Norchloro-fluoro-homoepibatidine (NCFHEB) - a promising radioligand for neuroimaging nicotinic acetylcholine receptors with PET

Evaluation of an Image-Derived Input Function for Kinetic Modeling of Nicotinic Acetylcholine Receptor-Binding PET Ligands in Mice

Positron emission tomography (PET) radioligands that bind with high-affinity to α4β2-type nicotinic receptors (α4β2Rs) allow for in vivo investigations of the mechanisms underlying nicotine addiction and smoking cessation. Here, we investigate the use of an image-derived arterial input function and the cerebellum for kinetic analysis of radioligand binding in mice. Two radioligands were explored: 2-[18F]FA85380 (2-FA), displaying similar pKa and binding affinity to the smoking cessation drug varenicline (Chantix), and [18F]Nifene, displaying similar pKa and binding affinity to nicotine. Time-activity curves of the left ventricle of the heart displayed similar distribution across wild type mice, mice lacking the β2-subunit for ligand binding, and acute nicotine-treated mice, whereas reference tissue binding displayed high variation between groups. Binding potential estimated from a two-tissue compartment model fit of the data with the image-derived input function were higher than estimates from reference tissue-based estimations. Rate constants of radioligand dissociation were very slow for 2-FA and very fast for Nifene. We conclude that using an image-derived input function for kinetic modeling of nicotinic PET ligands provides suitable results compared to reference tissue-based methods and that the chemical properties of 2-FA and Nifene are suitable to study receptor response to nicotine addiction and smoking cessation therapies.

Nicotinic Acetylcholine Receptors and Microglia as Therapeutic and Imaging Targets in Alzheimer's Disease

Amyloid-β (Aβ) accumulation and tauopathy are considered the pathological hallmarks of Alzheimer’s disease (AD), but attenuation in choline signaling, including decreased nicotinic acetylcholine receptors (nAChRs), is evident in the early phase of AD. Currently, there are no drugs that can suppress the progression of AD due to a limited understanding of AD pathophysiology. For this, diagnostic methods that can assess disease progression non-invasively before the onset of AD symptoms are essential, and it would be valuable to incorporate the concept of neurotheranostics, which simultaneously enables diagnosis and treatment. The neuroprotective pathways activated by nAChRs are attractive targets as these receptors may regulate microglial-mediated neuroinflammation. Microglia exhibit both pro- and anti-inflammatory functions that could be modulated to mitigate AD pathogenesis. Currently, single-cell analysis is identifying microglial subpopulations that may have specific functions in different stages of AD pathologies. Thus, the ability to image nAChRs and microglia in AD according to the stage of the disease in the living brain may lead to the development of new diagnostic and therapeutic methods. In this review, we summarize and discuss the recent findings on the nAChRs and microglia, as well as their methods for live imaging in the context of diagnosis, prophylaxis, and therapy for AD.

Bidirectional Associations among Nicotine and Tobacco Smoke, NeuroHIV, and Antiretroviral Therapy

