Brain imaging of nicotinic receptors in Alzheimer's disease

What We Have Gained from Ibogaine: α3β4 Nicotinic Acetylcholine Receptor Inhibitors as Treatments for Substance Use Disorders

For decades, ibogaine─the main psychoactive alkaloid found in Tabernanthe iboga─has been investigated as a possible treatment for substance use disorders (SUDs) due to its purported ability to interrupt the addictive properties of multiple drugs of abuse. Of the numerous pharmacological actions of ibogaine and its derivatives, the inhibition of α3β4 nicotinic acetylcholine receptors (nAChRs), represents a probable mechanism of action for their apparent anti-addictive activity. In this Perspective, we examine several classes of compounds that have been discovered and developed to target α3β4 nAChRs. Specifically, by focusing on compounds that have proven efficacious in pre-clinical models of drug abuse and have been evaluated clinically, we highlight the promising potential of the α3β4 nAChRs as viable targets to treat a wide array of SUDs. Additionally, we discuss the challenges faced by the existing classes of α3β4 nAChR ligands that must be overcome to develop them into therapeutic treatments.

Evidence for a protective effect of the loss of α4-containing nicotinic acetylcholine receptors on Aβ-related neuropathology in Tg2576 mice

Introduction

Loss of cholinergic neurons as well as α4β2* (* = containing) nicotinic acetylcholine receptors (nAChRs) is a prominent feature of Alzheimer's disease (AD). Specifically, amyloid β (Aβ), the principal pathogenic factor of AD, is a high affinity ligand for nAChRs. Yet, the pathophysiological role of nAChRs in AD is not well established.

Methods

In the present study, we have investigated the effects of the loss of α4* nAChRs on the histological alterations of the Tg2576 mouse model of AD (APPswe) crossing hemizygous APPswe mice with mice carrying the genetic inactivation of α4 nAChR subunit (α4KO).

Results

A global decrease in Aβ plaque load was observed in the forebrain of APPswe/α4KO mice in comparison with APPswe mice, that was particularly marked in neocortex of 15 month-old mice. At the same age, several alterations in synaptophysin immunoreactivity were observed in cortico-hippocampal regions of APPswe mice that were partially counteracted by α4KO. The analysis of the immunoreactivity of specific astroglia (glial fibrillary acidic protein, GFAP) and microglia (ionized calcium-binding adapter molecule, Iba1) markers showed an increase in the number as well as in the area occupied by these cells in APPswe mice that were partially counteracted by α4KO.

Conclusion

Overall, the present histological study points to a detrimental role of α4* nAChRs that may be specific for Aβ-related neuropathology.

Recent developments in the design and synthesis of benzylpyridinium salts: Mimicking donepezil hydrochloride in the treatment of Alzheimer's disease

Background: Alzheimer's disease (AD) is an advanced and irreversible degenerative disease of the brain, recognized as the key reason for dementia among elderly people. The disease is related to the reduced level of acetylcholine (ACh) in the brain that interferes with memory, learning, emotional, and behavior responses. Deficits in cholinergic neurotransmission are responsible for the creation and progression of numerous neurochemical and neurological illnesses such as AD. Aim: Herein, focusing on the fact that benzylpyridinium salts mimic the structure of donepezil hydrochlorideas a FDA-approved drug in the treatment of AD, their synthetic approaches and inhibitory activity against cholinesterases (ChEs) were discussed. Also, molecular docking results and structure-activity relationship (SAR) as the most significant concept in drug design and development were considered to introduce potential lead compounds. Key scientific concepts: AChE plays a chief role in the end of nerve impulse transmission at the cholinergic synapses. In this respect, the inhibition of AChE has been recognized as a key factor in the treatment of AD, Parkinson's disease, senile dementia, myasthenia gravis, and ataxia. A few drugs such as donepezil hydrochloride are prescribed for the improvement of cognitive dysfunction and memory loss caused by AD. Donepezil hydrochloride is a piperidine-containing compound, identified as a well-known member of the second generation of AChE inhibitors. It was established to treat AD when it was assumed that the disease is associated with a central cholinergic loss in the early 1980s. In this review, synthesis and anti-ChE activity of a library of benzylpyridinium salts were reported and discussed based on SAR studies looking for the most potent substituents and moieties, which are responsible for inducing the desired activity even more potent than donepezil. It was found that linking heterocyclic moieties to the benzylpyridinium salts leads to the potent ChE inhibitors. In this respect, this review focused on the recent reports on benzylpyridinium salts and addressed the structural features and SARs to get an in-depth understanding of the potential of this biologically improved scaffold in the drug discovery of AD.

