Efficacy of succimer chelation for reducing brain lead in a primate model of human lead exposure

Animal Models of Childhood Exposure to Lead or Manganese: Evidence for Impaired Attention, Impulse Control, and Affect Regulation and Assessment of Potential Therapies

Behavioral disorders involving attention and impulse control dysfunction, such as ADHD, are among the most prevalent disorders in children and adolescents, with significant impact on their lives. The etiology of these disorders is not well understood, but is recognized to be multifactorial, with studies reporting associations with polygenic and environmental risk factors, including toxicant exposure. Environmental epidemiological studies, while good at establishing associations with a variety of environmental and genetic risk factors, cannot establish causality. Animal models of behavioral disorders, when properly designed, can play an essential role in establishing causal relationships between environmental risk factors and a disorder, as well as provide model systems for elucidating underlying neural mechanisms and testing therapies. Here, we review how animal model studies of developmental lead or manganese exposure have been pivotal in (1) establishing a causal relationship between developmental exposure and lasting dysfunction in the domains of attention, impulse control, and affect regulation, and (2) testing the efficacy of specific therapeutic approaches for alleviating the lasting deficits. The lead and manganese case studies illustrate how animal models can advance knowledge in ways that are not possible in human studies. For example, in contrast to the Treatment of Lead Poisoned Children (TLC) human clinical trial evaluating succimer chelation efficacy to improve cognitive functioning in lead-exposed children, our developmental lead exposure animal model showed that succimer chelation can produce lasting cognitive benefits if chelation sufficiently reduces brain lead levels. In addition, this study revealed that succimer treatment in the absence of lead exposure produces lasting cognitive dysfunction, highlighting potential risks of chelation in off-label uses, such as the treatment of autistic children without a history of lead exposure. Our animal model of developmental manganese exposure has demonstrated that manganese can cause lasting attentional and sensorimotor deficits, akin to an ADHD-inattentive behavioral phenotype, thereby providing insights into the role of environmental exposures as contributors to ADHD. These studies have also shown that oral methylphenidate (Ritalin) can fully alleviate the deficits produced by early developmental Mn exposure. Future work should continue to focus on the development and use of animal models that appropriately recapitulate the complex behavioral phenotypes of behavioral disorders, in order to determine the mechanistic basis for the behavioral deficits caused by developmental exposure to environmental toxicants, and the efficacy of existing and emerging therapies.

The management of embedded metal fragment patients and the role of chelation Therapy: A workshop of the Department of Veterans Affairs-Walter Reed National Medical Center

Exposure to retained metal fragments from war-related injuries can result in increased systemic metal concentrations, thereby posing potential health risks to target organs far from the site of injury. Given the large number of veterans who have retained fragments and the lack of clear guidance on how to medically manage these individuals, the Department of Veterans Affairs (VA) convened a meeting of chelation experts and clinicians who care for embedded fragment patients to discuss current practices and provide medical management guidance. Based on this group's clinical expertise and review of published literature, the evidence presented suggests that, at least in the case of lead fragments, short-term chelation therapy may be beneficial for embedded fragment patients experiencing acute symptoms associated with metal toxicity; however, in the absence of clinical symptoms or significantly elevated blood lead concentrations (greater than 80 µg/dL), chelation therapy may offer little to no benefit for individuals with retained fragments and pose greater risks due to remobilization of metals stored in bone and other soft tissues. The combination of periodic biomonitoring to assess metal body burden, longitudinal fragment imaging, and selective fragment removal when metal concentrations approach critical injury thresholds offers a more conservative management approach to caring for patients with embedded fragments.

Neurotoxicants, Micronutrients, and Social Environments

SUMMARY—Systematic research evaluating the separate and interacting impacts of neurotoxicants, micronutrients, and social environments on children's cognition and behavior has only recently been initiated. Years of extensive human epidemiologic and animal experimental research document the deleterious impact of lead and other metals on the nervous system. However, discrepancies among human studies and between animal and human studies underscore the importance of variations in child nutrition as well as social and behavioral aspects of children's environments that mitigate or exacerbate the effects of neurotoxicants.

In this monograph, we review existing research on the impact of neurotoxic metals, nutrients, and social environments and interactions across the three domains. We examine the literature on lead, mercury, manganese, and cadmium in terms of dispersal, epidemiology, experimental animal studies, effects of social environments, and effects of nutrition.

