Species comparisons in evaluating carcinogenicity in humans

Model Selection in Toxicology: Principles and Practice

The key assumptions underlying modern toxicology are that (1) animals can serve as accurate predictive models of toxicity in humans, (2) that selection of an appropriate model to use is the key to accurate prediction in humans, and (3) that understanding the strengths and weaknesses of any particular model is essential to understanding the relevance of specific findings to humans. It must first be made clear, however, that the term model encompasses more than a test organism. Then the basic principles underlying these assumptions and the general limitations of models (both in vivo and in vitro) can be critically examined. The actual practice of model selection and why it deviates from the generally accepted principles are overviewed, along with the issue of errors induced by the “high-class model” problem. Historical results as to the accuracy of prediction of human effects are summarized. Finally, the reasons that models fail to predict effects in humans in some cases are examined, along with methods and problems in cross-species extrapolation and the issue of species peculiarities.

Relevance of animal carcinogenesis findings to human cancer predictions and prevention

Use of laboratory animals to identify carcinogenic potential of chemicals, mixtures, and other agents has a modern history of greater than 40 years from which much useful scientific and public health information can be derived. While laboratory animals differ from humans in some respects that may affect responses to hazardous exposures, use of such models is based on experimental evidence indicating that there are more genetic, genomic, physiological, biochemical, and metabolic similarities than differences among mammalian species. Issues of concordance of responses between rodent species and between rodents and humans as well as repeatability and site-specificity are important considerations in evaluating laboratory animal carcinogenicity results. Variables in experimental design such as animal strain, diet, route of exposure, and study, duration as well as single-site versus multisite carcinogenic responses all influence interpretation and intelligent use of study data. Similarities and differences in site-specific laboratory animal and corresponding human cancers should also be considered in study evaluation. Recent attempts to explore genetically engineered mice and to humanize the mouse for more relevant identification of carcinogen hazard identification have yielded mixed results. In the end we are confronted by the realization that virtually all animal cancer models are useful but imperfect surrogates for humans. Assuming the percentage of chemicals currently in commerce that are estimated to be potent animal or human carcinogens is quite low, the task of identifying agents with significant carcinogenic potential is daunting and important. The biological conundrum of scientific debate regarding the relevance of carcinogenicity studies in laboratory animals is likely to continue. Nonetheless public health considerations must take precedence when deciding human safety issues.

Opposing effects of prostaglandin E(2)and F(2 alpha) on rat liver-associated natural killer cell activity in vitro

Strain differences in cancer incidence are proposed to be due partly to differences in immune function. As potential cancer-associated immunological regulators, the concentrations of hepatic prostaglandins E(2)(PGE(2 alpha)and F(2 alpha)(PGF(2 alpha)) were compared in 9-week-old male and female F344/N and Sprague-Dawley (SD) rats. There were no strain or gender differences in the concentrations of hepatic PGE(2). No strain difference was found in the concentration of hepatic PGF(2 alpha), but the hepatic PGF(2 alpha)concentration in female rats was two-fold that of the male rat (130 vs 60 ng/g). PGE(2)significantly inhibited hepatic natural-killer cell (NK) activity in vitro compared with untreated cells from both genders and strains (P<0.05), 25 ng PGE(2)/ml inhibited NK activity significantly more than did 10 ng PGE(2)/ml (P<0.05). In contrast, 50 ng PGF(2 alpha)/ml and 100 ng PGF(2 alpha)/ml significantly stimulated hepatic NK activity compared with untreated hepatic cells from both F344/N and SD rats. This study suggests that prostaglandins may have a negligible net effect on NK activity associated with rat liver, and may be unlikely to mediate cancer-related immune function.

Cancer risk--does anyone really care?

Congress has enacted numerous laws in an attempt to protect people from carcinogens. But neither regulators nor oncologists can hope to accomplish this protection via studies in epidemiology. Nor can they hope to accomplish this using unverified models in carcinogenesis bioassays. The only viable hope is to test and utilize models of carcinogenicity that will tell us what we really need to know; i.e., does this substance pose a reasonable risk of carcinogenesis in man? And if so, what is a quantitative estimate of that risk? There is a substantial number of known human carcinogens. Yet, the most elaborate cancer-testing facility in the world continues to operate without the benefit of validating its results using positive controls (known human carcinogens). Without such controls to gauge the potency of the response in the test animal, Congress can pass 10,000 more laws and still the public will remain unprotected.

Uncertainty in health risk assessments

Considerable scientific evidence has accumulated in the area of risk assessment. Using physiologically based pharmacokinetic models and biologically based dose-response models, more precise estimates of risk are becoming available. Uncertainty analysis performed at three steps of the process will enhance a clearer understanding of the assumptions being made regarding a public health decision. Recommendations are made on how uncertainty in risk assessments can be addressed and the types of future research that are required to this end.

Therapeutic achlorhydria and risk of gastric cancer

New, powerful gastric secretory inhibitors, such as omeprazole, produce gastric cancer in rats. The mechanism by which the drugs elicit gastric carcinogenesis is considered to depend on the production of therapeutic achlorhydria, with subsequent release in to the circulation of peptides (such as gastrin) which are trophic to the gastric mucosa. It has been argued that the drugs do not pose a carcinogenic risk to man because the neoplastic response to gastric inhibitors in rats is a reaction to a 'toxic' insult; or because rats and humans react differently to the drugs; or because the mechanisms of gastric carcinogenesis are different in the two species. In any case, since most of the powerful gastric secretory inhibitors produce carcinoid tumours in rats, and carcinoid tumours of the human stomach are rare and largely benign, there would be no risk even if the drugs did produce proliferative abnormalities of the human stomach. Not one of the above hypotheses has been confirmed or, indeed, even satisfactorily tested. The mechanisms of the drug-induced gastric carcinogenesis in rats has not been defined and consequently it is not even possible to attempt to guess the risk to man. Until information is available about the effects of the powerful gastric secretory inhibitors on the proliferative indices and patterns of the human gastric mucosa, the drugs must be categorized as too dangerous to use therapeutically, especially since the proposed therapeutic benefits are minimal.