Evaluation of protein formulation and its viscosity with DSC, DLS, and microviscometer

The viscosity of highly concentrated protein solutions was evaluated using lysozyme as model protein. Viscosity profiles of lysozyme were examined with the effect of buffer and pH-value at various concentrations. The viscosity of lysozyme dissolved in water increased continuously with the concentration as the slope of shear stress against shear rate increased with the concentration. In addition, the viscosity of lysozyme was higher in histidine buffer than in acetate buffer at selected pH ranges. The effect of various excipient concentrations was also investigated in means of unfolding transition temperature (T_m), viscosity, hydrodynamic size and zeta potential by using differential scanning calorimetry (DSC), microviscometer and dynamic light scattering (DLS). The selected excipients except surfactants increased the viscosity of protein solution with their concentration. Carbohydrates increased the viscosity relatively higher than amino acids and also they increased the conformational stability (T_m) by enhancing the protein molecule more in compact form. Also amino acids increased the viscosity but decreased the conformational stability since they seemed to be only dispersed in the solution avoiding protein-protein interactions, resulting in a decrease of zeta potential. Consequently, the applied methods-DSC, DLS and microviscometer demonstrated the potential to develop a highly concentrated protein formulation to decrease the high viscosity effect with acceptable conformational stability.