Mechanism of protein stabilization by sugars during freeze-drying and storage: native structure preservation, specific interaction, and/or immobilization in a glassy matrix?

The purpose of this study is to investigate the mechanism of protein stabilization by sugars in the solid state. That is, explore whether the stabilization is controlled by "glass dynamics" or by native structure preservation through "specific interaction" between sugars and protein. The IgG1 antibody (150 kD) and recombinant human serum albumin (rHSA) (65 kD) were formulated with sorbitol, trehalose, and sucrose. Degradation of lyophilized formulations was quantified using size exclusion (SEC) and ion-exchange chromatography (IEX). The secondary structure of the protein in these formulations was characterized using Fourier Transform Infrared (FTIR) spectroscopy. The molecular mobility, as measured by the stretched relaxation time (tau(beta)) was obtained by fitting the modified stretched exponential (MSE) equation to the calorimetric data from the Thermal Activity Monitor (TAM). Compared with sucrose and trehalose, sorbitol could only slightly protect the protein against aggregation and had no effect on chemical degradation. The chemical degradation and aggregation rates of the protein decreased when the weight ratio of sucrose to protein increased from 0 to 2:1. Storage stability of the proteins showed a reasonably good correlation with the degree of retention of native structure of protein during drying as measured by the spectral correlation coefficient for FTIR spectra. The plots of tau(beta) as a function of fraction of sucrose passed through a maximum at 1:1 weight ratio of sucrose to protein. That is, the molecular mobility did not correlate with the stability of protein at high levels of sucrose content. Although the glass transition appears to be an important parameter for stability, protein stabilization by sugars in the solid state cannot be wholly explained by the glass dynamics mechanism, at least as measured by tau(beta). However, it is possible that the beta-relaxations rather than the alpha-relaxations (i.e., the tau we measured) are critical to stability. The data show that storage stability correlates best with "structure" as determined by FTIR spectroscopy. However, while a specific interaction between stabilizer and protein might be responsible for the preservation of native structure, the evidence supporting this position is not compelling.