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The type of coating technique strongly affects the film microstructure and, thus, the release mechanism and rate from pellets coated with polymer blends.

The aim of this study was to coat theophylline-loaded spherical beads with pH-sensitive polymer blends to control the resulting drug release kinetics. Various mixtures of ethylcellulose (water-insoluble) and Eudragit L (methacrylic-acid-ethyl-acrylate-copolymer; water-insoluble/water-soluble below/above pH 5.5) were used as coating materials. Two types of theophylline cores were studied: pure drug matrixes and theophylline-layered sugar cores. Importantly, the type of core significantly affected the resulting drug release patterns. Interestingly, not only the slope, but also the shape of the release curves was altered, indicating changes in the underlying mass transport mechanisms, despite of the identical composition of the polymeric coatings. The observed differences could be explained based on the physicochemical properties of the film coatings and the swelling behavior of the beads upon exposure to the release media. Using this knowledge the development/optimization of this type of drug delivery system can be facilitated and the safety of the pharmacotherapies be improved.