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Succimer (meso-2,3-dimercaptosuccinic acid) is an orally active metal chelator that is approved by the Food and Drug Administration (FDA) for the treatment of lead poisoning in children with blood lead concentrations greater than 45 μg/dL. Succimer is also used to treat patients poisoned with arsenic and organic and inorganic mercury. Succimer has a wider therapeutic index and exhibits many advantages over dimercaprol and edetate calcium disodium (CaNa2EDTA), the two other chelators used for the same clinical problems. Animal studies suggest that succimer does not redistribute lead or arsenic to the central nervous system (CNS). The role of succimer alone and in conjunction with other chelators to treat lead encephalopathy continues to be defined.61,74

Succimer was initially synthesized in 1949 in England.72 In 1954, antimony-a,a′-dimercaptopotassium succinate (TWSb) was developed to treat schistosomiasis.43 TWSb is antimony bound to the potassium salt of succimer in a 2:3 ratio, forming a water-soluble xenobiotic with 50 times less toxicity than the previously used antimony compound, tartar emetic. Several years later, a group from Shanghai demonstrated the ability of the sodium salt of succimer to increase the median lethal dose for 50% of test subjects (LD50) of tartar emetic 16-fold in mice.109 An early review of the Chinese experience with intravenous (IV) succimer in the treatment of occupational lead and mercury poisoning suggested efficacy similar to IV CaNa2EDTA in increasing urinary lead and to intramuscular (IM) DMPS (racemic-2,3-dimercapto-1-propanesulfonic acid, unithiol) for mercury, with little observed toxicity.105 This experience, the subsequent widespread use in Asia76,79,90,105,106,111 and Europe,17,34,41,44,64,99 and the realization that succimer could be used orally,7,50 led to US-based animal experiments, human trials, and FDA approval in 1991 for the treatment of lead-poisoned children.

Succimer is a white crystalline powder with a molecular weight of 182 daltons and a characteristic sulfur odor and taste.6 Succimer is the meso form of 2,3-dimercaptosuccinic acid; the racemic form is being investigated.38 Because it contains four ionizable hydrogen ions, succimer has four different pKas—2.31, 3.69, 9.68, and 11.14—with the dissociation of the two lower values representing the carboxyl groups and the two higher values the sulfur groups.4 Lead and cadmium bind to the adjoining sulfur and oxygen atoms, whereas arsenic and mercury bind to the two sulfur moieties, forming pH-dependent water-soluble complexes (see Fig. A27–1).86 Succimer is highly protein bound to albumin through a disulfide bond. Subhuman primate studies of IV and oral22C succimer indicate that following an IV dose, radiolabel is eliminated almost exclusively via the kidney, with only trace amounts (less than 1%) excreted via feces or expired air.72 Following the administration of a single oral dose of 10 mg/kg, succimer is rapidly and extensively metabolized.69 Approximately 20% of the administered oral dose is recovered in the urine, ...

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