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.     

Fully automated,high yielding production of N‐succinimidyl 4‐[18F]fluorobenzoate ([18F]SFB), and its use in microwave‐enhanced radiochemical coupling reactions

A General Electric Medical Systems (GEMS) Tracerlab FX_FN fluorine‐18 synthesis module has been reconfigured to allow rapid (45 min), fully automated production of N‐succinimidyl 4‐[¹⁸F]fluorobenzoate ([¹⁸F]SFB) using the established three‐step, one‐pot synthesis procedure. Purification is by sep‐pak only and [¹⁸F]SFB is routinely obtained in 38% non‐decay corrected yield,>1 Ci/µmol specific activity, and >95% radiochemical purity (n=20). Moreover, this report includes our preliminary research efforts into improving peptide coupling reactions with [¹⁸F]SFB using microwave‐enhanced radiochemistry. Reaction times can be reduced by>90%, when compared with traditional thermal reactions, with no significant effect on radiochemical reaction yield. Copyright © 2010 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.     

Two‐step radiosynthesis of [18F]N‐succinimidyl‐4‐fluorobenzoate ([18F]SFB)

The acylation reagent [¹⁸F]N‐succinimidyl‐4‐fluorobenzoate (¹⁸F‐SFB) has been prepared using a new two‐step approach. The starting material p‐[¹⁸F]fluorobenzaldehyde (¹⁸F‐FBA) was obtained by an improved radiosynthesis with a decay‐corrected radiochemical yield of 66±6 % (n=3). Reaction of ¹⁸F‐FBA with (diacetoxyiodine)benzene and N‐hydroxysuccinimide and preparative HPLC purification furnished ¹⁸F‐SFB in an r.c.y. of 49±6 % (n=3), based on the starting radioactivity of ¹⁸F‐FBA. The radiochemical purity of ¹⁸F‐SFB was >99%. Alternatively, purification by solid phase extraction gave ¹⁸F‐SFB with an r.c.y. of 77±9% (n=4) and a radiochemical purity of 89±5% (n=4). This radiochemical synthesis only used non‐aqueous solvents, which simplifies the method and facilitates subsequent applications of ¹⁸F‐SFB. Copyright © 2009 John Wiley & Sons, Ltd.     

Radiosynthesis of novel pitavastatin derivative ([18F]PTV‐F1) as a tracer for hepatic OATP using a one‐pot synthetic procedure

Pitavastatin is an antihyperlipidemic agent, a potent inhibitor of 3‐hydroxymethyl‐glutaryl‐CoA reductase, which is selectively taken up into the liver mainly via hepatic organic anion transporting polypeptide 1B1 (OATP1B1). OATP1B1 can accept a variety of organic anions, and previous reports indicated that it is responsible for the hepatic clearance of several clinically used anionic drugs. Therefore, the pharmacokinetics and the hepatic distribution of pitavastatin provide an insight into the function of OATP1B1 in humans. For the development of the in vivo evaluation of OATP1B1 function by positron emission tomography imaging, we designed a novel [¹⁸F]pitavastatin derivative ([¹⁸F]PTV‐F1), in which a [¹⁸F]fluoroethoxy group is substituted for the [¹⁸F]fluoro group of [¹⁸F]pitavastatin, with the aim of convenient radiolabeling protocol and high radiochemical yield. In vitro studies suggested that transport activities of PTV‐F1 mediated by OATP1B1 and OATP1B3 were very similar to those of pitavastatin and PTV‐F1 was metabolically stable in human liver microsomes. In the radiosynthesis of [¹⁸F]PTV‐F1 from the tosylate precursor, nucleophilic fluorination and subsequent deprotection were performed using a one‐pot procedure. [¹⁸F]PTV‐F1 was obtained with a radiochemical yield of 45% ± 3% (n = 3), and the operating time for the radiosynthesis of [¹⁸F]PTV‐F1 is very short (30 minutes) compared with [¹⁸F]pitavastatin.     

