Production Of Radiotracers Research Articles

A Convenient Synthesis of Difluoroalkyl Ethers from Thionoesters Using Silver(I) Fluoride.

Herein we report the mild and rapid fluorodesulfurization of thionoesters using only silver(I) fluoride. This reaction demonstrates excellent functional group tolerance and complements existing strategies for difluoroalkyl ether synthesis, which rely on toxic and often dangerous reagents that demonstrate limited functional group compatibility. We additionally report the translation of this finding to the production of 18 F-labelled difluoroalkyl ethers using fluoride-derived [18 F]AgF. This new process should enable the synthesis of a wide range of difluoroalkyl ethers with applications in medicinal and materials chemistry, and radiotracer production.

Solid Phase 11C-Methylation, Purification and Formulation for the Production of PET Tracers.

Routine production of radiotracers used in positron emission tomography (PET) mostly relies on wet chemistry where the radioactive synthon reacts with a non-radioactive precursor in solution. This approach necessitates purification of the tracer by high performance liquid chromatography (HPLC) followed by reformulation in a biocompatible solvent for human administration. We recently developed a novel 11C-methylation approach for the highly efficient synthesis of carbon-11 labeled PET radiopharmaceuticals, taking advantage of solid phase cartridges as disposable "3-in-1" units for the synthesis, purification and reformulation of the tracers. This approach obviates the use of preparative HPLC and reduces the losses of the tracer in transfer lines and due to radioactive decay. Furthermore, the cartridge-based technique improves synthesis reliability, simplifies the automation process and facilitates compliance with the Good Manufacturing Practice (GMP) requirements. Here, we demonstrate this technique on the example of production of a PET tracer Pittsburgh compound B ([11C]PiB), a gold standard in vivo imaging agent for amyloid plaques in the human brains.

In-loop flow [11 C]CO2 fixation and radiosynthesis of N,N'-[11 C]dibenzylurea.

Cyclotron‐produced carbon‐11 is a highly valuable radionuclide for the production of positron emission tomography (PET) radiotracers. It is typically produced as relatively unreactive carbon‐11 carbon dioxide ([11C]CO2), which is most commonly converted into a more reactive precursor for synthesis of PET radiotracers.The development of [11C]CO2 fixation methods has more recently enabled the direct radiolabelling of a diverse array of structures directly from [11C]CO2, and the advantages afforded by the use of a loop‐based system used in 11C‐methylation and 11C‐carboxylation reactions inspired us to apply the [11C]CO2 fixation “in‐loop.” In this work, we developed and investigated a new ethylene tetrafluoroethylene (ETFE) loop‐based [11C]CO2 fixation method, enabling the fast and efficient, direct‐from‐cyclotron, in‐loop trapping of [11C]CO2 using mixed DBU/amine solutions. An optimised protocol was integrated into a proof‐of‐concept in‐loop flow radiosynthesis of N,N′‐[11C]dibenzylurea. This reaction exhibited an average 78% trapping efficiency and a crude radiochemical purity of 83% (determined by radio‐HPLC), giving an overall nonisolated radiochemical yield of 72% (decay‐corrected) within just 3 minutes from end of bombardment.This proof‐of‐concept reaction has demonstrated that efficient [11C]CO2 fixation can be achieved in a low‐volume (150 μL) ETFE loop and that this can be easily integrated into a rapid in‐loop flow radiosynthesis of carbon‐11–labelled products.This new in‐loop methodology will allow fast radiolabelling reactions to be performed using cheap/disposable ETFE tubing setup (ideal for good manufacturing practice production) thereby contributing to the widespread usage of [11C]CO2 trapping/fixation reactions for the production of PET radiotracers.

11C‐Acetylation of Amines with [11C]Methyl Iodide with Bis(cyclopentadienyldicarbonyliron) as the CO Source

We describe herein a novel approach for the direct 11C‐acetylation of amines. The carbonylative protocol is palladium‐mediated, uses bis(cyclopentadienyldicarbonyliron) as the CO source, and [11C]methyl iodide or [11C]methyl iodide‐D3 as a radioactive precursor. A set of functionalized primary and secondary amines was 11C‐labelled in radiochemical yields ranging from 7–85 %. The potential use of this method for positron emission tomography radiotracer production was additionally demonstrated by the radiosynthesis of [11C]lacosamide, [11C]melatonine, and [11C]acecainide in 44–55 % RCY.

