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The Role of Apoptosis in 2',2'-Difluoro-2'-deoxycytidine (Gemcitabine)-mediated Radiosensitization¹

Department of Radiation Oncology, University of Michigan Medical Center, Ann Arbor, Michigan 48109-0582

The nucleoside analogue Gemcitabine [2',2'-difluoro-2'-deoxycytidine (dFdCyd)] is active against a wide variety of solid tumors and is a potent radiation sensitizer. Because apoptosis has been shown to be an important mechanism of cell death for many cancers, we wished to investigate the role of apoptosis in dFdCyd-mediated radiosensitization. We evaluated HT29 colon cancer cells, UMSCC-6 head and neck cancer cells, and A549 lung cancer cells, which differ substantially in the ability to undergo radiation-induced apoptosis. We hypothesized that if dFdCyd produced radiosensitization by potentiating preexisting death pathways, then only the apoptotic-prone HT29 cells would show a substantial increase in apoptosis when treated with the combination of dFdCyd and radiation and that UMSCC-6 cells and A549 cells would be radiosensitized through nonapoptotic mechanisms. We found that the radiosensitization of HT29 cells (enhancement ratio, 1.81 ± 0.16) was accompanied by an increase in apoptosis and by caspase activation and that inhibition of this activation by the caspase inhibitor Z-Asp-Glu-Val-Asp-fluoromethylketone (DEVD) significantly decreased radiosensitization (to 1.36 ± 0.24; P < 0.05). In contrast, UMSCC-6 cells and A549 cells were modestly radiosensitized (enhancement ratio, 1.47 ± 0.24 and 1.31 ± 0.04, respectively) via a nonapoptotic mechanism. These findings suggest that although apoptosis can contribute significantly to dFdCyd-mediated radiosensitization, the role of apoptosis in dFdCyd-mediated radiosensitization depends on the cell line rather than representing a general property of the drug.

Commentary

The Clinical Implications of Gemcitabine Radiosensitization

Todd H. Doyle, Francoise Mornex, and W. Gillies McKenna
Clin. Cancer Res. 2001 7: 226-228. [Full Text] [PDF]

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