Research documenting the negative impact of lead on cognition and behavior influenced reductions by the Center for Disease Control in child lead-screening guidelines from 30 micrograms per deciliter (μg/dL) in 1975 to 25 μg/dL in 1985 and to 10 μg/dL in 1991. A further reduction is currently being considered. Experimental animal research documents lead's alteration of glutamate-neurotransmitter (particularly N-methyl-D-aspartate) activity vital to learning and memory. In addition, lead induces changes in cholinergic and dopaminergic activity. Elevated lead concentrations in the blood are more common among children living in poverty and there is some evidence that socioeconomic status influences associations between lead and child outcomes. Micronutrients that influence the effects of lead include iron and zinc.

Research documenting the negative impact of mercury on children (as well as adults) has resulted in a reference dose (RfD) of 0.1 microgram per kilogram of body weight per day (μg/kg/day). In animal studies, mercury interferes with glutamatergic, cholinergic, and dopaminergic activity. Although evidence for interactions of mercury with children's social contexts is minimal, researchers are examining interactions of mercury with several nutrients.

Research on the effects of cadmium and manganese on child cognition and behavior is just beginning. Experimental animal research links cadmium to learning deficits, manganese to behaviors characteristic of Parkinson's disease, and both to altered dopaminergic functioning.

We close our review with a discussion of policy implications, and we recommend interdisciplinary research that will enable us to bridge gaps within and across domains.

The scientific basis for chelation: animal studies and lead chelation

This presentation summarizes several of the rodent and non-human studies that we have conducted to help inform the efficacy and clinical utility of succimer (meso-2,3-dimercaptosuccincinic acid) chelation treatment. We address the following questions: (1) What is the extent of body lead, and in particular brain lead reduction with chelation, and do reductions in blood lead accurately reflect reductions in brain lead? (2) Can succimer treatment alleviate the neurobehavioral impacts of lead poisoning? And (3) does succimer treatment, in the absence of lead poisoning, produce neurobehavioral deficits? Results from our studies in juvenile primates show that succimer treatment is effective at accelerating the elimination of lead from the body, but chelation was only marginally better than the complete cessation of lead exposure alone. Studies in lead-exposed adult primates treated with a single 19-day course of succimer showed that chelation did not measurably reduce brain lead levels compared to vehicle-treated controls. A follow-up study in rodents that underwent one or two 21-day courses of succimer treatment showed that chelation significantly reduced brain lead levels, and that two courses of succimer were significantly more efficacious at reducing brain lead levels than one. In both the primate and rodent studies, reductions in blood lead levels were a relatively poor predictor of reductions in brain lead levels. Our studies in rodents demonstrated that it is possible for succimer chelation therapy to alleviate certain types of lead-induced behavioral/cognitive dysfunction, suggesting that if a succimer treatment protocol that produced a substantial reduction of brain lead levels could be identified for humans, a functional benefit might be derived. Finally, we also found that succimer treatment produced lasting adverse neurobehavioral effects when administered to non-lead-exposed rodents, highlighting the potential risks of administering succimer or other metal-chelating agents to children who do not have elevated tissue lead levels. It is of significant concern that this type of therapy has been advocated for treating autism.

Switching selectivity between Pb2+ and Hg2+ ions through variation of substituents at xanthene end; ‘turn-on’ signalling responses by FRET modulation

Substitution at amino groups attached to the xanthene core modulates the stereo-electronic situation to switch selectivity; a FRET mediated signalling with a distinctive coupling of fluorophores.

Lead neurotoxicity in children: decomposing the variability in dose-effect relationships

BACKGROUND

Enormous progress has been made in recent decades in our understanding of lead neurotoxicology in children, but an important obstacle to additional progress is the striking variability that is evident in any plot of a lead biomarker versus a health endpoint.

METHODS

In this article, three potential sources of variability are identified: (1) errors or imprecision in characterizing dose (and/or outcome); (2) incomplete characterization of endpoint variance attributable to factors other than lead; and (3) inter-individual differences in susceptibility to lead neurotoxicity.

RESULTS

Strategies are suggested for reducing the variability contributed by these sources, including the development of validated PBPK models and biomarkers of early biological effects; the development of more comprehensive models of outcome variance and, specifically, the application of multi-level models that incorporate supra-individual and supra-family risk factors; and the use of study designs that permit assessments of the effect modifying influence of contextual factors on the form and severity of neurotoxicity.

CONCLUSION

Decomposing the variability in the distribution of observed scores around the best-fit lines that describe the dose-effect relationships for lead neurotoxicity in children is a major research need.