People living with HIV (PLWH) in the antiretroviral therapy (ART) era may lose more life-years to tobacco use than to HIV. Yet, smoking rates are more than twice as high among PLWH than the general population, contributing not just to mortality but to other adverse health outcomes, including neurocognitive deficits (neuroHIV). There is growing evidence that synergy with chronic inflammation and immune dysregulation that persists despite ART may be one mechanism by which tobacco smoking contributes to neuroHIV. This review will summarize the differential effects of nicotine vs tobacco smoking on inflammation in addition to the effects of tobacco smoke components on HIV disease progression. We will also discuss biomarkers of inflammation via neuroimaging as well as biomarkers of nicotine dependence (e.g., nicotine metabolite ratio). Tobacco smoking and nicotine may impact ART drug metabolism and conversely, certain ARTs may impact nicotine metabolism. Thus, we will review these bidirectional relationships and how they may contribute to neuroHIV and other adverse outcomes. We will also discuss the effects of tobacco use on the interaction between peripheral organs (lungs, heart, kidney) and subsequent CNS function in the context of HIV. Lastly, given the dramatic rise in the use of electronic nicotine delivery systems, we will discuss the implications of vaping on these processes. Despite the growing recognition of the importance of addressing tobacco use among PLWH, more research is necessary at both the preclinical and clinical level to disentangle the potentially synergistic effects of tobacco use, nicotine, HIV, cognition and immune dysregulation, as well as identify optimal approaches to reduce tobacco use. Graphical Abstract Proposed model of the relationships among HIV, ART, smoking, inflammation, and neurocognition. Solid lines represent relationships supported by evidence. Dashed lines represent relationships for which there is not enough evidence to make a conclusion. (a) HIV infection produces elevated levels of inflammation even among virally suppressed individuals. (b) HIV is associated with deficits in cognition function. (c) Smoking rates are higher among PLWH, compared to the general population. (d) The nicotine metabolite ratio (NMR) is associated with smoking behavior. (e) HIV and tobacco use are both associated with higher rates of psychiatric comorbidities, such as depression, and elevated levels of chronic stress. These factors may represent other mechanisms linking HIV and tobacco use. (f) The relationship between nicotine, tobacco smoking, and inflammation is complex, but it is well-established that smoking induces inflammation; the evidence for nicotine as anti-inflammatory is supported in some studies, but not others. (g) The relationship between tobacco use and neurocognition may differ for the effects of nicotine (acute nicotine use may have beneficial effects) vs. tobacco smoking (chronic use may impair cognition). (h) Elevated levels of inflammation may be associated with deficits in cognition. (i) PLWH may metabolize nicotine faster than those without HIV; the mechanism is not yet known and the finding needs validation in larger samples. We also hypothesize that if HIV-infection increases nicotine metabolism, then we should observe an attenuation effect once ART is initiated. (j) It is possible that the increase in NMR is due to ART effects on CYP2A6. (k) We hypothesize that faster nicotine metabolism may result in higher levels of inflammation since nicotine has anti-inflammatory properties.

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.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

Evaluation of [(18)F]-(-)-norchlorofluorohomoepibatidine ([(18)F]-(-)-NCFHEB) as a PET radioligand to image the nicotinic acetylcholine receptors in non-human primates

INTRODUCTION

The aims of the present study were to develop an optimized microfluidic method for the production of the selective nicotinic acetylcholine α4β2 receptor radiotracer [(18)F]-(-)-NCFHEB ([(18)F]-Flubatine) and to investigate its receptor binding profile and pharmacokinetic properties in rhesus monkeys in vivo.

METHODS

[(18)F]-(-)-NCFHEB was prepared in two steps, a nucleophilic fluorination followed by N-Boc deprotection. PET measurements were performed in rhesus monkeys including baseline and preblocking experiments with nicotine (0.24 mg/kg). Radiometabolites in plasma were measured using HPLC.

RESULTS

[(18)F]-(-)-NCFHEB was prepared in a total synthesis time of 140 min. The radiochemical purity in its final formulation was >98% and the mean specific radioactivity was 97.3 ± 16.1 GBq/μmol (n = 6) at end of synthesis (EOS). In the monkey brain, radioactivity concentration was high in the thalamus, moderate in the putamen, hippocampus, frontal cortex, and lower in the cerebellum. Nicotine blocked 98-100% of [(18)F]-(-)-NCFHEB specific binding, and the non-displaceable distribution volume (VND) was estimated at 5.9 ± 1.0 mL/cm(3) (n = 2), or 6.6 ± 1.1 mL/cm(3) after normalization by the plasma free fraction fP. Imaging data are amenable to kinetic modeling analysis using the multilinear analysis (MA1) method, and model-derived binding parameters display good test-retest reproducibility. In rhesus monkeys, [(18)F]-(-)-NCFHEB can yield robust regional binding potential (BPND) values (thalamus = 4.1 ± 1.5, frontal cortex = 1.2 ± 0.2, putamen = 0.96 ± 0.45, and cerebellum = 0.10 ± 0.29).

CONCLUSION

An efficient microfluidic synthetic method was developed for preparation of [(18)F]-(-)-NCFHEB. PET examination in rhesus monkeys showed that [(18)F]-(-)-NCFHEB entered the brain readily and its regional radioactivity uptake pattern was in accordance with the known distribution of α4β2 receptors. Estimated non-displaceable binding potential (BPND) values in brain regions were better than those of [(18)F]2-FA and comparable to [(18)F]AZAN. These results confirm previous findings and support further examination of [(18)F]-(-)-NCFHEB in humans.

