Factors affecting the permeability of skin. The relation between in vivo and in vitro observations

At the same temperature and with adequate circulation of blood or receptor solution beneath it the permeability of the stratum corneum of the rabbit ear to T2O or to 32P-TPP was the same in vivo as in vitro. When skin permeability was measured in vitro, the subcutaneous adipose tissue present in the full-thickness skin of the rat delayed the penetration of CR, a lipophilic substance with a low water solubility, and decreased the permeability constant by nearly 3x. The retardant solvent PEG 300 did not penetrate the stratum corneum; it formed a hydrogen-bonded complex with the cholinesterase inhibitor VX, thereby reducing the thermodynamic activity and penetration rate of this compound through the stratum corneum. The accelerant solvent DMSO removed protein components from the stratum corneum; electron microscope studies showed that the cells of stratum corneum so treated became separated from one another, and their contents became stainable in bulk with Pb++, indicating the creation of new diffusion pathways. When the temperature, clearance of penetrant from the lower surface of the stratum corneum and penetrant formulation were the same in vivo as in vitro, and the surface of the stratum corneum was saturated with the penetrant or its solution, the results of permeability measurements made in vivo were similar to those made in vitro.