The concept of identifying threshold exposures to chemicals below which the risk to human health is negligible was originally proposed in the late 1960's. The induction of skin sensitization has been shown to demonstrate a dose response, and thus, a threshold concentration, or exposure, required to induce sensitization can be derived. The idea of a Dermal Sensitization Thresholds (DST) for chemicals has been investigated since 2008. However, DST developments have so far focused on individual sensitizing chemicals, rather than complex mixtures or botanical or naturally derived substances (BNS). With an increased interest in the use of BNS, which often contain low levels but unidentified sensitizing substances, the development of a sensitization threshold for BNS could support their safety assessments. In this work, we propose a site-of-contact exposure threshold (SoCET) of 10 μg/cm2 for BNS using a constituent-based approach and demonstrate that it would be protective in a risk assessment against the induction of skin sensitization. The 10 μg/cm2 SoCET can be used as the exposure-based waiving threshold in Tier 0 of the skin sensitization Next Generation Risk Assessment (NGRA) framework for BNS.

BACKGROUND:Allergic contact dermatitis (ACD) is an immune response to chemical haptens, involving induction and elicitation phases. Quantitative risk assessment (QRA) reviewing the skin sensitisation potential for all ingredients and using chemical-specific induction thresholds for skin allergens remains the current best approach for the safety evaluation of consumer products. However, elicitation thresholds have utility in secondary protection for pre-sensitised consumers. OBJECTIVES:The objectives were to (1) review the current elicitation threshold database, (2) evaluate whether predicted relationships between chemical induction and elicitation potencies enable elicitation threshold prediction and (3) discuss elicitation threshold utility as a secondary method of consumer protection. METHODS:In total, 28 publications (with 39 elicitation datasets) were included in non-linear regression analysis to derive elicitation thresholds for 22 allergens, including fragrances, preservatives and metals. RESULTS:The median elicitation threshold (ED10) for the expanded database was 0.95 μg/cm2, which supports previously published general elicitation thresholds. The derived ED10 for each chemical had only weak to moderate correlation with corresponding induction thresholds, thus limiting suitability as the predictive model. CONCLUSIONS:Assessment of current EU fragrance allergen labelling and sensitiser labelling under the globally harmonised system in the context of general and chemical-specific elicitation thresholds shows that current practices are health protective.

Continuous potency assessment is crucial for conducting quantitative risk assessment (QRA) of sensitizers. Quantitative regression models, based on in vitro methods, have been developed to calculate points of departure for use in skin sensitization QRA. These models calculate a point of departure as a predicted value for Local Lymph Node Assay (LLNA) EC3 or potency value (PV), integrating data from the kinetic Direct Peptide Reactivity Assay (kDPRA), KeratinoSens (KS) assay, and human Cell Line Activation Test (h-CLAT). The goal of this study was to determine how in vitro predicted EC3s and PVs compare to published reference data. In vitro data were combined in point of departure regression models to predict EC3s and PVs. These points of departure were then grouped into sensitization potency categories, such as extreme, strong, moderate, weak, very weak, or non-sensitizer, as previously described. Trends in potency distribution and high concordance between predicted EC3 and PV categories and published potency categories were observed. Furthermore, the median absolute fold-misprediction ranged between 1.8 and 2.5 for models predicting EC3 and between 1.7 and 3.4 for those predicting PVs. These regression models are a promising animal alternative for determining sensitization quantitative potency for fragrance ingredients, thereby facilitating QRA.

Wearable devices are in contact with the skin for extended periods. As such, the device constituents should be evaluated for their skin sensitization potential, and a Point of Departure (PoD) should be derived to conduct a proper risk assessment. Without historical in vivo data, the PoD must be derived with New Approach Methods (NAMs). To accomplish this, regression models trained on LLNA data that use data inputs from OECD-validated in vitro tests were used to derive a predicted EC3 value, the LLNA value used to classify skin sensitization potency, for three adhesive monomers (Isobornyl acrylate (IBOA), N, N- Dimethylacrylamide (NNDMA), and Acryloylmorpholine (ACMO) and one dye (Solvent Orange 60 (SO60)). These chemicals can be used as constituents of wearable devices and have been associated with causing allergic contact dermatitis (ACD). Using kinetic DPRA and KeratinoSens (TM) data, the PoDs obtained with the regression model were 180, 215, 1535, and 8325 mu g/cm(2) for IBOA, SO60, ACMO, and NNDMA, respectively. The PoDs derived with the regression model using NAMs data will enable a proper skin sensitization risk assessment without using animals.

