Irreversible coupling of immunoglobulin fragments to preformed vesicles. An improved method for liposome targeting.

Rabbit Fab' antibody fragments were covalently couple to preformed large unilamellar vesicles using a new sulfhydryl-reactive phospholipid derivative N-[4-(p-maleimidophenyl)butyryl]phosphatidylethanolamine (MPB-PE). A highly efficient reaction between the sulfhydryl group on each Fab' fragment and the maleimide moiety of MPB-PE molecules incorporated at a low concentration in vesicle bilayers led to the formation of a highly stable Fab'-vesicle linkage. Coupling ratios in excess of 250 micrograms of Fab'/mumol of vesicle phospholipid were reproduciably obtained without vesicle aggregation. Bound Fab' fragments did not elute from vesicles in serum or in the presence of reducing agents (dithiothreitol or mercaptoethanol). Vesicles bearing Fab' fragments raised against specific human erythrocyte surface determinants bound selectively to human erythrocytes under physiological conditions (isotonic medium containing 50% human serum, pH 7.4) with minimal leakage of vesicle contents. Advantages of the present coupling method are discussed in relationship to our effects to optimize the properties of liposomes as a carrier system.