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...