Synthesis of [18F]‐labeled N‐3(substituted) thymidine analogues: N‐3([18F]fluorobutyl) thymidine ([18F]‐FBT) and N‐3([18F]fluoropentyl) thymidine ([18F]‐FPT) for PET

Syntheses of N‐3(substituted) analogues of thymidine, N‐3([¹⁸F]fluorobutyl)thymidine ([¹⁸F]‐FBT) and N‐3([¹⁸F]fluoropentyl)thymidine ([¹⁸F]‐FPT) are reported. 1,4‐Butane diol and 1,5 pentane diol were converted to their tosyl derivatives 2 and 3 followed by conversion to benzoate esters 4 and 5, respectively. Protected thymidine 1 was coupled separately with 4 and 5 to produce 6 and 7 , which were hydrolyzed to 8 and 9 , then converted to their mesylates 10 and 11 , respectively. Compounds 10 and 11 were fluorinated with n‐Bu₄N[¹⁸F] to produce 12 and 13 , which by acid hydrolysis yielded 14 and 15 , respectively. The crude products were purified by HPLC to obtain [¹⁸F]‐FBT and [¹⁸F]‐FPT. The radiochemical yields were 58–65% decay corrected (d.c.) for 14 and 46–57% (d.c.) for 15 with an average of 56% in three runs per compound. Radiochemical purity was >99% and specific activity was >74 GBq/µmol at the end of synthesis (EOS). The synthesis time was 65–75 min from the end of bombardment (EOB). Copyright © 2006 John Wiley & Sons, Ltd.     

A simplified protocol for the automated production of succinimidyl 4‐[18F]fluorobenzoate on an IBA Synthera module

The important peptide labelling reagent succinimidyl 4‐[¹⁸F]fluorobenzoate ([¹⁸F]SFB) has been synthesised in 75–85% decay corrected radiochemical yield using the IBA Synthera platform (IBA Cyclotron Solutions, Louvain‐la‐neuve, Belgium) with the fluorodeoxyglucose‐integrated fluidic processor nucleophilic and only four reagent vials in a single reactor. (4‐ethoxycarbonylphenyl) trimethylammonium triflate was used as the labelling precursor and 1 M aqueous tetramethylammonium hydroxide for the hydrolysis of the intermediate ethyl 4‐[¹⁸F]fluorobenzoate. N,N,N′,N′‐tetramethyl‐O‐(N‐succinimidyl)uronium tetrafluoroborate (TSTU) was then used to form [¹⁸F]SFB from 4‐[¹⁸F]fluorobenzoate. By omitting the addition of acetic acid and introducing a combined hydrolysis/water removal step, the synthesis time was shortened to 58 minutes. After SepPak purification, the radiochemical purity of [¹⁸F]SFB was 95.8–98.2%. These simplifications might be of significance to users of other automated synthesis modules.     

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.     

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.     

Radiochemical synthesis of 2′‐[18F]‐labelled and 3′‐[18F]‐labelled nucleosides for positron emission tomography imaging

This review article considers 2′‐labelled and 3′‐labelled nucleosides, which are of great importance as positron emission tomography (PET) probes in clinical diagnostics and PET research. Although the radiochemical preparation of several [¹⁸F]‐labelled nucleosides such as [¹⁸F]fluorothymidine or [¹⁸F](fluoroarabinofuranosyl)cytosine has been accomplished within the last two decades, a number of potentially interesting nucleoside‐based biomarkers are not yet available for automated good manufacturing practice production due to the lack of fast and efficient synthetic methods for late‐stage [¹⁸F]‐introduction. In order to meet recent demands for new PET‐based biomarkers in various clinical applications, appropriate precursors that can easily be fluorinated and deprotected need to be developed.     

Simplified synthesis of N‐(3‐[18F]fluoropropyl)‐2β‐carbomethoxy‐3β‐(4‐fluorophenyl)nortropane ([18F]β‐CFT‐FP) using [18F]fluoropropyl tosylate as the labelling reagent

A synthesis method has been developed for the labelling of N‐(3‐[¹⁸F]fluoropropyl)‐2β‐carbomethoxy‐3β‐(4‐fluorophenyl)nortropane ([¹⁸F]β‐CFT‐FP), a potential radioligand for visualization of the dopamine transporters by positron emission tomography. The two‐step synthesis includes preparation of [¹⁸F]fluoropropyl tosylate and its use without purification in the fluoroalkylation of 2β‐carbomethoxy‐3β‐(4‐fluorophenyl)nortropane (nor‐β‐CFT). The final product is purified by HPLC. Optimization of the two synthesis steps resulted in a greater than 30% radiochemical yield of [¹⁸F]β‐CFT‐FP (decay corrected to end of bombardment). The synthesis time including HPLC‐purification was approximately 90 min. The radiochemical purity of the final product was higher than 99% and the specific radioactivity at the end of synthesis was typically 20 GBq/µmol. In comparison to alkylation by [¹⁸F]fluoropropyl bromide, the procedure described here results in an improved overall radiochemical yield of [¹⁸F]β‐CFT‐FP in a shorter time. Copyright © 2005 John Wiley & Sons, Ltd.     

