Novel Gold(I)– and Gold(III)–N-Heterocyclic Carbene Complexes: Synthesis and Evaluation of Their Anticancer Properties

Design, Synthesis and Bioactivity Evaluation of Ag(I)-, Au(I)- and Au(III)-Quinoxaline-Wingtip N-Heterocyclic Carbene Complexes Against Antibiotic Resistant Bacterial Pathogens

Intending to homogenize the biological activities of both quinoxaline and imidazole moieties, the proligand, 1-methyl-3-quinoxaline-imidazolium hexaflurophosphate (1.HPF6), and [Ag(1)2][PF6], (2); [Au(1)2][PF6], (3); and [Au(1)Cl3], (4) NHC complexes were synthesized. All the synthesized compounds were characterized by elemental analysis, NMR, and UV-Vis spectroscopy. Finally, single crystal X-ray structures revealed a linear geometry for complex 2 whereas a square planar geometry for complex 4. The formation of complex 3 was confirmed and supported by its MS spectra. The antibacterial activities of all the synthesized complexes were investigated against gram-positive bacteria and gram-negative bacteria. The Au(III)-NHC complex, 4 showed the highest antibacterial activity with extremely low MIC values against both the bacterial strains (0.24 μg mL-1). Monitoring of zeta potential supports the higher activity of complex 4 compared to 2 and 3. ROS production by complex 4 has also been measured in vitro in the CT26 cancer cell lines, which is directly responsible for targetting and killing the bacterial pathogens. Cell cytotoxicity assay using 293T cell lines has been performed to investigate the biocompatibility nature of complex 4. Also, an excellent hemocompatibility was assigned to it from its hemolytic studies, which provide valuable insights into the design of novel antibacterial agents.

Protein Metalation by Medicinal Gold Compounds: Identification of the Main Features of the Metalation Process through ESI MS Experiments

Gold compounds form a new class of promising anticancer agents with innovative modes of action. It is generally believed that anticancer gold compounds, at variance with clinically established platinum drugs, preferentially target proteins rather than nucleic acids. The reactions of several gold compounds with a few model proteins have been systematically explored in recent years through ESI MS measurements to reveal adduct formation and identify the main features of those reactions. Here, we focus our attention on a group of five gold compounds of remarkable medicinal interest, i.e., Auranofin, Au(NHC)Cl, [Au(NHC)₂]PF₆, Aubipyc, and Auoxo6, and on their reactions with four different biomolecular targets, i.e., the proteins HEWL, hCA I, HSA and the C-terminal dodecapeptide of the enzyme thioredoxin reductase. Complete ESI MS data are available for those reactions due to previous experimental work conducted in our laboratory. From the comparative analysis of the ESI MS reaction profiles, some characteristic trends in the metallodrug-protein reactivity may be identified as detailed below. The main features are described and analyzed in this review. Overall, all these observations are broadly consistent with the concept that cytotoxic gold drugs preferentially target cancer cell proteins, with a remarkable selectivity for the cysteine and selenocysteine proteome. These interactions typically result in severe damage to cancer cell metabolism and profound alterations in the redox state, leading to eventual cancer cell death.

Inorganic Nanomaterials Used in Anti-Cancer Therapies:Further Developments

In this article, we provide an overview of the progress of scientists working to improve the quality of life of cancer patients. Among the known methods, cancer treatment methods focusing on the synergistic action of nanoparticles and nanocomposites have been proposed and described. The application of composite systems will allow precise delivery of therapeutic agents to cancer cells without systemic toxicity. The nanosystems described could be used as a high-efficiency photothermal therapy system by exploiting the properties of the individual nanoparticle components, including their magnetic, photothermal, complex, and bioactive properties. By combining the advantages of the individual components, it is possible to obtain a product that would be effective in cancer treatment. The use of nanomaterials to produce both drug carriers and those active substances with a direct anti-cancer effect has been extensively discussed. In this section, attention is paid to metallic nanoparticles, metal oxides, magnetic nanoparticles, and others. The use of complex compounds in biomedicine is also described. A group of compounds showing significant potential in anti-cancer therapies are natural compounds, which have also been discussed.

