Synthesis, evaluation and metabolic studies of radiotracers containing a 4-(4-[18F]-fluorobenzyl)piperidin-1-yl moiety for the PET imaging of NR2B NMDA receptors

State of the art procedures towards reactive [18F]fluoride in PET tracer synthesis

BACKGROUND

Positron emission tomography (PET) is a powerful, non-invasive preclinical and clinical nuclear imaging technique used in disease diagnosis and therapy assessment. Fluorine-18 is the predominant radionuclide used for PET tracer synthesis. An impressive variety of new 'late-stage' radiolabeling methodologies for the preparation of 18F-labeled tracers has appeared in order to improve the efficiency of the labeling reaction.

MAIN BODY

Despite these developments, one outstanding challenge into the early key steps of the process remains: the preparation of reactive [18F]fluoride from oxygen-18 enriched water ([18O]H2O). In the last decade, significant changes into the trapping, elution and drying stages have been introduced. This review provides an overview of the strategies and recent developments in the production of reactive [18F]fluoride and its use for radiolabeling.

CONCLUSION

Improved, modified or even completely new fluorine-18 work-up procedures have been developed in the last decade with widespread use in base-sensitive nucleophilic 18F-fluorination reactions. The many promising developments may lead to a few standardized drying methodologies for the routine production of a broad scale of PET tracers.

A Glutamate N-Methyl-d-Aspartate (NMDA) Receptor Subunit 2B-Selective Inhibitor of NMDA Receptor Function with Enhanced Potency at Acidic pH and Oral Bioavailability for Clinical Use

We describe a clinical candidate molecule from a new series of glutamate N-methyl-d-aspartate receptor subunit 2B-selective inhibitors that shows enhanced inhibition at extracellular acidic pH values relative to physiologic pH. This property should render these compounds more effective inhibitors of N-methyl-d-aspartate receptors at synapses responding to a high frequency of action potentials, since glutamate-containing vesicles are acidic within their lumen. In addition, acidification of penumbral regions around ischemic tissue should also enhance selective drug action for improved neuroprotection. The aryl piperazine we describe here shows strong neuroprotective actions with minimal side effects in preclinical studies. The clinical candidate molecule NP10679 has high oral bioavailability with good brain penetration and is suitable for both intravenous and oral dosing for therapeutic use in humans. SIGNIFICANCE STATEMENT: This study identifies a new series of glutamate N-methyl-d-aspartate (NMDA) receptor subunit 2B-selective negative allosteric modulators with properties appropriate for clinical advancement. The compounds are more potent at acidic pH, associated with ischemic tissue, and this property should increase the therapeutic safety of this class by improving efficacy in affected tissue while sparing NMDA receptor block in healthy brain.

GluN2B/N-methyl-D-aspartate Receptor Antagonists: Advances in Design, Synthesis, and Pharmacological Evaluation Studies

Selective GluN2B/N-methyl-D-aspartate receptor (NMDAR) antagonists have exposed their clinical effectiveness in a cluster of neurodegenerative diseases, such as epilepsy, Alzheimer's disease, Parkinson's disease, pain, and depression. Hence, GluN2B/NMDARs are considered to be a prospective target for the management of neurodegenerative diseases. Here, we have discussed the current results and significance of subunit selective GluN2B/NMDAR antagonists to pave the way for the establishment of new, safe, and economical drug candidates in the near future. By using summarized data of selective GluN2B/NMDAR antagonists, medicinal chemists are certainly a step closer to the goal of improving the therapeutic and side effect profile of selective antagonists. Outlined summary of designing strategies, synthetic schemes, and pharmacological evaluation studies reinvigorate efforts to identify, modify, and synthesize novel GluN2B/NMDAR antagonists for treating neurodegenerative diseases.

N-Methyl-D-Aspartate (NMDA) receptor modulators: a patent review (2015-present)

INTRODUCTION

- The NMDA receptor is implicated in various diseases including neurodegenerative, neurodevelopmental and mood disorders. However, only a limited number of clinically approved NMDA receptor modulators are available. Today, apparent NMDA receptor drug development strategies entail 1) exploring the unknown chemical space to identify novel scaffolds; 2) using the clinically available NMDA receptor modulators to expand the therapeutic indication space; 3) and to trace physiological functions of the NMDA receptor.

