Vol. 62, Issue 5, 1160-1166, November 2002

Inhibition of the Multidrug Resistance Protein 1 (MRP1) by Peptidomimetic Glutathione-Conjugate Analogs

Danny Burg, Peter Wielinga, Noam Zelcer, Tohru Saeki,¹ Gerard J. Mulder, and Piet Borst

Division of Toxicology, Leiden/Amsterdam Center for Drug Research, Leiden University, Leiden, the Netherlands (D.B., G.J.M.); and Division of Molecular Biology and Centre of Biomedical Genetics, the Netherlands Cancer Institute, Amsterdam, the Netherlands (P.W., N.Z., T.S., P.B.)

Inhibition of multidrug resistance protein 1 (MRP1) mediated cytostatic drug efflux might be useful in the treatment of drug resistant tumors. Because the glutathione (GSH) conjugate of ethacrynic acid (EA), GS-EA, is a good substrate of MRP1, GS-EA derivatives are expected to be good inhibitors of MRP1. To study structure-activity relationships of MRP1 inhibition, a series of novel GS-EA analogs was synthesized in which peptide bonds of the GSH backbone were replaced by isosteric groups [Bioorg Med Chem 10:195-205, 2002]. Several of these compounds were effective inhibitors of MRP1-mediated [³H]GS-EA and [³H]E₂17G transport, as studied in membrane vesicles prepared from MRP1-overproducing Sf9 cells. The modifications of the peptide backbone have distinct implications for recognition by MRP1: the -glutamyl-cysteine peptide bond is important for binding, whereas the cysteinyl-glycine amide does not seem essential. When the -glutamyl-cysteine peptide bond (C-CO-N) is replaced by a urethane isostere (O-CO-N), an effective competitive MRP1-inhibitor (K_i = 11 µM) is obtained. After esterification of this compound to improve its cellular uptake, it inhibited MRP1-mediated efflux of calcein from 2008 ovarian carcinoma cells overexpressing MRP1. This compound also partially reversed the resistance of these cells to methotrexate. Because the urethane isostere is stable toward -glutamyl transpeptidase-mediated breakdown, it is an interesting lead-compound for the development of in vivo active MRP1 inhibitors.