Radiosynthesis of N-(4-chloro-3-[(11)C]methoxyphenyl)-2-picolinamide ([(11)C]ML128) as a PET radiotracer for metabotropic glutamate receptor subtype 4 (mGlu4)

Fully Automated Radiosynthesis of [18F]mG4P027 for mGluR4 Imaging

Fluorine-18 labeled N-(4-chloro-3-(((fluoro-¹⁸F)methyl-d₂)thio)phenyl)picolinamide, [¹⁸F]mG4P027, is a potent positron emission tomography (PET) radiotracer for metabotropic glutamate receptor 4 (mGluR4). Our previous in vitro and in vivo evaluations have demonstrated that this tracer is promising for further translational studies. To automate the radiosynthesis of [¹⁸F]mG4P027, significant modifications were made to the manual process by carefully examining this process and addressing the root causes of the challenges associated with its automation. We successfully implemented its automated radiosynthesis using the TRACERlab FX2N module and consequently, obtained a high-purity radiolabeled [¹⁸F]mG4P027 in high yield, meeting the requirements for future human studies.

Detailed radiosynthesis of [18 F]mG4P027 as a positron emission tomography radiotracer for mGluR4

We report here the detailed radiosynthesis of [18 F]mG4P027, a metabotropic glutamate receptor 4 (mGluR4) PET radiotracer, which showed superior properties to the currently reported mGluR4 radiotracers. The radiosynthesis in the automated system has been challenging, therefore we disclose here the major limiting factors for the synthesis via step-by-step examination. And we hope this thorough study will help its automation for human use in the future.

A hybrid approach unveils drug repurposing candidates targeting an Alzheimer pathophysiology mechanism

The high number of failed pre-clinical and clinical studies for compounds targeting Alzheimer disease (AD) has demonstrated that there is a need to reassess existing strategies. Here, we pursue a holistic, mechanism-centric drug repurposing approach combining computational analytics and experimental screening data. Based on this integrative workflow, we identified 77 druggable modifiers of tau phosphorylation (pTau). One of the upstream modulators of pTau, HDAC6, was screened with 5,632 drugs in a tau-specific assay, resulting in the identification of 20 repurposing candidates. Four compounds and their known targets were found to have a link to AD-specific genes. Our approach can be applied to a variety of AD-associated pathophysiological mechanisms to identify more repurposing candidates.

•

Drug-repurposing approach that combines in silico analyses and in vitro screenings

•

A drug- and mechanism-oriented model, the Human Brain Pharmacome (HBP) was created

•

The HBP was used to mine data related to drugs and targets to generate a hypothesis

•

Experimental evidence validated predicted drug-target combinations

Owing to current setbacks in the discovery and development of novel treatments tackling Alzheimer disease (AD), a re-evaluation of research and development (R&D) strategies is underway. Here, we present a holistic pharmacological approach that combines drug-target information with knowledge graphs that represent essential pathophysiology mechanisms. The resulting Human Brain Pharmacome (HBP) embeds hundreds of relevant drug-target interactions in the context of disease mechanisms governing AD. We demonstrate how such a tool can be used to aid AD research by identifying already-approved drugs that have the potential to treat the disease, thereby bypassing the expensive and time-consuming task of researching and developing a new drug. In our study, we identified new drug-target combinations and provided mechanistic explanations that help to improve our understanding of AD pathology and support future development of effective therapeutic strategies.

