Multiscale Diffusion, Dynamic Cluster Formation, and Intermolecular Interactions in Pharmaceutically Relevant Monoclonal Antibody Formulations

Due to their specificity and versatility, monoclonal antibodies (mAbs) are the most popular class of biopharmaceuticals typically administered via intravenous injection. One of the current pharmaceutical challenges concerns mAb formulations for subcutaneous (SC) injection, which is gaining importance as an alternative administration route offering convenience to patients by allowing self-administration compared to other parenteral delivery methods. With volumes lower than 1–2 mL being better tolerated in the subcutaneous space, highly concentrated mAb formulations are needed to achieve significant therapeutic effects, potentially increasing the solution viscosity and altering drug injectability. The main challenge is to maintain the solution viscosity below the SC injectability threshold (15–20 mPa·s) while preserving solution stability. Since the understanding of macroscopic viscosity requires in-depth knowledge on protein multiscale diffusion, mutual interactions, and aggregation, we employ two complementary neutron scattering techniques to investigate 9 different mAbs of IgG1/IgG4 subtypes in aqueous solution as a function of protein concentration and temperature. The synergy between neutron spin-echo (NSE), a spectroscopy technique providing dynamic information, and small-angle neutron scattering (SANS), a time-averaged static technique, enables us to probe the short-time collective diffusion of different mAbs, explore their self-association into small transient clusters, their intermolecular interactions, and ultimately access their internal dynamics. This study builds on previous neutron backscattering (NBS) findings, bridging a critical gap between the time scales probed by NBS and viscometry. It also confirms that the formation of short-lived clusters comprising more than two monomers is a key factor driving high solution viscosity, phase separation, and opalescence.