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Introduction: 5-Fluorouracil is a commonly prescribed antineoplastic drug. Historically, methods to determine this compound in biological fluids have been time consuming or not sensitive enough. Materials and Methods: In this report, a method using simple and rapid sample preparation with good sensitivity, precision and accuracy for the quantitative determination of 5-fluorouracil in human plasma by LC-MS/MS is established and validated. By the judicious combination of a small amount of injected sample, low flow rate, careful control of gradient elution profile, use of a HILIC pre-column to further remove residual phospholipids and an isotopically labelled internal standard (ILIS), the method has successfully overcome matrix effect with protein precipitation sample pre-treatment without evaporating to dryness and reconstituting process. Conclusion: This newly developed method can be used to measure patient plasma concentrations of 5- fluorouracil, as low as 0.8 ng/mL and as high as 100 µg/mL, with less than 15% both intra- and interday accuracy and precision. Such a methodology will be very convenient of being used in routine therapeutic drug monitoring.

C-S lyase enzymes catalyse the generation of mutagenic and/or cytotoxic thiols from cysteine conjugated xenobiotics. These cysteine conjugates are produced subsequent to glutathione conjugations as a metabolic step in the mercapturic acid pathway, traditionally thought of as a pathway solely associated with detoxification. Human Chang liver (HCL) cells were challenged with a range of cysteine conjugates demonstrated to be substrates for human hepatic C-S lyases. The cellular toxicity of these compounds was determined and it was observed that the rank order of substrate toxicity obtained for the HCL cells followed the rank order of C-S lyase activity of the substrates in a freshly isolated mitochondrial fraction of human tissue. The presence of C-S lyase activity was also established in this cell line.