Preparation of protein-loaded sustained-release microspheres via ‘solid-in-oil-in-hydrophilic oil-in-ethanol (S/O/hO/E)’ emulsification

While formulating proteins into solid particles prior to microsphere preparation is regarded as an effective way to stabilize such macromolecules, the protein particles may still contact the aqueous continuous phase and be re-dissolved. Dissolved proteins may not only leak into the aqueous continuous phase (resulting in reduced loading efficiency), but also contact water–oil (the hydrophobic polymer solution) interfaces, factors known to denature proteins. To avoid dissolution of solidified protein particles, we developed a microencapsulation procedure involving a hydrophilic “oil” (hO) continuous phase to which the hydrophobic solution of the controlled-release polymer was dispersed. The hydrophilic “oil” phase was a glycerol-based liquid mixed with ethylene glycol and polyvinyl alcohol solution to adjust viscosity and surface tension. This non-water hydrophilic continuous phase is immiscible with the hydrophobic polymer solution yet unable to dissolve pre-formulated protein particles. After the embryonic microspheres loaded with the protein particles were formed in this hydrophilic “oil” phase, the formulation was transferred into a cold ethanol bath where the microspheres were immediately hardened due to extracting the organic solution by ethanol. This method was examined by microencapsulating bovine serum albumin (BSA) and β-galactosidase (β-gal) into polylactide-co-glycolide (PLGA) microspheres for encapsulation efficiency, release kinetics and bioactivity preservation. As measured using size exclusion chromatography (SEC–HPLC), up to 90% added BSA was encapsulated in microspheres, and the release kinetics of the protein was adjusted by selecting surfactants used in microencapsulation emulsification. The assay of enzymatic activity of β-galactosidase in hydrolysis of o-nitrophenyl-beta-d-galactopyranoside (ONPG) indicated that over 90% of the protein recovered from the microspheres was active.