AREAS COVERED

- The current review reflects on the functional and pharmacological facets of NMDA receptors and the current clinical status quo of NMDA receptor modulators. Patent literature covering 2015 till April 2020 is discussed with emphasis on new indications.

EXPERT OPINION

- Supporting evidence shows that subtype-selective NMDA receptor antagonists show an improved safety profile compared to broad-spectrum channel blockers. Although GluN2B-selective antagonists are by far the most extensively investigated subtype-selective modulators, they have shown only modest clinical efficacy so far. To overcome the limitations that have hampered the clinical development of previous subtype-selective NMDA receptor antagonists, future studies with improved animal models that better reflect human NMDA receptor pathophysiology are warranted. The increased availability of subtype-selective probes will allow target engagement studies and proper dose finding in future clinical trials.

Evaluation of the Novel PET Tracer [11C]HACH242 for Imaging the GluN2B NMDA Receptor in Non-Human Primates

PURPOSE

There are currently no positron emission tomography (PET) radiotracers for the GluN2B (NR2B) binding sites of brain N-methyl-D-aspartate (NMDA) receptors. In rats, the GluN2B antagonist Ro25-6981 reduced the binding of N-((5-(4-fluoro-2-[¹¹C]methoxyphenyl)pyridin-3-yl)methyl)cyclopentanamin ([¹¹C]HACH242). This paper reports the evaluation of [¹¹C]HACH242 PET in non-human primates at baseline and following administration of the GluN2B negative allosteric modulator radiprodil.

PROCEDURES

Eight 90-min dynamic [¹¹C]HACH242 PET scans were acquired in three male anaesthetised rhesus monkeys, including a retest session of subject 1, at baseline and 10 min after intravenous 10 mg/kg radiprodil. Standardised uptake values (SUV) were calculated for 9 brain regions. Arterial blood samples were taken at six timepoints to characterise pharmacokinetics in blood and plasma. Reliable input functions for kinetic modelling could not be generated due to variability in the whole-blood radioactivity measurements.

RESULTS

[¹¹C]HACH242 entered the brain and displayed fairly uniform uptake. The mean (± standard deviation, SD) T_max was 17 ± 7 min in baseline scans and 24 ± 15 min in radiprodil scans. The rate of radioligand metabolism in plasma (primarily to polar metabolites) was high, with mean parent fractions of 26 ± 10 % at 20 min and 8 ± 5 % at 85 min. Radiprodil increased [¹¹C]HACH242 whole-brain SUV in the last PET frame by 25 %, 1 %, 3 and 17 % for subjects 1, 2, 3 and retest of subject 1, respectively. The mean brain to plasma ratio was 5.4 ± 2.6, and increased by 39 to 110 % in the radiprodil condition, partly due to lower parent plasma radioactivity of -11 to -56 %.

CONCLUSIONS

The present results show that [¹¹C]HACH242 has a suitable kinetic profile in the brain and low accumulation of lipophilic radiometabolites. Radiprodil did not consistently change [¹¹C]HACH242 brain uptake. These findings may be explained by variations in cerebral blood flow, a low fraction of specifically bound tracer, or interactions with endogenous NMDA receptor ligands at the binding site. Further experiments of ligand interactions are necessary to facilitate the development of radiotracers for in vivo imaging of the ionotropic NMDA receptor.

Innovative Molecular Imaging for Clinical Research, Therapeutic Stratification, and Nosography in Neuroscience

Over the past few decades, several radiotracers have been developed for neuroimaging applications, especially in PET. Because of their low steric hindrance, PET radionuclides can be used to label molecules that are small enough to cross the blood brain barrier, without modifying their biological properties. As the use of 11C is limited by its short physical half-life (20 min), there has been an increasing focus on developing tracers labeled with 18F for clinical use. The first such tracers allowed cerebral blood flow and glucose metabolism to be measured, and the development of molecular imaging has since enabled to focus more closely on specific targets such as receptors, neurotransmitter transporters, and other proteins. Hence, PET and SPECT biomarkers have become indispensable for innovative clinical research. Currently, the treatment options for a number of pathologies, notably neurodegenerative diseases, remain only supportive and symptomatic. Treatments that slow down or reverse disease progression are therefore the subject of numerous studies, in which molecular imaging is proving to be a powerful tool. PET and SPECT biomarkers already make it possible to diagnose several neurological diseases in vivo and at preclinical stages, yielding topographic, and quantitative data about the target. As a result, they can be used for assessing patients' eligibility for new treatments, or for treatment follow-up. The aim of the present review was to map major innovative radiotracers used in neuroscience, and explain their contribution to clinical research. We categorized them according to their target: dopaminergic, cholinergic or serotoninergic systems, β-amyloid plaques, tau protein, neuroinflammation, glutamate or GABA receptors, or α-synuclein. Most neurological disorders, and indeed mental disorders, involve the dysfunction of one or more of these targets. Combinations of molecular imaging biomarkers can afford us a better understanding of the mechanisms underlying disease development over time, and contribute to early detection/screening, diagnosis, therapy delivery/monitoring, and treatment follow-up in both research and clinical settings.

