title: Comparison of biological stability and metabolism of CCK2 receptor targeting peptides, a collaborative project under COST BM0607.
abstract: OBJECTIVE
  Stability of radiolabelled cholecystokinin 2 (CCK2) receptor targeting peptides has been a major limitation in the use of such radiopharmaceuticals especially for targeted radionuclide therapy applications, e.g. for treatment of medullary thyroid carcinoma (MTC). The purpose of this study was to compare the in vitro stability of a series of peptides binding to the CCK2 receptor [selected as part of the COST Action on Targeted Radionuclide Therapy (BM0607)] and to identify major cleavage sites.
  
  
  METHODS
  Twelve different 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA)-minigastrin/CCK conjugates were provided within an European COST Action (BM0607) by different laboratories and radiolabelled with (177)Lu. Their in vitro stabilities were tested in fresh human serum. Radiochemical yields (RCY) and intact radioligands for half-life calculations were determined by radio-HPLC. Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) analysis of metabolites was performed to identify cleavage products using conjugates labelled with excess stable (nat)Lu, incubated in serum at 37°C. Urine metabolite analysis after injection in normal mice was performed by radio-HPLC analysis.
  
  
  RESULTS
  Variable stability in human serum was found for the different peptides with calculated half-lives between 4.5 ± 0.1 h and 198 ± 0.1 h (n = 2). In urine of normal mice only metabolised peptide fragments were detected even at short times after injection for all peptides. MALDI-TOF MS revealed a major cleavage site of all minigastrin derivatives between Asp and Phe-NH(2) at the C-terminal end.
  
  
  CONCLUSIONS
  Development of CCK2 receptor ligands especially for therapeutic purposes in patients with MTC or small cell lung cancer (SCLC) is still ongoing in different laboratories. This comparative study provided valuable insight into the importance of biological stability especially in the context of other results of this comparative trial within the COST Action BM0607.
authors.affiliation: Clinical Department of Nuclear Medicine, Innsbruck Medical University, Anichstrasse 35, 6020 Innsbruck, Austria.
authors.forename: Meltem
authors.initials: M
authors.lastname: Ocak
authors.forename: Anna
authors.initials: A
authors.lastname: Helbok
authors.forename: Christine
authors.initials: C
authors.lastname: Rangger
authors.forename: Petra Kolenc
authors.initials: PK
authors.lastname: Peitl
authors.forename: Berthold A
authors.initials: BA
authors.lastname: Nock
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Morelli
authors.forename: Annemarie
authors.initials: A
authors.lastname: Eek
authors.forename: Jane K
authors.initials: JK
authors.lastname: Sosabowski
authors.forename: Wout A P
authors.initials: WA
authors.lastname: Breeman
authors.forename: Jean Claude
authors.initials: JC
authors.lastname: Reubi
authors.forename: Clemens
authors.initials: C
authors.lastname: Decristoforo
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D003299:Cooperative Behavior; D005260:Female; D006801:Humans; D008715:Methionine / Q000378:metabolism; D051379:Mice; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D010455:Peptides / Q000097:blood / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D055550:Protein Stability; D043887:Receptor, Cholecystokinin B / Q000378:metabolism*
chemical_list: D010455:Peptides; D043887:Receptor, Cholecystokinin B; D008715:Methionine