Selective transfer of a lipophilic prodrug of 5-fluorodeoxyuridine from immunoliposomes to colon cancer cells

A monoclonal antibody against the rat colon carcinoma CC531 was covalently coupled to liposomes containing a dipalmitoylated derivative of the antican cer drug FUdR as a prodrug in their bilayers. We investigated the in vitro interaction of these liposomes with CC531 target cells and the mechanism by which they deliver the active drug FUdR intracellularly to the cells by mo nitoring the fate of the liposomal bilayer markers cholesterol-[C-14]oleate and [H-3]cholesteryloleylether as well as the H-3-labeled prodrug and coll oidal gold as an encapsulated liposome marker. After binding of the immunol iposomes to the cell surface, only limited amounts were internalized as dem onstrated by a low level of hydrolysis of liposomal cholesterol ester and b y morphological studies employing colloidal gold-labeled immunoliposomes. B y contrast, already within 24 h immunoliposome-incorporated FUdR-dP was hyd rolyzed virtually completely to the parent drug FUdR intracellularly. This process was inhibited by a variety of endocytosis inhibitors, indicating th at the prodrug enters and is processed by the cells by a mechanism involvin g an endocytic process, resulting in intracellular FUdR concentrations up t o 3000-fold higher than those in the medium. Immunoliposomes containing pol y(ethyleneglycol) (PEG) chains on their surface, with the antibody coupled either directly to the bilayer or at the distal end of the PEG chains were able to deliver the prodrug into the tumor cells at the same rate as immuno liposomes without PEG. Based on these observations, we tentatively conclude that during the interaction of the immunoliposomes with the tumor cells th e lipophilic prodrug FUdR-dP is selectively transferred to the cell surface and subsequently internalized by constitutive endocytic or pinocytic invag inations of the plasma membrane, thus ultimately delivering the prodrug to a lysosomal compartment where hydrolysis and release of parent drug takes p lace. This concept allows for an efficient delivery of a liposome-associate d drug without the need for the liposome as such to be internalized by the cells. (C) 1999 Elsevier Science B.V. All rights reserved.