Iontophoretic transdermal delivery of salicylic acid dissolved in ethanol-water mixture in rats.

The usefulness of iontophoresis is restricted to highly water-soluble compounds, since drugs are generally applied as an aqueous solution in a drug electrode. In the present study, salicylic acid (SA) dissolved in ethanol-water mixture was loaded in a drug electrode, and the effect of ethanol on the iontophoretic transdermal delivery of SA was evaluated. Ethanol at a concentration of 10 or 30% showed no significant effect on the iontophoretic transdermal delivery of SA compared to that in the absence of ethanol, but 40 or 70% ethanol increased it significantly. The current density passing through in vivo during iontophoretic treatment decreased with increase in ethanol concentrations. These results suggested that the enhanced transdermal absorption of SA iontophoretically by the presence of ethanol in a drug solution is not due to the increased current density in vivo, but probably due to the direct action of ethanol on the stratum corneum. In conclusion, addition of ethanol to a drug solution at an appropriate concentration was proved to enhance the iontophoretic transdermal delivery of SA. A mixture of ethanol and water can dissolve many poorly water-soluble drugs, and therefore it would be able to expand the application of iontophoresis to include many drugs that are poorly soluble in water.