PET imaging of cholinergic deficits in rats using [18F]fluoroethoxybenzovesamicol ([18F]FEOBV)

Cholinergic neurotransmission in the anterior cingulate cortex is associated with cognitive performance in healthy older adults: Baseline characteristics of the Improving Neurological Health in Aging via Neuroplasticity-based Computerized Exercise (INHANCE) trial

Aging is associated with dysfunction in the cholinergic system, including degeneration of basal forebrain cholinergic terminals that innervate the cortex, which directly contributes to age- and disease-related cognitive decline. In this study, we used [18F]fluoroethoxybenzovesamicol ([18F]FEOBV) positron emission tomography (PET) imaging to assess the effect of age on cholinergic terminal integrity in predefined regions of interest and its relationship to cognitive performance in healthy older adults who underwent neuropsychological assessment and FEOBV PET brain imaging. Our results showed age-related reductions in FEOBV binding, particularly in the anterior cingulate cortex-the primary region of interest-as well as in the striatum, posterior cingulate cortex, and primary auditory cortex. Notably, FEOBV binding in the anterior cingulate cortex was positively correlated with cognitive performance on the NIH EXAMINER Executive Composite Score. These findings suggest that [18F] FEOBV PET imaging can be used as a reliable biomarker to assess cholinergic changes in the human brain and indicate that preserving the cholinergic integrity of the basal forebrain may help maintain cognitive function and protect against age-related cognitive decline.

Memory network and cognitive reserve are associated with preserved and stimulated cholinergic neurotransmission

The cholinergic system plays a central role in cognition and neural function, and, in Alzheimer disease (AD) and Lewy body disease (LBD), it has profound implications for cognitive impairment and dementia. The cholinergic forebrain pathway, innervating the neocortex and limbic system, is crucial for learning, memory, and other essential aspects of cognition and plays a wider role in promoting neuronal plasticity. Given the neuroplasticity processes characterizing the cholinergic regions, this system may be sensitive to modulatory phenomena such as cognitive reserve (CR). The concept of CR has been introduced to account for the fact that individual clinical presentation might be milder than expected based on neuropathology. This mismatch can be explained by individual brain reserve (BR) buildup on life experiences, lifestyles, and neurobiologic factors that are associated with resilience. Sparse findings exist suggesting that the CR might result in an increased or preserved function of the cholinergic system in AD patients, and compensatory mechanisms in the early stages of LBD. The limited availability of effective treatment for neurodegenerative dementia emphasizes the importance of CR and BR, as they play a major role in delaying or slowing disease onset and progression. This chapter will describe the involvement of the cholinergic system in neurodegenerative diseases and the tools for the in vivo assessment, focusing specifically on the evidence suggesting the possibility of its modulation by CR.

Cholinergic neurotransmission in the anterior cingulate cortex is associated with cognitive performance in healthy older adults: Baseline characteristics of the Improving Neurological Health in Aging via Neuroplasticity-based Computerized Exercise (INHANCE) trial

Aging is associated with dysfunction in the cholinergic system, including degeneration of basal forebrain cholinergic terminals that innervate the cortex, which directly contributes to age- and disease-related cognitive decline. In this study, we used [18F]fluoroethoxybenzovesamicol ([18F]FEOBV) positron emission tomography (PET) imaging to assess the effect of age on cholinergic terminal integrity in predefined regions of interest and its relationship to cognitive performance in healthy older adults who underwent neuropsychological assessment and FEOBV PET brain imaging. Our results showed age-related reductions in FEOBV binding, particularly in the anterior cingulate cortex-our primary region of interest-as well as in the striatum, posterior cingulate cortex, and primary auditory cortex. Notably, FEOBV binding in the anterior cingulate cortex was positively correlated with cognitive performance on the NIH EXAMINER Executive Composite Score. These findings suggest that [18F]FEOBV PET imaging can be used as a reliable biomarker to assess cholinergic changes in the human brain and indicate that preserving the cholinergic integrity of the basal forebrain may help maintain cognitive function and protect against age-related cognitive decline.

