There are marked differences in carcinogenicity across sexes, strains and species. Often, the-same chemical will cause one kind of cancer in one experiment and another kind in another experiment Indeed, the most hard-bitten advocates of animal experiments do not claim to be able to predict which organ will be affected in humans by a chemical that is carcinogenic in animals.
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From a comparison of the results of the present study with those of other studies, one can draw the following conclusions: compounds that cause hepatomas (liver cancers) in mice, may not always be carcinogenic in rats, or if carcinogenic in both species, the compounds may have different primary target sites. The extrapolation of such results to define any risk to humans remains a troublesome problem.'-
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[Regarding tamoxifen, an anti-cancer drug] 'Experimentally, tamoxifen has carcinogenic potential. In some strains of rat, but not mouse or hamster, tamoxifen can cause liver cancers at doses as low as 5mg/kg per day... However, there are doubts about the correlation of [the results] with the risk of malignant disease even in rats, let alone in other rodents, mammals, or human beings. There are many uncertainties in extrapolating these experimental data from rats to women. The effect depends on bioavailability, hepatic [liver] blood flow, and hepatic [liver] metabolism to active genotoxic carcinogens, all of which differ enormously between rat and man.
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Let us look at some animal carcinogens – gold, DDT, clofibrate and bromocriptine. There is no doubt that all of these can rightly be regarded as carcinogenic for rodents, and yet there is really quite good evidence that they are not carcinogenic to man.
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