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Tonnis K, Kasting GB, Jaworska J. Impact of solvent dry down, phase change, vehicle pH and slowly reversible keratin binding on skin penetration of cosmetic relevant compounds: II. Solids. Int J Pharm 2024; 661:124451. [PMID: 38992735 DOI: 10.1016/j.ijpharm.2024.124451] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2024] [Revised: 06/11/2024] [Accepted: 07/07/2024] [Indexed: 07/13/2024]
Abstract
We extended a mechanistic, physics-based framework of the dry down process, previously developed for liquids and electrolytes, to solids and coded it into the latest UB/UC/P&G skin permeation model, herein renamed DigiSkin. The framework accounts for the phase change of the permeant from dissolved in a solvent (liquid) to precipitated on the skin surface (solid). The evaporation rate for the solid is reduced due to lower vapor pressure for the solid state versus subcooled liquid. These vapor pressures may differ by two orders of magnitude. The solid may gradually redissolve and penetrate the skin. The framework was tested by simulating the in vitro human skin permeation of the 38 cosmetically relevant solid compounds reported by Hewitt et al., J. Appl. Toxicol. 2019, 1-13. The more detailed handling of the evaporation process greatly improved DigiSkin evaporation predictions (r2 = 0.89). Further, we developed a model reliability prediction score classification using diverse protein reactivity data and identified that 15 of 38 compounds are out of model scope. Dermal delivery predictions for the remaining chemicals have excellent agreement with experimental data. The analysis highlighted the sensitivity of water solubility and equilibrium vapor pressure values on the DigiSkin predictions outcomes influencing agreement with the experimental observations.
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Affiliation(s)
- Kevin Tonnis
- College of Engineering and Applied Science, The University of Cincinnati, Cincinnati, OH 45221, USA
| | - Gerald B Kasting
- The James L. Winkle College of Pharmacy, The University of Cincinnati, Cincinnati, OH 45267-0514, USA
| | - Joanna Jaworska
- The Procter & Gamble Company, Discovery Innovation Platforms, Brussels Innovation Center, Belgium.
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2
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Stevens JN, Prockter AK, Fisher HA, Tran H, Evans MV. A database of chemical absorption in human skin with mechanistic modeling applications. Sci Data 2024; 11:755. [PMID: 38987285 PMCID: PMC11237069 DOI: 10.1038/s41597-024-03588-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2023] [Accepted: 07/01/2024] [Indexed: 07/12/2024] Open
Abstract
Whether from environmental and occupational hazards or from topical pharmaceuticals, the human skin comes into contact with various chemicals every day. In vivo experiments not only require large investments of both time and money, but in vivo experiments can also be unethical due to the need to intentionally or incidentally expose humans or animals to toxic chemicals. Comparatively, in vitro experiments offer ethical and financial advantages when combined with the opportunity to selectively choose chemicals for experimentation. With in vivo experimentation being so infeasible, many scientists have chosen to make their in vitro data available publicly. Using these data, a detailed database containing 73 chemicals was created with a robust set of descriptors to be used in connection with mathematical modeling to predict diffusion, permeability, and partition coefficients. This resulting database is tailored to be easily used in various coding languages.
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Affiliation(s)
- Jessica N Stevens
- Department of Mathematics, North Carolina State University, Raleigh, NC, USA.