Reversal of lead-induced neuronal apoptosis by chelation treatment in rats: role of reactive oxygen species and intracellular Ca(2+)

Lead, a ubiquitous and potent neurotoxicant causes several neurophysiological and behavioral alterations. Toxic properties of lead have been attributed to its capability to mimic calcium and alter calcium homeostasis. In this study, we have addressed the following issues: 1) whether chelation therapy could circumvent the altered Ca(2+) homeostasis and prevent neuronal death in chronic lead-intoxicated rats, 2) whether chelation therapy could revert altered biochemical and behavioral changes, 3) whether combinational therapy using two different chelating agents was more advantageous over monotherapy in lead-treated rats, and 4) what could be the mechanism of neuronal apoptosis. Results indicated that lead caused a significant increase in reactive oxygen species, neuronal nitric-oxide synthetase, and intracellular free calcium levels along with altered behavioral abnormalities in locomotor activity, exploratory behavior, learning, and memory that were supported by changes in neurotransmitter levels. A fall in membrane potential, release of cytochrome c, and altered bcl(2)/bax ratio indicated mitochondrial-dependent apoptosis. Most of these alterations reverted toward normal level following combination therapy over monotherapy with calcium disodium EDTA (CaNa(2)EDTA) or monoisoamyl meso-2,3-dimercaptosuccinic acid (MiADMSA). It could be concluded from our present results that combined therapy with CaNa(2)EDTA and MiADMSA might be a better treatment protocol than monotherapy with these chelators in lead-induced neurological disorders. We for the first time report the role of Ca(2+) in regulating neurological dystrophy caused by chronic lead exposure in rats and its recovery with a two-course treatment regime of mono or combination therapy.

Recommendations for medical management of adult lead exposure

Research conducted in recent years has increased public health concern about the toxicity of lead at low dose and has supported a reappraisal of the levels of lead exposure that may be safely tolerated in the workplace. In this article, which appears as part of a mini-monograph on adult lead exposure, we summarize a body of published literature that establishes the potential for hypertension, effects on renal function, cognitive dysfunction, and adverse female reproductive outcome in adults with whole-blood lead concentrations < 40 microg/dL. Based on this literature, and our collective experience in evaluating lead-exposed adults, we recommend that individuals be removed from occupational lead exposure if a single blood lead concentration exceeds 30 microg/dL or if two successive blood lead concentrations measured over a 4-week interval are > or = 20 microg/dL. Removal of individuals from lead exposure should be considered to avoid long-term risk to health if exposure control measures over an extended period do not decrease blood lead concentrations to < 10 microg/dL or if selected medical conditions exist that would increase the risk of continued exposure. Recommended medical surveillance for all lead-exposed workers should include quarterly blood lead measurements for individuals with blood lead concentrations between 10 and 19 microg/dL, and semiannual blood lead measurements when sustained blood lead concentrations are < 10 microg/dL. It is advisable for pregnant women to avoid occupational or avocational lead exposure that would result in blood lead concentrations > 5 microg/dL. Chelation may have an adjunctive role in the medical management of highly exposed adults with symptomatic lead intoxication but is not recommended for asymptomatic individuals with low blood lead concentrations.

Succimer chelation normalizes reactivity to reward omission and errors in lead-exposed rats

This study evaluated the efficacy of a 3-week course of succimer treatment to alleviate behavioral deficits in rats exposed to lead (Pb) for the first 4 weeks of life. A 3 x 2 factorial design was used: three levels of lead exposure (No Pb, Moderate, and High Pb) and two levels of chelation (succimer or vehicle). Behavioral testing was conducted following chelation therapy, from 2 to 9 months of age; this report presents the results of two of the administered tasks: (1) a conditional olfactory discrimination task (baseline task), and (2) a conditional olfactory discrimination task with periodic reward omission on some correct trials (RO task). In the RO task, the performance disruption produced by committing an error on the previous trial was significantly greater for both unchelated lead-exposed groups than for controls. The High Pb rats were also more sensitive to reward omission than controls, providing converging evidence for impaired regulation of arousal or emotion. Importantly, succimer treatment was effective in normalizing the heightened reactivity of the lead-exposed animals to both errors and reward omission. In addition, non-lead-exposed rats that were treated with succimer tended to be more affected by a prior error than controls in their latency to respond on post-error trials. In sum, these findings provide new evidence that succimer chelation can significantly lessen the lasting neurobehavioral dysfunction produced by early lead exposure, but also suggest that there may be risks of administering the drug to individuals without elevated blood lead levels.

Succimer chelation improves learning, attention, and arousal regulation in lead-exposed rats but produces lasting cognitive impairment in the absence of lead exposure

BACKGROUND

There is growing pressure for clinicians to prescribe chelation therapy at only slightly elevated blood lead levels. However, very few studies have evaluated whether chelation improves cognitive outcomes in Pb-exposed children, or whether these agents have adverse effects that may affect brain development in the absence of Pb exposure.

OBJECTIVES

The present study was designed to answer these questions, using a rodent model of early childhood Pb exposure and treatment with succimer, a widely used chelating agent for the treatment of Pb poisoning.

