Use of in vitro 3D tissue models in genotoxicity testing: Strategic fit, validation status and way forward. Report of the working group from the 7th International Workshop on Genotoxicity Testing (IWGT)

Interpretation of in vitro concentration-response data for risk assessment and regulatory decision-making: Report from the 2022 IWGT quantitative analysis expert working group meeting

Quantitative risk assessments of chemicals are routinely performed using in vivo data from rodents; however, there is growing recognition that non-animal approaches can be human-relevant alternatives. There is an urgent need to build confidence in non-animal alternatives given the international support to reduce the use of animals in toxicity testing where possible. In order for scientists and risk assessors to prepare for this paradigm shift in toxicity assessment, standardization and consensus on in vitro testing strategies and data interpretation will need to be established. To address this issue, an Expert Working Group (EWG) of the 8th International Workshop on Genotoxicity Testing (IWGT) evaluated the utility of quantitative in vitro genotoxicity concentration-response data for risk assessment. The EWG first evaluated available in vitro methodologies and then examined the variability and maximal response of in vitro tests to estimate biologically relevant values for the critical effect sizes considered adverse or unacceptable. Next, the EWG reviewed the approaches and computational models employed to provide human-relevant dose context to in vitro data. Lastly, the EWG evaluated risk assessment applications for which in vitro data are ready for use and applications where further work is required. The EWG concluded that in vitro genotoxicity concentration-response data can be interpreted in a risk assessment context. However, prior to routine use in regulatory settings, further research will be required to address the remaining uncertainties and limitations.

The frontline of alternatives to animal testing: novel in vitro skin model application in drug development and evaluation

The FDA Modernization Act 2.0 has brought nonclinical drug evaluation into a new era. In vitro models are widely used and play an important role in modern drug development and evaluation, including early candidate drug screening and preclinical drug efficacy and toxicity assessment. Driven by regulatory steering and facilitated by well-defined physiology, novel in vitro skin models are emerging rapidly, becoming the most advanced area in alternative testing research. The revolutionary technologies bring us many in vitro skin models, either laboratory-developed or commercially available, which were all built to emulate the structure of the natural skin to recapitulate the skin's physiological function and particular skin pathology. During the model development, how to achieve balance among complexity, accessibility, capability, and cost-effectiveness remains the core challenge for researchers. This review attempts to introduce the existing in vitro skin models, align them on different dimensions, such as structural complexity, functional maturity, and screening throughput, and provide an update on their current application in various scenarios within the scope of chemical testing and drug development, including testing in genotoxicity, phototoxicity, skin sensitization, corrosion/irritation. Overall, the review will summarize a general strategy for in vitro skin model to enhance future model invention, application, and translation in drug development and evaluation.

Assessing the Genotoxicity of Cellulose Nanomaterials in a Co-Culture of Human Lung Epithelial Cells and Monocyte-Derived Macrophages

Cellulose micro/nanomaterials (CMNMs) are innovative materials with a wide spectrum of industrial and biomedical applications. Although cellulose has been recognized as a safe material, the unique properties of its nanosized forms have raised concerns about their safety for human health. Genotoxicity is an endpoint that must be assessed to ensure that no carcinogenic risks are associated with exposure to nanomaterials. In this study, we evaluated the genotoxicity of two types of cellulose micro/nanofibrils (CMF and CNF) and one sample of cellulose nanocrystals (CNC), obtained from industrial bleached Eucalyptus globulus kraft pulp. For that, we exposed co-cultures of human alveolar epithelial A549 cells and THP-1 monocyte-derived macrophages to a concentration range of each CMNM and used the micronucleus (MN) and comet assays. Our results showed that only the lowest concentrations of the CMF sample were able to induce DNA strand breaks (FPG-comet assay). However, none of the three CMNMs produced significant chromosomal alterations (MN assay). These findings, together with results from previous in vitro studies using monocultures of A549 cells, indicate that the tested CNF and CNC are not genotoxic under the conditions tested, while the CMF display a low genotoxic potential.

Development of a micronucleus test using the EpiAirway™ organotypic human airway model

BACKGROUND

The use of organotypic human tissue models in genotoxicity has increased as an alternative to animal testing. Genotoxicity is generally examined using a battery of in vitro assays such as Ames and micronucleus (MN) tests that cover gene mutations and structural and numerical chromosome aberrations. At the 7th International Workshop on Genotoxicity Testing, working group members agreed that the skin models have reached an advanced stage of maturity, while further efforts in liver and airway models are needed [Pfuhler et al., Mutat. Res. 850-851 (2020) 503135]. Organotypic human airway model is composed of fully differentiated and functional respiratory epithelium. However, because cell proliferation in organotypic airway models is thought to be less active, assessing their MN-inducing potential is an issue, even in the cytokinesis-blocking approach using cytochalasin B (CB) [Wang et al., Environ. Mol. Mutagen. 62 (2021) 306-318]. Here, we developed a MN test using EpiAirway™ in which epidermal growth factor (EGF) was included as a stimulant of cell division.

