Disposition of intact liposomes of different compositions and of liposomal degradation products

Small unilamellar liposomes containing bovine serum albumin were prepared by a new double-emulsion technique and administered to mice and rats in intravenous injections. The elimination of intact liposomes, the association of phospholipid marker with lipoproteins, and the appearance of released internal marker and its degradation products were followed by column chromatography of plasma samples. In vitro labelled lipoproteins were administered to the animals in intravenous injections together with free bovine serum albumin and the elimination of the two substances was studied by closely related techniques. The clearance of intact PC:PS (4:1) liposomes from plasma was biphasic and much faster than that of labelled lipoproteins and bovine serum albumin either originating from liposomes or injected as such. The second elimination phase for these liposomes was barely detectable by our analytical methods. In contrast, DSPC:CHOL (2:1) liposomes showed a very significant second-phase elimination, with a half-life of 12 hr for the intact liposomes. In tissue distribution studies in mice, the major accumulation of liposomal markers was found in the liver and spleen, and less in the kidneys and intestinal wall. Uptake into liver and spleen appeared to be due to the uptake of intact liposomes, whereas the uptake into kidneys and gut wall was caused by the uptake of liposomal degradation products. The uptake of PC:PS (4:1) liposomes into the liver was higher than that of DSPC:CHOL (2:1) liposomes; the opposite was the case with their uptake into the spleen. In rats, too, liposomes of different compositions showed significant variations in stability and in plasma half-lives of intact liposomes. Generally, there was a considerable increase in the liposomal stability in the presence of cholesterol and when use was made of a phospholipid with a high transition temperature.