Robust <scp>UPLC</scp>–<scp>MS</scp>/<scp>MS</scp> Method With Acetonitrile for Precise Intracellular Quantification of Tacrolimus in <scp>PBMCs</scp>: A Step Toward Clinical Integration

Monitoring whole blood tacrolimus concentrations is standard in clinical practice; however, it may not fully reflect its therapeutic effects, as tacrolimus primarily acts within lymphocytes. While various intracellular quantification methods have been developed, many involve complex procedures such as evaporation, reconstitution, or specialized tools (e.g., magnetic beads, online solid-phase extraction), limiting their accessibility. This study aimed to develop and validate a streamlined, sensitive method for measuring intracellular tacrolimus concentrations using 5×10⁵ peripheral blood mononuclear cells (PBMCs). Tacrolimus concentrations were quantified using liquid chromatography-tandem mass spectrometry (LC-MS/MS). PBMCs were aliquoted into 50 μL volumes containing 5×10⁵ cells and prepared via acetonitrile-based protein precipitation. Chromatographic separation was performed using a Luna C18 column with a gradient mobile phase consisting of water with 20 mM ammonium acetate, 0.1% formic acid, and methanol at a flow rate of 0.4 mL/min. The method demonstrated excellent linearity between 0.1 and 25 ng/mL, corresponding to intracellular concentrations of 1-250 pg/5×10⁵ cells (r² = 0.999). Intra- and interday accuracy ranged from 98.1% to 109.8%, with precision between 2.08% and 8.70% across validation runs. Extraction recovery was high (93.0%-97.2%), with minimal matrix effects (100.9% at low QC and 111.6% at high QC). This validated LC-MS/MS method provides a rapid, reliable, and sensitive approach for pharmacokinetic studies and clinical applications, facilitating intracellular tacrolimus monitoring in transplant patients.