Evaluation of the skin sensitizing potency of chemicals by using the existing methods and considerations of relevance for elicitation

Use of guinea pig data to obtain starting points for skin sensitisation risk assessment - A commentary

The increasing drive to understand the likelihood of skin sensitisation from plant protection products (PPPs) in workers and the general public has resulted in recent initiatives to establish a quantitative risk assessment (QRA) methodology applicable to these products and their exposure scenarios. The effective evaluation of skin sensitising substances requires not only the identification of that toxicological hazard, but also determination of relative sensitising potency. Typically, this has been achieved by interpretation of local lymph node assay (LLNA) dose response data, delivering what is known as the EC3 value. This permitted regulatory division of skin sensitisers into defined potency sub-categories, but more importantly enabled derivation of a no expected sensitisation induction level (NESIL) as the point of departure for QRA. However, for many existing substances there is no LLNA data, only older guinea pig results exist. To avoid additional (in vivo) testing, an approach has been outlined to employ guinea pig data and existing regulatory guidelines on the determination of potency sub-categorisation to provide a guinea pig based NESIL. The approach adopts a conservative extrapolation from LLNA NESIL benchmarks to deliver points of departure as the basis for the type of QRA process already in successful use by other industries.

The BMDC model, a performant cell-based test to assess the sensitizing potential and potency of chemicals including pre/pro-haptens

BACKGROUND

Chemical-induced allergies at workplace represent a significant occupational health issue. These substances must be properly identified as sensitizers. In previous studies, an original model using mouse bone marrow-derived dendritic cells (BMDC) was developed for this purpose.

OBJECTIVES

The aim of this study was to evaluate the predictive capacity of the BMDC model with a large panel of sensitizers (including pre- and pro-haptens) and non-sensitizers.

METHODS

The readout from the BMDC model is based on expression levels of six phenotypic markers measured by flow cytometry.

RESULTS

The results indicate that 29 of the 37 non-sensitizers, and 81 of the 86 sensitizers were correctly classified compared to the Local Lymph Node Assay (LLNA). Statistical analysis revealed the BMDC model to have a sensitivity of 94%, a specificity of 78%, and an accuracy of 89%. The EC2 (Effective Concentration) values calculated with this model allow sensitizers to be categorized into four classes: extreme, strong, moderate and weak.

CONCLUSIONS

These excellent predictive performances show that the BMDC model discriminates between sensitizers and non-sensitizers with outstanding precision equal to or better than existing validated alternative models. Moreover, this model allows to predict sensitization potency of chemicals. The BMDC test could therefore be proposed as an additional tool to assess the sensitizing potential and potency of chemicals.

Assessing the risk of induction of skin sensitization to plant protection products: A quantitative approach

Exposure to skin sensitizers is common and regulated in many industry sectors. For cosmetics, a risk-based approach has been implemented, focused on preventing the induction of sensitization. First, a No Expected Sensitization Induction Level (NESIL) is derived, then modified by Sensitization Assessment Factors (SAFs) to derive an Acceptable Exposure Level (AEL). The AEL is used in risk assessment, being compared with an estimated exposure dose, specific to the exposure scenario. Since in Europe there is increased concern regarding exposure towards potentially sensitizing pesticides via spray drift, we explore how existing practice can be modified to allow Quantitative Risk Assessment (QRA) of pesticides for bystanders and residents. NESIL derivation by the Local Lymph Node Assay (LLNA), the globally required in vivo assay for this endpoint, is reviewed alongside consideration of appropriate SAFs. Using a case study, the principle that the NESIL in μg/cm² can be derived by multiplying LLNA EC3% figure by a factor of 250 is adopted. The NESIL is then reduced by an overall SAF of 25 to establish an exposure level below which there is minimal bystander and resident risk. Whilst this paper focuses on European risk assessment and management, the approach is generic and universally applicable.

Artificial Intelligence That Predicts Sensitizing Potential of Cosmetic Ingredients with Accuracy Comparable to Animal and In Vitro Tests-How Does the Infotechnomics Compare to Other "Omics" in the Cosmetics Safety Assessment?

