Gold(I) NHC-based homo- and heterobimetallic complexes: synthesis, characterization and evaluation as potential anticancer agents

Dimetal-Binding Scaffold 2-(Pyridin-2-yl)imidazo [1,5-b]pyridazine-7-ylidene: Synthesis of Trinuclear Heterobimetallic Complexes Involving Gold-Metal Interactions

As a dimetal-binding rigid scaffold, 2-(pyridin-2-yl)imidazo[1,5-b]pyridazine-7-ylidene was introduced. The scaffold was first converted into a meridional Au,N,N-tridentate ligand through binding of a Au(I)Cl moiety at the carbene center. The Au(I) center and the N,N-chelating moiety were expected to function as metallophilic and 4e-σ-donative interaction sites, respectively, in the binding of the second metal center. In this manner, various trinuclear heterobimetallic complexes were synthesized with different 3d-metal sources, such as cationic CuI , CuII , NiII , and CoII salts. SC-XRD analysis showed that the mono-3d-metal di-gold(I) trinuclear heterobimetallic complexes were constructed through gold(I)-metal interactions. Metallophilic interactions were also investigated by quantum chemical calculations including the AIM and IGMH methods.

Luminescent Heterobimetallic PtII-AuI Complexes Bearing N-Heterocyclic Carbenes (NHCs) as Potent Anticancer Agents

This study aims to probe into new series of heterobimetallic PtII-AuI complexes with a general formula of [Pt(p-MeC6H4)(dfppy)(μ-dppm)Au(NHC)]OTf, NHC = IPr, 2; IMes, 3; dfppy = 2-(2,4-difluorophenyl)pyridinate; dppm = 1,1-bis(diphenylphosphino)methane, which are the resultant of the reaction between [Pt(p-MeC6H4)(dfppy)(κ1-dppm)], 1, with [AuCl(NHC)], NHC = IPr, B; IMes, C, in the presence of [Ag(OTf)]. In the heterobimetallic complexes, the dppm ligand is settled between both metals as an unsymmetrical bridging ligand. Several techniques are employed to characterize the resulting compounds. Moreover, the photophysical properties of the complexes are investigated by means of UV-vis and photoluminescence spectroscopy. Furthermore, the experimental study is enriched by ab initio calculations (density functional theory (DFT) and time-dependent DFT (TD-DFT)) to assess the role of Pt and Au moieties in the observed optical properties. It is revealed that 1-3 is luminescent in the solid state and solution at different temperatures. In addition, the achieved results indicate the emissive properties of 1-3 are originated from a mixed 3IL/3MLCT excited state with major contribution of intraligand charge transfer (dfppy). A comparative study is conducted into the cytotoxic activities of starting materials and 1-3 against different human cancer cell lines such as the pancreas (MIA-PaCa2), breast (MDA-MB-231), cervix (HeLa), and noncancerous breast epithelial cell line (MCF-10A). The achieved results suggest the heterobimetallic PtII-AuI species as optimal compounds that signify the existence of cooperative and synergistic effects in their structures. The complex 3 is considered as the most cytotoxic compound with the maximum selectivity index in our screened complex series. Moreover, it is disclosed that 3 effectively causes cell death by inducing apoptosis in MIA-PaCa2 cells. Furthermore, the finding results by fluorescent cell microscopy manifest cytoplasmic staining of 3 rather than nucleus.

Homobimetallic Au(I)-Au(I) and Heterotrimetallic Au(I)-Fe(II)-Au(I) Complexes with Dialkyldithiophosphates and Phosphine Ligands: Structural Characterization, DFT Analysis, and Tyrosinase Inhibitory and Biological Effects

