Stabilization of a model formalinized protein antigen encapsulated in poly(lactide-co-glycolide)-based microspheres

A formaldehyde-mediated aggregation pathway (FMAP) has been shown to be primarily responsible for the solid-state aggregation of lyophilized formalinized protein antigens [e.g., tetanus toxoid (TT) and formalinized bovine serum albumin (f-BSA)] in the presence of moisture and physiological temperature. Coincorporation of the formaldehyde-interacting amino acid, histidine, strongly inhibits the FMAP. The purpose of this study was to test whether previous solid-state data are applicable toward the stabilization of formalinized antigens encapsulated in poly(lactide-co-glycolide) (PLGA)-based microspheres. Formaldehyde-treated bovine serum albumin (f-BSA) and BSA were selected as a model formalinized protein antigen and a nonformalinized control, respectively. As in the solid state, we found that the FMAP was dominant in the aggregation of f-BSA encapsulated in PLGA 50/50 microspheres, whereas the aggregation mechanism of encapsulated BSA was mostly converted from thiol-disulfide interchange to an acid-catalyzed noncovalent pathway. The lack of noncovalent aggregation in encapsulated f-BSA could be explained by its higher thermodynamic stability after formalinization, which inhibits protein unfolding. Targeting the FMAP, coencapsulation of histidine and trehalose successfully inhibited the aggregation of f-BSA in microspheres. By combining the use of an optimized oil-in-oil (o/o) encapsulation method, coencapsulation of histidine and trehalose, and use of low-acid-content poly(D,L-lactide) (PLA) and poly(ethylene glycol) (PEG) blends, a 2-month continuous release of f-BSA was achieved with the absence of aggregation.