High-concentration (>100 g/L) solutions of monoclonal antibodies (mAbs) are typically characterized by anomalously large solution viscosity and shear thinning behavior for strain rates ≥10 3 s −1 . Here, the link between protein−protein interactions (PPIs) and the rheology of concentrated solutions of COE-03 and COE-19 mAbs is studied by means of static and dynamic light scattering and microfluidic rheometry. By comparing the experimental data with predictions based on the Baxter sticky hard-sphere model, we surprisingly find a connection between the observed shear thinning and the predicted percolation threshold. The longest shear relaxation time of mAbs was much larger than that of model sticky hard spheres within the same region of the phase diagram, which is attributed to the anisotropy of the mAb PPIs. Our results suggest that not only the strength but also the patchiness of short-range attractive PPIs should be explicitly accounted for by theoretical approaches aimed at predicting the shear rate-dependent viscosity of dense mAb solutions.

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Molecular crowding in highly concentrated monoclonal antibody (mAb) solutions results in significant increases in viscosity, which complicates fill-finish steps and patient administration by subcutaneous injection. As viscosity measurements for optimization of the mAb formulation require significant amounts of material not always available in early development, fluorescence correlation spectroscopy (FCS) is evaluated as a potential ultralow volume technique for viscosity measurement of high concentration protein solutions assuming the Generalised Stokes Einstein relation (GSE) remains valid. Using like-charge fluorescent tracers of different sizes, FCS provided measurements of microviscosities which were compared to the macroviscosity. After parametrising the protein concentration dependence of the viscosity by the exponential coefficient (k) of a simple exponential model, FCS derived k-values of like-size tracer to the crowder followed the same ordering as the macroviscosity derived k-values with respect to solvent conditions. Furthermore, k and the diffusion-derived protein-protein interaction parameter, kD, are linked, and, attractive conditions for mAbs result in a stronger concentration dependence of the viscosity. For tracers and crowders of like-size, a key result is negative deviations from the GSE relation are observed in presence of strong attractive interactions between crowder molecules. These data demonstrate that FCS has application to the screening of high concentration mAb solutions for formulation selection.

Being able to predict and control concentrated solution properties for solutions of monoclonal antibodies (mAbs) is critical for developing therapeutic formulations. At higher protein concentrations, undesirable solution properties include high viscosities, opalescence, particle formation, and precipitation. The overall aim of this work is to understand the relationship between commonly measured dilute solution parameters, the reduced osmotic second virial coefficient b 22 and the diffusion interaction parameter k D and liquid−liquid phase separation, which occurs at higher protein concentrations. For globular proteins such as lysozyme or γB-crystallin, the location of the liquid−liquid coexistence curve is controlled by the net protein−protein interaction, which is related to b 22 . Because many mAbs undergo reversible self-association due to forming highly directional interactions, it is not known if b 22 can be used as a reliable predictor for LLPS since increasing the anisotropy in the interaction potential causes phase separation to occur at much stonger net protein−protein attractions or lower values of b 22 . Here, we map the coexistence curves for three mAbs, referred to as COE-01, COE-07, and COE-19, in terms of b 22 and k D values. The measurements are carried out at a low salt condition near the pI, where protein−protein interactions are expected to be anisotropic due to the presence of electrostatic attractions, and under salting-out conditions at high ammonium sulfate concentrations, which is expected to reduce the anisotropy by screening electrostatic interactions. We also show that deviations from a linear correlation between b 22 and k D can be used as an indicator of reversible self-association. Each of the mAbs under salting-out conditions follows the correlation supporting the hypothesis that protein−protein interactions are nonspecific, while deviations from the correlation occur for COE-01 and COE-19 under low salt conditions indicating the mAbs undergo reversible self-association. For five out of the six conditions, the onset of phase separation, as reflected by the reduced virial coefficient at the critical point b 22 c occurs in a small window −1.6 > b 22 c > −2.3, which is similar to what has been observed for lysozyme and for bovine γB-crystallin. Under low salt conditions, b 22 c ≈ −5.1 for COE-19, which we previously showed to self-associate into small oligomers. Our findings suggest that under conditions where mAb interactions are weakly anisotropic, such as occur at high salt conditions, phase separation will begin to occur in a small window of b 22 . Deviations from the window can occur when mAbs undergo reversible self-association, although this is not always the case and likely depends upon whether or not highly directional interactions are passivated in the oligomer formation. We expect fitting LLPS measurements to simplified interaction models for mAbs will provide additional insight into the nature of the protein−protein interactions and guide their development for calculating ...