The N.C.A. nucleophilic 18F-fluorination of 1,N-disubstituted alkanes as fluoroalkylation agents

Open-source flow setup for rapid and efficient [18 F]fluoride drying for automation of PET tracer syntheses

One of the key strategies for radiochemical research facilities is the automation of synthesis processes. Unnecessary manual operations increase the radiation exposure of personnel, while simultaneously threatening the reliability of syntheses. We have previously reported an affordable open-source system comprising 3D-printed continuous flow reactors, a custom syringe pump, and a pressure regulator that can be used to perform radiofluorinations. In this paper, we address additional essential processes that are needed for radiotracer development and synthesis, with the aim of making laboratory work safer and research more efficient. We have designed and evaluated a fully automated system for rapidly and effectively processing and drying aqueous [18 F]fluoride that can be directly connected to the cyclotron. This process relies on triflyl fluoride gas generation and allows nucleophilic [18 F]fluoride to be prepared safely in a hotcell within 10 min and an activity recovery of 91.7 ± 1.6% (n = 5). Owing to the need for convenient radiofluorinated prosthetic ligands, we have adapted our continuous flow system to produce [18 F]fluoroethyl tosylate (FEOTs) and [18 F]fluoroethyl triflate (FEOTf), prosthetic groups that are widely used for late-stage fluoroethylation of PET tracers. The processes as well as the radiolabeling of different groups are compared and comprehensively discussed. Having a method providing [18 F]fluoroethyl tosylate (FEOTs) as well as [18 F]fluoroethyl triflate (FEOTf) quickly and highly efficiently is beneficial for radiochemical research.

Radiosynthesis of 18F-fluoroethylated tracers via a simplified one-pot 18F-fluoroethylation method using [18F]fluoroethyl tosylate

Recently, a straightforward one-pot method for ¹⁸F-fluoroethylation without azeotropic drying of cyclotron-produced [¹⁸F]F^- was developed. In this study, we have attempted to simplify the automated radiosynthesis of two [¹⁸F]fluoroethylated tracers, [¹⁸F]FEDAC and [¹⁸F]FET, using a desmethyl labeling precursor and [¹⁸F]fluoroethyl tosylate, based on the above-mentioned method. The radiochemical yields of [¹⁸F]FEDAC and [¹⁸F]FET were 26 ± 3.7% (n = 5) and 14 ± 2.2% (n = 4), respectively, based on total [¹⁸F]F^- at the end of irradiation.

Reagents for Selective Fluoromethylation: A Challenge in Organofluorine Chemistry

The introduction of a monofluoromethyl moiety has undoubtedly become a very important area of research in recent years. Owing to the beneficial properties of organofluorine compounds, such as their metabolic stability, the incorporation of the CH2 F group as a bioisosteric substitute for various functional groups is an attractive strategy for the discovery of new pharmaceuticals. Furthermore, the monofluoromethyl unit is also widely used in agrochemistry, in pharmaceutical chemistry, and in fine chemicals. The problems associated with climate change and the growing need for environmentally friendly industrial processes mean that alternatives to the frequently used CFC and HFBC fluoromethylating agents (CH2 FCl and CH2 FBr) are urgently needed and also required by the Montreal Protocol. This has recently prompted many researchers to develop alternative fluoromethylation agents. This Minireview summarizes both the classical and new generation of fluoromethylating agents. Reagents that act via electrophilic, nucleophilic, and radical pathways are discussed, in addition to their precursors.

