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

I appreciate the opportunity to respond to the letter from Johnson and Huff on the May 2002 Spheres of Influence, "Assessing Assays" (Schmidt 2002). Several points brought up by Johnson and Huff need to be clarified and some need to be challenged:

It is disappointing that Johnson and Huff see this article as "bashing the bioassay." Criticism of the rodent bioassay as performed by the National Toxicology Program (NTP) does not imply that contributions to human hazard identification and risk assessment have not been made by this methodology over the 40 years that it has been employed, but is it "bad science" to seek ways to improve this process with new technology? It would seem that, as concerned scientists, our debate should not center around which assay is "better" but rather how we can best identify and use data from a variety of sources to improve our ability to prospectively identify chemicals that pose human risk.

The Spheres article (Schmidt 2002) described in some detail the efforts to assess the response to a list of well-characterized chemicals in several newly developed transgenic and knockout mice. Although many groups are evaluating these assays now, the efforts of the International Life Sciences Institute/Health and Environmental Sciences Institute (ILSI/HESI) Alternatives to Carcinogenicity Testing Committee were highlighted in the article. The specific intent of this effort by the ILSI/HESI was to gain experience with these newer assays in a carefully controlled experimental manner. The goal was not to replace the bioassay, as Johnson and Huff seem to suggest, but rather to see how (or if) data from these assays could be used in the process of assessing human risk of cancer from chemical exposure (Cohen et al. 2001). Research with these newer assays in the HESI initiative was focused on improving the process of human hazard identification and the use of the information from these assays in a global weight-of-evidence approach to risk assessment; it clearly was not focused on simply replacing one method with another.

Johnson and Huff reacted to my suggestion that conventional rodent models predict human cancer no better than "a flip of the coin." Perhaps this was a bit casual and even provocative, but is it really disingenuous? There is no argument that the rodent bioassay can detect human carcinogens. The problem is that it also identifies many other chemicals that are generally not regarded as human carcinogenic risks (Cohen and Ellwein 1991; MacDonald et al. 1994; McClain 1994). The data from the use of this assay are clear: approximately 50% of the chemicals tested in either the rat or mouse bioassay have yielded positive results (Contrera et al. 1997; Davies and Monro 1995; Gold et al. 1984; Van Oosterhout et al. 1997). Although this number has come down as fewer suspected carcinogens (genotoxic agents) are tested in the NTP Rodent Bioassay Program (Fung et al. 1995), there is still a very high rate of positive outcomes. Unless we are expecting a wave of carcinogens to be identified in the next 10-15 years from extensive human use of these agents, we can consider most of these responses as false-positive results. This may not be quite a flip of the coin, but it is not a great improvement for the significant expenditure of time and money for this assay.

Johnson and Huff state in their letter that alternative assays for carcinogen hazard identification such as the transgenic assays cannot be accepted for regulatory use without validation through a process such as the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM). Two important points should be mentioned here. First, the rodent bioassay, which is held up as the "gold standard," has never been validated through a rigorous process such as that proposed by ICCVAM. Given some of the concerns raised above, one wonders if even this well-established assay could satisfy our current demands for validation. Second, the International Conference on Harmonization has published guidelines (International Conference on Harmonization 1995) that clearly state that, for pharmaceutical chemicals, the appropriate use of alternative assays such as transgenic or knock-out mice can suffice to replace the use of one or another of the rodent bioassays. As this practice is occurring with pharmaceuticals in the approval process, we will come to better understand the utility (if not the validity) of these data just as we did with the rodent bioassay.

In their letter, Johnson and Huff point out an important, and appropriate, criticism of the current transgenic and knock-out mouse models: there are several protocol issues that need to be addressed. I participated in the ILSI/HESI initiative for about 6 years as this group struggled to understand how best to apply some of the then new assays to the problem of human hazard identification, specifically in the area of pharmaceutical chemicals. As the results from this effort suggest, data from these alternative assays can be used appropriately, but several important questions remain to be answered. (MacDonald et al. 2001) Whether these are being adequately addressed with currently ongoing research or whether additional, focused research efforts would be appropriate is the subject of ongoing discussions with the HESI group and a broad cross-section of stakeholders. It is again important to emphasize that the focus of this effort was not and is not replacement of the rodent bioassay, but a better understanding of how data from newer models might aid in our ability to detect potential human carcinogens.

Johnson and Huff also point out appropriately that additional fundamental research on the genetics of human cancer has the potential to improve our ability to reliably detect human carcinogens before these chemicals are released into our environment. Studying transgenic animals and carefully assessing whether the nature of the transgene alters the response to particular chemicals should not be construed as failing to realize the importance of fundamental research, but rather as a central component of this important process.

Assessment of the human risk of cancer is perhaps one of the most difficult tasks in toxicology. I think we can all agree that valid and appropriate information from a broad variety of sources should be brought to bear on this important question. Although data from the rodent bioassay have made important contributions in the past, we continue to compare the performance of new assays to this so-called standard. If we are trying to improve how we assess human risk of cancer from chemical exposure, we need to change this focus on how well we predict cancer in rats and mice.

James S. MacDonald
Drug Safety and Metabolism
Schering Plough Research Institute
Kenilworth, New Jersey
E-mail: [e-mail]

REFERENCES

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Van Oosterhout JPJ, Van der Laan JW, De Waal EJ, Olejniczak K, Hilgenfeld M, Schmidt V, et al. 1997. The utility of two rodent species in carcinogenic risk assessment of pharmaceuticals in Europe. Regul Toxicol Pharmacol 25:6-17.