General Method for the 11C‐Labeling of 2‐Arylpropionic Acids and Their Esters: Construction of a PET Tracer Library for a Study of Biological Events Involved in COXs Expression

Cyclooxygenase (COX) is a critical enzyme in prostaglandin biosynthesis that modulates a wide range of biological functions, such as pain, fever, and so on. To perform in vivo COX imaging by positron emission tomography (PET), we developed a method to incorporate ¹¹C radionuclide into various 2‐arylpropionic acids that have a common methylated structure, particularly among nonsteroidal anti‐inflammatory drugs (NSAIDs). Thus, we developed a novel ¹¹C‐radiolabeling methodology based on rapid C‐[¹¹C]methylation by the reaction of [¹¹C]CH₃I with enolate intermediates generated from the corresponding esters under basic conditions. One‐pot hydrolysis of the above [¹¹C]methylation products also allows the synthesis of desired ¹¹C‐incorporated acids. We demonstrated the utility of this method in the syntheses of six PET tracers, [¹¹C]Ibuprofen, [¹¹C]Naproxen, [¹¹C]Flurbiprofen, [¹¹C]Fenoprofen, [¹¹C]Ketoprofen, and [¹¹C]Loxoprofen. Notably, we found that their methyl esters were particularly useful as proradiotracers for a study of neuroinflammation. The microPET studies of rats with lipopolysaccharide (LPS)‐induced brain inflammation clearly showed that the radioactivity of PET tracers accumulated in the inflamed region. Among these PET tracers, the specificity of [¹¹C]Ketoprofen methyl ester was demonstrated by a blocking study. Metabolite analysis in the rat brain revealed that the methyl esters were initially taken up in the brain and then underwent hydrolysis to form pharmacologically active forms of the corresponding acids. Thus, we succeeded in general ¹¹C‐labeling of 2‐arylpropionic acids and their methyl esters as PET tracers of NSAIDs to construct a potentially useful PET tracer library for in vivo imaging of inflammation involved in COXs expression.     

Yield ratio of [11C]-CO2, [11C]-CO and [11C]-CH4 from the irradiation of N2/H2-mixtures in a gas target

The¹⁴N/p, /¹¹C-reaction was studied in different N₂/H₂-mixtures. The products are [¹¹C]-CO₂, [¹¹C]-CO and [¹¹C]-CH₄. The yield ratio may be controlled by varying the bombardment conditions. High pressure, high H₂-content, high beam current and high proton energy shift the ratio towards [¹¹C]-CH₄. Lower beam current and lower proton energy increase the yield of [¹¹C]-CO₂. The production of [¹¹C]-CO is constant over a wide range of conditions /about 10%/. For the production of [¹¹C]-CH₄ in good yield a target gas holder for high pressures has been developed. Details are given in Fig. 7. This target gas holder was filled with 5% H₂ in N₂ at 3×10⁶ Pa. Proton irradiation of the mixture gives a typical yield of [¹¹C]-CH₄ of 400–500 mCi at a beam current of 15–20 A within 20 min. Only traces of other¹¹C-labelled compounds could be detected under these conditions.     

Preliminary Assessment of the Anti-inflammatory Activity of New Structural Honokiol Analogs with a 4′-O-(2-Fluoroethyl) Moiety and the Potential of Their 18F-Labeled Derivatives for Neuroinflammation Imaging

