Copper-Mediated Radiofluorination of Arylstannanes with [¹⁸F]KF

A copper-mediated nucleophilic radiofluorination of aryl- and vinylstannanes with [¹⁸F]­KF is described. This method is fast, uses commercially available reagents, and is compatible with both electron-rich and electron-deficient arene substrates. This method has been applied to the manual synthesis of a variety of clinically relevant radiotracers including protected [¹⁸F]­F-phenylalanine and [¹⁸F]­F-DOPA. In addition, an automated synthesis of [¹⁸F]­MPPF is demonstrated that delivers a clinically validated dose of 200 ± 20 mCi with a high specific activity of 2400 ± 900 Ci/mmol

Copper(II)-Mediated [¹¹C]Cyanation of Arylboronic Acids and Arylstannanes

A copper-mediated method for the transformation of diverse arylboron compounds and arylstannanes to aryl-[¹¹C]-nitriles is reported. This method is operationally simple, uses commercially available reagents, and is compatible with a wide variety of substituted aryl- and heteroaryl substrates. This method is applied to the automated synthesis of high specific activity [¹¹C]­perampanel in 10% nondecay-corrected radiochemical yield (RCY)

Cu-Mediated C–H ¹⁸F‑Fluorination of Electron-Rich (Hetero)arenes

This communication describes a method for the nucleophilic radiofluorination of electron-rich arenes. The reaction involves the initial C­(sp²)–H functionalization of an electron-rich arene with MesI­(OH)­OTs to form a (mesityl)­(aryl)­iodonium salt. This salt is then used in situ in a Cu-mediated radiofluorination with [¹⁸F]­KF. This approach leverages the stability and availability of electron-rich arene starting materials to enable mild late-stage radiofluorination of toluene, anisole, aniline, pyrrole, and thiophene derivatives. The radiofluorination has been automated to access a 41 mCi dose of an ¹⁸F-labeled nimesulide derivative in high (2800 ± 700 Ci/mmol) specific activity

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

Synthesis and Evaluation of [¹⁸F]RAGER: A First Generation Small-Molecule PET Radioligand Targeting the Receptor for Advanced Glycation Endproducts

The receptor for advanced glycation endproducts (RAGE) is a 35 kDa transmembrane receptor that belongs to the immunoglobulin superfamily of cell surface molecules. Its role in Alzheimer’s disease (AD) is complex, but it is thought to mediate influx of circulating amyloid-β into the brain as well as amplify Aβ-induced pathogenic responses. RAGE is therefore of considerable interest as both a diagnostic and a therapeutic target in AD. Herein we report the synthesis and preliminary preclinical evaluation of [¹⁸F]­RAGER, the first small molecule PET radiotracer for RAGE (<i>K</i>_d = 15 nM). Docking studies proposed a likely binding interaction between RAGE and RAGER, [¹⁸F]­RAGER autoradiography showed colocalization with RAGE identified by immunohistochemistry in AD brain samples, and [¹⁸F]­RAGER microPET confirmed CNS penetration and increased uptake in areas of the brain known to express RAGE. This first generation radiotracer represents initial proof-of-concept and a promising first step toward quantifying CNS RAGE activity using PET. However, there were high levels of nonspecific [¹⁸F]­RAGER binding <i>in vitro</i>, likely due to its high log <i>P</i> (experimental log <i>P</i> = 3.5), and rapid metabolism of [¹⁸F]­RAGER in rat liver microsome studies. Therefore, development of second generation ligands with improved imaging properties would be advantageous prior to anticipated translation into clinical PET imaging studies

Synthesis of Diverse ¹¹C‑Labeled PET Radiotracers via Direct Incorporation of [¹¹C]CO₂

