A highly sensitive method for determination of tacrolimus in peripheral blood mononuclear cells by nano liquid chromatography–high resolution accurate mass spectrometry

The determination of intracellular tacrolimus concentration in peripheral blood mononuclear cells (PBMCs) is crucial for assessing the effect-site concentration of tacrolimus. Analytical methods previously reported required a minimum of 3 mL of whole blood sample for measuring the tacrolimus concentration. In this study, we developed a highly sensitive method using EASY nLC 1200 combined with Q Exactive orbitrap mass spectrometer for detecting tacrolimus in PBMCs, requiring only 0.5–2 mL of sample. Furthermore, we compared two primary normalization methods for PBMCs tacrolimus concentration using Passing–Bablok regression, Bland–Altman analysis, Spearman's rank correlation, and Mountain plot. The newly established method was employed to compare tacrolimus concentrations in whole blood and PBMCs among 194 lung transplant recipients. The developed method exhibited high sensitivity with a lower limit of quantitation at 5 pg/mL, and excellent intra- and inter-days accuracy and precision. The comparison between different normalization methods for PBMCs tacrolimus concentration revealed a strong correlation between PBMCs count and intracellular protein amount within these cells. This finding suggests that both PBMCs count and intracellular protein amount can be used for normalizing intracellular tacrolimus levels and can be mutually converted. However, a weaker correlation was observed between PBMCs and whole-blood tacrolimus concentrations in lung transplant recipients, warranting further investigation. The method reported herein enables the quantification of PBMCs tacrolimus concentration using smaller volumes of whole blood samples, which has significant implications for both patients and laboratory personnel.