Comparative effects of N,N-disubstituted dithiocarbamates and dimercaptosuccinate on mobilization of methylmercury in mice

Diethyldithiocarbamate (DDTC) and five of its N,N-disubstituted analogs were compared with dimercaptosuccinate (DMSA) for effectiveness in mobilizing and promoting excretion of methylmercury (MeHg) following administration of a sublethal dose of MeHg labeled with Me203Hg. The previously reported effectiveness of DMSA was confirmed. Of the dithiocarbamates assessed, only di(carboxymethyl)-dithiocarbamate (DCDC) was active in mobilizing MeHg, and its activity on a molar dose basis was less than that of DMSA. The disparate structural requirements of dithiocarbamates as antagonists of MeHg and of cadmium were discussed.

DMSA, DMPS, and DMPA--as arsenic antidotes

meso-Dimercaptosuccinic acid (DMSA), 2,3-dimercapto-1-propanesulfonic acid, Na salt (DMPS), and N-(2,3- dimercaptopropyl )- phthalamidic acid (DMPA) are water soluble analogs of 2,3-dimercapto-1-propanol (BAL). The relative effectiveness or therapeutic index of these dimercapto compounds in protecting mice from the lethal effects of an LD99 of sodium arsenite is DMSA greater than DMPS greater than DMPA greater than BAL in the magnitude of 42:14:4:1, respectively. DMPS, DMPA, or DMSA will mobilize tissue arsenic. BAL, however, increases the arsenic content of the brain of rabbits injected with sodium arsenite. These results raise the question as to the appropriateness of BAL as the treatment for systemic arsenic poisoning. Either DMSA or DMPS, when given sc or po, will protect rabbits against the lethal systemic effects of subcutaneously administered Lewisite . DMPS and DMSA have promise as prophylactics for the prevention of the vesicant action of Lewisite . The sodium arsenite inhibition of the pyruvate dehydrogenase (PDH) complex can be prevented and reversed in vitro or in vivo by DMPS, DMSA, DMPA, or BAL. Of them all, DMPS is most potent and BAL appears to be the least potent. The usefulness of all these dimercapto compounds would be enhanced by a careful study of their metabolism and biotransformation. These dimercapto compounds are in a great many respects orphan drugs. At this stage of their development, it is very difficult for the clinician to obtain funds to study them clinically even though they appear to be useful for treatment of poisoning by any one of the heavy metals.

Effect of thiol chelators on trace metal levels

Influence of 2,3 dimercaptosuccinic acid, diethyldithiocarbamate and DL-penicillamine on the levels of essential metals in liver, kidney and serum was investigated. The hepatic zinc increased while renal copper decreased significantly upon treatment with these commonly used thiol metal chelators. On the other hand, hepatic copper decreased on treatment with DL-penicillamine. The renal zinc and serum copper and zinc remained practically unaffected on treatment with these metal chelators.

In vivo screening of potential antidotes for chronic cadmium intoxication

Nineteen chelating agents have been screened under identical conditions of metal loading in an attempt to establish their relative ability to mobilize cadmium from the liver and kidney in mice with chronic cadmium intoxication. The compounds investigated were divided into five groups: polyaminocarboxylic acids, monothiols, dithiols, macrocycles, and a miscellaneous category. Only 2,3-dimercaptopropanol (BAL) and sodium diethyldithiocarbamate (NaDDTC) were able to produce a statistically significant (at the 95% level) reduction in the cadmium content of the kidney. The closely related dithiols sodium 2,3-dimercaptopropane-1-sulfonate and 2,3-dimercaptosuccinic acid produced statistically significant increases in the liver cadmium contents, as did N-(2-mercaptopropionyl)-glycine. The reduction in kidney cadmium levels produced by both BAL and NaDDTC was just under 40%.