Synthesis and biological evaluation of both enantiomers of [(18)F]flubatine, promising radiotracers with fast kinetics for the imaging of α4β2-nicotinic acetylcholine receptors

Both enantiomers of the epibatidine analogue flubatine display high affinity towards the α4β2 nicotinic acetylcholine receptor (nAChR) in vitro, accompanied by negligible interactions with diverse off-target proteins. Extended single dose toxicity studies in rodent indicated a NOEL (No Observed Effect Level) of 6.2μg/kg for (-)-flubatine and 1.55μg/kg for (+)-flubatine. We developed syntheses for both flubatine enantiomers and their corresponding precursors for radiolabeling. The newly synthesized trimethylammonium precursors allowed for highly efficient (18)F-radiolabelling in radiochemical yields >60% and specific activities >750GBq/μmol, thus making the radioligands practical for clinical investigation.

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.

(-)-[(18) F]Flubatine: evaluation in rhesus monkeys and a report of the first fully automated radiosynthesis validated for clinical use

(-)-[(18) F]Flubatine was selected for clinical imaging of α4 β2 nicotinic acetylcholine receptors because of its high affinity and appropriate kinetic profile. A fully automated synthesis of (-)-[(18) F]flubatine as a sterile isotonic solution suitable for clinical use is reported, as well as the first evaluation in nonhuman primates (rhesus macaques). (-)-[(18) F]Flubatine was prepared by fluorination of the Boc-protected trimethylammonium iodide precursor with [(18) F]fluoride in an automated synthesis module. Subsequent deprotection of the Boc group with 1-M HCl yielded (-)-[(18) F]flubatine, which was purified by semi-preparative HPLC. (-)-[(18) F]Flubatine was prepared in 25% radiochemical yield (formulated for clinical use at end of synthesis, n = 3), >95% radiochemical purity, and specific activity = 4647 Ci/mmol (171.9 GBq/µmol). Doses met all quality control criteria confirming their suitability for clinical use. Evaluation of (-)-[(18) F]flubatine in rhesus macaques was performed with a Concorde MicroPET P4 scanner (Concorde MicroSystems, Knoxville, TN). The brain was imaged for 90 min, and data were reconstructed using the 3-D maximum a posteriori algorithm. Image analysis revealed higher uptake and slower washout in the thalamus than those in other areas of the brain and peak uptake at 45 min. Injection of 2.5 µg/kg of nifene at 60 min initiated a slow washout of [(18) F]flubatine, with about 25% clearance from the thalamus by the end of imaging at 90 min.

Fully automated radiosynthesis of both enantiomers of [18F]Flubatine under GMP conditions for human application

A fully automatized radiosynthesis of (+)- and (-)-[(18)F]Flubatine ((+)- and (-)NCFHEB) by means of a commercially available synthesis module (TRACERlab FX FN) under GMP conditions is reported. Radiochemical yields of 30% within an overall synthesis time of 40 min were achieved in more than 70 individual syntheses. Specific activities were approximately 3000 GBq/μmol and radiochemical purity was determined to be at least 97%.

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.

Radiosynthesis of racemic and enantiomerically pure (-)-[18F]flubatine--a promising PET radiotracer for neuroimaging of α4β2 nicotinic acetylcholine receptors

(-)-[(18)F]flubatine is a promising agent for visualization by PET of cerebral α4β2 nicotinic acetylcholine receptors (nAChRs), which are implicated in psychiatric and neurodegenerative disorders. Here, we describe a substantially improved two-step radiosynthesis strategy for (-)-[(18)F]flubatine, based on the nucleophilic radiofluorination of an enantiomerically pure precursor followed by deprotection of the intermediate. An extensive leaving group/protecting group library of precursors was tested. Application of a trimethylammonium-iodide precursor with a Boc-protecting group provided the best results: labeling efficiencies of 80-95%, RCY of 60±5%, radiochemical purity of >98%, and a specific activity of >350GBq/μmol. The radiosynthesis is easily transferable to an automated synthesis module.

Exploiting nutrient transporters at the blood-brain barrier to improve brain distribution of small molecules

The blood-brain barrier (BBB) is a major physiological barrier for drugs that target CNS receptors or enzymes. Several methods exist by which permeability to the CNS can be increased, one of which is using native nutrient transporters to carry these drugs through the endothelial cells of the BBB. In this review, we focus on work that characterizes the use of nutrient transporters of the BBB in delivering drugs to the CNS.

