Synthesis and evaluation of new imaging agent for central nicotinic acetylcholine receptor α7 subtype

https://doi.org/10.1016/j.nucmedbio.2009.11.007Get rights and content

Abstract

Introduction

The nicotinic acetylcholine receptor (nAChR) α7 subtype (α7 nAChR) is one of the major nAChR subtypes in the brain. We synthesized C-11 labeled α7 nAChR ligands, (R)-2-[11C]methylamino-benzoic acid 1-aza-bicyclo[2.2.2]oct-3-yl ester ([11C](R)-MeQAA) and its isomer (S)-[11C]MeQAA, for in vivo investigation with positron emission tomography (PET). Then, the potential of (R)- and (S)-[11C]MeQAA for in vivo imaging of α7 nAChR in the brain was evaluated in mice and monkeys.

Methods

The binding affinity for α7 nAChR was measured using rat brain. Biodistribution and in vivo receptor blocking studies were undertaken in mice. Dynamic PET scans were performed in conscious monkeys.

Results

The affinity for α7 nAChR was 41 and 182 nM for (R)- and (S)-MeQAA, respectively. The initial uptake in the mouse brain was high ([11C](R)-MeQAA: 7.68 and [11C](S)-MeQAA: 6.65 %dose/g at 5 min). The clearance of [11C](R)-MeQAA was slow in the hippocampus7 nAChR-rich region) but was rapid in the cerebellum7 nAChR-poor region). On the other hand, the clearance was fast for [11C](S)-MeQAA in all regions. The brain uptake of [11C](R)-MeQAA was decreased by methyllycaconitine7 nAChR antagonist) treatment. In monkeys, α7 nAChRs were highly distributed in the thalamus and cortex but poorly distributed in the cerebellum. The high accumulation was observed in the cortex and thalamus for [11C](R)-MeQAA, while the uptake was rather homogeneous for [11C](S)-MeQAA.

Conclusions

[11C](R)-MeQAA was successfully synthesized and showed high uptake to the brain. However, since the in vivo selectivity for α7 nAChR was not enough, further PET kinetic analysis or structure optimization is needed for specific visualization of brain α7 nAChRs in vivo.

Introduction

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels and are composed of α-(α210) and β-(β24) subunits [1], [2]. The major subtypes of nAChRs in the brain are α4β2, α3β2 and α7 [3]. The α7 nAChR is a homo-pentamer of α7 subunits and has high calcium permeability [4]. Some autoradiographical and tissue homogenate investigations using postmortem human brain suggest that α7 nAChR is implicated in Alzheimer's disease, dementia with Lewy bodies and schizophrenia [5], [6], [7]. The radiolabeled α7 nAChR ligands and in vivo imaging of this receptor would be useful to provide new information and insights on the role and function of the α7 nAChR system in the brain. α-Bungarotoxin and methyllycaconitine (MLA) are the specific ligands to α7 nAChRs. However, the radiolabelled ligand cannot be used for in vivo study due to the large molecular weight and poor blood-brain-barrier permeability [8], [9], [10]. Pomper et al. developed the low-molecular-weight radioiodinated ligands with high affinity to α7 nAChRs [11]. This compound showed a specific binding fraction in the brain. However, the brain uptake was not high enough for in vivo imaging, and the selectivity for α7 nAChR was low since this compound had affinity for 5-hydroxytryptamine type 3 (5-HT3) receptors instead, in which the sequence homology to α7 nAChR was high [12]. Recently, we developed the radioiodinated ligand with higher selectivity for α7 nAChR and higher brain uptake [13]. However, despite its high affinity and selectivity, it did not appear to be a suitable tracer for in vivo α7 nAChR imaging due to its high non-specific binding. [11C]CHIBA-1001 also appeared to have high affinity for 5-HT3 receptor [14]. A very recently developed compound, [18F]NS10743, showed promising results with high selectivity for α7 nAChR, although the brain uptake was moderate and the clearance from the brain was fast [15]. In this study, in order to develop a more appropriate ligand for α7 nAChR imaging in vivo, we designed a new radiolabelled compound for positron emission tomography (PET) and evaluated its availability.

Section snippets

General

1H-NMR spectrums were recorded on a JEOL GSX-400 spectrometer (400 MHz), using tetramethylsilane as an internal standard. [125I]α-Bungarotoxin (specific activity: 5.5 GBq/μmol) was obtained from Amersham Biosciences (Piscataway, NJ, USA). All other chemicals used were reagent grade. Male ddY mice and Wistar rats were supplied by (Sizuoka, Japan). The present animal study was approved by the Animal Care and Use Committee of the Hamamatsu University School of Medicine.

(S)-1-aza-bicyclo[2.2.2]octan-3-ol [(S)-3-quinuclidinol, 1]

(R, S)-3-Quinuclidinol (15g,

Radiosynthesis

Both [11C](R)- and (S)-MeQAA were successfully radiolabeled by treating with [11C]CH3OTf and NaOHaq in 2-butanone. The overall synthesis time for [11C]MeQAA was 45 min after the end of bombardment, and the radiochemical yield was 28% (decay corrected). The radiochemical purity was more than 99% and the specific radioactivity was over 74.6 GBq/μmol.

In vitro receptor binding studies

The Ki values for α7 nAChR, 5-HT3 receptor, and mAChR (M1, M2) are summarized in Table 1. (R)-MeQAA showed high binding affinity for the α7 nAChR.

Discussion

In vivo molecular imaging technique should provide great opportunity to figure out the mechanism of functional brain disorders. Several molecular imaging agents for α7 nAChR have recently been developed by some groups [11], [13], [14], [15], [22], [23]. These compounds have been tested in rodents, and some demonstrated promising results. Toyohara et al. applied their compound, [11C]CHIBA-1001 in humans. It showed a high brain uptake, and the regional differences were small [23].

