Specific α4β2 nicotinic acetylcholine receptor binding of [F-18]nifene in the rhesus monkey

How can I measure brain acetylcholine levels in vivo? Advantages and caveats of commonly used approaches

The neurotransmitter acetylcholine (ACh) plays a central role in the regulation of multiple cognitive and behavioral processes, including attention, learning, memory, motivation, anxiety, mood, appetite, and reward. As a result, understanding ACh dynamics in the brain is essential for elucidating the neural mechanisms underlying these processes. In vivo measurements of ACh in the brain have been challenging because of the low concentrations and rapid turnover of this neurotransmitter. Here, we review a number of techniques that have been developed to measure ACh levels in the brain in vivo. We follow this with a deeper focus on use of genetically encoded fluorescent sensors coupled with fiber photometry, an accessible technique that can be used to monitor neurotransmitter release with high temporal resolution and specificity. We conclude with a discussion of methods for analyzing fiber photometry data and their respective advantages and disadvantages. The development of genetically encoded fluorescent ACh sensors is revolutionizing the field of cholinergic signaling, allowing temporally precise measurement of ACh release in awake, behaving animals. Use of these sensors has already begun to contribute to a mechanistic understanding of cholinergic modulation of complex behaviors.

Abnormal [18 F]NIFENE binding in transgenic 5xFAD mouse model of Alzheimer's disease: In vivo PET/CT imaging studies of α4β2* nicotinic acetylcholinergic receptors and in vitro correlations with Aβ plaques

Since cholinergic dysfunction has been implicated in Alzheimer's disease (AD), the effects of Aβ plaques on nicotinic acetylcholine receptors (nAChRs) α4β2* subtype were studied using the transgenic 5xFAD mouse model of AD. Using the PET radiotracer [¹⁸ F]nifene for α4β2* nAChRs, in vitro autoradiography and in vivo PET/CT studies in 5xFAD mice were carried out and compared with wild-type (C57BL/6) mice. Ratios of [¹⁸ F]nifene binding in brain regions versus cerebellum (CB) in 5xFAD mice brains were for thalamus (TH) = 17, hippocampus-subiculum = 7, frontal cortex (FC) = 5.5, and striatum = 4.7. [¹²⁵ I]IBETA and immunohistochemistry (IHC) in 5xFAD brain slices confirmed Aβ plaques. Nicotine and acetylcholine displaced [¹⁸ F]nifene in 5xFAD mice (IC₅₀ nicotine = 31-73 nM; ACh = 38-83 nM) and C57BL/6 (IC₅₀ nicotine = 16-18 nM; ACh = 34-55 nM). Average [¹⁸ F]nifene SUVR (CB as reference) in 5xFAD mice was significantly higher in FC = 3.04 compared to C57BL/6 mice FC = 1.92 (p = .001), whereas TH difference between 5xFAD mice (SUVR = 2.58) and C57BL/6 mice (SUVR = 2.38) was not significant. Nicotine-induced dissociation half life (t_1/2 ) of [¹⁸ F]nifene for TH were 37 min for 5xFAD mice and 26 min for C57BL/6 mice. Dissociation half life  for FC in C57BL/6 mice was 77 min , while no dissociation of [¹⁸ F]nifene occurred in the medial prefrontal cortex (mFC) of 5xFAD mice. Coregistration of [¹⁸ F]nifene PET with MR suggested that the mPFC, and anterior cingulate (AC) regions exhibited high uptake in 5xFAD mice compared to C57BL/6 mice. Ex vivo [¹⁸ F]nifene and in vitro [¹²⁵ I]IBETA Aβ plaque autoradiography after in vivo PET/CT scan of 5xFAD mouse brain were moderately correlated (r² = 0.68). In conclusion, 5xFAD mice showed increased non-displaceable [¹⁸ F]nifene binding in mPFC.

