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An Efficient Method for Loading Indium-111 into Liposomes Using Acetylacetone

University of Washington, Seattle, Washington 98195

Correspondence: For reprints contact: Karl J. Hwang. Dept. of Pharmaceutics, Univ. of Washington, Seattle, WA 98195.

ABSTRACT

The efficient loading of high levels on 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 1 mM nitrilotriacetic acid. The loading process is carried out in isotonic saline, 10 mM Tris-HCl, 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 sphingomyelin/cholesterol, our results indicate that the loaded liposomes retain the entrapped internally bound In-111 even after incubation with serum at 37°C for 24 hr.