RESULTS

Pb exposure produced lasting impairments in learning, attention, inhibitory control, and arousal regulation, paralleling the areas of dysfunction seen in Pb-exposed children. Succimer treatment of the Pb-exposed rats significantly improved learning, attention, and arousal regulation, although the efficacy of the treatment varied as a function of the Pb exposure level and the specific functional deficit. In contrast, succimer treatment of rats not previously exposed to Pb produced lasting and pervasive cognitive and affective dysfunction comparable in magnitude to that produced by the higher Pb exposure regimen.

CONCLUSIONS

These are the first data, to our knowledge, to show that treatment with any chelating agent can alleviate cognitive deficits due to Pb exposure. These findings suggest that it may be possible to identify a succimer treatment protocol that improves cognitive outcomes in Pb-exposed children. However, they also suggest that succimer treatment should be strongly discouraged for children who do not have elevated tissue levels of Pb or other heavy metals.

Monensin improves the effectiveness of meso-dimercaptosuccinate when used to treat lead intoxication in rats

Among divalent cations, the ionophore monensin shows high activity and selectivity for the transport of lead ions (Pb2+) across phospholipid membranes. When coadministered to rats that were receiving meso-dimercaptosuccinate for treatment of Pb intoxication, monensin significantly increased the amount of Pb removed from femur, brain, and heart. It showed a tendency to increase Pb removal from liver and kidney but had no effect of this type in skeletal muscle. Tissue levels of several physiologic (calcium, cobalt, copper, iron, magnesium, manganese, molybdenum, zinc) and nonphysiologic (arsenic, cadmium, chromium, nickel, strontium) elements were also determined after the application of these compounds. Among the physiologic elements, a number of significant changes were seen, including both rising and falling values. The size of these changes was typically around 20% compared with control values, with the largest examples seen in femur. These changes often tended to reverse those of similar size that had occurred during Pb administration. Among the nonphysiologic elements, which were present in trace amounts, the changes were smaller in number but larger in size. None of these changes appears likely to be significant in terms of toxicity, and there were no signs of overt toxicity under any of the conditions employed. Monensin may act by cotransporting Pb2+ and OH- ions out of cells, in exchange for external sodium ions. The net effect would be to shuttle intracellular Pb2+ to extracellular dimercaptosuccinic acid thereby enhancing its effectiveness. Thus, monensin may be useful for the treatment of Pb intoxication when applied in combination with hydrophilic Pb2+ chelators.

The Effects of Early Lead Exposure on the Brains of Adult Rhesus Monkeys: A Volumetric MRI Study

Little is known about direct effects of exposure to lead on central nervous system development. We conducted volumetric MRI studies in three groups of 17-year-old rhesus monkeys: (1) a group exposed to lead throughout gestation (n = 3), (2) a group exposed to lead through breast milk from birth to weaning (n = 4), and (3) a group not exposed to lead (n = 8). All fifteen monkeys were treated essentially identically since birth with the exception of lead exposure. The three-dimensional MRI images were segmented on a computer workstation using pre-tested manual and semi-automated algorithms to generate brain volumes for white matter, gray matter, cerebrospinal fluid, and component brain structures. The three groups differed significantly in the adjusted (for total brain size) volumes of the right cerebral white matter and the lateral ventricles. A significant reduction was noted in right cerebral white matter in prenatally exposed monkeys as compared to controls (p = 0.045). A similar reduction was detected in the white matter of the contralateral hemisphere; however, this difference did not achieve statistical significance (p = 0.143). Prenatally exposed monkeys also had larger right (p = 0.027) and left (p = 0.040) lateral ventricles. Depending on the timing of exposure during development, lead may exhibit differential effects with resultant life-long alterations in brain architecture.

The effects of meso-2,3-dimercaptosuccinic acid and oligomeric procyanidins on acute lead neurotoxicity in rat hippocampus

Oxidative stress is considered to be a mechanism involved in lead neurotoxicity. Apoptosis is also thought to relate to lead neurotoxicity. The present study, focused on the hippocampus, was designed to investigate the two possible mechanisms involved in lead neurotoxicity and the potential protective effects of 2,3-dimercaptosuccinic acid (DMSA) and oligomeric procyanidins (OPC). It was proved that reactive oxygen species and oxidative damage were implicated in the induction of apoptosis induced by lead in the hippocampus. Administration of DMSA attenuated the oxidative stress and apoptosis in addition to having strong chelating and lead-removing capacity. OPC alone had antioxidant protective effects in the hippocampus but no removing capacity for lead in vivo despite showing higher affinity and stronger chelating ability for Pb(2+) than DMSA in vitro. It is suggested that OPC chelates Pb(2+) but does not discharge it from the body and even accumulates Pb(2+) in some organs. At the same time, a reasonable deduction can also be made that the complex of OPC-Pb(2+) prevents or at least weakens the neurotoxicity of Pb(2+). Whether this complex displays toxicity over a long time span should be studied further.

