Determination and metabolism of dithiol-chelating agents: electrolytic and chemical reduction of oxidized dithiols in urine

Plasma and urine dimercaptopropanesulfonate concentrations after dermal application of transdermal DMPS (TD-DMPS)

2,3-Dimercaptopropane-1-sulfonate (DMPS) is a metal chelator approved in Europe for oral or intravenous use for heavy metal poisoning. Transdermally applied DMPS (TD-DMPS) is used by some alternative practitioners to treat autism, despite the absence of evidence for its efficacy. We found no literature evaluating the pharmacokinetics of the transdermal route of delivery or the ability of TD-DMPS to enhance urinary mercury elimination. We hypothesized that TD-DMPS is not absorbed. Eight adult volunteers underwent application of 1.5-3 drops/kg of TD-DMPS. Subjects provided 12-h urine collections the day before and day of application. Subjects underwent blood draws at 0, 30, 60,90, 120, and 240 min after TD-DMPS application. Plasma and urine were assayed for the presence of DMPS. Urine was assayed for any change in urinary mercury excretion after DMPS. One control subject ingested 250 mg of oral DMPS and underwent the same urine and blood collections and analyses. No subject had detectable urine DMPS or increased urine mercury excretion after TD-DMPS. One subject had detectable levels of DMPS in the 30-min plasma sample, suspected to be contamination. All other samples for that subject and the other seven subjects showed no detectable plasma DMPS. The control subject had detectable urine and plasma DMPS levels and increased urine mercury excretion. These results indicate that TD-DMPS is not absorbed. There was no increase in urine mercury excretion after TD-DMPS. Our results argue that TD-DMPS is an ineffective metal chelator.

The renal Na(+)-dependent dicarboxylate transporter, NaDC-3, translocates dimethyl- and disulfhydryl-compounds and contributes to renal heavy metal detoxification

The active transport of Krebs cycle intermediates, such as succinate, alpha-ketoglutarate, and citrate, is mediated by sodium-coupled transporters found in the luminal (NaDC-1) and basolateral plasma membranes (NaDC-3) of proximal tubule cells. This study used the two-electrode voltage clamp technique to examine steady-state currents associated with the influx of three sodium ions and one divalent dicarboxylate into oocytes expressing the sodium-dicarboxylate transporter from winter flounder kidney, fNaDC-3. The substrate concentration, where half-maximal current was observed (K(0.5)), was 30 micro M for succinate. Besides 2,2-dimethylsuccinate, fNaDC-3 also accepted 2,3-dimethylsuccinate and the oral lead-chelating agent, meso-2,3-dimercaptosuccinate (DMSA or Succimer). Whereas the K(0.5) for succinate and 2,2-dimethylsuccinate was independent of membrane voltage within -90 and -10 mV, K(0.5) for 2,3-dimethylsuccinate and 2,3-dimercaptosuccinate increased with decreasing voltage, indicating a critical role of the position of the methyl- or sulfhydryl-group in voltage-sensitive affinity. In addition to meso-2,3-dimercaptosuccinate, fNaDC-3 translocated dimercaptopropane-1-sulfonate (DMPS or Dimaval), an oral chelator for the treatment of mercury intoxication. The chelates formed by HgCl(2) and DMSA or DMPS and by Pb(NO(3))(2) and DMSA, however, were not translocated by fNaDC-3. The data suggest that NaDC-3 is an essential component in the delivery of uncomplexed antidotes for renal heavy metal detoxification.

Determination of lipoic acid by precolumn derivatization with monobromobimane and reversed-phase high-performance liquid chromatography

A new method for the quantitation of lipoic acid in plasma and tissues based on the selective precolumn derivatization of thiols with the fluorescent label monobromobimane is described. After extraction with diethyl ether, the dithiolane ring of lipoic acid is opened by reduction with NaBH4 before the free thiols can react with the label. Separation and quantitation was achieved by reversed-phase HPLC and fluorescence detection. The concentration-response curve was linear from 20 to 3000 nM in plasma. The recovery as determined with [3H]lipoic acid was 60.9% from plasma and 61.4% from rat heart tissue. The pretreatment of samples with N-ethylmaleimide makes it possible to differentiate between reduced and oxidized forms of lipoic acid.

Mobilization of heavy metals by newer, therapeutically useful chelating agents

Four chelating agents that have been used most commonly for the treatment of humans intoxicated with lead, mercury, arsenic or other heavy metals and metalloids are reviewed as to their advantages, disadvantages, metabolism and specificity. Of these, CaNa2EDTA and dimercaprol (British anti-lewisite, BAL) are becoming outmoded and can be expected to be replaced by meso-2,3-dimercaptosuccinic acid (DMSA, succimer) for treatment of lead intoxication and by the sodium salt of 2,3-dimercapto-1-propanesulfonic acid (DMPS, Dimaval) for treating lead, mercury or arsenic intoxication. Meso-2,3-DMSA and DMPS are biotransformed differently in humans. More than 90% of the DMSA excreted in the urine is found in the form of a mixed disulfide in which each of the sulfur atoms of DMSA is in disulfide linkage with an L-cysteine molecule. After DMPS administration, however, acyclic and cyclic disulfides of DMPS are found in the urine. The Dimaval-mercury challenge test holds great promise as a diagnostic test for mercury exposure, especially for low level mercurialism. Urinary mercury after Dimaval challenge may be a better biomarker of low level mercurialism than unchallenged urinary mercury excretion.

