Quantitation of 5-fluorouracil (5-FU) in human plasma by liquid chromatography/electrospray ionization tandem mass spectrometry

5-Fluorouracil (5-FU) has long had a place in the treatment of many malignancies. 5-FU plasma concentrations have been correlated with toxicity and efficacy, and therapeutic drug monitoring has been reported to result in an improved response/toxicity balance. We report validation, according to FDA guidelines, of a hydrophilic interaction chromatography (HILIC) liquid chromatography/tandem mass spectrometry (LC/MS/MS) assay for the sensitive, accurate and precise quantitation of 5-FU in human plasma. The assay employed an isotopically labeled 5-FU internal standard and ethyl acetate extraction. Separation was achieved with an amino column and an isocratic mobile phase of 0.1% formic acid in acetonitrile/water (97:3, v/v), followed by a wash. Detection consisted of electrospray, negative-mode ionization tandem mass spectrometry in the multiple reaction monitoring (MRM) mode. The accuracy was 96.0-102.2%, and precision was 2.1-7.5% in the concentration range of 10-10 000 ng/mL. Recovery from plasma was 46.0-72.6%, and ion suppression was 9.8-25.7%. Plasma freeze/thaw stability was 87.5-104.3%, and stability for 4 h at room temperature was 98.7-100.0%. This assay is currently being used to quantitate 5-FU in human plasma samples.