Influence of buffer solution composition on drug release from pellets coated with neutral and quaternary acrylic polymers and on swelling of free polymer films

Aqueous dispersions of the acrylic resins poly(ethylacrylate-methylmethacrylate-trimethylammonioethyl methacrylate chloride) (quaternary PMMA, Eudragit RS30D) and poly(ethylacrylate-methylmethacrylate) (neutral PMMA, Eudragit NE30D) were used to cast free films and to coat theophylline pellets. The release of theophylline from pellets coated with Eudragit RS showed a great dependence on the composition of the buffer solution. The highest release rates were observed in formate buffers; they were intermediate in phosphate buffers and low in citrate buffers and in all buffers containing chloride ions. The rates of release correspond with the extent of swelling of free Eudragit RS films, measured as weight gain in the respective solutions; a high water content in the swollen film lead to a fast release. However, the extent of ion exchange between the chloride ions of the polymer and the anions of the buffer solutions was of the same order of magnitude. In contrast, the release of theophylline from pellets coated with Eudragit NE and the swelling of free Eudragit NE films was not significantly affected by the composition of the buffer solutions.