The effect of cyanide on the uptake of gold by red blood cells

Anti-fibrotic effect of aurocyanide, the active metabolite of auranofin

Drug repositioning has gained significant attention over the past several years. The anti-rheumatoid arthritis drug auranofin has been investigated for the treatment of other diseases, including liver fibrosis. Because auranofin is rapidly metabolized, it is necessary to identify the active metabolites of auranofin that have detectable levels in the blood and reflect its therapeutic effects. In the present study, we investigated whether aurocyanide as an active metabolite of auranofin, can be used to evaluate the anti-fibrotic effects of auranofin. Incubation of auranofin with liver microsomes showed that auranofin was susceptible to hepatic metabolism. Previously, we found that the anti-fibrotic effects of auranofin are mediated via system x_c^--dependent inhibition of the NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome. Therefore, we tried to identify active metabolites of auranofin based on their inhibitory effects on system x_c^- and NLRP3 inflammasome in bone marrow-derived macrophages. Among the seven candidate metabolites, 1-thio-β-D-glycopyrano-sato-S-(triethyl-phosphine)-gold(I) and aurocyanide potently inhibited system x_c^- and NLRP3 inflammasome. A pharmacokinetics study on mice detected significant plasma levels of aurocyanide after auranofin administration. Oral administration of aurocyanide significantly prevented thioacetamide-induced liver fibrosis in mice. Moreover, the in vitro anti-fibrotic effects of aurocyanide were assessed in LX-2 cells, where aurocyanide significantly decreased the migratory ability of the cells. In conclusion, aurocyanide is metabolically stable and detectable in plasma, and has inhibitory effects on liver fibrosis, suggesting that it is a potential marker of the therapeutic effects of auranofin.

Honorary Professor Garry Graham

An appreciation of the contribution of Professor Gary Graham to anti-inflammatory and antirheumatic pharmacology and clinical pharmacology.

A Spectroscopic Study of Interaction of Auricyanide with n-Acetylcysteine

Interaction of auricyanide, an important metabolite of anti-arthritic gold-based drug auranofin, was studied in vitro with a pharmacologically active ligand n-acetylcysteine with a view to understand reactivity of gold in vivo. Formation of reduction product aurocyanide occurred through mono- and di-n-acetylcysteine-substituted intermediates. The product and intermediates were identified and monitored spectrophotometrically and by electrospray ionization mass spectrometry. This study suggests successive substitution with n-acetylcysteine through trans effect. At equimolar concentrations of auricyanide and n-acetylcysteine, only mono-substituted mixed-ligand complex was formed. Substitution of the data obtained to various kinetic models suggested that the reaction orders are 0.6 in terms of n-acetylcysteine, 1.5 in terms of auricyanide, and 2 overall. The intermediates detected in this work may help to synthesize more effective and less toxic gold drugs.

Redox and ligand exchange reactions of potential gold(I) and gold(III)-cyanide metabolites under biomimetic conditions

Biomimetic pathways for the oxidation of [Au(CN)(2)](-), a gold metabolite, and further cyanation of the gold(III) products to form Au(CN)(4)(-) were investigated using 13C NMR and UV-Visible spectroscopic methods. Hypochlorite ion, an oxidant released during the oxidative burst of immune cells, was employed. The reaction generates mixed dicyanoaurate(III) complexes, trans-[Au(CN)(2)X(2)](-), where X(-) represents equilibrating hydroxide and chloride ligands, and establishes the chemical feasibility of dicyanoaurate oxidation by OCl(-) to gold(III) species. This oxidation reaction suggests a new procedure for synthesis of H[Au(CN)(2)Cl(2)]. Reaction of trans-[Au(CN)(2)X(2)](-) (X(-)=Cl(-) and Br(-)) or [AuCl(4)](-) with HCN in aqueous solution at pH 7.4 leads directly to [Au(CN)(4)](-) without detection of the anticipated [Au(CN)(x)X(4-x)](-)intermediates, which is attributed to the cis- and trans-accelerating effects of the cyanides. The reduction of [Au(CN)(4)](-) by glutathione and other thiols is a complex, pH-dependent process that proceeds through two intermediates and ultimately generates [Au(CN)(2)](-). These studies provide further insight into the possible mechanisms of an immunogenically generated gold(I)/gold(III) redox cycle in vivo.