Volume Analysis of Brain Cognitive Areas in Alzheimer's Disease: Interim 3-Year Results from the ASCOMALVA Trial

Background:

Cerebral atrophy is a common feature of several neurodegenerative disorders, including Alzheimer’s disease (AD). In AD, brain atrophy is associated with loss of gyri and sulci in the temporal and parietal lobes, and in parts of the frontal cortex and cingulate gyrus.

Objective:

The ASCOMALVA trial has assessed, in addition to neuropsychological analysis, whether the addition of the cholinergic precursor choline alphoscerate to treatment with donepezil has an effect on brain volume loss in patients affected by AD associated with cerebrovascular injury.

Methods:

56 participants to the randomized, placebo-controlled, double-blind ASCOMALVA trial were assigned to donepezil + placebo (D + P) or donepezil + choline alphoscerate (D + CA) treatments and underwent brain magnetic resonance imaging and neuropsychological tests every year for 4 years. An interim analysis of 3-year MRI data was performed by voxel morphometry techniques.

Results:

The D + P group (n = 27) developed atrophy of the gray and white matter with concomitant increase in ventricular space volume. In the D + CA group (n = 29) the gray matter atrophy was less pronounced compared to the D + P group in frontal and temporal lobes, hippocampus, and amygdala. These morphological data are consistent with the results of the neuropsychological tests.

Conclusion:

Our findings indicate that the addition of choline alphoscerate to standard treatment with the cholinesterase inhibitor donepezil counters to some extent the loss in volume occurring in some brain areas of AD patients. The observation of parallel less pronounced decrease in cognitive and functional tests in patients with the same treatment suggests that the morphological changes observed may have functional relevance.

The Cholinergic System, the Adrenergic System and the Neuropathology of Alzheimer's Disease

Neurodegenerative diseases are a major public health problem worldwide with a wide spectrum of symptoms and physiological effects. It has been long reported that the dysregulation of the cholinergic system and the adrenergic system are linked to the etiology of Alzheimer’s disease. Cholinergic neurons are widely distributed in brain regions that play a role in cognitive functions and normal cholinergic signaling related to learning and memory is dependent on acetylcholine. The Locus Coeruleus norepinephrine (LC-NE) is the main noradrenergic nucleus that projects and supplies norepinephrine to different brain regions. Norepinephrine has been shown to be neuroprotective against neurodegeneration and plays a role in behavior and cognition. Cholinergic and adrenergic signaling are dysregulated in Alzheimer’s disease. The degeneration of cholinergic neurons in nucleus basalis of Meynert in the basal forebrain and the degeneration of LC-NE neurons were reported in Alzheimer’s disease. The aim of this review is to describe current literature on the role of the cholinergic system and the adrenergic system (LC-NE) in the pathology of Alzheimer’s disease and potential therapeutic implications.

Selective coactivation of α7- and α4β2-nicotinic acetylcholine receptors reverses beta-amyloid-induced synaptic dysfunction

