Johannes Ermert

Sleep Deprivation Increases A1 Adenosine Receptor Binding in the Human Brain: A Positron Emission Tomography Study

It is currently hypothesized that adenosine is involved in the induction of sleep after prolonged wakefulness. This effect is partially reversed by the application of caffeine, which is a nonselective blocker of adenosine receptors. Here, we report that the most abundant and highly concentrated A1 subtype of cerebral adenosine receptors is upregulated after 24 h of sleep deprivation. We used the highly selective A1 adenosine receptor (A1AR) radioligand [18F]CPFPX ([18F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine) and quantitative positron emission tomography to assess cerebral A1ARs before and after sleep deprivation in 12 healthy volunteers and a control group (n = 10) with regular sleep. In sleep deprived subjects, we found an increase of the apparent equilibrium total distribution volume in a region-specific pattern in all examined brain regions with a maximum increase in the orbitofrontal cortex (15.3%; p = 0.014). There were no changes in the control group with regular sleep. This is the first molecular imaging study that provides in vivo evidence for an A1AR upregulation in cortical and subcortical brain regions after prolonged wakefulness, indicating that A1AR expression is contributing to the homeostatic sleep regulation.

Three-Step, “One-Pot” Radiosynthesis of 6-Fluoro-3,4-Dihydroxy-l-Phenylalanine by Isotopic Exchange

The 18F-labeled aromatic amino acid 6-fluoro-3,4-dihydroxy-l-phenylalanine (6-18F-fluoro-l-DOPA) is widely used as a radiopharmaceutical in neurologic and oncologic PET. In this study, a novel approach to the preparation of carrier-added (CA) 6-18F-fluoro-l-DOPA in 3 radiosynthesis steps was developed and evaluated; in this approach, direct nucleophilic 18F fluorination of a protected amino acid derivative was used. The method currently used for the routine preparation of 6-18F-fluoro-l-DOPA by electrophilic labeling is limited to the production of small amounts of activity at high costs. Alternative syntheses based on the advantage of large-scale production of nucleophilic 18F-fluoride, however, either have resulted in insufficient enantiomeric purity or are difficult to automate because of the complexity of the necessary multiple steps. Methods: An isotopic exchange reaction on the precursor (2S,5S)-tert-butyl-5-(4-benzyloxy-2-fluoro-5-formylbenzyl)-2-tert-butyl-3-methyl-4-oxoimidazolidine-1-carboxylate was used. The formyl group served as the activating group in the 18F-for-19F exchange with tetrabutylammonium bicarbonate for anion activation in N,N-dimethylformamide. The intermediate was converted to a hydroxy group by Baeyer–Villiger oxidation with meta-chloroperbenzoic acid. After final deprotection with hydrobromic acid, CA 6-18F-fluoro-l-DOPA was isolated by high-performance liquid chromatography. Results: The precursor was obtained by an 11-step organic synthesis. The optimized isotopic 18F exchange proceeded with a radiochemical yield of about 50%. The complete preparation and isolation of CA 6-18F-fluoro-l-DOPA thus far are possible with a radiochemical yield of about 22%, within a synthesis time of 105 min, and at a much higher specific activity than with the electrophilic method. The enantiomeric excess of the desired l-isomer was greater than 96%.Conclusion: The pathway to 6-18F-fluoro-l-DOPA by isotopic exchange not only is more efficient but also is suited to automation as a “one-pot” procedure.

Iodonium ylides for one-step, no-carrier-added radiofluorination of electron rich arenes, exemplified with 4-(([18F]fluorophenoxy)-phenylmethyl)piperidine NET and SERT ligands

Iodonium ylide precursors of electron rich arenes, i.e. the NET and SERT ligands 4-((3- and 4-fluorophenoxy)phenylmethyl)piperidine, served as model compounds for the direct substitution with n.c.a. [18F]fluoride. Good radiochemical yields of about 20% were obtained in reaction times of ca. 130 minutes with a molar activity of the labelled ligands of more than 50 GBq μmol−1. Those failed as in vivo probes in first evaluation studies. Several important insights, however, were gained into the reaction of ylides, e.g. an unexpected formation of regioisomers. The results clearly demonstrate that aryliodonium ylides are a promising alternative to the well-known diaryliodonium salts for the direct preparation of complex, electron rich n.c.a. [18F]fluoroarenes.

