Kinetics and Mechanism of the Reaction between Serum Albumin and Auranofin (and Its Isopropyl Analogue) in Vitro

Gold-NHC complexes: from synthetic aspects to anti-cancer activity

Recent advancements in Au(I)-N-heterocyclic carbene (NHC) complexes have demonstrated significant potential for developing novel anticancer agents. These complexes exhibit unique properties, such as a strong affinity for thiol and selenol-containing biomolecules, which enable the selective targeting of cancer cells while minimising effects on healthy tissues. Recent studies have explored various structural modifications to enhance the anticancer efficacy of Au(I)-NHC complexes, including ligand substitution, incorporation of bioactive moieties, and hybridisation with other metal complexes. Mechanistic investigations have revealed that these complexes induce apoptosis through multiple pathways, such as inhibition of thioredoxin reductase (TrxR), disruption of mitochondrial function, and generation of reactive oxygen species (ROS). The introduction of NHC ligands is particularly advantageous, as they provide stability and tunability to the Au(I) centre, allowing for the optimisation of pharmacokinetic and pharmacodynamic properties. Moreover, the emergence of Au(I)-NHC complexes with dual-action mechanisms, combining anticancer activity with antiangiogenic or anti-inflammatory properties, has further broadened their therapeutic potential. This review article highlights the most recent breakthroughs in the design, synthesis, and biological evaluation of Au(I)-NHC complexes, emphasizing their promise as a new class of targeted anticancer therapeutics. While primarily focused on Au(I) complexes, it also includes a brief discussion of selected Au(III) complexes for comparison.

Exploring antiviral and antiparasitic activity of gold N-heterocyclic carbenes with thiolate ligands

Gold(I) N-heterocyclic carbenes have been explored for their therapeutic potential against several diseases. Neglected tropical diseases, including leishmaniasis, Chagas disease, and viral infections, such as zika, mayaro, and chikungunya, urgently require new treatment options. The emergent SARS-CoV-2 also demands significant attention. Gold complexes have shown promise as alternative treatments for these conditions. Previously, gold(I)(1,3-bis(mesityl)imidazole-2-ylidene)Cl (AuIMesCl) demonstrated significant leishmanicidal and anti-Chikungunya virus activities. In this study, we synthesized and fully characterized a series of gold(I)(1,3-bis(mesityl)imidazole-2-ylidene)(SR) complexes, where SR includes thiolate donor species such as 1,3-thiazolidine-2-thione, 1,3-benzothiazole-2-thione, 2-mercaptopyrimidine, and 2-thiouracil. These compounds were stable in solution, and ligand exchange reactions with N-acetyl-L-cysteine indicated that complexes with SR ligands are more labile than those with chloride. Although the reactions are rapid, they reach equilibrium at varying molar ratios depending on the SR ligand. The increased lability of these compounds results in higher cytotoxicity to host cells, such as Vero E6 and bone marrow-differentiated macrophages, compared to AuIMesCl. Despite this, the compounds effectively inhibited viral replication, achieving 95.5% inhibition of Zika virus replication at 2 μM with 96% host cell viability. Although active at low concentrations (∼2 μM) against Leishmania (L.) amazonensis and Trypanosoma cruzi, their high cytotoxicity for macrophages confirmed AuIMesCl as a better candidate with a higher selectivity index. This work correlates the coordination chemistry of pyrimidines and thiazolidines with their in vitro biological activities against significant diseases.

Strategies for the design of analogs of auranofin endowed with anticancer potential

INTRODUCTION

Auranofin (AF) is a well-established, FDA-approved, antiarthritic gold drug that is currently being reevaluated for a variety of therapeutic indications through drug repurposing. AF has shown great promise as a potential anticancer agent and has been approved for a few clinical trials in cancer. The renewed interest in AF has led to extensive research into the design, preparation and biological evaluation of auranofin analogs, which may have an even better pharmacological profile than the parent drug.

AREAS COVERED

This article reviews the strategies for chemical modification of the AF scaffold. Several auranofin analogs have been prepared and characterized for medical application in the field of cancer treatment over the last 20 years. Some emerging structure-function relationships are proposed and discussed.