RESULTS

By incubating EpiAirway™ tissue with medium containing various concentrations of CB, we found that the percentage of binucleated cells (%BNCs) almost plateaued at 3 μg/mL CB for 72 h incubation. Additionally, we confirmed that EGF stimulation with CB incubation produced an additional increase in %BNCs with a peak at 5 ng/mL EGF. Transepithelial electrical resistance measurement and tissue histology revealed that CB incubation caused the reduced barrier integrity and cyst formation in EpiAirway™. Adenylate kinase assay confirmed that the cytotoxicity increased with each day of culture in the CB incubation period with EGF stimulation. These results indicated that chemical treatment should be conducted prior to CB incubation. Under these experimental conditions, it was confirmed that the frequency of micronucleated cells was dose-dependently increased by apical applications of two clastogens, mitomycin C and methyl methanesulfonate, and an aneugen, colchicine, at the subcytotoxic concentrations assessed in %BNCs.

CONCLUSIONS

Well-studied genotoxicants demonstrated capability in an organotypic human airway model as a MN test system. For further utilization, investigations of aerosol exposure, repeating exposure protocol, and metabolic activation are required.

High-throughput micronucleus assay using three-dimensional HepaRG spheroids for in vitro genotoxicity testing

The in vitro micronucleus (MN) assay is a component of most test batteries used in assessing potential genotoxicity. Our previous study adapted metabolically competent HepaRG cells to the high-throughput (HT) flow-cytometry-based MN assay for genotoxicity assessment (Guo et al. in J Toxicol Environ Health A 83:702-717, 2020b, https://doi.org/10.1080/15287394.2020.1822972 ). We also demonstrated that, compared to HepaRG cells grown as two-dimensional (2D) cultures, 3D HepaRG spheroids have increased metabolic capacity and improved sensitivity in detecting DNA damage induced by genotoxicants using the comet assay (Seo et al. in ALTEX 39:583-604, 2022, https://doi.org/10.14573/altex.22011212022 ). In the present study, we have compared the performance of the HT flow-cytometry-based MN assay in HepaRG spheroids and 2D HepaRG cells by testing 34 compounds, including 19 genotoxicants or carcinogens and 15 compounds that show different genotoxic responses in vitro and in vivo. 2D HepaRG cells and spheroids were exposed to the test compounds for 24 h, followed by an additional 3- or 6-day incubation with human epidermal growth factor to stimulate cell division. The results demonstrated that HepaRG spheroids showed generally higher sensitivity in detecting several indirect-acting genotoxicants (require metabolic activation) compared to 2D cultures, with 7,12-dimethylbenzanthracene and N-nitrosodimethylamine inducing higher % MN formation along with having significantly lower benchmark dose values for MN induction in 3D spheroids. These data suggest that 3D HepaRG spheroids can be adapted to the HT flow-cytometry-based MN assay for genotoxicity testing. Our findings also indicate that integration of the MN and comet assays improved the sensitivity for detecting genotoxicants that require metabolic activation. These results suggest that HepaRG spheroids may contribute to New Approach Methodologies for genotoxicity assessment.

Combined Toxic Effects of BPA and Its Two Analogues BPAP and BPC in a 3D HepG2 Cell Model

Bisphenol A (BPA) is one of the most commonly used substances in the manufacture of various everyday products. Growing concerns about its hazardous properties, including endocrine disruption and genotoxicity, have led to its gradual replacement by presumably safer analogues in manufacturing plastics. The widespread use of BPA and, more recently, its analogues has increased their residues in the environment. However, our knowledge of their toxicological profiles is limited and their combined effects are unknown. In the present study, we investigated the toxic effects caused by single bisphenols and by the combined exposure of BPA and its two analogues, BPAP and BPC, after short (24-h) and prolonged (96-h) exposure in HepG2 spheroids. The results showed that BPA did not reduce cell viability in HepG2 spheroids after 24-h exposure. In contrast, BPAP and BPC affected cell viability in HepG2 spheroids. Both binary mixtures (BPA/BPAP and BPA/BPC) decreased cell viability in a dose-dependent manner, but the significant difference was only observed for the combination of BPA/BPC (both at 40 µM). After 96-h exposure, none of the BPs studied affected cell viability in HepG2 spheroids. Only the combination of BPA/BPAP decreased cell viability in a dose-dependent manner that was significant for the combination of 4 µM BPA and 4 µM BPAP. None of the BPs and their binary mixtures studied affected the surface area and growth of spheroids as measured by planimetry. In addition, all BPs and their binary mixtures studied triggered oxidative stress, as measured by the production of reactive oxygen species and malondialdehyde, at both exposure times. Overall, the results suggest that it is important to study the effects of BPs as single compounds. It is even more important to study the effects of combined exposures, as the combined effects may differ from those induced by single compounds.