The role of bridging and terminal ligand electronic and steric properties on the structure and antiproliferative activity of two-coordinated gold(I) complexes was investigated on seven novel binuclear and trinuclear gold(I) complexes synthesized by the reaction of either Au₂(dppm)Cl₂, Au₂(dppe)Cl₂, or Au₂(dppf)Cl₂ with potassium diisopropyldithiophosphate, K[(S-OⁱPr)₂], potassium dicyclohexyldithiophosphate, K[(S-OCy)₂], or sodium bis(methimazolyl)borate, Na(S-Mt)₂, which afforded air-stable gold(I) complexes. In 1-7, the gold(I) centers adopt a two-coordinated linear geometry and are structurally similar. However, their structural features and antiproliferative properties highly depend upon subtle ligand substituent changes. All complexes were validated by ¹H, ¹³C{¹H}, ³¹P NMR, and IR spectroscopy. The solid-state structures of 1, 2, 3, 6, and 7 were confirmed using single-crystal X-ray diffraction. A density functional theory geometry optimization calculation was used to extract further structural and electronic information. To investigate the possible cytotoxicities of 2, 3, and 7, in vitro cellular tests were carried out on the human cancerous breast cell line MCF-7. 2 and 7 show promising cytotoxicity.

Asymmetrically Coordinated Heterodimetallic Ir-Ru System: Synthesis, Computational, and Anticancer Aspects

Herein, we present an unprecedented formation of a heterodinuclear complex [{(ppy)₂Ir^III}(μ-phpy){Ru^II(tpy)}](ClO₄)₂ {[1](ClO₄)₂} using terpyridyl/phenylpyridine as ancillary ligands and asymmetric phpy as a bridging ligand. The asymmetric binding mode (N^∧N-∩-N^∧N^∧C^-) of the phpy ligand in {[1](ClO₄)₂} is confirmed by ¹H, ¹³C, ¹H-¹H correlated spectroscopy (COSY), high-resolution mass spectrum (HRMS), single-crystal X-ray crystallography techniques, and solution conductivity measurements. Theoretical investigation suggests that the highest occupied molecular orbital (HOMO) and the least unoccupied molecular orbital (LUMO) of [1]²⁺ are located on iridium/ppy and phpy, respectively. The complex displays a broad low energy charge transfer (CT) band within 450-575 nm. The time-dependent density functional theory (TDDFT) analysis suggests this as a mixture of metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT), where both ruthenium, iridium, and ligands are involved. Complex {[1](ClO₄)₂} exhibits Ru^IIIr^III/Ru^IIIIr^III- and Ru^IIIIr^III/Ru^IIIIr^IV-based oxidative couples at 0.83 and 1.39 V, respectively. The complex shows anticancer activity and selectivity toward human breast cancer cells (IC₅₀; MCF-7: 9.3 ± 1.2 μM, and MDA-MB-231: 8.6 ± 1.2 μM) over normal breast cells (MCF 10A: IC₅₀ ≈ 21 ± 1.3 μM). The Western blot analysis and fluorescence microscopy images suggest that combined apoptosis and autophagy are responsible for cancer cell death.

A naphthalene-based heterobimetallic triazolylidene IrIII/PdII complex: regioselective to regiospecific C-H activation, tandem catalysis and a copper-free Sonogashira reaction

Heterobimetallic complexes featuring mesoionic carbene (MIC) donor ligands are gaining enormous popularity in tandem catalysis owing to the combined action of two different metal centers during catalysis. A rare version of the heterobimetallic Pd^II/Ir^III complex possessing a cyclometalated mesoionic carbene (MIC) ligand is presented along with the analogous homodinuclear Pd^II complex. A sterically controlled regiospecific cyclometalation towards the formation of a six-membered ring complex over a five-membered ring complex has been performed using a naphthalene-based bis-MIC ligand platform. The interplay between regioselective vs. regiospecific C-H bond activation for the synthesis of cyclometalated Ir^III complexes has also been demonstrated using the corresponding naphthyl-derived mono-imidazolylidene ligand. Both homodinuclear Pd^II and heterobimetallic Pd^II/Ir^III complexes have been characterized using standard spectroscopic techniques including ¹H, ¹³C{¹H}, 2D correlation NMR spectroscopy and ESI mass spectrometry. The structure of the cyclometalated heterobimetallic complex has been established by single crystal XRD. The heterobimetallic complex has been employed as a pre-catalyst in the tandem Suzuki-Miyaura/transfer hydrogenation reaction and the homobimetallic Pd^II complex has been successfully employed as a catalyst in both the Sonogashira coupling and α-arylation of 1-methyl-2-oxindole.

