No abstract

The arnphipathic anthracycline base doxorubicin (DXR) was accumulated in the aqueous phase of the liposomes where it reached a level as high as 100-fold its concentration in the remote loading medium. Most of the intraliposomal D X R was present in an aggregated state. Eflicient (>90%) and stable loading into the liposomes' and ligandoliposomes' aqueous phase was obtained by using gradients of ammonium sulfate in which the ammonium sulfate concentration in the liposomes was higher than its concentration in the extraliposomal medium [(NH,),SO,)lip. > > [(NH,),SO,)med.]. The "remote" loading is a result of the D X R exchange with ammonia from (NH,),SO,. Both the ammonium and sulfate contribute to high level and stability of the loading. The ammonium sulfate gradient method differs from most other chemical approaches used for remote loading of liposomes since it neither requircs to prepare the liposomes in ijcidic pH, nor to alkalinize the extraliposomal aqucous phase. Although most of the intraliposomal D X R is present in an aggregated gel-like state, the drug is bioavailable. This approach permits the preparation of DXRloaded liposomes of a broad spectrum of types, sizes, and composition, including sterically-stabilized liposomes, irnmunoliposomes, and sterically-stabilized immunoliposomes. Due to the long shelf stability (>6 mo), no "bedside" remote 455

In this study, we examine the phase behavior as well as lateral diffusion and percolation in the region of coexisting gel and fluid phases in binary mixtures composed of dimyristoylphosphatidylcholine and one of two totally synthetic D-erythro-sphingomyelins (having either C16 or C24 acyl chains, both having similar gel to liquid-crystalline phase transition temperatures). This study stems from the uniqueness of sphingomyelins having gel to liquid-crystalline transition temperatures in the range of physiological interest, and the fact that more than 50% of the naturally occurring sphingomyelin species have a chain mismatch. The presence of sphingomyelin in biological membranes can thus be expected to give rise to a complex phase structure. Fluorescence recovery after photobleaching, differential scanning calorimetry, and electron microscopy are used to show that, despite similarity in the temperature range of the gel to liquid-cystalline phase transition of the two sphingomyelins, the two differ in their phase structure. Also they differ to a large extent in their mixing with dimyristoylphosphatidylcholine. Dimyristoylphosphatidylcholine and C16 sphingomyelin mix nearly ideally, with the percolation threshold locus lying close to the liquidus on the phase diagram. In contrast, the C24 sphingomyelin and dimyristoylphosphatidylcholine mix nonideally, with the percolation threshold locus lying close to the solidus. In addition, mixtures containing C24 sphingomyelin have a complex thermotropic behavior which may be related to the observation that these dispersions contain several types of particles, some of which are not multilamellar vesicles. These studies suggest that the degree of sphingomyelin chain mismatch is an important factor in determining lateral organization in the membrane.

The kinetics of the spontaneous exchange of [3H]cholesterol between small unilamellar vesicles of phosphatidylcholine has been reexamined. Although first-order exchange kinetics were observed (k = 0.0117 min-1), in good agreement with previous studies, about 20% of the total cholesterol was found to be nonexchangeable in the 8-h time frame of the experiments. The size of this nonexchangeable pool was found to depend on the type of phospholipid and the temperature. It seems probable that the two pools of cholesterol defined in these experiments reflect the complex phase structure of the cholesterol-phosphatidylcholine vesicles.