Evaluation of Chronic Toxicity and Carcinogenicity of Flavored E-Vapor Aerosols in an 18-Month Inhalation Study in A/J Mice

Background and Purpose: There is limited toxicological information to evaluate the chronic inhalation toxicity of e-vapor aerosols containing flavors, humectants, and nicotine. In this study, A/J mice were tested to evaluate and compare the local and systemic toxicity after lifetime exposure to cigarette smoke (CS) and aerosols from a prototype e-liquid formulation containing 38 selected flavors.

Methods: Mice were exposed via whole-body inhalation for 6 h/day for 5 days/week for up to 18 months to different aerosol compositions: aerosol from propylene glycol (PG) and vegetable glycerol (VG); PG/VG with nicotine (N, 2% [w/w]); PG/VG/N with flavors (F) at low, medium, and high concentrations (1.2-18.6% [w/w]); PG/VG/F-High; or CS from the 3R4F reference cigarette. Primary endpoints were evaluated based on the Organisation for Economic Co-operation and Development (OECD) Test Guideline 453, including in-life measurements, lung function, histopathology, and clinical pathology.

Results: Chronic inhalation exposure to PG/VG/N/F e-vapors did not impact mortality, lung function, clinical pathology, or macroscopic findings. Slightly suppressed body weight gain was noted in the male and female PG/ VG/N/F-H groups without a noticeable reduction in food consumption. In contrast to CS-exposed groups, lung histological changes were absent or minimal in the tested e-vapor groups, with notable microscopic findings limited in the larynx and nose. Laryngeal changes were minimal to mild in the PG/VG/N/F-High groups, were of lower severity than in 3R4F groups, and were considered adaptive changes. Olfactory epithelial (nasal) findings were observed in 3R4F and PG/VG/N/F groups and are considered degenerative changes, potentially related to irritation from repeated exposures. Among all treated groups, the CS groups showed the most notable lung inflammation, lung emphysema, increase in lung tumor and laryngeal papilloma development, moderate to severe respiratory epithelia (nasal) findings, altered lung function, serum liver clinical pathology parameters, erythrocyte parameters, and atrophy of the thymus. Findings in non-respiratory organs, such as the glandular stomach and skeletal system, were strain-specific and not likely treatment related.

Conclusions: Chronic exposure to e-vapor aerosols up to 18 months did not induce notable toxicity or carcinogenicity in the lung compared to air controls. E-vapor exposure resulted in either no exposure-related toxicological changes or, specifically in the PG/VG/N/F-H groups, changes were significantly reduced (larynx) compared to or comparable (olfactory epithelia) to CS-exposed mice. The study results support the potential reduced toxicity and carcinogenicity potential of e-vapor aerosols compared to combustible CS and the potential role of e-vapor products in tobacco harm reduction.