Liposome disposition in vivo. III. Dose and vesicle-size effects

The effect of lipid dose (4,3-512.8 mumol total lipid/kg body weight), administered intravenously as liposomes encapsulating radioactive inulin, upon the ability of mouse organs to bind and/or take-up the radioactive label has been studied in vivo. Three different liposome diameters were investigated: 0.46 micrometers (L), 0.16 micrometers (M) and 0.058 micrometers(S). All liposomes were negatively charged with lipid composition of phosphatidylcholine/phosphatidic acid/cholesterol/alpha-tocopherol in the molar ration 4 : 1 : 5 : 0.1 or 4 : 1 : 1 : 0.05. Overall radioactive label disposition after 2 h was consistent with localization predominantly in the reticuloendothelial system. A saturation of liver with increasing lipid dose was demonstrated for all three sizes, together with a corresponding increase in blood levels. Spleen radioactivity increased with increasing dose of L- and M-liposomes, but decreased for increasing doses of S-liposomes. Levels in residual carcass exhibited no trend. It was noted that by adjusting liposomal lipid dose and vesicle diameter the percentage of administered dose present in blood could be varied 733-fold, that in spleen 9-fold, liver 4-fold. Stability in vivo was ranked L greater than M greater than S-liposomes. Correction for differences of in vivo stability reduced the differences in organ accumulation between the three liposome sizes. The organ accumulation pattern suggested a dose- and diameter-dependent mechanism for liposome disposition. It was expected that when doses of fixed liposome composition were expressed as number of liposomes or their total surface area, organ saturation patterns would be similar. However, re-plotting the percent dose values for liver and spleen versus the number of liposomes administered revealed a saturation pattern for L-, M- and S-liposomes which was different in each case. Plotting the data versus the total surface area of the dose revealed a similar disposition pattern for L-, M- and S-liposomes in liver and L- and M-liposomes in spleen. The data indicate that in addition to composition, the lipid dose, total liposomal surface area and effective mean diameter are important pharmacokinetic variables. Further, the optimization of the therapeutic index of an encapsulated agent or target-tissue delivery via liposomes will require consideration of both the surface area and diameter of the liposome doses together with liposome composition.