11C]Choline as a potential PET marker for imaging of breast cancer athymic mice

[11C]Choline has been evaluated as a potential positron emission tomography (PET) marker for imaging of breast cancer. The biodistribution of [11C]choline was determined at 45 min post iv injection in MCF-7's transfected with IL-1alpha implanted athymic mice and MDA-MB-435 implanted athymic mice. The results showed the uptake of [11C]choline in these tumors was high, 2.0% dose/g in MCF-7's transfected with IL-1alpha implanted mice and 1.8% dose/g in MDA-MB-435 implanted mice; the ratios of tumor/muscle (T/M) and tumor/blood (T/B) were 1.7 (T/M, MCF-7's), 2.1 (T/M, MDA-MB-435) and 6.9 (T/B, MCF-7's), 12.5 (T/B, MDA-MB-435), respectively; the tumor/muscle ratios are moderate, and the tumor/blood ratios are high. The micro-PET imaging of [11C]choline in both breast cancer athymic mice was acquired for 15 min from a MCF-7's transfected with IL-1alpha and/or MDA-MB-435 implanted mouse at 45 min post iv injection of 1 mCi of the tracer using a dedicated high resolution (<3 mm full-width at half-maximum) small FOV (field-of-view) PET imaging system, Indy-PET II scanner, developed in our laboratory, which showed the uptake of [11C]choline in MCF-7's transfected with IL-1alpha tumor or MDA-MB-435 tumor implanted in a nude athymic mouse. These results suggest that [11C]choline may be a potential PET breast cancer imaging agent.     

Synthesis, biodistribution and micro-PET imaging of a potential cancer biomarker carbon-11 labeled MMP inhibitor (2R)-2-[[4-(6-fluorohex-1-ynyl)phenyl]sulfonylamino]-3-methylbutyric acid [11C]methyl ester

(2R)-2-[[4-(6-fluorohex-1-ynyl)phenyl]sulfonylamino]-3-methylbutyric acid [(11)C]methyl ester ([(11)C]FMAME), a novel carbon-11 labeled matrix metalloproteinase (MMP) inhibitor, has been synthesized for evaluation as new potential positron emission tomography (PET) cancer biomarker. [(11)C]FMAME was prepared by appropriate precursor (2R)-2-[[4-(6-fluorohex-1-ynyl)phenyl]sulfonylamino]-3-methylbutyric acid (FMA), which was synthesized in six steps from (D)-valine in 71% chemical yield. This acid precursor was labeled by [(11)C]methyl triflate through O-[(11)C]methylation method under basic conditions and isolated by solid-phase extraction (SPE) purification to produce pure target compound in 40-55% radiochemical yield, based on (11)CO(2), decay corrected to end of bombardment, and 15-20 min synthesis time. The biodistribution of [(11)C]FMAME was determined at 30 min post IV injection in breast cancer animal models MCF-7 transfected with IL-1 alpha implanted athymic mice and MDA-MB-435 implanted athymic mice. The results showed the uptakes of [(11)C]FMAME in these tumors were 1.13% dose/g in MCF-7 transfected with IL-1 alpha implanted mice and 1.37% dose/g in MDA-MB-435 implanted mice, respectively; the ratios of tumor/muscle (T/M) and tumor/blood (T/B) were 1.05 +/- 0.29 (T/M, MCF-7's), 0.77 +/- 0.20 (T/B, MCF-7's) and 0.99 +/- 0.35 (T/M, MDA-MB-435), 1.44 +/- 0.69 (T/B, MDA-MB-435), respectively. Pretreatment of MCF-7 transfected with IL-1 alpha tumor-bearing mice with MMP inhibitor FMA had no effect on [(11)C]FMAME biodistribution. Likewise, pretreatment of MDA-MB-435 tumor-bearing mice with FMA also showed no effect on [(11)C]FMAME biodistribution. The micro-PET images were acquired for 15 min from a MCF-7 transfected with IL-1 alpha tumor-bearing mouse or a MDA-MB-435 tumor-bearing mouse at 30 min post IV injection of 1 mCi of [(11)C]FMAME using a dedicated high resolution (<3 mm full-width at half-maximum) PET imaging system (Indy-PET II scanner). The initial dynamic micro-PET images of [(11)C]FMAME in a MCF-7 transfected with IL-1 alpha tumor-bearing mouse during different time periods of 0-15, 15-30, 30-45 and 45-60 min were performed by Indy-PET II. The PET images clearly showed both tumors were visible with [(11)C]FMAME. These results suggest that the localization of [(11)C]FMAME in the tumor is mediated by non-specific processes, and the visualization of [(11)C]FMAME on the tumor using the Indy-PET II scanner is related to non-specific binding.     

