Toxicology

40 Toxicology Harrell E. Hurst and Michael D. Martin

K E Y I N F O R M AT I O N • Dose is the major determinant that distinguishes therapeutic effects from toxicity with exposure to drugs and chemicals. • Safety, toxicity, hazard, and risk are terms with specific meanings relevant to toxicology. • Consideration of cause and effect requires careful assessment of criteria for causal relationships. • Toxicity varies widely with differences in biologic systems, timecourse of exposure, and chemistry of toxicants. • The dental practitioner has a responsibility to protect both patients and office staff from toxic exposures from environmental and therapeutic sources.

• The dental practitioner should understand and be familiar with principles for managing cases of poisoning. • Mercury, although an important dental therapeutic agent, is toxic and its use must be managed carefully. • A great variety of toxic chemical agents, including gases, solvents, pesticides, and natural products, are used in modern society and exposure is virtually inevitable; safety depends on careful use to minimize hazards and reduce risk.

 

CASE STUDY Three male dentists, who are within 2 years of each other in age and have been in the same group practice for over 20 years, together attend the ­American Dental Association annual meeting at which they each have urinary mercury level testing performed. On returning home, they each receive the results. When comparing notes, they are astonished to find that although they each work essentially the same schedule, work in the same environment, and utilize all of the same supplies and techniques, one of the three has a urinary Hg level approximately double that of the others. All three are relatively low, with the two lower at 3.3 and 3.5 μg/L and the higher at 6.1 μg/L. All three are healthy, with no kidney disease or reduced kidney function. Following a discussion of seafood ingestion, a known source of dietary Hg, and a review of their individual practice methods regarding use of mercury amalgam, they cannot determine why one would have such a different urinary Hg level. What might be additional details of a possible cause for this disparity?

Toxicology is a basic science that is concerned with poisonous substances and their toxic actions. This discipline draws on biology, chemistry, and medicine to inform knowledge regarding toxic materials. Toxicology strives to understand key features of biology relevant to adverse interactions of chemicals with living systems. A principal objective is to promote safe use of chemicals, whether encountered as medicines, as food additives or contaminants, as industrial materials, as household products, or in the environment. Topics of interest to toxicologists include management of poisoning, analysis of toxic agents, identification of toxic effects, elucidation of mechanisms of toxicity, characterization of potential chemical risks, forensic and legal applications, and timely application of knowledge to prevent potentially dire consequences of chemical use.

The toxicology of therapeutic agents related to their pharmacologic effects at elevated doses is described in the corresponding chapters of this text. This chapter reviews general principles of toxicology, summarizes key organ systems that are susceptible to toxic effects, and outlines prevention and management of acute poisoning. Toxic materials not described elsewhere in the text are reviewed, and relevant topics related to dental practice are discussed.

GENERAL PRINCIPLES All chemical substances can cause harm or kill if encountered at sufficiently large concentrations over crucial periods of time. This statement embodies insight articulated by Paracelsus in the 16th century: that dose is the major determinant of toxicity. A subset of substances has relatively specific toxic effects, however. These are considered very harmful based on human experience and are considered poisons or toxins. Beyond this base of experience exists a vast number of uncharacterized, potential toxicants. At this time, there are more than 32 million organic and inorganic substances, over 245,000 inventoried or regulated substances, and some 15 million commercially available chemicals. Because many of these are potentially toxic, this array dictates that toxicologists use some means of triage toward assessment of potential toxicants. At present, selection of chemicals for toxicologic testing is dictated by their potential for use, by funding of basic research on the chemicals, and by evidence of specific adverse effects. The ultimate aim of toxicologic science in society is to guide safe use of chemicals. The definitions in Box 40-1 can assist in understanding and promoting concise communication in the approach to this objective. Safety is a negative entity—that is, the absence of threat of injury. As such, safety cannot be proved directly. Society often simplistically considers chemicals “safe” or “toxic.” Such naive characterization can preclude the rational judgment that enables safe uses of

603

604

CHAPTER 40  Toxicology

BOX 40-1  Definitions Relevant to

BOX 40-2  Hill Criteria for Consideration of

Safety Toxicity

Criterion

Explanation

Strength of association

Observed magnitude of the association compared with other relevant observations should be considered as a primary indicator in assessment of cause and effect Association of cause and effect can be observed repeatedly by others under appropriate circumstances Particular conditions produce the effect, or a specific group is affected; bounds of causal relationship should be delimited Causation generally occurs before effect, whereas correlational effects can vary in temporality Demonstration of a fundamental dose relationship provides convincing evidence of cause and effect Some basis in previous knowledge provides a means of common understanding (remember, however, that all phenomena were novel at some point) Care should be taken that interpretation of cause and effect does not unduly conflict with scientifically established facts of biology and medicine Manipulation of accessible variables in the potential cause-and-effect relationship has an effect Previously understood examples provide basis for formulation of testable hypotheses

Principles of Toxicology

Hazard Risk

Condition of being secure from threat of danger, harm, or injury Property of grave harmfulness or deadliness associated with a chemical that is expressed on biologic exposure Threat of danger directly related to circumstances of use of a chemical Expected frequency of occurrence of an adverse effect in a given situation

Causal Relationships

Consistency

Specificity

chemicals. Critical judgment requires understanding of the distinction between the terms toxicity and hazard to enable assessment of risks (see Box 40-1). Toxicologic studies promote safety by defining hazardous situations of use so that the unsafe use of chemicals can be avoided. A primary concern of toxicology is evaluation of risk. All useful chemicals have some degree of risk associated with their use. Toxicologic science has developed testing paradigms to define toxicity and assess potential risk. Benefits also must be considered relative to the risk of use. A high degree of risk may be acceptable when benefits are great (e.g., use of toxic but potentially life-extending drugs such as chemotherapeutic agents). Otherwise, risk may be unacceptable for less essential uses (e.g., food coloring). In contrast to the science inherent in testing methods, judgment of risk acceptability involves policy. Such judgment invokes economic, social, and ethical values and should consider factors such as needs met by a chemical under consideration, alternative solutions and their risks, anticipated extent of use and public exposure, effects on environmental quality, and conservation of natural resources. Within such considerations is an issue of major importance to toxicology and to society in general, which is determination of causeand-effect relationships. This objective of epidemiologic studies is elusive for chronic diseases, such as many types of cancer. Such diseases may involve confounded potential causes, such as chemical or viral exposure and genetic susceptibility factors. Uncritical publication of unscientific observations or incomplete studies lead the public to inappropriate conclusions, which should be characterized more correctly as hypotheses. Adequate processes for determination of causation, as opposed to simple unrelated association or correlation, require scientific discipline and judgment based on considerable experience. The criteria developed by Sir Austin Bradford Hill (see reference) provide a sound basis for consideration of causal relationships and should be considered a touchstone for expert opinion regarding cause and effect (Box 40-2). None of these criteria should be considered as absolutely essential, and they cannot be considered as proof of causal relationships. Their careful application during evaluation of potential cause-and-effect relationships can assist, however, in organizing knowledge toward a weight-of-evidence judgment and may provide an alternative interpretation for consideration.

Dose–Response Relationships The relationship between dose and toxic response is the fundamental axiom of the science of toxicology. Studies are designed to ascertain dose–response functions associated with specific adverse effects. When simple all-or-nothing criteria, such as death, are used, quantitation of response is simple. More often, objectives require subtler means of assessment that are less readily quantified. Beyond simple indication of the quantity of material required for the toxic effect, dose–response relationships provide strong evidence of the causal relationship between the observed effect and the chemical under study.

Temporality Biologic gradient

Plausibility

Coherence

Experiment Analogy

Data from Hill A: The environment and disease: association or causation? Proc Roy Soc Med 58:295, 1965.

Figure 40-1 presents three modes of display of idealized dose– response data to illustrate and describe the dose required for median response in subjects tested. These data are typical of quantal or all-ornothing responses such as lethality. In this example, the dose axis is logarithmically spaced, and the data describe a log-normal distribution. Responses that arise from mass action, such as reversible occupancy of receptor by drugs, often are most easily interpreted when plotted on a logarithmic axis. Alternatively, effects caused by limited biologic capacity, such as irreversible enzyme inhibition, can exhibit abrupt threshold-like effects and may be more easily analyzed on a linear dose axis. The rule is to plot the data to see what type axis is most informative. The lower panel of Figure 40-1 indicates distribution of responses across the dose axis, with a mean of 10 and standard deviation (SD) of one log10 unit. Response percentages include approximately 68.3% within ±1 SD of the mean, 95.5% within ±2 SD, and 99.7% within ±3 SD of the mean. The distribution indicates hypersusceptibility for individuals at the lowest doses and resistant responders at the highest doses. Such a plot gives a convenient way to visualize the distribution of responses across dose within the test groups. The middle panel of Figure 40-1 plots the cumulative response versus dose across all treated groups. Here the response data are practically linear in the range from −1 to +1 SD for these ideal data. This plot provides a convenient, accurate estimation of dose required for a 50% response, such as the median lethal dose (LD50). Real data are rarely so well behaved, however, as too few data may be available for adequate

CHAPTER 40  Toxicology

Factors That Change Dose–Response Relationships Dose–response relationships can vary with many factors, including differences within and among individuals. Factors responsible for dose– response variations within an individual over time may include age and nutritional status, environmental influences, functional status of organs of excretion, concomitant disease, and various combinations of factors. Changes in pharmacokinetics of toxicants are a frequent basis for altered dose–response relationships. Known influences include increased toxicant bioactivation by enzyme induction, such as occurs in certain variants of cytochrome P450 with exposure to phenobarbital or polychlorinated biphenyls. Conversely, inhibition of metabolic clearance is possible with interacting chemicals, increasing the pharmacodynamic action of drugs and chemicals. The cytochrome P450 isozyme 3A4 is an important enzyme in human drug metabolism, and its presence in the gut and liver subjects it to inhibition by many drugs and dietary components, such as grapefruit juice. Conversely, substances are often less toxic by the oral route when administered with food as a consequence of less rapid absorption. The time and frequency of administration can be important in altering dose–response relationships through functional changes. Many compounds induce tolerance upon repeated administration, whereas others can become more toxic with closely repeated administration. Receptor densities and sensitivity may vary with time or as a consequence of previous exposure. An example of the latter is the wellknown tolerance that develops to long-term administration of opioids. Responses among individuals differ as a consequence of different genetic traits. Recognition and understanding of relevant aspects of human diversity derived from functional genomic analyses offer potential for therapeutic gains. The rationale is to use appropriate

Cumulative Response (Probit)

8 Mean

7 6 5 4 3 2

Cumulative Response (%)

100 80 60 40 20 0 Frequency of Response (%)

definition of the sigmoid curve. Another disadvantage is that the sigmoid curve presents difficulty in estimating doses that elicit extremes of response, such as 1% or 99%. An alternative presented in the top panel of Figure 40-1 uses the probit transform for the cumulative response. Probit units are derived by conversion of cumulative response percentages to units of SD from the mean. The scale uses normal equivalent deviation units (NED), for which the mean is arbitrarily set to an NED value of 5 to give positive values along the axis. As is evident in the example, the probit transform linearizes the extreme values of the response function, which allows more accurate estimation of doses affecting 1% or 99% of subjects exposed. In addition, the probit transform facilitates determination of the slope, which enables comparison of the dose–response function with other agents or responses. Such plots are inadequate in dealing with issues of societal risk beyond mean responses, however, as policy often requires estimation of exposure posing a theoretic risk of one in one million, otherwise described as a 10−6 risk factor. Practical problems intervene, including the impracticality of experimental studies involving sufficient animals to define adequately the dose–response function at low response levels. A classic toxicologic experiment conducted at the National Center for Toxicological Research illustrates this point. Officially termed the ED01 Study (See reference, Gaylor, 1980), this experiment examined in detail the response function of mice treated with low doses of the experimental carcinogen 2-acetylaminofluorene. The study, sometimes termed the megamouse study, involved more than 24,000 mice to determine, with precision, the dose effective in producing a 1% tumor rate. This work advanced toxicologic understanding of the complexity of genotoxic and proliferative cellular events in chronic cancer bioassays. It also exhibited logistic difficulties in conducting statistical studies of low incidence and illustrated gaps in the evolving understanding of chemically induced cancer.