Measuring DNA modifications with the comet assay: a compendium of protocols

The comet assay is a versatile method to detect nuclear DNA damage in individual eukaryotic cells, from yeast to human. The types of damage detected encompass DNA strand breaks and alkali-labile sites (e.g., apurinic/apyrimidinic sites), alkylated and oxidized nucleobases, DNA-DNA crosslinks, UV-induced cyclobutane pyrimidine dimers and some chemically induced DNA adducts. Depending on the specimen type, there are important modifications to the comet assay protocol to avoid the formation of additional DNA damage during the processing of samples and to ensure sufficient sensitivity to detect differences in damage levels between sample groups. Various applications of the comet assay have been validated by research groups in academia, industry and regulatory agencies, and its strengths are highlighted by the adoption of the comet assay as an in vivo test for genotoxicity in animal organs by the Organisation for Economic Co-operation and Development. The present document includes a series of consensus protocols that describe the application of the comet assay to a wide variety of cell types, species and types of DNA damage, thereby demonstrating its versatility.

Assessing the in vitro toxicity of airborne (nano)particles to the human respiratory system: from basic to advanced models

Several studies have been conducted to address the potential adverse health risks attributed to exposure to nanoscale materials. While in vivo studies are fundamental for identifying the relationship between dose and occurrence of adverse effects, in vitro model systems provide important information regarding the mechanism(s) of action at the molecular level. With a special focus on exposure to inhaled (nano)particulate material toxicity assessment, this review provides an overview of the available human respiratory models and exposure systems for in vitro testing, advantages, limitations, and existing investigations using models of different complexity. A brief overview of the human respiratory system, pathway and fate of inhaled (nano)particles is also presented.

The State of the Art and Challenges of In Vitro Methods for Human Hazard Assessment of Nanomaterials in the Context of Safe-by-Design

The Safe-by-Design (SbD) concept aims to facilitate the development of safer materials/products, safer production, and safer use and end-of-life by performing timely SbD interventions to reduce hazard, exposure, or both. Early hazard screening is a crucial first step in this process. In this review, for the first time, commonly used in vitro assays are evaluated for their suitability for SbD hazard testing of nanomaterials (NMs). The goal of SbD hazard testing is identifying hazard warnings in the early stages of innovation. For this purpose, assays should be simple, cost-effective, predictive, robust, and compatible. For several toxicological endpoints, there are indications that commonly used in vitro assays are able to predict hazard warnings. In addition to the evaluation of assays, this review provides insights into the effects of the choice of cell type, exposure and dispersion protocol, and the (in)accurate determination of dose delivered to cells on predictivity. Furthermore, compatibility of assays with challenging advanced materials and NMs released from nano-enabled products (NEPs) during the lifecycle is assessed, as these aspects are crucial for SbD hazard testing. To conclude, hazard screening of NMs is complex and joint efforts between innovators, scientists, and regulators are needed to further improve SbD hazard testing.

Different Sensitivity of Advanced Bronchial and Alveolar Mono- and Coculture Models for Hazard Assessment of Nanomaterials

For the next-generation risk assessment (NGRA) of chemicals and nanomaterials, new approach methodologies (NAMs) are needed for hazard assessment in compliance with the 3R's to reduce, replace and refine animal experiments. This study aimed to establish and characterize an advanced respiratory model consisting of human epithelial bronchial BEAS-2B cells cultivated at the air-liquid interface (ALI), both as monocultures and in cocultures with human endothelial EA.hy926 cells. The performance of the bronchial models was compared to a commonly used alveolar model consisting of A549 in monoculture and in coculture with EA.hy926 cells. The cells were exposed at the ALI to nanosilver (NM-300K) in the VITROCELL^® Cloud. After 24 h, cellular viability (alamarBlue assay), inflammatory response (enzyme-linked immunosorbent assay), DNA damage (enzyme-modified comet assay), and chromosomal damage (cytokinesis-block micronucleus assay) were measured. Cytotoxicity and genotoxicity induced by NM-300K were dependent on both the cell types and model, where BEAS-2B in monocultures had the highest sensitivity in terms of cell viability and DNA strand breaks. This study indicates that the four ALI lung models have different sensitivities to NM-300K exposure and brings important knowledge for the further development of advanced 3D respiratory in vitro models for the most reliable human hazard assessment based on NAMs.