EXPERT OPINION

The chemical modification of the AF scaffold has been the subject of intense activity in recent years and this strategy has led to the preparation and evaluation of several AF analogs. The case of iodauranofin is a particularly promising example. The availability of homogeneous biological data for a group of AF derivatives allows some initial structure-function relationships to be proposed, which may inspire the design and synthesis of new and better AF analogs for cancer treatment.

Some remarks on the biological application of gold(III) complexes

Gold(III) complexes are widely studied as antitumor agents and show good results. The interaction with biologically active thiols (thiomalate, cysteine, glutathione (GSH) and human serum albumin) of a number of gold(III) complexes with N-containing polydentate ligands in aqueous solution with pH 7.4 and 0.2 M NaCl was studied. Complexes with 1,10-phenanthroline and 2,2'-bipyridyl, Au(phen)(OH)2+ and Au(bipy)(OH)2+, react fast with an excess of any of these thiols and in less than a few seconds transform into gold(I) bis-thiolate complexes. For complexes with deprotonated ethylenediamine and diethylenetriamine, Au(en)(en-H)2+ and Au(dien-H)(Cl,OH)+, at a significant excess of GSH, a relatively long-lived gold(III) complex AuIII(GSH)iLj is formed. At t = 37 °C, it transforms into the gold(I) bis-thiolate complex Au(GSH)2 by 90% in 4 h. However, for other thiols, the rate of decomposition of similar complexes is about 10 times higher. Some other complexes were also considered. In all cases, a fairly fast reduction of gold(III) to gold(I) occurs with the formation of the gold(I) bis-thiolates.

Low level of antioxidant capacity biomarkers but not target overexpression predicts vulnerability to ROS-inducing drugs

Despite a strong rationale for why cancer cells are susceptible to redox-targeting drugs, such drugs often face tumor resistance or dose-limiting toxicity in preclinical and clinical studies. An important reason is the lack of specific biomarkers to better select susceptible cancer entities and stratify patients. Using a large panel of lung cancer cell lines, we identified a set of "antioxidant-capacity" biomarkers (ACB), which were tightly repressed, partly by STAT3 and STAT5A/B in sensitive cells, rendering them susceptible to multiple redox-targeting and ferroptosis-inducing drugs. Contrary to expectation, constitutively low ACB expression was not associated with an increased steady state level of reactive oxygen species (ROS) but a high level of nitric oxide, which is required to sustain high replication rates. Using ACBs, we identified cancer entities with a high percentage of patients with favorable ACB expression pattern, making it likely that more responders to ROS-inducing drugs could be stratified for clinical trials.

Hetero- and homoleptic binuclear gold(I)-thiolate and -halide complexes - ligand exchange kinetics and supramolecular structures

