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.

Gold(I) ion and the phosphine ligand are necessary for the anti-Toxoplasma gondii activity of auranofin.

Auranofin, an FDA-approved drug for rheumatoid arthritis, has emerged as a promising antiparasitic medication in recent years. The gold(I) ion in auranofin is postulated to be responsible for its antiparasitic activity. Notably, aurothiomalate and aurothioglucose also contain gold(I), and, like auranofin, they were previously used to treat rheumatoid arthritis. Whether they have antiparasitic activity remains to be elucidated. Herein, we demonstrated that auranofin and similar derivatives, but not aurothiomalate and aurothioglucose, inhibited the growth of Toxoplasma gondii in vitro. We found that auranofin affected the T. gondii biological cycle (lytic cycle) by inhibiting T. gondii's invasion and triggering its egress from the host cell. However, auranofin could not prevent parasite replication once T. gondii resided within the host. Auranofin treatment induced apoptosis in T. gondii parasites, as demonstrated by its reduced size and elevated phosphatidylserine externalization (PS). Notably, the gold from auranofin enters the cytoplasm of T. gondii, as demonstrated by scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS).IMPORTANCEToxoplasmosis, caused by Toxoplasma gondii, is a devastating disease affecting the brain and the eyes, frequently affecting immunocompromised individuals. Approximately 60 million people in the United States are already infected with T. gondii, representing a population at-risk of developing toxoplasmosis. Recent advances in treating cancer, autoimmune diseases, and organ transplants have contributed to this at-risk population's exponential growth. Paradoxically, treatments for toxoplasmosis have remained the same for more than 60 years, relying on medications well-known for their bone marrow toxicity and allergic reactions. Discovering new therapies is a priority, and repurposing FDA-approved drugs is an alternative approach to speed up drug discovery. Herein, we report the effect of auranofin, an FDA-approved drug, on the biological cycle of T. gondii and how both the phosphine ligand and the gold molecule determine the anti-parasitic activity of auranofin and other gold compounds. Our studies would contribute to the pipeline of candidate anti-T. gondii agents.

Screening trial with the coordinated gold compound auranofin using mouse lymphocyte leukemia P388.

The coordinated gold compound, 2,3,4,6-tetra-O-acetyl-1-thio-beta-D-glucopyranosato-S-triethylphosphine-gold (auranofin) was found to be effective in increasing the life span of C57BL x DBA/2 F1 mice inoculated with the lymphocytic leukemia P388. A number of dose schedules were used, the lowest dose being 6 mg/kg every fourth day and the highest dose being 6.0 mg/kg twice daily for 9 days; the lowest and highest doses produced treated versus control ratios of 140 and 220%, respectively. All treatment groups achieved the minimum treated versus control ratio of 125%. Animal weights remained stable at twice-daily and high-dose-daily regimens. Increased life span and weight changes were both found to correlate with drug concentration and/or dose frequency.

Evaluation of the in vivo antitumor activity and in vitro cytotoxic properties of auranofin, a coordinated gold compound, in murine tumor models.

The coordinated gold compound, 2,3,4,6-tetra-O-acetyl-1-thio-beta-D-glucopyranosato-S-triethyl phosphine gold (auranofin; Ridaura), was evaluated for antitumor activity in a variety of mouse tumor models. Of the 15 tumor models evaluated, auranofin was found to be active only against i.p. P388 leukemia. A number of dose schedules was used to measure activity against P388 with optimal activity observed at 12 mg/kg given daily, i.p., on Days 1 to 5. Auranofin was active against i.p. P388 leukemia only when administered i.p.; the drug was completely inactive when administered i.v., s.c., or p.o. on Days 1 to 5. Evaluation of the effects of auranofin in vitro demonstrated that survival curves for B16 melanoma cells as measured by the clongenic and dye exclusion assays were exponential and monophasic; cell cycle distribution was not altered, and auranofin displayed no preferential cytotoxicity to logarithmic or plateau growth phase cell populations; auranofin inhibited DNA, RNA, and protein synthesis at cytotoxic concentrations but showed no selective effect; the cytotoxic activity and cellular association of gold from auranofin were dose, time, and temperature dependent; and binding of auranofin gold to serum proteins markedly decreased cellular uptake of gold and cytotoxicity of auranofin in vitro.