"Dynamical Docking" of Cyclic Dinuclear Au(I) Bis-N-heterocyclic Complexes Facilitates Their Binding to G-Quadruplexes

With the aim to improve the design of metal complexes as stabilizers of noncanonical DNA secondary structures, namely, G-quadruplexes (G4s), a series of cyclic dinuclear Au(I) N-heterocyclic carbene complexes based on xanthine and benzimidazole ligands has been synthesized and characterized by various methods, including X-ray diffraction. Fluorescence resonance energy transfer (FRET) and CD DNA melting assays unraveled the compounds' stabilization properties toward G4s of different topologies of physiological relevance. Initial structure-activity relationships have been identified and recognize the family of xanthine derivatives as those more selective toward G4s versus duplex DNA. The binding modes and free-energy landscape of the most active xanthine derivative (featuring a propyl linker) with the promoter sequence cKIT1 have been studied by metadynamics. The atomistic simulations evidenced that the Au(I) compound interacts noncovalently with the top G4 tetrad. The theoretical results on the Au(I) complex/DNA Gibbs free energy of binding were experimentally validated by FRET DNA melting assays. The compounds have also been tested for their antiproliferative properties in human cancer cells in vitro, showing generally moderate activity. This study provides further insights into the biological activity of Au(I) organometallics acting via noncovalent interactions and underlines their promise for tunable targeted applications by appropriate chemical modifications.

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.

Halo complexes of gold(i) containing glycoconjugate carbene ligands: synthesis, characterization, cytotoxicity and interaction with proteins and DNA model systems

New neutral Au(i) glycoconjugate carbene complexes show stability in aqueous solutions and interact with both DNA and protein model systems. Cytotoxicity studies demonstrate that the activity depends on the halide ancillary ligand.

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.

Potential of Gold Candidates against Human Colon Cancer

Development of novel metallodrugs with pharmacological profile plays a significant role in modern medicinal chemistry and drug design. Metal complexes have shown remarkable clinical results in current cancer therapy. Gold complexes have attained attention due to their high antiproliferative potential. Gold-based drugs are used for the treatment of rheumatoid arthritis. Gold-containing compounds with selective and specific targets are capable to assuage the symptoms of a range of human diseases. Gold (I) species with labile ligands (such as Cl in TEPAuCl) interact with isolated DNA; therefore, this biomolecule has been considered as a target for gold drugs. Gold (I) has a high affinity towards sulfur and selenium. Due to this, gold (I) drugs readily interact with cysteine or selenocysteine residue of the enzyme to form protein-gold(I) thiolate or protein-gold (I) selenolate complexes that lead to inhibition of the enzyme activity. Au(III) compounds due to their square-planner geometriesthe same as found in cisplatin, represent a good source for the development of anti-tumor agents. This article aims to review the most important applications of gold products in the treatment of human colon cancer and to analyze the complex interplay between gold and the human body.

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.

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.

Targeting Telomeres: Molecular Dynamics and Free Energy Simulation of Gold-Carbene Binding to DNA

