The role of vehicle interactions on permeation of an active through model membranes and human skin

Previous work from this group has focused on the molecular mechanism of alcohol interaction with model membranes, by conducting thermodynamic and kinetic analyses of alcohol uptake, membrane partitioning and transport studies of a model compound (i.e. methyl paraben) in silicone membranes. In this article, similar membrane transport and partitioning studies were conducted in silicone membranes to further extend the proposed model of alcohol interactions with silicone membranes to include other vehicles more commonly used in dermal formulations, that is, isopropyl myristate (IPM), dimethyl isosorbide (DMI), polyethylene glycol (PEG) 200, PEG 400 and Transcutol P^® (TC). More importantly, membrane partitioning studies were conducted using human SC to evaluate the application of the proposed model of solvent‐enhanced permeation in simple model membranes for the more complex biological tissue. The findings support a model of vehicle interactions with model membranes and skin where high solvent uptake promotes drug partitioning (i.e. K) by enabling the solute to exist within the solvent fraction/solvent‐rich areas inside the membrane or skin in a concentration equivalent to that in the bulk solvent/vehicle. High solvent sorption may also ultimately impact on the membrane diffusional characteristics, and thus the diffusion coefficient of the solute across the membrane. The implications for skin transport are that increased partitioning of a drug into the SC may be achieved by (i) selecting vehicles that are highly taken up by the skin and also (ii) by having a relatively high concentration (i.e. molar fraction) of the drug in the vehicle. It follows that, in cases where significant co‐transport of the solvent into and across the skin may occur, its depletion from the formulation and ultimately from the skin may lead to drug crystallization, thus affecting dermal absorption.