Liposomal targeting of leukaemia HL60 cells induced by transferrin-receptor endocytosis

A liposomal carrier system able to interact specifically with HL60 leukaemia cells was produced using small unilamellar liposomes made of pure phospholipids chemically cross-linked to human transferrin. The conjugation of transferrin to liposomes was carried out using N-succinimidyl 3-(2-pyridyldithio)-propionate and 2-iminothiolane as activating agents for the liposomes and the protein. The reaction occurred under conditions set to covalently link on the surface of a single vesicle a limited number (one to ten) of transferrin molecules, as verified by means of electron microscopy and immunoenzymic measurements. Before conjugation, the ultrastructure of the liposomes, and the content and distribution of the amino groups within the bilayer, were determined. The reactivity of the liposomes towards amino-derivatizing or thiolating compounds was also measured. Kinetic spectroscopic measurements confirmed that the distribution of the phosphatidylethanolamine in the vesicle bilayer is asymmetrical: 22% of phosphatidylethanolamine was found exposed to the external surface of the liposomes and accessible to the cross-linker. The modified liposomes were able to interact specifically with the cells and to be internalized by active receptor-mediated endocytosis, as demonstrated by the full inhibition of internalization induced by free transferrin.