Applied Radiation and Isotopes 64 (2006) 668–676 Influence of prosthetic radioiodination on the chemical and biological behavior of chemotactic peptides labeled at high specific activity Oscar R. Pozzi à , Elisa O. Sajaroff 1 , Martı´n M. Edreira 2 National Atomic Energy Commission, Ezeiza Atomic Centre, Buenos Aires, Argentina Received 9 June 2005; received in revised form 30 November 2005; accepted 4 January 2006 Abstract The influence of radioiodination made through prosthetic group N-succinimidyl-3-[ 131 I]iodo-benzoate ([ 131 I]SIB) on the behavior of small peptides was investigated using as model the chemotactic hexapeptide Na-for-Nle-Leu-Phe-Nle-Tyr-Lys. No carrier added labeled peptide was isolated by reverse-phase HPLC (RP-HPLC) with coupling efficiencies up to 59–75%. Biodistribution in normal and infected C57 mice showed mainly a hepatobiliary clearance, a very low thyroid uptake and the highest uptake at the infection site was within 1 h of injection. Superoxide production and competitive binding assays studies in human polymorphonuclear leukocytes showed a preserved biological activity and high-affinity specific binding. However, the results indicated that the changes observed in the receptor- binding properties with an IC 50 almost twice than the unlabeled peptide and the increasing in the hepatobiliary excretion could be the consequence of the increased lipophicity observed due to the presence of the prosthetic group together with a strong influence of the radioisotope per se. r 2006 Elsevier Ltd. All rights reserved. Keywords: Radioiodination; Labeling; Chemotactic peptides; Biological behavior; Receptor-binding 1. Introduction Small receptor-binding peptides are the agents of choice for diagnostic imaging and radiotherapy of several diseases due to their favorable pharmacokinetics. Although mole- cular modification techniques allow the synthesis of a variety of bioactive peptides with modification in their structure like the addition of linkers for radiolabeling without compromising their biological properties, the radiolabeling itself could finally affect its properties. Even if site-specific labeling techniques are directed toward residues of the peptide away from the active-site, the binding of chelating groups for radioisotopes like 99m Tc or 111 In, or prosthetic groups for 123/131 I or 18 F, could affect chemical properties such as charge or lipophilicity, respec- tively. These changes could result in the modification of biochemical processes such as metabolization or membrane solubility, which would finally lead to changes on the biological behavior of the labeled peptide, even if their specific interaction with the receptor seems unmodified. Our Institution has been involved in a Technical Cooperation Project with the International Atomic Energy Agency (IAEA) for the production of iodine-123 and its radiopharmaceuticals in the cyclotron (The Cyclotron Corporation model CP-42) at Ezeiza Atomic Center. The purpose of the present study was the evaluation of the influence of radioiodination on the biological behavior of peptides labeled with iodine-123 or iodine-131 and its possible consequences over diagnostic or treatment studies, respectively. In order to achieve this goal our work focused on the optimization of radioiodination at high specific activity; as well as quality control, in vitro, and in vivo studies of peptide-based iodine radiopharmaceuticals. In our model the selection of the peptide was based on two main criteria besides its availability; namely the ARTICLE IN PRESS www.elsevier.com/locate/apradiso 0969-8043/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.01.001 à Corresponding author. Present address: Department of Radiology, Duke University Medical Center, Box 3808, Durham, NC 27710, USA. Tel.: +1 919 684 7705; fax: +1 919 684 7122. E-mail address: oscar.pozzi@duke.edu (O.R. Pozzi). 1 Present address: Department of Pediatric, Pediatric Allergy and Immunology Division, Duke University Medical Center, Durham, NC 27710, USA 2 Present address: Department of Pharmacology, School of Medicine, University of Pittsburgh, Pittsburgh, PA 15261, USA