Synthesis and biological evaluation of novel carbon-11-labelled analogues of citalopram as potential radioligands for the serotonin transporter

Three serotonin reuptake inhibitors where the 5-cyano group in citalopram [1-(3-dimethylamino-propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile (1)] was replaced with a methyl, acetyl and piperidinyl carbonyl group, respectively, were synthesized. In a Stille reaction applying [(11)C]methyl iodide the labelled compound [5-methyl-(11)C][3-[1-(4-fluorophenyl)-5-methyl-1,3-dihydroisobenzofuran-1-yl]-propyl]-dimethylamine ([(11)C]-2) was synthesized in 60-90% radiochemical yield. [5-carbonyl-(11)C][1-[1-(3-dimethylaminopropyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-yl]-1-piperidin-1-yl-methanone] ([(11)C]-3) was synthesized in 62% radiochemical yield in a palladium mediated cross-coupling reaction utilizing [(11)C]carbon monoxide. The specific activity of [(11)C]-2 was highly dependent on whether the corresponding trimethyltin or tributyltin precursor was applied. In ex vivo rodent studies compound [(11)C]-2 exhibited a good blood-brain barrier (BBB) penetration whereas [(11)C]-3 did not. The brain distribution of [(11)C]-2 was investigated in a non-human primate using PET. There was a rapid uptake of radioactivity into the brain. Accumulation of the radiotracer was in agreement with the known distribution of serotonin transporters. The maximal thalamus to cerebellum ratio of 1.3 was reached after 85 min and the specific binding was partly blocked after pre-treatment with citalopram. Thus, [(11)C]-2 does not exhibit appropriate properties as radioligand for visualization of the serotonin transporter in vivo.     

Synthesis and in vivo evaluation of [O-methyl-11C] N-[3,5-dichloro-2-(methoxy)phenyl]-4-(methoxy)-3-(1-piperazinyl)benzenesulfonamide as an imaging probe for 5-HT6 receptors

The serotonin receptor 6 (5-HT(6)) is implicated in the pathophysiology of cognitive diseases, schizophrenia, anxiety and obesity and in vivo studies of this receptor would be of value for studying the pathophysiology of these disorders. Therefore, N-[3,5-dichloro-2-(methoxy)phenyl]-4-(methoxy)-3-(1-piperazinyl)benzenesulfonamide (SB399885), a selective and high affinity (pK(i)=9.11) 5-HT(6) antagonist, has been radiolabeled with carbon-11 by O-methylation of the corresponding desmethyl analogue with [(11)C]MeOTf in order to determine the suitability of [(11)C]SB399885 to quantify 5-HT(6)R in living brain using PET. Desmethyl-SB399885 was prepared, starting from 1-(2-methoxyphenyl) piperazine hydrochloride, in excellent yield. The yield obtained for radiolabeling of [(11)C]SB399885 was 30±5% (EOS) and the total synthesis time was 30min at EOB. PET studies with [(11)C]SB399885 in baboon showed fast uptake followed by rapid clearance in the brain. Highest uptake of radioactivity of [(11)C]SB399885 in baboon brain were found in temporal cortex, parahippocampal gyrus, pareital cortex, amygdala, and hippocampus. Poor brain entry and inconsistent brain uptake of [(11)C]SB399885 compared to known 5-HT(6)R distribution limits its usefulness for the in vivo quantification of 5-HT(6)R with PET.     

Facile synthesis and PET imaging of a novel potential heart acetylcholinesterase tracer N-[11C]methyl-3-[[(dimethylamino)carbonyl]oxy]-2-(2',2'-diphenylpropionoxymethyl)pyridinium

A new AChE tracer N-[(11)C]methyl-3-[[(dimethylamino)carbonyl]oxy]-2-(2',2'-diphenylpropionoxymethyl)pyridinium ([(11)C]MDDP, [(11)C]1) has been synthesized in 40-65% radiochemical yield. Initial PET dynamic studies of [(11)C]MDDP in rat heart showed rapid heart uptake and blood pool clearance to give high-quality heart images. Blocking studies of [(11)C]MDDP with pretreatment drug neostigmine in rats found only minor reductions in rat heart [(11)C]MDDP retention. The results suggest that [(11)C]MDDP delineates the heart very clearly, and the uptakes of [(11)C]MDDP in rat heart might be related to non-specific binding.     

