The Myeloid U937 Skin Sensitization Test (U-SENS) addresses the activation of dendritic cell event in the adverse outcome pathway for skin sensitization

Skin sensitisation testing in practice: Applying a stacking meta model to cosmetic ingredients

Recently, several non-animal approaches contributing to the identification of skin sensitisation hazard have been introduced. Their validation and acceptance has largely been directed towards regulatory classification. Considering the driving force for replacement of in vivo tests centred on cosmetics, it is reasonable to ask how well the new approaches perform in this respect. In the present study, 219 substances, largely cosmetic raw materials (including dyes, preservatives and fragrances), have been evaluated in our Defined Approach integrating a stacking meta model (version 5), incorporating the individual outcomes of 3 in vitro validated methods (Direct Peptide Reactivity Assay, Keratinosens™, U-SENS™), 2 in silico tools (TIMES SS, TOXTREE) and physicochemical parameters (volatility, pH). Stacking meta model outcomes were compared with existing local lymph node assay (LLNA) data. Non-sensitisers comprised 68/219; 86 were weak/moderate and 65 were stronger sensitisers. The model version revision demonstrate the gain to discriminate sensitizers to non-sensitiser when the in silico TIMES model is incorporated as input parameter. The 85% to 91% accuracy for the cosmetics categories, indicates the stacking meta model offers value for the next generation risk assessment framework. These results pinpoint the power of the stacking meta model relying on a confidence based on the probability given in any individual prediction.

Drug-induced skin toxicity: gaps in preclinical testing cascade as opportunities for complex in vitro models and assays

Skin is the largest organ of the body and serves as the principle barrier to the environment. Composed of multiple cell types arranged in stratified layers with highly specialized appendages, it serves sensory and immune surveillance roles in addition to its primary mechanical function. Several complex in vitro models of skin (i.e. microphysiological systems (MPS) including but not limited to 3D tissues, organ-on-a-chip, organoids), have been developed and assays validated for regulatory purposes. As such, skin is arguably the most advanced organ with respect to model development and adoption across industries including chemical, cosmetic, and to a somewhat lesser extent, pharmaceutical. Early adoption of complex skin models and associated assays for assessment of irritation and corrosion spurred research into other areas such as sensitization, absorption, phototoxicity, and genotoxicity. Despite such considerable advancements, opportunities remain for immune capabilities, inclusion of appendages such as hair follicles, fluidics, and innervation, among others. Herein, we provide an overview of current complex skin model capabilities and limitations within the drug development scheme, and recommendations for future model development and assay qualification and/or validation with the intent to facilitate wider adoption of use within the pharmaceutical industry.

Adjustment of a no expected sensitization induction level derived from Bayesian network integrated testing strategy for skin sensitization risk assessment

Skin sensitization is a key adverse effect to be addressed during hazard identification and risk assessment of chemicals, because it is the first step in the development of allergic contact dermatitis. Multiple non-animal testing strategies incorporating in vitro tests and in silico tools have achieved good predictivities when compared with murine local lymph node assay (LLNA). The binary test battery of KeratinoSens^TM and h-CLAT could be used to classify non-sensitizers as the first part of bottom-up approach. However, the quantitative risk assessment for sensitizing chemicals requires a No Expected Sensitization Induction Level (NESIL), the dose not expected to induce skin sensitization in humans. We used Bayesian network integrated testing strategy (BN ITS-3) for chemical potency classification. BN ITS-3 predictions were performed without a pre-processing step (selecting data from their physic-chemical applicability domains) or post-processing step (Michael acceptor chemistry correction), neither of which necessarily improve prediction accuracy. For chemicals within newly defined applicability domain, all under-predictions fell within one potency class when compared with LLNA results, indicating no chemicals that were incorrectly classified by more than one class. Considering the potential under-prediction by one class, a worst case value to each class from BN ITS-3 was used to derive a NESIL. When in vivo and human data from suitable analogs cannot be used to estimate the uncertainty, adjusting the NESIL derived from BN ITS-3 may help perform skin sensitization risk assessment. The overall workflow for risk assessment was demonstrated by incorporating the binary test battery of KeratinoSens^TM and h-CLAT.

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.

