Mammalian Toxicology, Session 4

Discussion Questions on History, Biochemical Kinetics, Chronic Toxicity & Carcinogenesis, Metabolism, Risk Assessment, and Toxicity Protection

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Discussion Questions:

Topic: Question 1. Questions arising from the historical background of toxicology.

Posted: by Kenneth Campbell

1. The idea of studying causality through careful observation and testing might give rise to the notion of occupational hazards because certain occupations expose workers to unique risks of exposure. If particular subsets of workers are all experiencing a similar disease or condition, the logical way to explain the symptoms is to carefully observe the working conditions. This would entail sampling the environment in search of the causative agent, or the "toxicon". Potential candidates could then be used in animal or in vitro tests to determine response to various dose levels, site of action and what types of symptoms are observed in the test subjects. They could also undergo biochemical analysis to determine structure/activity relationships. The information gathered could then be used to determine the causative agent and could also be used to infer other potential hazards. This type of testing could then also be used to test other potentially hazardous working environments before workers exhibit symptoms. This would allow for preventative measures to be taken against toxic exposure.

The process of simple observation from occupational or incidental exposures may be suitable for noncarcinogenic effects resulting from acute exposures; however, for chronic adverse effects or carcinogenesis, it is not as easy a matter as to pinpoint the potential culprit. Liver cancer, or heart disease, for example, may result from a lifetime of cumulative effects of numerous causative agents, which may or may not be obvious based on the individual's exposure.

"Watch and learn" tactics. In the past, people have noted adverse effects associated with exposure to particular substances. Likewise, it was noted that certain occupations had correspondingly higher incidences of diseases (e.g., "Mad Hatters" syndrome, resulting from mercury toxicity (neurotoxicity). Mercury was commonly used by hat makers…..). As for oncogenesis, the same premise holds: exposure to a particular substance eventually initiated the onset of cancer (e.g., the elevated incidence of lung cancer among smokers). One problem with this approach, however, is that a particular type of cancer (or, say, a nonspecific noncancer effect) may correlate with a substance's use, but may not necessarily be directly related to exposure. Simple observation does not take into account confounding factors such as the incidence of background cancer rates, or the numerous other entities in our lives that may also attribute to adverse effects (e.g., diet, acute exposures, etc.)

During the Renaissance, the idea of scientific experiments and replicability arose. It became understood that an event can be caused by something non-immediate. The spread of literacy and printed works made it easier for scientists and others to share observations and note patterns, such as the high occurrence of scrotal cancer in chimney sweepers. Likewise it was known that mining and covering building domes with gold leaf (I believe the gold was in a mercury solution) were dangerous occupations, because many of these processionals died relatively quickly of similar diseases, such as lung problems. While the exact mechanisms of lung injury were not known, the cause-effect relationship was established. Similarly, while initially oncogenesis was attributed to mystical causes, the observation of patterns of lifestyle and environment among victims contributed to the uncovering of certain cause-effect relationships, like the above-mentioned scrotal cancer of chimney sweepers and, more recently, lung cancer and emphysema among heavy smokers.

While scientific advancements of the cause and effect relationship of toxicants on the human system did occurred during the Middle Ages, one could say that these were great scientific alchemy experiments than the precursors of government regulatory bodies like OSHA, EPA, and the NIH. One pioneer of the Middle Ages, Catherine de Medici, tested toxic concoctions on men. Now, her horrific crimes can be interpreted to modern toxicology terms such as onset of action, specificity of action, and clinical significant. The fate for her evil ways was execution, but one could argue that her scientific studies are exactly what occurs during Phase I, II, and III clinical trials of new chemical entities therapies. Overall humans have tried to learn from there mistakes and this what has initiated governmental control Back in the 1900's one pharmaceutical company killed 108 children with their cough drop medicine. They had used ethylene gylcol instead of propylene glycol in their formulation. Their medicine was termed the Elixr of death and hence the FDA was then established soon afterwards.

Posted: Feb-15-03 at 4:19 PM by Student 4

Posted: Feb-15-03 at 4:38 PM by Student 5

During the Middle Ages and the Renaissance, the beginnings of careful observations and testing began. Looking at the cause and affects of a toxicant/toxin began. For example, Catherine de Medici looked at toxic effects on the poor by noting the rapidity of the toxic response (onset of action), the effectiveness of the compound (potency), the degree of response of the parts of the body (specificity, site of action) and the complaints of the victims (clinical signs and symptoms). As cruel as these experiments were, they were really an early version of the Phase I, II, and III testing. During the Renaissance, Paracelsus was the first to look at a toxicant as a chemical entity rather than a mixture of some sort. His 3 concepts were:

If one considers simple hunter-gathering an occupation, causality via observation and testing likely developed prior to the middle ages and Renaissance, during early hunting and foraging expeditions. Early man likely encountered poisonous plant, animal and insect species. Tribal leaders likely recorded the species of plant, animal and insect that caused a toxic response in a member; thus establishing early causality. Over time I imagine that unexplained and/or mysterious death previously attributed to mystical causes was later attributed to an exposure to a naturally occurring toxin/species delivering said toxin. Once studied and understood, it is evident from early manuscripts that early man was able to apply this toxicological knowledge to hunting and warfare.

