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CHAPTER 2

PRINCIPLES OF TOXICOLOGY

David L. Eaton and Curtis D. Klaassen

INTRODUCTION TO TOXICOLOGY

Different Areas of ToxicologySpectrum of Toxic Dose

CLASSIFICATION OF TOXIC AGENTS

CHARACTERISTICS OF EXPOSURE

Route and Site of ExposureDuration and Frequency of Exposure

SPECTRUM OF UNDESIRED EFFECTS

Allergic ReactionsIdiosyncratic ReactionsImmediate versus Delayed ToxicityReversible versus Irreversible Toxic EffectsLocal versus Systemic Toxicity

INTERACTION OF CHEMICALS

TOLERANCE

DOSE RESPONSE

Individual, or Graded, Dose–Response RelationshipsQuantal Dose–Response RelationshipsShape of the Dose–Response Curve

Essential Nutrients

nisms by which chemicals exert toxic effects on living organisms(see Chap. 3 for a detailed discussion of mechanisms of toxicity).The results of mechanistic studies are very important in many ar-eas of applied toxicology. In risk assessment, mechanistic data maybe very useful in demonstrating that an adverse outcome (e.g., can-cer, birth defects) observed in laboratory animals is directly rele-vant to humans. For example, the relative toxic potential oforganophosphate insecticides in humans, rodents, and insects canbe accurately predicted on the basis of an understanding of com-mon mechanisms (inhibition of acetylcholinesterase) and differ-ences in biotransformation for these insecticides among the dif-ferent species. Similarly, mechanistic data may be very useful inidentifying adverse responses in experimental animals that may notbe relevant to humans. For example, the propensity of the widelyused artificial sweetener saccharin to cause bladder cancer in ratsmay not be relevant to humans at normal dietary intake rates. Thisis because mechanistic studies have demonstrated that bladder can-cer is induced only under conditions where saccharin is at such ahigh concentration in the urine that it forms a crystalline precipi-tate (Cohen, 1999). Dose–response studies suggest that such highconcentrations would not be achieved in the human bladder evenafter extensive dietary consumption.

Mechanistic data also are useful in the design and productionof safer alternative chemicals and in rational therapy for chemicalpoisoning and treatment of disease. For example, the drug thalido-

HormesisAssumptions in Deriving the Dose–Response

RelationshipEvaluating the Dose–Response Relationship

Comparison of Dose ResponsesTherapeutic IndexMargins of Safety and ExposurePotency versus Efficacy

VARIATION IN TOXIC RESPONSES

Selective ToxicitySpecies DifferencesIndividual Differences in Response

DESCRIPTIVE ANIMAL TOXICITY TESTS

Acute LethalitySkin and Eye IrritationsSensitizationSubacute (Repeated-Dose Study)SubchronicChronicDevelopmental and Reproductive ToxicityMutagenicityOther Tests

INTRODUCTION TO TOXICOLOGY

Toxicology is the study of the adverse effects of chemicals on liv-ing organisms. A toxicologist is trained to examine the nature ofthose effects (including their cellular, biochemical, and molecularmechanisms of action) and assess the probability of their occur-rence. Thus, the principles of toxicology are integral to the properuse of science in the process commonly called risk assessment,where quantitative estimates are made of the potential effects onhuman health and environmental significance of various types ofchemical exposures (e.g., pesticide residues on food, contaminantsin drinking water). The variety of potential adverse effects and thediversity of chemicals in the environment make toxicology a verybroad science. Therefore, toxicologists often specialize in one areaof toxicology.

Different Areas of Toxicology

The professional activities of toxicologists fall into three main cat-egories: descriptive, mechanistic, and regulatory (Fig. 2-1). Al-though each has distinctive characteristics, each contributes to theother, and all are vitally important to chemical risk assessment (seeChap. 4).

A mechanistic toxicologist is concerned with identifying andunderstanding the cellular, biochemical, and molecular mecha-

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Copyright © 2001 by The McGraw-Hill Companies Retrieved from: www.knovel.com

mide was originally marketed in Europe as a sedative agent forpregnant women. However, it was banned for clinical use in 1962because of devastating birth defects that occurred if the drug wasingested during a critical period in pregnancy. But mechanisticstudies over the past several decades have demonstrated that thisdrug may have a unique molecular mechanism of action that in-terferes with the expression of certain genes responsible for bloodvessel formation (angiogenesis). With an understanding of thismechanism, thalidomide has been “rediscovered” as a valuabletherapeutic agent that may be highly effective in the treatment ofcertain infectious diseases (e.g., leprosy and AIDS), a variety ofinflammatory diseases, and some types of cancer. Obviously, itsuse in pregnant women is strictly prohibited, but the discovery ofthis mechanism of action may now allow this drug to be used safelyand effectively in the treatment of many other diseases (Lary et al.,1999). In addition to aiding directly in the identification, treatment,and prevention of chemical toxicity, an understanding of the mech-anisms of toxic action contributes to the knowledge of basic phys-iology, pharmacology, cell biology, and biochemistry. For exam-ple, studies on the toxicity of fluoroorganic alcohols and acidscontributed to the knowledge of basic carbohydrate and lipid me-tabolism, and knowledge of regulation of ion gradients in nerveaxonal membranes has been greatly aided by studies of biologicaltoxins such as tetrodotoxin and synthetic chemicals such as DDT(dichlorodiphenyltrichloroethane). The advent of new technologiesin molecular biology and genomics now provide mechanistic tox-icologists with the tools to explore exactly how humans may dif-fer from laboratory animals in their response to toxic substances.These same tools are also being utilized to identify individuals whoare genetically susceptible to factors in the environment. For ex-ample, it is now recognized that a small percentage of the popula-tion genetically lacks the ability to detoxify the chemotherapeuticdrug, 6-mercaptopurine, used in the treatment of some forms ofleukemia. Young children with leukemia who are homozygous forthis genetic trait may experience serious toxic effects from a stan-dard therapeutic dose of this drug. Genetic tests are now availablethat can identify genetically susceptible individuals in advance oftreatment (Weinshilboum et al., 1999). This new area of ‘toxi-cogenomics’ provides an exciting opportunity in the future formechanistic toxicologists to identify and protect genetically sus-

ceptible individuals from harmful environmental exposures, and tocustomize drug therapies that enhance efficacy and minimize tox-icity, based on their individual genetic makeup.

A descriptive toxicologist is concerned directly with toxicitytesting, which provides information for safety evaluation and reg-ulatory requirements. The appropriate toxicity tests (as describedlater in this chapter) in experimental animals are designed to yieldinformation that can be used to evaluate risks posed to humans andthe environment by exposure to specific chemicals. The concernmay be limited to effects on humans, as in the case of drugs andfood additives. Toxicologists in the chemical industry, however,must be concerned not only with the risk posed by a company’schemicals (insecticides, herbicides, solvents, etc.) to humans butalso with potential effects on fish, birds, and plants, as well as otherfactors that might disturb the balance of the ecosystem. Descrip-tive toxicology studies provide important clues to a chemical’smechanism of action, and thus contribute to the development ofmechanistic toxicology through hypothesis generation. Such stud-ies are also a key component of risk assessments that are used byregulatory toxicologists.

A regulatory toxicologist has the responsibility for deciding,on the basis of data provided by descriptive and mechanistic tox-icologists, whether a drug or another chemical poses a sufficientlylow risk to be marketed for a stated purpose. The Food and DrugAdministration (FDA) is responsible for allowing drugs, cosmet-ics, and food additives to be sold in the market according to theFederal Food, Drug and Cosmetic Act (FDCA). The U.S. Envi-ronmental Protection Agency (EPA) is responsible for regulatingmost other chemicals according to the Federal Insecticide, Fungi-cide and Rodenticide Act (FIFRA), the Toxic Substances ControlAct (TSCA), the Resource Conservation and Recovery Act(RCRA), the Safe Drinking Water Act, and the Clean Air Act. TheEPA is also responsible for enforcing the Comprehensive Envi-ronmental Response, Compensation and Liability Act [CERCLA,later revised as the Superfund Amendments Reauthorization Act(SARA)], more commonly called the Superfund. This regulationprovides direction and financial support for the cleanup of wastesites that contain toxic chemicals and may present a risk to humanhealth or the environment. The Occupational Safety and Health Ad-ministration (OSHA) of the Department of Labor was establishedto ensure that safe and healthful conditions exist in the workplace.The Consumer Product Safety Commission is responsible for pro-tecting consumers from hazardous household substances, whereasthe Department of Transportation (DOT) ensures that materialsshipped in interstate commerce are labeled and packaged in a man-ner consistent with the degree of hazard they present. Regulatorytoxicologists are also involved in the establishment of standardsfor the amount of chemicals permitted in ambient air, industrial at-mospheres, and drinking water, often integrating scientific infor-mation from basic descriptive and mechanistic toxicology studieswith the principles and approaches used for risk assessment (Chap.4). Some of the philosophic and legal aspects of regulatory toxi-cology are discussed in Chap. 34.

In addition to the above categories, there are other specializedareas of toxicology such as forensic, clinical, and environmentaltoxicology. Forensic toxicology is a hybrid of analytic chemistryand fundamental toxicological principles. It is concerned primarilywith the medicolegal aspects of the harmful effects of chemicalson humans and animals. The expertise of forensic toxicologists isinvoked primarily to aid in establishing the cause of death and de-termining its circumstances in a postmortem investigation (Chap.

12 UNIT 1 GENERAL PRINCIPLES OF TOXICOLOGY

Mechanistic Toxicology

DescriptiveToxicology

RegulatoryToxicology

RiskAssessment

Figure 2-1. Graphical representation of the interconnections between different areas of toxicology.

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31). Clinical toxicology designates an area of professional empha-sis in the realm of medical science that is concerned with diseasecaused by or uniquely associated with toxic substances (see Chap.32). Generally, clinical toxicologists are physicians who receivespecialized training in emergency medicine and poison manage-ment. Efforts are directed at treating patients poisoned with drugsor other chemicals and at the development of new techniques totreat those intoxications. Environmental toxicology focuses on theimpacts of chemical pollutants in the environment on biologicalorganisms. Although toxicologists concerned with the effects ofenvironmental pollutants on human health fit into this definition,it is most commonly associated with studies on the impacts ofchemicals on nonhuman organisms such as fish, birds, and terres-trial animals. Ecotoxicology is a specialized area within environ-mental toxicology that focuses more specifically on the impacts oftoxic substances on population dynamics in an ecosystem. Thetransport, fate, and interactions of chemicals in the environmentconstitute a critical component of both environmental toxicologyand ecotoxicology.

Spectrum of Toxic Dose

One could define a poison as any agent capable of producing adeleterious response in a biological system, seriously injuring func-tion or producing death. This is not, however, a useful working def-inition for the very simple reason that virtually every known chem-ical has the potential to produce injury or death if it is present ina sufficient amount. Paracelsus (1493–1541), a Swiss/German/Austrian physician, scientist, and philosopher, phrased this wellwhen he noted, “What is there that is not poison? All things arepoison and nothing [is] without poison. Solely the dose determinesthat a thing is not a poison.”

Among chemicals there is a wide spectrum of doses neededto produce deleterious effects, serious injury, or death. This isdemonstrated in Table 2-1, which shows the dosage of chemicalsneeded to produce death in 50 percent of treated animals (LD50).Some chemicals produce death in microgram doses and are com-monly thought of as being extremely poisonous. Other chemicals

may be relatively harmless after doses in excess of several grams.It should be noted, however, that measures of acute lethality suchas LD50 may not accurately reflect the full spectrum of toxicity, orhazard, associated with exposure to a chemical. For example, somechemicals with low acute toxicity may have carcinogenic or ter-atogenic effects at doses that produce no evidence of acute toxicity.

CLASSIFICATION OF TOXICAGENTS

Toxic agents are classified in a variety of ways, depending on theinterests and needs of the classifier. In this textbook, for example,toxic agents are discussed in terms of their target organs (liver, kid-ney, hematopoietic system, etc.), use (pesticide, solvent, food ad-ditive, etc.), source (animal and plant toxins), and effects (cancer,mutation, liver injury, etc.). The term toxin generally refers to toxicsubstances that are produced by biological systems such as plants,animals, fungi or bacteria. The term toxicant is used in speakingof toxic substances that are produced by or are a by-product of an-thropogenic (human-made) activities. Thus, zeralanone, producedby a mold, is a toxin, whereas “dioxin” [2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)], produced during the combustion of certain chlo-rinated organic chemicals, is a toxicant. Some toxicants can be pro-duced by both natural and anthropogenic activities. For example,polyaromatic hydrocarbons are produced by the combustion of or-ganic matter, which may occur both through natural processes (e.g.,forest fires) and through anthropogenic activities (e.g., combustionof coal for energy production; cigarette smoking). Arsenic, a toxicmetalloid, may occur as a natural contaminant of groundwater ormay contaminate groundwater secondary to industrial activities.Generally, such toxic substances are referred to as toxicants, ratherthan toxins, because, although they are naturally produced, theyare not produce by biological systems.

Toxic agents also may be classified in terms of their physicalstate (gas, dust, liquid), their chemical stability or reactivity (ex-plosive, flammable, oxidizer), general chemical structure (aromaticamine, halogenated hydrocarbon, etc.), or poisoning potential (ex-tremely toxic, very toxic, slightly toxic, etc.). Classification of toxicagents on the basis of their biochemical mechanisms of action (e.g.,alkylating agent, sulfhydryl inhibitor, methemoglobin producer) isusually more informative than classification by general terms suchas irritants and corrosives. But more general classifications such asair pollutants, occupation-related agents, and acute and chronic poi-sons can provide a useful focus on a specific problem. It is evidentfrom this discussion that no single classification is applicable tothe entire spectrum of toxic agents and that combinations of clas-sification systems or a classification based on other factors may beneeded to provide the best rating system for a special purpose. Nev-ertheless, classification systems that take into consideration boththe chemical and the biological properties of an agent and the ex-posure characteristics are most likely to be useful for legislative orcontrol purposes and for toxicology in general.