Assessment of antioxidant and antidiabetic properties of Agaricus blazei Murill extracts

Agaricus blazei Murill (ABM), a medicinal mushroom, has beneficial effects on various human metabolic diseases. The objective of this research was to evaluate the antioxidant and antidiabetic properties of ABM extracts (ethanol extract and ethyl acetate extract). The antioxidant activities of ABM ethanol extract (EE) and ethyl acetate extract (EA) were analyzed using 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and hydroxyl radical scavenging assays and the reducing power using K3Fe(CN)6 in vitro. Moreover, the effects of EE and EA on α-glucosidase inhibitory activity and improving glucose uptake by HepG2 cells were investigated in vitro. The EA showed stronger antioxidant activity, as well as inhibition of α-glucosidase, compared to EE. The analysis of glucose uptake by HepG2 cells showed that EA had significant glucose-lowering activity and exhibited no difference compared to metformin. The results suggest that ABM extracts could improve the glucose uptake by HepG2 cells and thereby alleviate postprandial hyperglycemia. This investigation provides a strong rationale for further studies on the application of ABM to control type 2 diabetes.

Hepatic Bioactivation of Skin-Sensitizing Drugs to Immunogenic Reactive Metabolites

The clinical use of some drugs, such as carbamazepine, phenytoin, and allopurinol, is often associated with adverse cutaneous reactions. The bioactivation of drugs into immunologically reactive metabolites by the liver is postulated to be the first step in initiating a downstream cascade of pathological immune responses. Current mechanistic understanding and the ability to predict such adverse drug cutaneous responses have been partly limited by the lack of appropriate cutaneous drug bioactivation experimental models. Although in vitro human liver models have been extensively investigated for predicting hepatotoxicity and drug-drug interactions, their ability to model the generation of antigenic reactive drug metabolites that are capable of eliciting immunological reactions is not well understood. Here, we employed a human progenitor cell (HepaRG)-derived hepatocyte model and established highly sensitive liquid chromatography-mass spectrometry analytical assays to generate and quantify different reactive metabolite species of three paradigm skin sensitizers, namely, carbamazepine, phenytoin, and allopurinol. We found that the generation of reactive drug metabolites by the HepaRG-hepatocytes was sensitive to the medium composition. In addition, a functional assay based on the activation of U937 myeloid cells into the antigen-presenting cell (APC) phenotype was established to evaluate the immunogenicity potential of the reactive drug metabolites produced by HepaRG-derived hepatocytes. We showed that the reactive drug metabolites of known skin sensitizers could significantly upregulate IL8, IL1β, and CD86 expressions in U937 cells compared to the metabolites from a nonskin sensitizer (i.e., acetaminophen). Thus, the extent of APC activation by HepaRG-hepatocytes conditioned medium containing reactive drug metabolites can potentially be used to predict their skin sensitization potential.

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.

RIFM fragrance ingredient safety assessment, methyl ionone (mixture of isomers), CAS registry number 1335-46-2

Methyl ionone (mixture of isomers) was evaluated for genotoxicity, repeated dose toxicity, developmental and reproductive toxicity, local respiratory toxicity, phototoxicity/photoallergenicity, skin sensitization, and environmental safety. Data from methyl ionone (mixture of isomers) show that the material is not genotoxic and provided a NESIL of 70,000 μg/cm² for the skin sensitization endpoint. Data provided a calculated MOE >100 for the repeated dose toxicity and developmental toxicity endpoints, and data from read-across material (E)-β-ionone (CAS # 79-77-6) provided a calculated MOE >100 for the reproductive toxicity endpoint. For the local respiratory endpoint, a calculated MOE >100 was provided by the read-across material β-ionone (CAS # 14901-07-6). The phototoxicity/photoallergenicity endpoints were evaluated based on data and UV spectra; the material is not phototoxic/photoallergenic. The environmental endpoints were evaluated with data from the target chemical and read-across material α-allylionone (CAS # 79-78-7), and the material was not found to be PBT; its risk quotients, based on current volume of use in Europe and North America (PEC/PNEC), are <1.