Three new positron emission tomography (PET) radiotracers of interest to our functional neuroimaging and translational oncology programs have been prepared through new developments in [¹¹C]­CO₂ fixation chemistry. [¹¹C]­QZ (glutaminyl cyclase) was prepared via a tandem trapping of [¹¹C]­CO₂/intramolecular cyclization; [¹¹C]­tideglusib (glycogen synthase kinase-3) was synthesized through a tandem trapping of [¹¹C]­CO₂ followed by an intermolecular cycloaddition between a [¹¹C]­isocyanate and an isothiocyanate to form the 1,2,4-thiadiazolidine-3,5-dione core; [¹¹C]­ibrutinib (Bruton’s tyrosine kinase) was synthesized through a HATU peptide coupling of an amino precursor with [¹¹C]­acrylic acid (generated from [¹¹C]­CO₂ fixation with vinylmagnesium bromide). All radiochemical syntheses are fully automated on commercial radiochemical synthesis modules and provide radiotracers in 1–5% radiochemical yield (noncorrected, based upon [¹¹C]­CO₂). All three radiotracers have advanced to rodent imaging studies and preliminary PET imaging results are also reported

Targeting Metal-Aβ Aggregates with Bifunctional Radioligand [¹¹C]L2‑b and a Fluorine-18 Analogue [¹⁸F]FL2‑b

Interest in quantifying metal-Aβ species <i>in vivo</i> led to the synthesis and evaluation of [¹¹C]­L2-b and [¹⁸F]­FL2-b as radiopharmaceuticals for studying the metallobiology of Alzheimer’s disease (AD) using positron emission tomography (PET) imaging. [¹¹C]­L2-b was synthesized in 3.6% radiochemical yield (nondecay corrected, <i>n</i> = 3), >95% radiochemical purity, from the corresponding desmethyl precursor. [¹⁸F]­FL2-b was synthesized in 1.0% radiochemical yield (nondecay corrected, <i>n</i> = 3), >99% radiochemical purity, from a 6-chloro pyridine precursor. Autoradiography experiments with AD positive and healthy control brain samples were used to determine the specificity of binding for the radioligands compared to [¹¹C]­PiB, a known imaging agent for β-amyloid (Aβ) aggregates. The <i>K</i>_d for [¹¹C]­L2-b and [¹⁸F]­FL2-b were found to be 3.5 and 9.4 nM, respectively, from those tissue studies. Displacement studies of [¹¹C]­L2-b and [¹⁸F]­FL2-b with PiB and AV-45 determined that L2-b binds to Aβ aggregates differently from known radiopharmaceuticals. Finally, brain uptake of [¹¹C]­L2-b was examined through microPET imaging in healthy rhesus macaque, which revealed a maximum uptake at 2.5 min (peak SUV = 2.0) followed by rapid egress (<i>n</i> = 2)

Development of Positron Emission Tomography Radiotracers for the GABA Transporter 1

In vivo positron emission tomography (PET) imaging of the γ-aminobutyric acid (GABA) receptor complex has been accomplished using radiolabeled benzodiazepine derivatives, but development of specific presynaptic radioligands targeting the neuronal membrane GABA transporter type 1 (GAT-1) has been less successful. The availability of new structure–activity studies of GAT-1 inhibitors and the introduction of a GAT-1 inhibitor (tiagabine, Gabatril) into clinical use prompted us to reinvestigate the syntheses of PET ligands for this transporter. Initial synthesis and rodent PET studies of N-[¹¹C]­methylnipecotic acid confirmed the low brain uptake of that small and polar molecule. The common design approach to improve blood–brain barrier permeability of GAT-1 inhibitors is the attachment of a large lipophilic substituent. We selected an unsymmetrical bis-aromatic residue attached to the ring nitrogen by a vinyl ether spacer from a series recently reported by Wanner and coworkers. Nucleophilic aromatic substitution of an aryl chloride precursor with [¹⁸F]­fluoride was used to prepare the desired candidate radiotracer (<i>R</i>,<i>E</i>/<i>Z</i>)-1-(2-((4-fluoro-2-(4-[¹⁸F]­fluorobenzoyl)­styryl)­oxy)­ethyl)­piperidine-3-carboxylic acid ((<i>R</i>,<i>E</i>/<i>Z</i>)-<b>[</b>^<b>18</b><b>F]­10</b>). PET studies in rats showed no brain uptake, which was not altered by pretreatment of animals with the P-glycoprotein inhibitor cyclosporine A, indicating efflux by Pgp was not responsible. Subsequent PET imaging studies of (<i>R</i>,<i>E</i>/<i>Z</i>)-[^<b>18</b><b>F]­10</b> in rhesus monkey brain showed very low brain uptake. Finally, to test if the free carboxylic acid group was the likely cause of poor brain uptake, PET studies were done using the ethyl ester derivative of (<i>R</i>,<i>E</i>/<i>Z</i>)-<b>[</b>^<b>18</b><b>F]­10</b>. Rapid and significant monkey brain uptake of the ester was observed, followed by a slow washout over 90 min. The blood–brain barrier permeability of the ester supports a hypothesis that the free acid function limits brain uptake of nipecotic acid-based GAT-1 radioligands, and future radiotracer efforts should investigate the use of carboxylic acid bioisosteres