Trapping of Nicotinic Acetylcholine Receptor Ligands Assayed by In Vitro Cellular Studies and In Vivo PET Imaging

A question relevant to nicotine addiction is how nicotine and other nicotinic receptor membrane-permeant ligands, such as the anti-smoking drug varenicline (Chantix), distribute in brain. Ligands, like varenicline, with high pKa and high affinity for α4β2-type nicotinic receptors (α4β2Rs) are trapped in intracellular acidic vesicles containing α4β2Rs in vitro Nicotine, with lower pKa and α4β2R affinity, is not trapped. Here, we extend our results by imaging nicotinic PET ligands in vivo in male and female mouse brain and identifying the trapping brain organelle in vitro as Golgi satellites (GSats). Two PET 18F-labeled imaging ligands were chosen: [18F]2-FA85380 (2-FA) with varenicline-like pKa and affinity and [18F]Nifene with nicotine-like pKa and affinity. [18F]2-FA PET-imaging kinetics were very slow consistent with 2-FA trapping in α4β2R-containing GSats. In contrast, [18F]Nifene kinetics were rapid, consistent with its binding to α4β2Rs but no trapping. Specific [18F]2-FA and [18F]Nifene signals were eliminated in β2 subunit knock-out (KO) mice or by acute nicotine (AN) injections demonstrating binding to sites on β2-containing receptors. Chloroquine (CQ), which dissipates GSat pH gradients, reduced [18F]2-FA distributions while having little effect on [18F]Nifene distributions in vivo consistent with only [18F]2-FA trapping in GSats. These results are further supported by in vitro findings where dissipation of GSat pH gradients blocks 2-FA trapping in GSats without affecting Nifene. By combining in vitro and in vivo imaging, we mapped both the brain-wide and subcellular distributions of weak-base nicotinic receptor ligands. We conclude that ligands, such as varenicline, are trapped in neurons in α4β2R-containing GSats, which results in very slow release long after nicotine is gone after smoking.SIGNIFICANCE STATEMENT Mechanisms of nicotine addiction remain poorly understood. An earlier study using in vitro methods found that the anti-smoking nicotinic ligand, varenicline (Chantix) was trapped in α4β2R-containing acidic vesicles. Using a fluorescent-labeled high-affinity nicotinic ligand, this study provided evidence that these intracellular acidic vesicles were α4β2R-containing Golgi satellites (GSats). In vivo PET imaging with F-18-labeled nicotinic ligands provided additional evidence that differences in PET ligand trapping in acidic vesicles were the cause of differences in PET ligand kinetics and subcellular distributions. These findings combining in vitro and in vivo imaging revealed new mechanistic insights into the kinetics of weak base PET imaging ligands and the subcellular mechanisms underlying nicotine addiction.

White Matter Integrity and Nicotine Dependence: Evaluating Vertical and Horizontal Pleiotropy

Tobacco smoking is an addictive behavior that supports nicotine dependence and is an independent risk factor for cancer and other illnesses. Its neurogenetic mechanisms are not fully understood but may act through alterations in the cerebral white matter (WM). We hypothesized that the vertical pleiotropic pathways, where genetic variants influence a trait that in turn influences another trait, link genetic factors, integrity of cerebral WM, and nicotine addiction. We tested this hypothesis using individual genetic factors, WM integrity measured by fractional anisotropy (FA), and nicotine dependence-related smoking phenotypes, including smoking status (SS) and cigarettes per day (CPDs), in a large epidemiological sample collected by the UK Biobank. We performed a genome-wide association study (GWAS) to identify previously reported loci associated with smoking behavior. Smoking was found to be associated with reduced WM integrity in multiple brain regions. We then evaluated two competing vertical pathways: Genes → WM integrity → Smoking versus Genes → Smoking → WM integrity and a horizontal pleiotropy pathway where genetic factors independently affect both smoking and WM integrity. The causal pathway analysis identified 272 pleiotropic single-nucleotide polymorphisms (SNPs) whose effects on SS were mediated by FA, as well as 22 pleiotropic SNPs whose effects on FA were mediated by CPD. These SNPs were mainly located in important susceptibility genes for smoking-induced diseases NCAM1 and IREB2. Our findings revealed the role of cerebral WM in the maintenance of the complex addiction and provided potential genetic targets for future research in examining how changes in WM integrity contribute to the nicotine effects on the brain.

Cognitive correlates of α4β2 nicotinic acetylcholine receptors in mild Alzheimer's dementia

