Formulation and process parameters affecting protein encapsulation into PLGA microspheres during ethyl acetate-based microencapsulation process

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Abstract
The objective of this study was to investigate formulation and process parameters affecting protein encapsulation into PLGA microspheres during an ethyl acetate-based double emulsion microencapsulation process. Lysozyme was used as a model protein throughout this study. An aqueous lysozyme solution was emulsified in ethyl acetate containing 0.6 ∼ 1.2 g PLGA75 : 25. The primary emulsion was then transferred quickly to an aqueous phase to make a water-in-oil-in-water emulsion. Ethyl acetate quenching was performed on the double emulsion stirred for 5, 15, 30 or 45 min. The resultant microspheres were further hardened, collected and dried overnight under vacuum. The bicinchoninic acid assay was carried out to determine the quantity of lysozyme present in the aqueous continuous phase and inside the microspheres. While the primary emulsion was stirred without quenching, lysozyme in the inner water phase continued diffusing across the ethyl acetate phase into the aqueous continuous phase. Emulsion droplets were also broken into smaller ones with ongoing stirring; this event also contributed to lysozyme leaking out of the inner water phase. The amount of lysozyme leaching to the aqueous continuous phase ranged from 4.79 ± 2.1 to 51.9 ± 5.3% under the experimental condition. Ethyl acetate quenching stopped the primary emulsion droplets from being fragmented into smaller ones and caused PLA75 : 25 precipitation to form microspheres. As a result, the rate of ethyl acetate removal influenced lysozyme encapsulation efficiency, as well as microsphere size. Depending on the timing of ethyl acetate quenching, lysozyme encapsulation efficiencies were found to be 9.89 ± 4.53 ∼ 75.82 ± 6.55%. Optimization of the onset of ethyl acetate quenching and formulations could permit attainment of a desirable protein encapsulation efficiency.

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