EFFECTS ON THE MURINE MONONUCLEAR PHAGOCYTE SYSTEM OF CHRONIC ADMINISTRATION OF LIPOSOMES CONTAINING CYTOTOXIC DRUG OR LIPID-A COMPARED WITH EMPTY LIPOSOMES

In experiments designed to examine the adverse effects of chronic liposome administration in vivo on the mononuclear phagocyte system (reticuloendothelial system), the presence of drug entrapped in the liposomes may increase the level of reticuloendothelial impairment. We have compared the effects on the mononuclear phagocyte system in mice of chronic administration of empty liposomes with the effects of liposomes containing the anti-leishmanial drug meglumine antimoniate. We have also examined the effect on the mononuclear phagocyte system of continued injections of liposomes containing lipid A, a component of bacterial lipopolysaccharide, which is responsible for macrophage activation. Ten intravenous injections of multilamellar liposomes composed of dipalmitoylphosphatidylcholine and cholesterol (1:0.75 M ratio) were given to ICR mice over a 25-day period. Two individual groups of mice received endotoxin-free liposomes in which meglumine antimoniate was either present or absent. One additional group received liposomes containing lipid A derived from Escherichia coli lipopolysaccharide. A control group received sterile saline injections. In each group, a depression of the phagocytic index, as measured by reduction of uptake of particulate carbon, was observed among some of the individual animals 24 h after the first injection. In many mice a marked splenomegaly was observed. A depressed phagocytic index and splenomegaly were most marked for mice receiving lipid A liposomes. However, there was a large individual variability among mice receiving these preparations and some mice in each group had normal spleen size and a nearly normal phagocytic index. Tissue distribution of liposomes containing [14C]dipalmitoylphosphatidylcholine as a phospholipid marker was examined in all groups in mice 24 h after the last injection. Mice receiving liposomes with entrapped meglumine antimoniate showed decreased liver and increased spleen uptake of radiolabel as compared with controls. This has previously been suggested to be an indication of reticuloendothelial blockade. Mice receiving liposomes without meglumine antimoniate had tissue distributions of liposomes similar to control mice, and mice receiving liposomes containing lipid A had greatly decreased spleen uptake and increased liver uptake of liposomes as compared with control mice and mice receiving endotoxin-free liposomes (an indication of reticuloendothelial stimulation), suggesting that liposomes are being treated differently from particulate carbon in the presence of lipid A.