Protein modification with poly(ethylene glycol): An approach towards protein stabilization during encapsulation and release from biocompatible polymers

Encapsulation and delivery of pharmaceutically relevant proteins from biodegradable polymers, such as, poly(lactide-co-glycolic) acid (PLGA) using the solid-in-oil-in-oil (s/o/o) and the solid-in-oil-in-water (s/o/w) encapsulation techniques has been established and is of major interest to health care sciences. However, the success of this approach is still hindered by the fact that proteins are sensitive and labile. This frequently causes structural perturbation, aggregation, and inactivation by encapsulation processes. In addition, low encapsulation efficiencies and huge initial burst release are major drawbacks of these techniques.; To overcome these problems I tested whether physical stability of model proteins could be improved by formulating them with poly(ethylene) glycol (PEG). PEG was either employed as an excipient or covalently attached to the protein surface. The model proteins bovine serum albumin (BSA), horseradish peroxidase (HRP) and alpha-chymotrypsin were modified or colyophilized with PEGs with different molecular weights at different molar ratios and encapsulated into PLGA microspheres using s/o/o and s/o/w protocols.; My results show that the use of PEG as excipient preserves the structural integrity of bovine serum albumin (BSA) during encapsulation by the s/o/o method. Size exclusion chromatography-high performance liquid chromatography result shows that PEG minimizes the formation of soluble BSA aggregates during the initial 24 h of in vitro release. Co-lyophilization of BSA with PEG affords high encapsulation efficiency of ca. >90% with microsphere sizes of few micrometers.; FTIR, UV/Vis, and Raman spectroscopic results show that HRP covalently modified with PEG resists lyophilization-induced structural changes and exhibits a native-like structure in organic solvents. Stability data shows that PEG-modification significantly improves HRP stability during s/o/o encapsulation and release from PLGA microspheres. For example, the amount of HRP insoluble aggregates dropped over 5 fold from ca. 5% to <1% and the retained specific activity increased from 50% to >95%. Furthermore, modification of HRP with PEG increases HRP encapsulation efficiency to >98% and reduces initial burst release from 70% to 23%. PEG-modification also improves HRP activity during in vitro release and reduces the amount of insoluble aggregates formed during release from 11% to 3%. (Abstract shortened by UMI.)...