A Series of 18F-Labelled Pyridinylphenyl Amides as Subtype-Selective Radioligands for the Dopamine D3 Receptor

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.

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.

Radioligands for the dopamine receptor subtypes

The actions of the predominant neurotransmitter in the brain, dopamine, are mediated by the postsynaptic dopamine receptors. The five dopamine receptor subtypes and their regulation have been associated with a large variety of psychiatric diseases. Therefore, positron emission tomography (PET) imaging studies using suitable and selective (18) F-labeled and (11) C-labeled dopamine receptor radioligands could provide valuable knowledge on the impact of receptor density on the pathogenesis and evolvement of neuropsychiatric and neurological diseases. This special issue subchapter provides a summary of the most important (18) F-labeled and (11) C-labeled radioligands for PET imaging of the dopamine receptor subtypes, their radiochemistry, and characteristics from in vitro and in vivo applications, considering not only the already established PET ligands but also the recently published preclinical work.

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

INTRODUCTION

Dopamine D3 receptors are implicated in various neuropsychiatric diseases, drug abuse and alcoholism, but specific agents for D3 molecular imaging are lacking. We evaluated two in vitro selective fluorine-18-labeled radioligand candidates ([(18)F]5 and [(18)F]6) for positron emission tomography (PET) imaging of D3 receptor availability in the brain.

METHODS

Biodistribution was evaluated in Sprague-Dawley rats using ex vivo autoradiography and small-animal PET. Protein binding studies were conducted in human plasma and cerebrospinal fluid.

RESULTS

[(18)F]5 showed rapid blood-brain barrier penetration and fast washout after intravenous injection, whereas the rat brain penetration of [(18)F]6 was lower. The total distribution volume (VT) of [(18)F]5 was 20-26 mL g(-1) throughout brain. Co-injection with the D3 antagonist BP897 resulted in globally increased cerebral washout of [(18)F]5 and [(18)F]6, but SUV analysis and parametric mapping of binding potential (BPND) relative to the cerebellum did not reveal specific binding of either ligand in D3-rich brain regions, i.e. the ventral striatum. However, there was substantial displaceable binding of [(18)F]5, and to a lesser extent [(18)F]6, in the pituitary.

CONCLUSION

These radioligands reveal dopamine D3 receptors in the pituitary, but are not suitable for PET imaging of in brain, possibly due to low specific signal relative to the globally high VT.

Synthesis, biological evaluation and radiolabelling by 18F-fluoroarylation of a dopamine D3-selective ligand as prospective imaging probe for PET

Radical (18)F-fluoroarylation with fluorine-18-labelled arenediazonium chlorides has been successfully applied to the radiochemical synthesis of the dopamine D(3)-selective ligand SH 317 ([(18)F]8). SH 317 has been evaluated as a new PET ligand candidate by in vivo experiments.