title: Preclinical characterization and in vivo imaging studies of an engineered recombinant technetium-99m-labeled metallothionein-containing anti-carcinoembryonic antigen single-chain antibody.
abstract: We describe the engineering of a novel single-chain fragment (scFv) metallothionein (MET) containing anti-carcinoembryonic antigen (CEA) antibody (referred to as MET-scFv) for use as a diagnostic imaging agent in colorectal cancer.
  
  
  METHODS
  Site-directed cloning of annealed oligonucleotides, containing both the MET and a c-myc tag sequence, into a pUC19-based expression vector enabled soluble secreted protein expression from Escherichia coli. Affinity purification was used to purify the protein using an anti-c-myc affinity column. The specificity of both the unlabeled and labeled MET-scFv for CEA was demonstrated by solid-phase enzyme-linked immunosorbent assay and radioimmunoassay and by fluorescence-activated cell sorting analysis on CEA-expressing human colorectal LS-174T cells. Technetium-99m labeling was achieved using a Zn2+ transchelation step, enabling direct 99mTc transfer without separate reduction of MET. In vitro stability was demonstrated by fast protein liquid chromatography analysis of labeled MET-scFv, incubated with bovine serum albumin (BSA), transferrin and mouse serum. Last, in vivo pharmacokinetics, biodistribution and imaging were performed.
  
  
  RESULTS
  Yields of 6 mg/liter induced culture purified protein were achieved. Successful site-specific labeling was demonstrated using a Zn2+ transchelation modification of a pretinning method, which also enabled lower amounts of the reducing agent to be used. The specificity for CEA was retained after labeling. Despite a rapid serum clearance (t(1/2alpha) = 2.8 min), adequate localization to tumor of 5.37% injected dose/g at 4 hr was demonstrated. Moreover, the short-lived t(1/2alpha) of scFv, its early tumor targeting and rapid blood-pool clearance gave tumor-to-blood ratios of 2.07 by 4 hr, enabling early gamma camera imaging. Successful and specific imaging was achieved using LS-174T xenografts in nude mice by 3-6 hr.
  
  
  CONCLUSIONS
  A recombinant MET containing scFv was successfully expressed, purified and labeled with 99Tc. The stable site-specific labeling of 99Tc, combined with the rapid plasma clearance of the scFv, led to successful early in vivo imaging of xenografted mice.
authors.affiliation: Austin Research Institute, Austin Hospital, Heidelberg, Victoria, Australia.
authors.forename: G A
authors.initials: GA
authors.lastname: Pietersz
authors.forename: M R
authors.initials: MR
authors.lastname: Patrick
authors.forename: K A
authors.initials: KA
authors.lastname: Chester
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000493:pharmacokinetics; D002272:Carcinoembryonic Antigen / Q000276:immunology*; D015179:Colorectal Neoplasms / Q000000981:diagnostic imaging*; D005260:Female; D005818:Genetic Engineering; D006801:Humans; D007553:Isotope Labeling; D008668:Metallothionein / Q000493:pharmacokinetics; D051379:Mice; D008807:Mice, Inbred BALB C; D008819:Mice, Nude; D016513:Mice, SCID; D009368:Neoplasm Transplantation; D016719:Radioimmunodetection; D011994:Recombinant Proteins / Q000493:pharmacokinetics; D013667:Technetium; D014018:Tissue Distribution
chemical_list: D000911:Antibodies, Monoclonal; D002272:Carcinoembryonic Antigen; D011994:Recombinant Proteins; D013667:Technetium; D008668:Metallothionein