p-[18F]fluorobenzyltrozamicol ([18F]FBT): molecular decomposition- reconstitution approach to vesamicol receptor radioligands for positron emission tomography

(-)-o-[11 C]methyl-trans-decalinvesamicol ((-)-[11 C]OMDV) as a PET ligand for the vesicular acetylcholine transporter

To develop a PET imaging agent to visualize brain cholinergic neurons and synaptic changes caused by Alzheimer's disease, (-)- and (+)-o-[¹¹ C]methyl-trans-decalinvesamicol ([¹¹ C]OMDV) were isolated and investigated for differences in not only their binding affinity and selectivity to vesicular acetylcholine transporter (VAChT), but also their in vivo activities. [¹¹ C]OMDV has a high binding affinity for VAChT both in vitro and in vivo. Racemic OMDV and o-trimethylstannyl-trans-decalinvesamicol (OTDV), which are precursors for synthesis of [¹¹ C]OMDV, were separated into (-)-optical isomers ((-)-OMDV and (-)-OTDV) and (+)-optical isomers ((+)-OMDV and (+)-OTDV) by HPLC. In the in vitro binding assay, (-)-OMDV(7.2 nM) showed eight times higher binding affinity (Ki) to VAChT than that of (+)-OMDV(57.5 nM). In the biodistribution study, the blood-brain barrier permeability of both enantiomers ((-)-[¹¹ C]OMDV and (+)-[¹¹ C]OMDV) was similarly high (about 1.0%ID/g) at 2 min post-injection. However, (+)-[¹¹ C]OMDV clearance from the brain was faster than (-)-[¹¹ C]OMDV. In the in vivo blocking study, accumulation of (-)-[¹¹ C]OMDV in the cortex was markedly decreased (approximately 30% of control) by coadministration of vesamicol, and brain uptake of (-)-[¹¹ C]OMDV was not significantly altered by coadministration of (+)-pentazocine or (+)-3-(3-hydroxyphenyl)-N-propylpiperidine ((+)-3-PPP). PET-CT imaging revealed inhibition of the rat brain uptake of (-)-[¹¹ C]OMDV by coadministration of vesamicol. In conclusion, (-)-[¹¹ C]OMDV, which is an enantiomer of OMDV, selectively binds to VAChT with high affinity in the rat brain in vivo. (-)-[¹¹ C]OMDV may be utilized as a potential PET ligand for studying presynaptic cholinergic neurons in the brain.

Molecular imaging of β-cells: diabetes and beyond

Since diabetes is becoming a global epidemic, there is a great need to develop early β-cell specific diagnostic techniques for this disorder. There are two types of diabetes (i.e., type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM)). In T1DM, the destruction of pancreatic β-cells leads to reduced insulin production or even absolute insulin deficiency, which consequently results in hyperglycemia. Actually, a central issue in the pathophysiology of all types of diabetes is the relative reduction of β-cell mass (BCM) and/or impairment of the function of individual β-cells. In the past two decades, scientists have been trying to develop imaging techniques for noninvasive measurement of the viability and mass of pancreatic β-cells. Despite intense scientific efforts, only two tracers for positron emission tomography (PET) and one contrast agent for magnetic resonance (MR) imaging are currently under clinical evaluation. β-cell specific imaging probes may also allow us to precisely and specifically visualize transplanted β-cells and to improve transplantation outcomes, as transplantation of pancreatic islets has shown promise in treating T1DM. In addition, some of these probes can be applied to the preoperative detection of hidden insulinomas as well. In the present review, we primarily summarize potential tracers under development for imaging β-cells with a focus on tracers for PET, SPECT, MRI, and optical imaging. We will discuss the advantages and limitations of the various imaging probes and extend an outlook on future developments in the field.

In Vivo and In Vitro Characteristics of Radiolabeled Vesamicol Analogs as the Vesicular Acetylcholine Transporter Imaging Agents

The vesicular acetylcholine transporter (VAChT), a presynaptic cholinergic neuron marker, is a potential internal molecular target for the development of an imaging agent for early diagnosis of neurodegenerative disorders with cognitive decline such as Alzheimer's disease (AD). Since vesamicol has been reported to bind to VAChT with high affinity, many vesamicol analogs have been studied as VAChT imaging agents for the diagnosis of cholinergic neurodeficit disorder. However, because many vesamicol analogs, as well as vesamicol, bound to sigma receptors (σ₁ and σ₂) besides VAChT, almost all the vesamicol analogs have been shown to be unsuitable for clinical trials. In this report, the relationships between the chemical structure and the biological characteristics of these developed vesamicol analogs were investigated, especially the in vitro binding profile and the in vivo regional brain accumulation.

Biocatalytic approaches towards the stereoselective synthesis of vicinal amino alcohols

The global need for clean manufacturing technologies and the management of hazardous chemicals and waste present new research challenges to both chemistry and biotechnology.

Small Molecule PET Tracers for Transporter Imaging

As the field of PET has expanded and an ever-increasing number and variety of compounds have been radiolabeled as potential in vivo tracers of biochemistry, transporters have become important primary targets or facilitators of radiotracer uptake and distribution. A transporter can be the primary target through the development of a specific high-affinity radioligand: examples are the multiple high-affinity radioligands for the neuronal membrane neurotransmitter or vesicular transporters, used to image nerve terminals in the brain. The goal of a radiotracer might be to study the function of a transporter through the use of a radiolabeled substrate, such as the application of 3-O-[¹¹C]methyl]glucose to measure rates of glucose transport through the blood-brain barrier. In many cases, transporters are required for radiotracer distributions, but the targeted biochemistries might be unrelated: an example is the use of 2-deoxy-2-[¹⁸F]FDG for imaging glucose metabolism, where initial passage of the radiotracer through cell membranes requires the action of specific glucose transporters. Finally, there are transporters such as p-glycoprotein that function to extrude small molecules from tissues, and can effectively work against successful uptake of radiotracers. The diversity of structures and functions of transporters, their importance in human health and disease, and their role in therapeutic drug disposition suggest that in vivo imaging of transporter location and function will continue to be a point of emphasis in PET radiopharmaceutical development. In this review, the variety of transporters and their importance for in vivo PET radiotracer development and application are discussed. Transporters have thus joined the other major protein targets such as G-protein coupled receptors, ligand-gated ion channels, enzymes, and aggregated proteins as of high interest for understanding human health and disease.

Do spiroindolines have the potential to replace vesamicol as lead compound for the development of radioligands targeting the vesicular acetylcholine transporter?

