Synthesis and in vitro evaluation of new benzovesamicol analogues as potential imaging probes for the vesicular acetylcholine transporter

A novel class of insecticidal alkylsulfones are potent inhibitors of vesicular acetylcholine transport

Pyridine alkylsulfone derivatives typified by oxazosulfyl (Sumitomo Chemical Company Ltd.) and compound A2 (Syngenta) represent a new class of insecticides, with potent activity against several insect orders. Whilst the MOA of this class has been attributed to interaction with the voltage-gated sodium channel (VGSC), here we present strong evidence that their toxicity to insects is mediated primarily through inhibition of the vesicular acetylcholine transporter (VAChT). Alkylsulfone intoxication in insects is characterised by (i) a reduction in cholinergic synaptic transmission efficiency demonstrated by a depression of cercal afferent activity in giant-interneurone preparations of American cockroach (Periplaneta americana), (ii) selective block of cholinergic-transmission dependent post-synaptic potentials in the Drosophila giant-fibre pathway and (iii) abolition of miniature excitatory post-synaptic currents (mEPSCs) in an identified synapse in Drosophila larvae. Ligand-binding studies using a tritiated example compound ([3H]-A1) revealed a single saturable binding-site, with low nanomolar Kd value, in membrane fractions of green bottle fly (Lucilia sericata). Binding is inhibited by vesamicol and by several examples of a previously identified class of insecticidal compounds known to target VAChT, the spiroindolines. Displacement of this binding by analogues of the radioligand reveals a strong correlation with insecticidal potency. No specific binding was detected in untransformed PC12 cells but a PC12 line stably expressing Drosophila VAChT showed similar affinity for [3H]-A1 as that seen in fly head membrane preparations. Previously identified VAChT point mutations confer resistance to the spiroindoline class of insecticides in Drosophila by Gal-4/UAS directed expression in cholinergic neurones and by CRISPR gene-editing of VAChT, but none of these flies show detectable cross-resistance to this new chemical class. Oxazosulfyl was previously shown to stabilise voltage-gated sodium channels in their slow-inactivated conformation with an IC50 value of 12.3μM but inhibits binding of [3H]-A1 with approximately 5000 times greater potency. We believe this chemistry class represents a novel mode-of-action with high potential for invertebrate selectivity.

Computational modeling of PET imaging agents for vesicular acetylcholine transporter (VAChT) protein binding affinity: application of 2D-QSAR modeling and molecular docking techniques

The neurotransmitter acetylcholine (ACh) plays a ubiquitous role in cognitive functions including learning and memory with widespread innervation in the cortex, subcortical structures, and the cerebellum. Cholinergic receptors, transporters, or enzymes associated with many neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD), are potential imaging targets. In the present study, we have developed 2D-quantitative structure-activity relationship (2D-QSAR) models for 19 positron emission tomography (PET) imaging agents targeted against presynaptic vesicular acetylcholine transporter (VAChT). VAChT assists in the transport of ACh into the presynaptic storage vesicles, and it becomes one of the main targets for the diagnosis of various neurodegenerative diseases. In our work, we aimed to understand the important structural features of the PET imaging agents required for their binding with VAChT. This was done by feature selection using a Genetic Algorithm followed by the Best Subset Selection method and developing a Partial Least Squares- based 2D-QSAR model using the best feature combination. The developed QSAR model showed significant statistical performance and reliability. Using the features selected in the 2D-QSAR analysis, we have also performed similarity-based chemical read-across predictions and obtained encouraging external validation statistics. Further, we have also performed molecular docking analysis to understand the molecular interactions occurring between the PET imaging agents and the VAChT receptor. The molecular docking results were correlated with the QSAR features for a better understanding of the molecular interactions. This research serves to fulfill the experimental data gap, highlighting the applicability of computational methods in the PET imaging agents' binding affinity prediction.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s40203-023-00146-4.