In vitro assessment of mechanistic events induced by structurally related chemical rubber sensitizers

Allergic contact dermatitis (ACD) is one of the most common forms of immunotoxicity, and increased understanding of how chemicals trigger these adverse reactions is needed in order to treat or design testing strategies to identify and subsequently avoid exposure to such substances. In this study, we investigated the cellular response induced by rubber chemicals in a dendritic cell (DC) model, focusing on the structurally similar chemicals diethylthiocarbamylbenzothiazole sulfide and dimethylthiocarbamylbenzothiazole sulfide, with regard to regulation of microRNA, and messenger RNA expression. Only a few miRNAs were found to be commonly regulated by both rubber chemicals, among them miR1973, while the overall miRNA expression profiles were diverse. Similarly, out of approximately 500 differentially regulated transcripts for each chemical, about 60% overlapped, while remaining were unique. The pathways predicted to be enriched in the cell model by stimulation with the rubber chemicals were linked to immunological events, relevant in the context of ACD. These results suggest that small structural differences can trigger specific activation of the immune system in response to chemicals. The here presented mechanistic data can be valuable in explaining the immunotoxicological events in DC activation after exposure to skin sensitizing chemicals, and can contribute to understanding, preventing and treating ACD.

Skin toxicology and 3Rs-Current challenges for public health protection

Driven by the fast paced development of complex test systems in vitro, mass spectrometry and omics, we finally have the tools to unravel the molecular events that underlie toxicological adversity. Yet, timely regulatory adaptation of these new tools continues to pose major challenges even for organs readily accessible such as skin. The reasons for this encompass a need for conservatism as well as the need of tests to serve an existing regulatory framework rather than to produce scientific knowledge. It is important to be aware of this in order to align regulatory skin toxicity with the 3R principles more readily. While most chemical safety testing is still based on animal data, regulatory frameworks have seen a strong push towards non-animal approaches. The endpoints corrosion, irritation, sensitisation, absorption and phototoxicity, for example, can now be covered in vitro with the corresponding test guidelines (TGs) being made available by the OECD. However, in vitro approaches tend to be more reductionist. Hence, a combination of several tests is usually preferable to achieve satisfying predictivity. Moreover, the test systems and their combined use need to be standardised and are therefore subject not only to validation but also to the ongoing development of so-called integrated approaches to testing and assessment (IATAs). Concomitantly, skin models are being refined to deliver the complexity required for increased applicability and predictivity. Given the importance of regulatory applicability for 3R-derived approaches to have a long-lasting impact, this review examines the state of regulatory implementation and perspectives, respectively.

Skin sensitization: Uncertainties, challenges, and opportunities for improved risk assessment

At the ESCD congress held in Manchester in 2016, a session was organized to encourage more dialogue between clinicians with expertise in skin sensitization and toxicologists seeking to provide effective risk assessment to prevent human health issues. That session focused on the remaining uncertainties regarding the induction and regulation of skin sensitization in humans, and the opportunities and challenges associated with the refinement and improvement of risk assessment methodologies. This short article, prompted by those discussions, debates what the authors regard as being among the most important and most intriguing uncertainties about skin sensitization and allergic contact dermatitis in humans, and the most significant opportunities for improving risk assessment. The aim has been to provide a basis for mapping out the areas that might benefit from a closer alignment between the relevant clinical community and toxicologists charged with the responsibility of ensuring that skin sensitization risks are understood and managed.

The activity of methacrylate esters in skin sensitisation test methods: A review

Skin sensitisation associated with allergic contact dermatitis is an important occupational and environmental disease. The identification of skin sensitisation hazards was traditionally performed using animal tests; originally guinea pig assays and subsequently the murine local lymph node assay (LLNA). More recently there has, for a variety of reasons, been an increased interest in, and requirement for, non-animal assays. There are now available both validated in vitro assays and a variety of approaches based on consideration of quantitative structure-activity relationships (QSAR). With the increased availability and use of non-animal alternatives for skin sensitisation testing there is a continuing need to monitor the performance of these approaches using series of chemicals that do not normally form part of validation exercises. Here we report studies conducted with 11 methacrylate esters and methacrylic acid in which results obtained with 3 validated in vitro tests for which there are OECD guidelines (the Direct Peptide Reactivity Assay, DPRA; ARE-Nrf2 luciferase test methods, and - with some chemicals - a dendritic cell activation test, the myeloid U937 Skin Sensitisation test [U-SENS] assay) have been compared with QSAR approaches (DEREK and TIMES-SS), and with LLNA and guinea pig maximisation test (GPMT) data. The conclusions drawn from these data are that - with this series of chemicals at least - there is a strong correlation between the results of animal tests and the in vitro assays considered, but not with either DEREK or TIMES-SS.

