Fluorine‐18 labelling of oligonucleotides: Prosthetic labelling at the 5′‐end using the N‐(4‐[18F]fluorobenzyl)‐2‐bromoacetamide reagent

Labelled oligonucleotides are new imaging tools to study gene expression at the nucleic acid and protein levels. We have previously developed a universal method to label oligonucleotides at their 3′‐end with radiohalogens and particularly with fluorine‐18, the most widely used positron‐emitter, t_1/2: 109.8 min. Using the same strategy, we herein report the fluorine‐18 labelling of oligonucleotides at their 5′‐end. A 18‐mer 2′O‐methyl modified oligoribonucleotide, bearing a phosphorothioate group at its 5′‐end, was conjugated to our fluorine‐18‐labelled reagent N‐(4‐[¹⁸F]fluorobenzyl)‐2‐bromoacetamide. The whole synthetic procedure yielded up to 1 GBq of fluorine‐18‐labelled oligonucleotide with a specific radioactivity of 37–74 GBq/μmol in 160 min. Copyright © 2003 John Wiley & Sons, Ltd.     

Radiosynthesis of 3‐[18F]fluoropropyl and 4‐[18F]fluorobenzyl triarylphosphonium ions

3‐[¹⁸F]Fluoropropyl‐, 4‐[¹⁸F]fluorobenzyl‐triphenylphosphonium and 4‐[¹⁸F]fluorobenzyltris‐4‐dimethylaminophenylphosphonium cations were synthesized in multi‐step reactions from no carrier added (nca) [¹⁸F]fluoride. The time for synthesis, purification, and formulation was 56, 82, and 79 min with an average radiochemical yield of 12, 6 and 15%, respectively (not corrected for decay). The average specific radioactivity for the three radiolabeled compounds was 14.9 GB q/µmole (403 mCi/µmole) at end of synthesis (EOS). Copyright © 2004 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.     

Automated radiosynthesis of N‐(4‐[18F]fluorobenzyl)‐2‐bromoacetamide: an F‐18‐labeled reagent for the prosthetic radiolabeling of oligonucleotides

The potential for radiolabeled antisense oligonucleotides to image gene expression combined with the enhanced resolution of positron‐emission tomography justifies the continued interest in the development of oligonucleotides tagged with positron‐emitting radionuclides. The radiolabeling of oligonucleotides is a multi‐step process and may require handling large amounts of radioactivity initially. A previously reported method for radiolabeling oligonucleotides with N‐(4‐[¹⁸F]fluorobenzyl)‐2‐bromoacetamide was adapted for use in a commercially available automated synthesis unit by linking two reaction trains. The yield of N‐(4‐[¹⁸F]fluorobenzyl)‐2‐bromoacetamide ranged from 3 to 18% and the synthesis was completed within 1 h. Challenges in using this unit included the maintenance of anhydrous conditions for the effective reduction of 4‐[¹⁸F]fluorobenzonitrile. Preliminary results indicated that a mean yield of 36% could be obtained upon incubation of an oligonucleotide with N–(4‐[¹⁸F]fluorobenzyl)‐2‐bromoacetamide. The entire synthesis could be performed within 3 h. Copyright © 2008 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.     

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.     

Application of n.c.a. 4‐[18F]fluorophenol in diaryl ether syntheses of 2‐(4‐[18F]fluorophenoxy)‐benzylamines

The availability of no‐carrier‐added (n.c.a.) 4‐[¹⁸F]fluorophenol offers the possibility of introducing the 4‐[¹⁸F]fluorophenoxy moiety into potential radiopharmaceuticals. Besides alkyl–aryl ether synthesis using n.c.a. 4‐[¹⁸F]fluorophenol the diaryl ether coupling is an attractive synthetic method to enlarge the spectrum of interesting labelling procedures. As examples the syntheses of n.c.a. 2‐(4‐[¹⁸F]fluorophenoxy)‐N,N‐dimethylbenzylamine and n.c.a. 2‐(4‐[¹⁸F]fluorophenoxy)‐N‐methylbenzylamine were realized by an Ullmann ether synthesis of corresponding 2‐bromobenzoic acid amides using tetrakis(acetonitrile)copper(I) hexafluorophosphate as catalyst and a subsequent reduction of the amides formed. The radiochemical yield of the coupling varied between 5 and 65% based on labelled 4‐[¹⁸F]fluorophenol. Both compounds are structural analogues of recently published radiotracers for imaging the serotonin reuptake transporter sites (SERT). However, in vitro binding assays of both molecules showed only a low affinity towards monoamine transporters. Copyright © 2004 John Wiley & Sons, Ltd.     