Posted: Feb-18-03 at 3:15 PM by Student 7

The idea of causality seems to have developed around the introduction of the physician-alchemist, Paracelsus. He promoted the idea of a dose-response relationship. Looking at the relation of cause (i.e. "toxicon" exposure) and effect (i.e. disease, death) parallels Paracelsus' studies of how chemical entities can elicit various responses in individuals- both neg. and positive.

www.iet.msu.edu/tox_for_public/riskasmt.htm

1) The idea of casuality would potentially give rise to the notions of occupational hazards and/or the nonmystical explanations for oncogenesis by one's observing and posing a plausible causal relationship between the environment (work or otherwise) and an adverse (cancer promoting ..etc) reaction in the exposed individual. Instead of attributing ailments to superstitious beliefs, they had begun to establish the fundamental beginnings of the scientific method.

1. Occupational hazards associated with metal working were recognized during the 15th century. Paracelsus published a major piece of work addressing the etiology of miner's disease, along with treatment and prevention strategies. Occupational diseases and causuality increased during the Industrial Revolution. The recognition of the basis behind scrotal cancer among chimney sweeps was the first reported example of polyaromatic hydrocarbon carcinogenity. These findings led to improved medical practices and prevention. Through careful observation, diseases that were prevelant among a certain group of workers or people led to testing and experimenting on metals, plants, and minerals that they might be exposed to or using in their daily activity. This led to the understanding and further development of determination of how workers were becoming ill and the specific cause of these diseases.

1. The occupational hazards and causalities were very commonly occuring in olden days.

Topic: Question 2. Questions arising out of the historical background of toxicology.

Posted: Mar-04-02 at 3:43 PM by Kenneth Campbell

1. The similarities between enzyme kinetics and toxic effects are apparent when comparing dose-response curves to the plot of the Michalis-Menton equation also called the rate equation. The concentration of substrate on the X-axis is analogous to the dose and the velocity of the enzymatic reaction on the Y-axis is analogous to the response (efficacy). The median lethal dose (LD₅₀) is analogous to the Km., the substrate concentration at which the reaction velocity is half-maximal, ½ V_max. A small K_m means that the enzyme achieves maximal catalytic efficiency at low substrate concentration. A small LD₅₀ means that 50% lethality is reached at a low dosage of the toxicant. This also applies to the effective dose (ED) and toxic dose (TD). The mechanisms that facilitate the toxicants delivery to its target site could potentially influence the LD₅₀ or ED₅₀. If the facilitators decrease the dosage needed to reach the half-mark, they would be akin to catalysts of an enzymatic reaction.

Toxicants affect their targets (enzymes) by binding to them in the same ways their normal substrates do: noncovalently and covalently. Examples: TCDD binding to aryl hydrocarbon receptor, and acridine yellow to DNA in a noncovalent manner; metal ions and free radicals are able to form covalent bonds with biomolecules.

Chemical absorption, transformation, elimination, etc. is variable not only among substances, but among individual receptors (as in a human receptor) as well as on a cellular or subcellular level. In other words, toxicity is a function of chemical application, uptake, and transfer to a target, each of which in turn are dependent on the organism's distinct physiology and biochemistry.

The dose-response curves of toxic substances on living organisms are similar in shape to virtually any plot of some effect of a chemical (in varying amounts) on a living organism, such as an enzyme and its product's concentration or even ingestion of glucose and and its level in the bloodstream. This is because the general mechanisms of any substance's function are similar: There is a minimal dosage at which an effect is found, when the substance's amount is sufficient to trigger a reaction, a subsequent somewhat linear relationship between dose and effect (though the slope will differ), and a later reduction in the rate of effect increase. This last may be due, depending on the type of effect observed, to a saturation of receptors for the chemical, or possibly to the death of the organism and shutdown of biological processes in the case of a highly toxic substance.

Posted: Feb-15-03 at 4:51 PM by Student 3

Biochemical kinetics refers to the rate at which a reaction takes place (Purich and Allsion). In mammalian systems, kinetic mechanisms play a vital function in metabolic pathways, the mechanistic action of enzymes, and the processing of genetic material. The biochemical kinetics covered in class was focused on a sigmoidal curve. While simple in structure, this shape pertains to a wide variety of biological and chemical systems; titration curves, isoelectric points of proteins, and curves exhibited by regulatory enzymes. This sigmoidal shape also exists in toxicology and is seen with the quantal-dose response relationship curve. The quantal dose response is used extensively in toxicology. First, the LD₅₀ is determined where 50% of animals tested die from a certain dose. Second, a range of doses are used in a larger number of animals. From this a sigmoidal curve can be obtained. As with all biochemical kinetics, a saturation point exists where a stimulus will no longer result in an effect. For enzyme kinetics, only so many active sites exist on the enzyme for the substrate to bind. For a cumulative mortality curve, only so much dose can be given before death occurs.

