Chapter 67: Environmental Toxicology: Carcinogens and Heavy Metals

Epidemiological Approaches to Risk Assessment

Epidemiologists use multiple approaches to assess the risks of human environmental exposures (Gordis, 2008). The disadvantage of these approaches is the variability between people in their exposures and genetics, which results in confounding effects. Epidemiologists cannot control the environment or the genetics of the populations being studied. For example, generally individuals are simultaneously exposed to more than one environmental toxicant.

Because of the difficulties in assessing human exposures and the long times required to clinically observe effects on health, epidemiologists rely on biomarkers in risk assessment. There are three different types of biomarkers: biomarkers of exposure, biomarkers of toxicity, and biomarkers of susceptibility. Biomarkers of exposure usually are measurements of toxicants or their metabolites in blood, urine, or hair. Blood and urine concentrations measure recent exposures, while hair levels measure exposure over a period of months. An example of an unusual exposure biomarker is X-ray fluorescent measurement of bone lead levels, which estimates lifetime exposure to lead. Biomarkers of toxicity are used to measure toxic effects at a subclinical level and include measurement of liver enzymes in the serum, changes in the quantity or contents of urine, and performance on specialized exams for neurological or cognitive function. Biomarkers of susceptibility are used to predict which individuals are likely to develop toxicity in response to a given chemical. Examples include single nucleotide polymorphisms in genes for metabolizing enzymes involved in the activation or detoxification of a toxicant. Some biomarkers simultaneously provide information on exposure, toxicity, and susceptibility. For example, the measurement in the urine of N7-guanine adducts from aflatoxin B₁ provides evidence of both exposure and a toxic effect (in this case, DNA damage). These types of biomarkers are particularly valuable because they can support a proposed mechanism of toxicity.

Several types of epidemiological studies are used to assess risks, each with its own set of strengths and weaknesses. Ecological studies correlate frequencies of exposures and health outcomes between different geographical regions. These studies are inexpensive and can detect rare outcomes but are prone to confounding variables, including population migration. Cross-sectional studies examine the prevalence of exposures and outcomes at a single point in time. Such studies are an inexpensive way to determine an association but do not provide a temporal relationship and are not effective for establishing causality. They also can be subject to bias, as a perceived health outcome might cause someone to eliminate his exposure. Case-control studies start with a group of individuals affected by a disease, which then is matched to another group of unaffected individuals for known confounding variables. Questionnaires often are used to evaluate past exposures. This method also is relatively inexpensive and is good for examining rare outcomes because the endpoint is known. However, case-control studies rely on assessments of past exposures that often are unreliable and can be subject to bias. Prospective cohort studies measure exposures in a large group of people and follow that group for a long time to measure health outcomes. These studies are not as susceptible to confounding variables and bias and are good at establishing causality. However, they are extremely expensive, particularly when measuring very rare outcomes, because a large study population is required to observe sufficient disease to obtain statistical significance. One of the key types of human studies used in drug development is the randomized clinical trial (discussed in Chapter 1). These studies cannot be used to directly measure the effects of environmental toxicants (for obvious ethical reasons) but can be used to examine the effectiveness of an interventional strategy for reducing both exposure and disease.

Toxicological Approaches to Risk Assessment

Toxicologists use model systems, including experimental animals, to examine the toxicity of chemicals and predict their effect on humans (Faustman and Omenn, 2008). The significance of these model systems to human health is not always established. Toxicologists also test chemicals at the high end of the dose-response curve in order to see enough occurrences of an outcome to obtain statistical significance. As a result, there often is uncertainty about the effects of very low doses of chemicals. To determine the applicability of model studies, toxicologists study the mechanisms involved in the toxic effects of chemicals, with the goal of predicting whether that mechanism would occur in humans.

To predict the toxic effects of environmental chemicals, toxicologists perform subchronic (3 months of treatment for rodents) and chronic studies (2 years for rodents) in at least two different animal models. Subchronic experiments provide a model for occupational exposures, while chronic experiments are used to predict effects from lifetime exposures to chemicals in food or the environment. Doses for these studies are based on shorter preliminary studies, with the goal of having one concentration that does not have a significant effect, one concentration that results in statistically significant toxicity at the low end of the dose-response curve, and one or more concentrations that will have moderate-to-high levels of toxicity. A theoretical dose-response curve for an animal study is shown in Figure 67–1. An animal study provides two numbers that estimate the risk from a chemical. The no observed adverse effect level (NOAEL) is the highest dose used that does not result in a statistically significant increase in negative health outcomes. The lowest observed adverse effect level (LOAEL) is the lowest dose that results in a significant increase in toxicity. The NOAEL is divided by 10 for each source of uncertainty to determine a reference dose (RfD), which is commonly used as a starting point for determining regulations on human exposures to chemicals. The modifiers used to determine the RfD are based on the uncertainties between the experimental and human exposure. The most common modifiers used are for interspecies variability (human to animal) and interindividual variability (human to human), in which case RfD = NOAEL/100. Other modifiers can be used to account for specific experimental uncertainties. When a NOAEL is unavailable, a LOAEL may be used, in which case another 10-fold uncertainty factor is used. The use of factors of 10 in the denominator for determination of RfD is an application of the "precautionary principle," which attempts to limit human exposure by assuming a worst-case scenario for each unknown variable (Faustman and Omenn, 2008).

A major concern with animal studies is that they do not detect effects at low concentrations. Typically, they are designed to obtain statistical significance with a 10% increase in an outcome. As a result, there is considerable uncertainty about what occurs below that level, as demonstrated in Figure 67–1. Toxicologists often assume that there is a threshold dose (T), below which there is no toxicity. This condition is true when there are cellular defenses that prevent toxicity at concentrations below a given level but that can be overwhelmed. Thresholds usually are observed when the toxic effects of a chemical are the result of direct cell death. Many carcinogens and other toxicants with specific molecular targets (e.g., lead) do not exhibit a threshold. Ideally, mechanistic studies should be done to predict which dose-response curve is most likely to fit a given chemical.

Toxicologists perform a variety of mechanistic studies to understand how a chemical might cause toxicity. Computer modeling using a compound's three-dimensional structure to determine quantitative structure-activity relationships (QSARs) is commonly performed on both drugs and environmental chemicals. QSAR approaches can determine which chemicals are likely to exhibit toxicities or bind to specific molecular targets. Cell-based approaches in prokaryotes and eukaryotes are used to determine whether a compound damages DNA or causes cytotoxicity. DNA damage and the resulting mutagenesis often are determined with the Ames test. The Ames test uses Salmonella typhimurium strains with specific mutations in the gene needed to synthesize histidine. These strains are treated with chemicals in the presence or absence of a metabolic activating system, usually the supernatant fraction from homogenized rat liver. If a compound is a mutagen in the Ames test, it reverts the mutation in the histidine operon and allows the bacteria to form colonies on plates with limited histidine. Gene chip microarrays assess gene expression in cells or tissues from animals treated with a toxicant and provide a very useful tool to identify the molecular targets and pathways altered by toxicant exposures. The susceptibility of knockout mice to a toxicant can help to determine whether the knocked-out genes are involved in the metabolic activation and detoxification of a given toxicant.

