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. Author manuscript; available in PMC: 2014 Jul 28.
Published in final edited form as: J Nucl Med. 2014 Feb 20;55(4):672–677. doi: 10.2967/jnumed.113.132068

[18F]ASEM, a radiolabeled antagonist for imaging the α7-nicotinic acetylcholine receptor (α7-nAChR) with positron emission tomography (PET)

Andrew G Horti 1, Yongjun Gao 1, Hiroto Kuwabara 1, Yuchuan Wang 1, Sofya Abazyan 2,3, Robert P Yasuda 4, Thao Tran 4, Yingxian Xiao 4, Niaz Sahibzada 4, Daniel P Holt 1, Kenneth J Kellar 4, Mikhail V Pletnikov 2,3, Martin G Pomper 1, Dean F Wong 1,2, Robert F Dannals 1
PMCID: PMC4112566  NIHMSID: NIHMS607225  PMID: 24556591

Abstract

The α7-nicotinic cholinergic receptor (α7-nAChR) is a key mediator of brain communication and has been implicated in a wide variety of central nervous system disorders. None of the currently available PET radioligands for α7-nAChR are suitable for quantitative PET imaging, mostly due to insufficient specific binding. The goal of this study was to evaluate the potential of [18F]ASEM ([18F]JHU82132) as an α7-nAChR radioligand for PET.

Methods

Inhibition binding assay and receptor functional properties of ASEM were assessed in vitro. The brain regional distribution of [18F]ASEM in baseline and blockade were evaluated in DISC1 mice (dissection) and baboons (PET).

Results

ASEM is an antagonist for the α7-nAChR with high binding affinity (Ki = 0.3 nM). [18F]ASEM readily entered the baboon brain and specifically labeled α7-nAChR. The in vivo specific binding of [18F]ASEM in the brain regions enriched with α7-nAChRs was 80–90%. SSR180711, an α7-nAChR selective partial agonist, blocked [18F]ASEM binding in the baboon brain in a dose-dependent manner, suggesting that the binding of [18F]ASEM was mediated by α7-nAChRs and the radioligand was suitable for drug evaluation studies. In the baboon baseline studies, the brain regional volume of distribution (VT) values for [18F]ASEM were 23 (thalamus), 22 (insula), 18 (hippocampus) and 14 (cerebellum), whereas in the binding selectivity (blockade) scan, all regional VT values were reduced to less than 4. The range of regional binding potential (BPND) values in the baboon brain was from 3.9 to 6.6. In vivo cerebral binding of [18F]ASEM and α7-nAChR expression in mutant DISC1 mice, a rodent model of schizophrenia, was significantly lower than in control animals, which is in agreement with previous post-mortem human data.

Conclusion

[18F]ASEM holds promise as a radiotracer with suitable imaging properties for quantification of α7-nAChR in the human brain.

Keywords: α7-nAChR, nicotine, PET, baboon, [18F]ASEM

Introduction

Nicotinic cholinergic receptors (nAChRs) are ionotropic cationic channels that are fundamental to physiology and, as a class, represent an important target for drug discovery. nAChRs are found in the central nervous system (CNS), autonomic and sensory ganglia, and various non-neuronal cells. In the CNS, nAChRs mediate fast excitatory post-synaptic responses to its cognate ligand acetylcholine (ACh) and other nicotinic agonists (1).

Cerebral nAChRs are composed of various α and β subunits that can assemble into pentamers with α4β2-nAChR and α7-nAChR subtypes representing the highest concentration of nAChRs in the CNS (2). The α7-nAChR is involved in pathogenesis of a variety disorders and conditions including schizophrenia (SCZ) and Alzheimer's disease (AD), inflammation and traumatic brain injury, cancer and macrophage chemotaxis (3–8).

Despite intense study, the role of α7-nAChRs in the brain is not fully understood. Positron emission tomography (PET) is the most advanced technique to map and quantify cerebral receptors and their occupancy by neurotransmitters and drugs in human subjects. The lack of PET radioligands for quantitative imaging the α7-nAChRs in human subjects represents a gap that hampers research of this receptor system and development of new drugs for this target.

Over the past decade, considerable effort has been expended toward the development of α7-nAChR ligands and more than 20 compounds have been radiolabeled for PET and SPECT, but previous efforts by several research groups (see for review (9–11)), including our own (12–14), to develop a clinically viable α7-nAChR tracer for PET or SPECT have proved unsuccessful. None of these radioligands manifested sufficiently high specific binding at α7-nAChRs in vivo. Even [11C]CHIBA-1001, the recent PET radioligand for human subjects, exhibited low α7-nAChR binding affinity and poor in vivo selectivity (15, 16). Accordingly, there is still a pressing scientific need for a practical PET radiotracer for quantification of α7-nAChRs.