CHARACTERISTICS OF EXPOSURE

Toxic effects in a biological system are not produced by a chemi-cal agent unless that agent or its metabolic breakdown (biotrans-formation) products reach appropriate sites in the body at a con-centration and for a length of time sufficient to produce a toxicmanifestation. Many chemicals are of relatively low toxicity in the

CHAPTER 2 PRINCIPLES OF TOXICOLOGY 13

Table 2-1Approximate Acute LD50s of Some Representative ChemicalAgents

AGENT LD50, mg/kg*

Ethyl alcohol 10,000Sodium chloride 4,000Ferrous sulfate 1,500Morphine sulfate 900Phenobarbital sodium 150Picrotoxin 5Strychnine sulfate 2Nicotine 1d-Tubocurarine 0.5Hemicholinium-3 0.2Tetrodotoxin 0.10Dioxin (TCDD) 0.001Botulinum toxin 0.00001

*LD50 is the dosage (mg/kg body weight) causing death in 50 percent of exposed an-imals.

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“native” form but, when acted on by enzymes in the body, are con-verted to intermediate forms that interfere with normal cellular bio-chemistry and physiology. Thus, whether a toxic response occursis dependent on the chemical and physical properties of the agent,the exposure situation, how the agent is metabolized by the sys-tem, and the overall susceptibility of the biological system or sub-ject. Thus, to characterize fully the potential hazard of a specificchemical agent, we need to know not only what type of effect itproduces and the dose required to produce that effect but also in-formation about the agent, the exposure, and its disposition by thesubject. The major factors that influence toxicity as it relates to theexposure situation for a specific chemical are the route of admin-istration and the duration and frequency of exposure.

Route and Site of Exposure

The major routes (pathways) by which toxic agents gain access tothe body are the gastrointestinal tract (ingestion), lungs (inhala-tion), skin (topical, percutaneous, or dermal), and other parenteral(other than intestinal canal) routes. Toxic agents generally producethe greatest effect and the most rapid response when given directlyinto the bloodstream (the intravenous route). An approximate de-scending order of effectiveness for the other routes would be inhalation, intraperitoneal, subcutaneous, intramuscular, intrader-mal, oral, and dermal. The “vehicle” (the material in which thechemical is dissolved) and other formulation factors can markedlyalter absorption after ingestion, inhalation, or topical exposure. Inaddition, the route of administration can influence the toxicity ofagents. For example, an agent that is detoxified in the liver wouldbe expected to be less toxic when given via the portal circula-tion (oral) than when given via the systemic circulation (inhala-tion).

Occupational exposure to toxic agents most frequently resultsfrom breathing contaminated air (inhalation) and/or direct and pro-longed contact of the skin with the substance (dermal exposure),whereas accidental and suicidal poisoning occurs most frequentlyby oral ingestion. Comparison of the lethal dose of a toxic sub-stance by different routes of exposure often provides useful infor-mation about its extent of absorption. In instances when the lethaldose after oral or dermal administration is similar to the lethal doseafter intravenous administration, the assumption is that the toxicagent is absorbed readily and rapidly. Conversely, in cases wherethe lethal dose by the dermal route is several orders of magnitudehigher than the oral lethal dose, it is likely that the skin providesan effective barrier to absorption of the agent. Toxic effects by anyroute of exposure also can be influenced by the concentration ofthe agent in its vehicle, the total volume of the vehicle and theproperties of the vehicle to which the biological system is exposed,and the rate at which exposure occurs. Studies in which the con-centration of a chemical in the blood is determined at various timesafter exposure are often needed to clarify the role of these and otherfactors in the toxicity of a compound. For more details on the ab-sorption of toxicants, see Chap. 5.

Duration and Frequency of Exposure

Toxicologists usually divide the exposure of experimental animalsto chemicals into four categories: acute, subacute, subchronic, andchronic. Acute exposure is defined as exposure to a chemical forless than 24 h, and examples of exposure routes are intraperitoneal,intravenous, and subcutaneous injection; oral intubation; and der-mal application. While acute exposure usually refers to a single ad-

ministration, repeated exposures may be given within a 24-h pe-riod for some slightly toxic or practically nontoxic chemicals.Acute exposure by inhalation refers to continuous exposure for lessthan 24 h, most frequently for 4 h. Repeated exposure is dividedinto three categories: subacute, subchronic, and chronic. Subacuteexposure refers to repeated exposure to a chemical for 1 month orless, subchronic for 1 to 3 months, and chronic for more than 3months. These three categories of repeated exposure can be by anyroute, but most often they occur by the oral route, with the chem-ical added directly to the diet.

In human exposure situations, the frequency and duration ofexposure are usually not as clearly defined as in controlled animalstudies, but many of the same terms are used to describe generalexposure situations. Thus, workplace or environmental exposuresmay be described as acute (occurring from a single incident orepisode), subchronic (occurring repeatedly over several weeks or months), or chronic (occurring repeatedly for many months oryears).

For many agents, the toxic effects that follow a single expo-sure are quite different from those produced by repeated exposure.For example, the primary acute toxic manifestation of benzene iscentral nervous system (CNS) depression, but repeated exposurescan result in bone marrow toxicity and an increased risk forleukemia. Acute exposure to agents that are rapidly absorbed islikely to produce immediate toxic effects but also can produce de-layed toxicity that may or may not be similar to the toxic effectsof chronic exposure. Conversely, chronic exposure to a toxic agentmay produce some immediate (acute) effects after each adminis-tration in addition to the long-term, low-level, or chronic effects ofthe toxic substance. In characterizing the toxicity of a specificchemical, it is evident that information is needed not only for thesingle-dose (acute) and long-term (chronic) effects but also for ex-posures of intermediate duration. The other time-related factor thatis important in the temporal characterization of repeated exposuresis the frequency of exposure. The relationship between eliminationrate and frequency of exposure is shown in Fig. 2-2. A chemicalthat produces severe effects with a single dose may have no effectif the same total dose is given in several intervals. For the chemi-cal depicted by line B in Fig. 2-2, in which the half-life for elim-ination (time necessary for 50 percent of the chemical to be re-moved from the bloodstream) is approximately equal to the dosingfrequency, a theoretical toxic concentration of 2 U is not reacheduntil the fourth dose, whereas that concentration is reached withonly two doses for chemical A, which has an elimination rate muchslower than the dosing interval (time between each repeated dose).Conversely, for chemical C, where the elimination rate is muchshorter than the dosing interval, a toxic concentration at the site oftoxic effect will never be reached regardless of how many dosesare administered. Of course, it is possible that residual cell or tis-sue damage occurs with each dose even though the chemical itselfis not accumulating. The important consideration, then, is whetherthe interval between doses is sufficient to allow for complete re-pair of tissue damage. It is evident that with any type of repeatedexposure, the production of a toxic effect is influenced not only bythe frequency of exposure but may, in fact, be totally dependenton the frequency rather than the duration of exposure. Chronic toxiceffects may occur, therefore, if the chemical accumulates in the bi-ological system (rate of absorption exceeds the rate of biotrans-formation and/or excretion), if it produces irreversible toxic effects,or if there is insufficient time for the system to recover from thetoxic damage within the exposure frequency interval. For additionaldiscussion of these relationships, consult Chaps. 5 and 7.


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SPECTRUM OF UNDESIREDEFFECTS

The spectrum of undesired effects of chemicals is broad. Some ef-fects are deleterious and others are not. In therapeutics, for exam-ple, each drug produces a number of effects, but usually only oneeffect is associated with the primary objective of the therapy; allthe other effects are referred to as undesirable or side effects ofthat drug for that therapeutic indication. However, some of theseside effects may be desired for another therapeutic indication. Forexample, the “first-generation” antihistamine diphenhydramine(Benadryl) is effective in reducing histamine responses associatedwith allergies, but it readily enters the brain and causes mild CNSdepression (drowsiness, delayed reaction time). With the advent ofnewer histamine (H1) receptor antagonists that do not cross theblood-brain barrier and thus do not have this CNS-depressant sideeffect, the drug is used less commonly today as an antihistamine.However, it is widely used as an “over the counter” sleep remedyin combination with analgesics (e.g., Tylenol PM, Excedrin PM,etc), taking advantage of the CNS-depressant effects. Some sideeffects of drugs are never desirable and are always deleterious tothe well-being of humans. These are referred to as the adverse,deleterious, or toxic effects of the drug.

Allergic Reactions

Chemical allergy is an immunologically mediated adverse reactionto a chemical resulting from previous sensitization to that chemi-cal or to a structurally similar one. The term hypersensitivity ismost often used to describe this allergic state, but allergic reaction

and sensitization reaction are also used to describe this situationwhen preexposure of the chemical is required to produce the toxiceffect (see Chap. 12). Once sensitization has occurred, allergic re-actions may result from exposure to relatively very low doses ofchemicals; therefore population-based dose–response curves for al-lergic reactions have seldom been obtained. Because of this omis-sion, some people assumed that allergic reactions are not dose-related. Thus, they do not consider the allergic reaction to be a truetoxic response. However, for a given allergic individual, allergicreactions are dose-related. For example, it is well known that theallergic response to pollen in sensitized individuals is related to theconcentration of pollen in the air. In addition, because the allergicresponse is an undesirable, adverse, deleterious effect, it obviouslyis also a toxic response. Sensitization reactions are sometimes verysevere and may be fatal.

Most chemicals and their metabolic products are not suffi-ciently large to be recognized by the immune system as a foreignsubstance and thus must first combine with an endogenous proteinto form an antigen (or immunogen). A molecule that must com-bine with an endogenous protein to elicit an allergic reaction iscalled a hapten. The hapten-protein complex (antigen) is then ca-pable of eliciting the formation of antibodies, and usually at least1 or 2 weeks is required for the synthesis of significant amountsof antibodies. Subsequent exposure to the chemical results in anantigen–antibody interaction, which provokes the typical manifes-tations of allergy. The manifestations of allergy are numerous. Theymay involve various organ systems and range in severity from mi-nor skin disturbance to fatal anaphylactic shock. The pattern of al-lergic response differs in various species. In humans, involvementof the skin (e.g., dermatitis, urticaria, and itching) and involvement


Figure 2-2. Diagrammatic view of the relationship between dose and concentration at the target site underdifferent conditions of dose frequency and elimination rate.

Line A. A chemical with very slow elimination (e.g., half-life of 1 year). Line B. A chemical with a rate of elim-ination equal to frequency of dosing (e.g., 1 day). Line C. Rate of elimination faster than the dosing frequency(e.g., 5 h). Blue-shaded area is representative of the concentration of chemical at the target site necessary toelicit a toxic response.

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of the eyes (e.g., conjunctivitis) are most common, whereas inguinea pigs, bronchiolar constriction leading to asphyxia is themost common. However, chemically induced asthma (character-ized by bronchiolar constriction) certainly does occur in some hu-mans, and the incidence of allergic asthma has increased substan-tially in recent years. Hypersensitivity reactions are discussed inmore detail in Chap. 12.

Idiosyncratic Reactions

Chemical idiosyncrasy refers to a genetically determined abnormalreactivity to a chemical (Goldstein et al., 1974; Levine, 1978). Theresponse observed is usually qualitatively similar to that observedin all individuals but may take the form of extreme sensitivity tolow doses or extreme insensitivity to high doses of the chemical.However, while some people use the term idiosyncratic as a catchallto refer to all reactions that occur with low frequency, it should notbe used in that manner (Goldstein et al., 1974). A classic exampleof an idiosyncratic reaction is provided by patients who exhibitprolonged muscular relaxation and apnea (inability to breathe) last-ing several hours after a standard dose of succinylcholine. Suc-cinylcholine usually produces skeletal muscle relaxation of onlyshort duration because of its very rapid metabolic degradation byan enzyme that is present normally in the bloodstream calledplasma butyrylcholinesterase (also referred to as pseudo-cholinesterase). Patients exhibiting this idiosyncratic reaction havea genetic polymorphism in the gene for the enzyme butyryl-cholinesterase, which is less active in breaking down succinyl-choline. Family pedigree and molecular genetic analyses havedemonstrated that the presence of low plasma butyrylcholinesteraseactivity is due to the presence of one or more single nucleotidepolymorphisms in this gene (Bartels et al., 1992). Similarly, thereis a group of people who are abnormally sensitive to nitrites andcertain other chemicals that have in common the ability to oxidizethe iron in hemoglobin to produce methemoglobin, which is inca-pable of carrying oxygen to the tissues. The unusual phenotype isinherited as an autosomal recessive trait and is characterized by adeficiency in NADH-cytochrome b5 reductase activity. The geneticbasis for this idiosyncratic response has been identified as a singlenucleotide change in codon 127, which results in replacement ofserine with proline (Kobayashi et al., 1990). The consequence ofthis genetic deficiency is that these individuals may suffer from aserious lack of oxygen delivery to tissues after exposure to dosesof methemoglobin-producing chemicals that would be harmless toindividuals with normal NADH-cytochrome b5 reductase activity.

