Sensitive high-performance liquid chromatographic method for the determination of 5-fluorouracil in plasma.

The fluorinated pyrimidine, 5-fluorouracil (FU), has been used in the treatment of a wide variety of solid tumors for more than two decades [l] . It is frequently used in the treatment of early and advanced colorectal carcinomas. Since the drug has a short biological half-life, the clinical pharmacokinetic studies require a sensitive assay procedure which can quantitate low ng/ml FU levels in plasma. Several gas chromatographic (GC) and high-performance liquid chromatographic (HPLC) methods for determining plasma FU levels have been reported in the literature [2--193. The GC methods have employed either packed columns [2] or capillary columns [3] and utilized the nitrogen-phosphorusselective detector. The sensitivity of the capillary GC method was reported to be 150 ng/ml FU. Another GC procedure described in the literature requires derivatization of FU before analysis [9]. Several gas chromatographic-mass spectrometric (GC-MS) assays for FU have also been reported [4, 16-191 which are both sensitive and specific, but the complexities of the assays and associated instrumentation prohibit their routine use. A HPLC method reported in the literature suffered from poor reproducibility [5], which was probably due to adsorption of FU on glass surfaces [8] . Modification of this assay has been reported to improve its reproducibility [6] but the assay employs thymidine as the internal standard which is endogenous in plasma. Another HPLC assay employed back-extraction of the organic extract into phosphate buffer (pH=ll), followed by neutralization with sulfuric acid, but the internal standard, 5-fluorocytosine, coeluted with an endogenous peak in blank (control) plasma [ 71. The assay procedure described in this report can attain detection limits around 25 ng/ml and is routinely used in the FU concentration range of 50-