Synthesis,bioconjugation and stability studies of [18F]ethenesulfonyl fluoride

Fluorine‐18 labelled prosthetic groups (PGs) are often necessary for radiolabelling sensitive biological molecules such as peptides and proteins. Several shortcomings, however, often diminish the final yield of radiotracer. In an attempt to provide higher yielding and operationally efficient tools for radiolabelling biological molecules, we describe herein the first radiochemical synthesis of [¹⁸F]ethenesulfonyl fluoride ([¹⁸F]ESF) and its Michael conjugation with amino acids and proteins. The synthesis of [¹⁸F]ESF was optimised using a microfluidic reactor under both carrier‐added (c.a.) and no‐carrier‐added (n.c.a.) conditions, affording, in a straightforward procedure, 30‐50% radiochemical yield (RCY) for c.a. [¹⁸F]ESF and 60‐70% RCY for n.c.a. [¹⁸F]ESF. The conjugation reactions were performed at room temperature using 10 mg/mL precursor in aqueous/organic solvent mixtures for 15 min. The radiochemical stability of the final conjugates was evaluated in injectable formulation and rat serum, and resulted strongly substrate dependent and generally poor in rat serum. Therefore, in this work we have optimised a straightforward synthesis of [¹⁸F]ESF and its Michael conjugation with model compounds, without requiring chromatographic purification. However, given the general low stability of the final products, further studies will be required for improving conjugate stability, before assessing the use of this PG for PET imaging.     

[18F]Fluoride labelling of macromolecules in aqueous conditions: silicon and boroaryl‐based [18F]fluorine acceptors, [18F]FDG conjugation and Al18F chelation

[¹⁸F]Fluorination is usually carried out by nucleophilic substitution reactions entailing the use of stringent conditions. A number of novel techniques including silicon and boron‐based fluoride acceptor molecules, [¹⁸F]fluoro‐2‐deoxy‐ d ‐glucose and chelation of the Al¹⁸F complex have been employed recently to achieve [¹⁸F]radiolabelling of macromolecules. These approaches are reviewed herein. Copyright © 2012 John Wiley & Sons, Ltd.     

Batch‐mode microfluidic radiosynthesis of N‐succinimidyl‐4‐[18F]fluorobenzoate for protein labelling

The batch microfluidic technology is a promising system for sequential chemical steps combining the advantages of micro‐scale reactions, while addressing some shortcomings of flow‐through systems. We report herein the convenient three‐step, one‐pot synthesis and purification of [¹⁸F]SFB. [¹⁸F]SFB is a radiolabelling agent that can be used to label sensitive biomolecules, which are not accessible by direct nucleophilic ¹⁸F‐fluorination. Five sequential steps were performed with a batch microfluidic device to obtain the short‐lived positron‐emitter‐labelled molecule. Aqueous [¹⁸F]fluoride was concentrated and further eluted to a microreactor for evaporation. Nucleophilic ¹⁸F‐fluorination of the precursor was carried out at high temperature, prior to hydrolysis and subsequent activation of the 4‐[¹⁸F]fluorobenzoyl group. Purification on miniaturized solid‐phase finally afforded [¹⁸F]SFB in 25 min and 55±6% yield (not decay‐corrected) and >98% radiochemical purity. In this study, microfluidic prepared [¹⁸F]SFB could be further successfully used for labelling the epidermal growth factor protein. These results illustrate how microfluidic batch devices are advantageous for producing radiotracers for molecular imaging, e.g. Positron emission tomography. The technology offers many benefits such as the possibility to use much smaller quantities of starting material, reduced reaction time combined with improved efficiency, and easier purification. Copyright © 2010 John Wiley & Sons, Ltd.     