The vesicular acetylcholine transporter (VAChT) is an important target for in vivo imaging of neurodegenerative processes using positron emission tomography (PET). So far the development of VAChT PET radioligands is based on the single known lead compound vesamicol. In this study we investigated a recently published spiroindoline based compound class (Sluder et al., 2012), which was suggested to have potential in the development of VAChT ligands. Therefore, we synthesized a small series of N,N-substituted spiro[indoline-3,4'-piperidine] derivatives and determined their in vitro binding affinities toward the VAChT. In order to investigate the selectivity, the off-target binding toward σ₁ and σ₂ receptors was determined. The compounds possessed VAChT affinities with K_i values in the range of 39-376nM. Binding affinities toward the σ₁ and σ₂ receptors are in a similar range indicating that the strong structural difference between the spiroindolines and vesamicol did not improve the selectivity. The observed potential to additionally bind to σ receptors let us assume that the herein investigated spiroindolines are not suitable to replace vesamicol as lead compound for the development of VAChT ligands.

In Vivo Differences between Two Optical Isomers of Radioiodinated o-iodo-trans-decalinvesamicol for Use as a Radioligand for the Vesicular Acetylcholine Transporter

PURPOSE

To develop a superior VAChT imaging probe for SPECT, radiolabeled (-)-OIDV and (+)-OIDV were isolated and investigated for differences in their binding affinity and selectivity to VAChT, as well as their in vivo activities.

PROCEDURES

Radioiodinated o-iodo-trans-decalinvesamicol ([125I]OIDV) has a high binding affinity for vesicular acetylcholine transporter (VAChT) both in vitro and in vivo. Racemic [125I]OIDV was separated into its two optical isomers (-)-[125I]OIDV and (+)-[125I]OIDV by HPLC. To investigate VAChT binding affinity (Ki) of two OIDV isomers, in vitro binding assays were performed. In vivo biodistribution study of each [125I]OIDV isomer in blood, brain regions and major organs of rats was performed at 2,30 and 60 min post-injection. In vivo blocking study were performed to reveal the binding selectivity of two [125I]OIDV isomers to VAChT in vivo. Ex vivo autoradiography were performed to reveal the regional brain distribution of two [125I]OIDV isomers and (-)-[123I]OIDV for SPECT at 60 min postinjection.

RESULTS

VAChT binding affinity (Ki) of (-)-[125I]OIDV and (+)-[125I]OIDV was 22.1 nM and 79.0 nM, respectively. At 2 min post-injection, accumulation of (-)-[125I]OIDV was the same as that of (+)-[125I]OIDV. However, (+)-[125I]OIDV clearance from the brain was faster than (-)-[125I]OIDV. At 30 min post-injection, accumulation of (-)-[125I]OIDV (0.62 ± 0.10%ID/g) was higher than (+)-[125I]OIDV (0.46 ± 0.07%ID/g) in the cortex. Inhibition of OIDV binding showed that (-)-[125I]OIDV was selectively accumulated in regions known to express VAChT in the rat brain, and ex vivo autoradiography further confirmed these results showing similar accumulation of (-)-[125I]OIDV in these regions. Furthermore, (-)-[123I]OIDV for SPECT showed the same regional brain distribution as (-)-[125I]OIDV.

CONCLUSION

These results suggest that radioiodinated (-)-OIDV may be a potentially useful tool for studying presynaptic cholinergic neurons in the brain.

Synthesis and evaluation of a new vesamicol analog o-[(11)C]methyl-trans-decalinvesamicol as a PET ligand for the vesicular acetylcholine transporter

INTRODUCTION

We focused on the vesicle acetyl choline transporter (VAChT) as target for early diagnosis of Alzheimer's diseases because the dysfunction of the cholinergic nervous system is closely associated with the symptoms of AD, such as problem in recognition, memory, and learning. Due to its low binding affinity for the sigma receptors (σ-1 and σ-2), o-methyl-trans-decalinvesamicol (OMDV) demonstrated a high binding affinity and selectivity for vesicular acetyl choline transporter (VAChT). [(11)C]OMDV was prepared and investigated the potential as a new PET ligand for VAChT imaging through in vivo evaluation.

METHOD

[(11)C]OMDV was prepared by a palladium-promoted cross-coupling reaction using [(11)C]methyl iodide, with a radiochemical yield of 60-75%, a radiochemical purity of greater than 98%, and a specific activity of 5-10 TBq/mmol 30 min after EOB. In vivo biodistribution study of [(11)C]OMDV in blood, brain regions and major organs of rats was performed at 2, 10, 30 and 60 min post-injection. In vivo blocking study and PET-CT imaging study were performed to check the binding selectivity of [(11)C]OMDV for VAChT.

RESULTS

In vivo studies demonstrated [(11)C]OMDV passage through the blood-brain barrier (BBB) and accumulation in the rat brain. The regional brain accumulation of [(11)C]OMDV was significantly inhibited by co-administration of vesamicol. In contrast, brain accumulation of [(11)C]OMDV was not significantly altered by co-administration of (+)-pentazocine, a selective σ-1 receptor ligand, or (+)-3-(3-hydroxyphenyl)-N-propylpiperidine [(+)-3-PPP], a σ-1 and σ-2 receptor ligand. PET-CT imaging revealed inhibition of [(11)C]OMDV accumulation in the brain by co-administration of vesamicol.

CONCLUSION

[(11)C]OMDV selectively binds to VAChT with high affinity in the rat brain in vivo, and that [(11)C]OMDV may be utilized in the future as a specific VAChT ligand for PET imaging.

Development of novel PET probe [¹¹C](R,R)HAPT and its stereoisomer [¹¹C](S,S)HAPT for vesicular acetylcholine transporter imaging: a PET study in conscious monkey

Carbon-11-labeled (R,R)trans-8-methyl-2-hydroxy-3-[4-[2-aminophenyl]piperizinyl]-tetralin ([(11)C](R,R)HAPT) and its stereoisomer [(11)C](S,S)HAPT were developed for imaging vesicular acetylcholine transporters (VAChTs), exclusively located in presynaptic cholinergic neurons. Both positron emission tomography (PET) probes were evaluated in the brain of conscious monkey (Macaca mulatta) using high-resolution PET. Time-activity curves (TACs) of [(11)C](R,R)HAPT peaked within 5 min after the injection in all regions except the caudate and putamen, both of which showed peaks around 20 min postinjection. The regional distribution patterns of [(11)C](R,R)HAPT determined as total distribution volume (V(t)) were highest in the putamen, high in the caudate, intermediate in the amygdala, hippocampus, and thalamus, lower in the cingulate gyrus and frontal, temporal, and occipital cortices, and lowest in the cerebellum. In contrast, the distribution and TACs of [(11)C](S,S)HAPT were homogeneous in all regions. The uptake of [(11)C](R,R)HAPT was reduced by 1 mg/kg (-)-vesamicol, a specific VAChT antagonist, in all regions except the cerebellum, but not by 0.1 mg/kg SA4503, a specific sigma-1 receptor agonist. These results well reflect the in vitro affinity assessments using rat cerebral membranes. They also demonstrate that [(11)C](R,R)HAPT is a potential PET probe for noninvasive and quantitative imaging of VAChT in the living brain.