N1'‐(p‐[18F]Fluorobenzyl)naltrindole (p‐[18F]BNTI) as a potential PET imaging agent for DOP receptors

The N1'‐(p‐fluorobenzyl)naltrindole 5 has been synthesized by reaction of 3‐O‐benzyl NTI 3 with p‐fluorobenzylbromide under phase transfer catalysis. The subsequent 3‐O‐benzyldeprotection of 4 in HBr/CH₃COOH gave the target compound 5 in three steps from naltrindole 2 . p‐FBNTI 5 is a novel delta opioid receptor antagonist (K_i=0.00312 nM) and antagonizes the delta opioid (DOP) agonist, DPDPE, with a K_e=1.55 nM in the mouse vas deferens preparation. Using the same synthetic strategy the synthesis of p‐[¹⁸F]BNTI 10 was undertaken. The final yield was 4% and the specific activity varied in a range of 250–400 mCi/µmol. 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.     

The synthesis of a new probe for PET imaging reporter gene HSV1‐tk: 2‐amino‐6‐[18F] fluoro‐9‐ (4‐hydroxy‐3‐hydroxymethylbutyl) purine (6‐[18F]fluoropenciclovir)

The one step radiosynthesis of 2‐amino‐6‐ [¹⁸F]fluoro‐9‐(4‐hydroxy‐3‐hydroxymethylbutyl) purine (6‐[¹⁸F]fluoropenciclovir) 6 is reported. Radiolabeled product 6‐[¹⁸F]fluoropenciclovir 6 was prepared by radiofluorination of compound 4 with [¹⁸F]KF and isolated by a silica Sep‐Pak cartridge. The radiochemical yield of compound 6 was 45–55% decay corrected (d.c.) in six runs with radiochemical purity >98% and the radiosynthesis time was 35–42 min from end of bombardment (EOB). Copyright © 2006 John Wiley & Sons, Ltd.     

Efficient synthesis of 6‐chloro‐3‐((2‐(S)‐azetidinyl)methoxy)‐5‐((E)‐2‐(2‐[18F]fluoropyridin‐4‐yl)vinyl)pyridine ([18F]NIDA 52289), a very high affinity radioligand for nicotinic acetylcholine receptors

6‐Chloro‐3‐((2‐(S)‐azetidinyl)methoxy)‐5‐((E)‐2‐(2‐[¹⁸F]fluoropyridin‐4‐yl)vinyl)pyridine ([¹⁸F]NIDA 52289), a very high affinity radioligand for studying nicotinic acetylcholine receptors (nAChRs) by positron‐emission tomography, was synthesized through Kryptofix 222 assisted no‐carrier‐added nucleophilic [¹⁸F]fluorination of 6‐chloro‐3‐((1‐(tert‐butoxycarbonyl)‐2‐(S)‐azetidinyl)methoxy)‐5‐((E)‐2‐(2‐bromopyridin‐4‐yl)vinyl)pyridine, followed by acidic deprotection. The overall radiochemical yield of the radiosynthesis was 10% (non‐decay‐corrected), the specific radioactivity was in the range of 93–326 GBq/µmol (2.5–8.8 mCi/µmol) and the radiochemical purity was greater than 99%. Copyright © 2004 John Wiley & Sons, Ltd.     

A new synthesis of the labeling precursor for [18F]‐fluoromisonidazole

[¹⁸F]‐Fluoromisonidazole is the most widely used radiopharmaceutical for imaging hypoxia in tumors. The precursor for [¹⁸F]‐fluoromisonidazole was prepared from 1,3‐dibromo‐2‐propanol in 5 steps from available materials and straightforward purification steps. The overall yield for this synthesis was 18%. Copyright © 2005 John Wiley & Sons, Ltd.     