Investigation on the impact of three different quaternary methyl ammonium cartridges on the radiosynthetic yields of [18 F]fluoromethyl tosylate

Our recent investigations for the radiosynthesis of [¹⁸ F]fluoromethyl tosylate have highlighted that choice of quaternary methyl ammonium (QMA) cartridge used during the radiosynthesis can significantly impact the radiochemical yields. Often the details of the QMA cartridge used in fluourine-18 syntheses are not fully described. However, our studies demonstrate that the type, the size, and nature (method by which it has been conditioned) of the QMA cartridge used during the radiosynthesis can make a significant impact in the labelling efficiency. This paper investigates the use of three QMA cartridges and demonstrates that radiochemical yield (decay corrected) of [¹⁸ F]fluoromethyl tosylate can increase from 46% to 60% by simply changing the QMA cartridge (and leaving all other reagents and labelling conditions exactly the same). These learnings may be applied to improve the radiochemical yields of a number of [¹⁸ F]-fluorinated tracers (and synthons), where the labelling step is base-sensitive to increase the radiochemical yield, thereby significantly benefiting the radiochemistry and nuclear medicine community. This paper also highlights the necessity of the radiochemistry community to ensure the details of QMA cartridges used in fluorine-18 chemistry are fully and accurately described, since this will improve the translation of radiochemical methods from one laboratory to another.

First in vivo evaluation of a potential SPECT brain radiotracer for the gonadotropin releasing hormone receptor

OBJECTIVES

In vivo evaluations of a gonadotropin releasing hormone-receptor single photon emission computed tomography radiotracer for non-invasive detection of gonadotropin releasing homone-receptors in brain.

RESULTS

We have used a simple, robust and high-yielding procedure to radiolabel an alpha-halogenated bioactive compound with high radiochemical yield. Literature findings showed similar alpha-halogenated compounds suitable for in vivo evaluations. The compound was found to possess nano molar affinity for the gonadotropin releasing hormone-receptor in a competition dependent inhibition study. Furthermore, liquid chromatography-mass spectrometry analysis in saline, human and rat serum resulted in 46%, 52% and 44% stability after incubation for 1 h respectively. In addition, rat brain single photon emission computed tomography and biodistribution studies gave further insight into the nature of the compound as a radiotracer.

Challenges in the automated synthesis of [18F]-1-fluoroethyl tryptophan: Formation of both O- and N-alkylated products

[¹⁸F]Fluoroethyl tosylate was synthesized using an automated "Synthra" module using ethylene di-tosylate and [¹⁸F]fluoride/K₂₂₂/K₂CO₃ in acetonitrile. [¹⁸F]Fluoroethyl tosylate was purified by semi-preparative HPLC followed by reformulation using a C18 Sep-Pak cartridge and eluted with DMF. Using this [¹⁸F]fluoroethyl tosylate, we attempted to alkylate protected tryptophan aiming to obtain the N-[¹⁸F]fluoroethyl-t-Boc-tryptophan methyl ester. Initial attempts resulted in the formation of the O-alkylated, rather than N-alkylated product. Manual removal of the cartridge from the automated module, followed by an extended drying of the cartridge under high flow nitrogen, was required to form the desired N-alkylated product. This demonstrates that the drying process in automated modules requires modification for sensitive N-alkylation of compounds and may be essential for compounds like tryptophan methyl ester that have multiple potential sites of alkylation in their chemical structure.

Small Prosthetic Groups in 18F-Radiochemistry: Useful Auxiliaries for the Design of 18F-PET Tracers

Prosthetic group (PG) applications in ¹⁸F-radiochemistry play a pivotal role among current ¹⁸F^-labeling techniques for the development and availability of ¹⁸F-labeled imaging probes for PET (Wahl, 2002) (¹). The introduction and popularization of PGs in the mid-80s by pioneers in ¹⁸F-radiochemistry has profoundly changed the landscape of available tracers for PET and has led to a multitude of new imaging agents based on simple and efficiently synthesized PGs. Because of the chemical nature of anionic ¹⁸F^- (apart from electrophilic low specific activity ¹⁸F-fluorine), radiochemistry before the introduction of PGs was limited to simple nucleophilic substitutions of leaving group containing precursor molecules. These precursors were not always available, and some target compounds were either hard to synthesize or not obtainable at all. Even with the advent of recently introduced "late-stage fluorination" techniques for the ¹⁸F-fluorination of deactivated aromatic systems, PGs will continue to play a central role in ¹⁸F-radiochemistry because of their robust and almost universal usability. The importance of PGs in radiochemistry is shown by its current significance in tracer development and exemplified by an overview of selected methodologies for PG attachment to PET tracer molecules. Especially, click-chemistry approaches to PG conjugation, while furthering the historical evolution of PGs in PET tracer design, play a most influential role in modern PG utilization. All earlier and recent multifaceted approaches in PG development have significantly enriched the contingent of modern ¹⁸F-radiochemistry procedures and will continue to do so.

