Synthesis and Cytotoxic Evaluation of Combretafurans, Potential Scaffolds for Dual-Action Antitumoral Agents

Design and synthesis of new trimethoxylphenyl-linked combretastatin analogues loaded on diamond nanoparticles as a panel for ameliorated solubility and antiproliferative activity

A new series of vinyl amide-, imidazolone-, and triazinone-linked combretastatin A-4 analogues have been designed and synthesised. These compounds have been evaluated for their cytotoxic activity against MDA-MB-231 breast cancer cells. The triazinone-linked combretastatin analogues (6 and 12) exhibited the most potent cytotoxic activity, in sub-micromolar concentration compared with combretastatin A-4 as a reference standard. The results of β-tubulin polymerisation inhibition assay appear to correlate well with the ability to inhibit β-tubulin polymerisation. Additionally, these compounds were subjected to biological assays relating to cell cycle aspects and apoptosis induction. In addition, the most potent compound 6 was loaded on PEG-PCL modified diamond nanoparticles (PEG-PCL-NDs) and F4 was picked as the optimum formula. F4 exhibited enhanced solubility and release over the drug suspension. In the comparative cytotoxic activity, PEG-PCL modified F4 was capable of diminishing the IC50 by around 2.89 times for nude F4, while by 3.48 times relative to non-formulated compound 6.

Photostability and Antiproliferative Activity of Furan Analogues of Combretastatin A-4

Cancer is one of the most serious health problems that usually require heavy medical treatment. It is important to ensure that no additional burden is placed on patients due to the modes of administration and/or poor quality of pharmaceuticals. In this regard, understanding, quantifying, and improving the photostability (resistance to UV light or sunlight) of drugs is among the important elements that can improve the patient's quality of life. In this work, the photochemical properties of a wide range of furanone analogues of combretastatin A-4 and their antiproliferative activity against A-431 epidermoid carcinoma cells were studied in a search for compounds with improved photostability and antiproliferative activity. It was found that the incorporation of an arylidene moiety led to a significant improvement in photostability, while the antiproliferative activity strongly depends on the nature of the aryl residue in the arylidene moiety. The high photostability of arylidenes was achieved due to the delocalization of the central double bond of the 1,3,5-hexatriene system, which limited the 6π-electrocyclization. The best results in terms of antiproliferative activity were obtained for thiophene arylidene (IC₅₀ = 0.6 μM) and 3,4-diarylfuran (IC₅₀ = 0.047 μM). The obtained results address the lack of data available now in scientific literature on the photodegradation of combretastatin A-4 analogues and should be taken into account in studies of the side effects of pharmaceuticals based on them.

Exploring Diverse-Ring Analogues on Combretastatin A4 (CA-4) Olefin as Microtubule-Targeting Agents

Combretastatin-4 (CA-4) as a tubulin polymerization inhibitor draws extensive attentions. However, due to its weak stability of cis-olefin and poor metabolic stability, structure modifications on cis-configuration are being performed. In this work, we constructed a series of novel CA-4 analogues with linkers on olefin containing diphenylethanone, cis-locked dihydrofuran, α-substituted diphenylethanone, cyclobutane and cyclohexane on its cis-olefin. Cytotoxic activity of all analogues was measured by an SRB assay. Among them, compound 6b, a by-product in the preparation of diphenylethanone analogues, was found to be the most potent cytotoxic agents against HepG2 cells with IC₅₀ values of less than 0.5 μM. The two isomers of 6b induced cellular apoptosis tested by Annexin V-FITC and propidium iodide (PI) double staining, arrested cells in the G2/M phase by PI staining analysis, and disrupted microtubule network by immunohistochemistry study in HepG2 cells. Moreover, 6b-(E) displayed a dose-dependent inhibition effect for tubulin assembly in in vitro tubulin polymerization assay. In addition, molecular docking studies showed that two isomers of 6b could bind efficiently at colchicine binding site of tubulin similar to CA-4.

Medicinal chemistry of vicinal diaryl scaffold: A mini review

The privileged structures have been widely used as a valuable template in new drug discovery. 1,2-Diaryl or vicinal diaryl is a simple scaffold found in many drugs and naturally occurring compounds. From synthetic point of view, the vicinal diaryl derivatives are easily accessible due to their facile and expedient syntheses. These scaffolds have shown numerous interesting pharmacological activities against various diseases with lot of clinical potentials. This review aims to highlight the evidence of vicinal diaryl motif as a privileged scaffold in COX-2 inhibitors and CA-4 analogs.

