Lead identification of conformationally restricted β-lactam type combretastatin analogues: synthesis, antiproliferative activity and tubulin targeting effects

3,4-Diarylisoxazoles-Analogues of Combretastatin A-4: Design, Synthesis, and Biological Evaluation In Vitro and In Vivo

Focusing on the molecular docking results, a series of 3,4-diarylisoxazoles, analogues of Combretastatin A4, bearing various substituents at the fifth position of the isoxazole ring and pharmacophore groups bioisosteric to methoxy substituent at ring B, were synthesized in good yields and high regioselectivity. Depending on the substituent at C5, three approaches were chosen for the construction of isoxazole ring, including nitrosation of gem-dihalocyclopropanes, nitrile oxide synthesis, and difluoromethoxylation of isoxazolone to afford 5-haloisoxazoles, 5-unsubstituted isoxazoles, and 5-difluoromethoxyisoxazoles, respectively. Isoxazoles 43 and 45 showed selective cytotoxicity and antitubulin inhibition properties in vitro, with pharmacodynamic profiles closely related to that of CA-4. Both of them slow down tumor growth (66-74%) in mouse xenografts and slightly exceed in effectiveness Combretastatin A4-phosphate itself.

Synthesis and Selective Anticancer Activity Evaluation of 2-phenylacrylonitrile Derivatives as Tubulin Inhibitors

OBJECTIVE

This study aimed at synthesizing 13 series of novel derivatives with 2-phenylacrylonitrile, evaluating antitumor activity both in vivo and in vitro, and obtaining novel tubulin inhibitors.

METHOD

The 13 series of 2-phenylacrylonitrile derivatives were synthesized by Knoevenagel condensation and the anti-proliferative activities were determined by MTT assay. The cell cycle and apoptosis were analyzed by flow cytometer. Quantitative cell migration was performed using 24-well Boyden chambers. The proteins were detected by western blotting. in vitro kinetics of microtubule assembly was measured using ELISA kit for Human β-tubulin (TUBB). Molecular docking was done by Discovery Studio (DS) 2017 Client online tool.

RESULTS

Among the derivatives, compound 1g2a possessed strong inhibitory activity against HCT116 (IC50 = 5.9 nM) and BEL-7402 (IC50 = 7.8 nM) cells. Compound 1g2a exhibited better selective antiproliferative activities and specificities than all the positive control drugs, including taxol. Compound 1g2a inhibited proliferation of HCT116 and BEL-7402 cells by arresting them in the G2/M phase of the cell cycle, inhibited the migration of HCT116 and BEL-7402 cells and the formation of cell colonies. Compound 1g2a showed excellent tubulin polymerization inhibitory activity on HCT116 and BEL-7402 cells. The results of molecular docking analyses showed that 1g2a may inhibit tubulin to exert anticancer effects.

CONCLUSION

Compound 1g2a shows outstanding antitumor activity both in vivo and in vitro and has the potential to be further developed into a highly effective antitumor agent with little toxicity to normal tissues.

Nitrogen Containing Heterocycles as Anticancer Agents: A Medicinal Chemistry Perspective

Cancer is one of the major healthcare challenges across the globe. Several anticancer drugs are available on the market but they either lack specificity or have poor safety, severe side effects, and suffer from resistance. So, there is a dire need to develop safer and target-specific anticancer drugs. More than 85% of all physiologically active pharmaceuticals are heterocycles or contain at least one heteroatom. Nitrogen heterocycles constituting the most common heterocyclic framework. In this study, we have compiled the FDA approved heterocyclic drugs with nitrogen atoms and their pharmacological properties. Moreover, we have reported nitrogen containing heterocycles, including pyrimidine, quinolone, carbazole, pyridine, imidazole, benzimidazole, triazole, β-lactam, indole, pyrazole, quinazoline, quinoxaline, isatin, pyrrolo-benzodiazepines, and pyrido[2,3-d]pyrimidines, which are used in the treatment of different types of cancer, concurrently covering the biochemical mechanisms of action and cellular targets.

Mechanistic Insights and Docking Studies of Phytomolecules as Potential Candidates in the Management of Cancer

BACKGROUND

Cancer is a leading risk of death globally. According to the World Health Organization, it is presently the second most important disease that causes death in both developing and developed countries. Remarkable progress has been made in the war against cancer with the development of numerous novel chemotherapy agents. However, it remains an immense challenge to discover new efficient therapeutic potential candidates to combat cancer.

OBJECTIVES

The majority of the currently used anticancer drugs are of natural origins, such as curcumin, colchicine, vinca alkaloid, paclitaxel, bergenin, taxols, and combretastatin. Concerning this, this review article presents the structure of the most potent molecules along with IC50 values, structure-activity relationships, mechanistic studies, docking studies, in silico studies of phytomolecules, and important key findings on human cancer cell lines.

METHODS

A viewpoint of drug design and development of antiproliferative agents from natural phytomolecules has been established by searching peer-reviewed literature from Google Scholar, PubMed, Scopus, Springer, Science Direct, and Web of Science over the past few years.

