Use of wet milling combined with temperature cycling to minimize crystal agglomeration in a sequential antisolvent-cooling crystallization

The objective of the research was to improve the process design of a combined anti-solvent/cooling crystallization to reduce the degree of agglomeration of a real active pharmaceutical ingredient product, manufactured using a crystallization stage employing a methanol/water solvent system. Knowledge was gained from use of process analytical technology (PAT) tools to monitor the process variables, allowing particle size, degree of agglomeration, solute concentration and supersaturation to be tracked throughout the process. Based on knowledge of the solubility behavior and interpretation of the PAT histories, changes were made to the sequences of antisolvent addition and cooling within the crystallization process, to reduce agglomeration in the final product. Different seed loadings and seeding addition points, were also investigated to maintain operation within lower supersaturation regions of the phase diagram, to limit agglomeration and avoid an undesired polymorphic transformation to an unstable form. The improved sequences of operations and seeding conditions did not provide sufficient improvement in product quality and so were augmented by applying wet milling for further deagglomeration, followed by temperature cycling to remove fine particles generated during milling. Open-loop heating and cooling cycles produced some limited improvements, whereas closed-loop direct nucleation control methods using FBRM as a feedback sensor for particle counts/s, were much more successful at producing high quality crystals of the desired polymorphic form. The work shows that understanding the trajectory of the process through the phase diagram to follow appropriate supersaturation profiles gives improved control of the various kinetic mechanisms and can be used to improve the quality of the final product.