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

Abstract 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 µ M ) for dFdC and 250 ng/ml for dFdU, and validated the assay up to the dFdC concentration of 6.0 µg/ml (22.8 µ M ). 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. Copyright © 2007 John Wiley & Sons, Ltd.