Toxicologic Pathology: An Introduction

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An Overview of Toxicologic Pathology

Citizens of industrialized societies generally have enjoyed a rising standard of living since the dawn of the Industrial Age in Great Britain during the mid-18th century and in other countries thereafter. This time of widespread economic and social improvements greatly enhanced both individual and communal health, longevity, and productivity. Substantial declines in life-threatening infections and malnutrition reflect not only better hygienic practices and increased food supplies but also the advent of more medical therapies, including drugs with reliable and reproducible efficacy. Concurrent progress in bulk synthetic chemistry led to fabrication of many new, energy- and time-saving products including fertilizers, pesticides, petrochemical fuels, rubber, and more recently clothing and plastics. The adoption of various chemicals, metals, and mixtures during this period emphasized their utility (efficacy) rather than their potential harmful effects (toxicity).

The large-scale production of chemicals, fuels, and metals in industrializing countries was (and is) accompanied by exposure of people, animals, and the environment to toxic materials in the diet, environment, home, and workplace. Such agents may be introduced purposefully (e.g., chlorine and fluorine added to water), encountered accidentally (e.g., pesticide and solvent residues in soil and water), experienced unavoidably (e.g., automotive and industrial exhaust particles in air), or consumed deliberately (e.g., alcohol, tobacco smoke). Each individual will be exposed throughout life, often beginning in utero, to a complex and unique “exposome”—a personalized menu of chemicals and metals ( ; )—that collectively has the capacity to alter normal biological activities as well as initiate and sustain unexpected physiological effects. Contaminant exposures in industrialized societies have been linked to such “modern” diseases as cancer (e.g., asbestos in ship workers, soot in chimney sweeps) and hexacarbon neuropathy (in people manufacturing solvents or sniffing glues).

Focus groups and the general public in many industrialized countries soon developed concern over environmental and occupational pollution and the demonstrated potential for a decreased quality of life, eventually leading to legislation to address such issues as the effects of chemical dispersal on people and the environment and the safety of products for consumption ( The Role of Pathology in Product Discovery and Safety Assessment, Vol 2, Chap 2). Such considerations led to the organized pursuit of toxicology—the study of toxic agents, their harmful effects, and their mechanisms—as a vital means for protecting human, animal, and environmental health ( ; ). In industrialized countries, this led to a society-wide enterprise to test new products for toxicity using many endpoints: in-life observations, in vitro screening, and especially evaluation of tissue and fluid samples. The need for a methodical and expert examination of tissues and fluids was the impetus that launched the toxicologic pathology profession as a recognized biomedical specialty.

Toxicologic pathology was practiced nonsystematically to a variable degree in industrialized societies during the 19th and early 20th centuries. More systematic and widespread practice of toxicologic pathology began in the 1970s in many countries in response to the society-wide concerns described above. In the United States, three major factors were responsible for this acceleration. The first was President Richard Nixon's declared “War on Cancer.” The second was organization of the U.S. National Toxicology Program to screen environmental chemicals for potential toxicity and carcinogenicity. The third was the increasing demand placed on industrial firms to show that their products were “safe” for humans (or sometimes animals) that might be exposed. This concern was raised by contemporary cases demonstrating that flawed safety testing has a significant economic and social impact, which was shown most clearly by the thousands of infants with limb malformations born in the 1960s to European women who took the antinausea drug thalidomide during early pregnancy. Concern regarding the safety assessment apparatus was amplified by the discovery of fraud in some chemical testing laboratories during the mid-1970s, which resulted in the late 1970s in the release of “Good Laboratory Practice for Nonclinical Laboratory Studies” (GLP) guidance (21 CFR Part 58) by the U.S. Food and Drug Administration (FDA) ( Pathology and GLPs, Quality Control and Quality Assurance in a Global Environment, Vol 1, Chap 27 ) to provide more rigor and reproducibility in toxicity study data sets. These FDA GLP guidelines, as well as similar GLP guidance given by other organizations (e.g., the U.S. Environmental Protection Agency, found under 40 CFR 160), provide general direction for optimizing the conduct of many aspects of animal toxicity testing, including toxicologic pathology practices.

