A concise radiosynthesis of the tau radiopharmaceutical, [18F]T807

Fluorine‐18 labeled 7‐(6‐fluoropyridin‐3‐yl)‐5H‐pyrido[4,3‐b]indole ([¹⁸F]T807) is a potent and selective agent for imaging paired helical filaments of tau and is among the most promising PET radiopharmaceuticals for this target in early clinical trials. The present study reports a simplified one‐step method for the synthesis of [¹⁸F]T807 that is broadly applicable for routine clinical production using a GE TRACERlab? FX_FN radiosynthesis module. Key facets of our optimized radiosynthesis include development and use of a more soluble protected precursor, tert‐butyl 7‐(6‐nitropyridin‐3‐yl)‐5H‐pyrido[4,3‐b]indole‐5‐carboxylate, as well as new HPLC separation conditions that enable a facile one‐step synthesis. During the nucleophilic fluorinating reaction with potassium cryptand [¹⁸F]fluoride (K[¹⁸F]/K₂₂₂) in DMSO at 130 °C over 10 min the precursor is concurrently deprotected. Formulated [¹⁸F]T807 was prepared in an uncorrected radiochemical yield of 14 ± 3%, with a specific activity of 216 ± 60 GBq/µmol (5837 ± 1621 mCi/µmol) at the end of synthesis (60 min; n = 3) and validated for human use. This methodology offers the advantage of faster synthesis in fewer steps, with simpler automation that we anticipate will facilitate widespread clinical use of [¹⁸F]T807. Copyright © 2013 John Wiley & Sons, Ltd.     

A fully automated synthesis of [18F]‐FEAU and [18F]‐FMAU using a novel dual reactor radiosynthesis module

2′‐Deoxy‐2′‐[¹⁸F]fluoro‐5‐substituted‐1‐β‐D ‐arabinofuranosyluracils, including 2′‐deoxy‐2′‐[¹⁸F]fluoro‐5‐methyl‐1‐β‐D ‐arabinofuranosyluracil [¹⁸F]FMAU and [¹⁸F]FEAU are established radiolabeled probes to monitor cellular proliferation and herpes simplex virus type 1 thymidine kinase (HSV1‐tk) reporter gene expression with positron emission tomography. For clinical applications, a fully automated CGMP‐compliant radiosynthesis is necessary for production of these probes. However, due to multiple steps in the synthesis, no such automated synthetic protocols have been developed. We report here a fully automated synthesis of [¹⁸F]‐FEAU and [¹⁸F]‐FMAU on a prototype dual reactor module TRACERlab FX FN. The synthesis was performed by using a computer‐programmed standard operating procedure, and the product was purified on a semipreparative high‐performance liquid chromatography (HPLC) integrated with the synthesis module using 12% EtOH in 50 mM Na₂HPO₄. Finally, the percentage of alcohol was adjusted to 7% by adding Na₂HPO₄ and filtered through a Millipore filter to make dose for human. The radiochemical yield on the fluorination was 40±10% (n=10), and the overall yields were 4±1% (d. c.), from the end of the bombardment; [¹⁸F]FEAU (n=7) and [¹⁸F]FMAU (n=3). The radiochemical purity was >99%, specific activity was 1200–1300 mCi/µmol. The synthesis time was 2.5 h. This automated synthesis should be suitable for production of [¹⁸F]FIAU, [¹⁸F]FFAU, [¹⁸F]FCAU, [¹⁸F]FBAU and other 5‐substitued thymidine analogues. Copyright © 2009 John Wiley & Sons, Ltd.     

Fine‐tuning of the automated [18F]PSMA‐1007 radiosynthesis

Radiolabeled prostate‐specific membrane antigen (PSMA) targeting PET‐tracers have become desirable radiopharmaceuticals for the imaging of prostate cancer (PC). Recently, the PET radiotracer [¹⁸F]PSMA‐1007 was introduced as an alternative to [⁶⁸Ga]Ga‐PSMA‐11, for staging and diagnosing biochemically recurrent PC. We incorporated a one‐step procedure for [¹⁸F]PSMA‐1007 radiosynthesis, using both Synthra RNplus and GE TRACERlab FxFN automated modules, in accordance with the recently described radiolabeling procedure. Although the adapted [¹⁸F]PSMA‐1007 synthesis resulted in repeatable radiochemical yields (55 ± 5%, NDC), suboptimal radiochemical purities of 87 ± 8% were obtained using both modules. As described here, modifications made to the radiolabeling and the solid‐phase extraction purification steps reduced synthesis time to 32 minutes and improved radiochemical purity to 96.10%, using both modules, without shearing the radiochemical yield.     

