Modern Poisons

A Brief Introduction to Contemporary Toxicology

Alan Kolok

• Publisher: Island Press
• Year: 2016
• Print ISBN: 9781610913829, 9781610913812
• eBook ISBN: 9781610916097
• Topic: Biological Sciences
• Subtopic: Diseases & Allergies
• Index: Biological Sciences

Traditional toxicology textbooks tend to be doorstops: tomes filled with important but seemingly abstract chemistry and biology. Meanwhile, magazine and journal articles introduce students to timely topics such as BPA and endocrine disruption or the carcinogenic effects of pesticides, but don't provide the fundamentals needed to understand the science of toxicity. Written by a longtime professor of toxicology, Modern Poisons bridges this gap.

This accessible book explains basic principles in plain language while illuminating the most important issues in contemporary toxicology. Kolok begins by exploring age-old precepts of the field such as the dose-response relationship and the concept, first introduced by Ambroise Paré in the sixteenth century, that a chemical's particular action depends on its inherent chemical nature. The author goes on to show exactly how chemicals enter the body and elicit their toxic effect, as well as the body's methods of defense.

With the fundamentals established, Kolok digs into advances in toxicology, tracing the field's development from World War II to the present day. The book examines both technical discoveries and their impacts on public policy. Highlights include studies of endocrine-disrupting chemicals in toiletries and prescriptions, the emerging science on prions, and our growing understanding of epigenetics.

Readers learn not only how toxic exposure affects people and wildlife, but about the long-term social and environmental consequences of our chemicals. Whether studying toxicology itself, public health, or environmental science, readers will develop a core understanding of—and curiosity about—this fast-changing field.

Chapter 1

The Dose Makes the Poison

All things are poison, and nothing is without poison; only the dose permits something not to be poisonous.

— Paracelsus

When I was in elementary school, conversations on the playground often took a fatalistic turn. Perhaps it was just an echo of the Cold War era, but I can recall chatting with my school chums about chemical compounds and death. We’d exclaim, “If you breathe too hard, it could kill you,” or “You could drink so much water that you would die!”

Today, the concern is less about the lethal quantity of these relatively benign substances, but rather about pollutants in our food, water, and air. Yet despite our lack of sophistication, my young friends and I weren’t actually so far off the mark. We didn’t realize it at the time, but we were channeling a sixteenth-century physician, Paracelsus. Considered the father of toxicology, Paracelsus is credited with the first and most important tenet of the field, the idea that the dose makes the poison: “All things are poison and nothing is without poison; only the dose makes a thing not a poison.” In other words, seemingly benign substances like water as well as obviously dangerous ones like arsenic can be deadly when administered in excess.

Paracelsus’s groundbreaking idea centers on the dose–response relationship: the fact that in most cases the greater the dose, the greater the adverse, or toxic, response. While humble in its simplicity, the concept provides a thematic platform upon which modern regulatory toxicology is based. Furthermore, the relationship is actually more interesting than it would first appear, as both dose and response are surprisingly nuanced.

When a chemical, toxic or benign, contacts a biological organism, the contact is known as an exposure. The exposure dose is the quantitative amount of a chemical that a person (wittingly or unwittingly) is exposed to, and this quantity can be either directly or indirectly measured. For common chemicals that are deliberately administered, such as pharmaceuticals, the route of administration is direct, and generally occurs via oral consumption or injection. For exposures of this type, the dose is generally given in terms of the mass (in grams, g, or milligrams, mg) of the chemical being administered. For example, a regular-strength aspirin pill, one of most commonly consumed pharmaceuticals, contains 325 mg of the active ingredient, acetylsalicylic acid. The tablet also contains a number of other inert chemicals, but the dose refers to the amount of the active ingredient. For injections, the dosage is expressed in the same way. An epinephrine auto-injector, for example, widely self-administered by individuals with food allergies, will administer a dose of 0.3 mg of epinephrine to the individual despite the fact that the injected solution contains other chemical compounds.

In the examples given above, the exposure route is direct and easily quantifiable, but what if, on the other hand, the exposure is indirect? Indirect exposures would include the exposure that results when a fish ventilates contaminated water across its gills, or a person inhales secondhand smoke into their lungs. In these cases, the quantitative dose of the chemical exposure is much less certain, and much more difficult to measure. Rather than determining the exposure dose, it is far easier to quantify the concentration of the chemical in the “environment” (the water that the fish is ventilating, or the air that the animal or person is breathing). Furthermore, since the amount of the compound that the animal ventilates or inhales is not known exactly, the exposure cannot be quantified in terms of mass, but rather is quantified in terms of its concentration (the amount of chemical found in a specific volume of air or water) in the local environment.