In early Alzheimer's dementia, there is a need for PET biomarkers of disease progression with close associations to cognitive dysfunction that may aid to predict further cognitive decline and neurodegeneration. Amyloid biomarkers are not suitable for that purpose. The α4β2 nicotinic acetylcholine receptors (α4β2-nAChRs) are widely abundant in the human brain. As neuromodulators they play an important role in cognitive functions such as attention, learning and memory. Post-mortem studies reported lower expression of α4β2-nAChRs in more advanced Alzheimer's dementia. However, there is ongoing controversy whether α4β2-nAChRs are reduced in early Alzheimer's dementia. Therefore, using the recently developed α4β2-nAChR-specific radioligand (-)-18F-flubatine and PET, we aimed to quantify the α4β2-nAChR availability and its relationship to specific cognitive dysfunction in mild Alzheimer's dementia. Fourteen non-smoking patients with mild Alzheimer's dementia, drug-naïve for cholinesterase therapy, were compared with 15 non-smoking healthy controls matched for age, sex and education by applying (-)-18F-flubatine PET together with a neuropsychological test battery. The one-tissue compartment model and Logan plot method with arterial input function were used for kinetic analysis to obtain the total distribution volume (VT) as the primary, and the specific binding part of the distribution volume (VS) as the secondary quantitative outcome measure of α4β2-nAChR availability. VS was determined by using a pseudo-reference region. Correlations between VT within relevant brain regions and Z-scores of five cognitive functions (episodic memory, executive function/working memory, attention, language, visuospatial function) were calculated. VT (and VS) were applied for between-group comparisons. Volume of interest and statistical parametric mapping analyses were carried out. Analyses revealed that in patients with mild Alzheimer's dementia compared to healthy controls, there was significantly lower VT, especially within the hippocampus, fronto-temporal cortices, and basal forebrain, which was similar to comparisons of VS. VT decline in Alzheimer's dementia was associated with distinct domains of impaired cognitive functioning, especially episodic memory and executive function/working memory. Using (-)-18F-flubatine PET in patients with mild Alzheimer's dementia, we show for the first time a cholinergic α4β2-nAChR deficiency mainly present within the basal forebrain-cortical and septohippocampal cholinergic projections and a relationship between lower α4β2-nAChR availability and impairment of distinct cognitive domains, notably episodic memory and executive function/working memory. This shows the potential of (-)-18F-flubatine as PET biomarker of cholinergic α4β2-nAChR dysfunction and specific cognitive decline. Thus, if validated by longitudinal PET studies, (-)-18F-flubatine might become a PET biomarker of progression of neurodegeneration in Alzheimer's dementia.

Human brain imaging of nicotinic acetylcholine α4β2* receptors using [18 F]Nifene: Selectivity, functional activity, toxicity, aging effects, gender effects, and extrathalamic pathways

Nicotinic acetylcholinergic receptors (nAChR's) have been implicated in several brain disorders, including addiction, Parkinson's disease, Alzheimer's disease and schizophrenia. Here we report in vitro selectivity and functional properties, toxicity in rats, in vivo evaluation in humans, and comparison across species of [¹⁸ F]Nifene, a fast acting PET imaging agent for α4β2* nAChRs. Nifene had subnanomolar affinities for hα2β2 (0.34 nM), hα3β2 (0.80 nM) and hα4β2 (0.83 nM) nAChR but weaker (27-219 nM) for hβ4 nAChR subtypes and 169 nM for hα7 nAChR. In functional assays, Nifene (100 μM) exhibited 14% agonist and >50% antagonist characteristics. In 14-day acute toxicity in rats, the maximum tolerated dose (MTD) and the no observed adverse effect level (NOAEL) were estimated to exceed 40 μg/kg/day (278 μg/m² /day). In human PET studies, [¹⁸ F]Nifene (185 MBq; <0.10 μg) was well tolerated with no adverse effects. Distribution volume ratios (DVR) of [¹⁸ F]Nifene in white matter thalamic radiations were ∼1.6 (anterior) and ∼1.5 (superior longitudinal fasciculus). Habenula known to contain α3β2 nAChR exhibited low levels of [¹⁸ F]Nifene binding while the red nucleus with α2β2 nAChR had DVR ∼1.6-1.7. Females had higher [¹⁸ F]Nifene binding in all brain regions, with thalamus showing >15% than males. No significant aging effect was observed in [¹⁸ F]Nifene binding over 5 decades. In all species (mice, rats, monkeys, and humans) thalamus showed highest [¹⁸ F]Nifene binding with reference region ratios >2 compared to extrathalamic regions. Our findings suggest that [¹⁸ F]Nifene PET may be used to study α4β2* nAChRs in various CNS disorders and for translational research.

Human biodistribution and dosimetry of [18F]nifene, an α4β2* nicotinic acetylcholine receptor PET tracer

INTRODUCTION

The α4β2* nicotinic acetylcholine receptor (nAChR) system is implicated in many neuropsychiatric pathologies. [¹⁸F]Nifene is a positron emission tomography (PET) ligand that has shown promise for in vivo imaging of the α4β2* nAChR system in preclinical models and humans. This work establishes the radiation burden associated with [¹⁸F]nifene PET scans in humans.