Synthesis and Characterization in Rodent Brain of the Subtype-Selective NR2B NMDA Receptor Ligand [11C]Ro04-5595 as a Potential Radiotracer for Positron Emission Tomography

The NR2B subunit of the N-methyl-d-aspartate (NMDA) receptor has been implicated in controlling synaptic plasticity, memory, and learning. Herein, we describe an 11C-labeled PET radiotracer based on 1-(4-chlorophenethyl)-6-methoxy-2-methyl-1,2,3,4-tetrahydroisoquinolin-7-ol, Ro04-5595. The radiotracer was evaluated in rats using PET. The PET study showed a good pharmacokinetic profile with rapid uptake and washout over 90 min. Complementary high-resolution autoradiographic images using [3H]Ro04-5595 demonstrated strong binding in NR2B receptor-rich regions and low binding in cerebellum where NR2B concentration is low. We conclude to have developed a selective NR2B receptor radioligand suitable for quantitative and qualitative imaging of a NR2B receptor distribution in vitro and in vivo.

Synthesis and characterization of 11 C-labeled benzyl amidine derivatives as PET radioligands for GluN2B subunit of the NMDA receptors

GluN2B-containing NMDA receptors (NMDARs) play fundamental roles in learning and memory, although they are also associated with various brain disorders. In this study, we synthesized and evaluated three ¹¹ C-labeled N-benzyl amidine derivatives 2-[¹¹ C]methoxybenzyl) cinnamamidine ([¹¹ C]CBA), N-(2-[¹¹ C]methoxybenzyl)-2-naphthamidine ([¹¹ C]NBA), and N-(2-[¹¹ C]methoxybenzyl)quinoline-3-carboxamidine ([¹¹ C]QBA) as PET radioligands for these receptors. The ¹¹ C-benzyl amidines were synthesized via conventional methylation of corresponding des-methyl precursors with [¹¹ C]CH₃ I. In vitro binding characteristics were examined in brain sagittal sections using various GluN2B modulators and off-target ligands. Further, in vivo brain distribution studies were performed in normal mice. The ¹¹ C-labeled benzyl amidines showed high-specific binding to the GluN2B subunit at in vitro. In particular, the quinoline derivative [¹¹ C]QBA had the best binding properties in terms of high-brain localization to GluN2B-rich regions and specificity to the GluN2B subunit. Conversely, these ¹¹ C-radioligands showed the brain distributions were inconsistent with GluN2B expression in biodistribution experiments. The majority of the radiolabeled compounds were identified as metabolized forms of which amido derivatives seemed to be the major species. Although these ¹¹ C-ligands had high-specific binding to the GluN2B subunit, significant improvement in metabolic stability is necessary for successful positron emission tomography (PET) imaging of the GluN2B subunit of NMDARs.

Fluorinated GluN2B Receptor Antagonists with a 3-Benzazepine Scaffold Designed for PET Studies

To analyze the N-methyl-d-aspartate (NMDA) receptor distribution in the central nervous system, fluorinated ligands that selectively address the ifenprodil binding site of GluN2B-subunit-containing NMDA receptors were developed. Various strategies to introduce a fluorine atom into the potent GluN2B ligand 2 (3-(4-phenylbutyl)-2,3,4,5-tetrahydro-1H-3-benzazepin-1,7-diol) were pursued, including replacement of the benzylic OH moiety with a fluorine atom (13) and introduction of fluoroethoxy moieties at various positions (14 (7-position), 17 (9-position), 18a-c (1-position)). With respect to GluN2B affinity and selectivity over related receptors, the fluoroethoxy derivatives 14 and 18a are the most promising ligands. Radiosynthesis of fluoroethoxy derivative [18 F]14 was performed by nucleophilic substitution of the phenol 2 with 2-[18 F]fluoroethyl tosylate. On rat brain slices the fluorinated PET tracer [18 F]14 accumulated in regions with high density of NMDA receptors containing GluN2B subunits. The bound radioactivity could not be replaced by (S)-glutamate. However, the GluN2B ligands eliprodil, Ro 25-6981, and the non-labeled 3-benzazepine 14 were able to abolish the specific binding of [18 F]14.