Synthesis and biological evaluation of 2beta,3alpha-(substituted phenyl)nortropanes as potential norepinephrine transporter imaging agents

A series of 2beta,3alpha-(substituted phenyl)nortropanes was synthesized and evaluated in vitro for human monoamine transporters. All compounds studied in this series exhibited nanomolar potency for the norepinephrine transporter (NET). Radiolabeling and nonhuman primate microPET brain imaging studies were performed with the most promising compound, [(11)C]1, to determine its utility as a NET imaging agent. Despite high in vitro affinity for the human NET, the high uptake of [(11)C]1 in the caudate and putamen excludes its use as an in vivo PET imaging agent for the NET.     

Synthesis and in vivo evaluation of a new PET radioligand for studying sigma-2 receptors

The cyclohexyl piperazine 1 (1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydro-naphthalen-1-yl)-propyl]-piperazine) has been shown to be a potent and selective sigma-2 receptor ligand. In the present study, we prepared [(11)C]1 by O-alkylation of the phenolic precursor 2 with [(11)C]CH(3)I. [(11)C]1 was obtained in a 29% non-decay corrected yield and specific activity of 9299 mCi/micromol calculated at end-of-synthesis. The biodistribution of [(11)C]1 in mouse brain demonstrated rapid and homogenous concentration in all brain structures, which included the cortex, thalamus, cerebellum and striatum. Co-administration of unlabelled 1 (1 mg/kg) or the sigma-2 selective ligand SM-21 (1 mg/kg) failed to show any significant inhibition of [(11)C]1 uptake in the mouse brain. The evaluation of this radioligand in vivo in the mouse clearly indicates that it does not possess the required properties for studying sigma-2 receptors in the brain using PET.     

Efficient sequential synthesis of PET Probes of the COX-2 inhibitor [11C]celecoxib and its major metabolite [11C]SC-62807 and in vivo PET evaluation

Synthesis of [(11)C]celecoxib, a selective COX-2 inhibitor, and [(11)C]SC-62807, a major metabolite of celecoxib, were achieved and the potential of these PET probes for assessing the function of drug transporter in biliary excretion was evaluated. The synthesis of [(11)C]celecoxib was achieved in one-pot by reacting [(11)C]methyl iodide with an excess of the corresponding pinacol borate precursor using Pd(2)(dba)(3), P(o-tolyl)(3), and K(2)CO(3) (1:4:9) in DMF. The radiochemical yield of [(11)C]celecoxib was 63±23% (decay-corrected, based on [(11)C]CH(3)I) (n=7) with a specific radioactivity of 83±23GBq/μmol (n=7). The average time of synthesis from end of bombardment including formulation was 30min with >99% radiochemical purity. [(11)C]SC-62807 was synthesized from [(11)C]celecoxib by further rapid oxidation in the presence of excess KMnO(4) with microwave irradiation. The radiochemical yield of [(11)C]SC-62807 was 55±9% (n=3) (decay-corrected, based on [(11)C]celecoxib) with a specific radioactivity of 39±4GBq/μmol (n=3). The average time of synthesis from [(11)C]celecoxib including formulation was 20min and the radiochemical purity was >99%. PET studies in rats and the metabolite analyzes of [(11)C]celecoxib and [(11)C]SC-62807 showed largely different excretion processes, and consequently, [(11)C]SC-62807 was rapidly excreted via hepatobiliary excretion without further metabolism. [(11)C]SC-62807 was shown to have a high potential as a PET probe for evaluating drug transporter function in biliary excretion.     

