Direct Observation of Methylmercury and Auranofin Binding to Selenocysteine in Thioredoxin Reductase

Comparison of the mechanism of antimicrobial action of the gold(I) compound auranofin in Gram-positive and Gram-negative bacteria

While highly effective at killing Gram-positive bacteria, auranofin lacks significant activity against Gram-negative species for reasons that largely remain unclear. Here, we aimed to elucidate the molecular mechanisms underlying the low susceptibility of the Gram-negative model organism Escherichia coli to auranofin when compared to the Gram-positive model organism Bacillus subtilis. The proteome response of E. coli exposed to auranofin suggests a combination of inactivation of thiol-containing enzymes and the induction of systemic oxidative stress. Susceptibility tests in E. coli mutants lacking proteins upregulated upon auranofin treatment suggested that none of them are directly involved in E. coli's high tolerance to auranofin. E. coli cells lacking the efflux pump component TolC were more sensitive to auranofin treatment, but not to an extent that would fully explain the observed difference in susceptibility of Gram-positive and Gram-negative organisms. We thus tested whether E. coli's thioredoxin reductase (TrxB) is inherently less sensitive to auranofin than TrxB from B. subtilis, which was not the case. However, E. coli strains lacking the low-molecular-weight thiol glutathione, but not glutathione reductase, showed a high susceptibility to auranofin. Bacterial cells expressing the genetically encoded redox probe roGFP2 allowed us to observe the oxidation of cellular protein thiols in situ. Based on our findings, we hypothesize that auranofin leads to a global disturbance in the cellular thiol redox homeostasis in bacteria, but Gram-negative bacteria are inherently more resistant due to the presence of drug export systems and high cellular concentrations of glutathione.IMPORTANCEAuranofin is an FDA-approved drug for the treatment of rheumatoid arthritis. However, it has also high antibacterial activity, in particular against Gram-positive organisms. In the current antibiotics crisis, this would make it an ideal candidate for drug repurposing. However, its much lower activity against Gram-negative organisms prevents its broad-spectrum application. Here we show that, on the level of the presumed target, there is no difference in susceptibility between Gram-negative and Gram-positive species: thioredoxin reductases from both Escherichia coli and Bacillus subtilis are equally inhibited by auranofin. In both species, auranofin treatment leads to oxidative protein modification on a systemic level, as monitored by proteomics and the genetically encoded redox probe roGFP2. The single largest contributor to E. coli's relative resistance to auranofin seems to be the low-molecular-weight thiol glutathione, which is absent in B. subtilis and other Gram-positive species.

Exploring mercury detoxification in fish: The role of selenium from tuna byproduct diets for sustainable aquaculture

Exposure to mercury (Hg) through fish consumption poses significant environmental and public health risks, given its status as one of the top ten hazardous chemicals. Aquaculture is expanding, driving a surge in demand for sustainable aquafeeds. Tuna byproducts, which are rich in protein, offer potential for aquafeed production, yet their use is challenged by the high content of heavy metals, particularly Hg. However, these byproducts also contain elevated levels of selenium (Se), which may counteract Hg adverse effects. This study examines the fate of dietary Hg and Se in an aquaculture model fish. Biomolecular speciation analyses through hyphenated analytical approaches were conducted on the water-soluble protein fraction of key organs of juvenile rainbow trout (Oncorhynchus mykiss) exposed to various combinations of Hg and Se species, including diets containing tuna byproducts, over a six-month period. The findings shed light on the dynamics of Hg and Se compounds in fish revealing potential Hg detoxification mechanisms through complexation with Hg-biomolecules, such as cysteine, glutathione, and metallothionein. Furthermore, the trophic transfer of selenoneine is demonstrated, revealing novel opportunities for sustainable aquafeed production. Understanding the interactions between Hg and Se in aquaculture systems is crucial for optimizing feed formulations and mitigating environmental risks. This research contributes to the broader goal of advancing sustainable practices in aquaculture while addressing food security challenges.

Targeting the redox vulnerability of acute myeloid leukaemia cells with a combination of auranofin and vitamin C

Acute myeloid leukaemia (AML) is a heterogeneous disease characterized by complex molecular and cytogenetic abnormalities. Pro-oxidant cellular redox status is a common hallmark of AML cells, providing a rationale for redox-based anticancer strategy. We previously discovered that auranofin (AUF), initially used for the treatment of rheumatoid arthritis and repositioned for its anticancer activity, can synergize with a pharmacological concentration of vitamin C (VC) against breast cancer cell line models. In this study, we observed that this drug combination synergistically and efficiently killed cells of leukaemic cell lines established from different myeloid subtypes. In addition to an induced elevation of reactive oxygen species and ATP depletion, a rapid dephosphorylation of 4E-BP1 and p70S6K, together with a strong inhibition of protein synthesis were early events in response to AUF/VC treatment, suggesting their implication in AUF/VC-induced cytotoxicity. Importantly, a study on 22 primary AML specimens from various AML subtypes showed that AUF/VC combinations at pharmacologically achievable concentrations were effective to eradicate primary leukaemic CD34+ cells from the majority of these samples, while being less toxic to normal cord blood CD34+ cells. Our findings indicate that targeting the redox vulnerability of AML with AUF/VC combinations could present a potential anti-AML therapeutic approach.

Recharacterization of RSL3 reveals that the selenoproteome is a druggable target in colorectal cancer

Ferroptosis is a non-apoptotic form of cell death resulting from the iron-dependent accumulation of lipid peroxides. Colorectal cancer (CRC) cells accumulate high levels of intracellular iron and reactive oxygen species (ROS) and are thus particularly sensitive to ferroptosis. The compound (S)-RSL3 ([1S,3R]-RSL3) is a commonly used ferroptosis inducing compound that is currently characterized as a selective inhibitor of the selenocysteine containing enzyme (selenoprotein) Gluathione Peroxidase 4 (GPx4), an enzyme that utilizes glutathione to directly detoxify lipid peroxides. However, through chemical controls utilizing the (R) stereoisomer of RSL3 ([1R,3R]-RSL3) that does not bind GPx4, combined with inducible genetic knockdowns of GPx4 in CRC cell lines, we revealed that GPx4 dependency does not always align with (S)-RSL3 sensitivity, questioning the current characterization of GPx4 as the central regulator of ferroptosis. Utilizing affinity pull-down mass spectrometry with chemically modified (S)-RSL3 probes we discovered that the effects of (S)-RSL3 extend far beyond GPx4 inhibition, revealing that (S)-RSL3 is a broad and non-selective inhibitor of selenoproteins. To further investigate the therapeutic potential of broadly disrupting the selenoproteome as a therapeutic strategy in CRC, we employed additional chemical and genetic approaches. We found that the selenoprotein inhibitor auranofin, an FDA approved gold-salt, chemically induced oxidative cell death and ferroptosis in both in-vitro and in-vivo models of CRC. Consistent with these data, we found that AlkBH8, a tRNA-selenocysteine methyltransferase required for the translation of selenoproteins, is essential for the in-vitro growth and xenograft survival of CRC cell lines. In summary, these findings recharacterize the mechanism of action of the most commonly used ferroptosis inducing molecule, (S)-RSL3, and reveal that broad inhibition of selenoproteins is a promising novel therapeutic angle for the treatment of CRC.

