Animal and human carcinogens.

Lewis et al. (1) state that compared to the fairly large number (> 400) of known rodent carcinogens, only a relatively small number of compounds (-20-30) have been shown to be carcinogenic in humans. From this comparison, the authors seem to infer that animal bioassay cancer findings overpredict for human carcinogens. Of course, for most animal carcinogens there are no available epidemiologic data, and none planned, or for a few the findings are either inadequate or of limited value. In addition, few studies are under way or even planned on many known and potent animal carcinogens. Thus, one should realize that any conclusions about comparative numbers between animals and humans that are based on an apparent absence of data from humans can only be misleading and basically unusable for making any meaningful comparisons. A more relevant correlation to be made centers on the number of chemicals known to cause cancer in humans that have also been tested either prospectively or retrospectively in laboratory animals. In any event, the numbers reported by these authors regarding known human carcinogens should be updated and modified using more current information. A proposed updated revision follows. approximately 75 agents known to be causatively associated with cancer in humans and another 60 considered "probably carcinogenic to humans" (Table 1). With few exceptions, agents in both these categories have varying evidence of cancer in humans. While attempting to plan or implement cancer prevention strategies, the additional 225 agents considered by IARC as "possibly carcinogenic to humans" and the 169 agents judged by the NTP as "reasonably anticipated to be carcinogenic to humans" must be given dose public health attention as well. Of course both lists must be compared to account for any duplications. I have no particular disagreement with the number of 400 chemicals as being carcinogenic in animals, other than to emphasize that not all carcinogens are equal. That is, some chemicals cause multiple site cancer in both sexes of each strain and species, whereas others may induce tumors in a single organ in one sex of only one strain of rodent (2). Thus, one must evaluate the carcinogenicity data for each chemical rather than simplistically group chemicals that have been tested into two clusters by "positive" or "negative" results. In the NTP for example, of the 500 chemicals evaluated for carcinogenicity, * 14% caused cancer in each of the four sex/species groups used for testing …