The stability and immunogenicity of a protein antigen encapsulated in biodegradable microparticles based on blends of lactide polymers and polyethylene glycol

Protein-loaded microparticles were produced from blends of poly(ethylene gl ycol) (PEG) with poly(L-lactide) (PLA) homopolymer or poly(DL-lactide co-gl ycolide) copolymers (PLG) using a water-in oil-in oil method. The stability of ovalbumin (OVA) associated with microparticles prepared using PEG and 5 0:50 PLG, 75:25 PLG and PLA, respectively, was analysed by SDS-PAGE and qua ntified by scanning densitometry following incubation in PBS at 37 degrees C for up to 1 month. Fragmentation and aggregation of OVA was detected with all 3 formulations. The extent of both processes correlated with the degra dation rate of the lactide polymer used and decreased in the order PLA < 75 :25 PLC < 50:50 PLG. Extensive degradation of the PLG/PEG microparticles al so occurred over 4 weeks whereas the use of PLA/PEG blends resulted in a st able microparticle morphology and much reduced fragmentation and aggregatio n of the associated protein. Following a single sub-cutaneous immunisation, high levels of specific serum IgG antibody were elicited by OVA associated with the PLA/PEG particles. Injection of OVA associated with the 75:25 PLG /PEG microparticles resulted in very low levels of specific antibody. A hig her response was induced by the 50:50 PLG/PEG formulation but there was ver y large inter-animal variation in this group. Antibody levels elicited by a ll 3 formulations were significantly higher than those elicited by a single injection of soluble OVA. Analysis of antigen specific IgG1 and IgG2a anti body subtype levels also revealed the greater efficacy of the PLA/PEG micro particles as an adjuvant system. The use of PLA/PEG microparticles shows im proved protein loading and delivery capacity while maintaining a high level of stability of the associated protein. These results indicate a strong co rrelation between the stability of microencapsulated antigen and the magnit ude of the immune response following sub-cutaneous immunisation. (C) 1999 E lsevier Science Ltd. All rights reserved.