18F fluoroethylations: different strategies for the rapid translation of 11C-methylated radiotracers

Effects of bilateral sequential theta-burst stimulation on 5-HT1A receptors in the dorsolateral prefrontal cortex in treatment-resistant depression: a proof-of-concept trial

Theta-burst stimulation (TBS) represents a brain stimulation technique effective for treatment-resistant depression (TRD) as underlined by meta-analyses. While the methodology undergoes constant refinement, bilateral stimulation of the dorsolateral prefrontal cortex (DLPFC) appears promising to restore left DLPFC hypoactivity and right hyperactivity found in depression. The post-synaptic inhibitory serotonin-1A (5-HT1A) receptor, also occurring in the DLPFC, might be involved in this mechanism of action. To test this hypothesis, we performed PET-imaging using the tracer [carbonyl-11C]WAY-100635 including arterial blood sampling before and after a three-week treatment with TBS in 11 TRD patients compared to sham stimulation (n = 8 and n = 3, respectively). Treatment groups were randomly assigned, and TBS protocol consisted of excitatory intermittent TBS to the left and inhibitory continuous TBS to the right DLPFC. A linear mixed model including group, hemisphere, time, and Hamilton Rating Scale for Depression (HAMD) score revealed a 3-way interaction effect of group, time, and HAMD on specific distribution volume (VS) of 5-HT1A receptor. While post-hoc comparisons showed no significant changes of 5-HT1A receptor VS in either group, higher 5-HT1A receptor VS after treatment correlated with greater difference in HAMD (r = -0.62). The results of this proof-of-concept trial hint towards potential effects of TBS on the distribution of the 5-HT1A receptor. Due to the small sample size, all results must, however, be regarded with caution.

Small Prosthetic Groups in 18F-Radiochemistry: Useful Auxiliaries for the Design of 18F-PET Tracers

Prosthetic group (PG) applications in ¹⁸F-radiochemistry play a pivotal role among current ¹⁸F^-labeling techniques for the development and availability of ¹⁸F-labeled imaging probes for PET (Wahl, 2002) (¹). The introduction and popularization of PGs in the mid-80s by pioneers in ¹⁸F-radiochemistry has profoundly changed the landscape of available tracers for PET and has led to a multitude of new imaging agents based on simple and efficiently synthesized PGs. Because of the chemical nature of anionic ¹⁸F^- (apart from electrophilic low specific activity ¹⁸F-fluorine), radiochemistry before the introduction of PGs was limited to simple nucleophilic substitutions of leaving group containing precursor molecules. These precursors were not always available, and some target compounds were either hard to synthesize or not obtainable at all. Even with the advent of recently introduced "late-stage fluorination" techniques for the ¹⁸F-fluorination of deactivated aromatic systems, PGs will continue to play a central role in ¹⁸F-radiochemistry because of their robust and almost universal usability. The importance of PGs in radiochemistry is shown by its current significance in tracer development and exemplified by an overview of selected methodologies for PG attachment to PET tracer molecules. Especially, click-chemistry approaches to PG conjugation, while furthering the historical evolution of PGs in PET tracer design, play a most influential role in modern PG utilization. All earlier and recent multifaceted approaches in PG development have significantly enriched the contingent of modern ¹⁸F-radiochemistry procedures and will continue to do so.

Effects of Silexan on the serotonin-1A receptor and microstructure of the human brain: a randomized, placebo-controlled, double-blind, cross-over study with molecular and structural neuroimaging

BACKGROUND

Recently, Silexan, a patented active substance comprised of an essential oil produced from Lavandula angustifolia flowers, has been authorized in Germany as a medicinal product for the treatment of states of restlessness related to anxious mood. Its efficacy has been shown in several forms of anxiety disorders. Findings from preclinical and clinical studies attribute a major role to the serotonin-1A receptor in the pathogenesis and treatment of anxiety.

METHODS

To elucidate the effect of Silexan on serotonin-1A receptor binding, 17 healthy men underwent 2 positron emission tomography measurements using the radioligand [carbonyl-(11)C]WAY-100635 following the daily intake of 160 mg Silexan or placebo for a minimum of 8 weeks (randomized, double-blind, cross-over design). Additionally, structural magnetic resonance imaging and voxel-based morphometry analysis was performed to determine potential effects on gray matter microstructure.