Regardless of the direct or indirect source of the exposure, the response of an animal to a chemical exposure is also generally expressed in one of two broad categories, either discrete or continuous. Organism death is the ultimate discrete response, in that animals can only be found in one of two states, dead or alive. While perhaps somewhat gruesome, death provides a very valuable (and oft-times used) endpoint for toxicological studies. In contrast, variable responses to an exposure can also occur. For example, the impairment of cognition due to alcohol consumption is a classic example of a continuous variable. The response to alcohol is not all-or-none, but rather increases in its impact as the administered dose increases. This is also true for other types of toxicological impairment, such as changes in genetic expression or alterations in the activity of proteins.

Interestingly, the way that an exposure dose is expressed, whether indirect or direct, and the way that the response is measured, whether discrete or continuous, do not affect the overall shape of the dose–response relationship. In the majority of cases, the shape of the dose–response curve remains sacrosanct regardless how the dose and response data are represented within it.

Quantifying the Dose–Response Relationship

The dose–response relationship is a very powerful tool, frequently used by regulatory agencies. A common approach used to test new chemicals, or chemicals used in novel ways, begins with the generation of dose–response relationships. Generally, the first battery of toxicity testing evaluates the capacity of a chemical to produce the discrete endpoint, death, which is exacted upon a population of experimental laboratory animals, such as mice.

A dose–response curve does not really focus upon death, but rather mortality. Death is the response of an individual organism, and clearly each individual can be in only one of two states: dead or alive. In contrast to death, mortality is the response of a population of individuals. The mortality rate describes the proportion of a population that dies in response to a calamitous exposure to toxic chemicals. To graphically illustrate the mortality of a group of animals that are exposed to the same dose of a toxic compound, we use the discrete dose–response curve. At one extreme of the toxicology curve, animals exposed to low doses survive (mortality rate is zero), whereas at the other extreme all of the animals exposed to higher doses of a chemical die (mortality rate is 100 percent).

In between total survivorship and total mortality, the dose–response relationship gets more interesting. In the vast majority of cases, the relationship between the two is a characteristic “S” or sigmoidal shape. At low chemical doses, an incremental increase in the concentration of the toxic substance does not lead to a very large increase in mortality. At intermediate doses, the impact of the compound on mortality increases dramatically, while at the highest doses, the increase in mortality from one dose to the next higher dose is again minimal.

An important point to help clarify the relationship is the inflection point. On the lower half of the curve, increases in dose lead not only to a greater number of animals dying, but also to an increase in the rate at which mortality increases from one dose to the next. In other words, the slope of the line from one concentration of a chemical to the next continues to increase until it reaches a maximum slope at the inflection point. Further increases in the dose of the compound continue to elicit a greater biological response, but the rate at which the response increases is now declining with each successive increase in dose administered. The inflection point always occurs at the midpoint of the curve, the point at which 50 percent mortality would occur in a lethal toxicity test, and, as will become apparent in a later section of the chapter, the inflection point has taken on considerable importance with respect to toxicological testing.

The transition from experimentally derived data to a useful dose–response relationship (one that allows points of interest along the curve to be quantified) is more difficult to come by than it may first appear. Filling in the gaps between a few, relatively scarce data points (experimentally collected) to a complete curve, requires the use of a mathematical equation that characterizes the relationship. Once that equation has been defined, it can then be used to identify any point along the curve, not just points where data has been collected.

Pragmatically, there are important experimental design issues that have to be resolved when elucidating the dose–response relationship for a chemical compound. For example, if a toxic compound is novel and has never been tested previously, then the researcher is flying blind and will need to generate a dose–response relationship that includes a wide range of chemical concentrations. Very often the range is so large that the x-axis of the dose–response relationship is not represented arithmetically (that is, 1, 2, 3, and so on) but rather is arranged geometrically (that is, 1, 10, 100, etc.). In this case, it is highly likely that the experiment will include one or more groups of animals that are exposed to chemical concentrations that generate no mortality, and one or more doses that cause total mortality. Importantly, these doses do not help to quantify the dose–response curve. After excluding these points from analysis, the number of data points remaining to assess the sigmoidal curve may become disturbingly small, thereby reducing the scientific confidence that the researcher may have in the results.