DL- and meso-dimercaptosuccinic acid: in vitro and in vivo studies with sodium arsenite

Dimercaptosuccinic acid (DMSA) has been receiving increasing interest as an antidote for poisoning by heavy metals. Of the various isomeric forms, meso-DMSA has been studied most extensively because its preparation and purification are relatively easy. Using a variety of in vitro and in vivo procedures, we have investigated and compared DL- and meso-DMSA as antidotes for sodium arsenite. The two forms of DMSA are equally effective in preventing or reversing, in vitro, the arsenite inhibition of the activity of mouse kidney pyruvate dehydrogenase (PDH) complex, DL-DMSA, however, is superior to meso-DMSA for the in vivo reversal of PDH activity as measured in vitro. The LD50 values of DL- and meso-DMSA in the mouse were 10.84 and 13.73 mmol/kg, ip, respectively. The ED50 values of the two DMSA forms were not significantly different in mice receiving a LD99 dose of sodium arsenite. DL- and meso-DMSA were equally effective in mobilizing tissue 74As of rabbits. The activity of DMSA as an arsenic antidote appeared to be independent of its isomeric structural configuration. There did not appear to be any great advantage in using DL-DMSA instead of meso-DMSA as an arsenic antidote.

Interaction of methylmercury with microtubules in cultured cells and in vitro

The effects of methylmercury (MeHg) on cytoplasmic microtubules in cultured fibroblasts and on the in vitro polymerization of microtubules were examined. MeHg caused disruption of cellular microtubules in a concentration- and time-dependent manner. Addition of the metal-chelating agent, dimercaptosuccinic acid (DMSA), both prevented and reversed the effect of MeHg. Comparisons of the cellular levels of mercury and microtubule integrity indicated that microtubules dissociated at levels higher than 0.6 microgram Hg/mg protein. In vitro polymerization was also directly inhibited by MeHg; this effect was prevented by the addition of DMSA.

Methylmercury removal in the dog during infusion of 2,3-dimercaptosuccinic acid (DMSA)

2,3-Dimercaptosuccinic acid (DMSA) has been utilized in chelation therapy and in extracorporal complexing hemodialysis therapy for experimental methylmercury intoxication. In the latter application, substantial excretion of methylmercury occurred by the urinary route. This prompted the current study of the effects of continuous intravenous DMSA infusion therapy on methylmercury kinetics in the dog. Animals previously dosed with 203Hg-labeled methylmercury at 2.5 mg Hg/kg received a priming dose of DMSA, followed by a continuous iv infusion at dose rates that resulted in DMSA concentrations in plasma similar to those observed during DMSA complexing hemodialysis therapy. The kinetics of 203Hg removal in DMSA-infused dogs was compared with both saline-infused controls and DMSA complexing hemodialysis treated dogs. DMSA infusion therapy resulted in a shift in 203Hg binding within systemic blood from the red-cell fraction into plasma. This was consistent with an observed association of DMSA with the plasma fraction. The shift in 203Hg from red cells into plasma was paralleled by an increase in urinary clearance of 203Hg during the DMSA infusion period. In four dogs treated in this fashion, an average of 6.5 micrograms of mercury was removed by the urinary route over the 5-h treatment period, compared to 0.007 micrograms in the saline-infused dogs. Although a similar magnitude of mercury output into urine was observed during DMSA complexing hemodialysis, an additional 5 micrograms was removed by the dialyzer, making that technique 1.5 times as effective as infusion therapy. Comparing 203Hg tissue concentration after DMSA infusion therapy with the saline-treated controls revealed a 6.5-fold decrease in liver, a 3-fold reduction in kidney, and a 27% reduction in cerebrum. No significant differences were observed in medulla or cerebellum. Histopathology revealed no consistent differences between DMSA-treated and saline-treated animals. The DMSA infusion therapy was effective in causing a rapid removal of 203Hg from animals previously dosed with 203Hg-labeled methylmercury. DMSA infusion therapy may provide a useful therapeutic alternative for methylmercury poisoning when rapid removal of the intoxicant is desired and hemodialysis equipment and expertise are not readily available.

