Intramolecular Friedel-Crafts Acylation of [11C]Isocyanates Enabling the Radiolabeling of [carbonyl-11C]DPQ

α,β-aromatic lactams are highly abundant in biologically active molecules, yet so far they cannot be radiolabeled with short-lived (t1/2=20.3 min), β+-decaying carbon-11, which has prevented their application as positron emission tomography tracers. Herein, we developed, optimized, and applied a widely applicable, one-pot, quick, robust and automatable radiolabeling method for α,β-aromatic lactams starting from [11C]CO2 using the reagent POCl3⋅AlCl3. This method proceeds via intramolecular Friedel-Crafts acylation of in situ formed [11C]isocyanates and shows a broad substrate scope for the formation of five- and six-membered rings. We implemented our developed labeling method for the radiosynthesis of the potential PARP1 PET tracer [carbonyl-11C]DPQ in a clinical radiotracer production facility following the standards of the European Pharmacopoeia.

Photocatalyzed radiosynthesis of 11 C-phenylacetic acids

Fast and straightforward incorporation of radionuclides into pharmaceutically relevant molecules is one of the main barriers to preclinical and clinical tracer research. Late-stage direct incorporation of cyclotron-produced [11 C]CO2 to afford carbon-11-labeled radiopharmaceuticals has the potential to provide ready-to-inject positron emission tomography agents in less than an hour. The present work describes photocatalyzed carboxylation of alkylbenzene derivatives to afford 11 C-phenylacetic acids. Reaction conditions and scope are investigated followed by application of this methodology to the preparative radiosynthesis of [11 C]fenoprofen, a nonsteroidal anti-inflammatory drug.

Pharmacological and metabolic parameters of [18F]flubrobenguane in clinical imaging populations

BACKGROUND

Cardiac sympathetic nervous system molecular imaging has demonstrated prognostic value. Compared with meta-[11C]hydroxyephedrine, [18F]flubrobenguane (FBBG) facilitates reliable estimation of SNS innervation using similar analytical methods and possesses a more convenient physical half-life. The aim of this study was to evaluate pharmacokinetic and metabolic properties of FBBG in target clinical cohorts.

METHODS

Blood sampling was performed on 20 participants concurrent to FBBG PET imaging (healthy = NORM, non-ischemic cardiomyopathy = NICM, ischemic cardiomyopathy = ICM, post-traumatic stress disorder = PTSD). Image-derived blood time-activity curves were transformed to plasma input functions using cohort-specific corrections for plasma protein binding, plasma-to-whole blood distribution, and metabolism.

RESULTS

The plasma-to-whole blood ratio was 0.78 ± 0.06 for NORM, 0.64 ± 0.06 for PTSD and 0.60 ± 0.14 for (N)ICM after 20 minutes. 22 ± 4% of FBBG was bound to plasma proteins. Metabolism of FBBG in (N)ICM was delayed, with a parent fraction of 0.71 ± 0.05 at 10 minutes post-injection compared to 0.53 ± 0.03 for PTSD/NORM. While there were variations in metabolic rate, metabolite-corrected plasma input functions were similar across all cohorts.

CONCLUSIONS

Rapid plasma clearance of FBBG limits the impact of disease-specific corrections of the blood input function for tracer kinetic modeling.

Quinazoline-2-Carboxamides as Selective PET Radiotracers for Matrix Metalloproteinase-13 Imaging in Atherosclerosis

Matrix metalloproteinase-13 (MMP-13) plays a critical role in the progression of unstable atherosclerosis. A series of highly potent and selective MMP-13 inhibitors were synthesized around a quinazoline-2-carboxamide scaffold to facilitate radiolabeling with fluorine-18 or carbon-11 positron-emitting nuclides and visualization of atherosclerotic plaques. In vitro enzyme inhibition assays identified three compounds as promising radiotracer candidates. Efficient automated radiosyntheses provided [¹¹C]5b, [¹¹C]5f, and [¹⁸F]5j and enabled pharmacokinetic characterization in atherosclerotic mice. The radiotracers displayed substantial differences in their distribution and excretion. Most favorably for vascular imaging, [¹⁸F]5j exhibited low uptake in metabolic organs with minimal retention of myocardial radioactivity, substantial renal clearance, and high metabolic stability in plasma. Ex vivo aortic autoradiography and competition studies revealed that [¹⁸F]5j specifically binds to MMP-13 within atherosclerotic plaques and localizes to lipid-rich regions. This study demonstrates the utility of the quinazoline-2-carboxamide scaffold for MMP-13 selective positron emission tomography (PET) radiotracer development and identifies [¹⁸F]5j for imaging atherosclerosis.