Fortunately, there are mathematical methods by which some of these difficulties can be circumvented. Probit analysis allows for mathematical gyrations to occur so that a sigmoidal relationship can be straightened into a line. As students of Euclidian geometry can testify, the shortest distance between any two points is a straight line, and conversely, any line can be described by only two points. As such, the entire dose–response curve can be accurately estimated using probit analysis when as few as two of the chemical doses provide data that lie somewhere between zero and total mortality. Furthermore, once the relationship is described by a linear equation (y = slope*x + y-intercept), any point on the line can be readily quantified by plugging a few numbers into the simple linear equation.

Dancing along the Dose–Response Relationship

The beauty of the linear dose–response relationship is that it provides a wealth of preliminary information regarding the interaction between the animal and the chemical. For example, the slope of the line provides information regarding the efficacy, or the capacity to produce a biological effect, of the toxic chemical. As the slope increases, the efficacy of the chemical compound also increases. Furthermore, if the efficacies of two compounds are similar, then dose–response relationships can yield a number of useful points that provide a shorthand, a single number, by which the toxicity of the different chemicals can be compared.

Now recall the inflection point that was discussed previously. The inflection point, known as the LD₅₀, is the chemical concentration at which 50 percent of the animals die due to the exposure. If animals are exposed to an environment (a noxious gas in the atmosphere for animals that breathe air, or a toxic compound in the water where fish live), the inflection point can still be evaluated, although it is given the moniker LC₅₀ (the chemical concentration in the organism’s environment at which 50 percent of the organisms have died). These inflection points provide a handy numeric index that can be used to compare the toxicity of different compounds.

The second point that can be gleaned from a dose–response relationship is the threshold concentration. The threshold concentration is the concentration at which the probability of an adverse impact (for example, one adverse case per million individuals) is low enough to be deemed acceptable. Interestingly, while the threshold dose can easily be located on the dose–response line once the acceptable rates of adverse impacts are agreed upon, the acceptable probability of adverse effect is socially or politically determined, as opposed to scientifically determined. This topic will be discussed in greater detail in chapter 14.

A third important point regarding the threshold dose is that it is a mathematical rather than an empirical construct. In other words, a threshold dose is not limited by the choices of the scientist conducting the test. If a scientist, for example, has injected rats with a chemical at five concentrations (0.01, 0.1, 1, 10, and 100 milligrams per kilogram), the threshold dose is not limited to those concentrations. The line derived from a probit dose–response relationship does not just describe the relationship for a few points on the line, but rather describes the relationship for all of the points on the line. This is not the case for two other commonly used endpoints, known as NOEC and LOEC.These endpoints have also been used to infer chemical safety, despite the fact that both metrics have fallen into considerable disfavor. The NOEC (no-observable-effect concentration) represents the greatest measured chemical concentration on the curve that does not yield a positive effect, whereas the LOEC (lowest-observable-effect concentration) represents the lowest measured concentration of the chemical that yields a biologically adverse effect. Importantly, these metrics are inherently biased. While the threshold dose is mathematically derived, using all of the points on the dose–response curve, NOEC and LOEC values only correspond to the points on the curve where the exposures were empirically conducted. As such, the number of exposure doses selected by the experimenter limits the total number of possible values for these points. For example, the scientist that injected rats with a chemical at five concentrations (0.01, 0.1, 1, 10, and 100 milligrams per kilogram) can only determine NOECs or LOECs at one of the five concentrations at which the experiment was actually conducted. Effectively, the results, rather than being mathematically derived and unlimited, are derived by the whim of the investigator and are extremely limited.

When a federal or state agency develops chemical safety standards, these are almost always lower than the threshold values generated from the dose–reponse relationship. The reason for this is purely pragmatic, as the results from toxicity testing are generally derived from rodent models (rats and mice) and applied to humans. Rodents can be either more or less sensitive to the chemical than humans; therefore, a safety factor is often applied in order to decrease the maximum contaminant level by an order of magnitude—in other words, a tenfold reduction in the contaminant-level goal. Furthermore, while one safety factor can be applied to safeguard against species differences, a second factor can be applied to take into account the enhanced risk of the chemical to sensitive subpopulations (infants, children, the elderly, and those with compromised immune systems).