Improved synthesis of 4‐[18F]fluoro‐m‐hydroxyphenethylguanidine using an iodonium ylide precursor

Fluorine‐18 labeled hydroxyphenethylguanidines were recently developed in our laboratory as a new class of PET radiopharmaceuticals for quantifying regional cardiac sympathetic nerve density in heart disease patients. Studies of 4‐[¹⁸F]fluoro‐m‐hydroxyphenethylguanidine ([¹⁸F]4F‐MHPG) and 3‐[¹⁸F]fluoro‐p‐hydroxyphenethylguanidine ([¹⁸F]3F‐PHPG) in human subjects have shown that these radiotracers can be used to generate high‐resolution maps of regional sympathetic nerve density using the Patlak graphical method. Previously, these compounds were synthesized using iodonium salt precursors, which provided sufficient radiochemical yields for on‐site clinical PET studies. However, we were interested in exploring new methods that could offer significantly higher radiochemical yields. Spirocyclic iodonium ylide precursors have recently been established as an attractive new approach to radiofluorination of electron‐rich aromatic compounds, offering several advantages over iodonium salt precursors. The goal of this study was to prepare a spirocyclic iodonium ylide precursor for synthesizing [¹⁸F]4F‐MHPG and evaluate its efficacy in production of this radiopharmaceutical. Under optimized automated reaction conditions, the iodonium ylide precursor provided radiochemical yields averaging 7.8% ± 1.4% (n = 8, EOS, not decay corrected), around threefold higher than those achieved previously using an iodonium salt precursor. With further optimization and scale‐up, this approach could potentially support commercial distribution of [¹⁸F]4F‐MHPG to PET centers without on‐site radiochemistry facilities.     

New radiolabelling chemistry: synthesis of phosphorus–[18F]fluorine compounds

The feasibility of synthesizing compounds containing the P–¹⁸F bond has been demonstrated by labelling the pesticide, cholinesterase inhibitor Dimefox (N,N,N′N′‐tetramethylphosphorodiamidic fluoride) with F‐18. Radiolabelling was achieved in high radiochemical yield (96%) by nucleophilic substitution of the chloro group attached to phosphorus, in the oxidation state P(V), by ¹⁸F^? (activated with tetrabutylammonium carbonate in acetonitrile). Given the large number of important biological molecules possessing phosphorus such as oligonucleotides, phospholipids as well as phosphorylated proteins, sugars and steroids, this new labelling chemistry may provide an additional route to radiolabelling these biologically important compounds for use in PET. Copyright © 2005 John Wiley & Sons, Ltd.     

[18F]Fluoropropylsulfonyl chloride: a new reagent for radiolabeling primary and secondary amines for PET imaging

In vivo molecular imaging with positron emission tomography (PET) requires the preparation of an appropriate positron‐emitting radiotracer. New methods for the introduction of F‐18 into biologically interesting molecules could increase the availability of specific PET radiotracers and increase the application of PET to the study of human diseases. In this work, [¹⁸F]fluoropropylsulfonyl chloride was synthesized from 3‐toluenesulfonyloxypropyl thiocyanate in two steps and was successfully incorporated into molecules containing a reactive amino group. Both a primary amine, L‐phenylalanine ethyl ester hydrochloride, and a secondary amine, 1‐(2‐methoxyphenyl)‐piperazine, were successfully radiolabeled by this method. The entire radiochemical synthesis required 90 min. The products were obtained in 25.7±2.3% (n=3) and 22.8±9.1% (n=6) (EOB). This method provides a useful and easy way to make new F‐18 labeled radiopharmaceuticals for PET imaging. Copyright © 2008 John Wiley & Sons, Ltd.     

The development of an automated and GMP compliant FASTlab™ Synthesis of [18F]GE‐180; a radiotracer for imaging translocator protein (TSPO)

The level of the translocator protein (TSPO) increases dramatically in microglial cells when the cells are activated in response to neuronal injury and insult. The radiotracer [¹⁸F]GE‐180 binds selectively and with high affinity to TSPO and can therefore be used to measure neuroinflammation in a variety of disease states. An optimized, automated synthesis of [¹⁸F]GE‐180 has been developed for the GE FASTlab? synthesizer. The entire process takes place on the single‐use cassette. The radiolabelling is performed by nucleophilic fluorination of the S‐ enantiomer mesylate precursor. The crude product is purified post‐radiolabelling using two solid‐phase extraction cartridges integrated on the cassette. Experimental design and multivariate data analysis were used to assess the robustness, and critical steps were optimized with respect to efficacy and quality. The average radiochemical yield is 48% (RSD 6%, non‐decay corrected), and the synthesis time including purification is approximately 43 min. The radiochemical purity is ≥95% for radioactive concentration ≤1100 MBq/mL. The total amount of precursor‐related chemical impurities is 1–2 µg/mL. The use of solid‐phase extraction purification results in a robust GMP compliant process with a product of high chemical and radiochemical purity and consistent performance across positron emission tomography (PET) centers.     