Recent years have seen an increase in the use of botanicals and natural substances (BNS) in consumer products such as cosmetics and household care products. Most work conducted to date to assess botanicals for human safety has focused their use as dietary supplements and thus on systemic toxicity. However, the induction of skin sensitization is a possible adverse effect of natural products in particular those that come into skin contact, especially for cosmetics that remain on the skin and are not rinsed off following use. Assessments of BNS ingredients are often challenging for a number of reasons: the BNS are complex mixtures that can be of mostly unknown composition; the composition can be highly variable even within the same plant species and dependent on how processed; the physical form of the BNS raw material can vary from a highly concentrated powdered extract to a liquid extract containing only a small percentage of the BNS; testing of the BNS raw materials in New Approach Methods (NAM) has uncertainty as these methods are often not developed or validated for complex mixtures. In this study, a reference set of 14 selected BNS which span the range of skin sensitization potential was complied. These data were used in a Weight of Evidence (WoE) approach to evaluate their skin sensitization potential with each of the data rich BNS being classified as either having strong evidence of inducing skin sensitization based on human topical use history, animal data, clinical data, composition data and NAM data, or having some but more limited (weak) evidence of inducing skin sensitization, or having strong evidence of no skin sensitization potential. When available data have sufficient potency related information, sensitization potency assessment is also provided based on WoE, classifying these BNS as either strong, moderate, or weak sensitizers, or non-sensitizers. An outline for a BNS skin sensitization risk assessment framework is proposed starting with exposure-based waiving and WoE assessment for higher exposures. In addition to demonstrating the application of the WoE approach, the reference set presented here provides a set of 'data rich' botanicals which cover a range of sensitization potencies that could be used for evaluating existing test methods or aid in the development of new predictive models for skin sensitization.

Consumer products containing botanicals or natural substances (BNS) are often preferred because there is a perception that ‘natural’ is safe. As with any product ingredient, a thorough safety assessment must be conducted, including a determination of skin sensitization potential. A modification of the Peroxidase Peptide Reactivity Assay (PPRA) was explored for screening BNS (B-PPRA) for their reactivity to a model cysteine peptide. The PPRA incorporates a horseradish peroxidase‑hydrogen peroxide (+HRP/P) oxidation system for the activation of potential pre- and pro-haptens. BNS test materials contained <2% botanical constituent in either glycerin/water or propylene glycol/water. Stock solutions prepared in acetonitrile were diluted to 8 working concentrations. Direct reactivity was determined in reaction mixtures containing peptide and deferoxamine in potassium phosphate buffer. Enzyme-mediated reactivity determinations were performed with addition of +HRP/P. Initial studies demonstrated that results were reproducible and impact of carrier low. To determine the sensitivity of the assay, experiments were conducted with chamomile extract spiked with three sensitizers. Peptide depletion was observed in the +HRP/P reaction mixtures with isoeugenol spikes as low as 0.05%. The B-PPRA shows promise as a screening method for skin sensitization potential and could become part of a framework for the skin sensitization safety assessment of BNS.

Potency determination of potential skin sensitizers in humans is essential for quantitative risk assessment and proper risk management. SENS-IS is an in vitro test based on a reconstructed human skin model, that was developed to predict the hazard and potency of potential skin sensitizers. The performance of the SENS-IS assay in potency prediction for 174 materials was evaluated for this work. The potency used as a benchmark was determined based on the weight of evidence approach, by collectively considering all well-established test data, including human, animal, in chemico, in vitro, and in silico data. Based on this weight of evidence approach, the dataset was composed of 5, 19, 34, 54, and 38 extreme, strong, moderate, weak, and very weak sensitizers, respectively, as well as 24 non-sensitizers. SENS-IS provided good prediction of the skin sensitization potency for 85% of this dataset, with precise and approximate prediction on 46% and 39% of the 174 materials, respectively. Our evaluation showed that SENS-IS provides a good approximation of the skin sensitization potency.