METHODS

Four human subjects (2M, 2F) underwent whole-body PET/CT scans to determine the human biodistribution of [¹⁸F]nifene. Source organs were identified and time-activity-curves (TACs) were extracted from the PET time-series. Dose estimates were calculated for each subject using OLINDA/EXM v1.1.

RESULTS

[¹⁸F]Nifene was well tolerated by all subjects with no adverse events reported. The mean whole-body effective dose was 28.4±3.8 mSv/MBq without bladder voiding, and 22.6±1.9 mSv/MBq with hourly micturition. The urinary bladder radiation dose limited the maximum injected dose for a single scan to 278 MBq without urinary bladder voiding, and 519 MBq with hourly voiding.

CONCLUSIONS

[¹⁸F]Nifene is a safe PET radioligand for imaging the α4β2* nAChR system in humans.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

This works presents human internal dosimetry for [¹⁸F]nifene in humans for the first time. These results facilitate safe development of future [¹⁸F]nifene studies to image the α4β2* nAChR system in humans.

[18 F]Nifene test-retest reproducibility in first-in-human imaging of α4β2* nicotinic acetylcholine receptors

The aim of this study was to examine the suitability of [¹⁸ F]nifene, a novel α4β2* nicotinic acetylcholine receptor (nAChR) radiotracer, for in vivo brain imaging in a first-in-human study.

METHODS

Eight healthy subjects (4 M,4 F;21-69,44 ± 21 yrs) underwent a [¹⁸ F]nifene positron emission tomography scan (200 ± 3.7 MBq), and seven underwent a second scan within 58 ± 31 days. Regional estimates of DVR were measured using the multilinear reference tissue model (MRTM2) with the corpus callosum as reference region. DVR reproducibility was evaluated with test-retest variability (TRV) and intraclass correlation coefficient (ICC).

RESULTS

The DVR ranged from 1.3 to 2.5 across brain regions with a TRV of 0-7%, and did not demonstrate a systematic difference between test and retest. The ICCs ranged from 0.2 to 0.9. DVR estimates were stable after 40 min.

CONCLUSION

The binding profile and tracer kinetics of [¹⁸ F]nifene make it a promising α4β2* nAChR radiotracer for scientific research in humans, with reliable DVR test-retest reproducibility.

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.

Pathologic role of neuronal nicotinic acetylcholine receptors in epileptic disorders: implication for pharmacological interventions

Accumulating evidence suggests that neuronal nicotinic acetylcholine receptors (nAChRs) may play a key role in the pathophysiology of some neurological diseases such as epilepsy. Based on genetic studies in patients with epileptic disorders worldwide and animal models of seizure, it has been demonstrated that nAChR activity is altered in some specific types of epilepsy, including autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) and juvenile myoclonic epilepsy (JME). Neuronal nAChR antagonists also have antiepileptic effects in pre-clinical studies. There is some evidence that conventional antiepileptic drugs may affect neuronal nAChR function. In this review, we re-examine the evidence for the involvement of nAChRs in the pathophysiology of some epileptic disorders, especially ADNFLE and JME, and provide an overview of nAChR antagonists that have been evaluated in animal models of seizure.

In Vivo Monitoring of Sevoflurane-induced Adverse Effects in Neonatal Nonhuman Primates Using Small-animal Positron Emission Tomography

Background

Animals exposed to sevoflurane during development sustain neuronal cell death in their developing brains. In vivo micro-positron emission tomography (PET)/computed tomography imaging has been utilized as a minimally invasive method to detect anesthetic-induced neuronal adverse effects in animal studies.

Methods

Neonatal rhesus monkeys (postnatal day 5 or 6, 3 to 6 per group) were exposed for 8 h to 2.5% sevoflurane with or without acetyl-l-carnitine (ALC). Control monkeys were exposed to room air with or without ALC. Physiologic status was monitored throughout exposures. Depth of anesthesia was monitored using quantitative electroencephalography. After the exposure, microPET/computed tomography scans using 18F-labeled fluoroethoxybenzyl-N-(4-phenoxypyridin-3-yl) acetamide (FEPPA) were performed repeatedly on day 1, 1 and 3 weeks, and 2 and 6 months after exposure.

