Baeyer‐Villiger oxidation tuned to chemoselective conversion of non‐activated [18F]fluorobenzaldehydes to [18F]fluorophenols

A reaction pathway via oxidation of [¹⁸F]fluorobenzaldehydes offers a very useful tool for the no‐carrier‐added radiosynthesis of [¹⁸F]fluorophenols, a structural motive of several potential radiopharmaceuticals. A considerably improved chemoselectivity of the Baeyer‐Villiger oxidation (BVO) towards phenols was achieved, employing 2,2,2‐trifluoroethanol as reaction solvent in combination with Oxone or m‐CPBA as oxidation agent. The studies showed the necessity of H₂SO₄ addition, which appears to have a dual effect, acting as catalyst and desiccant. For example, 2‐[¹⁸F]fluorophenol was obtained with a RCY of 97% under optimised conditions of 80°C and 30‐minute reaction time. The changed performance of the BVO, which is in agreement with known reaction mechanisms via Criegee intermediates, provided the best results with regard to radiochemical yield (RCY) and chemoselectivity, i.e. formation of [¹⁸F]fluorophenols rather than [¹⁸F]fluorobenzoic acids. Thus, after a long history of the BVO, the new modification now allows an almost specific formation of phenols, even from electron‐deficient benzaldehydes. Further, the applicability of the tuned, chemoselective BVO to the n.c.a. level and to more complex compounds was demonstrated for the products n.c.a. 4‐[¹⁸F]fluorophenol (RCY 95%; relating to 4‐[¹⁸F]fluorobenzaldehyde) and 4‐[¹⁸F]fluoro‐m‐tyramine (RCY 32%; relating to [¹⁸F]fluoride), respectively.     

[18F]Fluoride recovery via gaseous [18F]HF

Acidification of target water with H₂SO₄ in a specially constructed glassy carbon/polyethylene apparatus allowed for recovery of up to 82% of [¹⁸F]fluoride as [¹⁸F]HF gas. The [¹⁸F]HF distillate was found to be acid‐free but moist; when passed through a solution of ^tBuPh₂SiOTf, it yielded [¹⁸F]^tBuPh₂SiF. The multivariate design of experiment showed that the key to high yield of [¹⁸F]HF was the efficient degassing of the reaction mixture.     

A first synthesis of 18F‐radiolabeled lapatinib: a potential tracer for positron emission tomographic imaging of ErbB1/ErbB2 tyrosine kinase activity

Fluorine‐18‐labeled lapatinib has been successfully synthesized for the first time by the reaction of a dimethylformamide solution of meta‐[¹⁸F]fluorobenzylbromide with a Boc‐protected lapatinib precursor fragment. The reaction proceeded in the presence of K₂CO₃ at 110 °C for 10 min in a microwave and was followed by Boc‐group deprotection with trifluoroacetic acid. The overall radiochemical yield of the reaction starting from the radiofluorination of the iodonium salt was 8–12% (uncorrected, n = 6) in a 140‐min synthesis time.     

How to increase the reactivity of [18F]fluoroethyl bromide: [18F]fluoroethylation of amine,phenol and amide functional groups with [18F]FEtBr, [18F]FEtBr/NaI and [18F]FEtOTf

[¹⁸F]Fluoroethyl bromide ([¹⁸F]FEtBr) is a useful synthetic precursor to synthesize ¹⁸F‐labeled compounds. However, the lower reactivity of [¹⁸F]FEtBr with amine, phenol and amide functional groups than that of [¹¹C]CH₃I partly limits its wide application in the synthesis of [¹⁸F]fluoroethylated compounds. The aim of this study was to increase the reactivity of [¹⁸F]FEtBr with various nucleophilic substrates for PET tracers containing amine, phenol and amide moieties. The present strategies included (1) adding NaI into the reaction mixture of [¹⁸F]FEtBr and substrate, where [¹⁸F]FEtI is reversibly formed and becomes more reactive; (2) converting [¹⁸F]FEtBr into much more reactive [¹⁸F]FEtOTf, similar to conversion of [¹¹C]CH₃I into [¹¹C]CH₃OTf. By these efforts, the [¹⁸F]fluoroethylation efficiency of various substrates containing amine, phenol and amide groups with [¹⁸F]FEtBr/NaI and [¹⁸F]FEtOTf was significantly improved, compared with the corresponding reaction efficiency with [¹⁸F]FEtBr. Copyright © 2003 John Wiley & Sons, Ltd.     

