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

Abstract

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.

Development of 11C-Labeled ASEM Analogues for the Detection of Neuronal Nicotinic Acetylcholine Receptors (α7-nAChR)

“…125 I-α-bungarotoxin has been suggested as the in vitro gold-standard radioligand for α7 nAChR. 19 , 25 The α7 nAChRs are widely distributed in the mammalian brain, with highest receptor density in the hippocampus, hypothalamus, amygdala, and cerebral cortex and lowest receptor density in the cerebellum. 26 The regional binding of 3 H-KIn-83 was comparable with the pattern of 125 I-α-bungarotoxin binding earlier demonstrated by Härfstrand et al, 38 showing high specific binding in the hippocampus, hypothalamus, amygdala, and the cerebral cortex of the rat brain.…”

“…Development of more selective radioligands is significant for describing the binding properties and occupancy of molecules targeting the receptor. 19 …”

Novel PET Biomarkers to Disentangle Molecular Pathways across Age-Related Neurodegenerative Diseases

“…Therefore, while cholinergic dysfunction is likely present across several age-related neurodegenerative diseases, the degree of deficits, as well as the temporal and spatial presentations, is likely different. Future PET imaging of the vesicular acetylcholine transporters, using ( 18 F)FEOBV [231], and of the α7-nACh receptors using the novel tracer ( 18 F)ASEM [232,233], could provide further insights into the integrity of cholinergic nerve terminals in vivo.…”

Homomeric and Heteromeric α7 Nicotinic Acetylcholine Receptors in Health and Some Central Nervous System Diseases

“…It should be noted that most studies conducted on tissues where α7 and α7β2 nAChR are co-expressed, e.g., the basal forebrain, are not able to dissect the contribution of each subtype of receptors. This is because the ligands used to assay the homomeric receptors also bind heteromeric receptors [ 42 ]. Many conclusions about the role of homomeric α7 nAChRs should be revisited to consider the contribution of heteromeric α7β2 nAChR.…”

“…A549 adenocarcinoma, U87MG human grade-IV astrocytoma, and GBM5 glioblastoma cells lines express α7 and α9/α10 nAChRs subunits [ 23 , 48 ]. The application of agonists such as choline or nicotine activates Akt/ERK anti-apoptotic signals and promotes cell proliferation [ 23 , 42 ]. The doses used to obtain responses are below those required to record currents in electrophysiological experiments, suggesting that metabotropic signaling is involved in the former case.…”