title: 99mTc-chelator engineering to improve tumour targeting properties of a HER2-specific Affibody molecule.
abstract: OBJECTIVE
  Monitoring HER2 expression is crucial for selection of breast cancer patients amenable to HER2-targeting therapy. The Affibody molecule Z(HER2:342) binds to HER2 with picomolar affinity and enables specific imaging of HER2 expression. Previously, Z(HER2:342) with the additional N-terminal mercaptoacetyl-glycyl-glycyl-glycyl (maGGG) sequence was labelled with (99m)Tc and demonstrated specific targeting of HER2-expressing xenografts. However, hepatobiliary excretion caused high radioactivity accumulation in the abdomen. We investigated whether the biodistribution of Z(HER2:342) can be improved by substituting glycyl residues in the chelating sequence with more hydrophilic seryl residues.
  
  
  METHODS
  The Affibody molecule Z(HER2:342), carrying the chelators mercaptoacetyl-glycyl-seryl-glycyl (maGSG), mercaptoacetyl-glycyl-D: -seryl-glycyl [maG(D-S)G] and mercaptoacetyl-seryl-seryl-seryl (maSSS), were prepared by peptide synthesis and labelled with (99m)Tc. The differences in the excretion pathways were evaluated in normal mice. The tumour targeting capacity of (99m)Tc-maSSS-Z(HER2:342) was studied in nude mice bearing SKOV-3 xenografts and compared with the capacity of radioiodinated Z(HER2:342).
  
  
  RESULTS
  A shift towards renal excretion was obtained when glycine was substituted with serine in the chelating sequence. The radioactivity in the gastrointestinal tract was reduced threefold for the maSSS conjugate in comparison with the maGGG conjugate 4 h post injection (p.i.). The tumour uptake of (99m)Tc-maSSS-Z(HER2:342) was 11.5 +/- 0.5% IA/g 4 h p.i., and the tumour-to-blood ratio was 76. The pharmacokinetics and uptake characteristics of technetium-labelled Z(HER2:342) were better than those of radioiodinated Z(HER2:342).
  
  
  CONCLUSIONS
  The introduction of serine residues in the chelator results in better tumour imaging properties of the Affibody molecule Z(HER2:342) compared with glycyl-containing chelators and is favourable for imaging of tumours and metastases in the abdominal area.
authors.affiliation: School of Biotechnology, Royal Institute of Technology, Stockholm, Sweden.
authors.forename: Torun
authors.initials: T
authors.lastname: Engfeldt
authors.forename: Thuy
authors.initials: T
authors.lastname: Tran
authors.forename: Anna
authors.initials: A
authors.lastname: Orlova
authors.forename: Charles
authors.initials: C
authors.lastname: Widström
authors.forename: Joachim
authors.initials: J
authors.lastname: Feldwisch
authors.forename: Lars
authors.initials: L
authors.lastname: Abrahmsen
authors.forename: Anders
authors.initials: A
authors.lastname: Wennborg
authors.forename: Amelie Eriksson
authors.initials: AE
authors.lastname: Karlström
authors.forename: Vladimir
authors.initials: V
authors.lastname: Tolmachev
mesh_terms: D000818:Animals; D002614:Chelating Agents / Q000737:chemistry; D016503:Drug Delivery Systems / Q000379:methods*; D005260:Female; D008657:Metabolic Clearance Rate; D051379:Mice; D008807:Mice, Inbred BALB C; D008819:Mice, Nude; D009928:Organ Specificity; D010051:Ovarian Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism*; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000737:chemistry / Q000493:pharmacokinetics; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000493:pharmacokinetics*; D013667:Technetium / Q000737:chemistry / Q000493:pharmacokinetics*; D014018:Tissue Distribution
chemical_list: D002614:Chelating Agents; D019275:Radiopharmaceuticals; D011993:Recombinant Fusion Proteins; C515865:Z(HER2.4)2 affibody; D013667:Technetium