Determination of 5-fluorouracil (NSC-19893) plasma levels in rats and man by isotope dilution-mass fragmentography.

A highly sensitive and specific rapid assay for 5-fluorouracil (5-FU) in plasma samples at concentrations as low as 1 ng/ml was developed using mass fragmentography. Highly enriched C14-5-FU was empolyed as an internal standard. A two-compartmental open model for the disposition of free 5-FU in plasma is proposed. The beta-phase of 5-FU plasma elimination describes the pool size of anabolic metabolites, assuming this model is correct. Injections of 5-fu at 15, 40, and 90 mg/kg in rats resulted in an alpha-phase of elimination from plasma with a half-life of 10-18 minutes. The beta-phase could only be measured after the injection of 90 mg/kg of 5-FU and amounted to about 20 hours in two rats. Higher doses resulted in an unproportional increase in 5-FU plasma concentrations in the beta-phase, thus establishing nonlinear kinetics, presumably of the degradative metabolism. Plasma concentrations of 5-FU in man were measured after oral and intravenous administration of 15 mg/kg of 5-FU. Plasma elimination half-lives after intravenous doses were 12 minutes and dropped below measurable levels 2 hours after administration in three patients. After oral administration, peak plasma levels of 5-FU varied between 0.8 and 60 mug/ml of 5-FU in three patients after 10 minutes to 2 hours. Two of these patients showed rapid absorption and a measurable beta-phase of 5-FU plasma elimination possibly indicating a larger anabolic pool size of 5-FU-derived nucleosides and nucleotides when compared to patients receiving intravenous doses. This study may provide a pharmacokinetic rationale to monitor patients receiving 5-FU treatment.