RESULTS

Serotonin-1A receptor binding potential was shown to be significantly reduced following the intake of Silexan compared with placebo in 2 large clusters encompassing the temporal gyrus, the fusiform gyrus and the hippocampus on one hand as well as the insula and anterior cingulate cortex on the other hand. No effects of Silexan on gray matter volume could be detected in this investigation.

CONCLUSION

This positron emission tomography study proposes an involvement of the serotonin-1A receptor in the anxiolytic effects of Silexan. The study was registered in the International Standard Randomized Controlled Trial Number Register as ISRCTN30885829 (http://www.controlled-trials.com/isrctn/).

2-[18F]Fluoroethyl tosylate – a versatile tool for building18F-based radiotracers for positron emission tomography

The review highlights the role of 2-[¹⁸F]fluoroethyltosylate ([¹⁸F]FETs) in PET radiotracer design since it is a preferred labeling reagent according to its high reactivity to phenolic, amine, thiophenolic and carboxylic functions.

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

This brief review gives an overview of newer developments in (18)F-chemistry with the focus on small (18)F-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 (18)F-labelled intermediates show new possibilities for no-carrier-added ring-fluorinated arenes, novel intermediates for tri[(18)F]fluoromethylation reactions, and (18)F-fluorovinylation methods.

[18F]FMeNER-D2: reliable fully-automated synthesis for visualization of the norepinephrine transporter

PURPOSE

In neurodegenerative diseases and neuropsychiatric disorders dysregulation of the norepinephrine transporter (NET) has been reported. For visualization of NET availability and occupancy in the human brain PET imaging can be used. Therefore, selective NET-PET tracers with high affinity are required. Amongst these, [(18)F]FMeNER-D2 is showing the best results so far. Furthermore, a reliable fully automated radiosynthesis is a prerequisite for successful application of PET-tracers. The aim of this work was the automation of [(18)F]FMeNER-D2 radiolabelling for subsequent clinical use. The presented study comprises 25 automated large-scale syntheses, which were directly applied to healthy volunteers and adult patients suffering from attention deficit hyperactivity disorder (ADHD).

PROCEDURES

Synthesis of [(18)F]FMeNER-D2 was automated within a Nuclear Interface Module. Starting from 20-30 GBq [(18)F]fluoride, azeotropic drying, reaction with Br2CD2, distillation of 1-bromo-2-[(18)F]fluoromethane-D2 ([(18)F]BFM) and reaction of the pure [(18)F]BFM with unprotected precursor NER were optimized and completely automated. HPLC purification and SPE procedure were completed, formulation and sterile filtration were achieved on-line and full quality control was performed.

RESULTS

Purified product was obtained in a fully automated synthesis in clinical scale allowing maximum radiation safety and routine production under GMP-like manner. So far, more than 25 fully automated syntheses were successfully performed, yielding 1.0-2.5 GBq of formulated [(18)F]FMeNER-D2 with specific activities between 430 and 1707 GBq/μmol within 95 min total preparation time.

CONCLUSIONS

A first fully automated [(18)F]FMeNER-D2 synthesis was established, allowing routine production of this NET-PET tracer under maximum radiation safety and standardization.

Impact of COMT genotype on serotonin-1A receptor binding investigated with PET

Alterations of the inhibitory serotonin-1A receptor (5-HT1A) constitute a solid finding in neuropsychiatric research, particularly in the field of mood and anxiety disorders. Manifold factors influencing the density of this receptor have been identified, e.g., steroid hormones, sunlight exposure and genetic variants of serotonin-related genes. Given the close interactions between serotonergic and dopaminergic neurotransmission, we investigated whether a common single-nucleotide-polymorphism of the catechol-O-methyltransferase (COMT) gene (VAL158MET or rs4680) coding for a key enzyme of the dopamine network that is associated with the pathogenesis of mood disorders and antidepressant treatment response, directly affects 5-HT1A receptor binding potential. Fifty-two healthy individuals (38 female, mean age ± standard deviation = 40.48 ± 14.87) were measured via positron emission tomography using the radioligand [carbonyl-(11)C]WAY-100635. Genotyping for rs4680 was performed using DNA isolated from whole blood with the MassARRAY platform of the software SEQUENOM(®). Whole brain voxel-wise ANOVA resulted in a main effect of genotype on 5-HT1A binding. Compared to A carriers (AA + AG) of rs4680, homozygote G subjects showed higher 5-HT1A binding potential in the posterior cingulate cortex (F (2,49) = 17.7, p = 0.05, FWE corrected), the orbitofrontal cortex, the anterior cingulate cortex, the insula, the amygdala and the hippocampus (voxel-level: p < 0.01 uncorrected, t > 2.4; cluster-level: p < 0.05 FWE corrected). In light of the frequently reported alterations of 5-HT1A binding in anxiety and mood disorders, this study proposes a potential implication of the COMT genotype, more specifically the VAL158MET polymorphism, via modulation of the serotonergic neurotransmission.