Immediate versus Delayed Toxicity

Immediate toxic effects can be defined as those that occur or de-velop rapidly after a single administration of a substance, whereasdelayed toxic effects are those that occur after the lapse of sometime. Carcinogenic effects of chemicals usually have a long latencyperiod, often 20 to 30 years after the initial exposure, before tu-mors are observed in humans. For example, daughters of motherswho took diethylstilbestrol (DES) during pregnancy have a greatlyincreased risk of developing vaginal cancer, but not other types ofcancer, in young adulthood, some 20 to 30 years after their in uteroexposure to DES (Hatch et al., 1998). Also, delayed neurotoxicityis observed after exposure to some organophosphorus insecticidesthat act by covalent modification of an enzyme referred to as neu-ropathy target esterase (NTE), a neuronal protein with serine es-terase activity (Glynn et al., 1999). Binding of certain organophos-

phates (OP) to this protein initiates degeneration of long axons inthe peripheral and central nervous system. The most notorious ofthe compounds that produce this type of neurotoxic effect is tri-orthocresylphosphate (TOCP). The effect is not observed until atleast several days after exposure to the toxic compound. In con-trast, most substances produce immediate toxic effects but do notproduce delayed effects.

Reversible versus Irreversible ToxicEffects

Some toxic effects of chemicals are reversible, and others are ir-reversible. If a chemical produces pathological injury to a tissue,the ability of that tissue to regenerate largely determines whetherthe effect is reversible or irreversible. Thus, for a tissue such asliver, which has a high ability to regenerate, most injuries are re-versible, whereas injury to the CNS is largely irreversible becausedifferentiated cells of the CNS cannot divide and be replaced. Car-cinogenic and teratogenic effects of chemicals, once they occur,are usually considered irreversible toxic effects.

Local versus Systemic Toxicity

Another distinction between types of effects is made on the basisof the general site of action. Local effects are those that occur atthe site of first contact between the biological system and the tox-icant. Such effects are produced by the ingestion of caustic sub-stances or the inhalation of irritant materials. For example, chlo-rine gas reacts with lung tissue at the site of contact, causingdamage and swelling of the tissue, with possibly fatal conse-quences, even though very little of the chemical is absorbed intothe bloodstream. The alternative to local effects is systemic effects.Systemic effects require absorption and distribution of a toxicantfrom its entry point to a distant site, at which deleterious effectsare produced. Most substances except highly reactive materials pro-duce systemic effects. For some materials, both effects can bedemonstrated. For example, tetraethyl lead produces effects on skinat the site of absorption and then is transported systemically to pro-duce its typical effects on the CNS and other organs. If the localeffect is marked, there may also be indirect systemic effects. Forexample, kidney damage after a severe acid burn is an indirect sys-temic effect because the toxicant does not reach the kidney.

Most chemicals that produce systemic toxicity do not causea similar degree of toxicity in all organs; instead, they usually elicittheir major toxicity in only one or two organs. These sites are re-ferred to as the target organs of toxicity of a particular chemical.The target organ of toxicity is often not the site of the highest con-centration of the chemical. For example, lead is concentrated inbone, but its toxicity is due to its effects in soft tissues, particu-larly the brain. DDT is concentrated in adipose tissue but producesno known toxic effects in that tissue.

The target organ of toxicity most frequently involved in sys-temic toxicity is the CNS (brain and spinal cord). Even with manycompounds having a prominent effect elsewhere, damage to theCNS can be demonstrated by the use of appropriate and sensitivemethods. Next in order of frequency of involvement in systemictoxicity are the circulatory system; the blood and hematopoieticsystem; visceral organs such as the liver, kidney, and lung; and theskin. Muscle and bone are least often the target tissues for systemiceffects. With substances that have a predominantly local effect, thefrequency with which tissues react depends largely on the portalof entry (skin, gastrointestinal tract, or respiratory tract).


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INTERACTION OF CHEMICALS

Because of the large number of different chemicals an individualmay come in contact with at any given time (workplace, drugs,diet, hobbies, etc.), it is necessary, in assessing the spectrum of re-sponses, to consider how different chemicals may interact with eachother. Interactions can occur in a variety of ways. Chemical inter-actions are known to occur by a number of mechanisms, such asalterations in absorption, protein binding, and the biotransforma-tion and excretion of one or both of the interacting toxicants. Inaddition to these modes of interaction, the response of the organ-ism to combinations of toxicants may be increased or decreasedbecause of toxicologic responses at the site of action.

The effects of two chemicals given simultaneously produce aresponse that may simply be additive of their individual responsesor may be greater or less than that expected by addition of theirindividual responses. The study of these interactions often leads toa better understanding of the mechanism of toxicity of the chem-icals involved. A number of terms have been used to describe phar-macologic and toxicologic interactions. An additive effect occurswhen the combined effect of two chemicals is equal to the sum ofthe effects of each agent given alone (example: 2 � 3 � 5). Theeffect most commonly observed when two chemicals are given to-gether is an additive effect. For example, when two organophos-phate insecticides are given together, the cholinesterase inhibitionis usually additive. A synergistic effect occurs when the combinedeffects of two chemicals are much greater than the sum of the ef-fects of each agent given alone (example: 2 � 2 � 20). For ex-ample, both carbon tetrachloride and ethanol are hepatotoxic com-pounds, but together they produce much more liver injury than themathematical sum of their individual effects on liver at a givendose would suggest. Potentiation occurs when one substance doesnot have a toxic effect on a certain organ or system but when addedto another chemical makes that chemical much more toxic (exam-ple: 0 � 2 � 10). Isopropanol, for example, is not hepatotoxic, butwhen it is administered in addition to carbon tetrachloride, the he-patotoxicity of carbon tetrachloride is much greater than that whenit is given alone. Antagonism occurs when two chemicals admin-istered together interfere with each other’s actions or one interfereswith the action of the other (example: 4 � 6 � 8; 4 � (�4) � 0;4 � 0 � 1). Antagonistic effects of chemicals are often very de-sirable in toxicology and are the basis of many antidotes. Thereare four major types of antagonism: functional, chemical, disposi-tional, and receptor. Functional antagonism occurs when twochemicals counterbalance each other by producing opposite effectson the same physiologic function. Advantage is taken of this prin-ciple in that the blood pressure can markedly fall during severebarbiturate intoxication, which can be effectively antagonized bythe intravenous administration of a vasopressor agent such as nor-epinephrine or metaraminol. Similarly, many chemicals, whengiven at toxic dose levels, produce convulsions, and the convul-sions often can be controlled by giving anticonvulsants such as thebenzodiazepines (e.g., diazepam). Chemical antagonism or inacti-vation is simply a chemical reaction between two compounds thatproduces a less toxic product. For example, dimercaprol (Britishantilewisite, or BAL) chelates with metal ions such as arsenic, mer-cury, and lead and decreases their toxicity. The use of antitoxinsin the treatment of various animal toxins is also an example ofchemical antagonism. The use of the strongly basic low-molecu-lar-weight protein protamine sulfate to form a stable complex withheparin, which abolishes its anticoagulant activity, is another ex-ample. Dispositional antagonism occurs when the disposition—

that is, the absorption, biotransformation, distribution, or excretionof a chemical—is altered so that the concentration and/or durationof the chemical at the target organ are diminished. Thus, the pre-vention of absorption of a toxicant by ipecac or charcoal and theincreased excretion of a chemical by administration of an osmoticdiuretic or alteration of the pH of the urine are examples of dis-positional antagonism. If the parent compound is responsible forthe toxicity of the chemical (such as the anticoagulant warfarin)and its metabolic breakdown products are less toxic than the par-ent compound, increasing the compound’s metabolism (biotrans-formation) by administering a drug that increases the activity ofthe metabolizing enzymes (e.g., a “microsomal enzyme inducer”such as phenobarbital) will decrease its toxicity. However, if thechemical’s toxicity is largely due to a metabolic product (as in thecase of the organophosphate insecticide parathion), inhibiting itsbiotransformation by an inhibitor of microsomal enzyme activity(SKF-525A or piperonyl butoxide) will decrease its toxicity. Re-ceptor antagonism occurs when two chemicals that bind to thesame receptor produce less of an effect when given together thanthe addition of their separate effects (example: 4 � 6 � 8) or whenone chemical antagonizes the effect of the second chemical (ex-ample: 0 � 4 � 1). Receptor antagonists are often termed block-ers. This concept is used to advantage in the clinical treatment ofpoisoning. For example, the receptor antagonist naloxone is usedto treat the respiratory depressive effects of morphine and othermorphine-like narcotics by competitive binding to the same recep-tor. Another example of receptor antagonism is the use of the anti-estrogen drug tamoxifen to lower breast cancer risk among womenat high risk for this estrogen-related cancer. Tamoxifen competi-tively block estradiol from binding to its receptor. Treatment oforganophosphate insecticide poisoning with atropine is an exam-ple not of the antidote competing with the poison for the receptor(cholinesterase) but involves blocking the receptor (cholinergic re-ceptor) for the excess acetylcholine that accumulates by poisoningof the cholinesterase by the organophosphate (see Chap. 22).

TOLERANCE

Tolerance is a state of decreased responsiveness to a toxic effectof a chemical resulting from prior exposure to that chemical or toa structurally related chemical. Two major mechanisms are re-sponsible for tolerance: one is due to a decreased amount of toxi-cant reaching the site where the toxic effect is produced (disposi-tional tolerance), and the other is due to a reduced responsivenessof a tissue to the chemical. Comparatively less is known about thecellular mechanisms responsible for altering the responsiveness ofa tissue to a toxic chemical than is known about dispositional tol-erance. Two chemicals known to produce dispositional toleranceare carbon tetrachloride and cadmium. Carbon tetrachloride pro-duces tolerance to itself by decreasing the formation of the reac-tive metabolite (trichloromethyl radical) that produces liver injury(Chap. 13). The mechanism of cadmium tolerance is explained byinduction of metallothionein, a metal-binding protein. Subsequentbinding of cadmium to metallothionein rather than to criticalmacromolecules thus decreases its toxicity.

DOSE RESPONSE

The characteristics of exposure and the spectrum of effects cometogether in a correlative relationship customarily referred to as thedose–response relationship. Whatever response is selected formeasurement, the relationship between the degree of response of


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the biological system and the amount of toxicant administered as-sumes a form that occurs so consistently as to be considered themost fundamental and pervasive concept in toxicology.

From a practical perspective, there are two types of dose–-response relationships: (1) the individual dose–response relation-ship, which describes the response of an individual organism tovarying doses of a chemical, often referred to as a “graded” re-sponse because the measured effect is continuous over a range ofdoses, and (2) a quantal dose–response relationship, which char-acterizes the distribution of responses to different doses in a pop-ulation of individual organisms.

Individual, or Graded, Dose–ResponseRelationships

Individual dose–response relationships are characterized by a dose-related increase in the severity of the response. The dose related-ness of the response often results from an alteration of a specificbiochemical process. For example, Fig. 2-3 shows the dose–response relationship between different dietary doses of theorganophosphate insecticide chlorpyrifos and the extent of inhibi-tion of two different enzymes in the brain and liver: acetyl-cholinesterase and carboxylesterase. In the brain, the degree of in-hibition of both enzymes is clearly dose-related and spans a widerange, although the amount of inhibition per unit dose is differentfor the two enzymes. From the shapes of these two dose–responsecurves it is evident that, in the brain, cholinesterase is more easilyinhibited than carboxylesterase. The toxicologic response that re-sults is directly related to the degree of cholinesterase enzyme in-hibition in the brain. Thus, clinical signs and symptoms for chlor-pyrifos would follow a dose–response relationship similar to thatfor brain cholinesterase. However, for many chemicals, more thanone effect may result because of multiple different target sites indifferent tissues. Thus, the observed response to varying doses ofa chemical in the whole organism is often complicated by the factthat most toxic substances have multiple sites or mechanisms oftoxicity, each with its own “dose–response” relationship and sub-sequent adverse effect. Note that when these dose–response dataare plotted using the base 10 log of the dose on the abscissa(Fig. 2.3B), a better “fit” of the data to a straight line occurs. Thisis typical of many graded as well as quantal dose–response rela-tionships.

Quantal Dose–Response Relationships

In contrast to the “graded” or continuous-scale dose–response re-lationship that occurs in individuals, the dose–response relation-ships in a population are by definition quantal—or “all or none”—in nature; that is, at any given dose, an individual in the populationis classified as either a “responder” or a “nonresponder.” Althoughthese distinctions of “quantal population” and “graded individual”dose–response relationships are useful, the two types of responsesare conceptually identical. The ordinate in both cases is simply la-beled the response, which may be the degree of response in an in-dividual or system or the fraction of a population responding, andthe abscissa is the range in administered doses.

In toxicology, the quantal dose response is used extensively.Determination of the median lethal dose (LD50) is usually the firstexperiment performed with a new chemical. The LD50 is the sta-tistically derived single dose of a substance that can be expected

to cause death in 50 percent of the animals tested. Typically, groupsof animals are dosed at different levels, and the mortality that re-sults in each dose group is recorded. The top panel of Fig. 2-4shows that quantal dose responses such as lethality exhibit a nor-mal or gaussian distribution. The frequency histogram in this panelalso shows the relationship between dose and effect. The bars rep-resent the percentage of animals that died at each dose minus thepercentage that died at the immediately lower dose. One can clearlysee that only a few animals responded to the lowest dose and thehighest dose. Larger numbers of animals responded to doses in-termediate between these two extremes, and the maximum fre-quency of response occurred in the middle portion of the doserange. Thus, we have a bell-shaped curve known as a normal fre-quency distribution. The reason for this normal distribution is thatthere are differences in susceptibility to chemicals among individ-


Figure 2-3. Dose–response relationship between different doses of theorganophosphate insecticide chlorpyrifos and esterase enzyme inhibitionin the brain.