Constructing novel dihydrofuran and dihydroisoxazole analogues of isocombretastatin-4 as tubulin polymerization inhibitors through [3+2] reactions

[3+2] reactions play a key role in constructing various pharmaceutical moleculars. In this study, using Mn(OAc)₃ mediated and 1,3-dipolar [3+2] cyclization reactions, 38 novel dihydrofuran and dihydroisoxazole analogues of isoCA-4 were synthesized as inhibitors of tubulin polymerization. Among them, compound 6g was found to be the most potent cytotoxic agents against PC-3 cells with IC₅₀ value of 0.47μM, and compound 5p exhibted highest activity on HeLa cells with IC₅₀ vaule of 2.32µM. Tubulin polymerization assay revealed that 6g was a dose-dependent and effective inhibitor of tubulin assembly. Immunohistochemistry studies and cell cycle distribution analysis indicated that 6g severely disrupted microtubule network and significantly arrested most cells in the G2/M phase of the cell cycle in PC-3 cells. In addition, molecular docking studies showed that two chiral isomers of 6g can bind efficiently and similarly at colchicine binding site of tubulin.

Harnessing the Properties of Natural Products

Natural products (NPs) have been used as traditional medicines since antiquity. With more than 10⁶⁰ estimated compounds with molecular weights less than 500 Da representing chemical space, NPs occupy a very small percentage; however, they are significantly overrepresented in biologically relevant chemical space. The classical approach concentrates on identifying one or more NPs with biological activity from a source organism. There is much more to be learned from NPs than we can discover this narrow view. In this review, we discuss ways to harness the global properties of NPs.

Synthesis of Novel Antiproliferative 1,2,3-triazole Hybrids Using the Molecular Hybridisation Approach

A series of nine novel 1,2,3-triazole-chalcone derivatives were designed using the molecular hybridisation approach and synthesised by click chemistry. Most of the synthesised compounds exhibited moderate to good antiproliferative activity against oesophagus, gastric and neuroendocrine cancer cell lines, but a compound containing a p-bromo group in the A ring and a [(4,5-dihydrothiazol-2-yl)thio]methyl group attached at the 4-position of a p-[3-(1,2,3-triazol-1-yl)propyloxy] group in the B ring showed the highest activity with an IC50 value of 8.16 μM against neuroendocrine cancer cells. The structure activity relationships of all nine compounds were discussed.

Design and Antiproliferative Activity of N-Heterocycle-Chalcone Derivatives

Nine chalcone derivatives containing an N-heterocyclic compound attached by its nitrogen atom to one of the benzene rings were prepared and evaluated for their antiproliferative activity against liver, gastric and neuroendocrine cancer cell lines. Most of the synthesised compounds exhibited moderate to good activity against all three cancer cell lines, but in particular, a chalcone containing a 4-(imidazol-1-yl)phenyl group and a 3,4,5-trimethoxyphenyl group showed the highest antiproliferative activity with an IC50 value of 5.39 μM against liver cancer cells.

Triazole-curcuminoids: A new class of derivatives for 'tuning' curcumin bioactivities?

Curcumin is a unique blend of pharmacophores responsible for the pleiotropy of this natural pigment. In the present study we have replaced the 1,3-dicarbonyl moiety with a 1,2,3-triazole ring to furnish a new class of triazole-curcuminoids as a possible strategy to generate new compounds with different potency and selectivity compared to curcumin. We obtained a proof-of-principle library of 28 compounds tested for their cytotoxicity (SY-SY5Y and HeLa cells) and for their ability to inhibit NF-κB. Furthermore, we also generated 1,3-dicarbonyl curcuminoids of selected click compounds. Triazole-curcuminoids lost their ability to be Michael's acceptors, yet maintained some of the features of the parent compounds and disclosed new ones. In particular, we found that some compounds were able to inhibit NF-κB without showing cytotoxicity, while others, unlike curcumin, activated NF-κB signalling. This validates the hypothesis that click libraries can be used to investigate the biological activities of curcumin as well as generate analogs with selected features.