| | - Alyson K Prockter
- Department of Mathematics, North Carolina State University, Raleigh, NC, USA
| | - Hunter A Fisher
- Oak Ridge Associated Universities (ORAU) assigned to United States Environmental Protection Agency (USEPA), Office of Research and Development (ORD), Research Triangle Park, NC, USA
| | - Hien Tran
- Department of Mathematics, North Carolina State University, Raleigh, NC, USA
| | - Marina V Evans
- United States Environmental Protection Agency (USEPA), Center for Computational Toxicity and Exposure, Office of Research and Development (ORD), Research Triangle Park, NC, USA
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3
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Xu L, Kasting GB. Solvent and Crystallization Effects on the Dermal Absorption of Hydrophilic and Lipophilic Compounds. J Pharm Sci 2024; 113:948-960. [PMID: 37797884 DOI: 10.1016/j.xphs.2023.09.025] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2023] [Revised: 09/27/2023] [Accepted: 09/28/2023] [Indexed: 10/07/2023]
Abstract
This study probes the mechanisms by which volatile solvents (water, ethanol) and a nonionic surfactant (Triton X-100) influence the skin permeation of dissolved solutes following deposition of small doses onto unoccluded human skin. A secondary objective was to sharpen guidelines for the use of these and other simple solvent systems for dermal safety testing of cosmetic ingredients at finite doses. Four solutes were studied - niacinamide, caffeine, testosterone and geraniol - at doses close to that estimated to saturate the upper layers of the stratum corneum. Methods included tensiometry, visualization of spreading on skin, polarized light microscopy and in vitro permeation testing using radiolabeled solutes. Ethanol, aqueous ethanol and dilute aqueous Triton solutions all yielded surface tensions below 36 mN/m, allowing them to spread easily on the skin, unlike water (72.4 mN/m) which did not spread. Deposition onto skin of niacinamide (32 μg·cm-2) or caffeine (3.2 μg·cm-2) from water and ethanol led to crystalline deposits on the skin surface, whereas the same amounts applied from aqueous ethanol and 2 % Triton did not. Skin permeation of these compounds was inversely correlated to the extent of crystallization. A separate study with caffeine showed the absence of a dose-related skin permeability increase with Triton. Permeation of testosterone (8.2 μg·cm-2) was modestly increased when dosed from aqueous ethanol versus ethanol. Permeation of geraniol (2.9 μg·cm-2) followed the order aqueous ethanol > water ∼ 2 % Triton >> ethanol and was inversely correlated with evaporative loss. We conclude that, under the conditions tested, aqueous ethanol and Triton serve primarily as deposition aids and do not substantially disrupt stratum corneum lipids. Implications for the design of in vitro skin permeability tests are discussed.
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Affiliation(s)
- Lijing Xu
- James L. Winkle College of Pharmacy, The University of Cincinnati, Cincinnati, OH 45267-0514, USA
| | - Gerald B Kasting
- James L. Winkle College of Pharmacy, The University of Cincinnati, Cincinnati, OH 45267-0514, USA.
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4
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Fisher HA, Evans MV, Bunge AL, Hubal EAC, Vallero DA. A compartment model to predict in vitro finite dose absorption of chemicals by human skin. CHEMOSPHERE 2024; 349:140689. [PMID: 37963497 PMCID: PMC10842870 DOI: 10.1016/j.chemosphere.2023.140689] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/06/2023] [Revised: 10/30/2023] [Accepted: 11/09/2023] [Indexed: 11/16/2023]
Abstract
Dermal uptake is an important and complex exposure route for a wide range of chemicals. Dermal exposure can occur due to occupational settings, pharmaceutical applications, environmental contamination, or consumer product use. The large range of both chemicals and scenarios of interest makes it difficult to perform generalizable experiments, creating a need for a generic model to simulate various scenarios. In this study, a model consisting of a series of four well-mixed compartments, representing the source solution (vehicle), stratum corneum, viable tissue, and receptor fluid, was developed for predicting dermal absorption. The model considers experimental conditions including small applied doses as well as evaporation of the vehicle and chemical. To evaluate the model assumptions, we compare model predictions for a set of 26 chemicals to finite dose in-vitro experiments from a single laboratory using steady-state permeability coefficient and equilibrium partition coefficient data derived from in-vitro experiments of infinite dose exposures to these same chemicals from a different laboratory. We find that the model accurately predicts, to within an order of magnitude, total absorption after 24 h for 19 of these chemicals. In combination with key information on experimental conditions, the model is generalizable and can advance efficient assessment of dermal exposure for chemical risk assessment.
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Affiliation(s)
- H A Fisher
- Oak Ridge Associated Universities, Assigned to U.S. Environmental Protection Agency, Office of Research and Development, Center for Computational Toxicology and Exposure, Research Triangle Park, NC, USA
| | - M V Evans
- U.S. Environmental Protection Agency, Office of Research and Development, Center for Computational Toxicology and Exposure, Research Triangle Park, NC, USA
| | - A L Bunge
- Chemical and Biological Engineering, Colorado School of Mines, Golden, CO, USA
| | - E A Cohen Hubal
- U.S. Environmental Protection Agency, Office of Research and Development, Center for Public Health and Environmental Assessment, Research Triangle Park, NC, USA
| | - D A Vallero
- U.S. Environmental Protection Agency, Office of Research and Development, Center for Computational Toxicology and Exposure, Research Triangle Park, NC, USA.