A defined approach for predicting skin sensitisation hazard and potency based on the guided integration of in silico, in chemico and in vitro data using exclusion criteria

A decision tree-based defined approach (DA) has been designed using exclusion criteria based on applicability domain knowledge of in chemico/in vitro information sources covering key events 1-3 in the skin sensitisation adverse outcome pathway and an in silico tool predicting the adverse outcome (Derek Nexus). The hypothesis is that using exclusion criteria to de-prioritise less applicable assays and/or in silico outcomes produces a rational, transparent, and reliable DA for the prediction of skin sensitisation potential. Five exclusion criteria have been established: Derek Nexus reasoning level, Derek Nexus negative prediction, metabolism, lipophilicity, and lysine-reactivity. These are used to prioritise the most suitable information sources for a given chemical and results from which are used in a '2 out of 3' approach to provide a prediction of hazard. A potency category (and corresponding GHS classification) is then assigned using a k-Nearest Neighbours model containing human and LLNA data. The DA correctly identified the hazard (sensitiser/non-sensitiser) for 85% and 86% of a dataset with reference LLNA and human data. The correct potency category was identified for 59% and 68% of chemicals, and the GHS classification accurately predicted for 73% and 76% with reference LLNA and human data, respectively.

A liver-immune coculture array for predicting systemic drug-induced skin sensitization

Drug-induced skin sensitization is prevalent worldwide and can trigger life-threatening health conditions, such as Stevens Johnson Syndrome. However, existing in vitro skin models cannot adequately predict the skin sensitization effects of drugs administered into the systemic circulation because dermal inflammation and injury are preceded by conversion of parent drugs into antigenic reactive metabolites in the liver and subsequent activation of the immune system. Here, we demonstrate that recapitulation of these early tandem cellular processes in a compartmentalized liver-immune coculture array is sufficient to predict the skin sensitization potential of systemic drugs. Human progenitor cell (HepaRG)-derived hepatocyte spheroids and U937 myeloid cells, a representative antigen presenting cell (APC), can maintain their respective functions in 2 concentric micro-chambers, which are connected by a diffusion microchannel network. Paradigm drugs that are reported to cause severe cutaneous drug reactions (i.e. carbamazepine, phenytoin and allopurinol) can be metabolized into their reactive metabolites, which diffuse efficiently into the adjoining immune compartment within a 48 hour period. By measuring the extent of U937 activation as indicated by IL8, IL1β and CD86 upregulation upon drug administration, we show that the liver-immune coculture array more consistently and reliably distinguish all 3-paradigm skin sensitizing drugs from a non-skin sensitizer than conventional bulk Transwell coculture. Given its miniaturized format, design simplicity and prediction capability, this novel in vitro system can be readily scaled into a screenable platform to identify the skin sensitization potential of systemically-administered drugs.

Optimizing the cutoff for the identification of skin sensitizers by the HaCaSens assay: Introducing an ROC-analysis-based cutoff approach

The worldwide restricted use of animal testing makes it challenging to identify the skin sensitizing potentials of newly manufactured products. The HaCaSens assay has shown promise as an in vitro skin sensitizing assay comparable to existing assays, and is currently under pre-validation. However, there is little agreement on how to assess the results of the assay to discriminate sensitizers from non-sensitizers as the stimulation index (SI) cutoff value was arbitrarily chosen without appropriate statistical methods. Here, we investigated the SI cutoff values in identifying sensitizers to obtain the optimal value. Sensitivities and specificities were calculated for a set of 30 test substances, and plotted in receiver operator characteristics (ROC) curves. The SI cutoff values with the highest sum of sensitivity and specificity according to LLNA data were 2.2, 1.8 and 3.0 for interleukin 1α (IL-1α), interleukin 6 (IL-6), and the combination of the two cytokines respectively. Also, the same statistical analysis of human data demonstrated optimal SI cutoff values 2.0, 2.0 and 3.2 for the same respective parameters. When considering the predictive capacity of each possible SI cutoff value determined by ROC curves, the optimal value for HaCaSens is 3.0 for the combination of IL-1α and IL-6 as it had the highest sensitivity (90.9%), specificity (75.0%) and accuracy (86.7%) based on LLNA data. Thus, we recommend the wide use of the SI cutoff value of 3.0 to ensure consistent endpoints.

