The brain A2A adenosine receptor (A2AAR) participates with the dopamine D2 receptor in the control of movement and also might influence behavior. Because PET is an important tool for studying the roles of receptors in disease, a ligand for imaging the brain A2AAR is desirable. This report describes the synthesis and A2AAR antagonist activities of a panel of phenyl-substituted 7-amino-2-(2-furyl)-5-phenylethylamino-oxazolo[5,4-d]pyrimidines, 11aa-af, and their 3-furyl congeners, 11ba-bd. In competitive binding studies all compounds displaced [3H]CGS21680 from the A2AAR with Ki values of 14-33 nM with selectivity for the A2AAR over the A1AR of 5- to 94-fold. Autoradiography of brain sections showed a high level of unspecific binding that obscured specific binding. Thus, these compounds are not promising PET ligands.

The importance of the brain A2A adenosine receptor (A(2A)AR) in movement disorders urges the development of radiolabeled ligands for imaging those receptors by positron emission tomography (PET). This study evaluated one class of A(2A)AR antagonists, derivatives of 4-amino-6-benzylamino-1,2-dihydro-2-phenyl-1,2,4-triazolo[4,3-a]quinoxalin-2H-1-one, 10a, as agents for imaging brain A(2A)ARs by PET.. Modifications of a literature synthesis of 10a efficiently generated analogs 10b-s for pharmacological evaluation. Radioligand binding experiments showed affinities for the rat brain A(2A)AR in the low nanomolar range but similar affinities for the A1AR and substantial unspecific binding. Autoradiography employing [3H]10a, showing that high unspecific binding obscured specific binding to both the A1AR and A(2A)AR. Thus, compounds 10b-s are unsuitable as ligands for imaging brain A(2A)ARs by PET.

UNLABELLED:Adenosine plays a critical role in both tumor proliferation and the cerebral response to tumor invasion. We used 8-cyclopentyl-3-(3-18F-fluoropropyl)-1-propylxanthine (18F-CPFPX) PET to investigate A1 adenosine receptor (A1AR) density as a potential indicator of the local cerebral response to glioma invasion.METHODS:A1AR density in F98 glioma-bearing rats was examined by 18F-CPFPX and 3H-CPFPX using PET, quantitative in vitro and ex vivo double-label receptor autoradiography, and immunohistochemical analyses.RESULTS:For all imaging modalities, A1AR signal intensity was increased in a zone surrounding experimental tumors (136%-146% that in control tissue) (P < 0.01). Immunostaining identified activated astrocytes as the main origin of peritumoral A1AR upregulation. The results of a pilot 18F-CPFPX PET study on a patient with recurrent glioblastoma multiforme confirmed increases in A1AR density in the immediate vicinity of the tumor.CONCLUSION:18F-CPFPX PET is suitable for the detection of peritumoral changes in A1AR density. Molecular imaging with 18F-CPFPX PET may open novel possibilities for gaining experimental and clinical insights into the cerebral response to tumor invasion.

Adenosine is an important neuromodulator. Basic cerebral effects of adenosine are exerted by the A 1 adenosine receptor (A 1 AR), which is accessible in vivo by the novel ligand [ 18 F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ([ 18 F]CPFPX) and positron emission tomography (PET). The present study investigates the applicability of kinetic models to describe the cerebral kinetics of [ 18 F]CPFPX in order to quantify A 1 AR density in vivo. Six healthy volunteers underwent dynamic PET scanning and arterial blood sampling after bolus injection of [ 18 F]CPFPX. For quantitative analysis, a standard two-tissue compartment model (2TCM) was compared with a one-tissue compartment model (1TCM) and Logan's graphical analysis (GA). The 2TCM described the cerebral kinetics of [ 18 F]CPFPX significantly better than the 1TCM (in all regions and subjects examined). The estimated values of the regional total distribution volumes ( DV t ) correlated strongly between the 2TCM and GA (linear regression r 2 = 0.99, slope: 1.007). The DV t correlation between the 2TCM and the 1TCM was comparably high, but there was a significant bias towards lower DV t estimates given by the 1TCM (r 2 : 0.99, slope: 0.929). It is concluded that a 2TCM satisfactorily accounts for the cerebral kinetics of [ 18 F]CPFPX. GA represents an attractive alternative method of analysis.

