Long- and Short-Range Electrostatic Interactions Affect the Rheology of Highly Concentrated Antibody Solutions

To explain the differences in protein-protein interactions (PPI) of concentrated versus dilute formulations of a model antibody. High frequency rheological measurements from pH 3.0 to 12.0 quantitated viscoelasticity and PPI at high concentrations. Dynamic light scattering (DLS) characterized PPI in dilute solutions. For concentrated solutions at low ionic strength, the storage modulus, a viscosity component and a measure of PPI, is highest at the isoelectric point (pH 9.0) and lowest at pH 5.4. This profile flattens at higher ionic strength but not completely, indicating PPI consist of long-range electrostatics and other short-range attractions. At low concentrations, PPI are near zero at pI but become repulsive as the pH is shifted. Higher salt concentrations completely flatten this profile to zero, indicating that these PPI are mainly electrostatic. This discrepancy occurs because long-range interactions are significant at low concentrations, whereas both long- and short-range interactions are significant at higher concentrations. Computer modeling was used to calculate antibody properties responsible for long- and short-range interactions, i.e. net charge and dipole moment. Charge-charge interactions are repulsive while dipole-dipole interactions are attractive. Their net effect correlated with the storage modulus profile. However, only charge-charge repulsions correlated with PPI determined by DLS.