In vitro characterization of [3H]VAT in cells, animal and human brain tissues for vesicular acetylcholine transporter

Vesicular acetylcholine transporter plays a crucial role in the cholinergic system, and its alterations is implicated in several neurodegenerative disorders. We recently developed a PET imaging tracer [18F]VAT to target VAChT in vivo with high affinity and selectivity. Here we report in vitro characterization of [3H]VAT, a tritiated counterpart of [18F]VAT. Using human VAChT-rich cell membrane extracts, a saturated binding curve was obtained for [3H]VAT with Kd = 6.5 nM and Bmax = 22.89 pmol/mg protein. In the [3H]VAT competition-binding assay with a panel of CNS ligands, binding inhibition of [3H]VAT was observed using VAChT ligands, the Ki values ranged from 5.41 to 33.3 nM. No inhibition was detected using a panel of other CNS ligands. In vitro [3H]VAT autoradiography of rat brain sections showed strong signals in the striatum, moderate to high signals in vermis, thalamus, cortex, and hippocampus, and weak signals in cerebellum. Strong [3H]VAT ARG signals were also observed from striatal sections of normal nonhuman primates and human brains. Competitive ARG study with human striatal sections demonstrated strong ARG signals of [3H]VAT in caudate and putamen were blocked significantly by either VAChT ligand TZ659 or (-)-vesamicol, but not by the σ1 receptor ligand Yun-122. ARG study also indicated that signal in the striatal sections from PSP human brains was lower than normal human brains. These data provide solid evidence supporting [18F]VAT as a suitable PET radiotracer for quantitative assessment of VAChT levels in vivo.

Direct access to the optically active VAChT inhibitor vesamicol and its analogues via the asymmetric aminolysis of meso-epoxides with secondary aliphatic amines

First highly enantioselective synthesis of biologically important vesamicol, benzovesamicol, and their derivatives was achieved via the desymmetrization of meso-epoxides with secondary aliphatic amines (4-phenylpiperidine derivatives) using a chiral [salenCo(iii)-BF₄] catalyst at room temperature. All products were obtained in good yield and with excellent optical induction.

Synthesis, resolution, and in vitro evaluation of three vesicular acetylcholine transporter ligands and evaluation of the lead fluorine-18 radioligand in a nonhuman primate

The vesicular acetylcholine transporter (VAChT) is a reliable biomarker for assessing cholinergic dysfunction associated with dementia. We recently reported three new potent and selective carbon-11 labeled VAChT radiotracers. Herein, we report the resolution with a Chiralcel OD column of three additional fluorine containing VAChT ligands in which a fluoroethoxy or fluoroethylamino moiety was substituted for the methoxy group. An in vitro competitive binding assay showed that (-)-7 had high potency for VAChT (K_i-VAChT = 0.31 ± 0.03 nM) and excellent selectivity for VAChT versus σ receptors (K_i-σ1 = 1870 ± 250 nM, K_i-σ2 = 5480 ± 140 nM). Three different radiolabeling approaches were explored; the radiosynthesis of (-)-[¹⁸F]7 was successfully accomplished via a stepwise two-pot, three-step method with moderate yield (11 ± 2%) and high radiochemical purity (>98%). PET imaging studies in a nonhuman primate indicated that (-)-[¹⁸F]7 rapidly entered the brain and accumulated in the VAChT-enriched striatum. The uptake of (-)-[¹⁸F]7 in the target striatal area peaked at 10 min and displayed improved clearance kinetics compared to the VAChT tracer [¹⁸F]VAT, which has been approved by the Food and Drug Administration (FDA) for first-in-man studies. These studies justify further investigation of (-)-[¹⁸F]7 and exploration of the structure-activity relationships of these fluoroethoxy and fluoroethylamino analogs.

Synthesis and in vitro evaluation of 5-substituted benzovesamicol analogs containing N-substituted amides as potential positron emission tomography tracers for the vesicular acetylcholine transporter

Herein, new ligands for the vesicular acetylcholine transporter (VAChT), based on a benzovesamicol scaffold, are presented. VAChT is acknowledged as a marker for cholinergic neurons and a positron emission tomography tracer for VAChT could serve as a tool for quantitative analysis of cholinergic neuronal density. With an easily accessible triflate precursor, aminocarbonylations were utilized to evaluate the chemical space around the C5 position on the tetrahydronaphthol ring. Synthesized ligands were evaluated for their affinity and selectivity for VAChT. Small, preferably aromatic, N-substituents proved to be more potent than larger substituents. Of the fifteen compounds synthesized, benzyl derivatives (±)-7i and (±)-7l had the highest affinities for VAChT. Compound (±)-7i was chosen to investigate the importance of stereochemistry for binding to VAChT and selectivity toward the σ₁ and σ₂ receptors. Enantiomeric resolution gave (+)-7i and (-)-7i, and the eutomer showed seven times better affinity. Although racemate (±)-7i was initially promising, the affinity of (-)-7i for VAChT was not better than 56.7nM which precludes further preclinical evaluation. However, the nanomolar binding together with the ready synthesis of [¹¹C]-(±)-7i shows that (-)-7i can serve as a scaffold for future optimizations to provide improved ¹¹C-labelled VAChT PET tracers.

