The role of tight junctions in skin barrier function and dermal absorption.

The skin protects our body from external assaults like pathogens, xenobiotics or UV irradiation. In addition, it prevents the loss of water and solutes. To fulfill these important tasks, a complex barrier system has developed which comprises the stratum corneum, tight junctions, the microbiome, the chemical barrier and the immunological barrier. These barriers do not act separately, but influence each other e.g. after external manipulation or in skin diseases. Especially the two mechanical barriers, i.e. stratum corneum and tight junctions, are of great interest for drug delivery, because they are the first interaction partners of drug delivery systems and play the major role in skin absorption. Tight junctions are of special interest, as they are centrally localized in this complex barrier system in the outermost viable layer - the stratum granulosum of the interfollicular epidermis and the companion cell layer of the hair follicle - and because they can react very quickly to stimuli. We summarize here our current knowledge about tight junction barrier function in mammalian interfollicular epidermis and hair follicles, and the interaction of tight junctions with other skin barrier components in health and disease. Furthermore, we discuss their relevance for drug delivery and provide examples for tight junction modulators.

Detailed modeling of skin penetration--an overview.

In recent years, the combination of computational modeling and experiments has become a useful tool that is proving increasingly powerful for explaining biological complexity. As computational power is increasing, scientists are able to explore ever more complex models in finer detail and to explain very complex real world data. This work provides an overview of one-, two- and three-dimensional diffusion models for penetration into mammalian skin. Besides diffusive transport this includes also binding of substances to skin proteins and metabolism. These models are based on partial differential equations that describe the spatial evolution of the transport process through the biological barrier skin. Furthermore, the work focuses on analytical and numerical techniques for this type of equations such as discretization schemes or homogenization (upscaling) techniques. Finally, the work compares different geometry models with respect to the permeability.

Mathematical modeling of process liquid flow and acetoclastic methanogenesis under mesophilic conditions in a two-phase biogas reactor.

Acetoclastic methanogenesis in the second stage of a two-phase biogas reactor is investigated. A mathematical model coupling chemical reactions with transport of process liquid and with the variation of population of the microorganisms living on the plastic tower packing of the reactor is proposed. The evolution of the liquid is described by an advection-diffusion-reaction equation, while a monod-type kinetic is used for the reactions. Moreover, a new inhibition factor MO(max) is introduced, which hinders the growth of microorganisms when the plastic tower packing is overpopulated. After estimating the reaction parameters, the acetate outflow measured experimentally is in good agreement with that predicted by simulations. For coupling liquid transport with reaction processes, a spatial discretization of the reactor is performed. This yields essential information about the distribution of acetate and the production of methane in the reactor. This information allows for defining a measure of the effectiveness of the reactor.

Computational modeling of the skin barrier.

A simulation environment for the numerical calculation of permeation processes through human skin has been developed. In geometry models that represent the actual cell morphology of stratum corneum (SC) and deeper skin layers, the diffusive transport is simulated by a finite volume method. As reference elements for the corneocyte cells and lipid matrix, both three-dimensional tetrakaidecahedra and cuboids as well as two-dimensional brick-and-mortar models have been investigated. The central finding is that permeability and lag time of the different membranes can be represented in a closed form depending on model parameters and geometry. This allows a comparison of the models in terms of their barrier effectiveness at comparable cell sizes. The influence of the cell shape on the barrier properties has been numerically demonstrated and quantified. It is shown that tetrakaidecahedra in addition to an almost optimal surface-to-volume ratio also has a very favorable barrier-to-volume ratio. A simulation experiment was successfully validated with two representative test substances, the hydrophilic caffeine and the lipophilic flufenamic acid, which were applied in an aqueous vehicle with a constant dose. The input parameters for the simulation were determined in a companion study by experimental collaborators.

The role of corneocytes in skin transport revised--a combined computational and experimental approach.

PURPOSE: To investigate mechanisms of compound-corneocyte interactions in a combined experimental and theoretical approach. MATERIALS AND METHODS: Experimental methods are presented to investigate compound-corneocyte interactions in terms of dissolution within water of hydration and protein binding and to quantify the extent of the concurrent mechanisms. Results are presented for three compounds: caffeine, flufenamic acid, and testosterone. Two compartmental stratum corneum models M1 and M2 are formulated based on experimentally determined input parameters describing the affinity to lipid, proteins and water. M1 features a homogeneous protein compartment and considers protein interactions only via intra-corneocyte water. In M2 the protein compartment is sub-divided into a cornified envelope compartment interacting with inter-cellular lipids and a keratin compartment interacting with water. RESULTS: For the non-protein binding caffeine the impact of the aqueous compartment on stratum corneum partitioning is overestimated but is successfully modeled after introducing a bound water fraction that is non-accessible for compound dissolution. For lipophilic, keratin binding compounds (flufenamic acid, testosterone) only M2 correctly predicts a concentration dependence of stratum corneum partition coefficients. CONCLUSIONS: Lipophilic and hydrophilic compounds interact with corneocytes. Interactions of lipophilic compounds are probably confined to the corneocyte surface. Interactions with intracellular keratin may be limited by their low aqueous solubility.