Development of molecular tools based on the dopamine D3 receptor ligand FAUC 329 showing inhibiting effects on drug and food maintained behavior

Dopamine D₃ receptor-mediated networks have been associated with a wide range of neuropsychiatric diseases, drug addiction and food maintained behavior, which makes D₃ a highly promising biological target. The previously described dopamine D₃ receptor ligand FAUC 329 (1) showed protective effects against dopamine depletion in a MPTP mouse model of Parkinson's disease. We used the radioligand [¹⁸F]2, a [¹⁸F]fluoroethoxy substituted analog of the lead compound 1 as a molecular tool for visualization of D₃-rich brain regions including the islands of Calleja. Furthermore, structural modifications are reported leading to the pyrimidylpiperazine derivatives 3 and 9 displaying superior subtype selectivity and preference over serotonergic receptors. Evaluation of the lead compound 1 on cocaine-seeking behavior in non-human primates showed a substantial reduction in cocaine self-administration behavior and food intake.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

Syntheses and evaluation of carbon-11- and fluorine-18-radiolabeled pan-tropomyosin receptor kinase (Trk) inhibitors: exploration of the 4-aza-2-oxindole scaffold as Trk PET imaging agents

Tropomyosin receptor kinases (TrkA/B/C) are critically involved in the development of the nervous system, in neurological disorders as well as in multiple neoplasms of both neural and non-neural origins. The development of Trk radiopharmaceuticals would offer unique opportunities toward a more complete understanding of this emerging therapeutic target. To that end, we first developed [(11)C]GW441756 ([(11)C]9), a high affinity photoisomerizable pan-Trk inhibitor, as a lead radiotracer for our positron emission tomography (PET) program. Efficient carbon-11 radiolabeling afforded [(11)C]9 in high radiochemical yields (isolated RCY, 25.9% ± 5.7%). In vitro autoradiographic studies in rat brain and TrkB-expressing human neuroblastoma cryosections confirmed that [(11)C]9 specifically binds to Trk receptors in vitro. MicroPET studies revealed that binding of [(11)C]9 in the rodent brain was mostly nonspecific despite initial high brain uptake (SUVmax = 2.0). Modeling studies of the 4-aza-2-oxindole scaffold led to the successful identification of a small series of high affinity fluorinated and methoxy derivatized pan-Trk inhibitors based on our lead compound 9. Out of this series, the fluorinated compound 10 was selected for initial evaluation and radiolabeled with fluorine-18 (isolated RCY, 2.5% ± 0.6%). Compound [(18)F]10 demonstrated excellent Trk selectivity in a panel of cancer relevant kinase targets and a promising in vitro profile in tumors and brain sections but high oxidative metabolic susceptibility leading to nonspecific brain distribution in vivo. The information gained in this study will guide further exploration of the 4-aza-2-oxindole scaffold as a lead for Trk PET ligand development.

2-[18F]Fluoroethyl tosylate – a versatile tool for building18F-based radiotracers for positron emission tomography

The review highlights the role of 2-[¹⁸F]fluoroethyltosylate ([¹⁸F]FETs) in PET radiotracer design since it is a preferred labeling reagent according to its high reactivity to phenolic, amine, thiophenolic and carboxylic functions.