RESULTS

Our analysis revealed that this article would assist chemical biologists and medicinal chemists in industry and academia in gaining insights into the anticancer potential of phytomolecules.

CONCLUSION

In vitro and in silico studies present phytomolecules, such as curcumin, colchicine, vinca alkaloids, colchicine, bergenin, combretastatin, and taxol encompassing anticancer agents, offerings abundant sanguinity and capacity in the arena of drug discovery to inspire the investigators towards the continual investigations on these phytomolecules. It is extremely expected that efforts in this track will strengthen and grant some budding cancer therapeutics candidates in the near future.

Synthesis, Characterisation and Mechanism of Action of Anticancer 3-Fluoroazetidin-2-ones

The stilbene combretastatin A-4 (CA-4) is a potent microtubule-disrupting agent interacting at the colchicine-binding site of tubulin. In the present work, the synthesis, characterisation and mechanism of action of a series of 3-fluoro and 3,3-difluoro substituted β-lactams as analogues of the tubulin-targeting agent CA-4 are described. The synthesis was achieved by a convenient microwave-assisted Reformatsky reaction and is the first report of 3-fluoro and 3,3-difluoro β-lactams as CA-4 analogues. The β-lactam compounds 3-fluoro-4-(3-hydroxy-4-methoxyphenyl)-1-(3,4,5-trimethoxy phenyl)azetidin-2-one 32 and 3-fluoro-4-(3-fluoro-4-methoxyphenyl)-1-(3,4,5-trimethoxyphenyl)azetidin-2-one) 33 exhibited potent activity in MCF-7 human breast cancer cells with IC50 values of 0.075 µM and 0.095 µM, respectively, and demonstrated low toxicity in non-cancerous cells. Compound 32 also demonstrated significant antiproliferative activity at nanomolar concentrations in the triple-negative breast cancer cell line Hs578T (IC50 0.033 μM), together with potency in the invasive isogenic subclone Hs578Ts(i)8 (IC50 = 0.065 μM), while 33 was also effective in MDA-MB-231 cells (IC50 0.620 μM). Mechanistic studies demonstrated that 33 inhibited tubulin polymerisation, induced apoptosis in MCF-7 cells, and induced a downregulation in the expression of anti-apoptotic Bcl2 and survivin with corresponding upregulation in the expression of pro-apoptotic Bax. In silico studies indicated the interaction of the compounds with the colchicine-binding site, demonstrating the potential for further developing novel cancer therapeutics as microtubule-targeting agents.

Molecular interactions at the colchicine binding site in tubulin: An X-ray crystallography perspective

Tubulin is an important cancer drug target. Compounds that bind at the colchicine site in tubulin have attracted significant interest as they are generally less affected by multidrug resistance than other potential drugs. Modeling is useful in understanding the interactions between tubulin and colchicine binding site inhibitors (CBSIs), but because the colchicine binding site contains two flexible loops whose conformations are highly ligand-dependent, modeling has its limitations. X-ray crystallography provides experimental pictures of tubulin-ligand interactions at this challenging colchicine site. Since 2004, when the first X-ray structure of tubulin in complex with N-deacetyl-N-(2-mercaptoacetyl)-colchicine (DAMA-colchicine) was published, many X-ray crystal structures have been reported for tubulin complexes involving the colchicine binding site. In this review, we summarize the crystal structures of tubulin in complexes with various CBSIs, aiming to facilitate the discovery of new generations of tubulin inhibitors.

Synthesis and Antiproliferative Evaluation of 3-Chloroazetidin-2-ones with Antimitotic Activity: Heterocyclic Bridged Analogues of Combretastatin A-4

Antimitotic drugs that target tubulin are among the most widely used chemotherapeutic agents; however, the development of multidrug resistance has limited their clinical activity. We report the synthesis and biological properties of a series of novel 3-chloro-β-lactams and 3,3-dichloro-β-lactams (2-azetidinones) that are structurally related to the tubulin polymerisation inhibitor and vascular targeting agent, Combretastatin A-4. These compounds were evaluated as potential tubulin polymerisation inhibitors and for their antiproliferative effects in breast cancer cells. A number of the compounds showed potent activity in MCF-7 breast cancer cells, e.g., compound 10n (3-chloro-4-(3-hydroxy-4-methoxy-phenyl)-1-(3,4,5-trimethoxyphenyl)azetidin-2-one) and compound 11n (3,3-dichloro-4-(3-hydroxy-4-methoxyphenyl)-1-(3,4,5-trimethoxyphenyl)-azetidin-2-one), with IC50 values of 17 and 31 nM, respectively, and displayed comparable cellular effects to those of Combretastatin A-4. Compound 10n demonstrated minimal cytotoxicity against non-tumorigenic HEK-293T cells and inhibited the in vitro polymerisation of tubulin with significant G2/M phase cell cycle arrest. Immunofluorescence staining of MCF-7 cells confirmed that β-lactam 10n caused a mitotic catastrophe by targeting tubulin. In addition, compound 10n promoted apoptosis by regulating the expression of pro-apoptotic protein BAX and anti-apoptotic proteins Bcl-2 and Mcl-1. Molecular docking was used to explore the potential molecular interactions between novel 3-chloro-β-lactams and the amino acid residues of the colchicine binding active site cavity of β-tubulin. Collectively, these results suggest that 3-chloro-2-azetidinones, such as compound 10n, could be promising lead compounds for further clinical anti-cancer drug development.

