The development of IL-2 conjugated liposomes for therapeutic purposes

A unique immunoliposome has been developed as a drug delivery vehicle for immunotherapy. Human recombinant interleukin-2 (IL-2) has been chemically coupled to the external surface of small unilamellar vesicles (SUVs) containing methotrexate as a candidate immunosuppresive agent in order to specifically direct the drug-bearing liposome to activated T-cells expressing the high affinity IL-2 receptor. This drug delivery system is designed to deliver an immunosuppressive agent to those cells that actively participate in disorders such as graft rejection without delivering an effective but potentially toxic drug to all cells of the immune system as well as other healthy tissues. IL-2 was chemically modified with succinimidyl 4-[p-maleidophenyl butyrate](SMPB) while the receptor binding domain on IL-2 was protected by monoclonal anti-IL-2 bound to Protein A-Silica Gel. The antibody recognizes the receptor binding domain of the IL-2 molecule. The IL-2 was derivatized with S-succinimidyl-S-thioacetate (SATA) in order to add an acetyl thioester group to the lipid and create the complex. The derivatized lipid (SATA-PE) was then part of the liposome formulation containing DSPC:cholesterol: SATA-PE at a mole ratio of 1.5:1.0:0.26. SMPB-IL-2 was covalently coupled to the external surface of the SUV after deacetylation of the thioester moiety at pH 7.4 in PBS. Liposomes prepared by sonication or extrusion had an average diameter of 46-50 nm. SUV-IL-2 bound to the high affinity IL-2 receptor as measured by competitive binding assays and Scatchard analysis using 111InCl2-loaded liposomes The preparation exhibited a binding constant of 30 pM, consistent with values for free IL-2 cited in the literature. SUV IL-2 could be used as the sole source of IL-2 for the murine CTLL-2 T-cell line or for human mitogen-activated PBLs. The presence of IL-2 coupled to the surface was absolutely required for delivery of the drug to the cell. When methotrexate was encapsulated within the internal aqueous space, receptor-mediated endocytosis led to the inhibition of proliferation due to delivery of MTX to the cytoplasm of the cell. More than 90% of the methotrexate was retained within the liposome during storage over a 24-h period at 4 degrees C. This immunoliposome represents a new class of cell specific immunoliposomes whose entry into the cell is controlled by a cell surface receptor.