Effect of temperature-increase rate on drug release characteristics of dextran microspheres prepared by emulsion solvent evaporation process

Microspheres containing theophylline (TH) were prepared from a hydrophobic dextran derivative by an emulsion solvent evaporation process using an acetone/liquid paraffin system. The effects of solvent evaporation rate on particle properties and drug release characteristic of the microspheres were evaluated. The solvent evaporation rate was controlled by the rate of increase in temperature of the water bath, ranging 7.5–30 °C/h. Drug release from the microspheres was examined using JPXIV 2nd fluid (pH 6.8) containing 0.1% Tween 80, and was found to be greatly affected by the solvent evaporation rate. The percentage of drug released until 8 h varied; from 28% to 84% for 30 and 7.5 °C, respectively. Differential scanning calorimetry and powder X-ray diffraction studies revealed that TH partially interacted with the polymer and drug crystallinity was maintained intact in the microspheres. According to scanning electron microscopy observations, all microspheres showed a well-formed spherical particle with a solid interior. The appearances of the microspheres were, however, extremely different. Microspheres prepared at 30 °C/h had a very smooth surface, while those prepared at 7.5–15 °C/h had a rough surface with large craters. These findings demonstrated that the surface morphology and drug release characteristic were controlled by the rate of increase of temperature.