Evaluation of 11C-Me-NB1 as a Potential PET Radioligand for Measuring GluN2B-Containing NMDA Receptors, Drug Occupancy, and Receptor Cross Talk

Clinical and preclinical research with modulators at the N-methyl-d-aspartate (NMDA) receptor GluN2B N-terminal domain (NTD) aims for the treatment of various neurologic diseases. The interpretation of the results is hampered by the lack of a suitable NMDA PET tracer for assessing the receptor occupancy of potential drugs. We have developed 11C-Me-NB1 as a PET tracer for imaging GluN1/GluN2B-containing NMDA receptors and used it to investigate in rats the dose-dependent receptor occupancy of eliprodil, a GluN2B NTD modulator. Methods:11C-Me-NB1 was synthesized and characterized by in vitro displacement binding experiments with rat brain membranes, in vitro autoradiography, and blocking and displacement experiments by PET and PET kinetic modeling. Receptor occupancy by eliprodil was studied by PET with 11C-Me-NB1. Results:11C-Me-NB1 was synthesized at 290 ± 90 GBq/μmol molar activity, 7.4 ± 1.9 GBq total activity at the end of synthesis (n = 17), and more than 99% radiochemical purity. 11C-Me-NB1 binding in rat brain was blocked in vitro and in vivo by the NTD modulators Ro-25-6981 and eliprodil. Half-maximal receptor occupancy by eliprodil occurred at 1.5 μg/kg. At 1 mg/kg of eliprodil, a dose with reported neuroprotective effects, more than 99.5% of binding sites were occupied. In vitro, 11C-Me-NB1 binding was independent of the σ-1 receptor (Sigma1R), and the Sigma1R agonist (+)-pentazocine did not compete for high-affinity binding. In vivo, a 2.5 mg/kg dose of (+)-pentazocine abolished 11C-Me-NB1-specific binding, indicating an indirect effect of Sigma1R on 11C-Me-NB1 binding. Conclusion:11C-Me-NB1 is suitable for the in vivo imaging of NMDA GluN1/GluN2B receptors and the assessment of receptor occupancy by NTD modulators. GluN1/GluN2B NMDA receptors are fully occupied at neuroprotective doses of eliprodil. Furthermore, 11C-Me-NB1 enables imaging of GluN1/GluN2B NMDA receptor cross talk.

Selected PET Radioligands for Ion Channel Linked Neuroreceptor Imaging: Focus on GABA, NMDA and nACh Receptors

Positron emission tomography (PET) neuroimaging of ion channel linked receptors is a developing area of preclinical and clinical research. The present review focuses on recent advances with radiochemistry, preclinical and clinical PET imaging studies of three receptors that are actively pursued in neuropsychiatric drug discovery: namely the γ-aminobutyric acid-benzodiazapine (GABA) receptor, nicotinic acetylcholine receptor (nAChR), and N-methyl-D-aspartate (NMDA) receptor. Recent efforts to develop new PET radioligands for these targets with improved brain uptake, selectivity, stability and pharmacokinetics are highlighted.

2,2,2-Trifluoroethyl Chlorooxoacetate--Universal Reagent for One-Pot Parallel Synthesis of N(1)-Aryl-N(2)-alkyl-Substituted Oxamides

A one-pot parallel synthesis of N(1)-aryl-N(2)-alkyl-substituted oxamides with 2,2,2-trifluoroethyl chlorooxoacetate was developed. The synthesis of a library of 45 oxamides revealed higher efficiency of this reagent over the known ethyl chlorooxoacetate. The reagent was successfully used to prepare the known oxamide-containing HIV entry inhibitors.

