On the Different Mode of Action of Au(I)/Ag(I)-NHC Bis-Anthracenyl Complexes Towards Selected Target Biomolecules

A CD Study of a Structure-Based Selection of N-Heterocyclic Bis-Carbene Gold(I) Complexes as Potential Ligands of the G-Quadruplex-Forming Human Telomeric hTel23 Sequence

Herein, we report the structure-based selection via molecular docking of four N-heterocyclic bis-carbene gold(I) complexes, whose potential as ligands for the hTel23 G-quadruplex structure has been investigated using circular dichroism (CD) spectroscopy, CD melting, and polyacrylamide gel electrophoresis (PAGE). The complex containing a bis(1,2,3,4,6,7,8,9-octahydro-11H-11λ3-pyridazino[1,2-a]indazol-11-yl) scaffold induces a transition from the hybrid (3 + 1) topology to a prevalent parallel G-quadruplex conformation, whereas the complex featuring a bis(2-(2-acetamidoethyl)-3λ3-imidazo[1,5-a]pyridin-3(2H)-yl) moiety disrupted the original G-quadruplex structure. These results deserve particular attention in light of the recent findings on the pathological involvements of G-quadruplexes in neurodegenerative diseases.

Synthesis, Chemical Characterization, and Biological Evaluation of Hydrophilic Gold(I) and Silver(I) N-Heterocyclic Carbenes as Potential Anticancer Agents

A series of new gold(I) and silver(I) N-heterocyclic carbenes bearing a 1-thio-β-d-glucose tetraacetate moiety was synthesized and chemically characterized. The compounds' stability and solubility in physiological conditions were investigated employing a multitechnique approach. Interaction studies with biologically relevant proteins, such as superoxide dismutase (SOD) and human serum albumin (HSA), were conducted via UV-vis absorption spectroscopy and high-resolution ESI mass spectrometry. The biological activity of the compounds was evaluated in the A2780 and A2780R (cisplatin-resistant) ovarian cancer cell lines and the HSkMC (human skeletal muscle) healthy cell line. Inhibition studies of the selenoenzyme thioredoxin reductase (TrxR) were also carried out. The results highlighted that the gold complexes are more stable in aqueous environment and capable of interaction with SOD and HSA. Moreover, these carbenes strongly inhibited the TrxR activity. In contrast, the silver ones underwent structural alterations in the aqueous medium and showed greater antiproliferative activity.

Heteronuclear Complexes with Promising Anticancer Activity against Colon Cancer

This study investigates the activity of novel gold(I) and copper(I)/zinc(II) heteronuclear complexes against colon cancer. The synthesised heteronuclear Au(I)-Cu(I) and Au(I)-Zn(II) complexes were characterised and evaluated for their anticancer activity using human colon cancer cell lines (Caco-2). The complexes exhibited potent cytotoxicity, with IC50 values in the low micromolar range, and effectively induced apoptosis in cancer cells. In the case of complex [Cu{Au(Spy)(PTA)}2]PF6 (2), its cytotoxicity is ×10 higher than its mononuclear precursor, while showing low cytotoxicity towards differentiated healthy cells. Mechanistic studies revealed that complex 2 inhibits the activity of thioredoxin reductase, a key enzyme involved in redox regulation, leading to an increase in reactive oxygen species (ROS) levels and oxidative stress, in addition to an alteration in DNA's tertiary structure. Furthermore, the complexes demonstrated a strong binding affinity to bovine serum albumin (BSA), suggesting the potential for effective drug delivery and bioavailability. Collectively, these findings highlight the potential of the investigated heteronuclear Au(I)-Cu(I) and Au(I)-Zn(II) complexes as promising anticancer agents, particularly against colon cancer, through their ability to disrupt redox homeostasis and induce oxidative stress-mediated cell death.