Open circles and blue lines represent acetylcholinesterase activity andclosed circles represent carboxylesterase activity in the brains of pregnantfemale Long-Evans rats given 5 daily doses of chlorpyrifos. A. Dose–response curve plotted on an arithmetic scale. B. Same data plotted on asemi-log scale. (Data derived from Lassiter et al., Gestational exposure tochloryprifos: Dose response profiles for cholinesterase and carboxylesteraseactivity. Toxicol Sci 52:92–100, 1999, with permission.)

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uals; this is known as biological variation. Animals responding atthe left end of the curve are referred to as hypersusceptible, andthose at the right end of the curve are called resistant. If the num-bers of individuals responding at each consecutive dose are addedtogether, a cumulative, quantal dose–response relationship is ob-tained. When a sufficently large number of doses is used with alarge number of animals per dose, a sigmoid dose–response curveis observed, as depicted in the middle panel of Fig. 2-4. With thelowest dose (6 mg/kg), 1 percent of the animals die. A normallydistributed sigmoid curve such as this one approaches a responseof 0 percent as the dose is decreased and approaches 100 percentas the dose is increased; but—theoretically—it never passesthrough 0 and 100 percent. However, the minimally effective dose

of any chemical that evokes a stated all-or-none response is calledthe threshold dose even though it cannot be determinedexperimentally.

The sigmoid curve has a relatively linear portion between 16and 84 percent. These values represent the limits of 1 standard de-viation (SD) of the mean (and the median) in a population withtruly normal or gaussian distribution. However, it is usually notpractical to describe the dose–response curve from this type of plotbecause one does not usually have large enough sample sizes todefine the sigmoid curve adequately. In a normally distributed pop-ulation, the mean �1 SD represents 68.3 percent of the popula-tion, the mean �2 SD represents 95.5 percent of the population,and the mean �3 SD equals 99.7 percent of the population. Sincequantal dose–response phenomena are usually normally distrib-uted, one can convert the percent response to units of deviationfrom the mean or normal equivalent deviations (NEDs). Thus, theNED for a 50 percent response is 0; an NED of �1 is equated withan 84.1 percent response. Later, it was suggested (Bliss, 1957) thatunits of NED be converted by the addition of 5 to the value toavoid negative numbers and that these converted units be calledprobit units. The probit (from the contraction of probability unit),then, is an NED plus 5. In this transformation, a 50 percent re-sponse becomes a probit of 5, a �1 deviation becomes a probit of6, and a �1 deviation is a probit of 4.

The data given in the top two panels of Fig. 2-4 are replottedin the bottom panel with the mortality plotted in probit units. Thedata in the middle panel (which was in the form of a sigmoid curve)and the top panel (a bell-shaped curve) form a straight line whentransformed into probit units. In essence, what is accomplished ina probit transformation is an adjustment of mortality or other quan-tal data to an assumed normal population distribution, resulting ina straight line. The LD50 is obtained by drawing a horizontal linefrom the probit unit 5, which is the 50 percent mortality point, tothe dose–effect line. At the point of intersection, a vertical line isdrawn, and this line intersects the abscissa at the LD50 point. It isevident from the line that information with respect to the lethaldose for 90 percent or for 10 percent of the population also maybe derived by a similar procedure. Mathematically, it can be demon-strated that the range of values encompassed by the confidence lim-its is narrowest at the midpoint of the line (LD50) and widest atboth extremes (LD10 and LD90) of the dose–response curve (dot-ted lines in Fig. 2-5). In addition to the LD50, the slope of thedose–response curve can also be obtained. Figure 2-5 demonstratesthe dose–response curves for the mortality of two compounds.Compound A exhibits a “flat” dose–response curve, showing thata large change in dosage is required before a significant change inresponse will be observed. However, compound B exhibits a“steep” dose–response curve, where a relatively small change indosage will cause a large change in response. It is evident that theLD50 for both compounds is the same (8 mg/kg). However, theslopes of the dose–response curves are quite different. At one-halfof LD50 of the compounds (4 mg/kg), less than 1 percent of theanimals exposed to compound B would die but 20 percent of theanimals given compound A would die.

The quantal all-or-none response is not limited to lethality.Similar dose–response curves can be constructed for cancer, liverinjury, and other types of toxic responses as well as for beneficialtherapeutic responses such as anesthesia or lowering of blood pres-sure. This is usually performed by measuring a particular param-eter (e.g., blood pressure) in a large number of control animals anddetermining its standard deviation, which is a measure of its vari-


Figure 2-4. Diagram of quantal dose–response relationship.

The abscissa is a log dosage of the chemical. In the top panel the ordinateis mortality frequency, in the middle panel the ordinate is percent mortal-ity, and in the bottom panel the mortality is in probit units (see text).

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ability. Because the mean �3 SD represents 99.7 percent of the population, one can assign all animals that lie outside this range after treatment with a chemical as being affected and thoselying within this range as not being affected by the chemical. Using a series of doses of the chemical, one thus can construct aquantal dose–response curve similar to that described above forlethality.

In Figs. 2-4 and 2-5 the dosage has been given on a log ba-sis. Although the use of the log of the dosage is empiric, log-dosageplots for normally distributed quantal data provide a more nearlylinear representation of the data. It must be remembered, however,that this is not universally the case. Some radiation effects, for ex-ample, give a better probit fit when the dose is expressed arith-metically rather than logarithmically. There are other situations inwhich other functions (e.g., exponentials) of dosage provide a bet-ter fit to the data than does the log function. It is also conventionalto express the dosage in milligrams per kilogram. It might be ar-gued that expression of dosage on a mole-per-kilogram basis wouldbe better, particularly for making comparisons among a series ofcompounds. Although such an argument has considerable merit,dosage is usually expressed in milligrams per kilogram.

One might also view dosage on the basis of body weight as being less appropriate than other bases, such as surface area,which is approximately proportional to (body weight)2/3. In Table2-2 selected values are given to compare the differences in dosageby the two alternatives. Given a dose of 100 mg/kg, it can be seenthat the dose (milligrams per animal), of course, is proportional tothe dose administered by body weight. Surface area is not propor-tional to weight: While the weight of a human is 3500 times greaterthan that of a mouse, the surface area of humans is only about 390 times greater than that of mice. Chemicals are usually admin-istered in toxicologic studies as milligrams per kilogram. The samedose given to humans and mice on a weight basis (mg/kg) wouldbe approximately 10 times greater in humans than mice if thatdosage were expressed per surface area (mg/cm2). Cancerchemotherapeutic agents are usually administered on the basis ofsurface area.

Shape of the Dose–Response Curve

Essential Nutrients The shape of the dose–response relationshiphas many important implications in toxicity assessment. For ex-ample, for substances that are required for normal physiologic func-tion and survival (e.g., vitamins and essential trace elements suchas chromium, cobalt, and selenium), the shape of the “graded”dose–response relationship in an individual over the entire doserange is actually U-shaped (Fig. 2-6). That is, at very low doses,there is a high level of adverse effect, which decreases with an in-creasing dose. This region of the dose–response relationship foressential nutrients is commonly referred to as a deficiency. As thedose is increased to a point where the deficiency no longer exists,no adverse response is detected and the organism is in a state ofhomeostasis. However, as the dose is increased to abnormally highlevels, an adverse response (usually qualitatively different from thatobserved at deficient doses) appears and increases in magnitudewith increasing dose, just as with other toxic substances. Thus, itis recognized that high doses of vitamin A can cause liver toxicityand birth defects, high doses of selenium can affect the brain, andhigh doses of estrogens may increase the risk of breast cancer, eventhough low doses of all these substances are essential for life.

Hormesis There is considerable evidence to suggest that somenonnutritional toxic substances may also impart beneficial or stim-ulatory effects at low doses but that, at higher doses, they produceadverse effects. This concept of “hormesis” was first described for


Figure 2-5. Comparison of dose–response relationship for two differentchemicals, plotted on a log dose-probit scale.

Note that the slope of the dose–response is steeper for chemical B thanchemical A. Dotted lines represents the confidence limits for chemical A.

Table 2-2Comparison of Dosage by Weight and Surface Area

SURFACE

WEIGHT DOSAGE DOSE AREA DOSAGE

g mg/kg mg/animal cm2 mg/cm2

Mouse 20 100 2 46 0.043Rat 200 100 20 325 0.061Guinea pig 400 100 40 565 0.071Rabbit 1500 100 150 1270 0.118Cat 2000 100 200 1380 0.145Monkey 4000 100 400 2980 0.134Dog 12,000 100 1200 5770 0.207Human 70,000 100 7000 18,000 0.388

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radiation effects but may also pertain to certain chemical responses(Calabrese and Baldwin, 1999). Thus, in plotting dose versus re-sponse over a wide range of doses, the effects of hormesis mayalso result in a “U-shaped” dose–response curve. In its original de-velopment, the concept of hormesis pertained to the ability of sub-stances to stimulate biological systems at low doses but to inhibitthem at high doses. The application of the concept of hormesis towhole-animal toxicologic dose–response relationships may also berelevant but requires that the “response” on the ordinate be variantwith dose. For example, chronic alcohol consumption is well rec-ognized to increase the risk of esophageal cancer, liver cancer, andcirrhosis of the liver at relatively high doses, and this response isdose-related (curve A, Fig. 2-7). However, there is also substantialclinical and epidemiologic evidence that low to moderate con-sumption of alcohol reduces the incidence of coronary heart dis-ease and stroke (curve B, Fig. 2-7) (Hanna et al., 1997). Thus,when all responses are plotted on the ordinate, a “U-shaped” dose–response curve is obtained (curve C, Fig. 2-7). U-shaped dose–response relationships have obvious implications for the process oflow dose extrapolation in risk assessment.

Another important aspect of the dose–response relationship atlow doses is the concept of the threshold. It has long been recog-nized that acute toxicologic responses are associated with thresh-olds; that is, there is some dose below which the probability of anindividual responding is zero. Obviously, the identification of athreshold depends on the particular response that is measured, thesensitivity of the measurement, and the number of subjects stud-ied. For the individual dose–response relationship, thresholds formost toxic effects certainly exist, although interindividual vari-ability in response and qualitative changes in response pattern withdose make it difficult to establish a true “no effects” threshold forany chemical. The biological basis of thresholds for acute responsesis well established and frequently can be demonstrated on the ba-

sis of mechanistic information (Aldridge, 1986). The traditionalapproaches to establishing acceptable levels of exposure to chem-icals are inherently different for threshold versus nonthreshold re-sponses. The existence of thresholds for chronic responses is lesswell defined, especially in the area of chemical carcinogenesis. Itis, of course, impossible to scientifically prove the absence of athreshold, as one can never prove a negative. Nevertheless, for theidentification of “safe” levels of exposure to a substance, the ab-sence or presence of a threshold is important for practical reasons(Chap. 4). A classic example of the difficulty of establishing thresh-olds experimentally is provided by the “ED01” study, where over24,000 mice and 81 different treatment groups were used to de-termine the shape of the dose–response relationship for the proto-


Figure 2-6. Individual dose–response relationship for an essential sub-stance such as a vitamin or trace element.

It is generally recognized that, for most types of toxic responses, a thresh-old exists such that at doses below the threshold, no toxicity is evident. Foressential substances, doses below the minimum daily requirement, as wellas those above the threshold for safety, may be associated with toxic ef-fects. The blue-shaded region represents the “region of homeostasis”—thedose range that results in neither deficiency or toxicity.

Figure 2-7. Hypothetical dose–response relationship depicting charac-teristics of hormesis.

Hormetic effects of a substance are hypothesized to occur when relativelylow doses result in the stimulation of a beneficial or protective response(B), such as induction of enzymatic pathways that protect against oxida-tive stress. Although low doses provide a potential beneficial effect, athreshold is exceeded as the dose increases and the net effects will be detri-mental (A), resulting in a typical dose-related increase in toxicity. The com-plete dose–response curve (C ) is conceptually similar to the individualdose–response relationship for essential nutrients shown in Fig. 2-6.

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typical carcinogen 2-acetylaminofluorene (2-AAF). The study wasdesigned to identify a statistically significant response of 1 percent(0.01 probability). The mice were exposed to 2-AAF at one ofseven different doses in the dose range of 30 to 150 ppm (plus 0dose control) (Littlefield et al., 1979). Eight “sacrifice intervals”were used to determine how quickly tumors developed. Thedose–response relationship between 2-AAF exposure and liver and bladder cancer at 24 and 33 months of exposure are shown inFig. 2-8. Both types of tumors demonstrated increasing incidencewith increasing dose, but the shapes of the two curves aredramatically different. For liver tumors, no clear threshold was ev-ident, whereas for bladder tumors, an apparent threshold wasevident. However, the apparent threshold, or “no observable ad-verse effect level” (NOAEL), for bladder cancer was lower at 33months (45 ppm) than at 24 months (75 ppm). Of course, the abil-ity to detect a low incidence of tumors depends on the number ofanimals used in the study. Thus, although a threshold (a dose be-

low which no response occurs) appears evident for bladder tumorsin Fig. 2-8, one cannot say for certain that tumors would not oc-cur if more animals had been included in the lower-dose groups.(See Chap. 4 for more discussion on statistical issues related to ex-trapolation of dose–response curves and the determination ofNOAELs.)