Beta-amyloid (Aβ) has been recognized as an early trigger in the pathogenesis of Alzheimer's disease (AD) leading to synaptic and cognitive impairments. Aβ can alter neuronal signaling through interactions with nicotinic acetylcholine receptors (nAChRs), contributing to synaptic dysfunction in AD. The three major nAChR subtypes in the hippocampus are composed of α7-, α4β2-, and α3β4-nAChRs. Aβ selectively affects α7- and α4β2-nAChRs, but not α3β4-nAChRs in hippocampal neurons, resulting in neuronal hyperexcitation. However, how nAChR subtype selectivity for Aβ affects synaptic function in AD is not completely understood. Here, we showed that Aβ associated with α7- and α4β2-nAChRs but not α3β4-nAChRs. Computational modeling suggested that two amino acids in α7-nAChRs, arginine 208 and glutamate 211, were important for the interaction between Aβ and α7-containing nAChRs. These residues are conserved only in the α7 and α4 subunits. We therefore mutated these amino acids in α7-containing nAChRs to mimic the α3 subunit and found that mutant α7-containing receptors were unable to interact with Aβ. In addition, mutant α3-containing nAChRs mimicking the α7 subunit interact with Aβ. This provides direct molecular evidence for how Aβ selectively interacted with α7- and α4β2-nAChRs, but not α3β4-nAChRs. Selective coactivation of α7- and α4β2-nAChRs also sufficiently reversed Aβ-induced AMPA receptor dysfunction, including Aβ-induced reduction of AMPA receptor phosphorylation and surface expression in hippocampal neurons. Moreover, costimulation of α7- and α4β2-nAChRs reversed the Aβ-induced disruption of long-term potentiation. These findings support a novel mechanism for Aβ's impact on synaptic function in AD, namely, the differential regulation of nAChR subtypes.

Basal Forebrain Cholinergic Circuits and Signaling in Cognition and Cognitive Decline

Recent work continues to place cholinergic circuits at center stage for normal executive and mnemonic functioning and provides compelling evidence that the loss of cholinergic signaling and cognitive decline are inextricably linked. This Review focuses on the last few years of studies on the mechanisms by which cholinergic signaling contributes to circuit activity related to cognition. We attempt to identify areas of controversy, as well as consensus, on what is and is not yet known about how cholinergic signaling in the CNS contributes to normal cognitive processes. In addition, we delineate the findings from recent work on the extent to which dysfunction of cholinergic circuits contributes to cognitive decline associated with neurodegenerative disorders.

Molecular Pathogenesis of Alzheimer's Disease: An Update

Dementia is a chronic or progressive syndrome, characterized by impaired cognitive capacity beyond what could be considered a consequence of normal aging. It affects the memory, thinking process, orientation, comprehension, calculation, learning ability, language, and judgment; although awareness is usually unaffected. Alzheimer's disease (AD) is the most common form of dementia; symptoms include memory loss, difficulty solving problems, disorientation in time and space, among others. The disease was first described in 1906 at a conference in Tubingen, Germany by Alois Alzheimer. One hundred and ten years since its first documentation, many aspects of the pathophysiology of AD have been discovered and understood, however gaps of knowledge continue to exist. This literature review summarizes the main underlying neurobiological mechanisms in AD, including the theory with emphasis on amyloid peptide, cholinergic hypothesis, glutamatergic neurotransmission, the role of tau protein, and the involvement of oxidative stress and calcium.

Selected PET Radioligands for Ion Channel Linked Neuroreceptor Imaging: Focus on GABA, NMDA and nACh Receptors

Positron emission tomography (PET) neuroimaging of ion channel linked receptors is a developing area of preclinical and clinical research. The present review focuses on recent advances with radiochemistry, preclinical and clinical PET imaging studies of three receptors that are actively pursued in neuropsychiatric drug discovery: namely the γ-aminobutyric acid-benzodiazapine (GABA) receptor, nicotinic acetylcholine receptor (nAChR), and N-methyl-D-aspartate (NMDA) receptor. Recent efforts to develop new PET radioligands for these targets with improved brain uptake, selectivity, stability and pharmacokinetics are highlighted.

