Significant increase in residual DNA damage as a possible mechanism of radiosensitization by gemcitabine

Purpose: To investigate the effect of gemcitabine (dFdC), a promising radiosensitizing nucleoside analog, on the induction and repair of DNA double-strand breaks (dsbs) after ionizing radiation (RT) in a pancreatic tumor cell line. Material and Methods: BxPC3 pancreatic tumor cells were treated using different concentrations of gemcitabine with and without subsequent irradiation. DNA dsbs were detected by constant-field gel electrophoresis under neutral conditions. Results: With the addition of gemcitabine (0.5-1,000 μmol/l for 2 h prior to RT) to RT (0-75 Gy), a considerable and dose-dependent increase of remaining DNA damage after 24 h (5.4-fold for 0.5 μmol/l dFdC, 12.2-fold for 1,000 μmol/l dFdC at 25 Gy) was noted. Enhancement factors were inversely correlated with increasing X-ray dose (7.8-fold for 0.5 μmol/l dFdC at 1 Gy decreasing to 1.6-fold at 75 Gy). Conversely, the induction of DNA dsbs was not affected. Gemcitabine alone lead to a slight increase of initial DNA dsbs and only a modest elevation of residual DNA damage. Conclusion: These findings strengthen the hypothesis of DNA repair inhibition as a major mechanism of radiosensitization by gemcitabine. (orig.)