Synthesis and biodistribution of [11C]R107474, a new radiolabeled alpha2-adrenoceptor antagonist

R107474, 2-methyl-3-[2-(1,2,3,4-tetrahydrobenzo[4,5]furo[3,2-c]pyridin-2-yl)ethyl]-4H-pyrido[1,2-a]pyrimidin-4-one, was investigated using in vitro and in vivo receptor assays and proved to be a potent and relatively selective alpha(2)-adrenoceptor antagonist. Performed assays in vitro were inhibition of binding to a large number of neurotransmitter receptor sites, drug receptor binding sites, ion channel binding sites, peptide receptor binding sites, and the monoamine transporters in membrane preparations of brain tissue or of cells expressing the cloned human receptors. The compound has subnanomolar affinity for halpha(2A)- and halpha(2C)-adrenoceptors (K(i) = 0.13 and 0.15 nM, respectively) and showed nanomolar affinity for the halpha(2B)-adrenoceptors and 5-hydroxytryptamine(7) (h5-HT(7)) receptors (K(i) = 1 and 5 nM, respectively). R107474 interacted weakly (K(i) values ranging between 81 and 920 nM) with dopamine-hD(2L), -hD(3) and -hD(4), h5-HT(1D)-, h5-HT(1F)-, h5-HT(2A)-, h5-HT(2C)-, and h5-HT(5A) receptors. The compound, tested up to 10 microM, interacted only at micromolar concentrations or not at all with any of the other receptor or transporter binding sites tested in this study. In vivo alpha(2A)- and alpha(2C)-adrenoceptor occupancy was measured by ex vivo autoradiography 1h after subcutaneous (sc) administration of R107474. It was found that R107474 occupies the alpha(2A)- and alpha(2C)-adrenoceptors with an ED(50) (95% confidence limits) of 0.014 mg/kg sc (0.009-0.019) and 0.026 mg/kg sc (0.022-0.030), respectively. Radiolabeled 2-methyl-3-[2-([1-(11)C]-1,2,3,4-tetrahydrobenzo[4,5]furo[3,2-c]pyridin-2-yl)ethyl]-4H-pyrido[1,2-a]pyrimidin-4-one ([(11)C]R107474) was prepared and evaluated as a potential positron emission tomography (PET) ligand for studying central alpha(2)-adrenoceptors. [(11)C]R107474 was obtained via a Pictet-Spengler reaction with [(11)C]formaldehyde in 33 +/- 4% overall decay-corrected radiochemical yield. The total synthesis time was 55 min and the specific activity was 24-28 GBq/micromol. The biodistribution of [(11)C]R107474 in rats revealed that the uptake of [(11)C]R107474 after in vivo intravenous administration is very rapid; in most tissues (including the brain) it reaches maximum concentration at 5 min after tracer injection. In agreement with the known distribution of alpha(2)-adrenoceptors in the brain, highest uptake of radioactivity was observed in septum (3.54 +/- 0.52 ID/g, 5 min pi) and entorhinal cortex (1.57 +/- 0.10 ID/g, 5 min pi). Tissue/cerebellum concentration ratios for septum (5.38 +/- 0.45, 30 min pi) and entorhinal cortex (3.43+/-0.24, 30 min pi) increased with time due to rapid uptake followed by a slow washout. In vivo blocking experiments using the non-selective alpha(2)-adrenoceptor antagonist mirtazapine demonstrated specific inhibition of [(11)C]R107474 binding in selective brain areas. The receptor binding profile of mirtazapine is reported and the selectivity of inhibition of binding is discussed. These results suggest that [(11)C]R107474 deserves further investigation as a potential radioligand for studying alpha(2)-adrenoceptors using PET.     

Synthesis and radiopharmacological characterization of [11C]AL-438 as a nonsteroidal ligand for imaging brain glucocorticoid receptors

The radiosynthesis and the radiopharmacological characterization of [(11)C]AL-438 as a nonsteroidal ligand for the glucocorticoid receptor (GR) is described. Radiolabeling of the corresponding desmethyl precursor 10 with [(11)C]MeI gave [(11)C]AL-438 in decay-corrected radiochemical yields of 30+/-4% (based upon [(11)C]CO(2)) within 35 min at a specific radioactivity of 10-15 GBq/micromol at the end-of-synthesis. The radiopharmacological evaluation of [(11)C]AL-438 involved biodistribution and small animal PET imaging in rats, and autoradiography studies using rat brain sections. Biodistribution studies were performed in male Wistar rats and demonstrated high radioactivity uptake in pituitary and brain. However, the inability of high dose corticosterone to block binding would suggest that the radioactivity accumulation in the brain was not receptor-mediated.     