Tri-culture system for pro-hapten sensitizer identification and potency classification

Allergic contact dermatitis (ACD) is an inflammatory disease that impacts 15-20% of the general population and accurate screening methods for chemical risk assessment are needed. However, most approaches poorly predict pre- and pro-hapten sensitizers, which require abiotic or metabolic conversion prior to inducing sensitization. We developed a tri-culture system comprised of MUTZ-3-derived Langerhans cells, HaCaT keratinocytes, and primary dermal fibroblasts to mimic the cellular and metabolic environments of skin sensitization. A panel of non-sensitizers and sensitizers was tested and the secretome was evaluated. A support vector machine (SVM) was used to identify the most predictive sensitization signature and classification trees identified statistical thresholds to predict sensitizer potency. The SVM computed 91% tri-culture prediction accuracy using the top 3 ranking biomarkers (IL-8, MIP-1β, and GM-CSF) and improved the detection of pre- and pro-haptens. This in vitro assay combined with in silico data analysis presents a promising approach and offers the possibility of multi-metric analysis for enhanced ACD sensitizer screening.

Non-animal methods to predict skin sensitization (II): an assessment of defined approaches *

Skin sensitization is a toxicity endpoint of widespread concern, for which the mechanistic understanding and concurrent necessity for non-animal testing approaches have evolved to a critical juncture, with many available options for predicting sensitization without using animals. Cosmetics Europe and the National Toxicology Program Interagency Center for the Evaluation of Alternative Toxicological Methods collaborated to analyze the performance of multiple non-animal data integration approaches for the skin sensitization safety assessment of cosmetics ingredients. The Cosmetics Europe Skin Tolerance Task Force (STTF) collected and generated data on 128 substances in multiple in vitro and in chemico skin sensitization assays selected based on a systematic assessment by the STTF. These assays, together with certain in silico predictions, are key components of various non-animal testing strategies that have been submitted to the Organization for Economic Cooperation and Development as case studies for skin sensitization. Curated murine local lymph node assay (LLNA) and human skin sensitization data were used to evaluate the performance of six defined approaches, comprising eight non-animal testing strategies, for both hazard and potency characterization. Defined approaches examined included consensus methods, artificial neural networks, support vector machine models, Bayesian networks, and decision trees, most of which were reproduced using open source software tools. Multiple non-animal testing strategies incorporating in vitro, in chemico, and in silico inputs demonstrated equivalent or superior performance to the LLNA when compared to both animal and human data for skin sensitization.

Tools to investigate and avoid drug-hypersensitivity in drug development

INTRODUCTION

Drug hypersensitivity reactions (DHRs) are common adverse effects of pharmaceuticals that clinically resemble allergies, and which are becoming an important burden to healthcare systems. Alongside accurate diagnostic techniques, tools which can predict potential drug-inducing hypersensitivity reactions in the pre-clinical phase are critical. Despite the important adverse reactions linked to immune-mediated hypersensitivity, at present, there are no validated or required in vivo or in vitro methods to screen the sensitizing potential of drugs and their metabolites in the pre-clinical phase. Areas covered: Enhanced prediction in preclinical safety evaluation is extremely important. The purpose of this review is to assess the state of the art of tools available to assess the allergenic potential of drugs and to highlight our current understanding of the molecular mechanisms underlying inappropriate immune activation. Expert opinion: The knowledge that allergenic drugs share common mechanisms of immune cell activation with chemical allergens, and of the definition of the mechanistic pathway to adverse outcomes, can enhance targeting toxicity testing in drug development and hazard assessment of hypersensitivity. Additional efforts and extensive resources are necessary to improve preclinical testing methodologies, including optimization, better design and interpretation of data.

Predicting Chemically Induced Skin Sensitization by Using In Chemico / In Vitro Methods

Over the recent years development toward assessing skin sensitization hazard has moved toward non-animal testing methods. These methods are based on the key events as described in the OECD Adverse Outcome Pathway (AOP) for skin sensitization initiated by covalent binding to proteins. As these individual methods address mainly one mechanistic event (key event) in the initiation of skin sensitization, combination of different methods are needed to conclude on the skin sensitization hazard. Validated and regulatory adopted (EU and OECD) in chemico/in vitro methods are available for KEs 1-3 and are presented here. This chapter also illustrates how individual test methods can be combined by providing two examples of defined approaches to testing and assessment for skin sensitization hazard identification and assessment.