Exploring the interaction between a fluorescent Ag(I)-biscarbene complex and non-canonical DNA structures: a multi-technique investigation

Silver compounds are mainly studied as antimicrobial agents, but they also have anticancer properties, with the latter, in some cases, being better than their gold counterparts. Herein, we analyse the first example of a new Ag(I)-biscarbene that can bind non-canonical structures of DNA, more precisely G-quadruplexes (G4), with different binding signatures depending on the type of G4. Moreover, we show that this Ag-based carbene binds the i-motif DNA structure. Alternatively, its Au(I) counterpart, which was investigated for comparison, stabilises mitochondrial G4. Theoretical in silico studies elucidated the details of different binding modes depending on the geometry of G4. The two complexes showed increased cytotoxic activity compared to cisplatin, overcoming its resistance in ovarian cancer. The binding of these new drug candidates with other relevant biosubstrates was studied to afford a more complete picture of their possible targets. In particular, the Ag(I) complex preferentially binds DNA structures over RNA structures, with higher binding constants for the non-canonical nucleic acids with respect to natural calf thymus DNA. Regarding possible protein targets, its interaction with the albumin model protein BSA was also tested.

Gold(III) complexes with thiosemicarbazone ligands: insights into their cytotoxic effects on lung cancer cells

Cancer continues to pose a global threat, underscoring the urgent need for more effective and safer treatment options. Gold-based compounds have recently emerged as promising candidates due to their diverse range of biological activities. In this study, three gold(III) complexes derived from thiosemicarbazone ligands have been synthesized, fully characterized, including their X-ray crystal structures. We conducted initial mode-of-action studies on DNA and BSA, followed by a comprehensive investigation into the cytotoxic effects of these novel gold(III) complexes on lung cancer cells (A549, H2052, and H28). The results demonstrated a concentration-dependent cytotoxic response, with H28 cells exhibiting the highest sensitivity to the treatment. Furthermore, the analysis of the cell cycle revealed that these compounds induce cell cycle arrest and promote apoptosis as a response to treatment. We also observed distinct morphological changes and increased oxidative stress, contributing significantly to cell death. Notably, these complexes exhibited the ability to suppress interleukin-6 production in mesothelioma cell lines, and this highlights their anti-inflammatory potential. To gain an initial understanding of cytotoxicity on healthy cells, hemolysis tests were conducted against human blood cells, with no evidence of hemolysis. Furthermore, a toxicity assessment through the in vivo Galleria mellonella model underscored the absence of detectable toxicity. These findings prove that these complexes are promising novel therapeutic agents for lung cancer.

Reaction Behavior of [1,3-Diethyl-4,5-diphenyl-1H-imidazol-2-ylidene] Containing Gold(I/III) Complexes against Ingredients of the Cell Culture Medium and the Meaning on the Potential Use for Cancer Eradication Therapy

The reactivities of halido[1,3-diethyl-4,5-diphenyl-1H-imidazol-2-ylidene]gold(I) (chlorido (5), bromido (6), iodido (7)), bis[1,3-diethyl-4,5-diphenyl-1H-imidazol-2-ylidene]gold(I) (8), and bis[1,3-diethyl-4,5-diphenyl-1H-imidazol-2-ylidene]dihalidogold(III) (chlorido (9), bromido (10), iodido (11)) complexes against ingredients of the cell culture medium were analyzed by HPLC. The degradation in the RPMI 1640 medium was studied, too. Complex 6 quantitatively reacted with chloride to 5, while 7 showed additionally ligand scrambling to 8. Interactions with non-thiol containing amino acids could not be detected. However, glutathione (GSH) reacted immediately with 5 and 6 yielding the (NHC)gold(I)-GSH complex 12. The most active complex 8 was stable under in vitro conditions and strongly participated on the biological effects of 7. The gold(III) species 9-11 were completely reduced by GSH to 8 and are prodrugs. All complexes were tested for inhibitory effects in Cisplatin-resistant cells, as well as against cancer stem cell-enriched cell lines and showed excellent activity. Such compounds are of utmost interest for the therapy of drug-resistant tumors.

