In vivo visualization of central muscarinic receptors using [11C]quinuclidinyl benzilate and positron emission tomography in baboons

Development of simultaneous determination of dopamine 2, histamine 1, and muscarinic acetylcholine receptor occupancies by antipsychotics using liquid chromatography with tandem mass spectrometry

Receptor occupancy is an indicator of antipsychotic efficacy and safety. It is desirable to simultaneously determine the occupancy of multiple brain receptors as an indicator of the efficacy and central side effects of antipsychotics because many of these drugs have binding affinities for various receptors, such as dopamine 2 (D2), histamine 1 (H1), and muscarinic acetylcholine (mACh) receptors. The purpose of this study was to develop a method for the simultaneous measurement of multiple receptor occupancies in the brain by the simultaneous quantification of unlabeled tracer levels using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Rats were pre-administered with a vehicle, displacer, or olanzapine, and mixed solutions of raclopride, doxepin, and 3-quinuclidinyl benzilate (3-QNB) were administered (3, 10, and 30 μg/kg). The brain tissue and plasma tracer concentrations were quantified 45 min later using LC-MS/MS, and the binding potential was calculated. The highest binding potential was observed at 3 μg/kg raclopride, 10 μg/kg doxepin, and 30 μg/kg 3-QNB. Tracer-specific binding at these optimal tracer doses in the cerebral cortex was markedly reduced by pre-administration of displacers. D2, H1, and mACh receptor occupancy by olanzapine increased in a dose-dependent manner, reaching 70-95%, 19-43%, and 12-45%, respectively, at an olanzapine dose range of 3-10 mg/kg. These results suggest that simultaneous determination of in vivo D2, H1, and mACh receptor occupancy is possible using LC-MS/MS.

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.

First-in-Human Assessment of 11C-LSN3172176, an M1 Muscarinic Acetylcholine Receptor PET Radiotracer

This was a first-in-human study of the PET radiotracer ¹¹C-LSN3172176 for the muscarinic acetylcholine receptor subtype M1. The objectives of the study were to determine the appropriate kinetic model to quantify binding of the tracer to M1 receptors, and the reliability of the chosen quantification method. Methods: Six healthy subjects completed the test-retest protocol, and 5 healthy subjects completed the baseline-scopolamine blocking protocol. Multiple modeling methods were applied to calculate total distribution volume (V _T) and nondisplaceable binding potential (BP _ND) in various brain regions. The reference region was selected from the blocking study. The occupancy plot was applied to compute receptor occupancy by scopolamine and nondisplaceable distribution volume. Results: Tracer uptake was highest in the striatum, followed by neocortical regions and white matter, and lowest in the cerebellum. Regional time-activity curves were fitted well by all models. The 2-tissue-compartment (2TC) model fits were good, but the 2TC parameters often could not be reliably estimated. Because V _T correlated well between the 2TC and 1-tissue-compartment (1TC) models after exclusion of unreliable estimates, the 1TC model was chosen as the most appropriate. The cerebellum showed the lowest V _T, consistent with preclinical studies showing little to no specific binding in the region. Further, cerebellar V _T did not change between baseline and blocking scans, indicating that the cerebellum is a suitable reference region. The simplified reference tissue model (SRTM) slightly underestimated 1TC BP _ND, and the simplified reference tissue model 2 (SRTM2) improved BP _ND estimation. An 80-min scan was sufficient to quantify V _T and BP _ND The test-retest study showed excellent absolute test-retest variability for 1TC V _T (≤5%) and BP _ND (≤10%). In the baseline and blocking studies, occupancy values were lower in the striatum than in nonstriatal regions, as may be attributed to differences in regional acetylcholine concentrations. Conclusion: The 1TC and SRTM2 models are appropriate for quantitative analysis of ¹¹C-LSN3172176 imaging data. ¹¹C-LSN3172176 displayed excellent test-retest reproducibility and is a highly promising ligand to quantify M1 receptors in the human brain.

