(34d) Characterizing Protein-Protein Interactions in Highly Concentrated Monoclonal Antibody Solutions Using Small Angle X-Ray Scattering and Molecular Dynamics Simulations

Monoclonal antibodies (mAbs) show great promise as effective therapeutics against diseases such as cancer. Highly concentrated solutions of mAbs are especially desirable in the biopharmaceutical industry for subcutaneous delivery of these proteins in patients. However, at these high concentrations, protein-protein interactions (PPI) including anisotropic short-range attractions due to hydrogen bonding, dipole-dipole, and hydrophobic interactions often cause prohibitively high viscosities and low storage stability. Recent studies have established Small Angle X-ray Scattering (SAXS) as an effective tool for probing mAb solution structure at varying length-scales and extracting information about the PPI involved. In this work, we present a set of generalized coarse-grained models capable of mimicking mAb geometry and interaction anisotropy. Through molecular dynamics simulations, we successfully fit our models to experimental structure factor (S_eff(q)) data obtained for mAbs across a series of concentrations and co-solute systems. Furthermore, our models highlight the importance of the slope of S_eff(q) at low q for distinguishing between mAbs that exhibit weak diffuse attraction and mAbs that exhibit strong interactions between the Fab and Fc domains. Cluster size analysis shows that models with strong anisotropic attraction form large percolated networks in solution, and real systems fit by this model exhibit high viscosities, revealing an intriguing correlation between solution viscosity and cluster characteristics. Taken together, these findings provide a more fundamental understanding of the nature of PPI in highly concentrated mAb solutions.