Novel N-Substituted 3-Aryl-4-(diethoxyphosphoryl)azetidin-2-ones as Antibiotic Enhancers and Antiviral Agents in Search for a Successful Treatment of Complex Infections

A novel series of N-substituted cis- and trans-3-aryl-4-(diethoxyphosphoryl)azetidin-2-ones were synthesized by the Kinugasa reaction of N-methyl- or N-benzyl-(diethyoxyphosphoryl)nitrone and selected aryl alkynes. Stereochemistry of diastereoisomeric adducts was established based on vicinal H3-H4 coupling constants in azetidin-2-one ring. All the obtained azetidin-2-ones were evaluated for the antiviral activity against a broad range of DNA and RNA viruses. Azetidin-2-one trans-11f showed moderate inhibitory activity against human coronavirus (229E) with EC50 = 45 µM. The other isomer cis-11f was active against influenza A virus H1N1 subtype (EC50 = 12 µM by visual CPE score; EC50 = 8.3 µM by TMS score; MCC > 100 µM, CC50 = 39.9 µM). Several azetidin-2-ones 10 and 11 were tested for their cytostatic activity toward nine cancerous cell lines and several of them appeared slightly active for Capan-1, Hap1 and HCT-116 cells values of IC50 in the range 14.5-97.9 µM. Compound trans-11f was identified as adjuvant of oxacillin with significant ability to enhance the efficacy of this antibiotic toward the highly resistant S. aureus strain HEMSA 5. Docking and molecular dynamics simulations showed that enantiomer (3R,4S)-11f can be responsible for the promising activity due to the potency in displacing oxacillin at β-lactamase, thus protecting the antibiotic from undesirable biotransformation.

Developments in the Application of 1,2,3-Triazoles in Cancer Treatment

BACKGROUND

The impact of cancer on modern society cannot be emphasized enough in terms of both economic and human costs. Cancer treatments are known, unfortunately, for their side effects - frequently numerous and severe. Drug resistance is another issue medical professionals have to tackle when dealing with neoplastic illnesses. Cancer rates are rising worldwide due to various factors - low-quality nutrition, air and water pollution, tobacco use, etc. For those and many other reasons, drug discovery in the field of oncology is a top priority in modern medical science.

OBJECTIVE

To present the reader with the latest in cancer drug discovery with regard to 1,2,3-triazole- containing molecules in a clear, concise way so as to make the present review a useful tool for researchers.

METHODS

Available information present on the role of 1,2,3-triazoles in cancer treatment was collected. Data was collected from scientific literature, as well as from patents.

RESULTS

A vast number of triazole-containing molecules with antiproliferative properties have been proposed, synthesized and tested for anticancer activity both in vitro and in vivo. The substances vary greatly when considering molecular structure, proposed mechanisms of action and affected cancer cell types.

CONCLUSION

Triazole-containing molecules with anticancer activity are being widely synthesized and extensively tested. They vary significantly in terms of both structure and mechanism of action. The methods for their preparation and administration are well established and with proven reproducibility. These facts suggest that triazoles may play an important role in the discovery of novel antiproliferative medications with improved effectiveness and safety profile.

Combretastatins: An Overview of Structure, Probable Mechanisms of Action and Potential Applications

Combretastatins are a class of closely related stilbenes (combretastatins A), dihydrostilbenes (combretastatins B), phenanthrenes (combretastatins C) and macrocyclic lactones (combretastatins D) found in the bark of Combretum caffrum (Eckl. & Zeyh.) Kuntze, commonly known as the South African bush willow. Some of the compounds in this series have been shown to be among the most potent antitubulin agents known. Due to their structural simplicity many analogs have also been synthesized. Combretastatin A4 phosphate is the most frequently tested compounds in preclinical and clinical trials. It is a water-soluble prodrug that the body can rapidly metabolize to combretastatin A4, which exhibits anti-tumor properties. In addition, in vitro and in vivo studies on combretastatins have determined that these compounds also have antioxidant, anti-inflammatory and antimicrobial effects. Nano-based formulations of natural or synthetic active agents such as combretastatin A4 phosphate exhibit several clear advantages, including improved low water solubility, prolonged circulation, drug targeting properties, enhanced efficiency, as well as fewer side effects. In this review, a synopsis of the recent literature exploring the combretastatins, their potential effects and nanoformulations as lead compounds in clinical applications is provided.