Highlights of New Strategies to Increase the Efficacy of Transition Metal Complexes for Cancer Treatments

Although important progress has been made, cancer still remains a complex disease to treat. Serious side effects, the insurgence of resistance and poor selectivity are some of the problems associated with the classical metal-based anti-cancer therapies currently in clinical use. New treatment approaches are still needed to increase cancer patient survival without cancer recurrence. Herein, we reviewed two promising-at least in our opinion-new strategies to increase the efficacy of transition metal-based complexes. First, we considered the possibility of assembling two biologically active fragments containing different metal centres into the same molecule, thus obtaining a heterobimetallic complex. A critical comparison with the monometallic counterparts was done. The reviewed literature has been divided into two groups: the case of platinum; the case of gold. Secondly, the conjugation of metal-based complexes to a targeting moiety was discussed. Particularly, we highlighted some interesting examples of compounds targeting cancer cell organelles according to a third-order targeting approach, and complexes targeting the whole cancer cell, according to a second-order targeting strategy.

N-Heterocyclic Carbene (NHC) Silver Complexes as Versatile Chemotherapeutic Agents Targeting Human Topoisomerases and Actin

In recent years, the number of people suffering from cancer has risen rapidly and the World Health Organization and U.S. and European governments have identified this pathology as a priority issue. It is known that most bioactive anticancer molecules do not target a single protein but exert pleiotropic effects, simultaneously affecting multiple pathways. In our study, we designed and synthesized a new series of silver N-heterocyclic carbene (NHC) complexes [(NHC)2 Ag]+ [AgX2 ]- (X=iodide or acetate). The new complexes were active against two human breast cancer cell lines, MCF-7 and MDA-MB-231. These compounds showed multiple target actions as anticancer, by inhibiting in vitro the activity of the human topoisomerases I and II and interfering with the cytoskeleton dynamic, as also confirmed by in silico studies. Moreover, the antimicrobial activity of these silver complexes was studied against Gram-positive/negative bacteria. These dual properties provide a two-tiered approach, making these compounds of interest to be further deepened for the development of new chemotherapeutic agents.

Discriminating between Parallel, Anti-Parallel and Hybrid G-Quadruplexes: Mechanistic Details on Their Binding to Small Molecules

G-quadruplexes (G4) are now extensively recognised as a peculiar non-canonical DNA geometry that plays a prime importance role in processes of biological relevance whose number is increasing continuously. The same is true for the less-studied RNA G4 counterpart. G4s are stable structures; however, their geometrical parameters may be finely tuned not only by the presence of particular sequences of nucleotides but also by the salt content of the medium or by a small molecule that may act as a peculiar topology inducer. As far as the interest in G4s increases and our knowledge of these species deepens, researchers do not only verify the G4s binding by small molecules and the subsequent G4 stabilisation. The most innovative studies now aim to elucidate the mechanistic details of the interaction and the ability of a target species (drug) to bind only to a peculiar G4 geometry. In this focused review, we survey the advances in the studies of the binding of small molecules of medical interest to G4s, with particular attention to the ability of these species to bind differently (intercalation, lateral binding or sitting atop) to different G4 topologies (parallel, anti-parallel or hybrid structures). Some species, given the very high affinity with some peculiar G4 topology, can first bind to a less favourable geometry and then induce its conversion. This aspect is also considered.

Anthracenyl Functionalization of Half-Sandwich Carbene Complexes: In Vitro Anticancer Activity and Reactions with Biomolecules

N-Heterocyclic carbene (NHC) ligands are widely investigated in medicinal inorganic chemistry. Here, we report the preparation and characterization of a series of half-sandwich [M(L)(NHC)Cl₂] (M = Ru, Os, Rh, Ir; L = cym/Cp*) complexes with a N-flanking anthracenyl moiety attached to imidazole- and benzimidazole-derived NHC ligands. The anticancer activity of the complexes was investigated in cell culture studies where, in comparison to a Rh derivative with an all-carbon-donor-atom-based ligand (5a), they were found to be cytotoxic with IC₅₀ values in the low micromolar range. The Ru derivative 1a was chosen as a representative for stability studies as well as for biomolecule interaction experiments. It underwent partial chlorido/aqua ligand exchange in DMSO-d₆/D₂O to rapidly form an equilibrium in aqueous media. The reactions of 1a with biomolecules proceeded quickly and resulted in the formation of adducts with amino acids, DNA, and protein. Hen egg white lysozyme crystals were soaked with 1a, and the crystallographic analysis revealed an interaction with an l-aspartic acid residue (Asp119), resulting in the cleavage of the p-cymene ligand but the retention of the NHC moiety. Cell morphology studies for the Rh analog 3a suggested that the cytotoxicity is exerted via mechanisms different from that of cisplatin.

