Issues in the Design and Interpretation of Chronic Toxicity and Carcinogenicity Studies in Rodents: Approaches to Dose Selection

Abstract

For more than three decades chronic studies in rodents have been the benchmark for assessing the potential long-term toxicity, and particularly the carcinogenicity, of chemicals. With doses typically administered for about 2 years (18 months to lifetime), the rodent bioassay has been an integral component of testing protocols for food additives, pesticides, pharmaceuticals, industrial chemicals, and all manner of byproducts and environmental contaminants. Over time, the data from these studies have been used to address an increasing diversity of questions related to the assessment of human health risks, adding complexity to study design and interpretation. An earlier ILSI RSI working group developed a set of principles for the selection of doses for chronic rodent studies (ILSI, 1997). The present report builds on that work, examining some of the issues that arise and offering new perspectives and approaches for putting the principles into practice. Dose selection is considered both from the prospective viewpoint of the choosing of dose levels for a study and from the retrospective interpretation of study results in light of the doses used. A main theme of this report is that the purposes and objectives of chronic rodent studies vary and should be clearly defined in advance. Dose placement, then, should be optimized to achieve study objectives. For practical reasons, most chronic studies today must be designed to address multiple objectives, often requiring trade-offs and innovative approaches in study design. A systematic approach to dose selection should begin with recognition that the design of chronic studies occurs in the context of a careful assessment of the accumulated scientific information on the test substance, the relevant risk management questions, priorities and mandates, and the practical limitations and constraints on available resources. A stepwise process is described. The aim is to increase insofar as possible the utility of an expensive and time-consuming experiment. The kinds of data that are most commonly needed for dose selection and for understanding the dose-related results of chronic rodent studies, particularly carcinogenicity studies, are discussed as "design/interpretation factors." They comprise both the inherent characteristics of the test substance and indicators of biological damage, perturbation or stress among the experimental animals. They may be primary toxicity endpoints, predictors or indicators of appropriate dose selection, or indicators of conditions to be avoided in dose selection. The application and interpretation of design/interpretation factors is conditioned by the study objectives-what is considered desirable will depend on the strategy for choice of doses that is being followed. The challenge is to select doses that accommodate all of the issues raised by the relevant design/interpretation factors. Three case studies are presented here that illustrate the interplay between study objectives and the design and selection of doses for chronic rodent studies. These exa...

Evaluation of Possible Carcinogenic Risk to Humans Based on Liver Tumors in Rodent Assays

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“…The highest dose given is a maximum tolerated dose (MTD) and usually includes some evidence of toxicity, such as decreased weight gain (Rhomberg et al 2007). Extrapolation from the high doses used in these animal experiments to exposures in humans, particularly for nonpharmaceutical chemicals for which there frequently are several orders of magnitude difference in exposure between rodents in the bioassays and humans, has been the source of considerable controversy (Ames and Gold 1990;Rhomberg et al 2007;Flamm and Winbush 1984;Slikker et al 2004). Most importantly, during the past several years, several examples have been identified that raise the question of the relevance of the tumors induced in the rodent bioassay to assessing human risk (Cohen 2004; International Agency for Research on Cancer [IARC] 1999a; Meek et al 2003;Cohen et al 2004;Seed et al 2005;Boobis et al 2006Boobis et al , 2008.…”

Immunotoxicologic effects of cyclosporine on tumor progression in models of squamous cell carcinoma and B-cell lymphoma in C3H mice

“…The standard design of the 2-year bioassay includes a rigorous sampling of all masses and a tissue types (Sontag et al, 1976). It is also intended to test the maximum tolerated dose (MTD) of the test article (Rhomberg et al, 2007). However, these design elements favor detecting potent genotoxic carcinogens, agents with relatively high MTDs and detecting macroscopic lesions and tumors that decrease life-span (Gaylor, 2005).…”

Regulatory Forum Opinion Piece*

“…But NTP started adding a third lower test article-treated dose group to nullify the effects of overt toxicity and excess mortality in the high-dose groups and to provide a safety margin against overestimation of MTD. The third-dose group was also added because having only 2 test article-treated dose groups did not provide information on dose response relationship or trends and no observed effect level (NOEL) or no observed adverse effect level (NOAEL; Sontag, Page, and Saffiotti 1976;Haseman 1984Haseman , 1985Hayes et al 2011;Hess, Bretz, and Gfeller 1983;Portier and Hoel 1984;Rhomberg et al 2007). Since our findings clearly demonstrate that the high-dose group at MTD could not detect a carcinogenic effect while the mid and low doses served the purpose in detecting the true carcinogenic potential of the assay, a high dose set at MTD is not needed.…”