Synthesis and autoradiographic localization of muscarinic cholinergic antagonist (+)N-[11C]methyl-3-piperidyl benzilate as a potent radioligand for positron emission tomography

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.

Muscarinic receptor occupancy and cognitive impairment: a PET study with [11C](+)3-MPB and scopolamine in conscious monkeys

The muscarinic cholinergic receptor (mAChR) antagonist scopolamine was used to induce transient cognitive impairment in monkeys trained in a delayed matching to sample task. The temporal relationship between the occupancy level of central mAChRs and cognitive impairment was determined. Three conscious monkeys (Macaca mulatta) were subjected to positron emission tomography (PET) scans with the mAChR radioligand N-[(11)C]methyl-3-piperidyl benzilate ([(11)C](+)3-MPB). The scan sequence was pre-, 2, 6, 24, and 48 h post-intramuscular administration of scopolamine in doses of 0.01 and 0.03 mg/kg. Occupancy levels of mAChR were maximal 2 h post-scopolamine in cortical regions innervated primarily by the basal forebrain, thalamus, and brainstem, showing that mAChR occupancy levels were 43-59 and 65-89% in doses of 0.01 and 0.03 mg/kg, respectively. In addition, dose-dependent impairment of working memory performance was measured 2 h after scopolamine. A positive correlation between the mAChR occupancy and cognitive impairment 2 and 6 h post-scopolamine was the greatest in the brainstem (P<0.00001). Although cognitive impairment was not observed 24 h post-scopolamine, sustained mAChR occupancy (11-24%) was found with both doses in the basal forebrain and thalamus, but not in the brainstem. These results indicate that a significant degree of mAChRs occupancy is needed to produce cognitive impairment by scopolamine. Furthermore, the importance of the brainstem cholinergic system in working memory in monkey is described.

Validation of reference tissue model of PET ligand [¹¹C]+3-MPB for the muscarinic cholinergic receptor in the living brain of conscious monkey

N-[¹¹C]methyl-3-piperidyl benzilate ([¹¹C]+3-MPB) was developed as a positron emission tomography (PET) ligand for muscarinic cholinergic receptor (mAChR). The aim of the present study was to validate a Logan reference tissue method as an analytical method for in vivo binding of [¹¹C]+3-MPB to mAChR. Seven monkeys (Macaca mulatta) underwent [¹¹C]+3-MPB PET scans with an arterial blood sampling. Logan plot with arterial input function (Logan arterial input method) was performed to determine the binding potential (BP(ND)). The BP(ND) was also determined by Logan plot with the cerebellum as the reference region (Logan reference tissue method). BP(ND) values determined by Logan arterial input method and Logan reference tissue method showed a significant linear relationship. The present study suggests that the cerebellum is a suitable reference region for quantification of mAChR in the living brain with [¹¹C]+3-MPB and PET.

In vivo quantitative autoradiographic analysis of brain muscarinic receptor occupancy by antimuscarinic agents for overactive bladder treatment

We evaluated the effects of five clinically used antimuscarinic agents for overactive bladder (OAB) treatment on in vivo muscarinic receptor binding in rat brain by quantitative autoradiography. There was a dose-related decrease in in vivo specific +N-[11C]methyl-3-piperidyl benzilate ([11C](+)3-MPB) binding in each brain region of rats 10 min after i.v. injection of oxybutynin, propiverine, solifenacin, and tolterodine. Rank order of the i.v. dose for 50% receptor occupancy (RO(50)) of antimuscarinic agents in rat brain regions was propiverine > solifenacin > tolterodine, oxybutynin. There was a good linear relationship between in vivo (pRO(50) values in the rat hippocampus) and in vitro (pK(i) values in human M(1) receptors) receptor binding activities of propiverine, solifenacin, and tolterodine. The observed RO(50) value of oxybutynin was approximately five times smaller than the predicted in vitro K(i) value. The dose ratios of antimuscarinic agents for the brain receptor occupancy (RO(50)) to the inhibition of carbachol- and volume-induced increases in intravesical pressure (ID(50)), which reflects in vivo selectivity for the urinary bladder over the brain, were greater for solifenacin, tolterodine, and propiverine than oxybutynin. Darifenacin displayed only a slight decrease in specific [11C](+)3-MPB binding in the rat brain regions, and it was not dose-related. In conclusion, in vivo quantitative autoradiographic analysis of brain muscarinic receptor occupancy may provide fundamental basis for managing central nervous system (CNS) side effects in antimuscarinic therapy for OAB. It is suggested that in the treatment of OAB, CNS side effects can be avoided by antimuscarinic agents with high selectivity for the urinary bladder over the brain.

