Mode of action: moving toward a more relevant and efficient assessment paradigm

To advance the utility and predictability of safety evaluation, an integrated approach that relies on all existing knowledge to understand how agents perturb normal biological function or structure is needed to progress more focused evaluation strategies. The mode of action (MOA)-human relevance framework developed by the International Program for Chemical Safety and The International Life Sciences Institute provides a useful analytical approach where different lines of evidence (e.g., in vitro, in vivo) can be organized, linked, and integrated at different levels of biological organization into a more efficient, hypothesis-driven approach to safety evaluation. This framework provides a weight-of-evidence approach based on considerations for causality (as originally articulated by Bradford Hill), including dose response and temporal concordance, consistency, specificity, and biological plausibility and coherence. Once an animal MOA and its key events are established, qualitative and quantitative comparisons between experimental animals and humans are made based on the key events. This comparison enables a conclusion as to whether the MOA is likely operative in humans and, if so, whether it can result in a more refined hazard and dose-response assessment. This framework provides an important tool to promote and formalize the use of MOA data in safety evaluation regardless of whether the information comes from traditional or novel approaches, such as those recommended by the NRC in its 2007 report "Toxicity Testing in the 21st Century," which recommends moving away from traditional approaches of measuring adverse endpoints by using newer technologies to identify ways agents may considerably perturb cellular pathways to produce their toxicity.