Behavioral toxicology in the 21st century: challenges and opportunities for behavioral scientists. Summary of a symposium presented at the annual meeting of the neurobehavioral teratology society, June, 2009.

The National Research Council (NRC) of the National Academies of Science recently published a report of its vision of toxicity testing in the 21st century. The report proposes that the current toxicity testing paradigm that depends upon whole-animal tests be replaced with a strategy based upon in vitro tests, in silico models and evaluations of toxicity at the human population level. These goals are intended to set in motion changes that will transform risk assessment into a process in which adverse effects on public health are predicted by quantitative structure-activity relationship (QSAR) models and data from suites of high-throughput in vitro tests. The potential roles for whole-animal testing in this futuristic vision are both various and undefined. A symposium was convened at the annual meeting of the Neurobehavioral Teratology Society in Rio Grande, Puerto Rico in June, 2009 to discuss the potential challenges and opportunities for behavioral scientists in developing and/or altering this strategy toward the ultimate goal of protecting public health from hazardous chemicals. R. Kavlock described the NRC vision, introduced the concept of the 'toxicity pathway' (a central guiding principle of the NRC vision), and described the current status of an initial implementation this approach with the EPA's ToxCast(R) program. K. Crofton described a pathway based upon disruption of thyroid hormone metabolism during development, including agents, targets, and outcomes linked by this mode of action. P. Bushnell proposed a pathway linking the neural targets and cellular to behavioral effects of acute exposure to organic solvents, whose predictive power is limited by our incomplete understanding of the complex CNS circuitry that mediates the behavioral responses to solvents. B. Weiss cautioned the audience regarding a pathway approach to toxicity testing, using the example of the developmental toxicity of phthalates, whose effects on mammalian sexual differentiation would be difficult to identify based on screening tests in vitro. Finally, D. Rice raised concerns regarding the use of data derived from toxicity screening tests to human health risk assessments. Discussion centered around opportunities and challenges for behavioral toxicologists regarding this impending paradigm shift. Opportunities include: identifying and characterizing toxicity pathways; informing the conditions and limits of extrapolation; addressing issues of susceptibility and variability; providing reality-checks on selected positives and negatives from screens; and performing targeted testing and dose-response assessments of chemicals flagged during screening. Challenges include: predicting behavior using models of complex neurobiological pathways; standardizing study designs and dependent variables to facilitate creation of databases; and managing the cost and efficiency of behavioral assessments. Thus, while progress is being made in approaching the vision of 21st century toxicology, we remain a long way from replacing whole-animal tests; indeed, some animal testing will be essential for the foreseeable future at least. Initial advances will likely provide better prioritization tools so that animal resources are used more efficiently and effectively.