Human biodistribution and internal dosimetry of 4-[ 18F]fluorobenzyl-dexetimide: a PET radiopharmaceutical for imaging muscarinic acetylcholine receptors in the brain and heart

BACKGROUND

4-[18F] fluorobenzyl dexetimide (F-DEX) is the first non-subtype selective fluorine-18 labelled tracer for muscarinic receptors (mAChR) used in humans. A recent first-in-human study found high regional brain uptake with low variation in normal subjects. Disturbance of mAChR has been reported in Alzheimer's and Parkinson's disease, schizophrenia and depression and various cardiac diseases. The following work assesses the biodistribution, organ tracer kinetics and radiation dose associated with F-DEX.

METHOD

Dose calculations were based on activity uptake derived from multiple time point whole body PET CT imaging and the organ-specific dosimetric S-factors derived from the ICRP 133 standard man and woman mathematical phantoms. Effective doses were calculated using the latest ICRP tissue weighting factors.

RESULTS

Serial images and time activity curves demonstrate high brain and left ventricular myocardial uptake (5% and 0.65% of injected activity, respectively) with greater retention in brain than myocardium. The mean effective dose was in concordance with other 18F labelled tracers at 19.70 ± 2.27 μSv/MBq. The largest absorbed doses were in the liver (52.91 ± 1.46 μGy/MBq) and heart wall (43.94 ± 12.88 μGy/MBq) for standard man and the liver (61.66 ± 13.61 μGy/MBq) and lungs (40.93 ± 3.11 μGy/MBq) for standard woman. The absorbed dose to all organs, most notably, the red bone marrow (20.03 ± 2.89 μGy/MBq) was sufficiently low to ensure no toxicity after numerous follow-up procedures.

CONCLUSIONS

The radiation dose associated with an administration of F-DEX is comparable to that of other 18F labelled tracers such as FDG (19.0 μSv/MBq) and lower than tracers used for SPECT imaging of muscarinic receptors (I-DEX 28.5 μSv/MBq). Clinical use would likely result in an effective dose less than 4 mSv for the ICRP 133 standard phantoms after dose optimisation allowing justification for numerous follow-up procedures. Recent results from first in-human studies and a comparatively low radiation dose make F-DEX an attractive option for future applications of imaging muscarinic receptors in the brain. Further investigation of the potential of F-DEX for imaging parasympathetic innervation of the heart may be warranted.

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.

In Vitro Pharmacological Characterization and In Vivo Validation of LSN3172176 a Novel M1 Selective Muscarinic Receptor Agonist Tracer Molecule for Positron Emission Tomography

In the search for improved symptomatic treatment options for neurodegenerative and neuropsychiatric diseases, muscarinic acetylcholine M1 receptors (M1 mAChRs) have received significant attention. Drug development efforts have identified a number of novel ligands, some of which have advanced to the clinic. However, a significant issue for progressing these therapeutics is the lack of robust, translatable, and validated biomarkers. One valuable approach to assessing target engagement is to use positron emission tomography (PET) tracers. In this study we describe the pharmacological characterization of a selective M1 agonist amenable for in vivo tracer studies. We used a novel direct binding assay to identify nonradiolabeled ligands, including LSN3172176, with the favorable characteristics required for a PET tracer. In vitro functional and radioligand binding experiments revealed that LSN3172176 was a potent partial agonist (EC₅₀ 2.4-7.0 nM, E_max 43%-73%), displaying binding selectivity for M1 mAChRs (K_d = 1.5 nM) that was conserved across species (native tissue K_d = 1.02, 2.66, 8, and 1.03 at mouse, rat, monkey, and human, respectively). Overall selectivity of LSN3172176 appeared to be a product of potency and stabilization of the high-affinity state of the M1 receptor, relative to other mAChR subtypes (M1 > M2, M4, M5 > M3). In vivo, use of wild-type and mAChR knockout mice further supported the M1-preferring selectivity profile of LSN3172176 for the M1 receptor (78% reduction in cortical occupancy in M1 KO mice). These findings support the development of LSN3172176 as a potential PET tracer for assessment of M1 mAChR target engagement in the clinic and to further elucidate the function of M1 mAChRs in health and disease.

