Impact of sucrose level on storage stability of proteins in freeze-dried solids: I. Correlation of protein-sugar interaction with native structure preservation.

The purpose of this study is to investigate protein-sugar interactions in dried protein solids as a function of sucrose level using water sorption isotherm data and secondary structure information from Fourier transform infrared (FTIR) spectroscopy. Three IgG1 fusion proteins and two cytokines were freeze-dried with sucrose at different sucrose/protein mass ratios. The water monolayer of the colyophilized sucrose/protein samples, as determined by BET analysis of water sorption data, was found to be lower than that expected based on additive contributions of pure protein and pure sucrose. This negative deviation suggests the presence of a solid-state interaction between protein and sucrose that reduces the availability of total water-binding sites. The difference in water monolayer between colyophilized and a physical mixture of protein and sucrose reached a maximum value at sucrose/protein mass ratio of 1/1 for these proteins, suggesting saturation of the protein-sugar interaction at this ratio. In addition, for four proteins studied, the normalized peak height of the major band in the FTIR spectra reached a plateau at about a 1/1 mass ratio. Therefore, it appears that there is a coupling between the preservation of protein secondary structure and the protein-sugar interaction as measured by water sorption isotherms.