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The development of formulation concepts for improved skin tissue oxygenation, including methods for measuring oxygen (O 2 ) transport across biological barriers, are important research topics with respect to all processes that are affected by the O 2 concentration, such as radiation therapy in oncology treatments, wound healing, and the general health status of skin. In this work we approach this topic by a novel strategy based on the antioxidative enzyme catalase, which is naturally present in the skin organ where it enables conversion of the reactive oxygen species hydrogen peroxide (H 2 O 2 ) into O 2 . We introduce various applications of the skin covered oxygen electrode (SCOE) as an in-vitro tool for studies of catalase activity and function. The SCOE is constructed by placing an excised skin membrane directly on an O 2 electrode and the methodology is based on measurements of the electrical current generated by reduction of O 2 as a function of time (i.e. chronoamperometry). The results confirm that a high amount of native catalase is present in the skin organ, even in the outermost stratum corneum (SC) barrier, and we conclude that excised pig skin (irrespective of freeze-thaw treatment) represents a valid model for ex vivo human skin for studying catalase function by the SCOE setup. The activity of native catalase in skin is sufficient to generate considerable amounts of O 2 by conversion from H 2 O 2 and proof-of-concept is presented for catalase-based transdermal O 2 delivery from topical formulations containing H 2 O 2 . In addition, we show that this concept can be further improved by topical application of external catalase on the skin surface, which enables transdermal O 2 delivery from 50 times lower concentrations of H 2 O 2 . These important results are promising for development of novel topical or transdermal formulations containing low and safe concentrations of H 2 O 2 for skin tissue oxygenation. Further, our results indicate that the O 2 production by catalase, derived from topically applied S. epidermidis (a simple model for skin microbiota) is relatively low as compared to the O 2 produced by the catalase naturally present in skin. Still, the catalase activity derived from S. epidermidis is measurable. Taken together, this work illustrates the benefits and versatility of the SCOE as an in vitro skin research tool and introduces new and promising strategies for transdermal oxygen delivery, with simultaneous detoxification of H 2 O 2 , based on native or topically applied catalase., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Skin is attractive for drug therapy because it offers an easily accessible route without first-pass metabolism. Transdermal drug delivery is also associated with high patient compliance and through the site of application, the drug delivery can be locally directed. However, to succeed with transdermal drug delivery it is often required to overcome the low permeability of the upper layer of the skin, the stratum corneum (SC). One common strategy is to employ so-called penetration enhancers that supposedly act to increase the drug passage across SC. Still, there is a lack of understanding of the molecular effects of so-called penetration enhancers on the skin barrier membrane, the SC. In this study, we provide a molecular characterization of how different classes of compounds, suggested as penetration enhancers, influence lipid and protein components in SC. The compounds investigated include monoterpenes, fatty acids, osmolytes, surfactant, and Azone. We employ natural abundance (13)C polarization transfer solid-state nuclear magnetic resonance (NMR) on intact porcine SC. With this method it is possible to detect small changes in the mobility of the minor fluid lipid and protein SC components, and simultaneously obtain information on the major fraction of solid SC components. The balance between fluid and solid components in the SC is essential to determine macroscopic material properties of the SC, including barrier and mechanical properties. We study SC at different hydration levels corresponding to SC in ambient air and under occlusion. The NMR studies are complemented with diffusion cell experiments that provide quantitative data on skin permeability when treated with different compounds. By correlating the effects on SC molecular components and SC barrier function, we aim at deepened understanding of diffusional transport in SC, and how this can be controlled, which can be utilized for optimal design of transdermal drug delivery formulations., (Copyright © 2016 The Authors. Published by Elsevier B.V. All rights reserved.)

