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

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.

Radiosynthesis and evaluation of a fluorine-18 labeled radioligand targeting vesicular acetylcholine transporter

Vesicular acetylcholine transporter (VAChT) is a reliable biomarker for assessing the loss of cholinergic neurons in the brain that is associated with cognitive impairment of patients. 5-Hydrotetralin compound (±)-5-OH-VAT is potent (K_i = 4.64 ± 0.32 nM) and selective for VAChT (>1800-fold and 398-fold for σ₁ and σ₂ receptor, respectively) with favorable hydrophilicity (LogD = 1.78), while (-)-5-OH-VAT originally serves as the radiolabeling precursor of (-)-[¹⁸F]VAT, a promising VAChT radiotracer with a logD value of 2.56. To evaluate (-)-5-OH-[¹⁸F]VAT as a radiotracer for VAChT, we performed in vitro binding assay to determine the potency of the minus enantiomer (-)-5-OH-VAT and plus enantiomer (+)-5-OH-VAT, indicating that (-)-5-OH-VAT is a more potent VAChT enantiomer. Radiosynthesis of (-)-5-OH-[¹⁸F]VAT was explored using three strategies. (-)-5-OH-[¹⁸F]VAT was achieved with a good yield (24 ± 6%) and high molar activity (∼37 GBq/µmol, at the end of synthesis) using a microwave assisted two-step one-pot procedure that started with di-MOM protected nitro-containing precursor (-)-6. MicroPET studies in the brain of nonhuman primate (NHP) suggest that (-)-5-OH-[¹⁸F]VAT readily penetrated the blood brain barrier and specifically accumulated in the VAChT-enriched striatum with improved washout kinetics from striatum compared to [¹⁸F]VAT. Nevertheless, the lower target to non-target ratio may limit its use for in vivo measurement of the VAChT level in the brain.

Kinetic modeling of [18 F]VAT, a novel radioligand for positron emission tomography imaging vesicular acetylcholine transporter in non-human primate brain

Molecular imaging of vesicular acetylcholine transporter (VAChT) in the brain provides an important cholinergic biomarker for the pathophysiology and treatment of dementias including Alzheimer's disease. In this study, kinetics modeling methods were applied and compared for quantifying regional brain uptake of the VAChT-specific positron emission tomography radiotracer, ((-)-(1-(-8-(2-fluoroethoxy)-3-hydroxy-1,2,3,4-tetrahydronaphthalen-2-yl)piperidin-4-yl)(4-fluorophenyl)-methanone) ([¹⁸ F]VAT) in macaques. Total volume distribution (V_T ) estimates were compared for one-tissue compartment model (1TCM), two-tissue compartment model (2TCM), Logan graphic analysis (LoganAIF) and multiple linear analysis (MA1) with arterial blood input function using data from three macaques. Using the cerebellum-hemispheres as the reference region with data from seven macaques, three additional models were compared: reference tissue model (RTM), simplified RTM (SRTM), and Logan graphic analysis (LoganREF). Model selection criterion indicated that a) 2TCM and SRTM were the most appropriate kinetics models for [¹⁸ F]VAT; and b) SRTM was strongly correlated with 2TCM (Pearson's coefficients r > 0.93, p < 0.05). Test-retest studies demonstrated that [¹⁸ F]VAT has good reproducibility and reliability (TRV < 10%, ICC > 0.72). These studies demonstrate [¹⁸ F]VAT is a promising VAChT positron emission tomography tracer for quantitative assessment of VAChT levels in the brain of living subjects.

