Challenges and progress in characterizing in vitro dosimetry for air-liquid interface exposures
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Inhalation is one of the three primary modes of chemical exposure, but key challenges in developing reproducible exposure systems with robust analytical dosimetry methods have limited the use of in vitro models for the assessment of inhalable chemicals. In vitro inhalation assays have been improved with air-liquid interface (ALI) exposure systems which allow direct cell-toxicant interactions, but these systems are complex and vary in their geometry, flow rates, and operational parameters. Physicochemical properties of the inhaled substance must be carefully considered when selecting an ALI exposure system, optimizing its operational parameters, and developing methods to characterize exposure conditions. This presentation will provide an overview of commonly utilized ALI exposure methods along with critical considerations for their use. Best methods to quantify exposure conditions for gases, vapors, and aerosols will be shared along with a case study that highlights differences between volatile organic compound (VOC) and aerosol delivery within an ALI exposure system. Although direct quantification of VOC delivery to cell cultures remains challenging, soluble fluorescent tracers can be aerosolized as a tool to evaluate exposure systems by quantifying particle deposition and cellular uptake. We have characterized multiple ALI exposure systems with aerosolized sodium fluorescein and rhodamine 6G and have found that particle deposition varies across exposure systems in a chemical-dependent manner and across collection methods. Cell-free collection methods did not reliably estimate cell deposition in many scenarios. Both fluorescent tracers were also able to highlight differences in internal dose rates across direct liquid application studies compared to ALI exposures and revealed that in vitro mass balance model (IV-MBM) simulations may not accurately estimate cellular uptake in cell models grown at ALI. Overall, these results highlight opportunities to better characterize in vitro exposures to inhalable substances and improve dosimetry methods for in vitro to in vivo extrapolation. [Abstract does not reflect views or policies of the U.S. EPA.]