DNA sequences in regulatory regions and in telomers at the ends of chromosomes frequently contain tandem repeats of guanine nucleotides that can form stacked structures stabilized by Hoogsten pairing and centrally bound monovalent cations. The replication and elongation of telomeres requires the disruption of these G-quadruplex structures. Hence, drug molecules such as gold (Au)-carbene that stabilize G-quadruplexes may also interfere with the elongation of telomeres and, in turn, could be used to control cell replication and growth. To better understand the molecular mechanism of Au-carbene binding to G-quadruplexes, we employed molecular dynamics simulations and free energy simulations. Whereas very restricted mobility of two Au-carbene ligands was found upon binding as a doublet to one side of the G-quadruplex, much larger translational and orientational mobility was observed for a single Au-carbene binding at the second G-quadruplex surface. Comparative simulations on duplex DNA in the presence of Au-carbene ligands indicates a preference for the minor groove and weaker unspecific and more salt-dependent binding than to the G-quadruplex surface. Analysis of energetic contributions reveals a dominance of nonpolar and van der Waals interactions to drive binding. The simulations can also be helpful for proposing possible modifications that could improve Au-carbene affinity and specificity for G-quadruplex binding.

An Organometallic Gold(I) Bis-N-Heterocyclic Carbene Complex with Multimodal Activity in Ovarian Cancer Cells

The organometallic AuI bis-N-heterocyclic carbene complex [Au(9-methylcaffeine-8-ylidene)2 ]+ (AuTMX2 ) was previously shown to selectively and potently stabilise telomeric DNA G-quadruplex (G4) structures. This study sheds light on the molecular reactivity and mode of action of AuTMX2 in the cellular context using mass spectrometry-based methods, including shotgun proteomics in A2780 ovarian cancer cells. In contrast to other metal-based anticancer agents, this organogold compound is less prone to form coordinative bonds with biological nucleophiles and is expected to exert its drug effects mainly by non-covalent interactions. Global protein expression changes of treated cancer cells revealed a multimodal mode of action of AuTMX2 by alterations in the nucleolus, telomeres, actin stress-fibres and stress-responses, which were further supported by pharmacological assays, fluorescence microscopy and cellular accumulation experiments. Proteomic data are available via ProteomeXchange with identifier PXD020560.

A Bis-Chelating O N O ^ / N N ^ Ligand for the Synthesis of Heterobimetallic Platinum(II)/Rhenium(I) Complexes: Tools for the Optimization of a New Class of Platinum(II) Anticancer Agents

The two independent and N N ^ coordination sites of a newly synthesized bis[2-(hydroxyphenyl)-1,2,4-triazole] platform have been exploited to prepare four monometallic neutral ()Pt^II complexes carrying DMSO, pyridine, triphenylphosphine, or N-heterocyclic carbene as the fourth ligand. Then, the second N N ^ coordination site was used to introduce an IR-active rhenium tricarbonyl entity, affording the four corresponding heterobimetallic neutral Pt^II /Re^I complexes, as well as a cationic Pt^II /Re^I derivative. X-ray crystallographic studies showed that distortion of the organic platform occurred to accommodate the coordination geometry of both metal centers. No ligand exchange or transchelation occurred upon incubation of the Pt^II complexes in aqueous environment or in the presence of Fe^III , respectively. The antiproliferative activity of the ligand and complexes was first screened on the triple-negative breast cancer cell line MDA-MB-231. Then, the IC₅₀ values of the most active candidates were determined on a wider panel of human cancer cells (MDA-MB-231, MCF-7, and A2780), as well as on a nontumorigenic cell line (MCF-10A). Low micromolar activities were reached for the complexes carrying a DMSO ligand, making them the first examples of highly active, but hydrolytically stable, Pt^II complexes. Finally, the characteristic mid-IR signature of the {Re(CO)₃ } fragment in the Pt/Re heterobimetallic complexes was used to quantify their uptake in breast cancer cells.

New selective thiolate gold(i) complexes inhibit the proliferation of different human cancer cells and induce apoptosis in primary cultures of mouse colon tumors

New thiolate gold(i) complexes with P(NMe₂)₃ (HMPT) as phosphane group have been developed as proapoptotic and selective anticancer drugs.

Dinuclear gold(i) complexes: from bonding to applications

The last two decades have seen a veritable explosion in the use of gold(i) complexes bearing N-heterocyclic carbene (NHC) and phosphine (PR₃) ligands.

