title: The glucose-dependent insulinotropic polypeptide receptor: a novel target for neuroendocrine tumor imaging—first preclinical studies.
abstract: A new family of peptide receptors, the incretin receptor family, overexpressed on many neuroendocrine tumors (NETs) is of great importance because it may enable the in vivo peptide-based receptor targeting of a category of NETs that does not express the somatostatin receptor. Impressive in vivo diagnostic data were published for glucagonlike peptide 1 receptor-targeting radiopeptides. Recently, promising in vitro data have appeared for the second member of the incretin family, the glucose-dependent insulinotropic polypeptide (GIP) receptor. This prompted us to develop and evaluate a new class of radioligands with the potential to be used for the in vivo targeting of GIP receptor-positive tumors.
  
  
  METHODS
  GIP(1-42) was modified C-terminally, and the truncated peptides [Lys(30)(aminohexanoic acid [Ahx]-DOTA)]GIP(1-30)NH2 (EG1), [Lys(16)(Ahx-DOTA)]GIP(1-30)NH2 (EG2), and [Nle(14), Lys(30)(Ahx-DOTA)]GIP(1-30)NH2 (EG4) were conjugated with Ahx-DOTA via the Lys(16) and Lys(30) side chains. Their inhibitory concentration of 50% (IC50) was determined using [(125)I-Tyr(10)]GIP(1-30) as radioligand and GIP(1-30) as control peptide. The DOTA conjugates were labeled with (111)In and (68)Ga. In vitro evaluation included saturation and internalization studies using the pancreatic endocrine cell line INR1G9 transfected with the human GIP receptor (INR1G9-hGIPr). The in vivo evaluation consisted of biodistribution and PET imaging studies on nude mice bearing INR1G9-hGIPr tumors.
  
  
  RESULTS
  Binding studies (IC50 and saturation studies) showed high affinity toward GIP receptor for the GIP conjugates. Specific in vitro internalization was found, and almost the entire cell-associated activity was internalized (>90% of the cell-bound activity), supporting the agonist potency of the (111)In-vectors. (111)In-EG4 and (68)Ga-EG4 were shown to specifically target INR1G9-hGIPr xenografts, with tumor uptake of 10.4% ± 2.2% and 17.0% ± 4.4% injected activity/g, 1 h after injection, respectively. Kidneys showed the highest uptake, which could be reduced by approximately 40%-50% with a modified-fluid-gelatin plasma substitute or an inhibitor of the serine protease dipeptidyl peptidase 4. The PET images clearly visualized the tumor.
  
  
  CONCLUSIONS
  The evaluation of EG4 as a proof-of-principle radioligand indicated the feasibility of imaging GIP receptor-positive tumors. These results prompt us to continue the development of this family of radioligands for imaging of a broad spectrum of NETs.
authors.affiliation: German Cancer Consortium (DKTK), Heidelberg, Germany Department of Nuclear Medicine, University Hospital Freiburg, Freiburg, Germany German Cancer Research Center (DKFZ), Heidelberg, Germany.
authors.forename: Eleni
authors.initials: E
authors.lastname: Gourni
authors.affiliation: Department of Pathology, University Hospital Bern, Bern, Switzerland; and.
authors.forename: Beatrice
authors.initials: B
authors.lastname: Waser
authors.affiliation: Institut de la Santé et de la Recherche Médicale, Toulouse, France.
authors.forename: Pascal
authors.initials: P
authors.lastname: Clerc
authors.affiliation: Institut de la Santé et de la Recherche Médicale, Toulouse, France.
authors.forename: Daniel
authors.initials: D
authors.lastname: Fourmy
authors.affiliation: Department of Pathology, University Hospital Bern, Bern, Switzerland; and.
authors.forename: Jean Claude
authors.initials: JC
authors.lastname: Reubi
authors.affiliation: Department of Nuclear Medicine, University Hospital Freiburg, Freiburg, Germany helmut.maecke@uniklinik-freiburg.de.
authors.forename: Helmut R
authors.initials: HR
authors.lastname: Maecke
mesh_terms: D000818:Animals; D001692:Biological Transport; D006224:Cricetinae; D005260:Female; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry; D006801:Humans; D007205:Indium Radioisotopes; D007668:Kidney / Q000378:metabolism; D008024:Ligands; D051379:Mice; D008819:Mice, Nude; D018358:Neuroendocrine Tumors / Q000000981:diagnostic imaging*; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D049268:Positron-Emission Tomography / Q000379:methods*; D011964:Receptors, Gastrointestinal Hormone / Q000737:chemistry / Q000378:metabolism*; D014018:Tissue Distribution
chemical_list: D006573:Heterocyclic Compounds, 1-Ring; D007205:Indium Radioisotopes; D008024:Ligands; D010446:Peptide Fragments; D011964:Receptors, Gastrointestinal Hormone; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; C041086:gastric inhibitory polypeptide receptor
keywords: 111In; 68Ga; glucose-dependent insulinotropic polypeptide receptor (GIPr); imaging; neuroendocrine tumors (NETs)