The quantification of sirolimus by high-performance liquid chromatography-tandem mass spectrometry and microparticle enzyme immunoassay in renal transplant recipients

Background: Sirolimus, an immunosuppressive agent, is undergoing clinical trials in the prophylaxis of organ rejection. Objectives: The aim of this study was to compare the performance of the semi-automated prototype (mode 1A) microparticle enzyme immunoassay (MEIA) against a validated highperformance liquid chromatography-mass spectrometry (HPLC-MS) method for measuring sirolimus concentrations. A secondary objective was to identify potential factors that may influence sirolimus measurement. Methods: The comparison was based on predose samples (n = 841) from 74 renal transplant patients receiving sirolimus therapy. Samples were collected up to 12 months after transplantation. Results: The mean (+/- SD) overestimation by MEIA was 42.5% +/- 16.9%. Several variables were investigated to determine potential contributors to the observed overestimation. Stratification of the data based on the mean sirolimus concentrations determined by both assays yielded no statistically significant differences in bias between concentration subgroups within the clinically relevant range. Multiple linear regression analysis identified HPLC-MS sirolimus concentration (P = 0.03), hemoglobin concentration (P < 0.001), and time after transplantation (P < 0.001) as significant variables in the prediction of overestimation by MEIA. Analysis of the effect of time after transplantation on overestimation yielded a statistically significant difference up to 6 months after transplantation (35.6% to 46.4%) compared with 9 (23.9%) and 12 months (24.4%). A relationship between hemoglobin concentration and time after transplantation may explain the reduction in bias observed after 6 months. Conclusion: The MEIA overestimates sirolimus concentrations in renal transplant patients compared with HPLC-MS.