A review of challenges and prospects of 3D cell-based culture models used for studying drug induced liver injury during early phases of drug development

Drug-induced liver injury (DILI) is the leading cause of compound attrition during drug development. Over the years, a battery of in-vitro cell culture toxicity tests is being conducted to evaluate the toxicity of compounds prior to testing in laboratory animals. Two-dimensional (2D) in-vitro cell culture models are commonly used and have provided a great deal of knowledge; however, these models often fall short in mimicking natural structures of tissues in-vivo. Testing in humans is the most logical method, but unfortunately there are ethical limitations associated with human tests. To overcome these limitations better human-relevant, predictive models are required. The past decade has witnessed significant efforts towards the development of three-dimensional (3D) in-vitro cell culture models better mimicking in-vivo physiology. 3D cell culture has advantages in being representative of the interactions of cells in-vivo and when validated can act as an interphase between 2D cell culture models and in-vivo animal models. The current review seeks to provide an overview of the challenges that make biomarkers used for detection of DILI not to be sensitive enough during drug development and explore how 3D cell culture models can be used to address the gap with the current models.

Adverse (geno)toxic effects of bisphenol A and its analogues in hepatic 3D cell model

Bisphenol A (BPA) is one of the most widely used and versatile chemical compounds in polymer additives and epoxy resins for manufacturing a range of products for human applications. It is known as endocrine disruptor, however, there is growing evidence that it is genotoxic. Because of its adverse effects, the European Union has restricted its use to protect human health and the environment. As a result, the industry has begun developing BPA analogues, but there are not yet sufficient toxicity data to claim that they are safe. We investigated the adverse toxic effects of BPA and its analogues (BPS, BPAP, BPAF, BPFL, and BPC) with emphasis on their cytotoxic and genotoxic activities after short (24-h) and prolonged (96-h) exposure in in vitro hepatic three-dimensional cell model developed from HepG2 cells. The results showed that BPFL and BPC (formed by an additional ring system) were the most cytotoxic analogues that affected cell viability, spheroid surface area and morphology, cell proliferation, and apoptotic cell death. BPA, BPAP, and BPAF induced DNA double-strand break formation (γH2AX assay), whereas BPAF and BPC increased the percentage of p-H3-positive cells, indicating their aneugenic activity. All BPs induced DNA single-strand break formation (comet assay), with BPAP (≥0.1 μM) being the most effective and BPA and BPC the least effective (≥1 μM) under conditions applied. The results indicate that not all of the analogues studied are safer alternatives to BPA and thus more in-depth research is urgently needed to adequately evaluate the risks of BPA analogues and assess their safety for humans.

Genotoxicity study of 2-methoxyethanol and benzalkonium chloride through Comet assay using 3D cultured HepG2 cells

Though the key data in identifying carcinogenicity is experience in human, long-term carcinogenicity tests using experimental animals are more realistic. Because carcinogenicity tests require much time and cost, performing the test is minimized through pre-screening. Recently, as bioethics has been strengthened, it is required to minimize animal testing in screening tests as well as carcinogenicity tests. The replacement of the micronucleus assay in experimental animal is the beginning, and the ultimate goal is to replace the carcinogenicity test using experimental animals. The micronucleus assay and the comet assay in 3D culture system of human-derived cells is considered as the most applicable practical measures at this stage. This study was conducted to provide more diverse information in the evaluation of carcinogenicity by establishing the comet test method in a three-dimensional cell culture system. In this study, HepG2 cells were cultured for 4 days in hang-in drop method, and then cultured for 7 days on a low adhesion plate to prepare spheroids. The methods were confirmed by d-mannitol (negative control), ethylmethane sulfonate (positive control), and cyclophosphamide (positive control for metabolite). 2-methoxyethanol and benzalkonium chloride were selected as test substances. Though 2-methoxyethanol is positive in in vivo comet assay and in vitro mammalian chromosome aberration test, it is considered negative in the comprehensive genotoxicity evaluation based on negative in bacterial reverse mutation assay, in vitro mammalian cell gene mutation test and mammalian chromosome aberration test. Benzalkonium chloride has been questioned on carcinogenicity because it is a disinfectant ingredient that has become a social issue in Korea. As a result of the Comet assay for 2-methoxyethanol and benzalkonium chloride in the cultured HepG2 cell line, 2-methoxyethanol was evaluated as positive in the metabolic activation system, but benzalkonium chloride was evaluated as negative in both the presence and absence of the metabolic activation system. Therefore, in order to clarify the carcinogenic potential of 2-methoxyethanol, it is judged that additional studies based on mechanistic studies are needed.

Replacement of animal testing by integrated approaches to testing and assessment (IATA): a call for in vivitrosi

Alternative methods to animal use in toxicology are evolving with new advanced tools and multilevel approaches, to answer from one side to 3Rs requirements, and on the other side offering relevant and valid tests for drugs and chemicals, considering also their combination in test strategies, for a proper risk assessment.

