Toxicology

TOXICOLOGY

Toxicology is the science of poisons. Understanding the potential for toxicity of agents found in nature has been a necessity for human survival. Learning to use natural toxins for purposes such as hunting and warfare was as much a part of human adaptation of the environment as was the taming of fire. One of the first known examples of the unwanted toxicity of a manufactured product was the lead poisoning that occurred in Roman times as a result of lead plumbing and lead dishware. Today, the emphasis of toxicology is on detecting and preventing the unwanted effects of chemical and physical agents, although concerns about the intentional misuse of chemicals, including chemical warfare, will persist for the foreseeable future.

As a science, toxicology is at the interface between chemistry and biology. There are three "laws" of toxicology. The oldest, that "the dose makes the poison," is attributed to Paracelsus, a fifteenth-century German physician. The concept that all chemical agents are toxic at some dose is central to a respect for the inherent hazard of all chemicals. The second "law" of toxicology, that the biologic actions of chemicals are specific to each chemical, has been attributed to Ambroise Paré, a sixteenth-century French surgeon who recognized that toxic agents have different effects dependent upon their inherent nature. Understanding the specific action of chemicals, known as hazard identification, depends upon recognizing the structural determinants of the activity of chemicals, and the biological niches in which chemicals interact. Very subtle changes in chemical structure can make an enormous difference in biological effects. The third "law" is that humans are animals. Protection against the toxicity of chemicals today would be impossible without the ability to study the effects of toxic agents in laboratory animals. As a corollary, animal rights activists advocating a ban on all animal research present a major threat to environmental protection and public health.

Toxicologists generally consider two types of dose-response relationships. One has a threshold below which no effect is expected. For example, one drop of fuming sulfuric acid will burn a hole in skin, yet this same drop in a bathtub full of water dilutes the sulfuric acid to a level at which no effects will occur. This theoretical threshold (experimentally known as a "no-observed-effect level") is presumed to exist for all agents, except for those that produce their effects through mutation, most notably many cancer-causing agents. A mutation can theoretically occur through a single chemical molecule producing a specific change in the chemical structure of a DNA molecule, thereby altering the genetic code from that of a normal cell to that of a mutated cell. As a further simplification, two molecules have twice as much chance as causing this effect. This can be described as a linear one-hit relationship between the dose of a mutational agent and the likelihood that the mutation will occur. The theoretical risk for any one molecule causing a mutation is infinitely small—there are about 1 trillion molecules of benzene, a known cause of leukemia, in every breath taken in an average American city, yet very few people develop leukemia. There are also many defense mechanisms within cells, as well as DNA repair mechanisms, that can impact on the likelihood of chemical exposures causing cancer.

Extrapolation of data from laboratory animals to humans, and from high to low doses, is central to modern toxicology. In addition to understanding dose-response relationships, knowledge about differences among species in the uptake, metabolism, and disposition of chemicals is also of importance. There is a strong similarity among mammalian species. Where differences do exist, attention to the kinetics of the processes that determine how an external exposure level is translated to the dose of a chemical at a target organ provides information of value to cross-species extrapolation.

A major challenge in modern toxicology is to prevent unwanted effects of otherwise valuable chemicals, including therapeutic agents. Understanding chemical mutagenesis and carcinogenesis has permitted the development of bacterial mutagenesis assays, such as the Ames test. These and other short-term assays for toxic effects are routinely used during the development phase of new chemicals to screen out potential toxic agents. Before marketing, additional testing is often required, depending in part on the use of the chemical. For new pharmaceutical agents, extensive toxicity and efficacy data are required, including studies in humans. Such agents are expected, at anticipated human dose levels, to have a biological effect of benefit to the consumer. In contrast, the developers of consumer chemicals, such as a new paint, hope that no biological effects will occur at usual doses to humans.

There are intermediate agents, such as insecticides and herbicides, for which a biological effect is intended at usual doses—for these agents, protection of humans depends, in large part, on our different biology. Accordingly, premarket testing is usually less rigorous for consumer chemicals than for therapeutic agents, and there is more dependence on structure-activity relationships (SAR). SAR, in essence, is a comparative analysis of aspects of chemical structure in relation to the existing toxicological database—a useful, but not completely effective, approach. In the United States, the Toxic Substances Control Act requires premanufacturing notification of new chemicals to the U.S. Environmental Protection Agency, which has the option of asking for additional testing. Such tests might include a battery of shorter-term and longer-term tests for acute and chronic diseases, including cancer. The recognition of the dangers inherent in compounds that bioaccumulate or otherwise persist in the environment has led to tests to identify and exclude such compounds from commerce.

