Gamma-Irradiation of Non-Frozen, Frozen, and Freeze-Dried Liposomes

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Abstract
Purpose. The aim of this work was to investigate the possibilities and limitations of gamma-irradiation as a sterilisation method for non-frozen, frozen, and freeze-dried liposomes. Methods. Liposomes with an average size of 0.2 µm were irradiated with doses up to about 5 × 104 Gy in a nitrogen atmosphere. Results. Phospholipids in dipalmitoylphosphatidycholine/dipalmitoylphosphatidylglycerol (DPPC/DPPG) 10/1-liposomes and egg phosphatidylcholine/egg phosphatidylglycerol (EPC/EPG) 10/1-liposomes in 10 mM phosphate buffer (pH 7.4) without trehalose degraded considerably upon gamma-irradiation. Irradiation damage was reduced in the presence of 10% trehalose added as a cryoprotectant, but trehalose reacted with species induced by gamma-irradiation as demonstrated by large decreases in pH. Both pH decrease and oxidative damage of EPC/EPG 10/1-liposomes were strongly dependent on the physical state during irradiation (non-frozen, frozen or freeze-dried). No changes in liposomal size were found upon gamma-irradiation, and hardly any change was seen in bilayer rigidity. Differences in the gel-to-liquid phase transition of DPPC/DPPG 10/1-liposome dispersions before and after gamma-irradiation were small in the presence of 10% trehalose, but larger in the absence of trehalose. Conclusions. The degradation of trehalose limits the use of freezing or freeze-drying liposome dispersions as a way to minimise irradiation damage.