The aim of the current study was to develop an in silico model to predict the sensitizing potential of cosmetic ingredients based on their physicochemical characteristics and to compare the predictions with historical animal data and results from "omics"-based in vitro studies. An in silico model was developed with the use of WEKA machine learning software fed with physicochemical and structural descriptors of haptens and trained with data from published epidemiological studies compiled into estimated odds ratio (eOR) and estimated attributable risk (eAR) indices. The outcome classification was compared to the results of animal studies and in vitro tests. Of all the models tested, the best results were obtained for the Naive Bayes classifier trained with 24 physicochemical descriptors and eAR, which yielded an accuracy of 86%, sensitivity of 80%, and specificity of 90%. This model was subsequently used to predict the sensitizing potential of 15 emerging and less-studied haptens, of which 7 were classified as sensitizers: cyclamen aldehyde, N,N-dimethylacrylamide, dimethylthiocarbamyl benzothiazole sulphide, geraniol hydroperoxide, isobornyl acrylate, neral, and prenyl caffeate. The best-performing model (NaiveBayes eAR, 24 parameters), along with an alternative model based on eOR (Random Comittee eOR, 17 parameters), are available for further tests by interested readers. In conclusion, the proposed infotechnomics approach allows for a prediction of the sensitizing potential of cosmetic ingredients (and possibly also other haptens) with accuracy comparable to historical animal tests and in vitro tests used nowadays. In silico models consume little resources, are free of ethical concerns, and can provide results for multiple chemicals almost instantly; therefore, the proposed approach seems useful in the safety assessment of cosmetics.

Risk management of skin sensitisers: A commentary

Historically, allergic contact dermatitis (ACD) to chemicals encouraged hazard identification improvements, more sophisticated risk assessment and implementation of regulatory strategies, including banning of specific sensitising substances. The validation process applied to hazard identification methods demonstrates their accuracy; their use to characterise sensitiser potency facilitates quantitative and transparent risk assessment. Diagnostic patch testing at dermatology clinics worldwide delivers feedback showing where risk assessment/management has been insufficient or did not target the exposure of concern, thereby facilitating improvements. When urgent action to protect human health was required, regulations limited/banned, specific skin sensitisers. This can be seen in practice with the fragrance industry, a known source of ACD, thus requiring risk management, usually restrictions to limit allergy induction, and very rarely specific bans on ingredients. Experience and development of more sophisticated tools, e.g. to assess aggregate exposure from multitude of consumer product types, has led to repeated adaptation of risk assessment and promulgation of updated fragrance use limits. Although targeted control may not always lead to rapid change in the overall clinical picture, it is preferable to a blanket undifferentiated regulatory control of all sensitisers, resulting in unwarranted restrictions for many uses of no health concern, with consequent substantial socio-economic impacts.

Nature-Derived Epoxy Resin Monomers with Reduced Sensitizing Capacity─Isosorbide-Based Bis-Epoxides

Epoxy resin systems (ERSs) are a class of thermosetting resins that become thermostable and insoluble polymers upon curing. They are widely used as components of protective surfaces, adhesives, and paints and in the manufacturing of composites in the plastics industry. The diglycidyl ether of bisphenol A (DGEBA) is used in 75-90% of ERSs and is thus by far the most used epoxy resin monomer (ERM). Unfortunately, DGEBA is a strong skin sensitizer and it is one of the most common causes of occupational contact dermatitis. Furthermore, DGEBA is synthesized from bisphenol A (BPA), which is a petroleum-derived chemical with endocrine-disruptive properties. In this work, we have used isosorbide, a renewable and nontoxic sugar-based material, as an alternative to BPA in the design of ERMs. Three different bis-epoxide isosorbide derivatives were synthesized: the diglycidyl ether of isosorbide (1) and two novel isosorbide-based bis-epoxides containing either a benzoic ester (2) or a benzyl ether linkage (3). Assessment of the in vivo sensitizing potency of the isosorbide bis-epoxides in the murine local lymph node assay (LLNA) showed that all three compounds were significantly less sensitizing than DGEBA, especially 2 which was nonsensitizing up to 25% w/v. The peptide reactivity showed the same order of reactivity as the LLNA, i.e., 2 being the least reactive, followed by 3 and then 1, which displayed similar peptide reactivity as DGEBA. Skin permeation of 2 and 3 was compared to DGEBA using ex vivo pig skin and static Franz cells. The preliminary investigations of the technical properties of the polymers formed from 1-3 were promising. Although further investigations of the technical properties are needed, all isosorbide bis-epoxides have the potential to be less sensitizing renewable replacements of DGEBA, especially 2 that had the lowest sensitizing potency in vivo as well as the lowest peptide reactivity.