A comparison of two- and three-dimensional models for the simulation of the permeability of human stratum corneum.

The stratum corneum is the outermost layer of cells in mammalian epidermis. It is widely believed to play the key role for the barrier function of the skin. This study characterises how the cell geometry influences the permeability of the membrane. It is based on a diffusion model, which is evaluated using numerical simulation. Three different geometry concepts, i.e., ribbon, cuboid and tetrakaidekahedral type, in two and three space dimensions are compared. The results confirm that tetrakaidekahedral cells with an almost optimal surface-to-volume ratio provide a barrier, in which a minimal amount of mass is used very effectively. Additionally, the study supplies tools to quantify this and links the results to the theory of homogenization.

In-silico model of skin penetration based on experimentally determined input parameters. Part I: experimental determination of partition and diffusion coefficients.

Mathematical modeling of skin transport is considered a valuable alternative of in-vitro and in-vivo investigations especially considering ethical and economical questions. Mechanistic diffusion models describe skin transport by solving Fick's 2nd law of diffusion in time and space; however models relying entirely on a consistent experimental data set are missing. For a two-dimensional model membrane consisting of a biphasic stratum corneum (SC) and a homogeneous epidermal/dermal compartment (DSL) methods are presented to determine all relevant input parameters. The data were generated for flufenamic acid (M(W) 281.24g/mol; logK(Oct/H2O) 4.8; pK(a) 3.9) and caffeine (M(W) 194.2g/mol; logK(Oct/H2O) -0.083; pK(a) 1.39) using female abdominal skin. K(lip/don) (lipid-donor partition coefficient) was determined in equilibration experiments with human SC lipids. K(cor/lip) (corneocyte-lipid) and K(DSL/lip) (DSL-lipid) were derived from easily available experimental data, i.e. K(SC/don) (SC-donor), K(lip/don) and K(SC/DSL) (SC-DSL) considering realistic volume fractions of the lipid and corneocyte phases. Lipid and DSL diffusion coefficients D(lip) and D(DSL) were calculated based on steady state flux. The corneocyte diffusion coefficient D(cor) is not accessible experimentally and needs to be estimated by simulation. Based on these results time-dependent stratum corneum concentration-depth profiles were simulated and compared to experimental profiles in an accompanying study.

In-silico model of skin penetration based on experimentally determined input parameters. Part II: mathematical modelling of in-vitro diffusion experiments. Identification of critical input parameters.

This work describes a framework for in-silico modelling of in-vitro diffusion experiments illustrated in an accompanying paper [S. Hansen, A. Henning, A. Naegel, M. Heisig, G. Wittum, D. Neumann, K.-H. Kostka, J. Zbytovska, C.M. Lehr, U.F. Schaefer, In-silico model of skin penetration based on experimentally determined input parameters. Part I: experimental determination of partition and diffusion coefficients, Eur. J. Pharm. Biopharm. 68 (2008) 352-367 [corrected] A mathematical model of drug permeation through stratum corneum (SC) and viable epidermis/dermis is presented. The underlying geometry for the SC is of brick-and-mortar character, meaning that the corneocytes are completely embedded in the lipid phase. The geometry is extended by an additional compartment for the deeper skin layers (DSL). All phases are modelled with homogeneous diffusivity. Lipid-donor and SC-DSL partition coefficients are determined experimentally, while corneocyte-lipid and DSL-lipid partition coefficients are derived consistently with the model. Together with experimentally determined apparent lipid- and DSL-diffusion coefficients, these data serve as direct input for computational modelling of drug transport through the skin. The apparent corneocyte diffusivity is estimated based on an approximation, which uses the apparent SC- and lipid-diffusion coefficients as well as corneocyte-lipid partition coefficients. The quality of the model is evaluated by a comparison of concentration-SC-depth-profiles of the experiment with those of the simulation. Good agreements are obtained, and by an analysis of the underlying model, critical parameters of the models can be identified more easily.