Synthesis and evaluation of in vitro bioactivity for vesicular acetylcholine transporter inhibitors containing two carbonyl groups

To identify selective high-affinity ligands for the vesicular acetylcholine transporter (VAChT), we have incorporated a carbonyl group into the structures of trozamicol and prezamicol scaffolds, and also converted the secondary amines of the piperidines of trozamicols and prezamicols into amides. Of 18 new racemic compounds, 4 compounds displayed high affinity for VAChT (K(i)=10-20 nM) and greater than 300-fold selectivity for VAChT over σ(1) and σ(2) receptors, namely (4-(4-fluorobenzoyl)-4'-hydroxy-[1,3'-bipiperidin]-1'-yl)(3-methylthiophen-2-yl)methanone oxalate (9g) (K(i-VAChT)=11.4 nM, VAChT/σ(1)=1063, VAChT/σ(2)=370), (1'-benzoyl-4'-hydroxy-[1,3'-bipiperidin]-4-yl)(4-methoxyphenyl)methanone oxalate (10c) (K(i-VAChT)=15.4 nM, VAChT/σ(1)=374, VAChT/σ(2)=315), (4'-hydroxy-1'-(thiophene-2-carbonyl)-[1,3'-bipiperidin]-4-yl)(4-methoxyphenyl)methanone oxalate (10e) (K(i-VAChT)=19.0 nM, VAChT/σ(1)=1787, VAChT/σ(2)=335), and (4'-hydroxy-1'-(3-methylthiophene-2-carbonyl)-[1,3'-bipiperidin]-4-yl)(4-methoxyphenyl)methanone oxalate (10g) (K(i-VAChT)=10.2 nM, VAChT/σ(1)=1500, VAChT/σ(2)=2030). These four compounds can be radiosynthesized with C-11 or F-18 to validate their possibilities of serving as PET probes for quantifying the levels of VAChT in vivo.

Synthesis and in vitro and in vivo evaluation of 18F-labeled positron emission tomography (PET) ligands for imaging the vesicular acetylcholine transporter

A new class of vesicular acetylcholine transporter inhibitor that incorporates a carbonyl group into the benzovesamicol structure was synthesized, and analogues were evaluated in vitro. (+/-)-trans-2-Hydroxy-3-(4-(4-[(18)F]fluorobenzoyl)piperidino)tetralin (9e) has K(i) values of 2.70 nM for VAChT, 191 nM for sigma(1), and 251 nM for sigma(2). The racemic precursor (9d) was resolved via chiral HPLC, and (+/-)-[(18)F]9e, (-)-[(18)F]9e, and (+)-[(18)F]9e were respectively radiolabeled via microwave irradiation of the appropriate precursors with [(18)F]/F(-) and Kryptofix/K(2)CO(3) in DMSO with radiochemical yields of approximately 50-60% and specific activities of >2000 mCi/micromol. (-)-[(18)F]9e uptake in rat brain was consistent with in vivo selectivity for the VAChT with an initial uptake of 0.911 %ID/g in rat striatum and a striatum/cerebellum ratio of 1.88 at 30 min postinjection (p.i.). MicroPET imaging of macaques demonstrated a 2.1 ratio of (-)-[(18)F]9e in putamen versus cerebellum at 2 h p.i. (-)-[(18)F]9e has potential to be a PET tracer for clinical imaging of the VAChT.

Enhanced cholinergic response in pancreata of nonhuman primates with impaired glucose tolerance shown on [18F]fluorobenzyltrozamicol positron emission tomography

BACKGROUND

Islet cell adaptation to insulin resistance in type 2 diabetes mellitus may be due in part to increased stimulation of beta cells by the autonomic nervous system. The parasympathetic neurotransmitter acetylcholine (ACh) mediates insulin release via M3 muscarinic receptors on islet beta cells. The vesicular ACh transporter (VAChT) receptor correlates with cholinergic activity in vivo. The positron emission tomography (PET) radiotracer (+)-4-[18F]fluorobenzyltrozamicol ([18F]FBT) binds to the VAChT receptor on presynaptic cholinergic neurons and can be quantified by PET. In this study, we utilize [18F]FBT PET to demonstrate pancreatic cholinergic activity before and after dextrose infusion in nonhuman primates with normal (NGT) and impaired (IGT) glucose tolerance.

METHODS

Seven adult female vervet (Chlorocebus aethiops) monkeys were maintained on an atherogenic Western diet. They were divided into two groups: four with NGT and three with IGT. Each subject underwent [18F]FBT PET twice: first, a baseline PET under fasting conditions; and second, PET under fasting conditions but after intravenous infusion of dextrose solution. Quantitative analysis of pancreatic uptake at 60 min post-injection was performed.

RESULTS

There was no difference in pancreatic uptake of [18F]FBT on baseline scans between the two groups. Pancreatic uptake of [18F]FBT increased in every subject after dextrose infusion (P = 0.03). On post-dextrose PET scans, pancreatic uptake of [18F]FBT was significantly higher in IGT subjects compared with NGT subjects (P = 0.03). The post-dextrose to pre-dextrose uptake ratios were higher in IGT subjects (P = 0.08).

CONCLUSIONS

Acute increases in pancreatic cholinergic activity in vivo were detected in the pancreata of nonhuman primates with NGT and IGT after intravenous dextrose infusion on [18F]FBT PET. In subjects with IGT, this activity was significantly higher, suggesting increased autonomic nervous system stimulation of the pancreatic islets in insulin-resistant subjects.

