Mass spectrometric quantification of AcSDKP-NH2 in human plasma and urine and comparison with an immunoassay

Precise assessment of renal glomerular filtration rate (GFR) is essential for the early detection of chronic kidney disease. AcSDKP-NH(2), an analogue of the endogenous tetrapeptide AcSDKP, is not degraded in vivo and is freely filtered by the kidney and eliminated in urine; for that reason this analogue is an ideal candidate marker for the assessment of GRF after administration to humans. Proof-of-concept demonstration and lack of toxicity in animals have allowed an ongoing clinical study in which AcSDKP-NH(2) was administered intravenously at a dose of 100 µg and compared with currently available GFR markers. The use of the AcSDKP analogue in clinical practice requires that this novel marker be associated with an analytical method that combines specificity, robustness and high accuracy. We have developed a liquid chromatography/tandem mass spectrometry (LC/MS/MS) assay and compared it with an existing enzyme immunoassay (EIA) for AcSDKP-NH(2).

Human urine and plasma samples from the clinical study were analyzed by EIA and LC/MS/MS. Before LC/MS/MS assessment, AcSDKP-NH(2) was extracted using mixed-mode cation-exchange solid-phase extraction cartridges. Chromatographic separation was performed by hydrophilic interaction liquid chromatography (HILIC), before analysis with an electrospray ionization triple quadrupole mass spectrometer.

Mass spectrometry, through the use of an internal standard, tailored sample preparation and chromatographic separation, has better intra- and inter-assay precision (accuracies between 95 and 101% with CVs <8% for LC/MS/MS vs. accuracies between 90 and 115% with CVs <18% for EIA) and allows greater steadiness in intra-subject concentrations during the infusion (4.4% for LC/MS/MS vs. 8.6% for EIA). Moreover, the LC/MS/MS assay circumvents matrix effects observed in certain instances for the EIA and which may reduce its accuracy.

Although the EIA can provide sufficient information in most subjects, the LC/MS/MS assay associated with this new marker should be the reference method.