At normal conditions there is a substantial water gradient over the skin as it separates the water-rich inside of the body from the dry outside. This leads to a variation in the degree of hydration from the inside to the outside of skin and changes in this gradient may affect its structure and function. In this study we raise the question: How do changes in the water gradient across skin affect its permeability? We approach this problem in novel diffusion experiments that permit strict control of the gradient in the chemical potential of water and hence well-defined boundary conditions. The results demonstrate that a water gradient can be used to regulate transport of drugs with different lipophilic characteristics across the skin barrier. It is shown that the transport of metronidazole (log P(o/w)=0.0) and methyl salicylate (log P(o/w)=2.5) across skin increases abruptly at low water gradients, corresponding to high degrees of skin hydration, and that this effect is reversible. This phenomenon is highly relevant to drug delivery applications due to its potential of temporarily open the skin barrier for transdermal drug delivery and subsequently close the barrier after treatment. Further, the results contribute to the understanding of the occlusion effect and indicate the boundary conditions of the water gradient needed to make use of this effect., (Copyright 2010 Elsevier B.V. All rights reserved.)

In dermal/transdermal drug administration stratum corneum (SC) is often the rate-limiting step. Furthermore, the intercellular lipid domain of SC is nowadays widely accepted as the major contributor to the skin barrier. The current work investigates whether the difference in the level of topical efficacy of local anesthetic compounds correlates with the type of interaction between the drug and the intercellular lipids of SC. Therefore, local anesthetics of varying topical efficacy were evaluated with respect to their effect on the morphology of various model lipid systems using small and wide angle X-ray diffraction (SWAXD) and differential scanning calorimetry (DSC). The model lipids used were glyceryl monooleate, sphingomyelin and lipids isolated from human SC. Furthermore, partitioning into isolated human SC as well as permeation through isolated human SC and human tape-stripped skin were investigated in vitro. The results indicate that local anesthetics may act as their own permeation enhancers by increasing the degree of hydrocarbon chain fluidity of the intercellular lipids. Eventually these interactions may induce non-lamellar reversed types of liquid crystalline structures locally in SC, which further facilitate the drug mobility. The large difference in topical efficacy of the investigated local anesthetics could not be explained simply by looking at their effect on the phase behavior of lipid model systems. Despite the similarities in physicochemical properties of these substances, the in vitro skin permeability differed markedly (AD>EMLA>lidocaine>prilocaine>sameridine). Thus, it was concluded that sufficient drug permeability over SC is essential to obtain local anesthesia by blocking the superficial nociceptors.

Simplified skin barrier models are necessary to get a first hand understanding of the very complex morphology and physical properties of the human skin barrier. In addition, it is of great importance to construct relevant models that will allow for rational testing of barrier perturbing/occlusive effects of a large variety of substances. The primary objective of this work was to study the effect of lipid morphology on water permeation through various lipid mixtures (i.e., partly neutralised free fatty acids, as well as a skin lipid model mixture). In addition, the effects of incorporating Azone((R)) (1-dodecyl-azacycloheptan-2-one) into the skin lipid model mixture was studied. Small- and wide-angle X-ray diffraction was used for structure determinations. It is concluded that: (a) the water flux through a crystalline fatty acid-sodium soap-water mixture (s) is statistically significantly higher than the water flux through the corresponding lamellar (L(alpha)) and reversed hexagonal (H(II)) liquid crystalline phases, which do not differ between themselves; (b) the water flux through mixtures of L(alpha)/s decreases statistically significantly with increasing relative amounts of lamellar (L(alpha)) liquid crystalline phase; (c) the addition of Azone((R)) to a skin lipid model system induces a reduction in water flux. However, further studies are needed to more closely characterise the structural basis for the occlusive effects of Azone((R)) on water flux.

Small and wide-angle X-ray diffraction was used to study the phase behaviour of cholesterol-fatty acid mixtures in an attempt to understand lipid interaction occurring in the stratum corneum, the outermost layer of skin. The effect of the penetration enhancer Azone was investigated as well. It was found that equimolar mixtures of cholesterol, palmitic acid and oleic acid (with the acids neutralised to 41 mol%) in 25% (wt/wt) water typically showed three phases at room temperature, two crystalline and one gel phase. The crystalline phases consisted mainly of palmitic acid:soap and cholesterol, respectively. The water present was unevenly distributed and was associated with the gel phase. Both cholesterol and palmitic acid seemed to be depleted from their crystalline phases by Azone. The electrostatic effects on titration of fatty acids in lamellar aggregates were calculated in view of the present results, and the effects of phase separation were discussed.