Possible involvement of a hypothalamic dopaminergic receptor in development of genetic obesity in mice.

We have studied the binding of the dopaminergic agonist 2-[5,8-3H]amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene [( 3H]ADTN) to hypothalamic membranes from the genetically diabetic-obese (db/db) mice and their lean littermates. The specific binding of [3H]ADTN was defined as the difference between the radioligand binding to the membranes in the absence or presence of 1 microM (+)-butaclamol. In order to control nonspecific binding, all binding assays were performed in the presence of 0.1 mM catechol and 0.3 mM ascorbic acid. Binding of [3H]ADTN was rapid, dissociable, saturable and stereoselective. (+)-Butaclamol was very potent whereas (-)-butaclamol was ineffective in inhibiting the binding of this radioligand. Concentration-dependent binding experiments and Scatchard analysis of the data yielded dissociation constant (Kd) of 3.5-4.2 nM and number of binding sites (n) equivalent to 170-200 fmol/mg protein for lean mice. For db/db mice, the data yielded a Kd of 4.0-4.7 nM and an n of 400-500 fmol/mg protein. It was also shown that the anorexic drugs, amphetamine and fenfluramine, inhibited [3H]ADTN binding in a dose-dependent manner. Binding parameters, obtained using membranes from mice made obese by parenteral administration of gold thioglucose, were not significantly different from those obtained for the lean mice. It is concluded that the regulation of the hypothalamic dopaminergic receptors may be related to the lesion in the genetically obese mice.

Serum IgA and gold-induced toxic effects in patients with rheumatoid arthritis.

Serum Immunoglobulin concentrations were measured prospectively in 25 patients with rheumatoid arthritis at months 0, 1, 3, 6, and 12 of aurothloglucose treatment. Substantial lowering of IgA and IgM levels was found at month 3 and thereafter, and of IgG at month 12 only. When patients in whom drug-induced toxic effects developed at any time during treatment (toxic group) were compared with those who did not (nontoxic group), serum levels of IgA and to a lesser degree of IgG, but not of IgM, were found to be substantially lower in the toxic than in the nontoxic group, both at the onset and during treatment, except for IgG at month 12. When measured at the moment of toxic effect, only igA, but not IgG and IgM, was substantially lower than in serum samples of patients without toxic effects at that moment. The serum IgA concentration in patients with rheumatoid arthritis seems to be related to whether or not aurothioglucose-induced toxic effects occur.

Treatment of rheumatoid arthritis: a review including newer and experimental anti-inflammatory agents.

Treatment of rheumatoid arthritis is often frustrating for patient and physician alike because of its chronic nature and our lack of knowledge about precise cause. It can be confused by the vast array of anti-inflammatory drugs now available, with new agents being added all the time. We present here an outline for a systematic treatment program that must be tailored to each patient's needs. It emphasizes a broad approach on the part of the physician and indicates that drugs comprise only one part of treatment. We review the drugs currently used in the treatment of rheumatoid arthritis and include discussion of those newer antiinflammatory agents that should be available shortly. A rational scheme for the use of these drugs is proposed.

The quantitative distribution of gold in skin during chrysotherapy.

Gold concentrations in epidermis, dermis, and whole skin were measured by neutron activation analysis after formation of suction bullae in 8 patients who had received protracted cyrysotherapy. Epidermis contained 3% (median) of the gold content of whole skin. A direct correlation between cumulative gold dose and skin gold level was noted. These findings suggest that apparent gold concentrations in skin are influenced by the depth of the biopsy, that keratinous tissues have little affinity for gold, and that the gold storage capability of skin is not saturated by large cumulative doses of gold. The beneficial effect of gold in pemphigus may not be mediated at the site of blister formation.

Increased renal tubular cell excretion by patients receiving chronic therapy with gold and with nonsteroidal anti-inflammatory drugs.

Using rheumatoid arthritis patients who were receiving gold as models, we evaluated the renal effects of the chronic administration of very low doses of a nephrotoxic drug. The heavy metal gold has been shown to increase urinary enzyme excretion when it is given in usual doses for the treatment of rheumatoid arthritis. It is not clear whether the increased urine enzyme excretion caused by long-term drug therapy represents injury to the kidney or whether it is merely an effect of the drug. Urinary N-acetyl-beta-glucosaminidase and renal tubular cell excretion rates were measured in 19 patients who were receiving chronic treatment with gold and with nonsteroidal anti-inflammatory drugs for rheumatoid arthritis, in 10 patients who were receiving nonsteroidal anti-inflammatory drugs, and in 8 healthy control subjects. No subjects showed evidence of kidney disease. Both renal tubular cell and enzyme excretion rates were elevated in the gold-treated group. This showed that there was increased renal tubular cell turnover in this group, which suggests low level renal tubular injury and not merely an effect of the usual dose of gold.

