Integrated Testing Strategies (ITS) for safety assessment

Distinguishing classes of neuroactive drugs based on computational physicochemical properties and experimental phenotypic profiling in planarians

Mental illnesses put a tremendous burden on afflicted individuals and society. Identification of novel drugs to treat such conditions is intrinsically challenging due to the complexity of neuropsychiatric diseases and the need for a systems-level understanding that goes beyond single molecule-target interactions. Thus far, drug discovery approaches focused on target-based in silico or in vitro high-throughput screening (HTS) have had limited success because they cannot capture pathway interactions or predict how a compound will affect the whole organism. Organismal behavioral testing is needed to fill the gap, but mammalian studies are too time-consuming and cost-prohibitive for the early stages of drug discovery. Behavioral medium-throughput screening (MTS) in small organisms promises to address this need and complement in silico and in vitro HTS to improve the discovery of novel neuroactive compounds. Here, we used cheminformatics and MTS in the freshwater planarian Dugesia japonica-an invertebrate system used for neurotoxicant testing-to evaluate the extent to which complementary insight could be gained from the two data streams. In this pilot study, our goal was to classify 19 neuroactive compounds into their functional categories: antipsychotics, anxiolytics, and antidepressants. Drug classification was performed with the same computational methods, using either physicochemical descriptors or planarian behavioral profiling. As it was not obvious a priori which classification method was most suited to this task, we compared the performance of four classification approaches. We used principal coordinate analysis or uniform manifold approximation and projection, each coupled with linear discriminant analysis, and two types of machine learning models-artificial neural net ensembles and support vector machines. Classification based on physicochemical properties had comparable accuracy to classification based on planarian profiling, especially with the machine learning models that all had accuracies of 90-100%. Planarian behavioral MTS correctly identified drugs with multiple therapeutic uses, thus yielding additional information compared to cheminformatics. Given that planarian behavioral MTS is an inexpensive true 3R (refine, reduce, replace) alternative to vertebrate testing and requires zero a priori knowledge about a chemical, it is a promising experimental system to complement in silico cheminformatics to identify new drug candidates.

Development of machine learning models for the prediction of the skin sensitization potential of cosmetic compounds

Background

To enhance the accuracy of allergen detection in cosmetic compounds, we developed a co-culture system that combines HaCaT keratinocytes (transfected with a luciferase plasmid driven by the AKR1C2 promoter) and THP-1 cells for machine learning applications.

Methods

Following chemical exposure, cell cytotoxicity was assessed using CCK-8 to determine appropriate stimulation concentrations. RNA-Seq was subsequently employed to analyze THP-1 cells, followed by differential expression gene (DEG) analysis and weighted gene co-expression net-work analysis (WGCNA). Using two data preprocessing methods and three feature extraction techniques, we constructed and validated models with eight machine learning algorithms.

Results

Our results demonstrated the effectiveness of this integrated approach. The best performing models were random forest (RF) and voom-based diagonal quadratic discriminant analysis (voomDQDA), both achieving 100% accuracy. Support vector machine (SVM) and voom based nearest shrunken centroids (voomNSC) showed excellent performance with 96.7% test accuracy, followed by voom-based diagonal linear discriminant analysis (voomDLDA) at 95.2%. Nearest shrunken centroids (NSC), Poisson linear discriminant analysis (PLDA) and negative binomial linear discriminant analysis (NBLDA) achieved 90.5% and 90.2% accuracy, respectively. K-nearest neighbors (KNN) showed the lowest accuracy at 85.7%.

Conclusion

This study highlights the potential of integrating co-culture systems, RNA-Seq, and machine learning to develop more accurate and comprehensive in vitro methods for skin sensitization testing. Our findings contribute to the advancement of cosmetic safety assessments, potentially reducing the reliance on animal testing.

Comparative case study on NAMs: towards enhancing specific target organ toxicity analysis

Traditional risk assessment methodologies in toxicology have relied upon animal testing, despite concerns regarding interspecies consistency, reproducibility, costs, and ethics. New Approach Methodologies (NAMs), including cell culture and multi-level omics analyses, hold promise by providing mechanistic information rather than assessing organ pathology. However, NAMs face limitations, like lacking a whole organism and restricted toxicokinetic interactions. This is an inherent challenge when it comes to the use of omics data from in vitro studies for the prediction of organ toxicity in vivo. One solution in this context are comparative in vitro-in vivo studies as they allow for a more detailed assessment of the transferability of the respective NAM data. Hence, hepatotoxic and nephrotoxic pesticide active substances were tested in human cell lines and the results subsequently related to the biology underlying established effects in vivo. To this end, substances were tested in HepaRG and RPTEC/tERT1 cells at non-cytotoxic concentrations and analyzed for effects on the transcriptome and parts of the proteome using quantitative real-time PCR arrays and multiplexed microsphere-based sandwich immunoassays, respectively. Transcriptomics data were analyzed using three bioinformatics tools. Where possible, in vitro endpoints were connected to in vivo observations. Targeted protein analysis revealed various affected pathways, with generally fewer effects present in RPTEC/tERT1. The strongest transcriptional impact was observed for Chlorotoluron in HepaRG cells (increased CYP1A1 and CYP1A2 expression). A comprehensive comparison of early cellular responses with data from in vivo studies revealed that transcriptomics outperformed targeted protein analysis, correctly predicting up to 50% of in vivo effects.

A novel integrated testing strategy (ITS) for evaluating acute fish toxicity with new approach methodologies (NAMs)

Acute fish toxicity (AFT) tests are performed in aquatic risk assessments of chemical compounds globally. However, the specific endpoint of in vivo AFT is based on the lethal concentration 50 (LC50), which is a serious challenge in terms of animal welfare. To support the 3Rs principle of replacing, reducing, and refining use of animals, integrated testing strategies (ITS) have recently been developed for environmental risk assessment. ITS efficiently integrates multiple types of information, especially new approach methodologies (NAMs), and further supports regulatory decision-making. Currently, an effective ITS framework for evaluating aquatic toxicity is lacking. Therefore, we aimed to develop a promising ITS for AFT using in silico, in vitro, and in vivo data. We established the ITS via in silico (OECD QSAR Toolbox 4.6), fish cell line acute toxicity (FCT), and fish embryo acute toxicity (FET) tests and then validated the NAMs with AFT testing. The NAM data were derived from the European Chemicals Agency (ECHA) dossier, toxicology databases, peer-reviewed research articles, and this study. For the first step in the ITS process, we aimed to design a high-throughput screening tool to identify non-toxic and toxic chemicals. We found that results of in silico, FCT, and FET tests alone were strongly correlated with AFT. Among the models, the in silico model was most suitable for identifying toxicants due to its high sensitivity and minimal animal use. Next, considering regulatory purposes and flexibility, we determined the predictive LC50 of toxic chemicals by pursuing a preference-dependent strategy, sequential testing strategy, and sensitivity-dependent strategy. All the strategies demonstrated a predictive power equal to or greater than 73%. In addition, to meet user preferences, our ITS approach has high flexibility and supports animal welfare and environmental protection. We have therefore developed multiple powerful, flexible, and more humane ITS methods for acute fish toxicity assessment by integrating NAMs.

Current Challenges to Align Inflammatory Key Events in Animals and Lung Cell Models In Vitro

With numerous novel and innovative in vitro models emerging every year to reduce or replace animal testing, there is an urgent need to align the design, harmonization, and validation of such systems using in vitro-in vivo extrapolation (IVIVE) approaches. In particular, in inhalation toxicology, there is a lack of predictive and prevalidated in vitro lung models that can be considered a valid alternative for animal testing. The predictive power of such models can be enhanced by applying the Adverse Outcome Pathways (AOP) framework, which casually links key events (KE) relevant to IVIVE. However, one of the difficulties identified is that the endpoint analysis and readouts of specific assays in in vitro and animal models for specific toxicants are currently not harmonized, making the alignment challenging. We summarize the current state of the art in endpoint analysis in the two systems, focusing on inflammatory-induced effects and providing guidance for future research directions to improve the alignment.

