Cyclometalated gold(III)-hydride under oriented external electric fields: a new strategy to modulate its reactivity?

Selected gold complexes have been regarded as promising anti-cancer agents because they can bind with protein targets containing thiol or selenol moieties, but their clinical applications were hindered by the unbiased binding towards off-target thiol-proteins. Recently, a novel gold(III)-hydride complex (abbreviated as 1) with visible light-induced thiol reactivity has been reported as potent photo-activated anticancer agents (Angew. Chem. Int. Ed., 2020, 132, 11139). To explore new strategies to stimuli this potential antitumor drug, the effect of oriented external electric fields (OEEFs) on its geometric structure, electronic properties, and chemical reactivity was systematically investigated. Results reveal that imposing external electric fields along the Au-H bond of 1 can effectively activate this bond, which is conducive to its dissociation and the binding of Au site to potential targets. Hence, this study provides a new OEEF-strategy to activate this reported gold(III)-hydride, revealing its potential application in electrochemical therapy. We anticipate this work could promote the development of more electric field-activated anticancer agents. However, further experimental research should be conducted to verify the conclusions obtained in this work.

Development of Anticancer Ferric Complex Based on Human Serum Albumin Nanoparticles That Generate Oxygen in Cells to Overcome Hypoxia-Induced Resistance in Metal Chemotherapy

To achieve the remarkable therapeutic efficacy of a ferric (Fe) complex via a reactive oxygen species (ROS) mechanism in solid tumors, a therapeutic Fe-based Schiff-base complex (Fe1) was synthesized and encapsulated in human serum albumin (HSA) nanoparticles (NPs), which generated oxygen (O2) in cancer cells in situ. The HSA-Fe1-O2 NP (HSA-Fe1-O2NP) delivery system effectively overcame hypoxia-induced resistance in metal chemotherapy, alleviated the hypoxic condition of tumor tissues, and showed excellent tumor suppression by generating excess ROS and promoting the apoptosis of SK-N-MC tumor cells. The HSA-Fe1-O2NPs not only enhanced the ability of the Fe1 complex to target tumor cells but also decreased adverse effects in vivo.

Binuclear Biphenyl Organogold(III) Complexes: Synthesis, Photophysical and Theoretical Investigation, and Anticancer Activity

A series of four binuclear complexes of general formula [(C^C)Au(Cl)(L^L)(Cl)Au(C^C)], where C^C is 4,4'-diterbutylbiphenyl and L^L is either a bridging diphosphine or 4,4'-bipyridine, are synthetized with 52 to 72 % yield and structurally characterized by X-ray diffraction. The use of the chelating 1,2-diphenylphosphinoethane ligand in a 1 : 2 (P^P):Au stoichiometry leads to the near quantitative formation of a gold double-complex salt of general formula [(C^C)Au(P^P)][(C^C^)AuCl2 ]. The compounds display long-lived yellow-green phosphorescence with λem in the range of 525 to 585 nm in the solid state with photoluminescence quantum yields (PLQY) up to 10 %. These AuIII complexes are tested for their antiproliferative activity against lung adenocarcinoma cells A549 and results show that compounds 2 and 5 are the most promising candidates. The digold salt 5 shows anticancer activity between 66 and 200 nM on the tested cancer cell lines, whereas derivative 2 displays concentration values required to reduce by 50 % the cell viability (IC50 ) between 7 and 11 μM. Reactivity studies of compound 5 reveal that the [(C^C)Au(P^P)]+ cation is stable in the presence of relevant biomolecules including glutathione suggesting a structural mechanism of action.

Thioredoxin Reductase and Organometallic Complexes: A Pivotal System to Tackle Multidrug Resistant Tumors?

Cancers classified as multidrug-resistant (MDR) are a family of diseases with poor prognosis despite access to increasingly sophisticated treatments. Several mechanisms explain these resistances involving both tumor cells and their microenvironment. It is now recognized that a multi-targeting approach offers a promising strategy to treat these MDR tumors. Inhibition of thioredoxin reductase (TrxR), a key enzyme in maintaining redox balance in cells, is a well-identified target for this approach. Auranofin was the first inorganic gold complex to be described as a powerful inhibitor of TrxR. In this review, we will first recall the main results obtained with this metallodrug. Then, we will focus on organometallic complexes reported as TrxR inhibitors. These include gold(I), gold(III) complexes and metallocifens, i.e., organometallic complexes of Fe and Os derived from tamoxifen. In these families of complexes, similarities and differences in the molecular mechanisms of TrxR inhibition will be highlighted. Finally, the possible relationship between TrxR inhibition and cytotoxicity will be discussed and put into perspective with their mode of action.