Cholinergic innervation topography in GBA-associated de novo Parkinson's disease patients

The most common genetic risk factors for Parkinson's disease are GBA1 mutations, encoding the lysosomal enzyme glucocerebrosidase. Patients with GBA1 mutations (GBA-PD) exhibit earlier age of onset and faster disease progression with more severe cognitive impairments, postural instability and gait problems. These GBA-PD features suggest more severe cholinergic system pathologies. PET imaging with the vesicular acetylcholine transporter ligand 18F-F-fluoroethoxybenzovesamicol (18F-FEOBV PET) provides the opportunity to investigate cholinergic changes and their relationship to clinical features in GBA-PD. The study investigated 123 newly diagnosed, treatment-naïve Parkinson's disease subjects-with confirmed presynaptic dopaminergic deficits on PET imaging. Whole-gene GBA1 sequencing of saliva samples was performed to evaluate GBA1 variants. Patients underwent extensive neuropsychological assessment of all cognitive domains, motor evaluation with the Unified Parkinson's Disease Rating Scale, brain MRI, dopaminergic PET to measure striatal-to-occipital ratios of the putamen and 18F-FEOBV PET. We investigated differences in regional cholinergic innervation between GBA-PD carriers and non-GBA1 mutation carriers (non-GBA-PD), using voxel-wise and volume of interest-based approaches. The degree of overlap between t-maps from two-sample t-test models was quantified using the Dice similarity coefficient. Seventeen (13.8%) subjects had a GBA1 mutation. No significant differences were found in clinical features and dopaminergic ratios between GBA-PD and non-GBA-PD at diagnosis. Lower 18F-FEOBV binding was found in both the GBA-PD and non-GBA-PD groups compared to controls. Dice (P < 0.05, cluster size 100) showed good overlap (0.7326) between the GBA-PD and non-GBA-PD maps. GBA-PD patients showed more widespread reduction in 18F-FEOBV binding than non-GBA-PD when compared to controls in occipital, parietal, temporal and frontal cortices (P < 0.05, FDR-corrected). In volume of interest analyses (Bonferroni corrected), the left parahippocampal gyrus was more affected in GBA-PD. De novo GBA-PD show a distinct topography of regional cholinergic terminal ligand binding. Although the Parkinson's disease groups were not distinguishable clinically, in comparison to healthy controls, GBA-PD showed more extensive cholinergic denervation compared to non-GBA-PD. A larger group is needed to validate these findings. Our results suggest that de novo GBA-PD and non-GBA-PD show differential patterns of cholinergic system changes before clinical phenotypic differences between carriers versus non-carrier groups are observable.

FEOBV-PET to quantify cortical cholinergic denervation in AD: Relationship to basal forebrain volumetry

BACKGROUND AND PURPOSE

Fluorine-18-fluoroethoxybenzovesamicol([¹⁸ F]-FEOBV) is a PET radiotracer previously used in neurodegenerative diseases to quantify brain cholinergic denervation. The current exploratory study aimed at verifying the reliability of such an approach in Alzheimer's disease (AD) by demonstrating its concordance with MRI volumetry of the cholinergic basal forebrain (ChBF).

METHODS

The sample included 12 participants evenly divided between healthy volunteers and patients with AD. All participants underwent MRI ChBF volumetry and PET imaging with [¹⁸ F]-FEOBV. Comparisons were made between the two groups, and partial correlations were performed in the AD patients between [¹⁸ F]-FEOBV uptake in specific cortical regions of interest (ROIs) and volumetry of the corresponding ChBF subareas, which include the nucleus basalis of Meynert (Ch4), and the medial septum/vertical limb of the diagonal band of Broca (Ch1/2).

RESULTS

Patients with AD showed both lower ChBF-Ch4 volumetric values and lower [¹⁸ F]-FEOBV cortical uptake than healthy volunteers. Volumes of the Ch4 subdivision were significantly correlated with the [¹⁸ F]-FEOBV uptake values observed in the relevant ROIs. Volumes of the Ch1/2, which remains relatively unaffected in AD, did not correlate with [¹⁸ F]-FEOBV uptake in the hippocampus, nor in any cortical area.