Estrogen receptor/androgen receptor transcriptional activation of benzophenone derivatives and integrated approaches to testing and assessment (IATA) for estrogenic effects

Estrogen receptor (ER) and androgen receptor (AR) transactivation assays for the benzophenone compounds (BPs) were performed using hERα-HeLa-9903 cells for ER and MMTV/22Rv1_GR-KO cells for AR. Results showed that some BPs, such as BP-1, BP-2, 4OH-BP, 4DHB, and 4-MBP, showed agonistic activity on ER with a higher RPCmax than 1 nM 17-β estradiol. The other BPs (BP, BP-3, BP-6, BP-7, and BP-8) showed low RPCmax in accordance with the OECD Test guideline (TG) 455 criteria, with BP-4 as the only ER-negative. However, the potency of the BPs was at least 1000 times less than the reference chemical, 17-β-estradiol. None of the BPs exhibited agonistic activity on AR except BP-2 which showed a small increase in activity. For further evaluation of the estrogenic effect of BPs based on the integrated approaches to testing and assessment (IATA) approach, existing data on ER binding, steroidogenesis, MCF-7 cell proliferation, and in vivo uterotrophic assays were collected and evaluated. There seemed to be a close association between the in vitro data on BPs, especially ER transcriptional activity, and the in vivo results of increased uterine weight. This case study implied that integrated approaches using in vitro data can be a useful tool for the prediction of in vivo data for estrogenic effects, without the need for additional animal toxicity tests.

Quantum-Mechanics Calculations Elucidate Skin-Sensitizing Pharmaceutical Compounds

Skin sensitization is a critical end point in occupational toxicology that necessitates the use of fast, accurate, and affordable models to aid in establishing handling guidance for worker protection. While many in silico models have been developed, the scarcity of reliable data for active pharmaceutical ingredients (APIs) and their intermediates (together regarded as pharmaceutical compounds) brings into question the reliability of these tools, which are largely constructed using publicly available nonspecialty chemicals. Here, we present the quantum-mechanical (QM) Computer-Aided Discovery and REdesign (CADRE) model, which was developed with the bioactive and structurally complex chemical space in mind by relying on the fundamentals of chemical interactions in key events (versus structural attributes of training-set data). Validated in this study on 345 APIs and intermediates, CADRE achieved 95% accuracy, sensitivity, and specificity and a combined 79% accuracy in assigning potency categories compared to the mouse local lymph node assay data. We show how historical outcomes from CADRE testing in the pharmaceutical space, generated over the past 10 years on ca. 2500 chemicals, can be used to probe the relationships between sensitization mechanisms (or the underlying chemical classes) and the probability of eliciting a sensitization response in mice of a given potency. We believe this information to be of value to both practitioners, who can use it to quickly screen and triage their data sets, as well as to model developers to fine-tune their structure-based tools. Lastly, we leverage our experimentally validated subset of APIs and intermediates to show the importance of dermal permeability on the sensitization potential and potency. We demonstrate that common physicochemical properties used to assess permeation, such as the octanol-water partition coefficient and molecular weight, are poor proxies for the more accurate energy-pair distributions that can be computed from mixed QM and classical simulations using model representations of the stratum corneum.

An Integrated Testing Strategy (ITS) to assess the environmental compatibility of wood protection techniques

To quantify the possible impact of different wood protection techniques on the aquatic environment, we applied a tiered Integrated Testing Strategy (ITS) on leachates obtained from untreated (UTW) Norway spruce (Picea abies), specimens treated with a copper-ethanolamine-based preservative solution, complying with the Use Class 3 (UC3), and specimens thermally modified (TM). Different maturation times in water were tested to verify whether toxicant leaching is time-dependent. Tier I tests, addressing acute effects on Aliivibrio fischeri, Raphidocelis subcapitata, and Daphnia magna, evidenced that TM toxicity was comparable or even lower than in UTW. Conversely, UC3 significantly affected all species compared to UTW, also after 30 days of maturation in water, and was not considered an environmentally acceptable wood preservation solution. Tier II (effects on early-life stages of Lymnea auricularia) and III (chronic effects on D. magna and L. auricularia) performed on UTW and TM confirmed the latter as an environmentally acceptable treatment, with increasing maturation times resulting in decreased adverse effects. The ITS allowed for rapid and reliable identification of potentially harmful effects due to preservation treatments, addressed the choice for a less impacting solution, and can be effective for manufacturers in identifying more environmentally friendly solutions while developing their products.

Novel Strategy to Assess the Neurotoxicity of Organic Solvents Such as Glycol Ethers: Protocol for Combining In Vitro and In Silico Methods With Human-Controlled Exposure Experiments

BACKGROUND

Chemicals are not required to be tested systematically for their neurotoxic potency, although they may contribute to the development of several neurological diseases. The absence of systematic testing may be partially explained by the current Organisation for Economic Co-operation and Development (OECD) Test Guidelines, which rely on animal experiments that are expensive, laborious, and ethically debatable. Therefore, it is important to understand the risks to exposed workers and the general population exposed to domestic products. In this study, we propose a strategy to test the neurotoxicity of solvents using the commonly used glycol ethers as a case study.

OBJECTIVE

This study aims to provide a strategy that can be used by regulatory agencies and industries to rank solvents according to their neurotoxicity and demonstrate the use of toxicokinetic modeling to predict air concentrations of solvents that are below the no observed adverse effect concentrations (NOAECs) for human neurotoxicity determined in in vitro assays.

METHODS

The proposed strategy focuses on a complex 3D in vitro brain model (BrainSpheres) derived from human-induced pluripotent stem cells (hiPSCs). This model is accompanied by in vivo, in vitro, and in silico models for the blood-brain barrier (BBB) and in vitro models for liver metabolism. The data are integrated into a toxicokinetic model. Internal concentrations predicted using this toxicokinetic model are compared with the results from in vivo human-controlled exposure experiments for model validation. The toxicokinetic model is then used in reverse dosimetry to predict air concentrations, leading to brain concentrations lower than the NOAECs determined in the hiPSC-derived 3D brain model. These predictions will contribute to the protection of exposed workers and the general population with domestic exposures.

RESULTS

The Swiss Centre for Applied Human Toxicology funded the project, commencing in January 2021. The Human Ethics Committee approval was obtained on November 16, 2022. Zebrafish experiments and in vitro methods started in February 2021, whereas recruitment of human volunteers started in 2022 after the COVID-19 pandemic-related restrictions were lifted. We anticipate that we will be able to provide a neurotoxicity testing strategy by 2026 and predicted air concentrations for 6 commonly used propylene glycol ethers based on toxicokinetic models incorporating liver metabolism, BBB leakage parameters, and brain toxicity.

CONCLUSIONS

This study will be of great interest to regulatory agencies and chemical industries needing and seeking novel solutions to develop human chemical risk assessments. It will contribute to protecting human health from the deleterious effects of environmental chemicals.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

DERR1-10.2196/50300.

Making the Case for Quantum Mechanics in Predictive Toxicology─Nearly 100 Years Too Late?

The use of quantum mechanics (QM) has long been the norm to study covalent-binding phenomena in chemistry and biochemistry. The pharmaceutical industry leverages QM models explicitly in covalent drug discovery and implicitly to characterize short-range interactions in noncovalent binding. Predictive toxicology has resisted widespread adoption of QM, including in the pharmaceutical industry, despite its obvious relevance to the metabolic processes in the upstream of adverse outcome pathways and advances in both QM methods and computational resources, which support fit-for-purpose applications in reasonable timeframes. Here, we make the case for embracing QM as an indispensable part of a toxicologist's toolkit. We argue that QM provides the necessary orthogonality to alert-based expert systems and traditional QSARs, consistent with calls for animal-free integrated testing strategies for safety assessments of commercial chemicals. We outline existing roadblocks to this transition, including the need to train model developers in QM and the shift toward service-based toxicity models that utilize high-performance computing clusters. Lastly, we describe recent examples of successful implementations of QM in hazard assessments and propose how in silico toxicology can be further advanced by integrating QM with artificial intelligence.