Long-term animal assays, usually two-year studies in male and female rats and mice, are the mainstay of thorough safety assessment of chemicals, particularly those for which there is a concern about cancer or other chronic effects. The basic approach is to first perform a multiple-dose ninety-day study to choose the maximum tolerated dose (MTD). This dose is then used for a two-year study. Sole reliance on standard safety-assessment approaches carries a small but finite risk of missing a potentially toxic agent, a risk which is lessened if studies assessing the mechanism of toxicity of the chemical are also performed. A major goal of toxicological research is a better understanding of the processes by which chemical agents produce adverse biological effects, which will lead to the development of better safety-assessment tests.

The pathways of chemicals into and through the body are usually considered under the headings of absorption, distribution, metabolism, and excretion. Absorption, the process by which an external dose is converted to an internal dose, occurs by ingestion, inhalation, or through the skin. Distribution of a chemical depends in part on the pathway of entry and on specific chemical and biological factors; for example, only certain types of chemicals are able to penetrate the blood-brain barrier and enter the central nervous system.

Much emphasis has been placed on understanding chemical metabolism, as this is central both to the impact and to the detoxification of chemicals. The activity of many of the metabolic enzymes can vary greatly among individuals due to genetic and environmental factors, including types of food. Further, metabolic rates may vary within a given individual at different times due to induction of metabolic enzymes by these same environmental factors. Studies of resistance to cancer chemotherapeutic agents have led to an understanding of mechanisms by which toxic agents can be rapidly transported out of an otherwise susceptible cell, including specific transporter proteins, which can also be induced in response to environmental factors.

The major metabolic organ is the liver, but all organs have some level of metabolic enzymes. Certain chemicals, such as benzene, are harmless until they are metabolized by the body to form toxic chemical intermediates. Further, not all chemicals are metabolized; some pass through the body unchanged, while others react directly with biological targets.

Major excretory pathways are through urination, defecation, and exhalation. Lactation is also a means of excretion, particularly of fat-soluble chemicals, to the potential detriment of the infant.

Differential sensitivity to chemicals is an important subject to toxicologists for which modern molecular biology is providing new insights, particularly through the understanding of the human genome. For most human disease, genetics will determine what is necessary, but the environment, defined broadly, will determine what is sufficient. A reasonable estimate is that over two-thirds of human disease is environmentally determined. Many of the genetic and environmental factors responsible for disease operate at the level of modifying the absorption, distribution, metabolism, or excretion of exogenous chemicals, including food constituents.

Susceptibility to toxic agents is also conferred by factors such as age, gender, and concomitant conditions. Children, the elderly, and those with preexisting disease tend to be more susceptible to environmental toxins than are healthy adults. For example, the greater respiratory ventilation per unit of body mass in children accounts for the tragic finding of death due to carbon monoxide poisoning in the children, but not the adults, in a snowbound car.

So-called safety factors have traditionally been used in establishing public health and regulatory guidelines and standards based on toxicological data. These are based on no-observed-effect levels in animals, which are then reduced by a factor of ten to provide assurance that the animal data is protective of humans. In general, a tenfold factor is used to account for the possibility that humans are more sensitive than the animal species from which the data are obtained. Another tenfold factor is based on the greater diversity in susceptibility factors among humans than in inbred laboratory animals. The resultant hundredfold safety level has been used on a relatively routine basis for establishing acceptable daily intake (ADI) levels by the Food and Drug Administration, as well as for other regulatory standards. Additional factors of ten can be added based upon the toxic endpoint involved, or in order to protect children. Conversely, when there is a sufficiently robust database on humans, such as for certain air pollutants, routine factors of ten are not used, and scientific judgment contributes to the determination of an appropriate margin of safety.

The effect of toxic agents on ecosystems has become increasingly recognized as being important to human health. Traditionally, ecotoxicology (in relation to human health) has focused on contamination of the food chain, including the biomagnification and bioaccumulation of toxic agents within foods. The recognition of the role of ecosystems in overall planetary health, including feedback loops affecting climate, desertification, and crop yield, as well as the importance of the natural world and its animal and plant components to human well-being, has led to additional emphasis on understanding the toxicity of chemical and physical agents to components of nature.

BERNARD D. GOLDSTEIN

(SEE ALSO: Ames Test; Benzene; Carcinogenesis; Ecosystems; Environmental Determinants of Health; Environmental Protection Agency; Ethics of Public Health; Genes; Genetics and Health; Lead; Maximum Tolerated Dose; One-Hit Model; Risk Assessment, Risk Management; Safety Assessment; Safety Factors; Threshold)

BIBLIOGRAPHY

Goldstein, B. D., and Henifin, M. S. (2000). "Reference Guide on Toxicology." In Reference Manual on Scientific Evidence, 2nd edition. Washington, DC: Federal Judicial Center.

Klaassen, C. D., ed. (1996). Casarett and Doull's Toxicology: The Basic Science of Poisons, 5th edition. New York: McGraw-Hill.

Lippman, M., ed. (1999). Environmental Toxicants: Human Exposures and Their Health Effects, 2nd edition. New York: John Wiley.