Our discussion of biochemical kinetics touched upon the following:

1. the rate and/or response to a given toxin and/or toxicant could be graphically depicted as a dose-response curve;
2. the dose-response curve is most often sigmoidal;
3. curves may be individual or quantal;
4. the basal, middle and maximal points of the curve may correspond to a wide variety of endpoints;
5. cellular, tissue, organ, individual and population variability may be attributed to dosage, duration and frequency of dosage, route of exposure, chemical and/or physical interaction, genetics, environment and a wide variety of other factors
6. a greater level of confidence exists around the linear portion of the curve;
7. the lack of confidence at the asymptotic portions of the curve is due to factors including hypersusceptibility, the inability to define sub-threshold responces, and resistance.

2) The notion of biochemcial kinetics was encompassed in our discussion of dose-response relationships as well as routes of entry of toxins/toxicants. For instance genetic backgrounds of the specimen in question ( recipient of toxin/toxicant) will have an effect on the biochemical result depending on its underlying genetic makeup. Such variations include:susceptibility to toxins/toxicants as well as the ability to excrete/eliminate toxins/toxicants once they infiltrate through the biological system. The particular route of entry may be more significant dependent upon the genetic background of the individual organism.

Topic: Question 6. Chronic Toxicity & Carcinogenicity

Posted: Mar-04-02 at 3:59 PM by Kenneth Campbell

Certainly, as scientific methods and techniques get more advanced it will become much easier and cost-effective to assess various other effects of toxicants beside carcinogenesis. Why is this important? For instance, a particular signaling protein affected by a toxicant can cause an increase in cellular proliferation (then cancer) but this protein might also have other functions either in the same organ or in different organs. Toxicant might also affect more than one protein with similar binding clefts (e.g. ATP is a substrate for various enzymes). Activation of one tyrosine kinase receptor by a toxicant can have multiple effects via diverging signaling cascades. Genes with distinct functions could be affected by mutagenesis (see chapter 3 for many specific examples).Continuous displacement of homeostasis can therefore lead to various other outcomes that might not be readily observed under submaximal doses but require further investigation (as mentioned in Session 2 notes: "neural degeneration, arthritic conditions, vascular rigidity, loss of muscle mass, loss of bone mass, decline in immune function"). Animals that undergo toxicity testing for carcinogenesis should be used for further microscopic and biochemical examination. Are there any increases/decreases in protein expression in experimental group compared to controls, any changes in tissue/organ architecture? Do animals show any of the effects mentioned above? Recent advances in proteomics such as DNA, mRNA, and protein chips will allow faster analysis.

Although it remains important that the emphasis of chronic toxicology testing is on carcinogenesis and mutagenesis, other possible outcomes are similarly significant. One such example is chronic exposure to mycotoxins. Mycotoxins are secondary products of Fungi found primarily in food or animal feed due mainly to poor storage environments. Due to their lipophilic nature, these toxins are generally stored in fat cells. The various species produce a variety of mycotoxins of which a few have been studied extensively for toxicity in wide ranges of animal species (mainly farm and laboratory animals). Symptoms exhibited by mice and rats chronically exposed to mycotoxins included, along with carcinogenesis, such deleterious effects as extensive liver damage, gastric ulcers, and thymic depression. It has been shown that the mycotoxin, aflatoxin, is cytotoxic due to its ability to induce lipid peroxidation leading to oxidative damage in certain cell types. Other mycotoxins damage cells through action on cellular respiration indicated competitive inhibition of ATPase succinate dehydrogenase, and cytochrome C oxidase. Some examples inhibit protein and DNA synthesis, compromise immunity, and/or disrupt the endocrine system (above issues reviewed in Hussein and Brasel 2001) These effects could damage any number of essential functions for survival. The reason that it is important to seriously consider these effects alongside carcinogeneisis and mutagenesis is that they may be just as deadly. In the case of mycotoxins, chronic exposure to humans may come in the form of consuming milk or meat from intoxicated farm animals, consuming contaminated food directly, and inhaling air from contaminated buildings. Lately, the issue of infected buildings has repeatedly been in the news, and the report often includes contamination of a school with mold and many sick children.

Carcinogenesis is certainly emphasized as a selective endpoint for toxicity testing, although noncarcinogenic effects that may potentially impact one's life are by no means considered a weaker endpoint. Not all substances are carcinogenic; therefore, long-term, low-dose exposures to such substances obviously should be evaluated with regard to noncarcinogenic, nonlethal effects. (Furthermore, some substances shown to be carcinogenic in laboratory animals have not been proven to cause cancer in humans).

Chronic toxicity test usually requires at least 1,000 animals and 18 months to complete. It should have the same route of exposure that would occur in humans and needs a close watch every day. I think the current emphasis of chronic toxicity testing is pretty appropriate to check on carcinogenesis, but the test is expensive and there is no standard measure for chronic toxicity, unlike LD₅₀ for acute toxicity. Humans are generally more vulnerable than the tested animals, and different species have different way of responding to toxic substances. Today the estimation of human risk is still one of the difficult problems, since quantitative risk analysis is especially difficult to make relationships based on animal data, while qualitative risk analysis is relatively easier to develop by finding some negative toxic effects in the tested animals although the applicable toxicity use level is quite different in each country.

Chronic testing looks at periods from 6 months - two years. These tests look at exposure over most of the animal's lifetime. These tests require a minimum of 1000 animals. These animals have to free of any tumors. These studies look at MYD (maximum tolerable dose). These tests are fairly appropriate but can be costly and errors do occur. Humans have different responses than lab animals and generally are more vulnerable. Estimating responses to toxicants in humans is still difficult today since what might be toxic in lab animals isn't necessarily going to be toxic in humans and vice versa.

