Effect of pH, oxygenation, and temperature on the cytotoxicity and radiosensitization by etanidazole

The effect of etanidazole was examined in vitro and in vivo in the FSaIIC tumor system. At pH 7.40 and 37 degrees C, etanidazole at 5-500 microM for 1 hr was minimally cytotoxic. At 42 degrees C and 43 degrees C, however, the cytotoxicity of etanidazole increased. Etanidazole was more cytotoxic at pH 6.45 and 37 degrees than at pH 7.40 by about 1 log. Increasing the temperature to 42 degrees C or 43 degrees C at pH 6.45 during drug exposure, however, caused little increase in drug killing above the lethality of hyperthermia. When the radiosensitizing abilities of etanidazole were tested in vitro, there was a radiation dose modifying factor of 2.40 at pH 7.40, but only 1.70 at pH 6.45. In vivo, etanidazole (1 g/kg) produced a radiation dose modifying factor of 1.47, whereas 43 degrees C for 30 min produced a radiation dose modifying factor of 1.38. The combination resulted in a radiation dose modifying factor of 2.29. When the cytotoxicities of hyperthermia (43 degrees C x 30 min), etanidazole (500 mg/kg or 1 mg/kg), and radiation (10 Gy) combinations were assayed by Hoechst 33342 dye selected tumor subpopulations, 43 degrees C x 30 min increased the killing of irradiated dim cells by approximately 9.2-fold but by only 2.9-fold in bright cells. Etanidazole (1 g/kg) increased radiation killing of bright cells by about 3-fold and dim cells by about 4.3-fold. The combination of hyperthermia and etanidazole increased the killing of both dim and bright cells exposed to radiation by approximately 10-fold versus 10 Gy alone.