Oxidative cell death in cancer: mechanisms and therapeutic opportunities

Reactive oxygen species (ROS) are highly reactive oxygen-containing molecules generated as natural byproducts during cellular processes, including metabolism. Under normal conditions, ROS play crucial roles in diverse cellular functions, including cell signaling and immune responses. However, a disturbance in the balance between ROS production and cellular antioxidant defenses can lead to an excessive ROS buildup, causing oxidative stress. This stress damages essential cellular components, including lipids, proteins, and DNA, potentially culminating in oxidative cell death. This form of cell death can take various forms, such as ferroptosis, apoptosis, necroptosis, pyroptosis, paraptosis, parthanatos, and oxeiptosis, each displaying distinct genetic, biochemical, and signaling characteristics. The investigation of oxidative cell death holds promise for the development of pharmacological agents that are used to prevent tumorigenesis or treat established cancer. Specifically, targeting key antioxidant proteins, such as SLC7A11, GCLC, GPX4, TXN, and TXNRD, represents an emerging approach for inducing oxidative cell death in cancer cells. This review provides a comprehensive summary of recent progress, opportunities, and challenges in targeting oxidative cell death for cancer therapy.

Gold-Promoted Biocompatible Selenium Arylation of Small Molecules, Peptides and Proteins

A low pKa (5.2), high polarizable volume (3.8 Å), and proneness to oxidation under ambient conditions make selenocysteine (Sec, U) a unique, natural reactive handle present in most organisms across all domains of life. Sec modification still has untapped potential for site-selective protein modification and probing. Herein we demonstrate the use of a cyclometalated gold(III) compound, [Au(bnpy)Cl2 ], in the arylation of diselenides of biological significance, with a scope covering small molecule models, peptides, and proteins using a combination of multinuclear NMR (including 77 Se NMR), and LC-MS. Diphenyl diselenide (Ph-Se)2 and selenocystine, (Sec)2 , were used for reaction optimization. This approach allowed us to demonstrate that an excess of diselenide (Au/Se-Se) and an increasing water percentage in the reaction media enhance both the conversion and kinetics of the C-Se coupling reaction, a combination that makes the reaction biocompatible. The C-Se coupling reaction was also shown to happen for the diselenide analogue of the cyclic peptide vasopressin ((Se-Se)-AVP), and the Bos taurus glutathione peroxidase (GPx1) enzyme in ammonium acetate (2 mM, pH=7.0). The reaction mechanism, studied by DFT revealed a redox-based mechanism where the C-Se coupling is enabled by the reductive elimination of the cyclometalated Au(III) species into Au(I).

Exposure of human glioblastoma cells to thimerosal inhibits the thioredoxin system and decreases tumor growth-related factors

Glioblastoma multiforme (GBM) is the most common, aggressive, and fatal primary malignant brain tumor in adults. The therapeutic efficacy of temozolomide (TMZ) is limited owing to frequent treatment resistance. The latter is in part related to the overexpression of redox systems such as the thioredoxin system. This system is fundamental for cell survival and proliferation, regulating hypoxia inducible factor-1alpha (HIF-1α) activity, in turn controlling vascular endothelial growth factor (VEGF), which is indispensable for tumor invasiveness, angiogenesis and microenvironment maintenance. HIF-1α can also be regulated by the signal transducer and activator of transcription 3 (STAT3), an oncogene stimulated by pro-inflammatory cytokines and growth factors. The thioredoxin system has several known inhibitors including mercury compounds such as Thimerosal (TmHg) which readily crosses the blood-brain barrier (BBB) and accumulates in the brain. Though previously used in various applications epidemiological evidence on TmHg's neurotoxicity is lacking. The objective of this study was to verify whether thimerosal is a suitable candidate for hard repurposing to control glioblastoma; therefore, the effects of this molecule were evaluated in human GBM (U87) cells. Our novel results show that TmHg decreased cellular viability (>50%) and migration (up to 90% decrease in wound closure), reduced thioredoxin reductase (TrxR/TXNRD1) and thioredoxin (Trx) activity, and increased reactive oxygen species (ROS) generation. Moreover, TmHg reduced HIF-1α expression (35%) as observed by immunofluorescence. Co-exposure of U87 cells to TmHg and TMZ reduced HIF-1α, VEGF, and phosphorylated STAT3. Consequently, TmHg alone or combined with chemotherapeutic drugs can reduce neoangiogenesis and ameliorate glioblastoma progression and treatment.

Mechanism of Action of Antitumor Au(I) N-Heterocyclic Carbene Complexes: A Computational Insight on the Targeting of TrxR Selenocysteine

The targeting of human thioredoxin reductase is widely recognized to be crucially involved in the anticancer properties of several metallodrugs, including Au(I) complexes. In this study, the mechanism of reaction between a set of five N-heterocyclic carbene Au(I) complexes and models of the active Sec residue in human thioredoxin reductase was investigated by means of density functional theory approaches. The study was specifically addressed to the kinetics and thermodynamics of the tiled process by aiming at elucidating and explaining the differential inhibitory potency in this set of analogous Au(I) bis-carbene complexes. While the calculated free energy profile showed a substantially similar reactivity, we found that the binding of these Au(I) bis-carbene at the active CysSec dyad in the TrxR enzyme could be subjected to steric and orientational restraints, underlining both the approach of the bis-carbene scaffold and the attack of the selenol group at the metal center. A new and detailed mechanistic insight to the anticancer activity of these Au(I) organometallic complexes was thus provided by consolidating the TrxR targeting paradigm.