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.     

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.     

Radiosynthesis of 11‐[18F]fluoroundecyltriphenylphosphonium (MitoF) as a potential mitochondria‐specific positron emission tomography radiotracer

Changes in the magnitude of the mitochondrial membrane potential occur in a range of important pathologies. To assess changes in membrane potential in patients, we set out to develop an improved mitochondria‐targeted positron emission tomography probe comprising a lipophilic triphenylphosphonium cation attached to a fluorine‐18 radionuclide via an 11‐carbon alkyl chain, which is well‐established to effectively transport to and localise within mitochondria. Here, we describe the radiosynthesis of this probe, 11‐[¹⁸F]fluoroundecyl‐triphenylphosphonium (MitoF), from no‐carrier‐added [¹⁸F]fluoride and a fully automated synthetic protocol to prepare it in good radiochemical yields (2–3 GBq at end‐of‐synthesis) and radiochemical purity (97–99%). Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Two‐step radiosynthesis of [18F]FE‐β‐CIT and [18F]PR04.MZ

The cocaine‐derived dopamine reuptake inhibitors FE‐β‐CIT (8‐(2‐fluoroethyl)‐3‐(4‐iodophenyl)‐8‐azabicyclo[3.2.1]octane‐2‐carboxylic acid methyl ester) (1) and PR04.MZ(8‐(4‐fluorobut‐2‐ynyl)‐3‐p‐tolyl‐8‐azabicyclo[3.2.1]octane‐2‐carboxylic acid methyl ester) (2) were labelled with ¹⁸F‐fluorine using a two‐step route. 2‐[¹⁸F]Fluoroethyltosylate and 4‐[¹⁸F]fluorobut‐2‐yne‐1‐yl tosylate were used as labelling reagents, respectively. Radiochemically pure (>98%) [¹⁸F]FE‐β‐CIT and [¹⁸F]PRD04.MZ (32–86 GBq/µmol) were obtained after a synthesis time of 100 min in about 25% non‐decay‐corrected overall yield.     

A microwave radiosynthesis of the 4‐[18F]‐fluorobenzyltriphenylphosphonium ion

The 4‐[¹⁸F]‐fluorobenzyltriphenylphosphonium cation was synthesized by a series of microwave reactions from no carrier added [¹⁸F]‐fluoride. The microwave procedure reduced the quantity of reagents used and synthesis time when compared with the original synthesis. In addition, problematic solid phase extraction, sodium borohydride reduction by column and inconsistent yields with excessive precipitate formation during the bromination step were eliminated. The 4‐[¹⁸F]‐fluorobenzyltriphenylphosphonium cation was produced radiochemically pure in 8.3% yield with a specific radioactivity of 534.5 ± 371.4 GBq/µmole at end of synthesis.     

N3‐Substituted thymidine analogues III: radiosynthesis of N3‐[(4‐[18F]fluoromethyl‐phenyl)butyl]thymidine ([18F]‐FMPBT) and N3‐[(4‐[18F]fluoromethyl‐phenyl)pentyl] thymidine ([18F]‐FMPPT) for PET

Radiosyntheses of two N³‐substituted thymidine analogues, N³‐[(4[¹⁸F]fluoromethyl‐phenyl)butyl]thymidine ([¹⁸F]‐FMPBT) and N³‐[(4[¹⁸F]fluoromethyl‐phenyl)pentyl]thymidine ([¹⁸F]‐FMPPT), are reported. The precursor compounds 9 and 10 were synthesized in six steps and the standard compounds 13 and 14 were synthesized from these precursors. For radiosynthesis, compounds 9 and 10 were fluorinated with n‐Bu₄N[¹⁸F] to produce [¹⁸F]‐ 11 and [¹⁸F]‐ 12 , which by acid hydrolysis yielded [¹⁸F]‐ 13 and [¹⁸F]‐ 14 , respectively. The crude products were purified by high‐performance liquid chromatography to obtain [¹⁸F]‐FMPBT and [¹⁸F]‐FMPPT. The average decay‐corrected radiochemical yield for [¹⁸F]‐ 13 was 15% in five runs, and that for [¹⁸F]‐ 14 was 10% in four runs. The radiochemical purity was >99% and the specific activity was >74 GBq/µmol at the end of synthesis. The synthesis time was 80–90 min from the end of bombardment. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthesis of 3′‐deoxy‐3′‐[18F]fluoro‐1‐β‐D‐xylofuranosyluracil ([18F]‐FMXU) for PET