605

40 30 20 10 0 10–2

10–1

100

101 102 103 104 Dose FIG 40-1 Various techniques for graphic display of quantal response versus dose data, including frequency of response (bottom panel ), cumulative response (middle panel ), and cumulative response linearized by probit transformation (top panel ).

drugs in patients best suited to benefit and to reduce use in patients with genetic traits that might result in toxicity. These efforts have spawned new terms, including pharmacogenetics, representing characterized genetic differences in drug metabolism and disposition, and pharmacogenomics, used to describe the broad spectrum of genes that affect drug response (see Chapter 4). A summary is available that describes progress in determining genetic polymorphisms relevant to drug action and disposition. Known variants linked to altered drug effects in humans include phase I cytochrome P450 enzymes, phase II enzymes such as N-acetyltransferases and glutathione-S-transferases, small molecule transporters, drug and endogenous substrate receptors, and ion channel variants. Similar advances are likely to be applied to understand genetic differences that result in toxic effects aside from those that arise during drug therapy. Approximately 400 million individuals worldwide exhibit a heritable deficiency in the cytoplasmic enzyme glucose-6-phosphate dehydrogenase. Because this enzyme is essential to the cell’s capacity to withstand oxidant stress through production of reducing equivalents, sensitive individuals with this enzymatic deficiency have chemically mediated hemolytic anemia when exposed to oxidants.

606

CHAPTER 40  Toxicology

Of particular importance to the interpretation of toxicologic studies are interspecies differences, which may confound understanding and interpretation of results from animal models. Well-known differences in physiology, metabolic rates, pharmacokinetics of toxicant metabolism and excretion, and sites of toxicant action mediate these interspecies differences. Advances involving physiologically based pharmacokinetic modeling and use of predictive, mechanistically based biomarkers offer promise of augmenting, or in some cases obviating, conventional toxicity testing.

Acute versus Chronic Toxicity Toxicity can be classified by the amount of time required for development of the adverse effect. For this purpose, the term acute describes toxicity with a sudden onset, whereas chronic indicates a long latency or duration. In epidemiology, this classification typically describes the time between exposure and onset of toxicity. Intoxication is an acute effect that results from ingestion of a large quantity of ethanol over a brief time. Alternatively, the progressive, diffuse architectural damage to the liver known as cirrhosis occurs over years with chronic ethanol exposure. In experimental toxicology, these terms are used to refer to experimental paradigms involving the duration of treatment or exposure. Acute testing typically describes a single treatment, whereas chronic toxicity testing usually involves dosing or feeding a chemical over the lifetime of a species, as in a rodent carcinogenicity bioassay. If exposure occurs repeatedly at intervals more frequent than the time required to eliminate a toxicant, the material accumulates in the body throughout the duration of exposure. Although each exposure may be less than toxic, accumulation may produce toxic concentrations if exposure continues for sufficient time. The primary determinant is the rapidity of elimination relative to the frequency and magnitude of exposure. Slowly eliminated toxicants, such as lipophilic chemicals or materials readily bound in tissues, have the greatest potential for accumulation. Chronic toxicity may exhibit little or no apparent relationship to acute toxicity. In such cases, understanding of cause and effect requires careful study. Of the many examples of chronic toxicity, carcinogenesis currently is of great concern in society. Precancerous cellular changes occur and develop slowly and may remain undetected over long periods. Periodic dental examinations often play a significant role in detection of cancers of the oral cavity. Knowledge of patient habits with adverse potential health effects, such as the link between tobacco use and occurrence of oral lesions, assists the dental practitioner in being vigilant against such chronic toxicity.

Chemically Related Toxicants Understanding of chemical toxicity requires knowledge of related chemicals that may be present as impurities because of manufacturing or exist as a result of environmental effects. A classic example is 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin, or TCDD), which was discovered in the herbicide mixture known as Agent Orange used in the Vietnam War. Although dioxin existed at low part-per-million levels in the herbicide mixture, the extreme toxicity of this contaminant in certain species created grave concern for contaminated areas. This concern led to a ban on the use of the herbicide 2,4,5-trichlorophenoxyacetic acid because TCDD is formed through a condensation reaction involving two molecules of 2,4,5-trichlorophenol. Dioxin also can be formed from other sources, such as combustion of municipal waste, iron ore sintering, and wood pulp and paper mills. The toxic actions of TCDD are mediated through its binding to the aryl hydrocarbon nuclear receptor, which regulates transcription of genes encoding cytochrome P450 enzymes in the CYP1A subfamily and several other genes that regulate cell growth, differentiation, innate immunity,

and autoimmune diseases. Despite its extreme toxic potential in some species, epidemiologic studies regarding the effects of low-dose environmental TCDD exposure on humans have been inconclusive to date. High human exposures cause a dermatologic toxicity known as chloracne in some individuals. The consequences of metabolism of drugs and chemicals after ingestion are extremely important. The following example illustrates the importance of understanding toxic effects relative to drug metabolism. Terfenadine is a nonsedating histamine H1 receptor antagonist that was widely used for relief of symptoms of seasonal allergy. This drug was removed from the market because studies revealed cardiotoxicity when terfenadine was given with erythromycin. The toxic interaction was traced to the antibiotic’s inhibition of the high-affinity oxidative enzyme system CYP3A in human liver and intestinal membranes. This interaction inhibited normal clearance of terfenadine, and the abnormally elevated concentrations produced toxicity in the form of a prolonged cardiac QT interval and the arrhythmia torsades de pointes. This antihistamine has been replaced with its active metabolite, fexofenadine, which apparently does not elicit this toxicity.

Local versus Systemic Effects Toxic effects can occur at a site of exposure, such as dermal contact, or at some site remote from the point of chemical contact or entry. Local effects dependent on applied concentration are usually diminished by dilution with physiologic fluids and diffusion within tissue away from the site of application. The toxic effect depends on the nature of the interaction at the local site. If the effect is caused by reversible interaction with a receptor, such as that of a local anesthetic, the effect is attenuated by diffusion, and the system is returned to a more normal state as the drug dissociates from receptors. This phenomenon is routinely used in dental practice with the inclusion of epinephrine with local anesthetics, such as lidocaine, to restrict local blood flow and extend duration of local anesthetic action. For toxicants that act through destruction of normal cellular architecture, such as a caustic agent, return to normality requires repair of membranes and cellular structures. Systemic effects are facilitated by transport within the body fluids and may be influenced by metabolism. Depending on whether biotransformation activates a protoxicant or detoxifies a toxicant, the effects of systemic processing can increase or attenuate toxicity. Compounds may be more or less toxic by the oral route than by other means of systemic exposure, as the first-pass effect of intestine or liver serves to activate or remove toxicants before distribution in the systemic circulation. Alternative systemic exposures, such as inhalation, are not modulated in this manner because systemic exposure occurs directly without first-pass effect.

Target Organ Systems Most toxic chemicals exhibit specificity in their action on target tissues or organs because these targeted biologic systems reach crucial points in which their physiologic functions are interrupted under the influence of the chemical. This section presents crucial physiologic systems and their characteristics that are important in understanding organ-specific toxicity.

Nervous system Given the primary importance in control of integrated function, the central nervous system (CNS) is a target of paramount importance for many toxicants. Individual neurons exhibit high metabolic rates and are unable to rely on anaerobic glycolysis. These characteristics make these cells susceptible to toxicants that adversely affect cellular respiration and energy production and lead to neuronal damage when central

CHAPTER 40  Toxicology or peripherally acting toxicants interrupt neuronal metabolism, cerebral circulation, oxygen-carrying capacity of blood, or pulmonary ventilation. A remarkable cell-selective neurotoxicant is 1-methyl-4-­phenyl1,2,3,6-tetrahydropyridine (MPTP), an impurity discovered after attempted illicit synthesis and injection of a meperidine analogue resulted in Parkinson disease-like paralysis. This compound is a protoxicant for 1-methyl-4-phenylpyridinium, which is formed by monoamine oxidase and concentrated by high-affinity carrier into dopaminergic neurons. The molecular target of 1-methyl-4-phenylpyridinium is reduced nicotinamide adenine dinucleotide dehydrogenase, and the interaction blocks the cellular respiratory exchange of electrons in mitochondria of cells. Its toxic actions result in destruction of dopaminergic neurons in the substantia nigra. Death of these cells produces symptoms strikingly similar to Parkinson disease, leading to nonintentional motor actions. Loss of integrity of neuronal cell metabolism can alter neuronal architecture, particularly the myelin sheath of peripheral neurons. Such effects are common to many forms of toxicity expressed in the nervous system. Various compounds, such as tri-o-cresyl phosphate, acrylamide, and metabolites of hexane, cause degeneration of long axons that control neuromuscular activities. Termed distal axonopathy, this toxicity involves a “dying back” or retrograde degeneration of distal axons and leads to loss of control of motor functions such as gait. Other effects, such as sensory neuropathy and paresthesia, can result from similar effects of toxicants on small sensory fibers.

Blood and hematopoietic system Because of the crucial roles of the elements of blood in delivering oxygen and maintaining immune function, toxic effects on blood or the hematopoietic system can be life-threatening. Of these, perhaps no poisoning is more common, preventable, or treatable with timely therapy than the toxic interaction of carbon monoxide (CO) with hemoglobin (Hb). This interaction blocks the vital oxygen-carrying capacity by formation of carboxyhemoglobin (CO-Hb). Characteristics of CO and its toxic effect on various tissues sensitive to anoxia have been concisely reviewed. Details of treatment, which involves displacement of Hb-bound CO with oxygen, are provided in the comprehensive text Medical Toxicology. (See reference, Ellenhorn et al., 1997). In mild (CO-Hb < 30%) or moderate (CO-Hb 30% to 40%) cases, therapy includes use of 100% oxygen by nonrebreathing mask until CO-Hb is less than 5%. Severe poisoning can mandate hyperbaric use of oxygen to hasten the exchange. Another toxic effect that alters the oxygen-carrying capacity of erythrocytes is the formation of methemoglobin. In this toxicity, the heme iron is oxidized from the ferrous (Fe++) to ferric (Fe+++) state by exposure to oxidizing chemicals such as nitrites or aromatic amines. As with CO-Hb, methemoglobin is incapable of carrying molecular oxygen to tissues. Although the effects of resultant anoxia are similar, the treatment differs. Treatment involves use of methylene blue, as a precursor to its metabolite leukomethylene blue, a cofactor that enables erythrocytes to reduce methemoglobin in the presence of reduced nicotinamide adenine dinucleotide phosphate (NADPH). This therapy has complications of potential hemolysis for treatment of infants and individuals with glucose-6-phosphate dehydrogenase deficiency because this enzyme is essential in the production of NADPH. Other adverse actions affect the blood-forming cells of the bone marrow. Such effects can cause loss of immune functions mediated through leukocytes, as noted with induction of agranulocytosis during treatment with thioamide antithyroid drugs, such as propylthiouracil. Although rare, this adverse effect is devastating because it leaves the patient susceptible to sepsis. Aplastic anemia is

607

a potential complication of therapy with the antiepileptic drugs felbamate and carbamazepine. This condition is very serious because the marrow loses the ability to produce cells. This potential effect requires vigilance for signs of blood dyscrasias and requires laboratory monitoring of blood cell counts during the first months of treatment. Other adverse effects on the hematopoietic system include overexpression of certain types of cells, such as that noted in the development of acute myelogenous leukemia from benzene. Benzene is a toxicant commonly encountered in petroleum distillates such as gasoline and is considered a causative agent in human leukemia, probably through an active hydroquinone or benzoquinone metabolite. The process of leukemia development seems to involve preferential selection and clonal expansion of stem and progenitor cells through interaction of the toxic benzene metabolites by multiple independent genetic and epigenetic factors.