CONCLUSION

These results suggest that cortical cholinergic denervation as measured with [¹⁸ F]-FEOBV PET is proportional to ChBF atrophy measured by MRI-based volumetry, further supporting the reliability and validity of [¹⁸ F]-FEOBV PET to quantify cholinergic degeneration in AD.

Make a Left Turn: Cortico-Striatal Circuitry Mediating the Attentional Control of Complex Movements

BACKGROUND

In movement disorders such as Parkinson's disease (PD), cholinergic signaling is disrupted by the loss of basal forebrain cholinergic neurons, as well as aberrant activity in striatal cholinergic interneurons (ChIs). Several lines of evidence suggest that gait imbalance, a key disabling symptom of PD, may be driven by alterations in high-level frontal cortical and cortico-striatal processing more typically associated with cognitive dysfunction.

METHODS

Here we describe the corticostriatal circuitry that mediates the cognitive-motor interactions underlying such complex movement control. The ability to navigate dynamic, obstacle-rich environments requires the continuous integration of information about the environment with movement selection and sequencing. The cortical-attentional processing of extero- and interoceptive cues requires modulation by cholinergic activity to guide striatal movement control. Cue-derived information is "transferred" to striatal circuitry primarily via fronto-striatal glutamatergic projections.

RESULT

Evidence from parkinsonian fallers and from a rodent model reproducing the dual cholinergic-dopaminergic losses observed in these patients supports the main hypotheses derived from this neuronal circuitry-guided conceptualization of parkinsonian falls. Furthermore, in the striatum, ChIs constitute a particularly critical node for the integration of cortical with midbrain dopaminergic afferents and thus for cues to control movements.

CONCLUSION

Procholinergic treatments that enhance or rescue cortical and striatal mechanisms may improve complex movement control in parkinsonian fallers and perhaps also in older persons suffering from gait disorders and a propensity for falls. © 2021 International Parkinson and Movement Disorder Society.

First-in-human imaging and kinetic analysis of vesicular acetylcholine transporter density in the heart using [18F]FEOBV PET

In contrast to cardiac sympathetic activity which can be assessed with established PET tracers, there are currently no suitable radioligands to measure cardiac parasympathetic (cholinergic) activity. A radioligand able to measure cardiac cholinergic activity would be an invaluable clinical and research tool since cholinergic dysfunction has been associated with a wide array of pathologies (e.g., chronic heart failure, myocardial infarction, arrythmias). [18F]Fluoroethoxybenzovesamicol (FEOBV) is a cholinergic radiotracer that has been extensively validated in the brain. Whether FEOBV PET can be used to assess cholinergic activity in the heart is not known. Hence, this study aimed to evaluate the properties of FEOBV for cardiac PET imaging and cholinergic activity mapping. PET data were collected for 40 minutes after injection of 230 ± 50 MBq of FEOBV in four healthy participants (1 female; Age: 37 ± 10; BMI: 25 ± 2). Dynamic LV time activity curves were fitted with Logan graphical, 1-tissue compartment, and 2-tissue compartment models, yielding similar distribution volume estimates for each participant. Our initial data show that FEOBV PET has favorable tracer kinetics for quantification of cholinergic activity and is a promising new method for assessing parasympathetic function in the heart.

