A universal surrogate matrix assay for urea measurement in clinical pharmacokinetic studies of respiratory diseases.

In pharmacokinetic studies for respiratory diseases, urea is a commonly used dilution marker for volume normalization of various biological matrices, owing to the fact that urea diffuses freely throughout the body and is minimally affected by disease states. In this study, we developed a convenient liquid chromatography-tandem mass spectrometry (LC-MS/MS) surrogate matrix assay for accurate urea quantitation in plasma, serum and epithelial lining fluid. Different mass spectrometer platforms and ionization modes were compared in parallel. The LC method and mass spectrometer parameters were comprehensively optimized to reduce interferences, to smooth the baseline and to maximize the signal-to-noise ratio. Saline was selected as the surrogate matrix, and its suitability was confirmed by good parallelism and accurate quality control sample measurements. Reliable and robust assay performance was demonstrated by precision and accuracy, dilution integrity, sensitivity, recovery and stability, all of which met bioanalysis requirements to support clinical studies. The assay performance was also verified and better understood by comparing it with a colorimetric assay and to a surrogate analyte assay. The newly developed surrogate matrix assay has the potential to be further expanded for urea quantitation in numerous physiological matrices.