As part of our PET radioligand development program, we developed [18F]ASEM ([18F]JHU82132, 3-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-6-[18F]fluorodibenzo[b,d]thiophene 5,5-dioxide), an 18F-labeled, high affinity and selectivity α7-nAChR PET radioligand (Fig. 1) that showed excellent specific binding (BPND = 8) in control CD-1 mice (17). Here we describe further pre-clinical characterization of [18F]ASEM in vitro and in vivo in baboon PET imaging studies and in DISC1 mice, a rodent model of SCZ.

Figure 1.

Figure 1

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Chemical structure of the high binding affinity α7-nAChR – selective PET radioligand [18F]ASEM ([18F]JHU82132) (17)

MATERIALS AND METHODS

All experimental animal protocols were approved by the Animal Care and Use Committee of the Johns Hopkins Medical Institutions.

[18F]ASEM and unlabelled ASEM were synthesized as described by our lab (17). Briefly, the radiotracer was prepared by nucleophilic radiofluorination of the corresponding nitro-precursor. The final product was purified by high-performance liquid chromatography (HPLC) and formulated as a sterile, apyrogenic solution in saline containing 8% alcohol. The specific radioactivity of the tracer was in the range of 296 to 2180 GBq/μmol (8 – 59 Ci/μmol), calculated at the end of synthesis, and the radiochemical purity was greater than 99%. The average radiochemical yield was 15 ± 7% (n = 12)

In Vitro Inhibition Binding Assay of ASEM and Functional Electrophysiology Method

HEK 293 cell culture and stable transfections of α7-nAChR and the ASEM inhibition binding assay with ([125I]α-bungarotoxin ([125I]-α-BTX) was performed as described previously (18) (see for details Supplemental S.1).

Whole-cell voltage clamp (holding potential −70 mV) recordings from HEK293 cells stably transfecting the rat α7–nAChR were made with patch electrodes (5–6 MΩ) containing a solution (pH 7.2) composed of: potassium gluconate (145 mM); ethylene glycol tetraacetic acid (EGTA) (5 mM); MgCl2 (2.5 mM); HEPES (10 mM); ATP·Na (5 mM); and GTP·Na (0.2 mM). Cells were continuously perfused with recording solution having the following composition: NaCl (130 mM); KCl (5 mM); CaCl2 (2 mM); MgCl2 (2 mM); glucose (10 mM); and HEPES (10 mM), pH 7.4, at a temperature of 24°C. The patch pipette was coupled to an amplifier Axopatch 200B (Molecular Devices) and its signal filtered (5 kHz), digitized with a Digidata 1440A (Molecular Devices) and analyzed with pClamp 10 software (Molecular Devices, Sunnyvale, CA). Acetylcholine was delivered to the cells rapidly by pressure application (picospritzer; World Precision Instruments, Sarasota, FL) for 0.5 sec. A bath was applied to the compound ASEM for 2 minutes before and during the application of ACh by pressure application.

Biodistribution Study in Mutant DISC1 and Control Mice

Male, DISC1 (16–18 g) and control (17–19 g) mice both on a C57BL/6 background were generated as previously described (19) and were used for biodistribution studies with six animals per data point. The animals were sacrificed by cervical dislocation at 90 min following injection of [18F]ASEM (2.6 mBq, specific radioactivity ~300 GBq/μmol, in 0.2 mL saline) into a lateral tail vein. The brains were rapidly removed and dissected on ice. The brain regions of interest were weighed and their radioactivity content was determined in an automated γ-counter with a counting error below 3%. Aliquots of the injectate were prepared as standards and their radioactivity content was determined along with the tissue samples. The percent of injected dose per gram of tissue (%ID/g tissue) was calculated.