In evaluating the shape of the dose–response relationship inpopulations, it is realistic to consider inflections in the shape of thedose–response curve rather than absolute thresholds. That is, theslope of the dose–response relationship at high doses may be sub-stantially different from the slope at low doses, usually because ofdispositional differences in the chemical. Saturation of biotrans-formation pathways, protein-binding sites or receptors, and deple-tion of intracellular cofactors represent some reasons why sharpinflections in the dose–response relationship may occur. For ex-ample, the widely used analgesic acetaminophen has a very lowrate of liver toxicity at normal therapeutic doses. Even though atoxic metabolite [N-acetyl-p-benzoquinoneimine (NAPQI)] is pro-duced in the liver at therapeutic doses, it is rapidly detoxifiedthrough conjugation with the intracellular antioxidant glutathione.However, at very high doses, the level of intracellular glutathionein the liver is depleted and NAPQI accumulates, causing seriousand potentially fatal liver toxicity. This effect is analogous to therapid change in pH of a buffered solution that occurs when thebuffer capacity is exceeded. Some toxic responses, most notablythe development of cancer after the administration of genotoxiccarcinogens, are often considered to be linear at low doses and thusdo not exhibit a threshold. In such circumstances, there is no dosewith “zero” risk, although the risk decreases proportionately witha decrease in the dose. The existence or lack of existence of athreshold dose for carcinogens has many regulatory implicationsand is a point of considerable controversy and research in the fieldof quantitative risk assessment for chemical carcinogens (Chap. 4).

Assumptions in Deriving the Dose–Response Relationship

A number of assumptions must be considered before dose–responserelationships can be used appropriately. The first is that the re-sponse is due to the chemical administered. To describe the rela-tionship between a toxic material and an observed effect or re-sponse, one must know with reasonable certainty that therelationship is indeed a causal one. For some data, it is not alwaysapparent that the response is a result of chemical exposure. For ex-ample, an epidemiologic study might result in the discovery of an“association” between a response (e.g., disease) and one or morevariables. Frequently, the data are presented similarly to the pre-sentation of “dose response” in pharmacology and toxicology. Useof the dose response in this context is suspect unless other con-vincing evidence supports a causal connection between the esti-mated dose and the measured endpoint (response). Unfortunately,in nearly all retrospective and case-control studies and even inmany prospective studies, the dose, duration, frequency, and routesof exposure are seldom quantified, and other potential etiologicfactors are frequently present. In its most strict usage, then, thedose–response relationship is based on the knowledge that the ef-fect is a result of a known toxic agent or agents.

A second assumption seems simple and obvious: The magni-tude of the response is in fact related to the dose. Perhaps becauseof its apparent simplicity, this assumption is often a source of mis-


Figure 2-8. Dose–response relationship for carcinogens.

Eight groups of male mice were administered 2-acetylaminofluorine (2-AAF) in the diet from weaning. The percent of animals with liver (blueline) or bladder (black line) tumors at 24 months (A) or 33 months (B) areshown. Most of the animals in the high-dose group (150 ppm) did not sur-vive to 33 months; thus, those data are not shown in B.

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understanding. It is really a composite of three other assumptionsthat recur frequently:

1. There is a molecular target site (or sites) with which the chem-ical interacts to initiate the response.

2. The production of a response and the degree of response arerelated to the concentration of the agent at the target site.

3. The concentration at the site is, in turn, related to the dose ad-ministered.

The third assumption in using the dose–response relationshipis that there exists both a quantifiable method of measuring and aprecise means of expressing the toxicity. For any given dose–response relationship, a great variety of criteria or endpoints of tox-icity could be used. The ideal criterion would be one closely as-sociated with the molecular events resulting from exposure to thetoxicant. It follows from this that a given chemical may have afamily of dose–response relationships, one for each toxic endpoint.For example, a chemical that produces cancer through genotoxiceffects, liver damage through inhibition of a specific enzyme, andCNS effects via a different mechanism may have three distinctdose–response relationships, one for each endpoint. Early in theassessment of toxicity, little mechanistic information is usuallyavailable; thus establishing a dose–response relationship based onthe molecular mechanism of action is usually impossible. Indeed,it might not be approachable even for well-known toxicants. In theabsence of a mechanistic, molecular ideal criterion of toxicity, onelooks to a measure of toxicity that is unequivocal and clearly rel-evant to the toxic effect. Such measures are often referred to as“effects-related biomarkers.” For example, with a new compoundchemically related to the class of organophosphate insecticides, onemight approach the measurement of toxicity by measuring the in-hibition of cholinesterase in blood. In this way, one would be meas-uring, in a readily accessible system and using a technique that isconvenient and reasonably precise, a prominent effect of the chem-ical and one that is usually pertinent to the mechanism by whichtoxicity is produced.

The selection of a toxic endpoint for measurement is not al-ways so straightforward. Even the example cited above may bemisleading, as an organophosphate may produce a decrease inblood cholinesterase, but this change may not be directly relatedto its toxicity. As additional data are gathered to suggest a mech-anism of toxicity for any substance, other measures of toxicity maybe selected. Although many endpoints are quantitative and precise,they are often indirect measures of toxicity. Changes in enzymelevels in blood can be indicative of tissue damage. For example,alanine aminotransferase (ALT) and aspartate aminotransferase(AST) are used to detect liver damage. Use of these enzymes inserum is yet another example of an effects-related biomarker be-cause the change in enzyme activity in the blood is directly relatedto damage to liver cells. Much of clinical diagnostic medicine re-lies on effects-related biomarkers, but to be useful the relationshipbetween the biomarker and the disease must be carefully estab-lished. Patterns of isozymes and their alteration may provide in-sight into the organ or system that is the site of toxic effects.

Many direct measures of effects also are not necessarily re-lated to the mechanism by which a substance produces harm to anorganism but have the advantage of permitting a causal relation tobe drawn between the agent and its action. For example, meas-urement of the alteration of the tone of smooth or skeletal muscle

for substances acting on muscles represents a fundamental ap-proach to toxicological assessment. Similarly, measures of heartrate, blood pressure, and electrical activity of heart muscle, nerve,and brain are examples of the use of physiologic functions as in-dexes of toxicity. Measurement can also take the form of a stillhigher level of integration, such as the degree of motor activity orbehavioral change.

The measurements used as examples in the preceding discus-sion all assume prior information about the toxicant, such as itstarget organ or site of action or a fundamental effect. However,such information is usually available only after toxicologic screen-ing and testing based on other measures of toxicity. With a newsubstance, the customary starting point in toxicologic evaluationutilizes lethality as an index. Determination of lethality is precise,quantal, and unequivocal and is therefore useful in its own right,if only to suggest the level and magnitude of the potency of a sub-stance. Lethality provides a measure of comparison among manysubstances whose mechanisms and sites of action may be markedlydifferent. Furthermore, from these studies, clues to the direction offurther studies are obtained. This comes about in two importantways. First, simply recording a death is not an adequate means ofconducting a lethality study with a new substance. A key elementmust be a careful, disciplined, detailed observation of the intact an-imal extending from the time of administration of the toxicant tothe death of the animal. From properly conducted observations, im-mensely informative data can be gathered by a trained toxicolo-gist. Second, a lethality study ordinarily is supported by histologicexamination of major tissues and organs for abnormalities. Fromthese observations, one can usually obtain more specific informa-tion about the events leading to the lethal effect, the target organsinvolved, and often a suggestion about the possible mechanism oftoxicity at a relatively fundamental level.

Evaluating the Dose–ResponseRelationship

Comparison of Dose Responses Figure 2-9 illustrates a hypo-thetical quantal dose–response curve for a desirable effect of achemical effective dose (ED) such as anesthesia, a toxic dose (TD)effect such as liver injury, and the lethal dose (LD). As depicted inFig. 2-9, a parallelism is apparent between the (ED) curve and thecurve depicting mortality (LD). It is tempting to view the paralleldose–response curves as indicative of identity of mechanism—thatis, to conclude that the lethality is a simple extension of the ther-apeutic effect. While this conclusion may ultimately prove to becorrect in any particular case, it is not warranted solely on the ba-sis of the two parallel lines. The same admonition applies to anypair of parallel “effect” curves or any other pair of toxicity or lethal-ity curves.

Therapeutic Index The hypothetical curves in Fig. 2-9 illustratetwo other interrelated points: the importance of the selection of thetoxic criterion and the interpretation of comparative effect. Theconcept of the “therapeutic index,” which was introduced by PaulEhrlich in 1913, can be used to illustrate this relationship. Althoughthe therapeutic index is directed toward a comparison of the ther-apeutically effective dose to the toxic dose of a chemical, it isequally applicable to considerations of comparative toxicity. Thetherapeutic index (TI) in its broadest sense is defined as the ratioof the dose required to produce a toxic effect and the dose needed


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to elicit the desired therapeutic response. Similarly, an index ofcomparative toxicity is obtained by the ratio of doses of two different materials to produce an identical response or the ratio of doses of the same material necessary to yield different toxiceffects.

The most commonly used index of effect, whether beneficialor toxic, is the median dose—that is, the dose required to result ina response in 50 percent of a population (or to produce 50 percentof a maximal response). The therapeutic index of a drug is an ap-proximate statement about the relative safety of a drug expressedas the ratio of the lethal or toxic dose to the therapeutic dose:

TI � LD50/ED50

From Fig. 2-9 one can approximate a therapeutic index by us-ing these median doses. The larger the ratio, the greater the rela-tive safety. The ED50 is approximately 20, and the LD50 is about200; thus, the therapeutic index is 10, a number indicative of a rel-atively safe drug. However, the use of the median effective and me-dian lethal doses is not without disadvantages, because mediandoses tell nothing about the slopes of the dose–response curves fortherapeutic and toxic effects.

Margins of Safety and Exposure One way to overcome this de-ficiency is to use the ED99 for the desired effect and the LD1 forthe undesired effect. These parameters are used in the calculationof the margin of safety:

Margin of safety � LD1/ED99

The quantitative comparisons described above have been usedmainly after a single administration of chemicals. However, forchemicals for which there is no beneficial or effective dose and ex-posures are likely to occur repeatedly, the ratio of LD1 to ED99 haslittle relevance. Thus, for non drug chemicals, the term margin ofsafety has found use in risk-assessment procedures as an indicatorof the magnitude of the difference between an estimated “exposed

dose” to a human population and the NOAEL determined in ex-perimental animals.

A measure of the degree of accumulation of a chemical and/orits toxic effects can also be estimated from quantal toxicity data.The chronicity index of a chemical is a unitless value obtained bydividing its 1-dose LD50 by its 90-dose (90-day) LD50, with bothexpressed in milligrams per kilogram per day. Theoretically, if nocumulative effect occurs over the doses, the chronicity index willbe 1. If a compound were absolutely cumulative, the chronicity in-dex would be 90.

Statistical procedures similar to those used to calculate theLD50 can also be used to determine the lethal time 50 (LT50), orthe time required for half the animals to die (Litchfield, 1949). TheLT50 value for a chemical indicates the time course of the toxic ef-fects but does not indicate whether one chemical is more toxic thananother.

Frequently, dose–response curves from repeated-dose exper-imental animal studies (subacute, subchronic, or chronic) are usedto estimate the NOAEL, or some other “benchmark” measure ofminimal toxic response, such as the dose estimated to produce toxiceffects in 10 percent of the population (TD10) (see also Chap. 4).These estimates of minimal toxic dose, derived from quantaldose–response curves, can be used in risk assessment to derive a“margin of exposure” (MOE) index. This index compares the es-timated daily exposure, in milligrams per kilogram per day, thatmight occur under a given set of circumstances to some estimatedvalue from the quantal dose–response relationship (e.g., NOAELor TD10). Like the MOS, the MOE is often expressed as a ratio ofthese two values. Thus, for example, if an estimate of human ex-posure to a pesticide residue yielded a value of 0.001 mg/kg/day,and a TD10 of 1 mg/kg/day was determined for that same pesti-cide, the MOE would be 1000. This value indicates that the esti-mate of daily exposure under the described set of conditions is1000 times lower than the estimated daily dose that would causeevident toxicity in 10% of exposed animals. (See Chap. 4 for amore complete discussion of benchmark doses, NOAELs, andMOE.)

Potency versus Efficacy To compare the toxic effects of two ormore chemicals, the dose response to the toxic effects of eachchemical must be established. One can then compare the potencyand maximal efficacy of the two chemicals to produce a toxic ef-fect. These two important terms can be explained by reference toFig. 2-10, which depicts dose–response curves to four differentchemicals for the frequency of a particular toxic effect, such as theproduction of tumors. Chemical A is said to be more potent thanchemical B because of their relative positions along the dosageaxis. Potency thus refers to the range of doses over which a chem-ical produces increasing responses. Thus, A is more potent than Band C is more potent than D. Maximal efficacy reflects the limitof the dose–response relationship on the response axis to a certainchemical. Chemicals A and B have equal maximal efficacy,whereas the maximal efficacy of C is less than that of D.

VARIATION IN TOXIC RESPONSES

Selective Toxicity

Selective toxicity means that a chemical produces injury to one kindof living matter without harming another form of life even thoughthe two may exist in intimate contact (Albert, 1965, 1973). The


Figure 2-9. Comparison of effective dose (ED), toxic dose (TD), andlethal dose (LD).

The plot is of log dosage versus percentage of population responding inprobit units.

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living matter that is injured is termed the uneconomic form (or un-desirable), and the matter protected is called the economic form (ordesirable). They may be related to each other as parasite and hostor may be two tissues in one organism. This biological diversityinterferes with the ability of ecotoxicologists to predict the toxiceffects of a chemical in one species (humans) from experimentsperformed in another species (laboratory animals). However, bytaking advantage of the biological diversity, it is possible to de-velop agents that are lethal for an undesired species and harmlessfor other species. In agriculture, for example, there are fungi, in-sects, and even competitive plants that injure the crop, and thus se-lective pesticides are needed. Similarly, animal husbandry and hu-man medicine require agents, such as antibiotics, that areselectively toxic to the undesirable form but do not produce dam-age to the desirable form.