6) The emphasis on chronic toxicity with respect to carcinogenesis is presumably due to the many oncogenic promoting agents in the environment as well as the relatively easiness in measuring them. As our society becomes more sophisticated in terms of understanding, technology and preventative strategies, it is certain that the range of chronic toxicity testing will expand.

Topic: Question 7. Toxin Metabolism Considerations

Posted: Mar-04-02 at 4:00 PM by Kenneth Campbell

For a developing fetus the major route of exposure is the placenta. Along with the nutrients, various toxins and toxicants that enter the mother via standard routes move across this barrier against a concentration gradient. Though this barrier could consist of multiple layers, their number is not of great importance for the rate of diffusion of chemicals. It is rather the solubility of toxicants that is important so if a toxicant was able to rapidly diffuse across other body membranes it will do so across the placenta. Luckily, 50-50 equilibrium is not always established between the mother and the fetus because fetal tissue is usually less developed and less able to concentrate the toxicant (exception: brain).

In addition to the placental barrier, another important route to consider is the blood-testis barrier. This barrier prevents free exchange of chemicals/drugs between the blood and the fluid inside the seminiferous tubules. The barrier is composed of tightly fit epithelial cells, which may have sections with leaky junctions. Lesser-developed barriers in immature testes have more of this leakiness, thus leading to a greater chance for exposure. If a toxin does pass through this barrier, several mechanisms for metabolism exist in the testes. These include mixed-function oxidases, epoxide-degrading enzymes, gonadal cytochrome P₄₅₀ and AHH present in microsomes. This metabolism, however, can have adverse effects on spermatogenesis and/or steriodogenesis. Metabolites of toxicants alter testicular structures resulting in deleterious effects to the germ cells. Examples of this include the oxidization product of n-hexane, 2,5-HD, which alters the microtubules in Sertoli cells ultimately preventing the paracrine support for the germ cells. Other toxic metabolites cause abnormal sperm, testicular lesions, decreased sperm mobility and damage to the Sertoli and Leydig cell functions (see C&D 689-690). All of these factors can be deleterious to the possibility of reproduction.

Differences in a child's behavior and physiology are major factors that need to be considered when addressing toxicity. Children often have a much higher exposure potential (to environmental toxicants, in particular) than adults: they play in the dirt outside or on the carpet; they consume more food and water and have a higher inhalation rate per body weight; they orally explore their surroundings; they drink breast milk. Likewise, their uptake, metabolism, and excretion mechanisms vary from those of an adult. Because of their enhanced exposures, children are generally more susceptible to chemical insult than adults.

Some routes of exposure which negatively effects on reproductive process are inhalation, ingestion of food and water containing toxin/toxicant, and skin. As an example, lead can be found in many kinds of food or in leaded paint. Lead is a tetratogen that can cause fetal malformation, a mutagen that can impair fertility and affect both sperm and eggs. Through inhalation lead is almost completely absorbed after entering lower respiratory tract, and it affects major organs in the body, especially children's nervous system. Its absorption rate through ingestion for young children is 4 to 5 times higher than for adults, causing lead poisoning and permanent neurological, cognitive impairments.

In the growing offspring or developing fetus, a number of considerations need to be made. The growing offspring is more susceptible to toxicants due to its behavior as a young, i.e. it eats more, plays around and explores. Also, their methods of clearance, metabolism, detoxification, and excretion are not as developed as adults along with their immune systems. During development, the fetus absorbs nutrients from the mother from the placenta, so whatever toxicants the mother may be able to clear, the fetus may not be able to and absorbs it. Examples of this can include leaded paint, where in small amounts to adults is not toxic while can cause numerous developmental problems during development.

Posted: Feb-18-03 at 7:10 PM by Student 5

First of all, embryonic and fetal development in mammals makes use of the placenta, which is permeable to many toxins and toxicants present in the mother's circulation, as well as viruses and other pathogens. If the pregnancy happens at a difficult time for the organism, fat reserves may be mobilized, which may contain a significant amount of toxic substances that are then passed to the fetus. Fetuses and newborns often have underdeveloped metabolic pathways for clearing toxins and toxicants crossing the placental barrier or passed through mother's milk. Once infants are weaned at least partially, any other food is also a consideration, as it may contain substances harmful to the underdeveloped systems, which might go unnoticed in adult toxicity testing. The same goes for other routes of exposure: anything inhalable or in dermal contact with the baby which may be considered safe for adults may not be safe for the newborns. In addition, once children are mobile, they tend to ingest and come in contact with a wider variety of materials not intended for consumption or inhalation.

7) YES! Teratogenesis (i.e. abnormal development) is evidenced by mutations, chromosomal breaks, altered mitosis, altered nucleic function, osmolar imbalance..etc..etc.. All of which may be triggered by toxic agents delivered through the conceptus at the cellular level, the insult may occur through a direct effect on the embryo/fetus, indirectly through toxicity of the agent to the mother and/or the placenta or through a combination of direct and indirect routes of entry ( C & D, page 365).