Synthesis of [18F]fluoroacetaldehyde. Application to [18F]fluoroethylation of benzylamine under reductive alkylation conditions

The radiosynthesis of a new [¹⁸F]fluoroalkylating agent, [¹⁸F]fluoroacetaldehyde, is described. It was produced using the Kornblum method by oxidation with dimethyl sulphoxide of 2‐[¹⁸F]fluoroethyl p‐toluenesulphonate ([¹⁸F]FETos). In these conditions the oxidation proceeds smoothly and rapidly to the selective conversion of tosyl esters of primary alcohols to aldehydes with no carboxylic acids being produced. The chemical identity of [¹⁸F]fluoroacetaldehyde was determined by comparing its chromatographic properties as well as those of its 2,4‐dinitrophenylhydrazone (2,4‐DNPH) derivative with those of, respectively, the standard fluoroacetaldehyde and its 2,4‐DNPH derivative. Standard fluoroacetaldehyde was prepared by oxidation of fluoroethanol with pyridinium dichromate and characterized as its 2,4‐DNPH derivative by mass spectrometry. To test its reactivity with amines under reductive alkylation conditions, [¹⁸F]fluoroacetaldehyde was reacted with benzylamine used as model substrate. The chemical identity of the resulting radiolabelled product was determined to be [¹⁸F]N‐(2‐fluoroethyl)‐benzylamine by comparing its chromatographic properties with those of the synthesized standard N‐(2‐fluoroethyl)‐benzylamine characterized by ¹⁹F and ¹H NMR spectroscopy and mass spectrometry. This new fluorine‐18 labelled synthon may find applications in radiolabelling peptide, protein and antibody fragments as well as in aldol condensation or in the Mannich reaction. Copyright © 2008 John Wiley & Sons, Ltd.     

Efficient automated syntheses of high specific activity 6‐[18F]fluorodopamine using a diaryliodonium salt precursor

6‐[¹⁸F]Fluorodopamine (6‐[¹⁸F]F‐DA) is a positron emission tomography radiopharmaceutical used to image sympathetic cardiac innervation and neuroendocrine tumors. Imaging with 6‐[¹⁸F]F‐DA is constrained, in part, by the bioactivity and neurotoxicity of 6‐[¹⁹F]fluorodopamine. Furthermore, routine access to this radiotracer is limited by the inherent difficulty of incorporation of [¹⁸F]fluoride into electron‐rich aromatic substrates. We describe the simple and direct preparation of high specific activity (SA) 6‐[¹⁸F]F‐DA from no‐carrier‐added (n.c.a.) [¹⁸F]fluoride. Incorporation of n.c.a. [¹⁸F]fluoride into a diaryliodonium salt precursor was achieved in 50–75% radiochemical yields (decay corrected to end of bombardment). Synthesis of 6‐[¹⁸F]F‐DA on the IBA Synthera® and GE TRACERlab FX‐FN automated platforms gave 6‐[¹⁸F]F‐DA in >99% chemical and radiochemical purities after HPLC purification. The final non‐corrected yields of 6‐[¹⁸F]F‐DA were 25 ± 4% (n = 4, 65 min) and 31 ± 6% (n = 3, 75 min) using the Synthera and TRACERlab modules, respectively. Efficient access to high SA 6‐[¹⁸F]F‐DA from a diaryliodonium salt precursor and n.c.a. [¹⁸F]fluoride is provided by a relatively subtle change in reaction conditions – replacement of a polar aprotic solvent (acetonitrile) with a relatively nonpolar solvent (toluene) during the critical radiofluorination reaction. Implementation of this process on common radiochemistry platforms should make 6‐[¹⁸F]F‐DA readily available to the wider imaging community.     