The activation of gold complexes by cyanide produced by polymorphonuclear leukocytes. III. The formation of aurocyanide by myeloperoxidase

There is considerable evidence that the anti-rheumatic gold complexes are activated by their conversion to aurocyanide. In order to understand the mechanism of production of aurocyanide, we investigated the involvement of myeloperoxidase in the reaction. This haem enzyme of neutrophils and monocytes uses hydrogen peroxide to oxidise chloride and thiocyanate to hypochlorous acid and hypothiocyanite, respectively. When aurothiomalate (10 microM) was incubated with thiocyanate (200 microM), hydrogen peroxide (100 microM) and myeloperoxidase (20 nM), it was transformed to a product that was spectrally identical to authentic aurocyanide. Aurothiomalate was quantitatively converted to aurocyanide in about 10 min at pH 6.0 and in 40 min at pH 7.4. Aurocyanide formation occurred after myeloperoxidase had used all the hydrogen peroxide available to produce hypothiocyanite. Thus, the cyanide must have formed from the slow decomposition of hypothiocyanite. The rate of aurocyanide production was increased in the presence of 100 mM chloride, which indicates that hypochlorous acid accelerates the formation of cyanide. Hypochlorous acid (100 to 400 microM) reacted non-enzymatically with thiocyanate (200 microM) and aurothiomalate (10 microM) to produce aurocyanide. Thus, aurocyanide is produced by two processes, involving both the formation of hypothiocyanite and hypochlorous acid. Aurocyanide is an effective inhibitor of the respiratory burst of neutrophils and monocytes and the proliferation of lymphocytes. Therefore, aurothiomalate may attenuate inflammation by acting as a pro-drug which is reliant on neutrophils and monocytes to produce hypothiocyanite. When the hypothiocyanite decays to hydrogen cyanide, the pro-drug is converted to aurocyanide which then suppresses further oxidant production by these inflammatory cells.

15N NMR studies of the binding of C15N- with gold(I) drugs

15N NMR studies of the interaction of 15N cyanide ion with gold(I)-thiomalate (Autm) and gold(I)-thioglucose (Autg) have been carried out at pH* 7.40. The chemical shifts of the two 15N ions containing species Au(C15N)2- and RS-Au-C15N- (where RS- = tm- or tg-) were identified at 265.94 and 260.30 ppm, respectively. From the broadened 15N NMR signals, approximate life times of the RS-Au-CN- species were calculated.

Determination of gold-based antiarthritis drugs and their metabolites in urine by reversed-phase ion-pair chromatography with ICP-MS detection

A sensitive method for the determination of gold-based drugs auranofin, myochrysine, and their metabolites has been developed. These gold-containing compounds were separated by reversed-phase ion-pair chromatography with tetrabutylammonium chloride as the ion-pairing agent. Gold-specific on-line detection utilized inductively coupled plasma mass spectrometry (ICP-MS). The separation conditions of pH, methanol content, concentration of the ion-pairing agent and ionic strength have been investigated. The detection limit for auranofin, the last peak in the chromatogram, was 0.3 ng. These methods were applied to the analysis of gold-containing species in urine from arthritis patients on auranofin, myochrysine or solganol therapy. The recovery of the total gold-containing species from urine was greater than 90%. Dicyanogold(I) anion, [Au(CN)2]-, was detected in the urine of several patients.

The 13C NMR study of the binding of gold(I) thiomalate with ergothionine in aqueous solution

The interaction of gold(I) thiomalate (Autm) (Myocrysine) with ergothionine (ErSH) has been studied in aqueous solution at pH 7.4. It was found that ErSH forms a ternary complex of the type ErS-Au-tm at a 1:1 mole ratio; unlike other thiols (e.g., cysteine and glutathione) it does not eject thiomalate (Htm) as a free ligand. However, in the presence of glutathione (GtSH), both ligands, ErSH as well as Htm, from the ErS-Au-tm complex were freed. The 13C NMR chemical shifts of C-2 resonance of ergothionine in the presence of Autm shifts greater than imidazolidine-2-thione (Imt) and 1,3-Diazinane-2-thione (Diaz), indicating the stronger complexation of ErS-Au-tm compared to Imt-Au-tm and Diaz-Au-tm.

Gold(I) efflux from auranofin-treated red blood cells. Evidence for a glutathione-gold-albumin metabolite

The efflux of gold from red blood cells (RBCs) exposed to 10-100 microM auranofin [the second generation chrysotherapy agent, triethylphosphine-(2,3,4,6-tetra-O-acetyl-1-beta-D-glucopyranosato -S-)gold(I)] was studied. RBCs in whole blood were allowed to accumulate gold, and then were placed in fresh plasma or buffered saline solution. In plasma, the kinetics of efflux were first order in gold with an apparent rate constant of 0.81 +/- 0.18 hr-1. Serum albumin, in plasma or added to a buffered solution, shifted the equilibrium between intra- and extracellular gold in favor of the latter (compared to saline solution). [14C]Glutathione, generated by in situ labeling, also effluxed and associated with the albumin and gold, providing the first direct evidence that the albumin-gold-glutathione complex (AlbSAuSG) may be a circulating metabolite of auranofin formed after both of the original ligands of auranofin are displaced.