Gold Complexes in Anticancer Therapy: From New Design Principles to Particle-Based Delivery Systems

The discovery of the medicinal properties of gold complexes has fuelled the design and synthesis of new anticancer metallodrugs, which have received special attention due to their unique modes of action. Current research in the development of gold compounds with therapeutic properties is predominantly focused on the molecular design of drug leads with superior pharmacological activities, e.g., by introducing targeting features. Moreover, intensive research aims at improving the physicochemical properties of gold compounds, such as chemical stability and solubility in the physiological environment. In this regard, the encapsulation of gold compounds in nanocarriers or their chemical grafting onto targeted delivery vectors could lead to new nanomedicines that eventually reach clinical applications. Herein, we provide an overview of the state-of-the-art progress of gold anticancer compounds, andmore importantly we thoroughly revise the development of nanoparticle-based delivery systems for gold chemotherapeutics.

The role of tridentate ligands on the redox stability of anticancer gold(III) complexes

Gold(III) complexes are promising compounds for cancer chemotherapy, whose action depends on their redox stability. In this context, the choice of ligands is crucial to adjust their reactivity and biological response. The present study addressed the effect of the gold coordination sphere on the reduction potential (E^o) for ten gold(III) complexes containing five or six-membered rings tridentate ligands - [Au^III(trident)Cl]³⁺ⁿ (trident = N^N^N, C^N^N, C^C^N, C^N^C, and N^C^N). The calculated E^o covered a broad range of 2500 mV with the most stable complexes containing two AuC bonds (E^o = -1.85 V for [Au^III(C^C^N)Cl] - f). For complexes with one AuC bond, the N^C^N ligands stabilize the gold(III) complex more efficiently than N^N^C; however, the inclusion of the non-innocent ligand bipy (2,2'-bipyridine) in N^N portion provides an extra stabilization effect. Among the derivatives with one AuC bond, [Au^III(N^N^C)Cl]⁺ (N^N = bipy) (a) showed E^o = -1.20 V. For the complexes with N^N^N ligands, E^o was positive and almost constant (+0.60 V). Furthermore, the kinetics for ligand exchange reactions (Cl^-/H₂O, H₂O/Cys and Cl^-/Cys) were monitored for the most stable compounds and the energy profiles compared to the reduction pathways.

Subcellular delivery of lipid nanoparticles to endoplasmic reticulum and mitochondria

Primarily responsible for the biogenesis and metabolism of biomolecules, endoplasmic reticulum (ER) and mitochondria are gradually becoming the targets of therapeutic modulation, whose physiological activities and pathological manifestations determine the functional capacity and even the survival of cells. Drug delivery systems with specific physicochemical properties (passive targeting), or modified by small molecular compounds, polypeptides, and biomembranes demonstrating tropism for ER and mitochondria (active targeting) are able to reduce the nonselective accumulation of drugs, enhancing efficacy while reducing side effects. Lipid nanoparticles feature high biocompatibility, diverse cargo loading, and flexible structure modification, which are frequently used for subcellular organelle-targeted delivery of therapeutics. However, there is still a lack of systematic understanding of lipid nanoparticle-based ER and mitochondria targeting. Herein, we review the pathological significance of drug selectively delivered to the ER and mitochondria. We also summarize the molecular basis and application prospects of lipid nanoparticle-based ER and mitochondria targeting strategies, which may provide guidance for the prevention and treatment of associated diseases and disorders. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Biology-Inspired Nanomaterials > Lipid-Based Structures Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

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.

[(C C)Au(N N)]+ Complexes as a New Family of Anticancer Candidates: Synthesis, Characterization and Exploration of the Antiproliferative Properties

A library of eleven cationic gold(III) complexes of the general formula [(C C)Au(N N)]⁺ when C C is either biphenyl or 4,4'-ditertbutyldiphenyl and N N is a bipyridine, phenanthroline or dipyridylamine derivative have been synthesized and characterized. Contrasting effects on the viability of the triple negative breast cancer cells MDA-MB-231 was observed from a preliminary screening. The antiproliferative activity of the seven most active complexes were further assayed on a larger panel of human cancer cells as well as on non-cancerous cells for comparison. Two complexes stood out for being either highly active or highly selective. Eventually, reactivity studies with biologically meaningful amino acids, glutathione, higher order DNA structures and thioredoxin reductase (TrxR) revealed a markedly different behavior from that of the well-known coordinatively isomeric [(C N C)Au(NHC)]⁺ structure. This makes the [(C C)Au(N N)]⁺ complexes a new class of organogold compounds with an original mode of action.

