The study and characterization of chemicals and other agents for toxic properties is often undertaken on the basis of specific organs and organ systems. In this chapter, two targets have been selected for in-depth discussion: the immune system and the gene. These examples were chosen to represent a complex target organ system and a molecular target within cells. For more comprehensive discussion of the toxicology of target organs, the reader is referred to standard toxicology texts such as Casarett and Doull, and Hayes. The International Programme on Chemical Safety (IPCS) has also published several criteria documents on target organ toxicology, by organ system.
Target organ toxicology studies are usually undertaken on the basis of information indicating the potential for specific toxic effects of a substance, either from epidemiological data or from general acute or chronic toxicity studies, or on the basis of special concerns to protect certain organ functions, such as reproduction or foetal development. In some cases, specific target organ toxicity tests are expressly mandated by statutory authorities, such as neurotoxicity testing under the US pesticides law (see “The United States approach to risk assessment of reproductive toxicants and neurotoxic agents,” and mutagenicity testing under the Japanese Chemical Substance Control Law (see “Principles of hazard identification: The Japanese approach”).
As discussed in “Target organ and critical effects,” the identification of a critical organ is based upon the detection of the organ or organ system which first responds adversely or to the lowest doses or exposures. This information is then used to design specific toxicology investigations or more defined toxicity tests that are designed to elicit more sensitive indications of intoxication in the target organ. Target organ toxicology studies may also be used to determine mechanisms of action, of use in risk assessment (see “The United States approach to risk assessment of reproductive toxicants and neurotoxic agents”).
Methods of Target Organ Toxicity Studies
Target organs may be studied by exposure of intact organisms and detailed analysis of function and histopathology in the target organ, or by in vitro exposure of cells, tissue slices, or whole organs maintained for short or long term periods in culture (see “Mechanisms of toxicology: Introduction and concepts”). In some cases, tissues from human subjects may also be available for target organ toxicity studies, and these may provide opportunities to validate assumptions of cross-species extrapolation. However, it must be kept in mind that such studies do not provide information on relative toxicokinetics.
In general, target organ toxicity studies share the following common characteristics: detailed histopathological examination of the target organ, including post mortem examination, tissue weight, and examination of fixed tissues; biochemical studies of critical pathways in the target organ, such as important enzyme systems; functional studies of the ability of the organ and cellular constituents to perform expected metabolic and other functions; and analysis of biomarkers of exposure and early effects in target organ cells.
Detailed knowledge of target organ physiology, biochemistry and molecular biology may be incorporated in target organ studies. For instance, because the synthesis and secretion of small-molecular-weight proteins is an important aspect of renal function, nephrotoxicity studies often include special attention to these parameters (IPCS 1991). Because cell-to-cell communication is a fundamental process of nervous system function, target organ studies in neurotoxicity may include detailed neurochemical and biophysical measurements of neurotransmitter synthesis, uptake, storage, release and receptor binding, as well as electrophysiological measurement of changes in membrane potential associated with these events.
A high degree of emphasis is being placed upon the development of in vitro methods for target organ toxicity, to replace or reduce the use of whole animals. Substantial advances in these methods have been achieved for reproductive toxicants (Heindel and Chapin 1993).
In summary, target organ toxicity studies are generally undertaken as a higher order test for determining toxicity. The selection of specific target organs for further evaluation depends upon the results of screening level tests, such as the acute or subchronic tests used by OECD and the European Union; some target organs and organ systems may be a priori candidates for special investigation because of concerns to prevent certain types of adverse health effects.