The impact of inhalation toxicity from environmental chemicals is a global public health concern. The World Health Organization estimates that more than seven million deaths annually can be attributed to ambient air pollution, and although regulations associated with the Clean Air Act have dramatically reduced the levels of inhalation toxicants in the air, nearly half of Americans still live in counties that exceed the National Ambient Air Quality Standards. Exposure to air pollutants, as well as pulmonary toxicants, cause a range of adverse health effects within the airway (e.g., airway inflammation and asthma) and throughout the body (e.g., ischaemic heart disease, type II diabetes, and stroke).
Human exposure data represent the “gold standard” for understanding the effects of inhaled toxicants on health outcomes; however, in vivo human (controlled human exposure and epidemiology) studies can only be used to collect data for a small fraction of inhaled chemicals due to cost and a number of safety/ethical considerations. In vivo animal models have been used to fill data gaps, but the power of these animal models is limited by fundamental anatomical, physiological, and biochemical differences compared to their human counterparts. Recently, there has been a growing moral and ethical movement toward the development and implementation of alternative in vitro and computational methods when reasonable.
Traditionally, airway epithelial cell lines have been used in inhalation toxicity safety assessment. Most airway epithelial cell lines are not known to undergo differentiation; however, when grown at air-liquid-interface (ALI) in the presence of the appropriate medium, primary and select airway epithelial cell lines will polarize and form a pseudostratified epithelium that closely resembles the human in vivo airway epithelium. The advent of ALI has led to a number of key advances in the field of inhalation and respiratory toxicology; however, lack of standardization of ALI techniques (e.g., flow rate, humidity, temperature, and analytical concentration pre- and post-exposure) can complicate study findings and has been attributed to difficulties in identifying molecular targets for the purpose of pulmonary therapeutic development and assessment of risk to human health. Knowing the cell exposure concentration which results in an in vitro or ex vivo observed response is key to translating effects of inhaled toxicants on the cellular and molecular level to biomarkers for human disease. Advancements in cell culture model systems, such as the In Vitro Cell C/F or Gillings Sampler, have been developed to address some of these concerns through the incorporation of emerging technologies that can enhance particle and gas delivery to cells cultured on a porous membrane insert.
Development of adverse outcome pathways (AOP) which incorporate in silico modeling has also enhanced the ability to screen compounds for inhalational toxicity and drive endpoint development based on mechanistic events. A side-by-side comparison of compounds exposed in both in vitro and in vivo models offers a powerful tool for ascertaining the ability of alternative in vitro model systems for predicting impacts to human health following exposure to inhaled toxicants. Moreover, incorporation of additional airway cell types within these in vitro model systems (e.g., fibroblasts, endothelial, and immune cells) will aid in increasing physiological relevancy and complexity of these models.
Ultimately, the implementation of alternative in vitro models for inhalation toxicity safety assessment requires validation, including test relevance, reliability, and predictive capacity. Species differences and the limitations of in vitro model systems also need to be considered before widespread adoption across both regulatory and non-regulatory agencies.
This blog post resulted from the author’s attendance of the “In Vitro Test Methods to Model Local Respiratory Effects after Exposure to Pulmonary Toxicants: Not Just Smoke and Mirrors” Scientific Session during the 2018 Annual Meeting and ToxExpo.
This blog was prepared by an SOT Reporter. SOT Reporters are SOT members who volunteer to write about sessions and events they attend during the SOT Annual Meeting and ToxExpo. If you are interested in participating in the SOT Reporter program in the future, please email SOT Communications Director Michelle Werts.