Synthesis and evaluation of two fluorine‐18 labelled phenylbenzothiazoles as potential in vivo tracers for amyloid plaque imaging

Uncharged derivatives of thioflavin‐T have known in vitro and in vivo affinity for amyloid β. We synthesized and evaluated two derivatives with a fluorine‐18 labelled fluoropropoxy substituent either at the 6‐position or at the 2′‐position of the 2‐(4′‐aminophenyl)‐1,3‐benzothiazole core with the aim to get suitable radiotracers to perform amyloid plaque imaging. The fluorine‐18 labelled compounds were obtained by nucleophilic substitution of the corresponding tosyl precursors with [¹⁸F]fluoride with a radiochemical yield of 50%, yielding 6‐(3′′‐[¹⁸F]fluoropropoxy)‐2‐(4′‐aminophenyl)‐1,3‐benzothiazole ([¹⁸F]2) and 2‐[2′‐(3′′‐[¹⁸F]fluoropropoxy)‐4′‐aminophenyl]‐1,3‐benzothiazole ([¹⁸F]3) with a specific activity between 33 and 51 GBq/µmol. The identity of the radiolabelled compounds was confirmed using radio‐LC‐MS and by comparing retention times on RP‐HPLC. Biodistribution studies in healthy mice showed for both compounds a relatively high initial brain uptake, which was significantly higher for [¹⁸F]2 than for [¹⁸F]3 (4.5% ID/g versus 3.0% ID/g, p<0.05). Wash‐out from control brain was faster for [¹⁸F]3. In vitro binding affinity tests using human AD brain homogenates revealed that only compound 2 has affinity for fibrillar amyloid β (K_i=14.5 nM). This was confirmed by the incubation of transgenic APP mouse brain sections with the cold compounds, where 3 did not stain any structure whereas 2 stained amyloid plaques present in APP mouse brain. These data suggest that [¹⁸F]2 may be a useful tracer for in vivo visualization of fibrillar amyloid β. Copyright © 2009 John Wiley & Sons, Ltd.     

Automation of [18F]fluoroacetaldehyde synthesis: application to a recombinant human interleukin‐1 receptor antagonist (rhIL‐1RA)

[¹⁸F]Fluoroacetaldehyde is a biocompatible prosthetic group that has been implemented pre‐clinically using a semi‐automated remotely controlled system. Automation of radiosyntheses permits use of higher levels of [¹⁸F]fluoride whilst minimising radiochemist exposure and enhancing reproducibility. In order to achieve full‐automation of [¹⁸F]fluoroacetaldehyde peptide radiolabelling, a customised GE Tracerlab FX‐FN with fully programmed automated synthesis was developed. The automated synthesis of [¹⁸F]fluoroacetaldehyde is carried out using a commercially available precursor, with reproducible yields of 26% ± 3 (decay‐corrected, n = 10) within 45 min. Fully automated radiolabelling of a protein, recombinant human interleukin‐1 receptor antagonist (rhIL‐1RA), with [¹⁸F]fluoroacetaldehyde was achieved within 2 h. Radiolabelling efficiency of rhIL‐1RA with [¹⁸F]fluoroacetaldehyde was confirmed using HPLC and reached 20% ± 10 (n = 5). Overall RCY of [¹⁸F]rhIL‐1RA was 5% ± 2 (decay‐corrected, n = 5) within 2 h starting from 35 to 40 GBq of [¹⁸F]fluoride. Specific activity measurements of 8.11–13.5 GBq/µmol were attained (n = 5), a near three‐fold improvement of those achieved using the semi‐automated approach. The strategy can be applied to radiolabelling a range of peptides and proteins with [¹⁸F]fluoroacetaldehyde analogous to other aldehyde‐bearing prosthetic groups, yet automation of the method provides reproducibility thereby aiding translation to Good Manufacturing Practice manufacture and the transformation from pre‐clinical to clinical production.     