Drugs and other chemical agents used for selective toxic pur-poses are selective for one of two reasons. Either (1) the chemicalis equitoxic to both economic and uneconomic cells but is accu-mulated mainly by uneconomic cells or (2) it reacts fairly specif-ically with a cytological or a biochemical feature that is absentfrom or does not play an important role in the economic form (Al-bert, 1965, 1973). Selectivity resulting from differences in distri-bution usually is caused by differences in the absorption, bio-transformation, or excretion of the toxicant. The selective toxicityof an insecticide spray may be partly due to a larger surface areaper unit weight that causes the insect to absorb a proportionallylarger dose than does the mammal being sprayed. The effective-ness of radioactive iodine in the treatment of hyperthyroidism (aswell as its thyroid carcinogenicity) is due to the selective abilityof the thyroid gland to accumulate iodine. A major reason whychemicals are toxic to one but not to another type of tissue is thatthere are differences in accumulation of the ultimate toxic com-pound in various tissues. This, in turn, may be due to differencesin the ability of various tissues to biotransform the chemical intothe ultimate toxic product.

Selective toxicity caused by differences in comparative cy-tology is exemplified by a comparison of plant and animal cells.Plants differ from animals in many ways—for example, absenceof a nervous system, an efficient circulatory system, and musclesas well as the presence of a photosynthetic mechanism and cell

walls. The fact that bacteria contain cell walls and humans do nothas been utilized in developing selective toxic chemotherapeuticagents, such as penicillin and cephalosporins, that kill bacteria butare relatively nontoxic to mammalian cells.

Selective toxicity also can be a result of a difference in bio-chemistry in the two types of cells. For example, bacteria do notabsorb folic acid but synthesize it from p-aminobenzoic acid, glu-tamic acid, and pteridine, whereas mammals cannot synthesizefolic acid but have to absorb it from the diet. Thus, sulfonamidedrugs are selectively toxic to bacteria because the sulfonamides,which resemble p-aminobenzoic acid in both charge and dimen-sions, antagonize the incorporation of p-aminobenzoic acid into thefolic acid molecule—a reaction that humans do not carry out.

Species Differences

Although a basic tenet of toxicology is that “experimental resultsin animals, when properly qualified, are applicable to humans,” itis important to recognize that both quantitative and qualitative dif-ferences in response to toxic substances may occur among differ-ent species. As discussed above, there are many reasons for selec-tive toxicity among different species. Even among phylogeneticallysimilar species (e.g., rats, mice, guinea pigs, and hamsters), largedifferences in response may occur. For example, the LD50 for thehighly toxic dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD),differs by more than 1000-fold between guinea pigs and hamsters.Not only does the lethal dose for TCDD vary widely among species,so do the particular target organs affected. Species differences inresponse to carcinogenic chemicals represent an important issue inregulatory risk assessment. As discussed in Chap. 4, extrapolationof laboratory animal data to infer human cancer risk is currently akey component of regulatory decision making. The validity of thisapproach of course depends on the relevance of the experimentalanimal model to humans. Large differences in carcinogenic re-sponse between experimental animal species are not unusual. Forexample, mice are highly resistant to the hepatocarcinogenic ef-fects of the fungal toxin aflatoxin B1. Dietary doses as high as10,000 parts per billion (ppb) failed to produce liver cancer in mice,whereas in rats dietary doses as low as 15 ppb produced a signif-icant increase in liver tumors (Wogan et al., 1974). The mechanis-


Figure 2-10. Schematic representation of the difference in the dose–response curves for four chemicals (A–D),illustrating the difference between potency and efficacy (see text).

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tic basis for this dramatic difference in response appears to beentirely related to species differences in the expression of a par-ticular form of glutathione S-transferase (mGSTA3-3) that has un-usually high catalytic activity toward the carcinogenic epoxide ofaflatoxin (Eaton and Gallagher, 1994). Mice express this enzymeconstitutively, whereas rats normally express a closely related formwith much less detoxifying activity toward aflatoxin epoxide. In-terestingly, rats do possess the gene for a form of glutathione S-transferase with high catalytic activity toward aflatoxin epoxide(rGSTA5-5) that is inducible by certain dietary antioxidants anddrugs. Thus, dietary treatment can dramatically change the sensi-tivity of a species to a carcinogen.

Other examples in which large species differences in responseto carcinogens have been observed include the development of re-nal tumors from 2,3,5-trimethylpentane and d-limonene in malerats (Lehman-McKeeman and Caudill, 1992), the production ofliver tumors from “peroxisomal proliferators” such as the antilipi-demic drug clofibrate and the common solvent trichloroethylene(Roberts, 1999), and the induction of nasal carcinomas in rats af-ter inhalation exposure to formaldehyde (Monticello and Morgan,1997).

Identifying the mechanistic basis for species differences in re-sponse to chemicals is an important part of toxicology because onlythrough a thorough understanding of these differences can the rel-evance of animal data to human response be verified.

Individual Differences in Response

Even within a species, large interindividual differences in responseto a chemical can occur because of subtle genetic differences.Hereditary differences in a single gene that occur in more than1 percent of the population are referred to as genetic polymorphismand may be responsible for idiosyncratic reactions to chemicals,as discussed earlier in this chapter. However, genetic polymorphismmay have other important but less dramatic effects than those de-scribed for acute idiosyncratic responses (such as that occurring inpseudocholinesterase-deficient individuals after succinylcholineexposure). For example, it is recognized that approximately 50 per-cent of the Caucasian population has a gene deletion for the en-zyme glutathione S-transferase M1. This enzyme has no apparentsignificant physiologic function, and thus homozygotes for the genedeletion (e.g., those who lack both copies of the normal gene) arefunctionally and physiologically normal. However, epidemiologicstudies have indicated that smokers who are homozygous for thenull allele may be at slightly increased risk of developing lung can-cer compared with smokers who have one or both copies of thenormal gene (Houlston, 1999; Strange and Fryer, 1999). Chapter 6provides additional examples of genetic differences in biotrans-formation enzymes that may be important determinants of vari-ability in individual susceptibility to chemical exposures.

Genetic polymorphism in physiologically important genesmay also be responsible for interindividual differences in toxic re-sponses. For example, studies in transgenic mice have shown thatmice possessing one copy of a mutated p53 gene (a so-called tu-mor suppressor gene; see Chap. 8) are much more susceptible tosome chemical carcinogens than are mice with two normal copiesof the gene (Tennant et al., 1999). In humans, there is evidencethat possessing one mutated copy of a tumor suppressor genegreatly increases the risk of developing certain cancers. For ex-ample, retinoblastoma is a largely inherited form of cancer thatarises because of the presence of two copies of a defective tumor

suppressor gene (the Rb gene) (Wiman, 1993). Individuals withone mutated copy of the Rb gene and one normal copy are not des-tined to acquire the disease (as are those with two copies of themutated gene), although their chance of acquiring it is much greaterthan that of persons with two normal Rb genes. This is the casebecause both copies of the gene must be nonfunctional for the dis-ease to develop. With one mutated copy present genetically, theprobability of acquiring a mutation of the second gene (potentiallyfrom exposure to environmental mutagens) is much greater thanthe probability of acquiring independent mutations in both copiesof the gene as would be necessary in people with two normal Rballeles. (See Chap. 8 for additional discussion of tumor suppressorgenes.)

As our understanding of the human genome increases, more“susceptibility” genes will be discovered, and it is likely that theetiology of many chronic diseases will be shown to be related toa combination of genetics and environment. Simple blood tests mayultimately be developed that allow an individual to learn whetherhe or she may be particularly susceptible to specific drugs or en-vironmental pollutants. Although the public health significance ofthis type of information could be immense, the disclosure of suchinformation raises many important ethical and legal issues that mustbe addressed before wide use of such tests.

DESCRIPTIVE ANIMAL TOXICITYTESTS

Two main principles underlie all descriptive animal toxicity test-ing. The first is that the effects produced by a compound in labo-ratory animals, when properly qualified, are applicable to humans.This premise applies to all of experimental biology and medicine.On the basis of dose per unit of body surface, toxic effects in hu-mans are usually in the same range as those in experimental ani-mals. On a body weight basis, humans are generally more vulner-able than are experimental animals, probably by a factor of about10. When one has an awareness of these quantitative differences,appropriate safety factors can be applied to calculate relatively safedoses for humans. All known chemical carcinogens in humans,with the possible exception of arsenic, are carcinogenic in somespecies but not in all laboratory animals. It has become increas-ingly evident that the converse—that all chemicals carcinogenic inanimals are also carcinogenic in humans—is not true (Dybing andSanner, 1999; Grisham, 1997; Hengstler et al., 1999). However, forregulatory and risk assessment purposes, positive carcinogenicitytests in animals are usually interpreted as indicative of potentialhuman carcinogenicity. If a clear understanding of the mechanismof action of the carcinogen indicates that a positive response in an-imals is not relevant to humans, a positive animal bioassay may beconsidered irrelevant for human risk assessment (see Chap. 4). Thisspecies variation in carcinogenic response appears to be due inmany instances to differences in biotransformation of the procar-cinogen to the ultimate carcinogen (see Chap. 6).

The second principle is that exposure of experimental animalsto toxic agents in high doses is a necessary and valid method ofdiscovering possible hazards in humans. This principle is based onthe quantal dose–response concept that the incidence of an effectin a population is greater as the dose or exposure increases. Prac-tical considerations in the design of experimental model systemsrequire that the number of animals used in toxicology experimentsalways be small compared with the size of human populations atrisk. Obtaining statistically valid results from such small groups of


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animals requires the use of relatively large doses so that the effectwill occur frequently enough to be detected. However, the use ofhigh doses can create problems in interpretation if the response(s)obtained at high doses does not occur at low doses. Thus, for ex-ample, it has been shown that bladder tumors observed in rats fedvery high doses of saccharin will not occur at the much lower dosesof saccharin encountered in the human diet. At the high concen-trations fed to rats, saccharin forms an insoluble precipitate in thebladder that subsequently results in chronic irritation of bladderepithelium, enhanced cell proliferation, and ultimately bladder tu-mors (Cohen, 1998, 1999). In vitro studies have shown that pre-cipitation of saccharin in human urine will not occur at the con-centrations that could be obtained from even extraordinaryconsumption of this artificial sweetener. Examples such as this il-lustrate the importance of considering the molecular, biochemical,and cellular mechanisms responsible for toxicological responseswhen extrapolating from high to low dose and across species.

Toxicity tests are not designed to demonstrate that a chemi-cal is safe but to characterize the toxic effects a chemical can pro-duce. There are no set toxicology tests that have to be performedon every chemical intended for commerce. Depending on the even-tual use of the chemical, the toxic effects produced by structuralanalogs of the chemical, as well as the toxic effects produced bythe chemical itself, contribute to the determination of the toxicol-ogy tests that should be performed. However, the FDA, EPA, andOrganization for Economic Cooperation and Development (OECD)have written good laboratory practice (GLP) standards. Theseguidelines are expected to be followed when toxicity tests are con-ducted in support of the introduction of a chemical to the market.

The following sections provide an overview of basic toxicitytesting procedures in use today. For a detailed description of thesetests, the reader is referred to several authoritative texts on this sub-ject (Auletta, 1995; Hayes, 2001; Weathereholtz, 1997).

Acute Lethality

The first toxicity test performed on a new chemical is acute toxi-city. The LD50 and other acute toxic effects are determined afterone or more routes of administration (one route being oral or theintended route of exposure) in one or more species. The speciesmost often used are the mouse and rat, but sometimes the rabbitand dog are employed. Studies are performed in both adult maleand female animals. Food is often withheld the night before dos-ing. The number of animals that die in a 14-day period after a sin-gle dosage is tabulated. In addition to mortality and weight, dailyexamination of test animals should be conducted for signs of in-toxication, lethargy, behavioral modifications, morbidity, food con-sumption, and so on. Acute toxicity tests (1) give a quantitative es-timate of acute toxicity (LD50) for comparison with othersubstances, (2) identify target organs and other clinical manifesta-tions of acute toxicity, (3) establish the reversibility of the toxic re-sponse, and (4) provide dose-ranging guidance for other studies.

Determination of the LD50 has become a public issue becauseof increasing concern for the welfare and protection of laboratoryanimals. The LD50 is not a biological constant. Many factors in-fluence toxicity and thus may alter the estimation of the LD50 inany particular study. Factors such as animal strain, age and weight,type of feed, caging, pretrial fasting time, method of administra-tion, volume and type of suspension medium, and duration of ob-servation have all been shown to influence adverse responses totoxic substances. These and other factors have been discussed in

detail in earlier editions of this textbook (Doull, 1980). Because ofthis inherent variability in LD50 estimates, it is now recognized thatfor most purposes it is only necessary to characterize the LD50

within an order of magnitude range such as 5 to 50 mg/kg, 50 to500 mg/kg, and so on.

There are several traditional approaches to determining theLD50 and its 95 percent confidence limit as well as the slope ofthe probit line. The reader is referred to the classic works of Litch-field and Wilcoxon (1949), Bliss (1957), and Finney (1971) for adescription of the mechanics of these procedures. A computer pro-gram in BASIC for determining probit and log-probit or logit cor-relations has been published (Abou-Setta et al., 1986). These tra-ditional methods for determining LD50s require a relatively largenumber of animals (40 to 50). Other statistical techniques that re-quire fewer animals, such as the “moving averages” method ofThompson and Weill (Weil, 1952), are available but do not provideconfidence limits for the LD50 and the slope of the probit line.Finney (1985) has succinctly summarized the advantages and de-ficiencies of many of the traditional methods. For most circum-stances, an adequate estimate of the LD50 and an approximationof the 95 percent confidence intervals can be obtained with as fewas 6 to 9 animals, using the “up-and-down” method as modifiedby Bruce (1985). When this method was compared with traditionalmethods that typically utilize 40 to 50 animals, excellent agree-ment was obtained for all 10 compounds tested (Bruce, 1987). Inmice and rats the LD50 is usually determined as described above,but in the larger species only an approximation of the LD50 is ob-tained by increasing the dose in the same animal until serious toxiceffects are evident.

