Fluorine‐18 labeling of peptide nucleic acids

Peptide nucleic acids (PNAs) are a unique class of synthetic macromolecules, originally designed as ligands for the recognition of double stranded DNA. From a chemical point of view, the deoxyribose phosphate backbone of DNA is replaced by a pseudo‐peptide N‐(2‐aminoethyl)glycyl backbone, while the nucleobases of DNA (adenine, guanine, cytosine and thymine) are retained. Due to the increasing interest in the labeling of peptide nucleic acids (PNAs) as potent diagnostic agents in nuclear medicine, we have used and adapted the reliable methodology developed for the fluorine‐18 labeling of oligonucleotides and have now demonstrated that it is possible to label PNAs in sufficient quantity and with high specific radioactivity for PET studies in a time compatible with the half life of fluorine‐18. Copyright © 2002 John Wiley & Sons, Ltd.     

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.     

Fluorine‐18‐ and iodine‐125‐labelling of spiegelmers

Spiegelmers are high‐affinity l‐enantiomeric oligonucleotide ligands (aptamers) that display high resistance to enzymatic degradation compared to d‐oligonucleotides. Spiegelmers belong to the third generation of aptamers, and are currently extensively investigated as potential therapeutic agents. We have previously developed an original method to label natural oligonucleotides 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 labelling of Spiegelmers, both with fluorine‐18 for positron emission tomography imaging and iodine‐125 for high resolution autoradiography. Three 25‐mer l‐oligonucleotides have been used, differing (a) by the position of the terminal phosphorothioate monoester group (3′‐ or 5′‐end, and therefore differing by the position of the labelling on the macromolecule) and (b) by the nature of the backbone sugar moiety (2′‐OH or 2′‐H, therefore covering the RNA and DNA series, respectively). N‐(4‐[¹⁸F]fluorobenzyl)‐2‐bromoacetamide was synthesized in three radiochemical steps from 4‐cyano‐N,N,N‐trimethylanilinium trifluoromethanesulfonate and HPLC‐purified in 90 min (typical production: 2.2–2.4 GBq starting from a batch of 22–24 GBq of [¹⁸F]fluoride). N‐(4‐[¹²⁵I]iodobenzyl)‐2‐bromoacetamide was synthesized from the corresponding trimethylsilyl derivative (one pot, two radiochemical steps) and HPLC‐purified in 60 min (typical production: 24 MBq starting from 37 MBq of Na[¹²⁵I]I). Coupling of the Spiegelmers with the appropriate HPLC‐purified [radiolabelled]‐halobenzyl‐2‐bromoacetamide (MeOH/PBS (0.1 M, pH 8), 10 min, 120°C) gave the corresponding labelled conjugated Spiegelmers after RP‐HPLC purification. For fluorine‐18, the whole synthetic procedure yields up to 1.1 GBq of pure labelled Spiegelmers in 160 min with a specific radioactivity of 37–74 GBq/μmol at the end of synthesis starting from 22–24 GBq of [¹⁸F]fluoride. For iodine‐125, the whole synthetic procedure allows producing up to 7.4 MBq of pure labelled Spiegelmers in 100 min with a specific radioactivity of 11–37 GBq/μmol starting from 37 MBq of Na[¹²⁵I]I. Copyright © 2003 John Wiley & Sons, Ltd.     

Fluorine‐18 labelling of small interfering RNAs (siRNAs) for PET imaging

Small interfering RNAs (siRNAs), a class of macromolecules constituted by the association of two single‐stranded ribonucleic acids of short sequences, have been labelled with the positron‐emitter fluorine‐18 (T_1/2: 109.8 min). The strategy involves (1) prosthetic conjugation of a single‐stranded oligonucleotide with [¹⁸F]FPyBrA (N‐[3‐(2‐[¹⁸F]fluoropyridin‐3‐yloxy)‐propyl]‐2‐bromoacetamide) followed by (2) formation of the target duplex by annealing with the complementary sequence, therefore, permitting parallel and combinatorial preparation of [¹⁸F]siRNAs. Pure fluorine‐18‐labelled siRNAs (0.55–1.11 GBq, specific activity: 74–148 GBq/µmol at EOB) could be obtained within 165 min starting from 37.0 GBq of starting [¹⁸F]fluoride (1.5–3.0%, non‐decay‐corrected isolated yields). Copyright © 2007 John Wiley & Sons, Ltd.     