Antitumor Activities for Two Pt(II) Complexes of Tropolone and 8-Hydroxyquinoline Derivative

The reactions of cis-Pt(DMSO)2Cl2 and tropolone (HL) with 8-hydroxyquinoline (HQ) or 2-methyl-8-hydroxyquinoline (HMQ) gave [Pt(Q)(L)] (1) and [Pt(MQ)(L)] (2), which present mononuclear structures with their Pt(II) ions four-coordinated in square planar geometries. Their in vitro biological properties were evaluated by MTT assay, which showed a remarkable cytotoxic activity on the cancer cell lines. 1 shows higher cytotoxic activities on tumor cells such as T24, HeLa, A549, and NCI-H460 than complex 2 and cisplatin, with IC50 values <16 μM. Among them, an IC50 value of 3.6 ± 0.63 μM was found for complex 1 against T24 cells. It presented a tuning cytotoxic activity by substitution groups on 8-hydroxyquinoline skeleton. In our case, the substitution groups of -H are much superior to -CH3 against tumor cells. It revealed that both complexes can induce cell apoptosis by decreasing the potential of a mitochondrial membrane, enhancing reactive oxygen species and increasing Ca2+ levels of T24 cells. The T24 cell cycle can be arrested at G2 and G1 phases by complexes 1 and 2, respectively, with an upregulation for P21 and P27 expression levels and a down-regulation for cyclin A, CDK1, Cdc25A, and cyclin B expression levels. Furthermore, complex 1 exhibits satisfactory in vivo antitumor activity as revealed by the tumor inhibitory rate and the tumor weight change as well as by the cute toxicity assay and renal pathological examinations, which is close to cisplatin and much better than complex 2. All of these suggest that 1 might be a potential candidate for developing into a safe and effective anticancer agent.

Cancer molecular biology and strategies for the design of cytotoxic gold(I) and gold(III) complexes: a tutorial review

This tutorial review highlights key principles underpinning the design of selected metallodrugs to target specific biological macromolecules (DNA and proteins). The review commences with a descriptive overview of the eukaryotic cell cycle and the molecular biology of cancer, particularly apoptosis, which is provided as a necessary foundation for the discovery, design, and targeting of metal-based anticancer agents. Drugs which target DNA have been highlighted and clinically approved metallodrugs discussed. A brief history of the development of mainly gold-based metallodrugs is presented prior to addressing ligand systems for stabilizing and adding functionality to bio-active gold(I) and gold(III) complexes, particularly in the burgeoning field of anticancer metallodrugs. Concepts such as multi-modal and selective cytotoxic agents are covered where necessary for selected compounds. The emerging role of carbenes as the ligand system of choice to achieve these goals for gold-based metallodrug candidates is highlighted prior to closing the review with comments on some future directions that this research field might follow. The latter section ultimately emphasizes the importance of understanding the fate of metal complexes in cells to garner key mechanistic insights.

Selective cytotoxicity of cyclometalated gold(III) complexes on Caco-2 cells is mediated by G2/M cell cycle arrest

New cyclometalated gold(III) complexes with a general structure [Au(C^N)(SR)2] or [Au(C^N)Cl(SR)], where C^N is a biphenyl ligand such as 2-(p-tolyl)pyridinate (tpy), 2-phenylpyridinate (ppy) and 2-benzylpyridinate (bzp) (SR = Spym, S(Me)2pym, 2-thiouracil (2-TU) and thiourea), and also with ethynyl moieties of the type [Au(C^N)(C≡C-Ar)2] (Ar = p-toluene and 2-pyridine) have been synthesized. All of them have been characterized, including X-ray studies of complex [Au(bzp)Cl(Spym)], and these studies have permitted to elucidate that leaving chloride ligand is trans located to CAr atom. After the full characterization, physicochemical properties were measured by evaluating drug-like water solubility and cell permeability (partition coefficient). All these experiments pointed that our complexes present adequate properties to be used as anticancer drugs. Although not all the complexes showed antiproliferative effects on Caco-2 cells, those that did were more cytotoxic than cisplatin; and complex [Au(tpy)Cl(2-TU)] is even more active than auranofin. In addition to this effectiveness, no evidence of cytotoxic effects was observed on considered normal cells (with the exception of [Au(bzp)Cl(2-TU)]. Further action mechanisms studies were performed using these selective complexes, showing cell cycle arrest on the G2/M phase, a proapoptotic behaviour and also the modification of some genes involved in tumorigenesis. Thus, as a result of this investigation, we present a new family of 17 cyclometalated complexes, 6 of them being selective and possible candidates to be used against colon cancer.

A Cytotoxic Bis(1,2,3-triazol-5-ylidene)carbazolide Gold(III) Complex Targets DNA by Partial Intercalation

The syntheses of bis(triazolium)carbazole precursors and their corresponding coinage metal (Au, Ag) complexes are reported. For alkylated triazolium salts, di- or tetranuclear complexes with bridging ligands were isolated, while the bis(aryl) analogue afforded a bis(carbene) AuI -CNC pincer complex suitable for oxidation to the redox-stable [AuIII (CNC)Cl]+ cation. Although the ligand salt and the [AuIII (CNC)Cl]+ complex were both notably cytotoxic toward the breast cancer cell line MDA-MB-231, the AuIII complex was somewhat more selective. Electrophoresis, viscometry, UV-vis, CD and LD spectroscopy suggest the cytotoxic [AuIII (CNC)Cl]+ complex behaves as a partial DNA intercalator. In silico screening indicated that the [AuIII (CNC)Cl]+ complex can target DNA three-way junctions with good specificity, several other regular B-DNA forms, and Z-DNA. Multiple hydrophobic π-type interactions involving T and A bases appear to be important for B-form DNA binding, while phosphate O⋅⋅⋅Au interactions evidently underpin Z-DNA binding. The CNC ligand effectively stabilizes the AuIII ion, preventing reduction in the presence of glutathione. Both the redox stability and DNA affinity of the hit compound might be key factors underpinning its cytotoxicity in vitro.

