Poly(hydroxybutyrate-hydroxyvalerate) microspheres containing progesterone: Preparation, morphology and release properties

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Abstract
The biodegradable polyesters, poly(hydroxybutyrate) (PHB) and poly(hydroxybutyrate-hydroxyvalerate) (PHBV) were investigated for use as sustained delivery carriers of a model drug, progesterone. Spherical microspheres containing the drug were prepared by an emulsion solvent-evaporation method with gelatin as an emulsifier. Methylene chloride as the polymer solvent yielded smoother microspheres than chloroform. The surface texture was also dependent upon the temperature of the preparation and polymer used. Surface crystals were observed when the drug loading was increased beyond 5 per cent w/w. Thermograms of the microspheres did not show an endotherm corresponding to the melting of the drug because the drug dissolved in the melted polymer while heating. The amount of residual solvent in the microspheres (gas chromatographic assay) ranged from 3.4 to 58.4 ppm and was dependent on the processing temperature, concentration of the polymer in the solvent and the polymer composition. In vitro release of the drug was slowest from microspheres made from copolymer containing 9 per cent hydroxyvalerate. A less porous microsphere matrix was formed by this copolymer.