Gold(I) N-heterocyclic carbene complexes with an “activable” ester moiety: Possible biological applications

Functional utility of gold complexes with phosphorus donor ligands in biological systems

Metallo-phosphines are ubiquitous in organometallic chemistry with widespread applications as catalysts in various chemical transformations, precursors for organic electronics, and chemotherapeutic agents or chemical probes. Here, we provide a comprehensive review of the exploration of the current biological applications of Au complexes bearing phosphine donor ligands. The goal is to deepen our understanding of the synthetic utility and reactivity of Au-phosphine complexes to provide insights that could lead to the design of new molecules and enhance the cross-application or repurposing of these complexes.

Post-metallation functionalization of the [(C^C)Au(P^P)]+ scaffold through a hydrothiolation reaction

Complex 1 [(C^C)Au(dppv)]PF6 (dppv = cis-1,2-diphenylphosphinoethylene) is reported to react efficiently and selectively with aliphatic and aromatic thiols in the presence of a base. This methodology enables the smooth introduction of various functional groups on the [(C^C)Au(P^P)]+ scaffold.

Rh(III) and Ru(II) complexes with phosphanyl-alkylamines: inhibition of DNA synthesis induced by anticancer Rh complex

Aim: This investigation was designed to synthesize half-sandwich Rh(III) and Ru(II) complexes and study their antiproliferative activity in human cancer cell lines. Materials & methods: Nine compounds were prepared and tested by various assays for their anticancer activity and mechanism of action. Results: Hit Rh(III) complex 6 showed low-micromolar potency in cisplatin-sensitive (A2780) and -resistant (A2780cis) ovarian carcinoma cell lines, promising selectivity toward these cancer cells over normal lung fibroblasts and an unprecedented mechanism of action in the treated cells. DNA synthesis was decreased and CDKN1A expression was upregulated, but p21 expression was not induced. Conclusion: Rh complex 6 showed high antiproliferative activity, which is induced through a p21-independent mechanism of action.

Recent development of gold(I) and gold(III) complexes as therapeutic agents for cancer diseases

Metal complexes have demonstrated significant antitumor activities and platinum complexes are well established in the clinical application of cancer chemotherapy. However, the platinum-based treatment of different types of cancers is massively hampered by severe side effects and resistance development. Consequently, the development of novel metal-based drugs with different mechanism of action and pharmaceutical profile attracts modern medicinal chemists to design and synthesize novel metal-based agents. Among non-platinum anticancer drugs, gold complexes have gained considerable attention due to their significant antiproliferative potency and efficacy. In most situations, the gold complexes exhibit anticancer activities by targeting thioredoxin reductase (TrxR) or other thiol-rich proteins and enzymes and trigger cell death via reactive oxygen species (ROS). Interestingly, gold complexes were recently reported to elicit biochemical hallmarks of immunogenic cell death (ICD) as an ICD inducer. In this review, the recent progress of gold(I) and gold(III) complexes is comprehensively summarized, and their activities and mechanism of action are documented.

Conception and evaluation of fluorescent phosphine-gold complexes: from synthesis to in vivo investigations

A phosphine gold(I) and phosphine-phosphonium gold(I) complexes bearing a fluorescent coumarin moiety were synthesized and characterized. Both complexes displayed interesting photophysical properties: good molar absorption coefficient, good quantum yield of fluorescence, and ability to be tracked in vitro thanks to two-photon imaging. Their in vitro and in vivo biological properties were evaluated onto cancer cell lines both human and murine and into CT26 tumor-bearing BALB/c mice. They displayed moderate to strong antiproliferative properties and the phosphine-phosphonium gold(I) complex induced significant in vivo anti-cancer effect.

