Stable isotope dilution microquantification of creatine metabolites in plasma, whole blood and dried blood spots for pharmacological studies in mouse models of creatine deficiency
To develop an accurate stable isotope dilution assay for simultaneous quantification of creatine metabolites ornithine, arginine, creatine, creatinine, and guanidinoacetate in very small blood sample volumes to study creatine metabolism in mice. Liquid-chromatography (C18) tandem mass spectrometry with butylation was performed in positive ionization mode. Stable isotope dilution assay with external calibration was applied to three different specimen types, plasma, whole blood and dried blood spot (DBS). Analytical separation, sensitivity, accuracy, and linearity of the assay were adequate. The stable isotope dilution assay in plasma revealed no significant bias to gold standard methods for the respective analytes. Compared to plasma, we observed an overestimate of creatine and creatinine (2- to 5-fold and 1.2- to 2-fold, respectively) in whole-blood and DBS, and an underestimate of arginine (2.5-fold) in DBS. Validation of the assay in mouse models of creatine deficiency revealed plasma creatine metabolite pattern in good accordance with those observed in human GAMT and AGAT deficiency. Single dose intraperitoneal application of ornithine in wild-type mice lead to fast ornithine uptake (Tmax ≤ 10 min) and elimination (T1/2=24 min), and a decline of guanidinoacetate. The assay is fast and reliable to study creatine metabolism and pharmacokinetics in mouse models of creatine deficiency.