Optimization and synthesis of an (18) F-labeled dopamine D3 receptor ligand using [(18) F]fluorophenylazocarboxylic tert-butylester

Synthesis and biological evaluation of 18F-labelled dopamine D3 receptor selective ligands

The dopamine D₃ receptor (D₃R) is highly expressed in the limbic regions of the brain and closely related to a variety of neurological disorders including Parkinson's disease, schizophrenia and drug-seeking behavior. In vivo imaging of D₃R with radio-labelled tracers and positron emission tomography (PET) has become a powerful technique in related disorders. In this study, we synthesized three novel aromatically ¹⁸F-labelled phenylpiperazine-like D₃R selective radioactive ligands ([¹⁸F]5b, [¹⁸F]8b and [¹⁸F]11b) and developed a simple, rapid and efficient ¹⁸F-labelling method by condition optimization. Radiosynthesis of [¹⁸F]5b, [¹⁸F]8b and [¹⁸F]11b was achieved by ¹⁸F-fluorination from nitroarene precursors. Final radiochemical purities of [¹⁸F]5b, [¹⁸F]8b and [¹⁸F]11b solution were > 99% and remained good stability (> 98% for up to 6 h) in PBS and FBS. PET imaging and cellular binding studies revealed that [¹⁸F]8b had a higher D₃R affinity than [¹⁸F]5b and [¹⁸F]11b. Autoradiography and biodistribution studies of the brain showed that [¹⁸F]8b had medium intensity specific accumulation in the striatum and cortex. Meanwhile, the low skeletal uptake of [¹⁸F]8b revealed a good in vivo stability with negligible defluorination. These results indicated that [¹⁸F]8b might be a potential ¹⁸F-labelled D₃R PET imaging agent.

An Efficient Automated Radiosynthesis and Bioactivity Confirmation of VMAT2 Tracer [18F]FP-(+)-DTBZ

PURPOSE

The aim of this study was to optimize the radiolabeling method of [¹⁸F]fluoropropyl-(+)-dihydrotetrabenazine ([¹⁸F]FP-(+)-DTBZ) to fulfill the demand of preclinical and clinical application.

PROCEDURES

Optimized labeling conditions were performed by altering the molar ratio of precursor to base (P/B), base species, solvents, reaction temperature, reaction time, and precursor concentration through manual radiosynthesis of [¹⁸F]FP-(+)-DTBZ. The conditions with the highest radiochemical yield (RCY) were applied to automated radiosynthesis, and the crude product was purified with a Sep-Pak Plus C18 cartridge. Quality control and stability of [¹⁸F]FP-(+)-DTBZ were carried out by HPLC. In vitro cellular uptake and blocking assays were conducted in human neuroblastoma cell line SH-SY5Y. In vivo imaging with small animal positron emission tomography (microPET) was performed with Sprague-Dawley rats.

RESULTS

Under the optimized conditions (P/K₂CO₃ = 1:8, heating at 120 °C for 3 min in dimethyl sulfoxide), an RCY of 88.7 % was obtained with 1.0 mg precursor. The optimized reaction conditions were successfully applied to an automated module and gave a high activity yield (AY) of 30-55 % in about 40 min with a > 99.0 % radiochemical purity (RCP) and a > 44.4 GBq/μmol molar activity (A_m). Stability test displayed that the RCP retained > 98.0 % in 8 h in saline and in phosphate buffer saline (PBS, pH 7.4). In vitro cellular uptake assay showed accumulation of [¹⁸F]FP-(+)-DTBZ in SH-SY5Y cells, which could be significantly inhibited by vesicular monoamine transporter 2 (VMAT2) inhibitor DTBZ. MicroPET images of rat brain displayed that the striatum showed the highest uptake with a standardized uptake value (SUV) of 3.91 ± 0.30 at ~ 70 min. Co-injection with DTBZ (1.0 mg/kg) resulted in a 75 % decrease of the striatal SUV, confirming the specificity of [¹⁸F]FP-(+)-DTBZ to VMAT2.

CONCLUSIONS

We obtained an optimized radiolabeling method of [¹⁸F]FP-(+)-DTBZ and successfully applied it to a commercial available module. The automated synthesis gave a high AY and RCP of [¹⁸F]FP-(+)-DTBZ with high and specific binding to VMAT2, facilitating its routine application for VMAT2 tracing.