Neolignans honokiol and 4′-O-methylhonokiol (MH) and their derivatives have pronounced anti-inflammatory activity, as evidenced by numerous pharmacological studies. Literature data suggested that cyclooxygenase type 2 (COX-2) may be a target for these compounds in vitro and in vivo. Recent studies of [¹¹C]MPbP (4′-[¹¹C]methoxy-5-propyl-1,1′-biphenyl-2-ol) biodistribution in LPS (lipopolysaccharide)-treated rats have confirmed the high potential of MH derivatives for imaging neuroinflammation. Here, we report the synthesis of four structural analogs of honokiol, of which 4′-(2-fluoroethoxy)-2-hydroxy-5-propyl-1, 1′-biphenyl (F-IV) was selected for labeling with fluorine-18 (T_1/2 = 109.8 min) due to its high anti-inflammatory activity confirmed by enzyme immunoassays (EIA) and neuromorphological studies. The high inhibitory potency of F-IV to COX-2 and its moderate lipophilicity and chemical stability are favorable factors for the preliminary evaluation of the radioligand [¹⁸F]F-IV in a rodent model of neuroinflammation. [¹⁸F]F-IV was prepared with good radiochemical yield and high molar activity and radiochemical purity by ¹⁸F-fluoroethylation of the precursor with Boc-protecting group (15) with [¹⁸F]2-fluoro-1-bromoethane ([¹⁸F]FEB). Ex vivo biodistribution studies revealed a small to moderate increase in radioligand uptake in the brain and peripheral organs of LPS-induced rats compared to control animals. Pretreatment with celecoxib resulted in significant blocking of radioactivity uptake in the brain (pons and medulla), heart, lungs, and kidneys, indicating that [¹⁸F]F-IV is likely to specifically bind to COX-2 in a rat model of neuroinflammation. However, in comparison with [¹¹C]MPbP, the new radioligand showed decreased brain uptake in LPS rats and high retention in the blood pool, which apparently could be explained by its high plasma protein binding. We believe that the structure of [¹⁸F]F-IV can be optimized by replacing the substituents in the biphenyl core to eliminate these disadvantages and develop new radioligands for imaging activated microglia.     

Preparation of N-[11C]methyl-2,5-dimethoxy-4-bromo-amphetamine

For in vivo receptor studies N-[¹¹C]methyl-2,5-dimethoxy-4-bromo-amphetamine, [¹¹C]MDOB, was synthesized by the reaction of [¹¹C]CH₃I with DOB in acetonitrile. The labelling yield was determined in dependence on amount of precursor, time and temperature of the methylation reaction. During 10 to 15 minutes 60% to 70% of [¹¹C]MDOB has been obtained at 110°C.     

Pd0‐Mediated Rapid Cross‐Coupling Reactions,the Rapid C‐[11C]Methylations,Revolutionarily Advancing the Syntheses of Short‐Lived PET Molecular Probes

Positron emission tomography is a noninvasive method for monitoring drug (or diagnostic) behavior and its localization on the target molecules in the living systems, including the human body, using a short‐lived positron‐emitting radionuclide. New methodologies for introducing representative short‐lived radionuclides, ¹¹C and ¹⁸F, into the carbon frameworks of biologically active organic compounds have been established by developing rapid C‐[¹¹C]methylations and C‐[¹⁸F]fluoromethylations using rapid Pd⁰‐mediated cross‐coupling reactions between [¹¹C]methyl iodide (sp³‐hybridized carbon) and an excess amount of organotributylstannane or organoboronic acid ester having sp²(phenyl, heteroaromatic, or alkenyl), sp(alkynyl), or sp³(benzyl and cinnamyl)‐hybridized carbons; and [¹⁸F]fluoromethyl halide (iodide or bromide) and an organoboronic acid ester, respectively. These rapid reactions provide a firm foundation for an efficient and general synthesis of short‐lived ¹¹C‐ or ¹⁸F‐labeled PET molecular probes to promote in vivo molecular imaging studies.     

N-[11C]methyl-3,4-methylenedioxyamphetamine (Ecstasy) and 2-methyl-N-[11C]methyl-4,5-methylenedioxyamphetamine: Synthesis and biodistribution studies

In order to evaluate the neurobiological mechanism causing the psychogenic effects of methylenedioxy-derivatives of amphetamine, the carbon-11 labeled analogues of 3,4-methylenedioxymethamphetamine (MDMA),2 and 2,N-dimethyl-4,5-methylenedioxyamphetamine (MADAM-6)4 were prepared for application in in-vivo PET studies by methylation of 3,4-methylenedioxyamphetamine (MDA)1 and 2-methyl-4,5-methylenedioxyamphetamine3 with [¹¹C]CH₃I. The radiochemical yield was determined in dependence on time, temperature and amount of precursor. The best conditions for a fast labeling reaction with carbon-11 on a preparative scale were found to be a reaction time of 10 min using 1 mg of the corresponding dimethyl-precursors1 or3, thus obtaining radiochemical yields of 60% (based on produced [¹¹C]CH₃I). Biodistribution studies were performed in rats, a high brain to blood ratio of 7.5 was observed for [¹¹C]MDMA in contrast to a ratio of 3.7 for [¹¹C]MADAM-6.     