Effects of acute acetylcholinesterase inhibition on the cerebral cholinergic neuronal system and cognitive function: Functional imaging of the conscious monkey brain using animal PET in combination with microdialysis

This study demonstrated the effects of acute acetylcholinesterase (AChE) inhibition by donepezil (Aricept) on the cerebral cholinergic neuronal system in the brains of young (5.2 +/- 1.1 years old) and aged (20.3 +/- 2.6 years old) monkeys (Macaca mulatta) in the conscious state. Donepezil at doses of 50 and 250 microg/kg suppressed AChE activity, analyzed by metabolic rate (k(3)) of N-[(11)C]methyl-4-piperidyl acetate ([(11)C]MP4A), in all cortical regions in a dose-dependent manner in both age groups. However, the suppression degree was more marked in young than in aged monkeys. AChE inhibition by donepezil resulted in a dose-dependent increase in acetylcholine levels in the prefrontal cortex of young animals as measured by microdialysis. Binding of (+)N-[(11)C]propyl-3-piperidyl benzilate ([(11)C](+)3-PPB) to cortical muscarinic receptors was reduced by donepezil, probably in a competitive inhibition manner. Aged monkeys showed less reduction of [(11)C](+)3-PPB binding than young animals. As evaluated by an oculomotor delayed response task, aged monkeys showed impaired working memory performance compared to young monkeys, and the impaired performance was partly improved by the administration of donepezil, due to the facilitation of the cholinergic neuronal system by AChE inhibition. These results demonstrate that the PET imaging technique with specific labeled compounds in combination with microdialysis and a behavioral cognition task could be a useful method to clarify the mechanism of drugs in the living brains of experimental animals.

PET imaging of ischemic neuronal death in the hippocampus of living monkeys

The aim of the present study was to visualize postischemic hippocampal neuronal death in the living monkey brain, using a high-resolution positron emission tomography (PET) and novel radioligands. In preceding papers, we reported on postischemic hippocampal neuronal death in a model of Japanese monkeys (Macaca fuscata) undergoing a 20-min complete whole-brain ischemia. Using the same model here, we investigated the in vivo bindings of two radiotracers, [11C]Ro15-4513 (a type II benzodiazepine receptor ligand) and [11C](+)3-MPB (a muscarinic cholinergic receptor ligand), in the hippocampus on day 7 after ischemia, as compared to the normal hippocampus. A significant decrease in the in vivo binding of [11C]Ro154513 and [11C(+)3-MPB was observed in the postischemic monkey hippocampus on day 7 after ischemia compared to controls. Light and electron microscopic analyses of postischemic CA1 neurons showed typical features of coagulation necrosis, as associated with a marked reduction of postsynaptic densities and presynaptic vesicles. These results suggest that semiquantification of hippocampal neuronal death is possible in the living primate brain using PET, and that the same procedures can be applied for evaluating neuronal cell loss in patients with ischemic injuries and/or dementia.

Age-related changes in muscarinic cholinergic receptors in the living brain: a PET study using N-[11C]methyl-4-piperidyl benzilate combined with cerebral blood flow measurement in conscious monkeys

The effects of changes in regional cerebral blood flow (rCBF) with aging on muscarinic cholinergic receptor binding were evaluated with [15O]H(2)O and N-[11C]methyl-4-piperidyl benzilate (4-MPB) in the living brains of young (5.9+/-1.8 years old) and aged (19.0+/-3.3 years old) monkeys (Macaca mulatta) in the conscious state using high-resolution positron emission tomography (PET). For quantitative analysis of receptor binding in vivo with [11C]4-MPB, metabolite-corrected arterial plasma radioactivity curves were obtained as an input function into the brain, and graphical Patlak plot analysis was applied. In addition, two-compartment model analysis using the radioactivity curve in the cerebellum as an input function (reference analysis) was also applied to determine the distribution volume (DV=K(1)/k(2)') for [11C]4-MPB. With metabolite-corrected arterial input, Patlak plot analysis of [11C]4-MPB indicated a regionally specific decrease in muscarinic cholinergic receptor binding in vivo in the frontal and temporal cortices as well as the striatum in aged compared with young animals, showing no correlation with the degree of reduced rCBF. In contrast, on the reference analysis with cerebellar input of [11C]4-MPB, all regions assayed except the pons showed a significant age-related decrease of DV, and the degree of reduction of DV was correlated with that of rCBF. These results demonstrated the usefulness of kinetic analysis of [11C]4-MPB with metabolite-corrected arterial input, not with reference region's input, as an indicator of the aging process of cortical muscarinic cholinergic receptors in vivo measured by PET with less blood flow dependency.

