Abstract 818: Targeting the redox-adaptive response in tumors to potentiate cancer therapy
Authors: Hanigan Marie H. , Wickham Stephanie , Reagan Nicholas , West Matthew B. , Thai Justin , Cook Paul F. , Li Pui Kai 
Published: August 2012

Abstract
Resistance to chemotherapy is a major impediment to successful cancer treatment. To enhance therapeutic intervention, we are developing novel compounds that target the redox-adaptive response in tumors, a process that increases intracellular glutathione levels. Elevated levels of intracellular glutathione render tumors resistant to chemotherapy and radiation therapy by protecting against reactive oxygen species, inactivating electrophilic metabolites of chemotherapy drugs, and blocking Bcl-2-mediated apoptosis. Glutathione synthesis requires free intracellular cysteine. Cells under oxidative stress synthesize high levels of glutathione and the supply of cysteine becomes rate limiting for glutathione synthesis. We have developed a novel strategy to deplete the free cysteine available to tumor cells, thereby blocking the drug resistance in tumor cells that have undergone redox-adaption. To accomplish this in humans, two enzymes must be inhibited, gamma-glutamyl transpeptidase (aka gamma-glutamyl transferase, GGT1) and gamma-glutamyl leukotrienase (aka, GGT-rel, GGT5). In humans, both GGT1 and GGT5 cleave the gamma-glutamyl bond of extracellular glutathione initiating the breakdown of glutathione into its constituent amino acids, glutamate, cysteine and glycine. Inhibiting GGT1 and GGT5 depletes the body of cysteine by blocking the catabolism of glutathione in the kidney resulting in the excretion of glutathione in the urine. Inhibiting these enzymes also blocks the tumor's ability to use extracellular glutathione as a source of cysteine.Our approach not only reduces the ability of the tumor to synthesize glutathione, it also reduces the cysteine (and the total amount of reduced thiol) in the tumor. Previously we used high-throughput screening and identified OU749 (N-[5-(4-methoxybenzyl)-1,3,4-thiadiazol-2-yl]benzenesulfonamide), a novel inhibitor of GGT1. We have subsequently developed a new assay system, which provides an unambiguous method to evaluate the inhibition of the physiologic reactions catalyzed by both GGT1 and GGT5. Kinetic data obtained from the assay can be used to directly evaluate the potency and mechanism of action of each inhibitor. Synthesis of an extensive array of OU749 analogues and evaluation of the inhibitory activity of these analogues in our new assay system has resulted in the identification of two new lead compounds, HL-6 and HL-7. These new leads are uncompetitive inhibitors of both human GGT1 and GGT5. These compounds are orders of magnitude less toxic than the currently available GGT1 inhibitors. Continued optimization of these lead compounds will provide new drugs for potentiating cancer therapy.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 818. doi:1538-7445.AM2012-818