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.

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/cm2) 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/cm2 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/cm2 (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.

Characterization of the in vitro penetration and first-pass metabolism of genistein and daidzein using human and pig skin explants and Phenion full-thickness skin models.

OECD test guideline compliant skin penetration studies, which also comply with the SCCS basic criteria, are lacking for genistein and daidzein. Therefore, we have measured their penetration and metabolism using ex vivo explants of fresh (i.e., metabolically viable) pig skin, fresh and frozen human skin, and Phenion full-thickness (FT) models. Preliminary studies using fresh pig skin helped to define the optimal experimental conditions. The dermal absorption of 10 nmoles/cm2 genistein and daidzein in ethanol was comparable in all four models. A first-pass metabolism in skin to glucuronide and sulfate metabolites was demonstrated for both chemicals in all models except frozen human skin. The main difference between fresh skin models was the overall extent of metabolism and the relative ratio of each metabolite, for example, much lower sulfate conjugates were formed in pig skin incubations. The extent of parent chemical metabolized and the contribution of the glucuronide pathway were relatively lower in PhenionFT models than in fresh human skin, possibly due to a higher penetration rate in this model and differences in the expression of functional metabolizing enzymes. When metabolism in human skin was abolished by freezing, more radiolabelled chemical remained in the skin tissue but the overall dermal absorption was unchanged. In conclusion, this initial characterization study showed that all models tested indicated that genistein and daidzein extensively penetrated the skin when applied to skin in ethanol. All fresh skin models produced the same metabolites, with the known species difference in the sulfation pathway demonstrated in pig skin.

Next-generation risk assessment read-across case study: application of a 10-step framework to derive a safe concentration of daidzein in a body lotion.

Introduction: We performed an exposure-based Next Generation Risk Assessment case read-across study using New Approach Methodologies (NAMs) to determine the highest safe concentration of daidzein in a body lotion, based on its similarities with its structural analogue, genistein. Two assumptions were: (1) daidzein is a new chemical and its dietary intake omitted; (2) only in vitro data were used for daidzein, while in vitro and legacy in vivo data for genistein were considered. Methods: The 10-step tiered approach evaluating systemic toxicity included toxicokinetics NAMs: PBPK models and in vitro biokinetics measurements in cells used for toxicogenomics and toxicodynamic NAMs: pharmacology profiling (i.e., interaction with molecular targets), toxicogenomics and EATS assays (endocrine disruption endpoints). Whole body rat and human PBPK models were used to convert external doses of genistein to plasma concentrations and in vitro Points of Departure (PoD) to external doses. The PBPK human dermal module was refined using in vitro human skin metabolism and penetration data. Results: The most relevant endpoint for daidzein was from the ERα assay (Lowest Observed Effective Concentration was 100 ± 0.0 nM), which was converted to an in vitro PoD of 33 nM. After application of a safety factor of 3.3 for intra-individual variability, the safe concentration of daidzein was estimated to be 10 nM. This was extrapolated to an external dose of 0.5 µg/cm2 for a body lotion and face cream, equating to a concentration of 0.1%. Discussion: When in vitro PoD of 33 nM for daidzein was converted to an external oral dose in rats, the value correlated with the in vivo NOAEL. This increased confidence that the rat oral PBPK model provided accurate estimates of internal and external exposure and that the in vitro PoD was relevant in the safety assessment of both chemicals. When plasma concentrations estimated from applications of 0.1% and 0.02% daidzein were used to calculate bioactivity exposure ratios, values were >1, indicating a good margin between exposure and concentrations causing adverse effects. In conclusion, this case study highlights the use of NAMs in a 10-step tiered workflow to conclude that the highest safe concentration of daidzein in a body lotion is 0.1%.

Development of a non-target strategy for evaluation of potential biological effects of inhalable aerosols generated during purposeful room conditioning using an in vitro inhalation model.

OBJECTIVES: An integrated in vitro inhalation approach was outlined to estimate potential adverse acute inhalation effects of aerosols from commercial nebulizer applications used for purposeful room conditioning such as disinfection, scenting or others. Aerosol characterization, exposure estimation and evaluation of acute biological effects by in vitro inhalation were included to generate dose-response data, allowing for determination of in vitro lowest observable adverse effect levels (LOAELs). Correlation of these to estimates of human lung deposition was included for quantitative in vitro to in vivo extrapolation approach (QIVIVE) for acute effects during human exposure. METHODS: To test the proposed approach, a case study was undertaken using two realistic test materials. An acute in vitro inhalation setup with air-liquid interface A549-cells in an optimized exposure situation (P.R.I.T.® ExpoCube®) was used to expose cells and analysis of relevant biological effects (viability, mitochondrial membrane potential, stress, IL-8 release) was carried out. RESULTS: The observed dose-responsive effects in a sub-toxic dose-range could be attributed to the main component of one test material and its presence in the aerosol phase of the nebulized material. QIVIVE resulted in a factor of at least 256 between the in vitro LOAEL and the estimated acute human lung exposure for this test material. CONCLUSIONS: The case-study shows the value of the non-target in vitro inhalation testing approach especially in case of a lack of knowledge on complex product composition. It is expected that approaches like this will be of high value for product safety and environmental health in the future.

Design of a routine in vitro inhalation approach to estimate biological effects of nebulized products.Application in a case study on a potential real product for purposeful room conditioning by use of a commercial nebulizer.Combining results from aerosol characterization and in vitro inhalation experiments allowed for comprehensive correlation of product composition, aerosol properties and biological effects.Assignment of sub-toxic biological effects to a specific product component enabled identification of a product composition with potentially even less biological effect.Combined in vivo exposure estimation and in vitro LOAEL determination enabled a QIVIVE approach.

Incorporating integrated testing strategy (ITSv1) defined approach into read-across (RAx) in predicting skin sensitization potency: ITSv1-based RAx.

Recently, due to regulatory and ethical demands, new approach methodologies (NAMs), defined approaches (DAs), and read-across (RAx) have been used in the risk assessment of skin sensitization. Integrated testing strategy (ITS)v1 DA, adopted in OECD Guideline No. 497, can be used for skin sensitization potency categorization. However, ITSv1 DA alone is not used for further refinement of the potency prediction based on EC3 (the estimated concentration that produces a stimulation index of 3 in murine local lymph node assay) values. Moreover, there is no explicit approach to incorporating NAM/DA data into RAx to fill the data gap of EC3 values with high confidence. This study developed a strategy incorporating ITSv1 DA into RAx to predict skin sensitization potency: ITSv1-based RAx. To examine the reliability of this novel strategy, a case study with lilial, a fragrance material, was performed. Based on ITSv1-based RAx, the skin sensitization potency of lilial was determined by extrapolating the EC3 value of 9.5% for the suitable analogue bourgeonal, which was close to the historical EC3 value of 8.6%. The result suggested that the strategy can refine the prediction of EC3 values with high confidence and be useful for the risk assessment of skin sensitization.