In this study,

Conclusion

[11C](R)-MeQAA could be successfully synthesized from [11C]CH3OTf and showed high uptake to the brain. However, further PET kinetic analysis or structure optimization is needed for specific visualization of brain α7 nAChRs in vivo.

Acknowledgments

We thank Takeharu Kakiuchi, Ph.D., Hiroyuki Ohba, Ph.D., and Norihiro Harada, M.S., for excellent technical assistance.

This work was supported by the Grants-in-Aid for Scientific Research and by the Center of Excellence (COE) at Hamamatsu University School of Medicine “Medical Photonics” at Hamamatsu University School of Medicine from the Ministry of Education, Culture, Sports, Science and Technology, Japan; CREST, Japan Science and Technology Agency (JST), Saitama, Japan and a grant for the

References (30)

  • KulakJ.M. et al.

    Differences in alpha7 nicotinic acetylcholine receptor binding in motor symptomatic and asymptomatic MPTP-treated monkeys

    Brain Res

    (2004)
  • Karlin A and Akabas MH. Toward a structural basis for the function of...
  • AlbuquerqueE.X. et al.

    Properties of neuronal nicotinic acetylcholine receptors: pharmacological characterization and modulation of synaptic function

    J Pharmacol Exp Ther

    (1997)
  • CourtJ. et al.

    Neuronal nicotinic receptors in dementia with Lewy bodies and schizophrenia: alpha-bungarotoxin and nicotine binding in the thalamus

    J Neurochem

    (1999)
  • JamesR.W. et al.

    Characterization of iodinated derivatives of alpha-bungarotoxin

    Hoppe Seylers Z Physiol Chem

    (1980)
  • Cited by (27)

    • Uptake of nicotinic acetylcholine receptor imaging agent is reduced in the pro-inflammatory macrophage

      2021, Nuclear Medicine and Biology
      Citation Excerpt :

      Several PET imaging agents have been reported for α7 and α4β2 nAChRs. ( R)-2-[11C]methylamino-benzoic acid 1-aza-bicyclo[2.2.2]oct-3-yl ester ([11C]MeQAA) and 2-[18F]fluoro-3-(2(S)-azetidinylmethoxy) pyridine ([18F]2FA) are previously developed radioligands for α7 and α4β2 nAChRs, respectively [8,9]. The α7 and α4β2 nAChRs are highly expressed in the central nervous system, thereby [11C]MeQAA and [18F]2FA are mainly used in studies of neuroimaging applications.

    • Synergic effect of a quinuclidine benzamide complexed with borane, the LMA10233, in combination with seven pesticides

      2020, Pesticide Biochemistry and Physiology
      Citation Excerpt :

      Here, an original BH3-quinuclidine complex compound (±)-4 named LMA10233, was prepared from the first step of acylation reaction between 3-amino quinuclidine salt 2 and 2-methoxybenzoyl chloride 1, to give the quinuclidine benzamide (±)-3, as described in an earlier study (Mathe-Allainmat et al., 2013). In a second step, LMA10233 was efficiently prepared from the benzamide precursor (±)-3, with 80% overall yield, following the procedure described in Fig. 1B. Similar quinuclidine borane complexes have been mainly described as synthetic intermediates in the literature, and were proved to be stable in various non acidic conditions (Ogawa et al., 2010; Cook et al., 2017; Del Bello et al., 2017). The susceptibility of A. pisum larvae to LMA10233 was determined using artificial diet bioassay (Sadeghi et al., 2009; Taillebois et al., 2014a; Taillebois et al., 2014b).

    • Human Brain Imaging of Acetylcholine Receptors

      2014, Imaging of the Human Brain in Health and Disease
    • Synthesis and evaluation of new radioligands [<sup>11</sup>C]A-833834 and [<sup>11</sup>C]A-752274 for positron-emission tomography of α7-nicotinic acetylcholine receptors

      2013, Nuclear Medicine and Biology
      Citation Excerpt :

      While PET imaging of α4β2-nAChR in human subjects is a current reality (see for review [23–25]), a lack of PET radioligands for α7-nAChR in human subjects impedes progress in the neuroscience of this receptor subtype. A number of α7-nAChR PET radioligands have been developed, but a quality radiotracer remains to be synthesized [26–28] (see also recent reviews [25,29]). Abbott Laboratories has recently developed two α7-nAChR ligands (A-833834 and A-752274) with very high binding affinities (Fig. 1) [30,31].

    • Methyllycaconitine: A non-radiolabeled ligand for mapping α7 neuronal nicotinic acetylcholine receptors - In vivo target localization and biodistribution in rat brain

      2012, Journal of Pharmacological and Toxicological Methods
      Citation Excerpt :

      Many of ligands which were tried to develop for such cause did not achieve the required properties. CHIBA1001 had shown poor in vivo regional, pharmacological selectivity and accumulation in cerebellum of rodent brain (Davies et al., 1999; Jun et al., 2009; Ogawa et al., 2006; Ogawa et al., 2009; Ogawa et al., 2010; Tanibuchi et al., 2010; Villemagne et al., 1999). Blockade study conducted with selective α7 nAChR ligands demonstrated incomplete reduction in the binding of [11C] CHIBA1001 and [11C] NSI2857 due to lack of in vivo selectivity in preclinical animal species (Biton et al., 2007; Briggs et al., 2009; Ettrup et al., 2009; Hashimoto et al., 2008).

    View all citing articles on Scopus
    View full text