title: Pre-clinical evaluation of eight DOTA coupled gastrin-releasing peptide receptor (GRP-R) ligands for in vivo targeting of receptor-expressing tumors.
abstract: BACKGROUND
  Overexpression of the gastrin-releasing peptide receptor (GRP-R) has been documented in several human neoplasms such as breast, prostate, and ovarian cancer. There is growing interest in developing radiolabeled peptide-based ligands toward these receptors for the purpose of in vivo imaging and radionuclide therapy of GRP-R-overexpressing tumors. A number of different peptide sequences, isotopes, and labeling methods have been proposed for this purpose. The aim of this work is to perform a direct side-by-side comparison of different GRP-R binding peptides utilizing a single labeling strategy to identify the most suitable peptide sequence.
  
  
  METHODS
  Solid-phase synthesis of eight derivatives (BN1-8) designed based on literature analysis was carried out. Peptides were coupled to the DOTA chelator through a PEG4 spacer at the N-terminus. Derivatives were characterized for serum stability, binding affinity on PC-3 human prostate cancer cells, biodistribution in tumor-bearing mice, and gamma camera imaging at 1, 6, and 24 h after injection.
  
  
  RESULTS
  Serum stability was quite variable among the different compounds with half-lives ranging from 16 to 400 min at 37 °C. All compounds tested showed K d values in the nanomolar range with the exception of BN3 that showed no binding. Biodistribution and imaging studies carried out for compounds BN1, BN4, BN7, and BN8 showed targeting of the GRP-R-positive tumors and the pancreas. The BN8 compound (DOTA-PEG-DPhe-Gln-Trp-Ala-Val-NMeGly-His-Sta-Leu-NH2) showed high affinity, the longest serum stability, and the highest target-to-background ratios in biodistribution and imaging experiments among the compounds tested.
  
  
  CONCLUSIONS
  Our results indicate that the NMeGly for Gly substitution and the Sta-Leu substitution at the C-terminus confer high serum stability while maintaining high receptor affinity, resulting in biodistribution properties that outperform those of the other peptides.
authors.affiliation: Department of Pharmacy, CIRPeB, University of Naples "Federico II" and Invectors srl, Napoli, Italy.
authors.forename: Antonella
authors.initials: A
authors.lastname: Accardo
authors.affiliation: Nuclear Medicine Unit, Department of Medical-Surgical Sciences and of Translational Medicine, Faculty of Medicine and Psychology, "Sapienza" University of Roma, Rome, Italy.
authors.forename: Filippo
authors.initials: F
authors.lastname: Galli
authors.affiliation: Department of Nuclear Medicine, University Hospital Freiburg, Freiburg, Germany.
authors.forename: Rosalba
authors.initials: R
authors.lastname: Mansi
authors.affiliation: Department of Nuclear Medicine, University Hospital Freiburg, Freiburg, Germany.
authors.forename: Luigi
authors.initials: L
authors.lastname: Del Pozzo
authors.affiliation: Centro Ricerche Oncologiche Mercogliano, Istituto Nazionale Tumori "Fondazione G. Pascale"-IRCCS, Mercogliano (AV), Italy.
authors.forename: Michela
authors.initials: M
authors.lastname: Aurilio
authors.affiliation: Centro Ricerche Oncologiche Mercogliano, Istituto Nazionale Tumori "Fondazione G. Pascale"-IRCCS, Mercogliano (AV), Italy.
authors.forename: Anna
authors.initials: A
authors.lastname: Morisco
authors.affiliation: Department of Pharmacy, CIRPeB, University of Naples "Federico II" and Invectors srl, Napoli, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Ringhieri
authors.affiliation: Nuclear Medicine Unit, Department of Medical-Surgical Sciences and of Translational Medicine, Faculty of Medicine and Psychology, "Sapienza" University of Roma, Rome, Italy.
authors.forename: Alberto
authors.initials: A
authors.lastname: Signore
authors.affiliation: Department of Pharmacy, CIRPeB, University of Naples "Federico II" and Invectors srl, Napoli, Italy.
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Morelli
authors.affiliation: Struttura Complessa Medicina Nucleare, Istituto Nazionale Tumori "Fondazione G. Pascale"-IRCCS, Via M. Semmola, 52, Napoli, 80131, Italy. l.aloj@istitutotumori.na.it.
authors.forename: Luigi
authors.initials: L
authors.lastname: Aloj
keywords: Biodistribution; Gastrin-releasing peptide receptor; Prostate cancer; Radiolabeled peptides