Comparative studies of potential cancer biomarkers carbon-11 labeled MMP inhibitors (S)-2-(4'-[11C]methoxybiphenyl-4-sulfonylamino)-3-methylbutyric acid and N-hydroxy-(R)-2-[[(4'-[11C]methoxyphenyl)sulfonyl]benzylamino]-3-methylbutanamide

(S)-2-(4'-[11C]methoxybiphenyl-4-sulfonylamino)-3-methylbutyric acid ([11C]MSMA) and N-hydroxy-(R)-2-[[(4'-[11C]methoxyphenyl)sulfonyl]benzylamino]-3-methylbutanamide ([11C]CGS 25966), carbon-11 labeled matrix metalloproteinase (MMP) inhibitors, have been synthesized for evaluation as new potential positron emission tomography (PET) cancer biomarkers. [11C]MSMA was prepared by appropriate precursor (S)-2-(4'-hydroxybiphenyl-4-sulfonylamino)-3-methylbutyric acid tert-butyl ester, which was synthesized in eight steps from amino acid (L)-valine in 39.4% chemical yield. This precursor was labeled by [11C]methyl triflate through O-[11C]methylation method at the hydroxyl position of biphenol under basic conditions, followed by a quick acid hydrolysis and isolated by solid-phase extraction (SPE) purification to produce pure target compound [11C]MSMA in 35-55% radiochemical yield, based on 11CO2, decay corrected to end of bombardment (EOB), and 20-25 min synthesis time. [11C]CGS 25966 was prepared in our previous work starting from amino acid (D)-valine. The biodistribution of [11C]MSMA and [11C]CGS 25966 were determined at 45 min post iv injection in breast cancer animal models MCF-7's transfected with IL-1alpha implanted athymic mice and MDA-MB-435 implanted athymic mice. The results showed the uptakes of [11C]MSMA and [11C]CGS 25966 in these tumors were 0.95 and 0.42%dose/g in MCF-7's transfected with IL-1alpha implanted mice, 0.98 and 1.53%dose/g in MDA-MB-435 implanted mice, respectively; the ratios of tumor/muscle (T/M) and tumor/blood (T/B) were 1.21 and 1.09 (T/M, MCF-7's), 0.99 and 0.84 (T/B, MCF-7's), 1.38 and 1.27 (T/M, MDA-MB-435), 1.27 and 1.95 (T/B, MDA-MB-435), respectively. The micro-PET images of [11C]MSMA and [11C]CGS 25966 in both breast cancer athymic mice were acquired for 15 min from a MCF-7's transfected with IL-1alpha and/or MDA-MB-435 implanted mouse at 45 min post iv injection of 1 mCi of the tracer using a dedicated high resolution (<3 mm full-width at half-maximum) small FOV (field-of-view) PET imaging system, Indy-PET II scanner, developed in our laboratory, which showed both tumors were invisible with both tracers. The results were compared. From our results, we concluded that both [11C]MSMA and [11C]CGS 25966 might be unsuitable as PET tracers for cancer imaging.     

Highly efficient automated synthesis of [C]choline for multi dose utilization

[¹¹C]Choline has been under investigation as a PET ligand for imaging tumor tissue, especially prostate cancer. An improved, automated synthesis of the tracer now was established. [¹¹C] Choline was produced by labeling 2-(dimethylamino)-ethanol (DMAE) with [¹¹C]CH₃I in a Tefzel^® tube at room temperature without solvent. The product was purified using a cation exchange cartridge. Reaction conditions were optimized with respect to synthesis time and amount of DMAE, resulting in radiochemical yields higher than 80% using 60 μl of DMAE in 20 min, radiochemical purity was >99% and residual DMAE was below 10 ppm. After ¹¹C-production of 1 h at 50 μA [¹¹C]choline activities of 30.0±5.6 GBq (n=29) were obtained in sterile solution ready for intravenous administration.     

