Synthesis and imaging validation of [¹⁸F]MDL100907 enabled by Ni-mediated fluorination

One-Step Synthesis of [18F]Aromatic Electrophile Prosthetic Groups via Organic Photoredox Catalysis

To avoid the harsh conditions that are oftentimes adopted in direct radiofluorination reactions, conjugation of bioactive ligands with 18F-labeled prosthetic groups has become an important strategy to construct novel PET agents under mild conditions when the ligands are structurally sensitive. Prosthetic groups with [18F]fluoroarene motifs are especially appealing because of their stability in physiological environments. However, their preparation can be intricate, often requiring multistep radiosynthesis with functional group conversions to prevent the decomposition of unprotected reactive prosthetic groups during the harsh radiofluorination. Here, we report a general and simple method to generate a variety of highly reactive 18F-labeled electrophiles via one-step organophotoredox-mediated radiofluorination. The method benefits from high step-economy, reaction efficiency, functional group tolerance, and easily accessible precursors. The obtained prosthetic groups have been successfully applied in PET agent construction and subsequent imaging studies, thereby demonstrating the feasibility of this synthetic method in promoting imaging and biomedical research.

Concise and Scalable Radiosynthesis of (+)-[18F]MDL100907 as a Serotonin 5-HT2A Receptor Antagonist for PET

5-Hydroxytryptamine (5-HT2A) receptors play an important role in several psychiatric disorders. In order to investigate the serotonin (5-HT) receptor in vivo, reliable syntheses are required for positron emission tomography (PET) 5-HT radioligands. Owing to the excellent in vivo properties of [18F]MDL100907 for PET, there has been great interest to develop a novel synthetic route for [18F]MDL100907. Here, we report a highly efficient, scalable, and expedient synthesis for [18F]MDL100907. The radiofluorination was performed on a 18F-labeling boron pinacol ester precursor, which is synthesized using the Liebeskind-Srogl cross-coupling reaction as a key step. Our method is practically more suitable to employ late-stage Cu-mediated radiofluorination and facilitate the production of the [18F]MDL100907 radioligand in excellent decay-corrected RCY of 32 ± 10% (n = 7) within 60 min. We prepared [18F]MDL100907 in high molar activity (2.1 Ci/μmol) and compared it to [11C]MDL100907 in the brain of a nonhuman primate.

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

Radiotracers for the Central Serotoninergic System

This review lists the most important radiotracers described so far for imaging the central serotoninergic system. Single-photon emission computed tomography and positron emission tomography radiotracers are reviewed and critically discussed for each receptor.

Radiosynthesis and Biological Evaluation of [18F]R91150, a Selective 5-HT2A Receptor Antagonist for PET-Imaging

Serotonergic 5-HT_2A receptors in cortical and forebrain regions are an important substrate for the neuromodulatory actions of serotonin in the brain. They have been implicated in the etiology of many neuropsychiatric disorders and serve as a target for antipsychotic, antidepressant, and anxiolytic drugs. Positron emission tomography imaging using suitable radioligands can be applied for in vivo quantification of receptor densities and receptor occupancy for therapy evaluation. Recently, the radiosynthesis of the selective 5-HT_2AR antagonist [¹⁸F]R91150 was reported. However, the six-step radiosynthesis is cumbersome and time-consuming with low radiochemical yields (RCYs) of <5%. In this work, [¹⁸F]R91150 was prepared using late-stage Cu-mediated radiofluorination to simplify its synthesis. The detailed protocol enabled us to obtain RCYs of 14 ± 1%, and the total synthesis time was reduced to 60 min. In addition, autoradiographic studies with [¹⁸F]R91150 in rat brain slices revealed the typical uptake pattern of 5-HT_2A receptor ligands.