S-Se oxidative addition to auranofin derivatives: a DFT study

Oxidative addition of the S-Se bond to Au(I) complexes is discussed for a series of 26 auranofin (AF) derivatives. AF and its analogues are Au(I) complexes with recognized anticancer activity that act by binding and inhibiting the thioredoxin reductase (TrxR) enzyme. Generally, the oxidative addition to Au(I) is a sluggish reaction under mild conditions (i.e., a high activation barrier - ΔH‡), which is also verified here for AF, ΔH‡ = 33.0 kcal mol-1. However, we predicted that subtle changes in the AF ligands can make the process feasible under standard conditions. For instance, the exchange of -PEt3 by -P(Et2)(OEt), which is a weaker electron σ-donor, reduced the activation barrier to 17.1 kcal mol-1. Furthermore, substitution of the -SAtg ligand by -Cl- leads to a ΔH‡ value of 22.5 kcal mol-1. Overall, the reaction is driven by the nucleophilic attack of the S-Se bond on the Au(I) center, attributed mainly to the charge transfer (4p)Se → (6p)Au, which characterizes the addition step. At the transition state (TS) point, the (5d)Au → σ*(S-Se) charge transfer becomes relevant, facilitating the S-Se bond breakage and the oxidation step. In addition to the electron transfers, the strain energy to deform the linear Au(I) geometry to the tetracoordinated Au(III) arrangement in the TS structure plays a primary role in explaining the trends in the activation barriers. Finally, the activation barrier (ΔH‡) and reaction energy (ΔH°) were correlated for most of the complexes studied, which suggests that the reaction passes through a late or product-like TS and, therefore, the steric and electronic factors affecting ΔH‡ also act on ΔH°. Overall, the results presented here might open up a new field of investigation for interactions between AF derivatives and TrxR, which contributes to a full understanding of the biological mechanism of action of these species.

Should ebselen be considered for the treatment of mercury intoxication? A minireview

Mercury is a ubiquitous environmental contaminant and can be found in inorganic (Hg0, Hg+ and Hg2+) and organic forms (chiefly CH3Hg+ or MeHg+). The main route of human, mammals and bird exposure occurs via predatory fish ingestion. Occupational exposure to Hg0 (and Hg2+) can also occur; furthermore, in gold mining areas the exposure to inorganic Hg can also be high. The toxicity of electrophilic forms of Hg (E+Hg) is mediated by disruption of thiol (-SH)- or selenol (-SeH)-containing proteins. The therapeutic approaches to treat methylmercury (MeHg+), Hg0 and Hg2+ are limited. Here we discuss the potential use of ebselen as a potential therapeutic agent to lower the body burden of Hg in man. Ebselen is a safe drug for humans and has been tested in clinical trials (for instance, brain ischemia, noise-induce hearing loss, diabetes complications, bipolar disorders) at doses varying from 400 to 3600 mg per day. Two clinical trials with ebselen in moderate and severe COVID are also approved. Ebselen can be metabolized to an intermediate with -SeH (selenol) functional group, which has a greater affinity to electrophilic Hg (E+Hg) forms than the available thiol-containing therapeutic agents. Accordingly, as observed in vitro and rodent models in vivo, Ebselen exhibited protective effects against MeHg+, indicating its potential as a therapeutic agent to treat MeHg+ overexposure. The combined use of ebselen with thiol-containing molecules (e.g. N-acetylcysteine and enaramide)) is also commented, because they can have synergistic protective effects against MeHg+.

Synchrotron X-ray methods in the study of mercury neurotoxicology

In situ methods are valuable in all fields of research. In toxicology, the importance of dose is well known, elevating the need for in situ techniques to measure levels of toxicants and their byproducts in precise anatomically identifiable locations. More recently, additional emphasis has been placed on the value of techniques which can detect chemical form or speciation, which is equally important in the toxicology of a chemical compound. Many important but conventional methods risk losing valuable information due to extractions, digestions, or the general reliance on mobile phases. Few analytical tools possess the power and diversity of X-ray methods as in-situ methods. Here we present an overview, intended for toxicologists and pathologists, of a variety of synchrotron X-ray methods for determining in situ chemical form and distribution of heavier elements. The versatility and range of these synchrotron techniques, which are both established and emerging, is demonstrated in the context of the study of neurotoxicology of mercury, a global pollutant with the ability to harm both human health and the environment.

TXNL1 has dual functions as a redox active thioredoxin-like protein as well as an ATP- and redox-independent chaperone

TXNL1 (also named TRP32, for thioredoxin related protein of 32 kDa) is a cytosolic thioredoxin-fold protein expressed in all cell types and conserved from yeast to mammals, but with yet poorly known function. Here, we expressed and purified human TXNL1 together with several Cys-to-Ser variants, characterizing their enzymatic properties. TXNL1 could reduce disulfides in insulin, cystine and glutathione disulfide (GSSG) in reactions coupled to thioredoxin reductase (TXNRD1, TrxR1) using NADPH, similarly to thioredoxin (TXN, Trx1), but with lower catalytic efficacy due to at least one order of magnitude higher Km of TrxR1 for TXNL1 compared to Trx1. However, in sharp contrast to Trx1, we found that TXNL1 also had efficient chaperone activity that did not require ATP. TXNL1 made non-covalent complexes with reduced insulin, thereby keeping it in solution, and TXNL1 provided chaperone function towards whole cell lysate proteins by preventing their aggregation during heating. The chaperone activities of TXNL1 did not require its redox activity or any dithiol-disulfide exchange reactions, as revealed using Cys-to-Ser substituted variants, as well as a maintained chaperone activity of TXNL1 also in the absence of TrxR1 and NADPH. These results reveal that TXNL1 has dual functions, supporting TrxR1-driven redox activities in disulfide reduction reactions, as well as being an ATP-independent chaperone that does not require involvement of its redox activity.

Taming the Biological Activity of Pd(II) and Pt(II) Complexes with Triazolato "Protective" Groups: 1H, 77Se Nuclear Magnetic Resonance and X-ray Crystallographic Model Studies with Selenocysteine to Elucidate Differential Thioredoxin Reductase Inhibition

The biological activity of Pd(II) and Pt(II) complexes toward three different cancer cell lines as well as inhibition of selenoenzyme thioredoxin reductase (TrxR) was modulated in an unexpected way by the introduction of triazolate as a "protective group" to the inner metal coordination sphere using the iClick reaction of [M(N3)(terpy)]PF6 [M = Pd(II) or Pt(II) and terpy = 2,2':6',2″-terpyridine] with an electron-poor alkyne. In a cell proliferation assay using A549, HT-29, and MDA-MB-231 human cancer cell lines, the palladium compound was significantly more potent than the isostructural platinum analogue and exhibited submicromolar activity on the most responsive cell line. This difference was also reflected in the inhibitory efficiency toward TrxR with IC50 values of 0.1 versus 5.4 μM for the Pd(II) and Pt(II) complexes, respectively. UV/Vis kinetic studies revealed that the Pt compound binds to selenocysteine faster than to cysteine [k = (22.9 ± 0.2)·10-3 vs (7.1 ± 0.2)·10-3 s-1]. Selective triazolato ligand exchange of the title compounds with cysteine (Hcys) and selenocysteine (Hsec)─but not histidine (His) and 9-ethylguanine (9EtG)─was confirmed by 1H, 77Se, and 195Pt NMR spectroscopy. Crystal structures of three of the four ligand exchange products were obtained, including [Pt(sec)(terpy)]PF6 as the first metal complex of selenocysteine to be structurally characterized.