New β-carboline derivatives containing imidazolium as potential VEGFR2 inhibitors: synthesis, X-ray structure, antiproliferative evaluations, and molecular modeling

A series of new β-carboline derivatives containing an imidazolium moiety were designed and synthesized via the reaction of β-carboline-1-carboxaldehydes, acetyl chloride, primary amine, and formaldehyde. The antitumor activity of the synthesized compounds was examined against lung carcinoma (A549), gastric carcinoma (BGC-823), murine colon carcinoma (CT-26), liver carcinoma (Bel-7402) and breast carcinoma (MCF-7) cells. The results indicated that most compounds exhibited significant antiproliferative activity, in some cases greater than that of cisplatin, and compound 3z was found to be the most potent antiproliferative agent against A549, BGC823, CT-26, Bel-7402 and MCF-7 cell lines with an IC50 value of 2.7 ± 0.4, 2.7 ± 0.6, 2.4 ± 0.2, 3.2 ± 0.2, and 5.6 ± 0.3 μM, respectively. Combined with favorable in vitro potency, the antitumor efficacies of the selected compounds in mice were also evaluated. Compound 3z exhibited potent antitumor activity with a tumor inhibition rate of 48.6% in sarcoma 180 models. Preliminary investigations on the mechanisms of action revealed that compound 3z could dramatically inhibit EA.hy926 cell tube formation in a dose-dependent manner. Further investigation of the preliminary mechanism of action demonstrated that compound 3z had obvious angiogenesis inhibitory effects in the chicken chorioallantoic membrane (CAM) assay. The results of the docking study showed a good fitting of the new compounds 3o and 3z to the active site of VEGFR-2 with a docking score energy of -11.31 kcal per mole and -11.26 kcal per mole, respectively.

Metal-Based Complexes as Potential Anti-cancer Agents

Metal based therapy is no new in biomedical research. In early days the biggest limitation was the inequality among therapeutical and toxicological dosages. Ever since, Barnett Rosenberg discovered cisplatin, a new era has begun to treat cancer with metal complexes. Platinum complexes such as oxaliplatin, cisplatin, and carboplatin, seem to be the foundation of metal/s-based components to challenge malignancies. With an advancement in the biomolemoecular mechanism, researchers have started developing non-classical platinum-based complexes, where a different mechanistic approach of the complexes is observed towards the biomolecular target. Till date, larger number of metal/s-based complexes was synthesized by overhauling the present structures chemically by substituting the ligand or preparing the whole novel component with improved cytotoxic and safety profiles. Howsoever, due to elevated accentuation upon the therapeutic importance of metal/s-based components, a couple of those agents are at present on clinical trials and several other are in anticipating regulatory endorsement to enter the trial. This literature highlights the detailed heterometallic multinuclear components, primarily focusing on platinum, ruthenium, gold and remarks on possible stability, synergism, mechanistic studies and structure activity relationships.

Boosting Gold(I) Catalysis via Weak Interactions: New Fine-Tunable Impy Ligands

A series of modular ImPy-carbene-Au(I) complexes are synthesized and fully characterized both in the solid state and in solution. The presence of oligoaryl units (phenyl and thienyl rings) at the C5-position of the ImPy core (in close proximity to the gold center) imprints on the organometallic species fine-tunable and predictable catalytic properties. A marked accelerating effect was recorded in several [Au(I)]-catalyzed electrophilic activations of unsaturated hydrocarbons when a CF₃-containing aromatic ring was accommodated at the ImPy core.

