RIPL peptide (IPLVVPLRRRRRRRRC)-conjugated liposomes for enhanced intracellular drug delivery to hepsin-expressing cancer cells.

To facilitate selective drug delivery to hepsin (Hpn)-expressing cancer cells, the RIPL peptide (IPLVVPLRRRRRRRRC; 16mer; 2.1 kDa) was synthesized as a novel cell penetrating/homing peptide (CPHP) and conjugated to a liposomal carrier. RIPL peptide-conjugated liposomes (RIPL-Lipo) were prepared by conjugating RIPL peptides to maleimide-derivatized liposomal vesicles via the thiol-maleimide reaction. Vesicle size and zeta potential were examined using a Zetasizer. Intracellular uptake specificity of the RIPL peptide, or RIPL-Lipo, was assessed by measuring mean fluorescence intensity (MFI) after treatment with a fluorescent marker in various cell lines: SK-OV-3, MCF-7, and LNCaP for Hpn(+); DU145, PC3, and HaCaT for Hpn(-). FITC-dextran was used as a model compound. Selective translocational behavior of RIPL-Lipo to LNCaP cells was visualized by fluorescence microscopy and confocal laser scanning microscopy. Cytotoxicities of the RIPL peptide and RIPL-Lipo were evaluated by WST-1 assay. RIPL peptides exhibited significant Hpn-selectivity. RIPL-Lipo systems were of positively charged nanodispersion (165 nm in average; 6-24 mV depending on RIPL conjugation ratio). RIPL-Lipo with the conjugation of 2300 peptide molecules revealed the greatest MFI in all cell lines tested. Cellular uptake of RIPL-Lipo increased by 20- to 70-fold in Hpn(+) cells, and 5- to 7-fold in Hpn(-) cells, compared to the uptake of FITC-dextran. Cytosolic internalization of RIPL-Lipo was time-dependent: bound instantly; internalized within 30 min; distributed throughout the cytoplasm after 1 h. Cytotoxicities of RIPL peptide (up to 50 μM) and RIPL-Lipo (up to 10%) were minor (cell viability >90%) in LNCaP and HaCaT cells. By employing a novel CPHP, the RIPL-Lipo system was successfully developed for Hpn-specific drug delivery.