Isolation, semisynthesis, covalent docking and transforming growth factor beta-activated kinase 1 (TAK1)-inhibitory activities of (5Z)-7-oxozeaenol analogues

(5Z)-7-Oxozeanol and related analogues were isolated and screened to explore their activity as TAK1 inhibitors. Seven analogues were synthesized and more than a score of natural products isolated that examined the role that different areas of the molecule contribute to TAK1 inhibition. A novel nonaromatic difluoro-derivative was synthesized that had similar potency compared to the lead. This is the first example of a nonaromatic compound in this class to have TAK1 inhibition. Covalent docking for the isolated and synthesized analogues was carried out and found a strong correlation between the observed activities and the calculated binding.

In vitro and in vivo characterization of two C-11-labeled pet tracers for vesicular acetylcholine transporter

PURPOSE

The vesicular acetylcholine transporter (VAChT) is a specific biomarker for imaging presynaptic cholinergic neurons. Herein, two potent and selective (11)C-labeled VAChT inhibitors were evaluated in rodents and nonhuman primates for imaging VAChT in vivo.

PROCEDURES

For both (-)-[(11)C]2 and (-)-[(11)C]6, biodistribution, autoradiography, and metabolism studies were performed in male Sprague Dawley rats. Positron emission tomography (PET) brain studies with (-)-[(11)C]2 were performed in adult male cynomolgus macaques; 2 h dynamic data was acquired, and the regions of interest were drawn by co-registration of the PET images with the MRI.

RESULTS

The resolved enantiomers (-)-2 and (-)-6 were very potent and selective for VAChT in vitro (K i  < 5 nM for VAChT with >35-fold selectivity for VAChT vs. σ receptors); both radioligands, (-)-[(11)C]2 and (-)-[(11)C]6, demonstrated high accumulation in the VAChT-enriched striatum of rats. (-)-[(11)C]2 had a higher striatum to cerebellum ratio of 2.4-fold at 60 min; at 30 min, striatal uptake reached 0.550 ± 0.086 %ID/g. Uptake was also specific and selective; following pretreatment with (±)-2, striatal uptake of (-)-[(11)C]2 in rats at 30 min decreased by 50 %, while pretreatment with a potent sigma ligand had no significant effect on striatal uptake in rats. In addition, (-)-[(11)C]2 displayed favorable in vivo stability in rat blood and brain. PET studies of (-)-[(11)C]2 in nonhuman primates indicate that it readily crosses the blood-brain barrier (BBB) and provides clear visualization of the striatum; striatal uptake reaches the maximum at 60 min, at which time the target to nontarget ratio reached ~2-fold.

CONCLUSIONS

The radioligand (-)-[(11)C]2 has high potential to be a suitable PET radioligand for imaging VAChT in the brain of living subjects.

Synthesis and biological characterization of a promising F-18 PET tracer for vesicular acetylcholine transporter

Nine fluorine-containing vesicular acetylcholine transporter (VAChT) inhibitors were synthesized and screened as potential PET tracers for imaging the VAChT. Compound 18a was one of the most promising carbonyl-containing benzovesamicol analogs; the minus enantiomer, (-)-18a displayed high potency (VAChT Ki=0.59 ± 0.06 nM) and high selectivity for VAChT versus σ receptors (>10,000-fold). The radiosynthesis of (-)-[(18)F]18a was accomplished by a two-step procedure with 30-40% radiochemical yield. Preliminary biodistribution studies of (-)-[(18)F]18a in adult male Sprague-Dawley rats at 5, 30, 60 and 120 min post-injection (p.i.) were promising. The total brain uptake of (-)-[(18)F]18a was 0.684%ID/g at 5 min p.i. and by 120 min p.i. slowly washed out to 0.409 %ID/g; evaluation of regional brain uptake showed stable levels of ∼0.800 %ID/g from 5 to 120 min p.i in the VAChT-enriched striatal tissue of rats, indicating the tracer had crossed the blood brain barrier and was retained in the striatum. Subsequent microPET brain imaging studies of (-)-[(18)F]18a in nonhuman primates (NHPs) showed high striatal accumulation in the NHP brain; the standardized uptake value (SUV) for striatum reached a maximum value of 5.1 at 15 min p.i. The time-activity curve for the target striatal region displayed a slow and gradual decreasing trend 15 min after injection, while clearance of the radioactivity from the cerebellar reference region was much more rapid. Pretreatment of NHPs with 0.25mg/kg of the VAChT inhibitor (-)-vesamicol resulted in a ∼90% decrease of striatal uptake compared to baseline studies. HPLC metabolite analysis of NHP plasma revealed that (-)-[(18)F]18a had a good in vivo stability. Together, these preliminary results suggest (-)-[(18)F]18a is a promising PET tracer candidate for imaging VAChT in the brain of living subjects.

