Investigation of nanocapsules stabilization by amorphous excipients during freeze-drying and storage

Freeze-drying was recently applied to improve the long-term storage stability of nanoparticles. Nanocapsules have a thin polymeric envelope that may not withstand the stresses of such process. So, cryoprotectants and lyoprotectants are usually added to the formulation to protect these vectors during freezing and desiccation steps. The aim of this paper was to investigate the importance of the vitrification of cryoprotectants on the stabilization of nanocapsules during freezing, desiccation, and storage steps. Furthermore, the effect of stabilizer crystallization on the conservation of nanocapsules properties was studied. Finally, the effect of temperature storage and relative humidity on the stability of nanocapsules was tested through an accelerated stability study. Results indicate that nanocapsules stabilization during the different steps of freeze-drying requires their dispersion within a vitrified matrix of amorphous excipient to protect them against the stress of freezing and dehydration. The crystallization of this stabilizer during the freezing, the desiccation or the storage steps can destabilize these fragile particles. Electron spectroscopy for chemical analysis revealed the adsorption of nanocapsules at the interface ice/liquid during the freezing step. Such adsorption must be avoided in the case of freeze-drying of immuno-nanoparticles to preserve the native structure of proteins attached to their surface.