Surface-Modification of RIPL Peptide-Conjugated Liposomes to Achieve Steric Stabilization and pH Sensitivity

We have previously demonstrated that RIPL peptide-conjugated liposomes (RIPL-L) exhibited high hepsin (HPN) selectivity and enhanced intracellular drug delivery. In this study, surface modification of RIPL-L was performed to reduce plasma protein adsorption and off-target effects. For steric stabilization, distearoyl phosphatidylethanolamine (DSPE)-polyethylene glycol (PEG)2000 was used (5% molar ratio to total lipid) to prepare PEG-RIPL-L. Further, pH-sensitive oligopeptides [(HD)4 or (HE)4] were coupled to shield the RIPL polyarginine moiety, yielding (HD)4/PEG-RIPL-L and (HE)4/PEG-RIPL-L. All liposomal vesicles had a narrow and homogenous size distribution of approximately 140–150 nm, with zeta potentials varying from −15 to 36 mV. Increased plasma stability was observed upon quantifying the protein adsorbed onto liposomes by using a micro bicinchoninic acid assay. The (HD)4- and (HE)4-coupling capacity of PEG-RIPL-L was investigated by measuring the amount of oligopeptide involved in transient ionic complexation (TIC-oligopep) and zeta potential changes. As the molar ratio of (HD)4 and (HE)4 increased, TIC-oligopep increased and zeta potential decreased. (HE)4/PEG-RIPL-L were pH-sensitive, producing 1.6-fold greater cellular uptake of FITC-dextran by LNCaP cells at pH 6.8 than at pH 7.4. This result suggested that (HE)4/PEG-RIPL-L might provide a sterically stabilized, pH-sensitive drug carrier for HPN-specific cancer targeting.