Spatial topography of the basal forebrain cholinergic projections: Organization and vulnerability to degeneration

The basal forebrain (BF) cholinergic system constitutes a heterogeneous cluster of large projection neurons that innervate the entire cortical mantle and amygdala. Cholinergic neuromodulation plays a critical role in regulating cognition and behavior, as well as maintenance of cellular homeostasis. Decades of postmortem histology research have demonstrated that the BF cholinergic neurons are selectively vulnerable to aging and age-related neuropathology in degenerative diseases such as Alzheimer's and Parkinson's diseases. Emerging evidence from in vivo neuroimaging research, which permits longitudinal tracking of at-risk individuals, indicates that cholinergic neurodegeneration might play an earlier and more pivotal role in these diseases than was previously appreciated. Despite these advances, our understanding of the organization and functions of the BF cholinergic system mostly derives from nonhuman animal research. In this chapter, we begin with a review of the topographical organization of the BF cholinergic system in rodent and nonhuman primate models. We then discuss basic and clinical neuroscience research in humans, which has started to translate and extend the nonhuman animal research using novel noninvasive neuroimaging techniques. We focus on converging evidence indicating that the selective vulnerability of cholinergic neurons in Alzheimer's and Parkinson's diseases is expressed along a rostral-caudal topography in the BF. We close with a discussion of why this topography of vulnerability in the BF may occur and why it is relevant to the clinician.

Modeling of [18F]FEOBV Pharmacokinetics in Rat Brain

PURPOSE

[¹⁸F]Fluoroethoxybenzovesamicol ([¹⁸F]FEOBV) is a radioligand for the vesicular acetylcholine transporter (VAChT), a marker of the cholinergic system. We evaluated the quantification of [¹⁸F]FEOBV in rats in control conditions and after partial saturation of VAChT using plasma and reference tissue input models and test-retest reliability.

PROCEDURE

Ninety-minute dynamic [¹⁸F]FEOBV PET scans with arterial blood sampling were performed in control rats and rats pretreated with 10 μg/kg FEOBV. Kinetic analyses were performed using one- (1TCM) and two-tissue compartmental models (2TCM), Logan and Patlak graphical analyses with metabolite-corrected plasma input, reference tissue Patlak with cerebellum as reference tissue, standard uptake value (SUV) and SUV ratio (SUVR) using 60- or 90-min acquisition. To assess test-retest reliability, two dynamic [¹⁸F]FEOBV scans were performed 1 week apart.

RESULTS

The 1TCM did not fit the data. Time-activity curves were more reliably estimated by the irreversible than the reversible 2TCM for 60 and 90 min as the influx rate K_i showed a lower coefficient of variation (COV, 14-24 %) than the volume of distribution V_T (16-108 %). Patlak graphical analysis showed a good fit to the data for both acquisition times with a COV (12-27 %) comparable to the irreversible 2TCM. For 60 min, Logan analysis performed comparably to both irreversible models (COV 14-32 %) but showed lower sensitivity to VAChT saturation. Partial saturation of VAChT did not affect model selection when using plasma input. However, poor correlations were found between irreversible 2TCM and SUV and SUVR in partially saturated VAChT states. Test-retest reliability and intraclass correlation for SUV were good.

CONCLUSION

[¹⁸F]FEOBV is best modeled using the irreversible 2TCM or Patlak graphical analysis. SUV should only be used if blood sampling is not possible.

PET Imaging of Perceptual Learning-Induced Changes in the Aged Rodent Cholinergic System

The cholinergic system enhances attention and gates plasticity, making it a major regulator of adult learning. With aging, however, progressive degeneration of the cholinergic system impairs both the acquisition of new skills and functional recovery following neurological injury. Although cognitive training and perceptual learning have been shown to enhance auditory cortical processing, their specific impact on the cholinergic system remains unknown. Here we used [¹⁸F]FEOBV, a positron emission tomography (PET) radioligand that selectively binds to the vesicular acetylcholine transporter (VAChT), as a proxy to assess whether training on a perceptual task results in increased cholinergic neurotransmission. We show for the first time that perceptual learning is associated with region-specific changes in cholinergic neurotransmission, as detected by [¹⁸F]FEOBV PET imaging and corroborated with immunohistochemistry.

Cholinergic system changes of falls and freezing of gait in Parkinson's disease

Objective

Postural instability and gait difficulties (PIGDs) represent debilitating disturbances in Parkinson's disease (PD). Past acetylcholinesterase positron emission tomography (PET) imaging studies implicate cholinergic changes as significant contributors to PIGD features. These studies were limited in quantification of striatal cholinergic synapse integrity. Vesicular acetylcholine transporter (VAChT) PET ligands are better suited for evaluation of high binding areas. We examined associations between regional VAChT expression and freezing of gait (FoG) and falls.