Investigations of the reactivity, stability and biological activity of halido (NHC)gold(I) complexes

The significance of the halido ligand (Cl^-, Br^-, I^-) in halido[3-ethyl-4-phenyl-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(I) complexes (2-4) in terms of ligand exchange reactions, including the ligand scrambling to the bis[3-ethyl-4-phenyl-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(I) complex (5), was evaluated by HPLC in acetonitrile/water = 50:50 (v/v) mixtures. In the presence of 0.9% NaCl, the bromido (NHC)gold(I) complex 3 was immediately transformed into the chlorido (NHC)gold(I) complex 2. The iodido (NHC)gold(I) complex 4 converted under the same conditions during 0.5 h of incubation by 52.83% to 2 and by 8.77% to 5. This proportion remained nearly constant for 72 h. The halido (NHC)gold(I) complexes also reacted very rapidly with 1 eq. of model nucleophiles, e.g., iodide or selenocysteine (Sec). For instance, Sec transformed 3 in the proportion 73.03% to the (NHC)Au(I)Sec complex during 5 min of incubation. This high reactivity against this amino acid, present in the active site of the thioredoxin reductase (TrxR), correlates with the complete inhibition of the isolated TrxR enzyme at 1 μM. Interestingly, in cellular systems (A2780cis cells), even at a 5-fold higher concentration, no increased ROS levels were detected. The concentration required for ROS generation was about 20 μM. Superficially considered, the antiproliferative and antimetabolic activities of the halido (NHC)Au(I) complexes correlate with the reactivity of the Au(I)-X bond (2 < 3 < 4). However, it is very likely that degradation products formed during the incubation in cell culture medium participated in the biological activity. In particular, the high-cytotoxic [(NHC)₂Au(I)]⁺ complex (5) distorts the results.

Organometallic dendrimers based on Ruthenium(II) N-heterocyclic carbenes and their implication as delivery systems of anticancer small interfering RNA

With the purpose of obtaining a new dendritic system against cancer, this paper is focused on the synthesis of spherical carbosilane metallodendrimers of different generations holding Ru(II) N-heterocyclic carbene (NHC) on the periphery from the imidazolium precursors. Both imidazolium salt dendrimers and their metallodendrimers counterparts showed promising anticancer activity, similar to cisplatin, mainly at high generations. In addition, both families of second and third generations were able to form dendriplexes with anticancer small interfering RNA (siRNA), protecting the cargo against RNAse and being able to internalize it in HEPG2 (human liver cancer) tumour cells. The characterization and effectiveness of the dendriplexes were evaluated by various analytical techniques such as zeta potential, electrophoresis and circular dichroism, the stability of the system and the protective nature of the dendrimer estimated using RNAse and the internalization of dendriplexes by confocal microscopy. The major advantage observed with the ruthenium metallodendrimers with respect to the imidazolium salts precursors was in cellular uptake, where the internalization of Mcl-1-FITC siRNA (myeloid cell leukaemia-1 fluorescein labelled siRNA) proceeded more efficiently. Therefore, we propose here that both imidazolium and Ru metallodendrimers are interesting candidates in cancer due to their double action, as anticancer per se and as carrier for anticancer siRNA, providing in this way a combined action.

Dinuclear orthometallated gold(I)-gold(III) anticancer complexes with potent in vivo activity through an ROS-dependent mechanism

Increasingly explored over the last decade, gold complexes have shown great promise in the field of cancer therapeutics. A major obstacle to their clinical progression has been their lack of in vivo stability, particularly for gold(III) complexes, which often undergo a facile reduction in the presence of biomolecules such as glutathione. Herein, we report a new class of promising anticancer gold(I)-gold(III) complexes with the general formula [XAuI(μ-2-C6F4PPh2)(κ2-2-C6F4PPh2)AuIIIX] [X = Cl (1), Br (2), NO3 (3)] which feature two gold atoms in different oxidation states (I and III) in a single molecule. Interestingly, gold(I)-gold(III) complexes (1-3) are stable against glutathione reduction under physiological-like conditions. In addition, complexes 1-3 exhibit significant cytotoxicity (276-fold greater than cisplatin) toward the tested cancer cells compared to the noncancerous cells. Moreover, the gold(I)-gold(III) complexes do not interact with DNA-like cisplatin but target cellular thioredoxin reductase, an enzyme linked to the development of cisplatin drug resistance. Complexes 1-3 also showed potential to inhibit cancer and endothelial cell migration, as well as tube formation during angiogenesis. In vivo studies in a murine HeLa xenograft model further showed the gold compounds may inhibit tumor growth on par clinically used cisplatin, supporting the significant potential this new compound class has for further development as cancer therapeutic.

