Rotational Rheology of Bovine Serum Albumin Solutions: Confounding Effects of Impurities, Mechanistic Considerations and Potential Implications on Protein Formulation Development

To show and rationalize the confounding effects on the rotational/oscillatory rheology of surface active impurities in commercial protein formulations such as bovine serum albumin, BSA. Bulk and interfacial rotational/oscillatory rheology were used to study the viscosity, complex viscosity, storage/elastic modulus, G' and loss/viscous modulus, G, as a function of time of aqueous formulations of BSA and their purified components. Viscosity/time profiles at steady shear for different commercial BSA products and lots showed viscosity increase, decrease and time-independent profiles at low shear rates. All lots showed shear thinning. BSA monomer and dimers/aggregates, in general, showed similar profiles. Addition of low levels of surfactant or high shear rates rendered all solutions to be Newtonian-like. Interfacial viscosity studies paralleled those on the rotational rheometer. G' > G with viscosity increase and G' < G with viscosity decrease over time. We provide a rational explanation for the time-dependent and source-dependent rheological behavior of aqueous formulations of commercially available BSA proteins based on the migration of protein and surface active impurities to the air/water interface within the rheometer plates leading to the formation and breakdown of protein networks. Highly purified proteins is warranted in rheological studies of protein drug product candidates.