High-concentration monoclonal antibody (mAb) formulations face stability challenges due to viscosity issues arising from protein-protein interactions (PPIs), which remain poorly understood at elevated concentrations. To mitigate viscosity-related risks, it is crucial to identify low-viscosity antibody candidates early in the discovery and lead optimization stages. However, measuring viscosity at high concentrations typically requires substantial resources and large sample volumes—constraints that are often impractical during early development. While diffusion interaction parameters (kD) measured at dilute concentrations offer some insight into mAb behavior, they exhibit only moderate correlation with viscosity and do not directly reflect how interparticle interactions change with concentration. Small-Angle X-ray Scattering (SAXS) is a powerful technique for probing PPIs in solution by analyzing how X-rays scatter as they pass through protein samples. For antibodies, SAXS enables the detection of changes in scattering patterns that reflect repulsive or attractive interparticle interactions across concentrations, providing valuable insights into self-association and crowding effects. In this study, we conducted SAXS measurements on 23 mAbs across concentrations ranging from 1 to 150 mg/mL, generating scattering profiles at each concentration to closely examine concentration-dependent interaction behaviors. We observed that SAXS could detect interparticle interactions and signs of self-association at concentrations as low as 10 mg/mL. Notably, a SAXS-derived structure factor parameter measured at 10 mg/mL exhibited strong correlation with viscosity at 150 mg/mL, outperforming traditional kD-based correlations. Based on these findings, we propose a SAXS-based strategy for early identification of low viscosity mAbs at high concentrations, enabling high-throughput screening using minimal sample volumes.
