Freezing process influences cake appearance of a lyophilized amorphous protein formulation with low solid content and high fill configuration.

Low solid content and high fill drug product configuration pose special challenges for achieving elegant cake appearance after lyophilization. In this study, such a configuration for a protein formulation required lyophilization within a narrow primary drying operating space to obtain elegant cakes. Freezing process optimization was explored as a solution. A Design of Experiment (DoE) approach was used to evaluate the effect of shelf cooling rate, annealing temperature, and their interaction on cake appearance. The slope of product resistance (R) vs. dried layer thickness (L) was used as the quantitative response because elegant cake appearance correlated with a lower initial R and positive slope. As the R vs. L slope can be experimentally established within the first 1/6th of the total primary drying duration, partial lyophilization runs were executed, allowing for rapid screening. The DoE model revealed that a slow cooling rate (≤0.3 °C/min) and high annealing temperature (≥-10 °C) resulted in a better cake appearance. Furthermore, X-ray micro-computed tomography showed that elegant cakes exhibited uniform porous structure and larger pores, while inelegant cakes showed dense top layers with smaller pores. With the optimized freezing process, the primary drying operating space was broadened with improved cake appearance and batch homogeneity.