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Sandoval-Rivas D, Morales DV, Hepp MI. Toxicity evaluation of Pinus radiata D.Don bark wax for potential cosmetic application. Food Chem Toxicol 2023; 178:113896. [PMID: 37339695 DOI: 10.1016/j.fct.2023.113896] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2022] [Revised: 04/25/2023] [Accepted: 06/12/2023] [Indexed: 06/22/2023]
Abstract
Radiata pine bark is a widely available organic waste, requiring alternative uses due to its environmental impact on soil, fauna, and forest fires. Pine bark waxes could be used as cosmetic substitutes, but their toxicity requires evaluation since pine bark may contain toxic substances or xenobiotics, depending on the extraction process. This study evaluates the toxicity of radiata pine bark waxes obtained through various extraction methods on human skin cells grown in vitro. The assessment includes using XTT to evaluate mitochondrial activity, violet crystal dye to assess cell membrane integrity, and ApoTox-Glo triple assay to measure cytotoxicity, viability, and apoptosis signals. Pine bark waxes extracted via T3 (acid hydrolysis and petroleum ether incubation) and T9 (saturated steam cycle, alkaline hydrolysis, and petroleum ether incubation) exhibit non-toxicity up to 2% concentration, making them a potential substitute for petroleum-based cosmetic materials. Integrating the forestry and cosmetic industries through pine bark wax production under circular economy principles could promote development while replacing petroleum-based materials. Extraction methodology affects pine bark wax toxicity in human skin cells due to the retention of xenobiotic compounds including methyl 4-ketohex-5-enoate; 1-naphthalenol; dioctyl adipate; eicosanebioic acid dimethyl ester; among others. Future research will investigate whether the extraction methodology alters the molecular structure of the bark, affecting the release of toxic compounds in the wax mixture.
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Affiliation(s)
- Daniel Sandoval-Rivas
- Laboratorio de Investigación en Ciencias Biomédicas, Departamento de Ciencias Básicas y Morfología, Facultad de Medicina, Universidad Católica de la Santísima Concepción, Concepción, 4090541, Chile; Facultad de Ingeniería, Arquitectura y Diseño, Universidad San Sebastián, Lientur 1457, Concepción, Chile
| | - Daniela V Morales
- Departamento de Química Ambiental, Facultad de Ciencias, Universidad Católica de la Santísima Concepción, Concepción, 4090541, Chile; Centro de energía, Universidad Católica de la Santísima Concepción, Concepción, 4090541, Chile
| | - Matías I Hepp
- Laboratorio de Investigación en Ciencias Biomédicas, Departamento de Ciencias Básicas y Morfología, Facultad de Medicina, Universidad Católica de la Santísima Concepción, Concepción, 4090541, Chile.
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Liu F, Hutchinson RW. Semiquantitative sensitization safety assessment of extractable and leachables associated with parenteral pharmaceutical products. Regul Toxicol Pharmacol 2023; 138:105335. [PMID: 36608924 DOI: 10.1016/j.yrtph.2023.105335] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2022] [Revised: 11/11/2022] [Accepted: 01/03/2023] [Indexed: 01/07/2023]
Abstract
Extractable and leachables (E&Ls) associated with parenteral pharmaceutical products should be assessed for patient safety. One essential safety endpoint is local or systemic sensitization. However, there are no regulatory guidelines for quantitative sensitization safety assessment of E&Ls. A semiquantitative sensitization safety assessment workflow is developed to refine the sensitization safety assessment of E&Ls associated with parenteral pharmaceutical products. The workflow is composed of two sequential steps: local skin sensitization and systemic sensitization safety assessment. The local skin sensitization step has four tiers. The output from this step is the acceptable exposure level for local sensitization (AELls) and this safety threshold can be used for local sensitization safety assessment. From the derived AELls, the systemic sensitization safety assessment at step 2 proceeds in 2 tiers. The output from this workflow is the derivation of acceptable exposure level for systemic sensitization (AELss). When the estimated human daily exposure (HDE) is compared with the AELss, the margin of exposure is calculated to determine the sensitization safety of E&Ls following parenteral administration. The current work represents an initial effort to develop a scientifically robust process for sensitization safety assessment of E&Ls associated with parenteral pharmaceutical products.
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Affiliation(s)
- Frank Liu
- The Estée Lauder Companies, 155 Pinelawn Rd, Melville, NY, USA.
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Bazyleva A, Acree WE, Diky V, Hefter GT, Jacquemin J, Magalhães MCF, Magee JW, Nordstrom DK, O’Connell JP, Olson JD, Polishuk I, Schmidt KAG, Shaw JM, Trusler JPM, Weir RD. Reference materials for phase equilibrium studies. 2. Solid–liquid equilibria (IUPAC Technical Report). PURE APPL CHEM 2023. [DOI: 10.1515/pac-2021-1002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Abstract
Abstract
This article is the second of three projected IUPAC Technical Reports on reference materials for phase equilibrium studies. The goal of this project was to select reference systems with critically evaluated property values for the verification of instruments and techniques used in phase equilibrium studies of mixtures. This report proposes seven systems for solid–liquid equilibrium studies, covering the four most common categories of binary mixtures: aqueous systems with organic solutes, aqueous systems with inorganic solutes, non-aqueous systems, and systems with low solubility. For each system, the available literature sources, accepted data, smoothing equations, and estimated uncertainties are given.