Effect of chelation therapy on the neuropsychological and behavioral development of lead-exposed children after school entry

OBJECTIVE

Some children in the United States continue to be exposed to levels of lead that increase their risk for lowered intellectual functioning and behavior problems. It is unclear whether chelation therapy can prevent or reverse the neurodevelopmental sequelae of lead toxicity. The objective of this study was to determine whether chelation therapy with succimer (dimercaptosuccinic acid) in children with referral blood lead levels between 20 and 44 microg/dL (0.96-2.12 micromol/L) at 12 to 33 months of age has neurodevelopmental benefits at age 7 years.

METHODS

The Treatment of Lead-Exposed Children (TLC) study is a randomized, double-blind, placebo-controlled trial that was conducted between September 1994 and June 2003 in Philadelphia, PA; Newark, NJ; Cincinnati, OH; and Baltimore, MD. Of 1854 referred children who were between the ages of 12 to 33 months and screened for eligibility, 780 were randomized to the active drug and placebo groups stratified by clinical center, body surface area, blood lead level, and language spoken at home. At 7 years of age, 647 subjects remained in the study. Participants were randomly assigned to receive oral succimer or placebo. Up to 3 26-day courses of succimer or placebo therapy were administered depending on response to treatment in those who were given active drug. Eighty-nine percent had finished treatment by 6 months, with all children finishing by 13 months after randomization. All participants received residential lead hazard control measures before treatment. TLC subjects also received a daily multivitamin supplement before and after treatment(s) with succimer or placebo. Scores on standardized neuropsychological measures that tap cognition, behavior, learning and memory, attention, and neuromotor skills were measured.

RESULTS

Chelation therapy with succimer lowered average blood lead levels for approximately 6 months but resulted in no benefit in cognitive, behavioral, and neuromotor endpoints.

CONCLUSION

These new follow-up data confirm our previous finding that the TLC regimen of chelation therapy is not associated with neurodevelopmental benefits in children with blood lead levels between 20 and 44 microg/dL (0.96-2.17 micromol/L). These results emphasize the importance of taking environmental measures to prevent exposure to lead. Chelation therapy with succimer cannot be recommended for children with blood lead levels between 20 and 44 microg/dL (0.96-2.12 micromol/L).

Studies of urinary cystine precipitation in vitro: ontogeny of cystine nephrolithiasis and identification of meso-2,3-dimercaptosuccinic acid as a potential therapy for cystinuria

Children with fully recessive (Type I/I) cystinuria have a high risk of stone formation in the first decade of life. To assess the tendency for cystine to precipitate in individual urine samples, we developed an in vitro assay in which radiolabelled cystine (4mM) was dissolved in urine at 37 degrees C after alkalization to pH 10. Samples were then brought to pH 5, cooled, and centrifuged. The % decrease in supernatant cpm was used as a measure of cystine precipitation (CP). CP varied widely among normal children (74%+/-34) whereas variability of repeated determinations on a single adult individual was modest (64%+/-3.3). The assay was used to compare various potential therapies for cystinuria. Precipitation of exogenous cystine from normal urine was strongly inhibited by addition of D-penicillamine (CP: 8%+/-3) or dimercaptosuccinic acid (DMSA) (CP: 5%+/-1), at urinary concentrations attained by standard oral doses of each drug. Mercaptopropionylglycine (MPG) was moderately effective (CP: 43%+/-9), whereas captopril was a weak inhibitor (CP: 63%+/-12). Precipitation of endogenous cystine (2191 micromol/L) from a cystinuric patient showed that DMSA and D-penicillamine were again highly effective compared to the other agents. In addition DMSA and penicillamine added to the same patient's urine reduced the free cystine by 50% (as measured by automated amino acid analyzer) whereas MPG and captopril had no effect. In conclusion, DMSA is comparable to D-penicillamine as an in vitro inhibitor.

Effects of calcium disodium EDTA and meso-2,3-dimercaptosuccinic acid on tissue concentrations of lead for use in treatment of calves with experimentally induced lead toxicosis

OBJECTIVE

To compare the efficacy of calcium disodium EDTA (CaNa2EDTA) and meso-2,3-dimercaptosuccinic acid (DMSA) in reducing concentrations of lead in selected tissues for use in treatment of calves with experimentally induced lead toxicosis.

ANIMALS

19 sexually intact male Holstein calves that weighed 35 to 60 kg.