In Vitro and In Vivo Methods for Analysis of Nanoparticle Potential to Induce Delayed-Type Hypersensitivity Reactions

Delayed-type hypersensitivity (DTH) reactions are among the common reasons for drug withdrawal from clinical use during the post-marketing stage. Several in vivo methods have been developed to test DTH responses in animal models. They include the local lymph node assay (LLNA) and local lymph node proliferation assay (LLNP). While LLNA is instrumental in testing topically administered formulations (e.g., creams), the LLNP was proven to be predictive of drug-mediated DTH in response to small molecule pharmaceuticals. Global efforts in reducing the use of research animals lead to the development of in vitro models to predict test-material-mediated DTH. Two such models include analysis of surface marker expression in human cell lines THP-1 and U-937. These tests are known as the human cell line activation test (hCLAT) and myeloid U937 skin sensitization test (MUSST or U-SENS), respectively. Here we describe experimental procedures for all these methods, discuss their in vitro-in vivo correlation, and suggest a strategy for applying these tests to analyze engineered nanomaterials and nanotechnology-formulated drug products.

Applicability of an Integrated Testing Strategy consisting of in silico, in chemico and in vitro assays for evaluating the skin sensitization potencies of isocyanates

The skin sensitization potential of chemicals has been traditionally assessed using regulatory accepted in vivo methods, such as guinea pig maximization test or mouse local lymph node assays (LLNAs). A huge effort to reduce and replace the use of animals for safety assessments of chemicals because of regulatory requirements and ethical issues is presently underway, and alternative non-animal methods have been greatly developed. So far, a few studies have investigated the sensitization potencies of isocyanates which is a group of highly reactive chemicals that are known to be occupational allergens. The present study evaluated nine commonly used isocyanates using an in vivo LLNA and assessed the applicability of an Integrated Testing Strategy (ITS) consisting of an in silico Derek Nexus prediction, an in chemico direct peptide reactivity assay (DPRA), and an in vitro human Cell Line Activation Test (h-CLAT) to isocyanates. All nine isocyanates were evaluated as positive using the LLNA, Derek Nexus and DPRA, whereas seven chemicals tested positive using the h-CLAT: hexamethylene diisocyanate tested negative, and 1,5-diisocyanatonaphthalene could not be examined because of a solubility issue. When assessed using the ITS, the positive/negative evaluations of skin sensitization hazard were consistent with those assessed using the LLNA for all nine chemicals. However, the potency prediction results of the ITS tended to be underestimated, compared with those of the LLNA. The data presented in this work provide insights into the performance of non-animal testing approaches for evaluating the skin sensitization potencies of isocyanates.

Application of proteomics in the elucidation of chemical-mediated allergic contact dermatitis

Allergic contact dermatitis (ACD) is a widespread hypersensitivity reaction of the skin. The cellular mechanisms underlying its development are complex and involve close interaction of different cell types of the immune system. It is this very complexity which has long prevented straightforward replacement of the corresponding regulatory in vivo tests. Recent efforts have already resulted in the development of several in vitro testing alternatives that address key steps of ACD. Yet identification of suitable biomarkers is still a subject of intense research. Search strategies for the latter encompass transcriptomics, proteomics as well as metabolomics approaches. The scope of this review shall be the application and use of proteomics in the context of ACD. This includes highlighting relevant aspects of the molecular and cellular mechanisms underlying ACD, the exploitation of these mechanisms for testing and biomarkers (e.g., in the context of the OECD's adverse outcome pathway initiative) as well as an outlook on emerging proteome targets, for example during the allergen-induced activation of dendritic cells (DCs).

Comparison and validation of an in vitro skin sensitization strategy using a data set of 33 chemical references

Allergic contact dermatitis (ACD) is an adverse health effect that develops following repeated exposure to skin sensitizing chemicals. An animal testing ban has been applied in EU, leading to development of reliably predictive non-animal methods. Several in vitro methods have been developed as alternatives but one single non-animal test method is not been sufficient to fully address since the LLNA test ban. Here, we have selected an ITS (Integrated Testing Strategy) for skin sensitization which focuses on three in vitro methods that covered the first three steps of the AOP (DPRA, SENS-IS or h-CLAT). The aim of this study was to compare these three methods due to the WoE approach based on a 2-out-of-3-assessment. The results of 33 references were compared to in vivo data (especially human). We have shown that tested firstly DPRA and SENS-IS have permitted to conclude on 29 of 33 chemicals, whereas DPRA and h-CLAT on 25, and SENS-IS and h-CLAT on 23. With this sequence, DPRA and SENS-IS and then h-CLAT in case of equivocal results, we conclude more quickly by performing fewer tests. Thereby, we have shown that it is better to follow a preferential sequence than testing chemicals simultaneously with these three methods.