Simultaneous determination of 5'deoxy-5-fluorouridine, 5-fluorouracil and 5,6-dihydro-5-fluorouracil in plasma by gas chromatography- mass spectrometry

A gas chromatography-mass spectrometry (GC-MS) method for the simultaneous determination of 5'-deoxy-5- ftuorouridine (doxifluridine, 5'-dFUR), 5-fluorouracil (5-FU) and its main catabolite 5,6-dihydro-5-fluorouracil (5-FUH2) in human plasma has been developed. Sample preparation consisted of protein precipitation with ammonium sulphate followed by analyte extraction with ethyl acetate/isopropanol (90/10, v/v) mixture. Extracts were then analysed by GC-MS in positive electron impact mode after derivatization with N,O-bis(trimethylsilyl)trifluoroacetamide/pyridine (1/1, v/v). The trimethylsilyl (TMS) derivative of 5'-dFUR was proved to be stable so that no interference in 5-FU determination was observed. Mass spectra of TMS derivatives of 5'-dFUR, 5-FU and 5-FUH2 are discussed. Analyte response was linear over two decades of concentration and detection limits were typically 20 ng/ml of plasma.