Organocatalyst-assisted Ar-18F bond formation: a universal procedure for direct aromatic radiofluorination.

A metal free, phosphane-catalysed protocol for C-F bond formation based on ipso-elimination of iodanes is described. The reaction proceeds in significantly higher yield and with markedly reduced variability of yields (yield range) than previously described procedures. Rapid and efficient radiofluorination is achieved using unobjectionable chemical reagents with precedence in routine radiotracer production. With operational ease and mild conditions, the method promises a high potential for radiolabelling of biomolecules.

Production of Ga-68 radiotracers under GMP and regulatory aspects - a German perspective

Searchable abstracts of presentations at key conferences in endocrinology ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

Production of gaseous radiotracers for industrial applications

This paper describes prerequisite tests, analysis and the procedure for irradiation of gaseous targets and production of gaseous radioisotopes i.e. argon-41 (41Ar) and krypton-79 (79Kr) in a 100MWTh DHRUVA reactor located at Bhabha Atomic Research Center (BARC), Trombay, Mumbai, India. The produced radioisotopes will be used as radiotracers for tracing gas phase in industrial process systems. Various details and prequalification tests required for irradiation of gaseous targets are discussed. The procedure for regular production of 41Ar and 79Kr, and assay of their activity were standardized. Theoretically estimated and experimentally produced amounts of activities of the two radioisotopes, irradiated at identical conditions, were compared and found to be in good agreement. Based on the various tests, radiological safety analysis and standardization of the irradiation procedure, necessary approval was obtained from the competent reactor operating and safety authorities for regular production of gaseous radiotracers in DHRUVA reactor.

Synthesis of 18F-Arenes from Spirocyclic Iodonium(III) Ylides via Continuous-Flow Microfluidics.

Spirocyclic hypervalent iodine(III) ylides have proven to be synthetically versatile precursors for efficient radiolabelling of a diverse range of non-activated (hetero)arenes, highly functionalised small molecules, building blocks and radiopharmaceuticals from [18F]fluoride ion. Herein, we report the implementation of these reactions onto a continuous-flow microfluidic platform, thereby offering an alterative and automated synthetic procedure of a radiopharmaceutical, 3-[18F]fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ([18F]FPEB) and a routinely used building block for click-radiochemistry, 4-[18F]fluorobenzyl azide. This new protocol was applied to the synthesis of [18F]FPEB (radiochemical conversion (RCC) = 68 ± 5%) and 4-[18F]fluorobenzyl azide (RCC=68 ± 5%; isolated radiochemical yield = 24±0%). We anticipate that the high throughput microfluidic platform will accelerate the discovery and applications of 18F-labelled building blocks and labelled compounds prepared by iodonium ylide precursors as well as the production of radiotracers for preclinical imaging studies.

Improved synthesis and application of [(11) C]benzyl iodide in positron emission tomography radiotracer production.

Positron emission tomography has increased the demand for new carbon-11 radiolabeled tracers and building blocks. A promising radiolabeling synthon is [(11) C]benzyl iodide ([(11) C]BnI), because the benzyl group is a widely present functionality in biologically active compounds. Unfortunately, synthesis of [(11) C]BnI has received little attention, resulting in limited application. Therefore, we investigated the synthesis in order to significantly improve, automate, and apply it for labeling of the dopamine D2 antagonist [(11) C]clebopride as a proof of concept. [(11) C]BnI was synthesized from [(11) C]CO2 via a Grignard reaction and purified prior the reaction with desbenzyl clebopride. According to a one-pot procedure, [(11) C]BnI was synthesized in 11 min from [(11) C]CO2 with high yield, purity, and specific activity, 52 ± 3% (end of the cyclotron bombardment), 95 ± 3%, and 123 ± 17 GBq/µmol (end of the synthesis), respectively. Changes in the [(11) C]BnI synthesis are reduced amounts of reagents, a lower temperature in the Grignard reaction, and the introduction of a solid-phase intermediate purification. [(11) C]Clebopride was synthesized within 28 min from [(11) C]CO2 in an isolated decay-corrected yield of 11 ± 3% (end of the cyclotron bombardment) with a purity of >98% and specific activity (SA) of 54 ± 4 GBq/µmol (n = 3) at the end of the synthesis. Conversion of [(11) C]BnI to product was 82 ± 11%. The reliable synthesis of [(11) C]BnI allows the broad application of this synthon in positron emission tomography radiopharmaceutical development.

