Shear-Induced Crystallization and Rheological Analysis of a Therapeutic Protein

Crystallization is an attractive alternative for protein-based therapeutic formulations since the crystalline form offers improved stability, longer storage lifetime, and enables controlled release of the active ingredient. 90% of the active drug ingredients are produced in the crystalline form within the biopharmaceutical industry. However, several problems have recently emerged related to the process manufacturing (i.e., handling and delivering) of these formulations. The occurrence of aggregation/agglomeration events is frequent and may induce shear viscosity increases. Herein, a rheological characterization combined with the analysis of insulin behavior in solution is addressed in the presence of a precipitant solution: zinc-providing salt (zinc chloride) combined with a buffer (trisodium citrate) and a co-solvent (acetone). While the independent solutions (insulin and precipitant) exhibit a Newtonian behavior, their mixture results in a shear-thinning response within broad ranges of protein concentration and temperature. The transition to a Newtonian response is only captured at high temperature values. Rhombohedral crystals with variable size and number are produced for the studied working conditions, except at low precipitant concentrations, where aggregation/agglomeration events are observed, followed by sudden shear viscosity increases over time. Moreover, the critical shear rate to generate single large insulin crystals is optimized alongside the shear viscosity analysis. This has been observed for both single- and double-shearing experiments. During the double-shearing experiments, the protein solutions are exposed to sudden changes in shear rate, which might disturb the molecular arrangement. However, the crystallization outcome seems to be similar compared to the single-shearing experiments. Lastly, this work highlights a strategy to induce insulin crystallization under uniform shearing fields, which can be extended to other therapeutic proteins.