An efficient method for loading indium-111 into liposomes using acetylacetone.

The efficient loading of high levels of In-111 into the inner aqueous compartment of liposomes can be achieved using a water-soluble lipophilic chelate, acetylacetone. The loading method involves the incubation of the acetylacetone-In-111 complex with liposomes entrapping 1mM nitrilotriacetic acid. The loading process is carried out in isotonic saline, 10 mM Tris-HCI, pH 7.6, at room temperature. Results indicate that In-111 ions are concentrated in the internal aqueous compartment of liposomes by acetylacetone, which mediates the transport of In-111 ions across the outermost lipid bilayer of liposomes, permitting subsequent transfer to the encapsulated nitrilotriacetic acid. As much as 90% of the acetylacetone-chelated In-111 becomes internalized in the aqueous compartment of liposomes and bound by entrapped nitrilotriacetic acid. Using liposomes made from bovine brain shingomyelin/cholesterol, our results indicate that the loaded liposomes retain the entrapped internally bound In-111 even after incubation with serum at 37 degrees C for 24 hr.