Insights into drug discovery from natural products through structural modification

Natural products (NPs) have played a key role in drug discovery and are still a prolific source of novel lead compounds or pharmacophores for medicinal chemistry. Pharmacological activity and druggability are two indispensable components advancing NPs from leads to drugs. Although naturally active substances are usually good lead compounds, most of them can hardly satisfy the demands for druggability. Hence, these structural phenotypes have to be modified and optimized to overcome existing deficiencies and shortcomings. This review illustrates druggability optimization of NPs through structural modification with some successful examples.

Tubulin-interactive stilbene derivatives as anticancer agents

Microtubules are dynamic polymers that occur in eukaryotic cells and play important roles in cell division, motility, transport and signaling. They form during the process of polymerization of α- and β-tubulin dimers. Tubulin is a significant and heavily researched molecular target for anticancer drugs. Combretastatins are natural cis-stilbenes that exhibit cytotoxic properties in cultured cancer cells in vitro. Combretastatin A-4 (3′-hydroxy-3,4,4′, 5-tetramethoxy-cis-stilbene; CA-4) is a potent cytotoxic cis-stilbene that binds to β-tubulin at the colchicine-binding site and inhibits tubulin polymerization. The prodrug CA-4 phosphate is currently in clinical trials as a chemotherapeutic agent for cancer treatment. Numerous series of stilbene analogs have been studied in search of potent cytotoxic agents with the requisite tubulin-interactive properties. Microtubule-interfering agents include numerous CA-4 and transresveratrol analogs and other synthetic stilbene derivatives. Importantly, these agents are active in both tumor cells and immature endothelial cells of tumor blood vessels, where they inhibit the process of angiogenesis. Recently, computer-aided virtual screening was used to select potent tubulin-interactive compounds. This review covers the role of stilbene derivatives as a class of antitumor agents that act by targeting microtubule assembly dynamics. Additionally, we present the results of molecular modeling of their binding to specific sites on the α- and β-tubulin heterodimer. This has enabled the elucidation of the mechanism of stilbene cytotoxicity and is useful in the design of novel agents with improved anti-mitotic activity. Tubulin-interactive agents are believed to have the potential to play a significant role in the fight against cancer.

Design, synthesis, and biological evaluation of (E)-N-aryl-2-arylethenesulfonamide analogues as potent and orally bioavailable microtubule-targeted anticancer agents

A series of novel (E)-N-aryl-2-arylethenesulfonamides (6) were synthesized and evaluated for their anticancer activity. Some of the compounds in this series showed potent cytotoxicity against a wide spectrum of cancer cell-lines (IC50 values ranging from 5 to 10 nM) including all drug resistant cell-lines. Nude mice xenograft assays with compound (E)-N-(3-amino-4-methoxyphenyl)-2-(2',4',6'-trimethoxyphenyl)ethenesulfonamide (6t) showed dramatic reduction in tumor size, indicating their in vivo potential as anticancer agents. A preliminary drug development study with compound 6t is predicted to have increased blood-brain barrier permeability relative to many clinically used antimitotic agents. Mechanistic studies indicate that 6t and some other analogues disrupted microtubule formation, formation of mitotic spindles, and arrest of cells in mitotic phase. Compound 6t inhibited purified tubulin polymerization in vitro and in vivo and circumvented drug resistance mediated by P-glycoprotein. Compound 6t specifically competed with colchicine binding to tubulin and with similar avidity as podophylltoxin, indicating its binding site on tubulin.

(Z)-1-aryl-3-arylamino-2-propen-1-ones, highly active stimulators of tubulin polymerization: synthesis, structure-activity relationship (SAR), tubulin polymerization, and cell growth inhibition studies

Tubulin, the major structural component of microtubules, is a target for the development of anticancer agents. A series of (Z)-1-aryl-3-arylamino-2-propen-1-one (10) were synthesized and evaluated for antiproliferative activity in cell-based assay. The most active compound (Z)-1-(2-bromo-3,4,5-trimethoxyphenyl)-3-(3-hydroxy-4-methoxyphenylamino)prop-2-en-1-one (10ae) was tested in 20 tumor cell lines including multidrug resistant phenotype and was found to induce apoptosis in all these cell lines with similar GI(50) values. Flow cytometry studies showed that 10ae arrested the cells in G2/M phase of cell cycle. In addition to G2/M block, these compounds caused microtubule stabilization like paclitaxel and induced apoptosis via activation of the caspase family. The observations made in this investigation demonstrate that (Z)-1-Aryl-3-arylamino-2-propen-1-one (10) represents a new class of microtubule-stabilizing agents.