Alzheimer's Disease-Current Status and Future Directions

Alzheimer's disease (AD) is an age-related neurodegenerative disorder of the brain. The presence of amyloid-beta (Aβ) plaques, neurofibrillary tangles (NFTs), loss of neurons, synapses, and altered sensory perceptions, including memory loss, delineate AD. However, the cause of AD is not clearly known. Several genetic and nongenetic factors have been implicated in the disease. Of the genes, the ɛ4 allele of apolipoprotein E is the largest known genetic risk factor of AD. This review article focuses on the various genetic and other predisposing factors that account for AD, pathophysiology of the disease, and the mechanisms by which Aβ plaques and NFTs are formed and could affect AD brain. In addition, recent advances and current diagnostics available for AD patients are detailed. As oxidative stress has been implicated in the etiology of the disease, special emphasis is given for nutrition based antioxidant therapies and interventional strategies for reducing/treating AD.

123I-Iodobenzovesamicol SPECT Imaging of Cholinergic Systems in Dementia with Lewy Bodies

Cholinergic alterations in dementia with Lewy bodies (DLB) have been widely documented in postmortem studies, whereas in vivo studies are sparse, particularly at the subcortical level. We used ¹²³I-iodobenzovesamicol, a SPECT radiotracer of the vesicular acetylcholine transporter, to evaluate in vivo in DLB the integrity of the 3 main cholinergic pathways-the Ch1 (septohippocampal), the Ch4 (innominatocortical), and the Ch5 (pontothalamic) cholinergic pathways-as well as the striatal cholinergic interneurons. In addition, we assessed the involvement of the cholinergic system in cognitive and neuropsychiatric disorders in DLB patients.

METHODS

Twelve healthy volunteers (median age, 72 y; interquartile range, 6.25 y) and 11 DLB patients (median age, 76 y; interquartile range, 10.50 y) underwent a dynamic ¹²³I-iodobenzovesamicol SPECT scan and an MRI scan. MR images were automatically segmented, providing the volumes of several regions of interest, including the striatum and cholinergic terminals in Ch1 (hippocampus), Ch4 (cortical lobes), and Ch5 (thalamus). For each region of interest and each subject, pharmacokinetic modeling allowed calculation of the nondisplaceable binding potential (BP_ND) values for the binding of ¹²³I-iodobenzovesamicol to the vesicular acetylcholine transporter. A neuropsychological evaluation of participants was performed with the Mini-Mental State Examination and the Grober-Buschke, Set, visual discrimination, Benton, and Wechsler tests, and cognitive fluctuations and apathy were also assessed.

RESULTS

Compared with BP_ND values for healthy subjects, BP_ND values for DLB patients were significantly lower in the Ch4 terminal regions of the anterior cingulate cortex and the superior and inferior parietal cortices (P = 0.0006, 0.0015, and 0.0023, respectively), in the Ch5 terminal region of the thalamus (P = 0.0003), and in the striatum (P = 0.0042). All of the neuropsychological test scores were significantly lower in DLB patients than in healthy subjects. Four DLB patients with apathy and 4 DLB patients without apathy were identified. For the anterior cingulate cortex, compared with BP_ND values in healthy subjects, BP_ND values were significantly lower in patients with apathy (P = 0.004) and were unchanged in patients without apathy.

CONCLUSION

Our results confirm the existence in DLB of cholinergic alterations, reaching both cortical and subcortical levels, including the Ch5 pathway and the striatum. Alterations in cholinergic transmission in the anterior cingulate cortex could be closely associated with the development of apathy.

Simplified Quantification Method for In Vivo SPECT Imaging of the Vesicular Acetylcholine Transporter with 123I-Iodobenzovesamicol

(123)I-iodobenzovesamicol is a SPECT radioligand selective for the vesicular acetylcholine transporter (VAChT) and used to assess the integrity of cholinergic pathways in various neurologic disorders. The current noninvasive method for quantitative analysis of (123)I-iodobenzovesamicol, based on multilinear reference tissue model 2 (MRTM2), requires repeated scans for several hours, limiting its application in clinical trials. Our objective was to validate a simplified acquisition method based on a single (123)I-iodobenzovesamicol static scan preserving the quantification accuracy. Three acquisition times were tested comparatively to a kinetic analysis using MRTM2.