Respiratory system The effect of toxicants on the respiratory tract is largely determined by the area of intimate cellular exposure to inhaled chemicals. Such contact is dictated by the structure of the conducting airways and the physical and chemical properties of the toxicant. Larger particles and more water-soluble compounds deposit in the upper regions of the respiratory tract, whereas very fine particles and less soluble gases reach more deeply into the lungs. Compounds that are rapidly absorbed or highly caustic generally affect the nasal passages. Formaldehyde has a detectable pungent odor at concentrations greater than 0.5 ppm and is highly irritating to the nasal passages. The nasopharyngeal region serves as a filter for particles 10 to 30 μm in diameter. Many of these particles are cleared upward by mucociliary action. Highly water-soluble gases, such as sulfur dioxide, dissolve in moisture present in the upper respiratory membranes and form irritating sulfurous acid. Less soluble compounds, such as oxides of nitrogen and ozone, penetrate more deeply into lung and generally exert effects at membranes in the smallest airways or alveoli. Particles smaller than 5 μm may travel well down into the bronchiolar region, whereas fine particles of 1 μm, nominal size, reach the alveolar region. Lung toxicity typically involves damage to the delicate architecture vital for efficient gas exchange. Because lung tissues contain many cytokines and immunologic mechanisms for particle clearance and tissue repair, inflammation is a common result of inspired toxic gases such as ozone. With severe acute injury, an exudative phase may progress to pulmonary edema, which alters ventilation, diffusion of oxygen and carbon dioxide, and perfusion. Severity depends on the extent of damage to bronchiolar and alveolar cells and the resolution of inflammation through mitogenic or fibrinogenic processes. Chronic injury to the lung may result from inhalation of fine particles. Phagocytic mechanisms attempting to remove insoluble particles may produce tissue scarring and interstitial fibrosis, in which collagen fibers replace normal membranes and occupy alveolar interstitial space. This kind of injury is common with inhalation of particles such as asbestos. These actions produce inflexible tissue, diminish surface area, and lead to poor surfaces for gas exchange. Another chronic lung toxicity is emphysema; its major cause is cigarette smoking. This toxic effect produces distended, enlarged air spaces that are poorly compliant but without fibrosis. The pathogenesis of this condition is not fully understood, but an imbalance between proteolytic activities of lung elastase and antiproteases seems to be involved. Lung cancer became a major concern with the increase in popularity of smoking; this health scourge of today was a rare disease a century ago. Smoking is believed to be the most

608

CHAPTER 40  Toxicology

important risk factor for this disease, presenting a tenfold and twentyfold increase in risk for average and heavy smokers.

Organs of excretion The primary organs of toxicant elimination are the liver and kidneys. The liver provides the major site for metabolic transformation, rendering compounds generally more water-soluble and subject to more efficient excretion in urine by the kidney. The unique physiologic features of each organ provide crucial characteristics that are susceptible to toxic actions and subsequent adverse consequences of impaired function. The liver possesses remarkable capabilities for regeneration. Hepatotoxicity often results in necrosis and loss of the vital capacities of the liver, however. Essential functions include protein synthesis, nutrient homeostasis, biotransformation, particle filtration, and formation and excretion of bile. Impaired production of proteins such as albumin, clotting factors, and lipoproteins may cause hypoalbuminemia, hemorrhage, and fatty liver. Toxic actions that alter glucose synthesis and storage often lead to hypoglycemia and confusion, whereas effects on cholesterol uptake may produce hypercholesterolemia. Altered biotransformation or biliary excretion of endogenous substrates such as steroid hormones or bilirubin may affect a wide variety of hormonal functions or cause jaundice. As noted in Chapter 2, various membrane and cytosolic enzymes in the liver provide the essential metabolic functions of oxidation and glucuronide, sulfate, and mercapturate conjugation for removal of toxicants. These reactions usually detoxify compounds, but occasionally metabolic products exhibit enhanced toxicity. Interactions can occur among effects of toxicants within the liver through induction of enzymes or depletion of metabolic resources. Acetaminophen has been widely used as an over-the-counter analgesic without adverse effects on the liver at therapeutic doses. In circumstances of glutathione depletion, however, which occurs with large acetaminophen overdose, malnutrition, or CYP2E1 induction by long-term ethanol use, a reactive electrophilic intermediate forms in sufficient amounts to produce covalent adducts that severely damage the liver. The kidney plays a vital role in regulating extracellular fluid and excreting soluble wastes through filtration of blood, concentration of wastes, and elimination. To accomplish these vital functions, nephrons are composed of vascular, glomerular, and tubular components. The kidneys possess metabolic and regenerative capabilities, but these resources lead to renal failure when overwhelmed. Nephrotoxicity can be classified as acute or chronic. Acute renal failure can be caused by hypoperfusion from renal vasoconstriction, as elicited by the antifungal amphotericin B, or hypofiltration through glomerular injury resulting from cyclosporine and aminoglycosides. Numerous compounds, including nonsteroidal antiinflammatory drugs, various antibiotics, and heavy metals, cause acute renal failure by nephritis, acute tubular necrosis, or obstruction. Causes of chronic renal failure from many of these toxicants include nephritis from inflammatory and immunologic mechanisms and papillary necrosis through ischemia or cellular injury. Compensatory mechanisms may include hypertrophy and induction of metallothionein synthesis in response to heavy metal exposure.

PREVENTION AND MANAGEMENT OF POISONING In the practice of dentistry, the practitioner has a responsibility to help protect both office staff as well as patients from accidental poisoning. Steps can be taken by practitioners to limit the possibility of accidental poisoning. Patients should be encouraged to keep all medications out of the reach of children, and drugs should always be kept in child-resistant containers. Medications and toxic agents should be clearly labeled

BOX 40-3  First Aid for Poisoning 1. Summon help. 2. Stabilize the patient. 3. Evaluate the cause. 4. Terminate absorption. 5. Consider specific antidotes. 6. Enhance elimination. 7. Provide for supportive care.

and stored in secure locations. Information on the label of a prescribed drug should be understandable and include the name of the agent and clear directions for use. The prescribing physician or dentist should always indicate the purpose of the medication in the label information on the prescription. The statement of purpose of the medication is known as an “indication.” Writing an indication on the prescription helps reduce confusion about drugs in the medicine cabinet and facilitates rapid identification of the drug involved in cases of accidental ingestion. Patients should be instructed to discard unused medication rather than attempt self-medication with drugs remaining from a previous course of therapy. Diagnosis and treatment of poisoning are the purview of the physician. Principles of therapy for poisoning are summarized in Box 40-3 and apply to the management of any drug overdose. A dentist may be called on to provide emergency treatment of acute poisoning, however, within the practice environment or because of training as a health care professional.

Principles of Therapy for Poisoning Summon help

When acute poisoning is evident, help should be sought through the emergency 911 telephone service if available. For less critical situations, the community poison control center provides an invaluable service. These centers are equipped with extensive files describing the signs and symptoms of poisoning and recommended methods of treatment for most toxic substances distributed within the United States. Poison control centers can be reached by telephone on a 24-hour basis, and phone numbers are usually published inside the cover of telephone directories. If the toxic reaction is serious, expert medical assistance should be sought immediately. In addition, most major medical centers have drug information centers that provide information to practitioners about drugs and drug interactions.

Stabilize the patient Supportive therapy should be provided. Because hypoxia and shock are two common manifestations of serious toxicity, respiration and circulation must be monitored and assisted if required. For convulsions, physical protective measures may suffice along with the administration of oxygen to help avoid hypoxia. Intravenous diazepam is a drug of choice for pharmacologic control of continuing seizures.

Evaluate the cause Proper therapy to eliminate exposure to the toxin or reverse its effects depends on identifying the poison. Questioning the victim or the victim’s associates, searching for empty containers, or looking for physical signs on the patient (e.g., miosis or needle tracks for opiate or opioid overdose, burn marks in the mouth for ingestion of caustic chemicals) can be important in establishing the cause of poisoning.

Terminate absorption Any obvious means of contact with the poison should be removed. For dermal exposure to chemicals, removal of contaminated clothing

CHAPTER 40  Toxicology and repeated washing with soap and water are indicated. With ingested compounds other than petroleum products and corrosive substances, induction of vomiting had been suggested, but only in a conscious patient. Vomiting should not be induced for poisoning by petroleum products or agents producing loss of consciousness because of the risk of aspiration. Likewise, corrosive damage to the esophagus and gastric perforations may result from corrosive substances if emesis is induced. Moreover, modern practice is to avoid inducing vomiting because it does not reliably remove ingested poisons. Gastric lavage can be used by qualified personnel if care is taken to avoid aspiration of stomach contents by the victim. Prevention of absorption of many drugs within the gastrointestinal tract can be achieved by activated charcoal (10 to 50 g in water), and cathartics may be used to hasten the exit of drugs from the intestine.

Consider specific antidotes Specific antidotes are available to treat poisoning by certain classes of compounds. Antidotes may be useful in preventing the absorption of ingested agents (e.g., Ca++ salts for F−), increasing their rate of elimination (e.g., dimercaprol for inorganic mercury), blocking specific receptors (e.g., naloxone for morphine), or blocking other toxic activity (e.g., N-acetyl cysteine for acetaminophen overdose). One specific antidote should be remembered by dentists. For ingestion of toxic amounts of fluoride, which might occur with prescribed tablets or with topical liquids or gels, the local antidote to prevent absorption is Ca++ (in milk, calcium lactate, calcium gluconate, or lime water). If necessary, 2 to 10 mL of 10% calcium gluconate may be injected intravenously to bind fluoride and overcome hypocalcemia. Dentists who use benzodiazepines and opioid analgesics for conscious sedation must be familiar with the use of flumazenil and naloxone, respectively, to reverse respiratory depression caused by these drugs.

Enhance elimination

609

exposures to blood-borne pathogens and biologic agents in addition to chemicals. To assist in fulfilling the requirements of OSHA regulations, the American Dental Association has excellent online materials available for study. To meet OSHA regulations, drugs and chemicals must be labeled with the name of the chemical, appropriate risk warnings regarding the chemical, and name and address of the manufacturer or other responsible party. If a hazardous material is transferred to another container at any time other than for immediate use, an appropriate label must be affixed to the new container. Material safety data sheets, which provide information on handling, storage, cleanup, disposal, and emergency and first aid procedures, are central to the safety program and are the primary source of risk and hazard information. These sheets are required to be provided by the manufacturer on request and must identify the hazardous substances included in the preparation, the physical and chemical characteristics, the fire and explosion danger, and other health hazard data. These sheets must be present in the workplace and available to employees at all times. Dentists are required to provide appropriate training for employees in the use and management of hazardous substances when they are hired, whenever new hazardous substances are brought into the workplace, and when new information regarding the use of existing substances becomes available.

SPECIFIC TOXICANTS The toxic effects of several classes of substances are presented. Agents that illustrate general principles presented earlier and that have public health importance or importance in the practice of dentistry are described.