Discovery of a chiral fluorinated azetidin-2-one as a tubulin polymerisation inhibitor with potent antitumour efficacy

Inhibition of tubulin polymerisation with small molecules has been clinically validated as a promising therapy for multiple solid tumours. Herein, a series of chiral azetidin-2-ones were asymmetrically synthesised and biologically evaluated for antitumour activities. Among them, a chiral fluorinated azetidin-2-one, 18, was found to exhibit the most potent activities against five cancer cell lines, including a drug-resistant cell line, with IC₅₀ values ranging from 1.0 to 3.6 nM. Further mechanistic studies revealed that the compound 18 worked by disrupting tubulin polymerisation, blocking the cell cycle in the G₂/M phase, inducing cellular apoptosis, and suppressing angiogenesis. Additionally, 18 exhibited higher human-microsomal metabolic stability and aqueous solubility compared to those of combretastatin A-4. Finally, 18 was also found to effectively inhibit tumour growth in a xenograft mice model with low toxicity and thus might be a promising lead for further clinical development.

Recent Advances in β-lactam Derivatives as Potential Anticancer Agents

Cancer, accounts for around 10 million deaths annually, is the second leading cause of death globally. The continuous emergency of drug-resistant cancers and the low specificity of anticancer agents are the main challenges in the control and eradication of cancers, so it is imperative to develop novel anticancer agents. Immense efforts have been made in developing new lead compounds and novel chemotherapeutic strategies for the treatment of various forms of cancers in recent years. β-Lactam derivatives constitute versatile and attractive scaffolds for the drug discovery since these kinds of compounds possess a variety of pharmacological properties, and some of them exhibited promising potency against both drug-sensitive and drug-resistant cancer cell lines. Thus, β-lactam moiety is a useful template for the development of novel anticancer agents. This review will provide an overview of β-lactam derivatives with the potential therapeutic application for the treatment of cancers covering articles published between 2000 and 2020. The mechanisms of action, the critical aspects of design and structureactivity relationships are also discussed.

β-Lactams with antiproliferative and antiapoptotic activity in breast and chemoresistant colon cancer cells

A series of novel 1,4-diaryl-2-azetidinone analogues of combretastatin A-4 (CA-4) have been designed, synthesised and evaluated in vitro for antiproliferative activity, antiapoptotic activity and inhibition of tubulin polymerisation. Glucuronidation of CA-4 by uridine 5-diphosphoglucuronosyl transferase enzymes (UGTs) has been identified as a mechanism of resistance in cancer cells. Potential sites of ring B glucuronate conjugation are removed by replacing the B ring meta-hydroxy substituent of selected series of β-lactams with alternative substituents e.g. F, Cl, Br, I, CH₃. The 3-phenyl-β-lactam 11 and 3-hydroxy-β-lactam 46 demonstrate improved activity over CA-4 in CA-4 resistant HT-29 colon cancer cells (IC₅₀ = 9 nM and 3 nM respectively compared with IC₅₀ = 4.16 μM for CA-4), while retaining potency in MCF-7 breast cancer cells (IC₅₀ = 17 nM and 22 nM respectively compared with IC₅₀ = for 4 nM for CA-4). Compound 46 binds at the colchicine site of tubulin, and strongly inhibits tubulin assembly at micromolar concentrations comparable to CA-4. In addition, compound 46 induced mitotic arrest at low concentration in both cell lines MCF-7 and HT-29 together with downregulation of expression of antiapoptotic proteins Mcl-1, Bcl-2 and survivin in MCF-7 cells. These novel antiproliferative and antiapoptotic β-lactams are potentially useful scaffolds in the development of tubulin-targeting agents for the treatment of breast cancers and chemoresistant colon cancers.

Colchicine-Binding Site Inhibitors from Chemistry to Clinic: A Review

It is over 50 years since the discovery of microtubules, and they have become one of the most important drug targets for anti-cancer therapies. Microtubules are predominantly composed of the protein tubulin, which contains a number of different binding sites for small-molecule drugs. There is continued interest in drug development for compounds targeting the colchicine-binding site of tubulin, termed colchicine-binding site inhibitors (CBSIs). This review highlights CBSIs discovered through diverse sources: from natural compounds, rational design, serendipitously and via high-throughput screening. We provide an update on CBSIs reported in the past three years and discuss the clinical status of CBSIs. It is likely that efforts will continue to develop CBSIs for a diverse set of cancers, and this review provides a timely update on recent developments.

Synthesis of Exclusively 4-Substituted β-Lactams through the Kinugasa Reaction Utilizing Calcium Carbide

A new Kinugasa reaction protocol has been elaborated for the one-pot synthesis of 4-substituted β-lactams utilizing calcium carbide and nitrone derivatives. Calcium carbide is thereby activated by TBAF·3H₂O in the presence of CuCl/NMI. The ease of synthesis and use of inexpensive chemicals provides rapid access of practical quantities of β-lactams exclusively substituted at position 4.

