Relaxation Processes in Freeze-Dried Monoclonal Antibody–Sucrose Formulations: The Role of Water Content

Residual water in freeze-dried protein formulations plays a critical role in molecular relaxation behavior. In a previous study (Kumar et al., Mol Pharmaceutics 2025, 22, 4125-4136), we examined the impact of sucrose concentration on native structure retention in monoclonal antibody (mAb)-sucrose systems. The present work investigates how residual water content affects relaxation dynamics in this formulation system. Water affinity was first assessed via sorption isotherms across a range of water activities. There was an increase in residual water content as a function of mAb content in freeze-dried formulations. mAb-sucrose interactions were evident from detailed analyses of sorption data. There was a progressive decrease in the glass transition temperature with increasing water content. Dielectric spectroscopy was then employed to characterize the β- and γ-relaxation processes in the glassy state. While the β-relaxation time decreased with increasing water content, there was no significant effect on the corresponding activation energy. The effect of water content on the γ-relaxation was complex, with an increase in activation energies as a function of water content. In light of the influence of mobility on both physical and chemical stability, these results underscore the importance of controlling water content in optimizing the stability of freeze-dried mAb formulations.