Liquid Droplet of Protein-Polyelectrolyte Complex for High-Concentration Formulations

The formulation of high-concentration protein solutions is a challenging issue for achieving subcutaneous administration. Previously, we developed a method of precipitation-redissolution using polyelectrolyte as a precipitant to produce protein solutions at high concentrations. However, the redissolution yield of proteins was insufficient. This study aims to optimize the solution conditions for practical applications by combining IgG and poly-l-(glutamic acid) (polyE). A systematic analysis of solution pH and polyE size conditions revealed that an acidic condition favors precipitation, whereas neutral pH values are more effective for the redissolution. We find that the optimal size for polyE ranged from 15,000 to 50,000. This slight modification in the procedure in comparison with previous studies increased the precipitation and redissolution yields to nearly 100%, without irreversible protein denaturation. The fully reversible IgG-polyE complex formed as a droplet structure, which is similar to a condensate of liquid-liquid phase separation. The droplet structure plays an indispensable role in the salt-induced, redissolved state, which is pertinent to the new application that takes advantage of the methods to produce highly concentrated protein solutions.