Abstract
Drug delivery to specific cells by immunoliposomes represents a potentially attractive mode of therapy. However, though immunoliposomes are effective in specific binding to target cells in vitro, their targeting efficiency in vivo is relatively low. We have recently developed a new type of long-circulating immunoliposomes (Type C) which can effectively bind to the designated target site in vivo. This was achieved by the use of newly synthesized DSPE-PEG-COOH to couple antibodies directly to the distal terminal of PEG chains, which allow the liposome to evade the RES uptake and achieve prolonged circulation. Liposomes were prepared from ePC and CH (2 : 1, m/m)containing 6 mol% of DSPE-PEG-COOH, and a monoclonal IgG antibody, 34A, which is highly specific to pulmonary endothelial cells, was conjugated to the carboxyl groups of DSPE-PEG-COOH to give various amount of antibody molecules per liposome. PEG-COOH-liposomes without antibodies showed prolonged circulation time and reduced RES uptake owing to the presence of PEG. The degree of lung binding of 34A-Type C was increased with increasing coupling amount of antibody. Antibody density is an important factor for target binding. We established the preparation of adriamycin (ADR) encapsulated 34A-Type C immunoliposomes. ADR encapsulation was done by employing an ammonium sulfate (AS) gradient method. PEG-COOH liposomes were prepared in 120 mM AS and attached 34A to make 34A-Type C. External medium of liposome was replaced to 120 μM AS for the creation of AS gradient between inside and outside of liposome membrane and ADR solution was added into external medium. The ADR loading efficiency was 95%. ADR level in lung after i.v. injection of ADR-34A-Type C was much higher than that of control ADR liposome and free ADR.
