Inhalation is one of the primary occupational routes of exposure to nanomaterials. Chronic and sub-chronic animal inhalation studies are time-consuming, expensive, of questionable human relevance and raise ethical concerns. Recent efforts are aiming to promote the use and adoption of Adverse Outcome Pathways (AOPs) to demonstrate the safety of advanced materials and technologies such as nanomaterials. These toxicology frameworks outline key biological events (KEs) that ultimately result in significant adverse effects such as fibrosis in the lung. By targeted testing of KEs with alternative, cell-based methods, the safety of new materials can be evaluated without animal testing.

Barosova and colleagues have developed a three-dimensional cell model (EpiAlveolar) consisting of human primary alveolar epithelial cells, fibroblasts, and endothelial cells as a tool to predict long-term effects from breathing in nanomaterials. In their study, profibrotic and proinflammatory responses based on the AOP for lung fibrosis were investigated following repeated sub-chronic exposures to multiwalled carbon nanotubes (MWCNTs) and silica quartz particles. Researchers exposed the lung model to occupationally relevant doses of MWCNTs with repeated exposures over 3 weeks. Specific KEs leading to lung fibrosis, such as compromised barrier integrity and release of proinflammatory and profibrotic markers, were assessed. The results indicate that the lung model can robustly predict inflammatory and fibrotic responses upon exposure to MWCNTs, and was able to discern differences in toxicity between two forms of MWCNTs (Nanocyl and Mistsui-7), in line with available toxicological data. The study is a promising application of AOPs and related alternative testing strategies for evaluating the safety of nanomaterials.