Species sensitivities and prediction of teratogenic potential

Many chemicals shown to be teratogenic in laboratory animals are not known to be teratogenic in humans. However, it remains to be determined if the unresponsiveness of humans is due to lessened sensitivity, to generally subteratogenic exposure levels, or to the lack of an appropriate means of identifying human teratogens. On the other hand, with the exception of the coumarin anticoagulant drugs, those agents well accepted as human teratogens have been shown to be teratogenic in one or more laboratory species. Yet, no single species has clearly distinguished itself as being more advantageous in the detection of human teratogens over any other. Among the species used for testing, the rat and mouse most successfully model the human reaction, but the rabbit is less likely than other species to give a false positive finding. Among species less commonly used for testing, primates offered a higher level of predicability than others. Regarding concordance of target malformations, the mouse and rat produced the greatest number of concordant defects, but they also were responsible for the most noncorcordant responses as well. Since no other species is clearly more predictive of the human response, it is concluded that safety decisions should be based on all reproductive and developmental toxicity data in light of the agent's known pharmacokinetic, metabolic and toxicologic parameters.