Access to 18F‐labelled isoxazoles by ruthenium‐promoted 1,3‐dipolar cycloaddition of 4‐[18F]fluoro‐N‐hydroxybenzimidoyl chloride with alkynes

4‐[¹⁸F]Fluoro‐N‐hydroxybenzimidoyl chloride (¹⁸FBIC), an ¹⁸F‐labelled aromatic nitrile oxide, was developed as building block for Ru‐promoted 1,3‐dipolar cycloaddition with alkynes. ¹⁸FBIC is obtained in a one‐pot synthesis in up to 84% radiochemical yield (RCY) starting from [¹⁸F]fluoride with 4‐[¹⁸F]fluorobenzaldehyde (¹⁸FBA) and 4‐[¹⁸F]fluorobenzaldehyde oxime (¹⁸FBAO) as intermediates, by reaction of ¹⁸FBAO with N‐chlorosuccinimide (NCS). ¹⁸FBIC was found to be a suitable and stable synthon to give access to ¹⁸F‐labelled 3,4‐diarylsubstituted isoxazoles by [Cp*RuCl(cod)]‐catalysed 1,3‐dipolar cycloaddition with various alkynes. So the radiosynthesis of a fluorine‐18–labelled COX‐2 inhibitor [¹⁸F] 1b , a close derivative of valdecoxib, was performed with ¹⁸FBIC and 1‐ethynyl‐4‐(methylsulfonyl)benzene, providing [¹⁸F] 1b in up to 40% RCY after purification in 85 minutes. The application of ¹⁸FBIC as a building block in the synthesis of ¹⁸F‐labelled heterocycles will generally extend the portfolio of available PET radiotracers.     

Biological evaluation of new [18F]F‐labeled synthetic amino acid derivatives as oncologic radiotracers

The present study evaluated the tumoral uptake of the novel synthetic amino acid positron emission tomography (PET) tracers (S)‐2‐amino‐3‐(4‐([¹⁸F]fluoromethyl)‐1H‐1,2,3‐triazol‐1‐yl)propanoic acid (AMC‐101), (S)‐2‐amino‐4‐(4‐([¹⁸F]fluoromethyl)‐1H‐1,2,3‐triazol‐1‐yl)butanoic acid (AMC‐102), and (S)‐2‐amino‐5‐(4‐([¹⁸F]fluoromethyl)‐1H‐1,2,3‐triazol‐1‐yl)pentanoic acid (AMC‐103), all of which are (S)‐2‐amino‐(4‐([¹⁸F]fluoromethyl)‐1H‐1,2,3‐triazol‐1‐yl)alkyl acids. In vitro cellular uptake was investigated using the rat glioma cell lines 9L and C6. In vitro competitive inhibition tests were performed to identify the involvement of specific amino acid transporters. In vivo dynamic PET images of 9L xenograft tumor‐bearing model mice were acquired over 2 h after AMC administration. [¹⁸F]FDOPA PET studies were performed with and without S‐carbidopa pretreatment for comparison. All three AMCs exhibited good in vitro cell uptake through the L and alanine‐serine‐cysteine transporters and enabled clear tumor visualization on PET, leaving the brain devoid of the tracer. Thirty minutes after injection, the mean tumor standardized uptake values were 1.59 ± 0.05, 1.89 ± 0.27, and 1.74 ± 0.13 for AMC‐101, AMC‐102, and AMC‐103, respectively. Although the tumor uptake values of AMCs were lower than that of [¹⁸F]FDOPA with S‐carbidopa pretreatment, AMCs enabled higher contrast images with lower background activity compared with [¹⁸F]FDOPA with S‐carbidopa pretreatment. Our results indicate the potential uses of these new synthetic amino acids as oncologic radiotracers.     