The synthesis of a pyrimidine analog, 3′‐deoxy‐3′‐[¹⁸F]‐fluoro‐1‐β‐D ‐xylofuranosyluracil ([¹⁸F]‐FMXU) is reported. 5‐Methyluridine 1 was converted to its di‐methoxytrityl derivatives 2 and 3 as a mixture. After separation the 2′,5′‐di‐methoxytrityluridine 2 was converted to its 3′‐triflate 4 followed by derivatization to the respective N³‐t‐Boc product 5 . The triflate 5 was reacted with tetrabutylammonium[¹⁸F]fluoride to produce 6 , which by acid hydrolysis yielded compound 7 . The crude preparation was purified by HPLC to obtain the desired product [¹⁸F]‐FMXU. The radiochemical yields were 25–40% decay corrected (d. c.) with an average of 33% in four runs. Radiochemical purity was >99% and specific activity was >74 GBq/µmol at the end of synthesis (EOS). The synthesis time was 67–75 min from the end of bombardment (EOB). Copyright © 2005 John Wiley & Sons, Ltd.     

Copper‐mediated reduction of 2‐[18F]fluoroethyl azide to 2‐[18F]fluoroethylamine

¹⁸F‐labelled fluoroalkylamines are attractive reagents for the preparation of positron emission tomography tracers containing amine, amide, and N‐heterocyclic moieties. Herein, we report that 2‐[¹⁸F]fluoroethylamine can be obtained from 2‐[¹⁸F]fluoroethyl azide by reduction with elemental copper under acidic conditions. Azide to amine reduction was achieved in near quantitative analytical yields within 30 min by heating a solution of 2‐[¹⁸F]fluoroethyl azide in the presence of copper wire and aqueous trifluoroacetic acid. Subsequent reaction of 2‐[¹⁸F]fluoroethylamine with benzoyl chloride in the presence of triethylamine provided N‐[¹⁸F]fluoroethyl benzamide in 63% decay‐corrected radiochemical yield from 2‐[¹⁸F]fluoroethyl azide. The utility of the Cu(0)/H⁺ azide reduction method was further exemplified by preparation of the potential GABA_A tracer 9H‐β‐carboline N‐2‐[¹⁸F]fluoroethylamide, which was obtained in 46% decay‐corrected radiochemical yield by reaction of 2‐[¹⁸F]fluoroethylamine with the corresponding 9H‐β‐carboline pentafluorophenyl ester. Copyright © 2012 John Wiley & Sons, Ltd.     

Utility of azetidinium methanesulfonates for radiosynthesis of 3‐[18F]fluoropropyl amines

3‐Methanesulfonyloxypropyl tertiary amines were observed to cyclize to form azetidinium methanesulfonate moieties. Heat‐induced cyclization of 3‐methanesulfonyloxypropyl amines was utilized for preparation of azetidinium methanesulfonates. The azetidinium methanesulfonates were found to incorporate radioactive [¹⁸F]fluoride (decay‐corrected yields >60%) efficiently, resulting in an efficient synthesis of 3‐[¹⁸F]fluoropropyl tertiary amines. Copyright © 2004 John Wiley & Sons, Ltd.     

[18F]Fluoropyruvate: radiosynthesis and initial biological evaluation

The radiosynthesis of [¹⁸F]fluoropyruvate was investigated using numerous precursors were synthesized from ethyl 2,2‐diethoxy‐3‐hydroxypropanoate (5) containing different leaving groups: mesylate, tosylate, triflate, and nonaflate. These precursors were evaluated for [¹⁸F]fluoride incorporation with triflate being superior. The subsequent hydrolysis step was investigated, and an acidic hydrolysis was optimized. After establishing suitable purification and formulation methods, the [¹⁸F]fluoropyruvate could be isolated in ca. 50% d.c. yield. The [¹⁸F]fluoropyruvate was evaluated in vitro for its uptake into tumor cells using adenocarcinomic human alveolar basal epithelial cells (A549) and unfortunately showed an uptake of approximately 0.1% of the applied dose per 100,000 cells after 30 min. Initial pharmacokinetic properties were assessed in vivo using nude mice showed a high degree of bone uptake from defluorination, which will limit its potential as an imaging agent for metabolic processes. Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis of [18F]‐labeled adenosine analogues as potential PET imaging agents