Exceptions

But what if the sigmoidal relationship is not valid? For example, consider vitamin A. Vitamin A is actually a suite of compounds, including retinol, retinal, and a number of similarly structured carotenoids. An insufficient intake of dietary vitamin A leads to a deficiency that can cause impaired vision, particularly during low light levels. Yet vitamin A is a fat-soluble compound that cannot be excreted as readily from the body as water-soluble vitamins, such as vitamin C. Therefore, if one consumes too much vitamin A, there is a risk of toxicity. Chronically high levels of vitamin A are toxic, particularly with respect to fetal development during organogenesis, the time of development when the primary body organs are developing. In the case of vitamin A, the dose–response curve is not a sigmoidal curve that consistently slopes forward, but rather a bowl shape with adverse impacts occurring on the lower end, where vitamin A deficiencies occur, and on the upper end of the distribution, where overt toxicity occurs. In this case, the dose makes the poison in two different ways: at higher concentrations of vitamin A, the toxic effect prevails and the lower the level the better. At low concentrations of vitamin A, the compound acts as a micro nutrient, deficiency governs the impact, and the more vitamin A the better.

While deficiency is not really a toxic impact, there are examples of other chemicals where adverse impacts occur at both lower and higher exposure concentrations. The compound 17β-estradiol is a perfect example. At high levels, 17beta-estradiol increases the risk of carcinogenesis, and can be overtly toxic. However, as the dose decreases, the impact will attenuate according to a classic dose–response sigmoidal relationship. Doses lower than the threshold will not increase cancer risk. However, at doses much lower than the threshold, this sex steroid will also act as a cell signal that helps to govern vertebrate fetal development, among other functions. While estradiol is essential to the development of both males and females, unusually elevated levels of the compound at the wrong times can lead to toxicity, including inappropriate female pattern development in males. This is the genesis of unusual reproductive morphologies in some male animals, such as ovo-testes in which ovarian follicles develop within the testicular tissue of males. (The relationship between chemicals and fetal development will be revisited in later chapters of this book.)

Our view of toxic chemicals is governed by the edict of Paracelsus, a sixteenth-century physician. For many toxins, perhaps the vast majority, the dose does make the poison. This first law of toxicology has driven a great deal of research and safety regulation. In the world of modern poisons, many of the interactions between the molecule and the organism are defined by this simple yet elegant relationship.

Chapter 2

The Nature of a Chemical

Some things just aren’t meant to go together.

Things like oil and water.

Orange juice and toothpaste.

— Jim Butcher

The second rule of toxicology helps to explain why some chemicals are easily excreted from the body, while others are not. It also goes to the heart of why different toxic agents affect us in different ways. The second rule was first posed by Ambroise Paré, a sixteenth-century French surgeon, who realized that “Poison . . . kills by a certain specific antipathy contrary to our nature.” In other words, a chemical’s particular action depends on its inherent chemical nature.

This is a seemingly simple concept, but what exactly is meant by a chemical’s “inherent nature”? We now know that chemical compounds generate biologic effects based upon the structure of individual molecules. Toxicity occ...

Table of contents

• Cover
• About Island Press
• Title Page
• Copyright
• Contents
• Preface
• Acknowledgments
• Chapter 1: The Dose Makes the Poison
• Chapter 2: The Nature of a Chemical
• Chapter 3: The Human Animal
• Chapter 4: Chemical Journeys: Absorption
• Chapter 5: Bodily Defense
• Chapter 6: Wider Journeys: Pollution
• Chapter 7: Traveling Particles
• Chapter 8: Toxins, Poisons, and Venoms
• Chapter 9: Metals: Gift and Curse
• Chapter 10: Combustion
• Chapter 11: Drugs and the Toxicology of Addiction
• Chapter 12: 70,000 Years of Pesticides
• Chapter 13: The Origins of Regulation
• Chapter 14: Low-Dose Chemical Carcinogenesis
• Chapter 15: POPs and Silent Spring
• Chapter 16: Toxic Toiletries
• Chapter 17: Determining Sex: Chemicals and Reproduction
• Chapter 18: The Earliest Exposure: Transgenerational Toxicology
• Chapter 19: Natural Toxins Revisited
• Chapter 20: Chemical Resistance
• Afterword: Toxicology and Beyond
• References
• Index
• Island Press | Board of Directors