[carbonyl-C]Benzyl acetate: automated radiosynthesis via Pd-mediated [C]carbon monoxide chemistry and PET measurement of brain uptake in monkey

Gas Phase Transformations in Carbon-11 Chemistry

The short-lived positron-emitter carbon-11 (t1/2 = 20.4 min; β+, 99.8%) is prominent for labeling tracers for use in biomedical research with positron emission tomography (PET). Carbon-11 is produced for this purpose with a cyclotron, nowadays almost exclusively by the 14N(p,α)11C nuclear reaction, either on nitrogen containing a low concentration of oxygen (0.1-0.5%) or hydrogen (~5%) to produce [11C]carbon dioxide or [11C]methane, respectively. These primary radioactive products can be produced in high yields and with high molar activities. However, only [11C]carbon dioxide has some utility for directly labeling PET tracers. Primary products are required to be converted rapidly and efficiently into secondary labeling synthons to provide versatile radiochemistry for labeling diverse tracer chemotypes at molecular positions of choice. This review surveys known gas phase transformations of carbon-11 and summarizes the important roles that many of these transformations now play for producing a broad range of labeling synthons in carbon-11 chemistry.

[11C]deschloroclozapine is an improved PET radioligand for quantifying a human muscarinic DREADD expressed in monkey brain

Previous work found that [¹¹C]deschloroclozapine ([¹¹C]DCZ) is superior to [¹¹C]clozapine ([¹¹C]CLZ) for imaging Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). This study used PET to quantitatively and separately measure the signal from transfected receptors, endogenous receptors/targets, and non-displaceable binding in other brain regions to better understand this superiority. A genetically-modified muscarinic type-4 human receptor (hM₄Di) was injected into the right amygdala of a male rhesus macaque. [¹¹C]DCZ and [¹¹C]CLZ PET scans were conducted 2-24 months later. Uptake was quantified relative to the concentration of parent radioligand in arterial plasma at baseline (n = 3 scans/radioligand) and after receptor blockade (n = 3 scans/radioligand). Both radioligands had greater uptake in the transfected region and displaceable uptake in other brain regions. Displaceable uptake was not uniformly distributed, perhaps representing off-target binding to endogenous receptor(s). After correction, [¹¹C]DCZ signal was 19% of that for [¹¹C]CLZ, and background uptake was 10% of that for [¹¹C]CLZ. Despite stronger [¹¹C]CLZ binding, the signal-to-background ratio for [¹¹C]DCZ was almost two-fold greater than for [¹¹C]CLZ. Both radioligands had comparable DREADD selectivity. All reference tissue models underestimated signal-to-background ratio in the transfected region by 40%-50% for both radioligands. Thus, the greater signal-to-background ratio of [¹¹C]DCZ was due to its lower background uptake.

The development of 11C-carbonylation chemistry: A systematic view

The prospects for using carbon-11 labelled compounds in molecular imaging has improved with the development of diverse synthesis methods, including 11C-carbonylations and refined techniques to handle [11C]carbon monoxide at a nanomole scale. Facilitating biological research and molecular imaging was the driving force when [11C]carbon monoxide was used in the first in vivo application with carbon-11 in human (1945) and when [11C]carbon monoxide was used for the first time as a chemical reagent in the synthesis of [11C]phosgene (1978). This review examines a rich plethora of labelled compounds synthesized from [11C]carbon monoxide, their chemistry and use in molecular imaging. While the strong development of the 11C-carbonylation chemistry has expanded the carbon-11 domain considerably, it could be argued that the number of 11C-carbonyl compounds entering biological investigations should be higher. The reason for this may partly be the lack of commercially available synthesis instruments designed for 11C-carbonylations. But as this review shows, novel and greatly simplified methods to handle [11C]carbon monoxide have been developed. The next important challenge is to make full use of these technologies and synthesis methods in PET research. When there is a PET-tracer that meets a more general need, the incentive to implement 11C-carbonylation protocols will increase.