Ethanol triggers sphingosine 1-phosphate elevation along with neuroapoptosis in the developing mouse brain

Recent studies suggest that l-3,4 dihydroxyphenylalanine (L-DOPA)-induced dyskinesia (LID), a severe complication of conventional L-DOPA therapy of Parkinson's disease, may be caused by dopamine (DA) release originating in serotonergic neurons. To evaluate the in vivo effect of a 5-HT(1A) agonist [(±)-8-hydroxy-2-(dipropylamino) tetralin hydrobromide, 8-OHDPAT] on the L-DOPA-induced increase in extracellular DA and decrease in [(11) C]raclopride binding in an animal model of advanced Parkinson's disease and LID, we measured extracellular DA in response to L-DOPA or a combination of L-DOPA and the 5-HT(1A) agonist, 8-OHDPAT, with microdialysis, and determined [(11) C]raclopride binding to DA receptors, with micro-positron emission tomography, as the surrogate marker of DA release. Rats with unilateral 6-hydroxydopamine lesions had micro-positron emission tomography scans with [(11) C]raclopride at baseline and after two pharmacological challenges with L-DOPA?+?benserazide with or without 8-OHDPAT co-treatment. Identical challenge regimens were used with the subsequent microdialysis concomitant with ratings of LID severity. The baseline increase of [(11) C]raclopride-binding potential (BP(ND) ) in lesioned striatum was eliminated by the L-DOPA challenge, while the concurrent administration of 8-OHDPAT prevented this L-DOPA-induced displacement of [(11) C]raclopride significantly in lesioned ventral striatum and near significantly in the dorsal striatum. With microdialysis, the L-DOPA challenge raised the extracellular DA in parallel with the emergence of strong LID. Co-treatment with 8-OHDPAT significantly attenuated the release of extracellular DA and LID. The 8-OHDPAT co-treatment reversed the L-DOPA-induced decrease of [(11) C]raclopride binding and increase of extracellular DA and reduced the severity of LID. The reversal of the effect of L-DOPA on [(11) C]raclopride binding, extracellular DA and LID by 5-HT agonist administration is consistent with the notion that part of the DA increase associated with LID originates in serotonergic neurons.     

Radiosynthesis of [11C]BBAC and [11C]BBPC as potential PET tracers for orexin2 receptors

Radiosynthesis of [N-methyl-(11)C](S)-N-([1,1'-biphenyl]-2-yl)-1-(2-((1-methyl-1H-benzo[d]imidazol-2-yl)thio)acetyl)pyrrolidine-2-carboxamide ([(11)C]BBAC or [(11)C]3) and [N-methyl-(11)C] (S)-N-([1,1'-biphenyl]-2-yl)-1-(3-(1-methyl-1H-benzo[d]imidazol-2-yl)propanoyl)pyrrolidine-2-carboxamide ([(11)C]BBPC or [(11)C]-4), two potential PET tracers for orexin2 receptors are described. Syntheses of non-radioactive standards 3, 4 and corresponding desmethyl precursors 1, 2 were achieved from common intermediate (S)-2-([1,1'-biphenyl]-2-yl)-1-(pyrrolidin-2-yl)ethanone. Methylation using [(11)C]CH(3)OTf in the presence of base in acetone afforded [(11)C]3 and [(11)C]4 in 30±5% yield (EOS) with >99 % radiochemical purities with a specific activity ranged from 2.5±0.5 Ci/μmol (EOB). The logP of [(11)C]3 and [(11)C]4 were determined as 3.4 and 2.8, respectively. The total synthesis time was 30 min from EOB. However, PET scans performed in a rhesus monkey did not show tracer retention or appropriate brain uptake. Hence [(11)C]3 and [(11)C]4 cannot be used as PET tracers for imaging orexin2 receptors.     

