Biodistribution studies of two 18F-labeled pyridinylphenyl amides as subtype selective radioligands for the dopamine D3 receptor

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.

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 .

Challenges in the development of dopamine D2- and D3-selective radiotracers for PET imaging studies

The dopamine D2-like receptors (ie, D2/3 receptors) have been the most extensively studied CNS receptor with Positron Emission Tomography (PET). The 3 different radiotracers that have been used in these studies are [¹¹ C]raclopride, [¹⁸ F]fallypride, and [¹¹ C]PHNO. Because these radiotracers have a high affinity for both dopamine D2 and D3 receptors, the density of dopamine receptors in the CNS is reported as the D2/3 binding potential, which reflects a measure of the density of both receptor subtypes. Although the development of D2- and D3-selective PET radiotracers has been an active area of research for many years, this by and large presents an unmet need in the area of translational PET imaging studies. This article discusses some of the challenges that have inhibited progress in this area of research and the current status of the development of subtype selective radiotracers for imaging D3 and D2 dopamine receptors with PET.

Development of molecular tools based on the dopamine D3 receptor ligand FAUC 329 showing inhibiting effects on drug and food maintained behavior

Dopamine D₃ receptor-mediated networks have been associated with a wide range of neuropsychiatric diseases, drug addiction and food maintained behavior, which makes D₃ a highly promising biological target. The previously described dopamine D₃ receptor ligand FAUC 329 (1) showed protective effects against dopamine depletion in a MPTP mouse model of Parkinson's disease. We used the radioligand [¹⁸F]2, a [¹⁸F]fluoroethoxy substituted analog of the lead compound 1 as a molecular tool for visualization of D₃-rich brain regions including the islands of Calleja. Furthermore, structural modifications are reported leading to the pyrimidylpiperazine derivatives 3 and 9 displaying superior subtype selectivity and preference over serotonergic receptors. Evaluation of the lead compound 1 on cocaine-seeking behavior in non-human primates showed a substantial reduction in cocaine self-administration behavior and food intake.

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.

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

There is still no efficient fluorine-18-labeled dopamine D3 subtype selective receptor ligand for studies with positron emission tomography. We aim at improving the D3 selectivity and hydrophilicity of a candidate ligand by changing the substitution pattern to a 2,3-dichlorophenylpiperazine and hydroxylation of the butyl chain. The compound [(18) F]3 exhibited D3 affinity of Ki  = 3.6 nM, increased subtype selectivity (Ki (D2 /D3 ) = 60), and low affinity to 5-HT1A and α1 receptors (Ki (5-HT1A /D3 )  = 34; Ki (α1 /D3 ) = 100). The two-step radiosynthesis was optimized for analog [(18) F]4 by reducing the necessary concentration of the precursor amine (57 mM), which reacted with [(18) F]fluorophenylazocarboxylic tert-butylester under basic conditions. The optimization of the base (Cs 2 CO3 , 23 mM) and the adjustment of reaction temperature led to the radiochemical yield of 63% after 5 min at 35°C. The optimized reaction conditions were transferred on to the synthesis of [(18) F]3 with an overall non-decay corrected yield of 8-12% in a specific activity of 32-102 GBq/µmol after a total synthesis time of 30-35 min. This provides a D 3 radioligand candidate with improved attributes concerning selectivity and radiosynthesis for further preclinical studies.

Synthesis and evaluation of fluoro substituted pyridinylcarboxamides and their phenylazo analogues for potential dopamine D3 receptor PET imaging

A series of fluoro substituted pyridinylcarboxamides and their phenylazo analogues with high affinity and selectivity for the dopamine D3 receptor was synthesized by the use of 6-fluoropyridine-3-carbonyl chloride (1) and fluorophenylazocarboxylic ester (2). Several of these compounds (9a-e and 10a-h) have been evaluated in vitro, among which 9b, 10a, 10c and 10d proved to have at least single-digit nanomolar affinity for D3. They also exhibit considerable selectivity over the other dopamine receptor subtypes and noteworthy selectivity over the structurally related serotonin receptor subtypes 5-HT(1A) and 5-HT₂, offering potential radiotracers for positron emission tomography.

Imaging the D3 dopamine receptor across behavioral and drug addictions: Positron emission tomography studies with [(11)C]-(+)-PHNO

Chronic drug use has been associated with dopaminergic abnormalities, detectable in humans with positron emission tomography (PET). Among these, a hallmark feature is low D2 dopamine receptor availability, which has been linked to clinical outcomes, but has not yet translated into a therapeutic strategy. The D3 dopamine receptor on the other hand has gained increasing attention, as, in contrast to D2, chronic exposure to drugs has been shown to up-regulate this receptor subtype in preclinical models of addiction-a phenomenon linked to dopamine system sensitization and drug-seeking. The present article summarizes the literature to date in humans, suggesting that the D3 receptor may indeed contribute to core features of addiction such as impulsiveness and cognitive impairment. A particularly useful tool in investigating this question is the PET imaging probe [(11)C]-(+)-PHNO, which binds to D2/3 dopamine receptors but has preferential affinity for D3. This technique has been used to demonstrate D3 up-regulation in humans, and can be applied to assess pharmacological interventions for development of D3-targeted strategies in addiction treatment.

Imaging of α7 nicotinic acetylcholine receptors in brain and cerebral vasculature of juvenile pigs with [(18)F]NS14490

Background

The α7 nicotinic acetylcholine receptor (nAChR) is an important molecular target in neuropsychiatry and oncology. Development of applicable highly specific radiotracers has been challenging due to comparably low protein expression. To identify novel ligands as candidates for positron emission tomography (PET), a library of diazabicyclononane compounds was screened regarding affinity and specificity towards α7 nAChRs. From these, [¹⁸F]NS14490 has been shown to yield reliable results in organ distribution studies; however, the radiosynthesis of [¹⁸F]NS14490 required optimization and automation to obtain the radiotracer in quantities allowing dynamic PET studies in piglets.

Methods

Automated radiosynthesis of [¹⁸F]NS14490 has been performed by [¹⁸F]fluorination with the tosylate precursor in the TRACERlab™ FX F-N synthesis module (Waukesha, WI, USA). After optimization, the radiochemical yield of [¹⁸F]NS14490 was consistently approximately 35%, and the total synthesis time was about 90 min. The radiotracer was prepared with >92% radiochemical purity, and the specific activity at the end of the synthesis was 226 ± 68 GBq μmol⁻¹. PET measurements were performed in young pigs to investigate the metabolic stability and cerebral binding of [¹⁸F]NS14490 without and with administration of the α7 nAChR partial agonist NS6740 in baseline and blocking conditions.

Results

The total distribution volume relative to the metabolite-corrected arterial input was 3.5 to 4.0 mL g⁻¹ throughout the telencephalon and was reduced to 2.6 mL g⁻¹ in animals treated with NS6740. Assuming complete blockade, this displacement indicated a binding potential (BP_ND) of approximately 0.5 in the brain of living pigs. In addition, evidence for specific binding in major brain arteries has been obtained.

Conclusion

[¹⁸F]NS14490 is not only comparable to other preclinically investigated PET radiotracers for imaging of α7 nAChR in brain but also could be a potential PET radiotracer for imaging of α7 nAChR in vulnerable plaques of diseased vessels.