Combined effects of iontophoretic and chemical enhancement on drug delivery. II. Transport across human and murine skin.

This paper reports measurements of the release characteristics of the model drug salbutamol from a liquid crystalline vehicle across both human and hairless murine skin in vitro. The use of oleic acid and iontophoresis as penetration enhancement techniques, used separately and simultaneously, was also investigated. Over a period of 12h, salbutamol base did not diffuse from the vehicle across excised human skin while, in contrast, over a period of 2h, the drug passively transported across hairless murine skin. The diffusion co-efficient for the drug in this tissue was estimated to be 4.54+/-0.60x10(-9)cm(2)s(-1) with a permeability co-efficient of 7.03+/-0.83x10(-7)cms(-1). A current of density of 0.39mAcm(-2) facilitated a significant transport of salbutamol from the liquid crystalline vehicle across excised human skin but with a small (<0.1) transport number. The quantity of salbutamol transported across excised hairless murine skin under the same conditions was significantly greater with a transport number of 0.68. The alteration of the permeability of the tissue was less than that of the human skin and a full recovery of the pre-iontophoretic permeability of murine skin was consistently observed. The incorporation of either oleic or lauric acid into the monoglyceride component of the vehicle at a concentration of 0.1M had a marked effect on the transport of salbutamol across both human and murine skin. The initial passive permeation of the drug across the skin was not affected but the rate of drug delivery during iontophoresis was typically observed to increase by a factor greater than two. The post-iontophoretic transport of salbutamol across either tissue was also substantially enhanced in the presence of the fatty acid. The analogous use of stearic acid did not significantly influence the iontophoretic or the post-iontophoretic transport of salbutamol across excised human skin. The investigation also revealed a synergistic combination of the fatty acid and anodal iontophoresis to enhance the in vitro transport of other drug substances, including nicotine and diltiazem hydrochloride across murine skin. Oleic acid increased both the iontophoretic and post-iontophoretic transport of nicotine, so that the enhancement of drug delivery was greater than that caused by the current alone. The investigation also indicated that the barrier properties of the skin recover following the constant current iontophoresis in the presence of oleic or lauric acids.