Air-Liquid Interface In Vitro Models for Respiratory Toxicology Research: Consensus Workshop and Recommendations

In vitro and ex vivo models in inhalation biopharmaceutical research - advances, challenges and future perspectives

Oral inhalation results in pulmonary drug targeting and thereby reduces systemic side effects, making it the preferred means of drug delivery for the treatment of respiratory disorders such as asthma, chronic obstructive pulmonary disease or cystic fibrosis. In addition, the high alveolar surface area, relatively low enzymatic activity and rich blood supply of the distal airspaces offer a promising pathway to the systemic circulation. This is particularly advantageous when a rapid onset of pharmacological action is desired or when the drug is suffering from stability issues or poor biopharmaceutical performance following oral administration. Several cell and tissue-based in vitro and ex vivo models have been developed over the years, with the intention to realistically mimic pulmonary biological barriers. It is the aim of this review to critically discuss the available models regarding their advantages and limitations and to elaborate further which biopharmaceutical questions can and cannot be answered using the existing models.

Assessment of in vitro kinetics and biological impact of nebulized trehalose on human bronchial epithelium

Trehalose is added in drug formulations to act as fillers or improve aerosolization performance. Its characteristics as a carrier molecule have been explored; however, the fate of trehalose in human airway tissues has not been thoroughly investigated. Here, we investigated the fate of nebulized trehalose using in vitro human air-liquid bronchial epithelial cultures. First, a tracing experiment was conducted using ¹³C₁₂-trehalose; we measured trehalose distribution in different culture compartments (apical surface liquid, epithelial culture, and basal side medium) at various time points following acute exposure to ¹³C₁₂-labeled trehalose. We found that ¹³C₁₂-trehalose was metabolized into ¹³C₆-glucose. The data was then used to model the kinetics of trehalose disappearance from the apical surface of bronchial cultures. Secondly, we evaluated the potential adverse effects of nebulized trehalose on the bronchial cultures after they were acutely exposed to nebulized trehalose up to a level just below its solubility limit (50 g/100 g water). We assessed the ciliary beating frequency and histological characteristics. We found that nebulized trehalose did not lead to marked alteration in ciliary beating frequency and morphology of the epithelial cultures. The in vitro testing approach used here may enable the early selection of excipients for future development of inhalation products.

In vitro exposure to complete engine emissions - a mini-review

Outdoor air pollution is classified as carcinogenic to humans and exposure to it contributes to increased incidence of various diseases, including cardiovascular, neurological or pulmonary disorders. Vehicle engine emissions represent a significant part of outdoor air pollutants, particularly in large cities with high population density. Considering the potentially negative health impacts of engine emissions exposure, the application of reliable test systems allowing assessment of the biological effects of these pollutants is crucial. The exposure systems should use relevant, preferably multicellular, cell models that are treated with the complete engine exhaust (i.e. a realistic mixture of particles, chemical compounds bound to them and gaseous phase) at the air-liquid interface. The controlled delivery and characterization of chemical and/or particle composition of the exhaust should be possible. In this mini-review we report on such exposure systems that have been developed to date. We focus on a brief description and technical characterization of the systems, and discuss the biological parameters detected following exposure to a gasoline/diesel exhaust. Finally, we summarize and compare findings from the individual systems, including their advantages/limitations.

Recent advances in human respiratory epithelium models for drug discovery

The respiratory epithelium is intimately associated with the pathophysiologies of highly infectious viral contagions and chronic illnesses such as chronic obstructive pulmonary disorder, presently the third leading cause of death worldwide with a projected economic burden of £1.7 trillion by 2030. Preclinical studies of respiratory physiology have almost exclusively utilised non-humanised animal models, alongside reductionistic cell line-based models, and primary epithelial cell models cultured at an air-liquid interface (ALI). Despite their utility, these model systems have been limited by their poor correlation to the human condition. This has undermined the ability to identify novel therapeutics, evidenced by a 15% chance of success for medicinal respiratory compounds entering clinical trials in 2018. Consequently, preclinical studies require new translational efficacy models to address the problem of respiratory drug attrition. This review describes the utility of the current in vivo (rodent), ex vivo (isolated perfused lungs and precision cut lung slices), two-dimensional in vitro cell-line (A549, BEAS-2B, Calu-3) and three-dimensional in vitro ALI (gold-standard and co-culture) and organoid respiratory epithelium models. The limitations to the application of these model systems in drug discovery research are discussed, in addition to perspectives of the future innovations required to facilitate the next generation of human-relevant respiratory models.

Advanced human-relevant in vitro pulmonary platforms for respiratory therapeutics

Over the past years, advanced in vitro pulmonary platforms have witnessed exciting developments that are pushing beyond traditional preclinical cell culture methods. Here, we discuss ongoing efforts in bridging the gap between in vivo and in vitro interfaces and identify some of the bioengineering challenges that lie ahead in delivering new generations of human-relevant in vitro pulmonary platforms. Notably, in vitro strategies using foremost lung-on-chips and biocompatible "soft" membranes have focused on platforms that emphasize phenotypical endpoints recapitulating key physiological and cellular functions. We review some of the most recent in vitro studies underlining seminal therapeutic screens and translational applications and open our discussion to promising avenues of pulmonary therapeutic exploration focusing on liposomes. Undeniably, there still remains a recognized trade-off between the physiological and biological complexity of these in vitro lung models and their ability to deliver assays with throughput capabilities. The upcoming years are thus anticipated to see further developments in broadening the applicability of such in vitro systems and accelerating therapeutic exploration for drug discovery and translational medicine in treating respiratory disorders.

Emerging Technologies for In Vitro Inhalation Toxicology

Respiratory toxicology remains a major research area in the 21st century since current scenario of airborne viral infection transmission and pollutant inhalation is expected to raise the annual morbidity beyond 2 million. Clinical and epidemiological research connecting human exposure to air contaminants to understand adverse pulmonary health outcomes is, therefore, an immediate subject of human health assessment. Important observations in defining systemic effects of environmental contaminants on inhalation metabolic dysfunction, liver health, and gastrointestinal tract have been well explored with in vivo models. In this review, a framework is provided, a paradigm is established about inhalation toxicity testing in vitro, and a brief overview of breathing Lungs-on-Chip (LoC) as design concepts is given. The optimized bioengineering approaches and microfluidics with their fundamental pros, and cons are presented. There are different strategies that researchers apply to inhalation toxicity studies to assess a variety of inhalable substances and relevant LoC approaches. A case study from published literature and frame arguments about reproducibility as well as in vitro/in vivo correlations are discussed. Finally, the opportunities and challenges in soft robotics, systems inhalation toxicology approach integrating bioengineering, machine learning, and artificial intelligence to address a multitude model for future toxicology are discussed.

An Integrated Approach to Testing and Assessment to Support Grouping and Read-Across of Nanomaterials After Inhalation Exposure

Introduction: Here, we describe the generation of hypotheses for grouping nanoforms (NFs) after inhalation exposure and the tailored Integrated Approaches to Testing and Assessment (IATA) with which each specific hypothesis can be tested. This is part of a state-of-the-art framework to support the hypothesis-driven grouping and read-across of NFs, as developed by the EU-funded Horizon 2020 project GRACIOUS.

Development of Grouping Hypotheses and IATA: Respirable NFs, depending on their physicochemical properties, may dissolve either in lung lining fluid or in acidic lysosomal fluid after uptake by cells. Alternatively, NFs may also persist in particulate form. Dissolution in the lung is, therefore, a decisive factor for the toxicokinetics of NFs. This has led to the development of four hypotheses, broadly grouping NFs as instantaneous, quickly, gradually, and very slowly dissolving NFs. For instantaneously dissolving NFs, hazard information can be derived by read-across from the ions. For quickly dissolving particles, as accumulation of particles is not expected, ion toxicity will drive the toxic profile. However, the particle aspect influences the location of the ion release. For gradually dissolving and very slowly dissolving NFs, particle-driven toxicity is of concern. These NFs may be grouped by their reactivity and inflammation potency. The hypotheses are substantiated by a tailored IATA, which describes the minimum information and laboratory assessments of NFs under investigation required to justify grouping.