Synthesis of 11C-labelled (R)-OHDMI and CFMME and their evaluation as candidate radioligands for imaging central norepinephrine transporters with PET

(R)-1-(10,11-Dihydro-dibenzo[b,f]azepin-5-yl)-3-methylamino-propan-2-ol ((R)-OHDMI) and (S,S)-1-cyclopentyl-2-(5-fluoro-2-methoxy-phenyl)-1-morpholin-2-yl-ethanol (CFMME) were synthesized and found to be potent inhibitors of norepinephrine reuptake. Each was labelled efficiently in its methyl group with carbon-11 (t(1/2)=20.4 min) as a prospective radioligand for imaging brain norepinephrine transporters (NET) with positron emission tomography (PET). The uptake and distribution of radioactivity in brain following intravenous injection of each radioligand into cynomolgus monkey was examined in vivo with PET. After injection of (R)-[(11)C]OHDMI, the maximal whole brain uptake of radioactivity was very low (1.1% of injected dose; I.D.). For occipital cortex, thalamus, lower brainstem, mesencephalon and cerebellum, radioactivity ratios to striatum at 93 min after radioligand injection were 1.35, 1.35, 1.2, 1.2 and 1.0, respectively. After injection of [(11)C]CFMME, radioactivity readily entered brain (3.5% I.D.). Ratios of radioactivity to cerebellum at 93 min for thalamus, occipital cortex, region of locus coeruleus, mesencephalon and striatum were 1.35, 1.3, 1.3, 1.2 and 1.2, respectively. Radioactive metabolites in plasma were measured by radio-HPLC. (R)-[(11)C]OHDMI represented 75% of plasma radioactivity at 4 min after injection and 6% at 30 min. After injection of [(11)C]CFMME, 84% of the radioactivity in plasma represented parent at 4 min and 20% at 30 min. Since the two new hydroxylated radioligands provide only modest regional differentiation in brain uptake and form potentially troublesome lipophilic radioactive metabolites, they are concluded to be inferior to existing radioligands, such as (S,S)-[(11)C]MeNER, (S,S)-[(18)F]FMeNER-D(2) and (S,S)-[(18)F]FRB-D(4), for the study of brain NETs with PET in vivo.     

Synthesis, radiosynthesis and in vivo evaluation of 5-[3-(4-benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-[(11)C]one, as a potent NR(1A)/2B subtype selective NMDA PET radiotracer

Recently, a new series of potent and highly subtype-selective 1-(heteroarylalkynyl)-4-benzylpiperidine antagonists of the NMDA receptors has been described by Pfizer Laboratories. In this series, 5-[3-(4-benzylpiperidin-1-yl)prop-1-ynyl]-1,3-dihydrobenzoimidazol-2-one (1) was identified as a selective antagonist for the NR1(A)/2B subtype, displaying IC(50) values for inhibition of the NMDA responses of 5.3 nM for this subtype (compared to NR1(A)/2A: 35 microM and NR1(A)/2C>100 microM) and was active in rat at a relatively low dosage (10mg/kg po). Derivative 1 has been synthesized in four chemical steps in good overall yield and labelled with carbon-11 at its benzoimidazolone ring using [(11)C]phosgene. The pharmacological profile of [(11)C]-1 was evaluated in vivo in rats with biodistribution studies and brain radioactivity monitored with intracerebral radiosensitive beta-microprobes. The brain uptake of [(11)C]-1 was extremely low (0.07% I.D./mL on average at 30 min) and rather uniform across the different brain structures. This in vivo brain regional distribution of [(11)C]-1 did not match with autoradiographic or binding data obtained with other NR2B subtype-selective NMDA ligands. Competition studies with ifenprodil (20 mg/kg, ip, 30 min before the radiotracer injection) failed to demonstrate specific binding of the radiotracer in the brain. In view of these results, and especially considering the low brain penetration of the radiotracer, [(11)C]-1 does not have the required properties for imaging NMDA receptors using positron emission tomography.     