A fully automated radiosynthesis of [18F]fluoroethyl-diprenorphine on a single module by use of SPE cartridges for preparation of high quality 2-[18F]fluoroethyl tosylate

We have developed a new method for automated production of 2-[¹⁸F]fluoroethyl tosylate ([¹⁸F]FETos) that enables ¹⁸F-alkylation to provide PET tracers with high chemical purity. The method is based on the removal of excess ethylene glycol bistosylate precursor by precipitation and subsequent filtration and purification of the filtrate by means of solid phase extraction cartridges (SPE). The method is integrated to a single synthesis module and thereby provides the advantage over previous methods of not requiring HPLC purification, as demonstrated by the full radiosynthesis of the potent opioid receptor PET tracer [¹⁸F]fluoroethyldiprenorphine.

Synthesis and radiopharmacological characterisation of a fluorine-18-labelled azadipeptide nitrile as a potential PET tracer for in vivo imaging of cysteine cathepsins

A fluorinated cathepsin inhibitor based on the azadipeptide nitrile chemotype was prepared and selected for positron emission tomography (PET) tracer development owing to its high affinity for the oncologically relevant cathepsins L, S, K and B. Labelling with fluorine-18 was accomplished in an efficient and reliable two-step, one-pot radiosynthesis by using 2-[(18) F]fluoroethylnosylate as a prosthetic agent. The pharmacokinetic properties of the resulting radiotracer compound were studied in vitro, ex vivo and in vivo in normal rats by radiometabolite analysis and small-animal positron emission tomography. These investigations revealed rapid conjugate formation of the tracer with glutathione in the blood, which is associated with slow blood clearance. The potential of the developed (18) F-labelled probe to image tumour-associated cathepsin activity was investigated by dynamic small-animal PET imaging in nude mice bearing tumours derived from the human NCI-H292 lung carcinoma cell line. Computational analysis of the obtained image data indicated the time-dependent accumulation of the radiotracer in the tumours. The expression of the target enzymes in the tumours was confirmed by immunohistochemistry with specific antibodies. This indicates that azadipeptide nitriles have the potential to target thiol-dependent cathepsins in vivo despite their disadvantageous pharmacokinetics.

Efficient automated one-step synthesis of 2-[18F]fluoroethylcholine for clinical imaging: optimized reaction conditions and improved quality controls of different synthetic approaches

[(18)F]-labelled choline analogues, such as 2-[(18)F]fluoroethylcholine ((18)FECH), have suggested to be a new class of choline derivatives highly useful for the imaging of prostate and brain tumours. In fact, tumour cells with enhanced proliferation rate usually exhibit an improved choline uptake due to the increased membrane phospholipids biosynthesis. The aim of this study was the development of a high yielding synthesis of (18)FECH. The possibility of shortening the synthesis time by reacting all the reagents in a convenient and rapid one-step reaction was specially considered.

METHODS

(18)FECH was synthesized by reacting [(18)F]fluoride with 1,2-bis(tosyloxy)ethane and N,N-dimethylaminoethanol. The synthesis was carried out using both a one- and a two-step reaction in order to compare the two procedures. The effects on the radiochemical yield and purity by using different [(18)F]fluoride phase transfer catalysts, reagents amounts and purification methods were assessed. Quality controls on the final products were performed by means of radio-thin-layer chromatography, gas chromatography and high-performance liquid chromatography equipped with conductimetric, ultraviolet and radiometric detectors.

RESULTS

In the optimized experimental conditions, (18)FECH was synthesized with a radiochemical yield of 43+/-3% and 48+/-1% (not corrected for decay) when the two-step or the one-step approach were used, respectively. The radiochemical purity was higher than 99% regardless of the different synthetic pathways or purification methods adopted. The main chemical impurity was due to N,N-dimethylmorpholinium. The identity of this impurity in (18)FECH preparations was not previously reported.

CONCLUSION

An improved two-step and an innovative one-step reaction for synthesizing (18)FECH in a high yield were reported. The adaptation of a multistep synthesis to a single step process, opens further possibilities for simpler and more reliable automations.