Radiosynthesis of [ 11 C]ximelagatran via palladium catalyzed [ 11 C]cyanation Anu J. Airaksinen, aà Jan Andersson, a Phong Truong, a Olle Karlsson, b and Christer Halldin a N-hydroxyamidines (amidoximes) may be used in prodrug technology in improving oral bioavailability of drugs containing amidino functional groups. In the body, amidoximes are reduced quickly to amidines by enzymes that are present in several organs. Ximelagatran is a benzamidoxime and ethyl ester prodrug of melagatran, which is a thrombin inhibitor. Our aim was to develop a fast and efficient labeling route for the synthesis of [ 11 C]ximelagatran ([ 11 C]3) with a label in a metabolically stable position. [ 11 C]3 was synthesized via a two-step synthesis sequence, starting from palladium catalyzed [ 11 C]cyanation of its corresponding bromide precursor (2-[2-(4-bromo-benzylcarbamoyl)-azetidin-1-yl]-1-cyclohexyl-2-oxo- ethyl amino-acetic acid ethyl ester) (1), followed by a reaction with hydroxylamine. [ 11 C]3 was synthesized with 27 ± 17% total overall decay corrected yield (specific radioactivity of 2360 ± 165 Ci/mmol at EOS), with a total synthesis time of 45 min. A fast and efficient labeling route for the synthesis of [ 11 C]3 was developed. Keywords: N-hydroxyamidine; amidoxime; serine protease inhibitor; radiosynthesis; PET; ximelagatran Introduction N-hydroxyamidine functionalities (amidoximes) may be used in prodrug technology in improving oral bioavailability of drugs containing amidino functional groups. 1–4 Amidino groups are very strong bases and they are protonated under physiological conditions, which causes poor absorption of amidino-based drugs from the gastrointestinal tract. In N-hydroxyamidines, an oxygen atom is introduced on one of the nitrogen atoms of the amidino group, which leads to lower pK a and thus higher lipophilicity, increasing permeability across epithelial cells. In the body, the hydroxyamidine group is reduced quickly to its active amidino moiety by enzymes that are present in several organs. 1 Amidino groups are a common feature of many serine protease inhibitors. 5 The functional group binds to the enzymatic pocket of the protease, inhibiting binding of the endogenous substrate. 6,7 Many serine proteases, such as thrombin and factors VIIa, IXa, Xa, XIa and XIIa, are involved in the blood coagulation cascade. 8 Serine protease inhibitors targeting especially the inhibition of the function of thrombin have been popular targets in the recent development of novel drugs in the treatment of thromboembolic diseases. 9 Ximelagatran is a prodrug of an oral direct thrombin inhibitor melagatran. 10,11 Melagatran belongs to the class of serine protease inhibitors with a benzamidine functional group for binding to the arginine binding S1 pocket of thrombin (Figure 1). Melagatran is a peptidomimetic, mimicking the D-Phe-Pro-Arg sequence. The pK a ’s of the benzamidine (pK a 5 11.5) and the carboxylic acid (pK a 5 2.0) groups of melagatran cause its zwitterionic character at physiological pH, leading to poor oral bioavailability of the drug. 12 The prodrug ximelagatran (Exanta, AstraZeneca) was designed to overcome this problem by using ester and amidoxime (pK a 5.2) functionalities in order to avoid ionization of the drug at physiological pH. In the body, after absorption from the gastrointestinal tract, ximelagatran is metabolized quickly in two steps, via hydrolysis of the ethyl ester group and reduction of the hydroxyamidine moiety, to the active drug melagatran. Melagatran itself is eliminated through urine and faeces without further metabolism. 13 Positron emission tomography is a useful, but still rarely used tool in evaluating the bioavailability of prodrugs in vivo. 14 The technique enables simultaneous observation of the biodistribu- tion of the investigated prodrug, metabolized intermediates and the active drug, which provides important information about possible accumulation of the prodrug or the intermediates in non-target organs. The technique also enables analysis of kinetics of the metabolic processes liberating the drug from its prodrug moiety. All this may provide crucial information on in vivo behavior of a novel prodrug in early clinical stages. [ 11 C]Amidination of [ 11 C]nitriles was reported by Sime ´on et al. in the radiosynthesis of [ 11 C](Z,Z)-BABCH. 15 According to our knowledge, [ 11 C](Z,Z)-BABCH is the only 11 C-labeled benzami- dine reported so far. However, synthesis of [ 11 C]benzamidox- imes has not been reported before and our aim was to develop a fast and efficient labeling route for the synthesis of 11 C-labeled benzamidoximes with a label in a metabolically stable position. We used ximelagatran as a model compound. 1 Research Article Received 19 June 2007, Revised 13 August 2007, Accepted 21 August 2007 Published online 3 January 2008 in Wiley Interscience (www.interscience.wiley.com) DOI: 10.1002/jlcr.1461 J. Label Compd. Radiopharm 2008, 51 1–5 Copyright r 2008 John Wiley & Sons, Ltd. a Karolinska Institutet, Department of Clinical Neuroscience, Psychiatry Section, Karolinska Hospital, S-17176 Stockholm, Sweden b Astrazeneca R&D, Department of Medicinal Chemistry, S-431 83 Mo¨lndal, Sweden *Correspondence to: Anu J. Airaksinen, Karolinska Institutet, Department of Clinical Neuroscience, Psychiatry Section, Karolinska Hospital, S-17176 Stock- holm, Sweden. E-mail: anu.airaksinen@ki.se