Synthesis, conformational analysis and antibacterial activity of Au(i)–Ag(i) and Au(i)–Hg(ii) heterobimetallic N-heterocyclic carbene complexes

A family heterobimetallic Au(i)–Ag(i) and Au(i)–Hg(ii) complexes of bis-N-heterocyclic carbene ligands been prepared and their antibacterial properties evaluated.

Ex vivo toxicological evaluation of experimental anticancer gold(i) complexes with lansoprazole-type ligands

Gold-based compounds are of great interest in the field of medicinal chemistry as novel therapeutic (anticancer) agents due to their peculiar reactivity and mechanisms of action with respect to organic drugs. Despite their promising pharmacological properties, the possible toxic effects of gold compounds need to be carefully evaluated in order to optimize their design and applicability. This study reports on the potential toxicity of three experimental gold-based anticancer compounds featuring lansoprazole ligands (1-3) studied in an ex vivo model, using rat precision cut kidney and liver slices (PCKS and PCLS, respectively). The results showed a different toxicity profile for the tested compounds, with the neutral complex 2 being the least toxic, even less toxic than cisplatin, followed by the cationic complex 1. The dinuclear cationic gold complex 3 was the most toxic in both liver and kidney slices. This result correlated with the metal uptake of the different compounds assessed by ICP-MS, where complex 3 showed the highest accumulation of gold in liver and kidney slices. Interestingly compound 1 showed the highest selectivity towards cancer cells compared to the healthy tissues. Histomorphology evaluation showed a similar pattern for all three Au(i) complexes, where the distal tubular cells suffered the most extensive damage, in contrast to the damage in the proximal tubules induced by cisplatin. The binding of representative gold compounds with the model ubiquitin was also studied by ESI-MS, showing that after 24 h incubation only 'naked' Au ions were bound to the protein following ligands' loss. The mRNA expression of stress response genes appeared to be similar for both evaluated organs, suggesting oxidative stress as the possible mechanism of toxicity. The obtained results open new perspectives towards the design and testing of bifunctional gold complexes with chemotherapeutic applications.

Ruthenium(ii) and palladium(ii) homo- and heterobimetallic complexes: synthesis, crystal structures, theoretical calculations and biological studies

Four Ru-Pd heterobimetallic complexes, each one in two different coordination modes (NNSS and NS) having metals connected by a binucleating dialkyldithiooxamidate [N(R)SC-CS(R)N] [R = methyl, ethyl, n-butyl and isopropyl], were prepared by reacting the monochelate [(triⁿpropyl-phosphine)ClPd(HR₂C₂N₂S₂κ-S,S-Pd)] with [(η⁶-p-cymene)RuCl₂]₂. Furthermore, two palladium homobimetallic complexes having two (triⁿpropyl-phosphine)ClPd moieties joined by a diethyldithiooxamidate in both κ-N,S Pd, κ-N',S' Pd' and κ-N,N' Pd, κ-S,S' Pd' coordination modes were synthesized. For both kinds of complexes, homo- and heterobimetallic, at room temperature and in chloroform solution, the NNSS coordination mode (kinetic compounds) turns out to be unstable and therefore the resulting complexes rearrange into a thermodynamically more stable form (NS coordination mode). The crystal structures of [(triⁿpropyl-phosphine)ClPd]₂[μ-(ethyl)₂-DTO κ-N,S Pd, κ-N',S' Pd'] (2) and [(η⁶-p-cymene)ClRu][μ-(methyl)₂-DTO κ-N,S Ru, κ-N,S Pd] [(triⁿpropyl-phosphine)ClPd] (1c) were determined by solid state X-ray crystallography. Moreover, the higher stability of the thermodynamic species in the heterobimetallic complexes (Ru-Pd) was evaluated by means of computational studies in accordance with the maximum hardness principle. All stable NS complexes (i.e.1c-4c, 2 and the previously reported homobimetallic Ru complex 3) were tested against two leukemia cell lines, namely the drug-sensitive CCRF-CEM cell line and its multidrug-resistant sub-cell line CEM/ADR5000 showing anti-proliferative activity in the low micromolar range (∼1-5 μM) and micromolar range (∼10-25 μM), respectively. In addition, these complexes efficaciously block at least two out of the three proteolytic activities of the tumor target 20S proteasome, with heterobimetallic complex 3c and homobimetallic complex 3 possessing the best inhibitory profile.

