Everolimus TDM using Thermo Fisher QMS immunoassay on Indiko, Beckman DxC, AU680, and AU5800 analyzers

Development of an agglutination-enhancement immunoturbidimetric tacrolimus assay with improved sensitivity

Accurate quantification of low level of blood drugs by Latex enhanced immunoturbidimetric assay (LEITA) remains a challenge due to its inherent limited sensitivity. To deal with this problem, we designed a new agglutination-enhancement strategy, and applied it for development of a highly sensitive and accurate tacrolimus LEITA. By this principle, a very small amount of biotin labeled anti-tacrolimus monoclonal antibodies (BLATMA) can arise agglutination strong enough for accurate reading of the increased absorbance since the BLATMA bears multiple biotin molecules, and the agglutination mediated by BLATMA can be inhibited by a similarly small amount of tacrolimus when the drug binds to BLATMA, giving rise to an improved sensitivity. The limit of detection (LOD) and functional sensitivity obtained by the proposed tacrolimus LEITA was 0.22 ng/mL and 0.59 ng/mL, respectively, 8-20 times more sensitive than the conventional drug-latex or antibody-latex based direct inhibition LEITA formats. Good precision was observed in the whole range of clinically significant tacrolimus concentration. The reliability of the tacrolimus LEITA was demonstrated by its strong correlation with both liquid chromatography-tandem mass spectrometry (LC-MS/MS) (R² = 0.977, slope = 0.998) and the ABBOTT tacrolimus chemiluminescent magnetic immunoassay (CMIA) (R² = 0.982, slope = 1.01) in the analysis of 119 clinical samples. It's concluded that the agglutination-enhancement strategy can be applied to construct highly sensitive LEITA for accurate tacrolimus analysis; owing to the improved sensitivity, this technique can be expected not only to improve the reliability of LEITA for low-level drug monitoring, but also to broaden the scope of analytes detectable by LEITA.

Long-Term Performance of Laboratory-Developed Liquid Chromatography-Tandem Mass Spectrometry Tests and a Food and Drug Administration-Approved Immunoassay for the Therapeutic Drug Monitoring of Everolimus

BACKGROUND

Laboratory-developed tests (LDTs) are analytical tests developed and validated "in-house" for clinical diagnosis. Regulatory agencies, such as the United States Food and Drug Administration (FDA), encourage using regulatory-approved assays rather than LDTs. In the ongoing Zortracker everolimus study, samples were provided monthly to participating clinical laboratories that conduct therapeutic drug monitoring of everolimus. This allowed for the long-term (6-year) comparison of the performance of liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays, which are LDTs, to the FDA-approved everolimus Quantitative Microsphere System (QMS).

METHODS

Each laboratory received the same 3 blinded samples. LC-MS/MS and QMS assays were compared using Passing Bablok regression analysis. Data were analyzed in 12-month periods to detect trends over time.

RESULTS

The slopes of the Passing Bablok regression curves remained unchanged in 2013 and 2014 (reference LC-MS/MS; test QMS: slope = 0.934 and 1.008). However, by 2016, the slope increased significantly to between 1.111 and 1.320, then dropped to 0.980 in 2017 and 0.912 in 2018, suggestive of changes in QMS bias compared with LC-MS/MS over longer periods. Outliers did not affect these results. The interlaboratory variability of LC-MS/MS and QMS remained unchanged from 2013 to 2015, with coefficients of variation of 15.3%, 18.4%, and 17.2% for LC-MS/MS, and 13.0%, 13.1%, and 15.3% for QMS, respectively, per year. At the end of the observation period, the average coefficients of variation in LC-MS/MS laboratories dropped to 14.3%, 12.6%, and 14.2%, whereas the variability in QMS laboratories was 19.5%, 13.3%, and 19.6% in 2016, 2017, and 2018, respectively.

CONCLUSIONS

Initially, QMS everolimus concentrations in patient samples were comparable with those detected in LC-MS/MS laboratories. However, concentration bias of the QMS assay significantly changed within 6 years, emphasizing the need for long-term, independent performance tracking of therapeutic drug monitoring assays, including FDA-approved assays.