Dual radiotracer analysis of cholinergic neuronal changes in prediabetic mouse pancreas

BACKGROUND

Pancreatic neuronal changes associated with beta cell loss in type 1 diabetes mellitus are complex, involving, in part, parasympathetic mechanisms to compensate for preclinical hyperglycemia. The parasympathetic neurotransmitter acetylcholine (ACh) mediates insulin release via M3 muscarinic receptors on islet beta cells. The vesicular ACh transporter (VAChT) receptor has been shown to be a useful marker of cholinergic activity in vivo. The positron emission tomography (PET) radiotracer (+)-4-[(18)F]fluorobenzyltrozamicol ([(18)F]FBT) binds to the VAChT receptor on presynaptic cholinergic neurons and can be quantified by PET. The compound 4-diphenylacetoxy-N-methylpiperidine (4-DAMP), available in a tritiated form, binds to M3 muscarinic receptors on beta cells and is a potential target for assessing pancreatic beta cell mass. In this study, we investigate the feasibility of dual radiotracer analysis in identifying neurofunctional changes that may signify type 1 diabetes mellitus in its early preclinical state.

METHODS

Ex vivo determinations of pancreatic uptake were performed in prediabetic nonobese diabetic mice and controls after intravenous injection of [(18)F]FBT or 4-[(3)H]DAMP. Beta cell loss in prediabetic mice was confirmed using immunohistochemical methods.

RESULTS

[(18)F]FBT uptake was significantly higher in prediabetic pancreata than controls: 3.22 +/- 0.81 and 2.51 +/- 1.04, respectively (P < 0.03). 4-[(3)H]DAMP uptake was significantly lower in prediabetic pancreata than controls: 0.612 +/- 0.161 and 0.968 +/- 0.364, respectively (P = 0.01).

CONCLUSIONS

These data suggest that a combination of radiotracer imaging agents that bind to neuronal elements intimately involved in insulin production may be an effective method of evaluating changes associated with early beta cell loss using PET.

Ex vivo and in vivo evaluation of (2R,3R)-5-[(18)F]-fluoroethoxy- and fluoropropoxy-benzovesamicol, as PET radioligands for the vesicular acetylcholine transporter

Molecular imaging of the vesicular acetylcholine transporter (VAChT) using positron emission tomography (PET) may provide insights into early diagnosis and better understanding of Alzheimer's disease. We further characterized the VAChT ligand (2R,3R)-5-FEOBV (1) and developed new fluoropropoxy analogues. Ex vivo studies of the new nonradiolabeled analogues (2R,3R)-5-FPOBV (2) (k(D) = 0.7 nM) and (2S,3S)-5-FPOBV (3) (k(D) = 8.8 nM) were performed in rat brain and showed an enantioselective inhibition of (-)-5-[(125)I]-IBVM uptake in striatum, cortex, and hippocampus (e.g., 74% for 2 and only 54% for 3 in the cortex). Radiochemical procedures were developed to produce [(18)F]1 and [(18)F]2 as potential imaging agent for the VAChT. The radiochemistry was carried out in a one step procedure, with radiolabeling yields of 17 and 2.6% (range: 1-5.4), respectively, nondecay corrected with good specific activity: 124-338 GBq/micromol. The radiochemical purity was greater than 98%. The biological (ex vivo and in vivo) properties of these radioligands were evaluated in rats and showed a low (less then 0.1% of the injected dose) and homogeneous brain uptake. The in vivo PET study of [(18)F]2 performed in baboon also revealed rapid defluorination as the main problem. Therefore [(18)F]1 and [(18)F]2 appear to be unsuitable for in vivo imaging of the VAChT using PET.

Synthesis and evaluation of vesamicol analog (-)-o-[11C]methylvesamicol as a PET ligand for vesicular acetylcholine transporter

(-)-O-Methylvesamicol ((-)-OMV) exhibited in vitro a high affinity for vesicular acetylcholine transporter (VAChT) (Ki, 6.7 nM) and a relatively low affinity for sigmal receptor (Ki, 33.7 nM). We prepared (-)-[11C]OMV by a palladium-promoted cross-coupling reaction using [11C]methyl iodide, in a radiochemical yield of 38 +/- 6.9% (n=3), a radiochemical purity of 98 +/- 2.3% (n = 5), and a specific activity of 11 +/- 7.0 TBq/mmol at 30 minutes after EOB (n=5). Then, we evaluated in vivo whether (-)-[11C]OMV has properties as a PET radioligand for mapping VAChT. In rats, the brain uptake of (-)-[11C]OMV was 1.1%ID/g at 5 minutes postinjection, and was retained of a high level for 60 minutes. The brain uptake was significantly inhibited by the co-injection (500 nmol/kg) of cold (-)-OMV (58-66%), (-)-vesamicol (57-65%), and two sigma receptor ligands with modulate affinities for VAChTs: SA4503 (56-71%) and haloperidol (39-64%) in all of the brain regions, including the cerebellum with a low density of VAChTs, but not of sigmal-selective ligand (+)-pentazocine. However, the pretreatment with a large excess amount of (+/-)-pentazocine (50 micromol/kg) reduced the uptake in a different manner in the brain regions: 25% reduction in the striatum with a high density of VAChTs, and a 50-55% reduction in the other regions with a lower density of VAChTs. Ex vivo autoradiography using (-)-[11C]OMV showed a similar regional brain distribution of [3H](-)-vesamicol. In the PET study of the monkey brain, the regional brain distribution pattern of (-)-[11C]OMV was different from that of [11C]SA4503. The uptake of (-)-[11C]OMV was relatively higher in the striatum, was reversible, and an apparent equilibrium state was found at 20-40 minutes. In conclusion, (-)-[11C]OMV exhibited appropriate brain kinetics during the time frame of 11C-labeled tracers and bound mainly to VAChTs; however, the binding to sigmal receptors was not disregarded. Therefore, (-)-[11C]OMV-PET together with help of [11C]SA4503-PET may evaluate VAChTs.

Structural changes of benzylether derivatives of vesamicol and their influence on the binding selectivity to the vesicular acetylcholine transporter

18F labelled vesamicol analogues, which bind to the vesicular acetylcholine transporter (VAChT) in central cholinergic nerve terminals, are expected to be potential radioligands for the visualisation of cholinergic transmission deficits via positron emission tomography (PET). In this report the regioselective synthesis of five novel vesamicol analogues as well as their in vitro binding properties to the VAChT are described. Beside having the 4-fluorobenzylether-substitution at the cyclohexyl ring as an unique structural feature, the new compounds are additionally modified at the phenyl and piperidine moiety of the vesamicol skeleton. The affinity and selectivity to the VAChT were analysed by competitive binding studies using tritium labelled radioligands. The VAChT affinities (Ki-values) of the novel compounds were estimated ranging between 7.8+/-3.5 nM and 161.6+/-17.3 nM, thus some of them are binding with higher affinity to the transporter than vesamicol. However, the compounds tested demonstrated also affinities to the sigma receptors sigma1 and sigma2 ranging between 4.1+/-1.5 nM and 327.5+/-75.9 nM. Nevertheless, these data provide the basis for future structure-binding-studies and further underline the potential and usefulness of vesamicol analogues for imaging of the VAChT.