PET ligands [18F]LSN3316612 and [11C]LSN3316612 quantify O-linked-β-N-acetyl-glucosamine hydrolase in the brain

We aimed to develop effective radioligands for quantifying brain O-linked-β-N-acetyl-glucosamine (O-GlcNAc) hydrolase (OGA) using positron emission tomography in living subjects as tools for evaluating drug target engagement. Posttranslational modifications of tau, a biomarker of Alzheimer's disease, by O-GlcNAc through the enzyme pair OGA and O-GlcNAc transferase (OGT) are inversely related to the amounts of its insoluble hyperphosphorylated form. Increase in tau O-GlcNAcylation by OGA inhibition is believed to reduce tau aggregation. LSN3316612, a highly selective and potent OGA ligand [half-maximal inhibitory concentration (IC50) = 1.9 nM], emerged as a lead ligand after in silico analysis and in vitro evaluations. [3H]LSN3316612 imaged and quantified OGA in postmortem brains of rat, monkey, and human. The presence of fluorine and carbonyl functionality in LSN3316612 enabled labeling with positron-emitting fluorine-18 or carbon-11. Both [18F]LSN3316612 and [11C]LSN3316612 bound reversibly to OGA in vivo, and such binding was blocked by pharmacological doses of thiamet G, an OGA inhibitor of different chemotype, in monkeys. [18F]LSN3316612 entered healthy human brain avidly (~4 SUV) without radiodefluorination or adverse effect from other radiometabolites, as evidenced by stable brain total volume of distribution (VT) values by 110 min of scanning. Overall, [18F]LSN3316612 is preferred over [11C]LSN3316612 for future human studies, whereas either may be an effective positron emission tomography radioligand for quantifying brain OGA in rodent and monkey.

"In-loop" carbonylation-A simplified method for carbon-11 labelling of drugs and radioligands

Transition-metal mediated carbonylation with 11 C-labelled carbon monoxide ([11 C]CO) is a versatile method for introducing 11 C (t1/2 = 20.3 min) into drugs and radioligands for subsequent use in positron emission tomography (PET). The aim of the current study was to perform the 11 C-carbonylation reaction on the interior surface of a stainless-steel loop used for high performance liquid chromatography (HPLC). In the experimental setup, cyclotron produced 11 C-labelled carbon dioxide ([11 C]CO2 ) was converted to [11 C]CO by reduction over heated Molybdenum and swept into an HPLC loop pre-charged with the appropriate reaction mixture. Following a 5 min reaction, the radiochemical purity (RCP) and the trapping efficiency (TE) of the reaction mixture was determined. After optimization, [11 C]N-Benzylbenzamide was obtained in quantitative radiochemical yield (RCY) following a 5 min reaction at room temperature. The methodology was further applied to label [11 C]benzoic acid (RCP≥99%, TE>91%), [11 C]methyl benzoate (RCP≥99%, TE>93%) and [11 C]phthalide (RCP≥99%, TE>88%). A set of pharmaceuticals was finally radiolabelled using non-optimized conditions. Excellent yields were obtained for the histamine-3 receptor radioligand [11 C]AZ13198083, the oncology drug [11 C]olaparib and the dopamine D2 receptor radioligand [11 C]raclopride, whereas a moderate yield was observed for the high-affinity dopamine D2 receptor radioligand [11 C]FLB457. The presented "in-loop" process proved efficient for diverse 11 C-carbonylations, providing [11 C]amides, [11 C]esters and [11 C]carboxylic acids in moderate to excellent RCYs. Based on the advantages associated with performing the radiolabelling step as an integrated part of the purification system, this methodology may become a valuable addition to the toolbox of methodologies used for 11 C-carbonylation of drugs and radioligands for PET.

Development and implementation of ISAR, a new synthesis platform for radiopharmaceutical production

BACKGROUND

PET radiopharmaceutical development and the implementation of a production method on a synthesis module is a complex and time-intensive task since new synthesis methods must be adapted to the confines of the synthesis platform in use. Commonly utilized single fluid bus architectures put multiple constraints on synthesis planning and execution, while conventional microfluidic solutions are limited by compatibility at the macro-to-micro interface. In this work we introduce the ISAR synthesis platform and custom-tailored fluid paths leveraging up to 70 individually addressable valves on a chip-based consumable. The ISAR synthesis platform replaces traditional stopcock valve manifolds with a fluidic chip that integrates all fluid paths (tubing) and valves into one consumable and enables channel routing without the single fluid bus constraint. ISAR can scale between the macro- (10 mL), meso- (0.5 mL) and micro- (≤0.05 mL) domain seamlessly, addressing the macro-to-micro interface challenge and enabling custom tailored fluid circuits for a given application. In this paper we demonstrate proof-of-concept by validating a single chip design to address the challenge of synthesizing multiple batches of [13N]NH3 for clinical use throughout the workday.

