Trivalent hydroxyarsinothricn (R-AST-OH) is a covalent and irreversible inhibitor of kidney-type glutaminase (KGA). It binds to the glutamine binding site and forms a covalent bond with the cysteine residue at the active site. This compound selectively kills triple-negative breast cancer (TNBC) cells without being cytotoxic to control cell lines. KGA is an enzyme that regulates glutamine metabolism and is associated with tumor malignancy.