Reference Chemical Potency List (RCPL): A new tool for evaluating the accuracy of skin sensitisation potency measurements by New Approach Methodologies (NAMs)

Considerable progress has been made in the design of New Approach Methodologies (NAMs) for the hazard identification of skin sensitising chemicals. However, effective risk assessment requires accurate measurement of sensitising potency, and this has proven more difficult to achieve without recourse to animal tests. One important requirement for the development and adoption of novel approaches for this purpose is the availability of reliable databases for determining the accuracy with which sensitising potency can be predicted. Some previous approaches have relied on comparisons with potency estimates based on either human or animal (local lymph node assay) data. In contrast, we here describe the development of a carefully curated Reference Chemical Potency List (RCPL) which is based on consideration of the best available human and animal data. The RCPL is comprised of 33 readily available chemicals that span a wide range of chemistry and sensitising potency, and contain examples of both direct and indirect (pre- and pro-) haptens. For each chemical a potency value (PV) was derived, and chemicals ranked according to PV without the use of potency categories. It is proposed that the RCPL provides an effective resource for assessment of the accuracy with which NAMs can measure skin sensitising potency.

Nature-derived epoxy resins: Synthesis, allergenicity, and thermosetting properties of pinoresinol diglycidyl ether

We describe a novel nature-derived epoxy resin monomer (ERM) derived from the plant lignan pinoresinol. Epoxy resins are thermosetting materials in global usage owing to their excellent technical properties such as flexibility and durability. However, their adverse health effects are often not considered and affect users of epoxy resins worldwide. Components of epoxy resin systems are strong skin sensitizers and cause allergic contact dermatitis. The reported prevalence attributable to epoxy chemicals is between 11.7 and 12.5% of all cases of occupational allergic contact dermatitis. We are committed to developing epoxy resins with reduced allergenic effect, while maintaining their excellent properties. The novel ERM, pinoresinol diglycidyl ether (PinoDGE), was synthesized in one step from pinoresinol and epichlorohydrin in 88% yield. It was not classified as a skin sensitizer in the in vivo local lymph node assay, at concentrations up to 0.17 m, as it did not cause a stimulation index >3 compared to control. Pinoresinol diglycidyl ether reacted with the model peptide AcPHCKRM in a reactivity assay and was predicted to be a skin sensitizer in the KeratinoSens assay. Preliminary cross-linking studies indicate that it has promising properties compared to commercially used ERMs. Pinoresinol diglycidyl ether could be seen as a lead compound for further development of alternative ERMs with a better safety profile based on natural and renewable sources for construction of epoxy resin polymers.

Skin Sensitization Testing: The Ascendancy of Non-Animal Methods

A century ago, toxicology was an empirical science identifying substance hazards in surrogate mammalian models. Over several decades, these models improved, evolved to reduce animal usage, and recently have begun the process of dispensing with animals entirely. However, despite good hazard identification, the translation of hazards into adequately assessed risks to human health often has presented challenges. Unfortunately, many skin sensitizers known to produce contact allergy in humans, despite being readily identified as such in the predictive assays, continue to cause this adverse health effect. Increasing the rigour of hazard identification is inappropriate. Regulatory action has only proven effective via complete bans of individual substances. Since the problem applies to a broad range of substances and industry categories, and since generic banning of skin sensitizers would be an economic catastrophe, the solution is surprisingly simple—they should be subject to rigorous safety assessment, with the risks thereby managed accordingly. The ascendancy of non-animal methods in skin sensitization is giving unparalleled opportunities in which toxicologists, risk assessors, and regulators can work in concert to achieve a better outcome for the protection of human health than has been delivered by the in vivo methods and associated regulations that they are replacing.

Nickel: Intrinsic Skin Sensitization Potency and Relation to Prevalence of Contact Allergy

Nickel remains the most commonly identified contact allergen. However, it has proven difficult to demonstrate significant skin-sensitizing activity for nickel in toxicology tests, which typically have indicated a weak skin sensitization potential. Information indicates that in vivo assays are not predictive of dermal sensitization hazard or potency for nickel due to a human-specific mechanistic route for nickel sensitization that animals lack. A similar rationale will apply to in vitro alternatives-although these currently have limited ability to determine intrinsic potency. Generally, in silico methods are not designed for metal allergens and cannot contribute to the analysis. For ethical reasons, human experimental work has been limited, with a single study suggesting moderate potency. Accordingly, it seems reasonable to conclude that the high frequency of contact allergy to nickel in humans is a function of both its intermediate potency coupled with a high level of dermal exposure, particularly to damaged/inflamed skin.