Estrogen modulates cognitive and cholinergic processes in surgically menopausal monkeys

Estrogen deficiency in postmenopausal women is associated with changes in physiological processes. The extent to which estrogen loss is associated with cognitive changes noted by postmenopausal women has been more difficult to determine for a variety of reasons. Primate models of menopause are now being used to determine the effects of estrogen loss and replacement on cognitive abilities and to investigate the neural mechanisms by which estrogen may influence cognitive function. The present report presents data from cognitive and neurobiological studies in surgically menopausal monkeys that have examined how estrogen loss and replacement may be affecting cognitive abilities and the cholinergic system; a neural system that is known to influence memory and attention function. These studies are indicating that visuospatial attention function is especially sensitive to estrogen states in young monkeys, but that multiple cognitive domains are sensitive to estrogen states in middle-aged monkeys. In addition, anatomical and functional imaging studies indicate that the primate cholinergic system is modulated by estrogen, and pharmacological studies demonstrate that estrogen uses cholinergic muscarinic receptors to influence visuospatial attention. These studies demonstrate that estrogen influences cognitive abilities in monkey models of menopause and the cholinergic system may be one of the mechanisms by which estrogen modulates cognitive function. Given the current unknowns and concerns regarding the use of hormone replacement therapy in postmenopausal women, continued studies in monkey models of menopause are especially needed to further elucidate the effects of estrogen on cognitive and neurobiological processes, with particular emphasis on studies in middle-aged monkeys, determining the optimal aspects of ERT regimens, and identifying the relationships between estrogen effects on cognitive and neurobiological function.

Evaluation of radioiodinated (-)-o-iodovesamicol as a radiotracer for mapping the vesicular acetylcholine transporter

We evaluated the potencies of radioiodinated (-)-o-iodovesamicol [(-)-oIV] as a selective vesicular acetylcholine transporter (VAChT) mapping agent. (-)-[125I]oIV exhibited significant accumulation (about 2.8% of the injected dose) in rat brain. The regional brain distribution of radioactivity was similar for both (-)-[125I]olV and (-)-[3H]vesamicol. The accumulation of (-)-[125I]oIV in the brain was significant reduced by post-administration of unlabeled vesamicol (0.5 /micromol/kg(-1)) and (-)-oIV (0.5 micromol/kg(-1)). On the other hand, the post-administration of sigma ligands hardly affected the accumulation of (-)-[125I]oIV in the brain. These studies showed that (-)-[125I]oIV, as well as [3H] vesamicol, bound to VAChT with high affinity in the rat brain. Furthermore, (-)-[125I]oIV binding in the ipsilateral cortex to the lesion was significantly reduced by 17.0%, compared with that in the contralateral cortex in a unilateral NBM-lesioned rat. These results suggested that radioiodinated (-)-oIV may potentially be useful for the diagnosis of cholinergic neurodegenerative disorders.

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.

Synthesis and evaluation of radiolabeled piperazine derivatives of vesamicol as SPECT agents for cholinergic neurons

To diagnose and investigate neurodegenerative diseases affecting cholinergic neuron density, piperazine derivatives of vesamicol were synthesized and evaluated. Previously, we reported that trans-5-iodo-2-hydroxy-3-[4-phenylpiperazinyl] tetralin (DRC140, 1) possessed high selectivity for vesicular acetylcholine transporter (VAChT). In present study of the effect of alkyl substituents, we observed that the introduction of a methyl group into the ortho or meta positions of the phenyl group of 1 increased affinity for VAChT. trans-5-Iodo-2-hydroxy-3-[4-[2-methylphenyl] piperazinyl]tetralin (2) displayed high affinity and specificity for VAChT. The regional distributions of radioactivity in the rat brain correlated well with known patterns of central cholinergic innervation. [(123)I]2 is a potentially useful compound for SPECT imaging.

Morphine-induced spinal cholinergic activation: in vivo imaging with positron emission tomography

Positron emission tomography (PET) imaging of spinal cord in monkeys with a cholinergic tracer demonstrates increased spinal cholinergic activity in response to an analgesic dose of morphine, and this PET result correlates with measurement of acetylcholine spillover into spinal cord extracellular space induced by morphine, as measured by microdialysis. Previous studies in rats, mice, and sheep demonstrate activation of spinal cholinergic neurons by systemic opioid administration, and participation of this cholinergic activity in opioid-induced analgesia. Testing the relevance of this observation in humans has been limited to measurement of acetylcholine spillover into lumbar cerebrospinal fluid. The purpose of this study was to apply a recently developed method to image spinal cholinergic terminals non-invasively via PET and to test the hypothesis that the tracer utilized would reflect changes in local cholinergic activity. Following Animal Care and Use Committee approval, seven adult male rhesus monkeys were anesthetized on three separate occasions. On two of the occasions PET scans were performed using [(18)F] (+)-4-fluorobenzyltrozamicol ([(18)F]FBT), which selectively binds to the vesicular acetylcholine (ACh) transporter in the presynaptic cholinergic terminals. PET scans were preceded by injection of either saline or an analgesic dose of IV morphine (10 mg/kg). On the third occasion, microdialysis catheters were inserted in the spinal cord dorsal horn and acetylcholine concentrations in dialysates determined before and after IV morphine injection. Morphine increased cholinergic activity in the spinal cord, as determined by blood flow corrected distribution volume of [(18)F]FBT in the cervical cord compared to the cerebellum. Morphine also increased acetylcholine concentrations in microdialysates from the cervical cord dorsal horn. The one animal which did not show increased spinal cholinergic activity by PET from this dose of morphine also did not show increased acetylcholine from this morphine dose in the microdialysis experiment. These data confirm the ability to use PET to image spinal cholinergic terminals in the monkey spinal cord and suggest that acute changes in cholinergic activity can be imaged with this non-invasive technique. Following preclinical screening, PET scanning with [(18)F]FBT may be useful to investigate mechanisms of analgesic action in normal humans and in those with pain.

Cholinergic activity of aged rhesus monkeys revealed by positron emission tomography

In the present study, the radiotracer [(18)F] (+)-4-fluorobenzyltrozamicol ((+)-[(18)F]FBT) and positron emission tomography (PET) were used to examine the vesicular acetylcholine transporter and determine if presynaptic cholinergic activity was altered with age in 23 rhesus monkeys that varied in age from 10 to 37 years. Binding of (+)-[(18)F]FBT in the basal ganglia was reduced significantly with increasing age of the monkeys. However, there were individual differences noted in that some middle-aged and aged monkeys demonstrated levels of (+)-[(18)F]FBT binding that were comparable to the binding measured in adult monkeys. These data indicate that presynaptic cholinergic function may decrease with age, but that there may be a differential susceptibility of the cholinergic system to the aging process in different individuals.

