[Effect of morphological properties on drug release from biodegradable microspheres]

The morphological properties of poly(beta-hydroxybutyric acid) (PHB) or poly(L-lactic acid) microspheres loading flomoxef sodium (FMOX) were investigated with regard to FMOX release. The release profiles of FMOX from the microspheres could be divided into two types, a sustained release type and a burst one. Two representative PHB microspheres, the release profiles of which were quite different from those of FMOX, were compared in detail from a morphological point of view. The shapes of their surfaces and sections were observed by using scanning electron microscopy (SEM), and FMOX distribution was analyzed by using electron probe microanalysis. The crystallinity of polymers was further measured by powder X-ray diffratometry. There was little difference in the FMOX distribution and their microscopic properties such as sphere size, specific surface area, shape of surface and section. In contrast, water penetration into the inside of the microspheres was found to be clearly different by use of cryogenic SEM. A significant difference was also observed in the crystallinity of polymers forming the microspheres. The release of FMOX from the microspheres was affected by the crystallinity of polymers forming the microspheres, and burst phenomena occurred in case the polymer was highly crystallized. It was speculated that the crystallization of polymer induced micro voids in the microspheres which functioned as channels for water penetration.