Factors influencing cryoprotective activity and drug leakage from liposomes after freezing

The effects of freezing and thawing conditions and cryoprotective additives on release of streptomycin from lecithin liposomes following freeze-thaw cycles have been investigated. Drug retention was maximized by slow cooling (approx. 1 degree C min-1). At temperatures between 0 degree and -20 degrees C, the extent of drug loss was time-dependent indicating incomplete freezing; below -45 degrees C this effect was abolished and the system was stable. Osmotic gradients across the liposome membrane during freezing were found to have little effect on drug loss. Marked cryoprotective activity was shown by dimethylsulphoxide, glycerol, alanine and glycinebetaine at concentrations of 3% w/v or less. At this concentration sucrose and mannitol had little activity.