Synthesis of a phenolic precursor and its efficient O‐[18F]fluoroethylation with purified no‐carrier‐added [18F]2‐fluoroethyl brosylate as the labeling agent

[¹⁸F]2‐Fluoroethyl‐p‐toluenesulfonate also called [¹⁸F]2‐fluoroethyl tosylate has been widely used for labeling radioligands for positron emission tomography (PET). [¹⁸F]2‐Fluoroethyl‐4‐bromobenzenesulfonate, also called [¹⁸F]2‐fluoroethyl brosylate ([¹⁸F]F(CH₂)₂OBs), was used as an alternative radiolabeling agent to prepare [¹⁸F]FEOHOMADAM, a fluoroethoxy derivative of HOMADAM, by O‐fluoroethylating the phenolic precursor. Purified by reverse‐phase HPLC, the no‐carrier‐added [¹⁸F]F(CH₂)₂OBs was obtained in an average radiochemical yield (RCY) of 35%. The reaction of the purified and dried [¹⁸F]F(CH₂)₂OBs with the phenolic precursor was performed by heating in DMF and successfully produced [¹⁸F]FEOHOMADAM, after HPLC purification, in RCY of 21%. Copyright © 2013 John Wiley & Sons, Ltd.     

Hydroacylation of 4‐[18F]fluorobenzaldehyde: a novel method for the preparation of 4′‐[18F]phenylketones

To assess the potential of intermolecular hydroacylation reactions as a new fluorine‐18 labeling method, model reactions of [¹⁸F]fluorobenzaldehyde with three different olefins (1‐hexene ( 2a ), allylbenzene ( 2b ), and 3‐phenoxypropene ( 2c )) in the presence of Wilkinson's catalyst were performed. The procedure gave high radiochemical yields (38–62%) of [¹⁸F]fluorophenylketones with short reaction times (15 min). The intermolecular hydroacylation reaction provides a new method for the preparation of fluorine‐18 labeled compounds. Copyright © 2002 John Wiley & Sons, Ltd.     

Vacuum ultraviolet photon–mediated production of [18F]F2

The chemistry of F₂ and its derivatives are amenable to facile aliphatic or aromatic substitution, as well as electrophilic addition. The main limitation in the use of [¹⁸F]F₂ for radiopharmaceutical synthesis is the low specific activity achieved by the traditional methods of production. The highest specific activities, 55 GBq/μmol, for [¹⁸F]F₂ have been achieved so far by using electrical discharge in the post‐target production of [¹⁸F]F₂ gas from [¹⁸F]CH₃F. We demonstrate that [¹⁸F]F₂ is produced by illuminating a gas mixture of neon/F₂/[¹⁸F]CH₃F with vacuum ultraviolet photons generated by an excimer laser. We tested several illumination chambers and production conditions. The effects of the initial amount of [¹⁸F]F^‐, amount of carrier F₂, and number of 193‐nm laser pulses at constant power were evaluated regarding radiochemical yield and specific activity. The specific activity attained for [¹⁸F]F₂‐derived [¹⁸F]NFSi was 10.3 ± 0.9 GBq/μmol, and the average radiochemical yield over a wide range of conditions was 6.7% from [¹⁸F]F^‐. The production can be improved by optimization of the synthesis device and procedures. The use of a commercially available excimer laser and the simplicity of the process can make this method relatively easy for adaptation in radiochemistry laboratories.     

