeutic agents

Structural redesign of selected non-steroidal estrogen receptor binding compounds has previously been successful in the discovery of new inhibitors of tubulin assembly. Accordingly, tetra-substituted alkene analogues (21–30) were designed based in part on combinations of the structural and electronic components of tamoxifen and combretastatin A-4 (CA4). The McMurry coupling reaction was used as the key synthetic step in the preparation of these tri- and tetra-arylethylene analogues. The structural assignment of E, Z isomers was determined on the basis of 2D-NOESY experiments. The ability of these compounds to inhibit tubulin polymerization and cell growth in selected human cancer cell lines was evaluated. Although the compounds were found to be less potent than CA4, these analogues significantly advance the known structure–activity relationship associated with the colchicine binding site on β-tubulin.     

1,2,3-Triazole analogs of combretastatin A-4 as potential microtubule-binding agents

A series of cis-restricted 1,4- and 1,5-disubstituted 1,2,3-triazole analogs of combretastatin A-4 (1) have been prepared. Cytotoxicity and tubulin inhibition studies showed that 2-methoxy-5-((5-(3,4,5-trimethoxyphenyl)-1H-1,2,3-triazol-1-yl)methyl)aniline (5e) and 2-methoxy-5-(1-(3,4,5-trimethoxybenzyl)-1H-1,2,3-triazol-5-yl)aniline (6e) were two of the most active compounds. Molecular modeling studies revealed that the N-2 and N-3 atoms in the triazole rings in 5e and 6e did not form hydrogen bonds with the amino acids in the anticipated pharmacophore.     

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.     

New naphthylcombretastatins. Modifications on the ethylene bridge

Compounds with three aromatic systems, carrying a 2-naphthalene and a 3,4,5-trimethoxyphenyl moieties bonded to five-membered, six-membered or fused heterocycles display potent cytotoxic effect and inhibition of tubulin polymerization, in agreement with their structural similarity to combretastatins and their heterocyclic analogues.     

Synthesis and biological evaluation of novel heterocyclic derivatives of combretastatin A-4

A novel series of combretastatin A-4 heterocyclic analogues was prepared by replacement of the B ring with indole, benzofurane or benzothiophene, attached at the C2 position. These compounds were evaluated for their abilities to inhibit tubulin assembly: derivative cis 3b, having a benzothiophene, showed an activity similar to those of colchicine or deoxypodophyllotoxine. The antiproliferative and antimitotic properties of cis 3b against keratinocyte cancer cell lines were also evaluated and the intracellular organization of microtubules in the cells after treatment with both stereoisomers of 3b was also determined, using confocal microscopy.