Novel fluoroalkyl derivatives of selective kappa opioid receptor antagonist JDTic: Design, synthesis, pharmacology and molecular modeling studies

Novel N- and O-fluoroalkyl derivatives of the highly potent KOR antagonist JDTic were designed and synthesized. Their opioid receptor properties were compared in both in vitro binding assays and modeling approach. All compounds displayed nanomolar affinities for KOR. The fluoropropyl derivatives were more active than their fluoroethyl analogues. N-Fluoroalkylation was preferable to O-alkylation to keep a selective KOR binding. Compared to JDTic, the N-fluoropropyl derivative 2 bound to KOR with an only 4-fold lower affinity and a higher selectivity relative to MOR and DOR [Ki(κ) = 1.6 nM; Ki(μ)/Ki(κ) = 12; Ki(δ)/Ki(κ) = 159 for 2versus Ki(κ) = 0.42 nM; Ki(μ)/Ki(κ) = 9; Ki(δ)/Ki(κ) = 85 for JDTic]. Modeling studies based on the crystal structure of the JDTic/KOR complex revealed that fluorine atom in ligand 2 was involved in specific KOR binding. Ligand 2 was concluded to merit further development for KOR exploration.

Synthesis and evaluation of ethyleneoxylated and allyloxylated chalcone derivatives for imaging of amyloid β plaques by SPECT

We report radioiodinated chalcone derivatives as new SPECT imaging probes for amyloid β (Aβ) plaques. The monoethyleneoxy derivative 2 and allyloxy derivative 8 showed a high affinity for Aβ(1-42) aggregates with Ki values of 24 and 4.5 nM, respectively. Fluorescent imaging demonstrated that 2 and 8 clearly stained thioflavin-S positive Aβ plaques in the brain sections of Tg2576 transgenic mice. In vitro autoradiography revealed that [(125)I]2 displayed no clear accumulation toward Aβ plaques in the brain sections of Tg2576 mice, whereas the accumulation pattern of [(125)I]8 matched with the presence of Aβ plaques both in the brain sections of Tg2576 mice and an AD patient. In biodistribution studies using normal mice, [(125)I]2 showed preferable in vivo pharmacokinetics (4.82%ID/g at 2 min and 0.45%ID/g at 60 min), while [(125)I]8 showed only a modest brain uptake (1.62%ID/g at 2 min) with slow clearance (0.56%ID/g at 60 min). [(125)I]8 showed prospective binding properties for Aβ plaques, although further structural modifications are needed to improve the blood brain barrier permeability and washout from brain.

Heteroaromatic and aniline derivatives of piperidines as potent ligands for vesicular acetylcholine transporter

To identify suitable lipophilic compounds having high potency and selectivity for vesicular acetylcholine transporter (VAChT), a heteroaromatic ring or a phenyl group was introduced into the carbonyl-containing scaffold for VAChT ligands. Twenty new compounds with ALogD values between 0.53 and 3.2 were synthesized, and their in vitro binding affinities were assayed. Six of them (19a, 19e, 19g, 19k, and 24a-b) displayed high affinity for VAChT (Ki = 0.93-18 nM for racemates) and moderate to high selectivity for VAChT over σ1 and σ2 receptors (Ki = 44-4400-fold). These compounds have a methyl or a fluoro substitution that provides the position for incorporating PET radioisotopes C-11 or F-18. Compound (-)-[(11)C]24b (Ki = 0.78 nM for VAChT, 1200-fold over σ receptors) was successfully synthesized and evaluated in vivo in rats and nonhuman primates. The data revealed that (-)-[(11)C]24b has highest binding in striatum and has favorable pharmacokinetics in the brain.