Synthesis and Evaluation of [11C]MCC950 for Imaging NLRP3-Mediated Inflammation in Atherosclerosis

Overexpression of the NLRP3 inflammasome has been attributed to the progressive worsening of a multitude of cardiovascular inflammatory diseases such as myocardial infarction, pulmonary arterial hypertension, and atherosclerosis. The recently discovered potent and selective NLRP3 inhibitor MCC950 has shown promise in hindering disease progression, but NLRP3-selective cardiovascular positron emission tomography (PET) imaging has not yet been demonstrated. We synthesized [¹¹C]MCC950 with no-carrier-added [¹¹C]CO₂ fixation chemistry using an iminophosphorane precursor (RCY 45 ± 4%, >99% RCP, 27 ± 2 GBq/μmol, 23 ± 3 min, n = 6) and determined its distribution both in vivo and ex vivo in C57BL/6 and atherogenic ApoE^-/- mice. Small animal PET imaging was performed in both strains following intravenous administration via the lateral tail vein and revealed considerable uptake in the liver that stabilized by 20 min (7-8.5 SUV), coincident with secondary renal excretion. Plasma metabolite analysis uncovered excellent in vivo stability of [¹¹C]MCC950 (94% intact). Ex vivo autoradiography performed on excised aortas revealed heterogeneous uptake in atherosclerotic plaques of ApoE^-/- mice in comparison to C57BL/6 controls (48 ± 17 %ID/m² vs 18 ± 8 %ID/m², p = 0.002, n = 4-5). Treatment of ApoE^-/- mice with nonradioactive MCC950 (5 mg/kg, iv) 10 min prior to radiotracer administration increased uptake in the intestine (5.3 ± 1.8 %ID/g vs 11.0 ± 3.7 %ID/g, p = 0.04, n = 4-6) and in aortic lesions (48 ± 17 %ID/m² vs 104 ± 15 %ID/m², p = 0.0002, n = 5) by 108% and 117%, respectively, without significantly increasing plasma free fraction (f_p, 1.3 ± 0.4% vs 1.7 ± 0.8%, n = 2). These results suggest that [¹¹C]MCC950 uptake demonstrates specific binding and may prove useful for in vivo NLRP3 imaging in atherosclerosis.

Aldehyde-catalysed carboxylate exchange in α-amino acids with isotopically labelled CO2

The isotopic labelling of small molecules is integral to drug development and for understanding biochemical processes. The preparation of carbon-labelled α-amino acids remains difficult and time consuming, with established methods involving label incorporation at an early stage of synthesis. This explains the high cost and scarcity of C-labelled products and presents a major challenge in 11C applications (11C t1/2 = 20 min). Here we report that aldehydes catalyse the isotopic carboxylate exchange of native α-amino acids with *CO2 (* = 14, 13, 11). Proteinogenic α-amino acids and many non-natural variants containing diverse functional groups undergo labelling. The reaction probably proceeds via the trapping of *CO2 by imine-carboxylate intermediates to generate iminomalonates that are prone to monodecarboxylation. Tempering catalyst electrophilicity was key to preventing irreversible aldehyde consumption. The pre-generation of the imine carboxylate intermediate allows for the rapid and late-stage 11C-radiolabelling of α-amino acids in the presence of [11C]CO2.

The histone H3.1 variant regulates TONSOKU-mediated DNA repair during replication

The tail of replication-dependent histone H3.1 varies from that of replication-independent H3.3 at the amino acid located at position 31 in plants and animals, but no function has been assigned to this residue to demonstrate a unique and conserved role for H3.1 during replication. We found that TONSOKU (TSK/TONSL), which rescues broken replication forks, specifically interacts with H3.1 via recognition of alanine 31 by its tetratricopeptide repeat domain. Our results indicate that genomic instability in the absence of ATXR5/ATXR6-catalyzed histone H3 lysine 27 monomethylation in plants depends on H3.1, TSK, and DNA polymerase theta (Pol θ). This work reveals an H3.1-specific function during replication and a common strategy used in multicellular eukaryotes for regulating post-replicative chromatin maturation and TSK, which relies on histone monomethyltransferases and reading of the H3.1 variant.

Evaluation of Therapeutic Collagen-Based Biomaterials in the Infarcted Mouse Heart by Extracellular Matrix Targeted MALDI Imaging Mass Spectrometry

The goal of this study was to develop strategies to localize human collagen-based hydrogels within an infarcted mouse heart, as well as analyze its impact on endogenous extracellular matrix (ECM) remodeling. Collagen is a natural polymer that is abundantly used in bioengineered hydrogels because of its biocompatibility, cell permeability, and biodegradability. However, without the use of tagging techniques, collagen peptides derived from hydrogels can be difficult to differentiate from the endogenous ECM within tissues. Imaging mass spectrometry is a robust tool capable of visualizing synthetic and natural polymeric molecular structures yet is largely underutilized in the field of biomaterials outside of surface characterization. In this study, our group leveraged a recently developed matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) technique to enzymatically target collagen and other ECM peptides within the tissue microenvironment that are both endogenous and hydrogel-derived. Using a multimodal approach of fluorescence microscopy and ECM-IMS techniques, we were able to visualize and relatively quantify significantly abundant collagen peptides in an infarcted mouse heart that were localized to regions of therapeutic hydrogel injection sites. On-tissue MALDI MS/MS was used to putatively identify sites of collagen peptide hydroxyproline site occupancy, a post-translational modification that is critical in collagen triple helical stability. Additionally, the technique could putatively identify over 35 endogenously expressed ECM peptides that were expressed in hydrogel-injected mouse hearts. Our findings show evidence for the use of MALDI-IMS in assessing the therapeutic application of collagen-based biomaterials.

