Unusual radiation response of gemcitabine-treated HeLa cells: reduced and enhanced survival.

Gemcitabine, a widely reported radiosensitizer, was found to increase and decrease radiation survival of HeLa cells under different treatment conditions. A 24 h exposure at 10 or 20 nM increased S phase cells and radiation survival, with a dose modification factor (DMF) of 0.82 or 0.78, respectively. Shortening exposure time avoided cell cycle effect and eliminated increases of radiation survival. Treatment for 1 h showed no change in radioresponse (DMF of 1.03) at 20 nM versus radiosensitization at 1, 2, or 20 microM (DMF of 1.66). These results document an enhanced radiation survival while confirming reported radiosensitization under different conditions. Such findings may impact drug dosimetry.