This report describes the precursor synthesis and the no-carrier-added (nca) radiosynthesis of the new A(1) adenosine receptor (A(1)AR) antagonist [(18)F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (CPFPX), 3, with fluorine-18 (half-life = 109.6 min). Nucleophilic radiofluorination of the precursor tosylate 8-cyclopentyl-3-(3-tosyloxypropyl)-7-pivaloyloxymethyl-1-propylxanthine, 2, with nca [(18)F]KF under aminopolyether-mediated conditions (Kryptofix 2.2.2/K(2)CO(3)) followed by deprotection was straightforward and, after formulation, gave the radioligand ready for injection with a radiochemical yield of 45 +/- 7%, a radiochemical purity of >98% and a specific radioactivity of >270 GBq/micromol (>7.2 Ci/micromol). Preparation time averaged 55 min. The synthesis proved reliable for high batch yields ( approximately 7.5 GBq) in routine production (n = 120 runs). The radiotracer was pharmacologically evaluated in vitro and in vivo and its pharmacokinetics in rodents determined in detail. After iv injection a high accumulation of radioactivity occurred in several regions of mouse brain including thalamus, striatum, cortex, and cerebellum. Antagonism by the specific A(1)AR antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and N(6)-cyclopentyl-9-methyladenine (N-0840), but not with the A(2)AR antagonist 3,7-dimethyl-1-propargylxanthine (DMPX), indicated specific and reversible binding of the radioligand to A(1)AR in cortical and subcortical regions of interest. In mouse blood at least two polar metabolites formed rapidly (50% at 5 min after tracer application). However, chromatographic analyses of brain homogenate extracts taken 60 min pi showed that >98% of radioactivity was unchanged radioligand. Chromatographic isolation and reinjection of peripherally formed radioactive metabolites revealed no accumulation of radioactivity in mouse brain, probably due to the polarity of the metabolites. These preliminary results suggest that nca [(18)F]CPFPX is a useful radioligand for the noninvasive imaging of the brain A(1)AR.

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Positron emission tomography (PET) is a useful procedure for assessing the density (Phelps, 2000) and pharmacological properties of receptors in vivo (Merlet et al., 1993;Beugel et al., 1999). However, receptor-PET studies favor radioligands that are either stable in the body or that give rise to metabolites that do not interfere with the imaging of specifically bound ligand.The ligand (Holschbach et al., 2002) is used to image the A 1 adenosine receptor (A 1 AR) in human brain (Bauer et al., 2003). Because this ligand does not undergo degradation in the central nervous system, specifically bound ligand accounts for a very large fraction of brain radioactivity. However, such is not the case in peripheral tissues. The metabolism of [18 F]CPFPX in primates (Boy et al., 1998) and humans gives rise to at least three polar metabolites in blood, and studies illustrated the confounding effects of these metabolites. For example, the intravenous administration of [18 F]CPFPX to experimental animals caused intense labeling of the heart that was unaffected by the administration of unlabeled ligand, evidence for high unspecific binding of a metabolite (Holschbach et al., 1998).The physiological importance of the A 1 AR, its wide tissue distribution, and the success of PET-imaging A 1 ARs in the central nervous system urge extension of this technique to other organs. The design of more stable radioligands to achieve that end requires the kind of information about the metabolism of [18 F]CPFPX provided by this study.Measurements of receptor density by PET depend on compartmental analysis by mathematical models that are very sensitive to the concentration of native radioligand in blood perfusing the organ (the "input function"). Such measurements on the plasma of human subjects (Meyer et al., 2004) identified several [18 F]CPFPX metabolites in addition to unchanged ligand. Because the radiotracers for PET studies are prepared under no-carrier-added conditions, the amount of compound administered is in the low to subnanomolar range, making direct spectrometric identification of metabolites impossible. As this report describes, incubating CPFPX with human liver microsomes generated compounds that by HPLC had the same mobilities as the metabolites in plasma, and LC-MS tentatively identified them by measuring m/z of the [M ϩ H] ϩ ions. The literature contains little information about the metabolism of synthetic xanthines. The use of CPFPX in humans for diagnostic and research PET imaging necessitates the knowledge of its metabolism in vivo. To our knowledge, the present study of the biotransformation of CPFPX in humans is the first of its kind.Article, publication date, and citation information can be found at

The high affinity of 8-cyclopentyl-1,3-dipropylxanthine (CPX) for the A1 adenosine receptor (A1AR) provides a good lead for developing radioligands suitable for positron emission tomography (PET) and single-photon emission tomography (SPET). This study tested the hypothesis that the kinds of chemical modifications made in the synthesis of CPX analogues containing carbon-11, fluorine-18, or radioiodine will not alter affinity for the A1AR. This report describes the synthesis and radioligand binding assays of unlabeled CPX analogues having methyl, 2-methoxyethyl, 2-fluoropropyl, or 3-fluoropropyl substituents, respectively, at either N-1 (13a-d) or N-3 (8a-d) or an (E)-3-iodoprop-2-en-1-yl substituent at N-3 (8f). Compounds 8d,f and 13b,d antagonized the binding of [3H]CPX to the A1AR of rat brain with affinities similar to those of CPX; compound 8c was twice as potent as CPX. Analogues 8a,b and 13a were less potent than CPX, but for each the Ki of antagonism was > or = 0.5 nM. Attempts to iodinate the 8-(4-hydroxyphenyl) analogue of CPX failed, probably because the xanthine substituent strongly deactivated the phenol toward electrophilic iodination. In summary, several of the modifications of the propyl groups of CPX needed to produce ligands for imaging by PET and SPET preserve or enhance affinity for the A1AR.