The significance of mouse liver tumor formation for carcinogenic risk assessment: results and conclusions from a survey of ten years of testing by the agrochemical industry.

A survey was performed on the results of 138 carcinogenicity studies conducted in various mouse strains by the agrochemical industry over the period 1983-1993. Data for liver tumor incidence, liver weight, and histopathology were collected along with data on genotoxicity. Studies were judged positive or negative for liver tumor formation on the basis of apparent dose response, malignancy, and difference from historical control values using a weight of evidence approach. Thirty-seven studies were judged to be positive for liver tumorigenicity in one or both sexes. There was no evidence showing an influence of the mouse strain and the duration of the study on the proportion of positive studies. Although 8 of the chemicals tested in the 138 studies were positive in the Ames test, only one of these was judged positive for carcinogenicity. Only 6 of the 37 positive chemicals had any other reported positive genotoxicity findings. A clear relationship between hepatomegaly at 1 year after exposure and a positive tumorigenic outcome at 18 months or 2 years after exposure was demonstrated. Whereas the average relative liver weight of top dose animals was 110% of control in negative studies, it was 150% in positive studies. Likewise, very few negative studies demonstrated significant pathological findings after 1 year, whereas the majority of positive studies had significant liver pathology. The implications of these findings for extrapolation to humans are discussed.