Freeze-concentration of solutes during bulk freezing and its impact on protein stability

Storage of protein therapeutics in a frozen state provides operational flexibility for fill and finish of drug products and extends the shelf life of protein solutions. However, freezing of bulk protein solutions leads to progressive freeze-concentration of solutes which may result in protein instability. A novel method was designed to study the freeze-concentration of solutes in 1 L bottles. Lactate dehydrogenase (LDH) solutions in phosphate buffer, pH 7.0, with or without sucrose were studied and compared with the same solutions without LDH. These solutions were frozen at −30 °C and at different freezing rates. The contents of each frozen bottle were evenly sliced and forty-five representative samples were analyzed for solute concentrations and LDH quality attributes. A homogeneity score was devised to describe the overall homogeneity of the frozen bulk. In general, the solutes (i.e., LDH, phosphate, and sucrose) concentrated towards the center and bottom of the bottles during freezing. The homogeneity score was highly dependent upon the freezing rate with significant retention of LDH activity and tetrameric native structures observed at higher freezing rates. Lastly, multiple linear regression showed the significant impact of the freezing rate, sample location inside the bottle, and presence of sucrose on LDH stability, which provided insights into the stability of other proteins during bulk freezing.