The synthetic tripeptide arsenical 4-(N-(S-glutathionylacetyl) amino)p-phenylarsenoxide ( p-GSAO) is an angiogenesis inhibitor that inactivates mitochondrial adenine nucleotide translocase (ANT) by cross-linking a pair of matrix-facing cysteine residues. This causes an increase in superoxide levels and proliferation arrest of endothelial cells followed by mitochondrial depolarization and apoptosis. p-GSAO induces proliferation arrest in endothelial cells and is a selective inhibitor of endothelial cells compared with tumor cells. An analogue of p-GSAO has been made in which the arsenical moiety is at the ortho instead of the para position on the phenyl ring. o-GSAO, like p-GSAO, bound to ANT in a dithioldependent manner but was f8-fold more efficient than p-GSAO at triggering the mitochondria permeability transition in isolated mitochondria. o-GSAO was an f50-fold more potent inhibitor of endothelial and tumor cell proliferation than p-GSAO. The mechanism of this effect was a consequence of f300-fold faster rate of accumulation of o-GSAO in the cells, which is due, at least in part, to impaired export by the multidrug resistance-associated protein 1. Administration of o-GSAO to tumor-bearing mice delayed tumor growth by inhibiting tumor angiogenesis but there were side effects not observed with p-GSAO administration. (Cancer Res 2005; 65(24): 11729-34)

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