Cardiac Sympathetic Positron Emission Tomography Imaging with Meta-[18F]Fluorobenzylguanidine is Sensitive to Uptake-1 in Rats

Dysfunction of the cardiac sympathetic nervous system contributes to the development of cardiovascular diseases including ischemia, heart failure, and arrhythmias. Molecular imaging probes such as meta-[¹²³I]iodobenzylguanidine have demonstrated the utility of assessing neuronal integrity by targeting norepinephrine transporter (NET, uptake-1). However, current radiotracers can report only on innervation due to suboptimal kinetics and lack sensitivity to NET in rodents, precluding mechanistic studies in these species. The objective of this work was to characterize myocardial sympathetic neuronal uptake mechanisms and kinetics of the positron emission tomography (PET) radiotracer meta-[¹⁸F]fluorobenzylguanidine ([¹⁸F]mFBG) in rats. Automated synthesis using spirocyclic iodonium(III) ylide radiofluorination produces [¹⁸F]mFBG in 24 ± 1% isolated radiochemical yield and 30-95 GBq/μmol molar activity. PET imaging in healthy rats delineated the left ventricle, with monoexponential washout kinetics (k_mono = 0.027 ± 0.0026 min^-1, A_mono = 3.08 ± 0.33 SUV). Ex vivo biodistribution studies revealed tracer retention in the myocardium, while pharmacological treatment with selective NET inhibitor desipramine, nonselective neuronal and extraneuronal uptake-2 inhibitor phenoxybenzamine, and neuronal ablation with neurotoxin 6-hydroxydopamine reduced myocardial retention by 33, 76, and 36%, respectively. Clearance of [¹⁸F]mFBG from the myocardium was unaffected by treatment with uptake-1 and uptake-2 inhibitors following peak myocardial activity. These results suggest that myocardial distribution of [¹⁸F]mFBG in rats is dependent on both NET and extraneuronal transporters and that limited reuptake to the myocardium occurs. [¹⁸F]mFBG may therefore prove useful for imaging intraneuronal dysfunction in small animals.

Riboflavin Surface Modification of Poly(vinyl chloride) for Light-Triggered Control of Bacterial Biofilm and Virus Inactivation

Poly(vinyl chloride) (PVC) is the most used biomedical polymer worldwide. PVC is a stable and chemically inert polymer. However, microorganisms can colonize PVC producing biomedical device-associated infections. While surface modifications of PVC can help improve the antimicrobial and antiviral properties, the chemically inert nature of PVC makes those modifications challenging and potentially toxic. In this work, we modified the PVC surface using a derivative riboflavin molecule that was chemically tethered to a plasma-treated PVC surface. Upon a low dosage of blue light, the riboflavin tethered to the PVC surface became photochemically activated, allowing for Pseudomonas aeruginosa bacterial biofilm and lentiviral in situ eradication.

Selective Imaging of Matrix Metalloproteinase-13 to Detect Extracellular Matrix Remodeling in Atherosclerotic Lesions

PURPOSE

Overexpression and activation of matrix metalloproteinase-13 (MMP-13) within atheroma increases susceptibility to plaque rupture, a major cause of severe cardiovascular complications. In comparison to pan-MMP targeting [¹⁸F]BR-351, we evaluated the potential for [¹⁸F]FMBP, a selective PET radiotracer for MMP-13, to detect extracellular matrix (ECM) remodeling in vascular plaques possessing markers of inflammation.

PROCEDURES

[¹⁸F]FMBP and [¹⁸F]BR-351 were initially assessed in vitro by incubation with en face aortae from 8 month-old atherogenic ApoE^-/- mice. Ex vivo biodistributions, plasma metabolite analyses, and ex vivo autoradiography were analogously performed 30 min after intravenous radiotracer administration in age-matched C57Bl/6 and ApoE^-/- mice under baseline or homologous blocking conditions. En face aortae were subsequently stained with Oil Red O (ORO), sectioned, and subject to immunofluorescence staining for Mac-2 and MMP-13.

RESULTS

High-resolution autoradiographic image analysis demonstrated target specificity and regional concordance to lipid-rich lesions. Biodistribution studies revealed hepatobiliary excretion, low accumulation of radioactivity in non-excretory organs, and few differences between strains and conditions in non-target organs. Plasma metabolite analyses uncovered that [¹⁸F]FMBP exhibited excellent in vivo stability (≥74% intact) while [¹⁸F]BR-351 was extensively metabolized (≤37% intact). Ex vivo autoradiography and histology of en face aortae revealed that [¹⁸F]FMBP, relative to [¹⁸F]BR-351, exhibited 2.9-fold greater lesion uptake, substantial specific binding (68%), and improved sensitivity to atherosclerotic tissue (2.9-fold vs 2.1-fold). Immunofluorescent staining of aortic en face cross sections demonstrated elevated Mac-2 and MMP-13-positive areas within atherosclerotic lesions identified by [¹⁸F]FMBP ex vivo autoradiography.

CONCLUSIONS

While both radiotracers successfully identified atherosclerotic plaques, [¹⁸F]FMBP showed superior specificity and sensitivity for lesions possessing features of destructive plaque remodeling. The detection of ECM remodeling by selective targeting of MMP-13 may enable characterization of high-risk atherosclerosis featuring elevated collagenase activity.

Interrupted aza-Wittig reactions using iminophosphoranes to synthesize 11C-carbonyls

A direct CO2-fixation methodology couples structurally diverse iminophosphoranes with various nucleophiles to synthesize ureas, carbamates, thiocarbamates, and amides, and is amenable for 11C radiolabeling. This methodology is practical, as demonstrated by the synthesis of >35 products and isolation of the molecular imaging radiopharmaceuticals [11C]URB694 and [11C]glibenclamide.