Heteroleptic and homoleptic binuclear Au(I) complexes [Au2(μ-PAnP)(SPh)(X)] (X = Cl^- or Br^-), [Au2(μ-PAnP)(SPh)2] and [Au2(μ-PAnP)(SPhCO2H)2] (SPh = benzenethiolate and SPhCO₂H = 4-thiolatobenzoic acid) containing the bridging diphosphine, 9,10-bis(diphenylphosphino)anthracene (PAnP), were synthesized and characterized by single crystal X-ray diffraction. [Au2(μ-PAnP)(SPh)2] exists as a monomer in its crystals but [Au2(μ-PAnP)(SPhCO2H)2] polymerizes into zig-zag chains via intermolecular hydrogen bonding. [Au2(μ-PAnP)(SPh)(Cl)] forms cyclophane-like dimers of C_i symmetry in crystals via intermolecular aurophilic interactions (Au-Au distance = 3.3081(5) Å). Recrystallization of [Au2(μ-PAnP)(SPh)(Br)] invariably led to crystals composed of [Au2(μ-PAnP)(SPh)(Br)] and [Au2(μ-PAnP)(Br)2]. Despite the chemically different P atoms in the heteroleptic [Au2(μ-PAnP)(SPh)(Cl)] and [Au2(μ-PAnP)(SPh)(Br)], solutions of the complexes show only a single signal in their ³¹P{¹H} NMR spectra at room temperature which resolved into two singlets of equal intensity at 183 K. Identical signals which show the same thermal behavior were observed in solutions of [Au2(μ-PAnP)(SPh)2] and [Au2(μ-PAnP)(X)2] in 1 : 1 molar ratios, indicating that there are three exchanging species, [Au2(μ-PAnP)(SPh)(X)], [Au2(μ-PAnP)(SPh)2] and [Au2(μ-PAnP)(X)2], in solution. A solution of [Au2(μ-PAnP)(Cl)2] and [Au2(μ-PAnP)(Br)2] in 1 : 1 molar ratio shows two singlets, implying that the exchange is not due to the dissociation of either PAnP or halide ligands, but rather it involves the exchange of the thiolate and the halide ligands (SPh^- ↔ X^-). A mixture of [(PPh3)Au(SPh)] and [(PPh3)Au(Cl)] (1 : 1 molar ratio) showed only one signal in its room temperature ³¹P{¹H} NMR spectrum, indicating that the ligand exchange can happen intermolecularly. Self-exchange of SPh^- ligands is possible as the room temperature ³¹P NMR spectrum of a mixture of [Au2(μ-PAnP)(SPh)2] and [Au2(μ-PAnP)(SPhCO2H)2] displayed only one signal. The rate constants of the exchange were determined by fitting the line shapes of the ³¹P NMR signals at different temperatures. The activation energies (E_as), obtained from Arrhenius plots, for the SPh^- ↔ Cl^- and SPh^- ↔ Br^- exchange are 36.9 ± 0.7 and 33.7 ± 1.0 kJ mol^-1, respectively. The activation enthalpy and activation entropy, obtained from Eyring plots, for the SPh^- ↔ Cl^- and SPh^- ↔ Br^- exchange are 35.0 ± 0.7 kJ mol^-1 and -25.7 ± 3.2 J K^-1, and 32.0 ± 1.0 kJ mol^-1 and -21.8 ± 4.7 J K^-1, respectively. Based on the kinetic results, two possible mechanisms were proposed for the reactions.

Practical and Computational Studies of Bivalence Metal Complexes of Sulfaclozine and Biological Studies

In the search for novel, metal-based drug complexes that may be of value as anticancer agents, five new transition metal complexes of sulfaclozine (SCZ) with Cu(II), Co(II), Ni(II), Zn(II), and Fe(II) were successfully synthesized. The chemical structure of each complex was characterized using elemental analysis (CHN), IR spectroscopy, UV–Vis spectroscopy, thermogravimetric analysis (TGA), and electronic paramagnetic resonance (EPR) spectroscopy. IR spectra indicated that the donor atoms were one sulfonyl oxygen atom and one pyrazine nitrogen atom, which associated with the metal ions to form a stable hexagonal coordination ring. The metal–ligand stability constant (K_f) revealed that Cu(II) and Ni(II) have good coordination stability among the metal compounds. Theoretical studies using DFT/B3LYP were performed to further validate the proposed structures. The obtained results indicated that Cu(II) has a trigonal bipyramidal geometry, whereas Fe(II), Co(II), and Ni(II) have an octahedral structure, while Zn(II) has a tetrahedral arrangement. The bio-activities of the characterized complexes were evaluated using DNA binding titration and molecular docking. The binding constant values for the metal complexes were promising, with a maximum value for the copper metal ion complex, which was 9 × 10⁵ M^-1. Molecular docking simulations were also carried out to evaluate the interaction strength and properties of the synthesized metal complexes with both DNA and selected cancer-relevant proteins. These results were supported by in vitro cytotoxicity assays showing that the Cu(II) and Ni(II) complexes display promising antitumor activity against colon and breast cancer cell lines.

Integrated experimental/computational approaches to characterize the systems formed by vanadium with proteins and enzymes

An integrated instrumental/computational approach to characterize metallodrug–protein adducts at the molecular level is reviewed. A series of applications are described, focusing on potential vanadium drugs with a generalization to other metals.