Conclusion: The GRACIOUS hypotheses and tailored IATA for respiratory toxicity of inhaled NFs can be used to support decision making regarding Safe(r)-by-Design product development or adoption of precautionary measures to mitigate potential risks. It can also be used to support read-across of adverse effects such as pulmonary inflammation and subsequent downstream effects such as lung fibrosis and lung tumor formation after long-term exposure.

A survey of aerosol exposure systems relative to the analysis of cytotoxicity: A Cooperation Centre for Scientific Research Relative to Tobacco (CORESTA) perspective

During a Cooperation Centre for Scientific Research Relative to Tobacco (CORESTA) meeting, the in vitro toxicity testing Sub-Group (IVT SG) met to discuss the evolving field of aerosol exposure research. Given the diversity of exposure parameters and biological endpoints being used, it was considered a high priority to investigate and contextualise the responses obtained. This is particularly driven by the inability to compare between studies on different exposure systems due to user preferences and protocol differences. Twelve global tobacco and contract research companies met to discuss this topic and formulate an aligned approach on how this diverging field of research could be appropriately compared. Something that is becoming increasingly important, especially in the light of more focused regulatory scrutiny. A detailed and comprehensive survey was conducted on over 40 parameters ranging from aerosol generation, dilution and data analysis across eight geographically independent laboratories. The survey results emphasise the diversity of in vitro exposure parameters and methodologies employed across the IVT SG and highlighted pockets of harmonisation. For example, many of the biological protocol parameters are consistent across the Sub-Group. However, variables such as cell type and exposure time remain largely inconsistent. The next steps for this work will be to map parameters and system data against biological findings and investigate whether the observed inconsistencies translate into increased biological variability. The results from the survey provide improved awareness of parameters and nuances, that may be of substantial benefit to scientists in intersecting fields and in the development of harmonised approaches.

Impact of Nanocomposite Combustion Aerosols on A549 Cells and a 3D Airway Model

The use of nanomaterials incorporated into plastic products is increasing steadily. By using nano-scaled filling materials, thermoplastics, such as polyethylene (PE), take advantage of the unique properties of nanomaterials (NM). The life cycle of these so-called nanocomposites (NC) usually ends with energetic recovery. However, the toxicity of these aerosols, which may consist of released NM as well as combustion-generated volatile compounds, is not fully understood. Within this study, model nanocomposites consisting of a PE matrix and nano-scaled filling material (TiO₂, CuO, carbon nano tubes (CNT)) were produced and subsequently incinerated using a lab-scale model burner. The combustion-generated aerosols were characterized with regard to particle release as well as compound composition. Subsequently, A549 cells and a reconstituted 3D lung cell culture model (MucilAir™, Epithelix) were exposed for 4 h to the respective aerosols. This approach enabled the parallel application of a complete aerosol, an aerosol under conditions of enhanced particle deposition using high voltage, and a filtered aerosol resulting in the sole gaseous phase. After 20 h post-incubation, cytotoxicity, inflammatory response (IL-8), transcriptional toxicity profiling, and genotoxicity were determined. Only the exposure toward combustion aerosols originated from PE-based materials induced cytotoxicity, genotoxicity, and transcriptional alterations in both cell models. In contrast, an inflammatory response in A549 cells was more evident after exposure toward aerosols of nano-scaled filler combustion, whereas the thermal decomposition of PE-based materials revealed an impaired IL-8 secretion. MucilAir™ tissue showed a pronounced inflammatory response after exposure to either combustion aerosols, except for nanocomposite combustion. In conclusion, this study supports the present knowledge on the release of nanomaterials after incineration of nano-enabled thermoplastics. Since in the case of PE-based combustion aerosols no major differences were evident between exposure to the complete aerosol and to the gaseous phase, adverse cellular effects could be deduced to the volatile organic compounds that are generated during incomplete combustion of NC.

Use of Cause-and-Effect Analysis to Optimize the Reliability of In Vitro Inhalation Toxicity Measurements Using an Air-Liquid Interface

In vitro inhalation toxicology methods are increasingly being used for research and regulatory purposes. Although the opportunity for increased human relevance of in vitro inhalation methods compared to in vivo tests has been established and discussed, how to systematically account for variability and maximize the reliability of these in vitro methods, especially for assays that use cells cultured at an air-liquid interface (ALI), has received less attention. One tool that has been used to evaluate the robustness of in vitro test methods is cause-and-effect (C&E) analysis, a conceptual approach to analyze key sources of potential variability in a test method. These sources of variability can then be evaluated using robustness testing and potentially incorporated into in-process control measurements in the assay protocol. There are many differences among in vitro inhalation test methods including the use of different types of biological test systems, exposure platforms/conditions, substances tested, and end points, which represent a major challenge for use in regulatory testing. In this manuscript, we describe how C&E analysis can be applied using a modular approach based on the idea that shared components of different test methods (e.g., the same exposure system is used) have similar sources of variability even though other components may differ. C&E analyses of different in vitro inhalation methods revealed a common set of recommended exposure systems and biological in-process control measurements. The approach described here, when applied in conjunction with Good Laboratory Practices (GLP) criteria, should help improve the inter- and intralaboratory agreement of in vitro inhalation test results, leading to increased confidence in these methods for regulatory and research purposes.

A Tissue-Engineered Tracheobronchial In Vitro Co-Culture Model for Determining Epithelial Toxicological and Inflammatory Responses

Translation of novel inhalable therapies for respiratory diseases is hampered due to the lack of in vitro cell models that reflect the complexity of native tissue, resulting in many novel drugs and formulations failing to progress beyond preclinical assessments. The development of physiologically-representative tracheobronchial tissue analogues has the potential to improve the translation of new treatments by more accurately reflecting in vivo respiratory pharmacological and toxicological responses. Herein, advanced tissue-engineered collagen hyaluronic acid bilayered scaffolds (CHyA-B) previously developed within our group were used to evaluate bacterial and drug-induced toxicity and inflammation for the first time. Calu-3 bronchial epithelial cells and Wi38 lung fibroblasts were grown on either CHyA-B scaffolds (3D) or Transwell® inserts (2D) under air liquid interface (ALI) conditions. Toxicological and inflammatory responses from epithelial monocultures and co-cultures grown in 2D or 3D were compared, using lipopolysaccharide (LPS) and bleomycin challenges to induce bacterial and drug responses in vitro. The 3D in vitro model exhibited significant epithelial barrier formation that was maintained upon introduction of co-culture conditions. Barrier integrity showed differential recovery in CHyA-B and Transwell® epithelial cultures. Basolateral secretion of pro-inflammatory cytokines to bacterial challenge was found to be higher from cells grown in 3D compared to 2D. In addition, higher cytotoxicity and increased basolateral levels of cytokines were detected when epithelial cultures grown in 3D were challenged with bleomycin. CHyA-B scaffolds support the growth and differentiation of bronchial epithelial cells in a 3D co-culture model with different transepithelial resistance in comparison to the same co-cultures grown on Transwell® inserts. Epithelial cultures in an extracellular matrix like environment show distinct responses in cytokine release and metabolic activity compared to 2D polarised models, which better mimic in vivo response to toxic and inflammatory stimuli offering an innovative in vitro platform for respiratory drug development.