Discovery of a Novel Muscarinic Receptor PET Radioligand with Rapid Kinetics in the Monkey Brain

Positron emission tomography (PET), together with a suitable radioligand, is one of the more prominent methods for measuring changes in synaptic neurotransmitter concentrations in vivo. The radioligand of choice for such measurements on the cholinergic system is the muscarinic receptor antagonist N-[1-¹¹C]propyl-3-piperidyl benzilate (PPB). In an effort to overcome the shortcomings with the technically cumbersome synthesis of [¹¹C]PPB, we designed and synthesized four structurally related analogues of PPB, of which (S,R)-1-methylpiperidin-3-yl)2-cyclopentyl-2-hydroxy-2-phenylacetate (1) was found to bind muscarinic receptors with similar affinity as PPB (3.5 vs 7.9 nM, respectively). (S,R)-1 was radiolabeled via N-¹¹C-methylation at high radiochemical purity (>99%) and high specific radioactivity (>130 GBq/μmol). In vitro studies by autoradiography on human brain tissue and in vivo studies by PET in nonhuman primates demonstrated excellent signal-to-noise ratios and a kinetic profile in brain comparable to that of [¹¹C]PBB. (S,R)-[¹¹C]1 is a promising candidate for measuring changes in endogenous acetylcholine concentrations.

Positron emission tomography: ligand imaging

Since it was first used to image the brain in 1976, positron emission tomography (PET) has been utilized in a wide range of neurologic and psychiatric applications. From cerebral metabolism to receptor concentration, various PET imaging techniques involving a host of radiopharmaceuticals have provided insight into countless facets of both the normal and diseased brain. Although the majority of these radiopharmaceuticals are still limited to the realm of research, one PET ligand in particular has gained widespread clinical use: (18)F-fluorodeoxyglucose, a radiolabeled analog of glucose, has become an exceedingly prevalent clinical tool for the measurement of metabolism in organs throughout the body, including the brain. In recent years, a number of novel PET ligands have also made it through the US Food and Drug Administration approval process and been used clinically. However, gaining approval is by no means the only challenge facing these radiopharmaceuticals. Traversing the blood-brain barrier is a formidable obstacle in drug delivery, and accurately modeling tracer kinetics and correcting for the partial-volume effect are among the difficult tasks that remain once the ligand reaches its intended target. Even so, the use of PET imaging in neurology and psychiatry can be expected to expand in the coming years as novel radiopharmaceuticals continue to be developed.

[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.

Preclinical testing of N-[(11)c]-methyl-piperidin-4-yl 2-cyclohexyl-2-hydroxy-2-phenylacetate, a novel radioligand for detection of cerebral muscarinic receptors using PET

The muscarinic antagonist N-[(11)C]methyl-piperidin-4-yl 2-cyclohexyl-2-hydroxy-2-phenylacetate (VC-004) 1 was tested for visualization of muscarinic receptors in the brain. The active (R)-isomer (pKb = 10.92) was labeled by reacting [(11)C]-CH(3)I with the secondary amine precursor (40-60% decay-corrected radiochemical yield, specific activity 13.0-34.3 TBq/mmol, 45 min after end of bombardment). Biodistribution studies were performed in male Wistar rats. Brain uptake of (R)-[(11)C]-VC-004 was high, standard uptake values (SUVs) ranging from 1.6 in cerebellum to 3.3 in frontal cortex. In all brain regions, the nonsubtype selective muscarinic antagonist scopolamine (2.5 mg/kg) blocked (R)-[(11)C]-VC-004 binding to the same extent (84.6 +/- 3.3%) as nonlabeled (R)-VC-004 (2.0 mg/kg, 83.2 +/- 4.6%). In contrast, the fraction of [(11)C]VC-004 binding which was blocked by atropine (2.5 mg/kg) was significantly smaller (54 +/- 17%). The reduction of (R)-[(11)C]-VC-004 binding by low-dose atropine (0.5 mg/kg) was not significantly different from that caused by (R)-(-)-QNB (20 microg/kg). The decrease in specific binding of (R)-[(11)C]VC-004 after (R)-(-)-QNB block correlated well with literature values for the percentages of M(2) receptors in the brain regions studied. (R)-[(11)C]VC-004 was rapidly cleared from plasma (92% with a half-life of 0.27 min) and the fraction of total plasma radioactivity representing parent compound decreased from 99% to 42% at 10 min postinjection. Although (R)-[(11)C]VC-004 can visualize muscarinic receptors in the brain, it does not show selectivity for the M(2)-subtype. A low dose (0.5 mg/kg) of atropine seems to preferentially block M(2)-receptors in vivo, as has been reported for (R)-(-)-QNB.