Synthesis and biological characterization of stable and radioiodinated (+/-)-trans-2-hydroxy-3-P[4-(3-iodophenyl)piperidyl]-1,2,3,4-tetrahydronaphthalene (3'-IBVM)

The vesamicol analogue (+/-)-trans-2-Hydroxy-3-[4-(3-iodophenyl)piperidyl]-1,2,3,4-tetrahydronaphthalene (3'-IBVM), a potent ligand for the vesicular acetylcholine transporter (VAChT), was evaluated as a potential radiotracer for studying VAChT density in vivo. In radioligand binding experiments, 3'-IBVM displays subnanomolar affinity for VAChT and 100-fold selectivity for VAChT over sigma1 and sigma2 receptors. Consistent with this profile, radioiodinated (+/-)-3'-IBVM distributed heterogenously in the rat brain following a bolus IV injection, displaying high concentrations in the striatum and moderate to low concentrations in the cortex and cerebellum, respectively. However, co-injection of the radiotracer with the sigma ligand haloperidol resulted in significant reductions of radiotracer levels in all brain regions examined. Therefore, radioiodinated (+/-)-IBVM appears to bind to both VAChT and sigma receptors in vivo.

Piperazine analog of vesamicol: in vitro and in vivo characterization for vesicular acetylcholine transporter

The probes to detect vesicular acetylcholine transporter (VAChT) in vivo are important to evaluate the mapping and function in cholinergic system. To develop high-specific and high-affinity radiotracer for single photon emission computed tomography, we investigated piperazine analogs which replaced the piperidine ring of (-)-vesamicol with a piperazine ring. We found that the piperazine analog of iodobenzovesamicol, trans-5-iodo-2-hydroxy-3-[4-phenylpiperazinyl] tetralin (DRC140), had high affinity for VAChT in rat brain. We carried out binding assay in subcellular fraction of the rat brain. The highest B(max) for [(125)I]-DRC140 binding was observed in the synaptic vesicle fraction (1,751 fmol/mg protein), followed by the crude vesicle (821 fmol/mg protein) and the P2 fraction (187 fmol/mg protein). These K(d) values were similar to the affinity of highly purified synaptic vesicular fraction (K(d) = 0.3 nM) with a one-site model. The possibility that [(125)I]-DRC140 recognizes sigma receptor was excluded by our finding large inhibition constants (K(i) = 849 nM for haloperidol, K(i) = 3,052 nM for 1,3-di(2-tolyl)guanidine). In vivo distribution studies with the [(123)I]-DRC140 in rats showed a rapid brain uptake. The highest brain area was in striatum, followed by frontal cortex, occipital cortex, and hippocampus. The lowest brain area was cerebellum. The radioactivity of high-accumulated areas in ex vivo autoradiography was reduced by a preinjection of (-)-vesamicol and these levels were reduced to the radioactivity in cerebellum. These results show that [(125)I]-DRC140 can provide extremely high specific tracer with excellent brain permeability as a ligand for single photon emission computed tomography.

A new, convenient method for the preparation of 4-[18F]fluorobenzyl halides

A convenient method suitable for automated preparation of 4-[18F]fluorobenzyl halides from no-carrier-added [18F]fluoride has been developed. 4-[18F]Fluorobenzaldehyde, synthesized from [18F]fluoride by aromatic nucleophilic substitution on 4-trimethylammoniumbenzaldehyde triflate, was first retained on a C18 cartridge and there efficiently reduced to 4-[18F]fluorobenzyl alcohol simply by flowing an aqueous solution of NaBH4. The conversion of 4-[18F]fluorobenzyl alcohol to 4-[18F]fluorobenzyl halide was investigated using PBr3, PI3, P2I4, Ph3PBr2 and Ph3PI2 in CH2Cl2. 4-[18F]Fluorobenzyl halides were purified by passing through a disposable silica cartridge. The conversion rapidly proceeded in radiochemical yields of nearly 90% at 40 degrees C with P2I4 and almost quantitatively at room temperature with Ph3PBr2. With this last reagent 4-[18F]fluorobenzyl bromide was obtained in overall radiochemical yields of 50-60% within 30 min from EOB.

N-(3-Iodophenyl)trozamicol (IPHT) and related inhibitors of vesicular acetylcholine transport: synthesis and preliminary biological characterization

Four isomeric N-(halophenyl)trozamicol analogues (6a-d) were synthesized and evaluated as potential vesicular acetylcholine transporter (VAChT) ligands. Of the four compounds, N-(3-bromophenyl) trozamicol (6b) and N-(3-iodophenyl)trozamicol (6d) displayed the highest affinity for the VAChT in vitro, whereas the para-substituted compound 6c showed the lowest affinity for this transporter. Tissue distribution studies of N-(3-[125I]iodophenyl)trozamicol ([125I]6d, [125I)IPHT) suggest that the central distribution of the latter is consistent with cholinergic innervation. However, only moderate target-to-background ratios were obtained, suggesting little improvement over the N-(halobenzyl)trozamicols described previously.

(+)-p-([18F]fluorobenzyl)spirotrozamicol [(+)-[18F]spiro-FBT]: synthesis and biological evaluation of a high-affinity ligand for the vesicular acetylcholine transporter (VAChT)

(+)-1'-[4-Hydroxy-1-(4-fluorobenzyl)piperidin-3-yl]spiro[1H- indene-1,4'- piperidine] {(+)-Spiro-FBT}, a high-affinity vesicular acetylcholine transporter ligand, was labeled with fluorine-18, and evaluated in the rat and monkey. In the rat brain, (+)-[18F]Spiro-FBT accumulated preferentially in the striatum, hippocampus, and cortex, brains regions containing high-to-moderate densities of cholinergic terminals. However, due to rapid metabolism, no preferential accumulation of the radiotracer was observed in corresponding regions of the monkey brain. Consequently, rapid metabolism renders (+)-[18F]Spiro-FBT unsuitable for studying cholinergic function with positron emission tomography.

In vivo imaging of the spinal cord cholinergic system with PET

PURPOSE

Our goal was to demonstrate the feasibility of an in vivo noninvasive method for imaging spinal cord cholinergic terminals using (+)-4-[18F]fluorobenzyltrozamicol ([18F]FBT) and PET.