Air-liquid interface cultures of the healthy and diseased human respiratory tract: promises, challenges and future directions

Air-liquid interface (ALI) culture models currently represent a valid instrument to recreate the typical aspects of the respiratory tract in vitro in both healthy and diseased state. They can help reducing the number of animal experiments, therefore, supporting the 3R principle. This review discusses ALI cultures and co-cultures derived from immortalized as well as primary cells, which are used to study the most common disorders of the respiratory tract, in terms of both pathophysiology and drug screening. The article displays ALI models used to simulate inflammatory lung diseases such as chronic obstructive pulmonary disease (COPD), asthma, cystic fibrosis, lung cancer, and viral infections. It also includes a focus on ALI cultures described in literature studying respiratory viruses such as SARS-CoV-2 causing the global Covid-19 pandemic at the time of writing this review. Additionally, commercially available models of ALI cultures are presented. Ultimately, the aim of this review is to provide a detailed overview of ALI models currently available and to critically discuss them in the context of the most prevalent diseases of the respiratory tract.

Genetic toxicity testing using human in vitro organotypic airway cultures: Assessing DNA damage with the CometChip and mutagenesis by Duplex Sequencing

The organotypic human air-liquid-interface (ALI) airway tissue model has been used as an in vitro cell culture system for evaluating the toxicity of inhaled substances. ALI airway cultures are highly differentiated, which has made it challenging to evaluate genetic toxicology endpoints. In the current study, we assayed DNA damage with the high-throughput CometChip assay and quantified mutagenesis with Duplex Sequencing, an error-corrected next-generation sequencing method capable of detecting a single mutation per 107 base pairs. Fully differentiated human ALI airway cultures were treated from the basolateral side with 6.25 to 100 μg/mL ethyl methanesulfonate (EMS) over a period of 28 days. CometChip assays were conducted after 3 and 28 days of treatment, and Duplex Sequencing after 28 days of treatment. Treating the airway cultures with EMS resulted in time- and concentration-dependent increases in DNA damage and a concentration-dependent increase in mutant frequency. The mutations observed in the EMS-treated cultures were predominantly C → T transitions and exhibited a unique trinucleotide signature relative to the negative control. Measurement of physiological endpoints indicated that the EMS treatments had no effect on anti-p63-positive basal cell frequency, but produced concentration-responsive increases in cytotoxicity and perturbations in cell morphology, along with concentration-responsive decreases in culture viability, goblet cell and anti-Ki67-positive proliferating cell frequency, cilia beating frequency, and mucin secretion. The results indicate that a unified 28-day study can be used to measure several important safety endpoints in physiologically relevant human in vitro ALI airway cultures, including DNA damage, mutagenicity, and tissue-specific general toxicity.

Inter-laboratory variability of A549 epithelial cells grown under submerged and air-liquid interface conditions

In vitro cell models offer a unique opportunity for conducting toxicology research, and the human lung adenocarcinoma cell line A549 is commonly used for toxicology testing strategies. It is essential to determine whether the response of these cells grown in different laboratories is consistent. In this study, A549 cells were grown under both submerged and air-liquid interface (ALI) conditions following an identical cell seeding protocol in two independent laboratories. The cells were switched to the ALI after four days of submerged growth, and their behaviour was compared to submerged conditions. The membrane integrity, cell viability, morphology, and (pro-)inflammatory response upon positive control stimuli were assessed at days 3, 5, and 7 under submerged conditions and at days 5, 7, and 10 at the ALI. Due to the high variability of the results between the two laboratories, the experiment was subsequently repeated using identical reagents at one specific time point and condition (day 5 at the ALI). Despite some variability, the results were more comparable, proving that the original protocol necessitated improvements. In conclusion, the use of detailed protocols and consumables from the same providers, special training of personnel for cell handling, and endpoint analysis are critical to obtain reproducible results across independent laboratories.

A Custom-Made Device for Reproducibly Depositing Pre-metered Doses of Nebulized Drugs on Pulmonary Cells in vitro

The deposition of pre-metered doses (i.e., defined before and not after exposition) at the air-liquid interface of viable pulmonary epithelial cells remains an important but challenging task for developing aerosol medicines. While some devices allow quantification of the deposited dose after or during the experiment, e.g., gravimetrically, there is still no generally accepted way to deposit small pre-metered doses of aerosolized drugs or pharmaceutical formulations, e.g., nanomedicines. Here, we describe a straightforward custom-made device, allowing connection to commercially available nebulizers with standard cell culture plates. Designed to tightly fit into the approximately 12-mm opening of either a 12-well Transwell^® insert or a single 24-well plate, a defined dose of an aerosolized liquid can be directly deposited precisely and reproducibly (4.8% deviation) at the air-liquid interface (ALI) of pulmonary cell cultures. The deposited dose can be controlled by the volume of the nebulized solution, which may vary in a range from 20 to 200 μl. The entire nebulization-deposition maneuver is completed after 30 s and is spatially homogenous. After phosphate-buffered saline (PBS) deposition, the viability and barrier properties transepithelial electrical resistance (TEER) of human bronchial epithelial Calu-3 cells were not negatively affected. Straightforward in manufacture and use, the device enables reproducible deposition of metered doses of aerosolized drugs to study the interactions with pulmonary cell cultures grown at ALI conditions.

Release and cytotoxicity screening of the printer emissions of a CdTe quantum dots-based fluorescent ink

The fluorescent properties of cadmium telluride (CdTe) containing quantum dots (QDs) have led to novel products and applications in the ink and pigment industry. The toxic effects of the emissions associated to the use of printing ink containing CdTe QDs might differ from those of conventional formulations which do not integrate nanoparticles, as CdTe QDs might be emitted. Within this work, the airborne emissions of a water-soluble fluorescent ink containing polyethylene glycol (PEG)-coated CdTe QDs of 3-5 nm diameter have been characterized and studied under controlled conditions during household inkjet printing in a scenario simulating the use phase. Subsequently, the cytotoxicological potential of atomized CdTe QDs ink in an acute exposure regimen simulating an accidental, worse-case scenario has been evaluated in vitro at the air-liquid interface (ALI) using the pulmonary cell line BEAS-2B. Endpoints screened included cell viability, oxidative stress and inflammatory effects. We have observed that CdTe QDs ink at 54.7 ng/mL decreased cell viability by 25.6 % when compared with clean air after 1h of exposure; a concentration about 65 times higher was needed to observe a similar effect in submerged conditions. However, we did not observe oxidative stress or inflammatory effects. The present study integrates the development of scenarios simulating the use phase of nano-additivated inks and the direct cell exposure for in vitro effects assessment, thus implementing a life-cycle oriented approach in the assessment of the toxicity of CdTe QDs.

Stem Cells for Next Level Toxicity Testing in the 21st Century

The call for a paradigm change in toxicology from the United States National Research Council in 2007 initiates awareness for the invention and use of human-relevant alternative methods for toxicological hazard assessment. Simple 2D in vitro systems may serve as first screening tools, however, recent developments infer the need for more complex, multicellular organotypic models, which are superior in mimicking the complexity of human organs. In this review article most critical organs for toxicity assessment, i.e., skin, brain, thyroid system, lung, heart, liver, kidney, and intestine are discussed with regards to their functions in health and disease. Embracing the manifold modes-of-action how xenobiotic compounds can interfere with physiological organ functions and cause toxicity, the need for translation of such multifaceted organ features into the dish seems obvious. Currently used in vitro methods for toxicological applications and ongoing developments not yet arrived in toxicity testing are discussed, especially highlighting the potential of models based on embryonic stem cells and induced pluripotent stem cells of human origin. Finally, the application of innovative technologies like organs-on-a-chip and genome editing point toward a toxicological paradigm change moves into action.