A low cost and open access system for rapid synthesis of large volumes of gold and silver nanoparticles

Rapid synthesis of nanomaterials in scalable quantities is critical for accelerating the discovery and commercial translation of nanoscale-based technologies. The synthesis of metal nanogold and silver in volumes larger than 100 mL is not automatized and might require of the use of harsh conditions that in most cases is detrimental for the production of nanoparticles with reproducible size distributions. In this work, we present the development and optimization of an open-access low-cost NanoParticle Flow Synthesis System (NPFloSS) that allows for the rapid preparation of volumes of up to 1 L of gold and silver nanoparticle aqueous solutions.

Fast Carbon Isotope Exchange of Carboxylic Acids Enabled by Organic Photoredox Catalysis

Carbazole/cyanobenzene photocatalysts promote the direct isotopic carboxylate exchange of C(sp³) acids with labeled CO₂. Substrates that are not compatible with transition-metal-catalyzed degradation-reconstruction approaches or prone to thermally induced reversible decarboxylation undergo isotopic incorporation at room temperature in short reaction times. The radiolabeling of drug molecules and precursors with [¹¹C]CO₂ is demonstrated.

Fluorine-18-Labeled Fluorescent Dyes for Dual-Mode Molecular Imaging

Recent progress realized in the development of optical imaging (OPI) probes and devices has made this technique more and more affordable for imaging studies and fluorescence-guided surgery procedures. However, this imaging modality still suffers from a low depth of penetration, thus limiting its use to shallow tissues or endoscopy-based procedures. In contrast, positron emission tomography (PET) presents a high depth of penetration and the resulting signal is less attenuated, allowing for imaging in-depth tissues. Thus, association of these imaging techniques has the potential to push back the limits of each single modality. Recently, several research groups have been involved in the development of radiolabeled fluorophores with the aim of affording dual-mode PET/OPI probes used in preclinical imaging studies of diverse pathological conditions such as cancer, Alzheimer's disease, or cardiovascular diseases. Among all the available PET-active radionuclides, 18F stands out as the most widely used for clinical imaging thanks to its advantageous characteristics (t1/2 = 109.77 min; 97% β+ emitter). This review focuses on the recent efforts in the synthesis and radiofluorination of fluorescent scaffolds such as 4,4-difluoro-4-bora-diazaindacenes (BODIPYs), cyanines, and xanthene derivatives and their use in preclinical imaging studies using both PET and OPI technologies.

PET and SPECT Tracers for Myocardial Perfusion Imaging

Coronary artery disease has been the leading cause of death since the 1960s, which has motivated the research and development of myocardial perfusion imaging (MPI) agents for early diagnosis and to guide treatment. MPI with SPECT has been the clinical workhorse for MPI, but over the past two decades PET MPI is experiencing growth due to enhanced image quality that results in superior diagnostic accuracy over SPECT. Furthermore, dynamic PET imaging of the tracer distribution process from time of tracer administration to tracer accumulation in the myocardium has enabled routine quantification of myocardial blood flow (MBF) and myocardial flow reserve (MFR) in absolute units. MBF and MFR incrementally improve diagnostic and prognostic accuracy over MPI alone. In some cases (eg, rubidium PET imaging with pharmacologic stress) MPI, MBF, and MFR can be acquired simultaneously without incremental cost, radiation exposure, or significant processing time. Nuclear cardiology clinics have been looking to incorporate MBF quantification into clinical routine, but traditional SPECT and MPI tracers are inadequate for this challenge. Cardiac dedicated SPECT scanners can also perform dynamic imaging and have stimulated research into MBF quantification using SPECT tracers. New perfusion tracers must be tailored for emerging clinical needs (including MBF quantification), technical capabilities of imaging instrumentation, market constraints, and supply chain feasibility. Because these conditions have been evolving, tracers previously considered inferior may be reconsidered for future applications and some recently developed tracers may be suboptimal. This article reviews current, clinically-available tracers and those under development showing greatest potential. It discusses for each tracer the rationale for development, physiological mechanism of uptake by the myocardium, published evaluation results and development state. Finally, it gauges the suitability of each tracer for clinical application. The article demonstrates an acceleration in the pace of perfusion radiotracer development due to better understanding of the relevant physiology, better chemistry tools and small animal imaging. Consequently, bad tracers may fail faster and with less wasted investment, and good tracers may translate more efficiently from bench to bedside.

Development of [18F]Maleimide-Based Glycogen Synthase Kinase-3β Ligands for Positron Emission Tomography Imaging

Dysregulation of glycogen synthase kinase-3β (GSK-3β) is implicated in the pathogenesis of neurodegenerative and psychiatric disorders. Thus, development of GSK-3β radiotracers for positron emission tomography (PET) imaging is of paramount importance, because such a noninvasive imaging technique would allow better understanding of the link between the activity of GSK-3β and central nervous system disorders in living organisms, and it would enable early detection of the enzyme's aberrant activity. Herein, we report the synthesis and biological evaluation of a series of fluorine-substituted maleimide derivatives that are high-affinity GSK-3β inhibitors. Radiosynthesis of a potential GSK-3β tracer [¹⁸F]10a is achieved. Preliminary in vivo PET imaging studies in rodents show moderate brain uptake, although no saturable binding was observed in the brain. Further refinement of the lead scaffold to develop potent [¹⁸F]-labeled GSK-3 radiotracers for PET imaging of the central nervous system is warranted.

