IN VITRO AND IN VIVO STUDIES OF LEAD AND ITS COMPLEXES

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DESCRIPTION (Adapted from the Investigator's Abstract): Exposure of children to environmental sources of lead is a serious public health concern. Medical approaches to treatment of lead poisoning increasingly use chelation therapy with meso-DMSA (Succimer), a relatively non-toxic, orally available chelating agent granted Orphan Drug Status by the FDA. Clinical trials at four health care centers, including Kennedy Krieger Institute at Johns Hopkins, are evaluating the effectiveness of meso-DMSA. Despite the significance of this therapeutic agent, data on the absorption, distribution, metabolism and elimination of meso-DMSA and its Pb complexes are limited. Critical knowledge is still needed that could aid in the development of efficient treatment strategies. For example, meso-DMSA is presumed to complex accessible lead in vivo, allowing urinary excretion. However, this process has yet to be validated, and meso-DMSA may or may not be the active chelating agent for lead in vivo. We hypothesize that radioactive tracer technologies can clarify fundamental aspects of the formation of Pb complexes with meso-DMSA and its metabolites in vitro and in vivo. Our goal is to identify the complexes formed in vivo with lead upon administration of meso-DMSA. We propose to use radioactive lead (Pb-203), in conjunction with non-radioactive and isotopically labeled (C-13 and C-14) meso-DMSA, for in vitro and in vivo studies of the complexes involved in lead excretion in a murine model. Our work plan requires 1) the synthesis and chemical characterization of Pb-complexes with meso- and racemic DMSA, the 1:2 DMSA:cysteine adduct, and cysteine; 2) mouse biodistribution studies of radioactive Pb-complexes in vivo, including dual isotope (Pb-203/C-14) experiments; 3) analysis of lead complexes formed in vivo in mouse urine by relation to characterized standards and 4) analysis of lead complexes in urine from patients receiving chelation therapy. Preliminary studies include: 1) the synthesis of Pb-203 complexes of meso- and racemic DMSA by novel Chelex methods; 2) whole-body and regional brain biodistribution studies of "free" Pb-203 and Pb-203-DMSA complexes in mouse; 3) ex vivo autoradiography of "free" Pb-203 in mouse brain; and 4) establishment of a murine model of urinary excretion of lead in vivo, in the presence and absence of meso-DMSA, that should provide sufficient quantities of metabolites for analysis. Our work sets the stage for development of a pharmacokinetic model of chelation therapy.