Comparative biological evaluation and G-quadruplex interaction studies of two new families of organometallic gold(I) complexes featuring N-heterocyclic carbene and alkynyl ligands

Experimental organometallic gold(I) compounds hold promise for anticancer therapy. This study reports the synthesis of two novel families of gold(I) complexes, including N1-substituted bis-N-heterocyclic carbene (NHC) complexes of general formula [Au(N1-TBM)₂]BF₄ (N1-TBM = N1-substituted 9-methyltheobromin-8-ylidene) and mixed gold(I) NHC-alkynyl complexes, [Au(N1-TBM)alkynyl]. The compounds were fully characterised for their structure and stability in aqueous environment and in the presence of N-acetyl cysteine by nuclear magnetic resonance (NMR) spectroscopy. The structures of bis(1-ethyl-3,7,9-trimethylxanthin-8-ylidene)gold(I), (4-ethynylpyridine)(1,9-dimethyltheobromine-8-ylidene)gold(I) and of (2,8-Diethyl-10-(4-ethynylphenyl)-5,5-difluoro-1,3,7,9-tetramethyl-5H-4λ⁴,5λ⁴-dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinine)(1,3,7,9-tetramethylxanthin-8-ylidene)gold(I) were also confirmed by X-ray diffraction analysis. The compounds were studied for their properties as DNA G-quadruplex (G4 s) stabilizers by fluorescence resonance energy transfer (FRET) DNA melting. Only the cationic [Au(N1-TBM)₂]BF₄ family showed moderate G4 stabilization properties with respect to the previously reported benchmark compound [Au(9-methylcaffein-8-ylidene)₂]⁺ (AuTMX₂). However, the compounds also showed marked selectivity for binding to G4 structures with respect to duplex DNA in competition experiments. For selected complexes, the interactions with G4 s were also confirmed by circular dichroism (CD) studies. Furthermore, the gold(I) complexes were assessed for their antiproliferative effects in human cancer cells in vitro, displaying moderate activity. Of note, among the mixed gold(I) NHC-alkynyl compounds, one features a fluorescent boron-dipyrromethene (BODIPY) moiety which allowed determining its uptake into the cytoplasm of cancer cells by fluorescence microscopy.

A new amido-phosphine of dichloroacetic acid as an active ligand for metals of pharmaceutical interest. Synthesis, characterization and tests of antiproliferative and pro-apoptotic activity

We herein describe the synthesis and characterization of the new amido-phosphinic ligand 3,7‑bis(dichloroacetyl)‑1,3,7‑triaza‑5‑phosphabicyclo[3.3.1]nonane (DCP), a derivative of dichloroacetic acid (DCA), whose ability to reverse the suppressed mitochondrial apoptosis in cancer cells is known. DCP was obtained by a double N-acylation of PTA (1,3,5‑triaza‑7‑phosphaadamantane) occurring with loss of CH₂, in appropriate conditions_. Due to the hindered rotation around the amidic CN bonds, three rotameric forms of DCP were observed, whose ratio in solution was dependent on the solvent, while the X-ray crystal structure of DCP showed an opposite orientation of the two amidic carbonyl groups (anti rotamer). The lipophilic, air and thermally stable DCP was found able to act regiospecifically as a P-donor ligand toward soft metal ions. By ligand substitution on appropriate precursors, we obtained the complexes 1-9, where proapoptotic DCA is associated with metal ions of known cytotoxic activity on cancer cells (Pt²⁺, Pd²⁺, Ru²⁺, Re⁺, Au⁺). The antiproliferative activity of DCP and its complexes was tested in vitro, in comparison with cisplatin, on three human tumor cell lines: A2780 (ovarian cisplatin-sensitive), A2780cis (ovarian cisplatin-resistant) and K562 (erythroleukemic). The results showed that the simultaneous presence of DCP (containing two residues of proapoptotic DCA) and Pt(II) produces the best performances with respect to non-platinum complexes. Experiments of pro-apoptotic activity indicated that the antiproliferative activity of the most active DCP-Pt(II) complexes is associated with induction of apoptosis.