Integrated Risk Assessment and Risk Management

Regulatory bodies at local, state, national, and international levels all are involved in limiting the adverse effects of chemicals. In the U.S., several federal agencies are involved, depending on the source of the exposure. The U.S. Food and Drug Administration (FDA) regulates the safety of drugs, the food supply, cosmetics, and now tobacco products. The Occupational Safety and Health Administration (OSHA) regulates workplace exposures. The U.S. Environmental Protection Agency (EPA) regulates exposures from other environmental sources, particularly the air and water. Regulators use epidemiological and toxicological data to estimate the risks from an exposure and come up with a level they consider to be reasonably safe. Regulators can nominate toxicants for a comprehensive study by the National Toxicology Program if there are insufficient toxicological data. The final stages of risk assessment and risk management are not based entirely on science; politics and economics also help determine which regulations are enacted. Ultimately, the social, economic, and political benefits predicted to result from limiting human exposure to a given toxicant are weighed against the corresponding costs.

Benzo[a]pyrene, a key carcinogen in tobacco smoke, is an example of a genotoxic carcinogen that forms both direct DNA adducts and ROS. Benzo[a]pyrene is oxidized by CYPs to a 7,8-dihydrodiol, which represents a proximate carcinogen (a more carcinogenic metabolite). This metabolite can either undergo a second oxidation step by a CYP to form a diol epoxide, which readily reacts with DNA, or it can undergo oxidation by aldo-keto reductases to form a catechol, which will redox cycle to form ROS (Conney, 1982; Penning, 2009).

If DNA damage from a genotoxic carcinogen is not repaired prior to DNA replication, a mutation can result. If this mutation is in a key tumor suppressor gene or proto-oncogene, it provides advantages in proliferation or survival. Alternatively, if the mutation is in a DNA repair gene, the mutation increases the probability that other mutations will occur. Given mutations in enough key genes and the right environment, a cell will proliferate faster than surrounding normal cells and possibly develop into malignant cancer. Genotoxic carcinogens are tumor initiators.

One successful approach to chemoprevention is modification of nuclear receptor signaling. Promising preliminary data suggested that retinoids might be beneficial for preventing lung and other cancers. Retinoids reduce the incidence of head and neck cancers, which represented one of the first successful uses of chemoprevention in humans (William et al., 2009; Evans and Kaye, 1999). Retinoids also are effective for the treatment of acute promyelocytic leukemia (see Section VII). However, in large clinical trials, retinoids actually increased the incidence of lung cancer, particularly among women, and had other unacceptable toxicities (Omenn et al., 1996).

The selective ER modulators tamoxifen and raloxifene reduced the incidence of breast cancer in high-risk women in large phase 3 clinical trials and are approved for chemoprevention in these patients (Vogel et al., 2006). The success of selective estrogen-receptor modulators for chemoprevention provides a proof-of-principle that the development of compounds based on mechanistic predictions can lead to effective drugs for the prevention of cancer.

Absorption, Distribution, Biotransformation, and Excretion. Aflatoxin B₁ is readily absorbed from the GI tract and initially distributed to the liver, where it undergoes extensive first-pass metabolism (Guengerich et al., 1996). Aflatoxin B₁ is metabolized by CYPs, including 1A2 and 3A4, to yield either an 8.9-epoxide or products hydroxylated at the 9 position (aflatoxin M₁) or 3 position (aflatoxin Q₁; Figure 67–3). The hydroxylation products are less susceptible to epoxidation and are therefore detoxification products. The 8,9-epoxide is highly reactive toward DNA and is the reactive intermediate responsible for aflatoxin carcinogenesis. The 8,9-epoxide is short lived and undergoes detoxification via non-enzymatic hydrolysis or conjugation with GSH. Aflatoxin M₁ enters the circulation and is excreted in urine and milk. Hydroxylated aflatoxin metabolites also can undergo several additional phase 1 and phase 2 metabolic pathways prior to excretion in urine or bile.

Toxicity. Aflatoxin B₁ primarily targets the liver, although it also is toxic to the GI tract and hematological system. High-dose exposures result in acute necrosis of the liver, leading to jaundice and, in many cases, death. Acute toxicity from aflatoxin is relatively rare in humans and requires consumption of milligram quantities of aflatoxin per day for multiple weeks. An outbreak in Kenya in 2004 led to 317 cases and 125 deaths (Groopman et al., 2008). Humans seem to be relatively resistant to acute aflatoxicosis; human outbreaks usually are preceded by the deaths of dogs and other domestic animals. Chronic exposure to aflatoxins results in cirrhosis of the liver and immunosuppression.

The mechanism of aflatoxin carcinogenesis has been extensively studied (IARC, 2002). The 8,9-epoxide of aflatoxin B₁ readily reacts with amines in biological macromolecules. Aflatoxin B₁ 8,9-epoxide forms adducts with deoxyguanosine and albumin that can be detected in the blood or urine of humans and laboratory animals exposed to aflatoxin, providing evidence for the activity of this pathway in vivo. Aflatoxin primarily forms DNA adducts at deoxyguanosine residues, reacting at either the N1 or N7 position. The N7-guanine adduct mispairs with adenine, leading to G l T transversions. Human aflatoxin exposure is associated with hepatocellular carcinomas bearing an AGG to AGT mutation in codon 249 of the p53 tumor suppressor gene, resulting in the replacement of an arginine with cysteine. This mutation is almost never observed in geographical regions with limited aflatoxin exposure (Hussain et al., 2007).

The interaction between aflatoxin and hepatitis B that is responsible for the increased incidence of hepatocellular carcinoma is not well understood (Sylla et al., 1999). Hepatitis B influences the metabolism of aflatoxin B₁ by upregulating CYPs, including 3A4, and decreasing glutathione S-transferase activity. In addition, hepatocellular proliferation to repair damage done by hepatitis B infection increases the likelihood that aflatoxin-induced DNA adducts will cause mutations. The hepatotoxic and tumor-promoting effects of hepatitis B also could provide a more favorable environment for the proliferation and invasion of initiated cells.

Green tea polyphenols also have been used to modify aflatoxin metabolism in exposed human populations. Individuals receiving a daily dose of 500 or 1000 mg (equivalent to 1 or 2 L of green tea) demonstrated a small decline in the formation of aflatoxin– albumin adducts and a large increase in the excretion of aflatoxin N-acetylcysteine, consistent with a protective effect. A third approach to modifying aflatoxin metabolism has been to use a broccoli sprout tea containing high levels of the isothiocyanate R-sulforaphane. Because of interindividual variation in the absorption of R-sulforaphane, the protocol did not significantly alter aflatoxin biomarkers. However, there was a significant inverse correlation between excretion of sulforaphane metabolites in the urine and the levels of aflatoxin–N7-guanine adducts.

Yet another approach used for the chemoprevention of aflatoxin hepatocarcinogenesis is the use of "interceptor molecules." Chlorophyllin, an over-the-counter mixture of water-soluble chlorophyll salts, binds tightly to aflatoxin in the GI tract, forming a complex that is not absorbed. In vitro, chlorophyllin inhibits CYP activity and acts as an antioxidant. Oral doses are poorly absorbed in vivo, so the agent remains largely in the GI tract. In a phase 2 trial, administration of 100 mg of chlorophyllin with each meal reduced aflatoxin–N7-guanine adduct levels in the urine by >50% (Egner et al., 2001).

Chemoprevention of hepatocellular carcinoma in people exposed to aflatoxin also can be achieved by limiting infections with hepatitis B. Because of the strong interaction between hepatitis B and aflatoxin in carcinogenesis, the hepatitis B vaccine will reduce the sensitivity of people to the induction of cancer by aflatoxin. Primary prevention of aflatoxin exposure through hand or fluorescent sorting of crops to remove those with fungal contamination can also reduce human exposure. A more cost-effective primary prevention approach is to improve food storage to limit the spread of A. flavus, which requires a warm and humid environment.

