Abstract

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 13C 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.

Highlights

  • The human skin is a large interfacial film that separates regions with completely different properties

  • We investigate different groups of compounds, including monoterpenes, saturated and unsaturated fatty acids (DAC12:0, SAC18:0 and OAC18:1), one surfactant (SDS), Azone and small polar compounds

  • We chose to compare the samples with the same water content rather than the same relative humidity (RH) due to uncertainties associated with the equilibration of samples containing volatile compounds in vapor with controlled RH

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Summary

Introduction

The human skin is a large interfacial film that separates regions with completely different properties. This fact implies several simultaneous transport processes occurring across the skin. The skin barrier function is vital to prevent desiccation and to protect the body from uptake of foreign compounds. The skin constitutes a mechanical protection in that it tolerates deformation from physical strain and stress. The barrier function of the skin is mainly assured by its outermost layer, the stratum corneum (SC) This is a thin (ca. 20 μm) [1] and dry

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