Characterization of In Vivo Immunoliposome Targeting to Pulmonary Endothelium

Abstract

Two rat monoclonal antibodies, 34A and 201B, which specifically bind to a surface glycoprotein (gp112) of the pulmonary endothelial cell surface, have been coupled to unilamellar liposomes of ~0.25 μm in diameter. The 34A- and 201B-liposomes (monoclonal antibodies 273-34A and 411-201B, respectively), but not antibody-free liposomes and liposomes coupled to 14, a nonspecific monoclonal antibody, accumulate efficiently (~30% injected dose) in the lung of mice which have been injected via the tail vein. Immunoliposome targeting to lung is demonstrated both by using a 125l-labeled lipid marker and an entrapped water-soluble marker. Lung accumulation of 34A-liposomes is completely blocked by a preincubation of free antibody 34A, but not antibody 14, indicating that the immunoliposome accumulation at the target site is immunospecific. Time course studies have revealed that 34A-liposomes bind to lung antigens within 1min after injection, indicating that the target binding takes place during the first few passages of immunoliposomes through the lung capillary bed. Unbound immunoliposomes are taken up by liver and spleen within 3–5min after injection. The level of lung accumulation increases significantly as the protein:lipid ratio of the immunoliposome increases. Approximately 50% of injected dose is accumulated in lung for 34A-liposomes, with an average of 935 antibody molecules per liposome. Immunoliposomes of larger size accumulate in lung more significantly than those of smaller size. Injection with higher doses also enhances the level of lung accumulation. Although the lung accumulation of these immunmoliposomes is clearly not due to capillary embolism, conditions which favor slower passage of liposomes through the lung capillaries (larger liposome size and higher injection dose) result in high levels of lung binding. These considerations are quite different from immunoliposome binding to the target cells in vitro. Finally, repeated weekly injections (more than twice) of immunoliposomes into the same mouse have failed to show significant lung accumulation, probably due to the generation of anti-rat IgG antibodies by the mouse.