Gold (III) Derivatives in Colon Cancer Treatment

Cancer is one of the leading causes of morbidity and mortality worldwide. Colorectal cancer (CRC) is the third most frequently diagnosed cancer in men and the second in women. Standard patterns of antitumor therapy, including cisplatin, are ineffective due to their lack of specificity for tumor cells, development of drug resistance, and severe side effects. For this reason, new methods and strategies for CRC treatment are urgently needed. Current research includes novel platinum (Pt)- and other metal-based drugs such as gold (Au), silver (Ag), iridium (Ir), or ruthenium (Ru). Au(III) compounds are promising drug candidates for CRC treatment due to their structural similarity to Pt(II). Their advantage is their relatively good solubility in water, but their disadvantage is an unsatisfactory stability under physiological conditions. Due to these limitations, work is still underway to improve the formula of Au(III) complexes by combining with various types of ligands capable of stabilizing the Au(III) cation and preventing its reduction under physiological conditions. This review summarizes the achievements in the field of stable Au(III) complexes with potential cytotoxic activity restricted to cancer cells. Moreover, it has been shown that not nucleic acids but various protein structures such as thioredoxin reductase (TrxR) mediate the antitumor effects of Au derivatives. The state of the art of the in vivo studies so far conducted is also described.

Coinage Metal N-Heterocyclic Carbene Complexes: Recent Synthetic Strategies and Medicinal Applications

New weapons are constantly needed in the fight against cancer. The discovery of cisplatin as an anticancer drug prompted the search for new metal complexes. The successful history of cisplatin motivated chemists to develop a plethora of metal-based molecules. Among them, metal-N-heterocyclic carbene (NHC) complexes have gained significant attention because of their suitable qualities for efficient drug design. The enhanced applications of coinage metal-NHC complexes have encouraged a gradually increasing number of studies in the fields of medicinal chemistry that benefit from the fascinating chemical properties of these complexes. This review aims to present recent developments in synthetic strategies and medicinal applications of copper, silver and gold complexes supported by NHC ligands.

Synthesis, structural characterization, docking simulation and in vitro antiproliferative activity of the new gold(III) complex with 2-pyridineethanol

Gold(III) complex containing 2-pyridineethanol has been synthesized and characterized structurally by single crystal X-ray diffraction, vibrational spectroscopy, ¹H NMR spectroscopy, electrochemical study, and DFT calculations. The Au(III) ion is four coordinated with one N-donor ligand (L) and three Cl anions. The Okuniewski's (τ'₄₌0.018) has been used to estimate the angular distortion from ideal square planar geometry. The vibrational spectroscopy studies, in the solid state and DMSO solution and cyclic voltammetry, have been performed to determine its stability and redox activity, respectively. A complete assignment of the IR and Raman spectra has been made based on the calculated potential energy distribution (PED). The theoretical calculations have been made for two functionals and several basis sets. The compound has been evaluated for its antiproliferative properties in a human lung adenocarcinoma cell line (A549), mouse colon carcinoma (CT26), human breast adenocarcinoma (MCF-7), human prostate carcinoma derived from the metastatic site in the brain (DU-145), and PANC-1 human pancreas/duct carcinoma cell line and non-tumorigenic cell lines: HaCat (human keratinocyte), and HEK293T (human embryonic kidney). Au(III) complex cytotoxicity is significantly against A549 and MCF-7 cells as in the reference drug: cisplatin. Studies of the interactions of Au(III) complex with DNA, HSA (human serum albumin) have been performed. The results from modeling docking simulations indicate that the title complex exerts anticancer effects in vitro based on different mechanisms of action to compare with cisplatin.

Bioconjugation of Cyclometalated Gold(III) Lipoic Acid Fragments to Linear and Cyclic Breast Cancer Targeting Peptides

