An immunophilin-binding assay for sirolimus

Objective: This review examines the performance of 4 assays for sirolimus in terms of their ability to meet 6 guidelines determined by a panel of experts. Background: Four methods have been described to date for the analysis of sirolimus concentrations in whole blood: high-performance liquid chromatography-mass spectrometry (HPLC-MS); microparticle enzyme immunoassay (MEIA); p70 S6 kinase inhibition; and an immunophilin-binding assay (IBA). Methods: A MEDLINE(R) search of the literature was performed to identify relevant studies. Results: The HPLC methods suffer fi om precision problems because of the substantial specimen preparation required, and HPLC-MS methods are not practical for clinical use. Initial studies of the MEIA have found overestimation of sirolimus concentrations that may be caused by antibody cross-reactivity with sirolimus metabolites. Monitoring of sirolimus effects by p70 S6 kinase inhibition is as yet possible only theoretically, and the assay itself is not vet optimal. With the IBA, use of a T-cell protein that binds to sirolimus and that may be the intracellular target of the drug as the assay binding protein allows the assay to measure sirolimus selectively, even in the presence of structurally similar metabolites. Conclusion: More than 200 clinical samples have been analyzed by the IBA, and correlation with HPLC values has been good, with a regression line slope near 1.0. In addition, the assay is easier to perform and more precise than HPLC, and has the potential to be automated. Thus, the IBA appears to have certain clear advantages over the other assays.