Rollover Cyclometalation vs Nitrogen Coordination in Tetrapyridyl Anticancer Gold(III) Complexes: Effect on Protein Interaction and Toxicity

In this work, a pair of gold(III) complexes derived from the analogous tetrapyridyl ligands H₂biqbpy1 and H₂biqbpy2 was prepared: the rollover, bis-cyclometalated [Au(biqbpy1)Cl ([1]Cl) and its isomer [Au(biqbpy2)Cl ([2]Cl). In [1]⁺, two pyridyl rings coordinate to the metal via a Au-C bond (C^∧N^∧N^∧C coordination) and the two noncoordinated amine bridges of the ligand remain protonated, while in [2]⁺ all four pyridyl rings of the ligand coordinate to the metal via a Au-N bond (N^∧N^∧N^∧N coordination), but both amine bridges are deprotonated. As a result, both complexes are monocationic, which allowed comparison of the sole effect of cyclometalation on the chemistry, protein interaction, and anticancer properties of the gold(III) compounds. Due to their identical monocationic charge and similar molecular shape, both complexes [1]Cl and [2]Cl displaced reference radioligand [³H]dofetilide equally well from cell membranes expressing the K_v11.1 (hERG) potassium channel, and more so than the tetrapyridyl ligands H₂biqbpy1 and H₂biqbpy2. By contrast, cyclometalation rendered [1]Cl coordinatively stable in the presence of biological thiols, while [2]Cl was reduced by a millimolar concentration of glutathione into metastable Au(I) species releasing the free ligand H₂biqbpy2 and TrxR-inhibiting Au⁺ ions. The redox stability of [1]Cl dramatically decreased its thioredoxin reductase (TrxR) inhibition properties, compared to [2]Cl. On the other hand, unlike [2]Cl, [1]Cl aggregated into nanoparticles in FCS-containing medium, which resulted in much more efficient gold cellular uptake. [1]Cl had much more selective anticancer properties than [2]Cl and cisplatin, as it was almost 10 times more cytotoxic to human cancer cells (A549, A431, A375, and MCF7) than to noncancerous cells (MRC5). Mechanistic studies highlight the strikingly different mode of action of the two compounds: while for [1]Cl high gold cellular uptake, nuclear DNA damage, and interaction with hERG may contribute to cell killing, for [2]Cl extracellular reduction released TrxR-inhibiting Au⁺ ions that were taken up in minute amounts in the cytosol, and a toxic tetrapyridyl ligand also capable of binding to hERG. These results demonstrate that bis-cyclometalation is an appealing method to improve the redox stability of Au(III) compounds and to develop gold-based cytotoxic compounds that do not rely on TrxR inhibition to kill cancer cells.

A "Golden Age" for the discovery of new antileishmanial agents: Current status of leishmanicidal gold complexes and prospective targets beyond the trypanothione system

Leishmaniasis is one of the most neglected diseases worldwide and is considered a serious public health issue. The current therapeutic options have several disadvantages that make the search for new therapeutics urgent. Gold compounds are emerging as promising candidates based on encouraging in vitro and limited in vivo results for several Au^I and Au^III complexes. The antiparasitic mechanisms of these molecules remain only partially understood. However, a few studies have proposed the trypanothione redox system as a target, similar to the mammalian thioredoxin system, pointed out as the main target for several gold compounds with significant antitumor activity. In this review, we present the current status of the investigation and design of gold compounds directed at treating leishmaniasis. In addition, we explore potential targets in Leishmania parasites beyond the trypanothione system, taking into account previous studies and structure modulation performed for gold-based compounds.

Highly cytotoxic gold(i)-phosphane dithiocarbamate complexes trigger an ER stress-dependent immune response in ovarian cancer cells

Ovarian cancer is a highly aggressive disease which is treated by surgery and platinum chemotherapy. However, a significant proportion of treated patients develop resistance to platinum treatment resulting in tumor relapse. Acquired platinum resistance has been recently correlated with activation of pro-survival endoplasmic reticulum (ER) stress responses. We hypothesized that Au complexes that induce severe ER stress might counteract pro-survival cellular attempts leading to the ER stress-mediated apoptosis and reduced platinum resistance. In this work, we prepared a series of highly cytotoxic AuI-dialkyldithiocarbamate complexes and investigated their anticancer potential in ovarian cancer cells. Complexes demonstrated surprisingly low stability in chloroform, resulting in the formation of an Au chain polymer, which also displayed excellent cytotoxicity. Lead complex 2 induced oxidative stress and ER stress-mediated p53-independent apoptosis associated with PARP cleavage and cell cycle arrest at G2/M phase. Importantly, 2 caused the surface exposure of calreticulin (CRT), which is the first step in the activation of cellular immunogenic response.