Sequential Ir/Cu-Mediated Method for the Meta-Selective C-H Radiofluorination of (Hetero)Arenes

This article describes a sequential Ir/Cu-mediated process for the meta-selective C-H radiofluorination of (hetero)arene substrates. In the first step, Ir-catalyzed C(sp2)-H borylation affords (hetero)aryl pinacolboronate (BPin) esters. The intermediate organoboronates are then directly subjected to copper-mediated radiofluorination with [18F]tetrabutylammonium fluoride to afford fluorine-18 labeled (hetero)arenes in high radiochemical yield and radiochemical purity. This entire process is performed on a benchtop without Schlenk or glovebox techniques and circumvents the need to isolate (hetero)aryl boronate esters. The reaction was automated on a TracerLab FXFN module with 1,3-dimethoxybenzene and a meta-tyrosine derivative. The products, [18F]1-fluoro-3,5-dimethoxybenzene and an 18F-labeled meta-tyrosine derivative, were obtained in 37 ± 5% isolated radiochemical yield and >99% radiochemical purity and 25% isolated radiochemical yield and 99% radiochemical purity, and 0.52 Ci/μmol (19.24 GBq/μmol) molar activity (Am), respectively.

Direct incorporation of [18F] into Aliphatic Systems: A promising Mn-catalysed Labelling Technique for PET Imaging

BACKGROUND

One of the challenges in positron emission tomography (PET) is labelling complex aliphatic molecules.

OBJECTIVE

This study aimed to develop a method of metal-catalysed radiofluorination that is site-selective and works in moderate to good yields under facile conditions.

METHODS

Herein, we report on the optimisation of an aliphatic C-H to C-18F bond transformation catalysed by a Mn(porphyrin) complex.

RESULTS

The successful oxidation of 11 aliphatic molecules, including progesterone, is reported. Radiochemical Incorporations (RCIs) up to 69% were achieved within 60 min without the need for pre-activation or special equipment.

CONCLUSION

The method features mild conditions (60 °C) and promises to constitute a valuable approach to labelling of biomolecules and drug substances.

11C- and 18F-labelled tryptophans as PET-tracers for imaging of altered tryptophan metabolism in age-associated disorders

The ageing of the world’s population is the result of increased life expectancy observed in almost all countries throughout the world. Consequently, a rising tide of ageing-associated disorders, like cancer and neurodegenerative diseases, represents one of the main global challenges of the 21st century. The ability of mankind to overcome these challenges is directly dependent on the capability to develop novel methods for therapy and diagnosis of age-associated diseases. One hallmark of age-related pathologies is an altered tryptophan metabolism. Numerous pathological processes including neurodegenerative and neurological diseases like epilepsy, Parkinson’s and Alzheimer’s diseases, cancer and diabetes exhibit marked changes in tryptophan metabolism. Visualization of key processes of tryptophan metabolic pathways, especially using positron emission tomography (PET) and related hybrid methods like PET/CT and PET/MRI, can be exploited to early detect the aforementioned disorders with considerable accuracy, allowing appropriate and timely treatment of patients. Here we review the published 11C- and 18F-labelled tryptophans with respect to the production and also preclinical and clinical evaluation as PET-tracers for visualization of different branches of tryptophan metabolism.

The bibliography includes 159 references.

Site-Selective Late-Stage Aromatic [18 F]Fluorination via Aryl Sulfonium Salts

Site-selective functionalization of C-H bonds in small complex molecules is a long-standing challenge in organic chemistry. Herein, we report a broadly applicable and site-selective aromatic C-H dibenzothiophenylation reaction. The conceptual advantage of this transformation is further demonstrated through the two-step C-H [18 F]fluorination of a series of marketed small-molecule drugs.

Radiosynthesis of the norepinephrine transporter tracer [18 F]NS12137 via copper-mediated 18 F-labelling

[¹⁸F]NS12137 (exo‐3‐[(6‐[¹⁸F]fluoro‐2‐pyridyl)oxy]8‐azabicyclo[3.2.1]octane) is a highly selective norepinephrine transporter (NET) tracer. NETs are responsible for the reuptake of norepinephrine and dopamine and are linked to several neurodegenerative and neuropsychiatric disorders. The aim of this study was to develop a copper‐mediated ¹⁸F‐fluorination method for the production of [¹⁸F]NS12137 with straightforward synthesis conditions and high radiochemical yield and molar activity. [¹⁸F]NS12137 was produced in two steps. Radiofluorination of [¹⁸F]NS12137 was performed via a copper‐mediated pathway starting with a stannane precursor and using [¹⁸F]F⁻ as the source of the fluorine‐18 isotope. Deprotection was performed via acid hydrolysis. The radiofluorination reaction was nearly quantitative as was the deprotection based on HPLC analysis. The radiochemical yield of the synthesis was 15.1 ± 0.5%. Molar activity of [¹⁸F]NS12137 was up to 300 GBq/μmol. The synthesis procedure is straightforward and can easily be automated and adapted for clinical production.