Radiolabeling of [18F]-fluoroethylnormemantine and initial in vivo evaluation of this innovative PET tracer for imaging the PCP sites of NMDA receptors

INTRODUCTION

The N-methyl-D-aspartate receptor (NMDAr) is an ionotropic receptor that mediates excitatory transmission. NMDAr overexcitation is thought to be involved in neurological and neuropsychiatric disorders such as Alzheimer disease and schizophrenia. We synthesized [(18)F]-fluoroethylnormemantine ([(18)F]-FNM), a memantine derivative that binds to phencyclidine (PCP) sites within the NMDA channel pore. These sites are primarily accessible when the channel is in the active and open state.

METHODS

Radiosynthesis was carried out using the Raytest® SynChrom R&D fluorination module. Affinity of this new compound was determined by competition assay. We ran a kinetic study in rats and computed a time-activity curve based on a volume-of-interest analysis, using CARIMAS® software. We performed an ex vivo autoradiography, exposing frozen rat brain sections to a phosphorscreen. Adjacent sections were used to detect NMDAr by immunohistochemistry with an anti-NR1 antibody. As a control of the specificity of our compound for NMDAr, we used a rat anesthetized with ketamine. Correlation analysis was performed with ImageJ software between signal of autoradiography and immunostaining.

RESULTS

Fluorination yield was 10.5% (end of synthesis), with a mean activity of 3145 MBq and a specific activity above 355 GBq/μmol. Affinity assessment allowed us to determine [(19)F]-FNM IC50 at 6.1 10(-6)M. [(18)F]-FMN concentration gradually increased in the brain, stabilizing at 40 minutes post injection. The brain-to-blood ratio was 6, and 0.4% of the injected dose was found in the brain. Combined ex vivo autoradiography and immunohistochemical staining demonstrated colocalization of NMDAr and [(18)F]-FNM (r=0.622, p<0.0001). The highest intensity was found in the cortex and cerebellum, and the lowest in white matter. A low and homogeneous signal corresponding to unspecific binding was observed when PCP sites were blocked with ketamine.

CONCLUSIONS

[(18)F]-FNM appears to be a promising tracer for imaging NMDAr activity for undertaking preclinical studies in perspective of clinical detection of neurological or neuropsychological disorders.

Radiolabelling of 1,4-disubstituted 3-[18F]fluoropiperidines and its application to new radiotracers for NR2B NMDA receptor visualization

In order to develop a novel and useful building block for the development of radiotracers for positron emission tomography (PET), we studied the radiolabelling of 1,4-disubstituted 3-[(18)F]fluoropiperidines. Indeed, 3-fluoropiperidine became a useful building block in medicinal chemistry for the pharmacomodulation of piperidine-containing compounds. The radiofluorination was studied on substituted piperidines with electron-donating and electron-withdrawing N-substituents. In the instance of electron-donating N-substituents such as benzyl or butyl, configuration retention and satisfactory fluoride-18 incorporation yields up to 80% were observed. In the case of electron-withdrawing N-substituents leading to carbamate or amide functions, the incorporation yields depend on the 4-susbtitutent (2 to 63%). The radiolabelling of this building block was applied to the automated radiosynthesis of NR2B NMDA receptor antagonists and effected by a commercially available radiochemistry module. The in vivo evaluation of three radiotracers demonstrated minimal brain uptakes incompatible with the imaging of NR2B NMDA receptors in the living brain. Nevertheless, moderate radiometabolism was observed and, in particular, no radiodefluorination was observed which demonstrates the stability of the 3-position of the fluorine-18 atom. In conclusion, the 1,4-disubstituted 3-[(18)F]fluoropiperidine moiety could be of value in the development of other radiotracers for PET even if the evaluation of the NR2B NMDA receptor antagonists failed to demonstrate satisfactory properties for PET imaging of this receptor.

Synthesis and in vitro characterization of trans- and cis-[(18)F]-4-methylbenzyl 4-[(pyrimidin-2-ylamino)methyl]-3-fluoropiperidine-1-carboxylates as new potential PET radiotracer candidates for the NR2B subtype N-methyl-D-aspartate receptor

Diastereoisomeric compounds [(18)F]cis- and [(18)F]trans-4-methylbenzyl 4-[(pyrimidin-2-ylamino)methyl]-3-fluoro-piperidine-1-carboxylates were successfully synthesized as new subtype-selective PET radiotracers for imaging the NR2B subunit containing NMDA receptors. Rat brain section autoradiographies demonstrated a high specific binding in NR2B/NMDA receptor rich regions for both radioligands. The measured logD(7.4) values as well as B(max)/K(d) ratios indicated that both radiotracers possess the adequate properties required for PET radiotracers.