METHOD

In vitro and in vivo experiments in rats were conducted to demonstrate the specific binding characteristics, localization, and time course of [3H]FBT binding in the spinal cord. PET imaging was then performed on seven rhesus monkeys.

RESULTS

The rat studies demonstrate high specific binding in the spinal cord with a distribution coinciding with the known distribution of cholinergic terminals. In vivo tracer concentrations in the spinal cord and basal ganglia were of the same magnitude. With use of [18F]FBT and PET in the rhesus monkey, the spinal cord was clearly visualized, with tracer concentration in the spinal cord being approximately one-fourth of that seen in the basal ganglia.

CONCLUSION

This work demonstrates the feasibility of imaging cholinergic terminals in vivo in the spinal cord using [18F]FBT and PET.

[18F]fluoroethoxy-benzovesamicol, a PET radiotracer for the vesicular acetylcholine transporter and cholinergic synapses

Loss of cholinergic transmission in the cortex and hippocampus is a characteristic feature of Alzheimer's disease, and visualization of functional cholinergic synapses in the brain with PET could be a useful method for studying this degenerative condition in living humans. We investigated [18F]fluoroethoxybenzovesamicol, (-)-[18F] FEOBV,(-)-(2R,3R)-trans-2-hydroxy-3-(4-phenylpiperidino)-5-(2-[18F ]fluoroethoxy)-1,2,3,4-tetralin, a high affinity positron emitting ligand for the vesicular acetylcholine transporter, as a potential in vivo cholinergic synapse mapping agent. Rodent biodistribution, dosimetry, stereospecificity of biological effects, pharmacologic blocking studies, in vivo rodent brain autoradiography and metabolites were examined. (-)-[18F]FEOBV brain uptake following intravenous injection was robust, with 2.65% dose/brain in mice at 5 min, and the regional localization matched the known distributions of presynaptic cholinergic markers at later times. Both the cholinergic localization and curare-like effects of FEOBV were associated with the "(-)"-enantiomer exclusively. (-)-[18F]FEOBV regional brain distribution in rodents was changed little by pretreatment with haloperidol, (+)-3-PPP, or E-2020, indicating FEOBV, unlike other vesamicol analogs, did not interact in vivo with dopamine or sigma receptor systems. Autoradiography of rat brain 3 h following i.v. injection of (-)-[18F]FEOBV showed high localization in brain areas rich in presynaptic cholinergic elements. Metabolic defluorination in rodents was modest, and analysis of brain tissue following tracer administration found FEOBV as the only extractable radioactive species. (-)-[18F]FEOBV dosimetry calculated from rat data estimate 10 mCi doses can be given to humans. These studies show FEOBV maps cholinergic areas with high specificity in vivo, and may provide a noninvasive means to safely and accurately gauge the functional integrity of cholinergic synapses in man using PET.

Pharmacological characterization of the vesamicol analogue (+)-[(125)I]MIBT in primate brain

The vesamicol analogue, meta-[(125)I]iodobenzyltrozamicol [(+)-[(125)I]MIBT] was evaluated as a probe for the in vitro labeling of the vesicular acetylcholine transporter in primate brain. In the striatum, (+)-[(125)I]MIBT bound a single high-affinity site with a Kd value of 4.4 +/- 0.7 nM. Competition for (+)-[(125)I]MIBT binding to the striatum by a group of vesamicol analogues displayed a pharmacological profile similar to the rank order of potency previously observed for the vesicular acetylcholine transporter on Torpedo synaptic vesicles. High-affinity binding of (+)-[(125)I]MIBT in the occipital cortex was characterized by a Kd value of 4.6 +/- 1.1 nM. However, the rank order of potency for inhibition of (+)-[(125)I]MIBT binding to the occipital cortex by the same test compounds differed from that observed in the striatum. The results suggest that (+)-[(125)I]MIBT is a reliable probe of the vesicular acetylcholine transporter in primate striatum, but its binding in primate occipital cortex is more complex.

Vesicular acetylcholine transporter density and Alzheimer's disease

We have evaluated the vesamicol analogue meta-[125I]iodobenzyltrozamicol {(+)-[125I]MIBT} as a probe to assess cholinergic terminal integrity in the human temporal cortex. Saturation binding analysis, using 5-aminobenzovesamicol (ABV) to define nonspecific binding, revealed a high-affinity binding site with a Kd value of 4.3 +/- 1.2 nM in the temporal cortex of the young control subjects. Similar affinity values were observed for (+)-[125I]MIBT binding in aged control subjects (Kd = 3.4 +/- 0.5 nM) and AD patients (Kd = 3.0 +/- 0.8 nM). In contrast, Bmax values for young subjects, aged controls and AD patients were 31.2 +/- 6.3, 17.0 +/- 2.0 and 9.4 +/- 1.6 pmol/g, respectively, clearly reflecting significant reductions in (+)-[125I]MIBT binding site density with aging and age-related neuropathology. Moreover, the decrease in (+)-[125I]MIBT binding was correlated with choline acetyltransferase activities (r = 0.72) in the AD temporal cortex. These results suggest that when selective ligands are used, the vesicular acetylcholine transporter can be a useful marker protein for assessing the loss of cholinergic projections in AD and related disorders.

Imaging of cholinergic terminals using the radiotracer [18F](+)-4-fluorobenzyltrozamicol: in vitro binding studies and positron emission tomography studies in nonhuman primates

The goal of the present set of studies was to characterize the in vitro binding properties and in vivo tissue kinetics for the vesicular acetylcholine transporter (VAcChT) radiotracer, [18F](+)-4-fluorobenzyltrozamicol ([18F](+)-FBT). In vitro binding studies were conducted in order to determine the affinity of the (+)- and (-)-stereoisomers of FBT for the VAcChT as well as sigma (sigma 1 and sigma 2) receptors. (+)-FBT was found to have a high affinity (Ki = 0.22 nM) for the VAcChT and lower affinities for sigma 1 (21.6 nM) and sigma 2 (35.9 nM) receptors, whereas (-)-FBT had similar affinities for the VAcChT and sigma 1 receptors (approximately 20 nM) and a lower affinity for sigma 2 (110 nM) receptors. PET imaging studies were conducted in rhesus monkeys (n = 3) with [18F](+)-FBT. [18F](+)-FBT was found to have a high accumulation and slow rate of washout from the basal ganglia, which is consistent with the labeling of cholinergic interneurons in this brain region. [18F](+)-FBT also displayed reversible binding kinetics during the 3 h time course of PET and produced radiolabeled metabolites that did not cross the blood-brain barrier. The results from the current in vitro and in vivo studies indicate that [18F](+)-FBT is a promising ligand for studying cholinergic terminal density, with PET, via the VAcChT.