Radiosynthesis of 4‐[18F]fluoromethyl‐L‐phenylalanine and [18F]FET via a same strategy and automated synthesis module

Currently there is still a need for more potent amino acid analogues as tumour imaging agents for peripheral tumour imaging with PET as it was recently reported that the success of O‐(2′‐[¹⁸F]fluoroethyl)‐L ‐tyrosine ([¹⁸F]FET) is limited to brain, head and neck tumours. As the earlier described 2‐Amino‐3‐(2‐[¹⁸F]fluoromethyl‐phenyl)‐propionic acid (2‐[¹⁸F]FMP) suffered from intramolecular‐catalysed defluorination, we synthesized 2‐Amino‐3‐(4‐[¹⁸F]fluoromethyl‐phenyl)‐propionic acid (4‐[¹⁸F]FMP) as an alternative for tumour imaging with PET. Radiosynthesis of 4‐[¹⁸F]FMP, based on Br for [¹⁸F] aliphatic nucleophilic exchange, was performed with a customized modular Scintomics automatic synthesis hotbox^three system in a high overall yield of 30% and with a radiochemical purity of \gt 99%. 4‐[¹⁸F]FMP was found to be stable in its radiopharmaceutical formulation, even at high radioactivity concentrations. Additionally, for a comparative study, [¹⁸F]FET was synthesized using the same setup in 40% overall yield, with a radiochemical purity \gt 99%. The described automated radiosynthesis allows the production of two different amino acid analogues with minor alternations to the parameter settings of the automated system, rendering this unit versatile for both research and clinical practice. Copyright © 2009 John Wiley & Sons, Ltd.     

Development of novel PET probes, [18F]BCPP‐EF, [18F]BCPP‐BF,and [11C]BCPP‐EM for mitochondrial complex 1 imaging in the living brain

We developed three novel positron‐emission tomography (PET) probes, 2‐tert‐butyl‐4‐chloro‐5‐{6‐[2‐(2[¹⁸F]fluoroethoxy)‐ethoxy]‐pyridin‐3‐ylmethoxy}‐2H‐pyridazin‐3‐one ([¹⁸F]BCPP‐EF), 2‐tert‐butyl‐4‐chloro‐5‐[6‐(4‐[¹⁸F]fluorobutoxy)‐pyridin‐3‐ylmethoxy]‐2H‐pyridazin‐3‐one ([¹⁸F]BCPP‐BF), and 2‐tert‐butyl‐4‐chloro‐5‐{6‐[2‐(2‐[¹¹C]methoxy‐ethoxy)‐ethoxy]‐pyridin‐3‐ylmethoxy}‐2H‐pyridazin‐3‐one ([¹¹C]BCPP‐EM), for quantitative imaging of mitochondrial complex 1 (MC‐1) activity in vivo. These three PET probes were successfully labeled by nucleophilic [¹⁸F]fluorination or by [¹¹C]methylation of their corresponding precursor with sufficient radioactivity yield, good radiochemical purity, and sufficiently high specific radioactivity for PET measurement. The specificity of these probes for binding to MC‐1 was assessed with rotenone, a specific MC‐1 inhibitor, by a rat brain slice imaging method in vitro. Rat whole‐body imaging by small‐animal PET demonstrated that all probes showed high uptake levels in the brain as well as in the heart sufficient to image them clearly. The rank order of uptake levels in the brain and the heart just after injection was as follows: high in [¹⁸F]BCPP‐BF, intermediate in [¹¹C]BCPP‐EM, and low in [¹⁸F]BCPP‐EF. The kinetics of [¹⁸F]BCPP‐EF and [¹¹C]BCPP‐EM provided a reversible binding pattern, whereas [¹⁸F]BCPP‐BF showed nonreversible accumulation‐type kinetics in the brain and heart. Metabolite analyses indicated that these three compounds were rapidly metabolized in the plasma but relatively stable in the rat brain up to 60 min post‐injection. The present study demonstrated that [¹⁸F]BCPP‐EF could be a useful PET probe for quantitative imaging of MC‐1 activity in the living brain by PET.     