Repurposing Auranofin and Evaluation of a New Gold(I) Compound for the Search of Treatment of Human and Cattle Parasitic Diseases: From Protozoa to Helminth Infections

Neglected parasitic diseases remain a major public health issue worldwide, especially in tropical and subtropical areas. Human parasite diversity is very large, ranging from protozoa to worms. In most cases, more effective and new drugs are urgently needed. Previous studies indicated that the gold(I) drug auranofin (Ridaura^®) is effective against several parasites. Among new gold(I) complexes, the phosphole-containing gold(I) complex {1-phenyl-2,5-di(2-pyridyl)phosphole}AuCl (abbreviated as GoPI) is an irreversible inhibitor of both purified human glutathione and thioredoxin reductases. GoPI-sugar is a novel 1-thio-β-d-glucopyranose 2,3,4,6-tetraacetato-S-derivative that is a chimera of the structures of GoPI and auranofin, designed to improve stability and bioavailability of GoPI. These metal-ligand complexes are of particular interest because of their combined abilities to irreversibly target the essential dithiol/selenol catalytic pair of selenium-dependent thioredoxin reductase activity, and to kill cells from breast and brain tumors. In this work, screening of various parasites—protozoans, trematodes, and nematodes—was undertaken to determine the in vitro killing activity of GoPI-sugar compared to auranofin. GoPI-sugar was found to efficiently kill intramacrophagic Leishmania donovani amastigotes and adult filarial and trematode worms.

Mechanistic Insights Into the Anticancer Properties of the Auranofin Analog Au(PEt3)I: A Theoretical and Experimental Study

Au(PEt₃)I (AF-I hereafter), the iodide analog of the FDA-approved drug auranofin (AF hereafter), is a promising anticancer agent that produces its pharmacological effects through interaction with non-genomic targets such as the thioredoxin reductase system. AF-I is endowed with a very favorable biochemical profile showing potent in vitro cytotoxic activity against several cancer types including ovarian and colorectal cancer. Remarkably, in a recent publication, some of us reported that AF-I induces an almost complete and rapid remission in an orthotopic in vivo mouse model of ovarian cancer. The cytotoxic potency does not bring about highly severe side effects, making AF-I very well-tolerated even for higher doses, even more so than the pharmacologically active ones. All these promising features led us to expand our studies on the mechanistic aspects underlying the antitumor activity of AF-I. We report here on an integrated experimental and theoretical study on the reactivity of AF-I, in comparison with auranofin, toward relevant aminoacidic residues or their molecular models. Results point out that the replacement of the thiosugar moiety with iodide significantly affects the overall reactivity toward the amino acid residues histidine, cysteine, methionine, and selenocysteine. Altogether, the obtained results contribute to shed light into the enhanced antitumoral activity of AF-I compared with AF.

Structural Basis of Drug Recognition by Human Serum Albumin

Background:

Human serum albumin (HSA), the most abundant protein in plasma, is a monomeric multi-domain macromolecule with at least nine binding sites for endogenous and exogenous ligands. HSA displays an extraordinary ligand binding capacity as a depot and carrier for many compounds including most acidic drugs. Consequently, HSA has the potential to influence the pharmacokinetics and pharmacodynamics of drugs.

Objective:

In this review, the structural determinants of drug binding to the multiple sites of HSA are analyzed and discussed in detail. Moreover, insight into the allosteric and competitive mechanisms underpinning drug recognition, delivery, and efficacy are analyzed and discussed.

Conclusion:

As several factors can modulate drug binding to HSA (e.g., concurrent administration of drugs competing for the same binding site, ligand binding to allosteric-coupled clefts, genetic inherited diseases, and post-translational modifications), ligand binding to HSA is relevant not only under physiological conditions, but also in the pharmacological therapy management.

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.

Anticancer auranofin engages 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) as a target

Auranofin (AuRF) has been reported to display anticancer activity and has entered several clinical trials; however, its mechanism of action remains largely unknown. In this work, the anticancer mechanism of auranofin was investigated using a proteomics strategy entailing subcellular fractionation prior to mass spectrometric analysis. Bioinformatics analysis of the nuclear sub-proteomes revealed that tumor suppressor p14^ARF is a key regulator of transcription. Through independent analysis, we validated that up-regulation of p14^ARF is associated with E2F-dependent transcription and increased p53 expression. Our analyses further reveal that 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR), which is the rate-determining enzyme of the mevalonate pathway, is a novel target of auranofin with half maximal inhibitory concentration at micromolar levels. The auranofin-induced cancer cell death could be partially reverted by the addition of downstream products of the mevalonate pathway (mevalonolactone or geranyleranyl pyrophosphate (GGPP)), implying that auranofin may target the mevalonate pathway to exert its anticancer effect.