Not listed in Table 67–3 is the metal gold, which has its own uses and toxicities. Among heavy metals, perhaps only gold is addictive: gold has been used for centuries for relief of the itching palm, and many cannot get enough of its influence.

Exposure. In the U.S., paint containing lead for use in and around households was banned in 1978, while the use of tetraethyl lead in gasoline was phased out and eventually eliminated between 1976 and 1996. The economic benefit of the reduction in lead exposure due to these two measures is estimated at hundreds of billions of dollars per year (Grosse et al., 2002). Despite these bans, past use of lead carbonate and lead oxide in paint and tetraethyl lead in gasoline remain the primary sources of lead exposure. Lead is not degradable and remains throughout the environment in dust, soil, and the paint of older homes. Young children often are exposed to lead by nibbling sweet-tasting paint chips or eating dust and soil in and around older homes. Renovation or demolition of older buildings may cause substantial lead exposure. Tetraethyl lead was used as an anti-knock agent in gasoline, which resulted in high levels of lead in air pollution. Removal of lead from gasoline caused lead levels in air pollution to drop by >90% between 1982 and 2002. Lead was commonly used in plumbing and can leach into drinking water. Acidic foods and beverages dissolve lead when stored in containers with lead in their glaze or lead-soldered cans, which was a significant problem through the middle of the 20th century and remains a problem in developing countries. Lead exposure also has been traced to other sources such as lead toys, non-Western folk medicines, cosmetics, retained bullets, artists' paint pigments, ashes and fumes from painted wood, jewelers' wastes, home battery manufacture, and lead type (ATSDR, 2007b; Levin et al., 2008). Blood lead levels in the general population have steadily decreased since the 1970s. Between 1976 and 2002, mean blood levels in children 1-5 years of age dropped from 15-1.9 μg/dL. The Centers for Disease Control and Prevention (CDC) recommends screening of children at 6 months of age and the use of aggressive lead abatement for children with blood lead levels >10 μg/dL.

Occupational exposure to lead also has decreased markedly because of protective regulations. Occupational exposure generally is through inhalation of lead containing dust and lead fumes. Workers in lead smelters and in storage battery factories are at the greatest risk for lead exposure because fumes are generated and dust containing lead oxide is deposited in their environment. Other workers at risk for lead exposure are those associated with steel welding or cutting, construction, rubber and plastic industries, printing, firing ranges, radiator repair shops, and any industry where lead is flame soldered (ATSDR, 2007b).

Chemistry and Mode of Action. Lead exists in its metallic form and as divalent or tetravalent cations. Divalent lead is the primary environmental form; inorganic tetravalent lead compounds are not naturally found. Organo-lead complexes primarily occur with tetravalent lead and include the gasoline additive tetraethyl lead.

Lead toxicity results from molecular mimicry of other divalent metals (Garza et al., 2006). Lead takes the place of zinc or calcium in a number of important proteins. Because of its size and electron affinity, lead alters protein structure and can inappropriately activate or inhibit protein function. Specific molecular targets for lead are discussed below.

Absorption, Distribution, and Excretion. Lead exposure occurs through ingestion or inhalation. GI absorption of lead varies considerably with age and diet. Children absorb a much higher percentage of ingested lead (~40% on average) than adults (<20%). Absorption of ingested lead is drastically increased by fasting. Dietary calcium or iron deficiencies increase lead absorption, suggesting that lead is absorbed through divalent metal transporters. The absorption of inhaled lead generally is much more efficient (~90%), particularly with smaller particles. Tetraethyl lead is readily absorbed through the skin, but this is not a route of exposure for inorganic lead.

About 99% of lead in the bloodstream binds to hemoglobin. Lead initially distributes in the soft tissues, particularly in the tubular epithelium of the kidney and the liver. Over time, lead is redistributed and deposited in bone, teeth, and hair. About 95% of the adult body burden of lead is found in bone. Growing bones will accumulate higher levels of lead and can form lead lines visible by radiography. Bone lead is very slowly reabsorbed into the bloodstream, except when calcium levels are depleted, such as during pregnancy. Small quantities of lead accumulate in the brain, mostly in gray matter and the basal ganglia. Lead readily crosses the placenta.

Lead is excreted by humans primarily in the urine, although there also is some biliary excretion. The concentration of lead in urine is directly proportional to its concentration in plasma, but because most lead is in erythrocytes, only a small quantity of total lead is removed by filtration. Lead is excreted in milk and sweat and deposited in hair and nails. The serum t_1/2 of lead is 1-2 months, with a steady state achieved in ~6 months. Lead accumulates in bone, where its t_1/2 is estimated at 20-30 years.

Adults also develop encephalopathy from lead exposure, although they are less sensitive than children. Encephalopathy in adults requires blood lead levels >100 μg/dL. The symptoms are similar to those observed with children. Workers chronically exposed to lead can develop neuromuscular deficits, termed lead palsy. Symptoms of lead palsy, including wrist drop and foot drop, were commonly associated with painters and other lead-exposed workers during previous eras but are very rare today. Lead induces degeneration of motor neurons, usually without affecting sensory neurons. Studies in older adults have shown associations between lead exposure and decreased performance on cognitive function tests, suggesting that lead accelerates neurodegeneration due to aging (ATSDR, 2007b).

The neurodevelopmental effects of lead primarily result from inhibition of calcium transporters and channels and altered activities of calcium responsive proteins, including PKC and calmodulin (Garza et al., 2006; Bellinger and Bellinger, 2006). These actions limit the normal activation of neurons caused by calcium release and cause inappropriate production and/or release of neurotransmitters. Lead affects almost all the neurotransmitter pathways, with the dopaminergic, cholinergic, and glutamatergic systems receiving the most attention. Neurotransmitter release and PKC signaling determine which synapses are maintained and which are lost during development. At high concentrations, lead causes disruption of membranes, including the blood-brain barrier, increasing their permeability to ions. This effect is likely responsible for encephalopathy.

The exact mechanisms for the cardiovascular and renal effects of lead are not known. The cardiovascular effects of lead are thought to involve the production of reactive oxygen species by lead, through an unknown mechanism. Reactive oxygen species react with nitric oxide, which may contribute to the elevated blood pressure by reducing NO-induced vasodilation and contribute to cardiovascular toxicity through the formation of highly reactive peroxynitrite (Vaziri and Khan, 2007). Lead also forms inclusion bodies with various proteins, including metallothionein, in the kidney. The formation of these bodies greatly increases intracellular lead concentrations in the kidney but appears to be protective. It is not known how lead reduces glomerular filtration rate, although there is evidence that lead targets kidney mitochondria and may interfere with the electron transport chain (ATSDR, 2007b).

Inhibition of γ-aminolevulinate (γ-ALA) dehydratase and ferrochelatase is well documented. Ferrochelatase is responsible for incorporating the ferrous ion into protoporphyrin IX to form heme. When ferrochelatase is inhibited by lead, zinc is incorporated in place of iron, resulting in zinc-protoporphyrin, which is highly fluorescent and diagnostic of lead poisoning. γ-ALA dehydratase is the most sensitive of these enzymes to inhibition by lead; very low levels of lead increase urinary excretion of γ-ALA. Lead also causes both immunosuppression and increased inflammation, primarily through changes in helper T-cell and macrophage signaling (Dietert and Piepenbrink, 2006).

Chelation therapy is warranted for children and adults with high blood lead levels (>45 μg/dL and >70 μg/dL, respectively) and/or acute symptoms of lead poisoning (Ibrahim et al., 2006). Although chelation therapy is effective at lowering blood lead levels and relieving immediate symptoms, it does not reduce the chronic effects of lead beyond the benefit of lead abatement alone (Rogan et al., 2001). In rats, chelators enhance mobilization of lead from the soft tissues to the brain and may increase the adverse neurodevelopmental effects of lead (Andersen and Aaseth, 2002).

