Transdermal ion migration

Abstract

This chapter will focus on the influence of ion composition on the transdermal delivery of drug species through mammalian skin, due to an applied electric field. The pH of the drug-containing medium is shown to affect drug transport by altering the fraction of charged drug and the permselectivity of skin. The competitive transport of ions having the same charge as the drug ion (co-ions) and those having a charge opposite that of the drug ion (counter-ions), is discussed in detail. A model is derived to predict drug ion flux through a homogeneous non-ionic membrane at constant current, in the presence of one type of co-ion and counter-ion. Model values are compared to experimental data for ion migration through a synthetic aqueous membrane and through excised porcine skin. This comparison suggests that ion transport through pig skin occurs primarily along an aqueous pathway. In addition, a comparison of in vitro and in vivo data for several drug ions as well as comparison of in vivo results for transdermal sodium influx and efflux, suggests that the apparent sodium chloride concentration of the viable epidermis is hypotonic, having a value of about 0.09 M.