Endothelial cells in the angiogenic vessels of solid tumors over-express several proteins, which could be recognized by some peptide ligands. In this study, the targeting properties of two peptides, RGD （arginine-glycine-aspartic acid） and NGR （asparagine-glycine-arginine）, towards human umbilical vein endothelial cells （HUVEC） were compared in vitro using doxorubicin entrapped liposomes as vehicles. The doxorubicin-loaded sterically stabilized liposomes （SSL-DOX） and RGD or NGR modified liposomes （RGD-SSL-DOX or NGR-SSL-DOX） were prepared and characterized. The studied properties included particle size, zeta potential, encapsulation efficiency and in vitro release rate. Flow cytometry, confocal microscopy and SRB assay were used on HUVEC to assess the targeting effect of the two peptides towards endothelial cells of tumor vasculature. All of the liposomes prepared in this study were obtained with encapsulation efficiencies of above 98%, particle sizes of about 65-75 nm and slight negative surface charges. The in vitro release results demonstrated that the modification of RGD or NGR did not alter the release behaviors of liposomes. It was observed in flow cytometry that the uptake of doxorubicin by HUVEC from SSL-DOX, NGR-SSL-DOX, RGD-SSL-DOX and doxorubicin solution followed the order of doxorubicin solution〉RGD-SSL-DOX 〉NGR-SSL-DOX〉SSL-DOX, and the intemalized doxorubicin distributed in both nuclei and cytoplasm for ligand modified SSL and only in nuclei for non-targeted SSL. The order of cytotoxicity in SRB assay was the same as that of the uptake study. The characterization study indicated that modifications did not significantly change the properties of the sterically stabilized liposomes. HUVEC treated with both modified liposomes showed higher uptake of doxorubicin as compared to those with SSL-DOX as a result of the receptor-mediated endocytosis. Moreover, RGD-SSL-DOX exhibited better targeting effect than NGR-SSL-DOX.
