Radiosynthesis and in vitro validation of (3)H-NS14492 as a novel high affinity alpha7 nicotinic receptor radioligand

Advances in PET Imaging of α7 Nicotinic Receptors: From Radioligand Development to CNS Applications

Positron emission tomography (PET) has significantly advanced our understanding of the brain by enabling non-invasive imaging and quantification of molecular processes, including receptor binding. In this review, we explore the development and application of PET radioligands targeting the α7 nicotinic acetylcholine receptor (α7 nAChR), a receptor implicated in various central nervous system (CNS) diseases, such as Alzheimer's disease, schizophrenia and cognitive disorders. Despite challenges associated with the low density of α7 nAChRs and difficulties in achieving adequate brain penetration, several promising radioligands have been developed, including 11C-(R)-MeQAA, 11C-NS14492 and 18F-ASEM. These radioligands facilitate the evaluation of the 'three pillars of survival' in drug development: tissue accessibility, target engagement and downstream pharmacology. PET imaging offers critical insights into drug distribution across the blood-brain barrier, receptor occupancy and the pharmacodynamic effects of α7 nAChR-targeted therapies. By reviewing current radioligands and their applications, we highlight the potential of PET imaging to deepen our understanding of α7 nAChR-mediated signalling pathways and its implications for CNS drug discovery. Future innovations in radioligand development, including more selective and brain-penetrant compounds, will be key to fully realizing the potential of PET imaging in α7 nAChR-targeted research and treatment.

Characterizing the binding of TC-5619 and encenicline on the alpha7 nicotinic acetylcholine receptor using PET imaging in the pig

The alpha7 nicotinic acetylcholine receptor (α7-nAChR) has has long been considered a promising therapeutic target for addressing cognitive impairments associated with a spectrum of neurological and psychiatric disorders, including Alzheimer's disease and schizophrenia. However, despite this potential, clinical trials employing α7-nAChR (partial) agonists such as TC-5619 and encenicline (EVP-6124) have fallen short in demonstrating sufficient efficacy. We here investigate the target engagement of TC-5619 and encenicline in the pig brain by use of the α7-nAChR radioligand 11C-NS14492 to characterize binding both with in vitro autoradiography and in vivo occupancy using positron emission tomography (PET). In vitro autoradiography demonstrates significant concentration-dependent binding of 11C-NS14492, and both TC-5619 and encenicline can block this binding. Of particular significance, our in vivo investigations demonstrate that TC-5619 achieves substantial α7-nAChR occupancy, effectively blocking approximately 40% of α7-nAChR binding, whereas encenicline exhibits more limited α7-nAChR occupancy. This study underscores the importance of preclinical PET imaging and target engagement analysis in informing clinical trial strategies, including dosing decisions.

In silico studies of ASEM analogues targeting α7-nAChR and experimental verification

The α7 nicotinic acetylcholine receptor (α7-nAChR) is implicated in a variety of neurodegenerative and neuropsychiatric disorders, such as Alzheimer's disease (AD) and schizophrenia. The progress of these disorders can be studied using positron emission tomography (PET) with radiotracers for α7-nAChR. [18F]ASEM and [18F] para-ASEM (also referred to as [18F]DBT-10) are novel and potent α7-nAChR PET radiotracers which have successfully been used in human subjects and nonhuman primates, though further improvement of them is still a pressing task in the community of neurodegeneration research. In this work, we demonstrate the use of modern in silico techniques to predict the binding modes, binding strengths, and residence times for molecular PET tracers binding to proteins, using ASEM and DBT-10 as a showcase of the predictive and interpretational power of such techniques, in particular free energy perturbation theory. The corresponding compounds were synthesized and further tested by in vitro binding experiment for validation. Encouragingly, our in silico modeling can correctly predict the binding affinities of the ASEM analogues. The structure-activity relationships for the ortho- and para-substitutions are well explained at the atomistic level and provide structure-based guiding for the future development of PET tracers for α7-nAChR. A discussion is presented on the complementary use of in silico rational methods based on atomic and electronic principles for in vitro characterization of PET tracers.

In Vitro and In Vivo Characterization of Dibenzothiophene Derivatives [125I]Iodo-ASEM and [18F]ASEM as Radiotracers of Homo- and Heteromeric α7 Nicotinic Acetylcholine Receptors

The α7 nicotinic acetylcholine receptor (α7 nAChR) is involved in several cognitive and physiologic processes; its expression levels and patterns change in neurologic and psychiatric diseases, such as schizophrenia and Alzheimer’s disease, which makes it a relevant drug target. Development of selective radioligands is important for defining binding properties and occupancy of novel molecules targeting the receptor. We tested the in vitro binding properties of [¹²⁵I]Iodo-ASEM [(3-(1,4-diazabycyclo[3.2.2]nonan-4-yl)-6-(¹²⁵I-iododibenzo[b,d]thiopentene 5,5-dioxide)] in the mouse, rat and pig brain using autoradiography. The in vivo binding properties of [¹⁸F]ASEM were investigated using positron emission tomography (PET) in the pig brain. [¹²⁵I]Iodo-ASEM showed specific and displaceable high affinity (~1 nM) binding in mouse, rat, and pig brain. Binding pattern overlapped with [¹²⁵I]α-bungarotoxin, specific binding was absent in α7 nAChR gene-deficient mice and binding was blocked by a range of α7 nAChR orthosteric modulators in an affinity-dependent order in the pig brain. Interestingly, relative to the wild-type, binding in β2 nAChR gene-deficient mice was lower for [¹²⁵I]Iodo-ASEM (58% ± 2.7%) than [¹²⁵I]α-bungarotoxin (23% ± 0.2%), potentially indicating different binding properties to heteromeric α7β2 nAChR. [¹⁸F]ASEM PET in the pig showed high brain uptake and reversible tracer kinetics with a similar spatial distribution as previously reported for α7 nAChR. Blocking with SSR-180,711 resulted in a significant decrease in [¹⁸F]ASEM binding. Our findings indicate that [¹²⁵I]Iodo-ASEM allows sensitive and selective imaging of α7 nAChR in vitro, with better signal-to-noise ratio than previous tracers. Preliminary data of [¹⁸F]ASEM in the pig brain demonstrated principal suitable kinetic properties for in vivo quantification of α7 nAChR, comparable to previously published data.