Pre-validation of SENS-IS assay for in vitro skin sensitization of medical devices

According to ISO 10993-1:2018, the skin sensitization potential of all medical devices must be evaluated, and for this endpoint ISO 10993-10:2010 recommends the use of in vivo assays. The goal of the present study was to determine if the in vitro SENS-IS assay could be a suitable alternative to the current in vivo assays. The SENS-IS assay uses the Episkin Large and SkinEthic RHE reconstructed human epidermis models to evaluate marker genes. In our study, the SENS-IS assay correctly identified 13 sensitizers spiked in a non-polar solvent. In a subsequent analysis six medical device silicone samples previously impregnated with sensitizers were extracted with polar and non-polar solvents. The SENS-IS assay correctly identified five of these extracts, while a sixth extract, which contained the weak sensitizer phenyl benzoate, was classified as negative. However, when this extract was concentrated, or a longer exposure time was used, the assay was able to detect phenyl benzoate. The SENS-IS assay was transferred to a naïve laboratory which correctly identified sensitizers in six blinded silicone samples, including the one containing phenyl benzoate. In light of these results, we conclude that the SENS-IS assay is able to correctly identify the presence of sensitizers in medical devices extracts.

The activity of methacrylate esters in skin sensitisation test methods II. A review of complementary and additional analyses

Allergic contact dermatitis is an important occupational health issue, and there is a need to identify accurately those chemicals that have the potential to induce skin sensitisation. Hazard identification was performed initially using animal (guinea pig and mouse) models. More recently, as a result of the drive towards non-animal methods, alternative in vitro and in silico approaches have been developed. Some of these new in vitro methods have been formally validated and have been assigned OECD Test Guideline status. The performance of some of these recently developed in vitro methods, and of 2 quantitative structure-activity relationships (QSAR) approaches, with a series of methacrylate esters has been reviewed and reported previously. In this article that first review has been extended further with additional data and complementary analyses. Results obtained using in vitro methods (Direct Peptide Reactivity Assay, DPRA; ARE-Nrf2 luciferase test methods, KeratinoSens and LuSens; Epidermal Sensitisation Assay, EpiSensA; human Cell Line Activation Test, h-CLAT, and the myeloid U937 Skin Sensitisation test, U-SENS), and 2 QSAR approaches (DEREK™-nexus and TIMES-SS), with 11 methacrylate esters and methacrylic acid are reported here, and compared with existing data from the guinea pig maximisation test and the local lymph node assay. With this series of chemicals it was found that some in vitro tests (DPRA and ARE-Nrf2 luciferase) performed well in comparison with animal test results and available human skin sensitisation data. Other in vitro tests (EpiSensA and h-CLAT) proved rather more problematic. Results with DEREK™-nexus and TIMES-SS failed to reflect accurately the skin sensitisation potential of the methacrylate esters. The implications for assessment of skin sensitising activity are discussed.

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.

Development of New Epoxy Resin Monomers - A Delicate Balance between Skin Allergy and Polymerization Properties

Epoxy resin monomers (ERMs) are used as building blocks for thermosetting polymers in applications where strong, flexible, and lightweight materials are required. Most epoxy resins are polymers of diglycidyl ether of bisphenol A (DGEBA). It is highly allergenic and causes occupational allergic contact dermatitis and contact allergy in the general population. Thus, measures to prevent exposure by protective clothing and education are not enough. This work describes a continuation of our research aiming at reducing the skin-sensitizing potency of ERMs while maintaining the ability to form polymers. Alternative ERMs were designed and synthesized whereafter the sensitizing potency was determined using the murine local lymph node assay (LLNA). The reactivity of the diepoxides toward a nucleophilic peptide was investigated, and the differences in reactivity explained using computational studies. The diepoxides were reacted with triethylenetetramine, and the formed polymers were tested for technical applicability using thermogravimetric analysis. We had previously shown that the absence of an oxygen atom in the side chains or removal of aromaticity reduced the sensitizing potency compared to that of DGEBA. Thus, a cycloaliphatic analogue 1 of DGEBA without ether oxygen in the side chains was considered promising and was synthesized. As predicted, the sensitizing potency was considerably reduced (10 times) compared to that of DGEBA. However, the technical properties of the polymer of this compound were not considered sufficient. More polar aromatic analogues were investigated, but they could not compete with our previously described ERMs regarding polymerization properties and with 1 regarding low skin sensitization properties. Development of alternative epoxy materials is a delicate balance between allergenic activity and polymerization properties. Tuning of structural properties together with investigation of polymerization conditions combined with skin sensitization studies should be used in industrial research and development. ERM 1 could be used as a lead compound for further studies of aliphatic ERMs.