Pharmacological evaluation of [c]donepezil as tracer for visualization of acetylcholinesterase by PET

Donepezil is a highly potent and selective reversible achetylcholinesterase inhibitor. [¹¹C]Donepezil is prepared by methylation with [¹¹C]CH₃I of the corresponding 6′-O-desmethylprecursor. Tissue distribution in mice revealed a high uptake in brain and rapid clearance from the blood. Metabolization studies in mice indicated the formation of one ¹¹C-labeled polar metabolite that didn’t penetrate the blood-brain barrier. Regional brain distribution in rabbits didn’t reflect the measured achetylcholinesterase distribution in rabbit brain.     

Synthesis of C-labeled desipramine and its metabolite 2-hydroxydesipramine: Potential radiotracers for pet studies of the norepinephrine transporter

The antidepressant desipramine (DMI) and its principal metabolite 2-hydroxydesipramine (HDMI) have been radiolabeled with ¹¹C for PET studies. The normethyl precursors of DMI and HDMI were synthesized from iminodibenzyl in 35% and 11% overall yield, respectively. Direct methylation of the normethyl precursor with [¹¹C]CH₃I, followed by HPLC purification, provided [¹¹C]DMI and [¹¹C]HDMI in 18–30% and 15–23% decay-corrected radiochemical yields, respectively, in a 45 min synthesis time from end of bombardment. The specific activities of the two radiotracers were > 1459 Ci/mmol at the end of synthesis. [¹¹C]DMI and [¹¹C]HDMI have potential utility as PET radiotracers for the norepinephrine transporter.     

Derivatives of way 100635 as potential imaging agents for 5-ht1a receptors: syntheses, radiosyntheses, and in vitro and in vivo evaluation

Analogues of the potent and selective 5-HT_1A ligand, WAY 100635, were synthesized and examined as potential candidates for imaging 5-HT_1A receptors by positron emission tomography (PET). Several of the analogues displayed nanomolar affinity for the 5-HT_1A receptor, comparable to WAY 100635. Three of these were examined in a model of human liver metabolism vis-à-vis WAY 100635. All showed a markedly lower propensity for amide hydrolysis than WAY 100635. Radiolabelling of these three potential PET radiotracers with carbon-11 was readily achieved from [¹¹C]-iodomethane, and the newly synthesized radioligands were tested in vivo in rats for binding to 5-HT_1A receptors. Whereas two of the ligands failed to bind to 5-HT_1A receptors in vivo, one was successful. The latter, [¹¹C]-7 {4-(2′-methoxyphenyl)-1-[2′-[N-(2′-pyridinyl)-2-bicyclo[2.2.2]octanecarboxamido]ethyl]-piperazine}, showed good brain penetration, hippocampal:cerebellar ratios of 10:1 at 45 min postinjection. Blocking studies with a variety of drugs demonstrated that the binding of [¹¹C]-7 in vivo was selective for 5-HT_1A receptors. [¹¹C]-7 is a promising candidate as a ligand for imaging 5-HT_1A receptors by PET.     

[C]NNC 687 and [C]NNC 756, dopamine D-1 receptor ligands. Preparation, autoradiography and PET investigation in monkey

