Formation and Isolation of Spherical Fine Protein Microparticles Through Lyophilization of Protein-Poly(ethylene Glycol) Aqueous Mixture

Purpose. Preparation of spherical fine protein microparticles by the lyophilization of a protein-poly(ethylene glycol) (PEG) aqueous mixture was investigated. The main objective was to establish a method for preparing protein microparticles suitable for pharmaceutical production.Methods. Aqueous solutions containing bovine serum albumin (BSA) and PEG at various mixing ratios were freeze-dried. The lyophilizates were dispersed in methylene chloride and subjected to particle size analysis. Analogous studies were performed using several model proteins. A phase diagram of the PEG-BSA aqueous system was obtained by the titration method.Results. The particle size of BSA decreased as the PEG-BSA ratio increased. A bending point was observed in this relationship, at which the PEG-BSA ratio coincided with that of the critical point on the phase diagram of the PEG-BSA system. These results were explained by the freezing-induced condensation, followed by phase separation in the PEG-BSA system.Conclusions. Spherical fine protein microparticles were successfully obtained at high yield and without any activity loss under optimum conditions. This new technology could be applicable to proteins with a wide range of molecular weights, and is expected to be developed for dry powder inhalations or long-term sustained release microsphere formulations.