Benzophenanthridine Alkaloid Chelerythrine Elicits Necroptosis of Gastric Cancer Cells via Selective Conjugation at the Redox Hyperreactive C-Terminal Sec498 Residue of Cytosolic Selenoprotein Thioredoxin Reductase

Targeting thioredoxin reductase (TXNRD) with low-weight molecules is emerging as a high-efficacy anti-cancer strategy in chemotherapy. Sanguinarine has been reported to inhibit the activity of TXNRD1, indicating that benzophenanthridine alkaloid is a fascinating chemical entity in the field of TXNRD1 inhibitors. In this study, the inhibition of three benzophenanthridine alkaloids, including chelerythrine, sanguinarine, and nitidine, on recombinant TXNRD1 was investigated, and their anti-cancer mechanisms were revealed using three gastric cancer cell lines. Chelerythrine and sanguinarine are more potent inhibitors of TXNRD1 than nitidine, and the inhibitory effects take place in a dose- and time-dependent manner. Site-directed mutagenesis of TXNRD1 and in vitro inhibition analysis proved that chelerythrine or sanguinarine is primarily bound to the Sec498 residue of the enzyme, but the neighboring Cys497 and remaining N-terminal redox-active cysteines could also be modified after the conjugation of Sec498. With high similarity to sanguinarine, chelerythrine exhibited cytotoxic effects on multiple gastric cancer cell lines and suppressed the proliferation of tumor spheroids derived from NCI-N87 cells. Chelerythrine elevated cellular levels of reactive oxygen species (ROS) and induced endoplasmic reticulum (ER) stress. Moreover, the ROS induced by chelerythrine could be completely suppressed by the addition of N-acetyl-L-cysteine (NAC), and the same is true for sanguinarine. Notably, Nec-1, an RIPK1 inhibitor, rescued the chelerythrine-induced rapid cell death, indicating that chelerythrine triggers necroptosis in gastric cancer cells. Taken together, this study demonstrates that chelerythrine is a novel inhibitor of TXNRD1 by targeting Sec498 and possessing high anti-tumor properties on multiple gastric cancer cell lines by eliciting necroptosis.

Disclosing the Preferential Mercury Chelation by SeCys Containing Peptides over Their Cys Analogues

Methylmercury, mercury (II), and mercury (I) chlorides were found to react with vasopressin, a nonapeptide hormone cyclized by two cysteine residues, and its mono- and diselenium analogues to form several mercury-peptide adducts. The replacement of Cys by SeCys in vasopressin increased the reactivity toward methylmercury, with the predominant formation of -Se/S-Hg-Se-bridged structures and the consequent demethylation of methylmercury. In competitive experiments, CH3HgCl reacted preferentially with the diselenium analogue rather than with vasopressin. The diselenium peptide also showed the capability to displace the CH3Hg moiety bound to S in vasopressin. These results open a promising perspective for the use of selenopeptides for methylmercury chelation and detoxification strategies.

Novel gold(III)-dithiocarbamate complex targeting bacterial thioredoxin reductase: antimicrobial activity, synergy, toxicity, and mechanistic insights

Introduction

Antimicrobial resistance is a pressing global concern that has led to the search for new antibacterial agents with novel targets or non-traditional approaches. Recently, organogold compounds have emerged as a promising class of antibacterial agents. In this study, we present and characterize a (C^S)-cyclometallated Au(III) dithiocarbamate complex as a potential drug candidate.

Methods and results

The Au(III) complex was found to be stable in the presence of effective biological reductants, and showed potent antibacterial and antibiofilm activity against a wide range of multidrug-resistant strains, particularly gram-positive strains, and gram-negative strains when used in combination with a permeabilizing antibiotic. No resistant mutants were detected after exposing bacterial cultures to strong selective pressure, indicating that the complex may have a low propensity for resistance development. Mechanistic studies indicate that the Au(III) complex exerts its antibacterial activity through a multimodal mechanism of action. Ultrastructural membrane damage and rapid bacterial uptake suggest direct interactions with the bacterial membrane, while transcriptomic analysis identified altered pathways related to energy metabolism and membrane stability including enzymes of the TCA cycle and fatty acid biosynthesis. Enzymatic studies further revealed a strong reversible inhibition of the bacterial thioredoxin reductase. Importantly, the Au(III) complex demonstrated low cytotoxicity at therapeutic concentrations in mammalian cell lines, and showed no acute in vivo toxicity in mice at the doses tested, with no signs of organ toxicity.

Discussion

Overall, these findings highlight the potential of the Au(III)-dithiocarbamate scaffold as a basis for developing novel antimicrobial agents, given its potent antibacterial activity, synergy, redox stability, inability to produce resistant mutants, low toxicity to mammalian cells both in vitro and in vivo, and non-conventional mechanism of action.

Next Generation Gold Drugs and Probes: Chemistry and Biomedical Applications

The gold drugs, gold sodium thiomalate (Myocrisin), aurothioglucose (Solganal), and the orally administered auranofin (Ridaura), are utilized in modern medicine for the treatment of inflammatory arthritis including rheumatoid and juvenile arthritis; however, new gold agents have been slow to enter the clinic. Repurposing of auranofin in different disease indications such as cancer, parasitic, and microbial infections in the clinic has provided impetus for the development of new gold complexes for biomedical applications based on unique mechanistic insights differentiated from auranofin. Various chemical methods for the preparation of physiologically stable gold complexes and associated mechanisms have been explored in biomedicine such as therapeutics or chemical probes. In this Review, we discuss the chemistry of next generation gold drugs, which encompasses oxidation states, geometry, ligands, coordination, and organometallic compounds for infectious diseases, cancer, inflammation, and as tools for chemical biology via gold-protein interactions. We will focus on the development of gold agents in biomedicine within the past decade. The Review provides readers with an accessible overview of the utility, development, and mechanism of action of gold-based small molecules to establish context and basis for the thriving resurgence of gold in medicine.

