094 Application of the Kwei equation for predicting the Tg of stabilizing formulations containing sugars and salts

Abstract

Sugars such as trehalose and sucrose have been used in numerous formulations to improve the shelf-life of products ranging from foods to pharmaceuticals and more recently they have also been used as a key protectant for preserving cell-based products. Disaccharides are known to have high glass transition temperatures and thus can immobilize molecules at high storage temperatures. Frequently these sugars are used in combination with salts for the purpose of buffering, oncotic control, or to maintain membrane potentials. The effect of salts on the glass-forming sugars has not been comprehensively studied. To better understand the effect of salts on the potential for the storage of glassy compositions we investigated various predictive models for sugar/salt formulations using data from the literature as well as new data generated on binary mixtures of trehalose and choline dihydrogen phosphate (CDHP). CDHP has recently been shown to have promise as a stabilizing agent for proteins and DNA, and thus may have utility in preserving certain biospecimens. For example, it can increase the thermal stability and shelf life of model proteins such as Cytochrome c, lysozyme and interleukin-2. In the current study we determined the glass transition temperature (Tg) of mixtures of CDHP and trehalose using a Q-100 differential scanning calorimeter (TA Instruments). The calorimetric results show that the relationship between Tg and composition exhibited a pronounced deviation from the conventional Gordon-Taylor prediction. Specifically, there is a “fall-and-rise” trend within the range of 70 ∼ 90 wt% of trehalose. A general and accurate model for predicting the Tg can conserve experimental effort and lead to better insights about the interactions between sugars and salts. In this study we applied the Fox, Gordon-Taylor and Kwei equations to both literature data and new data acquired on trehalose/CDHP mixtures that are of interest in formulation science. The Kwei equation was found to have more generality and accuracy than the Fox and Gordon-Taylor equations. From a mathematical point of view, the curves of the Tg as a function of weight percentage modeled with the Kwei equation can be further grouped into three types (i.e., U, inverted U, and inverted S shapes) based on the values of its two parameters, k and q. It is believed that the deviations from the Gordon-Taylor prediction can be attributed to the strong interactions between the two components in a mixture as well as their structural and volumetric differences. Source of funding: This work was funded by grant #5RO1GM101796 from the National Institutes of Health. Conflict of interest: None declared. gdelliot@uncc.edu