Western Blot with DISC1 and Control Mice

Mice were euthanized at postnatal day 21 to evaluate expression of α7-nAChR in mutant DISC1 and control animals. Frontal cortices were quickly dissected out on ice-cold phosphate buffered saline (PBS) and frozen on dry ice and kept at −80 °C until used. These samples were assayed for expression of mutant DISC1 (19). Membranes were incubated overnight at 4°C with either mouse anti-myc antibody (Santa Cruz Biotechnology Inc., CA, USA; 1:1000) to assess expression of mutant DISC1 tagged with myc or rabbit polyclonal antibody to α7-nAChR (ab10096, Abcam Inc., MA; 1:500). Secondary antibodies were peroxidase-conjugated goat anti-mouse (Kierkegaard Perry Labs, 1:1000) or sheep anti-rabbit (GE Healthcare, 1:2500). The optical density of protein bands on each digitized image was normalized to the optical density of β-tubulin as a loading control (Cell signaling technology Inc., Beverly, MA, 1:10 000). Normalized values were used for statistical analyses.

Baboon PET Imaging and Baboon PET Data Analysis

PET experiments were performed on three male baboons (Papio anubis) weighing 20.1 – 26 kg, on the High Resolution Research Tomograph (CPS Innovations, Inc., Knoxville, TN). The animals were anesthetized and handled as described previously (20) (see Supplemental S.3). Three animals were scanned with [18F]ASEM in baseline scans. Dynamic PET acquisitions were performed in a 3-D list mode for 90 min following an intravenous bolus injection of [18F]ASEM (246 – 319 MBq; n = 3) with specific radioactivities in the range of 343 – 1,764 GBq/μmol. PET images were reconstructed as described in Supplemental S.4.

In two blocking scans, the blocker SSR180711 solution in saline was given as intravenous bolus doses (0.5 mg/kg or 5 mg/kg) 90 min before the radioligand [18F]ASEM injection (doses 147 and 251 MBq, specific radioactivity 462 and 1014 GBq/μmol). The blocking scans were performed in one of the baboons that were used in the baseline scans and separated at least 32 days from each other and the baseline scan.

A locally-developed VOI template was transferred to each animal's MRI image using spatial normalization parameters given by SPM5 (statistical parametric mapping (21); available at http://www.fil.ion.ucl.ac.uk/spm/software/spm5) and adjusted for anatomical details. Then, VOIs were transferred to the PET spaces of the baseline and blocking scans using MRI-to-PET coregistration module of SPM5 (22). Time-radioactivity curves (TACs) of regions were obtained by applying the VOIs on PET frames.

One- and two-tissue compartmental models (OTCM and TTCM) were employed for derivation of regional distribution volume (VT) for [18F]ASEM, with and without setting the K1-k2 ratio to the estimate of a large region (denoted as TTCM-C). Akaike information criteria (23) and the numbers of outliers were used to identify the optimal plasma input method for the radioligand. In addition, the plasma reference graphical analysis (PRGA) was evaluated (24). In blocking scans, occupancies of α7-nAChRs by SSR180711 were obtained as follows: Occupancy = ΔVT/(VT(baseline) - VND), where ΔVT was given by VT(baseline) - VT(blocking), and VND, distribution volume of non-displaceable radioligand was obtained as x-intercept of the Lassen plot (25) of ΔVT (=y) versus baseline VT.

[18F]ASEM Radiometabolite Analysis in Baboon and Mice

Baboon arterial blood samples were withdrawn at 5, 10, 20, 30, 60 and 90 min after [18F]ASEM injection, and plasma was analyzed by HPLC. Male CD1 mice (25–26 g) were injected via the lateral tail veins with 37 mBq of high specific activity [18F]ASEM. The mice were killed by cervical dislocation at 2 and 30 minutes post injection and a terminal blood sample was obtained. The mouse brains were rapidly removed and analyzed by HPLC (see for details Supplemental S.2).

RESULTS

Binding Affinity

In two experiments unlabeled ASEM exhibited high in vitro binding affinity to HEK293 cells stably transfected with rat α7-nAChR (Ki = 0.3, 0.3 nM) ([125I]α-bungarotoxin ([125I]-α-BTX)).

In Vitro Functional Assay

The functional activity of unlabeled ASEM was examined using whole cell voltage clamp measurements in HEK293 cells expressing α7-nAChRs. As shown in Fig. 2, acetylcholine at a concentration of 316 μM activates these receptors and ASEM at a concentration of 1 nM nearly completely blocked activation by acetylcholine. Moreover, a partial block persists during the short period of washing, probably because of the high affinity of ASEM.

Figure 2.

Figure 2

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The unlabeled compound ASEM inhibits the activation of ACh-stimulated rat α7-nAChRs. Whole-cell voltage clamp current activated with 316 μM ACh either before or during the bath application of 1 nM ASEM was determined in HEK293 cells stably transfected with rat α7-nAChRs. Bath application of ASEM for 2 min prior to and during the application of ACh inhibited subsequent ACh-induced whole-cell current. This current was restored to 60% of baseline after 12 min of washing.