Metals

Measures to facilitate elimination of toxicants are in the realm of emergency care physicians; they are mentioned briefly here for completeness. The renal excretion of weak electrolytes can often be accelerated by appropriate modification of urinary pH to promote ionization of the electrolyte. Administration of an osmotic diuretic in conjunction with large volumes of water is helpful in promoting urinary excretion and reducing the renal concentration of nephrotoxic poisons. In limited instances, peritoneal dialysis or hemodialysis may be useful.

Metals as a class are toxic primarily because of their ability to bind with biologic structures such as thiol groups in enzymes and other proteins. The major effect in humans is the inhibition of enzyme function. Because of this binding affinity, the effect of metals may be widespread in the organism, but usually a primary or most sensitive system in which clinical manifestations may be detected is evident. Metals as a class are important because of their ubiquitous nature in modern medicine and technology and in nature. Two metals of importance in public health and dental practice are mercury and lead.

Provide for supportive care

Mercury

Medical assessment of poisoning and continuing treatment, as needed, should be provided by medical staff in an appropriate health care facility.

Mercury is present virtually everywhere in the environment. An estimated 2700 to 6000 tons is released annually from the oceans and the earth’s crust into the atmosphere. An additional 2000 to 3000 tons is released through human activities, including the burning of fossil fuels. Mercury exists in three chemical classes: elemental mercury (Hg0), which is a liquid at room temperature and is used as a primary component in dental amalgam; inorganic mercury salts; and organic mercury salts. Inorganic mercury salts may exist as mercurous (Hg+) or mercuric (Hg++) forms. Of the many organic forms of mercury, methylmercury is the most important toxicologically because of its ability to permeate membranes and the blood–brain barrier, its potency for biologic damage, and its widespread use in human activities. Mercury toxicity provides an interesting example of several impor­ tant toxicologic principles. The first is that a single substance may produce differing effects depending on presentation to the organism. Hg0 is relatively nontoxic when ingested because of poor absorption in the gastrointestinal tract. It may be toxic, however, when injected subcutaneously. In addition, because of its high vapor pressure, it vaporizes readily and is easily inhaled. When inhaled, Hg0 is absorbed readily

OCCUPATIONAL SAFETY IN DENTISTRY Although dentistry is considered to be relatively “occupationally safe,” numerous potentially hazardous substances are used in the dental office or laboratory. In addition, dental environments may provide exposure to radiation or to blood-borne pathogens. Since 1988 the Occupational Safety and Health Administration (OSHA) has been writing, implementing, and enforcing regulations designed to ensure that employees are informed of hazardous materials in their work environment and given appropriate instruction in the risks and handling of these materials. The primary components of this program include (1) labeling of containers for materials, (2) on-site maintenance of material safety data sheets for materials used in the workplace that contain hazardous chemicals, and (3) employee education and training. For dentistry, OSHA regulations and guidelines include potential

CHAPTER 40  Toxicology

into the blood, with absorption rates estimated at 74% to nearly 100% of inhaled dose. When in the blood, it is oxidized and is available for binding to enzymes and other proteins, producing toxic effects. Another important principle exhibited by mercury is that when a substance can exist in different chemical forms, the forms may present strikingly different health effects. Organic mercury typically produces signs of toxicity that are neurologic in nature, whereas inorganic salts often produce gastrointestinal destruction and, secondarily, nephritis. These effects are discussed further subsequently. Inorganic mercury salts are used widely in industry; mercuric chloride is an example of a mercury compound with a wide variety of industrial uses. These compounds, in contrast to organic mercury compounds, are not well absorbed through the gastrointestinal tract and do not readily cross biologic membranes when absorbed. Only approximately 10% of an inorganic mercury dose is absorbed through the gastrointestinal tract compared with more than 90% of an ingested dose of the organic compound methylmercury. Nevertheless, inorganic salts such as mercuric chloride are severely corrosive to tissue and when absorbed produce toxic effects through binding of enzymes. Inorganic mercury compounds have been used medicinally and applied dermally in makeup for hundreds of years until recent times; calomel (a cathartic) and mercurochrome (an antiseptic) are common examples. Virtually all such uses have been discontinued. Organic mercury compounds represent the most important form of mercury from a toxicologic perspective. This is particularly true of methylmercury because of its widespread use and because it is a by-product of many industrial processes. Organic mercury is known to accumulate in the food chain, and this is particularly evident in seafood, where the pelagic and top-level predators accumulate significant amounts of methylmercury in their flesh. A number of tragic, inadvertent organic mercury poisonings have occurred in modern times. Two incidents are particularly well documented. From 1932 to 1968, the Chisso Corporation, a company located in Kumamoto, Japan, dumped an estimated 27 tons of mercury compounds into Minamata Bay. Kumamoto is a small town approximately 570 miles southwest of Tokyo. The town consists of mostly farmers and fisherman whose normal diet included fish from the bay. Symptoms of methylmercury poisoning unexpectedly developed in thousands of these people. The illness became known as Minamata disease. Methylmercury has also been widely used to prevent grain spoilage through its antifungal effect. The second major outbreak occurred in the early 1970s when more than 500 people died and many others were made severely ill in Iraq when grain seed treated with methylmercury was inadvertently ground into flour and consumed. In both of these instances, because organic mercury readily crosses the blood– brain and placental barriers, a significant number of fetal deaths and teratogenic results occurred. Elemental mercury is the form of concern in dentistry because it is a primary component of dental amalgam, constituting approximately 50% by volume of the material. The greatest risk of exposure from Hg0 is by inhalation of the vapor. Hg0 vapor is highly lipid-soluble and readily crosses membranes; this gives it ready access to the CNS and other body components, where it is easily oxidized to the mercuric form. Acute, high-level exposure to Hg0 vapor produces corrosive inflammation of the upper and lower respiratory tract and nephrotoxic and CNS effects. Long-term exposure to low or moderate levels of Hg0 vapor damages enzymes and structural proteins in the CNS, resulting in blockage of neuromuscular and synaptic transmission. Figure 40-2 shows the currently known range of effects based on urinary mercury concentrations. Urinary mercury concentration is considered a reasonable indicator of recent Hg0 exposure, but because mercury is

Pneumonitis 1000

Mercury in Urine (mg/g Creatinine)

610

Kidney inflammation Oral manifestations Pronounced tremor & nervous system disturbances

500 Irritability, depression, memory loss Minor tremor & nervous disturbance Early signs of altered kidney function 100

25

Male fertility effects Decreased nerve conduction response EEG changes Decreased verbal skills WHO occupational limit Preclinical neurobehavioral changes

0

FIG 40-2 Signs and symptoms of mercury toxicity relative to concentrations in urine. Blue areas correspond to the various signs and symptoms seen at that blood level. EEG, Electroencephalogram; WHO, World Health Organization.

sequestered in organ systems, urinary mercury concentration is not a true indicator of total body burden. Although the three forms of mercury (inorganic, organic, and elemental) produce differing toxicologic effects, the two major target organs of any mercury exposure are the CNS and the kidneys. Although the earliest indicators of CNS effects of mercury exposure are not always clinically evident, they are measurable with neurobehavioral testing. As exposure increases, behavioral changes may be noticed, such as irritability, memory disturbances, personality changes, drowsiness, or depression. Fine muscle tremors are noted, especially of the fingers, eyelids, and lips, and this loss of neuromuscular control increases as exposure levels increase. Renal damage in the form of tubular necrosis increases in a dose-dependent manner. Oral manifestations of mercury intoxication include hypersalivation, gingivitis, and gingival discoloration. Cases of periodontal destruction with tooth loss have been reported at high levels of exposure. Also present at high levels of exposure is a yellow-brown discoloration of the lens of the eye.

Mercury in dentistry Since the introduction of mercury amalgam into dentistry in the early 19th century, concerns about its safety have been expressed from time to time. Claims of toxic effects cover virtually the entire spectrum of disease, and a vocal “anti-amalgam” contingent currently exists. Much of the confusion regarding the potential health effects of mercury exposure from dental sources is caused by false claims and by flawed studies used to support these claims by anti-amalgam proponents. Two areas of potential concern have been the subject of more recent and ongoing studies. One is the potential occupational risk to dental personnel working with dental amalgam, and the other is to the consumer or patient who has mercury amalgam placed in the teeth as a treatment. The OSHA and the National Institute for Occupational Safety and Health (NIOSH) have recognized the need to set occupational thresholds over which exposure to mercury must not occur. NIOSH has set a threshold limit value of 50 μg of Hg0 per cubic meter of air

CHAPTER 40  Toxicology BOX 40-4  Recommended Guidelines for

Minimizing Mercury Exposure in the Dental Environment 1.  Use pre-capsulated amalgam preparations only. Reclose disposable capsules after use. 2. Do not use squeeze cloths for expressing mercury from amalgam mix. 3. Monitor office levels of Hg0 yearly or whenever contamination is suspected. 4.  Use exposure badges that sample the air for Hg0 ­concentration. 5. Provide periodic urinary mercury concentration testing for personnel. 6. If “free” mercury (rather than pre-capsulated) must be used to mix amalgam, store it away from heat in unbreakable, tightly sealed containers. 7. Store amalgam scrap in a sulfide solution (e.g., used X-ray fixer) or under water. 8. Do not touch amalgam with bare hands. 9. Use a rubber dam for restorative procedures. 10. Use a high-velocity vacuum when manipulating the amalgam and vacuum and water spray when removing old amalgam restorations. 11. In the event of a mercury spill (even a small one), use a mercury spill cleanup kit (commercially available). Do not vacuum the spill because this hastens the volatilization of the mercury into the air.

as a time-weighted average based on a 40-hour workweek, while the legally binding OSHA limit is 100 μg/m3. The World Health Organization has set a more restrictive threshold limit value of 25 μg/m3. Hg0, which is readily vaporized, can achieve concentrations of 2000 μg/m3 in a closed room. Studies of ambient air mercury concentrations in dental offices have shown that, under conditions of careless handling of mercury, the occupational threshold levels set by the above organizations can be exceeded. These high concentrations may occur after contamination through accidental spills of Hg0. Studies examining occupational exposure among dental personnel show that certain practices in dental offices—now considered outmoded—are the most significant contributors to occupational exposure. These practices include the use of squeeze cloths to express mercury from amalgam; dispensing mercury from a central supply, which leads to accidental spills; and the use of office-prepared capsules. Neurobehavioral changes have been noted in dentists exposed to mercury. Modern dental offices that have good hygiene practices with respect to mercury pose minimal risk to dental personnel, however. Excellent hygiene remains vital to preventing unnecessary mercury exposure. With regard to mercury exposure that patients receive from the placement of amalgams in the course of treatment, anecdotal claims of disease states of every sort attributed to such exposure have been reported. Although rare individuals may be sensitive to very low-level mercury exposure, little or no valid scientific evidence supports such claims in the general population. An important reason for the controversy is the reliance of some individuals on false claims and dubious studies about the dangers of dental amalgams. Two large-scale, randomized, prospective clinical trials have been completed that studied the effect of mercury exposure from dental amalgam in children. These studies each included more than 500 children who were randomly

611

assigned to dental treatment groups that would receive either amalgam or resin composites for necessary restorations in posterior teeth. The subjects were followed annually for five years in the study by Bellinger and colleagues and seven years in the study by DeRouen and colleagues. (See references.) Outcome measures included comprehensive batteries of neurobehavioral tests including IQ testing, neurologic examinations including nerve conduction velocities, and renal function tests. Although there were measurable differences in urinary mercury concentrations between the amalgam and non-amalgam treatment groups, there were no significant differences found for any of the outcome measures between groups, indicating that the level of mercury exposure from routine dental treatment with amalgam does not present an important health risk for the measured neurologic or renal outcomes. A study of 1663 adults participating in the ongoing Air Force Health Study of Vietnam-era veterans to determine possible associations between amalgam exposure and neurologic abnormalities found no association between amalgam exposure and neurologic signs or clinically evident peripheral neuropathy. The body of evidence also indicates no detectable negative effect on general health at the levels of mercury exposure produced by the presence of dental amalgam fillings except in rare cases of allergy to amalgams. (See reference, Dodes.) Following the mercury hygiene guidelines listed in Box 40-4 minimizes any exposure to patients beyond that which results from the amalgam itself.

