Analytical Validation of an LC-MS/MS Method for Simultaneous Quantification of Multiple Immunosuppressants in Microvolume Whole Blood.

Immunosuppressants are essential for preventing allograft rejection; however, they require therapeutic drug monitoring to maintain efficacy and to prevent severe complications such as opportunistic infections. Calcineurin inhibitors (CIs) are primarily distributed in red blood cells, whereas mycophenolic acid (MPA) and its metabolites are found in plasma. These differences necessitate separate analyses for each drug, increasing laboratory workload, analytical complexity, and patient burden. We developed a liquid chromatography-tandem mass spectrometry method for simultaneous quantification of CIs such as tacrolimus (Tac), everolimus (Eve), sirolimus (Sir), cyclosporine A (CycA) and MPA in 2.8-µL whole-blood samples, with a hematocrit-based correction to estimate plasma-equivalent MPA concentrations. Performance of this method was assessed by comparison with conventional immunoassay results using linear regression and Bland-Altman analyses, demonstrating excellent agreement, with strong linearity (R > 0.995) at <2 to 35 ng/mL for three CIs, 26.0 to 1866 ng/mL for CycA, and 0.1 to 50 μg/mL for MPA. Furthermore, MPA and tacrolimus concentrations closely aligned with routine clinical results (R > 0.900), indicating high accuracy and reproducibility. This new approach may be particularly beneficial for hospitalized patients with limited venous access, pediatric populations, and in remote care settings where frequent blood sampling is challenging because of simultaneous quantification and fewer sample volume requirements.