Long term stability of freeze-dried, lyoprotected doxorubicin liposomes

Abstract

The aim of the present study was to determine which parameters influence the long term stability of freeze-dried doxorubicin (DXR) liposomes. The DXR content, DXR retention and average vesicle size of the rehydrated liposomes were examined as a function of storage temperature, lyoprotectant (sucrose, maltose, lactose and trehalose), residual water content and onset temperature of the glass transition (T g,onset) of the freeze-dried cake. No significant physical instability or chemical degradation was observed in cakes containing less than 1% residual water after storage for 6 months at temperatures up to 30°C. However, a 25–50% decrease in the DXR content after rehydration was observed in samples stored at 50°C, which was accompanied by leakage of the encapsulated drug from the liposomes. All disaccharides selected for this study followed a similar pattern in this respect. Over the period of storage, no increases in average vesicle size (initial size around 0.1 μm) over 0.02 μm were observed upon rehydration of these cakes, except for DXR-liposome samples containing sucrose and stored at 50°C. These effects all occurred below the T g,onset of the freeze-dried cakes. The residual water content clearly affected the stability of the freeze-dried liposomes. In contrast, sucrose cakes containing circa 3.5% residual water showed a size increase, DXR degradation and leakage of encapsulated DXR already after storage at 30°C. Thermal analysis of the dry cakes showed clear differences between the intraliposomal phase and the extraliposomal matrix. Stability of the encapsulated DXR was primarily dependent on the physical states of the solids inside the liposomes. In conclusion, freeze-drying of DXR-liposomes resulted in formulations that are stable at 30°C for 6 months. In addition, the results suggest that the stability of liposome encapsulated drugs is largely determined by the characteristics of the intraliposomal solid, and only to a limited extent by the T g,onset of the extraliposomal matrix.