Simultaneous determination of gemcitabine and its main metabolite, dFdU, in plasma of patients with advanced non-small-cell lung cancer by high-performance liquid chromatography-tandem mass spectrometry

Gemcitabine, 2′,2′-difluoro-2′-deoxycytidine (dFdC) is a pyrimidine antimetabolite employed against several human malignancies. It undergoes intracellular activation to the pharmacologically active triphosphate form (dFdCTP) and metabolic inactivation to the metabolite 2′,2′- difluorodeoxyuridine (dFdU). In order to investigate the human plasma pharmacokinetics of dFdC and dFdU, we developed and validated an HPLC-MS/MS method, adding 2′-deoxycytidine as internal standard and simply precipitating the protein with acetonitrile. The method requires a small sample (125 μl), and it is rapid and selective, allowing good resolution of peaks from the plasma matrix in only 7 min. It is sensitive, precise and accurate, with overall precision, expressed as CV%, always less than 10.0% for both analytes and high recovery: ≥80%. The limits of detection for dFdC and dFdU were 0.1 and 1.1 ng/ml, but considering the high concentrations in the plasma of patients investigated, we set the limit of quantitation at 20 ng/ml (0.08 μM) for dFdC and 250 ng/ml for dFdU, and validated the assay up to the dFdC concentration of 6.0 μg/ml (22.8 μM). The method was successfully used to study the drug pharmacokinetics in patients with advanced non-small-cell lung cancer in a phase II trial with gemcitabine administered as a fixed dose-rate infusion.