Folate-liposome-mediated antisense oligodeoxynucleotide targeting to cancer cells: evaluation in vitro and in vivo

The objective of this study was to investigate the use of folate-targeted liposomes for the delivery of encapsulated oligonucleotides to folate receptor (FR)-positive tumor cells in vitro and in vivo. This project involved the synthesis and biological evaluation of many folate-PEG-lipid conjugates, where the chemical form of the folate moiety (pteroate) and the length of the PEG linker chain were varied widely. Folate-targeted oligonucleotide-containing liposomes were prepared using conventional methods, and the extent of cell uptake was evaluated using, among others, the FR positive KB cell line. Oligonucleotide-loaded folate-targeted liposomes were found to rapidly associate with the KB cells, and saturation was typically reached within the first hour of incubation at 37 degrees C. Nearly 100,000 liposomes per cell were bound or internalized at saturation. Importantly, cell association was blocked by a large excess folic acid, thus reflecting the FR-specific nature of the cell interaction. Full targeting potential was achieved with PEG linkers as low as 1000 in molecular weight, and pteroates bearing glycine or gamma-aminobutyryl residues juxtaposed to the pteroic acid moiety were also effective for targeting, provided that a terminal cysteine moiety was present at the distal end of the PEG chain for added hydrophilicity. When tested in vivo, folate-targeted liposomes were found to deliver approximately 1.8-fold more oligonucleotide to the livers of nude mice (relative to the nontargeted PEG-containing formulations); however, no improvement in KB tumor uptake was observed. We conclude from these results that folate liposomes can effectively deliver oligonucleotides into folate receptor-bearing cells in vitro, but additional barriers exist in vivo that prevent or decrease effective tumor uptake and retention.