Methods for 11C- and 18F-labelling of amino acids and derivatives for positron emission tomography imaging†

The different concepts realized for the synthesis of (11) C- and (18) F-labelled amino acids are summarized. Carbon-11 enables principally authentic radiolabelling of natural occurring amino acids by substituting one of the skeleton carbons by the radionuclide. Fluorine-18 is a foreign element for natural amino acids. Because of its advantageous nuclidic properties for positron emission tomography, however, it becomes increasingly important in molecular imaging, also with amino acid analogues. Especially in the last decade, considerable progress has been made with the radiosynthesis of (18) F-labelled amino acids that are now clinically approved, and thus assure their availability. In contrast, the synthetic possibilities with (11) C-labelled amino acids are more limited because of the short half-life of carbon-11 which also hampers their wide spread use.

Evaluation of 18F-labeled benzodioxine piperazine-based dopamine D4 receptor ligands: lipophilicity as a determinate of nonspecific binding.

Derivatization of the putative neuroleptic 1-(2,3-dihydrobenzo[1,4]dioxin-6-yl)-4-(4-fluorobenzyl)piperazine (3a) led to a series of new dopamine receptor D4 ligands displaying high affinity (Ki=1.1-15 nM) and D2/D4 subtype selectivities of about 800-6700. These ligands were labeled with the short-lived positron emitter fluorine-18 and analyzed for their potential application for imaging studies by positron emission tomography (PET). In vitro autoradiography was used to determine their nonspecific binding behavior as a result of their structural and thus physicochemical properties. The biodistribution, in vivo stability, and brain uptake of the most promising D4 radioligand candidate were determined. This proved to be 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-4-((6-fluoropyridin-3-yl)methyl)piperazine ([18F]3d), which revealed an excellent binding pattern with a high selectivity and limited nonspecific binding in vitro. This analogue also exhibited a high stability and an extremely high brain uptake in vivo with specific binding in hippocampus, cortex, colliculus, and cerebellum as determined by ex vivo autoradiography. Thus, [18F]3d appears as a suitable D4 radioligand for in vivo imaging, encouraging continued evaluation by PET studies.

18F-labelled intermediates for radiosynthesis by modular build-up reactions: newer developments.

This brief review gives an overview of newer developments in 18F-chemistry with the focus on small 18F-labelled molecules as intermediates for modular build-up syntheses. The short half-life (<2 h) of the radionuclide requires efficient syntheses of these intermediates considering that multistep syntheses are often time consuming and characterized by a loss of yield in each reaction step. Recent examples of improved synthesis of 18F-labelled intermediates show new possibilities for no-carrier-added ring-fluorinated arenes, novel intermediates for tri[18F]fluoromethylation reactions, and 18F-fluorovinylation methods.

Circadian variation of metabotropic glutamate receptor 5 availability in the rat brain

The metabotrophic subtype 5 glutamate receptor (mGluR5) plays a critical role in synaptic plasticity besides its involvement in numerous neurological disorders, such as depression. As mGluR5 availability in humans is altered in sleep deprivation, we hypothesized that mGluR5 availability underlies a circadian variation. To investigate whether mGluR5 underlies potential circadian changes we measured its density in a randomized fashion at six different daytimes in 11 adult Sprague–Dawley rats. mGluR5 density was quantified by positron emission tomography (PET) using the radioactive ligand [11C]ABP688. [11C]ABP688 uptake was quantified in nine regions of interest with a reference tissue model. Significant differences in the binding potential (BPND) and therefore mGluR5 availability between the different circadian times were found in cortex, cingulate cortex, amygdala, caudate putamen and nucleus accumbens. Further post‐hoc statistical analysis (Tukey–Kramer test) of the different time‐points revealed significant changes in BPND between 07:00 hours (start of light‐on phase) and 15:00 hours (last time‐point of the light‐on phase) in the caudate putamen. This study shows that mGluR5 availability is increased during the light‐on, or sleep phase, of rodents by approximately 10%. Given that altered mGluR5 densities play a role in psychiatric disorders, further investigation is warranted to evaluate their circadian involvement in mood changes in humans.