Toxicologic pathologists are critical to public health efforts in modern societies, for several reasons. First, toxicologic pathologists typically are experts in integrative (“whole organism”) biology, largely due to the veterinary medicine (or medicine) and pathology backgrounds of most practitioners. As such, they can assemble a composite picture incorporating both functional and morphological data to determine a subject's individual health or a treatment group's collective vulnerability following exposure to a novel entity (variously called a “test article” or “test item” or “test substance”). Second, toxicologic pathologists are well versed in comparative biology principles that encompass a range of laboratory animal species as well as humans. This training makes them adept at translating animal data to identify hazards, assess safety, and manage potential risks. Third, toxicologic pathologists work well in a team setting with scientists who are expert in other bioscience disciplines. This ability stems from the broad educational curriculum required to enter the toxicologic pathology field as well as the typical formative experiences encountered by pathologists in training and working at institutions that perform toxicology research (i.e., multidisciplinary projects requiring an integrated approach followed by regular presentations and reports to multiple audiences with variable levels of expertise in pathology and toxicology). This collaborative ability is essential for an organization's efficient and effective product development program as well as for the individual's own career.

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What Is Toxicologic Pathology?

Toxicologic pathology integrates the disciplines of medicine, pathology, and toxicology. Medical and veterinary medical practitioners study the dynamic basis of health and disease, and are well versed in the normal and abnormal structure (at the cell, tissue, organ, and system levels) and function (in terms of activity and biochemistry) of all elements within the body. Pathologists are biomedical practitioners with extra training and experience in investigating the nature of disease (pathophysiology). As such, pathologists evaluate changes produced in cells, tissues, organs, or body fluids in response to a “challenge,” whether that challenge arises internally (e.g., immune mediated, metabolic, or neoplastic cause) or externally (e.g., infectious, physical, or toxic agent). As with other etiologies, most toxic diseases leave significant “signatures” in cells, fluids, and tissues. On the other hand, toxicologists focus on the biochemical basis and metabolism of products with known or unknown toxic potential. While grounded in medicine, pathology, and toxicology, toxicologic pathology also requires familiarity with other related disciplines, such as molecular biology, experimental design, and statistics.

Pathologists are well versed in evaluating manifestations of diseases, whether they affect humans (medical pathologists) or animals (veterinary pathologists). The toxicologic pathologist must master both experimental and comparative pathology in the context of data interpretation and extrapolation from observations in animals to predicting possible responses in target human populations. This perspective differs from those of other pathologists (whether medical or veterinary medical). For example, a diagnostic pathologist examines changes in tissues and body fluids from an individual or small group to define the cause of disease or death in the affected individual or a wider population. Similarly, a forensic pathologist investigates death or disease that is unnatural or “suspicious” in nature. In contrast, the main role of a toxicologic pathologist is to determine the biological significance of changes in form, function, or both as manifested by altered structure of cells and tissues (typically termed “changes” or “findings” or—if judged to be adverse—“lesions”) and/or composition of body fluids (“biomarkers”) by a test article. Toxicologic pathology is an essential element of hazard identification, dose–response data generation, and risk characterization, which are essential for risk analysis and assessment as well as risk management of human and animal chemical exposure.

Since these activities are largely confined to the industrial setting, toxicologic pathology is sometimes referred to as “industrial” pathology. Indeed, most toxicologic pathologists are employed by industry, be it biopharmaceutical (or other biomedical product), agrochemical, chemical, or contract research organizations (CROs). However, toxicologic pathologists also may work at academic institutions, private research foundations, and regulatory agencies. Finally, many toxicologic pathologists function as independent consultants, typically after completing a stint or an entire career at one of the settings listed above. Thus, toxicologic pathology is a vibrant and far-reaching discipline offering numerous and varied opportunities for scientific engagement and societal service.

