Ultrafiltration of Highly Concentrated Monoclonal Antibody Solutions

Monoclonal antibodies are key bioproducts used extensively in the treatment of a variety of diseases including cancer.  Antibody products typically need to be delivered at large doses to achieve the desired therapeutic effect, requiring final formulations with product concentrations greater than 200 mg/mL due to the limited volumes that can be delivered by subcutaneous injection.  However, it is often difficult if not impossible to achieve these high concentrations using existing ultrafiltration systems.  The objective of this study is to develop a fundamental understanding of the critical factors that control filtrate flux and membrane fouling during the ultrafiltration of highly concentrated antibody solutions, including the effects of intermolecular protein-protein interactions. Experiments were performed using anti-Streptavidin monoclonal antibody solutions at concentrations as high as 300 g/L. Ultrafiltration data were obtained in Millipore Pellicon 3 tangential flow filtration modules with Ultracel membranes having a 30 kDa nominal molecular weight cut-off. Protein-protein interactions were evaluated independently using self-interaction chromatography and membrane osmometry. The filtrate flux attained a pressure-independent value at high transmembrane pressures, consistent with the classical stagnant film model. However, plots of the filtrate flux data versus the logarithm of the bulk protein concentration show considerable curvature due to protein-protein interactions. At low to moderate protein concentrations, the flux increased with decreasing solution ionic strength due to the increase in the effective protein diffusion coefficient, and in turn the mass transfer coefficient, associated with the increase in intermolecular electrostatic repulsion under these conditions. The behavior at very high protein concentrations was more complex due to the corresponding increase in the protein osmotic pressure.  The filtrate flux data were analyzed using appropriate mass transfer models accounting for the effects of protein-protein interactions using the measured values of the second virial coefficient (from the self-interaction chromatography) and osmotic pressure. These results provide important insights into the ultrafiltration behavior of highly concentrated protein solutions as well as a framework for the development of more effective ultrafiltration / diafiltration processes for the formulation of highly concentrated monoclonal antibody products.