Radiosynthesis and In Vivo Evaluation of Four Positron Emission Tomography Tracer Candidates for Imaging of Melatonin Receptors

Abstract

Melatonin is a neurohormone that modulates several physiological functions in mammals through the activation of melatonin receptor type 1 and 2 (MT1 and MT2). The melatonergic system is an emerging therapeutic target for new pharmacological interventions in the treatment of sleep and mood disorders; thus, imaging tools to further investigate its role in the brain are highly sought-after. We aimed to develop selective radiotracers for in vivo imaging of both MT1 and MT2 by positron emission tomography (PET). We identified four previously reported MT ligands with picomolar affinities to the target based on different scaffolds which were also amenable for radiolabeling with either carbon-11 or fluorine-18. [11C]­UCM765, [11C]­UCM1014, [18F]­3-fluoroagomelatine ([18F]­3FAGM), and [18F]­fluoroacetamidoagomelatine ([18F]­FAAGM) have been synthesized in high radiochemical purity and evaluated in wild-type rats. All four tracers showed moderate to high brain permeability in rats with maximum standardized uptake values (SUVmax of 2.53, 1.75, 3.25, and 4.47, respectively) achieved 1–2 min after tracer administration, followed by a rapid washout from the brain. Several melatonin ligands failed to block the binding of any of the PET tracer candidates, while in some cases, homologous blocking surprisingly resulted in increased brain retention. Two 18F-labeled agomelatine derivatives were brought forward to PET scans in non-human primates and autoradiography on human brain tissues. No specific binding has been detected in blocking studies. To further investigate pharmacokinetic properties of the putative tracers, microsomal stability, plasma protein binding, log D, and membrane bidirectional permeability assays have been conducted. Based on the results, we conclude that the fast first pass metabolism by the enzymes in liver microsomes is the likely reason of the failure of our PET tracer candidates. Nevertheless, we showed that PET imaging can serve as a valuable tool to investigate the brain permeability of new therapeutic compounds targeting the melatonergic system.

Zinc‐Mediated Radiosynthesis of Unprotected Fluorine‐18 Labelled α‐Tertiary Amides

“…An α‐[ 18 F]fluoroamide analogue of 30‐ 18 F was recently prepared as a candidate for the in vivo imaging of melatonin receptors to support therapeutic interventions for sleep and mood disorders. The current methodology presents an alternative radiosynthetic strategy to produce agomelatine analogues [28] . Moreover, imaging agents based on antiandrogens, such as flutamide, have promising prospects for prostate cancer imaging.…”

Zinc‐Mediated Radiosynthesis of Unprotected Fluorine‐18 Labelled α‐Tertiary Amides

“…The current methodology presents an alternative radiosynthetic strategy to produce agomelatine analogues. [28]…”

Luminogenic HiBiT Peptide-Based NanoBRET Ligand Binding Assays for Melatonin Receptors

“…Three other iodinated radioligands have been recently developed; 13 melatonin receptor tracers for positron emission tomography studies failed as blocking studies showed no specific binding in nonhuman primate brains, most likely because of the high hydrophobicity of the probes. 15 Several fluorescently labeled melatonin receptor ligands have been generated, some of which retained acceptable binding affinity for these receptors (see ref 16 for a review). Unfortunately, high nonspecific binding and intracellular uptake render these tracers unsuitable for fluorescence imaging and fluorescence-based binding studies.…”

“…Three other iodinated radioligands have been recently developed; two of them, [ 125 I]­DIV880) and [ 125 I]­S70254, are full and partial MT 2 -selective agonists, respectively . Attempts to develop [ 11 C]- and [ 18 F]-labeled melatonin receptor tracers for positron emission tomography studies failed as blocking studies showed no specific binding in nonhuman primate brains, most likely because of the high hydrophobicity of the probes . Several fluorescently labeled melatonin receptor ligands have been generated, some of which retained acceptable binding affinity for these receptors (see ref for a review).…”