Results

Critical physiologic metrics in neonatal monkeys remained within the normal range during anesthetic exposures. The uptake of [18F]-FEPPA in the frontal and temporal lobes was increased significantly 1 day or 1 week after exposure, respectively. Analyses of microPET images recorded 1 day after exposure showed that sevoflurane exposure increased [18F]-FEPPA uptake in the frontal lobe from 0.927 ± 0.04 to 1.146 ± 0.04, and in the temporal lobe from 0.859 ± 0.05 to 1.046 ± 0.04 (mean ± SE, P &lt; 0.05). Coadministration of ALC effectively blocked the increase in FEPPA uptake. Sevoflurane-induced adverse effects were confirmed by histopathologic evidence as well.

Conclusions

Sevoflurane-induced general anesthesia during development increases glial activation, which may serve as a surrogate for neurotoxicity in the nonhuman primate brain. ALC is a potential protective agent against some of the adverse effects associated with such exposures.

Nicotinic receptors in non-human primates: Analysis of genetic and functional conservation with humans

Nicotinic acetylcholine receptors (nAChRs) are highly conserved between humans and non-human primates. Conservation exists at the level of genomic structure, protein structure and epigenetics. Overall homology of nAChRs at the protein level is 98% in macaques versus 89% in mice, which is highly relevant for evaluating subtype-specific ligands that have different affinities in humans versus rodents. In addition to conservation at the protein level, there is high conservation of genomic structure in terms of intron and exon size and placement of CpG sites that play a key role in epigenetic regulation. Analysis of single nucleotide polymorphisms (SNPs) shows that while the majority of SNPs are not conserved between humans and macaques, some functional polymorphisms are. Most significantly, cynomolgus monkeys express a similar α5 nAChR Asp398Asn polymorphism to the human α5 Asp398Asn polymorphism that has been linked to greater nicotine addiction and smoking related disease. Monkeys can be trained to readily self-administer nicotine, and in an initial study we have demonstrated that cynomolgus monkeys bearing the α5 D398N polymorphism show a reduced behavioral sensitivity to oral nicotine and tend to consume it in a different pattern when compared to wild-type monkeys. Thus the combination of highly homologous nAChR, higher cortical functions and capacity for complex training makes non-human primates a unique model to study in vivo functions of nicotinic receptors. In particular, primate studies on nicotine addiction and evaluation of therapies to prevent or overcome nicotine addiction are likely to be highly predictive of treatment outcomes in humans.

Measuring α4β2* nicotinic acetylcholine receptor density in vivo with [(18)F]nifene PET in the nonhuman primate

[(18)F]Nifene is an agonist PET radioligand developed to image α4β2* nicotinic acetylcholine receptors (nAChRs). This work aims to quantify the receptor density (Bmax) of α4β2* nAChRs and the in vivo (apparent) dissociation constant (KDapp) of [(18)F]nifene. Multiple-injection [(18)F]nifene experiments with varying cold nifene masses were conducted on four rhesus monkeys with a microPET P4 scanner. Compartment modeling techniques were used to estimate regional Bmax values and a global value of KDapp. The fast kinetic properties of [(18)F]nifene also permitted alternative estimates of Bmax and KDapp at transient equilibrium with the same experimental data using Scatchard-like methodologies. Averaged across subjects, the compartment modeling analysis yielded Bmax values of 4.8±1.4, 4.3±1.0, 1.2±0.4, and 1.2±0.3 pmol/mL in the regions of antereoventral thalamus, lateral geniculate, frontal cortex, and subiculum, respectively. The KDapp of nifene was 2.4±0.3 pmol/mL. The Scatchard analysis based on graphical evaluation of the data after transient equilibrium yielded Bmax estimations comparable to the modeling results with a positive bias of 28%. These findings show the utility of [(18)F]nifene for measuring α4β2* nAChR Bmax in vivo in the rhesus monkey with a single PET experiment.

Acute nicotine administration effects on fractional anisotropy of cerebral white matter and associated attention performance

Introduction: Nicotinic acetylcholine receptors are present in the cerebral white matter (WM). We hypothesized that WM response to nicotine can be detected by diffusion tensor imaging (DTI); and that such responses may be associated with nicotine-led cognitive enhancement in sustained attention.

