Thermal stability is essential for the understanding of protein stability and is a critical quality attribute of therapeutic biologics, including enzymes, fusion proteins, monoclonal antibodies, etc. The commonly used analytical methods, such as differential scanning calorimetry (DSC), differential scanning fluorimetry (DSF), and circular dichroism (CD), have their limitations in measuring protein thermal stability. Through this work, we described a novel method to probe the thermal stability of proteins in various formulations using a temperature scanning viscometer. The viscosity of the material was plotted against the temperature, and the peak in the first derivative of the viscosity versus temperature was shown to be related to the protein melting temperature. The measured melting temperature of bovine serum albumin (BSA) at a concentration of 1 mg/mL in phosphate buffer was 63 °C, which was close to the value of 64 °C obtained by DSC. The unfolding of BSA was confirmed using orthogonal techniques of second derivative ultraviolet-visible (UV-vis) spectroscopy and dynamic light scattering (DLS). This method was also able to reveal the microenvironment changes of proteins, including formulation effects. Other multiple domains proteins including lysozyme and IgG were also tested using this method and showed comparable melting temperatures with DSC. This work showed the feasibility of using a temperature scanning viscometer to measure the thermal stability of proteins in diverse formulation matrices with wider protein concentration ranges.
Keywords: biologics; melting temperature; rheology; thermal stability; viscosity.