METABOLISM OF 2',2'-DIFLUORO-2'-DEOXYCYTIDINE AND RADIATION SENSITIZATION OF HUMAN COLON-CARCINOMA CELLS

Difluorodeoxycytidine (dFdCyd) is a new antimetabolite with clinical a ctivity in patients with solid tumors but not leukemias. We have studi ed the metabolism, cytotoxicity, and radiosensitizing properties of dF dCyd in HT-29 human colon carcinoma cells. The results demonstrated th at dFdCyd rapidly accumulated as the 5'-triphosphate dFdCTP in HT-29 c ells, which was eliminated slowly in the absence of dFdCyd with a half -life of >12 h. Accumulation of dFdCTP was associated with rapid deple tion of cellular dATP pools. Exposure to the concentration that reduce s cell survival by 50% of 30 nM dFdCyd decreased dATP levels by >80% w ithin 4 h. dGTP pools were depleted at higher concentrations of dFdCyd , whereas smaller decreases were effected in dTTP and dCTP pools. Thes e results contrast with previous reports in leukemic cells which demon strated that dFdCyd exposure depleted the endogenous dCTP pool to a gr eater extent than the dTTP, dATP, or dGTP pools. Based on these data, we suggest that the profound depletion by dFdCyd of dATP and dGTP pool s in HT-29 compared to leukemic cells accounts for the superiority of this agent in solid tumors versus leukemias. Additional studies demons trated that dFdCyd was a potent radiosensitizer in HT-29 cells. Maxima l radiosensitization was observed when cells were irradiated immediate ly following dFdCyd exposure instead of prior to or in the middle of d rug treatment. Radiation sensitization was dose and time dependent, wi th a noncytotoxic exposure to 10 nM dFdCyd for 24 h or 30 nM dFdCyd fo r 16 h producing a radiation enhancement ratio of approximately 2. Und er these conditions, only the cellular dATP pool was depleted. When ce lls were exposed to higher concentrations of dFdCyd for 4 h, equivalen t radiosensitization with a radiation enhancement ratio of 1.4 was obt ained using 0.1, 1.0, or 10 mu M dFdCyd. Despite a 15-fold increase in dFdCTP and depletion of dGTP and dCTP pools to <25% of the control va lue with 10 mu M compared to 0.1 mu M dFdCyd, no increase in radiosens itization was observed. These results suggest that dATP depletion is a n important factor in the radiosensitizing property of this promising new antitumor compound.