Novel synthesis of [18F]‐fluorobenzene and pyridinecarbohydrazide‐folates as potential PET radiopharmaceuticals

In an attempt to visualize folate receptors that over‐express on many cancers, [¹⁸F]‐fluorobenzene and pyridine carbohydrazide‐folates were synthesized using two different synthetic approaches starting from nucleophilic displacement reactions on ethyl‐trimethylammonium‐benzoate and pyridine carboxylate precursors. The intermediates ethyl [¹⁸F]‐fluorinated benzene and pyridine esters were reacted with hydrazine to produce the [¹⁸F]‐fluorobenzene and pyridine carbohydrazides followed by coupling with NHS‐folate 11 in the first approach. Whereas hydrazide‐folate 5 was reacted with 2,5‐dioxoazolidinyl [¹⁸F]‐fluorobenzenecarboxylate in the second approach. Based on starting [¹⁸F]‐fluoride, radiochemical yields and synthesis times were found to be around 80% (45 min) and 35% (80 min) for the first and the second approaches, respectively. The first synthetic approach holds considerable promise as a rapid and simple method for the radiofluorination of folic acid with high radiochemical yield and short time. Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis of N‐(piperidin‐1‐yl)‐5‐(4‐methoxyphenyl)‐1‐(2‐chlorophenyl)‐4‐[18F]fluoro‐1H‐pyrazole‐3‐carboxamide by nucleophilic [18F] fluorination: a PET radiotracer for studying CB1 cannabinoid receptors

The feasibility of nucleophilic displacement of bromide in the 4‐bromopyrazole ring with [¹⁸F]fluoride has been demonstrated by the synthesis of two radiolabeled compounds: N‐(piperidin‐1‐yl)‐5‐(4‐methoxyphenyl)‐1‐(2‐chlorophenyl)‐4‐[¹⁸F]fluoro‐1H‐pyrazole‐3‐carboxamide, ([¹⁸F] NIDA‐42033) 1b and 1‐(2‐chlorophenyl)‐4‐[¹⁸F]fluoro‐5‐(4‐methoxyphenyl)‐1H‐pyrazole‐3‐carboxylic acid, ethyl ester 4 . The radiochemical yields were in the range of 1–6%. [¹⁸F]NIDA‐42033, a potential radiotracer for the study of CB₁ cannabinoid receptors in the animal brain by positron emission tomography, has been synthesized in sufficient quantities with specific radioactivity greater than 2500 mCi/μmol and radiochemical purity >95%. Copyright © 2002 John Wiley & Sons, Ltd.     

4‐[18F]fluorophenyl ureas via carbamate‐4‐nitrophenyl esters and 4‐[18F]fluoroaniline

Four different no carrier added (n.c.a.) 4‐[¹⁸F]fluorophenylurea derivatives are synthesized as model compounds via two alternative routes. In both cases carbamate‐4‐nitrophenylesters are used as intermediates. Either n.c.a. 4‐[¹⁸F]fluoroaniline reacts with carbamates of several amines, or the carbamate of n.c.a. 4‐[¹⁸F]fluoroaniline is formed at first and an amine is added subsequently to yield the urea derivative. The choice of the appropriate way of reaction depends on the possibilities of precursor synthesis. The radiochemical yields reach up to 80% after 50 min of synthesis time while no radiochemical by‐products can be determined. These high yields were possible due to an optimized preparation of n.c.a. 4‐[¹⁸F]fluoroaniline with a radiochemical yield of up to 90%. From the various ways of its radiosynthesis, the substitution with n.c.a. [¹⁸F]fluoride on dinitrobenzene is chosen, using phosphorous acid and palladium black for reduction of the second nitro group. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis of N‐(3‐[18F]Fluoropropyl)‐2β‐carbomethoxy‐3β‐(4‐iodophenyl)nortropane ([18F]FP‐β‐CIT)

