Applicability of the DPRA on mixture testing: challenges and opportunities

The in chemico direct peptide reactivity assay (DPRA) is validated to assess protein reactivity of chemical compounds, relating to the molecular initiating event of skin sensitization induction. According to OECD TG 442C, the DPRA is technically applicable to test multi-constituent substances and mixtures of known composition, even though limited experimental data are publicly available. First, we assessed the DPRA's predictive capability for individual substances, but at concentrations other than the recommended 100 mM, i.e., based on the LLNA EC3 concentration (Experiment A). Next, the applicability of the DPRA to test unknown mixtures was assessed (Experiment B). Here, the complexity of unknown mixtures was reduced to mixtures containing either two known skin sensitizers with varying potencies, or a combination of a skin sensitizer with a non-skin sensitizer, or multiple non-sensitizers. Experiments A and B revealed that one extremely potent sensitizer (oxazolone) was incorrectly classified as a non-sensitizer when tested at its low EC3 concentration of 0.4 mM instead of the suggested molar excess conditions of 100 mM (Experiments A). For binary mixtures tested in experiments B, the DPRA was able to distinguish all skin sensitizers and the strongest skin sensitizer in the mixture was determinant for the overall peptide depletion of a sensitizer. In conclusion, we confirmed that the DPRA test method can be used efficiently for well-known characterized mixtures. However, when deviating from the recommended testing concentration of 100 mM, caution should be taken in case of negative results, limiting the DPRA's applicability for mixtures of unknown composition.

Implementation of a dermal sensitization threshold (DST) concept for risk assessment: structure-based DST and in vitro data-based DST

Skin sensitization resulting in allergic contact dermatitis represents an important toxicological endpoint as part of safety assessments. When available substance-specific sensitization data are inadequate, the dermal sensitization threshold (DST) concept has been proposed to set a skin exposure threshold to provide no appreciable risk of skin sensitization. Structure-based DSTs, which include non-reactive, reactive, and high potency category (HPC) DSTs, can be applied to substances with an identified chemical structures. An in vitro data-based "mixture DST" can be applied to mixtures based on data from in vitro test methods, such as KeratinoSens™ and the human Cell Line Activation Test. The purpose of this review article is to discuss the practical use of DSTs for conducting sound sensitization risk assessments to assure the safety of consumer products. To this end, several improvements are discussed in this review. For application of structure-based DSTs, an overall structural classification workflow was developed to exclude the possibility that "HPC but non-reactive" chemicals are misclassified as "non-reactive", because such chemicals should be classified as HPC chemicals considering that HPC rules have been based on the chemical structure of high potency sensitizers. Besides that, an extended application of the mixture DST principle to mixtures that either is cytotoxic or evaluated as positive was proposed. On a final note, we also developed workflows that integrate structure-based and in vitro-based mixture DST. The proposed workflows enable the application of the appropriate DST, which serves as a point of departure in the quantitative sensitization risk assessment.

Chemical Compounds Responsible for Skin Allergy to Complex Mixtures: How to Identify Them?

In the cosmetics industry, various natural complex mixtures such as botanical extracts and essential oils are used. In addition, finished consumer products may contain a number of constituents of natural origin but many products are derived from organic synthesis too. Hence, finding skin sensitizers within this myriad of chemicals is an arduous task. Nowadays, methods validated by European dedicated instances to evaluate the allergenicity of chemicals are incapable of predicting the sensitization potential of complex mixtures, although research has progressed a lot in this direction recently. In this context, precisely identifying the culprit(s) responsible for skin sensitization in these mixtures is essential for risk assessment. This review is a short summary of approaches that identify allergens in chemical mixtures such as bioassay-guided chemical fractionation, structure–activity relationship studies, and recent methods allowing identification of reactive intermediates in natural extracts exposed to air oxidation. It is shown that substantial progress has been made, although the identification of sensitizers in complex mixtures continues to be puzzling.