Modernizing persistence-bioaccumulation-toxicity (PBT) assessment with high throughput animal-free methods

The assessment of persistence (P), bioaccumulation (B), and toxicity (T) of a chemical is a crucial first step at ensuring chemical safety and is a cornerstone of the European Union's chemicals regulation REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals). Existing methods for PBT assessment are overly complex and cumbersome, have produced incorrect conclusions, and rely heavily on animal-intensive testing. We explore how new-approach methodologies (NAMs) can overcome the limitations of current PBT assessment. We propose two innovative hazard indicators, termed cumulative toxicity equivalents (CTE) and persistent toxicity equivalents (PTE). Together they are intended to replace existing PBT indicators and can also accommodate the emerging concept of PMT (where M stands for mobility). The proposed "toxicity equivalents" can be measured with high throughput in vitro bioassays. CTE refers to the toxic effects measured directly in any given sample, including single chemicals, substitution products, or mixtures. PTE is the equivalent measure of cumulative toxicity equivalents measured after simulated environmental degradation of the sample. With an appropriate panel of animal-free or alternative in vitro bioassays, CTE and PTE comprise key environmental and human health hazard indicators. CTE and PTE do not require analytical identification of transformation products and mixture components but instead prompt two key questions: is the chemical or mixture toxic, and is this toxicity persistent or can it be attenuated by environmental degradation? Taken together, the proposed hazard indicators CTE and PTE have the potential to integrate P, B/M and T assessment into one high-throughput experimental workflow that sidesteps the need for analytical measurements and will support the Chemicals Strategy for Sustainability of the European Union.

Using Machine Learning to make nanomaterials sustainable

Sustainable development is a key challenge for contemporary human societies; failure to achieve sustainability could threaten human survival. In this review article, we illustrate how Machine Learning (ML) could support more sustainable development, covering the basics of data gathering through each step of the Environmental Risk Assessment (ERA). The literature provides several examples showing how ML can be employed in most steps of a typical ERA.A key observation is that there are currently no clear guidance for using such autonomous technologies in ERAs or which standards/checks are required. Steering thus seems to be the most important task for supporting the use of ML in the ERA of nano- and smart-materials. Resources should be devoted to developing a strategy for implementing ML in ERA with a strong emphasis on data foundations, methodologies, and the related sensitivities/uncertainties. We should recognise historical errors and biases (e.g., in data) to avoid embedding them during ML programming.

New Worm on the Block: Planarians in (Neuro)Toxicology

Traditional mammalian testing is too time- and cost-intensive to keep up with the large number of environmental chemicals needing assessment. This has led to a dearth of information about the potential adverse effects of these chemicals, especially on the developing brain. Thus, there is an urgent need for rapid and cost-effective neurotoxicity and developmental neurotoxicity testing. Because of the complexity of the brain, metabolically competent organismal models are necessary to understand the effects of chemicals on nervous system development and function on a systems level. In this overview, we showcase asexual freshwater planarians as an alternative invertebrate ("non-animal") organismal model for neurotoxicology research. Planarians have long been used to study the effects of chemicals on regeneration and behavior. But they have only recently moved back into the spotlight because modern molecular and computational approaches now enable quantitative high-content and high-throughput toxicity studies. Here, we present a short history of the use of planarians in toxicology research, highlight current techniques to measure toxicity qualitatively and quantitatively in planarians, and discuss how to further promote this non-animal organismal system into mainstream toxicology research. The articles in this collection will help work towards this goal by providing detailed protocols that can be adopted by the community to standardize planarian toxicity testing. © 2022 Wiley Periodicals LLC.

Bioactivation and detoxification of organophosphorus pesticides in freshwater planarians shares similarities with humans

Organophosphorus pesticides (OPs) are a chemically diverse class of insecticides that inhibit acetylcholinesterase (AChE). Many OPs require bioactivation to their active oxon form via cytochrome P450 to effectively inhibit AChE. OP toxicity can be mitigated by detoxification reactions performed by carboxylesterase and paraoxonase. The relative extent of bioactivation to detoxification varies among individuals and between species, leading to differential susceptibility to OP toxicity. Because of these species differences, it is imperative to characterize OP metabolism in model systems used to assess OP toxicity. We have shown that the asexual freshwater planarian Dugesia japonica is a suitable model to assess OP neurotoxicity and developmental neurotoxicity via rapid, automated testing of adult and developing organisms in parallel using morphological and behavioral endpoints. D. japonica has two cholinesterase enzymes with intermediate properties between AChE and butyrylcholinesterase that are sensitive to OP inhibition. Here, we demonstrate that D. japonica contains the major OP metabolic machinery to be a relevant model for OP neurotoxicity studies. Adult and regenerating D. japonica can bioactivate chlorpyrifos and diazinon into their respective oxons. Significant AChE inhibition was only observed after in vivo metabolic activation but not when the parent OPs were directly added to planarian homogenate using the same concentrations and timing. Using biochemical assays, we found that D. japonica has both carboxylesterase (24 nmol/(min*mg protein)) and paraoxonase (60 pmol/(min*mg protein)) activity. We show that planarian carboxylesterase activity is distinct from cholinesterase activity using benzil and tacrine. These results further support the use of D. japonica for OP toxicity studies.

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.

Co-Culture of THP-1 Cells and Normal Human Epidermal Keratinocytes (NHEK) for Modified Human Cell Line Activation Test (h-CLAT)

To improve the accuracy of skin sensitization prediction of chemicals by conventional alternative methods using cells, it is important to reproduce the environment of skin in vitro, such as the crosstalk between keratinocytes and dendritic cells (DCs). We developed a skin sensitization test system based on the markers and criteria of the human cell line activation test (h-CLAT), which combines THP-1 cells as a surrogate for DCs and keratinized normal human epidermal keratinocytes (NHEK). After exposure to chemicals via keratinized NHEK, the cell surface expression of CD54 and CD86 on THP-1 was measured by flow cytometry. This co-culture system evaluated 2,4-dinitrochlorobenzene (DNCB), a typical sensitizer, as positive, lactic acid (LA), a non-sensitizer, as negative, and isoeugenol (IE), a prohapten that requires biological activation to acquire skin sensitization, as positive. However, the expression levels of CD54 and CD86 in DNCB-treated THP-1 were lower than those in normal h-CLAT. Therefore, we investigated the effects of the medium and secretion by NHEK cells on THP-1 cells. CD54 and CD86 expression was enhanced in monocultured THP-1 in the medium for keratinized NHEK and in the conditioned medium of keratinized NHEK. The increase in CD54 and CD86 by changes in the medium type was higher than that by the NHEK secretion; therefore, it was found that the medium composition has a large effect on the evaluation index among the experimental parameters in the co-culture system. It is necessary to find the optimal medium for immunotoxicity assessment in the co-culture system.