While stand-alone methods, have demonstrated to be applicable for some specific toxicological predictions with some limitations, the new strategy for the application of New Approach Methods (NAM), to solve complex toxicological endpoints is addressed by Integrated Approaches for Testing and Assessment (IATA), aka Integrated Testing Strategies (ITS) or Defined Approaches for Testing and Assessment (DA). The central challenge of evidence integration is shared with the needs of risk assessment and systematic reviews of an evidence-based Toxicology. Increasingly, machine learning (aka Artificial Intelligence, AI) lends itself to integrate diverse evidence streams.

In this article, we give an overview of the state of the art of alternative methods and IATA in toxicology for regulatory use for various hazards, outlining future orientation and perspectives. We call on leveraging the synergies of integrated approaches and evidence integration from in vivo, in vitro and in silico as true in vivitrosi.

HepG2 spheroids as a biosensor-like cell-based system for (geno)toxicity assessment

3D spheroids developed from HepG2 cells were used as a biosensor-like system for the detection of (geno)toxic effects induced by chemicals. Benzo(a)pyrene (B(a)P) and amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) with well-known mechanisms of action were used for system validation. HepG2 spheroids grown for 3 days were exposed to BaP and PhIP for 24 and 72 h. The growth and viability of spheroids were monitored by planimetry and Live/Dead staining of cells. Multi-parametric flow cytometric analysis was applied for simultaneous detection of specific end-effects including cell cycle analysis (Hoechst staining), cell proliferation (KI67 marker), and DNA double-strand breaks (ℽH2AX) induced by genotoxic compounds. Depending on the exposure concentration/time, BaP reduced spheroid growth, affected cell proliferation by arresting cells in S and G2 phase and induced DNA double-strand breaks (DSB). Simultaneous staining of ℽH2AX formation and cell cycle analysis revealed that after BaP (10 μM; 24 h) exposure 60% of cells in G0/G1 phase had DNA DSB, while after 72 h only 20% of cells contained DSB indicating efficient repair of DNA lesions. PhIP did not influence the spheroid size whereas accumulation of cells in the G2 phase occurred after both treatment times. The evaluation of DNA damage revealed that at 200 μM PhIP 50% of cells in G0/G1 phase had DNA DSB, which after 72-h exposure dropped to 40%, showing lower repair capacity of PhIP-induced DSB compared to BaP-induced. The developed approach using simultaneous detection of several parameters provides mechanistic data and thus contributes to more reliable genotoxicity assessment of chemicals as a high-content screening tool.

Three-Dimensional Skin Tissue Printing with Human Skin Cell Lines and Mouse Skin-Derived Epidermal and Dermal Cells

Since the skin covers most surfaces of the body, it is susceptible to damage, which can be fatal depending on the degree of injury to the skin because it defends against external attack and protects internal structures. Various types of artificial skin are being studied for transplantation to repair damaged skin, and recently, the production of replaceable skin using three-dimensional (3D) bioprinting technology has also been investigated. In this study, skin tissue was produced using a 3D bioprinter with human skin cell lines and cells extracted from mouse skin, and the printing conditions were optimized. Gelatin was used as a bioink, and fibrinogen and alginate were used for tissue hardening after printing. Printed skin tissue maintained a survival rate of 90% or more when cultured for 14 days. Culture conditions were established using 8 mM calcium chloride treatment and the skin tissue was exposed to air to optimize epidermal cell differentiation. The skin tissue was cultured for 14 days after differentiation induction by this optimized culture method, and immunofluorescent staining was performed using epidermal cell differentiation markers to investigate whether the epidermal cells had differentiated. After differentiation, loricrin, which is normally found in terminally differentiated epidermal cells, was observed in the cells at the tip of the epidermal layer, and cytokeratin 14 was expressed in the lower cells of the epidermis layer. Collectively, this study may provide optimized conditions for bioprinting and keratinization for three-dimensional skin production.

OUP accepted manuscript

BlueScreen HC is a mammalian cell-based assay for measuring the genotoxicity and cytotoxicity of chemical compounds and mixtures. The BlueScreen HC assay has been utilized at the Research Institute for Fragrance Materials in a safety assessment program as a screening tool to prioritize fragrance materials for higher-tier testing, as supporting evidence when using a read-across approach, and as evidence to adjust the threshold of toxicological concern. Predictive values for the BlueScreen HC assay were evaluated based on the ability of the assay to predict the outcome of in vitro and in vivo mutagenicity and chromosomal damage genotoxicity assays. A set of 371 fragrance materials was assessed in the BlueScreen HC assay along with existing or newly generated in vitro and in vivo genotoxicity data. Based on a weight-of-evidence approach, the majority of materials in the data set were deemed negative and concluded not to have the potential to be genotoxic, while only a small proportion of materials were determined to show genotoxic effects in these assays. Analysis of the data set showed a combination of high positive agreement but low negative agreement between BlueScreen HC results, in vitro regulatory genotoxicity assays, and higher-tier test results. The BlueScreen HC assay did not generate any false negatives, thereby providing robustness when utilizing it as a high-throughput screening tool to evaluate the large inventory of fragrance materials. From the perspective of protecting public health, it is desirable to have no or minimal false negatives, as a false-negative result may incorrectly indicate the lack of a genotoxicity hazard. However, the assay did have a high percentage of false-positive results, resulting in poor positive predictivity of the in vitro genotoxicity test battery outcome. Overall, the assay generated 100% negative predictivity and 3.9% positive predictivity. In addition to the data set of 371 fragrance materials, 30 natural complex substances were evaluated for BlueScreen HC, Ames, and in vitro micronucleus assay, and a good correlation in all three assays was observed. Overall, while a positive result may have to be further investigated, these findings suggest that the BlueScreen HC assay can be a valuable screening tool to detect the genotoxic potential of fragrance materials and mixtures.