Development of selective estrogen receptor modulator (SERM)-like activity through an indirect mechanism of estrogen receptor antagonism: defining the binding mode of 7-oxabicyclo[2.2.1]hept-5-ene scaffold core ligands

Previously, we discovered estrogen receptor (ER) ligands with a novel three-dimensional oxabicyclo[2.2.1]heptene core scaffold and good ER binding affinity act as partial agonists via small alkyl ester substitutions on the bicyclic core that indirectly modulate the critical switch helix in the ER ligand binding domain, helix 12, by interactions with helix 11. This contrasts with the mechanism of action of tamoxifen, which directly pushes helix 12 out of the conformation required for gene activation. We now report that a much larger substitution can be tolerated at this position of the bicyclic core scaffold, namely a phenyl sulfonate group, which defines a novel binding epitope for the estrogen receptor. We prepared an array of 14 oxabicycloheptene sulfonates, varying the phenyl sulfonate group. As with the parent compound, 5,6-bis-(4-hydroxyphenyl)-7-oxabicyclo[2.2.1]hept-5-ene-2-sulfonic acid phenyl ester (OBHS), these compounds showed preferential affinity for ERα, and the disposition and size of the phenyl substituents were important determinants of the binding affinity and selectivity of these compounds, with those having ortho substituents giving the highest, and para substituents the lowest affinities for ERα. A few analogues exhibit ERα binding affinities that are comparable to or, in the case of the ortho-chloro analogue, higher than that of OBHS itself. In cell-based studies, we found several compounds with activity profiles comparable to tamoxifen, but acting entirely as indirect antagonists, allosterically interfering with recruitment of coactivator proteins to the receptor. Thus, the OBHS binding epitope represents a novel approach to the development of estrogen receptor antagonists via an indirect mechanism of antagonism.

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.

Synthesis and in-vitro antitumor activities of some mannich bases of 9-alkyl-1,2,3,4-tetrahydrocarbazole-1-ones

A novel series of 2-substituted aminomethyl-9-alkyl-1,2,3,4-tetrahydrocarbazole-1-ones 5a-q was synthesized via aminomethylation of 9-alkyl-1,2,3,4-tetrahydrocarbazole-1-ones 4a-e with hydrochlorides of the respective amines 6a-m. The structures of these newly synthesized compounds were characterized by (1)H-NMR, MS, and elemental analysis. All the compounds were tested for their cytotoxic activity in vitro against four human tumor cell lines including human non-small lung cancer cells (A549), human gastric adenocarcinoma (SGC), human colon cancer cell (HCT116), human myeoloid leukemia cells (K562), and one multi-drug resistant subline (KB-VCR). Most compounds showed moderate to potent cytotoxic activity against the tested cell lines. Preliminary mechanism research indicated that the most promising compound, 2-diethylaminomethyl-9-methyl-1,2,3,4-tetrahydrocarbazole-1-one 5c, exhibited a potential inhibitory effect against microtubule.

Solution-phase parallel synthesis and biological evaluation of combretatriazoles

Combretastatin A-4 is an antitumoral and antitubulin agent that is active only in its cis configuration. In the present manuscript, we have synthesized cis-locked combretastatins containing a triazole ring (combretatriazoles). To achieve this, we have developed a column chromatography-free parallel solution-phase synthesis that exploits the reaction between azides and alpha-keto phosphorus ylids, which is known to regioselectively generate the 1,5-disubstituted triazoles. The prepared compounds were screened as antitubulinic agents, allowing us to identify three new compounds with high potency, two of which show a new mechanism of action that induces cells to appear multinucleated and display a high number of mitotic spindles.

Discovery of 4-amino and 4-hydroxy-1-aroylindoles as potent tubulin polymerization inhibitors

1-Aroylindoline, 1-aroyl-1,2,3,4-tetrahydroquinoline, and 1-aroylindole derivatives were synthesized and evaluated for anticancer activity. The 4-amino and 4-hydroxy-1-aroylindoles 26 and 27 with IC 50 of 0.9 and 0.6 microM, respectively, exhibited antitubulin activity superior or comparable to that of colchicine and combretastatin A-4. They also showed antiproliferative activity with IC 50 of 0.3-5.4 nM in a set of human cancer cell lines.

