Application of the preliminary developmental toxicity screen for chemical hazard identification under the Toxic Substances Control Act

Workshop on the qualitative and quantitative comparability of human and animal developmental neurotoxicity, Work Group IV report: triggers for developmental neurotoxicity testing

A Work Group was formed to evaluate the criteria considered important in determining when to require developmental neurotoxicity testing in animal studies (i.e., triggers for testing). The primary objective of the Work Group was to determine whether there is sufficient scientific evidence to support the triggers identified by the Environmental Protection Agency and determine whether there is sufficient evidence to use structure activity relationships (SAR) to trigger automatic testing of certain classes of chemicals. A weight of evidence (WOE) approach was recommended by the Work Group in order to assist in determining which agents should undergo developmental neurotoxicity testing and to what level of testing. Evaluation of biological effects, length and duration of exposure, and quality and quantity of data available on an agent should be used in the WOE approach. Agents that are teratogenic to the central nervous system (CNS) were considered of highest priority for developmental neurotoxicity testing, especially if there is the potential for a high degree of exposure. Neuropathic and neuroactive compounds, chemicals with hormone-like activity, and developmental toxicants (with effects other than structural abnormalities of the CNS) were also considered likely candidates for such testing. Although reluctant to recommend testing based solely on SAR or chemical class, the Work Group recognized the importance of considering SAR along with other toxicity data, pharmacokinetic data and potential human exposure in making final requirements or recommendations for further testing.(ABSTRACT TRUNCATED AT 250 WORDS)

A perspective on the significance of maternally mediated developmental toxicity

The significance of maternally mediated developmental toxicity has been controversial from both a biological and a regulatory point of view. The open literature has at times been interpreted to mean that a number of the effects seen in fetuses from dams exposed to maternally toxic doses of chemicals were secondary consequences of maternal toxicity rather than direct effects on the conceptus. Recent experimental studies, however, indicate that although certain relatively species-specific manifestations of developmental toxicity may at times be maternally mediated, most are not. On occasion, even severe maternal toxicity can apparently occur without causing readily discernible effects on the embryo/fetus. The most important concern of a regulatory agency with regard to developmental toxicity is the possibility of the causation of significant, irreversible harm to the offspring. In practical terms, the margin of safety for exposure to a developmental toxicant is of much more importance than whether or not the agent's effects are maternally mediated. For protection of the unborn, it is obviously the end result that matters, regardless of the mechanism. Safeguarding the conceptus from specific developmental toxicants (i.e., agents with relatively high A/D ratios) requires the use of safety factors based on the developmental toxicity NOEL. Protecting the conceptus against agents with A/D ratios near unity could be based on the maternal toxicity NOEL, however, as the true NOEL for developmental toxicity may be near that for the mother, but the adult NOEL is likely to be more readily determinable.