Fluorinated-NHC Transition Metal Complexes: Leading Characters as Potential Anticancer Metallodrugs

In the last 20 years, N-Heterocyclic Carbene (NHC) ligands have been ubiquitous in biological and medicinal chemistry. Part of their success lies in the tremendous number of topologies that can be synthesized and thus finely tuned that have been described so far. This is particularly true in the case of those derivatives, including fluorine or fluorinated fragments on their NHC moieties, gaining much attention due to their enhanced biological properties and turning them into excellent candidates for the development of novel metallodrugs. Thus, this review summarizes the development that fluorinated-NHC transition metal complexes have had and their impact on cancer treatment.

Synthesis, characterization and biological activity of bis[3-ethyl-4-aryl-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(i) complexes

A series of bis[3-ethyl-4-aryl-5-(2-methoxypyridin-5-yl)-1-propyl-1,3-dihydro-2H-imidazol-2-ylidene]gold(i) complexes (2a-f) containing methyl, fluoro or methoxy substituents at various positions in the 4-aryl ring was synthesized and evaluated for their anti-cancer properties in A2780 (wild-type and Cisplatin-resistant) ovarian carcinoma as well as LAMA 84 (imatinib-sensitive and -resistant) and HL-60 leukemia cell lines. The bis-NHC gold(i) complexes were more active compared to their related mono-NHC gold(i) analogues and reduced proliferation and metabolic activity in a low micromolar range. With the exception of 2d (3-F), the compounds displayed higher potency than the established drugs Auranofin and Cisplatin. The lack of effects against non-cancerous lung fibroblast SV-80 cells indicated a high selectivity towards tumor cells. All tested complexes generated reactive oxygen species in A2780cis cells; however, the induction of apoptosis was very low. Furthermore, thioredoxin reductase is not the main target of these complexes, because its inhibition pattern did not correlate with their biological activity.

Silver (I) N-Heterocyclic Carbenes Carbosilane Dendritic Systems and Their Imidazolium-Terminated Analogues as Antibacterial Agents: Study of Their Mode of Action

Spherical dendrimers and dendrons containing silver(I) N-heterocyclic carbenes (Ag(I)-NHC) and additionally bow-tie metal-free dendritic systems were synthesized in a simple and straightforward synthetic procedure and subsequently characterized. The antibacterial activity was evaluated, and in parallel, a comparative study with the cationic analogue precursors was performed to explore the effect of silver ions in the dendritic structure. Other parameters, such as topology, generation, and hydrophobicity, of the imidazole substituents were also studied. All these dendritic systems presented antibacterial activity against three different bacterial strains, two Gram-positive (Staphylococcus aureus and Bacillus subtilis) and one Gram-negative (Escherichia coli). Several assays were conducted to elucidate their mechanism of action against Bacillus subtilis, by using bacterial biosensors or specific probes and fluorescent proteins sensitive to changes in the cell membrane potential. These studies are specially focused on the role of the polyvalence of our systems containing silver atoms, which may provoke interesting effects in the mode of action.

Gold(I)-Coumarin-Caffeine-Based Complexes as New Potential Anti-Inflammatory and Anticancer Trackable Agents

Three new gold(I)-coumarin-based trackable therapeutic complexes and two non-trackable analogues have been synthesised and fully characterised. They all display anti-proliferative properties on several types of cancer cell lines, including those of colon, breast, and prostate. Two complexes displayed significant anti-inflammatory effects; one displayed pro-inflammatory behaviour; this highlights the impact of the position of the fluorophore on the caffeine scaffold. Additionally, the three coumarin derivatives could be visualised in vitro by two-photon microscopy.

Synthesis of hydroxyl-functionalized N-heterocyclic carbene gold(i) complexes and peptide conjugates

The synthesis and characterization of a series of new cationic gold(i) complexes with hydroxyl-functionalized N-heterocyclic carbene (NHC) ligands is described. They are valuable building blocks for further derivatization: as a first example, coupling with amino acids and a dipeptide, respectively, successfully results in amino acid and peptide conjugates that are hard to obtain by other synthetic routes.