Lymphadenopathy and lymph node infarction in the course of gold therapy.

Lymphadenopathy, as a complication of gold therapy, appears to be an uncommon occurrence. Lymph node infarction has not been previously documented in this setting. The unique pathologic features in this case may reflect: a phenomenon specific to gold; early examination of the infarcted node; or underlying leukocytoclastic angiitis secondary to gold toxicity. Polymorphonuclear leukocytic infiltration of an infarcted lymph node has apparently not been previously reported.

Auranofin versus placebo in the treatment of rheumatoid arthritis.

In a six-month, multicenter, double-blind study involving 340 patients, auranofin, 3 mg twice daily, was compared with placebo in the treatment of adult-onset rheumatoid arthritis. All patients were continued on a therapeutic regimen of salicylates and/or a newer nonsteroidal anti-inflammatory drug. Patients in both treatment groups who completed six months of therapy with coded medications showed significant improvement in the clinical features of rheumatoid arthritis (that is, number of tender and swollen joints, severity of pain, grip strength and duration of morning stiffness); however, the mean improvement was greater in the auranofin-treated group. Fifty-two percent of the auranofin-treated patients compared with 24 percent of the placebo-treated patients (p less than 0.05) were judged by their physician to have shown marked or moderate improvement. Only in the auranofin-treated patients was there significant improvement from baseline in the laboratory parameters of disease activity: erythrocyte sedimentation rate, IgA, IgG, and IgM. After at least three months of therapy, 30 percent (46 of 152) of the placebo-treated patients but only 9 percent (13 of 152) of the auranofin-treated patients (p less than 0.05) withdrew from coded medication due to insufficient therapeutic effect. Study medication was discontinued by 5 percent (eight of 152) of the auranofin-treated patients and 3 percent (four of 152) of the placebo-treated patients because of adverse therapy events (p = 0.24). This study demonstrates the efficacy of auranofin when added to salicylates and/or nonsteroidal anti-inflammatory drugs in the treatment of rheumatoid arthritis.

Comparison of auranofin, gold sodium thiomalate, and placebo in the treatment of rheumatoid arthritis. Subsets of responses.

A comparison of placebo, auranofin, and parenteral gold sodium thiomalate therapy in 209 patients with active rheumatoid arthritis was performed in a 21-week prospective, controlled, double-blind multicenter trial. When the 161 patients who completed at least 20 weeks of treatment were analyzed for different degrees of response, no remissions were identified. When 50 percent or greater improvement of pain/tenderness scores were compared for end of trial versus entry values, 9 percent of placebo-treated patients, 34 percent of auranofin-treated patients, and 48 percent of gold sodium thiomalate-treated patients showed important improvement that was statistically significant for both gold treatments. When 50 percent improvement for joint swelling was analyzed, 12 percent of the placebo-treated group, 28 percent in the auranofin-treated group, and 37 percent in the gold sodium thiomalate-treated group showed this degree of improvement. Auranofin almost achieved statistical significance for improvement in joint swelling when compared with placebo (p = 0.07), but gold sodium thiomalate was much better than placebo (p = 0.009). There was no statistically significant difference between the two gold treatments. Thus it appears that a subset of patients had an important response to gold therapy that would not be evident by the usual analyses of mean or median changes. Analysis for predictors of response did not discriminate between responders and nonresponders. Because the trial was limited to 21 weeks of therapy, no prediction of the longer-term-effects, especially for auranofin, should be inferred.

A 12-month comparative trial of auranofin and D-penicillamine in rheumatoid arthritis.