CuII and AuIII Complexes with Glycoconjugated Dithiocarbamato Ligands for Potential Applications in Targeted Chemotherapy

This work is focused on the synthesis, characterization, and preliminary biological evaluation of bio-conjugated Au^III and Cu^II complexes with the aim of overcoming the well-known side effects of chemotherapy by improving the selective accumulation of an anticancer metal payload in malignant cells. For this purpose, carbohydrates were chosen as targeting agents, exploiting the Warburg effect that accounts for the overexpression of glucose-transporter proteins (in particular GLUTs) in the phospholipid bilayer of most neoplastic cells. We linked the dithiocarbamato moiety to the C1 position of three different monosaccharides: d-glucose, d-galactose, and d-mannose. Altogether, six complexes with a 1:2 metal-to-ligand stoichiometry were synthesized and in vitro tested as anticancer agents. One of them showed high cytotoxic activity toward the HCT116 colorectal human carcinoma cell line, paving the way to future in vivo studies aimed at evaluating the role of carbohydrates in the selective delivery of whole molecules into cancerous cells.

Targeting the Thioredoxin System as a Strategy for Cancer Therapy

Thioredoxin reductase (TrxR) participates in the regulation of redox reactions in organisms. It works mainly via its substrate molecule, thioredoxin, to maintain the redox balance and regulate signal transduction, which controls cell proliferation, differentiation, death, and other important physiological processes. In recent years, increasing evidence has shown that the overactivation of TrxR is related to the development of tumors. The exploration of TrxR-targeted antitumor drugs has attracted wide attention and is expected to provide new therapies for cancer treatment. In this perspective, we highlight the specific relationship between TrxR and apoptotic signaling pathways. The cytoplasm and mitochondria both contain TrxR, resulting in the activation of apoptosis. TrxR activity influences reactive oxygen species (ROS) and further regulates the inflammatory signaling pathway. In addition, we discuss representative TrxR inhibitors with anticancer activity and analyze the challenges in developing TrxR inhibitors as anticancer drugs.

Mitochondria-Targeting Anticancer Metal Complexes

Background:

Since the serendipitous discovery of the antitumor activity of cisplatin there has been a continuous surge in studies aimed at the development of new cytotoxic metal complexes. While the majority of these complexes have been designed to interact with nuclear DNA, other targets for anticancer metallodrugs attract increasing interest. In cancer cells the mitochondrial metabolism is deregulated. Impaired apoptosis, insensitivity to antigrowth signals and unlimited proliferation have been linked to mitochondrial dysfunction. It is therefore not surprising that mitochondria have emerged as a major target for cancer therapy. Mitochondria-targeting agents are able to bypass resistance mechanisms and to (re-) activate cell-death programs.

Methods:

Web-based literature searching tools such as SciFinder were used to search for reports on cytotoxic metal complexes that are taken up by the mitochondria and interact with mitochondrial DNA or mitochondrial proteins, disrupt the mitochondrial membrane potential, facilitate mitochondrial membrane permeabilization or activate mitochondria-dependent celldeath signaling by unbalancing the cellular redox state. Included in the search were publications investigating strategies to selectively accumulate metallodrugs in the mitochondria.

Results:

This review includes 241 references on antimitochondrial metal complexes, the use of mitochondria-targeting carrier ligands and the formation of lipophilic cationic complexes.

Conclusion:

Recent developments in the design, cytotoxic potency, and mechanistic understanding of antimitochondrial metal complexes, in particular of cyclometalated Au, Ru, Ir and Pt complexes, Ru polypyridine complexes and Au-N-heterocyclic carbene and phosphine complexes are summarized and discussed.