Exposure. Inorganic mercury cations and metallic mercury are found in the Earth's crust, and mercury vapor is released naturally into the environment through volcanic activity and off-gassing from soils. Mercury also enters the atmosphere through human activities such as combustion of fossil fuels. Once in the air, metallic mercury is photo-oxidized to inorganic mercury, which can then be deposited in aquatic environments in rain. Microorganisms can then conjugate inorganic mercury to form methyl mercury. Methyl mercury concentrates in lipids and will bioaccumulate up the food chain so that concentrations in aquatic organisms at the top of the food chain, such as swordfish or sharks, are quite high (Figure 67–6; ATSDR, 1999).

The primary source of exposure to metallic mercury in the general population is vaporization of mercury in dental amalgam, which often contains >50% Hg⁰ mixed with silver and other metals. This release is enhanced by chewing. There also is limited exposure through broken thermometers and other mercury-containing devices. Human exposure to organic mercury primarily is through the consumption of fish. Other foods contain inorganic mercury at low levels (ATSDR, 1999).

Workers are exposed to metallic and inorganic mercury, most commonly though exposure to vapors. The highest risk for exposure is in the chloralkali industry (i.e., bleach) and in other chemical processes in which mercury is used as a catalyst. Mercury is a component of many devices, including alkaline batteries, fluorescent bulbs, thermometers, and scientific equipment, and exposure occurs during the production of these devices. Dentists also are exposed to mercury from amalgam. Mercury can be used to extract gold during mining, which results in substantial occupational exposure, because the last step involves vaporization of the mercury. This process is still commonly used in developing countries. Mercuric salts are used as pigments in paints (ATSDR,1999).

Mercury salts were once found in a number of medications, including antiseptics, antidiuretics, skin-lightening creams, and laxatives. Most of these uses have been replaced by safer and more effective drugs. Thimerosal is an antimicrobial agent used as a preservative in vaccines. Its use is controversial because it releases ethyl mercury, which is chemically similar to methyl mercury. Due to the concerns of some parents that thimerosal might be a cause of autism, the American Academy of Pediatrics and the U.S. Public Health Service issued a call for its replacement in vaccines to improve the prevalence of vaccination (Ball et al., 2001). However, studies have not found an association between thimerosal use in vaccines and negative outcomes, and it is still used in influenza vaccines (Heron and Golding, 2004).

Chemistry and Mode of Action. There are three general forms of mercury of concern to human health. Metallic, or elemental, mercury (Hg⁰) is the liquid metal found in thermometers and dental amalgam; it is quite volatile, and exposure is often to the vapor form. Inorganic mercury can be either monovalent (mercurous, Hg¹⁺) or divalent (mercuric, Hg²⁺) and forms a variety of salts. Organic mercury compounds consist of divalent mercury complexed with one or occasionally two alkyl groups. The organic mercury compound of most concern is methyl mercury (MeHg⁺), which is formed environmentally from inorganic mercury by aquatic microorganisms.

Both Hg²⁺ and MeHg⁺ readily form covalent bonds with sulfur, which causes most of the biological effects of mercury. At very low concentrations, mercury reacts with sulfhydryl residues on proteins and disrupts their functions. Given the large numbers of proteins with important cysteines, determination of the specific mechanism for cellular dysfunction has been difficult, and there are probably multiple pathways affected. One of these pathways involves the generation of oxidative stress in cells. Also, microtubules are particularly sensitive to the toxic effects of mercury, which disrupts their formation and can catalyze their disassembly (Clarkson, 2002). There also may be an autoimmune component to mercury toxicity.

Absorption, Distribution, Biotransformation, and Excretion. Hg⁰ vapor is readily absorbed through the lungs (~70-80%), but GI absorption of metallic mercury is negligible. Once absorbed, Hg⁰ distributes throughout the body and crosses membranes such as the blood-brain barrier and the placenta via diffusion. Hg⁰ is oxidized by catalase in the erythrocytes and other cells to form Hg²⁺. Shortly after exposure, some Hg⁰ is eliminated in exhaled air. After a few hours, distribution and elimination of Hg⁰ resemble the properties of Hg²⁺. After exposure to Hg⁰ vapor, it is oxidized to Hg²⁺ and retained in the brain (ATSDR, 1999).

GI absorption of mercury salts varies depending on the individual and on the particular salt and averages ~10-15%. Hg¹⁺ will form Hg⁰ or Hg²⁺ in the presence of sulfhydryl groups. Hg²⁺ primarily is excreted in the urine and feces; a small amount also can be reduced to Hg⁰ and exhaled. With acute exposure, the fecal pathway predominates, but following chronic exposure, urinary excretion becomes more important. All forms of mercury also are excreted in sweat and breast milk and deposited in hair and nails. The t_1/2 for inorganic mercury is approximately 1-2 months (ATSDR, 1999).

Orally ingested MeHg⁺ is almost completely absorbed from the GI tract. MeHg⁺ readily crosses the blood-brain barrier and the placenta and distributes fairly evenly to the tissues, although concentrations are highest in the kidneys (ATSDR, 1999). MeHg⁺ can be demethylated to form inorganic Hg²⁺. The liver and kidney exhibit the highest rates of demethylation, but this also occurs in the brain. MeHg⁺ is excreted in the urine and feces, with the fecal pathway dominating. The t_1/2 for MeHg⁺ is ~2 months. The pharmacokinetic properties of MeHg⁺ are thought to result from molecular mimicry. Complexes between MeHg⁺ and cysteine resemble methionine and can be recognized by transporters for that amino acid and taken across membranes (Ballatori, 2002).

The effects of low-dose methyl mercury exposure from routine consumption of fish are difficult to assess due to the opposing beneficial effects of ω-3 fatty acids found in fish oils, and studies have produced discrepant results (Grandjean et al., 1999; Myers et al., 2003; Myers et al., 2007).

Treatment. With exposure to metallic mercury, termination of exposure is critical and respiratory support may be required. Emesis may be used within 30-60 minutes of exposure to inorganic mercury, provided the patient is awake and alert and there is no corrosive injury. Maintenance of electrolyte balance and fluids is important for patients exposed to inorganic mercury. Chelation therapy is beneficial in patients with acute inorganic or metallic mercury exposure. There are limited treatment options for methyl mercury. Chelation therapy does not provide clinical benefits, and several chelators potentiate the toxic effects of methyl mercury (Rush, 2008). Nonabsorbed thiol resins may be beneficial by preventing reabsorption of methyl mercury from the GI tract.

Because of the conflicting effects of mercury and ω-3 fatty acids, there is considerable controversy regarding the restriction of fish intake in women of reproductive age and children. The EPA recommends limiting fish intake to 12 oz (two meals) per week. Many experts feel this recommendation is too conservative, and the FDA is considering revising their recommendation to state that the benefits of fish consumption outweigh the risks. The recommendation that women consume fish that is lower in mercury content (i.e., canned light tuna, salmon, pollock, catfish) and avoid top predators, such as swordfish, shark, and tilefish, is not controversial.