Cell-targeting peptides (CTPs) are increasingly used in the field of cancer research due to their high affinity and specificity to cell or tissue targets. In the search for novel metal-based drug candidates, our research group is particularly focused on bioconjugates by utilizing peptides to increase the selectivity of cytotoxic organometallic compounds. Motivated by the relatively high cytotoxic activity of gold complexes, such as Auranofin (approved to treat rheumatoid arthritis), for the treatment of various diseases, we anticipated that gold peptide bioconjugates would present interesting candidates for novel breast cancer therapies. For this, we investigate the use of the natural compound lipoic acid (Lpa) as a bioconjugation handle to link Au complexes in the oxidation state +III to peptides using the dithiol moiety. Using this strategy, we have synthesized Au(III) complex bioconjugates linked to the linear LTVSPWY peptide and two cyclic DfKRG and KTTHWGFTLG tumor-targeting peptides. Solid-phase peptide synthesis (SPPS) was used to prepare the peptides, with lipoic acid introduced N-terminally as a conjugation handle. After peptide cleavage, the metal complex was introduced in solution by first reducing the internal disulfide bond, followed by reaction with Au(ppy)Cl₂ (1, ppy: 2-phenyl-pyridine), to yield the Au(III)-Lpa-peptide bioconjugates. The new bioconjugates were successfully synthesized, purified by semi-preparative HPLC, and characterized by ESI-MS. Au(III)-peptide bioconjugates were tested as cytotoxic agents against two different human breast cancer cell lines (MCF-7 and MDA-MB-231) and normal human fibroblasts cells (GM5657T) and compared to cisplatin, the parent Au(III) dichloride complex, and metal-free peptides. These in vitro data show that the Au(III)-peptide bioconjugate 5, possessing the cyclic integrin-targeting RGD-derived peptide sequence in the structure, exhibits improved activity compared to the parent gold(III) compound Au(ppy)Cl₂ (1) as well as to cisplatin or the metal-free peptide. Moreover, the excellent targeting properties of 5 are supported by the fact that a Au(III)-peptide conjugate with the exact same peptide sequence, but a linear rather than the cyclic form of 5 exhibits 10 times lower cytotoxic activity.

Synthesis of Chiral Bifunctional NHC Ligands and Survey of Their Utilities in Asymmetric Gold Catalysis

The synthesis and characterization of the chiral bifunctional NHC ligands based on the imidazo[1,5-a]pyridine (ImPy) scaffold are described. These ligands possess a fluxional biaryl axis and a chiral center. The configurational stability of the biaryl axis in their gold(I) complexes is investigated. The application of these axially chiral ImPy-based AuCl complexes in a series of gold catalysis is explored, and varying degrees of asymmetric induction are observed. In most cases, the ligand (aS,R)-L8-H with its cyclohexyl group pointing to the reaction site and hence exerting asymmetric steric influence is more effective in asymmetric induction.

Silver carbene complexes: An emerging class of anticancer agents

Cancer is a major global health problem with large therapeutic challenges. Although substantial progress has been made in cancer therapy, there still remains a need to develop novel and effective treatment strategies to treat several relapsed and refractory cancers. Recently, there has been growing demand for considering organometallics as antineoplastic agents. This review is focused on a group of organometallics, silver N-heterocyclic carbene complexes (SCCs) and their anticancer efficacy in targeting multiple pathways in various in vitro cancer model systems. However, the precise molecular mechanism of SCCs anticancer properties remains unclear. Here, we discuss the SCCs chemistry, potential molecular targets, possible molecular mechanism of action, and their application in cancer therapies.

Antiproliferative effects of two gold(I)-N-heterocyclic carbene complexes in A2780 human ovarian cancer cells: a comparative proteomic study

Au(NHC) and Au(NHC)₂, i.e. a monocarbene gold(I) complex and the corresponding bis(carbene) complex, are two structurally related compounds, endowed with cytotoxic properties against several cancer cell lines. Herein, we explore the molecular and cellular mechanisms at the basis of their cytotoxicity in A2780 human ovarian cancer cells. Through a comparative proteomic analysis, we demonstrated that the number of modulated proteins is far larger in Au(NHC)₂-treated than in Au(NHC)-treated A2780 cells. Both gold compounds mainly affected proteins belonging to the following functional classes: protein synthesis, metabolism, cytoskeleton and stress response and chaperones. Particularly, Au(NHC)₂ gave rise to an evident upregulation of several glycolytic enzymes. Moreover, only Au(NHC)₂ triggered a net impairment of respiration and a metabolic shift towards glycolysis, suggesting that mitochondria are relevant cellular targets. We also found that both carbenes, similarly to the gold(I) compound auranofin, caused a strong inhibition of the seleno-enzyme thioredoxin reductase (TrxR). In conclusion, we highlighted that coordination of two carbene ligands to the same gold(I) center greatly enhances the antiproliferative effects of the resulting compound in comparison to the monocarbene derivative. Moreover, TrxR inhibition and metabolic impairment seem to play a major role in the Au(NHC)₂ cytotoxicity. Overall, these antiproliferative effects were also confirmed on other two human ovarian cancer cell lines (i.e. SKOV3 and IGROV1).