Methods: A randomized, nicotine-placebo patch, crossover, double-blind clinical trial in two non-overlapping cohorts of smokers was used to test the hypothesis. The discovery cohort consisted of 39 subjects (N = 20/19 controls/schizophrenic patients, age = 36.8 ± 10.1 years) and the replication cohorts consisted of 38 healthy smokers (31.7 ± 10.5 years). WM integrity was measured by fractional anisotropy (FA) values for the whole brain and nine preselected WM tracts using tract-based-spatial-statistics.

Results: Nicotine significantly enhanced FA values for the genu of corpus callosum compared with placebo (ΔFA_genu) (p = 0.01) in smokers with low recent smoking exposure as measured by low average cotinine level. This finding was replicated in the second cohort (p = 0.02). ΔFA_genu values explained 22% of variance in performance of a sustained attention task during the nicotine session (p = 0.006). However, this effect was limited to schizophrenia patients (r = 0.62 and 0.09; p = 0.003 and 0.7 for patients and controls, respectively).

Conclusion: Acute pharmacological influence of nicotine patch on WM integrity appeared present, but was dependent on nicotine intake from recent smoking. Change in the WM integrity in the genu of corpus callosum was associated with a significant proportion of variability of nicotine-led changes in sustained attention/working memory of the smokers. Further studies will be necessary to understand biophysical underpinning of the nicotine-related changes in FA.

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.

The effects of lobeline on α4β2* nicotinic acetylcholine receptor binding and uptake of [(18)F]nifene in rats

Lobeline is a potential smoking cessation drug with affinity for the α4β2 nicotinic acetylcholine receptor and may inhibit the blood-brain barrier (BBB) amine transporter. The goal of this work was to use PET imaging to evaluate the effects of lobeline on the kinetic properties of [(18)F]nifene in the rat brain.

METHODS

Direct α4β2* competition of lobeline with [(18)F]nifene was evaluated using imaging experiments with both displacing and blocking doses of lobeline (1mg/kg, i.v.) given between two injections of [(18)F]nifene separated by 50min. Inhibition of the BBB amine transporter was examined using a separate imaging protocol with three injections of [(18)F]nifene, first at baseline, then following (-)nicotine blocking, and finally following lobeline blocking.

RESULTS

Rapid displacement of [(18)F]nifene was observed in the α4β2*-rich thalamus following lobeline administration, suggesting direct competition of the drug at α4β2* sites. Slight decreases in BBB transport of [(18)F]nifene were observed when the α4β2* system was first saturated with (-)nicotine and then given lobeline. This perturbation may be due to inhibition of the BBB amine transporter by lobeline or reductions in blood flow. Significant cerebellar displacement of [(18)F]nifene was found following the administration of both lobeline and (-)nicotine, indicating detectable specific binding in the rat cerebellum.

CONCLUSION

The competition of lobeline with [(18)F]nifene is largely dominated at the α4β2* binding site and only small perturbations in BBB transport of [(18)F]nifene are seen at the 1mg/kg dose. Similar experiments could be used to study other drugs as therapeutic agents for smoking cessation with PET.

Nicotinic α4β2 receptor imaging agents. Part IV. Synthesis and biological evaluation of 3-(2-(S)-3,4-dehydropyrrolinyl methoxy)-5-(3'-¹⁸F-fluoropropyl)pyridine (¹⁸F-Nifrolene) using PET

Imaging agents for nicotinic α4β2 receptors in the brain have been under way for studying various CNS disorders. Previous studies from our laboratories have reported the successful development of agonist, ¹⁸F-nifene. In attempts to develop potential antagonists, ¹⁸F-nifrolidine and ¹⁸F-nifzetidine were previously reported. Further optimization of these fluoropropyl derivatives has now been carried out resulting in 3-(2-(S)-3,4-dehydropyrrolinylmethoxy)-5-(3'-Fluoropropyl)pyridine (nifrolene) as a new high affinity agent for nicotinic α4β2 receptors. Nifrolene in rat brain homogenate assays--labeled with ³H-cytisine--exhibited a binding affinity of 0.36 nM. The fluorine-18 analog, ¹⁸F-nifrolene, was synthesized in approximately 10%-20% yield and specific activity was estimated to be >2000 Ci/mmol. Rat brain slices indicated selective binding to anterior thalamic nuclei, thalamus, subiculum, striata, cortex and other regions consistent with α4β2 receptor distribution. This selective binding was displaced >90% by 300 μM nicotine. Thalamus to cerebellum ratio (>10) was the highest for ¹⁸F-nifrolene with several other regions showing selective binding. In vivo rat PET studies exhibited rapid uptake of ¹⁸F-nifrolene in the brain with specific retention in the thalamus and other brain regions while clearing out from the cerebellum. Thalamus to cerebellum ratio value in the rat was >4. Administration of nicotine caused a rapid decline in the thalamic ¹⁸F-nifrolene suggesting reversible binding to nicotinic receptors. PET imaging studies of ¹⁸F-nifrolene in anesthetized rhesus monkey revealed highest binding in the thalamus followed by regions of the lateral cingulated and temporal cortex. Cerebellum showed the least binding. Thalamus to cerebellum ratio in the monkey brain was >3 at 120 min. These ratios of ¹⁸F-nifrolene are higher than measured for ¹⁸F-nifrolidine and ¹⁸F-nifzetidine. ¹⁸F-Nifrolene thus shows promise as a new PET imaging agent for α4β2 nAChR.

