K. Maruyama et al., TARGETABILITY OF NOVEL IMMUNOLIPOSOMES MODIFIED WITH AMPHIPATHIC POLY(ETHYLENE GLYCOL)S CONJUGATED AT THEIR DISTAL TERMINALS TO MONOCLONAL-ANTIBODIES, Biochimica et biophysica acta. Biomembranes, 1234(1), 1995, pp. 74-80
Distearoyl-N-(3-carboxypropionoyl poly(ethylene glycol) succinyl)phosp
hatidylethanolamine (DSPE-PEG-COOH) was newly synthesized and used to
prepare novel immunoliposomes carrying monoclonal antibodies at the di
stal ends of the PEG chains (Type C). Liposomes were prepared from egg
phosphatidylcholine (ePC) and cholesterol (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 amounts of antibody m
olecules per liposome. Other immunoliposomes with PEG coating (Type B)
or without PEG coating (an earlier type of immunoliposome, Type A) we
re prepared for comparison. The average molecular weight of PEG in Typ
e B or C immunoliposomes was 2000. Type B and Type C liposomes without
antibodies showed prolonged circulation time and reduced reticulo-end
othelial system (RES) uptake owing to the presence of PEG. These three
different types of 34A-immunoliposomes with 30-35 antibody molecules
per vesicle were injected into mice to test the immunotargetability to
the lung. The efficiency of lung binding of 34A-Type B was one-half o
f that of 34A-Type A, though a large amount of 34A-Type B remained in
the blood circulation for a long time, suggesting that the steric hind
rance of PEG chains reduced not only the immunospecific antibody-antig
en binding, but also the RES uptake. The degree of lung binding of 34A
-Type C was about 1.3-fold higher than that of 34A-Type A, indicating
that recognition by the antibodies attached to the PEG terminal was no
t sterically hindered and that the free PEG (i.e., that not carrying a
ntibody) was effective in increasing the blood concentration of immuno
liposomes by enabling them to evade RES uptake. The latter phenomenon
was confirmed by using nonspecific antibody-Type C immunoliposomes (14
-Type C), which showed a high blood level for a long time. Our approac
h provides a simple means of conjugating antibodies directly to the di
stal end of PEG which is already bound to the liposome membrane, and s
hould contribute to the development of superior targetable drug delive
ry vehicles for use in diagnostics and therapy.