Distribution and Fate of Synthetic Lipid Vesicles in the Mouse: A Combined Radionuclide and Spin Label Study

Abstract

Single compartmental spherules of various lipid constituents (vesicles), enclosing (99m)TcO(4) (-) as a radioactive marker, were injected intravenously into C(3)H mice, and the distribution of radioactivity was studied. About 25% of the administered radioactivity was present in the liver 5 min and 30 min after the injection of vesicles composed of phosphatidylcholine and gangliosides, which were sonicated for 5 min (standard preparation). About 10-20% of the radioactivity remained in the circulation. By use of a nonradioactive spin label (tempocholine) enclosed within vesicles, intact vesicles were demonstrated in the circulation for 46 min after intravenous injection. The distribution of radioactivity from (99m)TcO(4) (-) inside vesicles is very different from that of free (99m)TcO(4) (-) or of (99m)Tc sulfur colloid. Increase in the length of sonication or incorporation of cholesterol into the wall of the vesicles enhanced hepatic levels and reduced blood levels of radioactivity. These same manipulations also slowed the rate of transfer of (99m)TcO(4) (-) out of vesicles in dialysis experiments in vitro. Addition of phosphatidic acid, phosphatidylethanolamine, or phosphatidylserine to the standard constituents did not greatly alter the distribution of radioactivity in vivo but did increase the number and type of active coupling sites on the outside of the vesicle. The results indicate that vesicles might be valuable as carriers of diagnostic or therapeutic agents.