Chemistry for Positron Emission Tomography: Recent Advances in 11 C-, 18 F-, 13 N-, and 15 O-Labeling Reactions

Positron emission tomography (PET) is a molecular imaging technology that provides quantitative information about function and metabolism in biological processes in vivo for disease diagnosis and therapy assessment. The broad application and rapid advances of PET has led to an increased demand for new radiochemical methods to synthesize highly specific molecules bearing positron-emitting radionuclides. This Review provides an overview of commonly used labeling reactions through examples of clinically relevant PET tracers and highlights the most recent developments and breakthroughs over the past decade, with a focus on 11 C, 18 F, 13 N, and 15 O.

An azeotropic drying-free approach for copper-mediated radiofluorination without addition of base

Copper-mediated radiofluorination provides a quick and versatile approach for ¹⁸ F-labeling of arenes and heteroarenes. However, this method is known to be base sensitive, which has been a barrier for preparative scale radiosynthesis. In this report, we provide an approach for copper-mediated radiofluorination without azeotropic drying or adding a base. [¹⁸ F]Fluoride trapped on a PS-HCO₃ Sep-Pak was quantitatively eluted with a solution of 4-dimethylaminopyridinium trifluoromethanesulfonate (DMAP·OTf) in anhydrous N,N-dimethylformamide (DMF). The eluted solution was directly used for copper-mediated radiofluorination. Twelve boronic ester substrates were tested, yielding fluorinated products in 27% to 83% radiochemical yield based on HPLC analysis. This approach was successfully applied to the radiosynthesis of [¹⁸ F]flumazenil, a well-known positron emission tomography (PET) tracer for imaging central benzodiazepine receptors, with a radiochemical yield of 47%. This highly efficient protocol significantly augments the powerful copper-mediated radiofluorination approach.

Optimization of 18 F-syntheses using 19 F-reagents at tracer-level concentrations and liquid chromatography/tandem mass spectrometry analysis: Improved synthesis of [18 F]MDL100907

Traditional radiosynthetic optimization faces the challenges of high radiation exposure, cost, and inability to perform serial reactions due to tracer decay. To accelerate tracer development, we have developed a strategy to simulate radioactive ¹⁸ F-syntheses by using tracer-level (nanomolar) non-radioactive ¹⁹ F-reagents and LC-MS/MS analysis. The methodology was validated with fallypride synthesis under tracer-level ¹⁹ F-conditions, which showed reproducible and comparable results with radiosynthesis, and proved the feasibility of this process. Using this approach, the synthesis of [¹⁸ F]MDL100907 was optimized under ¹⁹ F-conditions with greatly improved yield. The best conditions were successfully transferred to radiosynthesis. A radiochemical yield of 19% to 22% was achieved with the radiochemical purity >99% and the molar activity 38.8 to 53.6 GBq/ μmol (n = 3). The tracer-level ¹⁹ F-approach provides a high-throughput and cost-effective process to optimize radiosynthesis with reduced radiation exposure. This new method allows medicinal and synthetic chemists to optimize radiolabeling conditions without the need to use radioactivity.