Pharmacokinetics of meso-2,3-dimercaptosuccinic acid in patients with lead poisoning and in healthy adults

We compared the pharmacokinetics of meso-2,3-dimercaptosuccinic acid (DMSA) in three children with lead poisoning, three adults with lead poisoning, and five healthy adult volunteers. All subjects received DMSA orally. Maximum blood concentration and time to maximum blood concentration of total DMSA concentration were not statistically different among the groups. Unaltered DMSA was detected in the blood of all poisoned patients but in only one of five healthy volunteers. Elimination half-life of total DMSA (parent drug plus oxidized metabolites) was longer in children with lead poisoning (3.0 +/- 0.2 hours) than in adults with lead poisoning (1.9 +/- 0.4 hours) and healthy adults (2.0 +/- 0.2 hours). Renal clearance of total DMSA was greater in healthy adults (77.0 +/- 13.2 ml/min per square meter) than in either adults (24.7 +/- 3.3 ml/min per square meter) or children with lead poisoning (16.6 ml/min per square meter); renal clearance of the metabolites of DMSA was also greater in healthy adults (64.6 +/- 10.1 ml/min per square meter) than in either adults (35.4 +/- 8.4 ml/min per square meter) or children with lead poisoning (19.5 ml/min per square meter). The DMSA appeared to enter the erythrocytes of patients with lead poisoning to a greater extent than in healthy adults. We conclude that renal clearance of DMSA and its metabolites may be impaired and that the distribution of DMSA in children with lead poisoning may be different from that in adults.

Effects of oral meso-2,3-dimercaptosuccinic acid (DMSA) administration on late gestation and postnatal development in the mouse

The present study was conducted to evaluate the effects of meso-2,3-dimercaptosuccinic acid (DMSA) on late gestation and postnatal viability and growth in the mouse. DMSA was given po to four groups of pregnant Swiss mice at 0, 200, 400, and 800 mg/kg/day from day 14 of pregnancy until postnatal day 21. At birth, the following data were recorded: length of gestation, number of live, dead, and abnormal pups, sex, and individual pup weights. Each pup was weighed again on day 4, 14, and 21 of lactation. Pinna detachment, incisor eruption and eye opening were also monitored. No treatment-related signs of toxicity were noted in any of the dams during the study. No adverse effects on offspring survival or development were evident in the 200 or 400 mg DMSA/kg/day groups. However, on days 14 and 21 of lactation a significant decrease in pup body weight was observed in the 800 mg/kg/day group. Also, a significant increase in the relative weight of the brain was seen in this group. The "no observable effect level" (NOEL) for health hazards to the developing pup was greater than 400 mg/kg/day. This dose is higher than the amounts of DMSA usually given in the treatment of human heavy metal intoxications.

Determination and metabolism of dithiol chelating agents. IV. Urinary excretion of meso-2,3-dimercaptosuccinic acid and mercaptosuccinic acid in rabbits given meso-2,3-dimercaptosuccinic acid

The water-soluble dithiol chelating agents meso-2,3-dimercaptosuccinic acid (DMSA) and 2,3-dimercaptopropane-1-sulfonic acid (DMPS) are becoming of increasing importance for the treatment of lead, arsenic and mercury poisoning. There is, however, a paucity of data about their metabolic transformation. Male rabbits were given DMSA (0.20 mmol/kg) i.m., and urine was collected over a 6-hr period. Monobromobimane derivatization, HPLC separation, and fluorescence detection, along with [U-14C]DMSA data, demonstrated that the total 14C found in the urine was distributed as 73% unaltered DMSA, 7% mercaptosuccinic acid and 6 and 14% of two unknowns. Electrolytic reductive treatment of the urine did not increase the urinary content of DMSA, indicating that oxidative biotransformation is not a major pathway for DMSA in the rabbit. This latter result is strikingly different from that for DMPS in rabbit.

Determination and metabolism of dithiol chelating agents. VI. Isolation and identification of the mixed disulfides of meso-2,3-dimercaptosuccinic acid with L-cysteine in human urine

Virtually nothing is known about the biotransformation of the heavy metal chelating agent, meso-2,3-dimercaptosuccinic acid (DMSA). Two fasted, normal, young men were given 10.0 mg DMSA/kg po, and their urines were collected over a 14-hr period. Urine samples were analyzed, before and after electrolytic reductive treatment, for DMSA and its biotransformants using bromobimane derivatization, HPLC separation, and fluorescence detection. Metabolites were isolated by HPLC, ion-pairing extraction, ion-exchange extraction, and TLC. By 14 hr after DMSA administration, 87% of the total DMSA and 95% of the total L-cysteine found in urine consisted of altered forms of these compounds. The urinary excretion of altered DMSA, at 1, 2, 4, 6, 9, and 14 hr after administration of DMSA, when compared to the urinary excretion of altered L-cysteine had a correlation coefficient of 0.952 and p less than 0.003. Approximately 90% of the altered DMSA excreted in the 2- to 4-hr urine was found in disulfide linkage with L-cysteine. The remaining 10% was found as cyclic disulfides of DMSA. Of the mixed disulfides found in 4- to 6-hr urine, 97% consisted of two L-cysteine residues per one DMSA and the remaining 3% consisted of one L-cysteine per one DMSA. The 2:1 mixed disulfides (97%) were isolated as three distinct species by TLC, consisting of 77, 12, and 8% of the total mixed disulfides found. In addition to the novelty of these biotransformants of DMSA, the DMSA-cysteine mixed disulfides indicate a thiol-disulfide interchange between DMSA and L-cystine. The discovery of the formation of these water soluble DMSA-cysteine mixed disulfides should encourage the evaluation of DMSA in the treatment of cystinuria.