Development and evaluation of a radiobromine-labeled sigma ligand for tumor imaging

INTRODUCTION

Sigma receptors are appropriate targets for tumor imaging because they are highly expressed in a variety of human tumors. Previously, we synthesized a vesamicol analog, (+)-2-[4-(4-iodophenyl)piperidino]cyclohexanol ((+)-pIV), with high affinity for sigma receptors, and prepared radioiodinated (+)-pIV. In this study, to develop a radiobromine-labeled vesamicol analog as a sigma receptor imaging agent for PET, nonradioactive and radiobromine-labeled (+)-2-[4-(4-bromophenyl)piperidino]cyclohexanol ((+)-pBrV) was prepared and evaluated in vitro and in vivo. In these initial studies, (77)Br was used because of its longer half-life.

METHODS

(+)-[(77)Br]pBrV was prepared by a bromodestannylation reaction with radiochemical purity of 98.8% after HPLC purification. The partition coefficient of (+)-[(77)Br]pBrV was measured. In vitro binding characteristics of (+)-pBrV to sigma receptors were assayed. Biodistribution experiments were performed by intravenous administration of a mixed solution of (+)-[(77)Br]pBrV and (+)-[(125)I]pIV into DU-145 tumor-bearing mice.

RESULTS

The lipophilicity of (+)-[(77)Br]pBrV was lower than that of (+)-[(125)I]pIV. As a result of in vitro binding assay to sigma receptors, the affinities of (+)-pBrV to sigma receptors were competitive to those of (+)-pIV. In biodistribution experiments, (+)-[(77)Br]pBrV and (+)-[(125)I]pIV showed high uptake in tumor via sigma receptors. The biodistributions of both radiotracers showed similar patterns. However, the accumulation of radioactivity in liver after injection of (+)-[(77)Br]pBrV was significantly lower compared to that of (+)-[(125)I]pIV.

CONCLUSION

These results indicate that radiobromine-labeled pBrV possesses great potential as a sigma receptor imaging agent for PET.

Effect of the prosthetic group on the pharmacologic properties of 18F-labeled rhodamine B, a potential myocardial perfusion agent for positron emission tomography (PET)

We recently reported the development of the 2-[(18)F]fluoroethyl ester of rhodamine B as a potential positron emission tomography (PET) tracer for myocardial perfusion imaging. This compound, which was prepared using a [(18)F]fluoroethyl prosthetic group, has significant uptake in the myocardium in rats but also demonstrates relatively high liver uptake and is rapidly hydrolyzed in vivo in mice. We have now prepared (18)F-labeled rhodamine B using three additional prosthetic groups (propyl, diethylene glycol, and triethylene glycol) and found that the prosthetic group has a significant effect on the in vitro and in vivo properties of these compounds. Of the esters prepared to date, the diethylene glycol ester is superior in terms of in vitro stability and pharmacokinetics. These observations suggest that the prosthetic group plays a significant role in determining the pharmacological properties of (18)F-labeled compounds. They also support the value of continued investigation of (18)F-labeled rhodamines as PET radiopharmaceuticals for myocardial perfusion imaging.

Correlation of the vesicular acetylcholine transporter densities in the striata to the clinical abilities of women with Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disability characterized by mutations in the X-linked methyl-CpG-binding protein 2 located at the Xq28 region. The severity is modified in part by X chromosomal inactivation resulting in wide clinical variability. We hypothesized that the ability to perform the activities of daily living (ADL) is correlated with the density of vesicular acetylcholine transporters in the striata of women with RTT. The density of the vesicular acetylcholine transporters in the living human brain can be estimated by single-photon emission-computed tomography (SPECT) after the administration of (-)-5-[¹²³I]iodobenzovesamicol ([¹²³I]IBVM). Twenty-four hours following the intravenous injection of ∼333 MBq (9 mCi) [¹²³ I]IBVM, four women with RTT and nine healthy adult volunteer control participants underwent SPECT brain scans for 60 min. The Vesicular Acetylcholine Transporter Binding Site Index (Kuhl et al., 1994), a measurement of the density of vesicular acetylcholine transporters, was estimated in the striatum and the reference structure, the cerebellum. The women with RTT were assessed for certain ADL. Although the striatal Vesicular Acetylcholine Transporter Binding Site Index was not significantly lower in RTT (5.2 ± 0.9) than in healthy adults (5.7 ± 1.6), RTT striatal Vesicular Acetylcholine Transporter Binding Site Indices and ADL scores were linearly associated (ADL = 0.89*(Vesicular Acetylcholine Transporter Binding Site Index) + 4.5; R² = 0.93; P < 0.01), suggesting a correlation between the ability to perform ADL and the density of vesicular acetylcholine transporters in the striata of women with RTT. [¹²³I]IBVM is a promising tool to characterize the pathophysiological mechanisms of RTT and other neurodevelopmental disabilities.