Automated production and quality testing of [18F]labeled radiotracers using the BG75 system

The simplification and automation of clinical PET radiotracer production, from isotope production to quality control, can streamline the current manufacturing workflow and, at the same time minimize the investment needed. In this article we present pre-clinical and clinical results showing the feasibility for manufacture of [18F]fluoride labeled radiotracers such as [18F]FDG, [18F]NaF and [18F]FMISO under automated conditions using the BG75 system.

Smoothened titania particles to improve radionuclide separation and their application to the development of a novel [68Ge]/[68Ga] generator

Smoothened titania particles for a long-lived radiopharmaceutical grade 68Ge/68Ga generator.

Slow neutron reactions in inverse kinematics for isotope production: a primer

Slow neutron inverse-kinematic reactions offer unique advantages since their kinematical conditions can be tuned to utilize very high resonance cross-sections and to separate transmuted-isotopes online. New advances in plasma sciences and thermal neutron facilities can usher practical applications of this type of reactions in some special cases. Production and implantation of isotopically-pure short-lived and spin-polarized radiotracers has been identified as a probable first application. The production rates for 110m,gAg radio-nuclides in 109Ag(n, γ) capture, that occur when a beam of 109Ag ions traverses a thermal neutron column, have been calculated and the results are presented. To develop the idea of inverse reactions in physical grounds, further analyses and advances on the non-trivial technological issues related to the generation and control of high density and high volume particle beams, isotope separation methods, and high flux thermal neutron channels of matching parameters are required and a brief discussion on these aspects are also included.

Tolerance of Water in Microfluidic Radiofluorinations: A Potential Methodological Shift?

Nucleophilic [18F]-fluorination reactions traditionally include a drying step of the labeling agent in order to achieve a successful substitution. This passage extends the time and complexity required for the whole radiotracer production, with increased hardware and detrimental effects on the radioactive recovery of such a short-lived (t½=109 min) isotope. Because the performance of radiofluorination reactions conducted under microfluidic flow conditions have been demonstrated to be more effective in terms of reaction time and yields, we have tested the tolerance to water present in this specific reaction condition, in view of eliminating the drying step in the process. To this purpose, we tested different substrates selected from typical radiofluorination intermediates. Our results show that water could be tolerated in a microfluidic environment; in particular, we observed a slight decrease in the labeling of aromatic precursors and a significant increase for iodonium salts, whereas the radiochemical yie...

Automated Solid-Phase Radiofluorination Using Polymer-Supported Phosphazenes

The polymer supported phosphazene bases PS-P2tBu and the novel PS-P2PEG allowed for efficient extraction of [18F]F− from proton irradiated [18O]H2O and subsequent radiofluorination of a broad range of substrates directly on the resin. The highest radiochemical yields were obtained with aliphatic sulfonates (69%) and bromides (42%); the total radiosynthesis time was 35–45 min. The multivariate analysis showed that the radiochemical yields and purities were controlled by the resin load, reaction temperature, and column packing effects. The resins could be reused several times with the same or different substrates. The fully automated on-column radiofluorination methodology was applied to the radiosynthesis of the important PET radiotracers [18]FLT and [18F]FDG. The latter was produced with 40% yield on a 120 GBq scale and passed GMP-regulated quality control required for commercial production of [18F]FDG. The combination of compact form factor, simplicity of [18F]F− recovery and processing, and column reusability can make solid phase radiofluorination an attractive radiochemistry platform for the emerging dose-on-demand instruments for bedside production of PET radiotracers.

