Preparation of biodegradable PLGA microspheres for sustained local anesthesia and their in vitro release behavior

Fentanyl-loaded biodegradable poly(L-lactide-co-glycolide) (75:25 by mole ratio of lactide to glycolide, PLGA) microspheres (MSs) were prepared to study the possibility for long-acting local anesthesia. We developed the fentanyl base(FB, slightly water-soluble)-loaded PLGA MSs by means of conventional O/W solvent evaporation method. The size of MSs was in the range of 10-150 μm. The morphology of MSs was characterized by SEM, and the in vitro release amounts of FB were analyzed by HPLC. The lowest porous cross-sectional morphology and the highest encapsulation efficiency were obtained by using gelatin as an emulsifier. The influences of several preparation parameters, such as emulsifier types, molecular weights and concentrations of PLGA, and initial drug loading amount, etc., have been observed in the release patterns of FB. The release of FB in in vitro was more prolonged over 25 days, with close to zero-order pattern by controlling the preparation parameters. We also investigated the physicochemical properties of FB-loaded PLGA MSs by X-ray diffraction and differential scanning calorimeter.