UC San Diego

BACKGROUND: Therapeutic drug monitoring of the immunosuppressant tacrolimus is commonly performed by immunoassay or LC-MS/MS. Measurement biases between these methodologies have been characterized for immediate-release tacrolimus (IR-tac; Prograf) but have not been performed for extended-release formulations such as Envarsus. These discrepancies can impact patient care, as appropriate dosing is required to maintain therapeutic concentrations and immunosuppression. METHODS: Validation of a whole-blood LC-MS/MS method for the simultaneous quantification of tacrolimus and its major metabolite, desmethyl tacrolimus, was performed using traceable calibrators (tacrolimus, ERM-DA110a) and quality control (QC) material for tacrolimus and standard material for desmethyl tacrolimus. Tacrolimus concentrations were determined by LC-MS/MS and the ARCHITECT immunoassay in patients receiving either IR-tac or Envarsus for clinical care. RESULTS: External calibration curves for both tacrolimus and desmethyl tacrolimus were linear (R2 > 0.995), and the analytical measurement range (AMR) for tacrolimus spanned from 1.1 to 31.6 ng/mL. Calibrator/QC biases were within 15% of their spiked concentrations throughout the AMR, and within-run imprecision was <10%, except at the lower limit of quantification (n = 25). Between-run imprecision for low, mid, and high QC levels was ≤11% over a 2-week period (n = 5 days). Comparative biases between immunoassay and LC-MS/MS were significantly lower (P = 0.0074) for patients receiving Envarsus (n = 20 specimens) relative to patients receiving IR-tac (n = 32 specimens). CONCLUSIONS: Biases between immunoassay and LC-MS/MS tacrolimus measurements in patients receiving immediate-release vs extended-release formulations indicate that their distinct pharmacokinetic profiles impact measurement accuracy. These assay biases should be considered when interpreting tacrolimus concentration measurements.