The oral gold salt auranofin, 6 mg per day, was compared with oral d-penicillamine, 500 mg per day, in a single-blind trial in 40 patients suffering with definite or classic rheumatoid arthritis. The patients were randomly allocated into the two therapeutic regimens (19 patients auranofin; 21 patients d-penicillamine) and monitored at a minimum of four-week intervals during the first year of treatment. Significant diminution in rheumatoid disease activity, as assessed by numerous clinical and laboratory parameters, was observed in both the auranofin- and penicillamine-treated groups. No significant differences existed for these parameters between the two groups, either initially or at the end of the trial period. Ten patients were lost from the trial over the 52-week period. Three subjects were withdrawn from the auranofin-treated group (increasing severity of rheumatoid arthritis at four weeks; severe diarrhea at four weeks; probable drug-related erosive gastritis at 40 weeks). Seven subjects were permanently withdrawn from the penicillamine-treated group (four, skin rashes four to eight weeks; one, heavy proteinuria at 24 weeks; one, therapeutic failure at 32 weeks; one, compliance failure at eight weeks), and treatment was temporarily withheld in three further patients because of thrombocytopenia (two) and proteinuria (one). We conclude that both drugs are effective in rheumatoid arthritis and that the lesser toxicity with auranofin will make it a valuable addition to our therapeutic armamentarium.

Auranofin: bane or bonanza for the nonrheumatologist.

The availability of an oral gold preparation in the treatment of rheumatoid arthritis, with a claim to being effective yet safer than parenteral gold, will most certainly attract the attention of physicians and patients who previously would not have used chrysotherapy. Cardinal clinical features that may be especially helpful to the nonrheumatologist in the diagnosis of rheumatoid arthritis are outlined. The often unpredictable course of rheumatoid arthritis is stressed, emphasizing the need for an adequate trial of nonsteroidal anti-inflammatory drug therapy before instituting the use of a potentially more toxic remission-inducing drug such as gold. The absence of an immediate flare of synovitis, should the use of oral gold be sporadic, will aggravate the problem of patient noncompliance. Techniques to minimize this problem will be noted. Perseverence and careful record keeping by physicians not necessarily comfortable with the long-term management of chronic disease must be part of the therapeutic program.

Gold therapy in patients with systemic lupus erythematosus.

Despite a progressively more favorable prognosis in systemic lupus erythematosus (SLE) a need remains for therapeutic agents with greater benefit and less toxicity than corticosteroids and immunosuppressive drugs. Therefore, we treated 16 patients with SLE but without renal diseases with auranofin, a drug with proved efficacy and safety in rheumatoid arthritis. A modest diminution in overall disease activity, as judged by the investigators, and a reduction in maintenance corticosteroid dosage was achieved. However, neither laboratory assessments nor more objective clinical measurements of SLE disease activity disclosed any improvement over baseline. One case each of proteinuria and thrombocytopenia was observed, most likely related to underlying disease and not the drug, suggesting that auranofin may be safe in patients with SLE. A controlled trial, utilizing a broader spectrum of patients with SLE, may be warranted.

Roentgenographic findings during auranofin treatment.

Roentgenograms of hands and wrists from rheumatoid arthritis patients treated with auranofin for 12 and 24 months were scored for severity of erosive disease. Scores were analyzed by a method in which a calculated mean annual rate of erosion prior to treatment was compared with the annualized rate during therapy. The original films from a previous Myochrysine versus placebo study were reanalyzed in order to compare the results of different scoring methods and to provide an auranofin versus placebo comparison. Two readers found the rate of erosion during oral gold therapy to be less than that predicted by the analytical model. Interobserver differences in scoring were demonstrated. A breakdown of patient scores by duration of disease showed the greatest difference between scores observed during treatment with those predicted by the model in patients with early disease.

Early experiences with auranofin in juvenile rheumatoid arthritis.