Design, Synthesis, and Characterization of Benzimidazole Derivatives as Positron Emission Tomography Imaging Ligands for Metabotropic Glutamate Receptor 2

Three benzimidazole derivatives (13-15) have been synthetized as potential positron emission tomography (PET) imaging ligands for mGluR2 in the brain. Of these compounds, 13 exhibits potent binding affinity (IC₅₀ = 7.6 ± 0.9 nM), positive allosteric modulator (PAM) activity (EC₅₀ = 51.2 nM), and excellent selectivity against other mGluR subtypes (>100-fold). [¹¹C]13 was synthesized via O-[¹¹C]methylation of its phenol precursor 25 with [¹¹C]methyl iodide. The achieved radiochemical yield was 20 ± 2% (n = 10, decay-corrected) based on [¹¹C]CO₂ with a radiochemical purity of >98% and molar activity of 98 ± 30 GBq/μmol EOS. Ex vivo biodistribution studies revealed reversible accumulation of [¹¹C]13 and hepatobiliary and urinary excretions. PET imaging studies in rats demonstrated that [¹¹C]13 accumulated in the mGluR2-rich brain regions. Pre-administration of mGluR2-selective PAM, 17 reduced the brain uptake of [¹¹C]13, indicating a selective binding. Therefore, [¹¹C]13 is a potential PET imaging ligand for mGluR2 in different central nervous system-related conditions.

Synthesis and Characterization of Fluorine-18-Labeled N-(4-Chloro-3-((fluoromethyl-d2)thio)phenyl)picolinamide for Imaging of mGluR4 in Brain

We have synthesized and characterized [18F]-N-(4-chloro-3-((fluoromethyl-d2)thio)phenyl)-picolinamide ([18F]15) as a potential ligand for the positron emission tomography (PET) imaging of mGluR4 in the brain. Radioligand [18F]15 displays central nervous system drug-like properties, including mGluR4 affinity, potent mGluR4 PAM activity, and selectivity against other mGluRs, as well as sufficient metabolic stability. Radiosynthesis was carried out in two steps. The radiochemical yield of [18F]15 was 11.6 ± 2.9% (n = 7, decay corrected) with a purity of 99% and a molar activity of 84.1 ± 11.8 GBq/μmol. Ex vivo biodistribution studies showed reversible binding of [18F]15 in all investigated tissues including the brain, liver, heart, lungs, and kidneys. PET imaging studies in male Sprague Dawley rats showed that [18F]15 accumulates in the brain regions known to express mGluR4. Pretreatment with the unlabeled mGluR4 PAM compounds 13 (methylthio analogue) and 15 showed significant dose-dependent blocking effects. These results suggest that [18F]15 is a promising radioligand for PET imaging mGluR4 in the brain.

Improved Synthesis of the Thiophenol Precursor N-(4-Chloro-3-mercaptophenyl)picolinamide for Making the mGluR4 PET Ligand

Recently [11C]mG4P012 (previously [11C]KALB012 and presently named as [11C]PXT012253 by Prexton Therapeutics) had been used as a biomarker during the preclinical development of a potential therapeutic drug, PXT0002331 (an mGluR4 PAM), for PD and L-dopa-induced dyskinesia. [11C]mG4P012 was shown to be a promising PET radioligand for mGluR4 in the monkey brain and for further development in human subjects. However, the previously reported multi-step synthesis of the thiophenol precursor suffered from low yields and difficult workup procedures. To support the translational research of [11C]mG4P012 and the other potential applications, we have developed a new route for synthesis of the thiophenol precursor and optimized the reaction conditions. The synthesis of N-(4-chloro-3-mercaptophenyl)picolinamide from 1-chloro-4-nitrobenzene has been greatly improved from 8% to 52% total yield with easy handling and in gram scales.

Imaging the glutamate receptor subtypes-Much achieved, and still much to do

Functional imaging of glutamate receptors using PET imaging modality can be used to study numerous CNS disorders and also to select appropriate doses of clinically relevant glutamate-receptor-targeting candidate drugs. Great strides have been made in developing PET imaging probes for the non-invasive detection of glutamate receptors in the brain. This review highlights recent progress made towards the development of glutamatergic PET imaging agents. Focus is placed on PET imaging probes that have been labelled with either carbon-11 or fluorine-18.

