Considerations on dosimetry for in vitro assessment of e-cigarette toxicity

A Ventilated Three-Dimensional Artificial Lung System for Human Inhalation Exposure Studies

Traditional in vitro and in vivo models for inhalation toxicology studies often fail to replicate the anatomical and physiological conditions of the human lung. This limitation hinders our understanding of intrapulmonary exposures and their related health effects. To address this gap, we developed a ventilated artificial lung system that replicates human inhalation exposures in four key aspects: (1) facilitating continuous breathing with adjustable respiratory parameters; (2) distributing inhaled aerosols through transitional airflow fields in 3D-printed airway structures, which enables size-dependent particle deposition; (3) duplicating the warm and humid lung environment to promote inhaled aerosol dynamics, such as hygroscopic growth; and (4) supporting the cultivation of human airway epithelium for aerosol exposure and toxicological analyses. As a proof-of-concept application, we exposed human bronchial epithelial cells to electronic cigarette aerosols in the system. Our results show that electronic cigarette particles undergo significant hygroscopic growth within the artificial lung, leading to a 19% greater deposition dose compared to data collected at room temperature and relative humidity. Additionally, short-term exposure altered epithelial production of the chemokine Fractalkine in a nicotine-dependent manner, but no acute toxic effects were observed. This artificial lung system provides a more physiologically relevant in vitro model for studying inhalation exposures.

Acute exposure to electronic cigarette components alters mRNA expression of pre-osteoblasts

The use of traditional nicotine delivery products such as tobacco has long been linked to detrimental health effects. However, little work to date has focused on the emerging market of aerosolized nicotine delivery known as electronic nicotine delivery systems (ENDS) or electronic cigarettes, and their potential for new effects on human health. Challenges studying these devices include heterogeneity in the formulation of the common components of most available ENDS, including nicotine and a carrier (commonly composed of propylene glycol and vegetable glycerin, or PG/VG). In the present study, we report on experiments interrogating the effects of major identified components in e-cigarettes. Specifically, the potential concomitant effects of nicotine and common carrier ingredients in commercial "vape" products are explored in vitro to inform the potential health effects on the craniofacial skeleton through novel vectors as compared to traditional tobacco products. MC3T3-E1 murine pre-osteoblast cells were cultured in vitro with clinically relevant liquid concentrations of nicotine, propylene glycol (PG), vegetable glycerin (VG), Nicotine+PG/VG, and the vape liquid of a commercial product (Juul). Cells were treated acutely for 24 h and RNA-Seq was utilized to determine segregating alteration in mRNA signaling. Influential gene targets identified with sparse partial least squares discriminant analysis (sPLS-DA) implemented in mixOmics were assessed using the PANTHER Classification system for molecular functions, biological processes, cellular components, and pathways of effect. Additional endpoint functional analyses were used to confirm cell cycle changes. The initial excitatory concentration (EC50) studied defined a target concentration of carrier PG/VG liquid that altered the cell cycle of the calvarial cells. Initial sPLS-DA analysis demonstrated the segregation of nicotine and non-nicotine exposures utilized in our in vitro modeling. Pathway analysis suggests a strong influence of nicotine exposures on cellular processes including metabolic processes and response to stimuli including autophagic flux. Further interrogation of the individual treatment conditions demonstrated segregation by treatment modality (Control, Nicotine, Carrier (PG+VG), Nicotine+PG/VG) along three dimensions best characterized by: latent variable 1 (PLSDA-1) showing strong segregation based on nicotine influence on cellular processes associated with cellular adhesion to collagen, osteoblast differentiation, and calcium binding and metabolism; latent variable 2 (PLSDA-2) showing strong segregation of influence based on PG+VG and Control influence on cell migration, survival, and cycle regulation; and latent variable 3 (PLSDA-3) showing strong segregation based on Nicotine and Control exposure influence on cell activity and growth and developmental processes. Further, gene co-expression network analysis implicates targets of the major pathway genes associated with bone growth and development, particularly craniofacial (FGF, Notch, TGFβ, WNT) and analysis of active subnetwork pathways found these additionally overrepresented in the Juul exposure relative to Nicotine+PG/VG. Finally, experimentation confirmed alterations in cell count, and increased evidence of cell stress (markers of autophagy), but no alteration in apoptosis. These data suggest concomitant treatment with Nicotine+PG/VG drives alterations in pre-osteoblast cell cycle signaling, specifically transcriptomic targets related to cell cycle and potentially cell stress. Although we suspected cell stress and well as cytotoxic effects of Nicotine+PG/VG, no great influence on apoptotic factors was observed. Further RNA-Seq analysis allowed for the direct interrogation of molecular targets of major pathways involved in bone and craniofacial development, each demonstrating segregation (altered signaling) due to e-cigarette-type exposure. These data have implications directed toward ENDS formulation as synergistic effects of Nicotine+PG/VG are evidenced here. Thus, future research will continue to interrogate how varied formulation of Nicotine+PG/VG affects overall cell functions in multiple vital systems.

