Skin sensitization, false positives and false negatives: experience with guinea pig assays

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.

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.

Characterization of dermal sensitization potential for industrial or agricultural chemicals with EpiSensA

The regulatory community is transitioning to the use of nonanimal methods for dermal sensitization assessments; however, some in vitro assays have limitations in their domain of applicability depending on the properties of chemicals being tested. This study explored the utility of epidermal sensitization assay (EpiSensA) to evaluate the sensitization potential of complex and/or "difficult to test" chemicals. Assay performance was evaluated by testing a set of 20 test chemicals including 10 methacrylate esters, 5 silicone-based compounds, 3 crop protection formulations, and 2 surfactant mixtures; each had prior in vivo data plus some in silico and in vitro data. Using the weight of evidence (WoE) assessments by REACH Lead Registrants, 14 of these chemicals were sensitizers and, six were nonsensitizers based on in vivo studies (local lymph node assay [LLNA] and/or guinea pig studies). The EpiSensA correctly predicted 16/20 materials with three test materials as false positive and one silane as false negative. This silane, classified as weak sensitizer via LLNA, also gave a "false negative" result in the KeratinoSens™ assay. Overall, consistent with prior evaluations, the EpiSensA demonstrated an accuracy level of 80% relative to available in vivo WoE assessments. In addition, potency classification based on the concentration showing positive marker gene expression of EpiSensA was performed. The EpiSensA correctly predicted the potency for all seven sensitizing methacrylates classified as weak potency via LLNA (EC3 ≥ 10%). In summary, EpiSensA could identify dermal sensitization potential of these test substances and mixtures, and continues to show promise as an in vitro alternative method for dermal sensitization.

The LLNA: A Brief Review of Recent Advances and Limitations

Allergic contact dermatitis is the second most commonly reported occupational illness, accounting for 10% to 15% of all occupational diseases. This highlights the importance of developing rapid and sensitive methods for hazard identification of chemical sensitizers. The murine local lymph node assay (LLNA) was developed and validated for the identification of low molecular weight sensitizing chemicals. It provides several benefits over other tests for sensitization because it provides a quantitative endpoint, dose-responsive data, and allows for prediction of potency. However, there are also several concerns with this assay including: levels of false positive responses, variability due to vehicle, and predictivity. This report serves as a concise review which briefly summarizes the progress, advances and limitations of the assay over the last decade.