Mechanisms of toxicity and biomarkers of flavoring and flavor enhancing chemicals in emerging tobacco and non-tobacco products

Abstract

Tobacco products containing flavorings, such as electronic nicotine delivery devices (ENDS) or e-cigarettes, cigars/cigarillos, waterpipes, and heat-not-burn devices (iQOS) are continuously evolving. In addition to increasing the exposure of teenagers and adults to nicotine containing flavoring products and flavoring enhancers, chances of nicotine addiction through chronic use and abuse also increase. These flavorings are believed to be safe for ingestion, but little information is available about their effects on the lungs. In this review, we have discussed the in vitro and in vivo data on toxicity of flavoring chemicals in lung cells. We have further discussed the common flavoring agents, such as diacetyl and menthol, currently available detection methods, and the toxicological mechanisms associated with oxidative stress, inflammation, mucociliary clearance, and DNA damage in cells, mice, and humans. Finally, we present potential biomarkers that could be utilized for future risk assessment. This review provides crucial parameters important for evaluation of risk associated with flavoring agents and flavoring enhancers used in tobacco products and ENDS. Future studies can be designed to address the potential toxicity of inhaled flavorings and their biomarkers in users as well as in chronic exposure studies.

Keywords: Biomarkers; DNA damage; E-cigarette; Flavoring chemicals; Inflammation; Oxidative stress.

Figure 1. Currently known toxicity mechanisms in affected respiratory cells upon flavorant exposure:

The figure represents the pulmonary toxicity after inhalation of a flavoring chemical such as diacetyl. Once inhaled, the flavoring agent causes insult to the first line of defense (e.g. depletion of antioxidant thiols and barrier dysfunction) against particulates and antigens, the airway epithelium. Responsible for appropriate conduction of airflow, the airway epithelium usually consists of ciliated epithelium cells interspersed with mucus secreting goblet cells. Hence, the contact with flavoring chemicals (flavorant) results in induction of oxidative stress (as visible from increased ROS production and IL-8 secretion) as well as immune responses such as decreased lubrication, increased impedance and flow resistance (marked by coughing). While both mechanisms are capable of separately leading to cell toxicity (as observed in bronchiolitis obliterans in case of diacetyl inhalation), cytotoxicity may also proceed by yet not known mechanisms. Fibroblasts, present in the adjacent connective tissue, beneath the airway epithelium, are also affected. Upon oxidative stress and DNA damage, various cellular signaling cascades are activated, thereby leading to inflammatory responses.