Synthesis of tricarbonyl rhenium and technetium complexes of a 5 0 -carboxamide 5-ethyl-2 0 -deoxyuridine for selective inhibition of herpes simplex virus thymidine kinase 1 D. Desbouis a,b , P.A. Schubiger a,b , R. Schibli a,b, * a Center for Radiopharmaceutical Science, ETH-PSI-USZ, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland b Department of Chemistry and Applied Biosciences ETH Zurich, 8093 Zurich, Switzerland Received 1 September 2006; received in revised form 5 October 2006; accepted 6 October 2006 Available online 13 October 2006 Abstract Herpes simplex virus thymidine kinase type 1 (HSV1-TK) is frequently used as reporter protein in gene therapy. Our aim is to produce single photon emitting reporter probe based on technetium-99m. The synthesis of organometallic technetium and rhenium complexes of a5 0 -carboxamide 5-ethyl-2 0 -deoxyuridine derivative able to selectively inhibit HSV1-TK is presented. The 5-ethyl-2 0 -deoxyuridine func- tionalized with a suitable tridentate chelating system at position 5 0 was synthesized from commercial 2 0 -deoxyuridine in seven steps. The 5-ethyl-2 0 -deoxyuridine derivative was labeled with the fac-M(CO) 3 -core (M = Tc, Re). The resulting rhenium complex was found to be a selective competitive inhibitor of HSV1-TK (K i = 4.56 lM). Inhibition of the human cytosolic thymidine kinase (hTK1) previously reported with organometallic rhenium and technetium complexes of 5 0 -carboxamide thymidine derivative was not observed. The uptake of the technetium-99m complex in transfected cells expressing HSV1-TK has been evaluated to assess its possible use as reporter. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Deoxyuridine derivative; Technetium; Rhenium; Thymidine kinases 1. Introduction Gene therapy is characterized by the transfer of new genetic material encoding a therapeutic protein into the cells [1,2]. Often a reporter protein is co-expressed in order to con- trol the level of expression of the therapeutic protein [3]. Her- pes simplex virus thymidine kinase type 1 (HSV1-TK) has been used both as a reporter and/or suicide gene [4]. HSV1-TK is a multifunctional enzyme with a lower sub- strate specificity compared to the human cytosolic thymidine kinase (hTK1) [5]. Over the last decade, chemists and radio- chemists have been trying to produce selective substrates and inhibitors of HSV1-TK acting as prodrug and reporter probes [6–9]. Currently, the best representatives of Single Photon Emission Tomography (SPET) or Positron Emis- sion Tomography (PET) reporter probes are 131 I-labeled 2 0 fluoro-2 0 deoxy-1-b-D-arabinofuranosy-5-iodo-uracyl ([ 131 I]- FIAU) and 18 F-labeled 9-[4fluoro-3-(hydroxymethyl)butyl]- guanine ([ 18 F]FHBG), respectively [10]. The aim of the present work is the development of novel SPET reporter probes selective for HSV1-TK labeled with the low-cost radionuclide Technetium-99m. Herein the syn- thesis of the organometallic technetium/rhenium complexes of a 5 0 -carboxamide 5-ethyl-2 0 -deoxyuridine derivative 2 (Fig. 1) is described. The biological active entity was based on 5 0 -carboxamide 5-ethyl-2 0 -deoxyuridine derivatives func- tionalized with spacer and chelating system enabling the sta- ble coordination of the metal center. The (radio)labeling was performed with the organometallic precursor fac- [M(CO) 3 (H 2 O) 3 ] + (M = 99m Tc, nat Re), which has proven to form very inert and in vivo stable complexes [11–14]. Com- plex 2 has been evaluated in vitro for potency and selectivity toward both HSV1-TK and hTK1. The results are compared 0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2006.10.011 * Corresponding author. Address: Center for Radiopharmaceutical Science, ETH-PSI-USZ, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland. Tel.: +41 56 310 2837; fax: +41 56 310 2849. E-mail address: roger.schibli@psi.ch (R. Schibli). www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 692 (2007) 1340–1347