Stability of doxorubicin-liposomes on storage : as an aqueous dispersion, frozen or freeze-dried

Shelf-life of doxorubicin (DXR) containing negatively charged liposomes was studied under various conditions. The following parameters were monitored: DXR-latency, stability against aggregation or fusion, and chemical stability of bilayer components and associated DXR. An ion exchange resin was used to remove free DXR from liposome-associated DXR. As an alternative for storing aqueous dispersions, the liposomes were frozen and freeze-dried. Variables under investigation were: type of cryoprotectant (saccharides, polyvinylpyrrolidone and mannitol), particle size (about 0.1, 0.25 and 0.65 μm), physical state of the bilayer (“gel”- or “fluid”-like, inclusion of α-tocopherolacetate). A comparison was made between the behaviour of a bilayer interacting compound (DXR) and a non-interacting compound (carboxyfluorescein, CF). When stored as aqueous dispersions extruded multilamellar structures (0.25 μm) showed lower DXR leakage rates than sonicated vesicles (0.1 μm). In the presence of various saccharides freezing and freeze-drying resulted in similar DXR latencies on thawing or rehydration; they provided adequate protection against aggregation or fusion. Other cryoprotectants failed. Latency of a bilayer-interacting drug like DXR, was hardly sensitive to variation of the bilayer structure; in contrast to this observation, latency of CF strongly depended on the bilayer composition. Under the experimental conditions no effects of ageing could be observed in terms of changing leakage profiles, aggregation behaviour or chemical decomposition of DXR