Effect of radiation and tirapazamine (SR-4233) on three melanoma cell lines

In this study the response of three melanoma cell lines to single doses of radiation, to the bioreductive drug tirapazamine (SR-4233) and to the combination of radiation and tirapazamine was determined. Tirapazamine is a bioreductive drug with specific cytotoxicity in hypoxic conditions. Three melanoma cell lines (MM576, MM96L and murine B16-F10) were exposed to increasing concentrations of tirapazamine to assess cytotoxicity under aerobic and hypoxic conditions. Also, clonogenic survival after single doses of 4 MV X-rays was determined under aerobic and hypoxic conditions, with and without 20 microM tirapazamine. Tirapazamine was an effective hypoxic cytotoxin in the three cell lines. The concentrations of tirapazamine causing equal cell kill were 1000 mM for aerobic cells and 50 mM for hypoxic cells. The oxygen enhancement ratios for single X-ray doses were between 2 and 3 for all the cell lines. Addition of 20 microM tirapazamine to hypoxic cells 1 h before irradiation produced the same radiosensitivity as aerobic cells. Tirapazamine had a minimal effect on the radiosensitivity of aerobic cells. Since melanomas are known to contain hypoxic cells which may reduce their radiosensitivity, these in vitro results have demonstrated the potential of tirapazamine to overcome the radioresistance of hypoxia and give encouragement for further studies.