Lead Lead has been a toxicologic problem for humans from the earliest times. It was found in early utensils and food storage and preparation vessels. It has been used extensively in plumbing, contaminating drinking water. Occupational exposures to lead occur in miners, smelters, and lead acid battery workers, but the most common chronic exposure is through diet. Perhaps the best recognized sources of lead exposure are from lead-based paint and combustion products of tetraethyl lead antiknock compound added to gasoline before the change to unleaded gasoline. Although Congress produced legislation limiting the lead concentration in paint to 0.06% in the 1970s, many older buildings still contain significant amounts of lead-based paint with very high concentrations of lead. A relatively small chip of this paint may contain 100 mg of lead. When consumed by a child, this amount exceeds the daily allowable intake by a factor of at least 30. Because lead compounds that were included in paint formulas have a sweet taste, young children have frequently consumed these paint chips. (The condition of eating unnatural foods is called pica.) Adults absorb approximately 10% of dietary lead, although children may absorb significantly larger amounts. With normal renal function, absorbed lead is primarily excreted by the kidneys. In the body, lead primarily concentrates in the hard tissues such as bone and teeth. Similar to mercury, lead produces toxic effects primarily by binding with proteins necessary for cellular function. Toxic signs exhibited at various blood levels are illustrated in Figure 40-3. One early effect of lead exposure is inhibition of the heme biosynthetic pathway. Intermediary products of heme biosynthesis called porphyrins are excreted in the urine in a characteristic pattern indicative of lead poisoning. Chronic lead poisoning, known as plumbism, produces a spectrum of effects depending on the duration and severity of exposure. A microcytic hypochromic anemia may be produced early in exposure and cause lethargy and weakness. Neurologic effects may produce restlessness, irritability, hyperactivity, and impaired intellect. Chronic low-level lead exposure can produce deficits in gross and fine motor

612

CHAPTER 40  Toxicology

Death (C) 100

Lead Blood (mg/dL)

50 40

CH3

COOH

150

Encephalopathy Frank anemia; nephropathy (C) Decreased longevity Colic (C) Decreased hemoglobin synthesis Peripheral neuropathies Infertility (men); nephropathy

30

Increased systolic blood pressure (men) Decreased vitamin D metabolism (C) Decreased hearing acuity

20

Decreased nerve conduction velocity (C)

H2C SH

HC SH

HC SH

CH3 C SH

HC SH

NH2 CH

CH2OH

COOH

COOH Succimer

Dimercaprol

Penicillamine

O

O

NaOCCH2

CH2OCNa N

N Ca

O

O

O

O

Increased erythrocyte protoporphyrin 10

Calcium disodium edetate

Developmental toxicity (C) Decreased IQ, hearing, and growth (C) Transplacental transfer (C)

0 FIG 40-3 Signs and symptoms of lead toxicity relative to concentrations in blood. Children are represented at the more sensitive end of the designated ranges. (C) denotes observations in children. (Adapted from Ellenhorn M, Schonwald S, Ordog G et al: Metals and related compounds: lead. In Cooke D, editor: Ellenhorn’s medical toxicology: diagnosis and treatment of human poisoning, ed 2, Baltimore, 1997, Williams & Wilkins.)

development and in cognitive and intellectual development. Early detection and management of lead exposure is crucial to prevent these permanent effects in children. Peripheral neuropathies may be seen and are manifested as wristdrop, footdrop, and muscular weakness. Gastrointestinal signs such as intestinal spasms may progress to severe abdominal cramping with increased or continued exposure. The greatest threat from lead poisoning is encephalopathy, which occurs more often in children. Early neurologic signs and symptoms develop as described earlier and progress to delirium, convulsions, and coma. One-fourth of patients with lead encephalopathy do not survive, and 40% of survivors are left with severe neurologic dysfunction. Lead is toxic to the kidney, and reversible tubular damage and irreversible interstitial fibrosis may be seen. Long-term exposure to lead is classically associated with a blue-black line that appears along the gingival margin. This deposit of lead sulfide is known as Burton lines, and although associated with lead exposure, it may also be caused by exposures to other metals, such as silver, iron, or mercury. For treatment, removal of the subject from the source of lead exposure is paramount. Depending on the blood lead levels, chelation therapy is instituted according to protocols for the treatment of lead poisoning recommended by the U.S. Centers for Disease Control and Prevention and the American Academy of Pediatrics. Succimer, calcium disodium edetate, dimercaprol, and penicillamine all are effective, but they differ in advantages of routes of administration and specificities relative to other essential trace metals.

Treatment of poisoning: heavy metal chelators Chelators are compounds that form complexes with metal ions. The word chelator is derived from the Greek word chele, meaning “claw.” A chelator molecule binds a metal ion by two or more polar functions, such as sulfhydryl, carbonyl, amino, or hydroxyl groups. These form

OH H2N(CH2)5N

O

OH N

O

H

(CH2)5N

O N

O

OH (CH2)5N

H

CH3

O

Deferoxamine

FIG 40-4  Chemical structures of chelating agents.

bonds similar to the bonds of the protein functional units attacked by metal ions. Through this action, chelators spare endogenous ligands and promote excretion of metals as the chelator-metal complexes. Dimercaprol, succimer, and penicillamine are drugs currently marketed to promote the excretion of mercury, lead, and other metals. A few additional agents are available to treat poisoning by metals other than mercury, such as calcium disodium edetate for lead and cadmium and deferoxamine for iron. Structures of these chelators are shown in Figure 40-4. Selectivity for metal ions varies among chelators. Some, such as edetate, also aggressively remove vital nutrient metals, such as calcium and zinc. Selectivity is important in the choice of the chelator, which should be matched for the heavy metal and circumstances of therapy. Selectivity of chelators for specific heavy metals is presented in Table 40-1. Dimercaprol (2,3-dimercapto-1-propanol) was developed during World War II as an antidote for the arsenical gas lewisite, and it was formerly known as British Anti-Lewisite (BAL). Subsequently, dimercaprol was found to be an active chelator of various heavy metals. Dimercaprol is prepared as a 10% solution in a peanut oil vehicle (beware of peanut allergy!) and must be injected intramuscularly. It is maximally effective when given shortly after an acute exposure to mercury; however, it is valuable even in chronic mercurialism. Dimercaprol is used with calcium disodium edetate in protocols for treatment of lead poisoning. The drug is usually injected two to three times a day initially, with doses tapering off to once or twice a day over about 10 days. The dimercaprol–mercury complex (actually two dimercaprol molecules to a single mercury atom) is excreted in the urine, which must be kept alkaline to avoid dissociation of the conjugate. Succimer (meso-2,3-dimercaptosuccinic acid) is structurally similar to dimercaprol. This drug has the advantage of being effective after oral administration and being less toxic than dimercaprol. Succimer is more water-soluble and is the drug of choice for the treatment of lead poisoning because it is more specific for lead chelation than calcium disodium

CHAPTER 40  Toxicology

613

TABLE 40-1  Metals and Chelators That Enhance Excretion Metal

Chelator

Other Names

Administration

Arsenic

Succimer; dimercaprol

Oral; IM

Cadmium Copper Iron Lead

CaNa2 EDTA p-Penicillamine Deferoxamine Succimer; dimercaprol + CaNa2 EDTA; d-penicillamine Succimer; dimercaprol; penicillamine

Dimercaptosuccinic acid, DMSA; 2,3-dimercapto-1-propanol, BAL Calcium disodium edetate 3-Mercapto-d-valine

Mercury

IV infusion Oral IM Oral; IM + IV infusion; oral Oral; IM; oral

IM, Intramuscular; IV, intravenous.

edetate and removes fewer essential minerals such as calcium, copper, iron, and zinc. The dose for lead chelation is 10 mg/kg every 8 hours for 5 days, then 10 mg/kg every 12 hours for 14 days. In animal studies, succimer was more effective than dimercaprol in alleviating acute toxicity and preventing distribution of orally administered mercury from mercuric chloride, particularly to the brain. In addition, oral administration was more efficient than parenteral administration in reducing retention and organ deposition of oral mercuric chloride, probably because of decreased intestinal uptake of the mercuric chloride. Penicillamine (3-mercapto-d-valine) is a highly effective chelator of copper and is of primary importance in the management of Wilson disease (hepatolenticular degeneration). Although less effective against other metals, penicillamine is often a useful drug for asymptomatic patients with a moderate body burden of metal because it is orally effective. In general, 1 to 2 g/day is administered as needed for therapy of mercury poisoning. The penicillamine–mercury complex is excreted in the urine. Calcium disodium edetate complex is a chelator for divalent and trivalent metals that can displace calcium from the molecule. Typically, these metals include lead, zinc, cadmium, manganese, iron, and mercury. Calcium edetate disodium is poorly absorbed from the gastrointestinal tract and is given intramuscularly or intravenously. Calcium disodium edetate must be used carefully according to suppliers’ protocols because it can produce nephrotoxicity. Calcium disodium edetate can aggravate symptoms of severe lead poisoning, such as cerebral edema and renal tubular necrosis, and in high doses can lead to severe zinc deficiency. Deferoxamine is a specific chelating agent for iron. It is available only for parenteral administration. The preferred route is intramuscular; acute iron intoxication treatment involves 1 g as an initial dose, followed by 500 mg every 4 hours for two doses and additional doses of 500 mg every 4 to 12 hours as needed based on clinical response.

TABLE 40-2  Carboxyhemoglobin Blood

Levels and Symptoms CO-Hb Level (%)

Symptoms

0-10 10-20 20-30

No symptoms Mild headache and breathlessness Throbbing headache, irritability, emotional instability, impaired judgment and memory, rapid fatigue Severe headache, weakness, nausea, vomiting, dizziness, dimmed vision, confusion Increasing confusion, severe ataxia, accelerated respiration, possible hallucinations Syncope, coma, convulsions, tachycardia with weak pulse Increased depth of coma, incontinence Profound coma, thread pulse, death Rapid death

30-40 40-50 50-60 60-70 70-80 >80

Source: Von Burg R: Carbon monoxide, J Appl Toxicol 16:379-386, 1999.

general information on topics important in the control of air pollution is available from the Internet. The U.S. Environmental Protection Agency (EPA) uses six “criteria pollutants” as indicators of air quality and has established a maximum concentration for each to preclude adverse effects on human health. The four gaseous criteria pollutants are discussed subsequently; the remaining two are airborne lead and fine particulate material that is 10 μm or smaller in diameter.

Carbon monoxide

Therapy depends on the type of mercury poisoning. Exposure to elemental or inorganic mercury can be treated with dimercaprol (higher mercury levels) or penicillamine (lower mercury levels). Hemodialysis may be needed to protect the kidney. Succimer is also effective. For short-chain organic mercurials such as methylmercury, chelation therapy is ineffective, and dimercaprol is contraindicated because it concentrates mercury in the brain. Hemodialysis is ineffective. Methylmercury can possibly be bound in the gut with a polythiol resin.