In vitro toxicological evaluation of aerosols generated by a 4th generation vaping device using nicotine salts in an air-liquid interface system

BACKGROUND

Electronic cigarettes (EC) have gained popularity, especially among young people, with the introduction of fourth-generation devices based on e-liquids containing nicotine salts that promise a smoother vaping experience than freebase nicotine. However, the toxicological effects of nicotine salts are still largely unknown, and the chemical diversity of e-liquids limits the comparison between different studies to determine the contribution of each compound to the cytotoxicity of EC aerosols. Therefore, the aim of this study was to evaluate the toxicological profile of controlled composition e-liquid aerosols to accurately determine the effects of each ingredient based on exposure at the air-liquid interface.

METHODS

Human lung epithelial cells (A549) were exposed to undiluted aerosols of controlled composition e-liquids containing various ratios of propylene glycol (PG)/vegetable glycerin (VG) solvents, freebase nicotine, organic acids, nicotine salts, and flavoured commercial e-liquids. Exposure of 20 puffs was performed at the air-liquid interface following a standard vaping regimen. Toxicological outcomes, including cytotoxicity, inflammation, and oxidative stress, were assessed 24 h after exposure.

RESULTS

PG/VG aerosols elicited a strong cytotoxic response characterised by a 50% decrease in cell viability and a 200% increase in lactate dehydrogenase (LDH) production, but had no effects on inflammation and oxidative stress. These effects occurred only at a ratio of 70/30 PG/VG, suggesting that PG is the major contributor to aerosol cytotoxicity. Both freebase nicotine and organic acids had no greater effect on cell viability and LDH release than at a 70/30 PG/VG ratio, but significantly increased inflammation and oxidative stress. Interestingly, the protonated form of nicotine in salt showed a stronger proinflammatory effect than the freebase nicotine form, while benzoic acid-based nicotine salts also induced significant oxidative stress. Flavoured commercial e-liquids was found to be cytotoxic at a threshold dose of ≈ 330 µg/cm².

CONCLUSION

Our results showed that aerosols of e-liquids consisting only of PG/VG solvents can cause severe cytotoxicity depending on the concentration of PG, while nicotine salts elicit a stronger pro-inflammatory response than freebase nicotine. Overall, aerosols from fourth-generation devices can cause different toxicological effects, the nature of which depends on the chemical composition of the e-liquid.

A Practical Framework for Novel Electronic Nicotine Delivery System Evaluation: Chemical and Toxicological Characterization of JUUL2 Aerosol and Comparison with Reference Cigarettes

Electronic nicotine delivery systems (ENDSs) are designed as a non-combustible alternative to cigarettes, aiming to deliver nicotine without the harmful byproducts of tobacco combustion. As the category evolves and new ENDS products emerge, it is important to continually assess the levels of toxicologically relevant chemicals in the aerosols and characterize any related toxicology. Herein, we present a proposed framework for characterizing novel ENDS products (i.e., devices and formulations) and determining the reduced risk potential utilizing analytical chemistry and in vitro toxicological studies with a qualitative risk assessment. To demonstrate this proposed framework, long-term stability studies (12 months) analyzing relevant toxicant emissions from six formulations of a next-generation product, JUUL2, were conducted and compared to reference combustible cigarette (CC) smoke under both non-intense and intense puffing regimes. In addition, in vitro cytotoxicity, mutagenicity, and genotoxicity assays were conducted on aerosol and smoke condensates. In all samples, relevant toxicants under both non-intense and intense puffing regimes were substantially lower than those observed in reference CC smoke. Furthermore, neither cytotoxicity, mutagenicity, nor genotoxicity was observed in aerosol condensates generated under both intense and non-intense puffing regimes, in contrast to results observed for reference cigarettes. Following the proposed framework, the results demonstrate that the ENDS products studied in this work generate significantly lower levels of toxicants relative to reference cigarettes and were not cytotoxic, mutagenic, or genotoxic under these in vitro assay conditions.

Vaping product exposure system (VaPES): a novel in vitro aerosol deposition system

OBJECTIVE

Due to recent increases in the use of vaping devices, there is a high demand for research addressing the respiratory health effects of vaping products. Given the constantly changing nature of the vaping market with new devices, flavors, metals, and other chemicals rapidly emerging, there is a need for inexpensive and highly adaptable vaping device exposure systems. Here, we describe the design and validation of a novel in vitro aerosol exposure system for toxicity testing of vaping devices.

MATERIALS AND METHODS

We developed an inexpensive, open-source in vitro vaping device exposure system that produces even deposition, can be adapted for different vaping devices, and allows for experiments to be performed under physiological conditions. The system was then validated with deposition testing and a representative exposure with human bronchial epithelial cells (hBECs).

RESULTS

The Vaping Product Exposure System (VaPES) produced sufficient and uniform deposition for dose-response studies and was precise enough to observe biological responses to vaping exposures. VaPES was adapted to work with both pod and cartridge-based vaping devices.

