Salting Out-Assisted Liquid–Liquid Extraction for Liquid Chromatography–Tandem Mass Spectrometry Measurement of Tacrolimus, Sirolimus, Everolimus, and Cyclosporine a in Whole Blood

Background: Therapeutic drug monitoring of the immunosuppressants tacrolimus, sirolimus, everolimus, and cyclosporine A is effectively performed by analyzing whole-blood samples using liquid chromatography coupled with tandem mass spectrometry. Samples are usually prepared using simple protein precipitation (PPT) with methanol and zinc sulfate (ZnSO 4 ). Significant sample dilution is necessary to obtain clean extracts but may increase the limit of quantification of the method. Salting out–assisted liquid–liquid extraction (SALLE) was explored as a novel sample preparation method for measuring these drugs in blood. Method: SALLE, which simply consists of LLE with a water-miscible solvent where phase separation is achieved by adding salt, was used to analyze treated blood samples. Results: SALLE allowed direct injection of a 5-µL extract from the upper solvent phase into a reversed phase LC column, which would not be feasible using standard LLE. Compared with PPT, SALLE provided better extraction efficiencies and more ion enhancement, resulting in limit of quantification of 0.4, 1.4, 0.06, and 0.4 ng/mL for tacrolimus, sirolimus, everolimus, and cyclosporine A, respectively. Full-method validation was performed, including a comparison of results with those of another laboratory. A ≤10% bias was observed for tacrolimus and cyclosporine A, whereas further investigation of that for sirolimus (−12%) and everolimus (−18%) revealed that it was caused by the different calibrators used. Conclusions: This is the first report of the use of SALLE for the measurement of tacrolimus, sirolimus, everolimus, and cyclosporine A in whole blood. The advantages of SALLE over PPT and conventional LLE would make it an attractive sample preparation method for clinical laboratories.