Use of in vitro testing to identify an unexpected skin sensitizing impurity in a commercial product: a case study.

Due to regulatory constraints and ethical considerations, the quest for alternatives to animal testing has gained a new momentum. In general, animal welfare considerations and compliance with regulations are the key drivers for this research. Mechanistically based in vitro tests addressing specific toxicological questions can yield new information, for example on reactive components, and thus in certain cases the in vitro tests are not only second choice replacements of a 'gold standard' animal test but can also be used to develop safer products. Here we report a case study from the in vitro investigation on the commercial fragrance chemical Azurone. This compound was found to be a moderate skin sensitizer in the LLNA, whereas the structurally closely similar compound Calone is a non-sensitizer. A peptide reactivity assay indicated, that indeed Azurone yields peptide depletion, thus the in vitro assays confirmed the animal test result. LC-MS analysis of the peptide reactivity sample showed the presence of peptide adducts of unexpected molecular weight. They were consistent with the reaction of the peptide with a catechol related to Azurone. Detailed analytics indicated that indeed this catechol is present in the original batches as an impurity, but it has escaped quality control analysis, as it is not detectable in routine GC-analysis. A new purified batch was prepared, re-tested in the in vitro assays and predicted by the tests to be a non-sensitizer. A confirmatory LLNA test indeed yielded a significantly (10-fold) higher EC3 value of the new batch, but the LLNA was still positive. A dose-response study in the EpiSkin assay indicated that this molecule still has a significant skin irritation potential, which may generate the weak positive signal in the LLNA. This case study illustrates how the mechanistically based in vitro LC-MS peptide reactivity assay can be used to contribute to the understanding of the sensitization mechanism of a commercial product and help to define a safer product specification.