The origin of carbon monoxide (CO), a colorless, odorless gas, is incomplete combustion of carbon. The toxicity of CO results from its combination with hemoglobin (Hb) and exclusion of oxygen from this vital oxygen transfer mechanism. CO exhibits an affinity for Hb 210 to 300 times that of oxygen, and the resultant complex with reduced heme iron, carboxyhemoglobin (CO-Hb), is incapable of combining with oxygen. Moreover, the presence of CO-Hb makes the release of oxygen from Hb in tissues less efficient, which additionally compromises oxygen delivery. Typical symptoms associated with varying CO-Hb levels are presented in Table 40-2. Of the four gaseous criteria pollutants, CO is the most likely to be present in the dental office or home environment, and offices should be equipped with CO detectors.

Gases

Ozone

Perhaps no other toxic pollution issue stirs such universal concern as air pollution because gaseous pollutants are dispersed over broad regions, and inhalation exposure is insidious. Significant regulatory effort is devoted to decreasing air pollutants by the Clean Air Act, and

Ozone (O3) is an odorless, colorless gas composed of three oxygen atoms. Typically, O3 is not emitted into the air, but it is created at ground level by photochemical reactions among nitrogen oxides and volatile organic compounds in the presence of heat and sunlight. O3

Treatment of mercury poisoning

614

CHAPTER 40  Toxicology

occurs naturally in the stratosphere (approximately 10 to 20 miles above the earth) and forms a protective barrier that absorbs the sun’s harmful ultraviolet rays. In the earth’s lower atmosphere and at ground level, O3 is considered unhealthy because of its oxidative effects. Because of its relative insolubility, inspired O3 is carried deep into the lung, where it oxidizes membranes in the alveoli. O3 irritates lung airways and causes inflammation, reduced lung capacity, and increased susceptibility to respiratory illnesses such as pneumonia and bronchitis. Other symptoms include wheezing, coughing, and pain with deep breathing. Oxidation products arising from O3 reactions with lung proteins or lipids initiate numerous cellular responses, including generation of cytokines and expression of adhesion molecules. These responses promote an influx of inflammatory cells to the lung in the absence of a pathogenic challenge, resulting in modification of cellular tight junctions, increased lung permeability, and development of edema. Individuals with preexisting respiratory problems, such as asthma or chronic obstructive pulmonary disease, are most vulnerable. Repeated exposure to O3 pollution for several months may cause permanent lung damage.

Cl H

Cl

C

Cl

Cl

C

Cl

H

Cl

Dichloromethane

Carbon tetrachloride

O

O

CH2 CH3

C

C

OCH3

O Methyl Benzoquinone methacrylate FIG 40-5 Chemical structures of chlorinated solvents, the benzene metabolite benzoquinone, and the acrylic plastic monomer methyl methacrylate.

Sulfur dioxide Sulfur dioxide (SO2) is a colorless gas with a pungent, irritating odor. SO2 is used as a preservative of fruits and vegetables, a disinfectant in wineries and breweries, and a bleaching agent in paper and textile industries. It is generated as an air pollutant by industry, such as high-sulfur coal-fired electric power plants, and it is largely responsible for the environmental and public health impact of acid rain. In contrast to the properties and site of impact of O3, SO2 is highly soluble in aqueous fluids and affects the upper respiratory tract. On dissolution, it forms sulfurous acid, which is extremely irritating to the nasopharyngeal and respiratory tracts. Acute exposure causes dryness of the nose and throat and a decrease in tidal respiratory volume. Coughing, sneezing, choking, and nasal discharge occur. In dentistry, chronic exposure at SO2 levels causing these symptoms has been associated with dental caries and gingival and periodontal disorders. Patients have noted rapid dental destruction, loss of restorations, and increased sensitivity of teeth to temperature change.

Exposure most often occurs through inhalation; absorption through the skin is also a common route of exposure. Absorption from the gastrointestinal tract is variable. Compounds that are well absorbed, such as benzene or toluene, can produce significant systemic toxicity. Others, such as naphtha or gasoline, are not as well absorbed. A major risk from ingestion is the potential for pneumonitis as a result of emesis and aspiration. Regardless of the site of absorption, the great lipid solubility of this group of compounds allows them to cross the blood–brain barrier readily. Individuals exposed to high concentrations of organic solvents usually exhibit profound CNS depression. Chronic exposure to lower concentrations of these chemicals produces toxic effects characteristic of the individual compounds.

Chlorinated solvents

The organic liquid that presents the greatest risks to humans is ethanol. The toxicologic profile of this compound is unique among organic liquids and is presented in detail in ­Chapter  39. Considered in this section are the organic solvents, including hydrocarbons and chlorinated compounds, and methyl methacrylate (because of its common use in dentistry). Figure 40-5 shows structures of some of the compounds discussed below.

Dichloromethane, otherwise known as methylene chloride, is a common solvent in paint remover and is used for liquid–liquid extraction in laboratories. Acute toxicity is caused by CNS depression, and fatalities have resulted from exposure. Symptoms include mental confusion, fatigue, lethargy, headache, and chest pain. Dichloromethane is metabolized to carbon monoxide. Evidence of its carcinogenicity, obtained in mice, seems to be related to toxic metabolites formed by glutathione-S-transferase and may be specific to the very high activity and localization of this enzyme in this species. Carbon tetrachloride is metabolized in the liver to a highly reactive free radical metabolite that, in the presence of oxygen, reacts with proteins and lipids. The resulting hepatotoxicity may take days to develop and is accompanied by severe renal toxicity. Compounds that increase the rate of carbon tetrachloride biotransformation, such as cytochrome P450 enzyme inducers, increase the danger of toxicity. Substances that inhibit its metabolism are protective. In a similar manner, perchloroethylene (also known as tetrachloroethylene) has been found to produce reactive metabolites that are thought to produce renal toxicity. This compound has also been associated with an increased risk of oral, laryngeal, and esophageal cancer in workers occupationally exposed to dry-cleaning processes that use perchloroethylene. Its use has declined due to these adverse effects.

Solvents

Benzene

Although transient exposure to solvents may occur in the home, more significant exposure most commonly occurs in the workplace.

Benzene is another widely used industrial solvent commonly encountered in petroleum distillates such as gasoline. Benzene is considered

Nitrogen oxides Nitrogen dioxide (NO2) is a brownish, highly reactive gas that is present in all urban atmospheres. The major mechanism for the formation of NO2 in the atmosphere is the oxidation of the primary air pollutant nitric oxide (NO). Mixtures of nitrogen oxides (NOx) play a major role, together with volatile organic hydrocarbons, in complex atmospheric reactions that produce O3 and are important precursors to acid rain. NO2 is relatively insoluble in aqueous media and decomposes in water to form nitric acid (HNO3) and NO, a potent vasodilator. When inspired, it reaches deep into the lungs. NO2 can cause bronchitis, pneumonia, hemorrhagic pulmonary edema, and diffuse alveolar damage. Exposure also seems to reduce resistance to respiratory infections.

Liquids and Vapors

CHAPTER 40  Toxicology a causative agent in human leukemia, probably through active hydroquinone or benzoquinone metabolites formed at oxidation.

Methyl methacrylate Methyl methacrylate is widely used in dentistry for the production of prosthetic devices and in orthopedic medicine as a luting agent. Although properly cured polymers from methyl methacrylate seem to be biologically inert, numerous adverse effects have been associated with the monomer. Exposure to the monomer can lead to toxicity and allergic reactions. A slight, transient decrease in blood pressure has occasionally been reported when methyl methacrylate was used to cement orthopedic devices. The assumption in these cases was that the effects were caused by absorption of the monomer into the patient’s vasculature. Adverse effects have also been reported by personnel in operating rooms, where, because of improper mixing, concentrations of more than 200 ppm have been measured. Surgeons have developed contact dermatitis and paresthesias, and nurses have reported dizziness, nausea, and vomiting. A survey of dental laboratories suggests exposure to more moderate concentrations (≤5 ppm) of the monomer, although peak concentrations can be double that amount. Although the concentrations to which dental technicians are exposed are moderate, a study of dental technicians suggested that cutaneous absorption of the monomer, a result of dipping the fingers in the liquid to smooth and improve the finish of the polymer surface, caused a localized slowing of nerve conduction. Other studies have found more generalized neuropathies attributed to methyl methacrylate exposure in dental technicians. In addition, cutaneous reactions have been reported from monomer and “cured” methacrylate polymer. Numerous studies have confirmed more recently that dental resins and composites release methacrylates and many similar components that are known to have the potential for endocrine-disrupting effects. The term endocrine-disrupting refers to alterations in the natural biosynthesis, metabolism, or receptor occupancy of hormones such as estrogen and testosterone. Many of these endocrine-disrupting compounds found in dental filling materials are uncured monomers, such as bisphenol A (BPA) and bisphenol A glycidylmethacrylate (BisGMA), and chemically related compounds such as phthalates. BPA, which is present in many food-use containers, is also present in many dental sealants and restorative materials, and it has been shown to be detectable from these sources in children. Although little is definitively known about the effects of low-dose exposure of these components of dental composites and sealants, some evidence exists of the potential for detectable effects on human metabolic systems. Much of this evidence is from animal model and human in vitro studies, and little or no research has been reported that uses in vivo studies to examine these potential effects. Potential systemic effects on the organism of endocrine disruption are wide-ranging and biologically important. These may include developmental defects, behavioral effects, fertility problems, and tumorigenic effects. There have been no safety studies or randomized clinical trials in humans to examine the potential effects of low-dose exposures to these substances, such as one might get from dental sources, but available data suggest that such exposures produce minimal effects, if any. Two additional precautions can be used with sealants and composites in dentistry: removal of residual monomer during sealant/composite placement, and minimizing elective use of these materials during pregnancy. (See www.ada.org.)

Pesticides Pesticides play a unique societal role as these products are designed and produced for their toxic effects. Much research has been devoted to develop the concept of selective toxicity, in which products have toxic

615

actions on pests, while affording advantage of less toxicity to other species. Selective toxicity can be derived from differential metabolism between target and nontarget species or can be due to entirely different physiologic receptors that mediate toxicity in target organisms. The diversity of target receptors is detailed in an excellent comprehensive, short review detailing primary mechanisms of toxic actions of pesticides including insecticides, herbicides, and fungicides. (See reference, Casida, 2009.) In the United States, pesticides are regulated by the EPA under auspices of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). Before a pesticide can be legally used, it must be registered with the EPA Office of Pesticide Programs (OPP). Pesticide registration typically extends for 15 years. By this process the EPA examines the ingredients of a pesticide; the site or crop on which it is to be used; the amount, timing, and frequency of its use; and storage and disposal practices. The EPA OPP evaluates each pesticide to ensure no adverse effects on humans, nontarget species, or the environment under specified use before initial registration. Older, previously registered pesticides undergo re-registration to assess health effects as new information becomes available. Depending on the toxicity of the marketed product, pesticides are registered for general public use or are restricted for use only by or under the direct supervision of a certified pesticide applicator. Pesticides are restricted from residential or institutional use if the product, as diluted for use, has an oral LD50 of 1.5 g/kg or less and restricted for other uses if the product, as diluted for use, has an oral LD50 of 50 mg/kg or less. Certain pesticides have been banned or severely restricted for export or import through the auspices of the United Nations Environment Programme and the Food and Agriculture Organization, which developed international guidelines for exchange of information on banned or severely restricted industrial chemicals and pesticides. These guidelines eventually evolved into the United Nations Rotterdam Convention on the Prior Informed Consent Procedure, which lists banned and restricted pesticides. The Food Quality Protection Act of 1996 amended FIFRA to require evaluation of pesticide safety with consideration of potential aggregate exposures from nondietary and dietary routes. From this mandate, pesticide registrations are being revised. The current status of pesticides is available electronically via a Website that the EPA OPP maintains with extensive information regarding pesticide use, regulation, data sources, consumer alerts, and educational materials. The EPA OPP also has supported production of a manual, Recognition and Management of Pesticide Poisonings, available electronically, that is designed to provide health professionals with current information regarding health hazards of pesticides. (See reference, Roberts and Reigart, 2015.)

