Validation of a simple assay for the quantification of the capecitabine metabolites 5'-DFCR and 5'-DFUR for drug monitoring in patients receiving outpatient chemotherapy.

A simple and precise analytical method for the determination of 5'deoxy-5-fluorocytidine (DFCR) and 5'deoxy-5-fluorouridine (DFUR), the enzymatically formed metabolites of capecitabine in plasma, was developed using a reversed-phase high performance liquid chromatography gradient method with external standard method. Blood samples were analyzed after separation of DFCR/DFUR by solid-phase extraction from matrix compounds using a C16 amide reversed-phase column operated at a flow rate of 0.8 ml/min in gradient elution mode with a mobile phase composed of water-methanol (10 mM ammonium acetate in water; m/v). Excellent recoveries in plasma ranging from 77.5-99.12% for DFCR and 84.70-99.15% for DFUR, respectively, were obtained. For both compounds the calibration curves were linear over the range from 0.156 to 5.0 μg/ml. The present assay is robust, selective and sensitive, and is being applied in our laboratories to monitor plasma concentrations of DFCR and DFUR in clinical phase I and phase II studies.