Functional modulation of G-protein coupled receptors during Parkinson disease-like neurodegeneration

G-protein coupled dopamine and metabotropic glutamate receptors (mGlu) can modulate neurotransmission during Parkinson's disease (PD)-like neurodegeneration. PET imaging studies in a unilateral dopamine denervation model (6-OHDA) showed a significant inverse correlation of presynaptic mGlu4 and postsynaptic mGlu5 expression in the striatum and rapidly declining mGlu4 and enhanced mGlu5 expression in the hippocampus during progressive degeneration over time. Immunohistochemical studies verified the decreased mGlu4 expression in the hippocampus on the lesion side but did not show difference in mGlu5 expression between lesion and control side. Pharmacological MRI studies showed enhanced hemodynamic response in several brain areas on the lesion side compared to the control side after challenge with mGlu4 positive allosteric modulator or mGlu5 negative allosteric modulator. However, mGlu4 response was biphasic having short enhancement followed by negative response on both sides of brain. Studies in mGlu4 expressing cells demonstrated that glutamate induces cooperative increase in binding of mGlu4 ligands - especially at high glutamate levels consistent with in vivo concentration. This suggests that mGlu allosteric modulators as drug candidates will be highly sensitive to changes in glutamate concentration and hence metabolic state. These experiments demonstrate the importance of the longitudinal imaging studies to investigate temporal changes in receptor functions to obtain individual response for experimental drugs.

Radiosynthesis and evaluation of 5-methyl-N-(4-[(11)C]methylpyrimidin-2-yl)-4-(1H-pyrazol-4-yl)thiazol-2-amine ([(11)C]ADX88178) as a novel radioligand for imaging of metabotropic glutamate receptor subtype 4 (mGluR4)

ADX88178 (1) has been recently developed as a potent positive allosteric modulator for metabotropic glutamate receptor 4 (mGluR4). The aim of this study was to develop [(11)C]1 as a novel positron emission tomography ligand and to evaluate its binding ability for mGluR4. Using stannyl precursor 3, [(11)C]1 was efficiently synthesized by introducing an [(11)C]methyl group into a pyrimidine ring via C-(11)C coupling and deprotection reactions, in 16±6% radiochemical yield (n=10). At the end of synthesis, 0.54-1.10GBq of [(11)C]1 was acquired with >98% radiochemical purity and 90-120GBq/μmol of specific activity. In vitro autoradiography and ex vivo biodistribution study in rat brains showed specific binding of [(11)C]1 in the cerebellum, striatum, thalamus, cerebral cortex, and medulla oblongata, which showed dose-dependent decreases by administration with multi-dose of unlabeled 1.

Synthesis and evaluation of N-(methylthiophenyl)picolinamide derivatives as PET radioligands for metabotropic glutamate receptor subtype 4

In recent years, mGlu4 has received great research attention because of the potential benefits of mGlu4 activation in treating numerous brain disorders, such as Parkinson's disease (PD). A specific mGlu4 PET radioligand could be an important tool in understanding the role of mGlu4 in both healthy and disease conditions, and also for the development of new drugs. In this study, we synthesized four new N-(methylthiophenyl)picolinamide derivatives 11-14. Of these ligands, 11 and 14 showed high in vitro binding affinity for mGlu4 with IC50 values of 3.4nM and 3.1nM, respectively, and suitable physicochemical parameters. Compound 11 also showed enhanced metabolic stability and good selectivity to other mGluRs. [(11)C]11 and [(11)C]14 were radiolabeled using the [(11)C]methylation of the thiophenol precursors 20a and 20c with [(11)C]CH3I in 19.0% and 34.8% radiochemical yields (RCY), and their specific activities at the end of synthesis (EOS) were 496±138GBq/μmol (n=6) and 463±263GBq/μmol (n=4), respectively. The PET studies showed that [(11)C]11 accumulated fast into the brain and had higher uptake, slower washout and 25% better contrast than [(11)C]2, indicating improved imaging characteristics as PET radiotracer for mGlu4 compared to [(11)C]2. Therefore, [(11)C]11 will be a useful radioligand to investigate mGlu4 in different biological applications.

