Inhibitory effect of bromodeoxyuridine on long-term cell proliferation

The thymidine analog bromodeoxyuridine (BrdU) can be incorporated into newlysynthesized DNA and is commonly used to "birthdate" proliferative cells. It has been suggested that BrdU alters the stability of DNA thereby increasing the risk of sister-chromatid exchanges, mutations, and double-strand breaks. However, most of these effects are found only when BrdU incorporation is combined with secondary stressors. Early toxicity studies showed that BrdU can induce chromosomal breakage and increase the sensitivity of treated cells to ionizing radiation, and this radiosensitizing effect has continued to be pursued as an adjunctive therapy in the treatment of a variety of cancers. BrdU readily crosses the blood-brain barrier, and has been combined with conventional chemotherapy and radiation treatment in several clinical trials. While the clinical benefits of including BrdU as a radiosensitizer have been disappointing - showing, at best, modest improvements for some outcome measurements - it is possible that other therapeutic effects of BrdU were not appreciated, either because of interference by the other treatment modalities in these studies, or because finer analytical resolution is required to discern them.;Despite its extensive history there is no accepted consensus mechanism of action for BrdU. Surprisingly little attention has been focused on examining the influence that BrdU alone may exert on cellular function. We report a novel anti-proliferative effect of BrdU on populations of stem and cancer cells that is independent of its role in radiosensitization. A single, brief in vitro exposure to BrdU induces a profound and sustained reduction in the proliferation rate of all cells examined. Cells do not die but variably upregulate some senescence-associated proteins as they accumulate in the G1 phase of the cell cycle. BrdU also impairs the proliferative capacity of primary tumor-initiating human glioma cells and may, therefore, represent a means of targeting cancer stem cells. Finally, conservative in vivo BrdU regimens (in the absence of any other treatment) significantly suppress tumor progression in a highly aggressive, syngeneic rat glioma model. Results suggest that BrdU may have an important role as an adjunctive therapeutic for a wide variety of cancers based upon new insights into its effect as a negative regulator of cell cycle progression.