PROCEDURE

Calves were randomly assigned to 1 of 5 treatment groups: group 1, control calves; group 2, lead only; group 3, lead and EDTA; group 4, lead and DMSA; and group 5, lead, EDTA, and DMSA. Calves in groups 2 to 5 were dosed daily with lead (5 mg/kg, PO) for 10 days. Doses of EDTA (100 mg/kg) and DMSA (25 mg/kg) were administered IV once daily for 4 consecutive days beginning on day 11. Effects of the chelators on lead concentrations in the liver, kidneys, testes, muscles, bones, and brain were compared among the various groups.

RESULTS

Compared with the effects of EDTA, DMSA greatly reduced lead concentrations in renal and hepatic tissues. We did not detect significant differences for the effects of EDTA or DMSA on lead concentrations in the testes; there was an adverse interaction of EDTA with DMSA that caused an increase in lead concentrations in the testes.

CONCLUSIONS AND CLINICAL RELEVANCE

DMSA is much more effective than EDTA in removing lead from renal and hepatic tissues in calves. Use of DMSA in calves with lead intoxication appears to be a viable treatment option. Combining DMSA and EDTA as a treatment modality in calves did not offer any advantages.

Elevated blood lead resulting from maxillofacial gunshot injuries with lead ingestion

PURPOSE

The purpose of this study was to identify the contribution of ingested lead particles to elevated blood lead concentrations in victims of gunshot injury to the maxillofacial region.

PATIENTS AND METHODS

As part of a larger study of the effects of retained lead bullets on blood lead, a retrospective review of study findings was completed on 5 of 8 patients who sustained injuries to the maxillofacial region. These 5 patients were recruited into the larger study within 11 days of injury and showed a penetration path for the projectile that engaged the upper aerodigestive tract. All subjects were recruited from patients presenting for care of their gunshot injuries to a large inner-city trauma center with a retained bullet resulting from a gunshot injury. An initial blood lead level was measured for all recruited patients and repeated 1 to 17 weeks later. Medical history was taken along with a screening and risk factor questionnaire to determine other potential or actual sources (occupational/recreational) of lead exposure. (109)Cd K-shell x-ray fluorescence determinations of bone lead were completed to determine past lead exposure not revealed by medical history and risk factor questionnaire. Radiographs taken of the abdomen and chest, required as a part of the patient's hospital care, were retrospectively reviewed for signs of metallic fragments along the aerodigestive tract.

RESULTS

All 5 patients retained multiple lead pellets or fragments at the site of injury, sustained fractures of the facial bones, and showed increases in blood lead. Three of the 5 study subjects who sustained maxillofacial gunshot injuries involving the mouth, nose, or throat region showed metallic densities along the gastrointestinal tract indicative of ingested bullet fragments. Each patient with ingested bullet fragments showed rapid elevation of blood lead exceeding 25 microg/dL and sustained increases well beyond the time when all ingested fragments were eliminated. A 3-year follow-up on these 3 patients showed significantly sustained elevation of blood lead but less than that observed during the initial 6 months after injury. None of the 5 study subjects showed any evidence of metallic foreign bodies within the tracheobronchial regions indicative of aspiration.

CONCLUSION

Ingestion of lead fragments can result from gunshot injuries to the maxillofacial region and may substantially contribute to a rapid increase in blood lead level. Prompt diagnosis and elimination of ingested lead fragments are essential steps necessary to prevent lead being absorbed from the gastrointestinal tract. Increased blood lead in victims after gunshot injuries must be fully evaluated for all potential sources, including recent environmental exposure, absorption of lead from any remaining bullets in body tissues, and the possibility of mobilization of lead from long-term body stores such as bone.

Succimer (meso-2,3-dimercaptosuccinic acid (DMSA)) treatment of Andean children with environmental lead exposure

The authors studied children in Andean villages contaminated by a lead-glazing cottage industry. Mean blood lead (PbB) level in 35 exposed children, aged 3-14 years, a year before treatment, at the time of initiation of a comprehensive lead education and prevention program, was 53.4 microg/dL. PbB levels immediately before and three weeks after a ten-day regimen of succimer treatment of the 35 children were 43.4 microg/dL and 34.3 microg/dL, respectively, showing a 21% reduction and a significant difference between means (t = 5.09, p = 0.0001). PbB levels of the same children a year before treatment and immediately pre-treatment were also significantly different (t = 10.59, p = 0.0001). Thus, a ten-day course of succimer chelation effectively reduced PbB in children with moderate to severe Pb intoxication, and the education and prevention program, initiated with parents, health care providers, and educators, also contributed significantly to reducing PbB.

