Comparison of fluorescent polarization immunoassay (FPIA) versus HPLC to measure everolimus blood concentrations in clinical transplantation.

Clinical management of transplant patients depends on therapeutic drug monitoring (TDM) and regulation of immunosuppressive therapy. TDM of whole-blood concentrations is mandatory for everolimus (ERL) dosage individualisation. We compared the new semi-automated immunoassay (Innofluor Certican Assay System, Seradyn Inc) using FPIA technology on Abbott TDxFLx analyzers with established HPLC-UV as reference method. A total of 165 samples were analyzed from 52 transplant patients (40 kidney, 12 heart) receiving ERL or another immunosuppressive agent as part their routine care after organ transplantation. The correlation coefficient was r(2)=0.8229, and the regression equation (95% IC) yielded FPIA=1.111 x (HPLC)+0.378. FPIA compared to HPLC gave a positive bias of 1.19 ng/ml. The FPIA assay so appears to have a diagnostic efficacy comparable to HPLC for assessing the risk of acute rejection in transplant recipients. However, the values of the FPIA were higher than those calculated from HPLC measurements, because of the cross-reactivity of the antibody used in the FPIA assay with the ERL metabolite and/or with sirolimus; this cross-reactivity occurs frequently when transplant patients are switched from sirolimus to ERL.