Brain Distribution of [18F]ASEM in Mutant DISC1 and Control Mice

Mutant DISC1 mice provide a model for brain and behavioral phenotypes seen in SCZ (19). Comparison of regional brain uptake of [18F]ASEM in mutant DISC1 versus control mice demonstrated that the uptake in the mutant mice was significantly lower in all regions studied. Because of the difference in the mouse weight (up to 15%), the uptake values were corrected for the body weight (%ID/g tissue × body weight) (Fig. 3A).

Figure 3.

Figure 3

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Panel A: Comparison of regional uptake of [18F]ASEM in control (black bars) and DISC1 (white bars) mice at 90 min post-injection. There was a significant reduction of [18F]ASEM in DISC1 in the brain regions studied. Data: mean %ID/g tissue*body weight ± standard deviation (n=6). Abbreviations: Coll = superior and inferior colliculus; Hipp = hippocampus; Ctx = cortex. *P = 0.01 and **P < 0.01, significantly different from controls (ANOVA). Panel B: Western blot. Expression of α7-nAChR protein in P21 cortex of mutant DISC1 (n=5) is significantly lower than in that of control mice (n=3). *P = 0.035 (Student's t-test, t = 2.7)

Western blot analysis of expression of α7-nAChR in the cortical regions was in agreement with the biodistribution of [18F]ASEM. We found a significant decrease in levels of the receptor in the cortex of mutant DISC1 mice compared to control mice (Fig. 3B).

PET Imaging in Papio Anubis Baboons

Heterogeneous uptake of radioactivity into the baboon brain was observed in baseline experiments after bolus injection of [18F]ASEM in three baboons (Figs. 4, 5). The highest accumulation of radioactivity occurred in the thalamus, insula and anterior cingulate cortex. The intermediate uptake was observed in the putamen, hippocampus and several cortical regions. The lowest uptake was in the corpus callosum, pons and cerebellum. The time-activity curves (TACs) of the cerebellum peaked prior to 20 min, and decreased rapidly while TACs of other regions were slower with wider peaks and decreased relatively slowly (Fig. 4). In the three baseline experiments no blocking effect was observed due to the variation of [18F]ASEM specific activity from high (343 GBq/μmol) to very high (1,764 GBq/μmol).

Figure 4.

Figure 4

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Baseline cerebral time-activity curves (TACs) after bolus administration of [18F]ASEM in three baboons. The graph demonstrates a substantial heterogeneous brain uptake of [18F]ASEM that matches distribution of α7-nAChR in non-human primates (29, 30, 32) and reversible brain kinetics. Data: mean Standardized Uptake Values (%SUV) ± SD (n = 3). Symbols: Th = thalamus; In = insula; aCg = anterior cingulate cortex; Pu = putamen; Tp = temporal lobe; Pa = parietal lobe; Hp = hippocampus; Oc = occipital lobe; CC = corpus callosum; Po = pons; CB = cerebellum.

Figure 5.

Figure 5

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Averaged trans-axial %SUV PET images (10–90 min) of [18F]ASEM (upper row) at levels showing putamen (Pu/+; Column A), thalamus (Th/+; Column B), and cortices such as frontal (Fr/+) and parietal (Pa/x) cortices (Column C), as shown on MRI images (lower row).

The kinetics of [18F]ASEM in the brain fitted well to a two-tissue compartmental model (TTCM). The plasma reference graphical analysis (PRGA) plots reached an asymptote (the coefficient of determination, R2 > 0.995) at 30 min in all regions. Therefore, PRGA was employed for further analyses. Regional values of VT of [18F]ASEM in baboon are shown in Fig. 7. The thalamus, insula, and anterior cingulate cortex provided the highest VT values, and pons, corpus callosum, and cerebellum showed the lowest VT values.

Figure 7.

Figure 7

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Panel A: The Lassen plot for the 5 mg/kg dose experiment demonstrates that specific binding of [18F]ASEM is blocked by α7-nAChR selective ligand SSR180711. Data points showed a linear appearance (ΔVT = 0.82·VT - 0.66; R2 = 0.979; VND = 0.8 mL/mL). VND is given as the x-intercept in the plot. Panel B: Histogram of VT values of [18F]ASEM (plasma reference graphical analysis PRGA) in selected brain regions of one baboon at baseline and after administration of two different doses of SSR180711. The graph demonstrates that the regional binding of [18F]ASEM is specific and high and mediated by α7-nAChR.

