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.

Evaluation of in vitro testing strategies for hazard assessment of the skin sensitization potential of "real-life" mixtures: The case of henna-based hair-colouring products containing p-phenylenediamine.

BACKGROUND: Allergic contact dermatitis caused by henna-based hair-colouring products has been associated with adulteration of henna with p-phenylenediamine (PPD). OBJECTIVES: To develop a testing approach based on in vitro techniques that address key events within the skin sensitization adverse outcome pathway in order to evaluate the allergenic potential of hair-colouring products. METHODS: The following in vitro assays were used to test the sensitizing capacity of hair dye ingredients: the micro-direct peptide reactivity assay (mDPRA); the HaCaT keratinocyte-associated interleukin (IL)-18 assay; the U937 cell line activation test (U-SENS)/IL-8 levels; the blood monocyte-derived dendritic cell test; and genomic allergen rapid detection (GARD skin). Those techniques with better human concordance were selected to evaluate the allergenic potential of 10 hair-colouring products. RESULTS: In contrast to the information on the label, chromatographic analyses identified PPD in all products. The main henna biomarker, lawsone, was not detected in one of the 10 products. Among the techniques evaluated by testing hair dye ingredients, the mDPRA, the IL-18 assay, GARD skin and the U-SENS correlated better with human classification (concordances of 91.7%-100%) and were superior to the animal testing (concordance of 78.5%). Thus, these assays were used to evaluate hair-colouring products, which were classified as skin sensitizers by the use of different two-of-three approaches. CONCLUSIONS: Our findings highlight the toxicological consequences of, and risks associated with, the undisclosed use of PPD in henna-based "natural" "real-life" products.