Autoradiography on rat brain using tritiated (1*), mono- (2*) and di-radioiodinated (3*) derivatives of the A2A adenosine receptor antagonist ZM241,385 showed high receptor density in striatum. KDs of 1*, 2* and 3* were 0.4, 2.2 and 15 nM and nonspecific binding was 5, 40 and 50% of total binding. Striatal uptake of 2* in mice was ∼ 0.2% ID/g 60 min post-injection; blocking by 2 was insignificant. Poor penetration of the blood brain barrier and high nonspecific binding make 2* unsuitable for imaging striatal receptors.

The structures of 1-benzyl-4-nitroso-5-aminopyrazole (1) and its hydrochloride (1H+) have been determined in the solid state and in solution in DMSO, methanol, and ethanol. The free base exists in solution as a mixture of amino/nitroso tautomers 2a and 2b rather than in the imino/oxime tautomers 3. The conjugated cation 1H+ results from the protonation of the nitroso group. X-ray crystallography showed that both amino hydrogen atoms of 2a form NH...O=N hydrogen bonds: one is intramolecular, the other links adjacent molecules in an infinite chain.

8-Cyclopentyl-3-[(E)-3-[131I]iodoprop-2-en-1-yl]-1-propylxanthine (2*) was generated by iododestannylation of the tributyl-stannyl-precursor with [131I]NaI and chloramine T. The radiochemical yield of 2* was 82 ± 4%, and the purity exceeded 98%. The specific activity was 33 ± 19 GBq/μmol. Affinities for rat, pig and human A1 adenosine receptors (A1ARs) were in the low nanomolar range, but poor selectivity for the human A1AR over the A2AAR was found. Additionally, in vitro and ex vivo autoradiographic studies revealed high unspecific binding which makes this ligand unsuitable for SPECT imaging.

UNLABELLEDAdenosine modulates brain activity through 4 G protein-coupled receptors, primarily adenosine A(1) receptors (A(1)ARs). A(1)ARs are heterogeneously distributed throughout the brain and participate in many physiologic processes-for example, the induction of sleep and feedback inhibition of excitatory neurotransmission. There is also evidence that A(1)ARs are involved in brain pathologies, including cerebral ischemia, epilepsy, and neurodegeneration. Therefore, measuring A(1)ARs in the living brain has been a long-standing goal. This report describes the preclinical evaluation of (18)F-8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ((18)F-CPFPX), a novel A(1)AR PET ligand.METHODSCPFPX, a xanthine-based A(1)AR antagonist, was labeled with either (18)F or (3)H, maintaining identical chemical structures, and evaluated in rats as a putative radioligand for in vivo or in vitro imaging of brain A(1)ARs by quantitative receptor autoradiography and the combination of high-resolution small animal PET and MRI.RESULTS(3)H-CPFPX bound with nanomolar affinity (K(d), 4.4 nmol/L) to A(1)ARs and showed a distribution typical of cerebral A(1)ARs. In extensive in vitro competition studies, (3)H-CPFPX proved to be a highly selective and specific A(1)AR radioligand. Neither the nonxanthine-type adenosine A(2A) receptor antagonist ZM 241385 nor multiple cholinergic, serotoninergic, and glutamatergic receptor compounds competed for (3)H-CPFPX below the micromolar level. In vivo animal PET and ex vivo autoradiographic experiments measured radioactivity in discrete brain regions after intravenous injection of (18)F-CPFPX. (18)F-CPFPX had excellent in vivo stability and penetrated the blood-brain barrier immediately after injection due to its high lipophilicity. Brain uptake was rapid and particularly high in gray matter regions. Retention of (18)F-CPFPX was highest in the cerebellum, thalamus, and neocortex with evidence of saturable binding. Low binding potentials were found in the midbrain. In vivo displacement PET experiments with the A(1)AR antagonist 8-cyclopentyl-1,3-dipropylxanthine showed a 72% +/- 8% displacement of (18)F-CPFPX.CONCLUSION(18)F-CPFPX is a highly selective and specific ligand for A(1)ARs and a suitable radioligand for noninvasive PET imaging of A(1)ARs in the living brain. These studies also support the application of high-resolution animal PET as an effective in vivo imaging tool in the evaluation process of new radioligands.