Comparative effects of diethyldithiocarbamate, dimercaptosuccinate, and diethylenetriaminepentaacetate on organ distribution and excretion of cadmium

Diethyldithiocarbamate (DDTC), dimercaptosuccinate (DMSA), and diethylenetriaminepentaacetate (DTPA) were compared for their relative efficacies at equimolar doses in promoting mobilization of metallothionein-bound cadmium (Cd) from its sites of deposition several weeks after i.p. injection of a sublethal dose of Cd containing 109Cd. Routes and rates of excretion were also determined. The most rapid and extensive reduction of renal, hepatic, and splenic Cd was obtained with DDTC. Concurrently, however, DDTC caused moderate increases in lung, testicular, and heart Cd burdens, and increased the brain burden about ten-fold. Only renal and testicular levels of Cd were reduced by DTPA, and it was much less effective than DDTC in lowering the renal levels. No reduction of Cd levels in any organ was attained with DMSA treatment. Excretion of Cd following DDTC treatment was exclusively by the fecal route; DTPA promoted both urinary and fecal excretion, but the total amount excreted by both routes was considerably less than that observed following DDTC treatment. It was concluded that the effectiveness of a chelator in promoting mobilization and excretion of metallothionein-bound Cd cannot be predicted on the basis of its effectiveness in protecting mice against a lethal dose of Cd when given immediately after Cd and prior to induction of metallothionein synthesis. The possibility is discussed that the greater effectiveness of DDTC may be due to an interaction with nascent metallothionein rather than to chelation of Cd.

Neurotoxicity, anorexia and the preferential choice of antidote in methylmercury intoxicated rats

In the rat the first clinical signs of methylmercury intoxication is loss of appetite, which can be restored by the administration of dimercaptosuccinic acid (DMSA). In female rats which lost body weight as a result of methylmercury treatment, the anorexic effect of methylmercury was reversed even when DMSA was given in the drinking water. When intoxicated animals had the choice between DMSA supplemented water (2.5 mg DMSA/ml) and tap water, they preferred DMSA. This preference is related to the severity of intoxication and is abolished after one or two days. During this time the urinary mercury excretion is substantially increased and the body burden of methylmercury is decreased. The effect of methylmercury mobilization is fast, and can be detected in urine collected from cannulated ureters shortly after the IV administration of 10 or 20 mg/kg dimercaptosuccinic acid.

Reversible effects of monothiol (D-penicillamine) and dithiol (dimercaptosuccinic acid) chelating compounds on methylmercury-inhibited choline acetyltransferase activity and high affinity choline uptake

The effects of thiol compounds on methylmercury chloride (MMC)-inhibited choline acetyltransferase (ChAT) activity and MMC-inhibited high affinity choline uptake of rat brain tissue were studied in vitro. D-penicillamine (D-Pc) and dimercaptosuccinic acid (DMS) reversed the MMC-inhibited ChAT activity dose-dependently. Equilibrium dialysis of MMC-inhibited ChAT against the buffer containing 10(-3) M D-Pc reversed the ChAT activity almost completely. The reversal effect of D-Pc (monothiol compound) on MMC-inhibited ChAT was significantly more potent than that of DMS (dithiol compound). D-Pc and DMS almost equally reversed the MMC-inhibited high affinity choline uptake by synaptosomes in a dose dependent fashion. Washing with a solution containing D-Pc or DMS equally reversed the MMC-inhibited high affinity choline uptake in a dose-dependent fashion. Neither D-Pc nor DMS could reverse the hemicholinium-3-inhibited high affinity choline uptake.

Regulation of human globin gene expression in mouse erythroleukemia x human fibroblast hybrid cells

A somatic cell hybrid, XX-8, was obtained from a fusion of tetraploid mouse erythroleukemia cells with human Lesch-Nyhan skin fibroblasts. This hybrid cell was previously shown (1) to produce human beta- but no human gamma-globin mRNA sequences after induction with dimethylsulfoxide. In this study we show that: (a) human beta- and gamma-globin genes are present in XX-8 cells in approximately equal numbers; (b) no human gamma-globin mRNA sequences can be detected in either the cytoplasmic or nuclear RNA fractions even with several different inducers; (c) after induction the human beta-globin gene is converted from a DNase I insensitive or closed structure to a DNase I open configuration, while the human gamma-globin gene remains closed; and (d) no human beta-globin polypeptide can be detected in the intact induced cells, indicating that fibroblast globin genes, even when induced to make mRNA in an erythroid environment, do not synthesize an RNA that is translated efficiently.

