Employing Pancreatic Tumor γ-Glutamyltransferase for Therapeutic Delivery

Abstract

γ-Glutamyltransferase (γGT) is a cell surface enzyme that catalyzes hydrolysis of the bond linking the glutamate and cysteine residues of glutathione and glutathione-S-conjugates. We have observed that human pancreatic tumor cells and tumor-associated stellate cells express high levels of this enzyme when compared to normal pancreatic epithelial and stellate cells. Detection of the protein in tumor sections correlated with γGT activity on the surface of the cultured tumor and stellate cells. We tested whether the tumor γGT could be employed to deliver a therapeutic to the tumor endothelial cells. GSAO is a glutathione-S-conjugate of a trivalent arsenical that is activated to enter endothelial cells by γGT cleavage of the γ-glutamyl residue. The arsenical moiety triggers proliferation arrest and death of the endothelial cells by targeting the mitochondria. Human pancreatic tumor and stellate cell γGT activated GSAO in culture and γGT activity positively correlated with GSAO-mediated proliferation arrest and death of endothelial cells in Transwell and coculture systems. A soluble form of γGT is found in blood, and we measured the rate of activation of GSAO by this enzyme. We calculated that systemically administered GSAO would circulate through the pancreatic blood supply several times before appreciable activation by normal blood levels of γGT. In support of this finding, tumor γGT activity positively correlated with GSAO-mediated inhibition of pancreatic tumor angiogenesis and tumor growth in mice. Our findings indicate that pancreatic tumor γGT can be used to deliver a therapeutic to the tumor.