Since the 1980s therapeutic proteins have made a breakthrough in the treatment of chronic diseases and are now predominant among biological drugs. Compared to smaller synthetic molecules, they are characterized by high specificity and efficacy. Their weakness is almost always mandatory parenteral administration, with subcutaneous administration being the most desirable among patients. It is associated with formulation issues, which is often a consequence of the high protein concentration required to deliver the appropriate dose in a limited volume. Highly concentrated protein formulations are often accompanied by increased viscosity due to non-covalent intermolecular interactions. We studied the effect of type and concentration of protein and excipients (buffers, salt, amino acids, mannitol, sucrose) on the viscosity of formulations. We determined the protein concentration spectrophotometrically using a UV-VIS-spectrophotometer NanoDrop. Viscosity was measured on a mVROC viscometer using a microfluidic technology. The viscosity of albumin and monoclonal antibody formulations exponentially increases with protein concentration due to friction and interactions between protein molecules. The latter is the reason for the deviation of viscosity from mathematical models. No significant differences were observed between the viscosity of albumin formulations in phosphate (pH=7.4) or histidine (pH=6.0) buffer. The viscosity of monoclonal antibody formulations decreases with increasing histidine buffer concentration. NaCl caused a negligible increase in viscosity in most of the investigated formulations. Bulking agents (phenylalanine, isoleucine, mannitol) and sucrose also had no significant effect on the viscosity of mAb formulations. Amino acids arginine and proline had an unfavorable effect on the viscosity of albumin formulations, with arginine being more pronounced. We conclude that viscosity strongly depends on intermolecular interactions, which in turn depend on the net charge on the protein and consequently the formulation pH, protein’s and excipients’ isoelectric point, as well as intermolecular distances and consequently protein concentration. From the changes in viscosity, one can only infer about intermolecular interactions, so it would make sense to consider the results herein in conjunction with other parameters such as the second virial coefficient and interaction parameter. In addition, to ensure the quality, safety, and efficacy of biological drugs, it is necessary to check the conformational (in)stability of the protein drug with each formulation change.
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