Radiosynthesis and in Vivo Evaluation of Two PET Radioligands for Imaging α-Synuclein

Two α-synuclein ligands, 3-methoxy-7-nitro-10H-phenothiazine (2a, K_i = 32.1 ± 1.3 nM) and 3-(2-fluoroethoxy)-7-nitro-10H-phenothiazine (2b, K_i = 49.0 ± 4.9 nM), were radiolabeled as potential PET imaging agents by respectively introducing ¹¹C and ¹⁸F. The syntheses of [¹¹C]2a and [¹⁸F]2b were accomplished in a good yield with high specific activity. Ex vivo biodistribution studies in rats revealed that both [¹¹C]2a and [¹⁸F]2b crossed the blood-brain barrier (BBB) and demonstrated good brain uptake 5 min post-injection. MicroPET imaging of [¹¹C]2a in a non-human primate (NHP) confirmed that the tracer was able to cross the BBB with rapid washout kinetics from brain regions of a healthy macaque. The initial studies suggested that further structural optimization of [¹¹C]2a and [¹⁸F]2b is necessary in order to identify a highly specific positron emission tomography (PET) radioligand for in vivo imaging of α-synuclein aggregation in the central nervous system (CNS).

Synthesis of novel halo and tosyloxy nortropane derivatives as efficient precursors for the one-step synthesis of the dopamine transporter PET ligand [(18)F]FECNT

The fluorine-18 labeled nortropane derivative 2β-carbomethoxy-3β-(4-chlorophenyl)-8-(2-fluoroethyl)-nortropane (FECNT) is a dopamine transporter (DAT) ligand. Currently, it is considered as reference for positron emission tomography imaging. Herein, the synthesis of novel precursors (N-tosyloxy-, chloro-, and bromo- analogues) for one-step radiosynthesis of [(18)F]FECNT is reported. Using the N-mesyloxy- precursor in a one-step radiosynthesis, the crude [(18)F]FECNT was obtained with the radiolabeling yield of 45 ± 10%, confirming the practical efficiency of this approach in the design of novel precursors for labeling.

A fully automated radiosynthesis of [18F]fluoroethyl-diprenorphine on a single module by use of SPE cartridges for preparation of high quality 2-[18F]fluoroethyl tosylate

We have developed a new method for automated production of 2-[¹⁸F]fluoroethyl tosylate ([¹⁸F]FETos) that enables ¹⁸F-alkylation to provide PET tracers with high chemical purity. The method is based on the removal of excess ethylene glycol bistosylate precursor by precipitation and subsequent filtration and purification of the filtrate by means of solid phase extraction cartridges (SPE). The method is integrated to a single synthesis module and thereby provides the advantage over previous methods of not requiring HPLC purification, as demonstrated by the full radiosynthesis of the potent opioid receptor PET tracer [¹⁸F]fluoroethyldiprenorphine.

Radiosynthesis and first evaluation in mice of [(18)F]NS14490 for molecular imaging of α7 nicotinic acetylcholine receptors

[(18)F]NS14490, a new potential radiotracer for neuroimaging of α7 nicotinic acetylcholine receptors (α7 nAChRs), was synthesized and evaluated in vitro and in vivo. Radioligand binding studies using [(3)H]methyllycaconitine and NS14490 as competitor showed a good target affinity (K(i,α7) = 2.5 nM) and a high selectivity towards other nAChRs. Radiosynthesis of [(18)F]NS14490 was performed by two different labelling procedures: a two-step synthesis using a prosthetic group, which led to 7% labelling yield, and the convenient direct nucleophilic substitution of the corresponding tosylate precursor, which resulted in 70% labelling yield. After optimisation of the isolation, purification and formulation process, biodistribution studies were performed in CD-1 mice. The brain uptake of [(18)F]NS14490 was comparably low (0.16% ID g(-1) wet weight at 5 min p.i.). The radiotracer showed a high metabolic stability in plasma and brain. Also, the target specificity was proven by pre-administration of a highly affine α7 ligand providing a rationale basis for further in vivo evaluation.

