Lethal effects of fluorodeoxyuridine on cultured mammalian cells at various stages of the cell cycle

The lethal damage induced by the exposure of synchronized Chinese hamster cells to various concentrations of 5-fluoro-2′deoxyuridine (FUdR) was not selectively restricted to cells exposed during the period of DNA synthesis S. The colony survival fraction observed after treatment for one hour with 5 × 10^?5 M FUdR was very low (0.0001–0.0003) whether the drug was administered during early G₁, late G₁, early S or in middle S. The survival of cells treated with the same concentration of FUdR during mitosis, however, was significantly higher (0.62) showing that mitotic cells were less sensitive to FUdR. Administration of 10^?7M thymidine or “conditioned” medium for one hour reversed the lethal effect of FUdR or improved the survival, depending on the time after removal of the FUdR at which these substances were given.