The ribonucleoside diphosphate reductase inhibitor (E)-2 '-deoxy(fluoromethylene)cytidine as a cytotoxic radiosensitizer in vitro

(E)-2'-Deoxy-(fluoromethylene)cytidine (FMdC) is known as an inhibitor of r ibonucleoside diphosphate reductase, a key enzyme in the de novo pathway of DNA synthesis. FMdC was tested as a modifier of radiation response in vitr o on a human colon carcinoma cell line (WiDr), and the observed radiosensit ization was confirmed on two human cervix cancer cell lines (C33-A and SiHa ). Using the clonogenic assay, the effect ratio (ER) at a clinically releva nt dose level of 2 Gy was 2.10 (50 nM FMdC), 1.70 (30 nM PMdC), and 1.71 (4 0 nM FMdC) for the three cell lines WiDr, C33-A, and SiHa, respectively. A more detailed analysis of the importance of timing and concentration of FMd C was done on the WiDr cell line alone, yielding an increased ER(2Gy) with increasing concentration and duration of exposure to the drug, ranging from 1.0 (6 h) to 1.8 (72 h) at 30 nM FMdC and from 1.2 (6 h) to 3.5 (24 h) at 300 nM. We investigated the effect of FMdC on the cellular deoxynucleotide triphosphate pool in WiDr cells and demonstrated a marked depletion of dATP and a significant rise of TTP levels. Cell cycle analysis showed early S-p hase accumulation induced by FMdC alone, G(2)-M block induced by irradiatio n alone, and an increased accumulation of cells in G(2)-M if both modalitie s are used. Our data suggest that FMdC is a radiation response modifier in vitro on different cancer cell lines. The observed radiosensitization may i n part be explained by alteration of the deoxynucleotide triphosphate pool, which is consistent with the effect of FMdC on ribonucleoside diphosphate reductase.