Abstract 4546: Depleting cysteine: A novel approach to tumor therapy

Abstract

Abstract A cure remains elusive for many types of tumors that initially respond to chemotherapy but recur with drug-resistant disease. Cysteine, (or its oxidized form cystine) is an essential amino acid for tumors. Enhanced uptake of cysteine and/or cystine is required for tumors to maintain their intracellular glutathione (GSH) levels and redox balance. In addition, some stem cells within epithelial tumors express CD44v8-10, which stabilizes the cystine transporter, increasing cystine uptake into the cell and enhancing resistance to pro-oxidant therapy. Depleting cysteine during treatment has a profound effect on sensitizing the tumor to therapy. We have a novel strategy that reduces the cysteine (and cystine) concentration in serum and extracellular fluid. The strategy exploits the finding that a severe cysteine deficiency develops in mice lacking the enzyme gamma-glutamyl transpeptidase (GGT). This enzyme reclaims the cysteine from GSH as the blood is filtered through the kidney. Inhibiting GGT activity results in excretion of GSH in the urine and a precipitous drop in the concentration of cysteine and cystine in serum. The inhibitors of GGT that have been evaluated in the clinic are glutamate analogs and are too toxic for clinical use. We have discovered and are developing a new class of GGT inhibitors that are not glutamate analogs. These inhibitors are orders of magnitude less toxic than the glutamate analogs. We have determined the crystal structure of human GGT. In the current study we have co-crystallized human GGT with a series of inhibitors. We report the identification of residues within the active site that are critical for inhibitor binding. We have identified changes in the conformation of the active site upon substrate binding. We have also delineated movement within the active site during substrate cleavage. Site-directed mutagenesis and functional assays have confirmed the role of the key residues identified in the structural studies. These data provide new insights for continued development of a potent, specific, non-toxic inhibitor of GGT that can be used clinically in cancer therapy. Supported by an Institutional Development Award (IDeA) from the National Institutes of Health under grant P20GM103640. Citation Format: Marie H. Hanigan, Anthony W. G. Burgett, Nancy Wakeham, Simon S. Terzyan. Depleting cysteine: A novel approach to tumor therapy. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4546. doi:10.1158/1538-7445.AM2015-4546