Radiosynthesis and Radiotracer Properties of a 7-(2-[18F]Fluoroethoxy)-6-methoxypyrrolidinylquinazoline for Imaging of Phosphodiesterase 10A with PET

Phosphodiesterase 10A (PDE10A) is a key enzyme of intracellular signal transduction which is involved in the regulation of neurotransmission. The molecular imaging of PDE10A by PET is expected to allow a better understanding of physiological and pathological processes related to PDE10A expression and function in the brain. The aim of this study was to develop a new ¹⁸F-labeled PDE10A ligand based on a 6,7-dimethoxy-4-pyrrolidinylquinazoline and to evaluate its properties in biodistribution studies. Nucleophilic substitution of the 7-tosyloxy-analogue led to the 7-[¹⁸F]fluoroethoxy-derivative [¹⁸F]IV with radiochemical yields of 25% ± 9% (n = 9), high radiochemical purity of ≥99% and specific activities of 110–1,100 GBq/μmol. [¹⁸F]IV showed moderate PDE10A affinity (K_D,PDE10A = 14 nM) and high metabolic stability in the brain of female CD-1 mice, wherein the radioligand entered rapidly with a peak uptake of 2.3% ID/g in striatum at 5 min p.i. However, ex vivo autoradiographic and in vivo blocking studies revealed no target specific accumulation and demonstrated [¹⁸F]IV to be inapplicable for imaging PDE10A with PET.

Development of a new epidermal growth factor receptor positron emission tomography imaging agent based on the 3-cyanoquinoline core: synthesis and biological evaluation

The epidermal growth factor receptor (EGFR/c-ErbB1/HER1) is overexpressed in many cancers including breast, ovarian, endometrial, and non-small cell lung cancer. An EGFR specific imaging agent could facilitate clinical evaluation of primary tumors and/or metastases. To achieve this goal we designed and synthesized a small array of fluorine containing compounds based on a 3-cyanoquinoline core. A lead compound, 16, incorporating 2'-fluoroethyl-1,2,3-triazole was selected for evaluation as a radioligand based on its high affinity for EGFR kinase (IC50=1.81+/-0.18 nM), good cellular potency (IC50=21.97+/-9.06 nM), low lipophilicity and good metabolic stability. 'Click' labeling afforded [18F]16 in 37.0+/-3.6% decay corrected radiochemical yield based on azide [18F]14 and 7% end of synthesis (EOS) yield from aqueous fluoride. Compound [18F]16 was obtained with >99% radiochemical purity in a total synthesis time of 3 h. The compound showed good stability in vivo and a fourfold higher uptake in high EGFR expressing A431 tumor xenografts compared to low EGFR expressing HCT116 tumor xenografts. Furthermore, the radiotracer could be visualized in A431 tumor bearing mice by small animal PET imaging. Compound [18F]16 therefore constitutes a promising radiotracer for further evaluation for imaging of EGFR status.

Automated synthesis of an 18F-labelled pyridine-based alkylating agent for high yield oligonucleotide conjugation

Alkylating agents have been shown to be very promising for the radiolabelling of oligonucleotides with fluorine-18. In this report we describe the fully automated synthesis of 2-bromo-N-[3-(2-[(18)F]fluoropyridin-3-yloxy)propyl]acetamide ([(18)F]FPyBrA) utilizing a modular synthesis unit. Reaction conditions for the coupling of this pyridine-based alkylating agent at the 5' end of a fully phosphorothioated random 20-mer DNA sequence were optimized to achieve very high radiochemical yields (>90%) and a maximum specific activity of 5-6 GBq/micromoL. The potential for rapid purification by solid phase extraction without need of chromatographic isolation of the radiolabelled oligonucleotide presents an overall benefit for the application of oligonucleotides in preclinical studies and potential clinical applications.

18F-Labeled FAUC 346 and BP 897 derivatives as subtype-selective potential PET radioligands for the dopamine D3 receptor

