Radiosensitization by fluorodeoxyuridine: effects of thymidylate synthase inhibition and cell synchronization

The combination of fluoropyrimidines and radiation has resulted in increased control of colorectal cancer in the clinic, but the basic mechanism of the interaction is not understood clearly. Preliminary work in our laboratory showed that 2-h exposures of HT 29 human colon carcinoma cells to relatively low levels of 5-fluorodeoxyuridine resulted in extended thymidylate synthase inhibition after the drug was removed (up to 30 h after treatment with 0.5 microM 5-fluorodeoxyuridine). The low cytotoxicity associated with this treatment simplified efforts to test the effects of extended thymidylate synthase inhibition on radiosensitivity of HT 29 cells. Although thymidylate synthase was completely inhibited at the end of the 2-h exposure, an increase in the radiosensitivity of the cells was not evident until 16 h after the removal of drug. Flow cytometric analysis showed that cells accumulated in early S phase over time, and the increase in radiation sensitivity of the entire population followed the increase of the proportion of cells in early S, a relatively radiosensitive phase of the cell cycle. This treatment schedule was compared with 24-h continuous exposure, and we found that the same maximum increase in radiosensitivity was achieved by both treatment strategies. However, more cytotoxicity was associated with continuous exposure. This study provides evidence that radiosensitization by 5-fluorodeoxyuridine is in part due to alteration of cell kinetics and redistribution of cells throughout the cycle. This information may be useful in the design of less toxic combined chemo- and radiotherapy treatment strategies by limiting systemic exposure to fluoropyrimidines.