Nucleus-Targeted Organoiridium-Albumin Conjugate for Photodynamic Cancer Therapy

An organoiridium-albumin bioconjugate (Ir1-HSA) was synthesized by reaction of a pendant maleimide ligand with human serum albumin. The phosphorescence of Ir1-HSA was enhanced significantly compared to parent complex Ir1. The long phosphorescence lifetime and high 1 O2 quantum yield of Ir1-HSA are highly favorable properties for photodynamic therapy. Ir1-HSA mainly accumulated in the nucleus of living cancer cells and showed remarkable photocytotoxicity against a range of cancer cell lines and tumor spheroids (light IC50 ; 0.8-5 μm, photo-cytotoxicity index PI=40-60), while remaining non-toxic to normal cells and normal cell spheroids, even after photo-irradiation. This nucleus-targeting organoiridium-albumin is a strong candidate photosensitizer for anticancer photodynamic therapy.

A gold(i) biscarbene complex with improved activity as a TrxR inhibitor and cytotoxic drug: comparative studies with different gold metallodrugs

A stable gold(i) biscarbene complex with improved activity as a TrxR inhibitor and cytotoxic metallodrug.

Validation and Applications of Protein-Ligand Docking Approaches Improved for Metalloligands with Multiple Vacant Sites

Decoding the interaction between coordination compounds and proteins is of fundamental importance in biology, pharmacy, and medicine. In this context, protein- ligand docking represents a particularly interesting asset to predict how small compounds could interact with biomolecules, but to date, very little information is available to adapt these methodologies to metal-containing ligands. Here, we assessed the predictive capability of a metal-compatible parameter set for the docking program GOLD for metallo ligands with multiple vacant sites and different geometries. The study first presents a benchmark of 25 well-characterized X-ray metallo ligand-protein adducts. In 100% of the cases, the docking solutions are superimposable to the X-ray determination, and in 92% the value of the root-mean-square deviation between the experimental and calculated structures is lower than 1.5 Å. After the validation step, we applied these methods to five case studies for the prediction of the binding of pharmacological active metal species to proteins: (i) the anticancer copper(II) complex [Cu^II(Br)(2-hydroxy-1-naphthaldehyde benzoyl hydrazine)(indazole)] to human serum albumin (HSA); (ii) one of the active species of antidiabetic and antitumor vanadium compounds, V^IVO²⁺ ion, to carboxypeptidase; (iii) the antiarthritic species [Au^I(PEt₃)]⁺ to HSA; (iv) the antitumor oxaliplatin to ubiquitin; (v) the antitumor ruthenium(II) compound RAPTA-PentaOH to cathepsin B. The calculations suggested that the binding modes are in good agreement with the partial information retrieved from spectroscopic and spectrometric analysis and allowed us, in certain cases, to propose additional hypotheses. This method is an important update in protein-metallo ligand docking, which could have a wide field of application, from biology and inorganic biochemistry to medicinal chemistry and pharmacology.

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.

Dual targeting of the cancer antioxidant network with 1,4-naphthoquinone fused Gold(i) N-heterocyclic carbene complexes

To achieve a systems-based approach to targeting the antioxidant pathway, 1,4-naphthoquinone annulated N-heterocyclic carbene (NHC) [bis(1,3-dimesityl-4,5-naphthoquino-imidazol-2-ylidene)-gold(i)] [silver(i) dichloride] (1), [bis(1,3-dimesityl-4,5-naphthoquino-imidazol-2-ylidene)-gold(i)] chloride (2), and 1,3-dimesityl-4,5-naphthoquino-imidazol-2-ylidene)-gold(i) chloride (3)) were designed, synthesized, and tested for biological activity in a series of human cancer cell lines. The solution phase of complexes 1-3 were assigned using several spectroscopy techniques, including NMR spectroscopic analysis. Complexes 1 and 3 were further characterized by single crystal X-ray diffraction analysis. Electrochemical and spectroelectrochemical studies revealed that quinone reductions are reversible and that the electrochemically generated semiquinone and quinone dianions are stable under these conditions. Complex 1, containing two NHC-quinone moieties (to accentuate exogenous ROS via redox cycling) centered around a Au(i) center (to inactivate thioredoxin reductase (TrxR) irreversibly), was found to inhibit cancer cell proliferation to a much greater extent than the individual components (i.e., Au(i)-NHC alone or naphthoquinone alone). Treatment of A549 lung cancer cells with 1 produced a 27-fold increase in exogenous reactive oxygen species (ROS) which was found to localize to the mitochondria. The inhibition of TrxR, an essential mediator of ROS homeostasis, was achieved in the same cell line at low administrated concentrations of 1. TrxR inhibition by 1 was similar to that of auranofin, a gold(i) containing complex known to inhibit TrxR irreversibly. Complex 1 was found to induce cell death via an apoptotic mechanism as confirmed by annexin-V staining. Complex 1 was demonstrated to be efficacious in zebrafish bearing A549 xenografts. These results provide support for the suggestion that a dual targeting approach that involves reducing ROS tolerance while concurrently increasing ROS production can perturb antioxidant homeostasis, enhance cancer cell death in vitro, and reduce tumor burden in vivo, as inferred from preliminary zebra fish model studies.

