A drug name in an analytical method can identify a laboratory reagent. A rat-plasma assay for lesinurad used diazepam as an internal standard, making the sample-preparation record essential to interpreting its presence.
Zhou and colleagues developed a UHPLC-MS/MS method for lesinurad and applied it to a rat pharmacokinetic experiment. During preparation, the laboratory added a specified volume of diazepam solution to plasma before extraction with ethyl acetate. The study describes diazepam as the internal standard.
The method monitored separate precursor-to-product ion transitions for lesinurad and diazepam. Calibration related the lesinurad-to-internal-standard peak-area ratio to known lesinurad concentrations.
The investigators selected diazepam because of its extraction recovery, chromatographic performance, and detection sensitivity under their analytical conditions. The internal standard supplied a comparison signal within the preparation and measurement process.
The paper also examined blank plasma, spiked plasma, and samples collected after lesinurad administration. These different sample types answer different questions about interference, calibration, and measured exposure. Their identities should remain attached to any chromatogram used in a report.
For forensic interpretation, first establish which compounds were administered to the experimental subject and which were introduced during laboratory preparation. An internal-standard signal generated after a known addition is expected within that method. It cannot independently establish exposure before sample collection.
The validated matrix was rat plasma, and the application concerned lesinurad pharmacokinetics. Extending this method to another matrix or analyte would require appropriate validation. The broader lesson is to read preparation instructions, standards, controls, and analytical targets together before assigning biological meaning to a named peak.
Zhou XY and colleagues. Determination of lesinurad in rat plasma by a UHPLC-MS/MS assay. Chemistry Central Journal, 2017. Full text ยท DOI 10.1186/s13065-017-0353-6.