Radiosynthesis and biological evaluation of the M1 muscarinic acetylcholine receptor agonist ligand [11C]AF150(S)

Imaging Cholinergic Receptors in the Brain by Positron Emission Tomography

Cholinergic receptors represent a promising class of diagnostic and therapeutic targets due to their significant involvement in cognitive decline associated with neurological disorders and neurodegenerative diseases as well as cardiovascular impairment. Positron emission tomography (PET) is a noninvasive molecular imaging tool that has helped to shed light on the roles these receptors play in disease development and their diverse functions throughout the central nervous system (CNS). In recent years, there has been a notable advancement in the development of PET probes targeting cholinergic receptors. The purpose of this review is to provide a comprehensive overview of the recent progress in the development of these PET probes for cholinergic receptors with a specific focus on ligand structure, radiochemistry, and pharmacology as well as in vivo performance and applications in neuroimaging. The review covers the structural design, pharmacological properties, radiosynthesis approaches, and preclinical and clinical evaluations of current state-of-the-art PET probes for cholinergic receptors.

Update on PET Tracer Development for Muscarinic Acetylcholine Receptors

The muscarinic cholinergic system regulates peripheral and central nervous system functions, and, thus, their potential as a therapeutic target for several neurodegenerative diseases is undoubted. A clinically applicable positron emission tomography (PET) tracer would facilitate the monitoring of disease progression, elucidate the role of muscarinic acetylcholine receptors (mAChR) in disease development and would aid to clarify the diverse natural functions of mAChR regulation throughout the nervous system, which still are largely unresolved. Still, no mAChR PET tracer has yet found broad clinical application, which demands mAChR tracers with improved imaging properties. This paper reviews strategies of mAChR PET tracer design and summarizes the binding properties and preclinical evaluation of recent mAChR tracer candidates. Furthermore, this work identifies the current major challenges in mAChR PET tracer development and provides a perspective on future developments in this area of research.

In Vivo Receptor Visualization and Evaluation of Receptor Occupancy with Positron Emission Tomography

Positron emission tomography (PET) is useful for noninvasive in vivo visualization of disease-related receptors, for evaluation of receptor occupancy to determine an appropriate drug dosage, and for proof-of-concept of drug candidates in translational research. For these purposes, the specificity of the PET tracer for the target receptor is critical. Here, we review work in this area, focusing on the chemical structures of reported PET tracers, their K_i/K_d values, and the physical properties relevant to target receptor selectivity. Among these physical properties, such as cLogP, cLogD, molecular weight, topological polar surface area, number of hydrogen bond donors, and pK_a, we focus especially on LogD and LogP as important physical properties that can be easily compared across a range of studies. We discuss the success of PET tracers in evaluating receptor occupancy and consider likely future developments in the field.

Enhanced arecoline derivatives as muscarinic acetylcholine receptor M1 ligands for potential application as PET radiotracers

Supported by their involvement in many neurodegenerative disorders, muscarinic acetylcholine receptors (mAChRs) are an interesting target for PET imaging. Nevertheless, no radiotracer is established in clinical routine. Within this work we aim to develop novel PET tracers based on the structure of arecoline. Fifteen novel arecoline derivatives were synthesized, characterized and tested for their affinity to the mAChRs M1-M5 and the conceivable off-target acetylcholinesterase. Five arecoline derivatives and arecoline were labeled with carbon-11 in good yields. Arecaidine diphenylmethyl ester (3b), arecaidine bis(4-fluorophenyl)methyl ester (3c) and arecaidine (4-bromophenyl)(4-fluorophenyl)methyl ester (3e) showed a tremendous gain in mAChR affinity compared to arecoline and a pronounced subtype selectivity for M1. Metabolic stability and serum protein binding of [¹¹C]3b and [¹¹C]3c were in line with properties of established brain tracers. Nonspecific binding of [¹¹C]3c was prevalent in kinetic and endpoint experiment on living cells as well as in autoradiography on native mouse brain sections, which motivates us to decrease the lipophilicity of this substance class prior to in vivo experiments.

Evaluation of 11C-LSN3172176 as a Novel PET Tracer for Imaging M1 Muscarinic Acetylcholine Receptors in Nonhuman Primates