Rapid,one‐step,high yielding 18F‐labeling of an aryltrifluoroborate bioconjugate by isotope exchange at very high specific activity

A rapid, single step, aqueous ¹⁸F‐labeling method that proceeds under mild conditions to provide radiotracers in high radiochemical yield and at high specific activity represents a long‐standing challenge. Arylboronates capture aqueous ¹⁸F‐fluoride ion in buffered pH 2–3 at moderate temperature to provide a highly polar ¹⁸F‐ArBF₃^? anion. Similarly, ¹⁹F‐¹⁸F isotope exchange on a ¹⁹F‐ArBF₃^? should create an ¹⁸F‐ArBF₃^?. We hypothesized that this reaction would proceed in volumes that would be amenable to the high levels of ¹⁸F‐activity used in clinical hospitals. In order to measure both radiochemical and chemical yields, along with specific activity, we linked an alkyne‐¹⁹F‐ArBF₃^? to rhodamine azide by standard click chemistry to afford a precursor Rh‐¹⁹F‐ArBF₃^?. This precursor was aliquoted in portions of 50 nmol and lyophilized for on‐demand use. Using robotic manipulators in a hot cell, we combined >29.6 GBq (800 mCi) and 50 nmol of the Rh‐¹⁹F‐ArBF₃^? in aqueous dimethylformamide at buffered pH 2–3. Following mild heating (40 °C) for 10‐15 min, the reaction was quenched and analyzed. We observed radiochemical yields of 50% and specific activities of nearly 555 GBq/µmol (15 Ci/µmol). Similar radiochemical yields and slightly lower specific activities were also obtained with ~400 mCi (n = 2). With radiochemical yields in the hundreds of millicuries and specific activities that are 3–10‐fold higher than most radiotracers, this method is very attractive method for preparing clinically useful radiotracers. Moreover, the ability to produce tracers at extraordinarily high specific activities expands the distribution time window from production labs to distant positron emission tomography scanners. Copyright © 2012 John Wiley & Sons, Ltd.     

“In‐loop” 18F‐fluorination: A proof‐of‐concept study

There is a great demand to develop more cost‐efficient and robust manufacturing processes for fluorine‐18 (¹⁸F) labelled compounds and radiopharmaceuticals. Herein, we present to our knowledge the first radiofluorination “in‐loop,” where [¹⁸F]triflyl fluoride was used as the labelling agent. Initial development of the “in‐loop” [¹⁸F]fluorination method was optimized by reacting [¹⁸F]triflyl fluoride with 1,4‐dinitrobenzene to form [¹⁸F]1‐fluoro‐4‐nitrobenzene. This methodology was then applied for the syntheses of two well‐known radiopharmaceuticals, namely, [¹⁸F]T807 for imaging of tau protein and [¹⁸F]FEPPA for imaging the translocator protein 18 KDa. Both radiotracers were synthesized and formulated using an automated radiosynthesis module with nondecay corrected radiochemical yields of 27% and 29% (relative [¹⁸F]F^?), respectively. The overall syntheses times for [¹⁸F]T807 and [¹⁸F]FEPPA were 65 and 55 minutes, respectively. In these cases, our “in‐loop” radiofluorination methodology enabled us to obtain equal or superior yields compared with conventional reactions in a vial. The radiochemical purities were more than 99%, and the molar activities were more than 350 GBq/μmol at the end‐of‐synthesis for both radiotracers. This novel method is simple, efficient, and allows for a reliable production of radiofluorinated compounds and radiopharmaceuticals.     

Detailed radiosynthesis of [18F]mG4P027 as a positron emission tomography radiotracer for mGluR4

We report here the detailed radiosynthesis of [¹⁸F] mG4P027 , a metabotropic glutamate receptor 4 (mGluR4) PET radiotracer, which showed superior properties to the currently reported mGluR4 radiotracers. The radiosynthesis in the automated system has been challenging, therefore we disclose here the major limiting factors for the synthesis via step-by-step examination. And we hope this thorough study will help its automation for human use in the future.     