The activation of gold complexes by cyanide produced by polymorphonuclear leukocytes--I. The effects of aurocyanide on the oxidative burst of polymorphonuclear leukocytes

It has been suggested that the antiarthritic gold complex, aurothiomalate (Autm), is activated by its conversion to aurocyanide by polymorphonuclear leukocytes (PMN) which generate cyanide from thiocyanate. In an examination of this hypothesis, a study has been conducted on the effects of aurocyanide on the oxidative burst of polymorphonuclear leukocytes (PMN) and monocytes activated by phorbol myristate acetate (PMA). Aurocyanide produced delayed inhibition of the oxidative burst as shown by its effect on both lucigenin and luminol-dependent chemiluminescence and on the production of superoxide. It was a more potent inhibitor of luminol-dependent chemiluminescence than free thiomalate and other by-products of the reaction between Autm and cyanide. Aurocyanide had a biphasic effect on the PMA-stimulated hexose monophosphate shunt of PMN, with enhancement at 0.1 microM and inhibition at 10 and 100 microM. The activity of aurocyanide was also compared with that of auranofin, an orally active gold complex, which inhibits a variety of functions of PMN and monocytes. At low concentrations, auranofin produced delayed inhibition of chemiluminescence in a similar fashion to aurocyanide but at high concentrations was an immediate inhibitor of the oxidative burst.

The activation of gold complexes by cyanide produced by polymorphonuclear leukocytes--II. Evidence for the formation and biological activity of aurocyanide

Experiments have been conducted to investigate a possible mechanism which might explain why aurothiomalate (Autm), a gold complex used in the treatment of rheumatoid arthritis, is active in vivo but not in vitro, by testing the hypothesis that Autm is converted to aurocyanide by activated polymorphonuclear leukocytes (PMN) which generate cyanide from thiocyanate, an anion which is present in plasma at concentrations ranging from 20 to 200 microM. Two-stage experiments were conducted in which PMN, in the first stage, were activated by opsonized zymosan in the presence of Autm both with and without thiocyanate. Then, in the second stage, the effect of the drugs on superoxide (O2-) production stimulated by a further addition of zymosan was measured. Autm at concentrations of 10 and 100 microM decreased O2- production if thiocyanate was present, but not if it was absent. By contrast, preformed aurocyanide at 10 and 100 microM decreases O2- production by PMN stimulated by opsonized zymosan both in the presence and absence of thiocyanate. Changes in the ultraviolet spectra of the supernatants of PMN indicated that aurocyanide was formed by activated PMN in the presence of thiocyanate but not in its absence.

Control of oxidative damage in rheumatoid arthritis by gold(I)-thiolate drugs

The roles of anti-arthritic gold(I)-thiolate drugs such as disodium aurothiomalate ('Myocrisin') in the modulation or promotion of oxygen radical-mediated oxidative damage in vivo are reviewed. In particular, the precise molecular mechanisms by which these novel second-line agents exert their therapeutic effects are discussed in terms of (i) the direct and indirect control of enzymes involved in the generation or scavenging of reactive oxygen species (ROS) such as superoxide ion, hydrogen peroxide and hydroxyl radical, (ii) the protection of proteins and relevant enzyme systems against attack by ROS and (iii) their direct involvement in the production (at appropriate 'target' sites) or scavenging of ROS in vivo. In addition, the role of the orally-effective gold(I)-phosphine complex auranofin in the control of oxidative damage in rheumatoid arthritis is also discussed.

Acute poisoning with gold cyanide

A case of deliberate ingestion of an electroplating solution containing gold cyanide is described. Despite the use of an antidote, and supportive treatment for cyanide poisoning, the patient died after 13 hours. Sublethal cyanide and high red blood cell gold levels suggest acute gold toxicity as the most likely cause of death. Evidence for this is discussed and recommendations are made for the treatment of cyanide poisoning.

Progress in the characterization of gold drugs

As used clinically, Myocrisin appears to contain largely polymeric (Autm)n, in which thiomalate (tm) sulphurs bridge between Au(I) ions, together with small amounts of a more reactive gold-bound thiomalate, and free thiomalate, as well as glycerol, unidentified yellow-products from autoclaving and phenylmercury adducts. Solganal usually contains thioglucose sulphinic acid as an impurity. Auranofin, on the other hand, has been crystallised. It is monomeric and Au(I) is almost linearly coordinated by P and S from triethylphosphine and tetraacetyl-beta-D-thioglucose, the latter adopting the chair conformation in the solid state and in solution. The major reaction of auranofin in acidic aqueous solutions appears to be hydrolysis of the sugar acetyl groups but other products arise if methanol is also present in the medium. NMR methods are used to examine in vitro the partition of auranofin between plasma and blood cells. The displacement of the thioglucose ligand and release of PEt3 from gold as OPEt3 are discussed.