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Mechanism of Ionophoric Transport of Indium-111 Cations through a Lipid Bilayer Membrane

School of Pharmacy, University of Southern California, Los Angeles, California

Correspondence: For reprints contact: Karl J. Hwang, School of Pharmacy, 1985 Zonal Ave., University of Southern California, Los Angeles, CA 90033.

ABSTRACT

The use of mobile ionophores to facilitate the transport of ¹¹¹In through a lipid bilayer membrane has broad applications in liposome technology and cell labeling. However, the mechanism of such ionophore-mediated transport of ¹¹¹In through a lipid bilayer membrane is not completely clear. The present report describes the correlations of the behaviors of ionophoric loading of ¹¹¹In into liposomes with the lipophilicity and the indium-binding affinity of three ionophores, namely, 8-hydroxyquinoline, acetylacetone, and tropolone. Our results suggest that the mechanism of the ionophoric transport of ¹¹¹In through a lipid bilayer membrane involves the rapid exchange of ¹¹¹In cations among the ionophores in both the aqueous solution and the lipid bilayer. Furthermore, the effectiveness of an ionophore in facilitating the transport of ¹¹¹In from the external aqueous compartment to the entrapped nitrilotriacetic acid depends not only on the lipophilicity of the [¹¹¹In]ionophore complex, but also on the lipophilicity of the free ionophore itself and the competiton of ¹¹¹In between nitrilotriacetic acid inside the inner aqueous compartment of the liposome and the ionophore imbedded in the lipid bilayer membrane of the liposome.