Difluorodeoxycytidine (dFdCyd) is a new antimetabolite with clinical activity in patients with solid tumors but not leukemias. We have studied the metabolism, cytotoxicity, and radiosensitizing properties of dFdCyd in HT-29 human colon carcinoma cells. The results demonstrated that dFdCyd rapidly accumulated as the 5′-triphosphate dFdCTP in HT-29 cells, which was eliminated slowly in the absence of dFdCyd with a half-life of >12 h. Accumulation of dFdCTP was associated with rapid depletion of cellular dATP pools. Exposure to the concentration that reduces cell survival by 50% of 30 nm dFdCyd decreased dATP levels by >80% within 4 h. dGTP pools were depleted at higher concentrations of dFdCyd, whereas smaller decreases were effected in dTTP and dCTP pools. These results contrast with previous reports in leukemic cells which demonstrated that dFdCyd exposure depleted the endogenous dCTP pool to a greater extent than the dTTP, dATP, or dGTP pools. Based on these data, we suggest that the profound depletion by dFdCyd of dATP and dGTP pools in HT-29 compared to leukemic cells accounts for the superiority of this agent in solid tumors versus leukemias. Additional studies demonstrated that dFdCyd was a potent radiosensitizer in HT-29 cells. Maximal radiosensitization was observed when cells were irradiated immediately following dFdCyd exposure instead of prior to or in the middle of drug treatment. Radiation sensitization was dose and time dependent, with a noncytotoxic exposure to 10 nm dFdCyd for 24 h or 30 nm dFdCyd for 16 h producing a radiation enhancement ratio of approximately 2. Under these conditions, only the cellular dATP pool was depleted. When cells were exposed to higher concentrations of dFdCyd for 4 h, equivalent radiosensitization with a radiation enhancement ratio of 1.4 was obtained using 0.1, 1.0, or 10 µm dFdCyd. Despite a 15-fold increase in dFdCTP and depletion of dGTP and dCTP pools to <25% of the control value with 10 µm compared to 0.1 µm dFdCyd, no increase in radiosensitization was observed. These results suggest that dATP depletion is an important factor in the radiosensitizing property of this promising new antitumor compound.

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Supported by Grants CA-46452, CA-53440, and CA-46592 from the National Cancer Institute and Grant DHP-85A from the American Cancer Society.

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