Re-exploring the N-phenylpicolinamide derivatives to develop mGlu4 ligands with improved affinity and in vitro microsomal stability

In recent years, mGlu4 has received great attention and research effort because of the potential benefits of mGlu4 activation in treating numerous brain disorders, such as Parkinson's disease (PD). Many positive allosteric modulators of mGlu4 have been developed. To better understand the role of mGlu4 in healthy and disease conditions, we are interested in developing an mGlu4 selective radioligand for in vivo studies. Thus, we had synthesized and studied [(11)C]2 as a PET tracer for mGlu4, which demonstrated some promising features as a PET radioligand as well as the limitation need to be improved. In order to develop an mGlu4 ligand with enhanced affinity and improved metabolic stability, we have modified, synthesized and evaluated a series of new N-phenylpicolinamide derivatives. The SAR study has discovered a number of compounds with low nM affinity to mGlu4. The dideuteriumfluoromethoxy modified compound 24 is identified as a very promising mGlu4 ligand, which has demonstrated enhanced affinity, improved in vitro microsomal stability, good selectivity and good permeability.

Co-operative binding assay for the characterization of mGlu4 allosteric modulators

The interest in the role of metabotropic glutamate receptor 4 (mGlu4) in CNS related disorders has increased the need for methods to investigate the binding of allosteric drug candidates. Our aim is to present the first fully characterized in vitro binding assay of mGlu4 positive allosteric modulators (PAMs). Results suggest that mGlu4 PAMs have characteristic co-operative binding with orthosteric glutamate, which offers a notable insight to the further development of mGlu4 targeted therapies.

Development of PET and SPECT probes for glutamate receptors

l-Glutamate and its receptors (GluRs) play a key role in excitatory neurotransmission within the mammalian central nervous system (CNS). Impaired regulation of GluRs has also been implicated in various neurological disorders. GluRs are classified into two major groups: ionotropic GluRs (iGluRs), which are ligand-gated ion channels, and metabotropic GluRs (mGluRs), which are coupled to heterotrimeric guanosine nucleotide binding proteins (G-proteins). Positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging of GluRs could provide a novel view of CNS function and of a range of brain disorders, potentially leading to the development of new drug therapies. Although no satisfactory imaging agents have yet been developed for iGluRs, several PET ligands for mGluRs have been successfully employed in clinical studies. This paper reviews current progress towards the development of PET and SPECT probes for GluRs.

Radiosynthesis and evaluation of an 18F-labeled positron emission tomography (PET) radioligand for metabotropic glutamate receptor subtype 4 (mGlu4)

Four 4-phthalimide derivatives of N-(3-chlorophenyl)-2-picolinamide were synthesized as potential ligands for the PET imaging of mGlu₄ in the brain. Of these compounds, N-(3-chloro-4-(4-fluoro-1,3-dioxoisoindolin-2-yl)phenyl)-2-picolinamide (3, KALB001) exhibited improved binding affinity (IC₅₀ = 5.1 nM) compared with ML128 (1) and was subsequently labeled with ¹⁸F. When finally formulated in 0.1 M citrate buffer (pH 4) with 10% ethanol, the specific activity of [¹⁸F]3 at the end of synthesis (EOS) was 233.5 ± 177.8 GBq/μmol (n = 4). The radiochemical yield of [¹⁸F]3 was 16.4 ± 4.8% (n = 4), and the purity was over 98%. In vivo imaging studies in a monkey showed that the radiotracer quickly penetrated the brain with the highest accumulation in the brain areas known to express mGlu₄. Despite some unfavorable radiotracer properties like fast washout in rodent studies, [¹⁸F]3 is the first ¹⁸F-labeled mGlu₄ radioligand, which can be further modified to improve pharmacokinetics and brain penetrability for future human studies.