New Trends and Current Status of Positron-Emission Tomography and Single-Photon-Emission Computerized Tomography Radioligands for Neuronal Serotonin Receptors and Serotonin Transporter

The critical role of serotonin (5-hydroxytryptamine; 5-HT) and its receptors (5-HTRs) in the pathophysiology of diverse neuropsychiatric and neurodegenerative disorders render them attractive diagnostic and therapeutic targets for brain disorders. Therefore, the in vivo assessment of binding of 5-HT receptor ligands under a multitude of physiologic and pathologic scenarios may support more-accurate identification of disease and its progression and the patient's response to therapy as well as the screening of novel therapeutic strategies. The present Review aims to focus on the current status of radioligands used for positron-emission tomography (PET) and single-photon-emission computerized tomography (SPECT) imaging of human brain serotonin receptors. We further elaborate upon and emphasize the attributes that qualify a radioligand for theranostics on the basis of its frequency of use in clinics, its benefit to risk assessment in humans, and its continuous evolution, along with the major limitations.

Synthesis of [18 F]Fluoroarenes by Nucleophilic Radiofluorination of N-Arylsydnones

A practical method for radiofluorination of anilines with [18 F]fluoride via N-arylsydnone intermediates is described. These precursors are stable, easy to handle and facilitate direct and regioselective 18 F-labeling to prepare [18 F]fluoroarenes. The value of this methodology is further highlighted by successful application to prepare an 18 F-labeled neuropeptide.

18F-Deoxyfluorination of Phenols via Ru π-Complexes

The deficiency of robust and practical methods for 18F-radiofluorination is a bottleneck for positron emission tomography (PET) tracer development. Here, we report the first transition-metal-assisted 18F-deoxyfluorination of phenols. The transformation benefits from readily available phenols as starting materials, tolerance of moisture and ambient atmosphere, large substrate scope, and translatability to generate doses appropriate for PET imaging.

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.

Fluorine-18 patents (2009-2015). Part 2: new radiochemistry

Fluorine-18 ((18)F) is one of the most common positron-emitting radionuclides used in the synthesis of positron emission tomography radiotracers due to its ready availability, convenient half-life and outstanding imaging properties. In Part 1 of this review, we presented the first analysis of patents issued for novel radiotracers labeled with fluorine-18. In Part 2, we follow-up with a focus on patents issued for new radiochemistry methodology using fluorine-18 issued between January 2009 and December 2015.

Enhanced copper-mediated (18)F-fluorination of aryl boronic esters provides eight radiotracers for PET applications

[(18)F]FMTEB, [(18)F]FPEB, [(18)F]flumazenil, [(18)F]DAA1106, [(18)F]MFBG, [(18)F]FDOPA, [(18)F]FMT and [(18)F]FDA are prepared from the corresponding arylboronic esters and [(18)F]KF/K222 in the presence of Cu(OTf)2py4. The method was successfully applied using three radiosynthetic platforms, and up to 26 GBq of non-carrier added starting activity of (18)F-fluoride.

Development of a Fluorinated Class-I HDAC Radiotracer Reveals Key Chemical Determinants of Brain Penetrance

Despite major efforts, our knowledge about many brain diseases remains remarkably limited. Epigenetic dysregulation has been one of the few leads toward identifying the causes and potential treatments of psychiatric disease over the past decade. A new positron emission tomography radiotracer, [(11)C]Martinostat, has enabled the study of histone deacetylase in living human subjects. A unique property of [(11)C]Martinostat is its profound brain penetrance, a feature that is challenging to engineer intentionally. In order to understand determining factors for the high brain-uptake of Martinostat, a series of compounds was evaluated in rodents and nonhuman primates. The study revealed the major structural contributors to brain uptake, as well as a more clinically relevant fluorinated HDAC radiotracer with comparable behavior to Martinostat, yet longer half-life.

A Transmetalation Reaction Enables the Synthesis of [18F]5-Fluorouracil from [18F]Fluoride for Human PET Imaging

Translation of new ¹⁸F-fluorination reactions to produce radiotracers for human positron emission tomography (PET) imaging is rare because the chemistry must have useful scope and the process for ¹⁸F-labeled tracer production must be robust and simple to execute. The application of transition metal mediators has enabled impactful ¹⁸F-fluorination methods, but to date none of these reactions have been applied to produce a human-injectable PET tracer. In this article we present chemistry and process innovations that culminate in the first production from [¹⁸F]fluoride of human doses of [¹⁸F]5-fluorouracil, a PET tracer for cancer imaging in humans. The first preparation of nickel σ-aryl complexes by transmetalation from arylboronic acids or esters was developed and enabled the synthesis of the [¹⁸F]5-fluorouracil precursor. Routine production of >10 mCi doses of [¹⁸F]5-fluorouracil was accomplished with a new instrument for azeotrope-free [¹⁸F]fluoride concentration in a process that leverages the tolerance of water in nickel-mediated ¹⁸F-fluorination.

