Prolonged retention of high concentrations of 5-fluorouracil in human and murine tumors as compared with plasma

Concentrations of 5-fluorouracil (5-FU) and its active metabolite 5-fluoro-2′-deoxy-5′-monophosphate (FdUMP) were measured in biopsy specimens of tumor tissue, normal mucosa, metastatic liver nodules, and normal liver tissue obtained from 39 patients and in two murine colon tumors (colon 26 and colon 38) after a single injection of 5FU at a therapeutic dose (500 mg/m² and 100 mg/kg, respectively). These data were compared with plasma concentrations. Peak plasma concentrations (300-500 μm) of 5FU were comparable in human and murine plasma. The half-life of plasma elimination (during the period from 15 to 120 min) in both mouse and man ranged from 10 to 20 min, whereas at between 2 and 8 h, plasma concentrations varied from 0.1 to 1 μm, the half-life being about 100 min. In both species, 5FU could be measured in plasma at concentrations ranging from 0.01 to 1 μm for several days after 5FU treatment. 5FU concentrations in tissue samples obtained from 14 patients were measured during the time range of 1-6 h, those in samples taken from 7 patients, during the interval of 19-27 h; and those in samples obtained from 18 patients, within the interval of 40-48 h after injection. 5FU tumor concentrations varied between 0.78-21.6, 0.44-6.1, and 0.17-10.8 μmol/kg wet wt., respectively. Some of the 48-h samples were obtained from patients who had received leucovorin plus 5FU; coadministration of leucovorin did not alter 5FU tissue concentrations. At between 4 and 48 h, the tissue concentration/plasma concentration ratio was at least 10. 5FU concentrations in murine tumors were measured for up to 10 days after 5FU administration, with plateau 5FU tumor concentrations being about 50 μmol/kg wet wt. in colon 38 and about 200 μmol/kg wet wt. in colon 26 at 2 h after treatment; after 4 days, values of 0.5 and 4.8 μmol/kg, respectively, were obtained and after 10 days, respective concentrations of 0.1 and 0.07 μmol/kg were detected. The FdUMP concentrations measured in colon 26 and colon 38 tumors were 214 and 46 pmol/g, respectively, at 2 h after 5FU administration, and these values subsequently decreased to about 15 pmol/g in both tumors. In human tumors the initial FdUMP concentration ranged from 10 to 1000 pmol/g; at later time points the level of FdUMP was just above the detection limit of the assay. In liver metastases, high 5FU concentrations seemed to be related to high levels of FdUMP, which was likely of importance for the antitumor effect. The prolonged retention of 5FU should be taken into consideration in the design of biochemical modulation studies.