Non-Animal Strategies for Toxicity Assessment of Nanoscale Materials: Role of Adverse Outcome Pathways in the Selection of Endpoints

Faster, cheaper, sensitive, and mechanisms-based animal alternatives are needed to address the safety assessment needs of the growing number of nanomaterials (NM) and their sophisticated property variants. Specifically, strategies that help identify and prioritize alternative schemes involving individual test models, toxicity endpoints, and assays for the assessment of adverse outcomes, as well as strategies that enable validation and refinement of these schemes for the regulatory acceptance are needed. In this review, two strategies 1) the current nanotoxicology literature review and 2) the adverse outcome pathways (AOPs) framework, a systematic process that allows the assembly of available mechanistic information concerning a toxicological response in a simple modular format, are presented. The review highlights 1) the most frequently assessed and reported ad hoc in vivo and in vitro toxicity measurements in the literature, 2) various AOPs of relevance to inhalation toxicity of NM that are presently under development, and 3) their applicability in identifying key events of toxicity for targeted in vitro assay development. Finally, using an existing AOP for lung fibrosis, the specific combinations of cell types, exposure and test systems, and assays that are experimentally supported and thus, can be used for assessing NM-induced lung fibrosis, are proposed.

Toxicity assessment of industrial engineered and airborne process-generated nanoparticles in a 3D human airway epithelial in vitro model

The advanced ceramic technology has been pointed out as a potentially relevant case of occupational exposure to nanoparticles (NP). Not only when nanoscale powders are being used for production, but also in the high-temperature processing of ceramic materials there is also a high potential for NP release into the workplace environment. In vitro toxicity of engineered NP (ENP) [antimony tin oxide (Sb₂O₃•SnO₂; ATO); zirconium oxide (ZrO₂)], as well as process-generated NP (PGNP), and fine particles (PGFP), was assessed in MucilAir™ cultures at air-liquid interface (ALI). Cultures were exposed during three consecutive days to varying doses of the aerosolized NP. General cytotoxicity [lactate dehydrogenase (LDH) release, WST-1 metabolization], (oxidative) DNA damage, and the levels of pro-inflammatory mediators (IL-8 and MCP-1) were assessed. Data revealed that ENP (5.56 µg ATO/cm² and 10.98 µg ZrO₂/cm²) only caused mild cytotoxicity at early timepoints (24 h), whereas cells seemed to recover quickly since no significant changes in cytotoxicity were observed at late timepoints (72 h). No meaningful effects of the ENP were observed regarding DNA damage and cytokine levels. PGFP affected cell viability at dose levels as low as ∼9 µg/cm², which was not seen for PGNP. However, exposure to PGNP (∼4.5 µg/cm²) caused an increase in oxidative DNA damage. These results indicated that PGFP and PGNP exhibit higher toxicity potential than ENP in mass per area unit. However, the presence of a mucociliary apparatus, as it occurs in vivo as a defense mechanism, seems to considerably attenuate the observed toxic effects. Our findings highlight the potential hazard associated with exposure to incidental NP in industrial settings.

Pre-validation of a Calu-3 epithelium cytotoxicity assay for predicting acute inhalation toxicity of chemicals

Although in vivo inhalation toxicity tests have been widely conducted, the testing of many chemicals is limited for economic and ethical reasons. Therefore, we previously developed an in vitro acute inhalation toxicity test method. The goal of the present pre-validation study was to evaluate the transferability, reproducibility, and predictive capacity of this method. After confirming the transferability of the Calu-3 epithelium cytotoxicity assay, reproducibility was evaluated using 20 test substances at three independent institutions. Cytotoxicity data were analyzed using statistical methods, including the intra-class correlation coefficient and Bland-Altman plots for within- and between-laboratory reproducibility. The assay for the 20 test substances showed excellent agreement within and between laboratories. To evaluate the predictive capacity, 77 test substances were analyzed for acute inhalation toxicity. Accuracy was measured using a cutoff of 40%, and the relevance was analyzed as a receiver-operating characteristic (ROC) curve. An accuracy of 72.73% was obtained, and the area under the ROC curve was 0.77, indicating moderate performance. In this study, we found that the in vitro acute inhalation toxicity test method demonstrated good reliability and relevance for predicting the acute toxicity of inhalable chemicals. Hence, this assay has potential as an alternative test for screening acutely toxic inhalants.

An interlaboratory in vitro aerosol exposure system reference study

Given the complexity of inhaled substances, the aerosol exposure environment has seen diversification and development of setups in conjunction with the evolving in vitro toxicology space. Each laboratory uses its in vitro exposure system differently (different protocols, adaptations, and biological analysis). Unfortunately, as systems diversify, so does the complexity of comparing multiple systems in a “standardized” manner. As yet, no one has compared simply whether these diverse systems can all generate a consistent aerosol stream, which is paramount prior to transit and exposure. This study has compared, at source, aerosol generation (using nicotine as an exposure marker) in nine in vitro whole-aerosol exposure setups (seven different systems) across five distinct geographically independent locations, including the UK, the USA, Switzerland, Germany, and Japan. The results demonstrate that, despite system-wide differences (adaptations, nuances, and application), these systems—when appropriately maintained and used under a prescribed set of established conditions can all generate a consistent and statistically comparable aerosol stream. These data will be invaluable for new researchers and established laboratories, so they may benchmark against this study. Finally, this interlaboratory comparison combined with the wealth of transit and exposure interface data, may help the environment move towards a truly validated and consistent approach to aerosol exposure. Such an approach could be replicated for other aerosolized products, such as e-cigarettes and heated tobacco products.

Comparison of the biological impact of aerosol of e-vapor device with MESH® technology and cigarette smoke on human bronchial and alveolar cultures

Exposure to aerosol from electronic vapor (e-vapor) products has been suggested to result in less risk of harm to smokers than cigarette smoke (CS) exposure. Although many studies on e-vapor products have tested the effects of liquid formulations on cell cultures, few have evaluated the effects of aerosolized formulations. We examined the effects of acute exposure to the aerosol of an e-vapor device that uses the MESH® technology (IQOS® MESH, Philip Morris International) and to CS from the 3R4F reference cigarette on human organotypic bronchial epithelial culture and alveolar triculture models. In contrast to 3R4F CS exposure, exposure to the IQOS MESH aerosol (Classic Tobacco flavor) did not cause cytotoxicity in bronchial epithelial cultures or alveolar tricultures despite its greater concentrations of deposited nicotine (3- and 4-fold, respectively). CS exposure caused a marked decrease in the frequency and active area of ciliary beating in bronchial cultures, whereas IQOS MESH aerosol exposure did not. Global mRNA expression and secreted protein profiles revealed a significantly lower impact of IQOS MESH aerosol exposure than 3R4F CS exposure. Overall, our whole aerosol exposure study shows a clearly reduced impact of IQOS MESH aerosol relative to CS in bronchial and alveolar cultures, even at greater nicotine doses.

Air-Liquid Interface Exposure of Lung Epithelial Cells to Low Doses of Nanoparticles to Assess Pulmonary Adverse Effects

Reliable and predictive in vitro assays for hazard assessments of manufactured nanomaterials (MNMs) are still limited. Specifically, exposure systems which more realistically recapitulate the physiological conditions in the lung are needed to predict pulmonary toxicity. To this end, air-liquid interface (ALI) systems have been developed in recent years which might be better suited than conventional submerged exposure assays. However, there is still a need for rigorous side-by-side comparisons of the results obtained with the two different exposure methods considering numerous parameters, such as different MNMs, cell culture models and read outs. In this study, human A549 lung epithelial cells and differentiated THP-1 macrophages were exposed under submerged conditions to two abundant types of MNMs i.e., ceria and titania nanoparticles (NPs). Membrane integrity, metabolic activity as well as pro-inflammatory responses were recorded. For comparison, A549 monocultures were also exposed at the ALI to the same MNMs. In the case of titania NPs, genotoxicity was also investigated. In general, cells were more sensitive at the ALI compared to under classical submerged conditions. Whereas ceria NPs triggered only moderate effects, titania NPs clearly initiated cytotoxicity, pro-inflammatory gene expression and genotoxicity. Interestingly, low doses of NPs deposited at the ALI were sufficient to drive adverse outcomes, as also documented in rodent experiments. Therefore, further development of ALI systems seems promising to refine, reduce or even replace acute pulmonary toxicity studies in animals.

