Dipalmitoylation of a cellular uptake-mediating apolipoprotein E-derived peptide as a promising modification for stable anchorage in liposomal drug carriers

Liposomes equipped with cellular uptake-mediating peptidic vector compounds have attracted much attention as target-specific drug delivery systems. Aside from the development of the target recognition motif itself, vector coupling to liposomes while conserving the active conformation constitutes an important element in carrier development. To elucidate the most efficient way for adsorptive peptide binding to liposomes, we synthesized and characterized two-domain peptides comprising a cationic sequence derived from the binding domain of apolipoprotein E (apoE) for the low-density lipoprotein receptor and different lipid-binding motifs, that is, an amphipathic helix, a transmembrane helix, single fatty acids or two palmitoyl chains. Peptide properties considered relevant for peptide-liposome complexes to initiate an endocytotic cellular uptake such as lipid binding, helicity, stability of anchorage, bilayer-disturbing activity, and toxicity showed that the dipalmitoyl derivative was the most suitable to associate the apoE peptide to the surface of liposomes. The peptide showed pronounced lipid affinity and was stably anchored within the lipid bilayer on a time scale of at least 30 min. The helicity of about 40% in the lipid-bound state and the location of the amphipathic helix on the liposomal surface provided the prerequisites for interaction of the complex with the cell surface-located receptor. The concentration of the dipalmitoylated peptide to permeabilize neutral lipid bilayers (lipid concentration 25 μM) was 0.06 μM and a 2 μM concentration reduced cell viability to about 80%. Efficient internalization of liposomes bearing about 180 peptide derivatives on the surface into brain capillary endothelial cells was monitored by confocal laser scanning microscopy. The concept of complexation using dipalmitoylated peptides may offer an efficient substitute to covalent vector coupling and a prospective way to optimize the capacity of liposomes as drug delivery systems also for different targets.