Exposure. The primary source of exposure to arsenic is through drinking water. Arsenic naturally leaches out of soil and rocks into well and spring water (Mead, 2005). Levels of arsenic in drinking water average 2 μg/L (ppb) in the U.S. but can be >50 μg/L (five times the EPA standard) in private well water, particularly in California, Nevada, and Arizona. Drinking water from other parts of the world where well water has been promoted to prevent waterborne illness, particularly Taiwan, China, Argentina, Chile, Bangladesh, and eastern India, sometimes is contaminated with much higher levels of arsenic (sometimes several hundred micrograms per liter), and widespread poisonings have resulted (Figure 67–8). Studies in Bangladesh found that ~40% of water samples from across the country were contaminated with >50 μg/L of arsenic, and some samples had far higher levels (Mead, 2005; Chowdhury et al., 1999; BGS and DPHE, 2001). Arsenic also can enter the environment through human activities such as the use of arsenic-containing pesticides, mining, and burning of coal. Food, particularly seafood, often is contaminated with arsenic. Arsenic in seafood exists primarily as organic compounds (i.e., arsenobetaine) that are much less toxic than inorganic arsenic. The average daily human intake of arsenic is 10 μg/day, almost exclusively from food and water.

Before 2003, >90% of arsenic used in the U.S. was as a preservative in pressure-treated wood, but the lumber industry has voluntarily replaced arsenic with other preservatives. Arsenic-treated wood is thought to be safe unless burned (Hall, 2002). The major source of occupational exposure to arsenic is in the production and use of organic arsenicals as herbicides and insecticides. Exposure to metallic arsenic, arsine, arsenic trioxide, and gallium arsenide also occurs in high-tech industries, such as the manufacture of computer chips and semiconductors.

Chemistry and Mode of Action. Arsenic exists in its elemental form and trivalent (arsenites/arsenious acid) and pentavalent (arsenates/arsenic acid) states. Arsine is a gaseous hydride of trivalent arsenic that exhibits toxicities that are distinct from other forms. Organic compounds containing either valence state of arsenic are formed in animals. The toxicity of a given arsenical is related to the rate of its clearance from the body and its ability to concentrate in tissues. In general, toxicity increases in the sequence: organic arsenicals < As⁵⁺ < As³⁺ < arsine gas (AsH₃).

Like mercury, trivalent arsenic compounds form covalent bonds with sulfhydryl groups. The pyruvate dehydrogenase system is particularly sensitive to inhibition by trivalent arsenicals because the two sulfhydryl groups of lipoic acid react with arsenic to form a six-membered ring. Inorganic arsenate (pentavalent) inhibits the electron transport chain. It is thought that arsenate competitively substitutes for phosphate during the formation of adenosine triphosphate, forming an unstable arsenate ester that is rapidly hydrolyzed.

Absorption, Distribution, Biotransformation, and Excretion. The absorption of arsenic compounds is directly related to their solubility. Poorly water-soluble forms such as arsenic sulfide, lead arsenate, and arsenic trioxide are not well absorbed. Water-soluble arsenic compounds are readily absorbed from both inhalation and ingestion. GI absorption of arsenic dissolved in drinking water is >90% (ATSDR, 2007a).

At low doses, arsenic is fairly evenly distributed throughout the tissues of the body. Nails and hair, due to their high sulfhydryl content, exhibit high concentrations of arsenic. After an acute high dose of arsenic (i.e., fatal poisoning), arsenic is preferentially deposited in the liver and, to a lesser extent, kidney, with elevated levels also observed in the muscle, heart, spleen, pancreas, lungs, and cerebellum. Arsenic readily crosses the placenta and blood-brain barrier.

Arsenic undergoes biotransformation in humans and animals (Figure 67–9). Trivalent compounds can be oxidized back to pentavalent compounds, but there is no evidence for demethylation of methylated arsenicals. Biotransformation of arsenic varies greatly across species, with humans excreting much higher levels of monomethyl-arsenic (MMA) compounds than most other animals (ATSDR, 2007a). Because the pentavalent methylated arsenic compounds have greatly reduced toxicity, the methylation pathway was long thought to be a detoxification pathway. However, the trivalent methylated arsenicals actually are more toxic than inorganic arsenite, due to an increased affinity for sulfhydryl groups, and formation of MMA^III now is considered a bioactivation pathway (Aposhian and Aposhian, 2006).

Elimination of arsenicals by humans primarily is in the urine, although some is also excreted in feces, sweat, hair, nails, skin, and exhaled air. Compared to most other toxic metals, arsenic is excreted quickly, with a t_1/2 of 1-3 days. In humans, ingested inorganic arsenic in urine is a mixture of 10-30% inorganic arsenicals, 10-20% monomethylated forms, and 60-80% dimethylated forms.

Other Non-Cancer Toxicities. Acute and chronic arsenic exposures induce anemia and leukopenia. Arsenic likely causes both direct cytotoxic effects on blood cells and suppression of erythropoiesis through bone marrow toxicity. Arsenic also may inhibit heme synthesis. In the liver, arsenic causes fatty infiltrations, central necrosis, and cirrhosis of varying severity. The action of arsenic on renal capillaries, tubules, and glomeruli can cause severe kidney damage. Inhaled arsenic is irritating to the lungs, and ingested arsenic may induce bronchitis progressing to bronchopneumonia in some individuals. Chronic exposure to arsenic is associated with an increased risk of diabetes.

The developing fetus and young children may be at increased risk of arsenic carcinogenesis, because humans exposed to arsenic in utero and in early childhood have a greatly elevated risk of lung cancer (Smith et al., 2006). Studies in rodents also have observed increased cancer risks from in utero exposure and suggest that the second trimester of pregnancy represents a critical susceptibility window (Waalkes et al., 2007).

The mechanism of arsenic carcinogenesis is poorly understood. Arsenic is an unusual carcinogen in that evidence for human carcinogenesis is much stronger than for carcinogenesis in laboratory animals. Arsenic does not directly damage DNA; rather, arsenic is thought to work through changes in gene expression, DNA methylation, inhibition of DNA repair, generation of oxidative stress, and/or altered signal transduction pathways (Salnikow and Zhitkovich, 2008; Hartwig et al., 2002). Arsenic compounds can act as tumor promoters or co-carcinogens in rodents, particularly when combined with ultraviolet light (Burns et al., 2004). In humans, exposure to arsenic potentiates lung tumorigenesis from tobacco smoke. Smokers in regions with high concentrations of arsenic in the drinking water have a 5-fold increased risk of cancer over smokers living in low arsenic regions (Ferreccio et al., 2000). Arsenic co-carcinogenesis may involve inhibition of proteins involved in nucleotide excision repair (Salnikow and Zhitkovich, 2008; Hartwig et al., 2002). Arsenic also has endocrine-disrupting activities on several nuclear steroid hormone receptors, enhancing hormone-dependent transcription at very low concentrations and inhibiting it at slightly higher levels (Bodwell et al., 2006).

Arsine Gas. Arsine gas, formed by electrolytic or metallic reduction of arsenic, is a rare cause of industrial poisonings. Arsine induces rapid and often fatal hemolysis, which probably results from arsine combining with hemoglobin and reacting with oxygen. A few hours after exposure, patients can develop headache, anorexia, vomiting, paresthesia, abdominal pain, chills, hemoglobinuria, bilirubinemia, and anuria. Jaundice appears after 24 hours. Arsine induces renal toxicities that can progress to kidney failure. Approximately 25% of cases of arsine exposure result in death.

Treatment. Following acute exposure to arsenic, stabilize the patient and prevent further absorption of the poison. Close monitoring of fluid levels is important because arsenic can cause fatal hypovolemic shock. Chelation therapy is effective following short-term exposure to arsenic but has very little or no benefit in chronically exposed individuals. Exchange transfusion to restore blood cells and remove arsenic often is warranted following arsine gas exposure (Ibrahim et al., 2006).

