Sensitizing effects of gallium citrate on hyperthermic cell killing in vitro

The lethal effects of gallium citrate in combination with heat were studied using four cell lines, L5178Y, FM3A, P388 and HeLa. Cells were incubated with different concentrations (0.2 2 mM) of gallium citrate at 37 degrees C for 24 h and heated at a range of temperatures from 40-44 degrees C for various time periods up to 6 h in the absence of gallium citrate. Survival and cell viability were determined by clonogenic assay and the dye-exclusion test, respectively. All of the cell lines tested were insensitive to heat below 41 degrees C, but were very sensitive to heat above 43 degrees C. Gallium citrate was cytotoxic to these cell lines at different levels: P388 and HeLa were far more sensitive than L5178Y and FM3A. The killing effects of heat at 41 degrees C were greatly enhanced by gallium citrate in L5178Y and P388 cells. The Arrhenius analysis for the lethal effect of heat, determined by clonogenic assay, in L5178Y cells showed that the transition temperature was remarkably decreased for the gallium-treated cells from approximately 43 degrees C to 41 degrees C. The mechanism for this decrease in the transition temperature may be attributable to the additional effects of gallium citrate on energy metabolism. Preincubation with 0.05 mM gallium citrate at 37 degrees C for 7 days also enhanced heat sensitization at 41 degrees C in L5178Y. This preincubation condition may correspond to the condition for the continuous infusion of gallium that is clinically used for cancer treatment. In contrast, treatment with gallium did not greatly enhance the sensitivity of FM3A or HeLa cells to heat at 41 degrees C, but the effects of gallium were significant.