Metal-free 18F-labeling of aryl-CF2H via nucleophilic radiofluorination and oxidative C-H activation

A metal-free and selective method to form [18F]aryl-CF2H through nucleophilic radiofluorination of benzyl (pseudo)halides and oxidative C-H activation of benzylic C-H bonds has been developed. The method is operationally simple and tolerates a variety of electron-neutral/deficient arenes and heteroarenes.

Preclinical PET Neuroimaging of [11C]Bexarotene

Activation of retinoid X receptors (RXRs) has been proposed as a therapeutic mechanism for the treatment of neurodegeneration, including Alzheimer's and Parkinson's diseases. We previously reported radiolabeling of a Food and Drug Administration-approved RXR agonist, bexarotene, by copper-mediated [¹¹C]CO₂ fixation and preliminary positron emission tomography (PET) neuroimaging that demonstrated brain permeability in nonhuman primate with regional binding distribution consistent with RXRs. In this study, the brain uptake and saturability of [¹¹C]bexarotene were studied in rats and nonhuman primates by PET imaging under baseline and greater target occupancy conditions. [¹¹C]Bexarotene displays a high proportion of nonsaturable uptake in the brain and is unsuitable for RXR occupancy measurements in the central nervous system.

(11)C[double bond, length as m-dash]O bonds made easily for positron emission tomography radiopharmaceuticals

The positron-emitting radionuclide carbon-11 ((11)C, t1/2 = 20.3 min) possesses the unique potential for radiolabeling of any biological, naturally occurring, or synthetic organic molecule for in vivo positron emission tomography (PET) imaging. Carbon-11 is most often incorporated into small molecules by methylation of alcohol, thiol, amine or carboxylic acid precursors using [(11)C]methyl iodide or [(11)C]methyl triflate (generated from [(11)C]carbon dioxide or [(11)C]methane). Consequently, small molecules that lack an easily substituted (11)C-methyl group are often considered to have non-obvious strategies for radiolabeling and require a more customized approach. [(11)C]Carbon dioxide itself, [(11)C]carbon monoxide, [(11)C]cyanide, and [(11)C]phosgene represent alternative reactants to enable (11)C-carbonylation. Methodologies developed for preparation of (11)C-carbonyl groups have had a tremendous impact on the development of novel PET tracers and provided key tools for clinical research. (11)C-Carbonyl radiopharmaceuticals based on labeled carboxylic acids, amides, carbamates and ureas now account for a substantial number of important imaging agents that have seen translation to higher species and clinical research of previously inaccessible targets, which is a testament to the creativity, utility and practicality of the underlying radiochemistry.

Synthesis and Preclinical Evaluation of Sulfonamido-based [(11)C-Carbonyl]-Carbamates and Ureas for Imaging Monoacylglycerol Lipase

Monoacylglycerol lipase (MAGL) is a 33 kDa member of the serine hydrolase superfamily that preferentially degrades 2-arachidonoylglycerol (2-AG) to arachidonic acid in the endocannabinoid system. Inhibition of MAGL is not only of interest for probing the cannabinoid pathway but also as a therapeutic and diagnostic target for neuroinflammation. Limited attempts have been made to image MAGL in vivo and a suitable PET ligand for this target has yet to be identified and is urgently sought to guide small molecule drug development in this pathway. Herein we synthesized and evaluated the physiochemical properties of an array of eleven sulfonamido-based carbamates and ureas with a series of terminal aryl moieties, linkers and leaving groups. The most potent compounds were a novel MAGL inhibitor, N-((1-(1H-1,2,4-triazole-1-carbonyl)piperidin-4-yl) methyl)-4-chlorobenzenesulfonamide (TZPU; IC₅₀ = 35.9 nM), and the known inhibitor 1,1,1,3,3,3-hexafluoropropan-2-yl 4-(((4-chlorophenyl)sulfonamido) methyl)piperidine-1-carboxylate (SAR127303; IC₅₀ = 39.3 nM), which were also shown to be selective for MAGL over fatty acid amide hydrolase (FAAH), and cannabinoid receptors (CB1 & CB2). Both of these compounds were radiolabeled with carbon-11 via [¹¹C]COCl₂, followed by comprehensive ex vivo biodistribution and in vivo PET imaging studies in normal rats to determine their brain permeability, specificity, clearance and metabolism. Whereas TZPU did not show adequate specificity to warrant further evaluation, [¹¹C]SAR127303 was advanced for preliminary PET neuroimaging studies in nonhuman primate. The tracer showed good brain permeability (ca. 1 SUV) and heterogeneous regional brain distribution which is consistent with the distribution of MAGL.

Mechanistic Studies and Radiofluorination of Structurally Diverse Pharmaceuticals with Spirocyclic Iodonium(III) Ylides

Theoretical studies provide insight into radiofluorination of non-activated electron-rich and sterically hindered ¹⁸F-arenes using a new class of adamantyl-based spirocyclic iodonium(iii) ylide precursors.

