Oxprenolol-loaded bioadhesive microspheres: preparation and in vitro/in vivo characterization

Biologically adhesive delivery systems offer important advantages over conventional drug-delivery systems. In this paper, microspheres intended as a sustained release carrier for oral or nasal administration have been prepared by associating a known bioadhesive polymer, poly(acrylic acid), in gelatin microspheres. A model drug oxprenolol hydrochloride was chosen. It was found that some of the formulation variables can influence the characteristics of the beads in a controlled manner. The internal structure of the microspheres studied by X-ray diffraction, thermal analysis and optical microscopy showed the absence of drug crystals in microspheres and a lowering in the glass transition temperature. The dynamic swelling of the beads obeyed the square root of time and a shift from the diffusional to the relaxational process dependent on the content of poly(acrylic acid) in gelatin microspheres was observed. As expected, drug release from gelatin/poly(acrylic acid) microspheres was influenced by the poly(acrylic acid) content in beads, by the particle size of microspheres and by the pH of the medium. The mechanism of release was analysed by applying the empirical exponential equation and by calculation of the approximate contribution of the diffusional and relaxational mechanisms to the anomalous release process by fitting the data to the coupled Fickian/Case II equation. In vitro and in vivo experiments in rats showed good adhesive characteristics of the gelatin/poly(acrylic acid) microspheres, which were greater if the poly(acrylic acid) content was greater. A significant retardation in gastric and intestinal emptying time of the beads was observed. This was also suggested by the bioavailability of the model drug after intragastric and intranasal administration of the microspheres. The pharmacokinetic parameters after microsphere administration were more appropriate to a slow release drug-delivery system. The work suggests the potential of this pharmaceutical delivery system as an alternative controlled-release dosage form, either for oral or nasal administration.