Strategies for the Improvement of Metal-Based Chemotherapeutic Treatments

This article provides an overview of the various research approaches we have explored in recent years to improve metal-based agents for cancer or infection treatments. Although cisplatin, carboplatin, and oxaliplatin remain the cornerstones in tumor chemotherapy, the discovery and approval of novel inorganic anticancer drugs is a very slow process. Analogously, although a few promising inorganic drugs have found clinical application against parasitic or bacterial infections, their use remains relatively limited. Moreover, the discovery process is often affected by small therapeutic enhancements that are not attractive for the pharmaceutical industry. However, the availability of increasing mechanistic information for the modes of action of established inorganic drugs is fueling the exploration of various approaches for developing effective inorganic chemotherapy agents. Through a series of examples, some from our own research experience, we focus our attention on a number of promising strategies, including (1) drug repurposing, (2) the simple modification of the chemical structures of approved metal-based drugs, (3) testing novel drug combinations, and (4) newly synthesized complexes coupling different anticancer drugs. Accordingly, we aim to suggest and summarize a series of reliable approaches that are exploitable for the development of improved and innovative treatments.

A focus on the biological targets for coinage metal-NHCs as potential anticancer complexes

Metal complexes of N-heterocyclic carbene (NHC) ligands are the object of increasing attention for therapeutic purposes. Among the different metal centres, interest on Au-based compounds started with the application as anti-arthritis drugs. On the other hand, Ag(I) antimicrobial properties have been known for a long time. For Au(I)/Au(III)-NHC and Ag(I)-NHC anti-tumour and anti-proliferative properties have been quite recently demonstrated. In addition to these and as for Group 11, copper is a much less investigated metal centre, but a few papers underline its pharmacological potential. This review wants to focus on the different biological targets for these metal-based compounds. It is divided into chapters which are respectively devoted on: i) mitochondria and thiol oxidoreductase systems; ii) other relevant enzymes; iii) nucleic acids. Examples of representative coinage NHCs for each of the targets are provided together with significant references on recent advances on the topic. Moreover, a final comment summarises the aspects enlightened by each chapter and provides some hints to better understand the metal-NHCs mechanistic behaviour based on structure-activity relationships.

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.

Fluorescent palladium(II) and platinum(II) NHC/1,2,3-triazole complexes: antiproliferative activity and selectivity against cancer cells

Fluorescent Pd(ii) and Pt(ii) complexes bearing 4-methylene-7-methoxycoumarin (MMC) and 2,6-diispropylphenyl (Dipp) substituted NHC/1,2,3-triazole hybrid ligands are described. Depending on the reaction conditions two different ligand coordination modes are observed, i.e., bidentate solely coordinating via NHCs or tetradentate coordinating via NHCs and 1,2,3-triazoles. All Dipp substituted complexes show antiproliferative activity against cervix (HeLa) and breast (MCF-7) human carcinoma cells. The activity significantly depends on the coordination mode, with the tetradentate motif being notably more effective (HeLa: IC50 = 3.9 μM to 4.7 μM; MCF-7: IC50 = 2.07 μM to 2.35 μM). Amongst the MMC series, only the Pd(ii) complex featuring the bidentate coordination mode is active against HeLa (IC50 = 6.1 μM). In contrast to its structurally related Dipp derivative (SI = 0.6), it shows a high selectivity for HeLa (SI > 16) compared to healthy skin cells (HaCaT). According to fluorescence microscopy, this compound is presumably located in late endosomes or lysosomes.