3-Vinylazetidin-2-Ones: Synthesis, Antiproliferative and Tubulin Destabilizing Activity in MCF-7 and MDA-MB-231 Breast Cancer Cells

Microtubule-targeted drugs are essential chemotherapeutic agents for various types of cancer. A series of 3-vinyl-β-lactams (2-azetidinones) were designed, synthesized and evaluated as potential tubulin polymerization inhibitors, and for their antiproliferative effects in breast cancer cells. These compounds showed potent activity in MCF-7 breast cancer cells with an IC₅₀ value of 8 nM for compound 7s 4-[3-Hydroxy-4-methoxyphenyl]-1-(3,4,5-trimethoxyphenyl)-3-vinylazetidin-2-one) which was comparable to the activity of Combretastatin A-4. Compound 7s had minimal cytotoxicity against both non-tumorigenic HEK-293T cells and murine mammary epithelial cells. The compounds inhibited the polymerisation of tubulin in vitro with an 8.7-fold reduction in tubulin polymerization at 10 μM for compound 7s and were shown to interact at the colchicine-binding site on tubulin, resulting in significant G2/M phase cell cycle arrest. Immunofluorescence staining of MCF-7 cells confirmed that β-lactam 7s is targeting tubulin and resulted in mitotic catastrophe. A docking simulation indicated potential binding conformations for the 3-vinyl-β-lactam 7s in the colchicine domain of tubulin. These compounds are promising candidates for development as antiproiferative microtubule-disrupting agents.

Synthesis and characterisation of (Z)-styrylbenzene derivatives as potential selective anticancer agents

To identify anticancer agents with high potency and low toxicity, a series of (Z)-styrylbenzene derivatives were synthesised and evaluated for anticancer activities using a panel of nine cancer cell lines and two noncancerous cell lines. Most derivatives exhibited significant anti-proliferative activities against five cancer cell lines, including MGC-803 and BEL-7402. (Z)-3-(p-Tolyl)-2-(3,4,5-trimethoxyphenyl)acrylonitrile (6h) showed a strong inhibitory effect on MGC-803 cells (IC50 < 0.01 µM) and exhibited stronger anti-proliferative activity than taxol (IC50 < 0.06 ± 0.01 µM). The IC50 value of 6h in L-02 cells was 10,000-fold higher than in MGC-803 cells. Compound 6h inhibited proliferation of BEL-7402 cells by arresting at the G2/M phase through up-regulation of cyclin B1 expression, down-regulation of cyclin A and D1 expression, and induction of apoptosis. In addition, 6h inhibited the migration of BEL-7402 cells and the formation of cell colonies.

β-Lactam analogues of combretastatin A-4 prevent metabolic inactivation by glucuronidation in chemoresistant HT-29 colon cancer cells

Glucuronidation by uridine 5-diphosphoglucuronosyl transferase enzymes (UGTs) is a cause of intrinsic drug resistance in cancer cells. Glucuronidation of combretastatin A-4 (CA-4) was previously identified as a mechanism of resistance in hepatocellular cancer cells. Herein, we propose chemical manipulation of β-lactam bridged analogues of Combretastatin A-4 as a novel means of overcoming drug resistance associated with glucuronidation due to the expression of UGTs in the CA-4 resistant human colon cancer HT-29 cells. The alkene bridge of CA-4 is replaced with a β-lactam ring to circumvent potential isomerisation while the potential sites of glucuronate conjugation are deleted in the novel 3-substituted-1,4-diaryl-2-azetidinone analogues of CA-4. We hypothesise that glucuronidation of CA-4 is the mechanism of drug resistance in HT-29 cells. Ring B thioether containing 2-azetidinone analogues of CA-4 such as 4-(4-(methylthio)phenyl)-3-phenyl-1-(3,4,5-trimethoxyphenyl)azetidin-2-one (27) and 3-hydroxy-4-(4-(methylthio)phenyl)-1-(3,4,5-trimethoxyphenyl)azetidin-2-one (45) were identified as the most potent inhibitors of tumour cell growth, independent of UGT status, displaying antiproliferative activity in the low nanomolar range. These compounds also disrupted the microtubular structure in MCF-7 and HT-29 cells, and caused G₂/M arrest and apoptosis. Taken together, these findings highlight the potential of chemical manipulation as a means of overcoming glucuronidation attributed drug resistance in CA-4 resistant human colon cancer HT-29 cells, allowing the development of therapeutically superior analogues.

Biological Evaluation in Vitro and in Silico of Azetidin-2-one Derivatives as Potential Anticancer Agents

Potential anticancer activity of 16 azetidin-2-one derivatives was evaluated showing that compound 6 [N-(p-methoxy-phenyl)-2-(p-methyl-phenyl)-3-phenoxy-azetidin-2-one] presented cytotoxic activity in SiHa cells and B16F10 cells. The caspase-3 assay in B16F10 cells displayed that azetidin-2-one derivatives induce apoptosis. Microarray and molecular analysis showed that compound 6 was involved on specific gene overexpression of cytoskeleton regulation and apoptosis due to the inhibition of some cell cycle genes. From the 16 derivatives, compound 6 showed the highest selectivity to neoplastic cells, it was an inducer of apoptosis, and according to an in silico analysis of chemical interactions with colchicine binding site of human α/β-tubulin, the mechanism of action could be a molecular interaction involving the amino acids outlining such binding site.

Novel Natural Product- and Privileged Scaffold-Based Tubulin Inhibitors Targeting the Colchicine Binding Site

Tubulin inhibitors are effective anticancer agents, however, there are many limitations to the use of available tubulin inhibitors in the clinic, such as multidrug resistance, severe side-effects, and generally poor bioavailability. Thus, there is a constant need to search for novel tubulin inhibitors that can overcome these limitations. Natural product and privileged structures targeting tubulin have promoted the discovery and optimization of tubulin inhibitors. This review will focus on novel tubulin inhibitors derived from natural products and privileged structures targeting the colchicine binding site on tubulin.

