Short term stability of freeze-dried, lyoprotected liposomes

In the present study we examined the short term stability of liposomes in the freeze-dried state for different lipid compositions containing trehalose as a lyoprotectant. The retention of carboxyfluorescein and average vesicle size after rehydration were monitored as a function of the temperature to which the dry cakes were exposed for 0.5 h. The thermal behaviour of the cakes was analysed by modulated temperature differential scanning calorimetry, and acyl chain order and interaction between trehalose molecules and the phospholipid headgroups was studied by Fourier transform infrared spectroscopy. Induction of leakage, suppression of the (onset) bilayer transition temperature (Tm) and enhancement of the interaction between sugar and phospholipid molecules were observed below the glass transition temperature (Tg) for all lipid compositions studied. The above changes concurred with the melting transition of the bilayer. Two out of five lipid compositions showed no significant change in average vesicle size, indicating that leakage was not necessarily caused by vesicle fusion. In addition, leakage could not be explained in terms of a phase transition during rehydration of the liposomes. We conclude that for liposomes freeze-dried in trehalose the temperature range of the bilayer melting process is a better indicator than Tg for the maximal temperature to which liposomes may be exposed for a short period of time (0.5 h) without loss of stability.