Functional and neurobiological similarities of aging in monkeys and humans

Aging in humans may be accompanied by alterations in several functional abilities. However, there is a great deal of individual variability in the functions that may be altered with age within and across aged people. One potential source of age-related behavioral variation may lie in a differential vulnerability of neurobiological systems to the aging process in particular individuals. Aged monkeys demonstrate behavioral and brain alterations that have many parallels with those observed in aged humans and are valuable animal models in which to investigate the interrelationships between age, behavior and neurobiological measures. This review outlines the similarities of functional and neurobiological aging in monkeys and humans, notes the variability that exists in both behavioral and neural systems in aging, and identifies some of the areas of aging that are in need of further investigation.

The vesamicol receptor ligand (+)-meta-[125I]iodobenzyltrozamicol [(+)-[125I]-MIBT] reveals blunting of the striatal cholinergic response to dopamine D2 receptor blockade in the 6-hydroxydopamine (6-OHDA)-lesioned rat: possible implications for Parkinson's disease

Previous studies of radiolabelled vesamicol receptor (VR) ligands suggest that the latter may be used, in conjunction with dopamine D2 antagonists, to measure changes in striatal cholinergic function in vivo. In the present study, the radiolabelled VR ligand (+)-meta-[125I]iodobenzyltrozamicol {(+)-[125I]MIBT} was used to assess striatal cholinergic function in the unilateral 6-hydroxydopamine (6-OHDA)-treated rat. In control animals, the levels of this radiotracer monitored at 3 hr post injection displayed bilateral symmetry in the striatum, cerebral cortex and cerebellum. However, in animals pretreated with the dopamine antagonist spiperone (2 mg/kg ip), the radiotracer concentration in the striatal hemisphere ipsilateral to 6-OHDA lesion increased by 23% (p = 0.068) while the concentration in the contralateral striatum was elevated by 87% (p < 0.0001). Since the nigrostriatal dopaminergic system modulates striatal cholinergic function, and dopamine D2 receptor blockade is known to result in increased striatal cholinergic function, the refractoriness of striatal cholinergic neurons following the loss of nigrostriatal dopaminergic innervation confirms the existence of a dopaminergic-cholinergic imbalance in Parkinson's disease. Therefore the combination of a D2 antagonist and radiolabelled VR ligand may provide a potentially useful method for assessing the effects of dopamine depletion in Parkinson's disease.

Comparative tissue distribution of conformationally restricted radioiodinated vesamicol receptor ligands

Three conformationally restricted analogs of vesamicol, 1'-[1-(3-iodobenzyl)-4-hydroxypiperidin-3-yl]-spirol[1H-i nde ne-1,4'- piperidine] (5), 1'-[1-(3-iodobenzyl)-4-hydroxypiperidin-3-yl]-3,4- dihydrospiro[indene-1,4'-piperidine] (6) and 1'-[1-(3-iodobenzyl)-4-hydroxypiperidin-3-yl)-3,4- dihydrospiro[naphthalene-1(2H),4'-piperidine] (7), were labelled with iodine-125 and evaluated as potential radioligands for mapping vesamicol receptor (VR) density and cholinergic function in vivo. All compounds showed similar kinetics in most tissues. However, differences were observed in the brain. Although comparable levels of each corresponding enantiomeric pair were obtained initially in the brain, the levels of the dextrorotatory enantiomers (+)-5, (+)-6 and (+)-7 were found to decrease by 72-82% over a period of 3 h. In contrast, the brain levels of the corresponding levorotatory isomers were maintained throughout the duration of the experiment. Among the dextrorotatory isomers, (+)-6 showed the highest brain extraction, while (+)-7 showed the lowest. In tissue dissection experiments, the levels of (+)-5, (+)-6 and (+)-7 were highest in the striatum and moderate to low in the cortex and cerebellum. Co-administration of haloperidol with (+)-6 decreased the levels of the latter in the striatum by 27%, while the levels in the cortex and cerebellum were each reduced by 60%. In addition, haloperidol failed to affect the regional distribution of (+)-7 in the brain. However, both haloperidol and spiperone increased the striatal levels of (+)-5 by 67 and 76%, respectively, suggesting that the binding of this radioligand is related to cholinergic function.(ABSTRACT TRUNCATED AT 250 WORDS)

Vesamicol analogues as sigma ligands. Molecular determinants of selectivity at the vesamicol receptor

The present study compares the affinities of 2-(4-phenylpiperidino)cyclohexanol (vesamicol, 1) and selected analogues of the latter at the vesamicol receptor (VR) with the corresponding affinities at sigma 1 and sigma 2 binding sites. For this study, the parent structure 1 was divided into three fragments: A (cyclohexyl), B (piperidyl) and C (phenyl). Vesamicol analogues were then selected to reflect structural modifications in these fragments. Consistent with earlier reports, vesamicol was found to exhibit nanomolar affinities at the VR and sigma 1 and sigma 2 sites, resulting in poor selectivity for the VR over the sigma sites. Vesamicol analogues characterized by an acyclic A-fragment showed moderate to low affinities at the VR and moderate to high affinities at sigma 1 and sigma 2 sites. As a result, many of these analogues showed poor selectivity for the VR. Replacement of the C4 carbon of 1 with a halobenzyl amine resulted in higher affinities at the VR coupled with moderate to low affinities at sigma 1 and sigma 2 sites. The introduction of a benzofused substituent at the C4 and C5 positions of 1 (compound 2) resulted in a 200-fold increase in affinity at the VR accompanied by a 5- to 6-fold decrease in affinity at sigma 1 and sigma 2 sites relative to the parent structure. Consequently, compound 2 showed 12,000-fold higher affinity at the VR than at sigma sites. Restricting the rotation of fragment C relative to B (by means of alkyl and alkenyl bridges) generally yielded analogues with subnanomolar affinities at the VR. The corresponding affinities of these spirofused conformationally restricted analogues were moderate to poor at sigma 1 and sigma 2 sites when fragment A was preserved. In contrast, the affinities at sigma 1 and sigma 2 sites were decreased 3- to 11-fold when fragment A was modified at position C4 and decreased up to 100-fold with benzofusion at the C4 and C5 positions of fragment A. Consequently, the spirofused analogues 15-19 were among the most selective VR ligands examined. Thus, the effect of conformational restriction in fragments A and B-C is to increase affinity at the VR while decreasing affinity at sigma 1 and sigma 2 sites, and thereby increasing selectivity for the VR over the sigma sites.