Injection of SSR180771, a selective α7-nAChR partial agonist (Ki = 22 nM) (26), reduced the regional uptake of [18F]ASEM in the baboon brain in dose-dependent manner (Fig. 6). Regional VT values in baseline and blockade experiments are shown in Fig. 7.

Figure 6.

Figure 6

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Sagittal (Top row), and trans-axial (Middle and Bottom rows) views of VT images of [18F]ASEM in the same baboon for a baseline PET scan (B), and after administration of 0.5 mg/kg (C) and 5 mg/kg (D) of SSR180711, a selective α7-nAChR partial agonist. MR images (A) indicate locations of selected brain structures including the cingulate cortex (Cg), thalamus (Th), and caudate nucleus (CN), which are indicated by + in the VT images (D). The VT images were displayed using the same minimum and maximum values for all scanning conditions. These data demonstrate the dose dependent blockade of [18F]ASEM in baboon brain and provide evidence that [18F]ASEM is specific and mediated by α7-nAChR. The images also suggest that there is no reference region devoid of α7-nAChRs.

Lassen plots showed a linear arrangement for 0.5 and 5 mg/kg doses, as exemplified for the 5 mg/kg dose in Fig. 7-A (for the 0.5 mg/kg dose: ΔVT = 0.39·VT − 2.1; R2 = 0.643; VND = 5.4 mL/mL). Mean occupancy values increased from 38% with 0.5 mg/kg dose to 80.5% with a 5 mg/kg dose using individual VND values, and from 32.9% to 94.1% using the mean VND value of two doses. Although estimates of VND differed between two blocking scans, individual values were several folds lower than the lowest observed VT (14 mL/mL in pons) among the tested regions. This finding confirmed the lack of α7-nAChR – free regions in the baboon brain, and low non-specific binding of [18F]ASEM across regions (e.g., less than 30% in pons and cerebellum and lower in other regions), and explained consistent occupancy estimates. Regional BPND (=(VT/VND) − 1) values of [18F]ASEM in the baboon brain ranged from 3.9 to 6.6 (unitless), using the mean VND value of the two blocking scans.

Metabolism of [18F]ASEM in Mouse and Baboon

Radiometabolite analysis of blood samples from CD-1 mice and baboons by reverse-phase high-performance liquid chromatography (RP-HPLC) showed that the parent compound [18F]ASEM was metabolized to two major hydrophilic species. The combined radiometabolites in the plasma reached values of 70% in baboon and ~ 99% in mice at 90 min and 30 min post-injection, respectively (see Supplemental S.2 and Fig. S1). These radiometabolites do not enter the brain to an appreciable extent, because at least 95% of the unchanged parent [18F]ASEM was present in the mouse brain versus ~ 1% in the mouse blood after intravenous administration of [18F]ASEM. The amount of unchanged parent [18F]ASEM in the baboon brain should be even greater than that in mouse (> 95%) because the metabolism in baboon is slower. This observation suggests that modeling of the metabolites may not be necessary for quantification of α7-nAChR with [18F]ASEM.

DISCUSSION

Our previous in vitro binding assay studies demonstrated that ASEM exhibits high α7-nAChR binding affinity in rat brain membranes and excellent selectivity versus other heteromeric nAChR subtypes and 5-HT3 (17). Those studies demonstrated that ASEM exhibits at least an order of magnitude greater binding affinity than the previous α7-nAChR PET radioligands (17). In this report, we have confirmed the high α7-nAChR binding affinity of ASEM in the binding assay with HEK293 cell line expressing rat α7-nAChR (Ki = 0.3, 0.3 nM).

The functional assay demonstrated that ASEM is a powerful antagonist of α7-nAChR (Fig. 2). This is in accord with functional properties of des-fluoro-ASEM, 3-(1,4-diazabicyclo[3.2.2]nonan-4-yl)dibenzo[b,d]thiophene 5,5-dioxide, that was recently published by Abbott Labs (27). This functional property may be also advantageous from the stand point of safety if [18F]ASEM is used in human PET studies as it should not cause toxic effects that are common among nicotinic agonists (28).