Interest in the development of methods to evaluate the respiratory sensitization potential of low-molecular weight chemicals continues, but no method has yet been generally accepted or validated. A lack of chemical reference standards, together with uncertainty regarding relevant immunological mechanisms, has hampered method development. The first key event in the development of either skin or respiratory sensitization is the formation of stable adducts of the chemical with host proteins. This event is measured in the Direct Peptide Reactivity Assay using cysteine- and lysine-containing model peptides. It is hypothesized that protein reactivity and subsequent adduct formation may represent the earliest point of divergence in the pathways leading to either skin or respiratory sensitization. Direct Peptide Reactivity Assay data for 200 chemicals were compiled and grouped into respiratory, skin and nonsensitizers. Chemicals grouping was based on extensive literature research and expert judgment. To evaluate if chemical groups represent different peptide reactivity profiles, peptide reactivity data were clustered and compared with information on protein binding mechanisms and chemical categories available via the Organization for Economic Co-operation and Development. Toolbox. Respiratory sensitizers (n = 15) showed a significant (3-fold) higher lysine reactivity than skin sensitizers (n = 129). However, this difference was driven largely by the high representation of acid anhydrides among the respiratory sensitizers that showed clear lysine selectivity. Collectively, these data suggest that preferential reactivity for either cysteine or lysine is associated primarily with chemical structure, and that lysine preference is not a unifying characteristic of chemical respiratory allergens.

While the skin sensitization hazard of substances can be identified using non-animal methods, the classification of potency into UN GHS sub-categories 1A and 1B remains challenging. The kinetic direct peptide reactivity assay (kDPRA) is a modification of the DPRA wherein the reaction kinetics of a test substance towards a synthetic cysteine-containing peptide are evaluated. For this purpose, several concentrations of the test substance are incubated with the synthetic peptide for several incubation times. The reaction is stopped by addition of monobromobimane, which forms a fluorescent complex with the free cysteine of the model peptide. The relative remaining non-depleted amount of peptide is determined. Kinetic rate constants are derived from the depletion vs concentration and time matrix and used to distinguish between UN GHS sub-category 1A sensitizers and test substances in sub-category 1B/not classified test substances. In this study, we present a ring trial of the kDPRA with 24 blind-coded test substances in seven laboratories. The intra- and inter-laboratory reproducibility were 96% and 88%, respectively (both for differentiating GHS Cat 1A sensitizers from GHS Cat 1B/not classified). Following an independent peer review, the kDPRA was considered to be acceptable for the identification of GHS Cat 1A skin sensitizers. Besides GHS Cat 1A identification, the kDPRA can be used as part of a defined approach(es) with a quantitative data integration procedure for skin sensitization potency assessment. For this aim, next to reproducibility of classification, the quantitative reproducibility and variability of the rate constants were quantified in this study.

In 2008, a proposal for assessing the risk of induction of skin sensitization to fragrance materials Quantitative Risk Assessment 1 (QRA1) was published. This was implemented for setting maximum limits for fragrance materials in consumer products. However, there was no formal validation or empirical verification after implementation. Additionally, concerns remained that QRA1 did not incorporate aggregate exposure from multiple product use and included assumptions, e.g. safety assessment factors (SAFs), that had not been critically reviewed. Accordingly, a review was undertaken, including detailed re-evaluation of each SAF together with development of an approach for estimating aggregate exposure of the skin to a potential fragrance allergen. This revision of QRA1, termed QRA2, provides an improved method for establishing safe levels for sensitizing fragrance materials in multiple products to limit the risk of induction of contact allergy. The use of alternative non-animal methods is not within the scope of this paper. Ultimately, only longitudinal clinical studies can verify the utility of QRA2 as a tool for the prevention of contact allergy to fragrance materials.