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Affiliation(s)
- Ala Bazyleva
- Applied Chemicals and Materials Division, National Institute of Standards and Technology , Boulder , CO 80305-3337 , USA
| | - William E. Acree
- Department of Chemistry , University of North Texas , Denton , TX 76203-5070 , USA
| | - Vladimir Diky
- Applied Chemicals and Materials Division, National Institute of Standards and Technology , Boulder , CO 80305-3337 , USA
| | - Glenn T. Hefter
- Department of Chemistry , Murdoch University , Murdoch , WA 6150 , Australia
| | - Johan Jacquemin
- Laboratoire PCM2E, Université de Tours , Parc de Grandmont, 37200 Tours , France
- Materials Science and Nano-Engineering, Mohammed VI Polytechnic University , Lot 660-Hay Moulay Rachid, 43150 , Ben Guerir , Morocco
| | | | - Joseph W. Magee
- Applied Chemicals and Materials Division, National Institute of Standards and Technology , Boulder , CO 80305-3337 , USA
| | - D. Kirk Nordstrom
- Applied Chemicals and Materials Division, National Institute of Standards and Technology , Boulder , CO 80305-3337 , USA
- U.S. Geological Survey , Boulder , CO 80303 , USA
| | | | - James D. Olson
- Mid-Atlantic Technology, Research & Innovation Center (MATRIC) , P.O. Box 8396 , South Charleston , WV 25303 , USA
| | - Ilya Polishuk
- Department of Chemical Engineering , Ariel University , 40700 Ariel , Israel
| | - Kurt A. G. Schmidt
- Department of Chemical and Materials Engineering , University of Alberta , Edmonton , T6G 1H9 AB , Canada
| | - John M. Shaw
- Department of Chemical and Materials Engineering , University of Alberta , Edmonton , T6G 1H9 AB , Canada
| | - J. P. Martin Trusler
- Department of Chemical Engineering , Imperial College London , London SW7 2AZ , UK
| | - Ronald D. Weir
- Department of Chemistry and Chemical Engineering , Royal Military College of Canada , P.O. Box 17000, Stn Forces , Kingston , K7K 7B4 ON , Canada
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Tonnis K, Nitsche JM, Xu L, Haley A, Jaworska J, Kasting GB. Impact of solvent dry down, vehicle pH and slowly reversible keratin binding on skin penetration of cosmetic relevant compounds: I. Liquids. Int J Pharm 2022; 624:122030. [PMID: 35863596 DOI: 10.1016/j.ijpharm.2022.122030] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2022] [Revised: 06/29/2022] [Accepted: 07/14/2022] [Indexed: 11/19/2022]
Abstract
To measure progress and evaluate performance of the newest UB/UC/P&G skin penetration model we simulated an 18-compound subset of finite dose in vitro human skin permeation data taken from a solvent-deposition study of cosmetic-relevant compounds (Hewitt et al., J. Appl. Toxicol. 2019, 1-13). The recent model extension involved slowly reversible binding of solutes to stratum corneum keratins. The selected subset was compounds that are liquid at skin temperature. This set was chosen to distinguish between slow binding and slow dissolution effects that impact solid phase compounds. To adequately simulate the physical experiments there was a need to adjust the evaporation mass transfer coefficient to better represent the diffusion cell system employed in the study. After this adjustment the model successfully predicted both dermal delivery and skin surface distribution of 12 of the 18 compounds. Exceptions involved compounds that were cysteine-reactive, highly water-soluble or highly ionized in the dose solution. Slow binding to keratin, as presently parameterized, was shown to significantly modify the stratum corneum kinetics and diffusion lag times, but not the ultimate disposition, of the more lipophilic compounds in the dataset. Recommendations for further improvement of both modeling methods and experimental design are offered.
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Affiliation(s)
- Kevin Tonnis
- College of Engineering and Applied Science, The University of Cincinnati, Cincinnati, OH 45221, USA
| | - Johannes M Nitsche
- Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260-4200, USA
| | - Lijing Xu
- The James L. Winkle College of Pharmacy, The University of Cincinnati, Cincinnati, OH 45267-0514, USA
| | - Alison Haley
- College of Engineering and Applied Science, The University of Cincinnati, Cincinnati, OH 45221, USA
| | - Joanna Jaworska
- The Procter & Gamble Company, Data and Modeling Sciences, Brussels Innovation Center, Belgium
| | - Gerald B Kasting
- The James L. Winkle College of Pharmacy, The University of Cincinnati, Cincinnati, OH 45267-0514, USA.