The important roles played by the A1 adenosine receptor (A1AR) in brain physiology and pathology make this receptor a target for in vivo imaging. Here we describe the distribution of A1ARs in the living human brain with PET, made possible for the first time by the highly potent and selective A1AR antagonist 8-cyclopentyl-3-(3-[18F]fluoropropyl)-1-propylxanthine ([18F]CPFPX). In vivo data demonstrate a rapid cerebral uptake, peaking at 2.9 ± 0.6% injected dose/liter at 3.3 ± 1.3 min, followed by a gradual washout. Consistent with the results of autoradiography, high receptor densities occurred in the putamen and the mediodorsal thalamus. Neocortical regions showed regional differences in [18F]CPFPX binding, with high accumulation in temporal > occipital > parietal > frontal lobes and a lower level of binding in the sensorimotor cortex. Ligand accumulation was low in cerebellum, midbrain, and brain stem. Metabolism of [18F]CPFPX is rapid outside the central nervous system, but the metabolites do not penetrate the blood–brain barrier. In conclusion, in vivo application of [18F]CPFPX, a highly potent and selective PET ligand, for the first time allows the imaging of A1ARs in the living human brain.

This report describes a straightforward, high yield synthesis of a previously inaccessible N-protected diaminopyrazole in five steps starting from acrylonitrile, hydrazine and benzaldehyde.

The reduction of 1-allyl-8-cyclopentyl-3-(3-fluoropropyl)xanthine, 7, with tritium gas catalyzed by 10% Pd-C gave 8-cyclopentyl-3-(3-fluoropropyl)-1[2,3-H-3]propylxanthine ([H-3]CPFPX), 8*, a potent and selective antagonist for the A, adenosine receptor (AIAR). The synthesis of 7 proceeded from 6-aminouracil, 1, which underwent silylation and alkylation with allyl bromide to form 6-amino-3-allyluracil, 2. Nitrosation led to the 5-nitroso compound, 3, which underwent reduction to the 4,5-diaminouracil, 4, and carbodiimide-mediated acylation with cyclopentanecarboxylic acid produced 3-allyl-6amino-5-cyclopentylcarboxamidouracil, 6. Alkylation at N-1 with 3-fluoro-1-bromopropane and cyclization with alkali completed the synthesis of 7. [H-3]CPFPX had a radiochemical purity of > 98% and a specific activity of > 2.1 TBq/mmol (57 Ci/mmol). [H-3]CPFPX bound to the rat, pig and human A(1)AR with a K-D of 0.63, 1.37 and 0.71 nM, respectively. The K-D at the rat and human A(2A)AR was 812 and 940 nM, respectively, thus giving selectivities of > 1200- and > 700-fold. Copyright (C) 2003 John Wiley Sons, Ltd.

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.

In the last decade the field of purinergic pharmacology has continued to grow as the complexity of the receptor families and the various enzymes involved in purine metabolism have been defined in molecular terms. Adenosine receptors (ARs) are currently divided into the four subclasses A(1)-, A(2A)-, A(2B)- and A(3)AR. The most intensively studied subtypes are the high-affinity A(1) and A(2A) receptors, which are activated by adenosine in nano- to submicromolar concentrations. The clinical importance of the A(1) adenosine receptor (A(1)AR) and the A(2A)adenosine receptor (A(2A)AR) makes them attractive targets for radionuclide in vivo imaging. Positron Emission Tomography (PET) is an imaging modality which can determine biochemical and physiological processes in vivo in a quantitative way by using radiopharmaceuticals labeled with positron emitting radionuclides as (11)C, (13)N, (15)O and (18)F and by measuring the annihilation radiation using a coincidence technique. This includes also measurement of the pharmacokinetics of labeled drugs and the assessment of the effects of drugs on metabolism. In the present article we review the radioligands which are currently available for visualisation and quantification of ARs using PET with a special focus on the A(1)AR and A(2A)AR.

