Measurement of 5-fluoro-2'-deoxyuridine serum levels by gas chromatography chemical ionization mass spectrometry.

Serum levels of 5-fluoro-2′-deoxyuridine in cancer treated patients were measured by gas chromatography mass spectrometry under chemical ionization conditions; 1-(2-deoxy-β-D-lyxofuranosyl)-5-fluorouracil (3′-epi-5-fluoro-2′-deoxyuridine) was used as an internal standard. The drug and internal standard were quantitatively isolated from the serum sample by a mini-column anion exchange method and the extract permethylated using potassium-tert-butoxide in dimethylsulphoxide and methyl iodidie. The derivatized nucleosides were then re-extracted from the reaction mixture and analysed on a glass capillary column coated with Superox-4. The column was coupled directly to the chemical ionization source of the mass spectrometer; NH3 was used as the reagent gas. The gas chromatographic effluent was monitored at m/z 289, the [MH]+ ion of the N,O-permethyl derivatives of 5-fluoro-2′-deoxyuridine and the internal standard. Recovery of 5-fluoro-2′-deoxyuridine from serum in the 0–1 μg ml−1 concentration range averaged 93 ± 2% (SD); a linear detector response was observed up to 50 ng 5-fluoro-2′-deoxyuridine ml−1. At the 10 ng ml−1 level, a within-run assay precision of 10% (CV) (n = 5) was found, while a detection limit of about 1 ng 5-fluoro-2′-deoxyuridine ml−1 of serum was attained. The method was applied to the measurement of disappearance curves of 5-fluoro-2′-deoxyuridine in the serum of treated patients.