title: Sandwich assay for tacrolimus using 2 antitacrolimus antibodies.
abstract: 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.
authors.affiliation: Siemens Healthcare Diagnostics Inc., Newark, DE 19714.
authors.forename: Tie Q
authors.initials: TQ
authors.lastname: Wei
authors.forename: Yi F
authors.initials: YF
authors.lastname: Zheng
authors.forename: Michael
authors.initials: M
authors.lastname: Dubowy
authors.forename: Manoj
authors.initials: M
authors.lastname: Sharma
mesh_terms: D000911:Antibodies, Monoclonal / Q000737:chemistry*; D001666:Binding Sites, Antibody; D002853:Chromatography, Liquid; D003429:Cross Reactions; D000939:Epitopes; D006241:Haptens / Q000276:immunology; D006801:Humans; D007118:Immunoassay / Q000379:methods; D007166:Immunosuppressive Agents / Q000032:analysis* / Q000097:blood / Q000276:immunology; D016559:Tacrolimus / Q000031:analogs & derivatives* / Q000032:analysis* / Q000097:blood / Q000276:immunology; D053719:Tandem Mass Spectrometry
chemical_list: D000911:Antibodies, Monoclonal; D000939:Epitopes; D006241:Haptens; D007166:Immunosuppressive Agents; D016559:Tacrolimus