The redundancy of mouse carcinogenicity bioassays.

Testing of chemicals for carcinogenic potential usually involves studies in rats and mice. The approaches followed in recent years often were limited to assessing tumor incidences and rarely viewed such information from the perspective of other kinds of toxicity. This practice supports the notion that certain chemicals possess the inherent characteristic of carcinogenic activity, which once identified in one species must be regarded as potential oncogenic hazard to all, including man. As long as public policy is based upon that premise, the classification of chemicals depends upon the worst results obtained in any species. It is not obvious why substances must be tested in both rats and mice when confirmatory or contradictory responses have little impact. Recent experience was examined to build a basis for an alternate approach. Review of oncogenicity information obtained for 273 chemicals fed to rats and mice shows that both species responded similarly to the majority of the substances tested. This "redundancy" probably would be higher had modern protocols been used. In view of the high cost in scientific resources of chronic studies, tests in only one rodent would be more cost effective. Reasons are presented for favoring the rat as the species to be used. Some of the savings thus achieved should be expended to improve the design of the experiments to yield toxicology data more comprehensive than mere tumor counts, so that the proper perspective can be obtained on the results.