Radiosynthesis and in vivo study of [18F]1‐(2‐fluoroethyl)‐4‐[(4‐cyanophenoxy)methyl]piperidine: a promising new sigma‐1 receptor ligand

The novel sigma‐1 receptor PET radiotracer [¹⁸F]1‐(2‐fluoroethyl)‐4‐[(4‐cyanophenoxy)methyl]piperidine ([¹⁸F]WLS1.002, [¹⁸F]‐2) was synthesized (n=6) by heating the corresponding N‐ethylmesylate precursor in an anhydrous acetonitrile solution containing [¹⁸F]fluoride, Kryptofix K₂₂₂ and potassium carbonate for 15 min. Purification was accomplished by reverse‐phase HPLC methods, providing [¹⁸F]‐2 in 59±8% radiochemical yield (EOB), with specific activity of 2.89±0.80 Ci/µmol (EOS) and radiochemical purity of 98.3±2.1%. Rat biodistribution studies revealed relatively high uptake in many organs known to contain sigma‐1 receptors, including the lungs, kidney, heart, spleen, and brain. Good clearance from normal tissues was observed over time. Blocking studies (60 min) demonstrated high (>80%) specific binding of [¹⁸F]‐2 in the brain, with reduction also noted in other organs known to express these sites. Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis of 6‐chloro‐3‐((2‐(S)‐azetidinyl)methoxy)‐5‐(2‐[18F]fluoropyridin‐4‐yl)pyridine ([18F]NIDA 522131), a novel potential radioligand for studying extrathalamic nicotinic acetylcholine receptors by PET

6‐Chloro‐3‐((2‐(S)‐azetidinyl)methoxy)‐5‐(2‐[¹⁸F]fluoropyridin‐4‐yl)pyridine ([¹⁸F]NIDA 522131), a potential radioligand for studying extrathalamic nicotinic acetylcholine receptors by positron‐emission tomography, was synthesized via no‐carrier‐added nucleophilic [¹⁸F]fluorination of 6‐chloro‐3‐((1‐(tert‐butoxycarbonyl)‐2‐(S)‐azetidinyl)methoxy)‐5‐(2‐iodopyridin‐4‐yl)vinyl)pyridine, followed by acidic deprotection. The overall radiochemical yield of the radiosynthesis was 4–8% (non‐decay‐corrected), the specific radioactivity was in the range of 167–335 GBq/µmol (4500–9000 mCi/µmol) and the radiochemical purity was greater than 99%. Preparation of [¹⁸F]NIDA522131 via corresponding bromo‐derivative 2 is also described. Copyright © 2004 John Wiley & Sons, Ltd.     

First no‐carrier‐added radioselenation of an adenosine‐A1 receptor ligand

The precursor synthesis and the no‐carrier‐added (n.c.a.) radiosynthesis of the adenosine‐A₁ receptor ligand 5′‐(methyl[⁷⁵Se]seleno)‐N⁶‐cyclopentyladenosine ([⁷⁵Se] 1 ) are described in this report. A method was developed starting from elemental n.c.a. selenium‐75, followed by a three‐step polymer‐supported radioselenation and deprotection which gave the radioligand with a radiochemical yield of 30%, a radiochemical purity of > 99% and a specific radioactivity of > 300 GBq/mmol (8 Ci/mmol). Preparation time was 40 min. The nonradioactive compound 5′‐(methylseleno)‐N⁶‐cyclopentyladenosine ( 1 ) was pharmacologically evaluated in vitro and showed high affinity and selectivity for the adenosine‐A₁ receptor. These preliminary results suggest that this compound could be a useful radioligand for the noninvasive imaging of the brain adenosine‐A₁ receptors using positron emission tomography (PET) when labelled with the positron emitter selenium‐73 (half‐life: 7.1 h). Copyright © 2004 John Wiley & Sons, Ltd.     