The direct electrophilic no-carrier-added (n.c.a.) aromatic radioiodination was examined using various metal salts in trifluoroacetic acid (TFA) as in situ oxidation agents. Two different types of metal salts were used comprising TFA-soluble (Pb(CH3CO2)4, Mn(CH3CO2)3, KMnO4, Tl(CF3CO2)3, AgCF3SO3) and TFA-insoluble (Ce(CF3SO3)4, RuCl3, FeBr3, K2Cr2O7) salts. Optimization of both labelling systems has been performed using Pb(CH3CO2)4, Ce(CF3SO3)4and benzene as a model substrate. At room temperature, the one-pot synthesis was completed within 15 min, resulting in a radiochemical yield of 82% and 64% using Pb(CH3CO2)4and Ce(CF3SO3)4, respectively. Radioiodination of weakly activated monosubstituted benzene derivatives led to high radiochemical yields of about 80% and 60% of the corresponding ortho- and para-radioiodo-isomers using both salts. Weakly deactivated chlorobenzene could only be radioiodinated with Ce(CF3SO3)4as oxidant, forming exclusively the para-product with a radiochemical yield of about 35%. Using the optimized reaction parameters for the other TFA-soluble and -insoluble metal salts with benzene and toluene good radiochemical yields were obtained in all cases except for the manganese and silver salts. Apparently their oxidation power was not strong enough for the radioiodination of the non-activated benzenes. In situ formed trifluoroacetyl [131I]hypoiodite is discussed with regard to the reaction mechanism.

Journal of Labelled Compounds and RadiopharmaceuticalsVolume 42, Issue 6 p. 537-551 Research Article N.C.A.[11C]CO2 as a safe substitute for phosgene in the carbonylation of primary amines A. Schirbel, A. Schirbel Institut für Nuklearchemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, GermanySearch for more papers by this authorM. H. Holschbach, Corresponding Author M. H. Holschbach Institut für Nuklearchemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, GermanyInstitut für Nuklearchemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany.Search for more papers by this authorH. H. Coenen, H. H. Coenen Institut für Nuklearchemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, GermanySearch for more papers by this author A. Schirbel, A. Schirbel Institut für Nuklearchemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, GermanySearch for more papers by this authorM. H. Holschbach, Corresponding Author M. H. Holschbach Institut für Nuklearchemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, GermanyInstitut für Nuklearchemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany.Search for more papers by this authorH. H. Coenen, H. H. Coenen Institut für Nuklearchemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, GermanySearch for more papers by this author First published: 05 August 1999 https://doi.org/10.1002/(SICI)1099-1344(199906)42:6<537::AID-JLCR215>3.0.CO;2-3Citations: 25AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract An efficient one-pot synthesis of [11C]ureas and [11C]isocyanates via dehydration of intermediately formed carbamate salts is described as a general alternative to their formation via [11C]phosgene. After optimization of the reaction parameters, in-target produced n.c.a. [11C]CO2 can be used for labelling in a one pot reaction within a very short reaction time of 10 minutes resulting in good radiochemical yields. The developed method has been applied to the 11C-carbonylation of aniline, benzyl- and phenethylamine and 1,2-diaminobenzene yielding the appropriate n.c.a. [11C]ureas in about 65, 85, 25 and 70% radiochemical yield (RCY), respectively. The presented reaction sequence can be handled easily and safely and lends itself to simple automation. Copyright © 1999 John Wiley & Sons, Ltd. Citing Literature Volume42, Issue6June 1999Pages 537-551 RelatedInformation

An efficient one-pot synthesis of [C-11]ureas and [C-11]isocyanates via dehydration of intermediately formed carbamate salts is described as a general alternative to their formation via [C-11]phosgene. After optimization of the reaction parameters, in-target produced n.c.a. [C-11]CO2 can be used for labelling in a one pot reaction within a very short reaction time of 10 minutes resulting in good radiochemical yields. The developed method has been applied to the C-11-carbonylation of aniline, benzyl- and phenethylamine and 1,2-diaminobenzene yielding the appropriate n.c.a. [C-11]ureas in about 65, 85, 25 and 70% radiochemical yield (RCY), respectively. The presented reaction sequence can be handled easily and safely and lends itself to simple automation.

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.