Glutathione arsenoxide (GSAO) is a peptide trivalent arsenical that has potential anti-angiogenic capability, suggesting that it could be used for treatment in cancers where tumor metastasis relies on new blood vessel formation (angiogenesis). Endothelial cell proliferation drives new blood vessel formation. GSAO treatment causes a concentration-dependent increase in superoxide levels, ATP depletion, mitochondrial depolarization, and apoptosis in proliferating, but not endothelial cells. GSAO is able to block blood flow to solid tumors in mice, thereby inhibiting tumor growth in mice with no apparent toxicity at efficacious doses. Initial experiments have implicated GSAO in perturbing mitochondrial function. Other molecular effects of GSAO in human cells, for example on the phosphorylation of proteins, are still largely unknown. A phase I clinical trial has been terminated.