Synthesis and preliminary evaluation of a 18F‐labeled ethisterone derivative [18F]EAEF for progesterone receptor targeting

To develop a novel progesterone receptor‐targeting probe for positron emission tomography imaging, an ethisterone derivative [¹⁸F]EAEF was designed and prepared in high decay‐corrected radiochemical yield (30–35%) with good radiochemical purity (>98%). [¹⁸F]EAEF is a lipophilic tracer (logP = 0.53 ± 0.06) with very good stability in saline and serum. In the biodistribution study, high radioactivity accumulation of [¹⁸F]EAEF were found in uterus (5.73 ± 1.83% ID/g) and ovary (4.05 ± 0.73% ID/g) at 2 hr postinjection (p.i.), which have high progesterone receptor expression after treated with estradiol, while the muscle background has very low uptake (0.50 ± 0.17% ID/g). For positron emission tomography imaging, [¹⁸F]EAEF showed high uptake in progesterone receptor‐positive MCF‐7 tumor (3.15 ± 0.07% ID/g at 2 hr p.i.) with good tumor to muscle ratio (2.90), and obvious lower tumor uptakes were observed in MCF‐7 with EAEF blocking (1.84 ± 0.05% ID/g at 2 hr p.i.) or in progesterone receptor‐negative MDA‐MB‐231 tumor (1.80 ± 0.03% ID/g at 2 hr p.i.). Based on the good stability and specificity of [¹⁸F]EAEF, it may be a good candidate for imaging progesterone receptor and worth further investigation.     

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.     

An improved radiosynthesis of O‐(2‐[18F]fluoroethyl)‐O‐(p‐nitrophenyl)methylphosphonate: A first‐in‐class cholinesterase PET tracer

O‐(2‐Fluoroethyl)‐O‐(p‐nitrophenyl) methylphosphonate 1 is an organophosphate cholinesterase inhibitor that creates a phosphonyl‐serine covalent adduct at the enzyme active site blocking cholinesterase activity in vivo . The corresponding radiolabeled O‐(2‐[¹⁸F]fluoroethyl)‐O‐(p‐nitrophenyl) methylphosphonate, [ ¹⁸ F]1 , has been previously prepared and found to be an excellent positron emission tomography imaging tracer for assessment of cholinesterases in live brain, peripheral tissues, and blood. However, the previously reported [ ¹⁸ F]1 tracer synthesis was slow even with microwave acceleration, required high‐performance liquid chromatography separation of the tracer from impurities, and gave less optimal radiochemical yields. In this paper, we report a new synthetic approach to circumvent these shortcomings that is reliant on the facile reactivity of bis‐(O,O‐p‐nitrophenyl) methylphosphonate, 2 , with 2‐fluoroethanol in the presence of DBU. The cold synthesis was successfully translated to provide a more robust radiosynthesis. Using this new strategy, the desired tracer, [ ¹⁸ F]1 , was obtained in a non‐decay–corrected radiochemical yield of 8 ± 2% (n = 7) in >99% radiochemical and >95% chemical purity with a specific activity of 3174 ± 345 Ci/mmol (EOS). This new facile radiosynthesis routinely affords highly pure quantities of [ ¹⁸ F]1 , which will further enable tracer development of OP cholinesterase inhibitors and their evaluation in vivo .     

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.     

Synthesis of 2‐[18F]fluoroadenosine (2‐[18F]FAD) as potential radiotracer for studying malignancies by PET

2‐[¹⁸F]fluoroadenosine (2‐[¹⁸F]FAD), a potential radioligand for assessment of adenylate metabolism, was synthesized by carrier‐added and no‐carrier‐added procedures via nucleophilic radiofluorination of 2‐fluoroadenosine and 2‐iodoadenosine. The radiochemical yield, specific radioactivity and radiochemical purity of carrier‐added and no‐carrier‐added 2‐[¹⁸F]FAD were 5%, 22–30 mCi/µmol and 99%, and 0.5%, 1200–1700 mCi/µmol and 99%, respectively. Copyright © 2006 John Wiley & Sons, Ltd.