In vitro vs in vivo Pb effects on brain protein kinase C activity

Alteration of normal protein kinase C (PKC) function by environmental Pb exposure during neurodevelopment is hypothesized to be an important mechanism of toxicity underlying neurologic impairment. Previous studies have reported widely varying effects of Pb on PKC, possibly in part because of differences in in vitro and in vivo models used in those studies. Therefore, we tested the hypothesis that, with comparable tissue Pb levels, the effects of in vitro Pb exposure on brain PKC are the same as the effects caused by in vivo Pb exposure of intact animals. For chronic in vivo Pb exposure, female Long-Evans rats were exposed to Pb or vehicle from postnatal days 1 to 34-36 (n=10/treatment). For in vitro Pb exposure, homogenate of the frontal cortex region was exposed directly to Pb in an amount comparable to that accumulated in brain during chronic in vivo Pb exposure. Brain Pb levels were measured using ultraclean techniques and inductively coupled plasma mass spectrometry. PKC activity was subsequently determined in cytosolic and membrane subcellular fractions in the frontal cortex, hippocampus, and remaining brain regions. Results indicate that brain Pb levels following in vivo Pb exposure were increased approximately 20-fold above those of nonexposed animals (vehicle group [Pb] approximately 130ng Pb/g dry wt.). However, in vivo Pb exposure did not measurably alter brain PKC activity in the regions tested. In contrast, in vitro Pb exposure significantly increased PKC activity by approximately 20% in the frontal cortex homogenate membrane subcellular fraction. These results indicate that Pb added in vitro caused more dramatic effects than those produced by a comparable amount of Pb in the tissue from in vivo exposure. While the mechanisms underlying these outcomes are not clear, they suggest that in vitro models might not accurately reflect effects of chronic low-level in vivo Pb exposure.

Do children with falling blood lead levels have improved cognition?

OBJECTIVE

Exposure to lead at levels encountered by urban children impairs cognitive development. An observational study suggested improvement in IQ when blood lead level fell, but the only randomized trial of chelation showed no benefit in IQ.

METHODS

We did a new analysis of the data from the clinical trial using change in blood lead level as the independent variable. The 741 children began with blood lead levels between 20 and 44 microg/dL, and were 13 to 33 months old at randomization to chelation or placebo. Blood lead levels were measured repeatedly, and cognitive tests were given at baseline, 6 months, and 36 months follow-up.

RESULTS

By 6 months after randomization, blood lead levels had fallen by similar amounts in both chelated and placebo children, despite the immediate drops in the chelated group; there was no association between change in blood lead level and change in cognitive test score. Blood lead levels continued to fall. At 36 months follow-up, in the placebo group only, cognitive test scores had increased 4.0 points per 10 microg/dL fall in blood lead level from baseline to 36 months follow-up and 5.1 points from 6 to 36 months.

CONCLUSIONS

The improvement in scores in the placebo group only implies that factors other than declining blood lead levels per se are responsible for cognitive improvement; it is possible but less likely that succimer, the active drug, impairs cognition.

The effects of succimer chelation therapy on auditory function in rhesus monkeys

Sixty-six female rhesus monkeys were randomly assigned to three lead exposure conditions (none, from birth to 1 year, and from birth to 2 years) by two chelation treatment (succimer and no succimer) conditions. Blood lead levels were maintained at 35-40 microg/dl beginning shortly after birth and continuing for 1 or 2 years postnatally. There were two separate chelation regimes: 53 and 65 weeks of age. Lead and lead-vehicle dosing were discontinued while succimer was administered. Succimer (or placebo) was administered orally at a dose of 30 mg/kg/day (divided into three doses per day) for 5 days and for 14 additional days at 20 mg/kg/day (divided into two doses per day) for a total 19-day treatment regimen. Auditory function was assessed in these monkeys at least 1 year after lead intake had been discontinued. The outcome measures included tympanometry to assess middle ear function, OAEs to assess cochlear function, and ABRs to assess the auditory nerve and brainstem pathways. There were no significant differences as a function of succimer treatment for any of the tympanometric variables measured. Suprathreshold and threshold distortion product otoacoustic emissions were comparable among the succimer and vehicle groups. However, there was a nonsignificant trend to smaller amplitude distortion products at the highest frequencies assessed (6.4-10.0 kHz). Finally, the auditory evoked response at levels from the auditory nerve to the cerebral cortex did not significantly differ as a function of succimer treatment.