Role of JNK and NF-κB in mediating the effect of combretastatin A-4 and brimamin on endothelial and carcinoma cells

PURPOSE

The 4,5-diarylimidazole brimamin is an analog of the natural vascular-disrupting agent combretastatin A-4 (CA-4) with improved water solubility, tolerance by animals and efficacy in multidrug-resistant tumors. Here, we aimed at identifying the major mechanisms underlying the in vitro and in vivo actions of brimamin on endothelial and carcinoma cells, including vascularization.

METHODS

The contribution of specific signaling kinases to the effects of brimamin on cytoskeleton organization and the viability and differentiation of endothelial cells was assessed by MTT and tube formation assays in the presence or absence of specific kinase inhibitors. Changes in DNA affinity and expression of NF-κB in endothelial and carcinoma-derived cells and their solid tumors (xenografts) treated with brimamin were ascertained by electrophoretic mobility shift assays and Western blotting. The anti-vascular effect of brimamin in solid tumors was verified by CD31 immunostaining.

RESULTS

We found that brimamin can inhibit tubulin polymerization and cause a reorganization of F-actin in Ea.hy926 endothelial cells. Its inhibitory effect on tube formation was found to depend on functional Rho kinase and JNK. JNK inhibition was found to suppress the induction of endothelial cell apoptosis by brimamin. In CA-4-refractory human BxPC-3 pancreas carcinoma-derived and triple-negative MDA-MB-231 breast carcinoma-derived cells brimamin was found to inhibit growth and to induce apoptosis at low nanomolar concentrations by blocking NF-κB activation in a dose-dependent manner. Brimamin was also found to reduce the in vivo growth rate and vascularization of MDA-MB-231 xenografts in mice. Residual tumor cells of these treated xenografts showed a relatively low expression of the p65 subunit of NF-κB.

CONCLUSIONS

Our data indicate that cellular JNK and Rho kinase activities are crucial for the cytotoxic and cytoskeleton reorganizing effects of brimamin on endothelial cells. In addition, we found that in resistant carcinoma cells and xenografts brimamin can induce down-regulation of anti-apoptotic NF-κB expression and signaling. Its chemical properties and efficacy against clinically relevant cancer entities make brimamin a promising candidate vascular-disrupting agent.

Combretastatins: more than just vascular targeting agents?

Several prodrugs of the naturally occurring combretastatins have undergone extensive clinical evaluation as vascular targeting agents (VTAs). Their increased selectivity toward endothelial cells together with their innate ability to rapidly induce vascular shutdown and inhibit tumor growth at doses up to 10-fold less than the maximum tolerated dose led to the clinical evaluation of combretastatins as VTAs. Tubulin is well established as the molecular target of the combretastatins and the vast majority of its synthetic derivatives. Furthermore, tubulin is a highly validated molecular target of many direct anticancer agents routinely used as front-line chemotherapeutics. The unique vascular targeting properties of the combretastatins have somewhat overshadowed their development as direct anticancer agents and the delineation of the various cell death pathways and anticancer properties associated with such chemotherapeutics. Moreover, the ongoing clinical trial of OXi4503 (combretastatin-A1 diphosphate) together with preliminary preclinical evaluation for the treatment of refractory acute myelogenous leukemia has successfully highlighted both the indirect and direct anticancer properties of combretastatins. In this review, we discuss the development of the combretastatins from nature to the clinic. The various mechanisms underlying combretastatin-induced cell cycle arrest, mitotic catastrophe, cell death, and survival are also reviewed in an attempt to further enhance the clinical prospects of this unique class of VTAs.

Synthesis and biological evaluation of novel 4,5-disubstituted 2H-1,2,3-triazoles as cis-constrained analogues of combretastatin A-4

A series of combretastatin A-4 (CA-4) analogues have been prepared from (Z)-substituted diarylacrylonitriles (1a-1p) obtained in a two-step synthesis from appropriate arylaldehydes and acrylonitriles. The resulting 4,5-disubstituted 2H-1,2,3-triazoles were evaluated for their anti-cancer activities against a panel of 60 human cancer cell lines. The diarylacrylonitrile analogue 2l exhibited the most potent anti-cancer activity in the screening studies, with GI₅₀ values of <10 nM against almost all the cell lines in the human cancer cell panel and TGI values of <10 nM against cancer cell lines SF-539, MDA-MB-435, OVCAR-3 and A498. Furthermore, in silico docking studies of compounds 2l, 2e and 2h within the active site of tubulin were carried out in order to rationalize the mechanism of the anti-cancer properties of these compounds. From the in silico studies, compound 2e was predicted to have better affinity for the colchicine binding site on tubulin compared to compounds 2l and 2h. Analogue 2e was also evaluated for its anti-cancer activity by colony formation assay against 9LSF rat gliosarcoma cells and afforded an LD₅₀ of 7.5 nM. A cell cycle redistribution assay using analogue 2e was conducted to further understand the mechanism of action of these CA-4 analogues. From this study, analogues 2e and 2l were the most potent anti-cancer agents in this structural class, and were considered lead compounds for further development as anti-cancer drugs.