A Gold(III) Pincer Ligand Scaffold for the Synthesis of Binuclear and Bioconjugated Complexes: Synthesis and Anticancer Potential

Cyclometalated (C^N^C)Au^III complexes bearing functionalized N-heterocyclic carbene (NHC) ligands provide a high-yielding, modular route to bioconjugated and binuclear complexes. This methodology has been applied to the synthesis of bioconjugated complexes presenting biotin and 17α-ethynylestradiol vectors, as well as to the synthesis of bimetallic Au^III /Au^I complexes. The in vitro antiproliferative activities of these compounds against various cancer cells lines depend on the linker length, with the longer linker being the most potent. The estradiol conjugate AuC₆ Estra proved to be more toxic against the estrogen receptor positive (ER+) cancer cells than against the ER- cancer cells and non-cancer cells. The bimetallic complex AuC₆ Au was more selective for breast cancer cells with respect to a healthy cell standard than the monometallic complex AuNHC. The metal uptake study on cells expressing or not biotin and estrogen receptors revealed an improved and targeted delivery of gold for both the bioconjugated complexes AuC₆ Biot and AuC₆ Estra compared to the non-vectorised analogue AuNHC. The investigations of the interaction of the bioconjugates and bimetallic complexes with human telomeric G-quadruplex DNA using FRET-melting techniques revealed a reduced ability to stabilize this DNA structure with respect to the non-vectorised analogue AuNHC.

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.

Antiproliferative Activity of Gold(III) Complexes with Esters of Cyclohexyl-Functionalized Ethylenediamine-N,N’-Diacetate

Six gold(III) complexes with esters of cyclohexyl-functionalized ethylenediamine-N,N’-diacetate, general formula [AuCl2{(S,S)-R2eddch}]PF6, [(S,S)-eddch = (S,S)-ethylenediamine-N,N’-di-2-(3-cyclohexyl)propanoate, R = Me, Et, n-Pr, n-Bu, i-Bu, i-Am, 1–6, respectively], were tested against cancer cell lines such as human melanoma Fem-x, human colon carcinoma LS174T and non-small cell lung carcinoma A549 as well as a non-cancerous human embryonic lung fibroblasts MRC-5 using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay with the aim of assessing in vitro antitumoral activity and selectivity. All investigated complexes showed lower cytotoxicity and better or similar selectivity in comparison to cisplatin, used as reference compound. Complex [AuCl2{(S,S)-(i-Am)2eddch}]PF6 (6) demonstrated the highest activity against Fem-x (IC50 = 14.98 ± 0.34 μM). Additionally, the same complex expressed 4.5 times higher selectivity than cisplatin.

Platinum, palladium, gold and ruthenium complexes as anticancer agents: Current clinical uses, cytotoxicity studies and future perspectives

Metallodrugs offer potential for unique mechanism of drug action based on the choice of the metal, its oxidation state, the types and number of coordinated ligands and the coordination geometry. This review illustrates notable recent progress in the field of medicinal bioinorganic chemistry as many new approaches to the design of innovative metal-based anticancer drugs are emerging. Current research addressing the problems associated with platinum drugs has focused on other metal-based therapeutics that have different modes of action and on prodrug and targeting strategies in an effort to diminish the side-effects of cisplatin chemotherapy. Examples of metal compounds and chelating agents currently in clinical use, clinical trials or preclinical development are highlighted.

(C^Npz^C)AuIII complexes of acyclic carbene ligands: synthesis and anticancer properties

Synthesis of amino ester conjugated (C^N^pz^C)Au acyclic carbene complexes with low micromolar cytotoxicity on human cancer cells.