Topic: Question 10. Risk Strategies & Levels of Toxicity

Posted: Mar-04-02 at 4:04 PM by Kenneth Campbell

10. The distinctions between these categories of various levels of risks and the risk assessment strategies are that of result of exposure to a particular toxic substance. Assessing the risk of a lethal substance would include animal testing and determination of LD₅₀ under a varying set of conditions, such as length of exposure, site of exposure, number of test subjects, etc. Assessing the risk of a morbid agent would require more specific testing. The observations would now have to include clinical observation of symptoms and postmortem examination to determine the site of action and characterization of the specific effect of the chemical. These observations may be the occurrence of tumors, damage to particular organs, or interruption of a biological function. In the case of assessing the risk to reproductive capacity, the tests must focus specifically on the endocrine and reproductive systems of the organisms. The tests to determine the reproductive toxicity of an agent are performed on rats through six stages described by the International Commission on Harmony, from premating to weaning and sexual maturity of the offspring (Klaasen 2001). Quality of life risk assessment would need to test the ability of the agent to affect a condition of life that does not cause death or disease such are skin and eye irritants. Quality of environment impacts risk assessment must test all of these conditions as well as the effect of the agent on the natural habitat. This assessment might include the effect of an air pollutant on the human population (lethality, morbidity, reproductive capacity, quality of life) and on the wildlife (LD₅₀ of certain populations) and on the plant life (observation of decrease in growth).

On an individual basis, the first three terms reflect varying degrees of toxicity: lethality, obviously, is the most severe; morbidity may ultimately decrease lifespan; reproductive capacity in and of itself may not adversely affect an individual, although could negatively impact a population through decreased fecundity.

Risk assessment can be classified as a quantitative estimate of the potential effects on human health and the environmental significance of various types of chemical exposure (Klaassen). In identifying a hazardous agent, tests are performed to determine if the agent will cause an adverse effect through structure activity analysis, in vitro testing, animal bioassays, and epidemiology studies. Additional testing is performed to determine the dose-response relationship and the duration of exposure to the hazardous agent in question. These assessments form a Venn diagram that make up the three categories of toxicological studies; mechanistic, regulatory, and descriptive. While lethality, morbidity, reproductive capacity, quality of life, and quality of the environment all play significant roles in the study of toxicology, further distinctions can be made by the following; lethality is an acute test first performed on a NCE with a fixed dose on a small population where an LD₅₀ is determined. Morbidity can be considered a long term exposure study where a population is subjective to an agent for greater than 3 months where alterations on biological functions are addressed. Reproduction takes into consideration the effects of the reproductive system and the potential of the hazard to disrupt embryonic development. And furthermore, quality of life and quality of the environment assess the effects of a hazard on a large population and in nature where risk managers summarize the potential risks.

Looking at individuals, lethality which leads to death, morbidity which leads to a decrease in lifespan, and reproductive capacity all are the most serious. The last one could end up affecting a population as a whole in the long run. Quality of life toxicity issues wont necessary affect the organism in harmful ways, but may cause degradation in the quality of life for the organism. For example a annoying smell may occur in the organism or a scar which may be unpleasant but won't probably lead to death. But, the presence of an odor in an area may lead to fears of something harmful even though there is no harmful toxicant. Quality of environment is looking at the safety of the environment as a whole. It is important for risk assessments because it allows us to keep the public informed to what is a safe level of risk for products that are toxic such as household products. Also in what are side effects for medicines.

Posted: Feb-17-03 at 7:09 PM by Student 5

A risk assessment is an expert's opinion of the likelihood of a deleterious effect. Depending on what that effect might be, lethality(death), morbidity(disease), quality of life (level of comfort for the living organism), etc., the assessment will differ for the same toxicant. There may be a high risk of reduction in the quality of environment (how conducive the surroundings are to life) but not necessarily lethality. For example, an oil slick is a high risk for the beach by your house, but it carries a very low risk of death for you personally. Similarly, reproductive capacity of pond-living organisms may be severely reduced by a toxicant dumped into their pond, but have no effect on their own quality of life. Assessment strategies would thus have to be tailored to estimate the risk to a specific effect (i.e. if it is necessary to carry out an assessment of morbidity of tadpoles in Lake Erie, one should probably observe behavior and collect some specimens and inspect their tissues, rather than spend time observing a reproductive cycle and counting young.)

Risk assessment strategies require the following components:

-dose response assessment-includes prediction of exposure levels at which risk is likely to be negligible or nonexistent

-exposure assessment - a determination of how much of an agent people might be exposed to under various conditions such as use of a drug or a consumer product, environmental exposure at a hazardous waste site.

-risk characterization- explicit description of the assumptions and uncertainties that go into the risk assessment, and the overall confidence in the results of the analysis.

The variety of adverse effects of chemicals on humans clearly impacts risk assessment strategies. Bioassays and LD₅₀ aid in developing these strategies. When assessing risk of toxins, lethality (ability to cause death), morbidity (ability to cause disease), reproductive capacity, quality of life, and quality of environment do have distinct impacts on determining risk. In this case, since the question is quite vague, it is probably more helpful to look at an example: mold spores.

