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 t rials in the prophylaxis of organ rejection. Objectives: The aim of this study was to compare the performance of the sem i-automated prototype (mode 1A) microparticle enzyme immunoassay (MEIA) aga inst a validated highperformance liquid chromatography-mass spectrometry (H PLC-MS) method for measuring sirolimus concentrations. A secondary objectiv e was to identify potential factors that may influence sirolimus measuremen t. Methods: The comparison was based on predose samples (n = 841) from 74 rena l transplant patients receiving sirolimus therapy. Samples were collected u p to 12 months after transplantation. Results: The mean (+/- SD) overestimation by MEIA was 42.5% +/- 16.9%. Seve ral variables were investigated to determine potential contributors to the observed overestimation. Stratification of the data based on the mean sirol imus concentrations determined by both assays yielded no statistically sign ificant differences in bias between concentration subgroups within the clin ically 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 th e prediction of overestimation by MEIA. Analysis of the effect of time afte r transplantation on overestimation yielded a statistically significant dif ference 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 concent ration and time after transplantation may explain the reduction in bias obs erved after 6 months. Conclusion: The MEIA overestimates sirolimus concentrations in renal transp lant patients compared with HPLC-MS.