Fluorescent verapamil derivative for monitoring activity of the multidrug transporter.

Multidrug resistance to amphipathic natural product chemotherapeutic drugs is conferred on cancer cells by expression of the MDR1 gene, which encodes the 170-kDa multidrug transporter known as P glycoprotein. The P glycoprotein-mediated efflux of toxic chemotherapeutic drugs can be reversed by agents such as verapamil, which is a substrate for the multidrug transporter and appears to be a competitive inhibitor of the efflux pump. In this study, Bodipy-verapamil, a fluorescent derivative of verapamil, has been shown to be a substrate for the efflux pump activity of P glycoprotein. Single-cell fluorescence analysis reveals that Bodipy-verapamil accumulates in lysosomes of drug-sensitive NIH3T3 and KB cells but is rapidly effluxed from multidrug-resistant derivatives of these cell lines. Although Bodipy-verapamil is a substrate for the multidrug transporter, it is not an efficient inhibitor of the pump and does not reverse resistance to vinblastine and colchicine as effectively as does verapamil. This new derivative may be a useful tool for imaging of lysosomes in drug-sensitive cells and for rapid screening for the multidrug-resistant phenotype in other cell types.