Development of an assay pipeline for the discovery of novel small molecule inhibitors of human glutathione peroxidases GPX1 and GPX4

Selenoprotein glutathione peroxidases (GPX), like ubiquitously expressed GPX1 and the ferroptosis modulator GPX4, enact antioxidant activities by reducing hydroperoxides using glutathione. Overexpression of these enzymes is common in cancer and can be associated with the development of resistance to chemotherapy. GPX1 and GPX4 inhibitors have thus shown promise as anti-cancer agents, and targeting other GPX isoforms may prove equally beneficial. Existing inhibitors are often promiscuous, or modulate GPXs only indirectly, so novel direct inhibitors identified through screening against GPX1 and GPX4 could be valuable. Here, we developed optimized glutathione reductase (GR)-coupled GPX assays for the biochemical high-throughput screen (HTS) of almost 12,000 compounds with proposed mechanisms of action. Initial hits were triaged using a GR counter-screen, assessed for isoform specificity against an additional GPX isoform, GPX2, and were assessed for general selenocysteine-targeting activity using a thioredoxin reductase (TXNRD1) assay. Importantly, 70% of the GPX1 inhibitors identified in the primary screen, including several cephalosporin antibiotics, were found to also inhibit TXNRD1, while auranofin, previously known as a TXNRD1 inhibitor, also inhibited GPX1 (but not GPX4). Additionally, every GPX1 inhibitor identified (including omapatrilat, tenatoprazole, cefoxitin and ceftibuten) showed similar inhibitory activity against GPX2. Some compounds inhibiting GPX4 but not GPX1 or GPX2, also inhibited TXNRD1 (26%). Compounds only inhibiting GPX4 included pranlukast sodium hydrate, lusutrombopag, brilanestrant, simeprevir, grazoprevir (MK-5172), paritaprevir, navitoclax, venetoclax and VU0661013. Two compounds (metamizole sodium and isoniazid sodium methanesulfate) inhibited all three GPXs but not TXNRD1, while 2,3-dimercaptopropanesulfonate, PI4KIII beta inhibitor 3, SCE-2174 and cefotetan sodium inhibited all tested selenoproteins (but not GR). The detected overlaps in chemical space suggest that the counter screens introduced here should be imperative for identification of specific GPX inhibitors. With this approach, we could indeed identify novel GPX1/GPX2- or GPX4-specific inhibitors, thus presenting a validated pipeline for future identification of specific selenoprotein-targeting agents. Our study also identified GPX1/GPX2, GPX4 and/or TXNRD1 as targets for several previously developed pharmacologically active compounds.

Auranofin Targeting the NDM-1 Beta-Lactamase: Computational Insights into the Electronic Configuration and Quasi-Tetrahedral Coordination of Gold Ions

Recently, the well-characterized metallodrug auranofin has been demonstrated to restore the penicillin and cephalosporin sensitivity in resistant bacterial strains via the inhibition of the NDM-1 beta-lactamase, which is operated via the Zn/Au substitution in its bimetallic core. The resulting unusual tetrahedral coordination of the two ions was investigated via the density functional theory calculations. By assessing several charge and multiplicity schemes, coupled with on/off constraining the positions of the coordinating residues, it was demonstrated that the experimental X-ray structure of the gold-bound NDM-1 is consistent with either Au(I)-Au(I) or Au(II)-Au(II) bimetallic moieties. The presented results suggest that the most probable mechanism for the auranofin-based Zn/Au exchange in NDM-1 includes the early formation of the Au(I)-Au(I) system, superseded by oxidation yielding the Au(II)-Au(II) species bearing the highest resemblance to the X-ray structure.

Thioredoxin reductase selenoproteins from different organisms as potential drug targets for treatment of human diseases

Human thioredoxin reductase (TrxR) is a selenoprotein with a central role in cellular redox homeostasis, utilizing a highly reactive and solvent-exposed selenocysteine (Sec) residue in its active site. Pharmacological modulation of TrxR can be obtained with several classes of small compounds showing different mechanisms of action, but most often dependent upon interactions with its Sec residue. The clinical implications of TrxR modulation as mediated by small compounds have been studied in diverse diseases, from rheumatoid arthritis and ischemia to cancer and parasitic infections. The possible involvement of TrxR in these diseases was in some cases serendipitously discovered, by finding that existing clinically used drugs are also TrxR inhibitors. Inhibiting isoforms of human TrxR is, however, not the only strategy for human disease treatment, as some pathogenic parasites also depend upon Sec-containing TrxR variants, including S. mansoni, B. malayi or O. volvulus. Inhibiting parasite TrxR has been shown to selectively kill parasites and can thus become a promising treatment strategy, especially in the context of quickly emerging resistance towards other drugs. Here we have summarized the basis for the targeting of selenoprotein TrxR variants with small molecules for therapeutic purposes in different human disease contexts. We discuss how Sec engagement appears to be an indispensable part of treatment efficacy and how some therapeutically promising compounds have been evaluated in preclinical or clinical studies. Several research questions remain before a wider application of selenoprotein TrxR inhibition as a first-line treatment strategy might be developed. These include further mechanistic studies of downstream effects that may mediate treatment efficacy, identification of isoform-specific enzyme inhibition patterns for some given therapeutic compounds, and the further elucidation of cell-specific effects in disease contexts such as in the tumor microenvironment or in host-parasite interactions, and which of these effects may be dependent upon the specific targeting of Sec in distinct TrxR isoforms.

Voyage of selenium from environment to life: Beneficial or toxic?

Selenium (Se), a naturally occurring metalloid, is an essential micronutrient for life as it is incorporated as selenocysteine in proteins. Although beneficial at low doses, Se is hazardous at high concentrations and poses a serious threat to various ecosystems. Due to this contrasting 'dual' nature, Se has garnered the attention of researchers wishing to unravel its puzzling properties. In this review, we describe the impact of selenium's journey from environment to diverse biological systems, with an emphasis on its chemical advantage. We describe the uneven distribution of Se and how this affects the bioavailability of this element, which, in turn, profoundly affects the habitat of a region. Once taken up, the subsequent incorporation of Se into proteins as selenocysteine and its antioxidant functions are detailed here. The causes of improved protein function due to the incorporation of redox-active Se atom (instead of S) are examined. Subsequently, the reasons for the deleterious effects of Se, which depend on its chemical form (organo-selenium or the inorganic forms) in different organisms are elaborated. Although Se is vital for the function of many antioxidant enzymes, how the pro-oxidant nature of Se can be potentially exploited in different therapies is highlighted. Furthermore, we succinctly explain how the presence of Se in biological systems offsets the toxic effects of heavy metal mercury. Finally, the different avenues of research that are fundamental to expand our understanding of selenium biology are suggested.

