Aggregation of concentrated monoclonal antibody solutions studied by rheology and neutron scattering*

Protein solutions are studied using rheology and scattering techniques to investigate aggregation. Here we present a monoclonal antibody (mAb) that aggregates after incubation at 40 $^{\circ}$C (below its unfolding temperature), with a decrease in monomer purity of 6{\%} in 10 days. The mAb solution contains surfactant and behaves as a Newtonian fluid when reconstituted into solution from the lyophilized form (before incubation at 40 $^{\circ}$C). In contrast, mAb solutions incubated at 40 $^{\circ}$C for 1 month exhibit shear yielding in torsional bulk rheometers. Interfacial rheology reveals that interfacial properties are controlled by the surfactant, producing a negligible surface contribution to the bulk yield stress. These results provide evidence that protein aggregates formed in the bulk are responsible for the yield stress. Small-angle neutron scattering (SANS) measurements show an increase in intensity at low wavevectors (q \textless\ 4*10$^{-2}$ nm$^{-1})$ that we attribute to protein aggregation, and is not observed in solutions stored at 4 $^{\circ}$C for 3 days before the measurement. This work suggests a correlation between the aggregated state of the protein (stability) and the yield stress from rheology.