Liposomes of α,ε-N,N’-di-stearoyl lysine-derived amide lipid and phospholipid: Incorporation of lipid A-ligand for bacterial targeting and sialic acid as PEGylation alternative for phagocytosis resistance

Phospholipid-based lipid nanoparticles (LNPs) are the most common delivery system for a variety of therapeutics. However, the adsorption of serum proteins leads to their rapid clearance by the immune system. Although inclusion of PEGylated lipid into the formulation prolongs circulation, the observation of anti-PEG antibodies in LNP recipients suggests that other alternatives are desired. In addition, degradation of the O-ester linkage and the oxidation of the unsaturated fatty acid in phospholipids may affect the long-term stability of LNPs and therefore lower the efficacy. As an alternative, we report here the synthesis of α,ε-N,N’-di-stearoyl lysine based amide lipids, and their application in liposome formulation, including incorporation of lipid A-binding ligand, polymyxin B nonapeptide (PMBN), and sialic acid via amide lipids. PMBN-liposome specifically targeted Gram-negative Acinetobacter baumannii bacteria but not Gram-positive Staphylococcus aureus, and the binding between PMBN-liposome and bacteria can be interrupted by serum proteins. Studies on the uptake of the amide lipid/phospholipid co-formulated liposomes by human leukemia monocytic THP-1 cells showed the critical role played by surface modification. The amine group on the liposome surface enhanced the cellular uptake in comparison to the sialic acid modification, and such difference become even greater after the adsorption of serum protein. Additionally, the sialic acid-modified liposomes, similar to PEGylation, showed more favorable biodistribution and retention time in mice than the phospholipid-only liposomes. Those co-formulated liposomes distributed over whole mouse bodies and lasted over 48 hrs, whereas the phospholipid-only liposomes rapidly migrated to the liver (5–15 mins). Thus, the results demonstrate that the sialic acid-containing lipid could be an alternative to PEGylated lipids in future nanomedicine formulations.