Molecular Fates of Organometallic Mercury in Human Brain

Mercury is ubiquitous in the environment, with rising levels due to pollution and climate change being a current global concern. Many mercury compounds are notorious for their toxicity, with the potential of organometallic mercury compounds for devastating effects on the structures and functions of the central nervous system being of particular concern. Chronic exposure of human populations to low levels of methylmercury compounds occurs through consumption of fish and other seafood, although the health consequences, if any, from this exposure remain controversial. We have used high energy resolution fluorescence detected X-ray absorption spectroscopy to determine the speciation of mercury and selenium in human brain tissue. We show that the molecular fate of mercury differs dramatically between individuals who suffered acute organometallic mercury exposure (poisoning) and individuals with chronic low-level exposure from a diet rich in marine fish. For long-term low-level methylmercury exposure from fish consumption, mercury speciation in brain tissue shows methylmercury coordinated to an aliphatic thiolate, resembling the coordination environment observed in marine fish. In marked contrast, for short-term high-level exposure, we observe the presence of biologically less available mercuric selenide deposits, confirmed by X-ray fluorescence imaging, as well as mercury(II)-bis-thiolate complexes, which may be signatures of severe poisoning in humans. These differences between low-level and high-level exposures challenge the relevance of studies involving acute exposure as a proxy for low-level chronic exposure.

Biochemical and structural characterizations of thioredoxin reductase selenoproteins of the parasitic filarial nematodes Brugia malayi and Onchocerca volvulus

Enzymes in the thiol redox systems of microbial pathogens are promising targets for drug development. In this study we characterized the thioredoxin reductase (TrxR) selenoproteins from Brugia malayi and Onchocerca volvulus, filarial nematode parasites and causative agents of lymphatic filariasis and onchocerciasis, respectively. The two filarial enzymes showed similar turnover numbers and affinities for different thioredoxin (Trx) proteins, but with a clear preference for the autologous Trx. Human TrxR1 (hTrxR1) had a high and similar specific activity versus the human and filarial Trxs, suggesting that, in vivo, hTrxR1 could possibly be the reducing agent of parasite Trxs once they are released into the host. Both filarial TrxRs were efficiently inhibited by auranofin and by a recently described inhibitor of human TrxR1 (TRi-1), but not as efficiently by the alternative compound TRi-2. The enzyme from B. malayi was structurally characterized also in complex with NADPH and auranofin, producing the first crystallographic structure of a nematode TrxR. The protein represents an unusual fusion of a mammalian-type TrxR protein architecture with an N-terminal glutaredoxin-like (Grx) domain lacking typical Grx motifs. Unlike thioredoxin glutathione reductases (TGRs) found in platyhelminths and mammals, which are also Grx-TrxR domain fusion proteins, the TrxRs from the filarial nematodes lacked glutathione disulfide reductase and Grx activities. The structural determinations revealed that the Grx domain of TrxR from B. malayi contains a cysteine (C22), conserved in TrxRs from clade IIIc nematodes, that directly interacts with the C-terminal cysteine-selenocysteine motif of the homo-dimeric subunit. Interestingly, despite this finding we found that altering C22 by mutation to serine did not affect enzyme catalysis. Thus, although the function of the Grx domain in these filarial TrxRs remains to be determined, the results obtained provide insights on key properties of this important family of selenoprotein flavoenzymes that are potential drug targets for treatment of filariasis.

Mercury Lα1 High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: A Versatile Speciation Probe for Mercury

Mercury is in some sense an enigmatic element. The element and some of its compounds are a natural part of the biogeochemical cycle; while many of these can be deadly poisons at higher levels, environmental levels in the absence of anthropogenic contributions would generally be below the threshold for concern. However, mercury pollution, particularly from burning fossil fuels such as coal, is providing dramatic and increasing emissions into the environment. Because of this, the environmental chemistry and toxicology of mercury are of growing importance, with the fate of mercury being vitally dependent upon its speciation. X-ray absorption spectroscopy (XAS) provides a powerful tool for in situ chemical speciation, but is severely limited by poor spectroscopic energy resolution. Here, we provide a systematic examination of mercury Lα1 high energy resolution fluorescence detected XAS (HERFD-XAS) as an approach for chemical speciation of mercury, in quantitative comparison with conventional Hg LIII-edge XAS. We show that, unlike some lighter elements, chemical shifts in the Lα1 X-ray fluorescence energy can be safely neglected, so that mercury Lα1 HERFD-XAS can be treated simply as a high-resolution version of conventional XAS. We present spectra of a range of mercury compounds that may be relevant to the environmental and life science research and show that density functional theory can produce adequate simulations of the spectra. We discuss strengths and limitations of the method and quantitatively demonstrate improvements both in speciation for complex mixtures and in background rejection for low concentrations.

The emerging role of selenium metabolic pathways in cancer: New therapeutic targets for cancer

Selenium (Se) is incorporated into the body via the selenocysteine (Sec) biosynthesis pathway, which is critical in the synthesis of selenoproteins, such as glutathione peroxidases and thioredoxin reductases. Selenoproteins, which play a key role in several biological processes, including ferroptosis, drug resistance, endoplasmic reticulum stress, and epigenetic processes, are guided by Se uptake. In this review, we critically analyze the molecular mechanisms of Se metabolism and its potential as a therapeutic target for cancer. Sec insertion sequence binding protein 2 (SECISBP2), which is a positive regulator for the expression of selenoproteins, would be a novel prognostic predictor and an alternate target for cancer. We highlight strategies that attempt to develop a novel Se metabolism-based approach to uncover a new metabolic drug target for cancer therapy. Moreover, we expect extensive clinical use of SECISBP2 as a specific biomarker in cancer therapy in the near future. Of note, scientists face additional challenges in conducting successful research, including investigations on anticancer peptides to target SECISBP2 intracellular protein.