Synthesis of non-activated electron-rich and sterically hindered ¹⁸F-arenes remains a major challenge due to limitations of existing radiofluorination methodologies. Herein, we report on our mechanistic investigations of spirocyclic iodonium(iii) ylide precursors for arene radiofluorination, including their reactivity, selectivity, and stability with no-carrier-added [¹⁸F]fluoride. Benchmark calculations at the G2[ECP] level indicate that pseudorotation and reductive elimination at iodine(iii) can be modeled well by appropriately selected dispersion-corrected density functional methods. Modeling of the reaction pathways show that fluoride–iodonium(iii) adduct intermediates are strongly activated and highly regioselective for reductive elimination of the desired [¹⁸F]fluoroarenes (difference in barriers, ΔΔG^‡ > 25 kcal mol^–1). The advantage of spirocyclic auxiliaries is further supported by NMR spectroscopy studies, which bolster evidence for underlying decomposition processes which can be overcome for radiofluorination of iodonium(iii) precursors. Using a novel adamantyl auxiliary, sterically hindered iodonium ylides have been developed to enable highly efficient radiofluorination of electron-rich arenes, including fragments of pharmaceutically relevant nitrogen-containing heterocycles and tertiary amines. Furthermore, this methodology has been applied for the syntheses of the radiopharmaceuticals 6-[¹⁸F]fluoro-meta-tyrosine ([¹⁸F]FMT, 11 ± 1% isolated radiochemical yield, non-decay-corrected (RCY, n.d.c.), n = 3), and meta-[¹⁸F]fluorobenzylguanidine ([¹⁸F]mFBG, 14 ± 1% isolated RCY, n.d.c., n = 3) which cannot be directly radiolabeled using conventional nucleophilic aromatic substitution with [¹⁸F]fluoride.

Synthesis and Preliminary PET Imaging Studies of a FAAH Radiotracer ([¹¹C]MPPO) Based on α-Ketoheterocyclic Scaffold

Fatty acid amide hydrolase (FAAH) is one of the principle enzymes for metabolizing endogenous cannabinoid neurotransmitters such as anandamide, and thus regulates endocannabinoid (eCB) signaling. Selective pharmacological blockade of FAAH has emerged as a potential therapy to discern the endogenous functions of anandamide-mediated eCB pathways in anxiety, pain, and addiction. Quantification of FAAH in the living brain by positron emission tomography (PET) would help our understanding of the endocannabinoid system in these conditions. While most FAAH radiotracers operate by an irreversible ("suicide") binding mechanism, a FAAH tracer with reversibility would facilitate quantitative analysis. We have identified and radiolabeled a reversible FAAH inhibitor, 7-(2-[(11)C]methoxyphenyl)-1-(5-(pyridin-2-yl)oxazol-2-yl)heptan-1-one ([(11)C]MPPO) in 13% radiochemical yield (nondecay corrected) with >99% radiochemical purity and 2 Ci/μmol (74 GBq/μmol) specific activity. The tracer showed moderate brain uptake (0.8 SUV) with heterogeneous brain distribution. However, blocking studies with a potent FAAH inhibitor URB597 demonstrated a low to modest specificity to the target. Measurement of lipophilicity, metabolite, and efflux pathway analysis were also performed to study the pharmacokinetic profile of [(11)C]MPPO. In all, we reported an efficient radiolabeling and preliminary evaluation of the first-in-class FAAH inhibitor [(11)C]MPPO with α-ketoheterocyclic scaffold.

Ortho-Stabilized (18) F-Azido Click Agents and their Application in PET Imaging with Single-Stranded DNA Aptamers

Azido (18) F-arenes are important and versatile building blocks for the radiolabeling of biomolecules via Huisgen cycloaddition ("click chemistry") for positron emission tomography (PET). However, routine access to such clickable agents is challenged by inefficient and/or poorly defined multistep radiochemical approaches. A high-yielding direct radiofluorination for azido (18) F-arenes was achieved through the development of an ortho-oxygen-stabilized iodonium derivative (OID). This OID strategy addresses an unmet need for a reliable azido (18) F-arene clickable agent for bioconjugation reactions. A ssDNA aptamer was radiolabeled with this agent and visualized in a xenograft mouse model of human colon cancer by PET, which demonstrates that this OID approach is a convenient and highly efficient way of labeling and tracking biomolecules.

Practical Radiosynthesis and Preclinical Neuroimaging of [11C]isradipine, a Calcium Channel Antagonist

In the interest of developing in vivo positron emission tomography (PET) probes for neuroimaging of calcium channels, we have prepared a carbon-11 isotopologue of a dihydropyridine Ca²⁺-channel antagonist, isradipine. Desmethyl isradipine (4-(benzo[c][1,2,5]oxadiazol-4-yl)-5-(isopropoxycarbonyl)-2,6-dimethyl-1,4-dihydropyridine-3-carboxylic acid) was reacted with [¹¹C]CH₃I in the presence of tetrabutylammonium hydroxide in DMF in an HPLC injector loop to produce the radiotracer in a good yield (6 ± 3% uncorrected radiochemical yield) and high specific activity (143 ± 90 GBq·µmol⁻¹ at end-of-synthesis). PET imaging of normal rats revealed rapid brain uptake at baseline (0.37 ± 0.08% ID/cc (percent of injected dose per cubic centimeter) at peak, 15–60 s), which was followed by fast washout. After pretreatment with isradipine (2 mg·kg⁻¹, i.p.), whole brain radioactivity uptake was diminished by 25%–40%. This preliminary study confirms that [¹¹C]isradipine can be synthesized routinely for research studies and is brain penetrating. Further work on Ca²⁺-channel radiotracer development is planned.