The M₁ muscarinic acetylcholine receptor (mAChR) plays an important role in learning and memory, and therefore is a target for development of drugs for treatment of cognitive impairments in Alzheimer disease and schizophrenia. The availability of M₁-selective radiotracers for PET will help in developing therapeutic agents by providing an imaging tool for assessment of drug dose-receptor occupancy relationship. Here we report the synthesis and evaluation of ¹¹C-LSN3172176 (ethyl 4-(6-(methyl-¹¹ C)-2-oxoindolin-1-yl)-[1,4'-bipiperidine]-1'-carboxylate) in nonhuman primates. Methods: ¹¹C-LSN3172176 was radiolabeled via the Suzuki-Miyaura cross-coupling method. PET scans in rhesus macaques were acquired for 2 h with arterial blood sampling and metabolite analysis to measure the input function. Blocking scans with scopolamine (50 μg/kg) and the M₁-selective agent AZD6088 (0.67 and 2 mg/kg) were obtained to assess tracer binding specificity and selectivity. Regional brain time-activity curves were analyzed with the 1-tissue-compartment model and the multilinear analysis method (MA1) to calculate regional distribution volume. Nondisplaceable binding potential values were calculated using the cerebellum as a reference region. Results: ¹¹C-LSN3172176 was synthesized with greater than 99% radiochemical purity and high molar activity. In rhesus monkeys, ¹¹C-LSN3172176 metabolized rapidly (29% ± 6% parent remaining at 15 min) and displayed fast kinetics and extremely high uptake in the brain. Imaging data were modeled well with the 1-tissue-compartment model and MA1 methods. MA1-derived distribution volume values were high (range, 10-81 mL/cm³) in all known M₁ mAChR-rich brain regions. Pretreatment with scopolamine and AZD6088 significantly reduced the brain uptake of ¹¹C-LSN3172176, thus demonstrating its binding specificity and selectivity in vivo. The cerebellum appeared to be a suitable reference region for derivation of nondisplaceable binding potential, which ranged from 2.42 in the globus pallidus to 8.48 in the nucleus accumbens. Conclusion: ¹¹C-LSN3172176 exhibits excellent in vivo binding and imaging characteristics in nonhuman primates and appears to be the first appropriate radiotracer for PET imaging of human M₁ AChR.

[11C]AF150(S), an agonist PET ligand for M1 muscarinic acetylcholine receptors

Background

The M1 muscarinic acetylcholine receptor (M1ACh-R) is a G protein-coupled receptor that can occur in interconvertible coupled and uncoupled states. It is enriched in the basal ganglia, hippocampus, olfactory bulb, and cortical areas, and plays a role in motor and cognitive functions. Muscarinic M1 agonists are potential therapeutic agents for cognitive disorders. The aim of this study was to evaluate [¹¹C]AF150(S) as a putative M1ACh-R agonist PET ligand, which, owing to its agonist properties, could provide a tool to explore the active G protein-coupled receptor.

Methods

Regional kinetics of [¹¹C]AF150(S) in rat brain were measured using a high-resolution research tomograph, both under baseline conditions and following pre-treatment with various compounds or co-administration of non-radioactive AF150(S). Data were analysed by calculating standard uptake values and by applying the simplified reference tissue model (SRTM).

Results

[¹¹C]AF150(S) was rapidly taken up in the brain, followed by a rapid clearance from all brain regions. Analysis of PET data using SRTM revealed a binding potential (BP_ND) of 0.25 for the striatum, 0.20 for the hippocampus, 0.16 for the frontal cortical area and 0.15 for the posterior cortical area, all regions rich in M1ACh-R. BP_ND values were significantly reduced following pre-treatment with M1ACh-R antagonists. BP_ND values were not affected by pre-treatment with a M3ACh-R antagonist. Moreover, BP_ND was significantly reduced after pre-treatment with haloperidol, a dopamine D₂ receptor blocker that causes an increase in extracellular acetylcholine (ACh). The latter may compete with [¹¹C]AF150(S) for binding to the M1ACh-R; further pharmacological agents were applied to investigate this possibility. Upon injection of the highest dose (49.1 nmol kg⁻¹) of [¹¹C]AF150(S) diluted with non-radioactive AF150(S), brain concentration of AF150(S) reached 100 nmol L⁻¹ at peak level. At this concentration, no sign of saturation in binding to M1ACh-R was observed.

Conclusions

The agonist PET ligand [¹¹C]AF150(S) was rapidly taken up in the brain and showed an apparent specific M1ACh-R-related signal in brain areas that are rich in M1ACh-R. Moreover, binding of the agonist PET ligand [¹¹C]AF150(S) appears to be sensitive to changes in extracellular ACh levels. Further studies are needed to evaluate the full potential of [¹¹C]AF150(S) for imaging the active pool of M1ACh-R in vivo.

Reproducible Analysis of Rat Brain PET Studies Using an Additional [(18)F]NaF Scan and an MR-Based ROI Template

Background. An important step in the analysis of positron emission tomography (PET) studies of the brain is the definition of regions of interest (ROI). Image coregistration, ROI analysis, and quantification of brain PET data in small animals can be observer dependent. The purpose of this study was to investigate the feasibility of ROI analysis based on a standard MR template and an additional [¹⁸F]NaF scan. Methods. [¹⁸F]NaF scans of 10 Wistar rats were coregistered with a standard MR template by 3 observers and derived transformation matrices were applied to corresponding [¹¹C]AF150(S) images. Uptake measures were derived for several brain regions delineated using the MR template. Overall agreement between the 3 observers was assessed by interclass correlation coefficients (ICC) of uptake data. In addition, [¹¹C]AF150(S) ROI data were compared with ex vivo biodistribution data. Results. For all brain regions, ICC analysis showed excellent agreement between observers. Reproducibility, estimated by calculation of standard deviation of the between-observer differences, was demonstrated by an average of 17% expressed as coefficient of variation. Uptake of [¹¹C]AF150(S) derived from ROI analysis closely matched ex vivo biodistribution data. Conclusions. The proposed method provides a reproducible and tracer-independent method for ROI analysis of rat brain PET data.