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Gauging the value of carcinogen assays.

Gauging the value of carcinogen assays

Long-term rodent studies to establishthe potential human carcinogenicity of a chemical can take up to five years and generally cost well over $1 million. Since the carcinogenic potential of an estimated 50,000 widely used chemicals has not yet been established, researchers often turn to quick, in-vitro assays, like the "Ames test,' in deciding which chemicals to run through the expensive, but more conclusive animal tests first. These assays test a chemical's ability to induce DNA damage.

But how predictive are they of carcinogenicity--at least in rodents? Four of the most commonly used, including the Ames mutagenesis assay, are all about 60 percent predictive, according to researchers with the National Institute of Environmental Health Sciences (NIEHS) and Information Systems and Networks Corp., both in Research Triangle Park, N.C. Moreover, they report in the May 22 SCIENCE, little is gained by subjecting a chemical to a battery of more than one of these assays, even though each assay picked up somewhat different subsets of the chemicals shown to be carcinogenic in rodents. The reason, says NIEHS's Michael D. Shelby, is that while subsequent assays pick up some carcinogens missed by the first test, they also contribute additional false positives and false negatives.

"What you're really interested in,' hesays, "is not the number of carcinogens found, so much as [an accurate] discrimination between carcinogens and noncarcinogens.' After testing 73 chemicals, the researchers conclude that "no battery of tests constructed from these assays improved substantially on the overall performance of the [Ames] . . . assay.' The other assays tested for chromosome aberations, for swapping between chromosomes of one of their paired strands of genetic material, and for lymphoma-cell mutagenesis.

While describing the SCIENCE paper as"very good,' Carnegie Mellon University economist Lester B. Lave does challenge some of its conclusions. He and Gilbert S. Omenn, dean of the University of Washington School of Public Health in Seattle, have suggested the cost of a false-positive finding of chemical carcinogenicity might be $1 million, and the cost of a false-negative, $10 million--based on their estimates of costs associated with pulling a safe chemical from commerce or not protecting humans from one that is truly dangerous. If one weighs the four assays in the SCIENCE paper for their false-negative /false-positive rates, Lave says one can no longer conclude they all perform equally. While the Ames test still comes out ahead, he says, a battery of tests can now offer somewhat better discrimination of hazard risk than any one assay alone.

Work by Fanny K. Ennever and HerbertS. Rosenkranz at Case Western Reserve University Medical School in Cleveland have used assay-misidentification rates to determine which assay results are inconclusive. Ennever says their data show that if these inconclusive results are eliminated, in contrast to the NIEHS findings, "the battery does indeed do better than the Ames test alone.'

The Case Western scientists have alsofound that identifying true carcinogens is not always an assay's most valued asset. If the cost of false negatives is very high-- and it may be to a firm formulating a new consumer product--then the test that most reliably identifies true-negatives or noncarcinogens, would be most useful. On these grounds, Ennever says, the Ames test would not surpass the other three assays in the SCIENCE paper.
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Author:Raloff, Janet
Publication:Science News
Date:May 30, 1987
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