We have developed a new type of PEG-immunoliposome carrying monoclonal antibodies or their fragments (F(ab')2, Fab') at the distal ends of the PEG chains (Type C). Distearoylphosphatidylethanolamine derivatives of PEG with carboxyl group (DSPE-PEG-COOH) or dipalmitoyl phosphatidylethanolamine derivatives of PEG with maleimidyl group (DPPE-PEG-Mal) at the PEG terminal were newly synthesized. Small unilamellar liposomes (90-130 nm in diameter) were prepared from distearoyl phosphatidylcholine and cholesterol (2:1, mim) containing 6 mol% of DSPE-PEG-COOH or DPPE-PEG-Mal. To target to the vascular endothelial lung surface as a model accessible site, 34A antibody, which is highly specific to mouse pulmonary endothelial cells, was conjugated to PEG-liposome (34A-Type C). The degree of lung binding of 34A-Type C in BALB/c mice was significantly higher than that of the 34A-Type A which is an ordinary type immunoliposome (without PEG derivatives). To target to the solid tumor tissue as a model of the less accessible site, 21B2 antibody which is anti-human CEA and its Fab' fragment were used. The targeting ability of Fab'-Type C was examined by using CEA-positive human gastric cancer strain MKN-45 cells inoculated Into BALB/c nu/nu mice. Fab'-Type C showed the low RES uptake and the long circulation time, and resulted in enhanced accumulation of the liposomes in the solid tumor. The small Fab'-Type C could predominantly pass through the leaky tumor endothelium by passive convective transport.
