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Ultrasonic Storage Modulus as a Novel Parameter for Analyzing Protein-Protein Interactions in High Protein Concentration Solutions: Correlation with Static and Dynamic Light Scattering Measurements

Atul Saluja ^*, Advait V. Badkar , David L. Zeng , Sandeep Nema  and Devendra S. Kalonia ^*

^* Department of Pharmaceutical Sciences, School of Pharmacy, University of Connecticut, Storrs, Connecticut 06269; and Global Biologics, Pfizer Global Research and Development, Chesterfield, Missouri 63017

Correspondence: Address reprint requests to Devendra S. Kalonia, Dept. of Pharmaceutical Sciences, School of Pharmacy, University of Connecticut Unit-3092, 69 North Eagleville Road, Storrs, CT 06269. Tel.: 860-486-3655; Fax: 860-486-4998; E-mail: kalonia{at}uconn.edu.

The purpose of this work was to establish ultrasonic storage modulus (G') as a novel parameter for characterizing protein-protein interactions (PPI) in high concentration protein solutions. Using an indigenously developed ultrasonic shear rheometer, G' for 20–120 mg/ml solutions of a monoclonal antibody (IgG₂), between pH 3.0 and 9.0 at 4 mM ionic strength, was measured at frequency of 10 MHz. Our understanding of ultrasonic rheology indicated decrease in repulsive and increase in attractive PPI with increasing solution pH. To confirm this behavior, dynamic (DLS) and static (SLS) light scattering measurements were conducted in dilute solutions. Due to technical limitations, light scattering measurements could not be conducted in concentrated solutions. Mutual-diffusion coefficient, measured by DLS, increased with IgG₂ concentration at pH 4.0 and this trend reversed as pH was increased to 9.0. Second virial coefficient, measured by SLS, decreased with increasing pH. These observations were consistent with the nature of PPI understood from G' measurements. Ultrasonic rheology, DLS, and SLS measurements were also conducted under conditions of increased ionic strength. The consistency between rheology and light scattering analysis under various solution conditions established the utility of ultrasonic G' measurements as a novel tool for analyzing PPI in high protein concentration systems.