Age differences in muscarinic cholinergic receptors assayed with (+)N-[(11)C]methyl-3-piperidyl benzilate in the brains of conscious monkeys

Age-related changes in muscarinic cholinergic receptors were evaluated with the novel ligand (+)N-[(11)C]methyl-3-piperidyl benzilate ((+)3-MPB) in the living brains of young (5.9 +/- 1.8 years old) and aged (19.0 +/- 3.3 years old) monkeys (Macaca mulatta) in the conscious state using high-resolution positron emission tomography (PET). For quantitative analysis of receptor binding in vivo, metabolite-corrected arterial plasma radioactivity curves were obtained as an input function into the brain, and kinetic analyses using the three-compartment model and graphical Logan plot analysis were applied. Kinetic analyses of [(11)C](+)3-MPB indicated a regionally specific decrease in the receptor binding in vivo determined as binding potential (BP) = k(3)/k(4) in aged animals compared with young animals. Thus, the frontal and temporal cortices as well as the striatum showed age-related reduction of muscarinic cholinergic receptors in vivo, reflecting the reduced receptor density (B(max)) determined by Scatchard plot analysis in vivo. In the hippocampus, although BP of [(11)C](+)3-MPB indicated no significant age-related changes, it showed an inverse correlation with individual cortisol levels in plasma. When the graphical Logan plot analysis was applied, all regions assayed showed significant age-related decrease of [(11)C](+)3-MPB binding. These results demonstrate the usefulness of kinetic three-compartment model analysis of [(11)C](+)3-MPB with metabolite-corrected arterial plasma input as an indicator for the aging process of the cortical muscarinic cholinergic receptors in vivo as measured by PET.

Evaluation of PET ligands (+)N-[(11)C]ethyl-3-piperidyl benzilate and (+)N-[(11)C]propyl-3-piperidyl benzilate for muscarinic cholinergic receptors: a PET study with microdialysis in comparison with (+)N-[(11)C]methyl-3-piperidyl benzilate in the conscious monkey brain

We developed PET ligands (+)N-[(11)C]ethyl-3-piperidyl benzilate ([(11)C](+)3-EPB) and (+)N-[(11)C]propyl-3-piperidyl benzilate ([(11)C](+)3-PPB) for cerebral muscarinic cholinergic receptors. The distribution and kinetics of the novel ligands were evaluated for comparison with the previously reported ligand (+)N-[(11)C]methyl-3-piperidyl benzilate ([(11)C](+)3-MPB) in the monkey brain (Macaca mulatta) in the conscious state using high-resolution positron emission tomography (PET). At 60-91 min postinjection, regional distribution patterns of these three ligands were almost identical, and were consistent with the muscarinic receptor density in the brain as previously reported in vitro. However, the time-activity curves of [(11)C](+)3-EPB and [(11)C](+)3-PPB showed earlier peak times of radioactivity and a faster clearance rate than [(11)C](+)3-MPB in cortical regions rich in the receptors. Kinetic analysis using the three-compartment model with time-activity curves of radioactivity in metabolite-corrected arterial plasma as input functions revealed that labeling with longer [(11)C]alkyl chain length induced lower binding potential (BP = k(3)/k(4)), consistent with the rank order of affinity of these ligands obtained by an in vitro assay using rat brain slices and [(3)H]QNB. The cholinesterase inhibitor Aricept administered at doses of 50 and 250 microg/kg increased acetylcholine level in extracellular fluid of the frontal cortex and the binding of [(11)C](+)3-PPB with the lowest affinity to the receptors was displaced by the endogenous acetylcholine induced by cholinesterase inhibition, while [(11)C](+)3-MPB with the highest affinity was not significantly affected. Taken together, these observations indicate that the increase in [(11)C]alkyl chain length could alter the kinetic properties of conventional receptor ligands for PET by reducing the affinity to receptors, which might make it possible to assess the interaction between endogenous neurotransmitters and ligand-receptor binding in vivo as measured by PET.