Developing a Novel Gold(III) Agent to Treat Glioma Based on the Unique Properties of Apoferritin Nanoparticles: Inducing Lethal Autophagy and Apoptosis

Effective delivery of anticancer agents across the blood-brain barrier (BBB) required innovative strategies to achieve glioma regression. To resolve this problem, we proposed to develop a metal agent that target and treat glioma based on the unique property of apoferritin (AFt) nanoparticles (NPs). Thus, we synthesized a series of Au(III) 3-(4-metyl piperidine)thiosemicarbazides compounds and analyzed their structure-activity relationships, obtaining a Au agent (C6) with remarkable cytotoxicity in glioma. Moreover, we confirmed that C6 kills glioma cells by inducing lethal autophagy and apoptosis. Importantly, our results revealed that the successfully constructed apoferritin-C6 NPs (AFt-C6 NPs) can effectively cross the BBB, inhibit glioma growth, and selectively accumulate in tumors.

Exploring the In Vivo and In Vitro Anticancer Activity of Rhenium Isonitrile Complexes

The established platinum-based drugs form covalent DNA adducts to elicit their cytotoxic response. Although they are widely employed, these agents cause toxic side-effects and are susceptible to cancer-resistance mechanisms. To overcome these limitations, alternative metal complexes containing the rhenium(I) tricarbonyl core have been explored as anticancer agents. Based on a previous study ( Chem. Eur. J. 2019, 25, 9206), a series of highly active tricarbonyl rhenium isonitrile polypyridyl (TRIP) complexes of the general formula fac-[Re(CO)3(NN)(ICN)]+, where NN is a chelating diimine and ICN is an isonitrile ligand, that induce endoplasmic reticulum (ER) stress via activation of the unfolded protein response (UPR) pathway are investigated. A total of 11 of these TRIP complexes were synthesized, modifying both the equatorial polypyridyl and axial isonitrile ligands. Complexes with more electron-donating equatorial ligands were found to have greater anticancer activity, whereas the axial ICN ligands had a smaller effect on their overall potency. All 11 TRIP derivatives trigger a similar phenotype that is characterized by their abilities to induce ER stress and activate the UPR. Lastly, we explored the in vivo efficacy of one of the most potent complexes, fac-[Re(CO)3(dmphen)(ptolICN)]+ (TRIP-1a), where dmphen = 2,9-dimethyl-1,10-phenanthroline and ptolICN = para-tolyl isonitrile, in mice. The 99mTc congener of TRIP-1a was synthesized, and its biodistribution in BALB/c mice was investigated in comparison to the parent Re complex. The results illustrate that both complexes have similar biodistribution patterns, suggesting that 99mTc analogues of these TRIP complexes can be used as diagnostic partner agents. The in vivo antitumor activity of TRIP-1a was then investigated in NSG mice bearing A2780 ovarian cancer xenografts. When administered at a dose of 20 mg/kg twice weekly, this complex was able to inhibit tumor growth and prolong mouse survival by 150% compared to the vehicle control cohort.

Mercury-Free Synthesis of Pincer [C^N^C]AuIII Complexes by an Oxidative Addition/CH Activation Cascade

Starting from the commercially available dimethyl sulfide-gold(I) chloride complex (DMSAuCl) and diazonium salts in the presence of 2,6-di-tert-butyl-4-methylpyridine as base, symmetric and unsymmetric [C^N^C]Au^III Cl complexes were synthesized in a selective, photosensitizer-free, photochemical reaction using blue LED light. This new protocol provides the first mercury-free synthesis of these types of pincer-complexes in moderate-to-excellent yields, starting from a readily available gold(I) precursor. Owing to the extraordinary properties of the target compounds, like excellent luminescence and high anticancer activities, the synthesis of such complexes is a highly active field of research, which might make its way to an industrial application. Owing to the disadvantages of the known protocols, especially the toxicity and the selectivity issues in the case of unsymmetric complexes, avoiding the use of mercury, should further accelerate this ongoing development.

