This review aims to provide an overview of current knowledge on stabilization of proteins by sugars in the solid state in relation to stress conditions commonly encountered during drying and storage. First protein degradation mechanisms in the solid state (i.e. physical and chemical degradation routes) and traditional theories regarding protein stabilization (vitrification and water replacement hypotheses) will be briefly discussed. Secondly, refinements to these theories, such as theories focusing on local mobility and protein-sugar packing density, are reviewed in relationship to the traditional theories and their analogies are discussed. The last section relates these mechanistic insights to the stress conditions against which these sugars are used to provide protection (i.e. drying, temperature, and moisture). In summary sugars should be able to adequately form interactions with the protein during drying, thereby maintaining it in its native conformation and reducing both local and global mobility during storage. Generally smaller sugars (disaccharides) are better at forming these interactions and reducing local mobility as they are less inhibited by steric hindrance, whilst larger sugars can reduce global mobility more efficiently. The principles outlined here can aid in choosing a suitable sugar as stabilizer depending on the protein, formulation and storage condition-specific dominant route of degradation.

Inulin, a fructan-type polysaccharide, consists of (2→1) linked β-d-fructosyl residues (n=2-60), usually with an (1↔2) α-d-glucose end group. The applications of inulin and its hydrolyzed form oligofructose (n=2-10) are diverse. It is widely used in food industry to modify texture, replace fat or as low-calorie sweetener. Additionally, it has several applications in other fields like the pharmaceutical arena. Most notably it is used as a diagnostic agent for kidney function and as a protein stabilizer. This work reviews the physicochemical characteristics of inulin that make it such a versatile substance. Topics that are addressed include morphology (crystal morphology, crystal structure, structure in solution); solubility; rheology (viscosity, hydrodynamic shape, gelling); thermal characteristics and physical stability (glass transition temperature, vapor sorption, melting temperature) and chemical stability. When using inulin, the degree of polymerization and processing history should be taken into account, as they have a large impact on physicochemical behavior of inulin.

Protein-based biopharmaceuticals are generally produced as aqueous solutions and stored refrigerated to obtain sufficient shelf life. Alternatively, proteins may be freeze-dried in the presence of sugars to allow storage stability at ambient conditions for prolonged periods. However, to act as a stabilizer, these sugars should remain in the glassy state during storage. This requires a sufficiently high glass transition temperature (Tg). Furthermore, the sugars should be able to replace the hydrogen bonds between the protein and water during drying. Frequently used disaccharides are characterized by a relatively low Tg, rendering them sensitive to plasticizing effects of residual water, which strongly reduces the Tg values of the formulation. Larger sugars generally have higher Tgs, but it is assumed that these sugars are limited in their ability to interact with the protein due to steric hindrance. In this paper, the size and molecular flexibility of sugars was related to their ability to stabilize proteins. Four diverse proteins varying in size from 6 kDa to 540 kDa were freeze-dried in the presence of different sugars varying in size and molecular flexibility. Subsequently, the different samples were subjected to an accelerated stability test. Using protein specific assays and intrinsic fluorescence, stability of the proteins was monitored. It was found that the smallest sugar (disaccharide trehalose) best preserved the proteins, but also that the Tg of the formulations was only just high enough to maintain sufficient vitrification. When trehalose-based formulations are exposed to high relative humidities, water uptake by the product reduces the Tgs too much. In that respect, sugars with higher Tgs are desired. Addition of polysaccharide dextran 70 kDa to trehalose greatly increased the Tg of the formulation. Moreover, this combination also improved the stability of the proteins compared to dextran only formulations. The molecularly flexible oligosaccharide inulin 4 kDa provided better stabilization than the similarly sized but molecularly rigid oligosaccharide dextran 6 kDa. In conclusion, the results of this study indicate that size and molecular flexibility of sugars affect their ability to stabilize proteins. As long as they maintain vitrified, smaller and molecularly more flexible sugars are less affected by steric hindrance and thus better capable at stabilizing proteins.

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