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.     

Simple and effective method for producing [11C]phosgene using an environmental CCl4 gas detection tube

IntroductionCarbon-11-labeled phosgene is an important labeling precursor for PET molecular probes. Despite the usefulness of [¹¹C]phosgene, some difficulties, especially in the formation of [¹¹C]phosgene process from [¹¹C]CCl₄, hamper its use. The present article shows a simple preparation method for [¹¹C]phosgene.Method[¹¹C]CCl₄ was obtained using the conventional method by passing a mixture of [¹¹C]CH₄ and Cl₂ through a heated quartz tube. The [¹¹C]CCl₄ was transformed to [¹¹C]phosgene simply by passing through a pretreatment tube of a Kitagawa gas detection system for the working-environmental CCl₄ concentration measurement at room temperature with a flow rate of 50 ml/min.ResultThis tube successfully transformed [¹¹C]CCl₄ to [¹¹C]phosgene at room temperature. [¹¹C]Phosgene was obtained at nearly 80% radiochemical yield (EOB) in a short synthesis time with high reproducibility.ConclusionA high yield and reliable [¹¹C]phosgene production method using a gas detector tube system for working-environmental CCl₄ concentration measurement was developed.     

Synthesis and preclinical evaluation of [11C-carbonyl]PF-04457845 for neuroimaging of fatty acid amide hydrolase

IntroductionFatty acid amide hydrolase (FAAH) has a significant role in regulating endocannabinoid signaling in the central nervous system. As such, FAAH inhibitors are being actively sought for pain, addiction, and other indications. This has led to the recent pursuit of positron emission tomography (PET) radiotracers targeting FAAH. We report herein the preparation and preclinical evaluation of [¹¹C-carbonyl]PF-04457845, an isotopologue of the potent irreversible FAAH inhibitor.MethodsPF-04457845 was radiolabeled at the carbonyl position via automated [¹¹C]CO₂-fixation. Ex vivo brain biodistribution of [¹¹C-carbonyl]PF-04457845 was carried out in conscious rats. Specificity was determined by pre-administration of PF-04457845 or URB597 prior to [¹¹C-carbonyl]PF-04457845. In a separate experiment, rats injected with the title radiotracer had whole brains excised, homogenized and extracted to examine irreversible binding to brain parenchyma.ResultsThe title compound was prepared in 5 ± 1% (n = 4) isolated radiochemical yield based on starting [¹¹C]CO₂ (decay uncorrected) within 25 min from end-of-bombardment in > 98% radiochemical purity and a specific activity of 73.5 ± 8.2 GBq/μmol at end-of-synthesis. Uptake of [¹¹C-carbonyl]PF-04457845 into the rat brain was high (range of 1.2–4.4 SUV), heterogeneous, and in accordance with reported FAAH distribution. Saturable binding was demonstrated by a dose-dependent reduction in brain radioactivity uptake following pre-treatment with PF-04457845. Pre-treatment with the prototypical FAAH inhibitor, URB597, reduced the brain radiotracer uptake in all regions by 71–81%, demonstrating specificity for FAAH. The binding of [¹¹C-carbonyl]PF-04457845 to FAAH at 40 min post injection was irreversible as 98% of the radioactivity in the brain could not be extracted.Conclusions[¹¹C-carbonyl]PF-04457845 was rapidly synthesized via an automated radiosynthesis. Ex vivo biodistribution studies in conscious rodents demonstrate that [11C PF-04457845 is a promising candidate radiotracer for imaging FAAH in the brain with PET. These results coupled with the known pharmacology and toxicology of PF-04457845 should facilitate clinical translation of this radiotracer.     

Radiosynthesis of [2-11C-carbonyl]dantrolene using [11C]phosgene for PET

Automated radiosynthesis of [2-¹¹C-carbonyl]dantrolene, the substrate of breast cancer resistance protein (BCRP/ABCG2), was performed for the first time through a multi-step/one-pot labeling sequence that started with ethyl 2-{2-[5-(4-nitrophenyl)furfurylidene]hydrazino}acetate and used [¹¹C]phosgene as a labeling agent. After optimization of the automated synthesis conditions and parameters, [2-¹¹C-carbonyl]dantrolene was obtained at a radiochemical yield of 34.0±8.4% (decay-corrected). The radiochemical purity was greater than 98% and the specific activity was 46.8±15.2 GBq/μmol at the end of the synthesis.     