METHODS

Six healthy volunteers underwent a dynamic SPECT acquisition comprising 14 frames over 28 h and an MR imaging scan. MR images were automatically segmented, providing the volumes of 19 regions of interest (ROIs). SPECT datasets were coregistered with MR images, and regional time-activity curves were derived. For each ROI, a complete MRTM2 pharmacokinetic analysis, using the cerebellar hemispheres as the reference region, led to the calculation of a (123)I-iodobenzovesamicol-to-VAChT binding parameter, the nondisplaceable binding potential (BP(ND-MRTM2)). A simplified analysis was also performed at 5, 8, and 28 h after injection, providing a simplified BP(ND), given as BP(ND-t) = C(ROI) - C(cerebellar hemispheres)/C(cerebellar hemispheres), with C being the averaged radioactive concentration.

RESULTS

No significant difference was found among BP(ND-5 h), BP(ND-8 h), and BP(ND-MRTM2) in any of the extrastriatal regions explored. BP(ND-28 h) was significantly higher than BP(ND-5 h), BP(ND-8 h), and BP(ND-MRTM2) in 9 of the 17 regions explored (P < 0.05). BP(ND-5 h), BP(ND-8 h), and BP(ND-28 h) correlated significantly with BP(ND-MRTM2) (P < 0.05; ρ = 0.99, 0.98, and 0.92, respectively). In the striatum, BP(ND-28 h) was significantly higher than BP(ND-5 h) and BP(ND-8 h). BP(ND-5 h) differed significantly from BP(ND-MRTM2) (P < 0.05), with BP(ND-5 h) being 43.6% lower.

CONCLUSION

In the extrastriatal regions, a single acquisition at 5 or 8 h after injection provides quantitative results similar to a pharmacokinetic analysis. However, with the highest correlation and accuracy, 5 h is the most suitable time to perform an accurate (123)I-iodobenzovesamicol quantification. In the striatum, none of the 3 times has led to an accurate quantification.

[(123)I]-IBVM SPECT imaging of cholinergic systems in multiple system atrophy: A specific alteration of the ponto-thalamic cholinergic pathways (Ch5-Ch6)

We evaluated in vivo the integrity of brain cholinergic pathways in Multiple System Atrophy (MSA) and the relationship between cholinergic dysfunction and motor disturbances, by measuring the vesicular acetylcholine transporter (VAChT) expression using Single Photon Emission Computed Tomography (SPECT) and [(123)I]-iodobenzovesamicol ([(123)I]-IBVM).

METHODS

Nine patients with probable MSA and 12 healthy volunteers underwent a dynamic [(123)I]-IBVM SPECT-CT scan and a magnetic resonance imaging (MRI) scan. All patients were examined with the Unified MSA Rating Scale (UMSARS; subscale I = activities of daily living (ADL), II = motor and IV = disability). CT and MRI images were used to register the dynamic SPECT image to the Montreal Neurological Institute brain template, which includes the regions of interest (ROI) of striatum and Ch1 (medial septum nucleus-hippocampus), Ch4 (nucleus basalis of Meynert-cortex) and Ch5-Ch6 (pedunculopontine and laterodorsal tegmental nuclei-thalamus) cholinergic pathways. For each ROI, pharmacokinetic modeling of regional time activity curves led to the calculation of [(123)I]-IBVM to VAChT binding potential (BPND) value, proportional to VAChT expression.

RESULTS

When compared to controls, BPND values for MSA in Ch5-Ch6 were significantly decreased in both the pedunculopontine-laterodorsal nuclei and the thalamus (p = 0.004 and p = 0.006, respectively). Additionally, thalamus BPND values were correlated with UMSARS ADL (p = 0.006), motor (p = 0.002) and disability (p = 0.02) sub-scores. UMSARS motor subscale items 13 (postural instability) and 14 (gait) were also correlated with thalamus BPND values (p = 0.04).