Rapid synthesis of C-labeled FK506 for positron emission tomography

The present study describes a rapid synthesis method for labeled [¹¹C]FK506 for positron emission tomography (PET). A one-pot reaction from [¹¹C]CH₃I, involving a Wittig reaction as the key carboncarbon bond formation was developed. The chemical process was accomplished using a designed, fully automated synthetic apparatus, and an injectable solution of [¹¹C]FK506 was obtained in only 34 min from [¹¹C]CH₃I. The decay-corrected radiochemical yield based on [¹¹C]CH₃I was 11.9%, and the specific activity was 39.8 GBq/μmol.     

A new technique for labeling of [11C]-choline, a positron-emitting tracer for tumor imaging

Summary [¹¹C]-choline has been reported as a potential tracer for imaging a variety of human tumors with positron emission tomography (PET). A new labeling technique for [¹¹C]-choline was established depending on parameters optimized, such as reaction time, volume, temperature, and the quantity of DMAE.The synthesis yield was improved from 82.0% to 96.5% (EOB), while the consumption of DMAE precursor decreased from 60 to 2 mg. Absolute yield of [¹¹C]-choline was 2500 MBq for a 10-minute irradiation at15mA, and a total synthesis time of less than 8 minutes from [¹¹C]-CH₃I to [¹¹C]-choline.     

Radiosynthesis of New Carbon-11-labeled Nimesulide Analogs as Potential PET SAER Tracers for Imaging of Aromatase Expression in Breast Cancer

Carbon-11-labeled nimesulide analogs, N-[¹¹C]methyl-N-(2-benzyloxy-4-nitrophenyl)methanesulfonamide ([¹¹C]4a), N-[¹¹C]methyl-N-[2-(4′-methylbenzyloxy)-4-nitrophenyl]methanesulfonamide ([¹¹C]4b), N-[¹¹C]methyl-N-[2-(4′-fluorobenzyloxy)-4-nitrophenyl]methanesulfonamide ([¹¹C]4c), N-[¹¹C]methyl-N-[2-(4′-nitrobenzyloxy)-4-nitrophenyl]methanesulfonamide ([¹¹C]8a), N-[¹¹C]methyl-N-[2-(β-naphthylmethoxy)-4-nitrophenyl]methanesulfonamide ([¹¹C]8b), and N-[¹¹C]methyl-N-[2-(2′-phenylbenzyloxy)-4-nitrophenyl]methanesulfonamide ([¹¹C]8c), have been synthesized as new potential positron emission tomography (PET) selective aromatase expression regulator (SAER) radiotracers for imaging of aromatase expression in breast cancer. The target tracers were prepared by N-[¹¹C]methylation of their corresponding precursors using [¹¹C]CH₃OTf under basic conditions (NaH) and isolated by reversed-phase high-pressure liquid chromatography (HPLC) method in 30–50% radiochemical yields decay corrected to end of bombardment (EOB) with 25–30 min overall synthesis time and 111–148 GBq/μmol specific activity at end of synthesis (EOS).     

Preparation of N-[11C]methyl-2,5-dimethoxy-4-methylamphetamine

In order to evaluate the neurobiological mechanism causing the psychogenic effects of N-methyl-2,5-dimethoxy-4-methylamphetamine (MDOM), the¹¹C labelled analogue was prepared for application in in vivo PET studies by the reaction of 2,5-dimethoxy-4-methylamphetamine (DOM) with [¹¹C]CH₃I. The radiochemical yield was determined in dependence on time, temperature, solvent and amount of substrate. The best conditions for fast labelling reactions with¹¹C on a preparative scale were found to be a reaction time of 10 miutes at 110°C using 1 mg DOM in acetonitrile thus obtaining radiochemical yields of 80% (based on produced [¹¹C]CH₃I).     

Spezifisch14C-kernmarkierte Phenolderivate, 1. Mitt.: Synthese von [14C]-Guajacol

Zusammenfassung Für oxydative Abbauversuche wurden verschiedene spezifisch¹⁴C-kernmarkierte Phenolderivate benötigt. Guajacol, in verschiedenen Positionen des aromatischen Ringes markiert, wurde — teils von [1-¹⁴C]-Phenol, teils von [2-¹⁴C]-Aceton ausgehend — synthetisiert.

