Use of the adult developmental relationship in prescreening for developmental hazards

The overwhelming majority of chemicals already in commerce or brought into use each year have not been evaluated for their potential to adversely affect in utero development. Data from those that have been evaluated thus far in pregnant laboratory animals establish that most, but not all, were no more hazardous to the conceptus than they were to adult homeostasis. Most did not need standard developmental toxicity testing because avoidance of adult toxic exposure levels would have precluded abnormal in utero development. The six general principles of teratology when modified, expanded, and placed into this type of context of contemporary developmental toxicology allow an updating of the present testing sequence which was devised prior to 1966. The developmental hazard index (A/D ratio) calculated from the adult and developmental NOELs of standard Segment II evaluations is predicted by in vitro means. This determination, when coupled with adequate considerations of exposure can be used to prioritize chemicals for more elaborate developmental toxicity tests. Those chemicals with large ratios, i.e., disruptive of embryogenesis at treatment levels too low to produce overt effects in the mother and/or with significant concern regarding exposure, can be identified and tested in pregnant laboratory animals as high priority items. Those with low ratios and those for which there is a low level of concern regarding exposure potential also can be identified and are not high priority items for testing in pregnant animals. The proposed tier system establishes priorities of testing based on exposure and the concept of target organ toxicity applied to the embryo. It provides intensive in vivo evaluations of those chemicals for which developmental effects testing is most needed and avoids use of resources and animals for unnecessary testing of agents that do not pose threats to the conceptus.