A peptide reactivity assay with an activation component was developed for use in screening chemicals for skin sensitization potential. A horseradish peroxidase-hydrogen peroxide (HRP/P) oxidation system was incorporated into the assay for characterizing reactivity of hapten and pre-/prohapten sensitizers. The assay, named the Peroxidase Peptide Reactivity Assay (PPRA) had a predictive accuracy of 83% (relative to the local lymph node assay) with the original protocol and prediction model. However, apparent false positives attributed to cysteine depletion at relatively high chemical concentrations and, for some chemicals expected to react with the -NH2 group of lysine, little to no depletion of the lysine peptide were observed. To improve the PPRA, cysteine peptide reactions with and without HRP/P were modified by increasing the number of test concentrations and refining their range. In addition, removal of DL-dithiothreitol from the reaction without HRP/P increased cysteine depletion and improved detection of reactive aldehydes and thiazolines without compromising the assay's ability to detect prohaptens. Modification of the lysine reaction mixture by changing the buffer from 0.1 M ammonium acetate buffer (pH 10.2) to 0.1 M phosphate buffer (pH 7.4) and increasing the level of organic solvent from 1% to 25% resulted in increased lysine depletion for known lysine reactive chemicals. Refinement of the prediction model improved the sensitivity, specificity, and accuracy for hazard identification. These changes resulted in significant improvement of the PPRA making it is a reliable method for predicting the skin sensitization potential of all chemicals, including pre-/prohaptens and directly reactive haptens.

Use of botanicals and natural substances in consumer products has increased in recent years. Such extracts can contain protein that may theoretically represent a potential risk of IgE-mediated allergy. No method has yet been generally accepted or validated for assessment of the allergenic potential of proteins. For development of suitable methods datasets of allergenic and nonallergenic (or low allergenic) proteins are required that can serve, respectively, as positive and negative controls. However, data are unavailable on proteins that lack or have low allergenic potential. Here, low allergenic potential proteins are identified based on the assumption that proteins with established human exposure, but with a lack of an association with allergy, possess low allergenic potential. Proteins were extracted from sources considered to have less allergenic potential (corn, potato, spinach, rice, and tomato) as well as higher allergenic potential (wheat) regarding common allergenic foods. Proteins were identified and semi-quantified by label-free proteomic analysis conducted using mass spectrometry. Predicted allergenicity was determined using AllerCatPro (https://allercatpro.bii.a-star.edu.sg/). In summary, 9077 proteins were identified and semi-quantified from 6 protein sources. Within the top 10% of the most abundant proteins identified, 178 characterized proteins were found to have no evidence for allergenicity predicted by AllerCatPro and were considered to have low allergenic potential. This panel of low allergenic potential proteins provides a pragmatic approach to aid the development of alternative methods for robust testing strategies to distinguish between proteins of high and low allergenic potential to assess the risk of proteins from natural or botanical sources.

Quantitative guidelines to distinguish allergenic proteins from related, but non-allergenic ones are urgently needed for regulatory agencies, biotech companies and physicians. In a previous study, we found that allergenic proteins populate a relatively small number of protein families, as characterized by the Pfam database. However, these families also contain non-allergenic proteins, meaning that allergenic determinants must lie within more discrete regions of the sequence. Thus, new methods are needed to discriminate allergenic proteins within those families. Physical–Chemical Properties (PCP)-motifs specific for allergens within a Pfam class were determined for 17 highly populated protein domains. A novel scoring method based on PCP-motifs that characterize known allergenic proteins within these families was developed, and validated for those domains. The motif scores distinguished sequences of allergens from a large selection of 80,000 randomly selected non-allergenic sequences. The motif scores for the birch pollen allergen (Bet v 1) family, which also contains related fruit and nut allergens, correlated better than global sequence similarities with clinically observed cross-reactivities among those allergens. Further, we demonstrated that the average scores of allergen specific motifs for allergenic profilins are significantly different from the scores of non-allergenic profilins. Several of the selective motifs coincide with experimentally determined IgE epitopes of allergenic profilins. The motifs also discriminated allergenic pectate lyases, including Jun a 1 from mountain cedar pollen, from similar proteins in the human microbiome, which can be assumed to be non-allergens. The latter lacked key motifs characteristic of the known allergens, some of which correlate with known IgE binding sites.

For many endpoints in toxicology, an interspecies safety factor remains a standard requirement. However, for skin sensitization, the hazard and potency predictions, notably from the local lymph node assay (LLNA) have been shown to correlate well with human data. Despite this, there are always exceptions, both over and under predictions. For this reason it has been suggested that an interspecies factor of 15 would accommodate potential “errors”. An alternative approach is suggested in which an evidence-based strategy is taken: the large majority of the information indicates a human:LLNA ratio of 1, therefore a corrective factor would best be applied where our knowledge of the underlying chemistry of sensitization indicates that it is necessary.