Synthesis, mechanisms of action, and toxicity of novel aminophosphonates derivatives conjugated irinotecan in vitro and in vivo as potent antitumor agents

Twenty novel aminophosphonates derivatives (5a-5j and 6a-6j) conjugated irinotecan were synthesized through esterification reaction, and evaluated their anticancer activities using MTT assay. In vitro evaluation revealed that they displayed similar or superior cytotoxicity compared to the positive drug irinotecan against A549, MCF-7, SK-OV-3, MG-63, U2OS and multidrug-resistant (MDR) SK-OV-3/CDDP cancer cell lines. Among them, 9b displayed the most potent activity, with IC₅₀ values of 0.92-3.23 μM against five human cancer cells, which exhibited a 5.4-19.1-fold increase in activity compared to the reference drug irinotecan, respectively. Moreover, cellular mechanism studies suggested that 9b arrested cell cycle at S stage and induced cell apoptosis along with the decrease of mitochondrial membrane potential (MMP). Interestingly, 9b significantly inhibited tumor growth in SK-OV-3 xenograft models in vivo without apparent toxicity, which was better than the positive drug irinotecan. Taken together, 9b possessed potent antitumor activity and may be a promising candidate for the potential treatment of human ovarian cancer cells.

Endoplasmic reticulum stress: an arising target for metal-based anticancer agents

Metal anticancer agents are rapidly emerging as selective, potent therapeutics that exhibit anticancer activity by inducing endoplasmic reticulum stress.

Conjugation of a gold(iii) complex with vitamin B1 and chlorambucil derivatives: anticancer evaluation and mechanistic insights

A gold(iii) complex containing vitamin B1 and chlorambucil derivatives was investigated for mechanistic insights in colon and breast cancer treatment.

Cyclometallated Au(iii) dithiocarbamate complexes: synthesis, anticancer evaluation and mechanistic studies

A series of cationic mixed cyclometallated (C^N)Au(iii) dithiocarbamate complexes has been synthesized in good yields [HC^N = 2-(p-t-butylphenyl)pyridine]. The crystal structure of [(C^N)AuS2CNEt2]PF6 (3) has been determined. The cytotoxic properties of the new complexes have been evaluated in vitro against a panel of human cancer cell lines and healthy cells and compared with a neutral mixed (C^C)Au(iii) dithiocarbamate complex (C^C = 4,4'-di-t-butylbiphenyl-2,2'-diyl). The complexes appeared to be susceptible to reduction by glutathione but were stable in the presence of N-acetyl cysteine. The potential mechanism of action of this class of compounds has been investigated by measuring the intracellular uptake of some selected complexes, by determining their interactions with higher order DNA structures, and by assessing the ability to inhibit thioredoxin reductase. The complexes proved unable to induce the formation of reactive oxygen species. The investigations add to the picture of the possible mode of action of this class of complexes.

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.

Scope of organometallic compounds based on transition metal-arene systems as anticancer agents: starting from the classical paradigm to targeting multiple strategies

The advent of the clinically approved drug cisplatin started a new era in the design of metallodrugs for cancer chemotherapy. However, to date, there has not been much success in this field due to the persistence of some side effects and multi-drug resistance of cancer cells. In recent years, there has been increasing interest in the design of metal chemotherapeutics using organometallic complexes due to their good stability and unique properties in comparison to normal coordination complexes. Their intermediate properties between that of traditional inorganic and organic materials provide researchers with a new platform for the development of more promising cancer therapeutics. Classical metal-based drugs exert their therapeutic potential by targeting only DNA, but in the case of organometallic complexes, their molecular target is quite distinct to avoid drug resistance by cancer cells. Some organometallic drugs act by targeting a protein or inhibition of enzymes such as thioredoxin reductase (TrRx), while some target mitochondria and endoplasmic reticulum. In this review, we mainly discuss organometallic complexes of Ru, Ti, Au, Fe and Os and their mechanisms of action and how new approaches improve their therapeutic potential towards various cancer phenotypes. Herein, we discuss the role of structure-reactivity relationships in enhancing the anticancer potential of drugs for the benefit of humans both in vitro and in vivo. Besides, we also include in vivo tumor models that mimic human physiology to accelerate the development of more efficient clinical organometallic chemotherapeutics.

Metal-based antitumor compounds: beyond cisplatin

Despite improvements in the 5-year survival rate to over 80% in cancers, such as Hodgkin lymphoma and testicular cancer, more aggressive tumors including pancreatic and brain cancer still have extremely low survival rates. The establishment of chemoresistance, responsible for the reduction in treatment efficiency and cancer relapse, is one possible explanation for this setback. Metal-based compounds, a class of anticancer drugs, are largely used in the treatment of cancer. Herein, we will review the use of metal-based small molecules in chemotherapy, focusing on recent studies, and we will discuss how new nonplatinum-based agents are prompting scientists to increase drug specificity to overcome chemoresistance in cancer cells.

Acridine-decorated cyclometallated gold(iii) complexes: synthesis and anti-tumour investigations

(C^N) and (C^N^C) cyclometalated Au(iii) represent a highly promising class of potential anticancer agents. We report here the synthesis of seven new cyclometalated Au(iii) complexes with five of them bearing an acridine moiety attached via (N^O) or (N^N) chelates, acyclic amino carbenes (AAC) and N-heterocyclic carbenes (NHC). The antiproliferative properties of the different complexes were evaluated in vitro on a panel of cancer cells including leukaemia, lung and breast cancer cells. We observed a trend between the cytotoxicity and the intracellular gold uptake of some representative compounds of the series. Some of the acridine-decorated complexes were demonstrated to interact with ds-DNA using FRET-melting techniques.