The effects of early lead exposure on auditory function in rhesus monkeys

Thirty-one female rhesus monkeys were randomly assigned to three lead exposure conditions (none, birth to 1 year, and birth to 2 years). Blood lead levels were maintained at 35-40 microg/dl beginning shortly after birth and continuing for 1 or 2 years postnatally. Auditory function was assessed in these monkeys at least 1 year after exposure to lead. The outcome measures included tympanometry to assess middle ear function, otoacoustic emissions (OAEs) to assess cochlear function, and auditory brainstem-evoked responses (ABRs) to assess the auditory nerve and brainstem pathways. There were no significant differences among the three experimental groups for any of the tympanometric variables measured suggesting no effect of lead exposure on middle ear function. Suprathreshold and threshold distortion product OAEs (DPOAEs) were comparable among the three groups. Finally, the auditory-evoked response at levels from the auditory nerve to the cerebral cortex did not significantly differ as a function of lead exposure. The lead exposure in this study had little effect on auditory function.

Safety and efficacy of succimer in toddlers with blood lead levels of 20-44 microg/dL. Treatment of Lead-Exposed Children (TLC) Trial Group

Although lead encephalopathy has virtually disappeared from the United States, thousands of children still have sufficient lead exposure to produce cognitive impairment. It is not known whether treating children with blood lead levels < 45 microg/dL (2.2 microM) is beneficial and can be done with acceptable safety. We conducted a 780-child, placebo-controlled, randomized trial of up to three courses of succimer in children with blood lead levels of 20-44 microg/dL (1.0-2.1 microM). Children were aged 12-33 mo, 77% were African-American, 7% were Hispanic, and they lived in deteriorating inner city housing. Placebo-treated children had a gradual decrease in blood lead level. Succimer-treated children had an abrupt drop in blood lead level, followed by rebound. The mean blood lead level of the succimer-treated children during the 6 mo after initiation of treatment was 4.5 microg/dL (95% confidence intervals, 3.7 to 5.3 microg/dL; 0.22 microM, 0.18 to 0.26 microM) lower than that of placebo-treated children. There were more scalp rashes in succimer-treated children (3.5% versus 1.3%) and an unanticipated excess of trauma. Succimer lowers blood lead level with few side effects. The unanticipated excess of trauma requires confirmation.

Succimer and the reduction of tissue lead in juvenile monkeys

The extent to which succimer (2,3-dimercaptosuccinic acid, DMSA) chelation reduces target organ lead (Pb) levels, including the skeleton, relative to the cessation of Pb exposure is a primary consideration in evaluating its efficacy for reducing toxicity in children. Here, we utilized a rhesus monkey model of childhood Pb exposure and a sensitive stable (204)Pb isotope tracer methodology to determine the efficacy of succimer for reducing Pb in blood, liver, and skeletal tissues from chronic (>/=1 year) versus short-term (3-4 days) Pb exposures. Specific attention was paid to the efficacy of succimer treatment compared to the cessation of Pb exposure. Infant rhesus monkeys (n = 48) were exposed to Pb daily for 1 year or >1 year postpartum to reach and maintain a target blood Pb level of 35-40 microg/dL. Two successive 19-day succimer treatment regimens were administered at 53 and 65 weeks of age (30 mg/kg/day x 5 days followed by 20 mg/kg/day x 14 days). Blood was collected over the course of treatment, and liver and bone biopsy samples were collected on days 0, 5, and 20, relative to the start of treatment (day 0). Complete 24-h urine collections were conducted over the course of treatment. Results of the first chelation indicate that a single regimen of succimer treatment led to significant reductions in blood and liver Pb levels, relative to the placebo group. However, the cessation of Pb exposure alone (i.e., placebo) also led to significant reductions in blood and liver compared to pretreatment levels. Neither succimer nor the cessation of Pb exposure had a significant impact on bone lead levels. Blood Pb levels in the succimer-treated group rebounded within 5 days after treatment ended, becoming comparable with levels in the placebo group from that point on. Results from the second chelation indicate that succimer treatment is essentially equally efficacious in reducing blood Pb at moderate (20 microg/dL) levels where exposures ended >3 months previously and more elevated (40-50 microg/dL) levels where exposures ended just prior to treatment, relative to the placebo treatment. Finally, similar overall outcomes were observed for tissue Pb from recent exposures (i.e., (204)Pb tracer levels), indicating little or no apparent difference in the chelation of Pb from recent (3-4 days) versus long-term exposures. These data demonstrate that succimer does not reduce skeletal Pb levels, and they show that the efficacy of succimer for reducing blood Pb levels does not persist beyond the completion of treatment due to posttreatment rebounds in blood Pb from endogenous sources. They also demonstrate the relative benefit of eliminating Pb exposures, which serves to underscore the importance of primary prevention of Pb exposure. The extent to which these data reflect the efficacy of succimer for reducing neurocognitive impairment is not yet known, although those data are forthcoming.