Worth weighting for : studies on benchmark dose analysis in relation to animal ethics in toxicity testing

Abstract

A purpose of chemical health risk assessment is to characterize the nature and size of the health risk associated with exposure to chemicals, including identification of a dose below which toxic effects are not expected or negligible. This is usually based on analysis of dose-response data from toxicity studies on animals. Traditionally the dose-response in animals has been analyzed employing the No-Observed-Adverse-Effect-Level (NOAEL) approach, but because of the several flaws of this approach it is to a greater and greater extent being replaced by the so called Benchmark Dose (BMD) approach.

Previous evaluations of how to design studies in order to obtain as much information as possible from a limited number of experimental animals have revealed the importance of including high doses. However, these studies have not taken the distress of the laboratory animals, which is likely to be higher at high doses, into account. The overall aim of the present thesis was to examine how study designs, especially with dose groups of unequal size, affect the quality of BMD estimates and level of animal distress.

In Paper I our computer simulations concerning the appropriateness of using nested models in BMD modelling of continuous endpoints indicate that it is problematic to calculate BMD on the basis of simpler models and that they should be used with caution in connection with risk assessment as they may result in underestimations of the true BMD. In Paper II-III our computer simulations of toxicity testing with unequal group sizes showed that better information about dose-response can be obtained with designs that also reduce the level of animal distress. In Paper IV we interviewed members of the Swedish Animal Ethics Committees concerning how the number of animals used in toxicity tests might be weight against the distress of the individual animal. Their opinions concerning whether it is preferable to use fewer animals that suffer more rather than a large number of animals that suffer a little, differed considerably between individuals. However, there were no statistically significant differences in relation to the fact that respondent were either researchers, political representatives or representatives of animal welfare organizations. In Paper V the results from Paper IV and the simulation techniques in Paper II were combined to evaluate how toxicity tests could be designed to obtain as much information as possible at a limited ethical cost, with respect to both the number of animals used and their individual distress. The most ethically efficient design depended on what constituted the ethical cost and how large that ethical cost was.

In conclusion, this thesis describes the potential to use BMD-aligned study design as a mean for refinement of animal toxicity testing. In addition, new strategies for model selection and quantitative measures of ethical weights are presented.