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Simplified High‐Performance Liquid Chromatography‐Mass Spectrometry Assay for Measurement of Tacrolimus and Its Metabolites and Cross‐Validation with Microparticle Enzyme Immunoassay

Summary In this study, a modified, specific assay for measurement of tacrolimus and its metabolites in blood and urine from transplant patients using high-performance liquid chromatography (HPLC) linked to mass spectrometry (MS) is described. Samples were prepared for HPLC-MS by modified solid-liquid extraction. The original two-step washing procedure was replaced by a single washing step, and samples were eluted with acetonitrile/water instead of dichloromethane, thus avoiding an evaporation step. Samples were injected automatically every 3 min into the HPLC-MS system. Time-consuming gradient elution was replaced by isocratic elution. This procedure resulted in a lower limit of quantitation of 0.2 μg/L. The interassay variability was 14.5% for 5 μg/L and 15.8% for 25 μg/L. The intraassay variability was 11.2% for 5 μg/L and 4% for 25 μg/L. The recovery for tacrolimus in blood was 90.4% for 1 μg/L, 78.9% for 10 μg/L, and 81.3% for 25 μg/L. Measurement of tacrolimus and its metabolites in samples from various transplant patients showed that the main metabolites found in blood and urine are demethyl-tacrolimus, di-demethyl-tacrolimus and demethyl-hydroxy-tacrolimus. Cross validation of the modified HPLC-MS assay with a microparticle enzyme immunoassay showed a significant correlation between the two assays, with r = 0.915.