Protein stability and viscosity in molecularly crowded high-concentration biologics

The paradigm for administering protein biologics is increasingly shifting from intravenous infusion to high-concentration subcutaneous delivery, driven by the desire for patient-centric, sometimes self-administered therapies to better manage chronic diseases. However, this trend is constrained by the inter-related biophysical challenges of protein instability and high viscosity that typically emerge at protein concentrations exceeding 100 mg/mL. In this review, we elucidate the underlying mechanisms of protein instability in a molecularly crowded environment of high-concentration formulations, wherein the close proximity of molecules affect protein structure and function through complex, and often competing, interplay of steric excluded volume repulsion and soft interactions including electrostatic, hydrogen-bonding and hydrophobic forces, leading to reversible and irreversible self-association, increased viscosity and meta-stable association pathways such as liquid-liquid phase separation. Consequently, manipulation of these competing intermolecular interactions can enable the development of stable high-concentration protein therapeutics through rational molecular and formulation design approaches that preserve the native state and elevate the energy barrier for aggregation. Here, we explore the multi-faceted strategies to achieve this balance, including rational formulation design with buffers, excipients, and innovative viscosity-reducing agents, alongside protein engineering approaches to create inherently developable molecules. Moreover, the molecular determinants of solution viscosity arising from protein-protein interactions are discussed with particular focus on the role of arginine and its derivatives to disrupt these network-forming interactions and reduce viscosity in a concentration-dependent manner. The discussion extends to advanced delivery strategies, such as non-aqueous protein powder suspensions and aqueous crystalline or amorphous formulations, which circumvent traditional viscosity limits, in part, by reducing bulk solution protein-protein interactions. Finally, the critical interface between drug product and delivery device is examined, highlighting device innovations that enable the injection of viscous liquids and addressing stability risks from silicone oil and metal leachables in prefilled syringes. Ultimately, the successful development of stable, deliverable, high-concentration biologics combination drug products requires an integrated approach that combines mechanistic understanding, protein biophysics, formulation science, and device engineering.