Methods

Ninety‐four PD subjects underwent clinical assessment and VAChT ([¹⁸F]FEOBV) PET.

Results

Thirty‐five subjects (37.2%) reported a history of falls, and 15 (16%) had observed FoG. Univariate volume‐of‐interest analyses demonstrated significantly reduced thalamic (p = 0.0016) VAChT expression in fallers compared to nonfallers. VAChT expression was significantly reduced in the striatum (p = 0.0012) and limbic archicortex (p = 0.004) in freezers compared to nonfreezers. Whole‐brain voxel‐based analyses of FEOBV PET complemented these findings and showed more granular changes associated with falling history, including the right visual thalamus (especially the right lateral geniculate nucleus [LGN]), right caudate nucleus, and bilateral prefrontal regions. Freezers had prominent VAChT expression reductions in the bilateral striatum, temporal, and mesiofrontal limbic regions.

Interpretation

Our findings confirm and extend on previous PET findings of thalamic cholinergic deficits associated with falling history and now emphasize right visual thalamus complex changes, including the right LGN. FoG status is associated with reduced VAChT expression in striatal cholinergic interneurons and the limbic archicortex. These observations suggest different cholinergic systems changes underlying falls and FoG in PD. Ann Neurol 2019;85:538–549

Brain cholinergic alterations in idiopathic REM sleep behaviour disorder: a PET imaging study with 18F-FEOBV

BACKGROUND

REM sleep behaviour disorder (RBD) occurs frequently in patients with synucleinopathies such as Parkinson's disease, dementia with Lewy body, or multiple system atrophy, but may also occur as a prodromal stage of those diseases; and is termed idiopathic RBD (iRBD) when not accompanied by other symptoms. Cholinergic degeneration of the mesopontine nuclei have been described in synucleinopathies with or without RBD, but this has not yet been explored in iRBD. We sought to assess cholinergic neuronal integrity in iRBD using PET neuroimaging with the ¹⁸F-fluoroethoxybenzovesamicol (FEOBV).

METHODS

The sample included 10 participants evenly divided between healthy subjects and patients with iRBD. Polysomnography and PET imaging with FEOBV were performed in all participants. Standardized uptake value ratios (SUVRs) were compared between the two groups using voxel wise t-tests. Non-parametric correlations were also computed in patients with iRBD between FEOBV uptake and muscle tonic and phasic activity during REM sleep.

RESULTS

Compared with healthy participants, significantly higher FEOBV uptakes were observed in patients with iRBD. The largest differences were observed in specific brainstem areas corresponding to the bulbar reticular formation, pontine coeruleus/subcoeruleus complex, tegmental periacqueductal grey, and mesopontine cholinergic nuclei. FEOBV uptake in iRBD was also higher than in controls in the ventromedial area of the thalamus, deep cerebellar nuclei, and some cortical territories (including the paracentral lobule, anterior cingulate, and orbitofrontal cortex). Significant correlation was found between muscle activity during REM sleep, and SUVR increases in both the mesopontine area and paracentral cortex.

CONCLUSION

We showed here for the first time the brain cholinergic alterations in patients with iRBD. As opposed to the cholinergic depletion described previously in RBD associated with clinical Parkinson's disease, increased cholinergic innervation was found in multiple areas in iRBD. The most significant changes were observed in brainstem areas containing structures involved in the promotion of REM sleep and muscle atonia. This suggests that iRBD might be a clinical condition in which compensatory cholinergic upregulation in those areas occurs in association with the initial phases of a neurodegenerative process leading to a clinically observable synucleinopathy.

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.

