Optimizing double emulsion process to decrease the burst release of protein from biodegradable polymer microspheres

The process parameters such as the compositions of inner and outer aqueous phase and emulsification technique of the primary emulsion were optimized to decrease the burst release of BSA from biodegradable polymer microspheres in double emulsion method. It was found that diminished burst release of -14% was achieved for the microspheres produced by formulations, where no phosphate was present in the inner water phase (non-buffered system). Primary emulsion made by probe sonication rather than homogenization or mechanical stirring led to microspheres with insignificant burst effect. Microspheres obtained using 0.1% aqueous Tween 80 (R) solution as the outer aqueous phase, frequently exhibit reduced burst effect of 2.7%. Low microsphere yield (52.1%), however, was observed. Microsphere yield was, therefore, enhanced by addition of additive such as sodium chloride, glucose or mannitol into the outer aqueous phase. Decrease in BSA entrapment was observed in the presence of sodium chloride, but reduction in entrapment efficiency was observed in the case of glucose. Burst release increased from 2.7% to 9.5% or 3.4% as 2.5% sodium chloride or 7.5% glucose was added into the outer aqueous phase respectively. Marked burst release (>20%) was observed in the presence of additive of higher concentration independent of sodium chloride or glucose. As far as surfactant type was concerned, diminished burst was found when PVP or Tween 80 (R) rather than PVA was utilized as the surfactant during microsphere preparation. In addition to PLGA, the copolymers of L-lactide (LLA) and dimethyl trimethylene carbonate (DTC) or trimethylene carbonate (TMC) were also evaluated. Insignificant burst effect was found for the microspheres composed of DTC or TMC copolymers.