Toxicological responses of BEAS-2B cells to repeated exposures to benzene, toluene, m-xylene, and mesitylene using air-liquid interface method

In order to reduce exposure to toxic chemicals, the European REACH regulation (1907/2006) recommends substituting toxic molecules with compounds that are less harmful to human health and the environment. Toluene is one of the most frequently used solvents in industries despite its toxicity. The objective of this study is to better understand and compare the toxicity of toluene and its homologues in a bronchial cell model. Thus, human bronchial BEAS-2B cells were exposed to steams of toluene, m-xylene, mesitylene (1,3,5-trimethylbenzene), and benzene (20 and 100 ppm). Exposure was carried out using an air-liquid interface (ALI) system (Vitrocell) during 1 h/day for 1, 3, or 5 days. Cytotoxicity, xenobiotic metabolism enzyme gene expression, and inflammatory response were evaluated following cell exposures. BEAS-2B cell exposure to toluene and its homologues revealed the involvement of major (CYP2E1) and minor metabolic pathways (CYP1A1). A late induction of genes (EPHX1, DHDH, ALDH2, and ALDH3B1) was measured from Day 3 and can be linked to the formation of metabolites. An increase in the secretion level of inflammatory markers (TNF-α, IL-6, IL-8, MCP-1, and GM-CSF) was also observed. In parallel, regulation between inflammatory mediators and the expression of transmembrane glycoprotein mucin MUC1 was also studied. This in vitro approach with ALI system points out the relevance of conducting repeated exposures to detect potential late effects. The difference recorded after cell exposure to toluene and its homologues highlights the importance of substitution principle.

Correlation and clinical relevance of animal models for inhaled pharmaceuticals and biopharmaceuticals

Nonclinical studies are fundamental for the development of inhaled drugs, as for any drug product, and for successful translation to clinical practice. They include in silico, in vitro, ex vivo and in vivo studies and are intended to provide a comprehensive understanding of the inhaled drug beneficial and detrimental effects. To date, animal models cannot be circumvented during drug development programs, acting as surrogates of humans to predict inhaled drug response, fate and toxicity. Herein, we review the animal models used during the different development stages of inhaled pharmaceuticals and biopharmaceuticals, highlighting their strengths and limitations.

Parametric Optimization of an Air-Liquid Interface System for Flow-Through Inhalation Exposure to Nanoparticles: Assessing Dosimetry and Intracellular Uptake of CeO2 Nanoparticles

Air-liquid interface (ALI) systems have been widely used in recent years to investigate the inhalation toxicity of many gaseous compounds, chemicals, and nanomaterials and represent an emerging and promising in vitro method to supplement in vivo studies. ALI exposure reflects the physiological conditions of the deep lung more closely to subacute in vivo inhalation scenarios compared to submerged exposure. The comparability of the toxicological results obtained from in vivo and in vitro inhalation data is still challenging. The robustness of ALI exposure scenarios is not yet well understood, but critical for the potential standardization of these methods. We report a cause-and-effect (C&E) analysis of a flow through ALI exposure system. The influence of five different instrumental and physiological parameters affecting cell viability and exposure parameters of a human lung cell line in vitro (exposure duration, relative humidity, temperature, CO2 concentration and flow rate) was investigated. After exposing lung epithelia cells to a CeO2 nanoparticle (NP) aerosol, intracellular CeO2 concentrations reached values similar to those found in a recent subacute rat inhalation study in vivo. This is the first study showing that the NP concentration reached in vitro using a flow through ALI system were the same as those in an in vivo study.

Invited review: human air-liquid-interface organotypic airway tissue models derived from primary tracheobronchial epithelial cells-overview and perspectives

The lung is an organ that is directly exposed to the external environment. Given the large surface area and extensive ventilation of the lung, it is prone to exposure to airborne substances, such as pathogens, allergens, chemicals, and particulate matter. Highly elaborate and effective mechanisms have evolved to protect and maintain homeostasis in the lung. Despite these sophisticated defense mechanisms, the respiratory system remains highly susceptible to environmental challenges. Because of the impact of respiratory exposure on human health and disease, there has been considerable interest in developing reliable and predictive in vitro model systems for respiratory toxicology and basic research. Human air-liquid-interface (ALI) organotypic airway tissue models derived from primary tracheobronchial epithelial cells have in vivo–like structure and functions when they are fully differentiated. The presence of the air-facing surface allows conducting in vitro exposures that mimic human respiratory exposures. Exposures can be conducted using particulates, aerosols, gases, vapors generated from volatile and semi-volatile substances, and respiratory pathogens. Toxicity data have been generated using nanomaterials, cigarette smoke, e-cigarette vapors, environmental airborne chemicals, drugs given by inhalation, and respiratory viruses and bacteria. Although toxicity evaluations using human airway ALI models require further standardization and validation, this approach shows promise in supplementing or replacing in vivo animal models for conducting research on respiratory toxicants and pathogens.

Cell-specific toxicity of short-term JUUL aerosol exposure to human bronchial epithelial cells and murine macrophages exposed at the air-liquid interface

Backgroud

JUUL, an electronic nicotine delivery system (ENDS), which first appeared on the US market in 2015, controled more than 75% of the US ENDS sales in 2018. JUUL-type devices are currently the most commonly used form of ENDS among youth in the US. In contrast to free-base nicotine contained in cigarettes and other ENDS, JUUL contains high levels of nicotine salt (35 or 59 mg/mL), whose cellular and molecular effects on lung cells are largely unknown. In the present study, we evaluated the in vitro toxicity of JUUL crème brûlée-flavored aerosols on 2 types of human bronchial epithelial cell lines (BEAS-2B, H292) and a murine macrophage cell line (RAW 264.7).

Methods

Human lung epithelial cells and murine macrophages were exposed to JUUL crème brûlée-flavored aerosols at the air–liquid interface (ALI) for 1-h followed by a 24-h recovery period. Membrane integrity, cytotoxicity, extracellular release of nitrogen species and reactive oxygen species, cellular morphology and gene expression were assessed.

Results

Crème brûlée-flavored aerosol contained elevated concentrations of benzoic acid (86.9 μg/puff), a well-established respiratory irritant. In BEAS-2B cells, crème brûlée-flavored aerosol decreased cell viability (≥ 50%) and increased nitric oxide (NO) production (≥ 30%), as well as iNOS gene expression. Crème brûlée-flavored aerosol did not affect the viability of either H292 cells or RAW macrophages, but increased the production of reactive oxygen species (ROS) by ≥ 20% in both cell types. While crème brûlée-flavored aerosol did not alter NO levels in H292 cells, RAW macrophages exposed to crème brûlée-flavored aerosol displayed decreased NO (≥ 50%) and down-regulation of the iNOS gene, possibly due to increased ROS. Additionally, crème brûlée-flavored aerosol dysregulated the expression of several genes related to biotransformation, inflammation and airway remodeling, including CYP1A1, IL-6, and MMP12 in all 3 cell lines.

Conclusion

Our results indicate that crème brûlée-flavored aerosol causes cell-specific toxicity to lung cells. This study contributes to providing scientific evidence towards regulation of nicotine salt-based products.