Synthesis of [O-methyl-11C]1-(2-chlorophenyl)-5-(4-methoxyphenyl)-4-methyl-1H-pyrazole-3-carboxylic acid piperidin-1-ylamide: a potential PET ligand for CB1 receptors

Synthesis and in vitro evaluation of [O-methyl-(11)C]1-(2-chlorophenyl)-5-(4-methoxyphenyl)-4-methyl-1H-pyrazole-3-carboxylic acid piperidin-1-ylamide ([(11)C]-1), a potential imaging agent for CB(1) receptors using PET is described. 1-(2-Chlorophenyl)-5-(4-hydroxyphenyl)-4-methyl-1H-pyrazole-3-carboxylic acid piperidin-1-ylamide (5), the precursor for radiolabeling, was synthesized from 4-OTBDPS-propiophenone (2) in five steps with 30% overall yield. The reaction of alcohol 5 with [(11)C]MeOTf at 60 degrees C afforded [(11)C]-1 with an average radiochemical yield of 14.5% (EOS) and >2000 Ci/mmol specific activity. The radiotracer was found to selectively label CB(1) receptors in slide-mounted sections of postmortem human brain containing prefrontal cortex as demonstrated by in vitro autoradiography using phosphor imaging.     

Synthesis and biodistribution of [(11)C]R116301, a promising PET ligand for central NK(1) receptors

N1-(2,6-Dimethylphenyl)-2-(4-{(2R,4S)-2-benzyl-1-[3,5-di(trifluoromethyl)[carbonyl-(11)C]benzoyl]hexahydro-4-pyridinyl}piperazino)acetamide ([(11)C]R116301) was prepared and evaluated as a potential positron emission tomography (PET) ligand for investigation of central neurokinin(1) (NK(1)) receptors. 1-Bromo-3,5-di(trifluoromethyl)benzene was converted in three steps into 3,5-di(trifluoromethyl)[carbonyl-(11)C]benzoyl chloride, which was reacted with N1-(2,6-dimethylphenyl)-2-{4-[(2R,4S)-2-benzylhexahydro-4-pyridinyl]piperazino}acetamide providing [(11)C]R116301 in 45-57% decay-corrected radiochemical yield. The total synthesis time, from end of bombardment (EOB) to the formulated product, was 35 min. Specific activity (SA) was 82-172 GBq/micromol (n=10) at the end of synthesis. N1-([4-(3)H]-2,6-Dimethylphenyl)-2-(4-{(2R,4S)-2-benzyl-1-[3,5-di(trifluoromethyl)benzoyl]hexahydro-4-pyridinyl}piperazino)acetamide ([(3)H]R116301) was also synthesized (SA: 467 GBq/mmol). The B(max) for [(3)H]R116301 measured in vitro on Chinese hamster ovary cell membranes stably transfected with the human NK(1) receptor was 19.10+/-1.02 pmol/mg protein with an apparent dissociation constant of 0.08+/-0.01 nM. Ex vivo, in vivo and in vitro autoradiography studies with [(3)H]R116301 in gerbils demonstrated a preferential accumulation of the radioactivity in the striatum, olfactory tubercule, olfactory bulb and locus coeruleus. In vivo, the biodistribution of [(11)C]R116301 in gerbils revealed that the highest initial uptake is in the lung, followed by the liver and kidney. In the brain, maximum accumulation was found in the olfactory tubercules (1.10+/-0.08 injected dose (ID)/g 20 min post injection (p.i.)) and the nucleus accumbens (1.00+/-0.12ID/g 10 min p.i.). Tissue/cerebellum concentration ratios for striatum and nucleus accumbens increased with time due to rapid uptake followed by a slow wash out (1.29 and 1.64, respectively, 30 min p.i.). A tissue to cerebellum ratio of 1.33 and 1.62 was also observed for olfactory bulb and olfactory tubercules, respectively (20 min p.i.). In summary, [(11)C]R116301 appears to be a promising radioligand suitable for the visualization of NK(1) receptors in vivo using PET.     

