Synthesis of short polyoxyethylene-based heterobifunctional cross-linking reagents. Application to the coupling of peptides to liposomes

We describe the synthesis of [2-[2-[2-[(2-bromoacetyl)amino]ethoxy]et acetic acid (7), [2-[2-(2,5-dioxo-2,5-dihydropyrrol-1-yl)ethoxy acetic acid (11), and [2-[2-(pyridin-2-yldisulfanyl)-ethoxy]ethoxy] acetic acid (16), three new thiol-reactive heterobifunctional reagents, and the preparation of their corresponding dipalmitoylphosphatidylethanolamine derivatives (8, 12, and 17). Such phospholipid amide derivatives were aimed to be incorporated into the bilayers of liposomal constructs used for immunization with e.g. synthetic peptides. The spacer arms introduced by 8, 12, and 17 are hydrophilic polyoxyethylene chains of variable lengths that were expected to provide a good accessibility to their conjugates and have a lesser intrinsic immunogenicity than the spacer introduced by N-[4-(p-maleimidophenyl)butyryl]phosphatidylethanolamine (MPB-PE), a classical reagent used for conjugation of ligands to the surface of liposomes. Such an immunogenicity might be prejudicial (e.g. carrier-induced epitopic suppression) to the development of synthetic vaccination formulations. Moreover, the derivatives 8, 12, and 17 allowed the coupling of peptides, bearing a thiol function, to their liposomal carrier via two types of linkages, i.e. stable thio ether (8 and 12) and bioreducible disulfide (17) bonds; this might be of importance in the mechanism of antigen presentation by competent cells. Using CG-IRGERA as a model peptide, the rate of coupling to 8, 12, and 17 was assessed as a function of pH.