A Review on Mixing-Induced Protein Particle Formation: The Puzzle of Bottom-Mounted Mixers.

Mixing is an important unit operation in monoclonal antibody manufacturing. The goal is to achieve homogeneity without compromising product quality. Mixing-induced protein degradation and protein subvisible particle (SvP) formation, which impacts product quality, are associated with 2 common stress modes: mechanical shear and air-liquid interfacial stress, which can generally be overcome by formulation optimization. This review addresses a unique stress mechanism that caused SvP formation when using certain bottom-mounted mixers equipped with impellers propelled by magnetic or mechanical coupling with a drive unit. During use, the coupling assembly is submerged in the protein solution allowing liquid access into the gap between the 2 bearings. Based on data from studies of bottom-mounted mixers and other small-scale mixers, grinding of the 2 bearings is a condition for inducing particulate formation. Although grinding stress is an accepted cause, identifying the responsible stress mechanism is challenging. By applying small-scale models, researchers attempted to elucidate the modes of stress, which ranged from common stress (mechanical shear; interfacial stress/adsorption; cavitation) to more speculative hypotheses (nucleation from nano- and micro-particles; localized thermal stress). Recent literature was reviewed, and recommendations are offered to development scientists and process engineers regarding mixer design to reduce protein SvP formation during mixing of monoclonal antibody formulations.