Freeze-Anneal-Thaw Cycling of Unilamellar Liposomes: Effect on Encapsulation Efficiency

Purpose Freeze-thaw cycling is an important processing step in the preparation of liposomes that leads to the encapsulation of drug molecules. There is considerable variability in the number of freeze-thaw cycles reported in the literature. This work is designed to aid in liposomal formulation design by gaining an insight into the drug encapsulation process and an understanding of liposome stabilization during various thawing conditions. Methods The effects of different thawing temperatures, as well as "annealing" at subzero temperatures on a liposome formulation, are reported here. Results Two freeze-anneal-thaw (FA(NN)T) cycles (freezing to -196A degrees C, annealing at -1.4A degrees C for similar to 30 min, thawing at 65A degrees C) resulted in the maximum predicted encapsulation efficiency without causing any significant change in particle size or zeta potential. Annealing at -22A degrees C was shown to be destabilizing due to limited hydration of the liposomes in the frozen state. Conclusion It was shown that two important processes are occurring during the FA(NN)T cycling that affect liposome encapsulation efficiency. The first is drug diffusion in the frozen state and the second is fusion/destabilization of the liposomes. This is the first report on the annealing of liposomes and understanding the mechanism of drug encapsulation using the freeze-thaw cycling method.