title: A comparison of three (67/68)Ga-labelled exendin-4 derivatives for β-cell imaging on the GLP-1 receptor: the influence of the conjugation site of NODAGA as chelator.
abstract: BACKGROUND
  Various diseases derive from pathologically altered β-cells. Their function can be increased, leading to hyperinsulinism, or decreased, resulting in diabetes. Non-invasive imaging of the β-cell-specific glucagon-like peptide receptor-1 (GLP-1R) would allow the assessment of both β-cell mass and derived tumours, potentially improving the diagnosis of various conditions. We tested three new (67/68)Ga-labelled derivatives of exendin-4, an agonist of GLP-1R, in vitro and in vivo. We determined the influence of the chelator NODAGA conjugated to resident lysines either at positions 12 and 27 or the C-terminally attached lysine at position 40 on the binding and kinetics of the peptide.
  
  
  METHODS
  Binding and internalisation of (67)Ga-labelled Ex4NOD12, Ex4NOD27 and Ex4NOD40 were tested on Chinese hamster lung (CHL) cells stably transfected to express the GLP-1 receptor (GLP-1R). In vivo biodistribution of (68)Ga-labelled peptides was investigated in CD1 nu/nu mice with subcutaneous CHL-GLP-1R positive tumours; the specificity of the binding to GLP-1R was determined by pre-injecting excess peptide.
  
  
  RESULTS
  All peptides showed good in vitro binding affinities to GLP-1R in the range of 29 to 54 nM. (67/68)Ga-Ex4NOD40 and (67/68)Ga-Ex4NOD12 show excellent internalisation (>30%) and high specific uptake in GLP-1R positive tissue, but high activity was also found in the kidneys.
  
  
  CONCLUSIONS
  We show that of the three peptides, Ga-Ex4NOD40 and Ga-Ex4NOD12 demonstrate the most favourable in vitro properties and in vivo binding to GLP-1R positive tissue. Therefore, we conclude that the lysines at positions 12 and 40 might preferentially be utilised for modifying exendin-4.
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institute, OIPA/103, Villigen 5232, Switzerland.
authors.forename: Andreas
authors.initials: A
authors.lastname: Jodal
authors.affiliation: Faculty of Medicine, Department of Gastroenterology, Endocrinology and Metabolism, University of Marburg, Marburg 35037, Germany.
authors.forename: Brigitte
authors.initials: B
authors.lastname: Lankat-Buttgereit
authors.affiliation: Department of Radiology and Nuclear Medicine, Radboud University Medical Center, Nijmegen 6525, The Netherlands.
authors.forename: Maarten
authors.initials: M
authors.lastname: Brom
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institute, OIPA/103, Villigen 5232, Switzerland ; Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich 8092, Switzerland.
authors.forename: Roger
authors.initials: R
authors.lastname: Schibli
authors.affiliation: Center for Radiopharmaceutical Sciences ETH-PSI-USZ, Paul Scherrer Institute, OIPA/103, Villigen 5232, Switzerland.
authors.forename: Martin
authors.initials: M
authors.lastname: Béhé
keywords: Exendin-4; GLP-1 receptor; Insulinoma; PET; SPECT; β-cell imaging