If there is a reasonable likelihood of substantial exposure tothe material by dermal or inhalation exposure, acute dermal andacute inhalation studies are performed. When animals are exposedacutely to chemicals in the air they breathe or the water they (fish)live in, the dose the animals receive is usually not known. For thesesituations, the lethal concentration 50 (LC50) is usually determined;that is, the concentration of chemical in the air or water that causesdeath to 50 percent of the animals. In reporting an LC50, it is im-perative that the time of exposure be indicated. The acute dermaltoxicity test is usually performed in rabbits. The site of applicationis shaved. The test substance is kept in contact with the skin for24 h by wrapping the skin with an impervious plastic material. Atthe end of the exposure period, the wrapping is removed and theskin is wiped to remove any test substance still remaining. Ani-mals are observed at various intervals for 14 days, and the LD50 iscalculated. If no toxicity is evident at 2 g/kg, further acute dermaltoxicity testing is usually not performed. Acute inhalation studiesare performed that are similar to other acute toxicity studies ex-cept that the route of exposure is inhalation. Most often, the lengthof exposure is 4 h.

Although by themselves LD50 and LC50 values are of limitedsignificance, acute lethality studies are essential for characterizingthe toxic effects of chemicals and their hazard to humans. The mostmeaningful scientific information derived from acute lethality testscomes from clinical observations and postmortem examination ofanimals rather than from the specific LD50 value.

Skin and Eye Irritations

The ability of a chemical to irritate the skin and eye after an acuteexposure is usually determined in rabbits. For the dermal irritationtest (Draize test), rabbits are prepared by removal of fur on a sec-


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tion of the back by electric clippers. The chemical is applied to theskin (0.5 mL of liquid or 0.5 g of solid) under four covered gauzepatches (1 in. square; one intact and two abraded skin sites on eachanimal) and usually kept in contact for 4 h. The nature of the cov-ering patches depends on whether occlusive, semiocclusive, ornonocclusive tests are desired. For occlusive testing, the test ma-terial is covered with an impervious plastic sheet; for semiocclu-sive tests, a gauze dressing may be used. Occasionally, studies mayrequire that the material be applied to abraded skin. The degree ofskin irritation is scored for erythema (redness), eschar (scab) andedema (swelling) formation, and corrosive action. These dermal ir-ritation observations are repeated at various intervals after the cov-ered patch has been removed. To determine the degree of ocularirritation, the chemical is instilled into one eye (0.1 mL of liquidor 100 mg of solid) of each test rabbit. The contralateral eye isused as the control. The eyes of the rabbits are then examined atvarious times after application.

Controversy over this test has led to the development of al-ternative in vitro models for evaluating cutaneous and ocular tox-icity of substances. The various in vitro methods that have beenevaluated for this purpose include epidermal keratinocyte andcorneal epithelial cell culture models. These and other in vitro testshave been reviewed recently (Davila et al., 1998).

Sensitization

Information about the potential of a chemical to sensitize skin isneeded in addition to irritation testing for all materials that mayrepeatedly come into contact with the skin. Numerous procedureshave been developed to determine the potential of substances toinduce a sensitization reaction in humans (delayed hypersensitiv-ity reaction), including the Draize test, the open epicutaneous test,the Buehler test, Freund’s complete adjuvant test, the optimizationtest, the split adjuvant test, and the guinea pig maximization test(Maibach and Patrick, 2001; Rush et al., 1995). Although they dif-fer in regard to route and frequency of duration, they all utilize theguinea pig as the preferred test species. In general, the test chem-ical is administered to the shaved skin topically, intradermally, orboth and may include the use of adjuvant to enhance the sensitiv-ity of the assay. Multiple administrations of the test substance aregenerally given over a period of 2 to 4 weeks. Depending on thespecific protocol, the treated area may be occluded. Some 2 to 3weeks after the last treatment, the animals are challenged with anonirritating concentration of the test substance and the develop-ment of erythema is evaluated.

Subacute (Repeated-Dose Study)

Subacute toxicity tests are performed to obtain information on thetoxicity of a chemical after repeated administration and as an aidto establish doses for subchronic studies. A typical protocol is togive three to four different dosages of the chemicals to the animalsby mixing it in their feed. For rats, 10 animals per sex per dose areoften used; for dogs, three dosages and 3 to 4 animals per sex areused. Clinical chemistry and histopathology are performed after 14days of exposure, as described below in the section on subchronictoxicity testing.

Subchronic

The toxicity of a chemical after subchronic exposure is then de-termined. Subchronic exposure can last for different periods of

time, but 90 days is the most common test duration. The principalgoals of the subchronic study are to establish a NOAEL and to fur-ther identify and characterize the specific organ or organs affectedby the test compound after repeated administration. One may alsoobtain a “lowest observed adverse effect level” (LOAEL) as wellas the NOAEL for the species tested. The numbers obtained forNOAEL and LOAEL will depend on how closely the dosages arespaced and the number of animals examined. Determinations ofNOAELs and LOAELs have numerous regulatory implications. Forexample, the EPA utilizes the NOAEL to calculate the referencedose (RfD), which may be used to establish regulatory values for“acceptable” pollutant levels (Barnes and Dourson, 1988)(Chap. 4). An alternative to the NOAEL approach referred to as the benchmark dose uses all the experimental data to fit one or more dose–response curves (Crump, 1984). These curves are then used to estimate a benchmark dose that is definedas “the statistical lower bound on a dose corresponding to a specified level of risk” (Allen et al., 1994a). Although subchronicstudies are frequently the primary or sole source of experi-mental data to determine both the NOAEL and the bench-mark dose, these concepts can be applied to other types of toxicity testing protocols, such as that for chronic toxicity or de-velopmental toxicity (Allen et al., 1994a, 1994b; Faustman et al.,1994) (see also Chap. 4 for a complete discussion of the deriva-tion and use of NOAELs, RfDs, and benchmark doses). If chronicstudies have been completed, these data are generally used forNOAEL and LOAEL estimates in preference to data from sub-chronic studies.

A subchronic study is usually conducted in two species (ratand dog) by the route of intended exposure (usually oral). At leastthree doses are employed (a high dose that produces toxicity butdoes not cause more than 10 percent fatalities, a low dose that pro-duces no apparent toxic effects, and an intermediate dose) with 10to 20 rats and 4 to 6 dogs of each sex per dose. Each animal shouldbe uniquely identified with permanent markings such as ear tags,tattoos, or electronically coded microchip implants. Only healthyanimals should be used, and each animal should be housed indi-vidually in an adequately controlled environment. Animals shouldbe observed once or twice daily for signs of toxicity, includingchanges in body weight, diet consumption, changes in fur color ortexture, respiratory or cardiovascular distress, motor and behav-ioral abnormalities, and palpable masses. All premature deathsshould be recorded and necropsied as soon as possible. Severelymoribund animals should be terminated immediately to preservetissues and reduce unnecessary suffering. At the end of the 90-daystudy, all the remaining animals should be terminated and bloodand tissues should be collected for further analysis. The gross andmicroscopic condition of the organs and tissues (about 15 to 20)and the weight of the major organs (about 12) are recorded andevaluated. Hematology and blood chemistry measurements are usu-ally done before, in the middle of, and at the termination of expo-sure. Hematology measurements usually include hemoglobin con-centration, hematocrit, erythrocyte counts, total and differentialleukocyte counts, platelet count, clotting time, and prothrombintime. Clinical chemistry determinations commonly made includeglucose, calcium, potassium, urea nitrogen, alanine aminotrans-ferase (ALT), serum aspartate aminotransferase (AST), gamma-glutamyltranspeptidase (GGT), sorbitol dehydrogenase, lactic dehydrogenase, alkaline phosphatase, creatinine, bilirubin, triglyc-erides, cholesterol, albumin, globulin, and total protein. Urinalysisis usually performed in the middle of and at the termination of the


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testing period and often includes determination of specific gravityor osmolarity, pH, proteins, glucose, ketones, bilirubin, and uro-bilinogen as well as microscopic examination of formed elements.If humans are likely to have significant exposure to the chemicalby dermal contact or inhalation, subchronic dermal and/or inhala-tion experiments may also be required. Subchronic toxicity stud-ies not only characterize the dose–response relationship of a testsubstance after repeated administration but also provide data for amore reasonable prediction of appropriate doses for chronic expo-sure studies.

For chemicals that are to be registered as drugs, acute andsubchronic studies (and potentially additional special tests if achemical has unusual toxic effects or therapeutic purposes) mustbe completed before the company can file an Investigational NewDrug (IND) application with the FDA. If the application is ap-proved, clinical trials can commence. At the same time phase I,phase II, and phase III clinical trials are performed, chronic expo-sure of the animals to the test compound can be carried out in lab-oratory animals, along with additional specialized tests.

Chronic

Long-term or chronic exposure studies are performed similarly tosubchronic studies except that the period of exposure is longer than3 months. In rodents, chronic exposures are usually for 6 monthsto 2 years. Chronic studies in nonrodent species are usually for1 year but may be longer. The length of exposure is somewhat de-pendent on the intended period of exposure in humans. If the agentis a drug planned to be used for short periods, such as an antimi-crobial agent, a chronic exposure of 6 months may be sufficient,whereas if the agent is a food additive with the potential for life-time exposure in humans, a chronic study up to 2 years in dura-tion is likely to be required.

Chronic toxicity tests are performed to assess the cumulativetoxicity of chemicals, but the study design and evaluation often in-clude a consideration of the carcinogenic potential of chemicals sothat a separate lifetime feeding study that addresses carcinogenic-ity does not have to be performed. These studies usually are per-formed in rats and mice and extend over the average lifetime ofthe species (18 months to 2 years for mice; 2 to 2.5 years for rats).To ensure that 30 rats per dose survive the 2-year study, 60 ratsper group per sex are often started in the study. Both gross and mi-croscopic pathological examinations are made not only on animalsthat survive the chronic exposure but also on those that die pre-maturely.

Dose selection is critical in these studies to ensure that pre-mature mortality from chronic toxicity does not limit the numberof animals that survive to a normal life expectancy. Most regula-tory guidelines require that the highest dose administered be theestimated maximum tolerable dose (MTD). This is generally de-rived from subchronic studies, but additional longer studies (e.g.,6 months) may be necessary if delayed effects or extensive cumu-lative toxicity are indicated in the 90-day subchronic study. TheMTD has had various definitions (Haseman, 1985). The MTD hasbeen defined by some regulatory agencies as the dose that sup-presses body weight gain slightly (i.e., 10 percent) in a 90-day sub-chronic study (Reno, 1997). However, regulatory agencies may alsoconsider the use of parameters other than weight gain, such as phys-iological and pharmacokinetic considerations and urinary metabo-lite profiles, as indicators of an appropriate MTD (Reno, 1997).

Generally, one or two additional doses, usually fractions of theMTD (e.g., one-half and one-quarter MTD), and a control groupare tested.

The use of the MTD in carcinogenicity has been the subjectof controversy. The premise that high doses are necessary for test-ing the carcinogenic potential of chemicals is derived from the sta-tistical and experimental design limitations of chronic bioassays.Consider that a 0.5 percent increase in cancer incidence in theUnited States would result in over 1 million additional cancerdeaths each year—clearly an unacceptably high risk. However,identifying with statistical confidence a 0.5 percent incidence of cancer in a group of experimental animals would require a minimum of 1000 test animals, and this assumes that no tumorswere present in the absence of exposure (zero background inci-dence).

Figure 2-11 shows the statistical relationship between mini-mum detectable tumor incidence and the number of test animalsper group. This curve shows that in a chronic bioassay with 50 an-imals per test group, a tumor incidence of about 8 percent couldexist even though no animals in the test group had tumors. Thisexample assumes that there are no tumors in the control group.These statistical considerations illustrate why animals are tested atdoses higher than those which occur in human exposure. Becauseit is impractical to use the large number of animals that would berequired to test the potential carcinogenicity of a chemical at thedoses usually encountered by people, the alternative is to assumethat there is a relationship between the administered dose and thetumorigenic response and give animals doses of the chemical thatare high enough to produce a measurable tumor response in a rea-sonable size test group, such as 40 to 50 animals per dose. Thelimitations of this approach are discussed in Chap. 4.

Recently a new approach for establishing maximum doses foruse in chronic animal toxicity testing of drugs has been proposedfor substances for which basic human pharmacokinetic data areavailable (for example, new pharmaceutical agents which havecompleted phase I clinical trials). For chronic animal studies per-formed on drugs where single-dose human pharmacokinetic dataare available, it has been suggested that a daily dose be used thatwould provide an area under the curve (AUC) in laboratory ani-


Figure 2-11. Statistical limitations in the power of experimental animalstudies to detect tumorigenic effects.

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mals equivalent to 25 times the AUC in humans given the highest(single) daily dose to be used therapeutically. This value wouldthen be used in place of the traditional MTD for chronic bioassays.

