Quantification of gimeracil, tegafur, and 5-FU in human plasma via LC-MS/MS with a simplified pretreatment using flow-through extraction

Gimeracil, a component in S-1 (an oral anticancer agent comprising tegafur, a prodrug of 5-fluorouracil (5-FU), potassium oxonate, and gimeracil), inhibits metabolic enzymes, thereby impeding 5-FU degradation. Therefore, the blood level of gimeracil is closely associated with the disposition of 5-FU, and quantification of gimeracil can provide important information if a case shows an inappropriate 5-FU blood concentration. Nevertheless, methods for quantifying gimeracil in human plasma are rarely reported. Herein, we aimed to develop a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for quantifying gimeracil, in addition to tegafur and 5-FU, levels in human plasma using a clinically applicable simplified pretreatment process and faster elution time. Hence, an acetamide-functionalized monolith silica disk-packed spin column was used to extract gimeracil and internal standard (IS; nicotinamide), whereas diatomaceous earth-based solid phase for liquid-liquid extraction was used to extract tegafur, 5-FU, and IS (5-chlorouracil) from plasma. Each extract was analyzed within 4 min of elution via LC-MS/MS using a shared LC column and mobile phase. Accuracy and precision analyses indicated lower limits of quantification of 5, 10, and 2 ng/mL for gimeracil, tegafur, and 5-FU, respectively. The calibration curves showed good linearity between 5 and 500 ng/mL for gimeracil, 10 and 5000 ng/mL for tegafur, and 2 and 1000 ng/mL for 5-FU. We confirmed that the levels of all analytes in the plasma of patients with cancer undergoing S-1-inclusive therapy were within the calibration range for each analyte. Thus, this newly developed quantification method is likely to be useful for optimization of S-1 therapy.