Choice of Differentiation Media Significantly Impacts Cell Lineage and Response to CFTR Modulators in Fully Differentiated Primary Cultures of Cystic Fibrosis Human Airway Epithelial Cells

In vitro cultures of primary human airway epithelial cells (hAECs) grown at air–liquid interface have become a valuable tool to study airway biology under normal and pathologic conditions, and for drug discovery in lung diseases such as cystic fibrosis (CF). An increasing number of different differentiation media, are now available, making comparison of data between studies difficult. Here, we investigated the impact of two common differentiation media on phenotypic, transcriptomic, and physiological features of CF and non-CF epithelia. Cellular architecture and density were strongly impacted by the choice of medium. RNA-sequencing revealed a shift in airway cell lineage; one medium promoting differentiation into club and goblet cells whilst the other enriched the growth of ionocytes and multiciliated cells. Pathway analysis identified differential expression of genes involved in ion and fluid transport. Physiological assays (intracellular/extracellular pH, Ussing chamber) specifically showed that ATP12A and CFTR function were altered, impacting pH and transepithelial ion transport in CF hAECs. Importantly, the two media differentially affected functional responses to CFTR modulators. We argue that the effect of growth conditions should be appropriately determined depending on the scientific question and that our study can act as a guide for choosing the optimal growth medium for specific applications.

An Alternative Perspective towards Reducing the Risk of Engineered Nanomaterials to Human Health

To elucidate the impact of human exposure to engineered nanomaterials, advanced in vitro models are a valid non-animal alternative. Despite significant gains over the last decade, implementation of these approaches remains limited. This work discusses the current state-of-the-art and how future developments can lead to advanced in vitro models better supporting nano-hazard assessment.

In vitro Alternatives to Acute Inhalation Toxicity Studies in Animal Models-A Perspective

When assessing the risk and hazard of a non-pharmaceutical compound, the first step is determining acute toxicity, including toxicity following inhalation. Inhalation is a major exposure route for humans, and the respiratory epithelium is the first tissue that inhaled substances directly interact with. Acute inhalation toxicity testing for regulatory purposes is currently performed only in rats and/or mice according to OECD TG403, TG436, and TG433 test guidelines. Such tests are biased by the differences in the respiratory tract architecture and function across species, making it difficult to draw conclusions on the potential hazard of inhaled compounds in humans. Research efforts have been therefore focused on developing alternative, human-relevant models, with emphasis on the creation of advanced In vitro models. To date, there is no In vitro model that has been accepted by regulatory agencies as a stand-alone replacement for inhalation toxicity testing in animals. Here, we provide a brief introduction to current OECD test guidelines for acute inhalation toxicity, the interspecies differences affecting the predictive value of such tests, and the current regulatory efforts to advance alternative approaches to animal-based inhalation toxicity studies. We then list the steps that should allow overcoming the current challenges in validating In vitro alternatives for the successful replacement of animal-based inhalation toxicity studies. These steps are inclusive and descriptive, and should be detailed when adopting in house-produced 3D cell models for inhalation tests. Hence, we provide a checklist of key parameters that should be reported in any future scientific publications for reproducibility and transparency.

A novel TEM grid sampler for airborne particles to measure the cell culture surface dose

The applied surface dose is a key parameter for the measurement of toxic effects of airborne particles by air liquid interface exposure of human lung cells. Besides online measurement of the deposited particle mass by quartz crystal microbalance frequently other dose metrics such as particle size distribution, surface and agglomeration state are required. These particle properties and their spatial distribution can be determined by digital processing of micrographs obtained by transmission electron microscopy (TEM). Here, we report the development and characterization of a novel holder for film coated TEM copper grids, which allows for sampling under identical geometric and ambient conditions as in a cell culture chamber. The sample holder avoids artefacts by reliable grounding of the grids and improves handling of the grids to prevent damage of the sensitive film. This sample holder is applied during exposure experiments with titanium dioxide nanoparticles. The measured dose of 0.2 µg/cm² corresponds well to the mass loading signal of the quartz crystal microbalance. Additionally, the spatial distribution of particles on the sampling surface shows a good homogeneity of deposition. This novel sampling method allows verifying other dosimetry methods and gives additional information about particle properties and homogeneity of the dose.

Nanoparticle exposure and hazard in the ceramic industry: an overview of potential sources, toxicity and health effects

The ceramic industry is an industrial sector of great impact in the global economy that has been benefiting from advances in materials and processing technologies. Ceramic manufacturing has a strong potential for airborne particle formation and emission, namely of ultrafine particles (UFP) and nanoparticles (NP), meaning that workers of those industries are at risk of potential exposure to these particles. At present, little is known on the impact of engineered nanoparticles (ENP) on the environment and human health and no established Occupational Exposure Limits (OEL) or specific regulations to airborne nanoparticles (ANP) exposure exist raising concerns about the possible consequences of such exposure. In this paper, we provide an overview of the current knowledge on occupational exposure to NP in the ceramic industry and their impact on human health. Possible sources and exposure scenarios, a summary of the existing methods for evaluation and monitoring of ANP in the workplace environment and proposed Nano Reference Values (NRV) for different classes of NP are presented. Case studies on occupational exposure to ANP generated at different stages of the ceramic manufacturing process are described. Finally, the toxicological potential of intentional and unintentional ANP that have been identified in the ceramic industry workplace environment is discussed based on the existing evidence from in vitro and in vivo inhalation toxicity studies.

Toxicity and Gene Expression Profiling of Copper- and Titanium-Based Nanoparticles Using Air-Liquid Interface Exposure

To assess the toxicity of nanomaterials, most in vitro studies have been performed under submerged conditions, which do not reflect physiological conditions upon inhalation. An air-liquid interface (ALI) exposure may provide more reliable data on dosimetry and prevent interactions with cell culture media components. Therefore, an ALI exposure was combined with a high-throughput RT-qPCR approach to evaluate the toxicological potential of CuO and TiO₂ nanoparticles (NP) in A549 cells. While TiO₂ NP did not show any cytotoxicity or other effects compromising genomic stability up to 25.8 μg/cm², CuO NP revealed a dose-dependent cytotoxicity, starting at 4.9 μg/cm². Furthermore, CuO NP altered distinct gene expression patterns indicative for disturbed metal homeostasis, stress response, and DNA damage induction. Thus, induction of metal homeostasis associated genes (MT1X, MT2A) at 0.4 μg/cm² and higher suggested uptake and intracellular dissolution of CuO NP, which was verified by a dose-dependent increase in intracellular copper concentration. Starting at 4.9 μg/cm², oxidative stress markers (HMOX1, HSPA1A) were induced dose-dependently, supported by elevated ROS levels. Furthermore, a dose-dependent induction of genes associated with DNA damage response (DDIT3, GADD45A) was observed, in concordance with an increase in DNA strand breaks. Finally, transcriptional data suggested the induction of apoptosis at high doses, while flow cytometric analysis revealed increased numbers of either late apoptotic or necrotic cells and clearly necrotic cells at the highest concentrations. Thus, an ALI cell culture system was successfully combined with a comprehensive high-throughput RT-qPCR system, allowing the quantification of NP deposition and their impact on genomic stability. For CuO NP, in principle the data confirm observations made under submerged conditions with respect to intracellular copper ion release, as well as oxidative and genotoxic stress response. However, the results derived from ALI exposure allow the assessment of dose-response-relationships as well as the comparison of relative toxic potencies of different NP.

