Water Activity as an Indicator for Antibody Storage Stability in Lyophilized Formulations.

Lyophilization remains a key method for preserving sensitive biopharmaceuticals such as monoclonal antibodies. Traditionally, stabilization mechanisms have been explained by vitrification, which minimizes molecular mobility in the lyophilized cake, and water replacement, which restores molecular interactions disrupted by water removal. This study proposes a novel design strategy that combines water activity and glass-transition temperature as the main indicators to predict long-term stability in lyophilized formulations. The water activity, calculated as the product of water activity coefficient and (residual) water content, serves as a mutual indicator of molecular interactions and influence of residual water content in the lyophilizate. By predicting beneficial excipient combinations through activity coefficient calculations using the perturbed-chain statistical association fluid theory model and calculating T_g using the Gordon-Taylor equation, the study identifies favorable excipient systems, such as sucrose/ectoine mixtures, providing formulation windows that offer broad stability ranges. The approach was validated with stability studies, confirming that formulations within a water activity range of 0.025-0.25 exhibit high (long-term) stability. This work advances formulation development by integrating water-excipient interactions and residual moisture content into a predictive model, moving beyond traditional empirical methods and offering a robust pathway to the design of stable biopharmaceutical formulations. This makes it possible to achieve high/favorable water activities despite low residual moisture (thus, high glass-transition temperatures) with plausible excipient concentrations and combinations.