LC-MS/MS quantification of endogenous taurine in plasma and urine using surrogate matrix calibration and background subtraction: Application to renal transporter phenotyping.

Taurine is an abundant endogenous amino acid-related compound involved in several physiological processes, including osmoregulation, antioxidative defense, and cytoprotection. Its quantification in biological matrices is analytically challenging due to its high polarity, endogenous background levels, and the absence of a true blank biological matrix. In addition, taurine has emerged as a potential endogenous biomarker for renal transporter phenotyping, particularly for organic anion transporters 1 and 3 (OAT1/3). In this study, a rapid and robust LC-MS/MS method was developed and validated for the quantification of endogenous taurine in human plasma and urine using surrogate matrix calibration combined with background subtraction. Chromatographic separation was achieved on a Luna NH₂ 100 Å column (5 µm, 150 × 4.6 mm) with a total run time of 5.5 min. Calibration curves prepared in 2% bovine serum albumin and diluted urine were linear over concentration ranges of 0.05-50 µg/mL and 0.05-100 µg/mL, respectively (r² > 0.99). Precision and accuracy met regulatory acceptance criteria (≤15%, or ≤20% at the lower limit of quantification). No significant matrix effects were observed across independent plasma and urine sources, including lipemic and hemolyzed plasma samples. Matrix and dilutional parallelism were confirmed, and taurine remained stable under all tested conditions. The method was successfully applied to clinical plasma and urine samples, enabling taurine pharmacokinetic profiling and renal clearance estimation. This validated LC-MS/MS assay provides a reliable analytical platform for taurine quantification in biological matrices and supports future pharmacokinetic, biomarker, nutritional, and transporter phenotyping studies.