Succimer Other names: Dimercaptosuccinic acid

Succimer is a lead chelator; it forms water soluble chelates and, consequently, increases the urinary excretion of lead.

In a study performed in healthy adult volunteers, after a single dose of ¹⁴C-succimer at 16, 32, or 48 mg/kg, absorption was rapid but variable with peak blood radioactivity levels between one and two hours. On average, 49% of the radiolabeled dose was excreted: 39% in the feces, 9% in the urine and 1% as carbon dioxide from the lungs. Since fecal excretion probably represented nonabsorbed drug, most of the absorbed drug was excreted by the kidneys. The apparent elimination half-life of the radiolabeled material in the blood was about two days.

In other studies of healthy adult volunteers receiving a single oral dose of 10 mg/kg, the chemical analysis of succimer and its metabolites in the urine showed that succimer was rapidly and extensively metabolized. Approximately 25% of the administered dose was excreted in the urine with the peak blood level and urinary excretion occurring between two and four hours. Of the total amount of drug eliminated in the urine, approximately 90% was eliminated in altered form as mixed succimer-cysteine disulfides; the remaining 10% was eliminated unchanged. The majority of mixed disulfides consisted of succimer in disulfide linkages with two molecules of L-cysteine, the remaining disulfides contained one L-cysteine per succimer molecule.

Succimer has low acute oral toxicity, with oral median lethal doses in rodents in excess of 3.6 g/kg. In a 28-day toxicity study, dogs receiving 30 and 100 mg/kg/day had lower urinary specific gravity and an increase in renal tubular regenerative hyperplasia. No renal toxicity was noted in dogs given 50 mg/kg/day orally for 14 consecutive days. In a chronic 6-month oral toxicity study, one male dog died (out of 7) at a dose of 200 mg/kg/day attributed to associated renal toxicity. Treatment related renal tubule epithelial changes in this study were observed in dogs after chronic (6-month) exposure to 110 and 200 mg/kg/day for 17 days then to 80 and 140 mg/kg/day for the remainder of the study. These changes were dose-dependent and correlated with increased kidney weights in male and female dogs at the 10 mg/kg/day dose. Nephropathy was not observed in dogs treated at 10 mg/kg/day. Reduced platelet counts were noted in 5 of 7 dogs receiving either 80 or 140 mg/kg/day for 3 or 6 months, although group means were not statistically different from concurrent controls. Platelets had not been quantified in earlier studies. Normal megakaryocytes in the bone marrow, plus the absence of fibrin degradation products or histologic evidence for DIC, suggested an autoimmune-mediated thrombocytopenia, a finding common in dogs but not in other species. However, serum antibody tests were inconclusive. Rats dosed chronically to 500 mg/kg/day developed no evidence for nephropathy orthrombocytopenia.