Synthesis and in vivo studies of a specific monoamine oxidase B inhibitor: 5-[4-(benzyloxy)phenyl]-3-(2-cyanoethyl)-1,3,4-oxadiazol-[11C]-2(3H)-one

We report the radiochemical synthesis of a specific MAO B inhibitor, namely 5-[4-(benzyloxy)phenyl]-3-(2-cyanoethyl)-1,3,4-oxadiazol-[¹¹C]-2(3H)-one (2b) (in vitro IC₅₀=4nM and selectivity over 71000 for MAO B), by cyclization of its hydrazide precursor1 with [¹¹C]phosgene. The reaction occurred within 2 min. The product obtained after HPLC purification,2b, had a high specific activity (11.1–22.2 GBq/µmol), allowing its use in experiments as a radiotracer in vivo. Biodistribution of2b in the CNS and in the peripheral organs of the rat, and positron emission tomography (PET) studies in the living baboon brain, pretreated or not withl-deprenyl (1 mg/kg, 1 h), an irreversible MAO B-specific inhibitor, were undertaken. The results showed a good uptake of2b in all organs of the rat, with a rapid clearance from the blood (10 min). Metabolite analyses in plasma and in the diencephalon of the rat showed that2b was the only radioactive compound in brain structure whereas in plasma three other radioactive products appeared. PET experiments show that in thel-deprenyl-pretreated baboon brain, specific binding of2b represents around 70% of total radioactivity, whereas in the blood and plasma the radioactivity cleared rapidly (15 min).     

Radiosynthesis of 4-[(2-chloroethyl)(2-[C]ethyl)amino]-phenoxycarbonyl- -glutamic acid a half mustard prodrug as a potential probe for imaging antibody- and gene-directed enzyme prodrug therapy with positron emission tomography

The potential antibody directed prodrug therapy half-mustard prodrug 4-[(2-chloroethyl)(2-ethyl)amino]-phenoxycarbonyl-L-glutamic acid was synthesised by reductive alkylation of 4-[(2-chloroethyl)amino]-phenoxycarbonyl-L-glutamic acid using acetaldehyde. 4-[(2-chloroethyl)[(11)C](2-ethyl)amino]phenoxycarbonyl-L-glutamic acid was synthesized with 18-22% decay corrected radiochemical yield in 45 min from EOB by reductive alkylation of 4-[(2-chloroethyl)amino]-phenoxycarbonyl-L-glutamic acid using [(11)C]acetaldehyde. [(11)C]Acetaldehyde was prepared in 60% decay corrected radiochemical yield by oxidation of [(11)C]ethanol over heated copper oxide. The radiosynthesis of [(11)C]ethanol was re-examined and optimized. 4-[(2-chloroethyl)(2-ethyl)amino]-phenoxycarbonyl-L-glutamic acid was found to have affinity for carboxypeptidase G2; the K(m) and V(max) were 99.4-115.9 microM (n=3) and 3.6-5.0 microM/min, respectively, at a carboxypeptidase G2 concentration of 0.0247 U/ml.     

Remote synthesis of [11C]acetate

This remote synthesis greatly simplifies previously reported liquid—liquid extraction techniques for remote production of [¹¹C]acetate. The use of solid phase extraction has reduced the total volume of solvents employed while providing [¹¹C]acetate free of labeled by-products. Remotely actuated loop flow valves have been utilized to efficiently and quickly introduce reagents. [¹¹C]Acetate is obtained within 15 min from the end of bombardment in 43% (EOB) radiochemical yield and with radiochemical purity >98%.     

An improved synthesis of dopamine D2/D3 receptor radioligands [11C]fallypride and [18F]fallypride