Radiation dosimetry of the α4β2 nicotinic receptor ligand (+)-[18F]flubatine, comparing preclinical PET/MRI and PET/CT to first-in-human PET/CT results

BACKGROUND

Both enantiomers of [18F]flubatine are new radioligands for neuroimaging of α4β2 nicotinic acetylcholine receptors with positron emission tomography (PET) exhibiting promising pharmacokinetics which makes them attractive for different clinical questions. In a previous preclinical study, the main advantage of (+)-[18F]flubatine compared to (-)-[18F]flubatine was its higher binding affinity suggesting that (+)-[18F]flubatine might be able to detect also slight reductions of α4β2 nAChRs and could be more sensitive than (-)-[18F]flubatine in early stages of Alzheimer's disease. To support the clinical translation, we investigated a fully image-based internal dosimetry approach for (+)-[18F]flubatine, comparing mouse data collected on a preclinical PET/MRI system to piglet and first-in-human data acquired on a clinical PET/CT system. Time-activity curves (TACs) were obtained from the three species, the animal data extrapolated to human scale, exponentially fitted and the organ doses (OD), and effective dose (ED) calculated with OLINDA.

RESULTS

The excreting organs (urinary bladder, kidneys, and liver) receive the highest organ doses in all species. Hence, a renal/hepatobiliary excretion pathway can be assumed. In addition, the ED conversion factors of 12.1 μSv/MBq (mice), 14.3 μSv/MBq (piglets), and 23.0 μSv/MBq (humans) were calculated which are well within the order of magnitude as known from other 18F-labeled radiotracers.

CONCLUSIONS

Although both enantiomers of [18F]flubatine exhibit different binding kinetics in the brain due to the respective affinities, the effective dose revealed no enantiomer-specific differences among the investigated species. The preclinical dosimetry and biodistribution of (+)-[18F]flubatine was shown and the feasibility of a dose assessment based on image data acquired on a small animal PET/MR and a clinical PET/CT was demonstrated. Additionally, the first-in-human study confirmed the tolerability of the radiation risk of (+)-[18F]flubatine imaging which is well within the range as caused by other 18F-labeled tracers. However, as shown in previous studies, the ED in humans is underestimated by up to 50 % using preclinical imaging for internal dosimetry. This fact needs to be considered when applying for first-in-human studies based on preclinical biokinetic data scaled to human anatomy.

Automation of the Radiosynthesis of Six Different 18F-labeled radiotracers on the AllinOne

Background

Fast implementation of positron emission tomography (PET) into clinical and preclinical studies highly demands automated synthesis for the preparation of PET radiopharmaceuticals in a safe and reproducible manner. The aim of this study was to develop automated synthesis methods for these six ¹⁸F-labeled radiopharmaceuticals produced on a routine basis at the University of Pennsylvania using the AllinOne synthesis module.

Results

The development of automated syntheses with varying complexity was accomplished including HPLC purification, SPE procedures and final formulation with sterile filtration. The six radiopharmaceuticals were obtained in high yield and high specific activity with full automation on the AllinOne synthesis module under current good manufacturing practice (cGMP) guidelines.

Conclusion

The study demonstrates the versatility of this synthesis module for the preparation of a wide variety of ¹⁸F-labeled radiopharmaceuticals for PET imaging studies.

Application of cross-species PET imaging to assess neurotransmitter release in brain

RATIONALE

This review attempts to summarize the current status in relation to the use of positron emission tomography (PET) imaging in the assessment of synaptic concentrations of endogenous mediators in the living brain.

OBJECTIVES

Although PET radioligands are now available for more than 40 CNS targets, at the initiation of the Innovative Medicines Initiative (IMI) "Novel Methods leading to New Medications in Depression and Schizophrenia" (NEWMEDS) in 2009, PET radioligands sensitive to an endogenous neurotransmitter were only validated for dopamine. NEWMEDS work-package 5, "Cross-species and neurochemical imaging (PET) methods for drug discovery", commenced with a focus on developing methods enabling assessment of changes in extracellular concentrations of serotonin and noradrenaline in the brain.