MicroPET/CT Imaging of [18F]-FEPPA in the Nonhuman Primate: A Potential Biomarker of Pathogenic Processes Associated with Anesthetic-Induced Neurotoxicity

Background. The inhalation anesthetics nitrous oxide (N2O) and isoflurane (ISO) are used in surgical procedures for human infants. Injury to the central nervous system is often accompanied by localization of activated microglia or astrocytosis at the site of injury. The tracer that targets to the peripheral benzodiazepine receptor (PBR), [18F]N-2-(2-fluoroethoxy)benzyl)-N-(4-phenoxypyridin-3-yl)acetamide ([18F]-FEPPA), has been reported as a sensitive biomarker for the detection of neuronal damage/inflammation. Methods. On postnatal day (PND) 5 or 6 rhesus monkey neonates were exposed to a mixture of N2O/oxygen and ISO for 8 hours and control monkeys were exposed to room air. MicroPET/CT images with [18F]-FEPPA were obtained for each monkey 1 day, one week, three weeks, and 6 months after the anesthetic exposure. Results. The radiotracer quickly distributed into the brains of both treated and control monkeys on all scan days. One day after anesthetic exposure, the uptake of [18F]-FEPPA was significantly increased in the temporal lobe. One week after exposure, the uptake of [18F]-FEPPA in the frontal lobe of treated animals was significantly greater than that in controls. Conclusions. These findings suggest that microPET imaging is capable of dynamic detection of inhaled anesthetic-induced brain damage in different brain regions of the nonhuman primate.

Synthesis and evaluation of 3-¹²³I-iodo-5-[2-(S)-3-pyrrolinylmethoxy]-pyridine (niodene) as a potential nicotinic α4β2 receptor imaging agent

Nicotinic acetylcholine receptors (nAChRs) are downregulated in disease conditions such as Alzheimer's and substance abuse. Presently, (123)I-5-IA-85380 is used in human studies and requires over 6h of scanning time, thus increases patient discomfort. We have designed and synthesized 3-iodo-5-[2-(S)-3-pyrrolinylmethoxy]pyridine (niodene) with the aim to have faster binding kinetics compared to (123)I-5-IA-85380, which may reduce scanning time and help in imaging studies. Binding affinity K(i) of niodene for rat brain α4β2 receptors in brain homogenate assays using (3)H-cytisine was 0.27 nM. Niodene, 10nM displaced >95% of (18)F-nifene bound to α4β2 receptors in rat brain slices. By using the iododestannylation method, (123)I-niodene was obtained in high radiochemical purity (>95%) but with low radiochemical yield (<5%) and low specific activity (∼100 Ci/mmol). Autoradiograms show (123)I-niodene localized in the thalamus and cortex, which was displaced by nicotine (thalamus to cerebellum ratio=4; cortex to cerebellum ratio=1.6). Methods of radioiodination need to be further evaluated in order to obtain (123)I-niodene in higher radiochemical yields and higher specific activity of this potentially useful new SPECT imaging agent.

PET imaging of α4β2* nicotinic acetylcholine receptors: quantitative analysis of 18F-nifene kinetics in the nonhuman primate

The PET radioligand 2-fluoro-3-[2-((S)-3-pyrrolinyl)methoxy]pyridine ((18)F-nifene) is an α4β2* nicotinic acetylcholine receptor (nAChR) agonist developed to provide accelerated in vivo equilibrium compared with existing α4β2* radioligands. The goal of this work was to analyze the in vivo kinetic properties of (18)F-nifene with both kinetic modeling and graphical analysis techniques.