New Variations on the Theme of Gold(III) C∧N∧N Cyclometalated Complexes as Anticancer Agents: Synthesis and Biological Characterization

A series of novel (C^∧N^∧N) cyclometalated Au^III complexes of general formula [Au(bipy^dmb-H)X][PF₆] (bipy^dmb-H = C^∧N^∧N cyclometalated 6-(1,1-dimethylbenzyl)-2,2'-bipyridine) were prepared with a range of anionic ligands X in the fourth coordination position, featuring C (alkynyl)-, N-, O-, or S-donor atoms. The X ligands are varied in nature and include three coumarins, 4-ethynylaniline, saccharine, and thio-β-d-glucose tetraacetate, the tripeptide glutathione (GSH), and a coumarin-substituted amide derived from 4-ethynylaniline. The gold(I) complex [Au(C₂ArNHCOQ)(PPh₃)] (HC₂ArNHCOQ = N-(4-ethynylphenyl)-2-oxo-2 H-chromene-3-carboxamide) was also prepared for comparison. The new compounds were fully characterized by means of analytical techniques, including NMR, absorption, and emission spectroscopy. The crystal structures of three cyclometalated Au^III complexes and of the Au^I derivative were solved by single-crystal X-ray diffraction. The antiproliferative activity of the new Au^III cyclometalated derivatives was evaluated against cancer cells in vitro. According to the obtained results, only complexes 3-PF₆ and 5-PF₆, featuring coumarins as ancillary ligands and endowed with high redox stability in solution, display antiproliferative effects, with 5-PF₆ being the most potent, while all of the others are scarcely active to nonactive in the selected cell lines. In order to study the reactivity of the compounds with biomolecules, the interaction of complexes 3-PF₆ and 5-PF₆ with the protein cytochrome c and the amino acids cysteine and histidine was analyzed by electrospray ionization mass spectrometry (ESI MS), showing adduct formation only with Cys after at least 1 h incubation. Furthermore, the parent hydroxo complex [Au(bipy^dmb-H)(OH)][PF₆] (1OH-PF₆) was investigated in a competitive assay to determine the protein vs oligonucleotide binding preferences by capillary zone electrophoresis (CZE) coupled to ESI-MS. Of note, the compound was found to selectively form adducts with the oligonucleotide over the protein upon ligand exchange with the hydroxido ligand. Adduct formation occurred within the first 10 min of incubation, demonstrating the preference of 1OH-PF₆ for nucleotides in this setup. Overall, the obtained results point toward the possibility to selectively target DNA with gold(III) organometallics.

Synthesis of Gold(III) Complexes with Bidentate Amino-Thiolate Ligands as Precursors of Novel Bifunctional Acyclic Diaminocarbenes

Two neutral bis(pentafluorophenyl)thiolate gold(III) complexes with the unsymmetrical S^N ligands 2-aminothiophenol or cysteamine have been synthesized and their reactivity has been studied. Homo- and heterodinuclear compounds were obtained by their coordination to gold(I) or silver(I) derivatives through the sulfur atom. Interestingly, a tetranuclear derivative bearing short gold(I)···gold(I) and the more unusual gold(I)···gold(III) interactions has been prepared. These amino-thiolate derivatives can be used as precursors for the synthesis of novel gold(III) acyclic diaminocarbene complexes by reaction with isocyanides CNR. The nucleophilic attack of the amino group to isocyanide molecules affords the synthesis of unprecedented bidentate C^S acyclic diaminocarbene ligands. All of the complexes are air- and moisture-stable at room temperature and have been spectroscopically and structurally characterized.

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

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

Organometallic Gold(III) Complexes Similar to Tetrahydroisoquinoline Induce ER-Stress-Mediated Apoptosis and Pro-Death Autophagy in A549 Cancer Cells

Agents inducing both apoptosis and autophagic death can be effective chemotherapeutic drugs. In our present work, we synthesized two organometallic gold(III) complexes harboring C^N ligands that structurally resemble tetrahydroisoquinoline (THIQ): Cyc-Au-1 (AuL¹Cl₂, L¹ = 3,4-dimethoxyphenethylamine) and Cyc-Au-2 (AuL²Cl₂, L² = methylenedioxyphenethylamine). In screening their in vitro activity, we found both gold complexes exhibited lower toxicity, lower resistance factors, and better anticancer activity than those of cisplatin. The organometallic gold(III) complexes accumulate in mitochondria and induce elevated ROS and an ER stress response through mitochondrial dysfunction. These effects ultimately result in simultaneous apoptosis and autophagy. Importantly, compared to cisplatin, Cyc-Au-2 exhibits lower toxicity and better anticancer activity in a murine tumor model. To the best of our knowledge, Cyc-Au-2 is the first organometallic Au(III) compound that induces apoptosis and autophagic death. On the basis of our results, we believe Cyc-Au-2 to be a promising anticancer agent or lead compound for further anticancer drug development.