When evaluating consumer products for safety, one must consider the heterogeneity of the population that might use those products, including the potential for different sensitivity based on factors such as age, gender, and genetics. For both systemic endpoints and allergic contact dermatitis (ACD), quantitative safety evaluations typically include a default 10-fold uncertainty/assessment factor to account for inter-individual variability. While this factor is intended to include age, the adequacy of the default 10-fold factor has been questioned for infants, for whom a precautionary assumption is often made that they are more sensitive. In-depth evaluations of the adequacy of the 10-fold factor have been published for systemic endpoints, but relatively little has been published to substantiate this for ACD. This paper reviews the state of the science regarding the etiology of ACD and factors that suggest an overall decreased sensitivity associated with early life exposures, thus confirming the sufficiency of the 10-fold inter-individual factor to provide protection for children and infants. While it remains prudent for all age groups to avoid contact with sensitizers, it is concluded that the quantitative methods used in safety evaluation to prevent the induction of skin sensitization are protective for infants, including neonates and premature infants.

The currently available animal-free methods for the detection of skin sensitizing potential of chemicals seem promising. However, no single method is able to comprehensively represent the complexity of the processes involved in skin sensitization. To ensure a mechanistic basis and cover the complexity, multiple methods should be integrated into a testing strategy, in accordance with the adverse outcome pathway that describes all key events in skin sensitization. Although current majority voting testing strategies have proven effective, the performance of individual methods is not taken into account. To that end, we designed a tiered strategy based on complementary characteristics of the included methods, and compared it to a majority voting approach. This tiered testing strategy was able to correctly identify all 41 chemicals tested. In terms of total number of experiments required, the tiered testing strategy requires less experiments compared to the majority voting approach. On the other hand, this tiered strategy is more complex due the number of different alternative methods required, and predicted costs are similar for both strategies. Both the tiered and majority voting strategies provide a mechanistic basis for skin sensitization testing, but the strategy most suitable for regulatory decision-making remains to be determined.

Use of quantitative risk assessment (QRA) for assessing the skin sensitization potential of chemicals present in consumer products requires an understanding of hazard and product exposure. In the absence of data, consumer exposure is based on relevant habits and practices and assumes 100% skin uptake of the applied dose. To confirm and refine the exposure, a novel design for in vitro skin exposure measurements was conducted with the preservative, methylisothiazolinone (MI), in beauty care (BC) and household care (HHC) products using realistic consumer exposure conditions. A difference between measured exposure levels (MELs) for MI in leave-on versus rinse-off BC products, and lower MELs for MI in HHC rinse-off compared to BC products was demonstrated. For repeated product applications, the measured exposure was lower than estimations based on summation of applied amounts. Compared to rinse-off products, leave-on applications resulted in higher MELs, correlating with the higher incidences of allergic contact dermatitis associated with those product types. Lower MELs for MI in rinse-off products indicate a lower likelihood to induce skin sensitization, also after multiple daily applications. These in vitro skin exposure measurements indicate conservatism of default exposure estimates applied in skin sensitization QRA and might be helpful in future risk assessments.

Several non-animal methods are now available to address the key events leading to skin sensitization as defined by the adverse outcome pathway. The KeratinoSens assay addresses the cellular event of keratinocyte activation and is a method accepted under OECD TG 442D. In this study, the results of an inter-laboratory evaluation of the "me-too" LuSens assay, a bioassay that uses a human keratinocyte cell line harboring a reporter gene construct composed of the rat antioxidant response element (ARE) of the NADPH:quinone oxidoreductase 1 gene and the luciferase gene, are described. Earlier in-house validation with 74 substances showed an accuracy of 82% in comparison to human data. When used in a battery of non-animal methods, even higher predictivity is achieved. To meet European validation criteria, a multicenter study was conducted in 5 laboratories. The study was divided into two phases, to assess 1) transferability of the method, and 2) reproducibility and accuracy. Phase I was performed by testing 8 non-coded test substances; the results showed a good transferability to naïve laboratories even without on-site training. Phase II was performed with 20 coded test substances (performance standards recommended by OECD, 2015). In this phase, the intra- and inter-laboratory reproducibility as well as accuracy of the method was evaluated. The data demonstrate a remarkable reproducibility of 100% and an accuracy of over 80% in identifying skin sensitizers, indicating a good concordance with in vivo data. These results demonstrate good transferability, reliability and accuracy of the method thereby achieving the standards necessary for use in a regulatory setting to detect skin sensitizers.