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The Basis of Toxicologic Pathology

Toxicologic pathology is founded in the art and science of observation. Detailed descriptions of altered morphology still represent a vital component for understanding tissue changes induced by toxic agents. The quest to understand the causes of disease resulted in efforts to associate lesions with their cause(s). As the discipline of toxicologic pathology has grown, associations have been made between structural lesions, hematologic and serum chemistry findings, and potential etiologic agents or classes.

Observations of altered morphology initially were gained by examining macroscopic changes seen during an autopsy or necropsy (i.e., animal autopsy). With the development of the light microscope and later the electron microscope, morphologic observations could be extended to include changes at the cellular and subcellular levels ( Morphological Manifestations of Toxic Cell Injury , Vol 1, Chap 6 ). More recently, special techniques have been developed to quantify subtle structural changes as well as to correlate structural changes in tissues and cells with key functional and molecular changes associated with them ( Special Techniques in Toxicologic Pathology, Vol 1, Chap 11 ; In Vivo Small Animal Imaging: A Comparison to Gross and Histopathologic Observations in Animal Model s , Vol 1, Chap 13 ). Importantly, enhanced techniques for examination of altered components in blood and other body fluids from living animals have led to the development of biomarkers that can be used to predict the presence and severity of toxic injury ( Biomarkers: Discovery, Qualification, and Application, Vol 1 , Chap 14 ).

Proficiency in toxicologic pathology now requires a working knowledge of many elements that are more closely related to toxicology than pathology. The most important aspects address what the body does to a chemical ( Biochemical and Molecular Basis of Toxicity, Vol 1, Chap 2 ; ADME Principles in Sma ll Molecule Discovery and Development - An Industrial Perspective , Vol 1, Chap 3 ; and Biotherapeutics ADME and PK/PD Principles, Vol 1, Chap 4 ) and what the chemical can do to the body ( Principles of Pharmacodynamics and Toxicodynamics, Vol 1, Chap 5 ). From both perspectives, many factors impact those interactions. Examples include the composition or chemical form of the test article, the dose, and the route of exposure; these parameters are different for traditional small molecule chemicals, large biomolecules or macromolecular vectors, cells, implanted materials, and vaccines (see chapters for these specific biomedical product categories in Vol 2). The study design is dictated in part by these factors but also is influenced by biological attributes of various model species (Vol 1: Animal Models in Toxicologic Research : Rodents , Chap 17 ; Animal Models in Toxicologic Research : Rabbits , Chap 18 ; Animal Models in Toxicologic Research : Dogs , Chap 19 ; Animal Models in Toxicologic Research : Pigs , Chap 20 ; Animal Models in Toxicologic Research , Chap 21 ; and Animal Models in Toxicologic Research , Chap 22 ) including race (or breed/strain); individual variations in absorption, distribution, metabolism, and excretion (ADME); the intended uses of the product; and often the environment in which exposure will occur. Toxicologic pathologists need to be conversant in such disciplines in order to maximize their ability to integrate and interpret data compiled during toxicity studies.

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Challenges in Toxicologic Pathology

Toxicologic pathology is tasked with gathering data to identify and characterize hazards, evaluate safety, and assess risk associated with toxicant or toxin exposure. When contemplating the implications of these objectives, one is reminded of Albert Einstein's remark, “No amount of experimentation could ever prove me right; a single experiment can prove me wrong.” This comment epitomizes the fundamental dilemma of toxicologic pathology data: results may show that a test article has toxic and/or carcinogenic properties, but can never say with certainty that a material is safe ( Experimental Design and Statistical Analysis for Toxicologic Pathologists, Vol 1, Chap 16 ).

Toxicologic pathologists regularly encounter a series of challenges during the course of their daily practice. Some of them are related to the unique tasks that comprise the raison d’être for toxicologic pathology as a distinct scientific discipline. Other challenges are related to ancillary knowledge and roles that must be considered by toxicologic pathologists to maximize the value of their work to the entire research endeavor. With experience, proficient practitioners learn to recognize the opportunities for excellence hidden within these challenges.