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Specifically¹⁴C-labelled phenol derivatives, I: Synthesis of [¹⁴C]guaiacol

Various phenol derivatives,¹⁴C-labelled in different specified positions of the aromatic ring, were required for oxidative degradation studies. Guaiacol, ring-labelled in different positions, was prepared both from [1-¹⁴C]phenol and from [2-¹⁴C]-acetone.

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Herrn Prof. Dr.M. Pailer zum 60. Geburtstag in Freundschaft gewidmet.     

The synthesis of a new cardiac sympathetic nerve imaging agent N-[11C]CH3-dopamine and biodistribution study

In this study, we synthesized and characterized N-[¹¹C]methyl-dopamine ([¹¹C]MDA) for cardiac sympathetic nerve imaging. [¹¹C]MDA was synthesized by direct N-methylation of dopamine with [¹¹C]methyl iodide and purified by semi-preparation reverse high pressure liquid chromatography (HPLC). The total synthesis time was 45 min including HPLC purification. The radiochemical yields of [¹¹C]MDA was 20 ± 3 %, without decay correction. The radiochemical purity was >98 % and the specific activity was about 50 GBq/mmol. The biological properties of [¹¹C]MDA were evaluated by biodistribution study in normal mice. PET imaging was performed in healthy Chinese mini-swines. Biodistribution study showed that [¹¹C]MDA had high myocardium uptake. PET/CT imaging showed [¹¹C]MDA had clear and symmetrical myocardium uptake, which was blocked obviously by injecting imipramine hydrochloride. [¹¹C]MDA would be a promising candidate of radiotracer for cardiac sympathetic nervous system imaging.     

Synthesis of [18F]F-γ-T-3, a Redox-Silent γ-Tocotrienol (γ-T-3) Vitamin E Analogue for Image-Based In Vivo Studies of Vitamin E Biodistribution and Dynamics

Vitamin E, a natural antioxidant, is of interest to scientists, health care pundits and faddists; its nutritional and biomedical attributes may be validated, anecdotal or fantasy. Vitamin E is a mixture of tocopherols (TPs) and tocotrienols (T-3s), each class having four substitutional isomers (α-, β-, γ-, δ-). Vitamin E analogues attain only low concentrations in most tissues, necessitating exacting invasive techniques for analytical research. Quantitative positron emission tomography (PET) with an F-18-labeled molecular probe would expedite access to Vitamin E’s biodistributions and pharmacokinetics via non-invasive temporal imaging. (R)-6-(3-[¹⁸F]Fluoropropoxy)-2,7,8-trimethyl-2-(4,8,12-trimethyltrideca-3,7,11-trien-1-yl)-chromane ([¹⁸F]F-γ-T-3) was prepared for this purpose. [¹⁸F]F-γ-T-3 was synthesized from γ-T-3 in two steps: (i) 1,3-di-O-tosylpropane was introduced at C6-O to form TsO-γ-T-3, and (ii) reaction of this tosylate with [¹⁸F]fluoride in DMF/K₂₂₂. Non-radioactive F-γ-T-3 was synthesized by reaction of γ-T-3 with 3-fluoropropyl methanesulfonate. [¹⁸F]F-γ-T-3 biodistribution in a murine tumor model was imaged using a small-animal PET scanner. F-γ-T-3 was prepared in 61% chemical yield. [¹⁸F]F-γ-T-3 was synthesized in acceptable radiochemical yield (RCY 12%) with high radiochemical purity (>99% RCP) in 45 min. Preliminary F-18 PET images in mice showed upper abdominal accumulation with evidence of renal clearance, only low concentrations in the thorax (lung/heart) and head, and rapid clearance from blood. [¹⁸F]F-γ-T-3 shows promise as an F-18 PET tracer for detailed in vivo studies of Vitamin E. The labeling procedure provides acceptable RCY, high RCP and pertinence to all eight Vitamin E analogues.     

