Quantification of N-acetyl-seryl-aspartyl-lysyl-proline in hemodialysis patients administered angiotensin-converting enzyme inhibitors by stable isotope dilution liquid chromatography-tandem mass spectrometry

We developed a sensitive, selective and accurate method based on liquid chromatography with tandem mass spectrometry (LC-MS/MS) to determine N-terminal thymosin-β peptides of Ac-SDKP and Ac-ADKP in human plasma samples. Quantification of Ac-SDKP and Ac-ADKP was performed using solid phase extraction (SPE) based on C18, reversed phase LC separation, and stable isotope dilution electrospray ionization-MS/MS in multiple reaction-monitoring (MRM) mode. The Ac-SDKP-13C6, 15N2 and Ac-ADKP-d7 were synthesized for the internal standards. These MRM monitoring ions were m/z 488 → 129 (quantitative ion)/226 for Ac-SDKP, m/z 496 → 137 for Ac-SDKP-13C6, 15N2, m/z 472 → 129 (quantitative ion)/226 for Ac-ADKP, and m/z 479 → 129 for Ac-ADKP-d7, respectively. Lower limit of quantitation (LLOQ) of Ac-SDKP and Ac-ADKP was 0.1 ng/mL in human plasma. Recovery values were ranged from 94.7% to 106.3% for inter- (RSD: 0.6-3.5%) and intra- (RSD: 0.4-4.9%) day assays. Plasma Ac-SDKP levels were significantly higher in hemodialyzed subjects treated with angiotensin-converting enzyme inhibitors of enalapril (27.3 ± 24.6 ng/mL, n = 10) and trandolapril (12.3 ± 16.9 ng/mL, n = 18) than healthy (0.4 ± 0.2 ng/mL, n = 7) and hemodialyzed subjects (0.6 ± 0.2 ng/mL, n = 34). This analytical method would be useful to measure N-terminal thymosin-β peptides in human plasma for the clinical study.