title: Technetium-99m radiolabeling using a phage-derived single-chain Fv with a C-terminal cysteine.
abstract: Single-chain Fv (scFv) antibody fragments have potential for clinical imaging because of their rapid tumor penetration and high tumor-to-tissue ratios at early time points. ScFvs clear rapidly from the circulation so radiolabels such as 99mTc which have short half-lives are desirable, but the free thiol groups necessary for labeling with 99mTc are not normally found on these molecules.
  
  
  METHODS
  We constructed a vector which enabled a free cysteine to be linked to the C-terminus of scFvs. MFE-23, a scFv directed against carcinoembryonic antigen (CEA), was cloned into this vector and cys-tagged MFE-23 was labeled with 99mTc using a D-glucarate transfer method.
  
  
  RESULTS
  The radiolabeled product was stable in vivo and in vitro and showed favorable tumor-to-blood ratios in vivo at early time points (4:1 at 24 hr and 8:1 at 48 hr), although high kidney levels were also detected.
  
  
  CONCLUSIONS
  Our study demonstrates an effective method to enable scFvs radiolabeling with 99mTc and also shows the potential of using a 99mTc-labeled scFv for clinical imaging studies.
authors.affiliation: CRC Laboratories, Department of Clinical Oncology, Royal Free Hospital School of Medicine, London, United Kingdom.
authors.forename: M J
authors.initials: MJ
authors.lastname: Verhaar
authors.forename: P A
authors.initials: PA
authors.lastname: Keep
authors.forename: R E
authors.initials: RE
authors.lastname: Hawkins
authors.forename: L
authors.initials: L
authors.lastname: Robson
authors.forename: J L
authors.initials: JL
authors.lastname: Casey
authors.forename: B
authors.initials: B
authors.lastname: Pedley
authors.forename: J A
authors.initials: JA
authors.lastname: Boden
authors.forename: R H
authors.initials: RH
authors.lastname: Begent
authors.forename: K A
authors.initials: KA
authors.lastname: Chester
mesh_terms: D000230:Adenocarcinoma / Q000000981:diagnostic imaging; D000818:Animals; D003110:Colonic Neoplasms / Q000000981:diagnostic imaging; D006801:Humans; D007128:Immunoglobulin Fragments; D007553:Isotope Labeling; D051379:Mice; D009368:Neoplasm Transplantation; D016719:Radioimmunodetection; D013667:Technetium; D014018:Tissue Distribution; D014183:Transplantation, Heterologous; D014407:Tumor Cells, Cultured
chemical_list: D007128:Immunoglobulin Fragments; D013667:Technetium