N‐(3‐[¹⁸F]fluoropropyl)‐2β‐carbomethoxy‐3β‐(4‐iodophenyl)nortropane ([¹⁸F]FP‐β‐CIT) was synthesized in a two‐step reaction sequence. In the first reaction, 1‐bromo‐3‐(nitrobenzene‐4‐sulfonyloxy)‐propane was fluorinated with no‐carrier‐added fluorine‐18. The resulting product, 1‐bromo‐3‐[¹⁸F]‐fluoropropane, was distilled into a cooled reaction vessel containing 2β‐carbomethoxy‐3β‐(4‐iodophenyl)‐nortropane, diisopropylethylamine and potassium iodide. After 30 min, the reaction mixture was subjected to a preparative HPLC purification. The product, [¹⁸F]FP‐β‐CIT, was isolated from the HPLC eluent with solid‐phase extraction and formulated to yield an isotonic, pyrogen‐free and sterile solution of [¹⁸F]FP‐β‐CIT. The overall decay‐corrected radiochemical yield was 25 ± 5%. Radiochemical purity was > 98% and the specific activity was 94 ± 50 GBq/µmol at the end of synthesis. Copyright © 2006 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.     

One‐step synthesis of 4‐[18F]fluorobenzyltriphenylphosphonium cation for imaging with positron emission tomography

4‐[¹⁸F]Fluorobenzyltriphenylphosphonium cation (¹⁸F‐FBnTP) is a promising negative membrane potential targeting positron emission tomography tracer. However, the reported multistep radiolabeling approach for the synthesis of ¹⁸F‐FBnTP poses a challenge for routine clinical applications. In this study, we demonstrated that ¹⁸F‐FBnTP can be prepared in good conversion yields (~60%, nondecay corrected) in just one step via a copper‐mediated ¹⁸F‐fluorination reaction using a pinacolyl arylboronate precursor. In addition, our data suggest that ¹⁸F‐labeled (phosphonium) cations can be efficiently prepared via a copper‐mediated ¹⁸F‐fluoronation by using triflate as the counterion.     

Synthesis of [18F]‐6‐deoxy‐6‐fluoro‐D‐glucose ([18F]6FDG), a potential tracer of glucose transport

With the goal of developing a PET radioligand for the in vivo assessment of glucose transport, 6-deoxy-6-[¹⁸F]fluoro-D -glucose ([¹⁸F]6FDG) was prepared in two steps from ¹⁸F⁻. Starting with D -glucose, the tosyl- and mesyl-derivatives of 3,5-O-benzylidene-1,2-O-isopropylidene-α-D -glucofuranose were prepared by known methods. Reaction of either of these precursors with ¹⁸F⁻ resulted in the formation of 3,5-O-benzylidene-6-deoxy-6-[¹⁸F]-fluoro-1,2-O-isopropylidene-α-D -glucofuranose in high yield. Subsequent hydrolysis resulted in the production of [¹⁸F]6FDG. Under optimal conditions, [¹⁸F]6FDG is produced 60–70 min after end of bombardment (EOB) in 71 ± 12% yield (decay corrected, based upon fluoride) with a radiochemical purity of ⩾96%. Preliminary experiments have indicated that [¹⁸F]6FDG may be a more representative in vivo tracer for the glucose transporter than 2FDG. Copyright © 2005 John Wiley & Sons, Ltd.     

Rapid and efficient synthesis of [18F]fluoronicotinamides, [18F]fluoroisonicotinamides and [18F]fluorobenzamides as potential pet radiopharmaceuticals for melanoma imaging

In an attempt to simplify nucleophilic radiofluorination reactions to be amenable for automation, a series of [¹⁸F]fluoronicotinamides, [¹⁸F]fluoroisonicotinamides and [¹⁸F]fluorobenzamides were synthesized using one‐step synthetic approach involving displacement reactions on trimethylammonium‐nicotinamide, trimethylammonium‐isonicotinamide and trimethylammonium‐benzamide precursors. Based on starting [¹⁸F]‐fluoride, radiochemical yields and purities were found to be greater than 90 and 97%, respectively, within 20 min synthesis time and, without high‐performance liquid chromatography purification. This synthetic approach holds great promise as a rapid and simple method for the automated radiofluorination of [¹⁸F]fluoronicotinamides, [¹⁸F]fluoroisonicotinamides and [¹⁸F]fluorobenzamides with high radiochemical yield and very short preparation time. Copyright © 2011 John Wiley & Sons, Ltd.