Properties and Biological Effects of Liposomes and their uses in Pharmacology and Toxicology

Liposome filtration. Dependence on transition temperature

Liposomes formed by vortexing and passed through polycarbonate surface retention membranes showed appreciable differences in filtration behavior depending on the temperature of filtration relation relative to the liposome gel-liquid crystal transition temperature. Below transition, liposomes were filterable and size distributions could be determined; the cumulative volume distributions were log-normal. Above transition, liposomes were not filterable: smaller liposomes were formed until a limiting size was reached. These results suggest that liquid crystal liposome size distributions cannot be determined by filtration. This filtration behavior is a physical property of liposomes, related to the gel-liquid crystal transition, not previously reported. This property could be exploited as a new method for controlling liposome size distributions, but the implications for lipid membranes, including biological membranes, are general.

Thin-layer chromatography with agarose gels. A quick, simple method for evaluating liposome size

Thin-layer gels can be made with agarose and used to assess within a few minutes the efficiency with which multilamellar vesicles are converted to small unilamellar ones by sonication. A fluroescent lipid marker or vesicle-encapsulated solute permits continuous monitoring of the chromatography. Advantages over agarose gel column chromatography include speed of analysis, small sample size, the possibility of running multiple samples and simultaneously, and direct accessibility to fluorescence microscopy. This approach should also be useful in the study of liposome-lipoprotein interactions and in affinity chromatography of liposomes.

Liposomes as vehicles for cellular incorporation of biologically active macromolecules

Lipid vesicles (liposomes) have recently been shown to be a useful vehicle for the delivery of a variety of compounds to cultured cells. Using large unilamellar vesicles composed of phosphatidylserine [LUV(PS)] we were able to encapsulate poliovirus and purified poliovirus ribonucleic acid (RNA) and show that it can be delivered efficiently to cells in an infectious form. LUV-entrapped poliovirus RNA produced infectious titers 100-fold higher than comparable RNA preparations delivered to cells by other techniques. We have made a quantitative analysis of the uptake and infectivity of the vesicle-encapsulated RNA by using various ratios of RNA copies per vesicle and by determining the percentage uptake of labelled lipid and RNA by HeLa cells.

Pulmonary localization of intravenously injected liposomes

Clearance from the circulation and tissue distribution of liposomes of differing size, surface charge, and composition injected IV into BALB/c mice have been examined. Large (500-3,400 nm diameter) liposomes are cleared from the circulation more rapidly than small sonicated liposomes (less than 100 nm diameter). Negatively charged liposomes are cleared more rapidly than neutral or positively charged liposomes of the same size. The major site at which liposomes injected IV accumulate, irrespective of their composition, is the liver. Accumulation of liposomes at extrahepatic sites, such as the lung, is influenced by liposome size, charge, and composition. Optimal localization and retention of liposomes in the lung were achieved with large negatively charged multilamellar and reversed evaporation phase liposomes containing phosphatidylserine (PS/PC, 3:7 mol ratio; PS/PC/LL 4.95:4.95:0.1 mol ratio).