Encapsulation of bovine serum albumin in poly(lactide-co-glycolide) microspheres by the solid-in-oil-in-water technique

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Abstract
Non-aqueous protocols to encapsulate pharmaceutical proteins into biocompatible polymers have gained much attention because they allow for the minimization of procedure-induced protein structural perturbations. The aim of this study was to determine if these advantages could be extended to a semi-aqueous encapsulation procedure, namely the solid-in-oil-in-water (s/o/w) technique. The model protein bovine serum albumin (BSA) was encapsulated into poly(lactide-co-glycolide) (PLG) microspheres by first suspending lyophilized BSA in methylene chloride containing PLG, followed by emulsification in a 1% aqueous solution of polyvinyl alcohol). By variation of critical encapsulation parameters (homogenization intensity, BSA:PLG ratio, emulsifier concentration, ratio of organic to aqueous phase) an encapsulation efficiency of > 90 % was achieved. The microspheres obtained showed an initial burst release of < 20 %, a sustained release over a period of about 19 days, and a cumulative release of at least 90% of the encapsulated BSA. Different release profiles were observed when using different encapsulation protocols. These differences were related to differences in the microsphere erosion observed using scanning electron microscopy. Release of BSA was mainly due to simple diffusion orto both diffusion and microsphere erosion. Fourier-transform infrared studies were conducted to investigate the secondary structure of BSA during the encapsulation. Quantification of the α-helix and β-sheet content as well as of overall structural changes showed that the secondary structure of encapsulated BSA was not more perturbed than in the lyophilized powder used initially. Thus, the encapsulation procedure did not cause detrimental structural perturbations in BSA. In summary, the results demonstrate that the s/o/w technique is an excellent alternative to the water-in-oil-in-water technique, which is still mainly used in the encapsulation of proteins in PLG microspheres.

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