Disturbances of neutrotransmission at the dopamine D3 receptor are related to several neuropsychiatric diseases and in particular to drug addiction. Herein, we report the computer-assisted prediction of D3 selectivities of new fluoroalkoxy-substituted receptor ligands by means of 3D-QSAR analysis. As close analogues of the D3-selective lead compound FAUC 346 and BP 879, the (19)F-substituted test compounds 4 a-d were synthesized and evaluated. In vitro investigation of their binding characteristics in transfected Chinese Hamster Ovary (CHO) cells led to excellent K(i) values between 0.12 and 0.69 nM at the dopamine D3 subtype. The benzothiophene-substituted carboxamide 4 a (K(i)=0.12 nM) displayed 133 and 283-fold selectivity over the structurally related D2(Long) and D4 subtypes, respectively. Mitogenesis assays showed the behavior of partial agonists. Based on these data, we synthesized the [(18)F]fluoroethoxy-substituted radioligands [(18)F]4 a-d. The N-[4-[4-(2-hydroxyphenyl)piperazin-1-yl]butyl]-2-carboxamides 3 a-d were prepared and labeled with 2-[(18)F]fluoroethyltosylate in a two-step procedure. Optimization of the (18)F-labeling conditions led to radiochemical yields between 24 and 65 %.

3-(4-(6-Fluoroalkoxy-3,4-dihydroisoquinoline-2(1H)-yl)cyclohexyl)-1H-indole-5-carbonitriles for SERT imaging: chemical synthesis, evaluation in vitro and radiofluorination

Aminocyclohexyl indoles bind with high affinity and specificity toward the serotonin transporter (SERT). Based on this structural lead, we designed fluoroalkoxydihydroisoquinoline-cyclohexyl indole carbonitriles for future application as (18)F-labeled tracers for SERT imaging by PET. Six compounds, three pairs of cis- and trans-isomer derivatives, respectively, were synthesized and evaluated in vitro. The chemistry of the new compounds, their affinity and specificity data, the general route to the phenolic precursor for labeling, and the successful (18)F-fluoroalkylation of one pair of compounds are described herein.

(18)F-labeled positron emission tomographic radiopharmaceuticals in oncology: an overview of radiochemistry and mechanisms of tumor localization

Molecular imaging is the visualization, characterization, and measurement of biological processes at the molecular and cellular levels in a living system. At present, positron emission tomography/computed tomography (PET/CT) is one the most rapidly growing areas of medical imaging, with many applications in the clinical management of patients with cancer. Although [(18)F]fluorodeoxyglucose (FDG)-PET/CT imaging provides high specificity and sensitivity in several kinds of cancer and has many applications, it is important to recognize that FDG is not a "specific" radiotracer for imaging malignant disease. Highly "tumor-specific" and "tumor cell signal-specific" PET radiopharmaceuticals are essential to meet the growing demand of radioisotope-based molecular imaging technology. In the last 15 years, many alternative PET tracers have been proposed and evaluated in preclinical and clinical studies to characterize the tumor biology more appropriately. The potential clinical utility of several (18)F-labeled radiotracers (eg, fluoride, FDOPA, FLT, FMISO, FES, and FCH) is being reviewed by several investigators in this issue. An overview of design and development of (18)F-labeled PET radiopharmaceuticals, radiochemistry, and mechanism(s) of tumor cell uptake and localization of radiotracers are presented here. The approval of clinical indications for FDG-PET in the year 2000 by the Food and Drug Administration, based on a review of literature, was a major breakthrough to the rapid incorporation of PET into nuclear medicine practice, particularly in oncology. Approval of a radiopharmaceutical typically involves submission of a "New Drug Application" by a manufacturer or a company clearly documenting 2 major aspects of the drug: (1) manufacturing of PET drug using current good manufacturing practices and (2) the safety and effectiveness of a drug with specific indications. The potential routine clinical utility of (18)F-labeled PET radiopharmaceuticals depends also on regulatory compliance in addition to documentation of potential safety and efficacy by various investigators.

Synthesis and biological evaluation of a fluorine-18 derivative of dasatinib

Tyrosine kinases often play pivotal roles in the pathogenesis of cancer and are good candidates for therapeutic intervention and targeted molecular imaging. The precursor synthesis, radiosynthesis, and biological characterization of a fluorine-18 analog of dasatinib, a multitargeted kinase inhibitor, are reported. Compound 5 potently inhibits Abl, Src, and Kit kinases and inhibits K562 and M07e/p210bcr-abl human leukemic cell growth. Using positron emission tomography, we visualized K562 tumor xenografts in mice with [18F]-5.

