Sandwich assay for tacrolimus using 2 antitacrolimus antibodies.

BACKGROUND: Although detection of natural haptens by antihapten antibodies in sandwich assay format has the theoretical advantages of high analytical specificity and sensitivity, this type of assay has not been reported because of the seemingly insurmountable task of avoiding steric hindrance between the 2 bindings. This is especially true for ring-structured hydrophobic haptens. The macrolide drug tacrolimus (FK506, Prograf®, 804 Da) is such a hapten. Here we show the detection of tacrolimus using 2 antitacrolimus monoclonal antibodies in a sandwich assay. METHODS: Both antibodies were developed by use of an intact tacrolimus molecule covalently linked to a carrier protein but via 2 different positions separated by 10 carbon atoms. Epitope analysis based on drug analog binding was used to show no overlap between the binding sites of the 2 antibodies, indicating the 10-carbon separation resulted in 2 distinct epitopes. The distinct epitopes suggested that the drug might be approachable by the antibodies from 2 separate directions, which predicted simultaneous binding as in sandwich formation. RESULTS: This prediction was confirmed in sandwich ELISA and affinity column-mediated immunoassay formats. The assay demonstrated good imprecision and significantly lower metabolite cross-reactivity than competitive assay counterparts. Comparison with liquid chromatography-tandem mass spectrometry (LC-MS/MS) using 55 whole-blood samples from transplant patients with tacrolimus concentrations ranging from 0.9 to 29.5 ng/mL showed a linear regression: sandwich = 0.99 × LC-MS/MS + 0.10 ng/mL, r = 0.991, Sy|x = 1.08 ng/mL. CONCLUSIONS: This work demonstrates that a highly specific sandwich assay using 2 antihapten antibodies is feasible for the measurement of a hapten drug.

Performance characteristics of a no-pretreatment, random access sirolimus assay for the Dimension RxL clinical chemistry system.

OBJECTIVES: Current therapeutic drug monitoring methods for sirolimus require a manual pre-treatment step and batch analysis. We describe and validate a no-pretreatment, random access sirolimus assay for the Dimension RxL clinical chemistry system from Siemens Healthcare Diagnostics Inc. DESIGN AND METHODS: Whole blood samples from renal transplant patients prescribed sirolimus were analyzed by the LC-MS/MS reference method, Abbott IMx and Dimension RxL methods in accordance with CLSI recommendations. RESULTS: The Dimension sirolimus assay had a functional sensitivity of 2.0 ng/mL and repeatability and within-laboratory imprecision less than 12.6% at 3 ng/mL and less than 5% at 11-12 ng/mL. Least squares linear regression demonstrated the following relationships: RxL=1.20(LC-MS/MS) - 0.70, r=0.95 and RxL=1.33(IMx) - 0.75, r=0.96. CONCLUSIONS: The Dimension sirolimus assay correlates well with the LC-MS/MS reference and IMx immunoassay methods and has the advantage of random access analysis without a manual pre-treatment step.

Performance of a no-pretreatment tacrolimus assay on the Dade Behring Dimension RxL clinical chemistry analyzer.

BACKGROUND: Therapeutic drug monitoring for tacrolimus is important for organ transplant patients receiving this immunosuppressant. Current available assays for tacrolimus require sample pre-treatment and operate in a batch mode. Here a no-pretreatment tacrolimus assay, performed on the Dade Behring Dimension analyzer is compared to the Abbott IMx tacrolimus assay and to an LC/MS/MS method. METHODS: Whole blood samples from 2 medical centers and different transplant types (kidney n=103, liver n=81, heart n=27, pancreas n=16, bone marrow n=9, [corrected] lung n=7), were obtained and tacrolimus quantified by each of the 3 assays. RESULTS: The lower limit of the linear range was 1.2 ng/ml on the Dimension assay. Total imprecision was 9.8% and within-run imprecision was 9.6% at a tacrolimus concentration of 3.4 ng/dL. Passing-Bablock regression analysis determined the following relationships: DIMN=(1.16) LC/MS - 0.43, r=0.90 and DIMN=(0.99)IMx - 0.35, r=0.87. CONCLUSIONS: The Dade Behring Dimension [corrected] Tacrolimus assay has adequate imprecision and correlates well with the reference method of LC/MS/MS. The assay appears suitable for clinical use, and has the advantages of not requiring a pretreatment step and the ability to be performed in a random-access mode.

Evaluation of a no-pretreatment cyclosporin A assay on the Dade Behring dimension RxL clinical chemistry analyzer.

BACKGROUND: Monitoring whole-blood concentrations of cyclosporin A (CsA) is common practice in the management of solid organ and bone marrow transplant recipients. In a multicenter study we evaluated a new, direct (no pretreatment) CsA assay on the Dade Behring Dimension RxL system and compared results with those from the Abbott TDx CsA immunoassay and a HPLC method. METHODS: Whole-blood samples from heart (n = 111; 35 patients), liver (n = 201; 44 patients), kidney (n = 279; 65 patients), and miscellaneous organ (n = 77; 12 lung, 12 bone marrow, 5 kidney/pancreas, and 1 pancreas patient) recipients were obtained from patient populations of the participating institutions. Routine clinical monitoring of CsA was performed using either the TDx method or HPLC. RESULTS: The minimum detectable concentration of CsA averaged 9.4 microg/L, and the lower limit of quantification was 30 microg/L. The method was linear from 30 to 500 microg/L. Cross-reactivity with seven different CsA metabolites ranged from 0.0% to 5.7% for the Dimension RxL assay compared with 0.4-15.9% for the TDx assay. Total imprecision (CV) averaged 6.2%, and within-run imprecision averaged 4.9%. Passing-Bablok linear regression analyses of all samples from two sites yielded the following: RxL = 0.81 x TDx - 16.8; and RxL = 1.12 x HPLC - 1.7. CONCLUSIONS: The Dade Behring CsA assay for the random-access Dimension platform offers adequate performance characteristics for routine clinical use, does not require a manual pretreatment step, and demonstrates less cross-reactivity with CsA metabolites than another commonly used immunoassay.