CONCLUSION

We have designed and validated a novel vaping device exposure system that will eliminate the need to use high-cost commercial exposure systems, lowering the barrier to entry of physiologically relevant vaping studies.

Adverse Biophysical Impact of e-Cigarette Flavors on Pulmonary Surfactant

The attractiveness and abundance of flavors are primary factors eliciting youth to use e-cigarettes. Emerging studies in recent years revealed the adverse health impact of e-cigarette flavoring chemicals, including disruption of the biophysical function of pulmonary surfactants in the lung. Nevertheless, a comprehensive understanding of the biophysical impact of various flavoring chemicals is still lacking. We used constrained drop surfactometry as a new alternative method to study the biophysical impact of flavored e-cigarette aerosols on an animal-derived natural pulmonary surfactant. The dose of exposure to e-cigarette aerosols was quantified with a quartz crystal microbalance, and alterations to the ultrastructure of the surfactant film were visualized using atomic force microscopy. We have systematically studied eight representative flavoring chemicals (benzyl alcohol, menthol, maltol, ethyl maltol, vanillin, ethyl vanillin, ethyl acetate, and ethyl butyrate) and six popular recombinant flavors (coffee, vanilla, tobacco, cotton candy, menthol/mint, and chocolate). Our results suggested a flavor-dependent inhibitory effect of e-cigarette aerosols on the biophysical properties of the pulmonary surfactant. A qualitative phase diagram was proposed to predict the hazardous potential of various flavoring chemicals. These results provide novel implications in understanding the environmental, health, and safety impacts of e-cigarette aerosols and may contribute to better regulation of e-cigarette products.

Can the impact of micro- and nanoplastics on human health really be assessed using in vitro models? A review of methodological issues

Because of the many advantages they offer (strength, low cost, durability, lightweight, resistance, etc.), plastics are integral part of our daily life with a production constantly rising. However, their waste management is still inadequate, resulting in their release and accumulation in the environment, representing a main source of pollution. Their degradation results in debris of variable size including microplastics (0.1 μm-5 mm) and even nanoplastics (<0.1 μm), whose potential impact on ecosystems and human health have raised concerns. The potential adverse effects they may cause have been evaluated using both in vitro and in vivo models. However, due to some specific characteristics of micro- and nanoplastics, there are challenging questions about whether conventional in vitro tests are appropriate for evaluating their toxicity. For example, low-density plastics float on the surface of the culture medium and cannot come into contact with cells adhering to the bottom of the culture plates, which prevents proper evaluation of potential adverse effects and leads to misinterpretation of toxicological assays. In this review, we discuss the main issues related to the evaluation of micro- and nanoplastics toxicity using conventional in vitro assays. A literature survey has allowed to propose some solutions to circumvent these issues including the use of mathematical models to accurately determine the dose of particles delivered to cells, advanced 3D models (organoids), inverted cell culture models, cell cultures at the air-liquid interface or under dynamic conditions. Finally, we propose some perspectives and recommendations for further research on the in vitro evaluation of micro- and nanoplastics toxicity, underlining the importance of using standardized protocols for comparison purposes and samples and experimental conditions more representative of real-life exposure.

Comparative study of the effects of cigarette smoke versus next-generation tobacco and nicotine product extracts on inflammatory biomarkers of human monocytes

Monocytes exhibiting a pro-inflammatory phenotype play a key role in adhesion and development of atherosclerotic plaques. As an alternative to smoking, next-generation tobacco and nicotine products (NGP) are now widely used. However, little is known about their pro-inflammatory effects on monocytes. We investigated cell viability, anti-oxidant and pro-inflammatory gene and protein expression in THP-1 monocytes after exposure to aqueous smoke extracts (AqE) of a heated tobacco product (HTP), an electronic cigarette (e-cig), a conventional cigarette (3R4F) and pure nicotine (nic). Treatment with 3R4F reduced cell viability in a dose-dependent manner, whereas exposure to alternative smoking products showed no difference to control. At the highest non-lethal dose of 3R4F (20%), the following notable mRNA expression changes were observed for 3R4F, HTP, and e-cig respectively, relative to control; HMOX1 (6-fold, < 2-fold, < 2-fold), NQO1 (3.5-fold, < 2-fold, < 2-fold), CCL2 (4-fold, 3.5-fold, 2.5-fold), IL1B (4-fold, 3-fold, < 2-fold), IL8 (5-fold, 2-fold, 2-fold), TNF (2-fold, 2-fold, < 2-fold) and ICAM1 was below the 2-fold threshold for all products. With respect to protein expression, IL1B (3-fold, < 2-fold, < 2-fold) and IL8 (3.5-fold, 2-fold, 2-fold) were elevated over the 2-fold threshold, whereas CCL2, TNF, and ICAM1 were below 2-fold expression for all products. At higher doses, greater inductions were observed with all extracts; however, NGP responses were typically lower than 3R4F. In conclusion, anti-oxidative and pro-inflammatory processes were activated by all products. NGPs overall showed lower responses relative to controls than THP-1 cells exposed to 3R4F AqE.