Why does PEG 400 co-encapsulation improve NGF stability and release from PLGA biodegradable microspheres?

Purpose. The aim of this work was to understand the mechanism by which co-e ncapsulated PEG 400 improved the stability of NGF and allowed a continuous release from PLGA 37.5/25 microspheres. Methods. Microparticles were prepared according to the double emulsion meth od. PEG 400 was added with NGF in the internal aqueous phase (PEG/PLGA rati o 1/1 and 1.8/1). Its effect was investigated through interfacial tension s tudies. Protein stability was assessed by ELISA. Results. A novel application of PEG in protein stabilization during encapsu lation was evidenced by adsorption kinetics studies. PEG 400 limited the pe netration of NGF in the interfacial film of the primary emulsion. Consequen tly, it stabilized the NGF by reducing the contact with the organic phase. In addition, it avoided the NGF release profile to level off by limiting th e irreversible NGF anchorage in the polymer layers. On the other hand, the amount of active NGF released in the early stages was increased. During mic roparticle preparation, NaCl could be added in the external aqueous phase t o modify the structure of microparticles. This allowed to reduce the initia l release rate without affecting the protein stability always encountered i n the absence of PEG. Conclusions. PEG 400 appeared of major interest to achieve a continuous del ivery of NGF over seven weeks from biodegradable microparticles prepared by the double emulsion technique.