Comparison of two methods for tumour-targeting peptide modification of liposomes

Liposomes decorated with tumour-targeting cell-penetrating peptides can enhance specific drug delivery at the tumour site. The TR peptide, c(RGDfK)-AGYLLGHINLHHLAHL(Aib)HHIL, is pH-sensitive and actively targets tumour cells that overexpress integrin receptor α_vβ₃, such as B16F10 melanoma cells. Liposomes can be modified with the TR peptide by two different methods: utilization of the cysteine residue on TR to link DSPE-PEG2000-Mal contained in the liposome formula (LIP^TR) or decoration of TR with a C18 stearyl chain (C18-TR) for direct insertion into the liposomal phospholipid bilayer through electrostatic and hydrophobic interactions (LIP^C18-TR). We found that both TR and C18-TR effectively reversed the surface charge of the liposomes when the systems encountered the low pH of the tumour microenvironment, but LIP^C18-TR exhibited a greater increase in the charge, which led to higher cellular uptake efficiency. Correspondingly, the IC₅₀ values of PTX-LIP^TR and PTX-LIP^C18-TR in B16F10 cells in vitro were 2.1-fold and 2.5-fold lower than that of the unmodified PTX-loaded liposomes (PTX-LIP), respectively, in an acidic microenvironment (pH 6.3). In B16F10 tumour-bearing mice, intravenous administration of PTX-LIP^TR and PTX-LIP^C18-TR (8 mg/kg PTX every other day for a total of 4 injections) caused tumour reduction ratios of 39.4% and 56.1%, respectively, compared to 20.8% after PTX-LIP administration. Thus, we demonstrated that TR peptide modification could improve the antitumour efficiency of liposomal delivery systems, with C18-TR presenting significantly better results. After investigating different modification methods, our data show that selecting an adequate method is vital even when the same molecule is used for decoration.