Synthesis and physicochemical evaluation of fluorinated lipopeptide precursors of ligands for microbubble targeting

Ligand-targeted microbubbles are focusing interest for molecular imaging and delivery of chemotherapeutics. Novel lipid-peptide conjugates (lipopeptides) that feature alternating serine-glycine (SG) n segments rather than the classical poly(oxyethylene) chains as linkers between the polar head of the lipid and a targeting ligand were recently proposed for liposome-mediated, selective delivery of anti-cancer drugs. Here, we report the synthesis of perfluoroalkylated lipopeptides (F-lipopeptides) bearing two hydrophobic chains (C n F2 n +1, n = 6, 7, 8, 1-3) grafted through a lysine moiety on a hydrophilic chain composed of a lysine-serine-serine (KSS) sequence followed by 5 SG sequences. These F-lipopeptides are precursors of targeting lipopeptide conjugates. A hydrocarbon counterpart with a C10H21 chain (4) was synthesized for comparison. The capacity for the F-lipopeptides to spontaneously adsorb at the air/water interface and to form monolayers in combination with dipalmitoylphosphatidylcholine (DPPC) was investigated. The F-lipopeptides 1-3demonstrated a markedly enhanced tendency to form monolayers at the air/water interface, with equilibrium surface pressures reaching ~7-10 mN m-1 as compared to less than 1 mN m-1 only for their hydrocarbon analog 4. The F-lipopeptides penetrate in the DPPC monolayers in both liquid expanded (LE) and liquid condensed (LC) phases without interfacial film destabilization. By contrast, the hydrocarbon analog provokes delipidation of the interfacial film. The commercial microbubble-based products used for contrast ultrasound imaging are all stabilized by a fluorocarbon gas. The ability of the F-lipopeptides to integrate fluorocarbon-stabilized phospholipid-shelled microbubbles was studied. Incorporation of F-lipopeptides 1-3 in microbubbles with a shell of DPPC and dipalmitoylphosphatidylethanolamine-PEG2000 decreased their mean diameter and increased their stability, the best results being obtained for the C8F17-bearing lipopeptide 3. By contrast, incorporation of the hydrocarbon lipopeptide led to microbubbles with a larger mean diameter, and significantly lower stability