Preparation and evaluation of 99m Tc-labeled cyclic arginine–glycine–aspartate (RGD) peptide for integrin targeting Dong-Eun Lee, Young-Don Hong, Kang-Hyuk Choi, So-Young Lee, Pil-Hoon Park, Sun-Ju Choi n Radioisotope Research Division, Basic Science and Technology Department, Korea Atomic Energy Research Institute (KAERI), Daejon 305-353, Republic of Korea article info Article history: Received 29 January 2010 Received in revised form 15 April 2010 Accepted 28 April 2010 Keywords: 99m Tc N 3 S 1 Cyclic RGD peptide Integrin Tumor targeting abstract Technetium coordination chemistry has been a subject of interest in the development of radio- pharmaceuticals, especially imaging radiotracers. Due to the extensive work done on developing chelates for 99m Tc, various chelators have been investigated and applied to radiopharmceuticals. Previous studies on the coordination chemistry of the [ 99m Tc ¼O] core have established peptide-derived sequences as effective chelating ligands. These observations led to the design of tetradentate ligands derived from amino acid sequences. Such amino acid sequences provide a tetradentate coordination site for chelation to the radionuclide and an effective functional group for conjugation to biomolecules using conventional solid-phase synthetic routes. A derivative of a novel tripeptide chelating sequence, Pro–Gly–Cys (PGC) has been developed where it is possible to form stable technetium complexes with the [ 99m Tc ¼O] via N 3 S 1 tetradentate coordination core that serves this function and can be readily incorporated into biomolecules using solid-phase synthesis techniques. As a model system, the RGD peptide was selected which has been well known to target the integrin receptor for angiogenesis and tumor imaging agents. The results of in vivo studies with these novel radiolabeled compounds in tumor xenografts demonstrated a distribution in tumor targeting and other organs, such as kidney, liver and intestines. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction In nuclear medicine, to use a receptor binding or other biological interactions is often called target-specific radiopharmaceuticals. Recent advances in the development of radiopharmaceuticals for imaging and therapy with radionuclides are intimately dependent on the successful application of target-specific biomolecules (Bakker et al. (1991); Liu, 2008). After the successful application of 111 In-labeled octreotide to the somatostatin receptor-targeting imaging agent (Bakker et al. (1991)), various researchers have intensively studied to find new target-specific radiopharmaceuti- cals based on small biomolecules, such as peptides and bioactive molecules. Many small biomolecules have been radiolabeled to evaluate their potential use as new imaging agents and therapy. These include radiolabeled receptor-specific peptides, namely, RGD derivatives, somatostatin analogues, a-MSH and folate receptor antagonists (Bakker et al. (1991); Leamon et al. (2002); Liu, 2008; Miao and Quinn (2008)). The rational strategy for incorporating radionuclides with favorite characteristics into target molecules has been the most considerable aspect in developing radiopharmaceuticals. For diagnostic radiopharmaceuticals, various gamma-emitting iso- topes including 99m Tc, 111 In, 62/64 Cu and 67/68 Ga have been used for SPECT or PET imaging (Liu, 2008). While 64 Cu and 68 Ga are particularly useful for PET, 99m Tc is the most widely used for SPECT imaging due to its optimal nuclear properties (T 1/2 ¼6 h, 140 keV gamma photons) and its convenient availability from commercial 99 Mo/ 99m Tc generator at low costs. This makes it attractive to use of 99m Tc for developing a radiotracer for cancer imaging. 99m Tc has diverse oxidation states which may make difficult for its application as a radionuclide for the development of radiopharmaceuticals. Among the core structure of 99m Tc, [ 99m Tc(V) ¼O] 3+ is the most extensively developed with tetra- dentate ligand chelates of the N x S 4 x system. For successful chelation of 99m Tc, the [ 99m Tc(V) ¼O] 3+ core with N x S 4 x tetra- dentate ligands have been extensively investigated, including N 2 S 2 (DADS, MAMA and DADT), N 3 S 1 triamidethiols which contain bioactive molecules with target-receptor affinity (Liu, 2008). Various peptide sequences such as, Gly–Gly–Cys (N 3 S 1 ) and Gly–Ala–Gly–Gly–Gly (N 4 ) have also been proposed as ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/apradiso Applied Radiation and Isotopes 0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.04.029 Abbreviations: Fmoc, 9-Fluorenylmethoxycarbonyl; Allyl, Allyl; Pbf, 2,2,4,6, 7-Pentamethyldihydrobenzofuran-5-sulfonyl; Mtt, 4-Methyltrityl; Trt, Trityl; HBTU, O-Benzotriazole-N, N,N 0 ,N 0 -tetramethyl-uronium-hexafluoro-phosphate; HOBt, N-Hydroxybenzotriazole; DIPEA, Diisopropylethylamine; DMF, Dimethyl- formamide; DCM, Dichloromethane; DIC, 1,3-Diisopropylcarbodiimide; TFA, Trifluoroacetic acid; TIS, Triisopropylsilane; EDT, Ethanedithiol n Corresponding author. Tel.: + 82 42 868 8449; fax: + 82 42 868 8448. E-mail address: choisj@kaeri.re.kr (S.-J. Choi). Applied Radiation and Isotopes 68 (2010) 1896–1902