Effects of lactone derivatives on aromatase (CYP19) activity in H295R human adrenocortical and (anti)androgenicity in transfected LNCaP human prostate cancer cells

Certain lactone-containing secondary plant metabolites display potent biological effects, including anti-tumor activities. This is of particular interest as these compounds appear effective against hormone-dependent cancers, such as those of breast and prostate, of which the incidence is on the rise. The mechanisms of anti-tumor action of these compounds are largely unknown. Thirteen synthetic lactone derivatives were evaluated for effects on aromatase activity and mRNA expression in H295R human adrenocortical carcinoma cells. Aromatase (CYP19) is a key enzyme in the synthesis of estrogens from androgens. Over-expression has been associated with increased risk of developing estrogen-dependent mammary tumors, and aromatase inhibitors are effective in their treatment. The androgen receptor is implicated in mediating hormone-dependent prostate tumor growth, and androgen antagonists are effective in the treatment of these cancers. Thus the (anti)androgenic effects of the lactones were also assessed in LNCaP human prostate cancer cells transfected with human androgen receptor and an androgen receptor-responsive luciferase reporter gene. Cells were exposed to lactones (0.1-100 microM) dissolved in dimethyl sulfoxide (0.1% in medium) for 24 h prior to measurement of aromatase activity using a tritiated water-release assay. Three (competitive) inhibitors of aromatase activity were identified (potencies in decreasing order): 3-(3,4-dimethoxy-phenyl)-4-(4-methoxy-phenyl)-5H-furan-2-one (CRI-7; IC(50)=1 microM; K(i)=1.0 microM), 3,4-bis-(3,4-dimethoxy-phenyl)-5H-furan-2-one (CRI-8; IC(50)=2 microM; K(i)=1.2 microM) and 3-(3,4-dimethoxy-phenyl)-4-(3,4,5-trimethoxy-phenyl)-5H-furan-2-one (CRI-9; IC(50)=3 microM; K(i)=6.8 microM). Several concentration-dependent inducers of aromatase (>2fold) were also identified (CRI-1, CRI-4 or Vioxx, CRI-11 and CRI-13). These lactones also induced pII promoter-specific CYP19 transcripts. In transfected LNCaP cells, the three aromatase inhibitors CRI-7, 8 and 9 demonstrated concentration-dependent anti-androgenicity above 0.1 microM in the presence of either 0.1 nM of dihydrotestosterone or the synthetic androgen R1881. The other lactones showed no consistent pro- or anti-androgenic effects in these LNCaP cells. Lactone moiety-containing molecules may form the structural basis for the development of potent drugs effective against hormone-dependent cancers.

4- and 5-aroylindoles as novel classes of potent antitubulin agents

A novel series of 4- and 5-aroylindole derivatives was prepared and evaluated for antitumor activity. Several compounds showed excellent antiproliferative activity as inhibitors of tubulin polymerization. Compounds 13, 14, 15, and 18, with IC50 values of 1.9, 1.1, 1.2, and 1.8 microM, respectively, exhibited more potent inhibition of tubulin polymerization than colchicine. They also displayed antiproliferative activity, with IC50 values ranging from 10 to 43 nM in a variety of human cell lines from different organs.

7-Aroyl-aminoindoline-1-sulfonamides as a novel class of potent antitubulin agents

A novel series of 7-aroyl-aminoindoline-1-benzenesulfonamides showed excellent activity as inhibitors of tubulin polymerization through binding with the colchicine binding site of microtubules. Compound 15 and 16 display IC50 values of 1.1 and 1.2 microM, respectively. Compound 15 inhibited the human cancer cell growth of KB, MKN45, H460, HT29, and TSGH, as well as one human-resistant cancer line of KB-vin 10, with an IC50 of 9.6, 8.8, 9.4, 8.6, 10.8, and 8.9 nM, respectively.

Structural simplification of bioactive natural products with multicomponent synthesis: dihydropyridopyrazole analogues of podophyllotoxin

A three-component condensation of 5-amino-3-methylpyrazole, tetronic acid, and various aromatic, heteroaromatic, and aliphatic aldehydes leads to the formation of dihydropyridopyrazole analogues of a cytotoxic lignan podophyllotoxin. This new heterocyclic scaffold-based library allows a drastic reduction of the structural complexity of the natural product with the retention of its potent cytotoxic properties. Similarly to podophyllotoxin, the presented analogues induce apoptosis in Jurkat cells.