RESULTS

Sharing the workload across institutions, we utilized in vitro techniques with cells and tissues, in vivo receptor binding and microdialysis techniques in rodents, and in vivo PET imaging in non-human primates and humans. Here, we discuss these efforts and review other recently published reports on the use of radioligands to assess changes in endogenous levels of dopamine, serotonin, noradrenaline, γ-aminobutyric acid, glutamate, acetylcholine, and opioid peptides. The emphasis is on assessment of the availability of appropriate translational tools (PET radioligands, pharmacological challenge agents) and on studies in non-human primates and human subjects, as well as current challenges and future directions.

CONCLUSIONS

PET imaging directed at investigating changes in endogenous neurochemicals, including the work done in NEWMEDS, have highlighted an opportunity to further extend the capability and application of this technology in drug development.

Internal dose assessment of (-)-18F-flubatine, comparing animal model datasets of mice and piglets with first-in-human results

(-)-(18)F-flubatine is a promising tracer for neuroimaging of nicotinic acetylcholine receptors (nAChRs), subtype α4β2, using PET. Radiation doses after intravenous administration of the tracer in mice and piglets were assessed to determine the organ doses (ODs) and the effective dose (ED) to humans. The results were compared with subsequent clinical investigations in human volunteers.

METHODS

Twenty-seven female CD1 mice (weight ± SD, 28.2 ± 2.1 g) received intravenous injection of 0.75 ± 0.33 MBq of (-)-(18)F-flubatine. Up to 240 min after injection, 3 animals per time point were sacrificed and the organs harvested, weighed, and counted in a γ counter to determine mass and activity, respectively. Furthermore, whole-body PET scans of 5 female piglets (age ± SD, 44 ± 3 d; weight ± SD, 13.7 ± 1.7 kg) and 3 humans (2 men and 1 woman; age ± SD, 59.6 ± 3.9 y; weight ± SD, 74.3 ± 3.1 kg) were obtained up to 236 min (piglets) and 355 min (humans) after injection of 186.6 ± 7.4 and 353.7 ± 10.2 MBq of (-)-(18)F-flubatine, respectively, using a PET/CT scanner. The CT was used for delineation of the organs. Exponential curves were fitted to the time-activity-data, and time and mass scales were adapted to the human anatomy. The ODs were calculated using OLINDA/EXM (version 1.0); EDs were calculated with the tissue-weighting factors of ICRP103.

RESULTS

After the injection of (-)-(18)F-flubatine, there were no adverse or clinically detectable pharmacologic effects in any of the subjects. The highest activities after injection were found in the kidneys, urinary bladder, and liver. The urinary bladder receives the highest OD in all investigated species, followed by the kidneys and the liver for animals and humans, respectively. On the basis of mouse, piglet, and human kinetic data, the projected human ED of (-)-(18)F-flubatine was estimated to be 12.5 μSv/MBq in mice, 14.7 ± 0.7 μSv/MBq in piglets, and 23.4 ± 0.4 μSv/MBq in humans.

CONCLUSION

As has been demonstrated for other PET radiotracers, preclinical (i.e., animal-derived) dosimetry underestimates the ED to humans, in the current case of (-)-(18)F-flubatine by 34%-44%.

Nicotinic acetylcholine receptors in attention circuitry: the role of layer VI neurons of prefrontal cortex

Cholinergic modulation of prefrontal cortex is essential for attention. In essence, it focuses the mind on relevant, transient stimuli in support of goal-directed behavior. The excitation of prefrontal layer VI neurons through nicotinic acetylcholine receptors optimizes local and top-down control of attention. Layer VI of prefrontal cortex is the origin of a dense feedback projection to the thalamus and is one of only a handful of brain regions that express the α5 nicotinic receptor subunit, encoded by the gene chrna5. This accessory nicotinic receptor subunit alters the properties of high-affinity nicotinic receptors in layer VI pyramidal neurons in both development and adulthood. Studies investigating the consequences of genetic deletion of α5, as well as other disruptions to nicotinic receptors, find attention deficits together with altered cholinergic excitation of layer VI neurons and aberrant neuronal morphology. Nicotinic receptors in prefrontal layer VI neurons play an essential role in focusing attention under challenging circumstances. In this regard, they do not act in isolation, but rather in concert with cholinergic receptors in other parts of prefrontal circuitry. This review urges an intensification of focus on the cellular mechanisms and plasticity of prefrontal attention circuitry. Disruptions in attention are one of the greatest contributing factors to disease burden in psychiatric and neurological disorders, and enhancing attention may require different approaches in the normal and disordered prefrontal cortex.