Exposure. In the general population, the primary source of exposure to cadmium is through food, with an estimated average daily intake of 50 μg/day. Cadmium also is found in tobacco; a cigarette contains 1-2 μg of cadmium (Jarup and Akesson, 2009). Workers in smelters and other metal-processing industries can be exposed to high levels of cadmium, particularly by inhalation.

Chemistry and Mode of Action. Cadmium exists as a divalent cation and does not undergo oxidation-reduction reactions. There are no covalent organometallic complexes of cadmium of toxicological significance. The mechanism of cadmium toxicity is not fully understood. Like lead and other divalent metals, cadmium can replace zinc in zinc-finger domains of proteins and disrupt them. Through an unknown mechanism, cadmium induces formation of reactive oxygen species, resulting in lipid peroxidation and glutathione depletion. Cadmium also upregulates inflammatory cytokines and may disrupt the beneficial effects of nitric oxide.

Absorption, Distribution, and Excretion. Cadmium is not well absorbed from the GI tract (1.5-5%) but is better absorbed via inhalation (~10%). Cadmium primarily distributes first to the liver and later the kidney, with those two organs accounting for 50% of the absorbed dose. Cadmium distributes fairly evenly to other tissues, but unlike other heavy metals, little cadmium crosses the blood-brain barrier or the placenta. Cadmium primarily is excreted in the urine and exhibits a t_1/2 of 10-30 years (ATSDR, 2008a).

Toxicity. Acute cadmium toxicity primarily is due to local irritation along the absorption route. Inhaled cadmium causes respiratory tract irritation with severe, early pneumonitis accompanied by chest pains, nausea, dizziness, and diarrhea. Toxicity may progress to fatal pulmonary edema. Ingested cadmium induces nausea, vomiting, salivation, diarrhea, and abdominal cramps; the vomitus and diarrhea often are bloody.

Symptoms of chronic cadmium toxicity vary by exposure route. The lung is an important target of inhaled cadmium, while the kidney is a major target of cadmium from both inhalation and ingestion.

Cadmium bound to metallothionein is transported to the kidney, where it can be released. The initial toxic effect of cadmium on the kidney is increased excretion of small-molecular-weight proteins, especially β₂ microglobulin and retinol-binding protein. Cadmium also causes glomerular injury, with a resulting decrease in filtration. Chronic occupational exposure to cadmium is associated with an increased risk of renal failure and death. There is no evidence for a threshold level for cadmium's effects on the kidney; cadmium levels consistent with normal dietary exposure can cause renal toxicity, including a reduction in glomerular filtration rate and creatinine clearance (Jarup and Akesson, 2009).

Workers with long-term inhalation exposure to cadmium exhibit decreased lung function. Symptoms initially include bronchitis and fibrosis of the lung, leading to emphysema. The exact cause of cadmium-induced lung toxicity is not known but may result from inhibition of the synthesis of α₁ antitrypsin. Chronic obstructive pulmonary disease causes increased mortality in cadmium-exposed workers.

When accompanied by vitamin D deficiency, cadmium exposure increases the risks for fractures and osteoporosis. This may be an effect of cadmium interfering with calcium and phosphate regulation due to its renal toxicity.

Carcinogenicity. Chronic occupational exposure to inhaled cadmium increases the risk of developing lung cancer (IARC, 1993; NTP, 2004). The mechanism of cadmium carcinogenesis is not fully understood. Cadmium causes chromosomal aberrations in exposed workers and treated animals and human cells. It also increases mutations and impairs DNA repair in human cells (NTP, 2004). Cadmium substitutes for zinc in DNA repair proteins and polymerases and may inhibit nucleotide excision repair, base excision repair, and the DNA polymerase responsible for repairing single-strand breaks (Hartwig et al., 2002). There is evidence that cadmium also alters cell signaling pathways and disrupts cellular controls of proliferation (Waisberg et al., 2003). Thus, cadmium acts as a non-genotoxic carcinogen.

Treatment. Treatment of cadmium poisoning is symptomatic. Patients suffering from inhaled cadmium may require respiratory support. Patients suffering from kidney failure due to cadmium poisoning may require a transplant. There is no evidence for clinical benefit from chelation therapy following cadmium poisoning, and chelation therapy may result in adverse effects (ATSDR, 2008a).

Exposure. Exposure to chromium in the general population primarily is through the ingestion of food, although there also is exposure from drinking water and air. Workers are exposed to chromium during chromate production, stainless steel production and welding, chromium plating, ferrochrome alloy and chrome pigment production, and in tanning industries (Ashley et al., 2003). Exposure usually is to a mixture of Cr^III and Cr^VI, except in chromium plating, which usually uses Cr^VI, and tanning, where Cr^III is used.

Chemistry and Mode of Action. Chromium occurs in its metallic state or in any valence state between divalent and hexavalent. Cr^III is the most stable and common form. Cr^VI is corrosive and is readily reduced to lower valence states. The primary reason for the different toxicological properties of Cr^III and Cr^VI is thought to be differences in their absorption and distribution. Hexavalent chromate resembles sulfate and phosphate and can be taken across membranes by anion transporters. Once inside the cell, Cr^VI undergoes a series of reduction steps, ultimately forming Cr^III, which is thought to cause most of the toxic effects. Trivalent chromium readily forms covalent interactions with DNA. Hexavalent chromium also induces oxidative stress and hypersensitivity reactions.

Absorption, Distribution, Biotransformation, and Excretion. Absorption of inhaled chromium depends on its solubility, valence state, and particle size. Smaller particles are better deposited in the lungs. Absorption into the bloodstream of hexavalent and soluble forms is higher than the trivalent or insoluble forms, with the remainder often retained in the lungs. Approximately 50-85% of inhaled Cr^VI particles (<5 μm) are absorbed. Absorption of ingested chromium is <10%. Soluble Cr^VI compounds are better absorbed from the GI tract than other forms. Cr^VI crosses membranes by facilitated transport, while Cr^III crosses by diffusion. Cr^VI is distributed to all of the tissues and crosses the placenta. The highest levels are attained in the liver, kidney, and bone; Cr^VI also is retained in erythrocytes, bound tightly to hemoglobin and other ligands. Excretion primarily is through urine, with small amounts also excreted in bile and breast milk and deposited in hair and nails. The t_1/2 of ingested Cr^VI is ~40 hours, while the t_1/2 of Cr^III is ~10 hours, reflecting the enhanced tissue retention of Cr^VI (ATSDR, 2008b).

Toxicity. Acute exposure to very high doses of chromium causes death via damage to multiple organs, particularly the kidney, where it causes tubular and glomerular damage. Chronic low-dose chromium exposure primarily causes toxicity at the site of contact. Workers exposed to inhaled chromium develop symptoms of lung and upper respiratory tract irritation, including epistaxis, chronic rhinorrhea, nasal itching and soreness, nasal mucosal atrophy, perforations and ulceration of the nasal septum, bronchitis, pneumonoconiosis, decreased pulmonary function, and pneumonia. Chronic exposure to chromium via ingestion, including after mucociliary clearance of inhaled particles, causes symptoms of GI irritation such as oral ulcer, diarrhea, abdominal pain, indigestion, and vomiting. Cr^VI is a dermal irritant and can cause ulceration or burns. Following low-dose exposure by any route, some individuals become sensitized to chromium and develop allergic dermatitis following dermal exposure to chromium, including products containing metallic chromium. Chromium-sensitized workers often also develop asthma following inhalation exposure (ATSDR, 2008b).