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

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

Benzothiazole-Based Cycloplatinated Chromophores: Synthetic, Optical, and Biological Studies

Cycloplatinated complexes based on 2-(4-substituted)benzothiazole ligands of type [Pt(R-PBT-κC,N)Cl(L)] (PBT=2-phenylbenzothiazole; R=Br (1), Me₂ N (2); L=dimethyl sulfoxide (DMSO; a), 1,3,5- triaza-7-phosphaadamantane (PTA; b), triphenylphosphine 3,3',3''-trisulfonate (TPPTS; c)) and [Pt(Br-PBT-κC)Cl(PTA)₂ ] (3) are presented. On the basis of the photophysical data and time-dependent (TD)-DFT calculations (1 a and 2 a), the low-lying transitions (absorption and emission) were associated with ligand-center (LC) charge transfer, with minor metal-to-ligand charge transfer (MLCT), and intraligand charge transfer (ILCT) [Me₂ N-PBT→PBT] excited states, respectively. Simultaneous fluorescence/phosphorescence bands were found in fluid solutions (and also in the solid state for 2 a), which become dominated by triplet emission bands in rigid media at 77 K. The effect of the concentration on emissive behavior of 2 a, b indicated the occurrence of aggregation-induced luminescence properties related to the occurrence of metal-metal and π⋅⋅⋅π interactions, which are more enhanced in 2 a because of the less bulky DMSO ligand. The behavior of 2 a toward para-toluenesulfonic acid (PTSA) in aerated acetonitrile and to hydrogen chloride gas in the solid state has been evaluated, thus showing a clear reversible change between the ¹ ILCT and ³ LC/³ MLCT states due to protonation of the NMe₂ group (theoretical calculations on 2 a-H⁺ ). Solid 2 a undergoes a surprising oxidation of the Pt^II center to Pt^IV with concomitant deoxygenation of DMSO, under prolonged reaction with hydrogen chloride gas to afford the Pt^IV /dimethyl sulfide complex (mer-[Pt(Me₂ N-PBT-κC,N)Cl₃ (SMe₂ )]; mer-4), which evolves in solution to fac-4, as confirmed by X-ray studies. Cytotoxic activity studies on A549 and HeLa cell lines indicated cytotoxic activity of 1 b and 2 a, b. In addition, fluorescent cell microscopy revealed cytoplasmic staining, more visible in perinuclear areas. Inhibition of tubulin polymerization by 1 b in both cells is presented as a preliminary mechanism of its cytotoxic action.

Cytotoxicity of Pyrazine-Based Cyclometalated (C^Npz^C)Au(III) Carbene Complexes: Impact of the Nature of the Ancillary Ligand on the Biological Properties

The synthesis of a series of cyclometalated gold(III) complexes supported by pyrazine-based (C^N^C)-type pincer ligands is reported, including the crystal structure of a cationic example. The compounds provide a new platform for the study of antiproliferative properties of gold(III) complexes. Seven complexes were tested: the neutral series (C^N^pz^C)AuX [X = Cl (1), 6-thioguanine (4), C≡CPh (5), SPh (6)] and an ionic series that included the N-methyl complex [(C^N^pzMe^C)AuCl]BF₄ (7) and the N-heterocyclic carbene complexes [(C^N^pz^C)AuL]⁺ with L = 1,3-dimethylbenzimidazol-2-ylidene (2) or 1,3,7,9-tetramethylxanthin-8-ylidene (3). Tests against human leukemia cells identified 1, 2, 3, and 4 as particularly promising, whereas protecting the noncoordinated N atom on the pyrazine ring by methylation (as in 7) reduced the cytotoxicity. Complex 2 proved to be the most effective of the entire series against the HL60 leukemia, MCF-7 breast cancer, and A549 lung cancer cell lines, with IC₅₀ values down to submicromolar levels, associated with a lower toxicity toward healthy human lung fibroblast cells. The benzimidazolylidene complex 2 accumulated more effectively in human lung cancer cells than its caffeine-based analogue 3 and the gold(III) chloride 1. Compound 2 proved to be unaffected by glutathione under physiological conditions for periods of up to 6 days and stabilizes the DNA G-quadruplex and i-motif structures; the latter is the first such report for gold compounds. We also show the first evidence of inhibition of MDM2–p53 protein–protein interactions by a gold-based compound and identified the binding mode of the compound with MDM2 using saturation transfer difference NMR spectroscopy combined with docking calculations.

We synthesized three new (C^N^pz^C)Au(III) complexes and screened them along with four other complexes as potential anticancer agents against leukemia cells. We tested the cellular uptake, the interaction with G4 and i-motif DNA structures, and the interaction with MDM2 protein. We highlight the very different biological behaviors of the compounds due to the different ancillary ligands.