The effects of thymidine (dThd) coadministration on the pharmacokinetics and metabolism of 5-fluorouracil (FUra) were investigated in 29 colorectal cancer patients. Five patients received 5-day i.v. infusion of FUra at 15 mg/kg/day and 24 patients received infusion of FUra (7.5 mg/kg/day, 5 days) and dThd (216 mg/kg/day, 6 days) preceded by a bolus dose of dThd (405 mg/kg). Plasma and urine concentrations of FUra, 5-fluorodeoxyuridine (FdUrd), thymine, and dThd were quantitated by a high-pressure liquid chromatographic assay. Concurrent dThd administration reduced the plasma clearance of FUra at steady state from 389.1 ± 153.5 (S.D.) to 56.0 ± 36.4 liters/kg/day. The mean steady-state plasma concentration of FUra in patients receiving FUra alone was 0.38 µm and was significantly lower than the 1.30 µm in patients receiving FUra-dThd. Plasma concentrations of FUra were linearly dependent on those of thymine. Furthermore, the metabolic and renal clearances of FUra decreased inversely with thymine concentrations indicating that the elimination of FUra was reduced by thymine. In contrast to the absence of FdUrd as a circulating metabolite in patients treated with FUra alone, µm concentrations of FdUrd were detected in plasma of most of the patients treated with FUra-dThd. This together with the linear correlation of FdUrd and dThd concentrations indicates that the interconversion of FUra to FdUrd was enhanced by dThd.

The incidence of dose-limiting leukopenia in the FUra-dThd combination therapy was 40%. There is an inverse correlation between the plasma clearance of FUra at steady state and hematological toxicity. The plasma clearance of FUra in the toxic population was 32.0 ± 16.8 liters/kg/day and was significantly lower than the clearance of 72.0 ± 37.3 liters/kg/day in the nontoxic population (p < 0.001). The corresponding critical toxic steady-state FUra plasma concentration was 1.5 µm.

The biochemical effects of dThd on the incorporation of FUra and FdUrd into RNA and into acid-soluble 5-fluorodeoxyuridine monophosphate (FdUMP) in human colon tumor cells were studied in vitro. At 100 µm, dThd increased the incorporation of FUra into RNA up to 4-fold but diminished the acid-soluble FdUMP pool. Similarly, the incorporation of FdUrd into acid-soluble FdUMP was inhibited by dThd. The response rate of colorectal carcinoma to FUra was not improved by coadministration of dThd; only one of the 11 patients who had no prior FUra therapy achieved partial remission. The lack of clinical response in these patients may be partly due to the inhibition of anabolism of FUra and FdUrd to FdUMP by dThd.

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This investigation was supported in part by USPHS Grant CA-21071 from the National Cancer Institute, NIH. A preliminary report has been presented (1).

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