Inhibiting the Inflammasome: A Chemical Perspective

Inflammasomes are high molecular weight complexes that sense and react to injury and infection. Their activation induces caspase-1 activation and release of interleukin-1β, a pro-inflammatory cytokine involved in both acute and chronic inflammatory responses. There is increasing evidence that inflammasomes, particularly the NLRP3 inflammasome, act as guardians against noninfectious material. Inappropriate activation of the NLRP3 inflammasome contributes to the progression of many noncommunicable diseases such as gout, type II diabetes, and Alzheimer's disease. Inhibiting the inflammasome may significantly reduce damaging inflammation and is therefore regarded as a therapeutic target. Currently approved inhibitors of interleukin-1β are rilonacept, canakinumab, and anakinra. However, these proteins do not possess ideal pharmacokinetic properties and are unlikely to easily cross the blood-brain barrier. Because inflammation can contribute to neurological disorders, this review focuses on the development of small-molecule inhibitors of the NLRP3 inflammasome.

Metal- and Semimetal-Containing Inhibitors of Thioredoxin Reductase as Anticancer Agents

The mammalian thioredoxin reductases (TrxRs) are a family of selenium-containing pyridine nucleotide disulfide oxidoreductases playing a central role in cellular redox homeostasis and signaling pathways. Recently, these selenoproteins have emerged as promising therapeutic targets for anticancer drug development, often being overexpressed in tumor cells and contributing to drug resistance. Herein, we summarize the current knowledge on metal- and semimetal-containing molecules capable of hampering mammalian TrxRs, with an emphasis on compounds reported in the last decade.

Electrochemical properties of a cobalt(II) complex with sulfadiazine and 1,3-bis(pyridin-4-yl)propane

catena-Poly[[bis{4-[(pyrimidin-2-ylazanidyl)sulfonyl]aniline}cobalt(II)]-bis[μ-1,3-bis(pyridin-4-yl)propane]], [Co(C10H8N4O4S2)2(C13H14N2)]n or [Co(L)2(bpp)]n, crystallizes as a one-dimensional polymeric structure which is further stabilized by intermolecular hydrogen bonding. The refined Flack parameter, -0.001 (10), indicates that the model represents the correct absolute structure. Investigation of the thermal stability shows that the complex is stable up to 543 K. The structure is of interest with respect to its electrochemical properties in the reduction reaction of H2O2 to H2O.

Binding of transition metal ions to albumin: sites, affinities and rates

BACKGROUND

Serum albumin is the most abundant protein in the blood and cerebrospinal fluid and plays a fundamental role in the distribution of essential transition metal ions in the human body. Human serum albumin (HSA) is an important physiological transporter of the essential metal ions Cu(2+), and Zn(2+) in the bloodstream. Its binding of metals like Ni(2+), Co(2+), or Cd(2+) can occur in vivo, but is only of toxicological relevance. Moreover, HSA is one of the main targets and hence most studied binding protein for metallodrugs based on complexes with Au, Pt and V.

SCOPE OF REVIEW

We discuss i) the four metal-binding sites so far described on HSA, their localization and metal preference, ii) the binding of the metal ions mentioned above, i.e. their stability constants and association/dissociation rates, their coordination chemistry and their selectivity versus the four binding sites iii) the methodology applied to study issues of items i and ii and iv) oligopeptide models of the N-terminal binding site.