Radiolabelling rituximab with 99mTc in three steps procedure

Lymphomas are the most frequent haematological malignancy. In non‐Hodgkin's lymphomas (NHL), more than 90% of tumor cells express the cluster of differentiation (CD) 20 antigen. At the end of frontline therapy, the evaluation of remission is based on computed tomography (CT) and positron emission tomography coupled with computer tomography (PET/CT) with [¹⁸F]‐fluorodeoxyglucose ([¹⁸F]FDG). Unfortunately, these techniques are not specific and cannot distinguish residual active tumor from inflammation. The aim of this study was to develop a specific radiotracer of NHL CD 20+ cells for clinical applications. The radiolabelling technique presented, based on the use of tricarbonyl compound, does not include an antibody reduction because this step could damage the protein. Actually, rituximab, an anti‐CD 20 chimeric antibody used for the treatment of these NHL, was radiolabelled with Isolink® ^99mTc‐tricarbonyl compound in a three‐step procedure without using a specific antibody reducer. Radiolabelling yield was greater than 97%. In vitro experiments showed a conservation of antibody integrity. In vivo experiments using Single‐photon emission computed tomography/CT showed significant tumor targeting 24 h after injection of the radiotracer. It was consequently possible to develop an immunoradiolabelling method to specifically detect the residual disease. As this procedure is fast, reproducible and gentle, it will be possible to comply with Good Manufacturing Practices.     

Radiosyntheses and reactivities of novel [18F]2‐fluoroethyl arylsulfonates

[¹⁸F]2‐Fluoroethyl tosylate ([¹⁸F]FEOX, X=Ts) is widely used for labeling radiotracers for positron emission tomography (PET). Little work has been reported on syntheses of other [¹⁸F]2‐fluoroethyl arylsulfonates ([¹⁸F]FEOX) that bear a less electron‐rich aryl group, even though these might offer enhanced reactivities. Thus, a series of novel [¹⁸F]FEOX (X=benzenesulfonyl, brosyl, nosyl, 3,4‐dibromobenzenesulfonyl) were synthesized and reactivities compared to [¹⁸F]FEOTs. Precursors for radiolabeling (bis‐ethylene glycol arylsulfonates) and reference FEOX were synthesized (alcohol+arylsulfonyl chloride+KOSiMe₃ in THF). Regardless of substitution pattern, [¹⁸F]FEOX (110°C, 5 min, acetonitrile) were obtained in similar decay‐corrected isolated radiochemical yields (RCY; 47–53%). All [¹⁸F]FEOX gave excellent RCYs (64–87%) of the dopamine uptake radioligand, [¹⁸F]FECNT (130°C, 10 min, acetonitrile). The 3,4‐dibromobenzensulfonate gave the highest RCY of [¹⁸F]FECNT (87%) and this HPLC‐purified labeling agent was used directly for efficient [¹⁸F]FECNT production. When the secondary aniline of an amyloid probe (HM‐IMPY) or p‐nitrophenol was reacted with [¹⁸F]FEOX, RCYs were appreciably higher for brosylate and nosylate than for tosylate, while 3,4‐dibromobenzenesulfonate again gave the highest RCY. Owing to the high reactivity of the new [¹⁸F]FEOX and their ease of syntheses via stable precursors, such agents (particularly 3,4‐dibromobenzenesulfonate) should be considered as alternatives to [¹⁸F]FEOTs. Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis of [18F]Fluoroalanine and [18F]Fluorotamoxifen for Imaging Breast Tumors

Abstract

To develop ligands for imaging breast tumors, [¹⁸F]fluoro analogue of tamoxifen and [¹⁸F]fluoro-alanine were radiosynthesized. In vivo biodistribution studies were performed in mammary tumor-bearing rats. In studies on the biodistribution of an [¹⁸F]fluoro analogue of tamoxifen, tumor uptake decreased when rats were pretreated with diethylstilbestrol (DES), suggesting that tracer uptake in tumors was receptor-mediated. An estrogen receptor assay indicated that tumors have a receptor density of 7.5 fmol/mg protein. Studies of the distribution of [¹⁸F]fluoroalanine in tissue showed that the tumor-to-tissue ratio increases as a function of time. Positron emission tomography (PET) images of tumor-bearing rats demonstrated that tumors can be visualized 1 h after rats are injected with an [¹⁸F]fluoro analogue of tamoxifen. PET imaging of pigs after injection of 10 mCi of [¹⁸F]fluoro analogue of tamoxifen showed uterine uptake that could be blocked by DES (50 mg). The findings suggest that both radiotracers are useful for imaging breast tumors.     