Frontiers in positron emission tomography imaging of the vulnerable atherosclerotic plaque

Rupture of vulnerable atherosclerotic plaques leading to an atherothrombotic event is the primary driver of myocardial infarction and stroke. The ability to detect non-invasively the presence and evolution of vulnerable plaques could have a huge impact on the future identification and management of atherosclerotic cardiovascular disease. Positron emission tomography (PET) imaging with an appropriate radiotracer has the potential to achieve this goal. This review will discuss the biological hallmarks of plaque vulnerability before going on to evaluate and to present PET imaging approaches which target these processes. The focus of this review will be on techniques beyond [18F]FDG imaging, some of which are clinically advanced, and others which are on the horizon. As inflammation is the primary driving force behind atherosclerotic plaque development, we will predominantly focus on approaches which either directly, or indirectly, target this process.

Brain PET Imaging of α7-nAChR with [18F]ASEM: Reproducibility, Occupancy, Receptor Density, and Changes in Schizophrenia

BACKGROUND

The α7 nicotinic acetylcholine receptor increasingly has been implicated in normal brain physiology, as well as in neuropsychiatric disorders. The highly cortical distribution of α7 nicotinic acetylcholine receptor suggests a role in cognition.

METHODS

We expanded the first-in-human PET imaging of α7 nicotinic acetylcholine receptor with [18F]ASEM from 5 to 21 healthy nonsmoking volunteers and added a feasibility study in 6 male patients with schizophrenia. Study aims included: (1) confirmation of test-retest reproducibility of [18F]ASEM binding, (2) demonstration of specificity by competition with DMXB-A, an α7 nicotinic acetylcholine receptor partial agonist, (3) estimation of [18F]ASEM binding potentials and α7 nicotinic acetylcholine receptor density in vivo in humans, and (4) demonstrating the feasibility of studying α7 nicotinic acetylcholine receptor as a target for schizophrenia.

RESULTS

Test-retest PET confirmed reproducibility (>90%) (variability ≤7%) of [18F]ASEM volume of distribution (VT) estimates in healthy volunteers. Repeated sessions of PET in 5 healthy subjects included baseline and effect of inhibition after oral administration of 150 mg DMXB-A. From reduction of binding potentials, we estimated the dose-dependent occupancy of α7 nicotinic acetylcholine receptor by DMXB-A at 17% to 49% for plasma concentrations at 60 to 200 nM DMXB-A. In agreement with evidence postmortem, α7 nicotinic acetylcholine receptor density averaged 0.67 to 0.82 nM and inhibitor affinity constant averaged 170 to 385 nM. Median VT in a feasibility study of 6 patients with schizophrenia was lower than in healthy volunteers in cingulate cortex, frontal cortex, and hippocampus (P = 0.02, corrected for multiple comparions, Mann-Whitney test).

CONCLUSIONS

The current results confirm the reproducibility of [18F]ASEM VT estimates and the specificity of the tracer for α7 nicotinic acetylcholine receptor. Preliminary findings from our feasibility study of [18F]ASEM binding in patients with schizophrenia are suggestive and provide guidance for future studies with more subjects.

Potential of α7 nicotinic acetylcholine receptor PET imaging in atherosclerosis

Atherosclerotic events are usually acute and often strike otherwise asymptomatic patients. Although multiple clinical risk factors have been associated with atherosclerosis, as of yet no further individual prediction can be made as to who will suffer from its consequences based on biomarker analysis or traditional imaging methods like CT, MRI or angiography. Previously, non-invasive imaging with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) PET was shown to potentially fill this niche as it offers high sensitive detection of metabolic processes associated with inflammatory changes in atherosclerotic plaques. However, ¹⁸F-FDG PET imaging of arterial vessels suffers from non-specificity and has still to be proven to reliably identify vulnerable plaques, carrying a high risk of rupture. Therefore, it may be regarded only as a secondary marker for monitoring treatment effects and it does not offer alternative treatment options or direct insight in treatment mechanisms. In this review, an overview is given of the current status and the potential of PET imaging of inflammation and angiogenesis in atherosclerosis in general and special emphasis is given to imaging of α7 nicotinic acetylcholine receptors (α7 nAChRs). Due to the gaps that still exist in our understanding of atherogenesis and the limitations of the available PET tracers, the search continues for a more specific radioligand, able to differentiate between stable atherosclerosis and plaques prone to rupture. The potential role of the α7 nAChR as imaging marker for plaque vulnerability is explored. Today, strong evidence exists that nAChRs are involved in the atherosclerotic disease process. They are suggested to mediate the deleterious effects of the major tobacco component, nicotine, a nAChR agonist. Mainly based on in vitro data, α7 nAChR stimulation might increase plaque burden via increased neovascularization. However, in animal studies, α7 nAChR manipulation appears to reduce plaque size due to its inhibitory effects on inflammatory cells. Thus, reliable identification of α7 nAChRs by in vivo imaging is crucial to investigate the exact role of α7 nAChR in atherosclerosis before any therapeutic approach in the human setting can be justified. In this review, we discuss the first experience with α7 nAChR PET tracers and developmental considerations regarding the "optimal" PET tracer to image vascular nAChRs.