Automated synthesis and purification of [18F]fluoro‐[di‐deutero]methyl tosylate

Automated synthetic procedures of [¹⁸F]fluoro‐[di‐deutero]methyl tosylate on a GE TRACERlab FX F‐N module and a non‐commercial synthesis module have been developed. The syntheses included azeotropic drying of the [¹⁸F]fluoride, nucleophilic ¹⁸F‐fluorination of bis(tosyloxy)‐[di‐deutero]methane, HPLC purification and subsequent formulation of the synthesized [¹⁸F]fluoro‐[di‐deutero]methyl tosylate (d₂‐[¹⁸F]FMT) in organic solvents. Automation shortened the total synthesis time to 50 min, resulting in an average radiochemical yield of about 50% and high radiochemical purity (>98%). The possible application of this procedure to commercially available synthesis modules might be of significance for the production of deuterated ¹⁸F‐fluoromethylated imaging probes in the future. Copyright © 2013 John Wiley & Sons, Ltd.     

A facile automated synthesis of N‐succinimidyl 4‐[18F]fluorobenzoate ([18F]SFB) for 18F‐labeled cell‐penetrating peptide as PET tracer

A fully automated synthesis of N‐succinimidyl 4‐[¹⁸F]fluorobenzoate ([¹⁸F]SFB) was carried out by a convenient three‐step, one‐pot procedure on the modified TRACERlab FX_FN synthesizer, including [¹⁸F]fluorination of ethyl 4‐(trimethylammonium triflate)benzoate as the precursor, saponification of the ethyl 4‐[¹⁸F]fluorobenzoate with aqueous tetrapropylammonium hydroxide instead of sodium hydroxide, and conversion of 4‐[¹⁸F]fluorobenzoate salt ([¹⁸F]FBA) to [¹⁸F]SFB treated with N,N,N′,N′‐tetramethyl‐O‐(N‐succinimidyl)uranium tetrafluoroborate (TSTU). The purified [¹⁸F]SFB was used for the labeling of Tat membrane‐penetrating peptide (containing the Arg‐Lys‐Lys‐Arg‐Arg‐Arg‐Arg‐Arg‐Arg‐Arg‐Arg‐Pro‐Leu‐Gly‐Leu‐Ala‐Gly‐Glu‐Glu‐Glu‐Glu‐Glu‐Glu‐Glu sequence, [¹⁸F]CPP) through radiofluorination of lysine amino groups. The uncorrected radiochemical yields of [¹⁸F]SFB were as high as 25–35% (based on [¹⁸F]fluoride) (n=10) with a synthesis time of～40 min. [¹⁸F]CPP was produced in an uncorrected radiochemical yields of 10–20% (n=5) within 30 min (based on [¹⁸F]SFB). The radiochemical purities of [¹⁸F]SFB and [¹⁸F]CPP were greater than 95%. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis of 18F‐labelled biphenyls via SUZUKI cross‐coupling with 4‐[18F]fluoroiodobenzene

The SUZUKI reaction of organoboron compounds with 4‐[¹⁸F]fluoroiodobenzene has been developed as a novel radiolabelling technique in ¹⁸F chemistry. The cross‐coupling reaction of p‐tolylboronic acid with 4‐[¹⁸F]fluoroiodobenzene was used to screen different palladium complexes, bases and solvents. Optimized reaction conditions (Pd₂(dba)₃, Cs₂CO₃, acetonitrile, 60°C for 5 min) were further applied to the synthesis of various ¹⁸F‐labelled biphenyls bearing different functional groups. The reaction proceeded in excellent radiochemical yields of up to 94% within 5 min while showing good compatibility to many functional groups. Copyright © 2006 John Wiley & Sons, Ltd.     