Advances on the Influence of Methylmercury Exposure during Neurodevelopment

Mercury (Hg) is a toxic heavy-metal element, which can be enriched in fauna and flora and transformed into methylmercury (MeHg). MeHg is a widely distributed environmental pollutant that may be harmful to fish-eating populations through enrichment of aquatic food chains. The central nervous system is a primary target of MeHg. Embryos and infants are more sensitive to MeHg, and exposure to MeHg during gestational feeding can significantly impair the homeostasis of offspring, leading to long-term neurodevelopmental defects. At present, MeHg-induced neurodevelopmental toxicity has become a hotspot in the field of neurotoxicology, but its mechanisms are not fully understood. Some evidence point to oxidative damage, excitotoxicity, calcium ion imbalance, mitochondrial dysfunction, epigenetic changes, and other molecular mechanisms that play important roles in MeHg-induced neurodevelopmental toxicity. In this review, advances in the study of neurodevelopmental toxicity of MeHg exposure during pregnancy and the molecular mechanisms of related pathways are summarized, in order to provide more scientific basis for the study of neurodevelopmental toxicity of MeHg.

Comparative reactivity of medicinal gold(I) compounds with the cyclic peptide vasopressin and its diselenide analogue

The reactions of the medicinal gold(I) compound auranofin and its close analogues with vasopressin and the diselenide analogue were comparatively investigated by LC-electrospray MS/MS. Evidence is gained of the possible cleavage of the S-S and Se-Se bridges induced by Au(I). Notably, we found that, in the absence of reducing agents, the sulfur and selenium atoms are metallated only at high temperature (70 °C) with the preferential binding of gold to selenium. The reaction with the S-S bridge can take place at physiological temperature (37 °C) under reducing conditions. The implications of these results are discussed in the general frame of the reactivity of biologically relevant soft Lewis acids with peptides and proteins.

Reactivity of antitumor coinage metal-based N-heterocyclic carbene complexes with cysteine and selenocysteine protein sites

The reaction of the antitumor M(I)-bis-N-heterocyclic carbene (M(I)-NHC) complexes, M = Cu, Ag, and Au, with their potential protein binding sites, i.e. cysteine and selenocysteine, was investigated by means of density functional theory approaches. Capped cysteine and selenocysteine were employed to better model the corresponding residues environment within peptide structures. By assuming the neutral or deprotonated form of the side chains of these amino acids and by considering the possible assistance of an external proton donor such as an adjacent acidic residue or the acidic component of the surrounding buffer environment, we devised five possible routes leading to the binding of the investigated M(I)-NHC scaffolds to these protein sites, reflecting their different location in the protein structure and exposure to the bulk. The targeting of either cysteine or selenocysteine in their neutral forms is a kinetically unfavored process, expected to be quite slow if observable at all at physiological temperature. On the other hand, the reaction with the deprotonated forms is much more favored, even though an external proton source is required to assist the protonation of the leaving carbene. Our calculations also show that all coinage metals are characterized by a similar reactivity toward the binding of cysteine and selenocysteine sites, although the Au(I) complex has significantly higher reaction and activation free energies compared to Cu(I) and Ag(I).

High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: An Analytical Method for Selenium Speciation

Selenium is in many ways an enigmatic element. It is essential for health but toxic in excess, with the difference between the two doses being narrower than for any other element. Environmentally, selenium is of concern due to its toxicity. As the rarest of the essential elements, its low levels often provide challenges to the analytical chemist. X-ray absorption spectroscopy (XAS) provides a powerful tool for in situ chemical speciation but is severely limited by poor spectroscopic resolution arising from core-hole lifetime broadening. Here we explore selenium Kα1 high energy resolution fluorescence detected XAS (HERFD-XAS) as a novel approach for chemical speciation of selenium, in comparison with conventional Se K-edge XAS. We present spectra of a range of selenium species relevant to environmental and life science studies, including spectra of seleno-amino acids, which show strong similarities with S K-edge XAS of their sulfur congeners. We discuss strengths and limitations of HERFD-XAS, showing improvements in both speciation performance and low concentration detection. We also develop a simple method to correct fluorescence self-absorption artifacts, which is generally applicable to any HERFD-XAS experiment.

Methylmercury Can Facilitate the Formation of Dehydroalanine in Selenoenzymes: Insight from DFT Molecular Modeling

Experimental studies have indicated that electrophilic mercury forms (e.g., methylmercury, MeHg⁺) can accelerate the breakage of selenocysteine in vitro. Particularly, in 2009, Khan et al. (Environ. Toxicol. Chem. 2009, 28, 1567-1577) proposed a mechanism for the degradation of a free methylmercury selenocysteinate complex that was theoretically supported by Asaduzzaman et al. (Inorg. Chem. 2010, 50, 2366-2372). However, little is known about the fate of methylmercury selenocysteinate complexes embedded in an enzyme, especially in conditions of oxidative stress in which methylmercury target enzymes operate. Here, an accurate computational study on molecular models (level of theory: COSMO-ZORA-BLYP-D3(BJ)/TZ2P) was carried out to investigate the formation of dehydroalanine (Dha) in selenoenzymes, which irreversibly impairs their function. Methylselenocysteine as well as methylcysteine and methyltellurocysteine were included to gain insight on the peculiar behavior of selenium. Dha forms in a two-step process, i.e., the oxidation of the chalcogen nucleus followed by a syn-elimination leading to the alkene and the chalcogenic acid. The effect of an excess of hydrogen peroxide, which may lead to the formation of chalcogenones before the elimination, and of MeHg⁺, a severe toxicant targeting selenoproteins, which leads to the formation of methylmercury selenocysteinate, are also studied with the aim of assessing whether these pathological conditions facilitate the formation of Dha. Indeed, elimination occurs after chalcogen oxidation and MeHg⁺ facilitates the process. These results indicate a possible mechanism of toxicity of MeHg⁺ in selenoproteins.

Review on the recent progress in the development of fluorescent probes targeting enzymes

Enzymes are very important for biological processes in a living being, performing similar or multiple tasks in and out of cells, tissues and other organisms at a particular location. The abnormal activity of particular enzyme usually caused serious diseases such as Alzheimer's disease, Parkinson's disease, cancers, diabetes, cardiovascular diseases, arthritis etc. Hence, nondestructive and real-time visualization for certain enzyme is very important for understanding the biological issues, as well as the drug administration and drug metabolism. Fluorescent cellular probe-based enzyme detectionin vitroandin vivohas become broad interest for human disease diagnostics and therapeutics. This review highlights the recent findings and designs of highly sensitive and selective fluorescent cellular probes targeting enzymes for quantitative analysis and bioimaging.

