Lysozyme stability in primary emulsion for PLGA microsphere preparation: Effect of recovery methods and stabilizing excipients

Purpose. To investigate the conformational stability of a model protein, lysozyme, in the primary emulsion phase of the microsphere preparation process. Methods. The conformational stability of lysozyme during primary emulsification was assessed by differential scanning calorimetry (DSC) and enzymatic activity assay. PEG 400 was used to separate lysozyme from water-in-oil (w/o) emulsion containing poly(lactide-co-glycolide) (PLGA). Results. No significant changes in the recovery of lysozyme were observed due to increasing sonication time from 20 to 60 s at 40 W or increasing intensity from 40 to 60 W for 20 s. By using the method involving PEG 400, lysozyme recovery in the presence of PLGA was increased from 11.8% to 70%. Hydroxypropyl-β-cyclodextrin (HP-β-CD) increased lysozyme recovery from 35% to 70% at low lysozyme concentration (20 mg/ml), and from 70% to 77% at high lysozyme concentration (100 mg/ml) in the presence of PLGA. Sugars such as trehalose and mannitol failed to increase lysozyme recovery. DSC results suggested the retention of the conformational structure of the recovered lysozyme, which was supported by an enzymatic activity assay. Conclusions. HP-β-CD was found to be a promising stabilizer that protected lysozyme during the primary emulsification. Protein recovery method with the help of PEG 400 allowed the study of protein stability in w/o emulsions in the presence of PLGA. DSC provided supplementary information on the conformational changes of lysozyme during emulsification.