Development of a new precursor‐minimizing base control method and its application for the automated synthesis and SPE purification of [18F]fluoromisonidazole ([18F]FMISO)

Bases such as potassium carbonate and potassium bicarbonate (KHCO₃) are essential for the elution of trapped [¹⁸F]fluoride from ion exchange cartridges and for the prevention of [¹⁸F]fluoride adsorption on the silica glass vial during the preparation of radiopharmaceuticals for positron emission tomography imaging. However, these bases promote the chemical decomposition of precursor compounds and the creation of unwanted organic impurities. Thus, the goal of the current study was to develop a new method for synthesizing [¹⁸F]fluoride‐labeled radiopharmaceuticals (e.g., [¹⁸F]fluoromisonizadole ([¹⁸F]FMISO)) that permits the fine control of base concentrations while minimizing adverse events. Non‐decay‐corrected radiochemical yields of 25.1 ± 5.0% and 13.3 ± 5.1% (n = 3) were achieved after solid‐phase extraction purification using automatic synthesis with GE TRACERlab MX and KHCO₃ at concentrations of 14.1 and 33.0 µmol, respectively, and 1 mg of precursor (1‐(2′‐nitro‐1′‐imidazolyl)‐2‐O‐tetra‐hydropyranyl‐3‐O‐toluenesulfonyl propanediol (NITTP)). The newly developed synthesis protocol with fine base control and low precursor content permits high radiochemical yields with minimal impurities. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis,labelling and first evaluation of [18F]R91150 as a serotonin 5‐HT2A receptor antagonist for PET

In psychiatric disorders such as anxiety, depression and schizophrenia, 5‐HT_2A receptors play an important role. In order to investigate them in vivo there is an increasing interest in selective and high‐affinity radioligands for receptor binding studies using positron emission tomography (PET). Since available radioligands have disadvantages, R91150, which is a selective and high‐affinity ligand for 5‐HT_2A receptors, was labelled with fluorine‐18. This was accomplished in six steps via 4‐[¹⁸F]fluorophenol and 1‐(3‐bromopropoxy)‐4‐[¹⁸F]fluorobenzene within 190 min starting from no‐carrier‐added [¹⁸F]fluoride. The overall radiochemical yield was 3.8±2% and the specific activity was at least 335 GBq/µmol at the end of the synthesis. First ex vivo studies in mice proved the uptake of [¹⁸F]R91150 in the brain. Radiometabolite studies revealed no radiometabolites in the brain, whereas in the plasma at least two could be detected 30 min p.i. Further preclinical studies are encouraged to evaluate the potential of this new 5‐HT_2A ligand as a radiotracer for PET. Copyright © 2008 John Wiley & Sons, Ltd.     

Fast [18F]FDG synthesis by alkaline hydrolysis on a low polarity solid phase support

The synthesis of 2‐deoxy‐2‐[¹⁸F]fluoro‐D ‐glucose ([¹⁸F]FDG) has been simplified by the use of a ^tC18 Sep Pak cartridge to effect purification and hydrolysis of the tetraacetylated [¹⁸F]fluoro‐glucose compound ([¹⁸F]TAG). After radiolabelling, this derivative was trapped on a solid phase extraction (SPE) cartridge and the residual reaction solvent (CH₃CN), reagents (K222, K₂CO₃,…) and by‐products removed by washing the support with water. After this cleaning step, the acetyl groups were cleaved on the same ^tC₁₈ column using 2N sodium hydroxide. This fast reaction proceeded near quantitatively (>98%) at room temperature in less than 2 min. The [¹⁸F]FDG was then recovered with a small amount of water, neutralized with a slight excess of 2N hydrochloric acid, buffered for pH with a citrate solution and finally purified on a neutral alumina oxide and a second ^tC18 column. After filtration, the radiochemical yield of this [¹⁸F]FDG isotonic solution after more than 100 production runs was found to be very reliable and reproducible (70±6% decay corrected). The synthesis time was about 22 min. Quality controls showed that the radiochemical purity was higher than 98% and in any case no [¹⁸F]FDM was detected. Only traces of 2‐chloro‐2‐deoxy‐glucose (ClDG) were found in the final sample (64±9 μg/ batch of 16 ml). [¹⁸F]FDG specific activity averaged between 1 and 20 Ci/µmol (EOS). No evaporation and use of ion retardation resin (AG11A8) are required. Moreover, this new approach is suitable for complete remote operation using available single use medical components. Copyright © 2002 John Wiley & Sons, Ltd.     

