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Disruption of P‐glycoprotein anticancer drug efflux activity by a small recombinant single‐chain Fv antibody fragment targeted to an extracellular epitope

Inherent and acquired MDR is characterized by simultaneous resistance to diverse anticancer drugs and continues to be a major impediment in the curative chemotherapy of cancer. The MDR1 gene product, Pgp, is an ATP‐driven efflux pump, which extrudes a variety of dissimilar hydrophobic cytotoxic compounds from MDR cells. Pgp overexpression results in MDR of tumor cell lines in vitro as well as of a variety of human malignancies. Thus, one major goal is to develop strategies aimed at specifically disrupting Pgp drug‐efflux activity. To this end, we have developed a small recombinant antibody capable of potent reversal of MDR, by disrupting Pgp drug‐efflux activity. Using a phage display approach, we isolated a small scFv recombinant antibody fragment that specifically reacts with the first extracellular loop of human Pgp. This scFv fragment binds specifically to various Pgp‐overexpressing human MDR carcinoma cell lines, consequently disrupts Pgp drug‐efflux function and thereby reverses the MDR phenotype. We have successfully disrupted anticancer drug‐extrusion pump activity in MDR cells using a small recombinant scFv fragment. We propose that these novel small Fv‐based recombinant antibody molecules may lead to the development of a new class of antibody fragment‐based agents that specifically inhibit Pgp drug extrusion. Hence, these small recombinant antibody fragments may be applied in combination chemotherapy to overcome MDR in various human cancers. © 2004 Wiley‐Liss, Inc.