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.

Significance of the mitochondrial thioredoxin reductase in cancer cells: An update on role, targets and inhibitors

Thioredoxin reductase 2 (TrxR2) is a key component of the mitochondrial thioredoxin system able to transfer electrons to peroxiredoxin 3 (Prx3) in a reaction mediated by thioredoxin 2 (Trx2). In this way, both the level of hydrogen peroxide and thiol redox state are modulated. TrxR2 is often overexpressed in cancer cells conferring apoptosis resistance. Due to their exposed flexible arm containing selenocysteine, both cytosolic and mitochondrial TrxRs are inhibited by a large number of molecules. The various classes of inhibitors are listed and the molecules acting specifically on TrxR2 are extensively described. Particular emphasis is given to gold(I/III) complexes with phosphine, carbene or other ligands and to tamoxifen-like metallocifens. Also chemically unrelated organic molecules, including natural compounds and their derivatives, are taken into account. An important feature of many TrxR2 inhibitors is provided by their nature of delocalized lipophilic cations that allows their accumulation in mitochondria exploiting the organelle membrane potential. The consequences of TrxR2 inhibition are presented focusing especially on the impact on mitochondrial pathophysiology. Inhibition of TrxR2, by hindering the activity of Trx2 and Prx3, increases the mitochondrial concentration of reactive oxygen species and shifts the thiol redox state toward a more oxidized condition. This is reflected by alterations of specific targets involved in the release of pro-apoptotic factors such as cyclophilin D which acts as a regulator of the mitochondrial permeability transition pore. Therefore, the selective inhibition of TrxR2 could be utilized to induce cancer cell apoptosis.

Bimetallic titanocene-gold phosphane complexes inhibit invasion, metastasis, and angiogenesis-associated signaling molecules in renal cancer

Following promising recent in vitro and in vivo studies of the anticancer efficacies of heterometallic titanocene-gold chemotherapeutic candidates against renal cancer, we report here on the synthesis, characterization, stability studies and biological evaluation of a new titanocene complex containing a gold-triethylphosphane fragment [(η-C₅H₅)₂TiMe(μ-mba)Au(PEt₃)] (4) Titanofin. The compound is more stable in physiological fluid than those previously reported, and it is highly cytotoxic against a line of human clear cell renal carcinoma. We describe here preliminary mechanistic data for this compound and previously reported [(η-C₅H₅)₂TiMe(μ-mba)Au(PPh₃)] (2) Titanocref which displayed remarkable activity in an in vivo mouse model. Mechanistic studies were carried out in the human clear cell renal carcinoma Caki-1 line for the bimetallic compounds [(η-C₅H₅)₂TiMe(μ-mba)Au(PR₃)] (PR₃ = PPh₃2 Titanocref and PEt₃4 Titanofin), the two monometallic gold derivatives [Au(Hmba)(PR₃)] (PR₃ = PPh₃1 cref; PEt₃3 fin), titanocene dichloride and Auranofin as controls. These studies indicate that bimetallic compounds Titanocref (2) and Titanofin (4) are more cytotoxic than gold monometallic derivatives (1 and 3) and significantly more cytotoxic than titanocene dichloride while being quite selective. Titanocref (2) and Titanofin (4) inhibit migration, invasion, and angiogenic assembly along with molecular markers associated with these processes such as prometastatic IL(s), MMP(s), TNF-α, and proangiogenic VEGF, FGF-basic. The bimetallic compounds also strongly inhibit the mitochondrial protein TrxR often overexpressed in cancer cells evading apoptosis and also inhibit FOXC2, PECAM-1, and HIF-1α whose overexpression is linked to resistance to genotoxic chemotherapy. In summary, bimetallic titanocene-gold phosphane complexes (Titanocref 2 and Titanofin 4) are very promising candidates for further preclinical evaluations for the treatment of renal cancer.