Insecticides Most insecticides in common use by the public today fall into three classes based on their mode of toxic action: neonicotinoids, which act as ligands for the insect nicotinic receptors; anticholinesterases, characterized by their inhibitory action on acetylcholinesterase; and pyrethroid insecticides, so named after their origin as pyrethrum extract from flowers of the genus Chrysanthemum. Organochlorine insecticides, such as DDT, were widely used from 1945 to 1969, but they have been banned for use in the United States because of their adverse effects, including their biologic and environmental persistence, biomagnification through diet in lipid tissues of higher organisms, demonstrated interaction with estrogen receptors, and enzyme-inducing properties. Neonicotinoids, the newest class insecticides developed during the past 25 years, (structure presented in Figure 40-6). Of these, imidacloprid is most widely used at present. These compounds exhibit selective

616

CHAPTER 40  Toxicology N N Cl N

C2H5O

O–

S

C2H5O

P

HN N+ O

O

P

NO2

Parathion H3C

O

CH3

O

CH3

H3C

H3C O O

S

Cl

CH3 CH3

C2H5O

CH3

N

P

N

O

CH3

Malathion

Diazinon

O

H N

toxicity through greater affinity for insect nicotinic receptors than those of mammals. Neonicotinoid insecticides have been described as partial agonists, super-agonists, or antagonists at nicotinic receptors, depending on the compound. As a result, they disrupt receptor function and cause paralysis and death. Although these compounds offer selectivity through differential affinity, this is not absolute, and mammalian toxicity ensues at higher concentrations. Acute human poisonings with neonicotinoids are increasing with increased use of this class of insecticides. The majority of case reports of neonicotinoid poisoning to date indicate oral ingestion in suicidal attempts, rather than inhalation or dermal exposure, as normal protective garments prevent applicator exposure. Syndromes of poisoning are typical of hyper-cholinergic stimulation at nicotinic receptors. CNS effects include dizziness, disorientation, and coma. Initially the autonomic nervous system is stimulated, and diaphoresis, tachycardia, and elevation of blood pressure occur. As the nervous system fails with exhaustive overstimulation, dyspnea or apnea, mydriasis, bradycardia, hypotension, and coma occur in severe poisoning cases. Neonicotinoids currently are under increasing regulatory scrutiny due to their toxicity to pollinating insects such as honey bees. Although the role of neonicotinoids in recent bee colony collapse syndrome is not yet clear, it is apparent that these insecticides are extremely toxic to these beneficial insects and must be carefully applied to prevent adverse environmental impact. The anticholinesterase insecticides are organophosphate or methylcarbamate esters. Representative structures are shown in Figure 40-7. The mechanism of action of anticholinesterase drugs is described in greater detail in Chapter 6. These compounds inhibit the hydrolytic action of the neurologically essential enzyme system, acetylcholinesterase, which is transiently acetylated during normal hydrolysis of acetylcholine. Anticholinesterases interact with the enzyme in a manner similar to the endogenous substrate, but with hydrolytic turnover numbers several orders of magnitude smaller than those of the natural substrate, acetylcholine. This interaction leaves the enzyme phosphorylated or carbamylated and incapable of physiologic function. Poisoning causes an overwhelming abundance of acetylcholine at cholinergic receptors in synapses of autonomic nerves, neuromuscular junctions, in the adrenal medulla, and in the CNS. As hydrolysis of acetylcholine is the controlling step in termination of synaptic cholinergic transmission, inactivation of acetylcholine is neurotoxic particularly in the autonomic nervous system of the CNS and the neuromuscular junctions of the somatic nervous system.

S

C2H5O

O

Permethrin (a Pyrethroid) FIG 40-6  Chemical structures of the neonicotinoid insecticides.

NO2

Paraoxon

O

O

O P S

Cl

O

C2H5O

C2H5O

Imidacloprid (a Neonicotinoid)

O

O

C

O

O

H

H3C

N H3C Carbaryl

C

CH3 O

N

CH

C

S

CH3

CH3 Aldicarb

FIG 40-7 Chemical structures of organophosphate and methylcarbamate anticholinesterase insecticides.

As many as 100 organophosphate-class insecticides have been used in the United States. Many are analogues of phosphorothioic acid and are relatively poor anticholinesterases. These are activated preferentially in insects to phosphate homologues by oxidative mechanisms. A classic example of differences in toxicity of thio versus oxo organophosphate homologues is exhibited by parathion versus paraoxon. Substitution of the sulfur atom with oxygen results in a tenfold increase in mammalian toxicity. The venerable insecticide malathion has been used widely for nonagricultural applications, and it is the active ingredient in Ovide lotion used topically to treat head lice. Other organophosphates such as chlorpyrifos, diazinon, and terbufos are now restricted to use only by certified applicators. Of some 20 methylcarbamates in use, carbaryl has been used most widely in home and garden applications. It is relatively nontoxic to mammals but is highly toxic to honeybees. In contrast, aldicarb, a methylcarbamate designed with molecular dimensions based on acetylcholine, is some 200 times more toxic to mammals. Aldicarb is available only to certified applicators; it is applied to the soil and taken up for systemic action in plants. Treatment of acute anticholinesterase poisoning by either organophosphates or methylcarbamates involves liberal use of anticholinergic drugs, particularly atropine, to antagonize muscarinic cholinergic signs. Pralidoxime has been used successfully to reverse cholinesterase inhibition when used early in treatment of cases of organophosphate poisoning, but it may aggravate poisoning with methylcarbamate insecticides. The pyrethroids consist of a group of natural or synthetic compounds that modify properties of ion channels in nerves. Pyrethroids maintain Na+ channels in the open state for prolonged periods, leading to hyperexcitation of the nervous system. These compounds elicit repetitive activity, particularly in sensory nerves, along with membrane depolarization, enhanced neurotransmitter release, and eventual block of excitation. These actions occur as a consequence of prolongation of

617

CHAPTER 40  Toxicology H3C

CH3

Cl

CCl3

O

Cl

Cl C

CH

Cl

Cl

COOCH2

Cl

Cl

Cl

Cl

p,p-DDT

Cl

Lindane

Permethrin

H3C CH3 O

Cl

Cl C Cl

CH

COOCH C

Cl

N

Cl Cl

Cl Cl Cl

Cl

cides.

Na+ ion current in voltage-dependent Na+ channels. The pyrethroids have remarkably selective toxicity for insects relative to mammals. They are largely contact insecticides with rapid “knock-down” properties. Natural pyrethroids (pyrethrin I, pyrethrin II) are short-lived as a consequence of rapid oxidation and photodegradation in the environment and are rapidly hydrolyzed or oxidized when taken orally. These properties have resulted in rapid acceptance with minimal risk from use, but disadvantages are short duration of action and expense of natural product isolation. Synthetic pyrethroids have been designed to be more persistent. These include two types, determined by the presence or absence of a cyano function; two examples are shown in Figure 40-8. Of these, permethrin is stable to light and has low toxicity in adult mammals, but it is more toxic to neonates with undeveloped hydrolytic and oxidative mechanisms. Other synthetic analogues substituted with a cyano group are more toxic. Occupational exposure to pyrethroid insecticides leads to temporary paresthesia and respiratory irritation. Treatment is generally supportive. Organochlorine insecticides, previously used extensively, are now only of historic importance in the United States, but some are still used in other regions of the world because of their low cost, stability, and efficacy. Figure 40-9 shows structures of some of these organochlorine insecticides. The Nobel Prize for Physiology and Medicine in 1948 was awarded to Paul Mueller, who discovered the insecticidal properties of DDT, the organochlorine prototype. DDT is a member of the dichlorodiphenylethane subclass of organochlorine insecticides, but it is now restricted under the UN PIC (Prior Informed Consent) procedure. The chlorinated cyclodiene structure subclass includes chlordane, dieldrin, and heptachlor, which also are restricted under the UN PIC procedure. Chlordecone and mirex represent another unique subgroup of cage-like, highly-chlorinated C10 structures that are restricted from use. The hexachlorocyclohexane-type compounds include lindane, a specific insecticidal isomer that is still used in lindane lotion as an ectoparasiticide and ovicide for crab and head lice. The toxic actions of organochlorines, similar to pyrethroids, alter conduction in the ion channels of nerves. DDT alters Na+ and K+ ion permeability, Na+, K+-ATPase and Ca++-dependent ATPase functions, and inhibition of calmodulin in nerves. These actions reduce the rate of nerve membrane repolarization and increase sensitivity to small stimuli. The chlorinated cyclodienes are different because their actions seem to be more localized within the CNS. These compounds inhibit Na+, K+-ATPase and Ca++-dependent ATPase and act as γ-aminobutyric acid antagonists, eliciting uncontrolled neurotoxic excitation.

Cl

O

Cl

Cl

Cl

Cl

Cl Cl

Cypermethrin Chlordane

FIG 40-8  Chemical structures of several synthetic pyrethroid insecti-

Cl

Cl Cl

Cl

Cl

Cl

Cl Cl

Cl

Cl Cl

Chlordecone

Cl

Cl Mirex

FIG 40-9  Chemical structures of several organochlorine insecticides.

N

(CH3)2CHNH N

Cl N

Cl OCH2COOH

Cl

NHCH2CH3 Atrazine

2,4-dichlorophenoxyacetic acid

OH HO

CH2NHCH2COOH H3C

P

+

+

N

N

CH3

O Glyphosate

Paraquat

FIG 40-10  Chemical structures of various herbicides.

