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Developmental toxicology: status of the field and contribution of the National Toxicology Program.

The NTP has conducted developmental toxicity studies on more than 50 chemicals, often in multiple species. Several chemicals caused developmental toxicity in the absence of any toxicity to the mother. Although hazard to humans is determined by the level of exposure to the chemical and its inherent toxicity, those agents that selectively disturb the development of the conceptus are of particular concern because other manifestations of toxicity would not warn the mother of overexposure. Whether the LOAEL (lowest-observed adverse effect level) for maternal toxicity was high or low did not correlate with the potential of chemicals to cause developmental toxicity. The form of developmental toxicity that determined the LOAEL most frequently was decreased body weight in mice and rats, but not rabbits, where the LOAEL was determined more often by an increase in resorptions. Several in vitro and short-term tests appear promising as screens to predict the outcome of developmental toxicity studies in mammals. However, the only screens that have undergone formal validation studies are those evaluated by the NTP. Improvements in our ability to predict risk to humans have been limited by our knowledge of the mechanisms by which agents cause developmental toxicity. Thus, future growth is dependent on a better understanding of the biological processes that regulate normal development, therein providing the necessary framework for understanding mechanisms of abnormal development.

Abnormalities, Drug-Induced↗

Genetic factors in toxicology: implications for toxicological screening.

Methods of assessing the toxicity of xenobiotics have improved substantially during the last decade. However, as compounds become generally safer, the problem of individual variation in response assumes increasing relative importance. Environmental factors such as age, health and nutritional status, and interactions with other xenobiotics account for some of this variation, but genetic differences between individuals and races have important implications. In a few cases, Mendelian loci which control drug susceptibility (e.g., to isoniazid) have been described. However, in most cases the exact mode of inheritance has not yet been determined due to the problems of carrying out genetic studies in man. It is well established that many loci that are polymorphic in man are also so in laboratory animals, so much of this genetic variation should be picked up in preclinical screening, and could be used to more accurately predict potential variation in toxicity in man. Unfortunately, most toxicologists use only a single stock of laboratory animals, which does not show whether the response to a given xenobiotic is under genetic control. The design of animal tests would be improved by using more than one strain of genetically defined animals, and by paying more attention to genetic variation in responses to xenobiotics, both in animals and man.

Animals↗