Quantification of brain cholinergic denervation in Alzheimer's disease using PET imaging with [18F]-FEOBV

¹⁸F-fluoroethoxybenzovesamicol (FEOBV) is a new PET radiotracer that binds to the vesicular acetylcholine transporter. In both animals and healthy humans, FEOBV was found sensitive and reliable to characterize presynaptic cholinergic nerve terminals in the brain. It has been used here for we believe the first time in patients with Alzheimer's disease (AD) to quantify brain cholinergic losses. The sample included 12 participants evenly divided in healthy subjects and patients with AD, all assessed with the Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA) cognitive scales. Every participant underwent three consecutive PET imaging sessions with (1) the FEOBV as a tracer of the cholinergic terminals, (2) the ¹⁸F-NAV4694 (NAV) as an amyloid-beta tracer, and (3) the ¹⁸F-Fluorodeoxyglucose (FDG) as a brain metabolism agent. Standardized uptake value ratios (SUVRs) were computed for each tracer, and compared between the two groups using voxel wise t-tests. Correlations were also computed between each tracer and the cognitive scales, as well as between FEOBV and the two other radiotracers. Results showed major reductions of FEOBV uptake in multiple cortical areas that were evident in each AD subject, and in the AD group as a whole when compared to the control group. FDG and NAV were also able to distinguish the two groups, but with lower sensitivity than FEOBV. FEOBV uptake values were positively correlated with FDG in numerous cortical areas, and negatively correlated with NAV in some restricted areas. The MMSE and MoCA cognitive scales were found to correlate significantly with FEOBV and with FDG, but not with NAV. We concluded that PET imaging with FEOBV is more sensitive than either FDG or NAV to distinguish AD patients from control subjects, and may be useful to quantify disease severity. FEOBV can be used to assess cholinergic degeneration in human, and may represent an excellent biomarker for AD.

Normal Striatal Vesicular Acetylcholine Transporter Expression in Tourette Syndrome

Considerable prior work suggests basal ganglia dysfunction in Tourette syndrome (TS). Analysis of a small number of postmortem specimens suggests deficits of some striatal interneuron populations, including striatal cholinergic interneurons. To assess the integrity of striatal cholinergic interneurons in TS, we used [¹⁸F]FEOBV positron emission tomography (PET) to quantify striatal vesicular acetylcholine transporter (VAChT) expression, a measure of cholinergic terminal density, in human TS and control subjects. We found no evidence of striatal cholinergic deficits. Discrepant imaging and postmortem analysis results may reflect agonal or postmortem changes, medication effects, or significant disease heterogeneity.

Sigma-1 Agonist Binding in the Aging Rat Brain: a MicroPET Study with [(11)C]SA4503

PURPOSE

Sigma-1 receptor ligands modulate the release of several neurotransmitters and intracellular calcium signaling. We examined the binding of a radiolabeled sigma-1 agonist in the aging rat brain with positron emission tomography (PET).

PROCEDURES

Time-dependent uptake of [(11)C]SA4503 was measured in the brain of young (1.5 to 3 months) and aged (18 to 32 months) Wistar Hannover rats, and tracer-kinetic models were fitted to this data, using metabolite-corrected plasma radioactivity as input function.

RESULTS

In aged animals, the injected probe was less rapidly metabolized and cleared. Logan graphical analysis and a 2-tissue compartment model (2-TCM) fit indicated changes of total distribution volume (V T) and binding potential (BP ND) of the tracer. BP ND was reduced particularly in the (hypo)thalamus, pons, and medulla.

CONCLUSIONS

Some areas showed reductions of ligand binding with aging whereas binding in other areas (cortex) was not significantly affected.