NNC 687 and NNC 756 [(+)-5-(2,3-dihydrobenzofuran-7-yl)-7-hydroxy-3-methyl-8-nitro-2,3,4,5-tetrahydro-1H-3-benzazepine and (+)-8-chloro-5-(2,3-dihydrobenzofuran-7-yl)-7-hydroxy-3-methyl-2,3,4,5-tetrahydro-1H-3-benzazepine] are two new potent dopamine D-1 receptor antagonists. [¹¹C]NNC 687 and [¹¹C]NNC 756 were both prepared by N-methylation of the corresponding desmethyl compounds with [¹¹C]methyl iodide. The reactions were performed in acetone with subsequent normal-phase semi-preparative HPLC and resulting in 50–60% radiochemical yield (from EOB and decay-corrected) with a total synthesis time of 30–35 min and a radiochemical purity higher than 99%. The specific radioactivity obtained at time of injection was about 1500 Ci/mmol (55 GBq/μmol). Autoradiographic examination of [¹¹C]NNC 687 and [¹¹C]NNC 756 binding in post-mortem human brain sections showed specific binding in the striatum, a region with high density of dopamine D-1 receptors. PET examination of the radioligands in a Cynomolgus monkey demonstrated accumulation of radioactivity predominantly in the striatum. The ratio between radioactivities in the striatum and the cerebellum was about 2 and 8 for [¹¹C]NNC 687 and [¹¹C]NNC 756 after 60 min. [¹¹C]NNC 756 should have potential as PET ligand for examination of central dopamine D-1 receptors in man.     

Synthesis of 6-Methyl[C]-2′-deoxyuridine and evaluation of Its In Vivo distribution in Wistar rats

In this study, 6-methyl-2′-deoxyuridine, an artificial nucleoside, was labeled with ¹¹C in the methyl position. Tissue distribution of 6-methyl[¹¹C]-2′-deoxyuridine was investigated in normal Wistar rats and compared to the behavior of the natural nucleoside [methyl-¹¹C]thymidine. High renal clearance, up to at least 20% of the injected activity, was noticed during the 20 min period following injection. Tissue distribution as determined by dynamic PET studies of both ¹¹C-labeled nucleosides was significantly different for most of the organs.     

Development and evaluation of muscarinic cholinergic receptor ligands N-[11C]ethyl-4-piperidyl benzilate and N-[11C]propyl-4-piperidyl benzilate: a PET study in comparison with N-[11C]methyl-4-piperidyl benzilate in the conscious monkey brain

The muscarinic cholinergic receptor ligands N-[¹¹C]ethyl-4-piperidyl benzilate (4-EPB) and N-[¹¹C]propyl-4-piperidyl benzilate (4-PPB) were developed and evaluated in comparison with N-[¹¹C]methyl-4-piperidyl benzilate (4-MPB) in the conscious monkey brain using positron emission tomography (PET). Time-activity curves of [¹¹C]4-EPB, unlike [¹¹C]4-MPB, showed peaks within 91 min in regions rich in muscarinic receptors. [¹¹C]4-PPB showed no specific binding even in the regions rich in these receptors. These observation demonstrated that increases in [¹¹C]alkyl chain length could alter the kinetic properties of receptor ligands for PET.     

Two routes to [11C-carbonyl]organo-isocyanates utilizing [11C]phosgene ([11C]organo-isocyanates from [11C]phosgene).

Two generic radiosynthetic routes for the preparation of [¹¹C-carbonyl]isocyanates have been developed. Reaction of N-organo-sulfinylamines; RNSO, (R = Me, Et, allyl, cyclohexyl and phenyl) with [¹¹C]phosgene gave the corresponding [¹¹C-carbonyl]isocyanates in good radiochemical yield (53–68%) from [¹¹C]phosgene (decay corrected) in ca 16 min from EOB. Alternatively, the reaction of [¹¹C]phosgene with N,N′-organo-ureas; (RNH)₂CO, (R = Me, Et, Pr and phenyl) also gave the corresponding [¹¹C-carbonyl]isocyanates in moderate radiochemical yield (9–37%) from [¹¹C]phosgene (decay corrected) in ca 16 min from EOB. For identification, the [¹¹C-carbonyl]organo-isocyanates were derivatized with 1-(2-methoxyphenyl)piperazine in situ to [¹¹C-carbonyl]carboxamides and the position of radiolabelling in the carbonyl group confirmed by [^11/13C]co-labeling and subsequent carbon-13 NMR spectroscopy.     