Biological chelation: 2,3-dimercapto-propanesulfonic acid and meso-dimercaptosuccinic acid

Water soluble analogs of British Anti-Lewisite that are active orally and less toxic than BAL are now available. These agents are 2,3-dimercapto-1-propanesulfonic acid and meso-dimercaptosuccinic acid. Evidence for their effectiveness in preventing the lethal effects of sodium arsenite in mice and lewisite in rabbits is presented. These analogs can be expected to replace BAL in the treatment of heavy metal poisoning.

The effects of dimercaptosuccinic acid and other chelating agents on the retention of platinum in the rat kidney after treatment with cisplatin

The ability of three chelating agents, dimercaptopropanesulphonate (DMPS), dimercaptosuccinic acid (DMSA), and diethylenetriaminepentaacetic acid (DPTA), to reduce the retention of platinum in the kidneys was tested in male Sprague-Dawley rats treated 24 h previously with Cisplatin. DMPS and DMSA, when given as four daily doses of 1 mMol/kg, increased the urinary excretion from 10.1% +/- 1.2% to 13.6% +/- 1.3% and 13.5% +/- 2.6%, respectively, but only DMSA caused a small but significant reduction in the kidney platinum content. DTPA was totally ineffective. It is concluded that none of these agents is likely to be useful for the reduction of Cisplatin-induced renal toxicity in the clinical situation.

The effect of heavy metal chelators on the renal accumulation of platinum after cis-dichlorodiammineplatinum II administration to the rat

1 Rats received a total of 18 mg/kg cis-dichlorodiammineplatinum (II) (CDDP) intravenously and were treated concomitantly with calcium-disodium ethylenediaminetetraacetic acid (CaNa2EDTA), 2,3-dimercaptopropanol (BAL), deferoxamine, 2,3-dimercaptosuccinic acid (DMS) or vehicle. In comparison to controls, renal platinum concentration was significantly reduced in the DMS and deferoxamine-treated groups. However, significant deterioration occurred in the deferoxamine-treated group. The hepatic platinum concentration was unaffected by the chelating agents. 2 Following a dose of 6 mg/kg CDDP intravenously, eight days of treatment with DMS, 50 mg/kg daily, had no effect on renal platinum excretion, while treatment with 100 or 200 mg/kg daily reduced renal platinum concentration by 50%. 3 In order to determine whether DMS could prevent the nephrotoxicity of CDDP, rats were given 6 mg/kg CDDP intravenously, followed by a four day course of DMS treatment at doses of 0, 50, 100 or 200 mg/kg daily begun 3 h after the CDDP dose. DMS failed to prevent renal toxicity as indicated by weight loss, serum creatinine concentration, renal histology, and the urinary excretion of N-acetyl-beta-glucosaminidase, a renal tubular enzyme.

The influence of chelating agents on the distribution and biotransformation of methylmercuric chloride in rats

Male albino rats were injected i.p. with [203Hg]methylmercuric chloride (MMC) and later at different time intervals they received N-acetylpenicillamine, 2,3-dimercaptopropane-1-sulfonate or dimercaptosuccinic acid (DMSA). The animals were sacrificed 24 h after the last chelant treatment and the content of 203Hg was determined in 10 organs. In liver and kidneys, [203Hg]MMC and 203Hg++ were measured separately. DMSA was most effective in removing the mercurial from all organs, except the kidneys, where 2,3-dimercaptopropane-1-sulfonate was better. N-acetylpenicillamine showed only marginal effectiveness. Separate determinations of [203Hg]MMC and 203Hg++ in liver and kidneys showed that DMSA removed more of the organic and 2,3-dimercaptopropane-1-sulfonate more of the inorganic Hg. DMSA injected i.p. or given p.o. removed almost equal amounts of MMC, indicating virtually 100% absorption of the chelating agent from the gastrointestinal tract.