Preparation of 3′‐deoxy‐3′‐[18F]fluorothymidine ([18F]FLT) in ionic liquid, [bmim][OTf]

Although 3′‐deoxy‐3′‐[¹⁸F]fluorothymidine ([¹⁸F]FLT) is a prospective radiopharmaceutical for the imaging of proliferating tumor cell, it is difficult to prepare large amount of [¹⁸F]FLT. We herein describe the preparation of [¹⁸F]FLT in an ionic liquid, [bmim][OTf] (1‐butyl‐3‐methyl‐imidazolium trifluoromethanesulfonate). At optimized condition, [¹⁸F]fluorinationin ionic liquid with 5 µl of 1 M KHCO₃ and 5 mg of the precursor yielded 61.5 ± 4.3% (n=10). Total elapsed time was about 70 min including HPLC purification. The rapid synthesis of [¹⁸F]FLT can be achieved by removing all evaporation steps. Overall radiochemical yield and radiochemical purity were 30 ± 5% and >95%, respectively. This method can use a small amount of a nitrobenzenesulfonate precursor and can be adapted for automated production. Copyright © 2006 John Wiley & Sons, Ltd.     

Comparison of pathways to the versatile synthon of no‐carrier‐added 1‐bromo‐4‐[18F]fluorobenzene

The availability of no‐carrier‐added (n.c.a.) 1‐bromo‐4‐[¹⁸F]fluorobenzene with high radiochemical yields is important for ¹⁸F‐arylation reactions using metallo‐organic 4‐[¹⁸F]fluorophenyl compounds (e.g. of lithium or magnesium) or Pd‐catalyzed coupling. In this study, different methods for the preparation of 1‐bromo‐4‐[¹⁸F]fluorobenzene by nucleophilic aromatic substitution reactions using n.c.a. [¹⁸F]fluoride were examined. Of six pathways compared, symmetrical bis‐(4‐bromphenyl)iodonium bromide proved most useful to achieve the title compound in a direct, one‐step nucleophilic substitution with a radiochemical yield (RCY) of 65% within 10 min. Copyright © 2004 John Wiley & Sons, Ltd.     

Automated cGMP‐compliant radiosynthesis of [18F]‐(E)‐PSS232 for brain PET imaging of metabotropic glutamate receptor subtype 5

(E)‐3‐(Pyridin‐2‐yl ethynyl)cyclohex‐2‐enone O‐(3‐(2‐[¹⁸F]‐fluoroethoxy)propyl) oxime ([¹⁸F]‐(E)‐PSS232, [¹⁸F] 2a ) is a recently developed radiotracer that can be used to visualize metabotropic glutamate receptor subtype 5 (mGlu₅) in vivo. The mGlu₅ has become an attractive therapeutic and diagnostic target owing to its role in many neuropsychiatric disorders. Several carbon‐11‐labeled and fluorine‐18‐labeled radiotracers have been developed to measure mGlu₅ receptor occupancy in the human brain. The radiotracer [¹⁸F] 2a , which is used as an analogue for [¹¹C]ABP688 ([¹¹C] 1 ) and has a longer physical half‐life, is a selective radiotracer that exhibits high binding affinity for mGlu₅. Herein, we report the fully automated radiosynthesis of [¹⁸F] 2a using a commercial GE TRACERlab? FX‐_FN synthesizer for routine production and distribution to nearby satellite clinics. Nucleophilic substitution of the corresponding mesylate precursor with cyclotron‐produced [¹⁸F]fluoride ion at 100°C in dimethyl sulfoxide (DMSO), followed by high‐performance liquid chromatography (HPLC) purification and formulation, readily provided [¹⁸F] 2a with a radiochemical yield of 40 ± 2% (decay corrected, n = 5) at the end of synthesis. Radiochemical purity for the [¹⁸F]‐(E)‐conformer was greater than 95%. Molar activity was determined to be 63.6 ± 9.6 GBq/μmol (n = 5), and the overall synthesis time was 70 minutes.