Synthesis and pre-clinical evaluation of a potential radiotracer for PET imaging of the dopamine D3 receptor

There is considerable interest in using positron emission tomography (PET) imaging to understand the function of dopamine D3 receptors. Due to high sequence homology with D2 receptors, development of D3-selective PET radiotracers has been challenging. In an effort to overcome this issue, we report the radiosynthesis of a new selective D3 ligand with carbon-11 ([11C]1 ), and its initial preclincial evaluation as a potential PET radiotracer for in vivo imaging of D3 receptors. [11C]1 was prepared via [11C]CO2 fixation in 0.1% non-corrected radiochemical yield, good radiochemical purity (>95%) and high specific activity (>2000 Ci mmol-1). [11C]1 exhibited specific binding to D3 receptors using ex vivo autoradiography experiments with rat brain, but only 14-fold selectivity over D2 receptors which is lower than the 1400-fold value reported previously for cell studies. Rodent PET imaging revealed reasonable uptake of the radiotracer in areas of the brain known to be rich in D3 receptors.

[18F]Fluorophenylazocarboxylates: Design and Synthesis of Potential Radioligands for Dopamine D3 and μ-Opioid Receptor

¹⁸F-Labeled building blocks from the type of [¹⁸F]fluorophenylazocarboxylic-tert-butyl esters offer a rapid, mild, and reliable method for the ¹⁸F-fluoroarylation of biomolecules. Two series of azocarboxamides were synthesized as potential radioligands for dopamine D3 and the μ-opioid receptor, revealing compounds 3d and 3e with single-digit and sub-nanomolar affinity for the D3 receptor and compound 4c with only micromolar affinity for the μ-opioid receptor, but enhanced selectivity for the μ-subtype in comparison to the lead compound AH-7921. A "minimalist procedure" without the use of a cryptand and base for the preparation of 4-[¹⁸F]fluorophenylazocarboxylic-tert-butyl ester [¹⁸F]2a was established, together with the radiosynthesis of methyl-, methoxy-, and phenyl-substituted derivatives ([¹⁸F]2b-f). With the substituted [¹⁸F]fluorophenylazocarbylates in hand, two prototype azocarboxylates radioligands were synthesized by ¹⁸F-fluoroarylation, namely the methoxy azocarboxamide [¹⁸F]3d as the D3 receptor radioligand and [¹⁸F]4a as a prototype structure of the μ-opioid receptor radioligand. By introducing the new series of [¹⁸F]fluorophenylazocarboxylic-tert-butyl esters, the method of ¹⁸F-fluoroarylation was significantly expanded, thereby demonstrating the versatility of ¹⁸F-labeled phenylazocarboxylates for the design of potential radiotracers for positron emission tomography .

In Vitro and In Vivo Characterization of Selected Fluorine-18 Labeled Radioligands for PET Imaging of the Dopamine D3 Receptor

Cerebral dopamine D3 receptors seem to play a key role in the control of drug-seeking behavior. The imaging of their regional density with positron emission tomography (PET) could thus help in the exploration of the molecular basis of drug addiction. A fluorine-18 labeled D3 subtype selective radioligand would be beneficial for this purpose; however, as yet, there is no such tracer available. The three candidates [¹⁸F]1, [¹⁸F]2a and [¹⁸F]2b were chosen for in vitro and in vivo characterization as radioligands suitable for selective PET imaging of the D3 receptor. Their evaluation included the analysis of radiometabolites and the assessment of non-specific binding by in vitro rat brain autoradiography. While [¹⁸F]1 and [¹⁸F]2a revealed high non-specific uptake in in vitro rat brain autoradiography, the D3 receptor density was successfully determined on rat brain sections (n = 4) with the candidate [¹⁸F]2b offering a B_max of 20.38 ± 2.67 pmol/g for the islands of Calleja, 19.54 ± 1.85 pmol/g for the nucleus accumbens and 16.58 ± 1.63 pmol/g for the caudate putamen. In PET imaging studies, the carboxamide 1 revealed low signal/background ratios in the rat brain and relatively low uptake in the pituitary gland, while the azocarboxamides [¹⁸F]2a and [¹⁸F]2b showed binding that was blockable by the D3 receptor ligand BP897 in the ventricular system and the pituitary gland in PET imaging studies in living rats.