Characterisation of [¹¹C]PR04.MZ in Papio anubis baboon: a selective high-affinity radioligand for quantitative imaging of the dopamine transporter

N-(4-fluorobut-2-yn-1-yl)-2β-carbomethoxy-3β-(4'-tolyl)nortropane (PR04.MZ, 1) is a PET radioligand for the non-invasive exploration of the function of the cerebral dopamine transporter (DAT). A reliable automated process for routine production of the carbon-11 labelled analogue [(11)C]PR04.MZ ([(11)C]-1) has been developed using GMP compliant equipment. An adult female Papio anubis baboon was studied using a test-retest protocol with [(11)C]-1 in order to assess test-retest reliability, metabolism and CNS distribution profile of the tracer in non-human primates. Blood sampling was performed throughout the studies for determination of the free fraction in plasma (f(P)), plasma input functions and metabolic degradation of the radiotracer [(11)C]-1. Time-activity curves were derived for the putamen, the caudate nucleus, the ventral striatum, the midbrain and the cerebellum. Distribution volumes (V(T)) and non-displaceable binding potentials (BP(ND)) for various brain regions and the blood were obtained from kinetic modelling. [(11)C]-1 shows promising results as a selective marker of the presynaptic dopamine transporter. With the reliable visualisation of the extra-striatal dopaminergic neurons and no indication on labelled metabolites, the tracer provides excellent potential for translation into man.     

Synthesis of [¹¹C]uric acid, using [¹¹C]phosgene, as a possible biomarker in PET imaging for diagnosis of gout

The synthesis and in vivo evaluation of (11)C -labeled uric acid ([(11)C]1), a potential imaging agent for the diagnosis of urate-related life-style diseases, was performed using positron emission tomography (PET) image analysis. First, the synthesis of [(11)C]1 was achieved by reacting 5,6-diaminouracil (2) with (11)C-labeled phosgene ([(11)C]COCl(2)). The radiochemical yield of [(11)C]1 was 37±7% (decay-corrected based on [(11)C]COCl(2)) with specific radioactivities of 96-152GBq/μmol at the end of synthesis (n=6). The average time of radiosynthesis from the end of bombardment, including formulation, was about 30min with >98% radiochemical purity. Second, the synthetic approach to [(11)C]1 was optimized using 5,6-diaminouracil sulfate (3) with [(11)C]COCl(2) in the presence of 1,8-bis(dimethylamino)naphthalene. [(11)C]1 was synthesized in 36±6% radiochemical yield, 89-142GBq/μmol of specific radioactivities, and 98% radiochemical purity by this method (n=5). This allowed the synthesis of [(11)C]1 to be carried out repeatedly and the radiochemical yield, specific radioactivities, average time of synthesis, and radiochemical purity of [(11)C]1 were similar to those obtained using 2. PET studies in rats showed large differences in the accumulation of radioligand in the limbs under normal and hyperuricemic conditions. Thus, an efficient and convenient automated synthesis of [(11)C]1 has been developed, and preliminary PET evaluation of [(11)C]1 confirmed the increased accumulation of radioactivity in the limbs of a rat model of hyperuricemia.     

Radiosynthesis of three [11C]ureido-substituted benzenesulfonamides as PET probes for carbonic anhydrase IX in tumors

Three ureido-substituted benzenesulfonamides 1a-c have been developed as potent inhibitors for carbonic anhydrase IX, which is overexpressed in hypoxic tumors. In this study, we labeled these unsymmetrical ureas 1a-c using [(11)C]phosgene ([(11)C]COCl(2)) as a labeling agent with the expectation that [(11)C]1a-c could become promising positron tomography probes for imaging carbonic anhydrase IX in tumors. The strategy for radiosynthesis of [(11)C]1a-c was to react hydrochloride of anilines 2a-c with [(11)C]COCl(2) to give isocyanate [(11)C]4a-c, followed by a reaction with 4-aminobenzenesulfonamide (3).     