The initial in vivo distribution studies in control mice have demonstrated that [18F]ASEM selectively labels α7-nAChR with very high specificity (BPND = 8) (17). Based on the favorable imaging properties identified in normal mice, we investigated here the [18F]ASEM cerebral binding in mutant DISC1 mice, a rodent model of SCZ (19). Previous post-mortem research demonstrated a significantly lower densities of α7-nAChR in the cortical and subcortical (hippocampus) brain regions of schizophrenic subjects versus controls (see for review (8)). In agreement with this in vitro human data, the brain regional distribution experiments with DISC1 mice showed a significant reduction of [18F]ASEM binding in the α7-nAChR – rich colliculus, cortex and hippocampus in comparison with control animals (Fig. 3A). Western blot data (Fig. 3B) of α7-nAChR protein expression in the cortex of DISC1 and control animals was in agreement with [18F]ASEM binding.

This result in DISC1 mice is consistent with previous post-mortem brain studies of subjects suffering from SCZ (8) and further emphasizes the potential utility of this new radioligand for imaging α7-nAChR in disease.

[18F]ASEM exhibited high (up to 500 %SUV) and reversible brain uptake in baboon brain experiments (Figs. 4 and 5). The cerebral α7-nAChR is heterogeneously distributed in primate brain with the highest concentration in the thalamus, putamen, several cortical regions and hippocampus (29–32). The observed PET regional distribution patterns of [18F]ASEM in the baboon brain (thalamus > putamen, cortex, hippocampus > caudate nucleus, globus pallidus > corpus callosum) are consistent with in vitro data in rhesus and cynomolgus macaque monkey (29, 30, 32). The existing quantitative non-human primate data describing the brain distribution of α7-nAChR using in vitro autoradiography are only detailed for subcortical regions, but limited for cortical regions or semi-quantitative (29, 30, 32). The PET-[18F]ASEM baboon experiments demonstrated that the lowest α7-nAChR uptake, albeit still considerable, was in cerebellum. The cerebellum was not assessed in the previous monkey autoradiography studies (29, 30, 32). It is noteworthy that the uptake of radioactivity in the baboon skull was very low which suggests little metabolism of [18F]ASEM to [18F]fluoride that can confound PET studies with 18F-labeled agents.

The dose-dependent blockade of [18F]ASEM with selective α7-nAChR partial agonist SSR180711 (Figs. 6 and 7) demonstrated that the binding of the radioligand in the baboon brain is specific (up to 80–90%) and mediated by α7-nAChR. The level of specific binding of [18F]ASEM is well above the conventional minimum of the required specific binding value (≥ 50%) for a clinically viable PET radioligand. [18F]ASEM is suitable for quantitative analysis and its BPND values (3.9 – 6.6 (unitless)) in the baboon brain are rather high. For comparison, the BPND values of all previously published α7-nAChR radioligands did not exceed 1 (see for review (9–11, 17)). This high specific binding of [18F]ASEM in combination with high brain uptake and VT values, reversible brain kinetics and absence of active metabolites make this radioligand an excellent candidate for further translation to human PET imaging of α7-nAChR's.

CONCLUSION

In conclusion, we have developed a new, specific α7-nAChR ligand, [18F]ASEM, that demonstrates suitable properties for imaging this important CNS target with PET in mice and baboons. Unlike its predecessors, [18F]ASEM has proved to be amenable to quantitative analysis with a useful degree of binding specificity (up to 80 – 90%) and high BPND values = 3.9 – 6.6 in the α7-nAChR - rich brain regions as demonstrated by in vivo receptor blockade studies in baboons.

The brain uptake of [18F]ASEM in an established rodent model of SCZ, DISC1 mutant mice, reflects reduced α7-nAChR binding, which has only been shown previously in postmortem human studies, lending further support to this model as well as suggesting high utility of [18F]ASEM for studying diseases of nicotinic transmission.

The [18F]ASEM radioligand holds considerable promise for human studies for understanding the role of α7-nAChRs in CNS disorders and will aid α7-nAChR targeted drug development. Experiments in relevant human populations are being vigorously pursued.

Supplementary Material

Supplemental data

ACKNOWLEDGEMENTS

We thank Dr. Richard Wahl for fruitful discussions. The authors thank Drs. Weiguo Ye, Asifa Zaidi and Jennifer Coughlin for injecting baboons with [18F]ASEM. We are grateful to Paige Finley, Heather Valentine, Gilbert Green and Chunxia Yang for their valuable help with baboon and mouse experiments, and David J. Clough and Karen Edmonds for PET scanner operation and Alimamy Kargbo for HPLC analysis of radiometabolites. We are thankful to Julia Buchanan for editorial help. This research was supported by NIH Grants MH079017 and AG037298 (AGH) and, in part, by the Division of Nuclear Medicine of Johns Hopkins University School of Medicine.

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