Gold-Based Medicine: A Paradigm Shift in Anti-Cancer Therapy?

A new era of metal-based drugs started in the 1960s, heralded by the discovery of potent platinum-based complexes, commencing with cisplatin [(H₃N)₂PtCl₂], which are effective anti-cancer chemotherapeutic drugs. While clinical applications of gold-based drugs largely relate to the treatment of rheumatoid arthritis, attention has turned to the investigation of the efficacy of gold(I) and gold(III) compounds for anti-cancer applications. This review article provides an account of the latest research conducted during the last decade or so on the development of gold compounds and their potential activities against several cancers as well as a summary of possible mechanisms of action/biological targets. The promising activities and increasing knowledge of gold-based drug metabolism ensures that continued efforts will be made to develop gold-based anti-cancer agents.

Cationic and Neutral N-Heterocyclic Carbene Gold(I) Complexes: Cytotoxicity, NCI-60 Screening, Cellular Uptake, Inhibition of Mammalian Thioredoxin Reductase, and Reactive Oxygen Species Formation

A structurally diverse library of 14 gold(I) cationic bis(NHC) and neutral mono(NHC) complexes (NHC: N-heterocyclic carbene) was synthesized and characterized in this work. Four of them were new cationic gold(I) complexes containing functionalized NHCs, and their X-ray crystal structures are presented herein. All of the complexes were investigated for their anticancer activities in four cancer cell lines, including a cisplatin-resistant variant, and a noncancerous cell line. Seven of the cationic gold(I) complexes were found to display high and specific cytotoxic activities toward cancer cells. Two of them were even able to overcome cisplatin resistance. Two highly potent cationic complexes (11 and 15) were also submitted to the NCI-60 cancer panel for further cytotoxicity evaluation. Complex 15 showed a surprisingly high potency toward leukemia among the nine examined cancer subtypes, particularly toward the CCRF-CEM leukemia cell line with a concentration for 50 % inhibition of growth down to 79.4 nm. In addition, cationic complex 13, which demonstrated a remarkable cytotoxicity against hepatocellular carcinoma, was selected to obtain insight into the mechanistic aspects in HepG2 cells. Cellular uptake measurements were indicative of good bioavailability. By various biochemical assays, this complex was found to effectively inhibit thioredoxin reductase and its cytotoxicity toward HepG2 cells was found to be reactive oxygen species dependent.

Cobaltocenylidene: A Mesoionic Metalloceno Carbene, Stabilized in a Gold(III) Complex

Oxidative addition of cobaltoceniumdiazonium bis(hexafluoridophosphate) with (pseudo)halide aurates gave gold(III) complexes containing zwitterionic cobaltoceniumide as a ligand. Its selenium derivative, cobaltoceniumselenolate, was obtained by an electrophilic aromatic substitution reaction of iodocobaltocenium iodide with Na2 Se. Spectroscopic and structural data in combination with DFT calculations showed that this cobaltocenylidene species is a mesoionic carbene quite different from common N-heterocyclic carbenes. Its ligand properties (TEP, singlet-triplet gap, nucleophilicity, π-acidity, Brønsted basicity) are in part comparable to those of cyclic (amino)(alkyl/aryl)carbenes. Electrochemistry data showed that the mesoionic cobaltoceniumides are more electron-rich than their parent ferrocenes. The reversible reduction of the tricyanido gold complex appears 50 mV negative of the cobaltocenium/cobaltocene couple, whereas that of the selenide derivative is shifted cathodically by 550 mV.