Pharmacologic management of juvenile rheumatoid arthritis is only one of several modalities necessary for effective control. The stepping stones to proper management include a planned long-range program, physical therapy with swimming, good health habits, and consultation with other health professionals who are part of the management team. Pharmacologic therapy includes nonsteroidal anti-inflammatory drugs initially, occasionally corticosteroids, and slow-acting antirheumatic drugs, including injectable gold when therapeutic response is inadequate. Early experiences with oral gold are reported here. Auranofin (triethylphosphine gold) was administered to 21 patients with juvenile rheumatoid arthritis during a segment I, open ended, open-label, noncontrolled trial designed to establish safety and preliminary efficacy. Initial dosage was 0.1 mg/kg per day; incremental increases to 0.2 mg/kg per day were allowed (with usual increase to 0.15 mg/kg per day). Aspirin (80 mg/kg per day) or tolmetin (20 to 40 mg/kg per day), or naproxen (400 to 600 mg/m2 per day) were allowed as rapidly acting antiinflammatory agents. Stable measurable plasma concentrations of gold were attained in all patients during the study. More than half the patients sustained clinically significant improvement (greater than 25 percent) with regard to the number and severity of joints with swelling, pain on motion, and tenderness. In nine of the 19 patients, the total number of joints with active arthritis decreased by at least 25 percent. All articular disease indices measured indicated improvement of group mean changes between the initial and final visit. Eleven of 16 patients with an elevated erythrocyte sedimentation rate showed decreases of at least 25 percent. The group given higher dosages had a greater proportion of responders in regard to decreases in erythrocyte sedimentation rate (nine of 11 patients). Four of six patients whose serums contained rheumatoid factor showed decreases in the titers. Discontinuation of auranofin was necessary in two patients: one because of headache and one because of hematuria and anemia associated with a severe flare-up of polyarticular disease. The results from this trial reveal sufficient patient improvement to plan a double-blind trial of auranofin in children with juvenile rheumatoid arthritis.

Auranofin. A preliminary review of its pharmacological properties and therapeutic use in rheumatoid arthritis.

Auranofin is the first orally active gold compound for the treatment of rheumatoid arthritis. Like other chrysotherapeutic agents, its exact mechanism of action is unknown, but it probably acts via immunological mechanisms and alteration of lysosomal enzyme activity. Although long term clinical experience with auranofin is limited, its efficacy appears to approach that of sodium aurothiomalate. Further comparative studies with aurothioglucose, hydroxychloroquine and D-penicillamine are required before definitive statements can be made regarding the relative efficacy of auranofin and these agents. While patients have demonstrated clinical remission of rheumatoid arthritis in response to auranofin therapy, radiological studies have been inconclusive regarding its effect on the occurrence or progression of erosive lesions. Auranofin is relatively well tolerated in most patients, but diarrhoea, skin rash, and pruritus are sometimes troublesome, and thrombocytopenia and proteinuria are potentially serious side effects which may occur during therapy. Whereas mucocutaneous side effects are more frequent with injectable gold compounds, gastrointestinal reactions are the most common adverse effect seen with auranofin. The frequency of side effects has been similar with auranofin and sodium aurothiomalate, but they are generally less severe with auranofin. While some of the side effects are controlled by a reduction in dosage, temporary or permanent withdrawal of auranofin may be necessary. Auranofin is clearly a useful addition to the limited list of agents with disease-modifying potential presently available for the treatment of rheumatoid arthritis. It will doubtless generate much interest as its final place in therapy becomes better defined through additional well-designed studies and wider clinical experience.

Disease-modifying drugs for progressive rheumatoid arthritis.

In patients with rheumatoid arthritis who do not respond to therapy with salicylates or nonsteroidal anti-inflammatory agents, stronger and potentially more toxic drugs are then considered. At the present time in the United States, this generally means a trial of hydroxychloroquine, gold, penicillamine, azathioprine, or cyclophosphamide. Each of these drugs is discussed with regard to pharmacology and possible modes of action, pertinent clinical studies, and toxicity.

Auranofin: a unique oral chrysotherapeutic agent.

Auranofin is a chemically unique gold coordination complex with demonstrated antiarthritic properties on oral administration. Its pharmacokinetic and immunologic profiles are distinct from injectable gold compounds. When auranofin is added to a regimen of salicylates and/or a nonsteroidal antiinflammatory drug for the treatment of RA, significant additional therapeutic benefit is observed. Published studies indicate that auranofin given 6 mg per day approaches the efficacy of parenteral gold salts in the treatment of rheumatoid disease. Noticeable improvement in clinical and laboratory parameters of disease activity has been observed by the third month of auranofin therapy. Further benefit occurs in some patients during the remainder of the first year of treatment. In the more than 3,000 patients treated with auranofin, the most frequently reported side effects were gastrointestinal (mainly diarrhea) and mucocutaneous. Most side effects were mild in nature and the withdrawal rate due to all adverse reactions averaged 11%. Auranofin differs from injectable gold by producing more gastrointestinal but fewer mucocutaneous reactions. The severity of these reactions is less with auranofin and causes fewer withdrawals from therapy.