<unk>  (SR-4233) is an experimental anticancer drug that is activated to a toxic radical only at very low levels of oxygen (hypoxia). Such levels are common in human solid tumors, a phenomenon known as tumor hypoxia. Thus,<unk>  is activated to its toxic form preferentially in the hypoxic areas of solid tumors. Cells in these regions are resistant to killing by radiotherapy and most anticancer drugs. Thus the combination of<unk>  with conventional anticancer treatments is particularly effective. As of 2006,<unk>  is undergoing phase III testing in patients with head and neck cancer and gynecological cancer, and similar trials are being undertaken for other solid tumor types.
Chemically it is an aromatic heterocycle di-N-oxide. Its full chemical name is 3-amino-1,2,4-benzotriazine-1,4 dioxide. Originally it was prepared in a programme screening for new herbicides in 1972. Its clinical use was first described by Zeman et al in 1986.
Tirapazamine
Thiopropazate
Fenpiprane
Fazadinium bromide
Levodropropizine
Tirilazad
Methyldopate
Benperidol