Significant amounts of soluble product aggregates were observed during low-pH viral inactivation (VI) scale-up for an IgG4 monoclonal antibody (mAb IgG4-N1), while small-scale experiments in the same condition showed negligible aggregation. Poor mixing and product exposure to low pH were identified as the root cause. To gain a mechanistic understanding of the problem, protein aggregation properties were studied by varying critical parameters including pH, hold time and protein concentration. Comprehensive biophysical characterization of product monomers and aggregates was performed using fluorescence-size-exclusion chromatography, differential scanning fluorimetry, fluorescence spectroscopy, and dynamic light scattering. Results showed IgG4-N1 partially unfolds at about pH 3.3 where the product molecules still exist largely as monomers owing to strong inter-molecular repulsions and favorable colloidal stability. In the subsequent neutralization step, however, the conformationally changed monomers are prone to aggregation due to weaker inter-molecular repulsions following the pH transition from 3.3 to 5.5. Surface charge calculations using homology modeling suggested that intra-molecular repulsions, especially between CH2 domains, may contribute to the IgG4-N1 unfolding at ≤ pH 3.3. Computational fluid dynamics (CFD) modeling was employed to simulate the conditions of pH titration to reduce the risk of aggregate formation. The low-pH zones during acid addition were characterized using CFD modeling and correlated to the condition causing severe product aggregation. The CFD tool integrated with the mAb solution properties was used to optimize the VI operating parameters for successful scale-up demonstration. Our research revealed the governing aggregation mechanism for IgG4-N1 under acidic conditions by linking its molecular properties and various process-related parameters to macroscopic aggregation phenomena. This study also provides useful insights into the cause and mitigation of low-pH-induced IgG4 aggregation in downstream VI operation.
Keywords: Low pH; antibody aggregation; computational fluid dynamics; viral inactivation.