Interaction of hyperthermia and radiation in murine cells: hypoxia and acidosis in vitro, tumor subpopulations in vivo.

To better understand the effect of the level of oxygenation and pH on the heat-radiation interaction, these factors were modeled in vitro using FSaIIC cells in monolayer and correlated with the response of Hoechst 33342 dye-defined FSaIIC tumor subpopulations treated in vivo. Exposure to both 42 degrees C and 43 degrees C for 1 h in culture prior to graded single fractions of radiation resulted in a striking decrease in the radiation oxygen enhancement ratio which was pH as well as temperature dependent. The oxygen enhancement ratio at 37 degrees C and pH 7.40 (or pH 6.45) was 2.9, but decreased to 1.4 at 42 degrees C at normal pH, 1.2 at low pH, and 1.0 at 43 degrees C at both pH values tested. This decrease in the oxygen enhancement ratio resulted from a far more marked decrease in Do values for the radiation survival curves of hypoxic cells compared to normally oxygenated cells at elevated temperatures. In addition, the shoulder region of the radiation survival curves was significantly decreased with increasing temperatures and the magnitude of the decrease was greatest in hypoxic cells at low pH. In vivo treatment followed by immediate tumor excision showed that bright cells (presumably oxygenated cells at normal pH) were approximately 2-fold more sensitive to 10 Gy of radiation than were dim cells (presumably hypoxic cells at low pH) but that dim cells were 2.5-fold more sensitive to 43 degrees C for 30 min hyperthermia. The combination of hyperthermia followed by radiation proved to be 1.8-fold more toxic to dim than to bright cells. Both hyperthermia alone and hyperthermia plus radiation, in contrast to radiation alone, were significantly more cytotoxic when tumors were left in situ for 24 h prior to excision as compared with immediate excision. These results indicate that hyperthermia markedly sensitizes hypoxic cells at low pH to the cytotoxic effects of radiation, as well as effectively killing cells in this tumor subpopulation.