Improved syntheses of dopamine D₂/D₃ receptor radioligands [¹¹C]Fallypride and [¹⁸F]Fallypride are reported. The phenolic precursor (9) for C-11 labeling and the Fallypride (10) reference standard were synthesized from the starting material 2-hydroxy-3-methoxy-5-(2-propenyl)benzoic acid methyl ester (1) in 7 and 8 steps with 16% and 5% overall chemical yields, respectively. The tosylated precursor (15) for F-18 labeling was synthesized from compound 1 in 5 steps with 32% overall chemical yield. An alternate synthetic approach for Fallypride has been developed using the same starting material 1 in 5 steps with 26% overall chemical yield. [¹¹C]Fallypride ([¹¹C]10) was prepared by O-[¹¹C]methylation of the phenolic precursor with [¹¹C]methyl triflate and purified with a semi-preparative HPLC method in 50–60% radiochemical yield, decay corrected to end of bombardment (EOB), based on [¹¹C]CO₂, and 370±185 GBq/μmol specific radioactivity at EOB. [¹⁸F]Fallypride ([¹⁸F]10) was prepared by nucleophilic substitution of the tosylated precursor with K[¹⁸F]F/Kryptofix 2.2.2 and HPLC combined with solid-phase extraction (SPE) purification in variable (up to 50%) decay corrected radiochemical yield from K[¹⁸F]F and 111–222 GBq/μmol specific activity at EOB.     

An automated SPE-based high-yield synthesis of [11C]acetate and [11C]palmitate: no liquid-liquid extraction, solvent evaporation or distillation required

Introduction

An automated method is described for the rapid and high-yield synthesis of two of the most commonly used radioactive fatty acids: [¹¹C]acetate and [¹¹C]palmitate.

Methods

Reaction of [¹¹C]CO₂ with the respective Grignard reagents in diethyl ether solution proceeded for 2 min at 40°C. Quenching of the reaction and liberation of nonreacted [¹¹C]CO₂ occurred upon addition of a fourfold molar excess of aqueous 0.1 M HCl (acetate) or nonaqueous HCl/Et₂O (palmitate). Labeled products were then purified by adsorption to an Alumina-N Sep-Pak Plus cartridge and eluted with either aqueous NaH₂PO₄ solution (acetate) or 100% ethanol (palmitate).

Results

High-performance liquid chromatography analysis confirmed that the radiochemical purity of each product was >98%, and decay-corrected radiochemical yields averaged 33% for [¹¹C]palmitate and 40% for [¹¹C]acetate.

Conclusion

The method requires no liquid–liquid extraction, solvent evaporation or distillation capabilities and can be readily adapted to existing radiosynthesis modules.     

A high-yield and simplified procedure for the synthesis of α-[C]Methyl-l-tryptophan

Alpha-[¹¹C]methyl-l-tryptophan (AMT) has been synthesized by stereoselective methylation with [¹¹C]methyl iodide of the lithium-enolate generated by treating dimethyl 2(S),3a(R),8a(S)-(+)-hexahydro-8(phenylsulfonyl) pyrrolo[2,3-b]indole-1,2-dicarboxylate (2) with lithium diisopropyl amide (LDA) at −55 °C, followed by ring opening using trifluoroacetic acid and alkaline hydrolysis of the protecting groups. The crude product was purified by a simple reverse-phase C-18 Sep-Pak procedure. The purified product was isolated with an average radiochemical yield of 53 ± 12% (decay corrected) in 30–35 min from [¹¹C]methyl iodide. At end of synthesis (EOS), 138 ± 35 mCi (n = 24) of product was collected with a specific activity of ca. 1–1.3 Ci/μmol (EOS) (4–5 Ci/μmol @ EOB) starting from 1.5 Ci (EOB) of [¹¹C]CO₂.     

An alternative synthesis of DL-[1-11C]Alanine from [11C]HCN

The synthesis of dl-[1-¹¹C]alanine from [1-¹¹C]HCN via dl-[1-¹¹C]2-aminopropanenitrile and its use in an enzymatic synthesis of [1-¹¹C]pyruvic acid is reported. The purified dl-[1-¹¹C]alanine was obtained in a radiochemical yield of 75% with a radiochemical purity higher than 98% within 40 min after end of bombardment (EOB). [1-¹¹C]Pyruvic acid was prepared by enzymatic synthesis from crude dl-[1-¹¹C]alanine using a previously reported procedure for the synthesis of [3-¹¹C]pyruvic acid. dl-[1-¹¹C]Alanine and [1-¹¹C]pyruvic acid prepared by these methods have been found to be sterile and free of pyrogens, and can thus be used in studies of the distribution of these radiopharmaceuticals in man using positron emission tomography (PET).     