Risks of Allergic Contact Dermatitis Elicited by Nickel, Chromium, and Organic Sensitizers: Quantitative Models Based on Clinical Patch Test Data

Risks of allergic contact dermatitis (ACD) from consumer products intended for extended (nonpiercing) dermal contact are regulated by E.U. Directive EN 1811 that limits released Ni to a weekly equivalent dermal load of ≤0.5 μg/cm² . Similar approaches for thousands of known organic sensitizers are hampered by inability to quantify respective ACD-elicitation risk levels. To help address this gap, normalized values of cumulative risk for eliciting a positive ("≥+") clinical patch test response reported in 12 studies for a total of n = 625 Ni-sensitized patients were modeled in relation to observed ACD-eliciting Ni loads, yielding an approximate lognormal (LN) distribution with a geometric mean and standard deviation of GM_Ni = 15 μg/cm² and GSD_Ni = 8.0, respectively. Such data for five sensitizers (including formaldehyde and 2-hydroxyethyl methacrylate) were also ∼LN distributed, but with a common GSD value equal to GSD_Ni and with heterogeneous sensitizer-specific GM values each defining a respective ACD-eliciting potency GM_Ni /GM relative to Ni. Such potencies were also estimated for nine (meth)acrylates by applying this general LN ACD-elicitation risk model to respective sets of fewer data. ACD-elicitation risk patterns observed for Cr(VI) (n = 417) and Cr(III) (n = 78) were fit to mixed-LN models in which ∼30% and ∼40% of the most sensitive responders, respectively, were estimated to exhibit a LN response also governed by GSD_Ni . The observed common LN-response shape parameter GSD_Ni may reflect a common underlying ACD mechanism and suggests a common interim approach to quantitative ACD-elicitation risk assessment based on available clinical data.

Sensitization and cross-reactivity patterns of contact allergy to diisocyanates and corresponding amines: investigation of diphenylmethane-4,4'-diisocyanate, diphenylmethane-4,4'-diamine, dicyclohexylmethane-4,4'-diisocyanate, and dicylohexylmethane-4,4'-diamine

Background

Isocyanates are used in polyurethane production. Dermal exposure to isocyanates can induce contact allergy. The most common isocyanate is diphenylmethane diisocyanate used for industrial purposes. The isomer diphenylmethane‐4,4′‐diisocyanate (4,4′‐MDI) is used in patch testing. Diphenylmethane‐4,4′‐diamine (4,4′‐MDA) is its corresponding amine. Concurrent reactions to 4,4′‐MDI and 4,4′‐MDA have been reported, as have concurrent reactions to 4,4′‐MDI and dicyclohexylmethane‐4,4′‐diisocyanate (4,4′‐DMDI).

Objectives

To investigate the sensitization capacities and the cross‐reactivity of 4,4′‐MDI, 4,4′‐MDA, 4,4′‐DMDI, and dicyclohexylmethane‐4,4′‐diamine (4,4′‐DMDA).

Methods

The guinea‐pig maximization test (GPMT) was used.

Results

The GPMT showed sensitizing capacities for all investigated substances: 4,4′‐MDI, 4,4′‐MDA, 4,4′‐DMDI, and 4,4′‐DMDA (all p < 0.001). 4,4′‐MDI‐sensitized animals showed cross‐reactivity to 4,4′‐MDA (p < 0.001) and 4,4′‐DMDI (all p < 0.05). 4,4′‐MDA‐sensitized animals showed cross‐reactivity to 4,4′‐DMDA (p = 0.008).

Conclusion

All of the investigated substances were shown to be strong sensitizers. Animals sensitized to 4,4′‐MDI showed cross‐reactivity to 4,4′‐MDA and 4,4′‐DMDI, supporting previous findings in the literature. The aromatic amine 4,4′‐MDA showed cross‐reactivity to the aliphatic amine 4,4′‐DMDA.

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.