An Integrated Testing Strategy for Ecotoxicity (ITS-ECO) Assessment in the Marine Environmental Compartment using Mytilus spp.: A Case Study using Pristine and Coated CuO and TiO2 Nanomaterials

An integrated testing strategy for ecotoxicity assessment (ITS-ECO) was developed to aid in the hazard and fate assessment of engineered nanomaterials (ENMs) deposited in marine environments using the bivalve Mytilus spp. as a test species. The ENMs copper(II) oxide (CuO) and titanium dioxide (TiO₂ ), either in pristine form (core) or with functionalized coatings (polyethylene glycol [PEG], carboxyl [COOH], and ammonia [NH₃ ]) were selected as case study materials based on their production levels and use. High-throughput in vitro testing in Tier 1 of the ITS-ECO revealed CuO ENMs to elicit cytotoxic effects on lysosomes of hemocytes of mussels, with the hazard potential CuO PEG > CuO COOH > CuO NH₃  > CuO core, whereas TiO₂ ENMs were not cytotoxic. Genotoxicity in hemocytes as well as gill cells of mussels following in vivo exposure (48 h) to CuO ENMs was also seen. Longer in vivo exposures in Tier 2 (48 h-21 days) revealed subacute and chronic oxidative effects for both CuO and TiO₂ ENMs, in some cases leading to lipid peroxidation (core TiO₂ ENMs). In Tier 3 bioaccumulation studies, distinct patterns of uptake for Cu (predominantly in gills) and Ti (predominantly in digestive glands) and between the different core and coated ENMs were found. Clear NM-specific and coating-dependent effects on hazard and fate were seen. Overall, using a tiered testing approach, the ITS-ECO was able to differentiate the hazard (acute, subacute, and chronic effects) posed by ENMs of different compositions and coatings and to provide information on fate for environmental risk assessment of these ENMs. Environ Toxicol Chem 2022;41:1390-1406. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Development of a 96-Well Electrophilic Allergen Screening Assay for Skin Sensitization Using a Measurement Science Approach

The Electrophilic Allergen Screening Assay (EASA) has emerged as a promising in chemico method to detect the first key event in the adverse outcome pathway (AOP) for skin sensitization. This assay functions by assessing the depletion of one of two probe molecules (4-nitrobenzenethiol (NBT) and pyridoxylamine (PDA)) in the presence of a test compound (TC). The initial development of EASA utilized a cuvette format resulting in multiple measurement challenges such as low throughput and the inability to include adequate control measurements. In this study, we describe the redesign of EASA into a 96-well plate format that incorporates in-process control measurements to quantify key sources of variability each time the assay is run. The data from the analysis of 67 TCs using the 96-well format had 77% concordance with animal data from the local lymph node assay (LLNA), a result consistent with that for the direct peptide reactivity assay (DPRA), an OECD test guideline (442C) protein binding assay. Overall, the measurement science approach described here provides steps during assay development that can be taken to increase confidence of in chemico assays by attempting to fully characterize the sources of variability and potential biases and incorporate in-process control measurements into the assay.

Multi-Strategy Assessment of Different Uses of QSAR under REACH Analysis of Alternatives to Advance Information Transparency

Under the Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH) analysis of alternatives (AoA) process, quantitative structure–activity relationship (QSAR) models play an important role in expanding information gathering and organizing frameworks. Increasingly recognized as an alternative to testing under registration. QSARs have become a relevant tool in bridging data gaps and supporting weight of evidence (WoE) when assessing alternative substances. Additionally, QSARs are growing in importance in integrated testing strategies (ITS). For example, the REACH ITS framework for specific endpoints directs registrants to consider non-testing results, including QSAR predictions, when deciding if further animal testing is needed. Despite the raised profile of QSARs in these frameworks, a gap exists in the evaluation of QSAR use and QSAR documentation under authorization. An assessment of the different uses (e.g., WoE and ITS) in which QSAR predictions play a role in evidence gathering and organizing remains unaddressed for AoA. This study approached the disparity in information for QSAR predictions by conducting a substantive review of 24 AoA through May 2017, which contained higher-tier endpoints under REACH. Understanding the manner in which applicants manage QSAR prediction information in AoA and assessing their potential within ITS will be valuable in promoting regulatory use of QSARs and building out future platforms in the face of rapidly evolving technology while advancing information transparency.

Models of Dendritic Cells to Assess Skin Sensitization

Allergic contact dermatitis (ACD) is a complex skin pathology occurring in reaction against environmental substances found in the workplace (cement, hair dyes, textile dyes), in the private environment (e.g., household products, cosmetic ingredients), or following skin exposure to drugs. Many cells are involved in the initiation of ACD during the sensitization phase. The four key events (KE) of skin sensitization AOP are covalent binding to skin proteins (KE1), keratinocyte activation (KE2), activation of DCs (KE3), and T-cell activation and proliferation (KE4), leading to the adverse outcome of ACD. Dendritic cells (DCs) are thus playing a key role in ACD pathophysiology. Indeed, in the presence of chemical sensitizers, DCs migrate from the skin to the draining lymph nodes and present peptide-chemical conjugates to T cells, leading to their activation and proliferation. In vitro methods have been actively developed to assess the activation of DCs by chemicals to establish a reliable in vitro sensitization test. Therefore, this review will detail the most used methods and protocols to develop DC models in vitro. Three different models of DCs will be addressed: 1) DCs derived from Cord Blood (CD34-DCs), 2) DCs derived from Monocytes (Mo-DCs), and 3) DCs derived from mice Bone-Marrow (BM-DCs). In addition, a model of exposition to contact sensitizers to assess KE3 of skin sensitization will be detailed for each of the models presented.

In Silico Models for Predicting Acute Systemic Toxicity

In this chapter, we give a brief overview of the regulatory requirements for acute systemic toxicity information in the European Union, and we review structure-based computational models that are available and potentially useful in the assessment of acute systemic toxicity. Emphasis is placed on quantitative structure-activity relationship (QSAR) models implemented by means of a range of software tools. The most recently published literature models for acute systemic toxicity are also discussed, and perspectives for future developments in this field are offered.

Probabilistic risk assessment - the keystone for the future of toxicology

Safety sciences must cope with uncertainty of models and results as well as information gaps. Acknowledging this uncer-tainty necessitates embracing probabilities and accepting the remaining risk. Every toxicological tool delivers only probable results. Traditionally, this is taken into account by using uncertainty / assessment factors and worst-case / precautionary approaches and thresholds. Probabilistic methods and Bayesian approaches seek to characterize these uncertainties and promise to support better risk assessment and, thereby, improve risk management decisions. Actual assessments of uncertainty can be more realistic than worst-case scenarios and may allow less conservative safety margins. Most importantly, as soon as we agree on uncertainty, this defines room for improvement and allows a transition from traditional to new approach methods as an engineering exercise. The objective nature of these mathematical tools allows to assign each methodology its fair place in evidence integration, whether in the context of risk assessment, sys-tematic reviews, or in the definition of an integrated testing strategy (ITS) / defined approach (DA) / integrated approach to testing and assessment (IATA). This article gives an overview of methods for probabilistic risk assessment and their application for exposure assessment, physiologically-based kinetic modelling, probability of hazard assessment (based on quantitative and read-across based structure-activity relationships, and mechanistic alerts from in vitro studies), indi-vidual susceptibility assessment, and evidence integration. Additional aspects are opportunities for uncertainty analysis of adverse outcome pathways and their relation to thresholds of toxicological concern. In conclusion, probabilistic risk assessment will be key for constructing a new toxicology paradigm - probably!

Human Oligodendrocytes and Myelin In Vitro to Evaluate Developmental Neurotoxicity

Neurodevelopment is uniquely sensitive to toxic insults and there are concerns that environmental chemicals are contributing to widespread subclinical developmental neurotoxicity (DNT). Increased DNT evaluation is needed due to the lack of such information for most chemicals in common use, but in vivo studies recommended in regulatory guidelines are not practical for the large-scale screening of potential DNT chemicals. It is widely acknowledged that developmental neurotoxicity is a consequence of disruptions to basic processes in neurodevelopment and that testing strategies using human cell-based in vitro systems that mimic these processes could aid in prioritizing chemicals with DNT potential. Myelination is a fundamental process in neurodevelopment that should be included in a DNT testing strategy, but there are very few in vitro models of myelination. Thus, there is a need to establish an in vitro myelination assay for DNT. Here, we summarize the routes of myelin toxicity and the known models to study this particular endpoint.