Powder X-ray study of racemic (2RS,3RS)-5-amino-3-[4-(3-methoxyphenyl)piperazin-1-yl]-1,2,3,4-tetrahydronaphthalen-2-ol

The structure of the title benzovesamicol analogue, C(21)H(27)N(3)O(2), an important compound for the diagnosis of Alzheimer's disease, has been determined by X-ray powder diffraction. The title compound was firstly synthesized and characterized by spectroscopic methods (FT-IR, and (13)C and (1)H NMR). The compound is a racemic mixture of enantiomers which crystallizes in the monoclinic system in a centrosymmetric space group (P2(1)/c). Crystallography, in particular powder X-ray diffraction, was pivotal in revealing that the enantio-resolution did not succeed. The piperazine ring is in a chair conformation, while the cyclohexene ring assumes a half-chair conformation. The crystal packing is dominated by intermolecular O-H···N hydrogen bonding which links molecules along the c direction.

Powder X-ray study of racemic (2RS,3RS)-5-amino-3-(4-phenylpiperazin-1-yl)-1,2,3,4-tetrahydronaphthalen-2-ol

The title compound, C(20)H(25)N(3)O, an important precursor for the preparation of benzovesamicol analogues for the diagnosis of Alzheimer's disease, has been synthesized and characterized by FT-IR, and (1)H and (13)C NMR spectroscopic analyses. The crystal structure was analysed using powder diffraction as no suitable single crystal was obtained. The piperazine ring has a chair conformation, while the cyclohexene ring assumes a half-chair conformation. The crystal packing is mediated by weak contacts, principally by complementary intermolecular N-H···O hydrogen bonds that connect successive molecules into a chain. Further stabilization is provided by weak C-H···N contacts and by a weak intermolecular C-H···π interaction.

2,2,2-Trifluoro-N-(1a,2,7,7a-tetrahydronaphtho[2,3-b]oxiren-3-yl)acetamide by X-ray powder diffraction

The title compound, C(12)H(10)F(3)NO(2), an important precursor in the preparation of benzovesamicol analogues for the diagnosis of Alzheimer's disease, was prepared by the epoxidation of 5,8-dihydronaphthalen-1-amine using 3-chloroperoxybenzoic acid. The structure was determined by X-ray powder diffraction, multinuclear NMR spectroscopy and FT-IR spectroscopy. A pair of molecules form intermolecular N-H...O hydrogen bonds, involving the amino and oxirene groups, to produce a dimer.

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.

Neuroimaging of the vesicular acetylcholine transporter by a novel 4-[18F]fluoro-benzoyl derivative of 7-hydroxy-6-(4-phenyl-piperidin-1-yl)-octahydro-benzo[1,4]oxazines

Phenylpiperidinyl-octahydro-benzo[1,4]oxazines represent a new class of conformationally restrained vesamicol analogues. Derived from this morpholine-fused vesamicol structure, a new fluorine-18-labeled 4-fluorobenzoyl derivative ([(18)F]FBMV) was synthesized with an average specific activity of 75 GBq/micromol and a radiochemical purity of 99%. The radiolabeling method included an exchange reaction of a 4-nitro group of the precursor by fluorine-18, a reduction procedure to eliminate excess of the nitro compound, followed by a high-performance liquid chromatography purification. [(18)F]FBMV demonstrates (i) a moderate lipophilic character with a logD(pH7.0) 1.8+/-0.10; (ii) a considerable binding affinity to the vesicular acetylcholine transporter (VAChT) (K(i)=27.5 nM), as determined using PC12 cells transfected with a VAChT cDNA, and a low affinity to sigma(1,2) receptors (K(i) >3000 nM); (iii) a good uptake into the rat and pig brains; (iv) a typical accumulation in the VAChT-containing brain regions; and (v) an approximately 20% reduction in cortical tracer binding after a specific cholinergic lesion using 192IgG-saporin. [(18)F]FBMV exhibits another PET marker within the group of vesamicol derivatives that demonstrates potentials in imaging brain cholinergic deficits, while its usefulness in clinical practice must await further investigation.