Synthesis and in vivo evaluation of [11C]SA6298 as a PET sigma1 receptor ligand

The potential of a ¹¹C-labeled selective sigma₁ receptor ligand, 1-(3,4-dimethoxyphenethyl)-4-[3-(3,4-dichlorophenyl)propyl]piperazine ([¹¹C]SA6298), was evaluated as a positron emission tomography (PET) ligand for mapping sigma₁ receptors in the central nervous system and peripheral organs. [¹¹C]SA6298 was synthesized by methylation of the desmethyl SA6298 with [¹¹C]CH₃I, with the decay-corrected radiochemical yield of 39 ± 5% based on [¹¹C]CH₃I and with the specific activity of 53 ± 17 TBq/mmol within 20 min from end of bombardment (EOB). In mice, the uptake of [¹¹C]SA6298 was significantly decreased by carrier loading in the brain, liver, spleen, heart, lung, small intestine, and kidney in which sigma receptors are present as well as in the skeletal muscle. Pretreatment with SA6298 also blocked the uptake of [¹¹C]SA6298 by these organs except for the small intestine, but significant displacement of [¹¹C]SA6298 by posttreatment with SA6298 was observed only in the heart, lung, and muscle. In the blocking study with one of the eight sigma receptor ligands, including haloperidol, SA6298, NE-100, (+)-pentazocine, SA4503, (−)-pentazocine, (+)-3-PPP, and (+)-SKF 10,047 (in the order of the affinity for sigma₁ receptor subtype), only SA6298 and an analog SA4503 significantly reduced the brain uptake of [¹¹C]SA6298 to approximately 80% of the control, but the other six ligands did not. Peripherally, the uptake of [¹¹C]SA6298 by the organs described above was decreased predominantly by SA6298 or SA4503, but the blocking effects of the other five ligands except for NE-100 depended on their affinity for sigma₁ receptors. The saturable brain uptake of [¹¹C]SA6298, approximately 20%, was also observed by tissue dissection method in rats and by PET in a cat. Ex vivo autoradiography of the rat brain showed a high uptake in the cortex and thalamus. In the cat brain a relatively high uptake was found in the cortex, thalamus, striatum, and cerebellum. These results have indicated a receptor-mediated uptake of the tracer to some extent in the brain and peripheral organs. However, the tracer has a limited potential for the PET study of the brain receptors because of a relatively high nonspecific binding.     

Radiosynthesis and ex vivo evaluation of [11C]-SIB-1553A as a PET radiotracer for beta4 selective subtype nicotinic acetylcholine receptor

[¹¹C]-SIB-1553A ((±)-4-[2-((N-[¹¹C]-methyl)-2-pyrrolidinyl)ethyl]thiophenol) was labelled with carbon-11 (t_1/2=20.4 min) and evaluated in vivo as potential radiotracer for noninvasive assessment of the β4 subunit nicotinic acetylcholine neurotransmission system with positron emission tomography (PET). The labelling precursor was obtained within five steps from N-Boc-prolinal in 45–56% overall yields. The radiosynthesis of [¹¹C]-SIB-1553A was achieved by a selective N-[¹¹C]-methylation in 32 min with a radiochemical purity greater than 97%, 7.5–30 GBq/μmol of specific radioactivity and 55–65% radiochemical yield (decay corrected, based on [¹¹C]methyl iodide). The ex vivo pharmacological profile of [¹¹C]-SIB-1553A was evaluated in rats with biodistribution studies in organs and in brain structures by autoradiography. The radiotracer uptake in the brain reached 0.49 %ID/g at 10 min and no brain radiometabolite was detected 40 min after intravenous injection. The quantification of radioactivity in various cerebral structures indicated a significantly higher radioactivity level at 15 min than at 30 min. Among the β4 nAChR subunit-rich structures studied in the rat brain, only the thalamus at 15 and 30 min and the hippocampus at 30 min showed significantly higher uptake. Moreover, competition studies performed with SIB-1553A (15 min before the radiotracer injection) revealed only a low specific binding estimated to 7% of the total binding at 15 min and 13% at 30 min.     