Progress on Reconstructed Human Skin Models for Allergy Research and Identifying Contact Sensitizers

Contact with the skin is inevitable or desirable for daily life products such as cosmetics, hair dyes, perfumes, drugs, household products, and industrial and agricultural products. Whereas the majority of these products are harmless, a number can become metabolized and/or activate the immunological defense via innate and adaptive mechanisms resulting in sensitization and allergic contact dermatitis upon following exposures to the same substance. Therefore, strict safety (hazard) assessment of actives and ingredients in products and drugs applied to the skin is essential to determine I) whether the chemical is a potential sensitizer and if so II) what is the safe concentration for human exposure to prevent sensitization from occurring. Ex vivo skin is a valuable model for skin penetration studies but due to logistical and viability limitations the development of in vitro alternatives is required. The aim of this review is to give a clear overview of the organotypic in vitro skin models (reconstructed human epidermis, reconstructed human skin, immune competent skin models incorporating Langerhans Cells and T-cells, skin-on-chip) that are currently commercially available or which are being used in a laboratory research setting for hazard assessment of potential sensitizers and for investigating the mechanisms (sensitization key events 1-4) related to allergic contact dermatitis. The limitations of the models, their current applications, and their future potential in replacing animals in allergy-related science are discussed.

Understanding the Development, Standardization, and Validation Process of Alternative In Vitro Test Methods for Regulatory Approval from a Researcher Perspective

Due to economic, practical, ethical, and scientific reasons, researchers, among others, are pushing for alternative in vitro test methods to replace or reduce existing animal experiments. In order for these tests to be more broadly used by the industrial sector and regulatory bodies, orchestrated efforts are required to show the robustness and reliability of in vitro methods, which can accelerate the use for early screening testing. Another way of increasing the use of alternatives is to coordinate validation studies, that is, multi-laboratory trials, and to gain regulatory approval and instatement as test guidelines or standard method. However, awareness of the exact standardization, validation, and approval process has been a major obstacle for many researchers. Herein, the process has been broken down into three main phases: i) test method development; ii) intra- and inter-laboratory validation; and iii) regulatory acceptance. This general process applies to all alternative methods seeking validation and approval, although the intricacies of different toxicological endpoints and/or chemical sectors may lead to additional work, particularly in the validation stage. The authors' aim is to provide insight in the development process of alternative methods with a focus on in vitro cell culture methods over validation to regulatory acceptance.

Strategy Combining Nonanimal Methods for Ocular Toxicity Evaluation

Historically, the ocular toxicity of manufactured consumer materials has been evaluated using the rabbit in vivo Draize rabbit eye test. The animal data obtained were used by the United Nations Globally Harmonized System of Classification and Labelling of Chemicals (UN GHS) to define the classification and labelling (C&L) for eye damage/irritation endpoint. However, the Draize test, a method which was never formally validated, has been widely criticized because of its technical limitations. In addition, ethical and economic issues and advances in scientific knowledge, and political and public pressures have made animal experimentation unsustainable. This scenario has consequently led to the development of nonanimal testing and protocols/approaches with considerable predictive value and relevance for humans. It is widely accepted that one single nonanimal method cannot cover all the criteria of damage/inflammation assessed by regulatory adopted in vivo animal testing. Thus, integrated testing strategies (ITS) have been proposed, including a tiered testing approach combining different nonanimal testing with different endpoints, which have been used for regulatory purposes, on a case-by-case basis and within integrated approaches to testing and assessment (IATA), to identify materials according to their ability to trigger eye damage. In particular, the top-down and bottom-up approaches have been recommended for the C&L of materials, which cause serious eye damage or eye irritation, respectively. This chapter describes detailed protocols for eye irritation testing based on cells (Short Time Exposure-STE, OECD No. 491/2017), a vascularized membrane (the Hen's Egg Test-Chorioallantoic Membrane-HET-CAM) and corneal tissue (Bovine Corneal Opacity and Permeability-BCOP, OECD No. 437/2017), which can be applied using top-down or bottom-up approaches. In addition, it suggests making a corneal histomorphometric evaluation as an additional parameter in the BCOP method to differentiate materials that cause serious eye tissue damage (UN GHS Cat. 1) from materials that have reversible eye irritation effects (UN GHS Cat. 2).

Multiple-endpoint in vitro carcinogenicity test in human cell line TK6 distinguishes carcinogens from non-carcinogens and highlights mechanisms of action

Current in vitro genotoxicity tests can produce misleading positive results, indicating an inability to effectively predict a compound's subsequent carcinogenic potential in vivo. Such oversensitivity can incur unnecessary in vivo tests to further investigate positive in vitro results, supporting the need to improve in vitro tests to better inform risk assessment. It is increasingly acknowledged that more informative in vitro tests using multiple endpoints may support the correct identification of carcinogenic potential. The present study, therefore, employed a holistic, multiple-endpoint approach using low doses of selected carcinogens and non-carcinogens (0.001-770 µM) to assess whether these chemicals caused perturbations in molecular and cellular endpoints relating to the Hallmarks of Cancer. Endpoints included micronucleus induction, alterations in gene expression, cell cycle dynamics, cell morphology and bioenergetics in the human lymphoblastoid cell line TK6. Carcinogens ochratoxin A and oestradiol produced greater Integrated Signature of Carcinogenicity scores for the combined endpoints than the "misleading" in vitro positive compounds, quercetin, 2,4-dichlorophenol and quinacrine dihydrochloride and toxic non-carcinogens, caffeine, cycloheximide and phenformin HCl. This study provides compelling evidence that carcinogens can successfully be distinguished from non-carcinogens using a holistic in vitro test system. Avoidance of misleading in vitro outcomes could lead to the reduction and replacement of animals in carcinogenicity testing.

Skin Sensitization Tests: The LLNA and the RhE IL-18 Potency Assay

Contact allergy is of considerable importance to the toxicologist, and regulatory authorities worldwide require testing for skin sensitization potential and appropriate hazard labeling to enable management of the risk to human health. Although traditionally the identification of skin-sensitizing chemicals has been carried out using animal models, in Europe legislative changes have promoted, and now require, the use of non-animal methods (i.e., Cosmetic Directive, REACH). Several in vitro alternatives for hazard identification have now been validated, but do not provide information on the potency of a skin sensitizer. Here, we describe an animal model, the local lymph node assay (LLNA), and an in vitro model, the RhE IL-18 potency assay, in the context of the identification and potency classification of skin sensitizers. These two assays have been chosen among the different available tests as representative of an alternative in vivo model (the LLNA) and a promising in vitro method with the potential of both hazard identification and potency classification.

Development and Validation of Disease Assays for Phenotypic Screening

Screening in phenotypic assays is an important strategy for the discovery of innovative drugs and novel targets. Here we present key strategies for developing successful phenotypic screens and prosecuting phenotypic drug discovery (PDD) programs. Successful screens incorporate physiological relevance through the use of human cell types and assay designs that have (1) strong mechanistic connection to clinical outcomes and (2) strong biological justification for both efficacy and safety. In addition to guidance for designing successful screens, we also propose incorporation of specific counterscreens at an early point in the program. The suggested counterscreens are based on analysis of 1000s of drugs and drug candidates profiled through a large set of human-based phenotypic assays. These assays include cytotoxicity in human primary vascular endothelial cells, proliferation of endothelial cells, and proliferation of lymphocytes, all under specific activation conditions. These counterscreens form a generic screening funnel to triage a large fraction of early-stage hits, binning compounds into those with undesirable mechanisms (associated with acute toxicity), mechanisms with utility for oncology indications, and mechanisms useful for autoimmune indications. The application of this screening funnel offers a standardized and more predictable path for prosecuting PDD programs, reducing the risk of failure, and improving program timelines.

Comparing the preclinical risk profile of inhalable candidate and potential candidate modified risk tobacco products: A bridging use case

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Heated tobacco products tested for reduced exposure and reduced risk properties.