PET imaging and optical imaging with D-luciferin [11C]methyl ester and D-luciferin [11C]methyl ether of luciferase gene expression in tumor xenografts of living mice

New carbon-11 labeled D-luciferin analogs D-luciferin [(11)C]methyl ester ([(11)C]LMEster, [(11)C]1) and D-luciferin [(11)C]methyl ether ([(11)C]LMEther, [(11)C]2) were synthesized in 25-55% radiochemical yield. PET studies with [(11)C]LMEster and [(11)C]LMEther demonstrate a lower retention of the C-11 label at 45 min post-injection in luciferase expression tumor. Optical imaging with unlabeled substrate D-luciferin and radiotracers [(11)C]LMEster and [(11)C]LMEther gave tumor luciferase images within a few minutes of photon counting.     

Towards the preparation of radiolabeled 1-aryl-3-benzyl ureas: Radiosynthesis of [(11)C-carbonyl] AR-A014418 by [(11)C]CO(2) fixation

The highly selective glycogen synthase kinase-3 (GSK-3) inhibitor N-(4-methoxybenzyl)-N'-(5-nitro-1,3-thiazol-2-yl)urea (AR-A014418) was radiolabeled with carbon-11 ((11)C; half-life=20.4min) at the urea moiety via [(11)C]CO(2) fixation. Reaction of [(11)C]CO(2) with 4-methoxybenzylamine in the presence of a CO(2) fixating base was followed by dehydration with POCl(3) and addition of 2-amino-5-nitrothiazole to prepare [(11)C-carbonyl] AR-A014418. This reaction resulted in an 8% uncorrected radiochemical yield, based on [(11)C]CO(2), with high specific activity (4Ci/μmol) within 30min. An in vitro GSK-3β enzyme activity assay revealed that AR-A014418 (K(i)=770nM) is not as potent as previously claimed. The [(11)C]CO(2) fixation methodology described herein should prove generally applicable to preparing 1-aryl-3-benzyl-[(11)C-carbonyl] ureas as radiotracers for positron emission tomography.     

Evaluation of [11C]hemicholinium-15 and [18F]hemicholinium-15 as new potential PET tracers for the high-affinity choline uptake system in the heart

[(11)C]Hemicholinium-15 ([(11)C]HC-15) and [(18)F]hemicholinium-15 ([(18)F]HC-15) have been synthesized as new potential PET tracers for the heart high-affinity choline uptake (HACU) system. [(11)C]HC-15 was prepared by N-[(11)C]methylation of the appropriate precursor, 4-methyl-2-phenyl-morpholin-2-ol, using [(11)C]CH(3)OTf in 55-70% radiochemical yield decay corrected to end of bombardment (EOB) and 2-3Ci/mumol specific activity at end of synthesis (EOS). [(18)F]HC-15 was prepared by N-[(18)F]fluoromethylation of the precursor using [(18)F]FCH(2)OTf in 20-30% radiochemical yield decay corrected to EOB and >1.0Ci/mumol specific activity at EOS. The biodistribution of both compounds was determined in rats at 20min post-intravenous injection, and the results show the heart region uptakes 1.32+/-0.75%ID/g in R-ventricle for [(11)C]HC-15 and 1.28+/-0.81%ID/g in L-ventricle for [(18)F]HC-15, respectively. The dynamic PET imaging studies of [(11)C]HC-15 in rats were acquired 60min post-intravenous injection of the tracer using the IndyPET-II scanner. For the blocking experiments, the rats were intravenously pretreated with 3.0mg/kg of unlabeled HC-15 prior to [(11)C]HC-15 injection. [(11)C]HC-15 rat heart PET studies show rapid heart uptake to give clear heart images. The rat heart PET blocking studies found no significant blocking effect. The dynamic PET studies in normal and ablated dogs were performed using Siemens PET scanner with [(13)N]NH(3), [(11)C]HC-15, and [(18)F]HC-15. PET studies in dogs of both [(11)C]HC-15 and [(18)F]HC-15 also show significant heart uptake and give images of the heart. However, there is no significant change in [(11)C]HC-15 L-ventricle uptake following radiofrequency ablation in the dog. These results suggest that the localization of HC-15 tracers in the heart is mediated by non-specific processes, and the visualization of HC-15 tracers on the heart is related to non-specific binding of HACU.