POLY(ETHYLENE GLYCOL)-GRAFTED LIPOSOMES WITH OLIGOPEPTIDE OR OLIGOSACCHARIDE LIGANDS APPENDED TO THE TERMINI OF THE POLYMER-CHAINS

Novel conjugates tailor-made for inclusion in liposomal formulations, containing distearoylphosphatidylethanolamine (DSPE) as a lipid anchor , heterobifunctional polyethylene glycol (PEG) with a molecular weight of 2000 as a linking moiety, and a biological cell adhesive ligand [Y IGSR peptide or Sialyl Lewis(x) oligosaccharide (SLX)], were synthesiz ed. They were characterized by NMR, chromatography, and matrix-assiste d laser desorption ionization-time of flight mass spectrometry (MALDI- TOFMS). Inclusion of either of the ligand-PEG-lipid conjugates (2 mol %) in a lecithin/cholesterol/methoxy-PEG(2000)-DSPE (55:40:3 mole rati o) lipid mixture followed by preparation of unilamellar vesicles (100 nm) resulted in positioning of 55% of the YIGSR and 63% of the SLX lig ands on the periphery of the outer surface-grafted polymeric ''brush'' , as determined by a combination of specific enzymatic alterations of each ligand and HPLC. Similar densities of ligand-bearing PEG chains w ere incorporated into liposomes by simply incubating (37 degrees C, 5 h) either one of the ligand-PEG-lipid conjugates with preformed lipid vesicles. This conjugate insertion process was aggregation free. Using enzymatic derivatization-HPLC, it was demonstrated that all the ligan ds incorporated into lipid membranes by this new approach were positio ned exclusively on the outer leaflet of the liposomal bilayers. Since liposomes of this type are intended for in vivo use as long-circulatin g, Ligand-presenting platforms, the insertion approach is preferable b ecause of the more efficient utilization of ligand-PEG-lipid conjugate s.