Use of EpiAlveolar Lung Model to Predict Fibrotic Potential of Multiwalled Carbon Nanotubes

Expansion in production and commercial use of nanomaterials increases the potential human exposure during the lifecycle of these materials (production, use, and disposal). Inhalation is a primary route of exposure to nanomaterials; therefore it is critical to assess their potential respiratory hazard. Herein, we developed a three-dimensional alveolar model (EpiAlveolar) consisting of human primary alveolar epithelial cells, fibroblasts, and endothelial cells, with or without macrophages for predicting long-term responses to aerosols. Following thorough characterization of the model, proinflammatory and profibrotic responses based on the adverse outcome pathway concept for lung fibrosis were assessed upon repeated subchronic exposures (up to 21 days) to two types of multiwalled carbon nanotubes (MWCNTs) and silica quartz particles. We simulate occupational exposure doses for the MWCNTs (1-30 μg/cm²) using an air-liquid interface exposure device (VITROCELL Cloud) with repeated exposures over 3 weeks. Specific key events leading to lung fibrosis, such as barrier integrity and release of proinflammatory and profibrotic markers, show the responsiveness of the model. Nanocyl induced, in general, a less pronounced reaction than Mitsui-7, and the cultures with human monocyte-derived macrophages (MDMs) showed the proinflammatory response at later time points than those without MDMs. In conclusion, we present a robust alveolar model to predict inflammatory and fibrotic responses upon exposure to MWCNTs.

Choosing the Right Differentiation Medium to Develop Mucociliary Phenotype of Primary Nasal Epithelial Cells In Vitro

In vitro differentiation of airway epithelium is of interest for respiratory tissue engineering and studying airway diseases. Both applications benefit from the use of primary cells to maintain a mucociliated phenotype and thus physiological functionality. Complex differentiation procedures often lack standardization and reproducibility. To alleviate these shortfalls, we compared differentiation behavior of human nasal epithelial cells in four differentiation media. Cells were differentiated at the air-liquid interface (ALI) on collagen-coated inserts. Mucociliary differentiation status after five weeks was analyzed by electron microscopy, histology and immunohistochemistry. The amount of ciliation was estimated and growth factor concentrations were evaluated using ELISA. We found that retinoic-acid-supplemented mixture of DMEM and Airway Epithelial Cell Growth Medium gave most promising results to obtain ciliated and mucus producing nasal epithelium in vitro. We discovered the balance between retinoic acid (RA), vascular endothelial growth factor (VEGF), epidermal growth factor (EGF) and fibroblast growth factor β (FGF-β) to be relevant for differentiation. We could show that low VEGF, EGF and FGF-β concentrations in medium correspond to absent ciliation in specific donors. Therefore, our results may in future facilitate donor selection and non-invasive monitoring of ALI cultures and by this contribute to improved standardization of epithelial in vitro culture.

Comparison of experimentally measured and computational fluid dynamic predicted deposition and deposition uniformity of monodisperse solid particles in the Vitrocell® AMES 48 air-liquid-interface in-vitro exposure system

Accurately determining the delivered dose is critical to understanding biological response due to cell exposure to chemical constituents in aerosols. Deposition efficiency and uniformity of deposition was measured experimentally using monodisperse solid fluorescent particles with mass median aerodynamic diameters (MMAD) of 0.51, 1.1, 2.2 and 3.3 μm in the Vitrocell® AMES 48 air-liquid-interface (ALI) in vitro exposure system. Experimental results were compared with computational fluid dynamic, (CFD; using both Lagrangian and Eulerian approaches) predicted deposition efficiency and uniformity for a single row (N = 6) of petri dishes in the Vitrocell® AMES 48 system. The average experimentally measured deposition efficiency ranged from 0.007% to 0.43% for 0.51-3.3 μm MMAD particles, respectively. There was good agreement between average experimentally measured and the CFD predicted particle deposition efficiency, regardless of approach. Experimentally measured and CFD predicted average uniformity of deposition was greater than 45% of the mean for all particle diameters. During this work a new design was introduced by the manufacturer and evaluated using Lagragian CFD. Lagragian CFD predictions showed better uniformity of deposition, but reduced deposition efficiency with the new design. Deposition efficiency and variability in particle deposition across petri dishes for solid particles should be considered when designing exposure regimens using the Vitrocell® AMES 48 ALI in vitro exposure system.

Dr. Daniel Acosta and In Vitro toxicology at the U.S. Food and Drug Administration's National Center for Toxicological Research

For the past five years, Dr. Daniel Acosta has served as the Deputy Director of Research at the National Center for Toxicological Research (NCTR), a principle research laboratory of the U.S. Food and Drug Administration (FDA). Over his career at NCTR, Dr. Acosta has had a major impact on developing and promoting the use of in vitro assays in regulatory toxicity and product safety assessments. As Dr. Acosta nears his retirement we have dedicated this paper to his many accomplishments at the NCTR. Described within this paper are some of the in vitro studies that have been conducted under Dr. Acosta's leadership. These studies include toxicological assessments involving developmental effects, and the development and application of in vitro reproductive, heart, liver, neurological and airway cell and tissue models.

Innovative preclinical models for pulmonary drug delivery research

Introduction: Pulmonary drug delivery is a complex field of research combining physics which drive aerosol transport and deposition and biology which underpins efficacy and toxicity of inhaled drugs. A myriad of preclinical methods, ranging from in-silico to in-vitro, ex-vivo and in-vivo, can be implemented.Areas covered: The present review covers in-silico mathematical and computational fluid dynamics modelization of aerosol deposition, cascade impactor technology to estimated drug delivery and deposition, advanced in-vitro cell culture methods and associated aerosol exposure, lung-on-chip technology, ex-vivo modeling, in-vivo inhaled drug delivery, lung imaging, and longitudinal pharmacokinetic analysis.Expert opinion: No single preclinical model can be advocated; all methods are fundamentally complementary and should be implemented based on benefits and drawbacks to answer specific scientific questions. The overall best scientific strategy depends, among others, on the product under investigations, inhalation device design, disease of interest, clinical patient population, previous knowledge. Preclinical testing is not to be separated from clinical evaluation, as small proof-of-concept clinical studies or conversely large-scale clinical big data may inform preclinical testing. The extend of expertise required for such translational research is unlikely to be found in one single laboratory calling for the setup of multinational large-scale research consortiums.

Differences in cytotoxicity of lung epithelial cells exposed to titanium dioxide nanofibers and nanoparticles: Comparison of air-liquid interface and submerged cell cultures

Air Liquid Interface (ALI) system has emerged as a useful tool for toxicity evaluation of nanomaterials related to inhalation since the system mimics the aerosol exposure. We compared the biological responses of lung epithelial cells exposed to titanium dioxide (TiO₂) nanofibers and nanoparticles in ALI and submerged cell cultures systems. Cells were exposed to 2 and 10 μg/cm² for 24 h, 48 h and 72 h and LDH release, TiO₂ internalization, DNA-double strand breaks (DSBs) and ROS production were assessed. LDH release was similar in both systems and particles had higher cytoplasmic uptake in submerged systems. Both TiO₂ types were located in the cytoplasm but nanofibers had nuclear uptake regardless to the system tested. Cells exposed to TiO₂ nanofibers had higher DSBs in the ALI system than in submerged cell cultures but cells exposed to TiO₂ nanoparticles had similar DSBs in both systems. ROS production was higher in cells exposed to TiO₂ nanofibers compared to cells exposed to TiO₂ nanoparticles. In conclusion, cytotoxicity of lung epithelial cells was similar in ALI or submerged cell cultures, however cells exposed to TiO₂ nanofibers displayed higher toxicity than cells exposed to TiO₂ nanoparticles.

Toward Rigorous Materials Production: New Approach Methodologies Have Extensive Potential to Improve Current Safety Assessment Practices

Advanced material development, including at the nanoscale, comprises costly and complex challenges coupled to ensuring human and environmental safety. Governmental agencies regulating safety have announced interest toward acceptance of safety data generated under the collective term New Approach Methodologies (NAMs), as such technologies/approaches offer marked potential to progress the integration of safety testing measures during innovation from idea to product launch of nanomaterials. Divided in overall eight main categories, searchable databases for grouping and read across purposes, exposure assessment and modeling, in silico modeling of physicochemical structure and hazard data, in vitro high-throughput and high-content screening assays, dose-response assessments and modeling, analyses of biological processes and toxicity pathways, kinetics and dose extrapolation, consideration of relevant exposure levels and biomarker endpoints typify such useful NAMs. Their application generally agrees with articulated stakeholder needs for improvement of safety testing procedures. They further fit for inclusion and add value in nanomaterials risk assessment tools. Overall 37 of 50 evaluated NAMs and tiered workflows applying NAMs are recommended for considering safer-by-design innovation, including guidance to the selection of specific NAMs in the eight categories. An innovation funnel enriched with safety methods is ultimately proposed under the central aim of promoting rigorous nanomaterials innovation.

