Freeze-concentration separates proteins and polymer excipients into different amorphous phases

Abstract

Purpose: To study the miscibility of proteins and polymer excipients in frozen solutions and freeze-dried solids as protein formulation models.

Methods: Thermal profiles of frozen solutions and freeze-dried solids containing various proteins (lysozyme, ovalbumin, BSA), nonionic polymers (Ficoll, polyvinylpyrrolidone [PVP]), and salts were analyzed by differential scanning calorimetry (DSC). The polymer miscibility was determined from the glass transition temperature of maximally freeze-concentrated solute (Tg') and the glass transition temperature of freeze-dried solid (Tg).

Results: Frozen Ficoll or PVP 40k solutions showed Tg' at -22 degrees C, while protein solutions did not show an apparent Tg'. All the protein and nonionic polymer combinations (5% w/w, each) were miscible in frozen solutions and presented single Tg's that rose with increases in the protein ratio. Various salts concentration-dependently lowered the single Tg's of the proteins and Ficoll combinations maintaining the mixed amorphous phase. In contrast, some salts induced the separation of the proteins and PVP combinations into protein-rich and PVP-rich phases among ice crystals. The Tg's of these polymer combinations were jump-shifted to PVP's intrinsic Tg' at certain salt concentrations. Freeze-dried solids showed varied polymer miscibilities identical to those in frozen solutions.

Conclusions: Freeze-concentration separates some combinations of proteins and nonionic polymers into different amorphous phases in a frozen solution. Controlling the polymer miscibility is important in designing protein formulations.