Applying modular design principles of adverse outcome pathways to experimental planning in toxicology

Adverse outcome pathways (AOPs) offer a modular framework that can transform experimental planning in toxicology beyond their current use in organising knowledge. This commentary highlights how adopting modular AOP principles, treating each experimental design element as an interconnected module, can bridge mechanistic understanding, improve data integration, and pave the way for more efficient, mechanism-based toxicity testing. The systematic application of these principles not only streamlines resource use and supports non-animal methods but also enhances the predictive and regulatory relevance of toxicological studies. Viewed this way, modular experimental planning complements AOP-informed Defined Approaches (DA) and Integrated Approaches to Testing and Assessment (IATA) by improving the design, reporting, and integration of evidence-generating modules; it is most immediately actionable in mechanistic research and NAM development that support regulatory decision-making.