Characterization of Water and a Model Lipophilic Compound in Human Stratum Corneum by NMR Spectroscopy and Equilibrium Sorption

Characterization of Water Self-Diffusion in Human Stratum Corneum

The stratum corneum (SC) is the outermost layer of human skin and primary barrier to water loss and chemical exposure. It consists of keratin-filled corneocytes of large aspect ratio surrounded by a thin matrix of highly organized lipophilic molecules. In the presence of water, the corneocytes swell and permeability for many chemicals increases. The role of hydration and SC structure on water self-diffusion was investigated using the pulsed-gradient stimulated echo nuclear magnetic resonance technique. Proton (¹H) self-diffusion, associated with water inside the corneocytes, was determined in human SC as a function of hydration, with and without lipid extraction, at 20°C to 40°C. SC layers were oriented either parallel or perpendicular to the field-gradient direction. Self-diffusion in the direction parallel to the long dimension of the corneocytes is unaffected by lipid extraction and consistent with a free-volume diffusion model. The effect of temperature corresponds with the activation energy of water in wool. Self-diffusion perpendicular to the long dimension of the corneocytes was less dependent on hydration and smaller than in the parallel direction, except at low hydration, when diffusion is insensitive to orientation. Corneocyte diffusion predicted by 2 microscopic SC models in common use are compared with our results.

How Predictable Are Human Stratum Corneum Lipid/Water Partition Coefficients? Assessment and Useful Correlations for Dermal Absorption

Partition coefficients between human stratum corneum lipids and water (K^sclip/w) are collected or deduced from a variety of sources in a manner that approximately doubles the available data compared to the current state-of-the-art model (Hansen et al., Adv Drug Deliv Rev. 2013;65(2):251-264). An additional datum for water itself in porcine SC that considerably extends the molecular size and lipophilicity range of the data set is considered. The data are analyzed in terms of an extended linear free energy relationship involving octanol/water partition coefficients, Abraham solvation parameters, and a secondary, power law molecular weight dependence. The optimum fit to log K^sclip/w for the full data set reduces the standard error of prediction from 0.50 for a Hansen-like model to 0.39; corresponding multiplicative errors in K^sclip/w are reduced from a factor of 3.1 to one of 2.5. The difference in performance is driven by the water datum, which requires a more complex dependence on molecular size than that afforded by Abraham parameters. In the absence of the water value, the Hansen-like model, which does not include a dependence on molecular size, is essentially optimum. A comparison is presented to fluid-phase phospholipid-water systems, which have a demonstrably different structure-property relationship.