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Bridging opportunities for nonclinical results from two heated tobacco products.

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Similarly reduced impact on apical and molecular endpoints relative to cigarettes.

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Evidence evaluated along a “causal chain of events leading to disease” (CELSD).

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Representative assays along CELSD could support nonclinical substantial equivalence.

Cigarette smoking causes major preventable diseases, morbidity, and mortality worldwide. Smoking cessation and prevention of smoking initiation are the preferred means for reducing these risks. Less harmful tobacco products, termed modified-risk tobacco products (MRTP), are being developed as a potential alternative for current adult smokers who would otherwise continue smoking. According to a regulatory framework issued by the US Food and Drug Administration, a manufacturer must provide comprehensive scientific evidence that the product significantly reduces harm and the risk of tobacco-related diseases, in order to obtain marketing authorization for a new MRTP. For new tobacco products similar to an already approved predicate product, the FDA has foreseen a simplified procedure for assessing “substantial equivalence”. In this article, we present a use case that bridges the nonclinical evidence from previous studies demonstrating the relatively reduced harm potential of two heat-not-burn products based on different tobacco heating principles. The nonclinical evidence was collected along a “causal chain of events leading to disease” (CELSD) to systematically follow the consequences of reduced exposure to toxicants (relative to cigarette smoke) through increasing levels of biological complexity up to disease manifestation in animal models of human disease. This approach leverages the principles of systems biology and toxicology as a basis for further extrapolation to human studies. The experimental results demonstrate a similarly reduced impact of both products on apical and molecular endpoints, no novel effects not seen with cigarette smoke exposure, and an effect of switching from cigarettes to either MRTP that is comparable to that of complete smoking cessation. Ideally, a subset of representative assays from the presented sequence along the CELSD could be sufficient for predicting similarity or substantial equivalence in the nonclinical impact of novel products; this would require further validation, for which the present use case could serve as a starting point.

Titanium salts tested in reconstructed human skin with integrated MUTZ-3-derived Langerhans cells show an irritant rather than a sensitizing potential

BACKGROUND

The nature of clinically related adverse reactions to titanium is still unknown.

OBJECTIVE

To determine whether titanium salts have irritant or sensitizing potential in a reconstructed human skin (RHS) model with integrated Langerhans cells (LCs).

METHODS

RHS-LCs (ie, reconstructed epidermis) containing primary differentiated keratinocytes and CFSE⁺ CD1a⁺ -LCs generated from the MUTZ-3 cell line on a primary fibroblast-populated collagen hydrogel (dermis) were topically exposed to titanium(IV) bis(ammonium lactato)dihydroxide (TiALH). LC migration and plasticity were determined.

RESULTS

TiALH resulted in CFSE⁺ CD1a⁺ -LC migration out of the epidermis. Neutralizing antibodies to CCL5 and CXCL12 showed that LC migration was CCL5 and not CXCL12 mediated. LCs accumulating within the dermis after TiALH exposure were CFSE⁺ Lang⁺ CD68⁺ which is characteristic of a phenotypic switch of MUTZ-LC to a macrophage-like cell. Furthermore, TiALH did not result in increased interleukin (IL)-1β or CCR7 messenger RNA (mRNA) in the dermis, but did result in increased IL-10 mRNA. In addition, monocultures of MUTZ-LCs failed to increase LC maturation biomarkers CD83, CD86, and CXCL-8 when exposed to noncytotoxic concentrations of four different titanium salts.

CONCLUSION

These results classify titanium salts as irritants rather than sensitizers and indicate that titanium implant-related complaints could be due to localized irritant-mediated inflammation arising from leachable agents rather than a titanium metal allergy.

Variability in in vivo studies: Defining the upper limit of performance for predictions of systemic effect levels

New approach methodologies (NAMs) for chemical hazard assessment are often evaluated via comparison to animal studies; however, variability in animal study data limits NAM accuracy. The US EPA Toxicity Reference Database (ToxRefDB) enables consideration of variability in effect levels, including the lowest effect level (LEL) for a treatment-related effect and the lowest observable adverse effect level (LOAEL) defined by expert review, from subacute, subchronic, chronic, multi-generation reproductive, and developmental toxicity studies. The objectives of this work were to quantify the variance within systemic LEL and LOAEL values, defined as potency values for effects in adult or parental animals only, and to estimate the upper limit of NAM prediction accuracy. Multiple linear regression (MLR) and augmented cell means (ACM) models were used to quantify the total variance, and the fraction of variance in systemic LEL and LOAEL values explained by available study descriptors (e.g., administration route, study type). The MLR approach considered each study descriptor as an independent contributor to variance, whereas the ACM approach combined categorical descriptors into cells to define replicates. Using these approaches, total variance in systemic LEL and LOAEL values (in log10-mg/kg/day units) ranged from 0.74 to 0.92. Unexplained variance in LEL and LOAEL values, approximated by the residual mean square error (MSE), ranged from 0.20-0.39. Considering subchronic, chronic, or developmental study designs separately resulted in similar values. Based on the relationship between MSE and R-squared for goodness-of-fit, the maximal R-squared may approach 55 to 73% for a NAM-based predictive model of systemic toxicity using these data as reference. The root mean square error (RMSE) ranged from 0.47 to 0.63 log10-mg/kg/day, depending on dataset and regression approach, suggesting that a two-sided minimum prediction interval for systemic effect levels may have a width of 58 to 284-fold. These findings suggest quantitative considerations for building scientific confidence in NAM-based systemic toxicity predictions.

The EU-ToxRisk method documentation, data processing and chemical testing pipeline for the regulatory use of new approach methods

Hazard assessment, based on new approach methods (NAM), requires the use of batteries of assays, where individual tests may be contributed by different laboratories. A unified strategy for such collaborative testing is presented. It details all procedures required to allow test information to be usable for integrated hazard assessment, strategic project decisions and/or for regulatory purposes. The EU-ToxRisk project developed a strategy to provide regulatorily valid data, and exemplified this using a panel of > 20 assays (with > 50 individual endpoints), each exposed to 19 well-known test compounds (e.g. rotenone, colchicine, mercury, paracetamol, rifampicine, paraquat, taxol). Examples of strategy implementation are provided for all aspects required to ensure data validity: (i) documentation of test methods in a publicly accessible database; (ii) deposition of standard operating procedures (SOP) at the European Union DB-ALM repository; (iii) test readiness scoring accoding to defined criteria; (iv) disclosure of the pipeline for data processing; (v) link of uncertainty measures and metadata to the data; (vi) definition of test chemicals, their handling and their behavior in test media; (vii) specification of the test purpose and overall evaluation plans. Moreover, data generation was exemplified by providing results from 25 reporter assays. A complete evaluation of the entire test battery will be described elsewhere. A major learning from the retrospective analysis of this large testing project was the need for thorough definitions of the above strategy aspects, ideally in form of a study pre-registration, to allow adequate interpretation of the data and to ensure overall scientific/toxicological validity.

Nonclinical regulatory immunotoxicity testing of nanomedicinal products: Proposed strategy and possible pitfalls

Various nanomedicinal products (NMPs) have been reported to induce an adverse immune response, which may be related to their tendency to accumulate in or target cells of the immune system. Therefore, before their market authorization, NMPs should be thoroughly evaluated for their immunotoxic potential. Nonclinical regulatory immunotoxicity testing of nonbiological medicinal products, including NMPs, is currently performed by following the guideline S8 “Immunotoxicity Studies for Human Pharmaceuticals” of the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use (ICH). However, this guideline does not cover all the immunotoxicity endpoints reported for NMPs in the literature, such as complement activation related pseudo allergy, hypersensitivity and immunosuppression. In addition, ICH‐S8 does not provide any nanospecific testing considerations, which is important given their tendency to interfere with many commonly used toxicity assays. We therefore propose a nonclinical regulatory immunotoxicity assessment strategy, which considers the immunotoxicity endpoints currently missing in the ICH‐S8. We also list the known pitfalls related to the testing of NMPs and how to tackle them. Next to defining the relevant physicochemical and pharmacokinetic properties of the NMP and its intended use, the proposed strategy includes an in vitro assay battery addressing various relevant immunotoxicity endpoints. A weight of evidence evaluation of this information can be used to shape the type and design of further in vivo investigations. The final outcome of the immunotoxicity assessment can be included in the overall risk assessment of the NMP and provide alerts for relevant endpoints to address during clinical investigation.