10) Lethality: death

Morbidity: diseased state, decreased life span

Reproductive capacity: ability to reproduce

Quality of Life: nonmedical threats that compromise one's ability to enjoy life

Quality of environment: degree of safety, comfort and security in surroundings

10. Lethality is related to death, morbidity is related to compromise in functionality. In individual lethality, it is not necessary that it means inability to reproduce. When talking about population lethality, the morbidity might be lethal. The toxins might change one's quality of life or living standards but it is not necessarily lethal. The quality of environment will effect each individual differently.

Posted: Mar-04-02 at 6:54 PM by Kenneth Campbell

The paradox of oral toxicity studies is that the absorption of a substance depends on a number of variable factors. Organic acids and bases are absorbed in the section of the GI tract where they exist in the nonionized form. The acidic environment of the stomach versus the neutral environment of the intestine can markedly affect the ability of a toxin to be absorbed. Differences in surface area of the stomach, small intestine and large intestine also influence absorbency, larger surface areas generally allowing for more opportunity for absorption. Another factor is that a number of toxicants are biotransformed in the GI tract into substances with either increased or decreased toxicity. The time a toxic agent spends in the GI tract can also influence absorption, the longer time spent, the more opportunity. Here is where dietary insoluble fiber can come into play. If the level of insoluble fiber is high, then movement through the GI tract is faster, thus decreasing the opportunity of a toxin to be absorbed. These situations can be used to treat acute poisonings by capitalizing on the absorptive properties of poison. If the nature of the poison is known, administrating ions that prevent the absorption could be used as a treatment. Another tactic may be to administer a substance that would decrease the solubility of the substance. If the poison cannot be dissolved, it won't be absorbed. The property of dietary insoluble fiber could be used to flush out the poison from the GI tract by essentially pushing it through before it could be absorbed at high enough concentrations to exhibit a toxic effect.

The paradox is that the oral route of exposure is probably the most common route (e.g., accidental or intentional ingestion), and intuitively seems like it would be the most direct route to toxicity, but toxicity generally will not occur through oral exposure until the toxicant is absorbed into the body (with the exception of caustic materials). In principle, this is not very different from dermal exposures-a chemical would have to penetrate the skin and be absorbed into the bloodstream in order for toxicity to result. Thus if absorption can be inhibited within the gastrointestinal (g.i.) tract, toxicity may be reduced or eliminated. Numerous factors influence a toxicant's absorption in the g.i. tract: pH, surface area, active and passive transport, pinocytosis, etc.

I am not sure about other substances that regulate absorption but in the case of dietary fiber continuos exposure is extremely beneficial. In the diets of many third world cultures between 150 and 175 grams of fiber is consumed daily (only 30 for US). Accordingly, they are not plagued with most of the "industrialized" disorders which include colon and breast cancer, heart disease, diabetes, arthritis, chronic fatigue syndrome, weakened immune systems, personality disorders, and all of the hormone-based female disorders. Most experts now agree that these nondigestible carbohydrates promote not only laxation, but blood cholesterol and glucose attenuation as well. The latter two functions are promoted by soluble fiber. Soluble fiber reduces cholesterol by reducing its absorption in the intestine and by complexing with bile acids (made from cholesterol in the liver) and preventing their reabsorption (so that liver must use additional cholesterol for making more bile acids). Soluble fibers also help normalize blood glucose levels by slowing the rate at which food leaves the stomach and by delaying the absorption of glucose following a meal. They also increase insulin sensitivity.

I couldn't agree more in the case of insoluble fiber-but I was suggesting, rather, that perhaps other substances (not noting any specifically) that may inhibit uptake of toxicants into the gi tract to treat chronic exposures may possibly have a negative effect on absorption of essential nutrients. I gave the example of Olestra, a synthetic fat that is not absorbed (and therefore is 'calorie-free'). However, it was noted that certain nutrients were also not absorbed. In the long-term, reduced absorption of essential nutrients will be detrimental to an organism's health.

Oral toxicity may involve transmucosal absorption and also chemical modification in the stomach acid, which degrades and hydrolyze the compound to make it easily degraded or absorbed. The paradox is that the body of animals, including humans, does not really resist toxic materials until they enter into the bloodstream. Most of animals have "oral tolerance"-- the immune system of the animals does not recognize potential immunogens if they are exposed orally in their childhood. Through an oral route those organisms invade the animal's intestinal tract and grow inside white blood cells without being killed.

Posted: Feb-17-03 at 7:17 PM by Student 8

Other answers have described in detail the paradox of the oral toxicity and pointed at intestine as the main site for entry of xenobiotics. Under normal conditions, xenobiotics are poorly absorbed within the mouth and esophagus, due mainly to the very short time that a substance resides within these parts of the GI tract. There are some exceptions though. Nicotine readily penetrates the mouth mucosa and nitroglycerin is placed under the tongue for immediate absorption and treatment of heart conditions. The sublingual mucosa under the tongue and in some other areas of the mouth is thin and highly vascularized so that some substances will be rapidly absorbed. Very little absorption takes place in the colon and rectum. As a general rule, if a xenobiotic has not been absorbed after passing through the stomach or small intestine, very little further absorption will occur. However, there are some exceptions, as some medicines may be administered as rectal suppositories with significant absorption. An example, Anusol is used for treatment of local inflammation which is partially absorbed (about 25%).