Performances of a TRACERlab FX C synthesis module using a Ni-nanopowder/molecular sieves mixed catalyst

The progress of positron emission tomography goes together with an increasing demand for new radiopharmaceuticals: among these, the development of radiopharmaceuticals labelled with carbon-11 is particularly interesting because these compounds are biologically indistinguishable from their stable analogues. These radiotracers are prepared starting from [¹¹C]carbon dioxide, the most common and versatile primary labelling precursor, or from secondary labelling precursors such [¹¹C]methyl iodide produced by “wet” or “gas-phase” method. The gas-phase is the most used method and consists in the radical reaction of iodine vapours with [¹¹C]methane, produced in target or from [¹¹C]carbon dioxide by reduction with hydrogen on nickel catalyst at high temperature. This second approach is frequently adopted in commercial automatic methylation modules, such as the TRACERlab FX C. When not performed in target, [¹¹C]CH₄ production represents a key step for the [¹¹C]CH₃I synthesis from which the outcome of the whole radiolabelling process depends. In order to improve the performance of the module, a new reduction catalyst made of a mixture of metallic Ni (nanopowder) and molecular sieves mixed in different ratios has been tested. Preliminary results demonstrated that not only the mixture of nanopowder-Ni and molecular sieves represents a valid reduction catalyst but also permits to trap [¹¹C]CO₂ and subsequently use it as labelling reagent, making TRACERlab FX C a module for both methylation and carboxylation.     

Broad Scope and High-Yield Access to Unsymmetrical Acyclic [11C]Ureas for Biomedical Imaging from [11C]Carbonyl Difluoride

Effective methods are needed for labelling acyclic ureas with carbon-11 (t_1/2=20.4 min) as potential radiotracers for biomedical imaging with positron emission tomography (PET). Herein, we describe the rapid and high-yield syntheses of unsymmetrical acyclic [¹¹C]ureas under mild conditions (room temperature and within 7 min) using no-carrier-added [¹¹C]carbonyl difluoride with aliphatic and aryl amines. This methodology is compatible with diverse functionality (e. g., hydroxy, carboxyl, amino, amido, or pyridyl) in the substrate amines. The labelling process proceeds through putative [¹¹C]carbamoyl fluorides and for primary amines through isolable [¹¹C]isocyanate intermediates. Unsymmetrical [¹¹C]ureas are produced with negligible amounts of unwanted symmetrical [¹¹C]urea byproducts. Moreover, the overall labelling method tolerates trace water and the generally moderate to excellent yields show good reproducibility. [¹¹C]Carbonyl difluoride shows exceptional promise for application to the synthesis of acyclic [¹¹C]ureas as new radiotracers for biomedical imaging with PET.     

N-[11C]methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine (MBDB): synthesis, quality control and biodistribution

N-[¹¹C]methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine ([¹¹C]MBDB) 3 was prepared by methylation of the demethyl precursor BDB with [¹¹C]CHI. The radiosynthesis was optimized with regard to temperature, reaction time and amount of precursor, best results (i.e., 84% radiochemical yield, based on [¹¹C]CH₃I activity) were obtained using 3 mg BDB at a reaction temperature of 130 °C in 8 minutes. With respect to a facilitated workup routine, productions were performed with 0.6 mg BDB at 110 °C for 10 minutes, yielding more than 50% of 3. The radiochemical purity of the final tracer solution was >98%, the specific activity was determined to be 300 GBq/mol (8000 Ci/mmol). Biodistribution, studies in rats showed two major metabolic pathways as indicated by an increasing liver uptake (9.1% ID/organ at 5 minutes to 21% ID/organ at 30 minutes) and a high urine activity (up to 16% ID/g). In brain tracer uptake was more than 1%, with a brain to blood ratio of almost 12 resulting from a very rapid blood clearance of 3.     

Synthesis of [carbonyl-C]acetophenone via the Stille cross-coupling reaction of [1-C]acetyl chloride with tributylphenylstannane mediated by Pd2(dba)3/P(MeNCH2CH2)3N·HCl

The Stille cross-coupling reaction of [1-¹¹C]acetyl chloride with tributylphenylstannane leading to [carbonyl-¹¹C]acetophenone was studied with the goal of developing a new ¹¹C-labeling method for positron emission tomography tracer synthesis. The coupled product [carbonyl-¹¹C]acetophenone was synthesized using the Pd₂(dba)₃/P(MeNCH₂CH₂)₃N·HCl system with a 60-61% radiochemical conversion from [1-¹¹C]acetyl chloride (decay-corrected, n = 3).     

