Immunoliposomes bearing polyethyleneglycol-coupled Fab' fragment show prolonged circulation time and high extravasation into targeted solid tumors in vivo.

We have developed a new type of long-circulating immunoliposome (Fab'-PEG immunoliposomes) which is efficiently extravasated into the targeted solid tumor in vivo. Small unilamellar liposomes (100-130 nm in diameter) were prepared from distearoylphosphatidylcholine (DSPC), cholesterol (CHOL) and a dipalmitoylphosphatidylethanolamine derivative of PEG with a terminal maleimidyl group (DPPE-PEG-Mal), and conjugated Fab' fragment of antibody. Inclusion of DPPE-PEG-Mal and linkage of the Fab' fragment instead of intact antibody to PEG terminals allowed the liposomes to evade RES uptake and remain in the circulation for a long time, resulting in enhanced accumulation of the liposomes in the solid tumor. Because of the ability of such Fab'-PEG immunoliposomes to target solid tumors, they appear highly attractive as carriers of not only chemotherapeutic agents, but also of macromolecular drugs.