Synthesis and evaluation of a series of resveratrol analogues as potent anti-cancer agents that target tubulin

A series of novel diarylacrylonitrile and trans-stilbene analogues of resveratrol has been synthesized and evaluated for their anticancer activities against a panel of 60 human cancer cell lines. The diarylacrylonitrile analogues 3b and 4a exhibited the most potent anticancer activity of all the analogues synthesized in this study, with GI50 values of < 10 nM against almost all the cell lines in the human cancer cell panel. Compounds 3b and 4a were also screened against the acute myeloid leukemia (AML) cell line, MV4-11, and were found to have potent cytotoxic properties that are likely mediated through inhibition of tubulin polymerization. Results from molecular docking studies indicate a common binding site for 4a and 3b on the 3,3-tubulin heterodimer, with a slightly more favorable binding for 3b compared to 4a; this is consistent with the results from the microtubule assays, which demonstrate that 4a is more potent than 3b in inhibiting tubulin polymerization in MV4-11 cells. Taken together, these data suggest that diarylacrylonitriles 3b and 4a may have potential as antitubulin therapeutics for treatment of both solid and hematological tumors.

β-Lactam estrogen receptor antagonists and a dual-targeting estrogen receptor/tubulin ligand

Twelve novel β-lactams were synthesized and their antiproliferative effects and binding affinity for the predominant isoforms of the estrogen receptor (ER), ERα and ERβ, were determined. β-Lactams 23 and 26 had the strongest binding affinities for ERα (IC50 values: 40 and 8 nM, respectively) and ERβ (IC50 values: 19 and 15 nM). β-Lactam 26 was the most potent in antiproliferative assays using MCF-7 breast cancer cells, and further biochemical analysis showed that it caused accumulation of cells in G2/M phase (mitotic blockade) and depolymerization of tubulin in MCF-7 cells. Compound 26 also induced apoptosis and downregulation of the expression of pro-survival proteins Bcl-2 and Mcl-1. Computational modeling predicted binding preferences for the dual ER/tubulin ligand 26. This series is an important addition to the known pool of ER antagonists and β-lactam 26 is the first reported compound that has dual-targeting properties for both the ER and tubulin.

Crystal structure of 4,5-bis-(3,4,5-tri-meth-oxy-phen-yl)-2H-1,2,3-triazole methanol monosolvate

The title compound, C20H23N3O6·CH3OH, was synthesized by [3 + 2] cyclo-addition of (Z)-2,3-bis-(3,4,5-tri-meth-oxy-phen-yl)acrylo-nitrile with sodium azide and ammonium chloride in DMF/water. The central nitro-gen of the triazole ring is protonated. The dihedral angles between the triazole ring and the 3,4,5-tri-meth-oxy-phenyl ring planes are 34.31 (4) and 45.03 (5)°, while that between the 3,4,5-tri-meth-oxy-phenyl rings is 51.87 (5)°. In the crystal, the mol-ecules, along with two methanol solvent mol-ecules are linked into an R (4) 4(10) centrosymmetric dimer by N-H⋯O and O-H⋯N hydrogen bonds.

Rational approaches, design strategies, structure activity relationship and mechanistic insights for anticancer hybrids

A Hybrid drug which comprises the incorporation of two drug pharmacophores in one single molecule are basically designed to interact with multiple targets or to amplify its effect through action on another bio target as one single molecule or to counterbalance the known side effects associated with the other hybrid part(.) The present review article offers a detailed account of the design strategies employed for the synthesis of anticancer agents via molecular hybridization techniques. Over the years, the researchers have employed this technique to discover some promising chemical architectures displaying significant anticancer profiles. Molecular hybridization as a tool has been particularly utilized for targeting tubulin protein as exemplified through the number of research papers. The microtubule inhibitors such as taxol, colchicine, chalcones, combretasatin, phenstatins and vinca alkaloids have been utilized as one of the functionality of the hybrids and promising results have been obtained in most of the cases with some of the tubulin based hybrids exhibiting anticancer activity at nanomolar level. Linkage with steroids as biological carrier vector for anticancer drugs and the inclusion of pyrrolo [2,1-c] [1,4]benzodiazepines (PBDs), a family of DNA interactive antitumor antibiotics derived from Streptomyces species in hybrid structure based drug design has also emerged as a potential strategy. Various heteroaryl based hybrids in particular isatin and coumarins have also been designed and reported to posses' remarkable inhibitory potential. Apart from presenting the design strategies, the article also highlights the structure activity relationship along with mechanistic insights revealed during the biological evaluation of the hybrids.

The chemistry and biological potential of azetidin-2-ones

Azetidin-2-ones, commonly referred as β-lactams, represent a unique ring system, with interesting chemistry and great biological potential. Besides its well known antibiotic activity, this ring system exhibits a wide range of activities, attracting the attention of researchers. The biological and pharmacological profile of azetidin-2-ones is reviewed here comprehensively with several examples under fourteen different activity heads. The chemistry and methods of synthesis have also been discussed.

