Therapeutic Monitoring of Tacrolimus Concentrations in Blood: Semi-Automated Extraction and Liquid Chromatography-Electrospray Ionization Mass Spectrometry

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Abstract
The authors describe a highly selective liquid chromatographic/mass spectrometric (LC/MS) method for measurement of the immunosuppressive drug tacrolimus. A protein-free supernatant of a patient's whole-blood sample (500 μL) is applied to an Empore styrene-divinylbenzene (SDB-XC) disk cartridge (Varian Sample Preparation Products; Harbor City, CA) to isolate tacrolimus and the internal standard, ascomycin. The entire extraction is automated on the Gilson ASPEC XL4 (Gilson; Middleton, WI). Separation takes place on an octyldecyl (C18) high-performance liquid chromatographic (HPLC) column maintained at 75°C with a simple mobile phase of acetonitrile/water (90/10, v/v). An atmospheric pressure ionization (API)-electrospray ionization(ESI)-mass spectrometer (MS) is used for analysis. Detection is by selected ion monitoring (SIM) of the positively charged sodium adduct of tacrolimus and ascomycin, m/z 826.5 and 814.5, respectively. Collision-induced dissociation (CID) fragment ions of tacrolimus and ascomycin (m/z 616.4 and 604.4, respectively) are also monitored to enhance overall selectivity. Total run time is 1 minute per injection. A plot of chromatographic peak height ratio of m/z 826.5 to m/z 814.5 vs tacrolimus blood concentration is linear from the lowest limit of quantitation (0.3 μg/L) to at least 120.0 μg/L. Metabolites of tacrolimus and other immunosuppressive and commonly prescribed drugs do not interfere. Analytical recovery is 94% to 113% over a range of 4.4 to 41.0 μg/L. Between-run precision coefficients of variation (CV) are 3.6% to 4.2% over a range of 8.1 to 32.4 μg/L. Comparison data (n = 156) of the LC/MS method versus a commercial immunoassay (Abbott Tacrolimus IMx MEIA II) (Abbott; Chicago, IL) demonstrated that the tacrolimus concentration as measured by LC/MS = (0.912 × MEIA concentration) − 0.049;r 2 = 0.968. This validated LC/MS method demonstrates significant advantages over conventional immunoassays and improves on the lower limit of detection, sensitivity, accuracy, and range of linearity. In the authors' laboratory, this method is approximately 60% less costly to perform than the most commonly implemented immunoassay. Together, the authors' daily clinical experience during 1 year and participation in a proficiency testing program has demonstrated that the method is robust and suitable for routine therapeutic monitoring of tacrolimus.

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