Repurposing of auranofin and honokiol as antifungals against Scedosporium species and the related fungus Lomentospora prolificans

The slowing-down de novo drug-discovery emphasized the importance of repurposing old drugs. This is particularly true when combating infections caused by therapy-refractory microorganisms, such as Scedosporium species and Lomentospora prolificans. Recent studies on Scedosporium responses to oxidative stress underscored the importance of targeting the underlying mechanisms. Auranofin, ebselen, PX-12, honokiol, and to a lesser extent, conoidin A are known to disturb redox-homeostasis systems in many organisms. Their antifungal activity was assessed against 27 isolates belonging to the major Scedosporium species: S. apiospermum, S. aurantiacum, S. boydii, S. dehoogii, S. minutisporum, and Lomentospora prolificans. Auranofin and honokiol were the most active against all Scedosporium species (mean MIC₅₀ values of 2.875 and 6.143 μg/ml, respectively) and against L. prolificans isolates (mean MIC₅₀ values of 4.0 and 3.563μg/ml respectively). Combinations of auranofin with voriconazole or honokiol revealed additive effects against 9/27 and 18/27 isolates, respectively. Synergistic interaction between auranofin and honokiol was only found against one isolate of L. prolificans. The effects of auranofin upon exposure to oxidative stress were also investigated. For all species except S. dehoogii, the maximal growth in the presence of auranofin significantly decreased when adding a sublethal dose of menadione. The analysis of the expression of genes encoding oxidoreductase enzymes upon exposure of S. apiospermum to honokiol unveiled the upregulation of many genes, especially those coding peroxiredoxins, thioredoxin reductases, and glutaredoxins. Altogether, these data suggest that auranofin and honokiol act via dampening the redox balance and support their repurposing as antifungals against Scedosporium species and L. prolificans.

Effect of Methylmercury Binding on the Peroxide-Reducing Potential of Cysteine and Selenocysteine

Methylmercury (CH3Hg+) binding to catalytically fundamental cysteine and selenocysteine of peroxide-reducing enzymes has long been postulated as the origin of its toxicological activity. Only very recently, CH3Hg+ binding to the selenocysteine of thioredoxin reductase has been directly observed [Pickering, I. J. Inorg. Chem., 2020, 59, 2711-2718], but the precise influence of the toxicant on the peroxide-reducing potential of such a residue has never been investigated. In this work, we employ state-of-the-art density functional theory calculations to study the reactivity of molecular models of the free and toxified enzymes. Trends in activation energies are discussed with attention to the biological consequences and are rationalized within the chemically intuitive framework provided by the activation strain model. With respect to the free, protonated amino acids, CH3Hg+ binding promotes oxidation of the S or Se nucleus, suggesting that chalcogenoxide formation might occur in the toxified enzyme, even if the actual rate of peroxide reduction is almost certainly lowered as suggested by comparison with fully deprotonated amino acids models.

Demethylation of Methylmercury in Bird, Fish, and Earthworm

Toxicity of methylmercury (MeHg) to wildlife and humans results from its binding to cysteine residues of proteins, forming MeHg-cysteinate (MeHgCys) complexes that hinder biological functions. MeHgCys complexes can be detoxified in vivo, yet how this occurs is unknown. We report that MeHgCys complexes are transformed into selenocysteinate [Hg(Sec)₄] complexes in multiple animals from two phyla (a waterbird, freshwater fish, and earthworms) sampled in different geographical areas and contaminated by different Hg sources. In addition, high energy-resolution X-ray absorption spectroscopy (HR-XANES) and chromatography-inductively coupled plasma mass spectrometry of the waterbird liver support the binding of Hg(Sec)₄ to selenoprotein P and biomineralization of Hg(Sec)₄ to chemically inert nanoparticulate mercury selenide (HgSe). The results provide a foundation for understanding mercury detoxification in higher organisms and suggest that the identified MeHgCys to Hg(Sec)₄ demethylation pathway is common in nature.

ESI MS studies highlight the selective interaction of Auranofin with protein free thiols

The clinically established gold drug Auranofin was reacted individually with a group of representative proteins, namely ubiquitin, ribonuclease A, carbonic anhydrase, haemoglobin and superoxide dismutase, and adduct formation was monitored in the various cases by ESI-MS analysis. We found that the reaction is highly selective for solvent exposed free cysteines that are modified through coordination of the AuPEt₃⁺ fragment. Indeed, ESI-Q-TOF MS spectra carried out on protein samples incubated with a three fold molar excess of Auranofin allowed direct detection of the native proteins bearing bound AuPEt₃⁺ fragments in the cases of carbonic anhydrase and haemoglobin. At variance, the two proteins that do not possess any free cysteine residue, i.e. ubiquitin and ribonuclease A, were unable to bind the gold fragment. In the case of superoxide dismutase, adduct formation is hindered by the scarce solvent accessibility of the free cysteine residue. These findings were further confirmed by a series of competition binding experiments with ebselen, a potent and selective cysteine-modifying reagent; we observed that pre-treatment with ebselen prevents the binding of the AuPEt₃⁺ fragment to both carbonic anhydrase and haemoglobin.

Cross-talk between Bcr-abl and the Thioredoxin System in Chronic Myeloid Leukaemia: Implications for CML Treatment

Chronic myeloid leukaemia (CML) is currently treated with inhibitors of the CML specific oncoprotein, bcr-abl. While this strategy is initially successful, drug resistance can become a problem. Therefore, new targets need to be identified to ensure the disease can be appropriately managed. The thioredoxin (Trx) system, comprised of Trx, thioredoxin reductase (TrxR), and NADPH, is an antioxidant system previously identified as a target for therapies aimed at overcoming drug resistance in other cancers. We assessed the effectiveness of TrxR inhibitors on drug resistant CML cells and examined links between TrxR and the bcr-abl cell-signalling pathway. Two TrxR inhibitors, auranofin and [Au(d2pype)2]Cl, increased intracellular ROS levels and elicited apoptosis in both sensitive and imatinib resistant CML cells. Inhibition of TrxR activity by these pharmacological inhibitors, or by specific siRNA, also resulted in decreased bcr-abl mRNA and protein levels, and lower bcr-abl downstream signalling activity, potentially enhancing the effectiveness of TrxR inhibitors as CML therapies. In addition, imatinib resistant CML cell lines showed upregulated expression of the Trx system. Furthermore, analysis of datasets showed that CML patients who did not respond to imatinib had higher Trx mRNA levels than patients who responded to treatment. Our study demonstrates a link between the Trx system and the bcr-abl protein and highlights the therapeutic potential of targeting the Trx system to improve CML patients' outcomes.