Interaction between drugs and pressure-sensitive adhesives in transdermal therapeutic systems

Release experiments with four drugs using representative pressure-sensitive adhesive (PSA) matrices were performed at 37 degrees C, and drug-PSA polymer interaction was determined by the Williams, Landel, and Ferry (WLF) equation. Two acrylic-type [2-ethylhexylacrylate and acrylic acid copolymer (2EHA/AA) or acrylamide copolymer (2EHA/AAm)], one rubber-type (a mixture of high and low molecular weight polyisobutylene), and one silicone-type PSA were used, and dipropylphthalate (PP), aminopyrine (AMP), ketoprofen (KP), and lidocaine (LC) were selected as model drugs because of their molecular size and functional groups. PSA containing acrylic acid (2EHA/AA) strongly interacted with the amide LC, with the tertiary amine AMP, and with the carboxylic acid KP; PSA-containing acrylamide (2EHA/AAm), however, did not interact with LC or AMP, although it markedly interacted with KP. The rubber-type and silicone-type PSAs, composed of no or only a few polar functional groups, did not interact with any of the drugs used in this experiment. Therefore, the diffusion coefficient of the drugs through PSA was influenced by the drug-PSA polymer interaction, and the extent of this interaction can be estimated by the relationship between the drug concentrations in the PSA and their diffusion coefficients.