Use of the EpiDerm TM 3D reconstructed skin micronucleus assay for fragrance materials

In order to evaluate the utility of the 3D reconstructed skin micronucleus assay (3DRSMN) to assess clastogenic/aneugenic potential of the fragrance chemicals, a set of 22 fragrance materials were evaluated in 3DRSMN assay. These materials evaluated were also evaluated in an in vitro as well as in vivo micronucleus assay, conducted as per Organisation for Economic Co-operation and Development guidelines. The results of the RSMN assay were in 100% agreement with the in vivo micronucleus assay results. From this dataset, 18 materials were positive in an in vitro micronucleus assay but were negative in an in vivo micronucleus assay. All these 18 materials were also concluded to be negative in 3DRSMN assay, stressing the importance of the assay to help minimize misleading positive outcomes from the in vitro assay. Since the highest exposure for fragrances is through the dermal route, the RSMN assay fits the applicability domain for testing. Thus, RSMN assay is an important alternative to animal testing for characterization of the genotoxicity potential of fragrance materials.

In vitro and integrated in vivo strategies to reduce animal use in genotoxicity testing

Scientific, financial, and ethical drivers have led to unprecedented interest in implementing human-relevant, mechanistic in vitro and in silico testing approaches. Further, as non-animal approaches are being developed and validated, researchers are interested in strategies that can immediately reduce the use of animals in toxicology testing. Here, we aim to outline a testing strategy for assessing genotoxicity beginning with standard in vitro methods, such as the bacterial reverse mutation test and the in vitro micronucleus test, followed by a second tier of in vitro assays including those using advanced 3D tissue models. Where regulatory agencies require in vivo testing, one demonstrated strategy is to combine genotoxicity studies traditionally conducted separately into a single test or to integrate genotoxicity studies into other toxicity studies. Standard setting organisations and regulatory agencies have encouraged such strategies, and examples of their use can be found in the scientific literature. Employing approaches outlined here will reduce animal use as well as study time and costs.

Validation of the 3D reconstructed human skin Comet assay, an animal-free alternative for following-up positive results from standard in vitro genotoxicity assays

As part of the safety assessment process, all industrial sectors employ genotoxicity test batteries, starting with well-established in vitro assays. However, these batteries have limited predictive capacity for the in vivo situation, which may result in unnecessary follow-up in vivo testing or the loss of promising substances where animal tests are prohibited or not desired. To address this, a project involving regulators, academia and industry was established to develop and validate in vitro human skin-based genotoxicity assays for topically exposed substances, such as cosmetics ingredients. Here, we describe the validation of the 3D reconstructed skin (RS) Comet assay. In this multicenter study, chemicals were applied topically three times to the skin over 48 h. Isolated keratinocytes and fibroblasts were transferred to slides before electrophoresis and the resulting comet formation was recorded as % tail DNA. Before decoding, results of the validation exercise for 32 substances were evaluated by an independent statistician. There was a high predictive capacity of this assay when compared to in vivo outcomes, with a sensitivity of 77 (80)%, a specificity of 88 (97)% and an overall accuracy of 83 (92)%. The numbers reflect the calls of the performing laboratories in the coded phase, whereas those in parenthesis reflect calls according to the agreed evaluation criteria. Intra- and inter-laboratory reproducibility was also very good, with a concordance of 93 and 88%, respectively. These results generated with the Phenion® Full-Thickness skin model demonstrate its suitability for this assay, with reproducibly low background DNA damage and sufficient metabolic capacity to activate pro-mutagens. The validation outcome supports the use of the RS Comet assay to follow up positive results from standard in vitro genotoxicity assays when the expected route of exposure is dermal. Based on the available data, the assay was accepted recently into the OECD test guideline development program.