Iodo-QNB cortical binding and brain perfusion: effects of a cholinergic basal forebrain lesion in the rat

This study deals with the question of whether in vivo application of [125I]iodo-quinuclidinyl-benzilate (QNB) is able to demonstrate changes in cortical muscarinic receptor density induced by a cholinergic immunolesion of the rat basal forebrain cholinergic system, and whether the potential effects on IQNB distribution in vivo are also associated with effects on regional cerebral perfusion. Immunolesioned and control animals were injected with (R,S) [125]iodo-QNB and with [99mTc]-d,l-hexamethylpropyleneamine oxime (HMPAO). The cerebral distribution of both tracers was imaged using double tracer autoradiography. Impaired cholinergic transmission was paralleled by a 10-15% increase of [125I]iodo-QNB binding in the regions of cortex and hippocampus. The local cerebral blood flow remained unchanged after cholinergic lesion.

An increase in [3H]QNB binding by proton-beam irradiation in intact rat brain: an apparent positive cooperativity of binding

After focal irradiation of rat brains with a beam of proton (dose, 30 Gy), [3H]quinuclidinyl benzilate (QNB) binding, both in vitro and in vivo, was measured, using either autoradiographic or tissue-dissection methods. No changes in in vitro [3H]QNB binding were seen in autoradiograms of brain slices from irradiated rat. The irradiated side of the brain showed a significant increase in [3H]QNB binding in vivo in the striatum and cerebral cortex 24 h after irradiation. This increase in binding was transient, and had almost disappeared 2 weeks after irradiation. These results indicate that early changes in receptor function as a results of radiation damage are only detected in in-vivo conditions. In the striatum of the intact rat brain, an apparent positive cooperativity of binding was observed, which was more pronounced on the irradiated side.

In vivo muscarinic binding selectivity of (R,S)- and (R,R)-[18F]-fluoromethyl QNB

We have developed a multistep radiochemical synthesis of two diastereomers of quinuclidinyl-4-[18F]-fluoromethyl-benzilate ([18F]-FMeQNB), a high-affinity ligand for muscarinic acetylcholine receptors. Previously, we have shown that the nonradioactive (R,R)-diastereomer displays an eightfold selectivity for M1 over M2 while the nonradioactive (R,S)-diastereomer displays a sevenfold selectivity for M2 over M1 in vitro. This paper reports the results of in vivo comparison studies. In the rat, uptake of (R,S)-[18F]-FMeQNB was nearly uniform in all brain regions following the concentration of M2 subtype. The uptake was reduced by 36-54% in all brain regions on coinjection with 50 nmol of unlabeled ligand. An injection of (R,S)-[18F]-FMeQNB followed at 60 min by injection of unlabeled ligand and subsequent sacrifice at 120 min displaced 30-50% of radioactivity in the pons, medulla, and cerebellum, which contain a high proportion of M2 subtype. The most dramatic displacement and inhibition of uptake on coinjection of (R,S)-[18F]-FMeQNB was observed in the heart. In rhesus monkey, the compound showed prolonged uptake and retention in the brain. In the blood, the parent compound degraded rapidly to a single radiolabeled polar metabolite believed to be fluoride. Within 30 min the parent compound represented less than 5% of the plasma activity. Displacement with (R)-QNB was generally slow, but was more rapid from those tissues which contain a higher proportion of M2 subtype. The results are consistent with the hypothesis that (R,S)-[18F]-FMeQNB is M2 selective in vivo. On the other hand, (R,R)-[18F]-FMeQNB showed higher uptake in those brain regions containing a higher concentration of M1 subtype. Uptake in the heart at 60 min was much lower than that observed with the (R,S)-diastereomer. Inhibition of uptake on coinjection with unlabeled (R,S)-FMeQNB is only significant in the heart, thalamus, and pons. Inhibition of uptake on coinjection with unlabeled (R,R)-FMeQNB is quite uniform in all brain regions. Displacement with (R)-QNB shows a more varying amount displaced. These results are consistent with (R,R)-[18F]-FMeQNB being M1 selective in vivo.