Fluorine‐18 labeling of 6,7‐disubstituted anilinoquinazoline derivatives for positron emission tomography (PET) imaging of tyrosine kinase receptors: synthesis of 18F‐Iressa and related molecular probes

Inhibitors of tyrosine kinase enzymatic activity represent a promising new class of antineoplastic agents. Although clinical studies performed over the last decade give more insight on the potential therapeutic applications of such drugs, identification of the individual patients who might benefit from them remains a major challenge. We have developed a synthetic strategy for the production of a wide variety of radiolabeled 6,7‐disubstituted 4‐anilinoquinazolines suitable for noninvasive imaging of tyrosine kinase receptors to predict therapy effectiveness. Three new F‐18 labeled radiopharmaceuticals based on the therapeutic agents Tarceva, Iressa, and ZD6474 were synthesized. Decay‐corrected yields varied between 25 and 40% for a total synthesis time of 120 min, thus providing F‐18 labeled tyrosine kinase inhibitors in quantities and times practical for use as PET radiopharmaceuticals. Copyright © 2005 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.     

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.     

Synthesis of an 18F‐fluorobenzoate idarubicin derivative as new potential PET radiotracer to predict chemotherapy resistance

Anthracyclines are among the most widely used antineoplastic agents in current clinical practice. Nevertheless, chemoresistance, which results in failure to eradicate the tumor, is often observed after administration of anthracyclines, and no assay system has yet been found to accurately predict tumor resistance to those antitumor agents. We sought to prepare an F‐18 labeled derivative of idarubicin, a 4‐demethoxy‐daunorubicin analogue, to use in helping to assess physiologic resistance to anthracyclines in vivo. Two different synthetic pathways, which required the preparation of the key intermediate [¹⁸F]fluorobenzoic acid ([¹⁸F]FBA), are advanced to label idarubicin with F‐18 on its primary amine. The first approach yielded the desired [¹⁸F]fluorobenzoate idarubicin derivative in two steps from [¹⁸F]FBA, while the second strategy consisted of a direct acylation of idarubicin by treatment with [¹⁸F]FBA in presence of diethyl cyanophosphonate. Although the first method led to fewer byproducts, it required more time to obtain the HPLC‐purified radiopharmaceutical (100 min vs 90 min) and resulted in lower radiochemical yields (8–25% vs 25–39% decay corrected from starting fluoride). Copyright © 2005 John Wiley & Sons, Ltd.     

18F‐Labeled benzylpiperazine derivatives as highly selective ligands for imaging σ1 receptor with positron emission tomography

We report the design, synthesis, and evaluation of a new series of benzylpiperazine derivatives as selective σ₁ receptor ligands. All seven ligands possessed low nanomolar affinity for σ₁ receptors (K_i(σ₁) = 0.31‐4.19 nM) and high subtype selectivity (K_i(σ₂)/K_i(σ₁) = 50‐2448). The fluoroethoxy analogues also exhibited high selectivity toward the vesicular acetylcholine transporter (K_i(VAChT)/K_i(σ₁) = 99‐18252). The corresponding radiotracers [¹⁸F] 13 , [¹⁸F] 14 , and [¹⁸F] 16 with high selectivity (K_i(σ₂)/K_i(σ₁) > 100, K_i(VAChT)/K_i(σ₁) > 1000) were prepared in 42% to 55% radiochemical yields (corrected for decay), greater than 99% radiochemical purity (RCP), and molar activity of about 120 GBq/μmol at the end of synthesis (EOS). All three radiotracers showed high initial brain uptake in mouse (8.37‐11.48% ID/g at 2 min), which was not affected by pretreatment with cyclosporine A, suggesting that they are not substrates for permeability‐glycoprotein (P‐gp). Pretreatment with SA4503 or haloperidol resulted in significantly reduced brain uptake (35%‐62% decrease at 30 min). In particular, [¹⁸F] 16 displayed high brain‐to‐blood ratios and high in vivo metabolic stability. Although it may not be an optimal neuroimaging agent because of its slow kinetics in the mouse brain, [¹⁸F] 16 can serve as a lead compound for further structural modifications to explore new potential radiotracers for σ₁ receptors.     

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.     

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.     