Syntheses of carbon-11 labeled piperidine esters as potential in vivo substrates for acetylcholinesterase

A series of carbon-11 labeled N-methylpiperidinyl esters were prepared as potential in vivo substrates for acetylcholinesterase (AChE). Target compounds were designed based on the structure of N-[¹¹C]methylpiperidin-4-yl propionate, an ester currently used to measure AChE enzymatic activity in the human brain, to examine the structure–activity relationship for in vivo enzymatic hydrolysis. Changes in steric bulk and in the ester order (“reverse” esters) were made. Addition of methyl groups was made to both the acid side chain (synthesis of N-[¹¹C]methylmethylpiperidin-4-yl isobutyrate) and to the piperidine ring (syntheses of N-[¹¹C]methyl-4-methylpiperidin-4-yl propionate, N-[¹¹C]methyl-4-methylpiperidin-4-yl acetate, and N-[¹¹C]methyl-3-methylpiperidin-4-yl propionate). Alterations of the order of the ester heteroatoms was accomplished through syntheses of the N-[¹¹C]methyl-2,3- and 4-piperidinecarboxylic acid ethyl esters. Finally, an additional piperidine-based ester (N-[¹¹C]methylpiperidin-2-yl)methyl propionate was also prepared. All carbon-11-labeled esters were prepared by N-[¹¹C]methylation reactions, using the desmethyl precursors and no-carrier-added [¹¹C]methyltriflate, and were obtained in decay-corrected yields (not optimized) of 10–40% and high specific activities.     

Biodistribution of 3,4-dihydro-5-[c]methoxy-1(2h)-isoquinolinone, a potential pet tracer for poly(adp-ribose) synthetase

Poly(adenosine diphosphate-ribose) synthetase (PARS) is a nuclear enzyme that is activated by deoxyribonucleic acid (DNA) strand breaks and participates in DNA repair. Excessive PARS activation, however, leads to cell death due to depletion of adenosine triphosphate (ATP). To evaluate whether it is possible to detect excessive activation of PARS with positron emission tomography (PET), we examined the pharmacokinetics of 3,4-dihydro-5-[¹¹C]methoxy-1(2H)-isoquinolinone ([¹¹C]MIQO), a potent poly(ADP-ribose) synthetase inhibitor, in the brain of rats and monkeys. Although the uptake of [¹¹C]MIQO in the brain of normal rats was low, [¹¹C]MIQO was rapidly incorporated into and then quickly washed out from the brain. The uptake of the radiotracer in the brain of normal monkeys was also low; however, [¹¹C]MIQO gave a distribution image that differed from that of cerebral blood flow obtained by [¹⁵O]water-PET. No localization of [¹¹C]MIQO in the brain of normal monkeys was observed. Low accumulation of some radioactivity was also observed in muscles surrounding the brain of monkeys, but did not seem to interfere with measurement of [¹¹C]MIQO uptake in the brain with PET. Thus, detection of [¹¹C]MIQO uptake with PET may be useful for detecting PARS activity in ischemic injury.     

In vivo evaluation of [C]-3-[2-[(3-methoxyphenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxylic acid ([C]3MPICA) as a PET radiotracer for the glycine site of the NMDA ion channel

Alterations in normal NMDA receptor composition, densities and function have been implicated in the pathophysiology of certain neurological and neuropsychiatric disorders such as Parkinson’s Disease, Huntington’s Chorea, schizophrenia, alcoholism and stroke. In our first effort to provide PET ligands for the NMDA/glycine site, we reported the synthesis of a novel high affinity glycine site ligand, 3-[2-[(3-methoxyphenylamino)carbonyl]ethenyl]-4,6-dichloroindole-2-carboxylic acid ((3MPICA), Ki = 4.8 ± 0.9 nM) and the corresponding carbon-11 labeled PET ligand, [¹¹C]3MPICA. We report here the in vivo evaluation of [¹¹C]3MPICA in rats. Biodistribution analysis revealed that [¹¹C]3MPICA exhibited low degree of brain penetration and high blood concentration. The average uptake at two minutes was highest in the cerebellum (0.19 ± 0.04 %ID/g) and thalamus (0.18 ± 0.05 %ID/g) and lower in the hippocampus (0.13 ± 0.03) and frontal cortex (0.11 ± 0.04 %ID/g). The radioactivity cleared quickly from all brain regions examined. Administration of unlabeled 3MPICA (1 mg/kg, i.v.) revealed at 60 minutes a small general reduction in regional brain radioactivity concentrations in treated animals versus controls, however, the blood radioactivity concentration was also lowered, confounding the assessment of the degree of saturable binding. Warfarin co-administration (100 mg/kg, i.v.) significantly lowered blood activity at 5 minutes post-injection (−27%, P < 0.01) but failed to significantly increase the brain uptake of the radiotracer. In view of these results, and especially considering the low brain penetration of this tracer, [¹¹C]3MPICA does not appear to be a promising PET radiotracer for in vivo use.     