Positron emission tomographic investigations of central muscarinic cholinergic receptors with three isomers of [76Br]BrQNP

We studied the potential of three radiobrominated isomers of BrQNP, (Z(-,-)-[76Br]BrQNP, E(-,-)-[76Br]BrQNP and E(-,+)-[76Br]BrQNP), as suitable radioligands for imaging of central muscarinic cholinergic receptors in the human brain. These radioligands were stereospecifically prepared by electrophilic radiobromodestannylation of the respective tributylstannyl precursors using no-carrier-added [76Br]BrNH4 and peracetic acid. Preliminary pharmacological characterizations were determined by biodistribution, autoradiography, competition, displacement and metabolite studies in rats. The (-,-)-configuration presented important specific uptakes in brain muscarinic cholinergic receptor (mAChR)-rich structures and in heart, low metabolization rates and an apparent M2 selectivity. The (-,+)-configuration revealed more rapid clearance, lower uptake, a higher metabolization rate and an apparent M1 selectivity. Reversibility of the binding was confirmed for the three radiotracers. Positron emission tomography in the living baboon brain revealed high and rapid uptake in the brain and accumulation in the mAChR-rich structures studied. At 30 min p.i., the E(-,-)-radiotracer reached a plateau in cortex, pons and thalamus with concentrations of 29%, 24% and 19% ID/l, respectively. Z(-,-)-[76Br]BrQNP also accumulated in these structures, reaching a maximal uptake (27% ID/l) in the cortex 2 h p.i. At 5 min p.i. a plateau (17% ID/l) was only observed in the cortex for the E(-, +)-[76Br]BrQNP; by contrast, the other structures showed slow washout. After 3 weeks, the (-,-)-radiotracers were studied in the same baboon pretreated with dexetimide (1 mg/kg), a well-known muscarinic antagonist. In all the mAChR structures, the highly reduced uptake observed after this preloading step indicates that these radiotracers specifically bind to muscarinic receptors. Z(-, -)-[76Br]BrQNP, which is displaced in higher amounts from M2 mAChR-enriched structures, reveals an M2 affinity. The two isomers having the (-,-)-configuration are potential probes for investigating central muscarinic receptors. The absolute configuration on the acetate chiral centre influences their muscarinic subtype selectivity and the cis-trans isomerism of the vinyl moiety affects their specific fixation.

In vivo effect of methyl-quinuclidinyl-benzylate on myocardial beta-adrenoceptor density

The muscarinic receptor antagonist methyl-quinuclidinyl-benzylate decreased myocardial beta-adrenoceptor density Bmax: 20.4 +/- 2.4 pmol/ml tissue versus 33.3 +/- 4 pmol/ml tissue in control dogs (P < 0.001), as assessed by using [11C]CGP-12177 (((2S)-4-(3-t-butyl-amino-2 hydroxypropoxy)-benzimidazol-2-one)) and positron emission tomography. In contrast, atropine did not induce any change in Bmax: 33.7 +/- 3.6 pmol/ml tissue. We hypothetized that methyl-quinuclidinyl-benzylate induced the release of norepinephrine from sympathetic nerve terminals, an effect which could be blocked by guanethidine. Guanethidine alone (10 mg/kg) did not change Bmax: 35.5 +/- 6 pmol/ml tissue. Guanethidine + methyl-quinuclidinyl-benzylate did not induce any significant change in Bmax: 31.5 +/- 5.1 pmol/ml tissue. Therefore, it seems likely that methyl-quinuclidinyl-benzylate acts at the presynaptic level, probably inducing the release of norepinephrine which then causes a down-regulation of beta-adrenoceptors.