New insights in Au-NHCs complexes as anticancer agents

Within the research field of antitumor metal-based agents alternative to platinum drugs, gold(I/III) coordination complexes have always been in the forefront due mainly to the familiarity of medicinal chemists with gold compounds, whose application in medicine goes back in the ancient times, and to the rich chemistry shown by this metal. In the last decade, N-heterocyclic carbene ligands (NHC), a class of ligands that largely resembles the chemical properties of phosphines, became of interest for gold(I) medicinal applications, and since then, the research on NHC-gold(I/III) coordination complexes as potential antiproliferative agents boosted dramatically. Different classes of gold(I/III)-NHC complexes often showed an outstanding in vitro antiproliferative activity, however up to now very few in vivo data have been reported to corroborate the in vitro results. This review summarizes all achievements in the field of gold (I/III) complexes comprising NHC ligands proposed as potential antiproliferative agents in the period 2004-2016, and critically analyses biological data (mainly IC₅₀ values) in relation to the chemical structures of Au compounds. The state of art of the in vivo studies so far described is also reported.

Influence of the substituent on the phosphine ligand in novel rhenium(i) aldehydes. Synthesis, computational studies and first insights into the antiproliferative activity

Cyrhetrenyl phosphine derivatives were synthesized and evaluated as potential anticancer agents. Electrochemical and computational studies were carried out.

Isoelectronic Pt(ii)– and Au(iii)–N-heterocyclic carbene complexes: a structural and biological comparison

To differentiate the intrinsic biological activities displayed by Pt vs. Au reagents, a series of square planar carbene complexes were compared.

Synthesis and cytotoxic characteristics displayed by a series of Ag(i)-, Au(i)- and Au(iii)-complexes supported by a common N-heterocyclic carbene

The design, synthesis and anticancer properties of a series of Ag(i), Au(i) and Au(iii)–NHC complexes supported by pyridyl[1,2-a]{2-acetylylphenylimidazol}-3-ylidene are described.

N-Heterocyclic Carbene-Gold(I) Complexes Conjugated to a Leukemia-Specific DNA Aptamer for Targeted Drug Delivery

This report describes the synthesis and characterization of novel N-heterocyclic carbene (NHC)-gold(I) complexes and their bioconjugation to the CCRF-CEM-leukemia-specific aptamer sgc8c. Successful bioconjugation was confirmed by the use of fluorescent tags on both the NHC-Au(I) complex and the aptamer. Cell-viability assays indicated that the NHC-Au(I) -aptamer conjugate was more cytotoxic than the NHC-gold complex alone. A combination of flow cytometry, confocal microscopy, and cell-viability assays provided clear evidence that the NHC-Au(I) -aptamer conjugate was selective for targeted CCRF-CEM leukemia cells.

Titanocene-Gold Complexes Containing N-Heterocyclic Carbene Ligands Inhibit Growth of Prostate, Renal, and Colon Cancers in Vitro

We report on the synthesis, characterization, and stability studies of new titanocene complexes containing a methyl group and a carboxylate ligand (mba = −OC(O)-p-C₆H₄-S−) bound to gold(I)–N-heterocyclic carbene fragments through the thiolate group: [(η⁵-C₅H₅)₂TiMe(μ-mba)Au(NHC)]. The cytotoxicities of the heterometallic compounds along with those of novel monometallic gold–N-heterocyclic carbene precursors [(NHC)Au(mbaH)] have been evaluated against renal, prostate, colon, and breast cancer cell lines. The highest activity and selectivity and a synergistic effect of the resulting heterometallic species was found for the prostate and colon cancer cell lines. The colocalization of both titanium and gold metals (1:1 ratio) in PC3 prostate cancer cells was demonstrated for the selected compound 5a, indicating the robustness of the heterometallic compound in vitro. We describe here preliminary mechanistic data involving studies on the interaction of selected mono- and bimetallic compounds with plasmid (pBR322) used as a model nucleic acid and the inhibition of thioredoxin reductase in PC3 prostate cancer cells. The heterometallic compounds, which are highly apoptotic, exhibit strong antimigratory effects on the prostate cancer cell line PC3.