Reference: http://www.sis.nlm.nih.gov/ToxTutor/Tox2/a22.htm

A paradox in the po route of exposure lies in the fact that a chemical passes through several environments (the mouth, esophagus, stomach, small, then large intestine, rectum), most of which are lined with mucosal layers and are functionally geared to eliminate the contents. The stomach and small intestine also radically change the pH of their contents, likely biotransforming the initial toxicant. Thus, most poisons do not do much damage in the alimentary canal, but only once they are absorbed from there into the bloodstream, even if they are broad cellular mechanism disruptors such as cyanide, which blocks the final step in the electron transport chain in aerobic respiration. Insoluble fiber promotes a speedy passage through the intestines and has, as someone mentioned, a "gentle brushing effect" on the walls, thereby removing mucosal cells that already contain the poison and would allow it to diffuse deeper and closer to the bloodstream. Though one might intuitively think that fiber "stops up" the flow as it does in a sink, it in fact promotes elimination. The faster the flow, the less material will get absorbed in the alimentary lining.

16) A paradox in the oral route of exposure is that of the alimentary canal. It is the route that serves both digestion and respiration…that in and of itself is a blatant paradox. In terms of toxicity however, the coupling of food which is required for maintenance and sustenance of the organism, may also be threatened by inadvertently taking in toxins/toxicants into the GI tract. Many environmental toxicants enter the food chain and are absorbed with food from the GI tract ( C & D page 111). Although the GI tract is within the body, its content may be considered "exterior" to the body thereby not typically producing injury following the entrance of noxious agents until they are actually absorbed. Absorption can take place "anywhere" along the GI tract (i.e., mouth to rectum) depending on the chemical composition of the toxic agent in question.

Topic: Question 22. Time, dose, shielding.

Posted: Mar-07-02 at 3:15 AM by Kenneth Campbell

    2. maximize distance from the source to minimize dose delivered

    3. optimize the amount and type of shielding used to minimize dose delivered.

    2. Maximize distance from source

    3. Optimize shielding (protective clothes, gloves, shields, behaviors)

These three cardinal rules can definitely be translated into considerations in toxicant exposure. The analog of time in chemical exposure is the duration and frequency of exposure. This includes the concepts of acute, subchronic and chronic exposure. The analog of dose is the applied amount of the toxic agent (mg agent/ kg test subject) that an individual is exposed to or more specifically the concentration of the agent at the target site. The analogs for shielding for chemical exposure include mechanisms for preventing absorption, presystematic elimination by biotransformation at exposure sites (lungs, stomach, skin), and prevention of agent's function through inhibitory binding. Shielding could also come in the form of protective gear specific to the route of exposure for the particular toxicant. The nature of the toxicant needs to be considered if the effect of the toxicant is increased by certain conditions. If the delivery of the toxicant were facilitated within the system, then the dosage amounts would be increased. Also if the toxicant were held in an inactive state after exposure until particular conditions released it, then the timing of exposure would also be altered.

Depending on the route of exposure biological responses to a toxicant will differ/its toxicity changes. A chemical may be severely toxic by inhalation but not toxic by oral or dermal exposure. Differences in toxicity arise due to several factors:

In terms of toxicant exposure, these same three rules likewise hold:

1. Minimize the time of exposure to the source. If you reduce your exposure frequency and duration, you reduce your risk of harm.
2. Maximize the distance from the source to minimize the dose delivered. In other words, avoid exposure, if possible. No exposure is equivalent to no toxicity.
3. Optimize the amount and type of shielding used to minimize dose delivered. We can reduce exposures further by wearing protective equipment (e.g., gloves, respirators, goggles) or by other measures such as washing exposed surfaces following contact.

As ionizing radiation passes through a substance it will lose energy, due to the fact it produces an electron and a positively charged atom residue. When humans are exposed to radiation for certain periods of time, DNA synthesis becomes disrupted as the radiation breaks apart the precursors of DNA replication (purines and pyrimidines molecules). The following cardinal rules for minimizing exposure to radiation can be translated into considerations to toxic exposure because they somewhat pertain to an individual dose response curve: (1) Deficiency and Toxicity will occur with no protection from the radiation source, and (2) the Region of Homeostasis will not be perturbed with protection from the radiation source.

The radiation exposure rules can definitely be translated into toxicant exposure rules. For chemical exposure, time of radiation exposure is analogous to the amount of chemical one is being exposed to, since the more time one spends exposed to radiation, the more particles have the potential to hit the body. The dose is analogous to the toxicity of the chemical, the relative speed and amount of damage done at target sites, or possibly route of exposure, since that will control how much of the chemical will get inside the body. Shielding can be compared to either the effectiveness of detoxification processes in the body or the physical barriers preventing the toxicant reaching the target site.

Yes, these can be translated into consideration in toxicant exposure.