Validation of the 3D reconstructed human skin micronucleus (RSMN) assay: an animal-free alternative for following-up positive results from standard in vitro genotoxicity assays

In vitro test batteries have become the standard approach to determine the genotoxic potential of substances of interest across industry sectors. While useful for hazard identification, standard in vitro genotoxicity assays in 2D cell cultures have limited capability to predict in vivo outcomes and may trigger unnecessary follow-up animal studies or the loss of promising substances where animal tests are prohibited or not desired. To address this problem, a team of regulatory, academia and industry scientists was established to develop and validate 3D in vitro human skin-based genotoxicity assays for use in testing substances with primarily topical exposure. Validation of the reconstructed human skin micronucleus (RSMN) assay in MatTek Epi-200™ skin models involved testing 43 coded chemicals selected by independent experts, in four US/European laboratories. The results were analysed by an independent statistician according to predefined criteria. The RSMN assay showed a reproducibly low background micronucleus frequency and exhibited sufficient capacity to metabolise pro-mutagens. The overall RSMN accuracy when compared to in vivo genotoxicity outcomes was 80%, with a sensitivity of 75% and a specificity of 84%, and the between- and within-laboratory reproducibility was 77 and 84%, respectively. A protocol involving a 72-h exposure showed increased sensitivity in detecting true positive chemicals compared to a 48-h exposure. An analysis of a test strategy using the RSMN assay as a follow-up test for substances positive in standard in vitro clastogenicity/aneugenicity assays and a reconstructed skin Comet assay for substances with positive results in standard gene mutation assays results in a sensitivity of 89%. Based on these results, the RSMN assay is considered sufficiently validated to establish it as a ‘tier 2’ assay for dermally exposed compounds and was recently accepted into the OECD’s test guideline development program.

Adaptation of the in vitro micronucleus assay for genotoxicity testing using 3D liver models supporting longer-term exposure durations

Following advancements in the field of genotoxicology, it has become widely accepted that 3D models are not only more physiologically relevant but also have the capacity to elucidate more complex biological processes that standard 2D monocultures are unable to. Whilst 3D liver models have been developed to evaluate the short-term genotoxicity of chemicals, the aim of this study was to develop a 3D model that could be used with the regulatory accepted in vitro micronucleus (MN) following low-dose, longer-term (5 days) exposure to engineered nanomaterials (ENMs). A comparison study was carried out between advanced models generated from two commonly used liver cell lines, namely HepaRG and HepG2, in spheroid format. While both spheroid systems displayed good liver functionality and viability over 14 days, the HepaRG spheroids lacked the capacity to actively proliferate and, therefore, were considered unsuitable for use with the MN assay. This study further demonstrated the efficacy of the in vitro 3D HepG2 model to be used for short-term (24 h) exposures to genotoxic chemicals, aflatoxin B1 (AFB1) and methyl-methanesulfonate (MMS). The 3D HepG2 liver spheroids were shown to be more sensitive to DNA damage induced by AFB1 and MMS when compared to the HepG2 2D monoculture. This 3D model was further developed to allow for longer-term (5 day) ENM exposure. Four days after seeding, HepG2 spheroids were exposed to Zinc Oxide ENM (0–2 µg/ml) for 5 days and assessed using both the cytokinesis-block MN (CBMN) version of the MN assay and the mononuclear MN assay. Following a 5-day exposure, differences in MN frequency were observed between the CBMN and mononuclear MN assay, demonstrating that DNA damage induced within the first few cell cycles is distributed across the mononucleated cell population. Together, this study demonstrates the necessity to adapt the MN assay accordingly, to allow for the accurate assessment of genotoxicity following longer-term, low-dose ENM exposure.

Organoids for toxicology and genetic toxicology: applications with drugs and prospects for environmental carcinogenesis

Advances in three-dimensional (3D) cell culture technology have led to the development of more biologically and physiologically relevant models to study organ development, disease, toxicology and drug screening. Organoids have been derived from many mammalian tissues, both normal and tumour, from adult stem cells and from pluripotent stem cells. Tissue organoids can retain many of the cell types and much of the structure and function of the organ of origin. Organoids derived from pluripotent stem cells display increased complexity compared with organoids derived from adult stem cells. It has been shown that organoids express many functional xenobiotic-metabolising enzymes including cytochrome P450s (CYPs). This has benefitted the drug development field in facilitating pre-clinical testing of more personalised treatments and in developing large toxicity and efficacy screens for a range of compounds. In the field of environmental and genetic toxicology, treatment of organoids with various compounds has generated responses that are close to those obtained in primary tissues and in vivo models, demonstrating the biological relevance of these in vitro multicellular 3D systems. Toxicological investigations of compounds in different tissue organoids have produced promising results indicating that organoids will refine future studies on the effects of environmental exposures and carcinogenic risk to humans. With further development and standardised procedures, advancing our understanding on the metabolic capabilities of organoids will help to validate their use to investigate the modes of action of environmental carcinogens.

