Effect of pH, Counter Ion, and Phosphate Concentration on the Glass Transition Temperature of Freeze-Dried Sugar-Phosphate Mixtures

Effect of pH, Counter Ion, and Phosphate Concentration on the Glass Transition Temperature of Freeze-Dried Sugar-Phosphate Mixtures
September 2004
Satoshi Ohtake, Carolina Schebor, Sean P. Palecek, Juan J. de Pablo
Pharmaceutical Research
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Abstract:
Purpose. The aim of the present work is to study the interaction of phosphate salts with trehalose and sucrose in freeze-dried matrices, particularly the effect of the salts on the glass transition temperature (Tg) of the sugars.
Methods. Freeze-dried trehalose and sucrose systems containing different amounts of sodium or potassium phosphate were analyzed by differential scanning calorimetry to determine the Tg and by Fourier-transform infrared spectroscopy (FTIR) analysis to evaluate the strength of the interaction between sugars and phosphate ions.
Results. Sucrose-phosphate mixtures show an increase in Tg up to 40?C in a broad pH range (4-9) compared to that of pure sucrose. Sucrose-phosphate mixtures exhibit a higher Tg than pure sucrose while retaining higher water contents. Trehalose-phosphate mixtures (having a Tg of 135?C at a pH of 8.8) are a better option than pure trehalose for preservation of labile materials. The -OH stretching of the sugars in the presence of phosphates decreases with increase in pH, indicating an increase in the sugar-phosphate interaction.
Conclusions. Sugar-phosphate mixtures exhibit several interesting features that make them useful for lyophilization of labile molecules; Tg values much higher than those observed for the pure sugars can be obtained upon the addition of phosphate.