RESULTS

ISAR was installed at an academic PET Center and used to manufacture [13N]NH3 in > 96% radiochemical yield. Up to 9 batches were manufactured with a single consumable chip having parallel paths without the need to open the hot-cell. Quality control testing confirmed the ISAR-based [13N]NH3 met existing clinical release specifications, and utility was demonstrated by imaging a rodent with [13N]NH3 produced on ISAR.

CONCLUSIONS

ISAR represents a new paradigm in radiopharmaceutical production. Through a new system architecture, ISAR integrates the principles of microfluidics with the standard volumes and consumables established in PET Centers all over the world. Proof-of-concept has been demonstrated through validation of a chip design for the synthesis of [13N]NH3 suitable for clinical use.

Ultra-compact, automated microdroplet radiosynthesizer

Application of microfluidics offers numerous advantages in the field of radiochemistry and could enable dramatic reductions in the cost of producing radiotracers for positron emission tomography (PET). Droplet-based microfluidics, in particular, requires only microgram quantities of expensive precursors and reagents (compared to milligram used in conventional radiochemistry systems), and occupies a more compact footprint (potentially eliminating the need for specialized shielding facilities, i.e. hot cells). However, the reported platforms for droplet radiosynthesis have several drawbacks, including high cost/complexity of microfluidic reactors, requirement for manual intervention (e.g. for adding reagents), or difficulty in precise control of droplet processes. We describe here a platform based on a particularly simple chip, where reactions take place atop a hydrophobic substrate patterned with a circular hydrophilic liquid trap. The overall supporting hardware (heater, rotating carousel of reagent dispensers, etc.) is very simple and the whole system could be packaged into a very compact format (about the size of a coffee cup). We demonstrate the consistent synthesis of [18F]fallypride with high yield, and show that protocols optimized using a high-throughput optimization platform we have developed can be readily translated to this device with no changes or re-optimization. We are currently exploring the use of this platform for routine production of a variety of 18F-labeled tracers for preclinical imaging and for production of tracers in clinically-relevant amounts by integrating the system with an upstream radionuclide concentrator.

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.

Synthesis and evaluation of two new candidate high-affinity full agonist PET radioligands for imaging 5-HT1B receptors

INTRODUCTION

The serotonin 1B receptor subtype is of interest in the pathophysiology and treatment of depression, anxiety, and migraine. Over recent years 5-HT_1B receptor binding in human brain has been examined with PET using radioligands that are partial but not full agonists. To explore how the intrinsic activity of a PET radioligand may affect imaging performance, two high-affinity full 5-HT_1B receptor agonists (AZ11136118, 4; and AZ11895987, 5) were selected from a large compound library and radiolabeled for PET examination in non-human primates.

METHODS

[¹¹C]4 was obtained through Pd(0)-mediated insertion of [¹¹C]carbon monoxide between prepared iodoarene and homochiral amine precursors. [¹¹C]5 was obtained through N-¹¹C-methylation of N-desmethyl precursor 6 with [¹¹C]methyl triflate. [¹¹C]4 and [¹¹C]5 were studied with PET in rhesus or cynomolgus monkey. [¹¹C]4 was studied with PET in mice and rats to measure brain uptake and specific binding. Ex-vivo experiments in rats were performed to identify whether there were radiometabolites in brain. Physiochemical parameters for [¹¹C]4 (pKa, logD and conformational energetics) were evaluated.

RESULTS

Both [¹¹C]4 and [¹¹C]5 were successfully produced in high radiochemical purity and in adequate amounts for PET experiments. After intravenous injection of [¹¹C]4, brain radioactivity peaked at a low level (0.2 SUV). Pretreatment with tariquidar, an inhibitor of the brain P-gp efflux transporter, increased brain exposure four-fold whereas pretreatment with a high pharmacological dose of the 5-HT_1B antagonist, AR-A000002, had no effect on the binding. Ex-vivo experiments in rats showed no radiometabolites entering brain. [¹¹C]5 also failed to enter monkey brain under baseline conditions.