The three cardinal rules for minimizing radiation exposure can be translated in toxicant exposure. Minimization of time for toxicant exposure reduces the time available to act on the toxicant's targets and primary molecules. The ability of the toxicant to bind and act on targets is minimized by minimizing time, as in radiation exposure as well. Maximizing distance of a toxicant relates to dose/potency/concentration. By maximizing distance from source of toxicant the dose/potency/concentration are minimized. Intensity of toxicant is inversely proportional to the distance squared. Toxicants can be minimized by optimizing shielding. This includes limiting the routes of exposure available to the toxicant. Vaccines may also act as an internal shield, which will optimize endogenous barriers or repair processes (Campbell, Lecture 3/7/02). There are also differences among route of exposure or nature of toxicant that need to be considered in minimizing toxicant exposure because limiting access to one route of exposure that is known to be a risk for toxicants may make another route of exposure more accessible to the toxicant after chronic response.

Analogs in chemical exposure:

Yes, these rules can be translated into considerations in toxicant exposure. Minimizing time means lowering the time of toxicant to interact with its target molecule. Dose/Potency/ Concentration can be a analog of dose and limiting routes of exposures via vaccines; optimize endogenous barrier of repair processes can be analog of shielding for chemical exposure. Well just like with any other toxicant; you want to choose a route of exposure that will not take a long time to eliminate that toxicant. You also want to choose a toxicant that can be easily discarded by the body. Here, we are talking about radiation, therefore, we have to be little more careful than if we are talking about some kind of drug because radiation can cause mutations rapidly and therefore, can cause developmental defects. Also, depending on the nature of the toxicant, you want to choose the route of exposure that is limited and will easily eliminate the toxicant. For example, if the toxicant can be degraded in liver, you want to choose the route of exposure that is limited to liver only.

The major project for this course is a project chosen by the student in collaboration with the instructor.  The project will be submitted as an HTML compatible document (as generated by a word processor) that explores a gap or questionable aspect of current toxicological practice or a topic that will not be covered fully within the context of the course, e.g., appropriateness for classifying toxicants as initiators, promotors, or progressors of carcinogenesis; current knowledge of the impact of phytoestrogens on apoptosis within the rat endometrium; the molecular relatedness of P450 enzymes involved in Type I metabolism across multiple species as ascertained using online molecular databases; or, comparisons of several related case studies of toxicant exposures.  This document can use figures, models, and tables as well as a narrative argument to make or illustrate points.  It should incorporate standard references for all texts and journal articles cited as well as complete URLs for Internet or Web resources.  No materials should be copied or reproduced without alteration in production of this document.  The penultimate version of the project should be submitted electronically one month prior to the end of the term.  This will be critiqued by the instructor and at least two student peers.  The final version of the project will be due at the time of the final and must be submitted electronically.  The documents may be added to the course Website at the end of the term including appropriate information on their authorship. If it deals with case studies, it should contain an evaluation and discussion of at least two related studies dealing with a substantive topic in mammalian toxicology.  Although there are no set limits on the size or extent of the document, it is probably realistic to approximate 10-20 total pages including 40 or so references and URLs.  The document should be hypertext linked so the studies or documents being addressed can be accessed. The project constitutes 40% of the grade for the course and should be approached accordingly. 

Possible Case Study Topics

1. DDT and persistent pesticide bans
2. Dredging of the upper Hudson for PCBs
3. Arsenic wells in northeast and eastern India and Bangladesh
4. Zero tolerance for Alar in apples
5. Labeling and restriction of milk from cows injected with or transgenic for bovine GH
6. Oral contraceptive contamination of municipal sewage effluents in Great Britain

These should take into account the following items:

A. Mammalian associations
B. How does toxicology play a role in these cases?
C. What are the assumptions, stated or unstated, that support the contrasting arguements in these debates?
D. Are risk assessments being used?  If so, are these realistic and well grounded?
E. What particular elements of toxicity testing and modeling are being employed? Give examples.

Additional project ideas, case study possibilities.  What about Hanford radiation ground water contamination case?  Suggestion was, in such situations, to contrast this with other similar risk assessment cases, e.g., storage site contaminations or studies on possible contamination in South Carolina or the new site in Nevada.  Alternatively, look at contaminations at Three Mile Island, PA versus Chernobyl, USSR perhaps focussing on only a single isotope like ¹³¹I.  The intent of contrasts for environmental cases is to see if the assessment strategies were similar and if the public and private responses to the cases were justified and/or adequate.  Other examples might be the groundwater contaminations in Woburn, MA, those on Cape Cod near Otis Airforce Base, those at Love Canal, NY, Bopal, India or those associated with the tanning industries or gasoline storage.  Sources to include in case studies would be books, journal articles, websites, newspaper articles, regulatory documents or laws, maps, and images resulting from the cases.

What about food safety history?  Upton Sinclair’s The Jungle which depicted conditions of workers in the Chicago meat packing industry in the early 1900's initiated a public response that resulted in Congressional action and the passage of the earliest pure food legislation.  This eventuated in the creation of the Food and Drug Administration and became the model for other regulatory agencies charged with limiting public risks.  Examination of recent cases or present day conditions in the poultry industry, meatpacking, and fish processing would seem very suitable as case studies as would an historical approach to development of any of several of the existing agencies charged with risk assessment, limitation, and reduction.  New opportunities to look at food safety might also involve the handling and management of transgenic organisms such as grains or animal products.  Do genetically modified foods/crops require FDA approval, for example?