The syntheses of adenosine analogues, 2′‐deoxy‐2′‐[¹⁸F]fluoro‐9‐β‐D ‐arabinofuranosyladenine ([¹⁸F]‐FAA) and 3′‐deoxy‐3′‐[¹⁸F]fluoro‐9‐β‐D ‐xylofuranosyladenine ([¹⁸F]‐FXA) are reported. Adenosine ( 1 ) was converted to its methoxytrityl derivatives 2 and 3 as a mixture. After separation, these derivatives were converted to their respective triflates 4 and 5 . Each triflate was reacted with tetrabutylammonium[¹⁸F]fluoride to produce 6b or 7b , which by acidic hydrolysis yielded compounds 8b and 9b . Crude preparations were purified by HPLC to obtain the desired pure products. The radiochemical yields were 10‐18% decay corrected (d. c.) for 8b and 30‐40% (d. c.) for 9b in 4 and 3 runs, respectively. Radiochemical purity was >99% and specific activity was >74 GBq/μmol at the end of synthesis (EOS). The synthesis time was 90‐95 min from the end of bombardment (EOB). Copyright © 2003 John Wiley & Sons, Ltd.     

Photoconjugation of 3‐azido‐5‐nitrobenzyl‐[18F]fluoride to an oligonucleotide aptamer

Photoconjugation techniques are well established in biochemistry and molecular biology applications. We report here the first application of photoconjugation to label an oligonucleotide with positron emitting fluorine‐18. ANBF, 3‐azido‐5‐nitrobenzyl fluoride, was tagged with fluorine‐18 in 30–50% yield from the corresponding tosylate. The HPLC purified [¹⁸F]ANBF was photochemically conjugated to a hexylamine modified 15‐base, single stranded, DNA aptamer with up to 20% radiolabeling efficiency, based on starting [¹⁸F]ANBF. Separation by gel exclusion chromatography and HPLC analysis of the DNA fractions indicated that the benzyl fluoride was covalently bound to the aptamer. The total reaction time from the start of radiosynthesis was 135 min. Based on these results, the feasibility to photolabel oligonucleotide molecules with fluorine‐18 labeled arylazides for PET applications has been established. Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis of 6‐acrylamido‐4‐(2‐[18F]fluoroanilino)quinazoline: a prospective irreversible EGFR binding probe

Acrylamido‐quinazolines substituted at the 6‐position bind irreversibly to the intracellular ATP binding domain of the epidermal growth factor receptor (EGFR). A general route was developed for preparing 6‐substituted‐4‐anilinoquinazolines from [¹⁸F]fluoroanilines for evaluation as EGFR targeting agents with PET. By a cyclization reaction, 2‐[¹⁸F]fluoroaniline was reacted with N′‐(2‐cyano‐4‐nitrophenyl)‐N,N‐dimethylimidoformamide to produce 6‐nitro‐4‐(2‐[¹⁸F]fluoroanilino)quinazoline in 27.5% decay‐corrected radiochemical yield. Acid mediated tin chloride reduction of the nitro group was achieved in 5 min (80% conversion) and subsequent acylation with acrylic acid gave 6‐acrylamido‐4‐(2‐[¹⁸F]fluoroanilino)quinazoline in 8.5% decay‐corrected radiochemical yield, from starting fluoride, in less than 2 h. Copyright © 2005 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.     

Radiosynthesis of 6‐([18F]fluoroacetamido)‐1‐hexanoicanilide ([18F]FAHA) for PET imaging of histone deacetylase (HDAC)

Radiosynthesis of a novel substrate for histone deacetylase (HDAC), 6‐([¹⁸F]fluoroacetamido)‐1‐hexanoicanilide ([¹⁸F]FAHA, [¹⁸F]‐ 3 ) is reported. For precursor synthesis, compound 1 (6‐amino‐1‐hexanoicanilide) was prepared by the reaction of 6‐amino hexanoic acid with thionyl chloride in dichloroethane followed by addition of aniline. Compound 1 was reacted with bromoacetic anhydride in tetrahydrofuran (THF) in the presence of triethylamine to produce the precursor compound 6‐(bromoacetamido)‐1‐hexanoicanilide 2 . Fluorination reactions were performed using tetrabutylammonium fluoride in various solvents at 80°C to prepare the unlabeled reference compound 3 . Radiofluorinations were performed using either n‐Bu₄N¹⁸F or K¹⁸F/kryptofix, and the crude product was purified by high performance liquid chromatography (HPLC). The radiochemical yields were 9–13% decay corrected (d.c.) with an average of 11% using K¹⁸F/kryptofix, and specific activity >2 GBq/µmol at the end of synthesis. The synthesis time was 67–75 min from the end of bombardment (EOB). Copyright © 2006 John Wiley & Sons, Ltd.