search is still needed on other aspects of absorption, distribution (including the multidrug transporter proteins), and excretion.

In general, this research is an exemplary area for the exchange of information across levels with the ever closer approximation of the fast, inexpensive level 1 tests to human metabolizing conditions.

1.4.2. Toxicodynamics: toxicant effects on developmental components—information about mechanism and susceptibility.

Other assays of toxicants should be used to focus on their effects on developmental processes, particularly on the intercellular signaling pathways and genetic regulatory circuits, which operate repeatedly and pervasively in the development of animals of all phyla. Some assays can be done at level 1, since many signaling components have been identified and their genes isolated. Relevant proteins can be produced for in vitro tests or single-cell tests. For example, to test for agents interfering with Hedgehog signaling, the signal transduction intermediates could be introduced into cultured cells. To some extent, cell functions—such as secretion, entry into mitosis, motility, or specific gene expression—that depend on signaling could be scored for interruption by chemicals. The availability of components for nonanimal assay systems is already considerable. Many signaling components have been mapped on the mouse genome, and their corresponding location in the human genome is predictable and will soon be sequenced.

The committee recognized at the outset that the information about chemical impacts on cell signaling components is of little use for risk assessment if there is no organ and mammalian relevancy. Therefore, a comprehensive approach was envisioned to allow this initial information to be placed directly into an overall assessment framework. For testing the effects of toxicants on the activity of these components in development, level 2 assays on Drosophila, C. elegans, and zebrafish development are incisive. Multiple pathways are used in the development of complex organs. One pathway at a time can be sensitized in a specific aspect of development by genetic means. An animal, when exposed to a toxicant affecting that pathway, will have altered development of that organ, whereas the rest of the animal will probably be unaffected or less affected. Altered development is, then, the scored end point of the toxicant’s effect on the specific pathway. Although the model organisms have different organs from those of humans, the signaling pathways and genetic regulatory circuits that operate in the development of that organ also operate in the development of mammalian organs of other kinds. Thus, the effects of chemicals on fundamental processes, such as signaling and transcription, can be detected. The signaling pathways operating in the various kinds of organogenesis in mammals are known; therefore, a prediction can be made and tested in mammals, using level 3 testing approaches. Because level 2 assays are inexpensive and fast, many compounds can be tested, and patterns of toxicity effects can be recognized in advance of the rodent tests. In the multilevel approach, level 3 tests are ones with specially modified mice (containing sensi-



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