This article is categorized under:

Toxicology and Regulatory Issues in Nanomedicine > Regulatory and Policy Issues in Nanomedicine

Toxicology and Regulatory Issues in Nanomedicine > Toxicology of Nanomaterials

Innovative strategy based on mechanisms to substitute animal testing for ocular toxicity assessment of agrochemical formulations market in Brazil

Considering the successful employment of alternative methods for eye toxicity assessment of products for regulatory purposes, and the recent advances in Brazilian legislative scenario, which adopted the UN GHS classification system for agrochemical formulations toxicity assessment, there is an emerging demand for strategies that allow the evaluation of such products. Based on this, the present study aimed to address the applicability of a mechanistic-based defined approach for eye toxicity assessment of agrochemical formulations. It was investigated the opacity/permeability, depth and location of corneal injury in bovine cornea, and vascular events in chorioallantoic membrane induced for different Brazilian agrochemicals using a Sequential Testing Strategy (STS). Cytotoxicity induced by the agrochemical formulations was evaluated by Short Time exposure (STE) (OECD TG 491) assay (step 1), corneal injury was investigated by standard Bovine Corneal Opacity and Permeability (BCOP) (OECD TG 437) followed by histopathological evaluation (step 2), and Hen Chorionic-allantoic Membrane test (HET-CAM) was used to evaluate vascular injury (step 3). The results demonstrated that the proposed defined approach enabled a classification corresponding UN GHS classification of agrochemical formulations while minimizing the use of live animals. Therefore, this approach may be useful for categorization of agrochemicals in Brazil according to the new regulatory scenario.

Going All In: A Strategic Investment in In Silico Toxicology

As vast numbers of new chemicals are introduced to market annually, we are faced with the grand challenge of protecting humans and the environment while minimizing economically and ethically costly animal testing. In silico models promise to be the solution we seek, but we find ourselves at crossroads of future development efforts that would ensure standalone applicability and reliability of these tools. A conscientious effort that prioritizes experimental testing to support the needs of in silico models (versus regulatory needs) is called for to achieve this goal. Using economic analogy in the title of this work, we argue that a prudent investment is to go all-in to support in silico model development, rather than gamble our future by keeping the status quo of a "balanced portfolio" of testing approaches. We discuss two paths to future in silico toxicology-one based on big-data statistics ("broadsword"), and the other based on direct modeling of molecular interactions ("scalpel")-and offer rationale that the latter approach is more transparent, is better aligned with our quest for fundamental knowledge, and has a greater potential to succeed if we are willing to transform our toxicity-testing paradigm.

The Overt and Hidden Use of Animal-Derived Products in Alternative Methods for Skin Sensitisation: A Systematic Review

In vitro methods that can replace animal testing in the identification of skin sensitisers are now a reality. However, as cell culture and related techniques usually rely on animal-derived products, these methods may be failing to address the complete replacement of animals in safety assessment. The objective of this study was to identify the animal-derived products that are used as part of in vitro methods for skin sensitisation testing. Thus, a systematic review of 156 articles featuring 83 different in vitro methods was carried out and, from this review, the use of several animal-derived products from different species was identified, with the use of fetal bovine serum being cited in most of the methods (78%). The use of sera from other animals, monoclonal antibodies and animal proteins were also variously mentioned. While non-animal alternatives are available and methods free of animal-derived products are emerging, most of the current methods reported used at least one animal-derived product, which raises ethical and technical concerns. Therefore, to deliver technically and ethically better in vitro methods for the safety assessment of chemicals, more effort should be made to replace products of animal origin in existing methods and to avoid their use in the development of new method protocols.

Development of a prioritization method for chemical-mediated effects on steroidogenesis using an integrated statistical analysis of high-throughput H295R data

Synthesis of 11 steroid hormones in human adrenocortical carcinoma cells (H295R) was measured in a high-throughput steroidogenesis assay (HT-H295R) for 656 chemicals in concentration-response as part of the US Environmental Protection Agency's ToxCast program. This work extends previous analysis of the HT-H295R dataset and model by examining the utility of a novel prioritization metric based on the Mahalanobis distance that reduced these 11-dimensional data to 1-dimension via calculation of a mean Mahalanobis distance (mMd) at each chemical concentration screened for all hormone measures available. Herein, we evaluated the robustness of mMd values, and demonstrate that covariance and variance of the hormones measured appear independent of the chemicals screened and are inherent to the assay; the Type I error rate of the mMd method is less than 1%; and, absolute fold changes (up or down) of 1.5 to 2-fold have sufficient power for statistical significance. As a case study, we examined hormone responses for aromatase inhibitors in the HT-H295R assay and found high concordance with other ToxCast assays for known aromatase inhibitors. Finally, we used mMd and other ToxCast cytotoxicity data to demonstrate prioritization of the most selective and active chemicals as candidates for further in vitro or in silico screening.

The 21st Century movement within the area of skin sensitization assessment: From the animal context towards current human-relevant in vitro solutions

In a regulatory context, skin sensitization hazard and risk evaluations of manufactured products and their ingredients (e.g. cosmetics) are mandatory in several regions. Great efforts have been made within the field of 21st Century Toxicology to provide non-animal testing approaches to assess the skin allergy potential of materials (e.g. chemicals, mixtures, nanomaterials, particles). Mechanistic understanding of skin sensitization process through the adverse outcome pathway (AOP) has promoted the development of in vitro methods, demonstrating accuracies superior to the traditional animal testing. These in vitro testing approaches are based on one of the four AOP key events (KE) of skin sensitization: formation of immunogenic hapten-protein complexes (KE-1 or the molecular initiating event, MIE), inflammatory keratinocyte responses (KE-2), dendritic cell activation (KE-3), and T-lymphocyte activation and proliferation (KE-4). This update provides an overview of the historically used in vivo methods as well as the current in chemico and in cell methods with and without OECD guideline designations to analyze the progress towards human-relevant in vitro test methods for safety assessment of the skin allergenicity potential of materials. Here our focus is to review 96 in vitro testing approaches directed to the KEs of the skin sensitization AOP.

Improved metal allergen reactivity of artificial skin models by integration of Toll-like receptor 4-positive cells

BACKGROUND

Reconstructed human epidermis (RhE) is widely used to replace animal models in order to assess the proinflammatory and allergenic effects of chemicals. Unfortunately, RhE lacks proinflammatory responsiveness for metal haptens, which are the most prevalent human contact allergens, raising concerns about its reliability for predicting skin allergens.

OBJECTIVES

To investigate whether this limitation of RhE might be attributable to a lack of functional expression of Toll-like receptor 4 (TLR4), which governs proinflammatory sensitivity to nickel and cobalt.

MATERIALS AND METHODS

RhE, dendritic cell (DC)-containing RhE and full-thickness skin equivalent (FTSE) were compared regarding their proinflammatory responsiveness to metal allergens.

RESULTS

The incorporation of dermal fibroblasts was sufficient to confer metal sensitivity to RhE. Unlike keratinocytes, normal human fibroblasts expressed high levels of TLR4 mRNA and induced interleukin-8 expression upon stimulation with nickel or cobalt. Consistently, dermal isolates from FTSE expressed considerable amounts of TLR4 mRNA, whereas RhE or epidermis isolated from FTSE, normal human epidermis or inflamed human epidermis failed to express TLR4. Similarly, co-culture with TLR4-positive DCs bestowed RhE with proinflammatory responsiveness to metals.