Comparison of noninvasive quantification methods of in vivo vesicular acetylcholine transporter using [123I]-IBVM SPECT imaging

Dementia with Lewy Body and Alzheimer's disease exhibit degeneration of the cholinergic neurons, and currently, the primary target of treatment is the cholinergic neurotransmitter system. [(123)I]-IBVM is a highly selective radioligand for in vivo visualization of the vesicular acetylcholine transporter (VAChT) using single photon emission computed tomography. This study compares different noninvasive methods using the occipital cortex as a reference region for the quantification of [(123)I]-IBVM binding in six older, healthy volunteers: two kinetic analyses based on one-tissue (1TCM) or two-tissue compartment model (2TCM), one linear and one multilinear analysis, and a simplified peak equilibrium analysis. Time-activity curves were well described by a 1TCM for all regions. The 2TCM converged reliably only in the striatum. Goodness of fit was not improved by using a 2TCM as compared with a 1TCM. The multilinear analysis gave binding potentials similar to the 1TCM while being more robust. The peak equilibrium method might prove to be a useful simplified analysis. The binding potentials obtained with reference region methods strongly correlated with results from invasive blood-sampling analysis. Noninvasive quantification of [(123)I]-IBVM data provides reliable estimates of VAChT binding, which is most valuable to study neurodegenerative diseases with specific cholinergic alteration.

A new 18F-labeled fluoroacetylmorpholino derivative of vesamicol for neuroimaging of the vesicular acetylcholine transporter

With the aim of producing selective radiotracers for in vivo imaging of the vesicular acetylcholine transporter (VAChT) using positron mission tomography (PET), here, we report synthesis and analysis of a new class of conformationally constrained vesamicol analogues with moderate lipophilicity. The sequential ring opening on trans-1,4-cyclohexadiene dioxide enabled an approach to synthesize 6-arylpiperidino-octahydrobenzo[1,4]oxazine-7-ols [morpholino vesamicols]. The radiosynthesis of the [18F]fluoroacetyl-substituted derivative ([18F]FAMV) was achieved starting from a corresponding bromo precursor [2-Bromo-1-[7-hydroxy-6-(4-phenyl-piperidin-1-yl)-octahydro-benzo[1,4]oxazin-4-yl]-ethanone] and using a modified commercial computer-controlled module system with a radiochemical yield of 27+/-4%, a high radiochemical purity (99%) and a specific activity of 35 GBq/micromol. In competitive binding assays using a PC12 cell line overexpressing VAChT and [3H]-(-) vesamicol, 2-fluoro-1-[7-hydroxy-6-(4-phenyl-piperidin-1-yl)-octahydro-benzo[1,4]oxazin-4-yl]-ethanone (FAMV) demonstrated a high selectivity for binding to VAChT (K(i): 39.9+/-5.9 nM) when compared to its binding to sigma 1/2 receptors (Ki>1500 nM). The compound showed a moderate lipophilicity (logD (pH 7)=1.9) and a plasma protein binding of 49%. The brain uptake of [18F]FAMV was about 0.1% injected dose per gram at 5 min after injection and decreased continuously with time. Notably, an increasing accumulation of radioactivity in the lateral brain ventricles was observed. After 1 h, the accumulation of [18F]FAMV, expressed as ratio to the cerebellum, was 4.5 for the striatum, 2.0 for the cortical and 1.5 for the hippocampal regions, measured on brain slices using ex vivo autoradiography. At the present time, 75% of [18F]FAMV in the plasma was shown to be metabolized to various hydrophilic compounds, as detected by high-performance liquid chromatography. The degradation of [18F]FAMV was also detected in brain extracts as early as 15 min post injection (p.i.) and increased to 50% at 1 h postinjection. In conclusion, although the chemical properties of [18F]FAMV and the selectivity of binding to VAChT appear to be promising indicators of a useful PET tracer for imaging VAChT, a low brain extraction, in combination with only moderate specific accumulation in cholinergic brain regions and an insufficient in vivo stability prevents the application of this compound for neuroimaging in humans.

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.