Pharmacological characterization and positron emission tomography evaluation of 4-[76Br]bromodexetimide and 4-[76Br]bromolevetimide for investigations of central muscarinic cholinergic receptors

4-[76Br]bromodexetimide and its inactive enantiomer 4-[76Br]bromolevetimide were prepared via electrophilic bromodesilylation using chloramine-T and no-carrier-added (NCA) [76Br]NH4. In vitro, Bmax measured on rat cortex membranes were 3.7 +/- 0.2 and < 0.07 pmol/mg protein for 4-[76Br]bromodexetimide and 4-[76Br]bromolevetimide, respectively. The kD of 4-[76Br]bromodexetimide was 1.9 +/- 0.3 nM. In vivo studies in rats showed specific uptake of 4-[76Br]bromodexetimide in cortex, striatum, thalamus and hippocampus. No specific uptake was observed with 4-[76Br]bromolevetimide. With [76Br]bromodexetimide, positron emission tomography (PET) studies in primates demonstrated a preferential accumulation of the radioactivity in the cortex and striatum which was displaced to the level of cerebellum by dexetimide. With 4-[76Br]bromolevetimide, the radioactivity concentrations in the cortex and striatum were similar to that of cerebellum.

In vivo studies of acetylcholinesterase activity using a labeled substrate, N-[11C]methylpiperdin-4-yl propionate ([11C]PMP)

Two esters, N-[11C]methylpiperidyl acetate ([11C]AMP) and N-[11C]methylpiperidyl propionate ([11C]PMP), were synthesized in no-carrier-added forms and evaluated as in vivo substrates for brain acetylcholinesterase (AChE). After peripheral injection in mice, each ester showed rapid penetration into the brain and a regional retention of radioactivity (striatum > cortex, hippocampus > cerebellum) reflecting known levels of AChE activity in the brain. Regional brain distributions after [11C]PMP administration showed better discrimination between regions of high, intermediate, and low AChE activities. Chromatographic analysis of blood and brain tissue extracts showed rapid and nearly complete hydrolysis of [11C]PMP within 10 min after injection. For both [11C]AMP and [11C]PMP, retention of radioactivity in all regions was reduced by pretreatment with diisopropylfluorophosphate (DFP), a specific irreversible AChE inhibitor. DFP treatment also significantly increased the proportions of unhydrolyzed ester in both blood and brain. Radioactivity localization in brain after peripheral injection was thus dependent on AChE-catalyzed hydrolysis to the hydrophilic product N-[11C]methylpiperidinol. PET imaging of [11C]AMP or [11C]PMP distributions in monkey brain showed clear accumulation of radioactivity in areas of highest AChE activity (striatum, cortex). These esters are thus in vivo substrates for brain AChE, with potential applications as in vivo imaging agents of enzyme action in the human brain. [11C]PMP, the ester with a slower rate of hydrolysis, appears to be the better candidate radiotracer for further development.