Syntheses and evaluation of carbon-11- and fluorine-18-radiolabeled pan-tropomyosin receptor kinase (Trk) inhibitors: exploration of the 4-aza-2-oxindole scaffold as Trk PET imaging agents

Tropomyosin receptor kinases (TrkA/B/C) are critically involved in the development of the nervous system, in neurological disorders as well as in multiple neoplasms of both neural and non-neural origins. The development of Trk radiopharmaceuticals would offer unique opportunities toward a more complete understanding of this emerging therapeutic target. To that end, we first developed [(11)C]GW441756 ([(11)C]9), a high affinity photoisomerizable pan-Trk inhibitor, as a lead radiotracer for our positron emission tomography (PET) program. Efficient carbon-11 radiolabeling afforded [(11)C]9 in high radiochemical yields (isolated RCY, 25.9% ± 5.7%). In vitro autoradiographic studies in rat brain and TrkB-expressing human neuroblastoma cryosections confirmed that [(11)C]9 specifically binds to Trk receptors in vitro. MicroPET studies revealed that binding of [(11)C]9 in the rodent brain was mostly nonspecific despite initial high brain uptake (SUVmax = 2.0). Modeling studies of the 4-aza-2-oxindole scaffold led to the successful identification of a small series of high affinity fluorinated and methoxy derivatized pan-Trk inhibitors based on our lead compound 9. Out of this series, the fluorinated compound 10 was selected for initial evaluation and radiolabeled with fluorine-18 (isolated RCY, 2.5% ± 0.6%). Compound [(18)F]10 demonstrated excellent Trk selectivity in a panel of cancer relevant kinase targets and a promising in vitro profile in tumors and brain sections but high oxidative metabolic susceptibility leading to nonspecific brain distribution in vivo. The information gained in this study will guide further exploration of the 4-aza-2-oxindole scaffold as a lead for Trk PET ligand development.

Nonamyloid PET biomarkers and Alzheimer's disease: current and future perspectives

ABSTRACT 

Recent advances in neurobiology and PET have helped redefine Alzheimer's disease (AD) as a dynamic pathophysiological process, clinically characterized by preclinical, mild cognitive impairment due to AD and dementia stages. Though a majority of PET studies conducted within these populations have to date focused on β-amyloid, various ‘nonamyloid’ radiopharmaceuticals exist for evaluating neurodegeneration, neuroinflammation and perturbations in neurotransmission across the spectrum of AD. Importantly, findings using such tracers have been shown to correlate with various clinical, cognitive and behavioral measures. In the context of a growing shift toward early diagnosis and symptomatic and disease-modifying clinical trials, nonamyloid PET radiotracers will prove of use, and, potentially, contribute to improved therapeutic prospects for AD.

Deficit in sustained attention following selective cholinergic lesion of the pedunculopontine tegmental nucleus in rat, as measured with both post-mortem immunocytochemistry and in vivo PET imaging with [¹⁸F]fluoroethoxybenzovesamicol

Cholinergic neurons of the pedunculopontine tegmental nucleus (PPTg) are thought to be involved in cognitive functions such as sustained attention, and lesions of these cells have been documented in patients showing fluctuations of attention such as in Parkinson's disease or dementia with Lewy Body. Animal studies have been conducted to support the role of these cells in attention, but the lesions induced in these animals were not specific to the cholinergic PPTg system, and were assessed by post-mortem methods remotely performed from the in vivo behavioral assessments. Moreover, sustained attention have not been directly assessed in these studies, but rather deduced from indirect measurements. In the present study, rats were assessed on the 5-Choice Serial Reaction Time Task (5-CSRTT), and a specific measure of variability in response latency was created. Animals were observed both before and after selective lesion of the PPTg cholinergic neurons. Brain cholinergic denervation was assessed both in vivo and ex vivo, using PET imaging with [(18)F]fluoroethoxybenzovesamicol ([(18)F]FEOBV) and immunocytochemistry respectively. Results showed that the number of correct responses and variability in response latency in the 5-CSRTT were the only behavioral measures affected following the lesions. These measures were found to correlate significantly with the number of PPTg cholinergic cells, as measured with both [(18)F]FEOBV and immunocytochemistry. This suggests the primary role of the PPTg cholinergic cells in sustained attention. It also allows to reliably use the PET imaging with [(18)F]FEOBV for the purpose of assessing the relationship between behavior and cholinergic innervation in living animals.

