"Bioassay bashing is bad science": Cohen's response. (Correspondence).

Thank you for the opportunity to respond to the comments made by Johnson and Huff regarding the article assessing alternative assays, which appeared in the May 2002 issue of EHP (Schmidt 2002). In the comments that I made to Schmidt, it was not my intent to bash the bioassay. Instead, it was my intent that the bioassay be put in perspective, which is why I used the quote from George Box indicating that all models are wrong, but that some are useful. Clearly, the 2-year bioassay has been useful. The difficulty is that, like all models, it is not perfect.

There are innumerable examples now of chemicals that have been identified as carcinogenic in the rat and/or mouse that for a variety of reasons are now considered not to be carcinogenic to humans, either because of qualitative differences in the mechanism of action or, more commonly, striking quantitative differences between species or between exposures. The mechanism of action for certain rodent chemical carcinogens has been identified in animal models and is not relevant to the human situation. Examples include [[alpha].sub.2u]-globulin-related male rat renal tumors (d-limonene), luteinizing hormone abnormalities and the induction of breast tumors in Sprague-Dawley rats (atrazine), and calcium phosphate-containing precipitate-related bladder tumors in rats (sodium saccharin, sodium ascorbate). Others are likely to be identified. This has led to the delisting of sodium saccharin from the National Toxicology Program's (NTP) congressionally mandated list of carcinogens. Also removed from that list of carcinogens was ethyl acrylate, a rat forestomach carcinogen for which much of the research was performed at the NTP. The International Agency for Research on Cancer has also made an effort to reclassify chemicals based on mechanism of action, and has done so for a variety of chemicals and known mechanisms. In addition to qualitative differences, numerous quantitative differences exist between carcinogenic effects in rodents and humans, such as urinary calculi (melamine, fosetyl-Al) and rat thyroid tumors (sulfamethazine), suggesting no potential carcinogenic risk to humans.

The intent of these studies is to try to predict human carcinogenicity. The same is true for the alternative models. I agree with Johnson and Huff in their statement that

   The question of how to predict human carcinogens
   more effectively would benefit from fundamental
   research.

The 2-year bioassay is useful and is likely to remain so for several years to come. However, it is imperfect, and additional research is needed not only to come up with better screening models but also to provide the necessary mechanism of action information that is needed to interpret the results in the screening assays.

The alternative models involving transgenic and knockout mice have many of the same imperfections as the 2-year bioassay. Johnson and Huff are correct in indicating that protocol variables need to be more fully evaluated and defined. Nevertheless, it is already clear that some of these alternative models can provide useful information that is as reliable as the standard 2-year mouse bioassay. The U.S. Food and Drug Administration and the European authorities are already utilizing results from these assays in their evaluation of pharmaceuticals.

The 2-year bioassay is nearly 40 years old. It has proven useful and will continue to do so. However, the intent is not to determine carcinogenicity in rats or mice, but ultimately to accurately predict carcinogenicity in humans. There is no doubt that sometime in the future assays will be developed that are better able to make these predictions than the 2-year bioassay. The development of the alternative animal models that were discussed in the Spheres of Influence article (Schmidt 2002) is a step in that direction. Additional research is required, but the 2-year bioassay should not be viewed as sacrosanct nor as the gold standard by which other assays are to be validated. Appropriate validation is the comparison with human carcinogenesis.

Samuel M. Cohen
Department of Pathology and Microbiology
University of Nebraska Medical Center
Omaha, Nebraska
E-mail: [e-mail]

REFERENCES

Schmidt CW. 2002. Assessing assays. Environ Health Perspect 110:A249-A251.