There are multiple potential mechanisms for chromium carcinogenicity (Salnikow and Zhitkovich, 2008). After uptake into cells, reduction of Cr^VI to Cr^III occurs with concomitant oxidation of cellular molecules. Ascorbate is the primary reductant, but other molecules, including glutathione, lipids, proteins, and DNA, also can be oxidized. Cr^III forms a large number of covalent DNA adducts, primarily at the phosphate backbone. The most common DNA adducts are either binary (DNA-Cr^III) adducts or cross-links to small molecules such as ascorbate and glutathione. The DNA adducts are not very mutagenic and are repaired by nucleotide excision repair. It is thought that the high level of nucleotide excision repair activity following chromium exposure contributes to carcinogenesis, either by preventing repair of mutagenic lesions formed by other carcinogens or through the formation of single-strand breaks due to incomplete repair. Chromium also forms toxic cross-links between DNA and protein. Chronic inflammation due to chromium-induced irritation also may promote tumor formation.

Treatment. There are no standard protocols for treatment of acute chromium poisoning. One approach that has shown promise in rodents is the use of reductants such as ascorbate, glutathione, or N-acetylcysteine to reduce Cr^VI to Cr^III after exposure but before absorption to limit bioavailability (ATSDR, 2008b). These compounds and EDTA also increase urinary excretion of chromium after high-dose exposure, particularly if given soon enough to prevent uptake into cells. Exchange transfusion to remove chromium from plasma and erythrocytes may be beneficial.

Chemistry and Mechanism of Action. The pharmacological effects of CaNa₂EDTA result from chelation of divalent and trivalent metals in the body. Accessible metal ions (both exogenous and endogenous) with a higher affinity for CaNa₂EDTA than Ca²⁺ will be chelated, mobilized, and usually excreted. Because EDTA is charged at physiological pH, it does not significantly penetrate cells. CaNa₂EDTA mobilizes several endogenous metallic cations, including those of zinc, manganese, and iron. Additional supplementation with zinc following chelation therapy may be beneficial. The most common therapeutic use of CaNa₂EDTA is for acute lead intoxication. CaNa₂EDTA does not provide clinical benefits for the treatment of chronic lead poisoning. There is evidence in rats that CaNa₂EDTA mobilizes lead from various tissues to the brain and liver, which may account for this observation (Sánchez-Fructuoso et al., 2002; Andersen and Aaseth, 2002).

CaNa₂EDTA is available as edetate calcium disodium (calcium disodium versenate). Intramuscular administration of CaNa₂EDTA results in good absorption, but pain occurs at the injection site; consequently, the chelator injection often is mixed with a local anesthetic or administered intravenously. For intravenous use, CaNa₂EDTA is diluted in either 5% dextrose or 0.9% saline and is administered slowly by intravenous drip. A dilute solution is necessary to avoid thrombophlebitis. To minimize nephrotoxicity, adequate urine production should be established prior to and during treatment with CaNa₂EDTA. However, in patients with lead encephalopathy and increased intracranial pressure, excess fluids must be avoided. In such cases, intramuscular administration of CaNa₂EDTA is recommended.

EDTA and its glycol congener, EGTA, are used in biological research to chelate and control the concentration of Ca²⁺ in biological buffer solutions. The 2008 Nobel laureate, Roger Tsien, and his colleagues used the EDTA/EGTA structure as a starting point in the development of fluorescent sensors of cellular [Ca²⁺] (Tsien et al., 1984).

Absorption, Distribution, and Excretion. Less than 5% of CaNa₂EDTA is absorbed from the GI tract. After intravenous administration, CaNa₂EDTA has a t_1/2 of 20-60 minutes. In blood, CaNa₂EDTA is found only in the plasma. CaNa₂EDTA is excreted in the urine by glomerular filtration, so adequate renal function is necessary for successful therapy. Altering either the pH or the rate of urine flow has no effect on the rate of excretion. There is very little metabolic degradation of EDTA. The drug is distributed mainly in the extracellular fluids; very little gains access to the spinal fluid (5% of the plasma concentration).

Toxicity. Rapid intravenous administration of Na₂EDTA causes hypocalcemic tetany. However, a slow infusion (<15 mg/minute) administered to a normal individual elicits no symptoms of hypocalcemia because of the availability of extracirculatory stores of Ca²⁺. In contrast, CaNa₂EDTA can be administered intravenously with no untoward effects because the change in the concentration of Ca²⁺ in the plasma and total body is negligible.

The principal toxic effect of CaNa₂EDTA is on the kidney. Repeated large doses of the drug cause hydropic vacuolization of the proximal tubule, loss of the brush border, and eventually, degeneration of proximal tubular cells. The early renal effects usually are reversible, and urinary abnormalities disappear rapidly with cessation of treatment. The most likely mechanism of toxicity is chelation of essential metals, particularly zinc, in proximal tubular cells.

Other side effects associated with CaNa₂EDTA include malaise, fatigue, and excessive thirst, followed by the sudden appearance of chills and fever and subsequent myalgia, frontal headache, anorexia, occasional nausea and vomiting, and rarely, increased urinary frequency and urgency. CaNa₂EDTA is teratogenic in laboratory animals, probably as a result of zinc depletion; it should be used in pregnant women only under conditions in which the benefits clearly outweigh the risks (Kalia and Flora, 2005). Other possible undesirable effects include sneezing, nasal congestion, and lacrimation; glycosuria; anemia; dermatitis with lesions strikingly similar to those of vitamin B₆ deficiency; transitory lowering of systolic and diastolic blood pressures; prolonged prothrombin time; and T-wave inversion on the electrocardiogram.

Chemistry and Mechanism of Action. The pharmacological actions of dimercaprol result from formation of chelation complexes between its sulfhydryl groups and metals. Dissociation of dimercaprol–metal complexes and oxidation of dimercaprol occurs in vivo. Furthermore, the sulfur–metal bond may be labile in the acidic tubular urine, which may increase the delivery of metal to renal tissue and increase toxicity. The dosage regimen should maintain a concentration of dimercaprol in plasma adequate to favor the continuous formation of the more stable 2:1 (BAL–metal) complex. However, because of pronounced and dose-related side effects, excessive plasma concentrations must be avoided. The concentration in plasma therefore must be maintained by repeated dosage until the metal is excreted.

Dimercaprol is most beneficial when given very soon after exposure to the metal because it is more effective in preventing inhibition of sulfhydryl enzymes than in reactivating them. Dimercaprol limits toxicity from arsenic, gold, and mercury, which form mercaptides with essential cellular sulfhydryl groups. It also is used in combination with CaNa₂EDTA to treat lead poisoning.

Dimercaprol is contraindicated for use following chronic exposures to heavy metals because it does not prevent neurotoxic effects. There is evidence in laboratory animals that dimercaprol mobilizes lead and mercury from various tissues to the brain (Andersen and Aaseth, 2002). This effect may be due to the lipophilic nature of dimercaprol and is not observed with its more hydrophilic analogs described later (see "Succimer" and "Sodium 2,3-Dimercaptopropane Sulfonate [DMPS]").

Absorption, Distribution, and Excretion. Dimercaprol cannot be administered orally; it is given by deep intramuscular injection as a 100 mg/mL solution in peanut oil and should not be used in patients who are allergic to peanuts or peanut products. Peak concentrations in blood are attained in 30-60 minutes. The t_1/2 is short, and metabolic degradation and excretion essentially are complete within 4 hours. Dimercaprol and its chelates are excreted in both urine and bile.

Toxicity. The administration of dimercaprol produces a number of side effects, which occur in ~50% of subjects receiving 5 mg/kg intramuscularly. One of the most consistent responses to dimercaprol is a rise in systolic and diastolic arterial pressures, accompanied by tachycardia. The rise in pressure may be as great as 50 mm Hg in response to the second of two doses (5 mg/kg) given 2 hours apart. The pressure rises immediately but returns to normal within 2 hours.