CONCLUSION

Ch5-Ch6 are the most affected cholinergic pathways in MSA at both cell bodies and thalamic cholinergic terminals. These results underscore the relevant role of [(123)I]-IBVM SPECT for improving our understanding of the pathophysiology in MSA.

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.

Progressive supranuclear palsy: in vivo SPECT imaging of presynaptic vesicular acetylcholine transporter with [123I]-iodobenzovesamicol

PURPOSE

To evaluate the integrity of brain cholinergic pathways in vivo in patients with progressive supranuclear palsy (PSP) by measuring the vesicular acetylcholine transporter expression at single photon emission computed tomography (SPECT) with [123I]-iodobenzovesamicol.

MATERIALS AND METHODS

All participants provided informed written consent according to institutional human ethics committee guidelines. Ten patients with PSP and 12 healthy volunteers underwent dynamic [123I]-iodobenzovesamicol SPECT and magnetic resonance (MR) imaging. CT and MR images were used to register the dynamic SPECT image to the Montreal Neurologic Institute brain template, which includes the regions of interest of the striatum and the septo-hippocampal, innominato-cortical, and ponto-thalamic cholinergic pathways. For each region of interest, pharmacokinetic modeling of regional time activity curves was used to calculate [123I]-iodobenzovesamicol to vesicular acetylcholine transporter binding potential value, proportional to vesicular acetylcholine transporter expression.

RESULTS

When compared with control participants, patients with PSP had binding potential values that were unchanged in the striatum and septohippocampal pathway, significantly lower in the anterior cingulate cortex (P=.017) in the innominatocortical pathway, and significantly decreased in the thalamus (P=.014) in the pontothalamic cholinergic pathway. In addition, binding potential values in the thalamus were positively correlated with those in the pedunculopontine nucleus (ρ=0.81, P<.004) and binding potential values in both the thalamus (ρ=-0.88, P<.001) and pedunculopontine nucleus (ρ=-0.80, P<.010) were inversely correlated with disease duration.

CONCLUSION

Cholinergic pathways were differentially affected in the PSP group, with a significant alteration of pontothalamic pathways that increased with disease progression at both cell body and terminal levels, while the innominatocortical pathway was only mildly affected, and the septohippocampal pathway and the striatum were both preserved.

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.

Synthesis and in vitro biological evaluation of carbonyl group-containing inhibitors of vesicular acetylcholine transporter

To identify selective high-affinity inhibitors of the vesicular acetylcholine transporter (VAChT), we have interposed a carbonyl group between the phenyl and piperidyl groups of the prototypical VAChT ligand vesamicol and its more potent analogues benzovesamicol and 5-aminobenzovesamicol. Of 33 compounds synthesized and tested, 6 display very high affinity for VAChT (K(i), 0.25-0.66 nM) and greater than 500-fold selectivity for VAChT over sigma(1) and sigma(2) receptors. Twelve compounds have high affinity (K(i), 1.0-10 nM) and good selectivity for VAChT. Furthermore, 3 halogenated compounds, namely, trans-3-[4-(4-fluorobenzoyl)piperidinyl]-2-hydroxy-1,2,3,4-tetrahydronaphthalene (28b) (K(i) = 2.7 nM, VAChT/sigma selectivity index = 70), trans-3-[4-(5-iodothienylcarbonyl)piperidinyl]-2-hydroxy-1,2,3,4-tetrahydronaphthalene (28h) (K(i) = 0.66 nM, VAChT/sigma selectivity index = 294), and 5-amino-3-[4-(p-fluorobenzoyl)piperidinyl]-2-hydroxy-1,2,3,4,-tetrahydronaphthalene (30b) (K(i) = 2.40 nM, VAChT/sigma selectivity index = 410) display moderate to high selectivity for VAChT. These three compounds can be synthesized with the corresponding radioisotopes so as to serve as PET/SPECT probes for imaging the VAChT in vivo.

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.