CONCLUSIONS

[¹¹C]4 and [¹¹C]5 show too low brain uptake and specific binding to be useful PET radioligands. Low brain uptake is partly ascribed to efflux transporter action as well as unfavorable conformations.

[Carboxyl-11 C]Labelling of Four High-Affinity cPLA2α Inhibitors and Their Evaluation as Radioligands in Mice by Positron Emission Tomography

Cytosolic phospholipase A2α (cPLA2α) may play a critical role in neuropsychiatric and neurodegenerative disorders associated with oxidative stress and neuroinflammation. An effective PET radioligand for imaging cPLA2α in living brain might prove useful for biomedical research, especially on neuroinflammation. We selected four high-affinity (IC₅₀ 2.1-12 nm) indole-5-carboxylic acid-based inhibitors of cPLA2α, namely 3-isobutyryl-1-(2-oxo-3-(4-phenoxyphenoxy)propyl)-1H-indole-5-carboxylic acid (1); 3-acetyl-1-(2-oxo-3-(4-(4-(trifluoromethyl)phenoxy)phenoxy)propyl)-1H-indole-5-carboxylic acid (2); 3-(3-methyl-1,2,4-oxadiazol-5-yl)-1-(2-oxo-3-(4-phenoxyphenoxy)propyl)-1H-indole-5-carboxylic acid (3); and 3-(3-methyl-1,2,4-oxadiazol-5-yl)-1-(3-(4-octylphenoxy)-2-oxopropyl)-1H-indole-5-carboxylic acid (4), for labelling in carboxyl position with carbon-11 (t_1/2 =20.4 min) to provide candidate PET radioligands for imaging brain cPLA2α. Compounds [¹¹ C]1-4 were obtained for intravenous injection in adequate overall yields (1.1-5.5 %) from cyclotron-produced [¹¹ C]carbon dioxide and with moderate molar activities (70-141 GBq μmol^-1 ) through the use of Pd⁰ -mediated [¹¹ C]carbon monoxide insertion on iodo precursors. Measured logD_7.4 values were within a narrow moderate range (1.9-2.4). After intravenous injection of [¹¹ C]1-4 in mice, radioactivity uptakes in brain peaked at low values (≤0.8 SUV) and decreased by about 90 % over 15 min. Pretreatments of the mice with high doses of the corresponding non-radioactive ligands did not alter brain time-activity curves. Brain uptakes of radioactivity after administration of [¹¹ C]1 to wild-type and P-gp/BCRP dual knock-out mice were similar (peak 0.4 vs. 0.5 SUV), indicating that [¹¹ C]1 and others in this structural class, are not substrates for efflux transporters.

Pd(0)-Mediated 11C-Carbonylation of Aryl(mesityl)iodonium Salts as a Route to [11C]Arylcarboxylic Acids and Derivatives

Pd(0)-mediated ¹¹C-carbonylation of aryl(mesityl)iodonium salts followed by suitable quench provides a rapid room-temperature two-pot procedure for labeling arylcarboxylic acids and amide derivatives with the short-lived positron emitter carbon-11 (t_1/2 = 20.4 min) in generally good to high yields (up to 71%). High product ring selectivity (≥13) was achieved when using mesityl as a spectator group in the diaryliodonium salt precursors. This process has potential for preparing new radiotracers for molecular imaging with positron emission tomography.

11C-Labeling of Aryl Ketones as Candidate Histamine Subtype-3 Receptor PET Radioligands through Pd(0)-Mediated 11C-Carbonylative Coupling

Pd(0)-mediated coupling between iodoarenes, [¹¹C]carbon monoxide and aryltributylstannanes has been used to prepare simple model [¹¹C]aryl ketones. Here, we aimed to label four 2-aminoethylbenzofuran chemotype based molecules ([¹¹C]1–4) in the carbonyl position, as prospective positron emission tomography (PET) radioligands for the histamine subtype 3 receptor (H3R) by adapting this methodology with use of aryltrimethylstannanes. Radiosynthesis was successfully performed on a platform equipped with a mini-autoclave and a liquid handling robotic arm, within a lead-shielded hot-cell. Candidate radioligands were readily formulated in saline containing ethanol (10%, v/v) and ascorbic acid (0.5 mg/10 mL). Yields for preclinical use were in the range of 5–9%, decay-corrected from cyclotron-produced [¹¹C]CO₂ and molar activities were >115 GBq/µmol at end of synthesis. Radiochemical purities exceeded >97%.