MAJOR CONCLUSIONS

Albumin has four partially selective metal binding sites with well-defined metal preferences. It is an important regulator of the blood transport of physiological Cu(II) and Zn(II) and toxic Ni(II) and Cd(II). It is also an important target for metal-based drugs containing Pt(II), V(IV)O, and Au(I).

GENERAL SIGNIFICANCE

The thorough understanding of metal binding properties of serum albumin, including the competition of various metal ions for specific binding sites is important for biomedical issues, such as new disease markers and design of metal-based drugs. This article is part of a Special Issue entitled Serum Albumin.

Protein metalation by metal-based drugs: reactions of cytotoxic gold compounds with cytochrome c and lysozyme

Protein metalation processes are crucial for the mechanism of action of several anticancer metallodrugs and warrant deeper characterisation. We have explored the reactions of three cytotoxic gold(III) compounds-namely [(bipy(2Me))(2)Au(2)(μ-O)(2)][PF(6)](2) (where bipy(2Me) is 6,6'-dimethyl-2,2'-bipyridine) (Auoxo6), [(phen(2Me))(2)Au(2)(μ-O)(2)][PF(6)](2) (where phen(2Me) is 2,9-dimethyl-1,10-phenanthroline) (Au(2)phen) and [(bipy(dmb)-H)Au(OH)][PF(6)] [where bipy(dmb)-H is deprotonated 6-(1,1-dimethylbenzyl)-2,2'-bipyridine] (Aubipyc)-with two representative model proteins, i.e. horse heart cytochrome c and hen egg white lysozyme, through UV-visible absorption spectroscopy and electrospray ionisation mass spectrometry (ESI MS) to characterise the inherent protein metalation processes. Notably, Auoxo6 and Au(2)phen produced stable protein adducts where one or more "naked" gold(I) ions are protein-coordinated; very characteristic is the case of cytochrome c, which upon reaction with Auoxo6 or Au(2)phen preferentially forms "tetragold" adducts with four protein-bound gold(I) ions. In turn, Aubipyc afforded monometalated protein adducts where the structural core of the gold(III) centre and its +3 oxidation state are conserved. Auranofin yielded protein derivatives containing the intact auranofin molecule. Additional studies were performed to assess the role played by a reducing environment in protein metalation. Overall, the approach adopted provides detailed insight into the formation of metallodrug-protein derivatives and permits trends, peculiarities and mechanistic details of the underlying processes to be highlighted. In this respect, electrospray ionisation mass spectrometry is a very straightforward and informative research tool. The protein metalation processes investigated critically depend on the nature of both the metal compound and the interacting protein and also on the solution conditions used; thus, predicting with accuracy the nature and the amounts of the adducts formed for a given metallodrug-protein pair is currently extremely difficult.

Gold-containing indoles as anticancer agents that potentiate the cytotoxic effects of ionizing radiation

This report describes the design and application of several distinct gold-containing indoles as anticancer agents. When used individually, all gold-bearing compounds display cytostatic effects against leukemia and adherent cancer cell lines. However, two gold-bearing indoles show unique behavior by increasing the cytotoxic effects of clinically relevant levels of ionizing radiation. Quantifying the amount of DNA damage demonstrates that each gold-indole enhances apoptosis by inhibiting DNA repair. Both Au(I)-indoles were tested for inhibitory effects against various cellular targets including thioredoxin reductase, a known target of several gold compounds, and various ATP-dependent kinases. While neither compound significantly inhibits the activity of thioreoxin reductase, both showed inhibitory effects against several kinases associated with cancer initiation and progression. The inhibition of these kinases provides a possible mechanism for the ability of these Au(I)-indoles to potentiate the cytotoxic effects of ionizing radiation. Clinical applications of combining Au(I)-indoles with ionizing radiation are discussed as a new strategy to achieve chemosensitization of cancer cells.