METHODS

Dynamic PET experiments were performed on 4 rhesus monkeys (female; age range, 9-13 y) using a small-animal PET scanner. Studies began with a high-specific-activity (18)F-nifene injection, followed by a coinjection of (18)F-nifene and unlabeled nifene at 60 min. Sampling of arterial blood with metabolite analysis was performed throughout the experiment to provide a parent radioligand input function. In vivo kinetics were characterized with both a 1-tissue-compartment model (1TCM) and a 2-tissue-compartment model, Logan graphical methods (both with and without blood sampling), and the multilinear reference tissue model. Total distribution volumes and nondisplaceable binding potentials (BP(ND)) were used to compare regional binding of (18)F-nifene. Regions examined include the anteroventral thalamus, lateral geniculate body, frontal cortex, subiculum, and cerebellum.

RESULTS

The rapid uptake and binding of (18)F-nifene in nAChR-rich regions of the brain was appropriately modeled using the 1TCM. No evidence for specific binding of (18)F-nifene in the cerebellum was detected on the basis of the coinjection studies, suggesting the suitability of the cerebellum as a reference region. Total distribution volumes in the cerebellum were 6.91 ± 0.61 mL/cm(3). BP(ND) values calculated with the 1TCM were 1.60 ± 0.17, 1.35 ± 0.16, 0.26 ± 0.08, and 0.30 ± 0.07 in the anteroventral thalamus, lateral geniculate body, frontal cortex, and subiculum, respectively. For all brain regions, there was a less than 0.04 absolute difference in the average BP(ND) values calculated with each of the 1TCM, multilinear reference tissue model, and Logan methods.

CONCLUSION

The fast kinetic properties and specific regional binding of (18)F-nifene promote extension of the radioligand into preclinical animal models and human subjects.

A cognitive phenotype for a polymorphism in the nicotinic receptor gene CHRNA4

Drawing on converging behavioral, electrophysiological, and imaging evidence, we advance an hypothesis for a cognitive phenotype of a SNP in the CHRNA4 gene encoding the α(4) subunit of α(4)β(2) nicotinic receptors. First, we review evidence that visuospatial attention can be decomposed into several component processes. Secondly, we consider evidence that one component, redirection of attention, is modulated by the nicotinic cholinergic system. Third, we review evidence that nicotinic stimulation exerts effects at the network level. Fourth, we consider evidence that normal variation in this SNP exerts nicotine-like modulatory effects on visuospatial attention. Fifth, we hypothesize that the cognitive phenotype of the CHRNA4 rs1044396 SNP is characterized by greater ability of T allele carriers to preferentially process events in the attentional focus compared to events outside the attentional focus. Finally, we consider effects of the CHNRA4 rs1044396 SNP on brain activity and cognition in light of our hypothesized cognitive phenotype. This hypothesis makes an important contribution to the development of cognitive phenomics by arguing for a cognitive phenotype of CHRNA4.

Nicotinic acetylcholine receptors in rat forebrain that bind ¹⁸F-nifene: relating PET imaging, autoradiography, and behavior

Nicotinic acetylcholine receptors (nAChRs) in the brain are important for cognitive function; however, their specific role in relevant brain regions remains unclear. In this study, we used the novel compound ¹⁸F-nifene to examine the distribution of nAChRs in the rat forebrain, and for individual animals related the results to behavioral performance on an auditory-cognitive task. We first show negligible binding of ¹⁸F-nifene in mice lacking the β2 nAChR subunit, consistent with previous findings that ¹⁸F-nifene binds to α4β2* nAChRs. We then examined the distribution of ¹⁸F-nifene in rat using three methods: in vivo PET, ex vivo PET and autoradiography. Generally, ¹⁸F-nifene labeled forebrain regions known to contain nAChRs, and the three methods produced similar relative binding among regions. Importantly, ¹⁸F-nifene also labeled some white matter (myelinated axon) tracts, most prominently in the temporal subcortical region that contains the auditory thalamocortical pathway. Finally, we related ¹⁸F-nifene binding in several forebrain regions to each animal's performance on an auditory-cued, active avoidance task. The strongest correlations with performance after 14 days training were found for ¹⁸F-nifene binding in the temporal subcortical white matter, subiculum, and medial frontal cortex (correlation coefficients, r > 0.8); there was no correlation with binding in the auditory thalamus or auditory cortex. These findings suggest that individual performance is linked to nicotinic functions in specific brain regions, and further support a role for nAChRs in sensory-cognitive function.