PET neuroimaging studies of [(18)F]CABS13 in a double transgenic mouse model of Alzheimer's disease and nonhuman primates

Fluorine-18 labeled 2-fluoro-8-hydroxyquinoline ([(18)F]CABS13) is a promising positron emission tomography (PET) radiopharmaceutical based on a metal chelator developed to probe the "metal hypothesis of Alzheimer's disease". Herein, a practical radiosynthesis of [(18)F]CABS13 was achieved by radiofluorination followed by deprotection of an O-benzyloxymethyl group. Automated production and formulation of [(18)F]CABS13 resulted in 19 ± 5% uncorrected radiochemical yield, relative to starting [(18)F]fluoride, with ≥95% chemical and radiochemical purities, and high specific activity (>2.5 Ci/μmol) within 80 min. Temporal PET neuroimaging studies were carried out in female transgenic B6C3-Tg(APPswe,PSEN 1dE9)85Dbo/J (APP/PS1) and age-matched wild-type (WT) B6C3F1/J control mice at 3, 7, and 10 months of age. [(18)F]CABS13 showed an overall higher uptake and retention of radioactivity in the central nervous system of APP/PS1 mice versus WT mice with increasing age. However, PET/magnetic resonance imaging in normal nonhuman primates revealed that the tracer had low uptake in the brain and rapid formation of a hydrophilic radiometabolite. Identification of more metabolically stable (18)F-hydroxyquinolines that can be readily accessed by the radiochemical strategy presented herein is underway.

Stereocontrolled disruption of the Ugi reaction toward the production of chiral piperazinones: substrate scope and process development

The factors determining diastereoselectivity observed in the multicomponent conversion of amino acids, aziridine aldehyde dimers, and isocyanides into chiral piperazinones have been investigated. Amino acid-dependent selectivity for either trans- or cis-substituted piperazinone products has been achieved. An experimentally determined diastereoselectivity model for the three-component reaction driven by aziridine aldehyde dimers has predictive value for different substrate classes. Moreover, this model is useful in reconciling the previously reported observations in multicomponent reactions between isocyanides, α-amino acids, and monofunctional aldehydes.

Shifting the energy landscape of multicomponent reactions using aziridine aldehyde dimers: a mechanistic study

A multicomponent reaction between an aziridine aldehyde dimer, isocyanide, and l-proline to afford a chiral piperazinone was studied to gain insight into the stereodetermining and rate-limiting steps of the reaction. The stereochemistry of the reaction was found to be determined by isocyanide addition, while the rate-limiting step was found to deviate from traditional isocyanide-based multicomponent reactions. A first-order rate dependence on aziridine aldehyde dimer and a zero-order rate dependence on all other reagents have been obtained. Computations at the MPWPW91/6-31G(d) level supported the experimental kinetic results and provide insight into the overall mechanism and the factors contributing to stereochemical induction. These factors are similar to traditional isocyanide-based multicomponent reactions, such as the Ugi reaction. The computations revealed that selective formation of a Z-iminium ion plays a key role in controlling the stereoselectivity of isocyanide addition, and the carboxylate group of l-proline mediates stereofacial addition. These conclusions are expected to be applicable to a wide range of reported stereoselective Ugi reactions and provide a basis for understanding the related macrocyclization of peptides with aziridine aldehydes.

PET imaging of fatty acid amide hydrolase with [(18)F]DOPP in nonhuman primates

Fatty acid amide hydrolase (FAAH) regulates endocannabinoid signaling. [(11)C]CURB, an irreversibly binding FAAH inhibitor, has been developed for clinical research imaging with PET. However, no fluorine-18 labeled radiotracer for FAAH has yet advanced to human studies. [(18)F]DOPP ([(18)F]3-(4,5-dihydrooxazol-2-yl)phenyl (5-fluoropentyl)carbamate) has been identified as a promising (18)F-labeled analogue based on rodent studies. The goal of this work is to evaluate [(18)F]DOPP in nonhuman primates to support its clinical translation. High specific activity [(18)F]DOPP (5-6 Ci·μmol(-1)) was administered intravenously (iv) to three baboons (2M/1F, 3-4 years old). The distribution and pharmacokinetics were quantified following a 2 h dynamic imaging session using a simultaneous PET/MR scanner. Pretreatment with the FAAH-selective inhibitor, URB597, was carried out at 200 or 300 μg/kg iv, 10 min prior to [(18)F]DOPP administration. Rapid arterial blood sampling for the first 3 min was followed by interval sampling with metabolite analysis to provide a parent radiotracer plasma input function that indicated ∼95% baseline metabolism at 60 min and a reduced rate of metabolism after pretreatment with URB597. Regional distribution data were analyzed with 1-, 2-, and 3-tissue compartment models (TCMs), with and without irreversible trapping since [(18)F]DOPP covalently links to the active site of FAAH. Consistent with previous findings for [(11)C]CURB, the 2TCM with irreversible binding was found to provide the best fit for modeling the data in all regions. The composite parameter λk3 was therefore used to evaluate whole brain (WB) and regional binding of [(18)F]DOPP. Pretreatment studies showed inhibition of λk3 across all brain regions (WB baseline: 0.112 mL/cm(3)/min; 300 μg/kg URB597: 0.058 mL/cm(3)/min), suggesting that [(18)F]DOPP binding is specific for FAAH, consistent with previous rodent data.

