Cryoconcentration modeling and experimental measurements for freezing and thawing of a biologic bulk drug substance

The freezing and thawing process is a key determinant of the cryoconcentration distribution in pharmaceutical and biological bulk drug substances (BDS), which has a direct and inevitable impact on protein stability. Due to the critical influence of cryoconcentration — either stabilizing or destabilizing the protein depending on its distribution — understanding the specific mechanisms and changes occurring during freezing and thawing is essential for ensuring the stability of the BDS and, by extension, the drug product (DP). This work presents computational modeling of protein cryoconcentration induced by freezing and thawing in a two-liter plastic container. The study utilized fixed-grid modeling with a species segregation model to investigate freezing and thawing dynamics that lead to a shift in protein concentration distribution inside the container, simulating typical processing for pharmaceutical bulk drug substance. The model incorporates a partition coefficient to represent protein distribution between the BDS solution phase and the frozen ice phase. The solute (such as protein) partitioning during ice formation and gravity settlement of protein-rich liquid from the ice–liquid interface towards the bottom of a container creates cryoconcentration during freezing in an ultra-low temperature freezer. Simulations of previously frozen solution subjected to water-bath thawing at $+23°C$ showed a further increase of protein concentration at the bottom of the container due to the inability of the free convective currents set up during the thawing process to overcome the density based segregation. The modeling results were consistent with experimental measurements of freezing and thawing of Human serum albumin solution at 60 mg/mL in 2 L bottles, showing promise for the utilization of computational techniques to design process and equipment that reduces protein cryoconcentration and segregation gradients during freeze–thaw operation improving pharmaceutical manufacturing and product quality.