Synthesis and evaluation of an 18F‐labeled trifluoroborate derivative of 2‐nitroimidazole for imaging tumor hypoxia with positron emission tomography

2‐Nitroimidazole‐based hypoxia imaging tracers such as ¹⁸F‐FMISO are normally imaged at late time points (several hours post‐injection) due to their slow clearance from background tissues. Here, we investigated if a hydrophilic zwitterion‐based ammoniomethyl‐trifluoroborate derivative of 2‐nitroimidazole, ¹⁸F‐AmBF₃‐Bu‐2NI, could have the potential to image tumor hypoxia at earlier time points. AmBF₃‐Bu‐2NI was prepared in 4 steps. ¹⁸F labeling was conducted via ¹⁸F‐¹⁹F isotope exchange reaction, and ¹⁸F‐AmBF₃‐Bu‐2NI was obtained in 14.8 ± 0.4% (n = 3) decay‐corrected radiochemical yield with 24.5 ± 5.2 GBq/μmol specific activity and >99% radiochemical purity. Imaging and biodistribution studies in HT‐29 tumor‐bearing mice showed that ¹⁸F‐AmBF₃‐Bu‐2NI cleared quickly from blood and was excreted via the hepatobiliary and renal pathways. However, the tumor was not visualized in PET images until 3 hours post‐injection due to low tumor uptake (0.54 ± 0.13 and 0.19 ± 0.04%ID/g at 1 and 3 hours post‐injection, respectively). The low tumor uptake is likely due to the highly hydrophilic motif of ammoniomethyl‐trifluoroborate that prevents free diffusion of ¹⁸F‐AmBF₃‐Bu‐2NI across the cell membrane. Our results suggest that highly hydrophilic ¹⁸F‐labeled ammoniomethyl‐trifluoroborate derivatives might not be suitable for imaging intracellular targets including nitroreductase, a common tumor hypoxia imaging target.     

Preparation of fluorine‐18‐labelled fluoromisonidazole using two different synthesis methods

¹⁸F‐labelled fluoromisonidazole [1H‐1‐(3‐[¹⁸F]fluoro‐2‐hydroxypropyl)‐2‐nitroimida‐zole; ([¹⁸F]FMISO)] is used as an in vivo marker of hypoxic cells in tumours and ischaemic areas of the heart and the brain. The compound plays an important role in evaluating the oxygenation status in tumours during radiotherapy. In this paper, we report experiments carried out in our laboratory in synthesizing [¹⁸F]FMISO using two different methods. The first method (I) for the [¹⁸F]FMISO synthesis was the fluorination of (2R)‐(?)‐glycidyl tosylate to [¹⁸F]epifluorohydrin. The subsequent nucleophilic ring opening, achieved with 2‐nitroimidazole, leads to labelled FMISO. The second method (II) was the fluorination of the protected precursor 1‐(2′‐nitro‐1′‐imidazolyl)‐2‐O‐tetrahydropyranyl‐3‐O‐toluenesulphonyl‐propanediol, followed by a rapid removal of the protecting group. With the first method, the radiochemical yield was about 10% at the end of the synthesis (EOS), and the radiochemical purity was over 99%. The radiochemical yield in the second method was 21% (EOS) on an average, and the radiochemical purity was over 97%. When an automated commercial synthesis module was used with method II, slightly better and more reproducible yields were achieved. The improvement in the synthesis yield with the automated apparatus will be valuable when working with high activities, and therefore it is under further development. Copyright © 2003 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.     

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.     

Fluorine‐18 labelling of a series of potential EGFRvIII targeting peptides with a parallel labelling approach using [18F]FPyME

There is growing interest in the use of radiolabelled peptides as receptor targeting agents for diagnostic imaging of various cancer types using positron emission tomography. In this work, 1‐[3‐(2‐[¹⁸F]fluoropyridin‐3‐yloxy)propyl]pyrrole‐2,5‐dione ([¹⁸F]FPyME) has been used for parallel fluorine‐18 labelling of PEPHC1, a peptide selective towards the cancer‐specific mutation of the epidermal growth factor receptor (EGFRvIII), and a number of truncated and mutated analogues. Conjugation of the peptides with [¹⁸F]FPyME was achieved within 10 min in non‐decay‐corrected radiochemical yields of 30–50%. The high yield of the conjugation reaction combined with its short synthesis time allows the labelling of several peptides from a single batch of [¹⁸F]FPyME. Copyright © 2010 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.     

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.     

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.     

Microwave applications in radiolabelling with short‐lived positron‐emitting radionuclides

The use of microwave dielectric heating to reduce reaction times in organic transformations is rapidly increasing worldwide. Besides the time gains from simply performing reactions faster, other advantages have been noted, e.g. cleaner reaction mixtures due to decreased sample decomposition and altered product distributions as well as improved chemical flexibility due to the ability to accelerate typically sluggish reactions of less activated substrates. Microwave applications in radiolabelling tracers for positron emission tomography, paralleling and sometimes preceding developments in other areas of microwave‐enhanced chemistry, are reviewed here. Copyright © 2002 John Wiley & Sons, Ltd.