[C]Formaldehyde revisited: considerable concurrent [C]formic acid formation in the low-temperature conversion of [C]carbon dioxide into [C]formaldehyde

The reduction of [¹¹C]carbon dioxide with lithium aluminium hydride in diethyl ether at temperatures ranging from −56°C to 19°C was studied. In contrast to what others have reported, considerable amounts of [¹¹C]formic acid were found at all studied temperatures.     

High yield synthesis of high specific activity R-(−)-[C]epinephrine for routine PET studies in humans

R-(−)-[¹¹C]Epinephrine ([¹¹C]EPI) has been synthesized from R-(−)-norepinephrine by direct methylation with [¹¹C]methyl iodide or [¹¹C]methyl triflate. The total synthesis time including HPLC purification was 35–40 min. The radiochemical yields (EOB) were 5–10% for [¹¹C]methyl iodide and 15–25% for [¹¹C]methyl triflate. Radiochemical purity was >98%; optical purity determined by radio-chiral HPLC was >97%. The [¹¹C]methyl triflate technique produces R-(−)-[¹¹C]epinephrine in quantities (80–170 mCi) sufficient for multiple positron emission tomography studies in humans. The two synthetic methods are generally applicable to the production of other N-[¹¹C]methyl phenolamines and N-[¹¹C]methyl catecholamines.     

A remote-controlled high pressure reactor for radiotracer synthesis with [C]carbon monoxide

[¹¹C]Carbon monoxide is a versatile building block for the synthesis of PET radiotracers. However, the difficulty of trapping [¹¹C]CO in a small solvent volume has limited its utility. We wish to report the details of a simple, remotely operated High Pressure Reactor (HiPR) system for trapping and reacting practical quantities of [¹¹C]CO. All parts used in the HiPR are commercially available, providing an inexpensive and easily assembled system. A number of compounds have been synthesized using the HiPR via palladium mediated reactions with [¹¹C]CO, an aryl halide, and a nucleophile dissolved in dioxane. For example, AMPA receptor modulator [¹¹C]CX546 was synthesized from its respective precursors in 37% isolated yield, uncorrected from trapped [¹¹C]CO.     

Synthesis and evaluation of [11C]FR194921 as a nonxanthine-type PET tracer for adenosine A1 receptors in the brain

This report describes the synthesis of [¹¹C]2-(1-methyl-4-piperidinyl)-6-(2-phenylpyrazolo[1,5-a]pyridin-3-yl)-3(2H)-pyridazinone ([¹¹C]FR194921), a highly selective, nonxanthine-type adenosine A₁ receptor antagonist, used in brain imaging in rats and conscious monkeys as a potential novel PET tracer. [¹¹C]FR194921 was successfully synthesized in 19 min after [¹¹C]CH₃I formation. The radiochemical yield was 38±3%; and radioactivity was 4.1±0.4 GBq, calculated from end of synthesis; radiochemical purity was higher than 99%; and the specific radioactivity was 25.0±8.1 GBq μmol⁻¹ (n=5). In a rat experiment, the distribution of [¹¹C]FR194921 was higher in the hippocampus, striatum and cerebellum regions. This accumulation was significantly decreased by approximately 50% by pretreatment with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an adenosine A₁ receptor antagonist, which indicated specific binding of the radioligand to adenosine A₁ receptors. In conscious monkey PET experiments, [¹¹C]FR194921 accumulated in several regions of the brain, especially in the occipital cortex, thalamus and striatum. These results suggest that [¹¹C]FR194921 can be used as an agent for imaging adenosine A₁ receptors in vivo by positron emission tomography (PET).