Synthesis of a [C]Methoxy derivative of α-Dihydrotetrabenazine: a radioligand for studying the vesicular monoamine transporter

The synthesis of [¹¹C]TBZOMe, a [¹¹C]methoxy derivative at the 2-hydroxy position of α-dihydrotetrabenazine, was carried out by an O-[¹¹C]methylation reaction. The product [¹¹C]TBZOMe (100–200 mCi) was obtained in 15–40% radiochemical yield (corrected for decay) within 37 min, and in high specific activity (2000–2500 Ci/mmol) and radiochemical purity (>97%). [¹¹C]TBZOMe is a potential new radioligand for studying the vesicular monoamine transporter using positron emission tomography.     

The first design and synthesis of [11C]MKC-1 ([11C]Ro 31-7453), a new potential PET cancer imaging agent

Bisindolylmaleimide MKC-1 (formerly known as Ro 31-7453) is a novel, orally active, small-molecule, cell cycle inhibitor with broad-spectrum antitumor effects. [¹¹C]MKC-1 ([¹¹C]Ro 31-7453) was first designed and synthesized as a new potential positron emission tomography cancer imaging agent through two different strategies. The first strategy was to prepare a carbon-11-labeled bisindolyl maleic anhydride intermediate followed by the conversion to maleimide. The second strategy involved the condensation of either carbon-11-labeled indole-3-acetamides with indole-3-glyoxalates, or indole-3-acetamides with carbon-11-labeled indole-3-glyoxalates. The radiochemical yields were 15–30%, decay corrected to end of bombardment (EOB), based on [¹¹C]CO₂. The specific activity was 222–296 GBq/μmol at EOB and 111–148 GBq/μmol at the end of synthesis, respectively.     

An automated radiosynthesis of 2-[11C]thymidine using anhydrous [11C]urea derived from [11C]phosgene.

2-[11C]Thymidine has been produced from [11C]methane via [11C]phosgene and [11C]urea. Anhydrous [11C]urea was prepared from [11C]phosgene by reaction with liquid ammonia. This novel approach avoids the problems associated with the synthesis of anhydrous [11C]urea from [11C]cyanide. A fully automated system based on a modular approach and under PLC control has been developed. The system provides 2-[11C]thymidine reliably and reproducibly for clinical PET studies. The radiosynthesis takes 45-50 min from [11C]methane and the average yield was 1.5-3.3 GBq (40-90 mCi). The specific radioactivity was typically in the range 29.6-51.8 GBq mumol-1 (0.8-1.4 Ci mumol-1) at EOS corresponding to 6-12 micrograms of stable thymidine. The radiochemical yield of 2-[11C]thymidine was ca. 14% from [11C]methane.     

Development of a modular system for the synthesis of PET [C]labelled radiopharmaceuticals

[¹¹C]labelled radiopharmaceuticals as N-[¹¹C]methyl-choline ([¹¹C]choline), l-(S-methyl-[¹¹C])methionine ([¹¹C]methionine) and [¹¹C]acetate have gained increasing importance in clinical PET and for the routine production of these radiopharmaceuticals, simple and reliable modules are needed to produce clinically relevant radioactivity. On the other hand, flexible devices are needed not only for the routine synthesis but also for more complex applications as the development of new tracers. The aim of this work was the adaptation of an Eckert Ziegler modular system for easy routine synthesis of [¹¹C]choline, [¹¹C]methionine and [¹¹C]acetate using components that account for straightforward scaling up and upgrades.     

PET studies with l-[1-11C]tyrosine,l-[methyl-11C]methionine and 18F-fluorodeoxyglucose in prolactinomas in relation to bromocryptine treatment

Aspects of metabolism in prolactinomas were investigated by positron emission tomography using l-[1-¹¹C]tyrosine, l-[methyl-¹¹C]methionine and ¹⁸F-fluorodeoxyglucose (¹⁸FDG). Using l-[1-¹¹C]tyrosine, four patients were monitored prior to and 18 h after an injection of 50 mg bromocryptine. At 18 h after bromocryptine intervention, l-[1-¹¹C]tyrosine uptake into tumour was reduced with 28% (P<0.07). A correlation analysis of the bromocryptine-induced decrease in l-[1-¹¹C]tyrosine uptake and the reduction of serum prolactin levels indicated that the action of bromocryptine on prolactin synthesis and prolactin release is not coupled. In the untreated situation, the four patients were investigated with ¹⁸FDG as well, but the prolactinomas could not be visualized. Three untreated patients were studied with l-[methyl-¹¹C]methionine. The tumour-imaging potential of l-[methyl-¹¹C]methionine and l-[1-¹¹C]tyrosine appeared to be nearly equivalent for prolactinomas. Unlike prolactinoma tissue, the salivary glands showed a pronounced preference for l-[1-¹¹C]tyrosine as compared to l-[methyl-¹¹C]methionine. l-[1-¹¹C]tyrosine is a valuable tool to obtain information on the metabolism and treatment of prolactinomas.     