Localisation of pre- and postsynaptic cholinergic markers in the human brain

The cholinergic neurotransmission in the central nervous system plays an important role in modulating cognitive processes such as learning, memory, arousal and sleep as well as in modulating locomotor activity. Dysfunction of the central cholinergic system is involved in numerous neuropsychiatric diseases. This review will provide a synopsis on the regional localisation of cholinergic and cholinoceptive structures within the adult human brain. On the cholinergic site data based on the distribution of choline acetyltransferase-immunoreactive structures are in the focus, complemented by data from acetylcholinesterase and vesicular acetylcholine transporter studies. On the cholinoceptive site, the distribution and localisation of receptors that transduce the acetylcholine message, i.e. the muscarinic and the nicotinic acetylcholine receptors is summarized. In addition to these data obtained on post mortem brain an overview of markers which allow for the in vivo monitoring of the cholinergic system in the brain is given. The detailed knowledge on the distribution and localisation of cholinergic markers in human brain will provide further information on the cholinergic circuits of neurotransmission - a prerequisite for the interpretation of in vivo imaging data and the development of selective diagnostic and therapeutic compounds.

Development of radioligands with optimized imaging properties for quantification of nicotinic acetylcholine receptors by positron emission tomography

AIMS

There is an urgent need for positron emission tomography (PET) imaging of the nicotinic acetylcholine receptors (nAChR) to study the role of the nicotinic system in Alzheimer's and Parkinson's diseases, schizophrenia, drug dependence and many other disorders. Greater understanding of the underlying mechanisms of the nicotinic system could direct the development of medications to treat these disorders. Central nAChRs also contribute to a variety of brain functions, including cognition, behavior and memory.

MAIN METHODS

Currently, only two radiotracers, (S)-3-(azetidin-2-ylmethoxy)-2-[(18)F]fluoropyridine (2-[(18)F]FA) and (S)-5-(azetidin-2-ylmethoxy)-2-[(18)F]fluoropyridine (6-[(18)F]FA), are available for studying nAChRs in human brain using PET. However, the "slow" brain kinetics of these radiotracers hamper mathematical modeling and reliable measurement of kinetic parameters since it takes 4-7 h of PET scanning for the tracers to reach steady state. The imaging drawbacks of the presently available nAChR radioligands have initiated the development of radioligands with faster brain kinetics by several research groups.

KEY FINDINGS

This minireview attempts to survey the important achievements of several research groups in the discovery of PET nicotinic radioligands reached recently. Specifically, this article reviews papers published from 2006 through 2008 describing the development of fifteen new nAChR (11)C-and (18)F-ligands that show improved imaging properties over 2-[(18)F]FA.

SIGNIFICANCE

The continuous efforts of radiomedicinal chemists led to the development of several interesting PET radioligands for imaging of nAChR including [(18)F]AZAN, a potentially superior alternative to 2-[(18)F]FA.

Acetylcholine receptors in dementia and mild cognitive impairment

PURPOSE

To clarify whether changes in the cholinergic transmission occur early in the course of Alzheimer's disease (AD), we carried out positron emission tomography (PET) with the radioligand 2-[(18)F]F-A-85380, which is supposed to be specific for alpha4beta2 nicotinic acetylcholine receptors (nAChRs).

METHOD

We included patients with moderate to severe AD and patients with amnestic mild cognitive impairment (MCI), presumed to present preclinical AD.

RESULTS

Both patients with AD and MCI showed significant reductions in alpha4beta2 nAChRs in brain regions typically affected by AD pathology. These findings indicate that a reduction in alpha4beta2 nAChRs occurs during early symptomatic stages of AD. The alpha4beta2 nAChR availability in these regions correlated with the severity of cognitive impairment, indicating a stage sensitivity of the alpha4beta2 nAChR status.

CONCLUSION

Together, our results provide evidence for the potential of 2-[(18)]F-A-85380 nAChR PET in the diagnosis of patients at risk for AD. Because of the extraordinary long acquisition time with 2-[(18)F]F-A-85380, we developed the new alpha4beta2 nAChR-specific radioligands (+)- and (-)-[(18)F]norchloro-fluoro-homoepibatidine (NCFHEB) and evaluated them preclinically. (-)-[(18)F]NCFHEB shows twofold higher brain uptake and significantly shorter acquisition times. Therefore, (-)-[(18)F]NCFHEB should be a suitable radioligand for larger clinical investigations.