Combretazet-3 a novel synthetic cis-stable combretastatin A-4-azetidinone hybrid with enhanced stability and therapeutic efficacy in colon cancer

In recent years an extensive series of synthetic combretastatin A-4 (CA-4)-azetidinone (β-lactam) hybrids were designed and synthesised with a view to improve the stability, therapeutic efficacy and aqueous solubility of CA-4. Lead compounds containing a 3,4,5-trimethoxy aromatic ring at position 1 and a variety of substitution patterns at positions 3 and 4 of the β-lactam ring were screened in three adenocarcinoma-derived colon cancer cell lines (CT-26, Caco-2 and the CA-4 resistant cell line, HT-29). In both CT-26 and Caco-2 cells all β-lactam analogues analysed displayed potent therapeutic efficacy within the nanomolar range. Substitution of the ethylene bridge of CA-4 with the β-lactam ring together with the aforementioned aryl substitutions improved the therapeutic efficacy of CA-4 up to 300‑fold in the combretastatin refractory HT-29 cells. The lead compound combretazet-3 (CAZ-3); chemical name [4-(3-hydroxy-4-methoxyphenyl)-3-(4-hydroxyphenyl)-1-(3,4,5-trimethoxyphenyl)azetidin-2-one] demonstrated improved chemical stability together with enhanced therapeutic efficacy as compared with CA-4 whilst maintaining the natural biological properties of CA-4. Furthermore, CAZ-3 demonstrated significant tumour inhibition in a murine model of colon cancer. Our results suggest that combretastatin-azetidinone hybrids represent an effective novel therapy for the treatment of combretastatin resistant carcinomas.

Synthesis and biochemical activities of antiproliferative amino acid and phosphate derivatives of microtubule-disrupting β-lactam combretastatins

The synthesis and biochemical activities of novel water-soluble β-lactam analogues of combretastatin A-4 are described. The first series of compounds investigated, β-lactam phosphate esters 7a, 8a and 9a, exhibited potent antiproliferative activity and caused microtubule disruption in human breast carcinoma-derived MCF-7 cells. They did not inhibit tubulin polymerisation in vitro, indicating that biotransformation was necessary for their antiproliferative and tubulin binding effects in MCF-7 cells. The second series of compounds, β-lactam amino acid amides (including 10k and 11l) displayed potent antiproliferative activity in MCF-7 cells, disrupted microtubules in MCF-7 cells and also inhibited the polymerisation of tubulin in vitro. This indicates that the β-lactam amides did not require metabolic activation to have antiproliferative effects, in contrast to the phosphate series. Both series of compounds caused mitotic catastrophe and apoptosis in MCF-7 cells. Molecular modelling studies indicated potential binding conformations for the β-lactam amino acid amides 10k and 11l in the colchicine-binding site of tubulin. Due to their aqueous solubility and potent biochemical effects, these compounds are promising candidates for further development as microtubule-disrupting agents.

Novel cis-restricted β-lactam combretastatin A-4 analogues display anti-vascular and anti-metastatic properties in vitro

Combretastatin A-4 (CA-4) is a naturally occurring microtubular-destabilising agent that possesses potent anti-tumour and anti-vascular properties both in vitro and in vivo. Clinical trials to date indicate that its water-soluble prodrug, combretastatin A-4 phosphate (CA-4P), is well tolerated at therapeutically useful doses. However, the stilbenoid structure of CA-4, consisting of two phenyl rings linked by an ethylene bridge, renders the compound readily susceptible to isomerisation from its biologically active cis-conformation to its more thermodynamically stable but inactive trans-isomer. To circumvent this problem, we synthesised a series of cis-restricted CA-4 analogues. Replacement of the ethylene bridge with a 1,4-diaryl-2-azetidinone (β-lactam) ring provided a rigid scaffold thus preventing cis-trans isomerisation. We previously documented that these tubulin-depolymerising β-lactam compounds potently induced cell cycle arrest and apoptosis in a variety of cancerous cell lines (including those displaying multidrug resistance) and ex vivo patient samples, whilst exerting only minimal toxicity to normal cells. The purpose of this study was to elucidate the effect of the β-lactam compounds on both tumour vascularisation and tumour cell migration, two critical elements that occur during the growth and metastatic progression of tumours. We established that two representative β-lactam compounds, CA-104 and CA-432, exerted both anti-endothelial effects [G2/M arrest and apoptosis of primary human umbilical vein endothelial cells (HUVECs)] and anti-angiogenic effects [inhibition of HUVEC migration and differentiation and reduced vascular endothelial growth factor (VEGF) release from MDA-MB-231 breast adenocarcinoma cells]. In addition, we established that lead analogue, CA-432, abrogated the migration of MDA-MB-231 cells indicating an anti-metastatic function for these compounds. In summary, our results to date collectively indicate that these cis-restricted β-lactam CA-4 analogues may prove to be useful alternatives to CA-4 in the treatment of cancer but with the added advantage of improved stability of the cis-isomer.