Release and Skin Permeation of Scopolamine From Thin Polymer Films in Relation to Thermodynamic Activity

The object was to demonstrate if the diffusional flux of the drug out of a drug-in-adhesive-type matrix and its subsequent permeation through an excised skin membrane is a linear function of the drug's thermodynamic activity in the thin polymer film. The thermodynamic activity, ap*, is defined here as the degree of saturation of the drug in the polymer. Both release and release/permeation of scopolamine base from 3 different poylacrylate pressure-sensitive adhesives (PSAs) were measured. The values for ap* were calculated using previous published saturation solubilities, wps, of the drug in the PSAs. Different rates of release and release/permeation were determined between the 3 PSAs. These differences could be accounted for quantitatively by correlating with ap* rather than the concentration of the drug in the polymer films. At similar values for ap* the same release or release/permeation rates from the different polymers were measured. The differences could not be related to cross-linking or presence of ionizable groups of the polymers that should influence diffusivity.