Toxicological assessment of nanoparticle interactions with the pulmonary system

Nanoparticle(NP)-based materials have breakthrough applications in many fields of life, such as in engineering, communications and textiles industries; food and bioenvironmental applications; medicines and cosmetics, etc. Biomedical applications of NPs are very active areas of research with successful translation to pharmaceutical and clinical uses overcoming both pharmaceutical and clinical challenges. Although the attractiveness and enhanced applications of these NPs stem from their exceptional properties at the nanoscale size, i.e. 1-1000 nm, they exhibit completely different physicochemical profiles and, subsequently, toxicological profiles from their parent bulk materials. Hence, the clinical evaluation and toxicological assessment of NPs interactions within biological systems are continuously evolving to ensure their safety at the nanoscale. The pulmonary system is one of the primary routes of exposure to airborne NPs either intentionally, via aerosolized nanomedicines targeting pulmonary pathologies such as cancer or asthma, or unintentionally, via natural NPs and anthropogenic (man-made) NPs. This review presents the state-of-the-art, contemporary challenges, and knowledge gaps in the toxicological assessment of NPs interactions with the pulmonary system. It highlights the main mechanisms of NP toxicity, factors influencing their toxicity, the different toxicological assessment methods and their drawbacks, and the recent NP regulatory guidelines based on literature collected from the research pool of NPs interactions with lung cell lines, in vivo inhalation studies, and clinical trials.

Mechanical strain mimicking breathing amplifies alterations in gene expression induced by SiO2 NPs in lung epithelial cells

The effects of engineered nanomaterials on human health are still intensively studied in order to facilitate their safe application. However, relatively little is known how mechanical strain as induced in alveolar epithelial cells by breathing movements modifies biological responses to nanoparticles (NPs). In this study, A549 cells as a model for alveolar epithelial cells were exposed to 25 nm amorphous colloidal silica NPs under dynamic and static culture conditions. Gene array data, qPCR, and ELISA revealed an amplified effect of NPs when cells were mechanically stretched in order to model the physiological mechanical deformation during breathing. In contrast, treatment of cells with either strain or NPs alone only led to minor changes in gene expression or interleukin-8 (IL-8) secretion. Confocal microscopy revealed that stretching does not lead to an increased internalization of NPs, indicating that elevated intracellular NP accumulation is not responsible for the observed effect. Gene expression alterations induced by combined exposure to NPs and mechanical strain showed a high similarity to those known to be induced by TNF-α. This study suggests that the inclusion of mechanical strain into in vitro models of the human lung may have a strong influence on the test results.

In vitro exposure of a 3D-tetraculture representative for the alveolar barrier at the air-liquid interface to silver particles and nanowires

BACKGROUND

The present study aimed to evaluate the potential differences in the biological effects of two types of spherical silver particles of 20 and 200 nm (Ag20 and Ag200), and of PVP-coated silver nanowires (AgNWs) with a diameter of 50 nm and length up to 50 μm, using a complex 3D model representative for the alveolar barrier cultured at air-liquid interface (ALI). The alveolar model was exposed to 0.05, 0.5 and 5 μg/cm2 of test compounds at ALI using a state-of-the-art exposure system (Vitrocell™Cloud System). Endpoints related to the oxidative stress induction, anti-oxidant defence mechanisms, pro-inflammatory responses and cellular death were selected to evaluate the biocompatibility of silver particles and nanowires (AgNMs) and to further ascribe particular biological effects to the different morphologic properties between the three types of AgNMs evaluated.

RESULTS

Significant cytotoxic effect was observed for all three types of AgNMs at the highest tested doses. The increased mRNA levels of the pro-apoptotic gene CASP7 suggests that apoptosis may occur after exposure to AgNWs. All three types of AgNMs increased the mRNA level of the anti-oxidant enzyme HMOX-1 and of the metal-binding anti-oxidant metallothioneins (MTs), with AgNWs being the most potent inducer. Even though all types of AgNMs induced the nuclear translocation of NF-kB, only AgNWs increased the mRNA level of pro-inflammatory mediators. The pro-inflammatory response elicited by AgNWs was further confirmed by the increased secretion of the 10 evaluated interleukins.

CONCLUSION

In the current study, we demonstrated that the direct exposure of a complex tetra-culture alveolar model to different types of AgNMs at ALI induces shape- and size-specific biological responses. From the three AgNMs tested, AgNWs were the most potent in inducing biological alterations. Starting from 50 ng/cm2, a dose representative for an acute exposure in a high exposure occupational setting, AgNWs induced prominent changes indicative for a pro-inflammatory response. Even though the acute responses towards a dose representative for a full-lifetime exposure were also evaluated, chronic exposure scenarios at low dose are still unquestionably needed to reveal the human health impact of AgNMs during realistic conditions.

Validation of the CULTEX® Radial Flow System for the assessment of the acute inhalation toxicity of airborne particles

The CULTEX® Radial Flow System (RFS) is a modular in vitro system for the homogenous exposure of cells to airborne particles at the air-liquid interface (ALI). A former pre-validation study successfully demonstrated the general applicability of the CULTEX® RFS and its transferability, stability and reproducibility. Based on these results, the methodology was optimized, validated and prediction models for acute inhalation hazards were established. Cell viability of A549 cells after ALI exposure to 20 pre-selected test substances was assessed in three independent laboratories. Cytotoxicity of test substances was compared to the respective incubator controls and used as an indicator of toxicity. Substances were considered to exert an acute inhalation hazard when viability decreased below 50% (prediction model (PM) 50%) or 75% (PM 75%) at any of three exposure doses (25, 50 or 100 μg/cm²). Results were then compared to existing in vivo data and revealed an overall concordance of 85%, with a specificity of 83% and a sensitivity of 88%. Depending on the applied PM, the within-laboratory and between-laboratory reproducibility ranged from 90 to 100%. In summary, the CULTEX® RFS was proven as a transferable, reproducible and well predictive screening method for the qualitative assessment of the acute pulmonary cytotoxicity of airborne particles.

Responsiveness assessment of a 3D tetra-culture alveolar model exposed to diesel exhaust particulate matter

The aim of the current study was to evaluate the responses of a 3D tetra-culture alveolar model cultivated at the air-liquid-interface (ALI) after apical exposure to diesel exhaust particulate matter (DEPM) based on the three-tiered oxidative stress concept. The alveolar model exposed to increasing doses of DEPM (1.75-5 μg/cm²) responded with increasing activity of the anti-oxidant defense mechanisms (Nrf2 translocation, increased gene expression for anti-oxidant proteins and increased HMOX-1 synthesis) (tier 1). Higher exposure generated a proinflammatory response (NF-kB translocation, increased gene expression of pro-inflammatory cytokines and adhesion molecules, and increased IL-6 and IL-8 synthesis) (tier 2) and, finally, the highest doses applied resulted in a decrease of cell viability due to necrosis (extra-cellular release of LDH) or apoptosis (increased expression of the pro-apoptotic genes CASP7 and FAS) (tier 3). Overall, the results of our study demonstrate that the 3D tetra-culture model when directly exposed to DEPM potently generates a realistic response according to the three-tiered oxidative stress concept. Further evaluation and benchmarking against currently used in vivo rodent models is needed to show its suitability, and to serve in the future as an alternative for in vivo studies in the hazard evaluation of inhalable irritants.