Targeting multidrug resistant tumor cells with a recombinant single-chain FV fragment directed to P-glycoprotein

The MDR1 gene product P-glycoprotein (Pgp) plays a key role in multidrug re sistance of cancer cells. Pgp is an ATP-driven efflux pump that extrudes a variety of dissimilar hydrophobic cytotoxic compounds. P-glycoprotein overe xpression results in multidrug resistance (MDR) of tumor cell lines in vitr o as well as in cancer patients. To selectively target and eliminate MDR tu mor cells, we have isolated a monoclonal antibody that specifically reacts with the first extracellular loop of the human Pgp. We have cloned the vari able domain genes of this antibody and assembled a functional single-chain Fv fragment capable of specifically targeting various Pgp-expressing MDR ca rcinoma cells lines. Targeting and specific elimination of Pgp-dependent MD R human cancer cells was achieved by constructing a single-chain immunotoxi n in which the scFv fragment was fused to a truncated form of Pseudomonas e xotoxin (PE38). We conclude that recombinant Fv-immunotoxins or other Fv-ba sed molecules armed with potent cytotoxins represent an effective tool in t argeted cancer therapy aimed at specific elimination of MDR tumor cell sub- populations. Recombinant antibody fragments targeting MDR proteins such as Pgp may be also used for intracellular expression and consequent phenotypic knockout of MDR. (C) 2001 Wiley-Liss, Inc.