Synthesis of no-carrier-added 4-[18F]fluorophenol from 4-benzyloxyphenyl-(2-thienyl)iodonium bromide.

4-[18F]Fluorophenol is a versatile synthon for the synthesis of more complex radiopharmaceuticals bearing a 4-[18F]fluorophenoxy moiety. In order to prepare 4-[18F]fluorophenol in no-carrier-added (n.c.a.) form only a nucleophilic labelling method starting from [18F]fluoride is suitable. In this paper a new, two step radiosynthesis starting from 4-benzyloxyphenyl-(2-thienyl)iodonium bromide and [18F]fluoride with subsequent deprotection is described, yielding n.c.a. [18F]fluorophenol in 34 to 36% radiochemical yield.

4-[18F]Fluorophenylpiperazines by improved Hartwig-Buchwald N-arylation of 4-[18F]fluoroiodobenzene, formed via hypervalent λ3-iodane precursors: application to build-up of the dopamine D4 ligand [18F]FAUC 316.

Substituted phenylpiperazines are often neuropharmacologically active compounds and in many cases are essential pharmacophores of neuroligands for different receptors such as D2-like dopaminergic, serotoninergic and other receptors. Nucleophilic, no-carrier-added (n.c.a.) 18F-labelling of these ligands in an aromatic position is desirable for studying receptors with in vivo molecular imaging. 1-(4-[18F]Fluorophenyl)piperazine was synthesized in two reaction steps starting by 18F-labelling of a iodobenzene-iodonium precursor, followed by Pd-catalyzed N-arylation of the intermediate 4-[18F]fluoro-iodobenzene. Different palladium catalysts and solvents were tested with particular attention to the polar solvents dimethylformamide (DMF) and dimethylsulfoxide (DMSO). Weak inorganic bases like potassium phosphate or cesium carbonate seem to be essential for the arylation step and lead to conversation rates above 70% in DMF which is comparable to those in typically used toluene. In DMSO even quantitative conversation was observed. Overall radiochemical yields of up to 40% and 60% in DMF and DMSO, respectively, were reached depending on the labelling yield of the first step. The fluorophenylpiperazine obtained was coupled in a third reaction step with 2-formyl-1H-indole-5-carbonitrile to yield the highly selective dopamine D4 ligand [18F]FAUC 316.

Labeling of benzodioxin piperazines with fluorine-18 as prospective radioligands for selective imaging of dopamine D4 receptors.

The D(4) receptor is of high interest for research and clinical application but puts high demands on appropriate radioligands to be useful tools for investigation. Search for adequate radioligands suitable for in vivo imaging is therefore still in progress. The potential neuroleptic drug 6-(4-[4-fluorobenzyl]piperazin-1-yl)benzodioxin shows high affinity and selectivity to the D(4) receptor. Derivatization of this lead structure by adding hydrophilic moieties was carried out in order to lower its lipophilicity what led to three new putative dopamine receptor D(4) ligands. A comprehensive description of the syntheses of standard compounds and corresponding labeling precursors is given which were obtained in satisfactory yields. Furthermore, the radiosyntheses by direct (18) F-labeling and build-up synthesis were compared. All derivatives of 6-(4-[4-fluorobenzyl]-piperazin-1-yl)benzodioxin were successfully synthesized in (18) F-labeled form with radiochemical yields of 9-35% and molar activities of 30-60 GBq/µmol using one-pot procedures.