Direct one-step labeling of cysteine residues on peptides with [11C]methyl triflate for the synthesis of PET radiopharmaceuticals

Radiolabeled peptides have emerged as an attractive platform for the diagnostic and therapeutic oncology. However, the (11)C-radiolabeling of peptides for positron emission tomography (PET) has been poorly explored, owing to the relatively short half-life of carbon-11 (t 1/2 = 20.3 min) and time-consuming multi-step radiochemical reactions. Existing methods have found limited use and are not routinely encountered in the production of radiotracers. Herein, we propose a facile one-step direct (11)C-methylation of cysteine residues in peptides using [(11)C]methyl triflate under ambient temperatures (20 °C) and short reaction times, on the order of seconds. Good regioselectivity of this method was demonstrated by HPLC in a simple peptide (glutathione, GSH) and a more complex test decapeptide (Trp-Tyr-Trp-Ser-Arg-Cys-Lys-Trp-Thr-Gly) bearing multiple nucleophilic sites. In addition, we extend this method towards the synthesis of [(11)C]Cys(Me)-[Tyr(3)-octreotate] as a demonstration of applicability for peptides of biological interest. This octreotate derivative was obtained in non-decay-corrected radiochemical yields of 11 ± 2 % (n = 3) with a synthesis time of approx. 30 min.

Kit-like 18F-labeling of RGD-19F-Arytrifluroborate in high yield and at extraordinarily high specific activity with preliminary in vivo tumor imaging

Positron Emission Tomography (PET) is a rapidly expanding, cutting edge technology for preclinical evaluation, cancer diagnosis and staging, and patient management. A one-step aqueous (18)F-labeling method, which can be applied to peptides to provide functional in vivo images, has been a long-standing challenge in PET imaging. Over the past few years, we have sought a rapid and mild radiolabeling method based on the aqueous radiosynthesis of in vivo stable aryltrifluoroborate (ArBF(3)(-)) conjugates. Recent access to production levels of (18)F-Fluoride led to a fluorescent-(18)F-ArBF(3)(-) at unprecedentedly high specific activities of 15Ci/μmol. However, extending this method to labeling peptides as imaging agents has not been explored. In order to extend these results to a peptide of clinical interest in the context of production-level radiosynthesis, we applied this new technology for labeling RGD, measured its specific activity by standard curve analysis, and carried out a preliminary evaluation of its imaging properties. RGD was labeled in excellent radiochemical yields at exceptionally high specific activity (~14Ci/μmol) (n = 3). Preliminary tumor-specific images corroborated by ex vivo biodistribution data with blocking controls show statistically significant albeit relatively low tumor uptake along with reasonably high tumor:blood ratios (n = 3). Isotope exchange on a clinically useful (18)F-ArBF(3)(-) radiotracer leads to excellent radiochemical yields and exceptionally high specific activities while the anionic nature of the aryltrifluoroborate prosthetic results in very rapid clearance. Since rapid clearance of the radioactive tracer is generally desirable for tracer development, these results suggest new directions for varying linker arm composition to slightly retard clearance rather than enhancing it. This work is the first to use production levels of (18)F-activity to directly label RGD at specific activities that are an order of magnitude higher than most reports and thereby increases the distribution window for radiotracer production and delivery.

Positron emission tomography oncology research in the UK

PET imaging with 18F-fluorodeoxyglucose (FDG) has become a valuable procedure in oncology patient management and drug development. The availability of non-FDG radiotracers to study different aspects of cancer biology presents new opportunities to improve healthcare outcomes and develop new therapeutics. An investigation has been carried out to determine the extent of the use of non-FDG tracers in the UK, assess overall oncology PET research activity and document current UK PET infrastructure. There has been significant recent investment in PET facilities, increasing the UK's capacity to undertake PET research. Nevertheless, the UK still has a lower level of PET equipment compared with Europe and the USA. Despite the increase in PET imaging capacity in UK research centres, there has not been an increase in non-FDG research, with most studies utilizing FDG at multiple sites. Most non-FDG trials are single-centre studies at well established centres. High tracer prices, a limited range of non-FDG tracers and restricted geographical availability make multicentre trials with non-FDG radiotracers difficult. Several solutions have been identified: formation of purchasing consortia for 18F-labelled radiotracers, production of radiotracers with longer half-lives and establishment of production at regional supply centres.