Dimercaprol also can cause anxiety and unrest, nausea and vomiting, headache, a burning sensation in the mouth and throat, a feeling of constriction or pain in the throat and chest, conjunctivitis, blepharospasm, lacrimation, rhinorrhea, salivation, tingling of the hands, a burning sensation in the penis, sweating, abdominal pain, and the occasional appearance of painful sterile abscesses at the injection site. The dimercaprol–metal complex breaks down easily in an acidic medium; production of an alkaline urine protects the kidney during therapy. Children react similarly to adults, although ~30% also may experience a fever that disappears on drug withdrawal. Dimercaprol is contraindicated in patients with hepatic insufficiency, except when this condition is a result of arsenic poisoning.

Absorption, Distribution, and Excretion. After its absorption in humans, succimer is biotransformed to a mixed disulfide with cysteine (Aposhian and Aposhian, 2006). Succimer lowers blood lead levels and attenuates lead toxicity. The succimer–lead chelate is eliminated in both urine and bile. The fraction eliminated in bile can undergo enterohepatic circulation.

Succimer has several desirable features over other chelators. It is orally bioavailable, and because of its hydrophilic nature, it does not mobilize metals to the brain or enter cells. It also does not significantly chelate essential metals such as zinc, copper, or iron. As a result of these properties, succimer exhibits a much better toxicity profile relative to other chelators. Animal studies suggest that succimer also is effective as a chelator of arsenic, cadmium, mercury, and other toxic metals (Andersen and Aaseth, 2002; Kalia and Flora, 2005).

Toxicity. Succimer is much less toxic than dimercaprol. Transient elevations in hepatic transaminases have been observed with succimer treatment. The most commonly reported adverse effects are nausea, vomiting, diarrhea, and loss of appetite. In a few patients, rashes necessitate discontinuation of therapy.

Succimer has been approved in the U.S. for treatment of children with blood lead levels >45 μg/dL. Because of its oral availability, improved toxicity profile, and selective chelation of heavy metals, succimer also is used off label for the treatment of adults with lead poisoning and for the treatment of arsenic and mercury intoxication, although no large clinical trials have been undertaken for these indications.

Chemistry and Mode of Action. DMPS is a clinically effective chelator of lead, arsenic, and especially mercury. It is orally available and is rapidly excreted, primarily through the kidneys. It is negatively charged and exhibits distribution properties similar to those of succimer. DMPS is less toxic than dimercaprol but mobilizes zinc and copper and thus is more toxic than succimer. In a small clinical trial, DMPS exhibited some clinical benefit for the treatment of chronic arsenic poisoning. Similar benefits were not observed with either dimercaprol or succimer, suggesting that DMPS might be effective for treatment of chronic heavy metal poisonings (Kalia and Flora, 2005). However, more thorough clinical studies are needed.

Absorption, Distribution, Biotransformation, and Excretion. Penicillamine (cuprimine, depen) is available for oral administration. For chelation therapy, the usual adult dose is 1-1.5 g/day in four divided doses. The drug should be given on an empty stomach to avoid interference by metals in food. In addition to its use as a chelating agent for the treatment of copper, mercury, and lead poisoning, penicillamine is used in Wilson's disease (hepatolenticular degeneration owing to an excess of copper), cystinuria, and rheumatoid arthritis (rarely). For the treatment of Wilson's disease, 1-2 g/day usually is administered in four doses. The urinary excretion of copper should be monitored to determine whether the dosage of penicillamine is adequate.

Penicillamine is well absorbed (40-70%) from the GI tract. Food, antacids, and iron reduce its absorption. Peak concentrations in blood are obtained between 1 and 3 hours after administration. Penicillamine is relatively stable in vivo compared to its unmethylated parent compound cysteine. Hepatic biotransformation primarily is responsible for degradation of penicillamine, and very little drug is excreted unchanged. Metabolites are found in both urine and feces. N-Acetylpenicillamine is more effective than penicillamine in protecting against the toxic effects of mercury, presumably because it is more resistant to metabolism.

Toxicity. With long-term use, penicillamine induces several cutaneous lesions, including urticaria, macular or papular reactions, pemphigoid lesions, lupus erythematosus, dermatomyositis, adverse effects on collagen, and other less serious reactions, such as dryness and scaling. Cross-reactivity with penicillin may be responsible for some episodes of urticarial or maculopapular reactions with generalized edema, pruritus, and fever that occur in as many as one-third of patients taking penicillamine. The hematological system also may be affected severely; reactions include leukopenia, aplastic anemia, and agranulocytosis. These may occur at any time during therapy and may be fatal, so patients should be monitored carefully.

Renal toxicity induced by penicillamine usually is manifested as reversible proteinuria and hematuria, but it may progress to nephrotic syndrome with membranous glomerulopathy. More rarely, fatalities have been reported from Goodpasture's syndrome. Toxicity to the pulmonary system is uncommon, but severe dyspnea has been reported from penicillamine-induced bronchoalveolitis. Myasthenia gravis also has been induced by long-term therapy with penicillamine. Penicillamine is a teratogen in laboratory animals, but for pregnant women with Wilson's disease, the benefits appear to outweigh the risks. Less serious side effects include nausea, vomiting, diarrhea, dyspepsia, anorexia, and a transient loss of taste for sweet and salt, which is relieved by supplementation of the diet with copper. Contraindications to penicillamine therapy include pregnancy, renal insufficiency, or a previous history of penicillamine-induced agranulocytosis or aplastic anemia.

Absorption, Distribution, and Excretion. Deferoxamine (deferoxamine mesylate, desferal) is poorly absorbed after oral administration, and parenteral administration is required. For severe iron toxicity (serum iron levels >500 μg/dL), the intravenous route is preferred. The drug is administered at 10-15 mg/kg/hour by constant infusion. Faster rates of infusion (45 mg/kg/hour) have been used in a few cases; rapid boluses usually are associated with hypotension. Deferoxamine may be given intramuscularly in moderately toxic cases (serum iron 350-500 μg/dL) at a dose of 50 mg/kg with a maximum dose of 1 g. Hypotension also can occur with the intramuscular route.

For chronic iron intoxication (e.g., thalassemia), an intramuscular dose of 0.5-1.0 g/day is recommended, although continuous subcutaneous administration (1-2 g/day) is almost as effective as intravenous administration. During blood transfusion, patients with thalassemia should be given 2 g deferoxamine (per unit of blood) by slow intravenous infusion (rate not to exceed 15 mg/kg/hour) by a separate line. Deferoxamine is not recommended in primary hemochromatosis; phlebotomy is the treatment of choice. Deferoxamine also has been used for the chelation of aluminum in dialysis patients.

Deferoxamine is metabolized by plasma enzymes, but the pathways have not yet been defined. The drug is excreted readily in the urine.

Toxicity. Deferoxamine causes a number of allergic reactions, including pruritus, wheals, rash, and anaphylaxis. Other adverse effects include dysuria, abdominal discomfort, diarrhea, fever, leg cramps, and tachycardia. Occasional cases of cataract formation have been reported. Deferoxamine may cause neurotoxicity during long-term, high-dose therapy for transfusion-dependent thalassemia major; both visual and auditory changes have been described. A "pulmonary syndrome" has been associated with high-dose (10-25 mg/kg/hr) deferoxamine therapy; tachypnea, hypoxemia, fever, and eosinophilia are prominent symptoms. Contraindications to the use of deferoxamine include renal insufficiency and anuria; during pregnancy, the drug should be used only if clearly indicated.