Preservation of dried liposomes in the presence of sugar and phosphate

dIt has been well established that sugars can be used to stabilize liposomes during drying by a mechanism that involves the formation of a glassy state by the sugars as well as by a direct interaction between the sugar and the phospholipid head groups. We have investigated the protective effect of phosphate on solute retention and storage stability of egg phosphatidylcholine (egg PC) liposomes that were dried (air-dried and freeze-dried) in the presence of sugars and phosphate. The protective effect of phosphate was tested using both glucose (low T,) and sucrose (high T,) by measuring leakage of carboxyfluorescein (CF), which was incorporated inside the vesicles. Liposomes that were dried with glucose or phosphate alone showed complete leakage after rehydration. However, approximately 30% CF-retention was obtained using mixtures of phosphate and glucose. Approximately 75% CF-retention was observed with liposomes that were dried with sucrose. The solute retention further increased to 85% using mixtures of phosphate and sucrose. The pH of the phosphate buffer prior to drying was found to have a strong effect on the solute retention. Fourier transform infrared spectroscopy studies showed that phosphate and sugars form a strong hydrogen bonding network, which dramatically increased the T-g. The HPO42- form of phosphate was found to interact stronger with sugars than the H2PO4- form. The increased solute retention of liposomes dried in the sugar phosphate mixtures did not coincide with improved storage stability. At temperatures below 60 degreesC the rate of solute-leakage was found to be strikingly higher in the presence of phosphate, indicating that phosphate impairs storage stability of dried liposomes. (C) 2004 Elsevier B.V. All rights reserved.