Microfluidic approach for fast labeling optimization and dose-on-demand implementation

The diffusion of PET as a pivotal molecular imaging modality has emphasized the need for new positron-emitting radiotracers to be used in diagnostic applications and research. Microfluidic represents an innovative approach, owing to its potential to increase radiochemical productivity in terms of yields, time reduction, precursor consumption and flexible experimental planning. We focused on fluorine-18 labeling and used a microfluidic platform to perform sequential reactions, by using the same batch of (18)F-labeling solution on one or more substrates, during the same experimental session. A solid-phase extraction (SPE) workup procedure was also implemented in the system to provide a repeatable purification step. We were able to quickly optimize the conditions for labeling of ethyl and propyl ditosylate and of a new cannabinoid type 2 (CB2) receptor agonist, CB41. In all substrates, we obtained good incorporation yields (60% to 85%) in short (<90 s) reaction times. Single dosages of the CB2 ligand were sequentially prepared, upon request, in satisfactory quantities and purity for small animal PET scanning. This work demonstrates the usefulness of a microfluidic-based system for a rapid optimization of temperature, flow rate of reactants and their relative ratio in the labeling of different precursors by using the same (18)F-fluoride batch. This approach was used to obtain in sequence several injectable doses of a novel CB2 ligand, thus providing the proof of principle that microfluidic systems permit a dose-on-demand production of new radiotracers.

Fast and repetitive in-capillary production of [18F]FDG

The increasing demand for radiopharmaceuticals to be provided reproducibly and flexibly with high frequency for clinical application and animal imaging would be better met by improved or even new strategies for automated tracer production. Radiosynthesis in microfluidic systems, i.e. narrow tubing with a diameter of approximately 50-500 microm, holds promise for providing the means for repetitive multidose and multitracer production. In this study, the performance of a conceptually simple microfluidic device integrated into a fully automated synthesis procedure for in-capillary radiosynthesis (ICR) of clinical grade [(18)F]FDG was evaluated. The instrumental set-up consisted of pumps for reagent and solvent delivery into small mixing chambers, micro-fluidic capillaries, in-process radioactivity monitoring, solid-phase extraction and on-column deprotection of the (18)F-labelled intermediate followed by on-line formulation of [(18)F]FDG. In-capillary(18)F-fluorination of 2.1 micromol 1,3,4,6-tetra-O-acetyl-2-O-trifluoromethanesulphonyl-beta-D-mannopyranose (TATM; precursor for [(18)F]FDG) in acetonitrile (MeCN) at a flow rate of 0.3 ml/min within 40 s and subsequent on-line hydrolysis of the intermediate by treatment with 0.3 M NaOH for 1 min at 40 degrees C resulted in a radiochemical yield of 88 +/- 4% within <7 min. Reproducibility, robustness and suitability as a fast and efficient radiopharmaceutical research tool for (18)F-fluorination was demonstrated by eight independent, sequentially performed ICRs which provided identical tracer quality (radiochemical purity >97%, MeCN <5 microg/ml) and similar absolute yields (approximately 1.4 GBq). The described ICR process is a simple and efficient alternative to classic radiotracer production systems and provides a comparatively cheap instrumental methodology for the repetitive production of [(18)F]FDG with remarkably high efficiency and high yield under fully automated conditions. Although the results concerning the levels of activity need to be confirmed after installation of the equipment in a suitable GMP hot-cell environment, we expect the instrumental design to allow up-scaling without major difficulties or fundamental restrictions. Furthermore, we are convinced that similar or nearly identical procedures, and thus instrumentation, will allow ICR of other (18)F-labelled radiopharmaceuticals.

Chemistry with [18F]Fluoride Ion (Eur. J. Org. Chem. 17/2008)

AbstractThe cover picture shows the “Chemistry with [18F]Fluoride Ion?”in PET radiotracer production. The procedure includes: (i) production of aqueous [18F]fluoride ion from the cyclotron‐promoted 18O(p,n)18F nuclear reaction on 18O‐enriched water, (ii) conversion of the aqueous [18F]fluoride ion into a more nucleophilic species by complexation with kryptand‐K+, (iii) incorporation of fluorine‐18 into a radiotracer, namely [18F]SP203, for brain mGluR5 receptors, (iv) positron emission tomography (PET) of the radiotracer in a human subject, and (v) generation of the corresponding PET image of the brain mGluR5 receptor distribution. These steps are presented over a clock background to stress the necessity to work quickly and efficiently with short‐lived fluorine‐18 (t1/2 = 109.7 min) and according to a tight time schedule from radionuclide production through to PET imaging. Details are presented in the Microreview by L. S. Cai, S. Y. Lu and V. W. Pike on p. 2853 ff. The authors thank Dr. A. K. Brown and other staff of the Molecular Imaging Branch at the National Institute of Mental Health (Bethesda, Maryland, USA) for the generation of the PET image.