Synthesis of [11C]PBR170, a novel imidazopyridine,for imaging the translocator protein with PET

The translocator protein (TSPO) ligand 2-(6,8-dichloro-2-(4-ethoxyphenyl)imidazo[1,2-a]pyridin-3-yl)–N-(2-fluoropyridin-3-yl)–N-methylacetamide (PBR170), is a novel imidazopyridineacetamide with high affinity (2.6 nm) and selectivity for the TSPO. The synthesis of [¹¹C]PBR170 was accomplished by N-methylation of the corresponding desmethyl precursor with [¹¹C]methyl iodide in the presence of sodium hydroxide in dimethylformamide. [¹¹C]PBR170 was produced in 30–45% radiochemical yield (decay-corrected, based on [¹¹C]methyl iodide) with a radiochemical purity >98% and a specific activity of 90–190 GBq/μmol after 35 min of synthesis time.     

Synthesis of DL-[3-11C]valine using [2-11C]isopropyl iodide,and preparation of L-[3-11C]valine by treatment with D-amino acid oxidase

DL-[3-¹¹C]Valine, synthesized by phase-transfer alkylation of N-(diphenylmethylene)glycine t-butyl ester with [2-¹¹C]isopropyl iodide, followed by acidic hydrolysis, was obtained in 20–30% radiochemical yield (decay corrected and calculated on the amount of [¹¹C]carbon dioxide used) and with 93–99% radiochemical purity with a total synthesis time of 50 min. Following treatment with immobilized D-amino acid oxidase, L-[3-¹¹C]valine was obtained in 90–99% enantiomeric excess with a total synthesis time of 85 min. [2-¹¹C]Isopropyl iodide was obtained in 40 and 90% radiochemical yield and purity respectively, within 12 min calculated from [¹¹C]carbon dioxide. In a typical experiment starting with 150 mCi of [¹¹C]carbon dioxide, 7 mCi of DL-[3-¹¹C]valine and 0.8 mCi of L-[3-¹¹C]valine were obtained.     

Development and evaluation of muscarinic cholinergic receptor ligands n-[c]ethyl-4-piperidyl benzilate and n-[c]propyl-4-piperidyl benzilate: a pet study in comparison with n-[c]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.     

Evaluation of [C]laniquidar as a tracer of P-glycoprotein: radiosynthesis and biodistribution in rats

At present, P-glycoprotein (P-gp) function can be studied using positron emission tomography (PET) together with a labelled P-gp substrate such as (R)-[¹¹C]verapamil. Such a tracer is, however, less suitable for investigating P-gp (over)expression. Laniquidar is a third-generation P-gp inhibitor, which has been used in clinic trials for modulating multidrug resistance transporters. The purpose of the present study was to develop the radiosynthesis of [¹¹C]laniquidar and to assess its suitability as a tracer of P-gp expression.The radiosynthesis of [¹¹C]laniquidar was performed by methylation of the carboxylic acid precursor with [¹¹C]CH₃I. The product was purified by HPLC and reformulated over a tC₁₈ Seppak, yielding a sterile solution of [¹¹C]laniquidar in saline. For evaluating [¹¹C]laniquidar, rats were injected with 20 MBq [¹¹C]laniquidar via a tail vein and sacrificed at 5, 15, 30 and 60 min after injection. Several tissues and distinct brain regions were dissected and counted for radioactivity. In addition, uptake of [¹¹C]laniquidar in rats pretreated with cyclosporine A and valspodar (PSC 833) was determined at 30 min after injection. Finally, the metabolic profile of [¹¹C]laniquidar in plasma was determined.[¹¹C]Laniquidar could be synthesized in moderate yields with high specific activity. Uptake in brain was low, but significantly increased after administration of cyclosporine A. Valspodar did not have any effect on cerebral uptake of [¹¹C]laniquidar. In vivo rate of metabolism was relatively low. Further kinetic studies are needed to investigate the antagonistic behaviour of [¹¹C]laniquidar at tracer level.