(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.

Small Molecule Radiopharmaceuticals - A Review of Current Approaches

Radiopharmaceuticals are an integral component of nuclear medicine and are widely applied in diagnostics and therapy. Though widely applied, the development of an “ideal” radiopharmaceutical can be challenging. Issues such as specificity, selectivity, sensitivity, and feasible chemistry challenge the design and synthesis of radiopharmaceuticals. Over time, strategies to address the issues have evolved by making use of new technological advances in the fields of biology and chemistry. This review presents the application of few advances in design and synthesis of radiopharmaceuticals. The topics covered are bivalent ligand approach and lipidization as part of design modifications for enhanced selectivity and sensitivity and novel synthetic strategies for optimized chemistry and radiolabeling of radiopharmaceuticals.

Exploration of the labeling of [11C]tubastatin A at the hydroxamic acid site with [11C]carbon monoxide

We aimed to label tubastatin A (1) with carbon-11 (t1/2 = 20.4 min) in the hydroxamic acid site to provide a potential radiotracer for imaging histone deacetylase 6 in vivo with positron emission tomography. Initial attempts at a one-pot Pd-mediated insertion of [(11)C]carbon monoxide between the aryl iodide (2) and hydroxylamine gave low radiochemical yields (<5%) of [(11) C]1. Labeling was achieved in useful radiochemical yields (16.1 ± 5.6%, n = 4) through a two-step process based on Pd-mediated insertion of [(11)C]carbon monoxide between the aryl iodide (2) and p-nitrophenol to give the [(11)C]p-nitrophenyl ester ([(11)C]5), followed by ultrasound-assisted hydroxyaminolysis of the activated ester with excess hydroxylamine in a DMSO/THF mixture in the presence of a strong phosphazene base P1-t-Bu. However, success in labeling the hydroxamic acid group of [(11)C]tubastatin A was not transferable to the labeling of three other model hydroxamic acids.

Efficient nitrogen-13 radiochemistry catalyzed by a highly stable immobilized biocatalyst

In the present work, an unprecedented strategy for the reduction of [¹³N]NO₃⁻ to [¹³N]NO₂⁻ using a heterogeneous biocatalyst will be presented.

Improvement of brain uptake for in vivo PET imaging of astrocytic oxidative metabolism using benzyl [1-(11)C]acetate

Brain uptake of acetate is insufficient for obtaining a quantitative image of astrocytic oxidative metabolism. To improve the brain uptake of [1-(11)C]acetate, we synthesized benzyl [1-(11)C]acetate ([1-(11)C]BA) and conducted a positron emission tomography (PET) study assessing astrocytic oxidative metabolism. The brain uptake of [1-(11)C]BA was markedly higher compared with [1-(11)C]acetate, and disappeared with a half-life of 20 min in all regions studied. The brain uptake of [1-(11)C]BA was significantly decreased by fluorocitrate. The results indicate that [1-(11)C]BA could be a useful PET probe for assessing astrocytic oxidative metabolism.

Cross-coupling reactions as valuable tool for the preparation of PET radiotracers

The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of new radiotracers and novel radiolabeling procedures with the most prominent short-lived positron emitters carbon-11 and fluorine-18. Radiolabeling with these radionuclides represents a remarkable challenge. Special attention has to be paid to synthesis time and specific labeling techniques due to the short physical half life of the respective radionuclides ¹¹C (t_1/2 = 20.4 min) and ¹⁸F (t_1/2 = 109.8 min). In the past, numerous transition metal-catalyzed reactions were employed in organic chemistry, even though only a handful of these coupling reactions were adopted in radiochemical practice. Thus, the implementation of modern synthesis methods like cross-coupling reactions offers the possibility to develop a wide variety of novel radiotracers. The introduction of catalysts based on transition metal complexes bears a high potential for rapid, efficient, highly selective and functional group-tolerating incorporation of carbon-11 and fluorine-18 into target molecules. This review deals with design, application and improvement of transition metal-mediated carbon-carbon as well as carbon-heteroatom cross-coupling reactions as a labeling feature with the focus on the preparation of radiolabeled compounds for molecular imaging.