First-order nucleation and subsequent growth promote liquid-liquid phase separation of a model IgG1 mAb

Although protein aggregation is commonly encountered during the manufacturing and storage of bio-therapeutics, the actual aggregation mechanism remains unclear, and little has been reported about the protein aggregation kinetics from time zero under particular solution conditions. In this study, we used real-time dynamic light scattering (DLS) to continuously monitor the time-dependent evolution of the Z-average hydrodynamic radius of a model IgG1 (JM2) immediately after the JM2 solution was subjected to various low temperatures (0-4 °C). We observed that JM2 aggregated to form nuclei first, and then it subsequently grew to small liquid droplets via a two-step, first-order, reversible process without causing irreversible structural changes: a slow first step defined as the "nucleation" step, wherein nuclei formed slowly until reaching a transitional time point (t_onset), and a much faster second step initiated after t_onset and the nucleus size of the protein increased rapidly, which eventually caused liquid droplet formation and liquid-liquid phase separation (LLPS). The "nucleation" rate constant (K_nucleation) and particle growth rate constant (K_growth), as well as t_onset, were found to be temperature, pH and concentration dependent. The aggregation of JM2 could be universally described by these two-step first-order kinetics: under conditions where JM2 aggregated very slowly, the second step was not observed within the experimental time scale, while under conditions where JM2 aggregated very rapidly, the first step could not be recorded. We believe that these three parameters, K_nucleation, K_growth, and t_onset, can be used to quantify and compare the aggregation kinetics of JM2 under different solution and temperature conditions and, furthermore, serve as a theoretical base to account for the key characteristics of the aggregation kinetics of JM2 and other protein therapeutics under conditions of interest.