Evaluation of [¹¹C]<i>N</i>‑Methyl Lansoprazole as a Radiopharmaceutical for PET Imaging of Tau Neurofibrillary Tangles

[¹¹C]<i>N</i>-Methyl lansoprazole ([¹¹C]­NML, <b>3</b>) was synthesized and evaluated as a radiopharmaceutical for quantifying tau neurofibrillary tangle (NFT) burden using positron emission tomography (PET) imaging. [¹¹C]­NML was synthesized from commercially available lansoprazole in 4.6% radiochemical yield (noncorrected RCY, based upon [¹¹C]­MeI), 99% radiochemical purity, and 16095 Ci/mmol specific activity (<i>n</i> = 5). Log <i>P</i> was determined to be 2.18. A lack of brain uptake in rodent microPET imaging revealed [¹¹C]­NML to be a substrate for the rodent permeability-glycoprotein 1 (PGP) transporter, but this could be overcome by pretreating with cyclosporin A to block the PGP. Contrastingly, [¹¹C]­NML was not found to be a substrate for the primate PGP, and microPET imaging in rhesus revealed [¹¹C]­NML uptake in the healthy primate brain of ∼1600 nCi/cc maximum at 3 min followed by rapid egress to 500 nCi/cc. Comparative autoradiography between wild-type rats and transgenic rats expressing human tau (hTau +/+) revealed 12% higher uptake of [¹¹C]­NML in the cortex of brains expressing human tau. Further autoradiography with tau positive brain samples from progressive supranuclear palsy (PSP) patients revealed colocalization of [¹¹C]­NML with tau NFTs identified using modified Bielschowsky staining. Finally, saturation binding experiments with heparin-induced tau confirmed <i>K</i>_d and Bmax values of [¹¹C]­NML as 700 pM and 0.214 fmol/μg, respectively

High Affinity Radiopharmaceuticals Based Upon Lansoprazole for PET Imaging of Aggregated Tau in Alzheimer’s Disease and Progressive Supranuclear Palsy: Synthesis, Preclinical Evaluation, and Lead Selection

Abnormally aggregated tau is the hallmark pathology of tauopathy neurodegenerative disorders and is a target for development of both diagnostic tools and therapeutic strategies across the tauopathy disease spectrum. Development of carbon-11- or fluorine-18-labeled radiotracers with appropriate affinity and specificity for tau would allow noninvasive quantification of tau burden using positron emission tomography (PET) imaging. We have synthesized [¹⁸F]­lansoprazole, [¹¹C]<i>N</i>-methyl lansoprazole, and [¹⁸F]<i>N</i>-methyl lansoprazole and identified them as high affinity radiotracers for tau with low to subnanomolar binding affinities. Herein, we report radiosyntheses and extensive preclinical evaluation with the aim of selecting a lead radiotracer for translation into human PET imaging trials. We demonstrate that [¹⁸F]<i>N</i>-methyl lansoprazole, on account of the favorable half-life of fluorine-18 and its rapid brain entry in nonhuman primates, favorable kinetics, low white matter binding, and selectivity for binding to tau over amyloid, is the lead compound for progression into clinical trials