Controlling the Size and Shape of Monoclonal Antibody Particles through Crystallization Process Design

The crystallization of monoclonal antibodies is deemed particularly challenging due to their large size and structural flexibility. An additional challenge is represented by the tendency of mAb crystals to rapidly lose their crystallinity when removed from liquid and exposed to air or nitrogen, which complicates filtration. This study aims to develop crystallization design strategies for mAbs using the caffeine cocrystal of pembrolizumab as a model. Optimal crystallization conditions are identified by minimizing cycle times for crystallization and filtration. Metrics are introduced to rank the filtration performance of the different crystalline particle populations. This work focuses on the crystallization of caffeine cocrystal-1 of pembrolizumab, which is consistently generated upon charge of the crystallizing agent poly(ethylene glycol) 3350 (PEG) and of mediating salts, among others, dextran sodium sulfate (DSS). This study also demonstrates that seeding is feasible in the crystallization of mAbs and consistently shortens the cycle times for crystallization and improves the robustness of the process. As a proof of concept, seeded semicontinuous crystallizations are performed to enhance supersaturation control during crystallization when stringent impurity rejection and particle attribute specifications must be met. The isolation of crystalline mAb particles presents considerable challenge, and there are no documented studies that have established precedent for this aspirational goal. Finally, to avoid amorphization of the particles during isolation, a conservative approach is adopted to only partially deliquor the filter cake and to leave enough supernatant to fully submerge the filter cake to prevent air or nitrogen exposure. Assuming multiple filtration steps may be required to meet drug substance specifications, this study demonstrates that the crystal form can be retained through successive filtration and dilution/wash cycles with the supernatant composition adjusted after each filtration. This outcome demonstrates that the particles are resistant to changes in the solvent composition, such that the final product could be swapped from the matrix best suited for crystallization to a matrix more suitable for product delivery.