Spirocyclic hypervalent iodine(III)-mediated radiofluorination of non-activated and hindered aromatics

Fluorine-18 (t½=109.7 min) is the most commonly used isotope to prepare radiopharmaceuticals for molecular imaging by positron emission tomography (PET). Nucleophilic aromatic substitution reactions of suitably activated (electron-deficient) aromatic substrates with no-carrier-added [(18)F]fluoride ion are routinely carried out in the synthesis of radiotracers in high specific activities. Despite extensive efforts to develop a general (18)F-labelling technique for non-activated arenes there is an urgent and unmet need to achieve this goal. Here we describe an effective solution that relies on the chemistry of spirocyclic hypervalent iodine(III) complexes, which serve as precursors for rapid, one-step regioselective radiofluorination with [(18)F]fluoride. This methodology proves to be efficient for radiolabelling a diverse range of non-activated functionalized arenes and heteroarenes, including arene substrates bearing electron-donating groups, bulky ortho functionalities, benzylic substituents and meta-substituted electron-withdrawing groups. Polyfunctional molecules and a range of previously elusive (18)F-labelled building blocks, compounds and radiopharmaceuticals are synthesized.

Rapid microfluidic flow hydrogenation for reduction or deprotection of 18F-labeled compounds

We have combined the benefits of both microfluidics and flow hydrogenation to provide facile access to previously underutilized reduction and protecting group chemistries for PET imaging applications. The rapid removal of an O-benzyl protecting group to prepare 2-[(18)F]fluoroquinolin-8-ol and the reduction of a nitro group in the synthesis of 4-[(18)F]fluoroaniline were achieved within 3 minutes.

Alternative approaches for PET radiotracer development in Alzheimer's disease: imaging beyond plaque

Alzheimer's disease (AD) and related dementias show increasing clinical prevalence, yet our understanding of the etiology and pathobiology of disease-related neurodegeneration remains limited. In this regard, noninvasive imaging with radiotracers for positron emission tomography (PET) presents a unique tool for quantifying spatial and temporal changes in characteristic biological markers of brain disease and for assessing potential drug efficacy. PET radiotracers targeting different protein markers are being developed to address questions pertaining to the molecular and/or genetic heterogeneity of AD and related dementias. For example, radiotracers including [(11) C]-PiB and [(18) F]-AV-45 (Florbetapir) are being used to measure the density of Aβ-plaques in AD patients and to interrogate the biological mechanisms of disease initiation and progression. Our focus is on the development of novel PET imaging agents, targeting proteins beyond Aβ-plaques, which can be used to investigate the broader mechanism of AD pathogenesis. Here, we present the chemical basis of various radiotracers which show promise in preclinical or clinical studies for use in evaluating the phenotypic or biochemical characteristics of AD. Radiotracers for PET imaging neuroinflammation, metal ion association with Aβ-plaques, tau protein, cholinergic and cannabinoid receptors, and enzymes including glycogen-synthase kinase-3β and monoamine oxidase B amongst others, and their connection to AD are highlighted.

11CO2 fixation: a renaissance in PET radiochemistry

Carbon-11 labelled carbon dioxide is the cyclotron-generated feedstock reagent for most positron emission tomography (PET) tracers using this radionuclide. Most carbon-11 labels, however, are installed using derivative reagents generated from [(11)C]CO2. In recent years, [(11)C]CO2 has seen a revival in applications for the direct incorporation of carbon-11 into functional groups such as ureas, carbamates, oxazolidinones, carboxylic acids, esters, and amides. This review summarizes classical [(11)C]CO2 fixation strategies using organometallic reagents and then focuses on newly developed methods that employ strong organic bases to reversibly capture [(11)C]CO2 into solution, thereby enabling highly functionalized labelled compounds to be prepared. Labelled compounds and radiopharmaceuticals that have been translated to the clinic are highlighted.

Comparison of Benzene, Nitrobenzene, and Dinitrobenzene 2-Arylsulfenylpyrroles

The effectiveness of the 2,4-dinitrobenzenesulfenyl and 4-nitrobenzenesulfenyl groups as masking and directing groups at the 2-position of pyrrole has been investigated and compared to that of 2-phenylthiopyrrole. The presence of the nitro group(s) enhances stability of the corresponding pyrrole toward acid and does not significantly decrease the ability of the pyrrolic unit to undergo electrophilic aromatic substitution reactions in the form of formylation, nitration, and condensation with aldehydes. The synthetic utility of 2-(2,4-dinitrobenzenesulfenyl)pyrrole was demonstrated through the synthesis of meso-substituted dipyrromethanes. The sulfoxides 2-(2,4-dinitrobenzenesulfinyl)pyrrole and 2-(4-nitrobenzenesulfinyl)pyrrole underwent neither formylation nor nitration, and the increasing presence of nitro groups within the moiety at the 2-position resulted in decreased stability under acidic conditions.