Comparative sensitizing potencies of fragrances, preservatives, and hair dyes

The local lymph node assay (LLNA) is used for assessing sensitizing potential in hazard identification and risk assessment for regulatory purposes. Sensitizing potency on the basis of the LLNA is categorized into extreme (EC3 value of ≤0.2%), strong (>0.2% to ≤2%), and moderate (>2%). To compare the sensitizing potencies of fragrance substances, preservatives, and hair dye substances, which are skin sensitizers that frequently come into contact with the skin of consumers and workers, LLNA results and EC3 values for 72 fragrance substances, 25 preservatives and 107 hair dye substances were obtained from two published compilations of LLNA data and opinions by the Scientific Committee on Consumer Safety and its predecessors. The median EC3 values of fragrances (n = 61), preservatives (n = 19) and hair dyes (n = 59) were 5.9%, 0.9%, and 1.3%, respectively. The majority of sensitizing preservatives and hair dyes are thus strong or extreme sensitizers (EC3 value of ≤2%), and fragrances are mostly moderate sensitizers. Although fragrances are typically moderate sensitizers, they are among the most frequent causes of contact allergy. This indicates that factors other than potency need to be addressed more rigorously in risk assessment and risk management.

State of the art in non-animal approaches for skin sensitization testing: from individual test methods towards testing strategies

The hazard assessment of skin sensitizers relies mainly on animal testing, but much progress is made in the development, validation and regulatory acceptance and implementation of non-animal predictive approaches. In this review, we provide an update on the available computational tools and animal-free test methods for the prediction of skin sensitization hazard. These individual test methods address mostly one mechanistic step of the process of skin sensitization induction. The adverse outcome pathway (AOP) for skin sensitization describes the key events (KEs) that lead to skin sensitization. In our review, we have clustered the available test methods according to the KE they inform: the molecular initiating event (MIE/KE1)-protein binding, KE2-keratinocyte activation, KE3-dendritic cell activation and KE4-T cell activation and proliferation. In recent years, most progress has been made in the development and validation of in vitro assays that address KE2 and KE3. No standardized in vitro assays for T cell activation are available; thus, KE4 cannot be measured in vitro. Three non-animal test methods, addressing either the MIE, KE2 or KE3, are accepted as OECD test guidelines, and this has accelerated the development of integrated or defined approaches for testing and assessment (e.g. testing strategies). The majority of these approaches are mechanism-based, since they combine results from multiple test methods and/or computational tools that address different KEs of the AOP to estimate skin sensitization potential and sometimes potency. Other approaches are based on statistical tools. Until now, eleven different testing strategies have been published, the majority using the same individual information sources. Our review shows that some of the defined approaches to testing and assessment are able to accurately predict skin sensitization hazard, sometimes even more accurate than the currently used animal test. A few defined approaches are developed to provide an estimate of the potency sub-category of a skin sensitizer as well, but these approaches need further independent evaluation with a new dataset of chemicals. To conclude, this update shows that the field of non-animal approaches for skin sensitization has evolved greatly in recent years and that it is possible to predict skin sensitization hazard without animal testing.

Human Monocytoid THP-1 Cell Line versus Monocyte-Derived Human Immature Dendritic Cells as in Vitro Models for Predicting the Sensitising Potential of Chemicals

Immature dendritic cells (DCs) modulate differentiation markers following in vitro exposure to chemicals generating contact allergies. THP-1 is a monocytoid cell line maintaining some differentiating plasticity. In this study, human DCs and THP-1 cells were compared as in vitro models to predict contact sensitisation of chemicals with different sensitising potential. Expression of CD80 and CD86 was assessed by flow cytometry after exposure to subtoxic concentrations of potent (2,4-dinitrochlorobenzene, DNCB and p-phenylendiamine, PPD), strong (thimerosal, TMS), moderate (sodium tetrachloroplatinate, Na2PtCl4) sensitising compounds as well as of non-sensitising, irritating sodium dodecyl sulphate (SDS) as compared to a vehicle of sensitising substances (dimethyl sulphoxide, DMSO). Up-regulation of CD86 following in vitro incubation of DCs and THP-1 cells with DNCB, PPD, TMS and Na2PtCl4 but not with SDS, was observed. The CD80 membrane marker was up-regulated on DCs following in vitro incubation with DNCB and PPD, but not with TMS, Na2PtCl4 and SDS. On THP-1 cells, only DNCB up-regulated CD80 expression. In conclusion, both the cell line THP-1 and DCs are promising in vitro models for assays aiming at predicting the sensitisation potential of chemicals. THP-1 cell line is by far easier to handle and offers relevant advantages from the practical point of view.

Continuous usage of a hair dye product containing 2-methoxymethyl-para-phenylenediamine by hair-dye-allergic individuals

BACKGROUND

Despite a positive patch test reaction to para-phenylenediamine (PPD) and/or toluene-2,5-diamine (PTD), many people attempt to continue dyeing their hair with products containing PPD or its derivatives.

