Targeting of small unilamellar liposomes to the galactose receptor in vivo.

Intravenously administered phospholipid liposomes are known to interact with plasma high-density lipoproteins, which remove phospholipid molecules from the bilayers, thus leading to vesicle destabilization. Consequently, drugs that may have been originally entrapped leak out into the circulation to acquire their own clearance rates and tissue distribution. The destabilized liposomes and drugs that are still entrapped are removed by the reticulo-endothelial system, mostly liver and spleen (Gregoriadis, 1983; Ryman & Tyrrell, 1980). Any attempt to deliver drugs to the cells of the reticulo-endothelial system quantitatively must therefore employ liposomes that, while in the circulation, retain most or all of their contents. Interaction of alternative accessible target cells expressing receptors on their surface with liposomes bearing related ligands may also occur quantitatively if the liposomes, in addition to being stable, exhibit a rate of clearance that is prolonged enough to decrease their interception by the reticulo-endothelial system. Long-lived liposomes can also be used as carriers of agents for which the extended presence in the circulation is required (e.g. haemoglobin) or which are expected to act after their slow release (e.g. antitumour agents; Kirby & Gregoriadis, 1983; Large & Gregoriadis, 1983). Work in vivo with large (Gregoriadis & Davis, 1979) or small (Kirby et al., 1980; Senior & Gregoriadis, 1982a) liposomes has shown that high-density-lipoprotein-mediated phospholipid loss from the vesicles (Krupp et al., 1976; Scherphof et al., 1978) can be curtailed or abolished altogether by incorporating into liposomes cholesterol, phospholipids with a high liquid-crystalline transition temperature or sphingomyelin, which forms intermolecular bonding with other phospholipids (e.g. egg phosphatidylcholine; Allen, 1981). Such manipulations of lipid composition not only decrease entrapped-solute loss from circulating liposomes, but they also decrease vesicle clearance rates (Gregoriadis & Senior, 1980). For instance, half-lives of up to 20h (Fig. lb) have been achieved with the very stable (Fig. la) small unilamellar liposomes composed of equimolar distearoyl phosphatidylcholine and cholesterol injected intravenously into mice at a total lipid dose which does not saturate the reticulo-endothelial system. This inverse relationship of liposomal stability and clearance rate from the circulation is discussed in detail elsewhere (Senior & Gregoriadis, 19826; Gregoriadis, 1983). Although there have been several successful attempts selectively to associate liposomes with specific cells in uitro (e.g. Gregoriadis & Neerunjun, 1975; Barbet et al., 1981 ; Gregoriadis & Meehan, 1981; Bragman et al., 1983), targeting of liposomes in viuo has so far been limited either to circulating molecules (Gregoriadis et al., 1981 ; Barratt ef al., 1983) or to the hepatic parenchymal cells expressing the galactose receptor (Gregoriadis & Neerunjun, 1975; Ghosh & Bachhawat, 1980). The latter studies were, however, carried out with preparations of multilamellar liposomes which were heterogeneous in size. Therefore only vesicles with a diameter small enough to allow passage through the fenestrations would have been able to interact selectively with hepatic parenchymal cells. In more recent studies, small unilamellar liposomes incorporating lactosylceramide were used (Szoka & Mayhew, 1983; Spanjer & Scherphof, 1983), but their half-lives (without the lactosylceramide component) in the circulation, reflecting non-specific uptake by the liver and spleen, may have been too short for maximum time of exposure to the target cells. Further, as