CONCLUSION

Our data suggest that FTSE or DC/RhE co-culture models can circumvent the shortcomings of RhE assays, and combine the benefits of complex and monoculture-based test systems in a single assay.

Nonanimal Models for Acute Toxicity Evaluations: Applying Data-Driven Profiling and Read-Across

BACKGROUND

Low-cost, high-throughput in vitro bioassays have potential as alternatives to animal models for toxicity testing. However, incorporating in vitro bioassays into chemical toxicity evaluations such as read-across requires significant data curation and analysis based on knowledge of relevant toxicity mechanisms, lowering the enthusiasm of using the massive amount of unstructured public data.

OBJECTIVE

We aimed to develop a computational method to automatically extract useful bioassay data from a public repository (i.e., PubChem) and assess its ability to predict animal toxicity using a novel bioprofile-based read-across approach.

METHODS

A training database containing 7,385 compounds with diverse rat acute oral toxicity data was searched against PubChem to establish in vitro bioprofiles. Using a novel subspace clustering algorithm, bioassay groups that may inform on relevant toxicity mechanisms underlying acute oral toxicity were identified. These bioassays groups were used to predict animal acute oral toxicity using read-across through a cross-validation process. Finally, an external test set of over 600 new compounds was used to validate the resulting model predictivity.

RESULTS

Several bioassay clusters showed high predictivity for acute oral toxicity (positive prediction rates range from 62-100%) through cross-validation. After incorporating individual clusters into an ensemble model, chemical toxicants in the external test set were evaluated for putative acute toxicity (positive prediction rate equal to 76%). Additionally, chemical fragment -in vitro-in vivo relationships were identified to illustrate new animal toxicity mechanisms.

CONCLUSIONS

The in vitro bioassay data-driven profiling strategy developed in this study meets the urgent needs of computational toxicology in the current big data era and can be extended to develop predictive models for other complex toxicity end points. https://doi.org/10.1289/EHP3614.

Environmental hazard and risk assessment of thiochemicals. Application of integrated testing and intelligent assessment strategies (ITS) to fulfil the REACH requirements for aquatic toxicity

REACH requires information on hazardous properties of substances to be generated avoiding animal testing where possible. It is the objective of the present case study with thiochemicals to extract as much information as possible from available experimental data with fish, daphnia and algae and to fill data gaps for analogues to be registered under REACH in 2018. Based on considerations of chemical similarity and common mode of action (MOA) the data gaps regarding the aquatic toxicity of the thiochemicals were largely closed by trend analysis ("category approach") and read-across within the same group, for example, thioglycolates or mercaptopropionates. Among 16 thiochemicals to be registered by 2018 there are only 2 substances with sufficient data. 36 data gaps for 14 thiochemicals were identified. Most of the required data (>60%) could be estimated by in silico methods. Only 14 tests (6 algae, 6 daphnia, 1 limit fish test and 1 acute fish test) were proposed. When the results of these tests are available it has to be discussed whether 2 further fish (limit) tests are required. For two substances (exposure-based) waiving was suggested. The relatively high toxicity of the thiochemicals is manifested in low predicted no-effect concentrations (PNECs). Only preliminary predicted environmental concentrations (PECs) could be derived for the thiochemicals for which a risk assessment has to be performed (production rate >10 t/y). The preliminary PEC/PNEC ratios indicate no risk for the aquatic compartment at the production site. PECs due to down-stream use must not exceed the estimated PNECs.

Perspectives on In Vitro to In Vivo Extrapolations

Quantitative in vitro to in vivo extrapolation (QIVIVE) is broadly considered a prerequisite bridge from in vitro findings to a dose paradigm. Quality and relevance of cell systems are the first prerequisite for QIVIVE. Information-rich and mechanistic endpoints (biomarkers) improve extrapolations, but a sophisticated endpoint does not make a bad cell model a good one. The next need is reverse toxicokinetics (TK), which estimates the dose necessary to reach a tissue concentration that is active in vitro. The Johns Hopkins Center for Alternatives to Animal Testing (CAAT) has created a roadmap for animal-free systemic toxicity testing, in which the needs and opportunities for TK are elaborated, in the context of different systemic toxicities. The report was discussed at two stakeholder forums in Brussels in 2012 and in Washington in 2013; the key recommendations are summarized herein. Contrary to common belief and the Paracelsus paradigm of everything is toxic, the majority of industrial chemicals do not exhibit toxicity. Strengthening the credibility of negative results of alternative approaches for hazard identification, therefore, avoids the need for QIVIVE. Here, especially the combination of methods in integrated testing strategies is most promising. Two further but very different approaches aim to overcome the problem of modeling in vivo complexity: The human-on-a-chip movement aims to reproduce large parts of living organism's complexity via microphysiological systems, that is, organ equivalents combined by microfluidics. At the same time, the Toxicity Testing in the 21st Century (Tox-21c) movement aims for mechanistic approaches (adverse outcome pathways as promoted by Organisation for Economic Co-operation and Development (OECD) or pathways of toxicity in the Human Toxome Project) for high-throughput screening, biological phenotyping, and ultimately a systems toxicology approach through integration with computer modeling. These 21st century approaches also require 21st century validation, for example, by evidence-based toxicology. Ultimately, QIVIVE is a prerequisite for extrapolating Tox-21c such approaches to human risk assessment.

Comparison of the skin sensitization potential of 3 red and 2 black tattoo inks using interleukin-18 as a biomarker in a reconstructed human skin model

BACKGROUND

During the last decade, the number of people with ≥1 tattoo has increased noticeably within the European population. Despite this, limited safety information is available for tattoo inks.

OBJECTIVES

To test the skin sensitization potential of 5 tattoo inks in vitro by using reconstructed human skin (RHS) and the contact sensitization biomarker interleukin (IL)-18.

METHODS

Two red and 3 black tattoo inks, 1 additive (Hamamelis virginiana extract) and 1 irritant control (lactic acid) were tested. The culture medium of RHS (reconstructed epidermis on a fibroblast-populated collagen hydrogel) was supplemented with test substances in a dose-dependent manner for 24 hours, after which cytotoxicity (histology; thiazolyl blue tetrazolium bromide assay) and skin sensitization potential (IL-18 secretion; enzyme-linked immunosorbent assay) were assessed.

RESULTS

All but 1 ink showed cytotoxicity. Notably, 1 red ink and 1 black ink were able to cause an inflammatory response, indicated by substantial release of IL-18, suggesting that these inks may be contact sensitizers.

CONCLUSIONS

The in vitro RHS model showed that 4 tattoo inks were cytotoxic and 2 were able to cause an inflammatory IL-18 response, indicating that an individual may develop allergic contact dermatitis when exposed to these tattoo inks, as they contain contact sensitizers.

Skin sensitization testing needs and data uses by US regulatory and research agencies

United States regulatory and research agencies may rely upon skin sensitization test data to assess the sensitization hazards associated with dermal exposure to chemicals and products. These data are evaluated to ensure that such substances will not cause unreasonable adverse effects to human health when used appropriately. The US Consumer Product Safety Commission, the US Environmental Protection Agency, the US Food and Drug Administration, the Occupational Safety and Health Administration, the National Institute for Occupational Safety and Health, and the US Department of Defense are member agencies of the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM). ICCVAM seeks to identify opportunities for the use of non-animal replacements to satisfy these testing needs and requirements. This review identifies the standards, test guidelines, or guidance documents that are applicable to satisfy each of these agency's needs; the current use of animal testing and flexibility for using alternative methodologies; information needed from alternative tests to fulfill the needs for skin sensitization data; and whether data from non-animal alternative approaches are accepted by these US federal agencies.