OBJECTIVES

Investigation of elicitation reactions among PPD/PTD-allergic individuals to hair dye products containing the less sensitizing PPD derivative 2-methoxymethyl (ME)-PPD.

METHODS

Elicitation reactions were studied in 43 PPD/PTD-allergic individuals by a 45-min pretest with an ME-PPD-containing hair dye on their forearm. Upon a negative result this was followed by exposure to subsequent hair colour treatment(s).

RESULTS

Overall, 38 of 43 PPD/PTD-allergic individuals did not develop an elicitation reaction during the pretest with ME-PPD-containing hair dye products, and were eligible for subsequent hair colour treatments. Of these 38 PPD/PTD-allergic individuals, 29 tolerated subsequent hair dyeing with ME-PPD-containing hair dye products, while seven showed mild and two showed moderate/marked allergic reactions upon the first hair colour treatment.

CONCLUSIONS

Hair dye products with the less sensitizing ME-PPD were tolerated by 29 of 43 (67%) PPD/PTD-allergic individuals throughout continued hair dyeing with an average of nine treatments per year. Five individuals reacted upon pretesting, while only mild-to-moderate/marked skin reactions occurred upon hair dyeing in nine individuals who were not identified by the pretest. To our knowledge this is the first study among PPD/PTD-allergic individuals indicating that a negative 45-min pretest with a hair dye product helps to avoid severe allergic reactions.

Peptide Reactivity of Isothiocyanates--Implications for Skin Allergy

Skin allergy is a chronic condition that affects about 20% of the population of the western world. This disease is caused by small reactive compounds, haptens, able to penetrate into the epidermis and modify endogenous proteins, thereby triggering an immunogenic reaction. Phenyl isothiocyanate (PITC) and ethyl isothiocyanate (EITC) have been suggested to be responsible for allergic skin reactions to chloroprene rubber, the main constituent of wetsuits, orthopedic braces, and many types of sports gear. In the present work we have studied the reactivity of the isothiocyanates PITC, EITC, and tetramethylrhodamine-6-isothiocyanate (6-TRITC) toward peptides under aqueous conditions at physiological pH to gain information about the types of immunogenic complexes these compounds may form in the skin. We found that all three compounds reacted quickly with cysteine moieties. For PITC and 6-TRITC the cysteine adducts decomposed over time, while stable adducts with lysine were formed. These experimental findings were verified by DFT calculations. Our results may suggest that the latter are responsible for allergic reactions to isothiocyanates. The initial adduct formation with cysteine residues may still be of great importance as it prevents hydrolysis and facilitates the transport of isothiocyanates into epidermis where they can form stable immunogenic complexes with lysine-containing proteins.

Contact sensitizers in commercial hair dye products sold in Thailand

BACKGROUND

Hair dyes are known to contain potent contact allergens for which sensitization rates have increased over the last decade.

OBJECTIVE

To examine the type and frequency of potent contact sensitizers labelled on hair dyes sold in metropolitan Bangkok, Thailand.

METHODS

During the 2013-2014 study period, labelled ingredient information from home use and professional hair dye products was collected.

RESULTS

Two hundred and fifty-two hair dye products were evaluated. One hundred and forty-nine products from 48 brands were domestically produced in Thailand, and 103 products were from 23 multinational brands produced in countries other than Thailand. Two hundred and fourteen of 252 (84.9%) hair dye products were found to contain strong skin sensitizers, with 118 (46.8%) being found in domestically produced products, and 96 (38.1%) being found in multinational brand products. Thirty-eight hair dye products (15.1%) were free of potent skin sensitizers. The number of domestically produced products (31, 20.8%) that were free of potent skin sensitizers was significantly higher (p = 0.002) than the number of multinational brand products (7, 6.8%).

CONCLUSIONS

p-Phenylenediamine was the most prevalent potent sensitizer found among domestically produced hair dyes available on the market. Our findings indicate regional differences in hair dye allergen exposure.

Prevalence of potent skin sensitizers in oxidative hair dye products in Korea

OBJECTIVES

The objective of the present study was to elucidate the prevalence of potent skin sensitizers in oxidative hair dye products manufactured by Korean domestic companies.