In vivo binding, pharmacokinetics and metabolism of the selective M2 muscarinic antagonists [3H]AF-DX 116 and [3H]AF-DX 384 in the anesthetized rat

The pharmacokinetics, in vivo binding and metabolism of two M2 muscarinic receptor antagonists, [3H]AF-DX 116 and [3H]AF-DX 384, were studied in anesthetized rats, which received either the tracer alone or following a saturating injection of atropine. Both radioligands were cleared from the circulation with distribution half-lives of 17 and 14 sec and elimination half-lives of 17 and 40 min for [3H]AF-DX 116 and [3H]AF-DX 384, respectively. A radioactive distribution, predominant in peripheral organs when compared to brain, was found at each time studied after tracer injection. Atropine-displaceable tracer uptake was evidenced at 20-40 min in brain (31%), submandibular glands (26%), spleen (37%) and notably heart (55%) for [3H]AF-DX 116 but only in heart (50%) for [3H]AF-DX 384 at 10-20 min. Regional brain sampling revealed a relatively uniform distribution of [3H]AF-DX 384 and a -45% atropine saturation effect (i.e., specific binding) in the thalamus 20 min after injection. Sequential thin-layer chromatographic studies performed on tissue extracts demonstrated the rapid appearance of labeled metabolites of both radiotracers in brain (but less so in liver) and especially in cardiac tissues, where almost 70% of total radioactivity still corresponded to authentic tracer 40 min after injection. Thus, based on their low blood-brain barrier permeability and the high presence of labeled metabolites in the central nervous system, AF-DX 116 and AF-DX 384 might be more helpful in the study of M2 muscarinic receptors present in heart rather than brain. Labeled with positron emittors, these M2 antagonists might be applicable to the pathophysiological study of disease states, such as cardiomyopathies.

Cholinergic neurotransmission studied in vivo using positron emission tomography or single photon emission computerized tomography

During the past decade, considerable efforts have been made in the development of radiopharmaceuticals for the in vivo study of the cholinergic neurotransmission using positron emission tomography or single photon emission computerized tomography. The main cholinergic radioligands, labelled with positron- or gamma-photon-emitting radionuclides, are reviewed with respect to use as in vivo markers of either acetylcholinesterase, vesicular acetylcholine transporter, brain and heart muscarinic receptors, or cholinergic nicotinic receptors. The main results obtained in the in vivo study of the physiology, pharmacology or pathology of the different steps of the cholinergic neurotransmission using single photon emission computerized tomography and positron emission tomography are discussed.

Synthesis, in vivo biodistribution and dosimetry of [11C]N-methylpiperidyl benzilate ([11C]NMPB), a muscarinic acetylcholine receptor antagonist

4-N-Methylpiperidyl benzilate (NMPB), a high affinity antagonist for the muscarinic cholinergic receptor, has been synthesized in carbon-11-labeled form through the N-[11C]methylation of 4-piperidylbenzilate. The product was isolated by HPLC, and obtained in yields (> 100 mCi) and specific activities (500-3000 Ci/mmol) sufficient for in vivo evaluation in small animals. Time-dependent regional brain distributions in rats and mice showed high radiotracer uptake and retention in striatum and cortex, and low in cerebellum, consistent with muscarinic cholinergic receptor distributions. Radiotracer retention in tissues could be significantly reduced by pretreatment of animals with a large dose of a competing antagonist, quiniclidinyl benzilate. Whole body biodistribution in rats was used to calculate the expected human internal radiation dosimetry for this new radiopharmaceutical. These animal experiments formed the basis for subsequent introduction of [11C]NMPB into human use with positron emission tomography.

Effects of trihexyphenidyl and L-dopa on brain muscarinic cholinergic receptor binding measured by positron emission tomography

The effects of pharmacological intervention on brain muscarinic cholinergic receptor (mAChR) binding were assessed in seven patients with Parkinson's disease by positron emission tomography and carbon-11 labelled N-methyl-4-piperidyl benzilate ([11C]NMPB). [11C]NMPB was injected twice, approximately 2 hours apart, in each patient, to assess the effect of single doses of 4 mg of trihexyphenidyl (n = 5) or 400 mg of L-dopa with 57 mg of benserazide (n = 2) on the binding parameter of mAChRs (K3). There was a mean 28% inhibition of K3 values in the brain in the presence of trihexyphenidyl, which was assumed to reflect mAChR occupancy. No significant change in K3 was observed in the presence of L-dopa. This study demonstrates the feasibility of measuring mAChR occupancy by an anticholinergic medication with PET.