Cytotoxic gold compounds: synthesis, biological characterization and investigation of their inhibition properties of the zinc finger protein PARP-1

The new gold(III) complexes: [Au{2-(2'-pyridyl)imidazolate}Cl(2)] and [Au{2,6-bis(2'-benzimidazolate)pyridine}(OCOCH(3))] and the mono- and binuclear gold(I) complexes: [Au{2-(2'-pyridyl)imidazole}(PPh(3))](PF(6)), [Au(2-phenylimidazolate)(DAPTA)] (DAPTA = 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane), [(PPh(3)Au)(2)(2-R-imidazolate)](PF(6)) (R = 2-C(5)H(4)N, Ph) have been synthesized and characterized. The structure of the [(PPh(3)Au)(2){2-(2'-pyridyl)imidazolate)](PF(6)) complex was also characterized by X-ray crystallography. The antiproliferative properties of the complexes were assayed against human ovarian carcinoma cell lines, either sensitive (A2780) or resistant to cisplatin (A2780cisR), human mammary carcinoma cells (MCF7) and non-tumorigenic human kidney (HEK293) cells. Most of the studied compounds showed important cytotoxic effects. Interestingly, the compounds containing the 2-(2'-pyridyl)imidazolate ligand showed selectivity towards cancer cells with respect to the non-tumorigenic ones, with the dinuclear compound [(PPh(3)Au)(2){2-(2'-pyridyl)imidazolate)](PF(6)) being the most active. Some compounds were also screened for their inhibitory effect of the zinc-finger protein PARP-1, essential for DNA repair and relevant to the mechanisms of cancer cell resistance to cisplatin. Interaction studies of the compounds with the model protein ubiquitin were undertaken by electrospray ionization mass spectrometry (ESI MS). The results are discussed in relation to the putative mechanisms of action of the cytotoxic gold compounds.

Analysis of local polarity change around Cys34 in bovine serum albumin during N-->B transition by a polarity-sensitive fluorescence probe

The change trend of the local environment of Cys34 domain in bovine serum albumin has been studied as a function of pH value by using thiol-specific and polarity-sensitive fluorescent probe 3-(4-chloro-6-p-maleimidylphenoxyl-1,3,5-triazinylamino)-7-dimethylamino-2-methyl-phenazine. The local polarity of the Cys34 domain is found to rise with the increase of pH values, and the corresponding dielectric constant is raised from 12.8 at pH 6.0 to 23.3 at pH 9.1. The result shows that the environment of the Cys34 domain is rather hydrophobic in normal state at pH 6.0 and becomes a little hydrophilic in the course of N-->B transition, which may be attributed to the slight unfolding of the protein and thus the increasing of exposure of the previously relatively buried Cys34. In addition, the increased dielectric constant (23.3) is much lower than that (80.1) of water, suggesting that the unfolding of bovine serum albumin does not cause the full exposure of the Cys34 to the aqueous media during the transition.

Cancer cell death induced by phosphine gold(I) compounds targeting thioredoxin reductase

The thioredoxin system, composed of thioredoxin reductase (TrxR), thioredoxin (Trx), and NADPH (nicotinamide adenine dinucleotide phosphate), plays a central role in regulating cellular redox homeostasis and signaling pathways. TrxR, overexpressed in many tumor cells and contributing to drug resistance, has emerged as a new target for anticancer drugs. Gold complexes have been validated as potent TrxR inhibitors in vitro in the nanomolar range. In order to obtain potent and selective TrxR inhibitors, we have synthesized a series of linear, 'auranofin-like' gold(I) complexes all containing the [Au(PEt(3))](+) synthon and the ligands: Cl(-), Br(-), cyanate, thiocyanate, ethylxanthate, diethyldithiocarbamate and thiourea. Phosphine gold(I) complexes efficiently inhibited cytosolic and mitochondrial TrxR at concentrations that did not affect the two related oxidoreductases glutathione reductase (GR) and glutathione peroxidase (GPx). The inhibitory effect of the redox proteins was also observed intracellularly in cancer cells pretreated with gold(I) complexes. Gold(I) compounds were found to induce antiproliferative effects towards several human cancer cells some of which endowed with cisplatin or multidrug resistance. In addition, they were able to activate caspase-3 and induce apoptosis observed as nucleosome formation and sub-G1 cell accumulation. The complexes with thiocyanate and xanthate ligands were particularly effective in inhibiting thioredoxin reductase and inducing apoptosis. Pharmacodynamic studies in human ovarian cancer cells allowed for the correlation of intracellular drug accumulation with TrxR inhibition that leads to the induction of apoptosis via the mitochondrial pathway.