The cryopreservation of liposomes. 1. A differential scanning calorimetry study of the thermal behavior of a liposome dispersion containing mannitol during freezing/thawing.

The thermal behavior of water in liposome dispersions and in liposome dispersions containing mannitol at subzero temperatures was investigated with differential scanning calorimetry (DSC). The cooling curves from 20 down to -60 degrees C for a liposome dispersion (bilayer composition PL100H/DCP), monitored at cooling rates of 5 and 10 degrees C/min, showed several heat flows related to water crystallization. All lipid-containing dispersions showed water crystallization at temperatures below -40 degrees C. The magnitude of this heat flow strongly depended on the experimental variables. Cooling rate, particle size, lipid concentration, and location and nature of the cryoprotectant all influenced the water crystallization behavior as shown in the DSC cooling curve. Different fractions of water--presumably related to their location in the dispersion--could be distinguished. It is concluded that DSC provides a valuable tool for the detection of changes in the physical state of water in liposome dispersions during freezing/thawing. The insights gained from these DSC studies may make it possible to select--on the basis of rational considerations rather than by trial and error--optimum conditions for the cryopreservation of liposomes containing water-soluble drugs.