Efficient intracellular delivery of 5-fluorodeoxyuridine into colon cancer cells by targeted immunoliposomes

Immunoliposomes, liposomes with monoclonal antibodies attached, are being developed for targeting the anti-cancer drug 5-fluoro-2'-deoxyuridine (FUdR) to colon cancer cells. A monoclonal antibody against the rat colon carcinoma CC531 was covalently coupled to liposomes containing a dipalmitoylated derivative of the anti-cancer drug FUdR (FUdR-dP) as a prodrug in their bilayers. We studied the association with the tumor cells of different types of immunoliposomes varying in the position and orientation of the antibody at the liposome surface. We also assessed the in vitro anti-tumor activity of these liposomes and the mechanism by which the active drug FUdR is delivered intracellularly. Specific binding of the immunoliposomes to the tumor cells was observed. Immunoliposomes containing FUdR-dP caused a much stronger inhibition of CC531 cell growth in vitro than FUdR-dP in non-targeted liposomes. After binding to the cell surface only limited amounts of the immunoliposomes were internalized. By contrast, already within 24 h immunoliposome-incorporated FUdR-dP was hydrolyzed virtually completely to the parent drug FUdR, intracellularly. The mechanism of intracellular delivery of the drug most likely involves a selective transfer of the lipophilic prodrug from the liposomes to the cell membrane and subsequent intracellular processing. In conclusion, we developed a targeted liposomal formulation, which is able to deliver FUdR to colon carcinoma cells intracellularly with high efficiency, without the need for the cells to internalize the liposomes as such. This approach may be attractive for other lipophilic anti-cancer (pro)drugs. In this sense our system also serves as a model for the development of new lipid-based drug delivery systems for anti-cancer therapy.