Simultaneous determination of capecitabine and its metabolite 5-fluorouracil by column switching and liquid chromatographic/tandem mass spectrometry.

A liquid chromatographic/tandem mass spectrometric method was developed and validated for the quantitation of capecitabine and its metabolite 5-fluorouracil in human plasma. The simultaneous determination of both analytes was achieved by a column switching method using a trapping column and two analytical columns with different stationary phases. Isocratic elution was used for the separation of capecitabine on a C18 column whereas 5-fluorouracil was separated using gradient elution on an non-polar carbon phase. The calibration curves were linear for both compounds with a correlation factor (R2) > 0.9993 for 5-fluorouracil and >0.9942 for capecitabine. The assay was validated in the concentration range 5.00-1000 ng ml(-1) for both compounds. The intra-day precision was better than 10% for 5-fluorouracil and better than 11% for capecitabine whereas the inter-day precision was better than 8% for 5-fluorouracil and better than 14% for capecitabine.