Regional brain imaging of vesicular acetylcholine transporter using o-[125 I]iodo-trans-decalinvesamicol as a new potential imaging probe

In this study, the regional rat brain distribution of radioiodinated o-iodo-trans-decalinvesamicol ([(125) I]OIDV) was determined in vivo to evaluate its potential as a single-photon emission computed tomography (SPECT) imaging probe for vesicular acetylcholine transporter (VAChT). Following intravenous injection, [(125) I]OIDV passed freely across the blood-brain barrier and accumulated in rat brain. The accumulation of [(125) I]OIDV in rat brain was significantly reduced by coadministration of (+/-)-vesamicol (0.125 µmol). In contrast, the coadministration of σ-receptor ligands, such as (+)-pentazocine (0.125 µmol) as a σ-1 receptor ligand and (+)-3-(3-hydroxyphenyl)-N-propylpiperidine (0.125 µmol) as a σ-1 and σ-2 receptor ligands, barely affected the accumulation of [(125) I]OIDV in rat brain. These findings in vivo were corroborated by autoradiographic analysis ex vivo. The authors found that the tracer binds with pharmacological selectivity to VAChT in rat brain and predicted that it may likewise serve in translational SPECT imaging studies of this marker in the integrity of cholinergic innervations.

Cholinergic Depletion in Alzheimer's Disease Shown by [ (18) F]FEOBV Autoradiography

Rationale. Alzheimer's Disease (AD) is a neurodegenerative condition characterized in part by deficits in cholinergic basalocortical and septohippocampal pathways. [¹⁸F]Fluoroethoxybenzovesamicol ([¹⁸F]FEOBV), a Positron Emission Tomography ligand for the vesicular acetylcholine transporter (VAChT), is a potential molecular agent to investigate brain diseases associated with presynaptic cholinergic losses. Purpose. To demonstrate this potential, we carried out an [¹⁸F]FEOBV autoradiography study to compare postmortem brain tissues from AD patients to those of age-matched controls. Methods. [¹⁸F]FEOBV autoradiography binding, defined as the ratio between regional grey and white matter, was estimated in the hippocampus (13 controls, 8 AD) and prefrontal cortex (13 controls, 11 AD). Results. [¹⁸F]FEOBV binding was decreased by 33% in prefrontal cortex, 25% in CA3, and 20% in CA1. No changes were detected in the dentate gyrus of the hippocampus, possibly because of sprouting or upregulation toward the resilient glutamatergic neurons of the dentate gyrus. Conclusion. This is the first demonstration of [¹⁸F]FEOBV focal binding changes in cholinergic projections to the cortex and hippocampus in AD. Such cholinergic synaptic (and more specifically VAChT) alterations, in line with the selective basalocortical and septohippocampal cholinergic losses documented in AD, indicate that [¹⁸F]FEOBV is indeed a promising ligand to explore cholinergic abnormalities in vivo.

Concordance between in vivo and postmortem measurements of cholinergic denervation in rats using PET with [18F]FEOBV and choline acetyltransferase immunochemistry

BACKGROUND

Fluorine-18 fluoroethoxybenzovesamicol ([18F]FEOBV) is a radioligand for the selective imaging of the vesicular acetylcholine transporter with positron emission tomography (PET). The current study demonstrates that pathological cortical cholinergic deafferentation can be quantified in vivo with [18F]FEOBV PET, yielding analogous results to postmortem histological techniques.

METHODS

Fifteen male rats (3 months old) underwent a cerebral infusion of 192 IgG-saporin at the level of the nucleus basalis magnocellularis. They were scanned using [18F]FEOBV PET, then sacrificed, and their brain tissues collected for immunostaining and quantification of cholinergic denervation using optical density (OD).

RESULTS

For both PET binding and postmortem OD, the highest losses were found in the cortical areas, with the highest reductions in the orbitofrontal, sensorimotor, and cingulate cortices. In addition, OD quantification in the affected areas accurately predicts [18F]FEOBV uptake in the same regions when regressed linearly.

CONCLUSIONS

These findings support [18F]FEOBV as a reliable imaging agent for eventual use in human neurodegenerative conditions in which cholinergic losses are an important aspect.