Assessing the skin sensitization potential of fragrance ingredients in consumer products using the peroxidase peptide reactivity assay (PPRA) as an additional weight of evidence

A fragrance ingredient's potential to induce skin sensitization is an important factor in risk assessment. The key events in the skin sensitization adverse outcome pathway (AOP) can be evaluated with new approach methodologies (NAMs) for hazard identification, potency, and point of departure (PoD) derivation, distinguishing between sensitizers and non-sensitizers. The first key event is the covalent binding of an electrophilic material to skin proteins, which can be assessed using peptide reactivity assays (e.g., the Direct Peptide Reactivity Assay (DPRA)). Some sensitizers are not directly electrophilic and require biotic (pro-hapten) or abiotic (pre-hapten) activation before reacting with proteins. The Peroxidase Peptide Reactivity Assay (PPRA) was developed to characterize the reactivity of certain pre-/pro-haptens. The PPRA incorporates a horseradish peroxidase-hydrogen peroxide (+HRP/P) oxidation system to mimic the activation required for some pre-/pro-haptens to become protein-reactive and therefore potentially sensitizing. An evaluation of chemical structures for their suitability as substrates for +HRP/P resulted in the development of 5 structural activity groups that were defined based on the ability of a chemical to act as an HRP/P substrate. The goal of this study was to apply these sub-groups to understand the applicability domain of the PPRA in analyzing the sensitization potential of pre-/pro-hapten fragrance ingredients. The PPRA expanded hazard identification for 20 of the 88 known sensitizers of the 99 pre-/pro-hapten fragrance ingredients analyzed.