Evaluation of [18F]Nifene biodistribution and dosimetry based on whole-body PET imaging of mice

INTRODUCTION

[(18)F]Nifene is a novel radiotracer specific to the nicotinic acetylcholine α4β2 receptor class. In preparation for using this tracer in humans we have performed whole-body PET studies in mice to evaluate the in vivo biodistribution and dosimetry of [(18)F]Nifene.

METHODS

Seven BALB/c mice (3 males, 4 females) received IV tail injections of [(18)F]Nifene and were scanned for 2 h in an Inveon dedicated PET scanner. Each animal also received a high resolution CT scan using an Inveon CT. The CT images were used to draw volume of interest (VOI) on the following organs: brain, large intestine, small intestine, stomach, heart, kidneys, liver, lungs, pancreas, bone, spleen, testes, thymus, uterus and urinary bladder. All organ time activity curves had the decay correction reversed and were normalized to the injected activity. The area under the normalized curves was then used to compute the residence times in each organ. The absorbed doses in mouse organs were computed using the RAdiation Dose Assessment Resource (RADAR) animal models for dose assessment. The residence times in mouse organs were converted to human values using scale factors based on differences between organ and body weights. OLINDA 1.1 software was used to compute the absorbed human doses in multiple organs for both female and male phantoms.

RESULTS

The highest mouse residence times were found in urinary bladder, liver, bone, small intestine and kidneys. The largest doses in mice were found in urinary bladder and kidneys for both females and males. The elimination of radiotracer was primarily via kidney and urinary bladder with the urinary bladder being the limiting organ. The projected human effective doses were 1.51E-02 mSv/MBq for the adult male phantom and 1.65E-02 mSv/MBq for the adult female model phantom.

CONCLUSION

This study indicates that the whole-body mouse imaging can be used as a preclinical tool for initial estimation of the absorbed doses of [(18)F]Nifene in humans.

Occupancy of α7 Nicotinic Acetylcholine Receptors in the Brain by Tropisetron: A Positron Emission Tomography Study Using [(11)C]CHIBA-1001 in Healthy Human Subjects

OBJECTIVE

Agonists of α7-nicotinic acetylcholine receptors (nAChRs) have been developed as potential therapeutic drugs for neuropsychiatric diseases such as schizophrenia and Alzheimer's disease. Positron emission tomography (PET) is a noninvasive brain imaging technique to measure receptor occupancy in the living human brain. Although much effort has been expended to create specific PET radioligands for α7-nAChRs in the brain, only 4-[(11)C]methylphenyl-1,4-diazabicyclo[3.2.2.]nonane-4-carboxylate ([(11)C]CHIBA-1001) is currently available for clinical studies. In contrast, two 5-hydroxytryptamine-3 (5-HT(3)) receptor antagonists, tropisetron and ondansetron, have been used to treat patients with chemotherapy-induced or postoperative nausea and vomiting. Furthermore, tropisetron, but not ondansetron, possesses high affinity for α7-nAChRs. In the present study, we evaluated the receptor occupancy in the human brain after a single oral administration of tropisetron and ondansetron using [(11)C]CHIBA-1001 and PET.

METHODS

Two serial dynamic PET scans using [(11)C]CHIBA-1001 in healthy non-smoking male subjects were performed before and after receiving an oral administration of these medications.

RESULTS

A single oral administration of tropisetron, but not ondansetron, decreased the total distribution volume of [(11)C]CHIBA-1001 in the human brain.

CONCLUSION

This study shows that tropisetron, but not ondansetron, could bind to α7-nAChRs in the human brain after a single oral administration. Therefore, [(11)C]CHIBA-1001 may be a useful PET radioligand to measure the occupancy of α7-nAChRs in the human brain.