Fully automated synthesis and coupling of [18F]FBEM to glutathione using the iPHASE FlexLab module

Site‐specific radiolabelling of peptides or antibodies using [¹⁸F]FBEM is often preferred over non‐site‐specific radiolabelling with [¹⁸F]SFB because it does not affect the affinity of the antibody to its target. Unfortunately, the synthesis of [¹⁸F]FBEM and its conjugation to thiol containing macromolecules requires some manual intervention, which leads to radiation exposure of the radiochemist. In this publication, we report on the complete automation of [¹⁸F]FBEM production and its subsequent conjugation to glutathione using a slightly modified iPHASE FlexLab module. [¹⁸F]FBEM was produced in 1.185 ± 0.168 GBq (15–20%; n = 10; 0.75 ± 0.106 GBq non‐decay corrected) with a specific activity of 57 ± 10 GBq/µmol. Radiochemical purity was 97 ± 1% and the synthesis time including HPLC purification and reformulation was 70 min. After evaporation to dryness, [¹⁸F]FBEM was conjugated to glutathione in PBS buffer pH 7.4 in quantitative yields. This fully automated method does not require any manual intervention and therefore reduces the radiation exposure to the operator.     

The first enzymatic method for C–18F bond formation: the synthesis of 5′‐[18F]‐fluoro‐5′‐deoxyadenosine for imaging with PET

The use of the key enzyme involved in carbon–fluorine bond formation in Streptomyces cattleya catalysing the formation of 5′‐fluoro‐5′‐deoxyadenosine (5′‐FDA) from fluoride ion and S‐adenosyl‐l‐methionine (SAM) was explored for its potential application in fluorine‐18 labelling of the adenosine derivative. Enzymatic radiolabelling of [¹⁸F]‐5′‐FDA was successfully carried out starting from SAM and [¹⁸F]HF when the concentration of the enzyme preparation was increased from sub‐mg/ml values to mg/ml values. The purity of the enzyme had no measurable effect on the radiochemical yield of the reaction and the radiochemical purity of [¹⁸F]‐5′‐FDA. Copyright © 2003 John Wiley & Sons, Ltd.     

cGMP production of the radiopharmaceutical [18F]MK‐6240 for PET imaging of human neurofibrillary tangles

Fluorine‐18–labelled 6‐(fluoro)‐3‐(1H‐pyrrolo[2,3‐c]pyridin‐1‐yl)isoquinolin‐5‐amine ([¹⁸F]MK‐6240) is a novel potent and selective positron emission tomography (PET) radiopharmaceutical for detecting human neurofibrillary tangles, which are made up of aggregated tau protein. Herein, we report the fully automated 2‐step radiosynthesis of [¹⁸F]MK‐6240 using a commercially available radiosynthesis module, GE Healthcare TRACERlab FX_FN. Nucleophilic fluorination of the 5‐diBoc‐6‐nitro precursor with potassium cryptand [¹⁸F]fluoride (K[¹⁸F]/K₂₂₂) was performed by conventional heating, followed by acid deprotection and semipreparative high‐performance liquid chromatography under isocratic conditions. The isolated product was diluted with formulation solution and sterile filtered under Current Good Manufacturing Practices, and quality control procedures were established to validate this radiopharmaceutical for human use. At the end of synthesis, 6.3 to 9.3 GBq (170‐250 mCi) of [¹⁸F]MK‐6240 was formulated and ready for injection, in an uncorrected radiochemical yield of 7.5% ± 1.9% (relative to starting [¹⁸F]fluoride) with a specific activity of 222 ± 67 GBq/μmol (6.0 ± 1.8 Ci/μmol) at the end of synthesis (90 minutes; n = 3). [¹⁸F]MK‐6240 was successfully validated for human PET studies meeting all Food and Drug Administration and United States Pharmacopeia requirements for a PET radiopharmaceutical. The present method can be easily adopted for use with other radiofluorination modules for widespread clinical research use.