Structural requirement of C11b chirality of tetrabenazine analogs as VMAT2 imaging ligands: synthesis and in vivo evaluation

We studied on the structural requirement of C11b chirality of tetrabenazine (TBZ) analogs as vesicular monoamine transporter 2 (VMAT2) ligands. TBZ analogs (2, 6a, 6b) and ¹⁸F-radiolabeled [¹⁸F]6a and [¹⁸F]6b with eliminated C11b chirality were synthesized and characterized. Competition studies demonstrated that 2, 6a and 6b displayed much lower in vivo VMAT2 bindings than TBZ. MicroPET imaging studies of [¹⁸F]6a and [¹⁸F]6b showed negligible accumulation in VMAT2-enriched regions as compared with the known VMAT2 ligand ¹⁸F-FP-(+)-DTBZ. These results suggest that C11b chirality of TBZ analogs is essential for in vivo VMAT2 binding bioactivity.     

Identification of PET radiometabolites by cytochrome P450, UHPLC/Q-ToF-MS and fast radio-LC: applied to the PET radioligands [11C]flumazenil, [18F]FE-PE2I, and [11C]PBR28

A general method is presented for the identification of radiometabolites in plasma of human and monkey subjects after administration of positron emission tomography (PET) radioligands. The radiometabolites are first produced in vitro, using liver microsomes, subsequently separated using fast radio-liquid chromatography (radio-LC), and individually collected and identified by ultra high-performance liquid chromatography/quadrupole-time of flight-mass spectrometry in MS and MS^E mode. Fast radio-LC provided superior resolution compared to conventional radio-LC, resulting in separation of a greater number of metabolites. The radiometabolites produced in vivo are then compared to and identified based on the in vitro results. This approach was applied to three PET radioligands, [¹¹C]flumazenil, [¹⁸F]FE-PE2I, and [¹¹C]PBR28, resulting in the identification of five, two, and one radiometabolites, respectively. This procedure can easily be adopted to identify the radiometabolites produced in vivo from a variety of PET radioligands.     

Molecular and Functional Imaging Studies of Psychedelic Drug Action in Animals and Humans

Hallucinogens are a loosely defined group of compounds including LSD, N,N-dimethyltryptamines, mescaline, psilocybin/psilocin, and 2,5-dimethoxy-4-methamphetamine (DOM), which can evoke intense visual and emotional experiences. We are witnessing a renaissance of research interest in hallucinogens, driven by increasing awareness of their psychotherapeutic potential. As such, we now present a narrative review of the literature on hallucinogen binding in vitro and ex vivo, and the various molecular imaging studies with positron emission tomography (PET) or single photon emission computer tomography (SPECT). In general, molecular imaging can depict the uptake and binding distribution of labelled hallucinogenic compounds or their congeners in the brain, as was shown in an early PET study with N¹-([¹¹C]-methyl)-2-bromo-LSD ([¹¹C]-MBL); displacement with the non-radioactive competitor ketanserin confirmed that the majority of [¹¹C]-MBL specific binding was to serotonin 5-HT_2A receptors. However, interactions at serotonin 5HT_1A and other classes of receptors and pleotropic effects on second messenger pathways may contribute to the particular experiential phenomenologies of LSD and other hallucinogenic compounds. Other salient aspects of hallucinogen action include permeability to the blood–brain barrier, the rates of metabolism and elimination, and the formation of active metabolites. Despite the maturation of radiochemistry and molecular imaging in recent years, there has been only a handful of PET or SPECT studies of radiolabeled hallucinogens, most recently using the 5-HT_2A/2C agonist N-(2[¹¹CH₃O]-methoxybenzyl)-2,5-dimethoxy- 4-bromophenethylamine ([¹¹C]Cimbi-36). In addition to PET studies of target engagement at neuroreceptors and transporters, there is a small number of studies on the effects of hallucinogenic compounds on cerebral perfusion ([¹⁵O]-water) or metabolism ([¹⁸F]-fluorodeoxyglucose/FDG). There remains considerable scope for basic imaging research on the sites of interaction of hallucinogens and their cerebrometabolic effects; we expect that hybrid imaging with PET in conjunction with functional magnetic resonance imaging (fMRI) should provide especially useful for the next phase of this research.