N-heterocyclic carbene metal complexes as bio-organometallic antimicrobial and anticancer drugs

Late transition metal complexes that bear N-heterocyclic carbene (NHC) ligands have seen a speedy growth in their use as both, metal-based drug candidates and potentially active homogeneous catalysts in a plethora of C-C and C-N bond forming reactions. This review article focuses on the recent developments and advances in preparation and characterization of NHC-metal complexes (metal: silver, gold, copper, palladium, nickel and ruthenium) and their biomedical applications. Their design, syntheses and characterization have been reviewed and correlated to their antimicrobial and anticancer efficacies. All these initial discoveries help validate the great potential of NHC-metal derivatives as a class of effective antimicrobial and anticancer agents.

In vitro and in vivo evaluation of organometallic gold(I) derivatives as anticancer agents

Alkyne gold(I) derivatives with the water soluble phosphanes PTA (1,3,5-triaza-7-phosphaadamantane) and DAPTA (3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane) were described and their anticancer potential against the colon cancer cell line Caco-2 (PD7 and TC7 clones) was studied. Strong antiproliferative effects are found, for all the new complexes, to be even more pronounced than for the reference drug cisplatin, and similar to auranofin. The interaction of these derivatives with bovine serum albumin (BSA) was studied by fluorescence spectroscopy. The types of quenching and binding constants were determined by a fluorescence quenching method. Moderate values of the binding constants are calculated for the tested derivatives indicating that these complexes can be stored and carried easily by this protein in the body. The study of the thermodynamic parameters in the case of [Au(C[triple bond, length as m-dash]CCH2Spyridine)(PTA)] points out to the presence of van der Waals interactions or hydrogen bonding between the metallic complex and the protein. In addition, the complex [Au(C[triple bond, length as m-dash]CCH2Spyridine)(PTA)] has shown inhibition in colon cancer proliferation of HTC-116-luc2 cell lines via the apoptotic pathway and S-phase arrest of the cell cycle. Intraperitoneal injection of this derivative in athymic nude mice inoculated with HTC-116-luc2 cells prolonged their survival and displayed moderate inhibition of the tumour growth with no subsequent organ (kidney and liver) damage after treatment.

Synthesis, characterization and theoretical calculations of (1,2-diaminocyclohexane)(1,3-diaminopropane)gold(III) chloride complexes: in vitro cytotoxic evaluations against human cancer cell lines

The gold(III) complexes of the type (1,2-diaminocyclohexane)(1,3-diaminopropane)gold(III) chloride, [(DACH)Au(pn)]Cl3, [where DACH = cis-, trans-1,2- and S,S-1,2-diaminocyclohexane and pn = 1,3-diaminopropane] have been synthesized and characterized using various spectroscopic and analytical techniques including elemental analysis, UV-Vis and FTIR spectroscopy; solution as well as solid-state NMR measurements. The solid-state (13)C NMR shows that 1,2-diaminocyclohexane (1,2-DACH) and 1,3-diaminopropane (pn) are strongly bound to the gold(III) center via N donor atoms. The stability of the mixed diamine ligand gold(III) was checked by UV-Vis spectroscopy and NMR measurements. The molecular structure of compound 1 (containing cis-1,2-DACH) was determined by X-ray diffraction analysis. The structure of 1 consists of [(cis-DACH)Au(pn)](3+) complex ion and chloride counter ions. Each gold atom in the complex ion adopts a distorted square-planar geometry. The structural details and relative stabilities of the four possible isomers of the complexes were also estimated at the B3LYP/LANL2DZ level of theoretical calculations. The computational study demonstrates that trans- conformations are slightly more stable than the cis- conformations. The antiproliferative effects and cytotoxic properties of the mixed ligand gold(III) complexes were evaluated in vitro on human gastric SGC7901 and prostate PC3 cancer cells using MTT assay. The antiproliferative study of the gold(III) complexes on PC3 and SGC7901 cells indicate that complex 3 (containing 1S,2S-(+)-1,2-(DACH)) is the most effective antiproliferative agent. The IC50 data reveal that the in vitro cytotoxicity of complex 3 against SGC7901 cancer cells manifested similar and very pronounced cytotoxic effects with respect to cisplatin. Moreover, the electrochemical behavior, and the interaction of complex 3 with two well-known model proteins, namely, hen egg white lysozyme and bovine serum albumin is also reported.