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Models of the outer epithelia of the human body - namely the skin, the intestine and the lung - have found valid applications in both research and industrial settings as attractive alternatives to animal testing. A variety of approaches to model these barriers are currently employed in such fields, ranging from the utilization of ex vivo tissue to reconstructed in vitro models, and further to chip-based technologies, synthetic membrane systems and, of increasing current interest, in silico modeling approaches. An international group of experts in the field of epithelial barriers was convened from academia, industry and regulatory bodies to present both the current state of the art of non-animal models of the skin, intestinal and pulmonary barriers in their various fields of application, and to discuss research-based, industry-driven and regulatory-relevant future directions for both the development of new models and the refinement of existing test methods. Issues of model relevance and preference, validation and standardization, acceptance, and the need for simplicity versus complexity were focal themes of the discussions. The outcomes of workshop presentations and discussions, in relation to both current status and future directions in the utilization and development of epithelial barrier models, are presented by the attending experts in the current report. [ABSTRACT FROM AUTHOR]

Skin exposure to sensitizing chemicals can induce allergic reactions. Certain chemicals, so called pro-sensitizers, need metabolic activation to become allergenic. Their metabolic activation occurs in skin cells such as keratinocytes or dendritic cells. These cell types are also incorporated into dermal in vitro test systems used to assess the sensitizing potential of chemicals for humans. In vitrosystems range from single cell cultures to organotypic multi-cellular reconstructed skin models. Until now, their metabolic competence to unmask sensitizing potential of pro-sensitizers was rarely investigated. This review aims to summarize current information on available skin in vitro models and the relevance of xenobiotic metabolizing enzymes for the activation of pro-sensitizers such as eugenol, 4-allylanisole, and ethylendiamine. Among others, these chemicals are discussed as performance standards to validate new coming in vitro systems for their potential to identify pro-sensitizers. [ABSTRACT FROM AUTHOR]

Abstract: The fate of nano-TiO2 particles in the body was investigated after inhalation exposure or intravenous (i.v.) injection, and compared with pigmentary TiO2 and quartz. For this purpose, a 5-day inhalation study (6h/day, head/nose exposure) was carried out in male Wistar rats using nano-TiO2 (100mg/m3), pigmentary TiO2 (250mg/m3) and quartz dust DQ 12 (100mg/m3). Deposition in the lung and tissue distribution was evaluated, and histological examination of the respiratory tract was performed upon termination of exposure, and 2 weeks after the last exposure. Broncho-alveolar lavage (BAL) was carried out 3 and 14 days after the last exposure. Rats were also injected with a single intravenous dose of a suspension of TiO2 in serum (5mg/kg body weight), and tissue content of TiO2 was determined 1, 14 and 28 days later. The majority of the inhaled nano-TiO2 was deposited in the lung. Translocation to the mediastinal lymph nodes was also noted, although to smaller amounts than following inhalation of pigmentary TiO2, but much higher amounts than after exposure to quartz. Systemically available nano-TiO2, as simulated by the i.v. injection, was trapped mainly in the liver and spleen. The (agglomerate) particle size of lung deposited nano-TiO2 was virtually the same as in the test atmosphere. Changes in BAL fluid composition and histological examination indicated mild neutrophilic inflammation and activation of macrophages in the lung. The effects were reversible for nano- and pigmentary TiO2, but progressive for quartz. The effects observed after 5-day inhalation exposure to nano-TiO2 were qualitatively similar to those reported in sub-chronic studies. [Copyright &y& Elsevier]

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., (© 2024 John Wiley & Sons Ltd.)

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/cm 2 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., (© 2024 John Wiley & Sons Ltd.)

The goal of the present study was to assess the predictive performance of a minimal generic rat physiologically based kinetic (PBK) model based on in vitro and in silico input data to predict peak plasma concentrations (Cmax) upon single oral dosing. To this purpose, a dataset was generated of 3960 Cmax predictions for 44 compounds, applying different combinations of in vitro and in silico approaches for chemical parameterization, and comparison of the predictions to reported in vivo data. Best performance was obtained when (1) the hepatic clearance was parameterized based on in vitro measured intrinsic clearance values, (2) the method of Rodgers and Rowland for calculating partition coefficients, and (3) in silico calculated fraction unbound plasma and Papp values (the latter especially for very lipophilic compounds). Based on these input data, the median Cmax of 32 compounds could be predicted within 10-fold of the observed Cmax, with 22 out of these 32 compounds being predicted within 5-fold, and 8 compounds within 2-fold. Overestimations of more than 10-fold were observed for 12 compounds, whereas no underestimations of more than 10-fold occurred. Median Cmax predictions were frequently found to be within 10-fold of the observed Cmax when the scaled unbound hepatic intrinsic clearance (Clint,u) was either higher than 20 l/h or lower than 1 l/h. Similar findings were obtained with a test set of 5 in-house BASF compounds. Overall, this study provides relevant insights in the predictive performance of a minimal PBK model based on in vitro and in silico input data., (© The Author(s) 2021. Published by Oxford University Press on behalf of the Society of Toxicology.)

The goal of the present study was to assess the predictive performance of a generic human physiologically based kinetic (PBK) model based on in vitro and in silico input data and the effect of using different input approaches for chemical parameterization on those predictions. For this purpose, a dataset was created of 38,772 Cmax predictions for 44 compounds by applying different combinations of in vitro and in silico approaches for chemical parameterization, and these predicted Cmax values were compared to reported in vivo data. Best results were achieved when the hepatic clearance was parameterized based on in vitro (i.e., hepatocytes or liver S9) measured intrinsic clearance values, the method of Rodgers and Rowland for calculating tissue:plasma partition coefficients, and the method of Lobell and Sivarajah for calculating the fraction unbound in plasma. With these parameters, the median Cmax values of 34 out of the 44 compounds were predicted within 5-fold of the observed Cmax, and the Cmax values of 19 compounds were predicted within 2-fold. The median Cmax values of 10 compounds were more than 5-fold overestimated. Underestimations (> 5-fold) did not occur. A comparison of the current generic PBK model structure with chemical-specific PBK models available in literature was made to identify possible kinetic processes not included in the generic PBK model that might explain the overestimations. Overall, the results provide crucial insights into the predictive performance of PBK models based on in vitro and in silico input and the influence of different input approaches on the model predictions.

Metabolomics, the comprehensive analysis of metabolites in a biological system, provides detailed information about the biochemical/physiological status of a biological system, and about the changes caused by chemicals. Metabolomics analysis is used in many fields, ranging from the analysis of the physiological status of genetically modified organisms in safety science to the evaluation of human health conditions. In toxicology, metabolomics is the -omics discipline that is most closely related to classical knowledge of disturbed biochemical pathways. It allows rapid identification of the potential targets of a hazardous compound. It can give information on target organs and often can help to improve our understanding regarding the mode-of-action of a given compound. Such insights aid the discovery of biomarkers that either indicate pathophysiological conditions or help the monitoring of the efficacy of drug therapies. The first toxicological applications of metabolomics were for mechanistic research, but different ways to use the technology in a regulatory context are being explored. Ideally, further progress in that direction will position the metabolomics approach to address the challenges of toxicology of the 21st century. To address these issues, scientists from academia, industry, and regulatory bodies came together in a workshop to discuss the current status of applied metabolomics and its potential in the safety assessment of compounds. We report here on the conclusions of three working groups addressing questions regarding 1) metabolomics for in vitro studies 2) the appropriate use of metabolomics in systems toxicology, and 3) use of metabolomics in a regulatory context.