Novel radiation sensitizers targeting tissue hypoxia.

That hypoxic tissues are more resistant to the effects of radiation than well-oxygenated tissues has been known for many decades, and repeated in vitro demonstrations have confirmed that to achieve the same degree of cytotoxicity, hypoxic cells require about three times the radiation dose that well-oxygenated cells need. Hypoxic cell sensitizers enhance the tissue response to standard radiation, generally by mimicking the effects of oxygen, which induces the formation and stabilization of toxic DNA radicals. Although many hypoxic cell sensitizers like the nitroimidazoles have been evaluated in combination with radiation, these agents have had no or only minimal therapeutic impact due to either their limited potency or their toxicity at biologically relevant concentrations. This article reviews several new modalities that either increase oxygen delivery or sensitize hypoxic tissues. These modalities, all currently in early clinical evaluations, include: (1) tirapazamine, a bioreductive agent; (2) gadolinium texaphyrin, a hypoxic cell sensitizer with biolocalization properties using magnetic resonance imaging; (3) RSR13, an allosteric modifier of hemoglobin; and (4) bovine hemoglobin modified by the attachment of polyethylene glycol polymers.