(18)F-Labeling of Arenes and Heteroarenes for Applications in Positron Emission Tomography

Diverse radiochemistry is an essential component of nuclear medicine; this includes imaging techniques such as positron emission tomography (PET). As such, PET can track diseases at an early stage of development, help patient care planning through personalized medicine and support drug discovery programs. Fluorine-18 is the most frequently used radioisotope in PET radiopharmaceuticals for both clinical and preclinical research. Its physical and nuclear characteristics (97% β(+) decay, 109.8 min half-life, 635 keV positron energy) and high specific activity make it an attractive nuclide for labeling and molecular imaging. Arenes and heteroarenes are privileged candidates for (18)F-incorporation as they are metabolically robust and therefore widely used by medicinal chemists and radiochemists alike. For many years, the range of (hetero)arenes amenable to (18)F-fluorination was limited by the lack of chemically diverse precursors, and of radiochemical methods allowing (18)F-incorporation in high selectivity and efficiency (radiochemical yield and purity, specific activity, and radio-scalability). The appearance of late-stage fluorination reactions catalyzed by transition metal or small organic molecules (organocatalysis) has encouraged much research on the use of these activation manifolds for (18)F-fluorination. In this piece, we review all of the reactions known to date to install the (18)F substituent and other key (18)F-motifs (e.g., CF3, CHF2, OCF3, SCF3, OCHF2) of medicinal relevance onto (hetero)arenes. The field has changed significantly in the past five years, and the current trend suggests that the radiochemical space available for PET applications will expand rapidly in the near future.

Synthesis of [18F]Arenes via the Copper-Mediated [18F]Fluorination of Boronic Acids

A copper-mediated radiofluorination of aryl- and vinylboronic acids with K¹⁸F is described. This method exhibits high functional group tolerance and is effective for the radiofluorination of a range of electron-deficient, -neutral, and -rich aryl-, heteroaryl-, and vinylboronic acids. This method has been applied to the synthesis of [¹⁸F]FPEB, a PET radiotracer for quantifying metabotropic glutamate 5 receptors.

PET tracers for serotonin receptors and their applications

Serotonin receptors (5-HTRs) are implicated in the pathophysiology of a variety of neuropsychiatric and neurodegenerative disorders and are also targets for drug therapy. In the CNS, most of these receptors are expressed in high abundance in specific brain regions reflecting their role in brain functions. Quantifying binding to 5-HTRs in vivo may permit assessment of physiologic and pathologic conditions, and monitoring disease progression, evaluating treatment response, and for investigating new treatment modalities. Positron emission tomography (PET) molecular imaging has the sensitivity to quantify binding of 5-HTRs in CNS disorders and to measure drug occupancy as part of a process of new drug development. Although research on PET imaging of 5-HTRs have been performed more than two decades, the successful radiotracers so far developed for human studies are limited to 5-HT₁AR, 5-HT₁BR, 5-HT₂AR, 5-HT₄R and 5-HT₆R. Herein we review the development and application of radioligands for PET imaging of 5-HTRs in living brain.

Applications of Fluorine in Medicinal Chemistry

The role of fluorine in drug design and development is expanding rapidly as we learn more about the unique properties associated with this unusual element and how to deploy it with greater sophistication. The judicious introduction of fluorine into a molecule can productively influence conformation, pKa, intrinsic potency, membrane permeability, metabolic pathways, and pharmacokinetic properties. In addition, (18)F has been established as a useful positron emitting isotope for use with in vivo imaging technology that potentially has extensive application in drug discovery and development, often limited only by convenient synthetic accessibility to labeled compounds. The wide ranging applications of fluorine in drug design are providing a strong stimulus for the development of new synthetic methodologies that allow more facile access to a wide range of fluorinated compounds. In this review, we provide an update on the effects of the strategic incorporation of fluorine in drug molecules and applications in positron emission tomography.