METHODS

A database on hair dye products made by domestic companies and selling in the Korean market in 2013 was used to obtain information on company name, brand name, quantity of production, and ingredients. The prevalence of substances categorized as potent skin sensitizers was calculated using the hair dye ingredient database, and the pattern of concomitant presence of hair dye ingredients was analyzed using network analysis software.

RESULTS

A total of 19 potent skin sensitizers were identified from a database that included 99 hair dye products manufactured by Korean domestic companies. Among 19 potent skin sensitizers, the four most frequent were resorcinol, m-aminophenol, p-phenylenediamine (PPD), and p-aminophenol; these four skin-sensitizing ingredients were found in more than 50% of the products studied. Network analysis showed that resorcinol, m-aminophenol, and PPD existed together in many hair dye products. In 99 products examined, the average product contained 4.4 potent sensitizers, and 82% of the products contained four or more skin sensitizers.

CONCLUSION

The present results demonstrate that oxidative hair dye products made by Korean domestic manufacturers contain various numbers and types of potent skin sensitizers. Furthermore, these results suggest that some hair dye products should be used with caution to prevent adverse effects on the skin, including allergic contact dermatitis.

Risk Assessment of residual monomer migrating from acrylic polymers and causing Allergic Contact Dermatitis during normal handling and use

Acrylic, Poly Methyl Methacrylate (PMMA) based polymers are found in many industrial, professional and consumer products and are of low toxicity, but do contain very low levels of residual monomers and process chemicals that can leach out during handling and use. Methyl Methacrylate, the principle monomer is of low toxicity, but is a recognized weak skin sensitizer. The risk of induction of contact allergy in consumers was determined using a method based upon the Exposure-based Quantitative Risk Assessment approach developed for fragrance ingredients. The No Expected Sensitization Induction Level (NESIL) was based on the threshold to induction of sensitization (EC3) in the Local Lymph Node Assay (LLNA) since no Human Repeat Insult Patch Test (HRIPT) data were available. Categorical estimation of Consumer Exposure Level was substituted with a worst case assumption based upon the quantitative determination of MMA monomer migration into simulants. Application of default and Chemical-Specific Adjustment Factors results in a Risk Characterization Ratio (RCR) of 10,000 and a high Margin of Safety for induction of Allergic Contact Dermatitis (ACD) in consumers handling polymers under conservative exposure conditions. Although there are no data available to derive a RCR for elicitation of ACD it is likely to be lower than that for induction.

Methylisothiazolinone: a case of perianal dermatitis caused by wet wipes and review of an emerging pediatric allergen

Methylisothiazolinone (MI) is a preservative found in cosmetic, personal hygiene, and industrial products. It has been characterized as a moderate to strong sensitizer and is an emerging allergen in the pediatric population. We discuss a case of perianal dermatitis in a child caused by contact allergy to MI-containing wet wipes.

Tissue reaction to the nickel implants in the guinea pigs

OBJECTIVES

The aim of the study was the assessment of local tolerance to nickel implants during 9 months observation in guinea pigs sensitized to nickel before implantation and non-sensitized ones.

MATERIALS AND METHODS

Three groups of guinea pigs were included in the study: 10 sensitized to nickel by the guinea pig maximization test; 10 previously non-sensitized and 10 in control group. In 20 animals (except control group) the nickel implants were inserted in the muscle of the back. After 9 months of observation, the animals were patch-tested with 5% nickel sulfate. Also percentage of eosinophils in peripheral blood was examined. Next, the tissue surrounding the implant and skin from the area of patch tests were collected for the histological examination.

RESULTS

In 70% of previously sensitized animals, the patch test confirmed the sensitivity to nickel. In 60% of previously non-sensitized animals, a positive reaction to nickel occurred. The results of patch tests in control group were negative. Percentage of eosinophils in peripheral blood was fourfold higher in animals sensitized to nickel than in control group. In histological examination, in the tissue surrounding the implant a dissimilarity concerning the intensity of cellular infiltration was observed between animals previously allergic and non-allergic to nickel. In the 2 of 10 previously sensitized guinea pigs quite severe inflammatory reactions in the inside of connective tissue capsule were noted which may indicate a local allergic reaction. The histological images of skin collected from the positive patch test site corresponded with the typical allergic contact dermatitis.

CONCLUSIONS

Nickel implants may cause primary sensitization to nickel. The nature of the histological changes in the tissues around the implants in guinea pigs sensitized to nickel may correspond to an allergic reaction. The examination of percentage of eosinophils in blood of guinea pigs may be useful in assessing the allergenic activity of metal alloys containing nickel.