Effect of vehicle on the in vitro penetration and metabolism of genistein and daidzein in ex vivo skin explants and the Phenion full‐thickness skin model

Abstract In a read‐across assessment of the safety of genistein and daidzein in cosmetic products, additional information was required to account for differences in their systemic exposure after topical application in a typical body lotion formulation. Therefore, we measured the penetration and metabolism of two doses (3 and 30 nmoles/cm 2 ) of genistein and daidzein applied in ethanol and in a body formulation to fresh pig skin, fresh and frozen human skin, and PhenionFT models. Both chemicals readily penetrated all skin models when applied in ethanol. The same sulfate and glucuronide metabolites were formed in fresh pig skin, fresh human skin, and PhenionFT models, which also all demonstrated that (a) these pathways could be saturated between 3 and 30 nmoles/cm 2 and (b) the extent of metabolism of daidzein was lower than genistein. Although the relative amounts of radiolabeled chemical in human skin and medium compartments were altered by freezing, their overall bioavailability was not affected. The greatest impact on the bioavailability and distribution of both chemicals was observed when they were applied in the formulation. Most of the dose applied in the formulation was retained on the skin surface, especially at 30 nmoles/cm 2 (60%–90%), resulting in much lower amounts in the medium and/or skin. In conclusion, all four skin models demonstrated first‐pass metabolism of genistein and daidzein and a marked alteration in their disposition by applying them in a body lotion formulation. This supports the use of fresh pig skin and PhenionFT models as alternatives to human skin for investigating skin metabolism and formulation effects for these two chemicals. The results were used to develop the dermal module of a PBPK model and dose setting for organ‐on‐chip experiments. They could also be used to refine internal exposure estimates in regulatory safety assessments.