Solution pH that minimizes self-association of three monoclonal antibodies is strongly dependent on ionic strength

Monoclonal antibodies display highly variable solution properties such as solubility and viscosity at elevated concentrations (>50 mg/mL), which complicates antibody formulation and delivery. To understand this complex behavior, it is critical to measure the underlying protein self-interactions that govern the solution properties of antibody suspensions. We have evaluated the pH-dependent self-association behavior of three monoclonal antibodies using self-interaction chromatography for a range of pH values commonly used in antibody formulations (pH 4.4-6). At low ionic strength (<25 mM), we find that each antibody is more associative at near-neutral pH (pH 6) than at low pH (pH 4.4). At high ionic strength (>100 mM), we observe the opposite pH-dependent pattern of antibody self-association. Importantly, this inversion in self-association behavior is not unique to multidomain antibodies, as similar pH-dependent behavior is observed for some small globular proteins (e.g., ribonuclease A and α-chymotrypsinogen). We also find that the opalescence of concentrated antibody solutions (90 mg/mL) is minimized at low ionic strength at pH 4.4 and high ionic strength at pH 6, in agreement with the self-interaction measurements conducted at low antibody concentrations (5 mg/mL). Our results highlight the complexity of antibody self-association and emphasize the need for systematic approaches to optimize the solution properties of concentrated antibody formulations.