An immunoassay for the measurement of sirolimus

This study assessed the performance characteristics of a new microparticle enzyme immunoassay (MEIA) for the determination of sirolimus in whole blood.

In clinical investigatory studies, dose adjustments of the immunosuppressive drug sirolimus have been carried out using either high-performance liquid chromatography (HPLC) or, more recently, this investigational immunoassay kit based on the MEIA technique.

Calibration was made over the linear range 0 to 30 ng/mL. Inaccuracy and imprecision were assessed by means of 3 control samples supplied with the kit (5, 11, and 22 ng/mL) and dilution of an above-quantitation-limit sample (154 ng/mL). Specificity was determined by the addition of 2 sirolimus metabolites to sirolimus-free human whole blood or to I of the control samples supplied with the kit. In addition, whole-blood samples from patients receiving either cyclosporine or tacrolimus (N = 24) were analyzed for sirolimus. A comparison of the MEIA and a validated HPLC/MS/MS assay analyzed both pooled samples from patients receiving sirolimus and spiked samples (sirolimus 2-60 ng/mL). In a more extensive comparison of patient samples measured by the MEIA assay, a validated HPLC assay with UV detection (HPLC-UV) was used (HPLC-UV sirolimus 7-64 ng/mL).

Inaccuracy (between-run) was < or =16.2% at all 4 concentrations (N = 5). Within-assay imprecision (repeatability) was <6% (N = 5), and between-assay imprecision (reproducibility) for the same samples was < 11% (N = 5). Recovery, assessed by means of 3 in-house control samples prepared in both fresh and previously frozen sirolimus-free human whole blood, ranged from 93.9% to 109.5%. The limit of detection, determined by dilution of the lowest nonzero calibrator (3 ng/mL), was set at 1 ng/mL, at which repeatability was 20.5% (N = 5). Five ng/mL of hydroxysirolimus cross-reacted with the assay by a mean of between 44% and 50% (N = 4); 5 ng/mL of 41-O-demethylsirolimus cross-reacted with the assay by a mean of between 86% and 127% (N = 4). Assay specificity was further challenged by ethylenediamine-tetraacetic acid (EDTA)-whole-blood samples from transplant patients not receiving sirolimus. These samples had tacrolimus and cyclosporine concentrations of 7.8 to 15.9 ng/mL and 38 to 485 microg/L, respectively. The median result was 0 ng/mL (third quartile, 0.7 ng/mL; maximum, 1.4 ng/mL); no value was above the lowest nonzero calibrator. The results of the comparison between the MEIA and the HPLC/MS/MS assay showed mean positive biases of 21% and 8% for the MEIA in measuring sirolimus in pooled patient samples and spiked samples, respectively. The results of the comparison of the MEIA and HPLC-UV median sirolimus concentrations were 18.2 and 20.1. Whole-blood samples anticoagulated with EDTA and containing sirolimus were stable for analysis by MEIA for 3 freeze-thaw cycles when stored at -20 degrees C and for 10 days when stored at 4 degrees C or at ambient temperature. A decline in sirolimus concentration occurred when samples were stored at 37 degrees C.

The MEIA showed suitable precision across a clinically relevant concentration range. In terms of patient management, the practical significance of cross-reactivity with sirolimus metabolites remains to be assessed.