In Vitro assessments of ENDS toxicity in the respiratory tract: Are we there yet?

Approximately 4.6 % of U.S. adults over the age of 18 use e-cigarettes, which are a type of electronic nicotine delivery system (ENDS). Over 2.5 million U.S. middle and high school students also use both disposable and/or flavored ENDS products. The health impacts of ENDS use by adults and adolescents are considered a controversial topic in the social media partially due to misperceptions surrounding ENDS toxicity compared to that of combustible cigarettes. There is growing evidence that ENDS, particularly their product composition and design, individual and combined ingredients, and produced aerosols, are toxic to human health. Animal studies have been critical for defining the pathophysiologic outcomes resulting from ENDS use. However, <i>in vitro</i> approaches using human cells can measure the potential toxicity of ENDS e-liquids and aerosols on a shorter timeline and are in keeping with recent statements to replace, reduce and refine the use of animals in biomedical research and regulatory decision making. This review examines current research related to cell culture models of the respiratory tract and exposure methodologies for ENDS use and compares known <i>in vivo</i> parameters of injury and inflammation associated with ENDS to different <i>in vitro</i> systems developed to replicate the inhaled toxicant outcomes. The design and interpretation of exposure methodologies and technological gaps in the evaluation of ENDS aerosols are also discussed. Given the ongoing evolution and popularity of ENDS products, <i>in vitro</i> assessments for measuring respiratory tract injury and inflammation resulting from ENDS use provide a critical scientific platform for rapid evaluation of potential inhalation toxicity in tobacco regulatory science.