Safety Testing of Cosmetic Products: Overview of Established Methods and New Approach Methodologies (NAMs)

Cosmetic products need to have a proven efficacy combined with a comprehensive toxicological assessment. Before the current Cosmetic regulation N°1223/2009, the 7th Amendment to the European Cosmetics Directive has banned animal testing for cosmetic products and for cosmetic ingredients in 2004 and 2009, respectively. An increasing number of alternatives to animal testing has been developed and validated for safety and efficacy testing of cosmetic products and cosmetic ingredients. For example, 2D cell culture models derived from human skin can be used to evaluate anti-inflammatory properties, or to predict skin sensitization potential; 3D human skin equivalent models are used to evaluate skin irritation potential; and excised human skin is used as the gold standard for the evaluation of dermal absorption. The aim of this manuscript is to give an overview of the main in vitro and ex vivo alternative models used in the safety testing of cosmetic products with a focus on regulatory requirements, genotoxicity potential, skin sensitization potential, skin and eye irritation, endocrine properties, and dermal absorption. Advantages and limitations of each model in safety testing of cosmetic products are discussed and novel technologies capable of addressing these limitations are presented.

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.

Experiments in the EpiDerm 3D Skin In Vitro Model and Minipigs In Vivo Indicate Comparatively Lower In Vivo Skin Sensitivity of Topically Applied Aneugenic Compounds

Risk management of in vitro aneugens for topically applied compounds is not clearly defined because there is no validated methodology to accurately measure compound concentration in proliferating stratum basale keratinocytes of the skin. Here, we experimentally tested several known aneugens in the EpiDerm reconstructed human skin in vitro micronucleus assay and compared the results to flow cytometric mechanistic biomarkers (phospho-H3; MPM2, DNA content). We then evaluated similar biomarkers (Ki-67, nuclear area) using immunohistochemistry in skin sections of minipigs following topical exposure the potent aneugens, colchicine, and hesperadin. Data from the EpiDerm model showed positive micronucleus responses for all aneugens tested following topical or direct media dosing with similar sensitivity when adjusted for applied dose. Quantitative benchmark dose-response analysis exhibited increases in the mitotic index biomarkers phospho-H3 and MPM2 for tubulin binders and polyploidy for aurora kinase inhibitors are at least as sensitive as the micronucleus endpoint. By comparison, the aneugens tested did not induce histopathological changes, increases in Ki-67 immunolabeling or nuclear area in skin sections from the in vivo minipig study at doses in significant excess of those eliciting a response in vitro. Results indicate the EpiDerm in vitro micronucleus assay is suitable for the hazard identification of aneugens. The lack of response in the minipig studies indicates that the barrier function of the minipig skin, which is comparable to human skin, protects from the effects of aneugens in vivo. These results provide a basis for conducting additional studies in the future to further refine this understanding.

Characterization of In Vitro 3D Cell Model Developed from Human Hepatocellular Carcinoma (HepG2) Cell Line

In genetic toxicology, there is a trend against the increased use of in vivo models as highlighted by the 3R strategy, thus encouraging the development and implementation of alternative models. Two-dimensional (2D) hepatic cell models, which are generally used for studying the adverse effects of chemicals and consumer products, are prone to giving misleading results. On the other hand, newly developed hepatic three-dimensional (3D) cell models provide an attractive alternative, which, due to improved cell interactions and a higher level of liver-specific functions, including metabolic enzymes, reflect in vivo conditions more accurately. We developed an in vitro 3D cell model from the human hepatocellular carcinoma (HepG2) cell line. The spheroids were cultured under static conditions and characterised by monitoring their growth, morphology, and cell viability during the time of cultivation. A time-dependent suppression of cell division was observed. Cell cycle analysis showed time-dependent accumulation of cells in the G0/G1 phase. Moreover, time-dependent downregulation of proliferation markers was shown at the mRNA level. Genes encoding hepatic markers, metabolic phase I/II enzymes, were time-dependently deregulated compared to monolayers. New knowledge on the characteristics of the 3D cell model is of great importance for its further development and application in the safety assessment of chemicals, food products, and complex mixtures.

Genotoxicity of Nanomaterials: Advanced In Vitro Models and High Throughput Methods for Human Hazard Assessment-A Review

Changes in the genetic material can lead to serious human health defects, as mutations in somatic cells may cause cancer and can contribute to other chronic diseases. Genotoxic events can appear at both the DNA, chromosomal or (during mitosis) whole genome level. The study of mechanisms leading to genotoxicity is crucially important, as well as the detection of potentially genotoxic compounds. We consider the current state of the art and describe here the main endpoints applied in standard human in vitro models as well as new advanced 3D models that are closer to the in vivo situation. We performed a literature review of in vitro studies published from 2000–2020 (August) dedicated to the genotoxicity of nanomaterials (NMs) in new models. Methods suitable for detection of genotoxicity of NMs will be presented with a focus on advances in miniaturization, organ-on-a-chip and high throughput methods.