Copper-mediated aromatic radiofluorination revisited: efficient production of PET tracers on a preparative scale

Two novel methods for copper-mediated aromatic nucleophilic radiofluorination were recently reported. Evaluation of these methods reveals that, although both are efficient in small-scale experiments, they are inoperative for the production of positron emission tomography (PET) tracers. Since high base content turned out to be responsible for low radiochemical conversions, a "low base" protocol has been developed which affords (18)F-labeled arenes from diaryliodonium salts and aryl pinacol boronates in reasonable yields. Furthermore, implementation of our "minimalist" approach to the copper-mediated [(18)F]-fluorination of (mesityl)(aryl)iodonium salts allows the preparation of (18)F-labeled arenes in excellent RCCs. The novel radiofluorination method circumvents time-consuming azeotropic drying and avoids the utilization of base and other additives, such as cryptands. Furthermore, this procedure enables the production of clinically relevant PET tracers; [(18)F]FDA, 4-[(18)F]FPhe, and [(18)F]DAA1106 are obtained in good isolated radiochemical yields. Additionally, [(18)F]DAA1106 has been evaluated in a rat stroke model and demonstrates excellent potential for visualization of translocator protein 18 kDa overexpression associated with neuroinflammation after ischemic stroke.

Late-stage [18F]Fluorination: New Solutions to Old Problems

The last 2-3 years have seen numerous relationships develop between organometallic chemists, fluorine chemists and PET Centers around the world. These collaborations have led to the development of many new strategies for the late-stage introduction of fluorine-18 into complex bioactive molecules. In this perspective we highlight recent developments and key milestones since 2011.

A philosophy for CNS radiotracer design

Decades after its discovery, positron emission tomography (PET) remains the premier tool for imaging neurochemistry in living humans. Technological improvements in radiolabeling methods, camera design, and image analysis have kept PET in the forefront. In addition, the use of PET imaging has expanded because researchers have developed new radiotracers that visualize receptors, transporters, enzymes, and other molecular targets within the human brain.

However, of the thousands of proteins in the central nervous system (CNS), researchers have successfully imaged fewer than 40 human proteins. To address the critical need for new radiotracers, this Account expounds on the decisions, strategies, and pitfalls of CNS radiotracer development based on our current experience in this area.

We discuss the five key components of radiotracer development for human imaging: choosing a biomedical question, selection of a biological target, design of the radiotracer chemical structure, evaluation of candidate radiotracers, and analysis of preclinical imaging. It is particularly important to analyze the market of scientists or companies who might use a new radiotracer and carefully select a relevant biomedical question(s) for that audience. In the selection of a specific biological target, we emphasize how target localization and identity can constrain this process and discuss the optimal target density and affinity ratios needed for binding-based radiotracers. In addition, we discuss various PET test–retest variability requirements for monitoring changes in density, occupancy, or functionality for new radiotracers.

In the synthesis of new radiotracer structures, high-throughput, modular syntheses have proved valuable, and these processes provide compounds with sites for late-stage radioisotope installation. As a result, researchers can manage the time constraints associated with the limited half-lives of isotopes. In order to evaluate brain uptake, a number of methods are available to predict bioavailability, blood–brain barrier (BBB) permeability, and the associated issues of nonspecific binding and metabolic stability. To evaluate the synthesized chemical library, researchers need to consider high-throughput affinity assays, the analysis of specific binding, and the importance of fast binding kinetics. Finally, we describe how we initially assess preclinical radiotracer imaging, using brain uptake, specific binding, and preliminary kinetic analysis to identify promising radiotracers that may be useful for human brain imaging. Although we discuss these five design components separately and linearly in this Account, in practice we develop new PET-based radiotracers using these design components nonlinearly and iteratively to develop new compounds in the most efficient way possible.