Evaluation of novel PET ligands (+)N-[11C]methyl-3-piperidyl benzilate ([11C](+)3-MPB) and its stereoisomer [11C](-)3-MPB for muscarinic cholinergic receptors in the conscious monkey brain: a PET study in comparison with

The novel muscarinic cholinergic ligands (+)N-[11C]methyl-3-piperidyl benzilate ([11C](+)3-MPB) and its stereoisomer [11C](-)3-MPB were evaluated in comparison with [11C]4-MPB in the brains of conscious monkeys (Macaca mulatta) using high-resolution positron emission tomography (PET). The regional distribution patterns of [11C](+)3-MPB and [11C]4-MPB at 60-91 min postinjection were almost identical: highest in the striatum and occipital cortex; intermediate in the temporal and frontal cortices, cingulate gyrus, hippocampus, and thalamus; lower in the pons; and lowest in the cerebellum. The uptake of [11C](+)3-MPB in all regions was higher and the dynamic range of regional uptake differences of [11C](+)3-MPB was better than those of [11C]4-MPB. The levels of [11C](-)3-MPB were much lower in all regions of the brain than [11C](+)3-MPB and [11C]4-MPB. Administration of scopolamine, a muscarinic cholinergic antagonist, at a dose of 50 microg/kg reduced the radioactivity of [11C](+)3-MPB and [11C]4-MPB in all regions except the cerebellum. Time-activity curves of [11C](+)3-MPB peaked in all regions, while those of [11C]4-MPB showed gradual increases with time in all regions except the thalamus, pons, and cerebellum. Two graphical analyses (Logan plot and Patlak plot) with plasma radioactivity as an input function into the brain were applied to evaluate receptor binding in vivo. [11C](+)3-MPB showed linear regression curves on Logan plot analysis and nonlinear curves on Patlak plot in all regions, suggesting that [11C](+)3-MPB bound reversibly to the muscarinic receptors. The in vivo binding parameters as well as uptake at 60-91 min postinjection of [11C](+)3-MPB were consistent with muscarinic receptor density in the brain as reported in vitro.

Synthesis, (18)F-labeling, and biological evaluation of piperidyl and pyrrolidyl benzilates as in vivo ligands for muscarinic acetylcholine receptors

A series of 31 compounds based on the piperidyl or pyrrolidyl benzilate scaffold were prepared from methyl benzilate and 4-piperidinol, (R)-(+)-3-piperidinol, or (R)-(+)-3-pyrrolidinol. Amine substituents included alkyl and aralkyl groups. In vitro K(i) values ranged from 0.05 nM to >100 nM. (R)-N-(2-Fluoroethyl)-3-piperidyl benzilate (3-FEPB, 22, K(i) = 12.1 nM) and N-(2-fluoroethyl)-4-piperidyl benzilate (4-FEPB, 8, K(i) = 1. 83 nM) were selected for radiolabeling with fluorine-18. Using alkylation with 2-[(18)F]fluoroethyl triflate, 3-[(18)F]FEPB (42) and 4-[(18)F]FEPB (43) were produced in 7-9% radiochemical yield and >97% radiochemical purity. For in vivo studies, retention was moderate in mouse brain for 42; however, blocking with scopolamine showed that uptake was not muscarinic cholinergic receptor-mediated. Conversely, 43 exhibited high, receptor-mediated retention in mouse brain, with significant clearance after 1 h. These results suggest that 43 could have applications as an in vivo probe for measuring endogenous acetylcholine levels.

Development and evaluation of muscarinic cholinergic receptor ligands N-[11C]ethyl-4-piperidyl benzilate and N-[11C]propyl-4-piperidyl benzilate: a PET study in comparison with N-[11C]methyl-4-piperidyl benzilate in the conscious monkey brain

The muscarinic cholinergic receptor ligands N-[(11)C]ethyl-4-piperidyl benzilate (4-EPB) and N-[(11)C]propyl-4-piperidyl benzilate (4-PPB) were developed and evaluated in comparison with N-[(11)C]methyl-4-piperidyl benzilate (4-MPB) in the conscious monkey brain using positron emission tomography (PET). Time-activity curves of [(11)C]4-EPB, unlike [(11)C]4-MPB, showed peaks within 91 min in regions rich in muscarinic receptors. [(11)C]4-PPB showed no specific binding even in the regions rich in these receptors. These observation demonstrated that increases in [(11)C]alkyl chain length could alter the kinetic properties of receptor ligands for PET.