Micro-reactors for PET tracer labeling

Miniaturization of PET radiosynthesis devices (micro-reactors or microfluidic systems) is an emerging area that has the potential to deliver many advantages, such as more efficient use of hot-cell space for production of multiple radiotracers; use of less non-radioactive precursor for saving precious material and a reduced separation challenge; highly controlled, reproducible and reliable radiotracer production; and cheap, interchangeable, disposable and quality-assured radiochemistry processors. Several 'proof of principle' examples along with basics of micro-reactor flow control, mixing principle and design, and device fabrication are discussed in this chapter.

Fluorine-18 labeling methods: Features and possibilities of basic reactions

Many experimental and established tracers make fluorine- 18 the most widely used radionuclide in positron emission tomography with an increasing demand for new or simpler 18F-labeling procedures. After a brief summary of the advantages of the nuclide and its major production routes, the basic features of the principal radiofluorination methods are described. These comprise direct electrophilic and nucleophilic processes, or in case of more complex molecules, the labeling of synthons and prosthetic groups for indirect built-up syntheses. While addressing the progress of no-carrier-added 18F-labeling procedures, the following chapters on more specific topics in this book are introduced. Emphasis is given to radiofluorination of arenes--especially with iodonium leaving groups. Examples of radiopharmaceutical syntheses are mentioned in order to illustrate strategic concepts of labeling with fluorine-18.

Synthesis, enantiomeric resolution, F-18 labeling and biodistribution of reboxetine analogs: promising radioligands for imaging the norepinephrine transporter with positron emission tomography

Racemic and enantiomerically pure ((S,S) and (R,R)) 2-[alpha-(2-(2-[(18)F]fluoroethoxy)phenoxy)benzyl]morpholine ([(18)F]FRB) and its tetradeuterated form [(18)F]FRB-D(4), analogs of the highly selective norepinephrine reuptake inhibitor reboxetine (2-[alpha-(2-ethoxyphenoxy)benzyl]morpholine, RB), have been synthesized for studies of norepinephrine transporter (NET) system with positron emission tomography (PET). The [(18)F]fluorinated precursor, (S,S)/(R,R)-N-tert-butyloxycarbonyl-2-[alpha-(2-hydroxyphenoxy)benzyl]morpholine ((S,S)/(R,R)-N-Boc-desethylRB), was prepared by the N-protection of (S,S)/(R,R)-2-[alpha-(2-hydroxyphenoxy)benzyl]morpholine ((S,S)/(R,R)-desethylRB) with a tert-butyloxycarbonyl (Boc) group followed by enantiomeric resolution with chiral HPLC to provide both (S,S) and (R,R) enantiomers with >99% enantiomeric purity. These compounds were then used for radiosynthesis to prepare enantiomerically pure [(18)F]FRB and [(18)F]FRB-D(4) via the following three-step procedure: (1) formation of 1-bromo-2-[(18)F]fluoroethane ([(18)F]BFE or [(18)F]BFE-D(4)) by nucleophilic displacement of 2-bromoethyl triflate (or D(4) analog) with no-carrier added [(18)F]F(-) in THF; (2) reaction of [(18)F]BFE (or [(18)F]BFE-D(4)) with N-Boc-desethylRB in DMF in the presence of excess base; and (3) deprotection with trifluoroacetic acid. The racemates, (S,S) and (R,R) enantiomers of [(18)F]FRB and [(18)F]FRB-D(4) were obtained in 11-27% (decay corrected to the end of bombardment, EOB) in 120-min synthesis time with a radiochemical purity of >98% and specific activities of 21-48 GBq/micromol (EOB). The results of the whole-body biodistribution studies with (S,S)-[(18)F]FRB-D(4) were similar to those with (S,S)-[(18)F]FRB but showed relatively faster blood clearance and no significant in vivo defluorination. Positron emission tomography studies in baboon brain also showed that (S,S)-[(18)F]FRB-D(4) may be a potentially useful ligand for imaging NET with PET.