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Lipsticks History, Formulations, and Production: A Narrative Review. COSMETICS 2022. [DOI: 10.3390/cosmetics9010025] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/10/2022] Open
Abstract
A considerable amount of literature has been published on several aspects of lipsticks production. To date, there is no collation of studies related to lipsticks production that has been published. This review was conducted to examine information about the history of lipsticks; ingredients used in the preparation of lipsticks, focusing on the natural and chemical ingredients; methods of preparation for the lipsticks; and the characterization of the lipsticks. A literature search for English language articles was conducted by searching electronic databases including Web of Science, Scopus, PubMed, and Google Scholar. Overall, the evidence indicates that lipsticks have been used since ancient times and are among the highest demand cosmetics. The findings of this review summarize those of earlier studies that explained the use of different types of ingredients in the manufacturing processes of lipsticks. It highlights the importance of using green technology and ingredients to fabricate lipsticks to avoid potential side effects such as skin irritation and allergy reaction.
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Ellison CA, Tankersley KO, Obringer CM, Carr GJ, Manwaring J, Rothe H, Duplan H, Géniès C, Grégoire S, Hewitt NJ, Jamin CJ, Klaric M, Lange D, Rolaki A, Schepky A. Partition coefficient and diffusion coefficient determinations of 50 compounds in human intact skin, isolated skin layers and isolated stratum corneum lipids. Toxicol In Vitro 2020; 69:104990. [DOI: 10.1016/j.tiv.2020.104990] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2020] [Revised: 08/18/2020] [Accepted: 08/27/2020] [Indexed: 11/24/2022]
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Eilstein J, Grégoire S, Fabre A, Arbey E, Géniès C, Duplan H, Rothe H, Ellison C, Cubberley R, Schepky A, Lange D, Klaric M, Hewitt NJ, Jacques‐Jamin C. Use of human liver and EpiSkin™ S9 subcellular fractions as a screening assays to compare the in vitro hepatic and dermal metabolism of 47 cosmetics‐relevant chemicals. J Appl Toxicol 2020; 40:416-433. [DOI: 10.1002/jat.3914] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2019] [Revised: 09/24/2019] [Accepted: 09/24/2019] [Indexed: 11/09/2022]
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12
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Géniès C, Jacques-Jamin C, Duplan H, Rothe H, Ellison C, Cubberley R, Schepky A, Lange D, Klaric M, Hewitt NJ, Grégoire S, Arbey E, Fabre A, Eilstein J. Comparison of the metabolism of 10 cosmetics-relevant chemicals in EpiSkin™ S9 subcellular fractions and in vitro human skin explants. J Appl Toxicol 2019; 40:313-326. [PMID: 31701564 DOI: 10.1002/jat.3905] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2019] [Revised: 08/13/2019] [Accepted: 08/27/2019] [Indexed: 11/08/2022]
Abstract
An understanding of the bioavailability of topically applied cosmetics ingredients is key to predicting their local skin and systemic toxicity and making a safety assessment. We investigated whether short-term incubations with S9 from the reconstructed epidermal skin model, EpiSkin™, would give an indication of the rate of chemical metabolism and produce similar metabolites to those formed in incubations with human skin explants. Both have advantages: EpiSkin™ S9 is a higher-throughput assay, while the human skin explant model represents a longer incubation duration (24 hours) model integrating cutaneous distribution with metabolite formation. Here, we compared the metabolism of 10 chemicals (caffeine, vanillin, cinnamyl alcohol, propylparaben, 4-amino-3-nitrophenol, resorcinol, 4-chloroaniline, 2-amino-3-methyl-3H-imidazo[4,5-F]quinoline and 2-acetyl aminofluorene) in both models. Both models were shown to have functional Phase 1 and 2 enzymes, including cytochrome P450 activities. There was a good concordance between the models with respect to the level of metabolism (stable vs. slowly vs. extensively metabolized chemicals) and major early metabolites produced for eight chemicals. Discordant results for two chemicals were attributed to a lack of the appropriate cofactor (NADP+ ) in S9 incubations (cinnamyl alcohol) and protein binding influencing chemical uptake in skin explants (4-chloroaniline). These data support the use of EpiSkin™ S9 as a screening assay to provide an initial indication of the metabolic stability of a chemical applied topically. If required, chemicals that are not metabolized by EpiSkin™ S9 can be tested in longer-term incubations with in vitro human explant skin to determine whether it is slowly metabolized or not metabolized at all.
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