An immunophilin-binding assay for sirolimus

Objective: This review examines the performance of 4 assays for sirolimus i n terms of their ability to meet 6 guidelines determined by a panel of expe rts. Background: Four methods have been described to date for the analysis of si rolimus concentrations in whole blood: high-performance liquid chromatograp hy-mass spectrometry (HPLC-MS); microparticle enzyme immunoassay (MEIA); p7 0 S6 kinase inhibition; and an immunophilin-binding assay (IBA). Methods: A MEDLINE(R) search of the literature was performed to identify re levant studies. Results: The HPLC methods suffer fi om precision problems because of the su bstantial specimen preparation required, and HPLC-MS methods are not practi cal for clinical use. Initial studies of the MEIA have found overestimation of sirolimus concentrations that may be caused by antibody cross-reactivit y with sirolimus metabolites. Monitoring of sirolimus effects by p70 S6 kin ase 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 sir olimus and that may be the intracellular target of the drug as the assay bi nding protein allows the assay to measure sirolimus selectively, even in th e presence of structurally similar metabolites. Conclusion: More than 200 clinical samples have been analyzed by the IBA, a nd 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 tha n HPLC, and has the potential to be automated. Thus, the IBA appears to hav e certain clear advantages over the other assays.