Herbicides Herbicides are the most widely used type of pesticides. Given the broad use of these pesticides with apparently low relative risk in normal use, some herbicides in common use are presented, with selection based on high usage or significant toxicity where evident. Research efforts by crop scientists in recent decades have produced diverse structures, many of which offer selective toxicity against weeds, while sparing economic crops. An example is the use of herbicides in “no-till” production of grains, in which fields are sprayed to kill grasses, and seeds are planted without the need for plowing fields. Structures of some herbicides are illustrated in Figure 40-10. Atrazine is a member of the class of chemically similar compounds known as the triazine herbicides that block photosynthesis in plants. Atrazine is one of the most widely used agricultural pesticides in the United States, as approximately 80 million pounds of the atrazine active ingredient are applied annually to control broadleaf weeds in field corn and sorghum, in lawns and turf, and after production of wheat. Epidemiologic studies of workers exposed in chemical plants and farming populations have not shown a significant incidence of disease related to atrazine use, and little acute toxicity is evident in suicide attempts with atrazine. Atrazine is undergoing review, however, for re-registration by the EPA Health Effects Division. This decision for re-review was based on the high volume of use, persistence of atrazine in surface and ground water,

618

CHAPTER 40  Toxicology

and more recent research indicating that atrazine diminished secretion of hypothalamic gonadotropin-releasing hormone in rats. Previous work had indicated that atrazine given by gavage in high doses altered luteinizing hormone and prolactin serum levels in two strains of female rats by altering the hypothalamic control of these hormones. Subsequent studies at more relevant concentrations in amphibians did not find that atrazine adversely affected amphibian gonadal development. Glyphosate has broad-spectrum herbicidal activity, sometimes called “total kill,” against a wide range of weeds. Glyphosate kills plants by inhibiting an essential plant enzyme involved in biosynthesis of aromatic amino acids, which is absent in nonplant life forms. As a result, under normal use, glyphosate is practically nontoxic to mammals, aquatic organisms, and avian species. Irritation of the oral mucous membrane and gastrointestinal tract was frequently reported with ingestion of the concentrate. Other effects recorded were pulmonary dysfunction, oliguria, metabolic acidosis, hypotension, leukocytosis, and fever. Various reviews, which indicated absence of toxicity in longterm animal studies of glyphosate, have been summarized. Chlorophenoxy compounds, typified by 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-(2-methyl-4-chlorophenoxy)propionic acid, are used to control broadleaf weeds. They act as stimulants of uncontrolled, and unsustainable, growth in plants by mimicking and disrupting the actions of plant growth regulators such as indole acetic acid, and leading to plant death. In animals 2,4-D exhibits various mechanisms of toxicity, including uncoupling of oxidative phosphorylation, damage to cell membranes, and disruption of acetyl coenzyme A metabolism. Ingestion of large doses can cause nausea, gastrointestinal hemorrhage, hypotension, muscular twitching

and stiffness, metabolic acidosis, and renal failure. Significant dermal exposure and occupational inhalation are associated with progressive sensory and motor peripheral neuropathy. Nitrophenolic compounds formerly used as herbicides, such as dinitroocresol and dinitrophenol, are highly toxic to humans and animals. These stimulate energy metabolism in mitochondria by uncoupling cellular oxidative phosphorylation; this leads to hyperthermia, causing profuse sweating, fever, thirst, and tachycardia. Because of this toxicity, the registrations for herbicidal uses of dinitroocresol and dinitrophenol and similar compounds have been canceled. In contrast, certain dinitroaminobenzene herbicides, including butralin, oryzalin, and pendimethalin, and fluorodinitrotoluidine derivatives, such as benfluralin, dinitramine, fluchloralin, and trifluralin, do not uncouple oxidative phosphorylation or generate methemoglobinemia. These herbicides inhibit cell division in plants. Paraquat is the most important dipyridyl herbicide for toxicologic consideration because it has delayed, severe, and specific pulmonary toxicity. Paraquat exhibits its particular and unique toxicity in part because of its selective accumulation in lung tissue by a diamine transport system located in the alveolar epithelium. In addition, paraquat is involved in a single-electron cyclic reduction–oxidation reaction that attacks unsaturated lipids in membranes to form lipid peroxides. The oxidative destruction and subsequent fibrotic lesions developed during reparative processes lead to severely diminished lung function, anoxia, and death days after ingestion of paraquat. The comprehensive treatise by Ellenhorn and associates (See references) presents pharmacokinetic plots indicating likely survival or death based on blood concentrations versus time after ingestion of paraquat.

Hemoglobin CN

 Hydroxocobalamin

FeO2

NaNO2

Cyanocobalamin

Cytochrome oxidase

Methemoglobin Na2S2O3

FeCN

FeCN

SCN

FIG 40-11  Treatment of cyanide poisoning. Cyanide (CN−), whether inhaled or ingested, combines with ferric ions (Fe+++) in cytochrome oxidase to inhibit cellular respiration. Therapy is aimed at eliminating cyanide from the cells by a two-step process: (1) sodium nitrite (NaNO2) is administered intravenously to oxidize the iron in hemoglobin from the ferrous (Fe++) to the ferric state; the methemoglobin that is formed competes for cyanide, freeing cytochrome oxidase from attack by cyanide. (2) Cyanide is inactivated by the administration of sodium thiosulfate (Na2S2O3) to yield thiocyanate (SCN−), which is readily excreted in the urine. Cyanide can also be removed by the use of hydroxocobalamin as shown. Experimentally, these steps reduce the lethal potency of cyanide by 80%.

CHAPTER 40  Toxicology Cyanide Sodium cyanide, which liberates hydrogen cyanide, is occasionally used against predatory animals. this is the only current registered use for sodium cyanide as a pesticide. This use is controversial. Because of its extreme toxicity, sodium cyanide is restricted to use only by trained applicators. Cyanide inactivates cellular oxidative phosphorylation by binding to the Fe+++ in the cytochrome oxidase. The inability of cells to use oxygen, particularly in the brain and heart, is rapidly lethal to warm-blooded animals. Therapy for poisoning involves treatment with 100% oxygen and rapid provision of an alternative, less critical source of Fe+++ for cyanide binding. This is accomplished through induction of methemoglobinemia by administering amyl nitrite or sodium nitrite. Treatment with sodium thiosulfate solution follows to assist conversion of cyanide to thiocyanate by the mitochondrial enzyme rhodanese (Fig. 40-11). A newer treatment of cyanide poisoning involves the use of hydroxocobalamin to form cyanocobalamin and reduce cyanide complexed with the cytochrome complex. The cyanocobolamin is excreted in the urine.

Rodenticides Various compounds have been used to attack rodents. Some are quite toxic to rodents, humans, and wildlife through acute exposure, whereas others require multiple doses to elicit significant toxicity. Most of these act as anticoagulants, and their structures are shown in Figure 40-12.

O

O

CH3

OH

O

O

O

Diphacinone

Warfarin

OH

O

O Br Brodifacoum

OH

O

The oldest, warfarin, is a coumarin derivative that has been used as a rodenticide since 1950. Warfarin derives its action through antagonism of vitamin K action as a cofactor in synthesis of coagulation factors (see Chapter 26). Resistant strains of rodents have emerged, which has led to development of new hydroxycoumarin derivatives (so-called “superwarfarins”) that are much more potent and do not require repeated doses to kill. Brodifacoum and bromadiolone are characterized as single dose in use. Necropsies after poisonings support the diagnosis of coagulopathy with findings of hemoperitoneum, hemothorax, and pulmonary hemorrhage. Because of the increased potency and increased duration of action in some of these newer rodenticides, poisoning has occurred in pets, wildlife, and exposed humans. Treatment is based on assessment of prothrombin time, which should be monitored at 24 and 48 hours after ingestion. If prothrombin time is elevated at these times, treatment with phytonadione (phylloquinone, vitamin K1) should be instituted with continued assessment of prothrombin time over 4 to 5 days.



REPRESENTATIVE CHEMICAL TOXICANTS Metals Lead Mercury Gases Carbon monoxide Ozone

O

OH

O Br Bromadiolone

FIG 40-12  Chemical structures of anticoagulant rodenticides.

619

Liquids and solvents Benzene Carbon tetrachloride Dichloromethane Methyl methacrylate Pesticides Anticholinesterases  Carbaryl  Diazinon  Malathion Chlorinated hydrocarbons  Chlordane  DDT  Lindane Neonicotinoids  Acetamiprid  Imidacloprid Pyrethroids  Cypermethrin  Permethrin Herbicides   2,4-Dichlorophenoxyacetic acid (2,4-D)  Glyphosate  Atrazine  Paraquat Rodenticide  Warfarin

620

CHAPTER 40  Toxicology

CASE DISCUSSION This case of three dentists of the same age, gender, health status, and occupational exposure to Hg allows several principles of toxicology to be examined. One is that of exposures. The primary source of Hg exposure in a dental office is via absorption through the lungs. Elemental Hg, which is the form used in dental offices, whether pre-capsulated or mixed on-site, produces Hg vapor which is readily absorbed through the lungs. All three dentists in this case use approximately the same amount of amalgam in their practice, all use the same pre-capsulated brand, and all are exposed to the same ambient levels of Hg vapor since they all work the same number of hours. The degree to which each dentist scavenged mercury vapor during operative procedures was presumed to be similar although this is another relevant variable. Because elemental Hg is absorbed so readily through the lungs, control of Hg vapor in the dental environment is important. A second toxicologic principle is that of consideration of multiple exposures and how one exposure might influence another. In this case, one of the dentists, abstains from alcohol. The other two each have, on average, two drinks of alcohol per day. As demonstrated in a study (Martin and Naleway, 2004), the amount of ethanol consumed by dentists was inversely related to urinary Hg concentrations. The biologic and biochemical basis for this relationship demonstrates another principle of toxicology, that “route of absorption” is crucial. Elemental Hg is somewhat poorly absorbed through the gut, and poorly absorbed through the skin in comparison to the very effective absorption via the lungs. The inhibition of Hg vapor exposure through an antioxidant effect of ethanol is thought to take place primarily in the lungs, at the primary site of absorption for Hg vapor, preventing the Hg from entering the individual’s bloodstream. There may be a secondary inhibition that occurs within the bloodstream itself, but this is uncertain. (This is not to encourage the consumption of alcohol, but rather to address the issue of Hg absorption.)

GENERAL REFERENCES 1. Bellinger DC, Trachtenberg F, Barregard L, Tavares M, Cernichiari E, Daniel D, McKinlay S: Neuropsychological and renal effects of dental amalgam in children: a randomized clinical trial, JAMA 295:1775–1783, 2006. 2. Bradshaw TD, Bell DR: Relevance of the aryl hydrocarbon receptor (AhR) for clinical toxicology, Clin Toxicol 47:632–642, 2009.

3. Casida JE: Pest Toxicology: the primary mechanisms of pesticide action, Chemical Research in Toxicology 22:609–619, 2009. 4. Danielson PB: The cytochrome P450 superfamily: biochemistry, evolution and drug metabolism in humans, Current Drug Metabolism 3:561–597, 2002. 5. DeRouen TA, Martin MD, Leroux BG, Townes BD, Woods JS, Leitao J, Castro-Caldas A, Luis H, Bernardo M, Rosenbaum G, Martins IP: Neurobehavioral effects of dental amalgam in children: a randomized clinical trial, JAMA 295:1784–1792, 2006. 6. Dodes JE: The amalgam controversy. An evidence-based analysis, Journal of the American Dental Association 132:348–356, 2001. 7. Dresser GK, Spence JD, Bailey DG: Pharmacokinetic-pharmacodynamic consequences and clinical relevance of cytochrome P450 3A4 inhibition, Clinical Pharmacokinetics 38:41–57, 2000. 8. Ellenhorn MJ, Schonwald S, Ordog G, Wasserberger J, editors: Ellenhorn’s Medical Toxicology; Diagnosis and Treatment of Human Poisoning, ed 2, Baltimore, 1997, Williams & Wilkins. 9. Gaylor DW: The ED01 study: summary and conclusions, Journal of Environmental Pathology and Toxicology 3:179–183, 1980. 10. Hanieh H: Toward understanding the role of aryl hydrocarbon receptor in the immune system: current progress and future trends, BioMed Research International 2014, 520763. 11. Hill AB: The environment and disease: association or causation? Proceedings of the Royal Society of Medicine 58:295–300, 1965. 12. Kingman A, Albers JW, Arezzo JC, Garabrant DH, Michalek JE: Amalgam exposure and neurological function, Neurotoxicology 26:241–255, 2005. 13. Langston JW, Ballard P, Tetrud JW, Irwin I: Chronic Parkinsonism in humans due to a product of meperidine-analog synthesis, Science 219:979–980, 1983. 14. Martin MD, Naleway C: The inhibition of mercury absorption by dietary ethanol in humans: cross-sectional and case-control studies, Occupational and Environmental Medicine 61:e8, 2004. 15. McKinney C, Rue T, Sathyanarayana S, Martin M, Seminario AL, DeRouen T: Dental sealants and restorations and urinary bisphenol A concentrations in children in the 2003-2004 National Health and Nutrition Examination Survey, Journal of the American Dental Association 145:745–750, 2014. 16. Roberts JR, Reigart JR: Recognition and Management of Pesticide Poisonings, ed 6, www2.epa.gov, 2015.