Abstract 683: Sensitizing tumors to pro-oxidant therapy by inhibiting gamma-glutamyl transpeptidase

Abstract

Abstract We have reported that 73% of ovarian tumors express the cell surface enzyme gamma-glutamyl transpeptidase (GGT). Clinical studies have shown a poorer prognosis for ovarian tumors that are GGT-positive compared to GGT-negative tumors. GGT-positive tumors are able to use the glutathione in the interstitial fluid as an additional source of cysteine, the limiting component of intracellular glutathione synthesis in tumors. The concentration of cysteine/cystine in standard tissue culture media is more than 3 times the concentration in interstitial fluid. In this study we show that in studies of cultured cells, the high concentration of cysteine in tissue culture media obscured the role of GGT expression in drug resistance. However, culturing human ovarian tumor cells in tissue culture media containing physiological concentrations of cystine and glutathione demonstrated that the supply of cysteine/cystine regulates intracellular glutathione concentrations. In these studies, inhibition of GGT blocked the use of extracellular glutathione as a source of cysteine and sensitized the tumors to cisplatin, carboplatin and melphalan. Inhibiting GGT in patients would also inhibit GGT in the kidney resulting in the excretion of glutathione in the urine and depletion of cysteine from serum and interstitial fluid. As a result, inhibition of GGT in patients would sensitize both GGT-positive and GGT-negative tumors to pro-oxidant chemotherapy. Clinical application of this therapeutic strategy is limited by the fact that most inhibitors of GGT are glutamate analogs and too toxic for use in humans. We discovered a non-glutamate analog that is an uncompetitive inhibitor of GGT, and are developing more potent, non-toxic GGT inhibitors. We recently published the crystal structure of human GGT. We have now also solved the structure of the substrate-bound enzyme, the form of the enzyme to which our uncompetitive inhibitors bind. Using structure-based drug design and our physiologically relevant tissue culture system we are designing new GGT inhibitors and evaluating them for their ability to sensitize tumors to chemotherapy regimens used in the clinic. Supported by an Institutional Development Award (IDeA) from the National Institutes of Health under grant P20GM103640. Citation Format: Marie H. Hanigan, Nancy Wakeham, Stephanie Wickham, Simon S. Terzyan. Sensitizing tumors to pro-oxidant therapy by inhibiting gamma-glutamyl transpeptidase. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 683. doi:10.1158/1538-7445.AM2014-683