Chronic toxicity assays are commonly used to evaluate thepotential oncogenicity of test substances (Huff, 1999). Most regu-latory guidelines require that both benign and malignant tumors bereported in the evaluation. Statistical increases above the controlincidence of tumors (either all tumors or specific tumor types) inthe treatment groups are considered indicative of carcinogenic po-tential of the chemical unless there are qualifying factors that sug-gest otherwise (lack of a dose response, unusually low incidenceof tumors in the control group compared with “historic” controls,etc.). Thus, the conclusion as to whether a given chronic bioassayis positive or negative for carcinogenic potential of the test sub-stance requires careful consideration of background tumor inci-dence. Properly designed chronic oncogenicity studies require thata concurrent control group matched for variables such as age, diet,housing conditions be used. For some tumor types, the “back-ground” incidence of tumors is surprisingly high. Figure 2-12shows the background tumor incidence for various tumors in maleand female F-344 rats used in 27 National Toxicology Program 2-year rodent carcinogenicity studies. The data shown represent thepercent of animals in control (nonexposed) groups that developedthe specified tumor type by the end of the 2-year study. These stud-


ies involved more than 1300 rats of each sex. Figure 2-13 showssimilar data for control (nonexposed) male and female B6C3F1mice from 30 recent NTP 2-year carcinogenicity studies and in-cludes data from over 1400 mice of each sex. There are severalkey points that can be derived from these summary data:

1. Tumors, both benign and malignant, are not uncommon eventsin animals even in the absence of exposure to any known car-cinogen.

2. There are numerous different tumor types that develop “spon-taneously” in both sexes of both rats and mice, but at differ-ent rates.

3. Background tumors that are common in one species may beuncommon in another (for example, testicular interstitial celladenomas are very common in male rats but rare in male mice;liver adenomas/carcinomas are about 10 times more prevalentin male mice than in male rats).

4. Even within the same species and strain, large gender differ-ences in background tumor incidence are sometimes observed(for example, adrenal gland pheochromocytomas are aboutseven times more prevalent in male F344 rats than in femaleF344 rats; lung and liver tumors are twice as prevalent in maleB6C3F1 mice as in female B6C3F1 mice).

5. Even when the general protocols, diets, environment, strain andsource of animals, and other variables are relatively constant,

Figure 2-12. Most frequently occurring tumors in untreated control ratsfrom recent NTP 2-year rodent carcinogenicity studies.

The values shown represent the mean �SD of the percentage of animalsdeveloping the specified tumor type at the end of the 2-year study. The val-ues were obtained from 27 different studies involving a combined total ofbetween 1319 and 1353 animals per tumor type.

Figure 2-13. Most frequently occurring tumors in untreated control micefrom recent NTP 2-year rodent carcinogenicity studies.

The values shown represent the mean �SD of the percentage of animalsdeveloping the specified tumor type at the end of the 2-year study. The val-ues were obtained from 30 different studies involving a total of between1447 and 1474 animals per tumor type.

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background tumor incidence can vary widely, as shown by therelatively large standard deviations for some tumor types in theNTP bioassay program. For example, the range in liver ade-noma/carcinoma incidence in 30 different groups of unexposed(control) male B6C3F1 mice went from a low of 10 percent toa high of 68 percent. Pituitary gland adenomas/carcinomasranged from 12 to 60 percent and 30 to 76 percent in unex-posed male and female F344 rats, respectively, and from 0 to36 percent in unexposed female B6C3F1 mice.

Taken together, these data demonstrate the importance of in-cluding concurrent control animals in such studies. In addition,comparisons of the concurrent control results to “historic” con-trols accumulated over years of study may be important in iden-tifying potentially spurious “false-positive” results. The relativelyhigh variability in background tumor incidence among groups ofhealthy, highly inbred strains of animals maintained on nutri-tionally balanced and consistent diets in rather sterile environ-ments highlights the dilemma in interpreting the significance ofboth positive and negative results in regard to the human popu-lation, which is genetically diverse, has tremendous variability indiet, nutritional status, and overall health; and lives in an envi-ronment full of potentially carcinogenic substances, both naturaland human-made.

Developmental and ReproductiveToxicity

The effects of chemicals on reproduction and development alsoneed to be determined. Developmental toxicology is the study ofadverse effects on the developing organism occurring anytime dur-ing the life span of the organism that may result from exposure tochemical or physical agents before conception (either parent), dur-ing prenatal development, or postnatally until the time of puberty.Teratology is the study of defects induced during development be-tween conception and birth (see Chap. 10). Reproductive toxicol-ogy is the study of the occurrence of adverse effects on the maleor female reproductive system that may result from exposure tochemical or physical agents (see Chap. 20).

Several types of animal tests are utilized to examine the po-tential of an agent to alter development and reproduction. Althoughdifferent countries have often had different testing requirementsfor reproductive and developmental toxicity, efforts to “harmonize”such testing protocols have resulted in more flexible guidelines forreproductive toxicity testing strategies [see (Christian, 1997) for asummary description of the ICH guidelines]. General fertility andreproductive performance (segment I) tests are usually performedin rats with two or three doses (20 rats per sex per dose) of thetest chemical (neither produces maternal toxicity). Males are giventhe chemical 60 days and females 14 days before mating. The an-imals are given the chemical throughout gestation and lactation.Typical observations made include the percentage of the femalesthat become pregnant, the number of stillborn and live offspring,and the weight, growth, survival, and general condition of the off-spring during the first 3 weeks of life.

The potential of chemicals to disrupt normal embryonicand/or fetal development (teratogenic effects) is also determinedin laboratory animals. Current guidelines for these segment IIstudies call for the use of two species, including one nonrodentspecies (usually rabbits). Teratogens are most effective when ad-

ministered during the first trimester, the period of organogene-sis. Thus, the animals (usually 12 rabbits and 24 rats or mice pergroup) are usually exposed to one of three dosages duringorganogenesis (day 7 to 17 in rodents and days 7 to 19 in rab-bits), and the fetuses are removed by cesarean section a day be-fore the estimated time of delivery (gestational days 29 for rab-bit, 20 for rat, and 18 for mouse). The uterus is excised andweighed and then examined for the number of live, dead, andresorbed fetuses. Live fetuses are weighed; half of each litter isexamined for skeletal abnormalities and the remaining half forsoft tissue anomalies.

The perinatal and postnatal toxicities of chemicals also are of-ten examined (segment III). This test is performed by administer-ing the test compound to rats from the 15th day of gestationthroughout delivery and lactation and determining its effect on thebirthweight, survival, and growth of the offspring during the first3 weeks of life.

In some instances a multigenerational study may be chosen,often in place of segment III studies, to determine the effects ofchemicals on the reproductive system. At least three dosage levelsare given to groups of 25 female and 25 male rats shortly afterweaning (30 to 40 days of age). These rats are referred to as theF0 generation. Dosing continues throughout breeding (about 140days of age), gestation, and lactation. The offspring (F1 generation)have thus been exposed to the chemical in utero, via lactation, andin the feed thereafter. When the F1 generation is about 140 daysold, about 25 females and 25 males are bred to produce the F2 gen-eration, and administration of the chemical is continued. The F2

generation is thus also exposed to the chemical in utero and vialactation. The F1 and F2 litters are examined as soon as possibleafter delivery. The percentage of F0 and F1 females that get preg-nant, the number of pregnancies that go to full term, the litter size,the number of stillborn, and the number of live births are recorded.Viability counts and pup weights are recorded at birth and at 4, 7,14, and 21 days of age. The fertility index (percentage of matingresulting in pregnancy), gestation index (percentage of pregnanciesresulting in live litters), viability index (percentage of animals thatsurvive 4 days or longer), and lactation index (percentage of ani-mals alive at 4 days that survived the 21-day lactation period) arethen calculated. Gross necropsy and histopathology are performedon some of the parents (F0 and F1), with the greatest attention be-ing paid to the reproductive organs, and gross necropsy is per-formed on all weanlings.

The International Commission on Harmonization (ICH)guidelines provide for flexible guidelines that address six “ICHstages” of development: premating and conception (stage A), con-ception to implantation (stage B), implantation to closure of thehard palate (Stage C), closure of the hard palate to end of preg-nancy (stage D), birth and weaning (stage E), and weaning to sex-ual maturity (stage F). All of these stages are covered in the seg-ment I to segment III studies described above (Christian, 1997).

Numerous short-term tests for teratogenicity have been de-veloped (Faustman, 1988). These tests utilize whole-embryo cul-ture, organ culture, and primary and established cell cultures to examine developmental processes and estimate the potential ter-atogenic risks of chemicals. Many of these in utero test systemsare under evaluation for use in screening new chemicals for ter-atogenic effects. These systems vary in their ability to identify spe-cific teratogenic events and alterations in cell growth and differ-entiation. In general, the available assays cannot identify functionalor behavioral teratogens (Faustman, 1988).

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Mutagenicity

Mutagenesis is the ability of chemicals to cause changes in the ge-netic material in the nucleus of cells in ways that allow the changesto be transmitted during cell division. Mutations can occur in eitherof two cell types, with substantially different consequences. Germi-nal mutations damage DNA in sperm and ova, which can undergomeiotic division and therefore have the potential for transmission ofthe mutations to future generations. If mutations are present at thetime of fertilization in either the egg or the sperm, the resulting com-bination of genetic material may not be viable, and the death mayoccur in the early stages of embryonic cell division. Alternatively, themutation in the genetic material may not affect early embryogenesisbut may result in the death of the fetus at a later developmental pe-riod, resulting in abortion. Congenital abnormalities may also resultfrom mutations. Somatic mutations refer to mutations in all other celltypes and are not heritable but may result in cell death or transmis-sion of a genetic defect to other cells in the same tissue through mi-totic division. Because the initiating event of chemical carcinogene-sis is thought to be a mutagenic one, mutagenic tests are often usedto screen for potential carcinogens.

Numerous in vivo and in vitro procedures have been devised totest chemicals for their ability to cause mutations. Some genetic al-terations are visible with the light microscope. In this case, cytoge-netic analysis of bone marrow smears is used after the animals have


been exposed to the test agent. Because some mutations are incom-patible with normal development, the mutagenic potential of a chem-ical can also be measured by the dominant lethal test. This test is usu-ally performed in rodents. The male is exposed to a single dose ofthe test compound and then is mated with two untreated femalesweekly for 8 weeks. The females are killed before term, and the num-ber of live embryos and the number of corpora lutea are determined.

The test for mutagens that has received the widest attention isthe Salmonella/microsome test developed by Ames and colleagues(Ames et al., 1975). This test uses several mutant strains of Salmo-nella typhimurium that lack the enzyme phosphoribosyl ATP syn-thetase, which is required for histidine synthesis. These strains areunable to grow in a histidine-deficient medium unless a reverse orback-mutation to the wild type has occurred. Other mutations in thesebacteria have been introduced to enhance the sensitivity of the strainsto mutagenesis. The two most significant additional mutations en-hance penetration of substances into the bacteria and decrease theability of the bacteria to repair DNA damage. Since many chemicalsare not mutagenic or carcinogenic unless they are biotransformed toa toxic product by enzymes in the endoplasmic reticulum (micro-somes), rat liver microsomes are usually added to the medium con-taining the mutant strain and the test chemical. The number of re-verse mutations is then quantitated by the number of bacterial coloniesthat grow in a histidine-deficient medium.

Strains of yeast have recently been developed that detect geneticalterations arising during cell division after exposure to non-geno-toxic carcinogens as well as mutations that arise from directly geno-toxic carcinogens. This test identifies deletions of genetic materialthat occur during recombination events in cell division that may re-sult from oxidative damage to DNA, direct mutagenic effects, alter-ations in fidelity of DNA repair, and/or changes in cell cycle regula-tion (Galli and Schiestl, 1999). Mutagenicity is discussed in detail inChap. 9.

Other Tests

Most of the tests described above will be included in a “standard”toxicity testing protocol because they are required by the variousregulatory agencies. Additional tests also may be required or in-cluded in the protocol to provide information relating a specialroute of exposure (inhalation) or a special effect (behavior). In-halation toxicity tests in animals usually are carried out in a dy-namic (flowing) chamber rather than in static chambers to avoidparticulate settling and exhaled gas complications. Such studiesusually require special dispersing and analytic methodologies, de-pending on whether the agent to be tested is a gas, vapor, or aerosol;additional information on methods, concepts, and problems asso-ciated with inhalation toxicology is provided in Chaps. 15 and 28.A discussion of behavioral toxicology can be found in Chap. 16.The duration of exposure for both inhalation and behavioral toxi-city tests can be acute, subchronic, or chronic, but acute studiesare more common with inhalation toxicology and chronic studiesare more common with behavioral toxicology. Other special typesof animal toxicity tests include immunotoxicology, toxicokinetics(absorption, distribution, biotransformation, and excretion), thedevelopment of appropriate antidotes and treatment regimes forpoisoning, and the development of analytic techniques to detectresidues of chemicals in tissues and other biological materials.The approximate costs of some descriptive toxicity tests are givenin Table 2-3.

Table 2-3Typical Costs of Descriptive Toxicity Tests

TEST COST, $

General Acute toxicityAcute toxicity (rat; two routes) 6,500Acute dermal toxicity (rabbit) 3,500Acute inhalation toxicity (rat) 10,000Acute dermal irritation (rabbit) 2,000Acute eye irritation (rabbit) 1,500Skin sensitization (guinea pig) 5,000

Repeated dose toxicity14-day exposure (rat) 45,00090-day exposure (rat) 110,0001-year (diet; rat) 250,0001-year (oral gavage; rat) 300,0002-year (diet; rat) 685,0002-year (oral gavage; rat) 860,000

Genetic toxicology testsBacterial reverse mutation (Ames test) 7,000Mammalian cell forward mutation 25,000In vitro cytogenetics (CHO cells) 20,000In vivo micronucleus (mouse) 11,000In vivo chromosome aberration (rat) 22,500Dominant lethal (mouse) 85,000Drosophila sex-linked recessive lethal 55,000Mammalian bone marrow cytogenetics

(in vivo; rat) 22,500Reproduction

Segment I (rat) 90,000Segment II (rat) 63,000Segment II (rabbit) 72,000Segment III (rat) 160,000

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