Background

[PubMed]

The nicotinic acetylcholine receptors (nAChR) are pentameric ligand-gated cation channels that are found primarily in the central nervous system of mammals and are composed of α and β subunits arranged in various homomeric or heteromeric stoichiometric arrangements (for details, see dos Santos Coura and Granon (1)). The nAChRs have a high affinity for both nicotine and acetylcholine; binding of a ligand to the receptor facilitates the passage of cations into the cell, which alters the polarity of the cell membrane and activates the signal transduction pathways that culminate in a cellular response (2). These receptors are believed to be involved in a variety of cognitive functions and memory (1), neuroprotection in Parkinson’s disease (3), Alzheimer’s disease (AD) (4), and the progression of certain cancers (5). The heteromeric α4β2 and the homomeric α7 are the most commonly expressed nAChRs in the brain, and the β2 subunit is a component of most heteromeric nicotinic receptors in the central nervous system. There is much evidence that α4β2-nAChR is involved in normal cognitive functions in animals; it is also implicated in addiction (e.g., smoking) and the cognitive impairments observed in AD, attention deficit hyperactivity disorder, and schizophrenia (6). α7-nAChR participates in cognition and has a reduced expression in the human schizophrenic brain, and there are indications that it has a therapeutic role in the treatment of AD (7).

Although some radiolabeled compounds can be used with positron emission tomography (PET) to visualize α4β2-nAChR in the human brain these radio-ligands exhibit slow kinetics and a low binding potential in the brain which limits their use to investigate nAChR in humans (8). Therefore, no suitable tracer(s) are available for the noninvasive imaging of α7-nAChR in humans (7). In an effort to develop new ligands for α7-nAChR two compounds, 5-(6-(5-methylhexahydropyrrolo[3,4-c]pyrrol-2(1H)-yl)pyridazin-3-yl)-1H-indole (A-833834) and 2-(6-methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-7-(6-methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-9H-fluoren-9-one (A-752274) were synthesized, labeled with ¹¹C (to obtain [¹¹C]A-833834 and [¹¹C]A-752274, respectively), and evaluated in mice (7). The biodistribution of both tracers was investigated in the mouse brain, and [¹¹C]A-752274 was also evaluated for use with in vivo PET imaging of a baboon brain. This chapter presents results obtained with [¹¹C]A-833834. The biodistribution and PET imaging characteristics of [¹¹C]A-752274 are discussed in a separate chapter in MICAD (www.micad.nih.gov) (9).

Synthesis

[PubMed]

The synthesis of A-833834 and its labeling with ¹¹C have been described by Horti et al. (7). The total time required for the synthesis of [¹¹C]A-833834 was reported to be 37 min. The identity of [¹¹C]A-833834 was confirmed with coinjection of the tracer with the corresponding non-radiolabeled compound for analysis on a high-performance liquid chromatography (HPLC) column. The radiochemical yield of [¹¹C]A-833834 with respect to [¹¹C]methyl iodide (used to label the receptor ligand) was 32 ± 3%. The specific activity of [¹¹C]A-833834 at the end of synthesis was 403 ± 70 GBq/μmol (14.9 ± 2.58 Ci/μmol), with a radiochemical purity of 96% as determined with HPLC analysis. The total synthesis time for [¹¹C]A-833834 was 37 min (including the production of [¹¹C]methyl iodide and quality control of the tracer).

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

The binding affinity of A-833834 for α7-nAChR was determined in a displacement assay that used rat brain membrane preparations (without the cerebellum) with [³H]-(S,S)-2,2-dimethyl-5-(6-phenyl-pyridazin-3-yl)-5-aza-2-azonia-bicyclo[2.2.1]heptane iodide as the radioligand (7). The K_i of A-833834 was reported to be 1.53 nM compared with 0.092 nM for A-752272. This indicated that the binding affinity of A-833834 was ~16-fold lower than that of A-752274 (7).

Animal Studies

Human Studies

[PubMed]

No reference is currently available.

Supplemental Information

[Disclaimers]

No information is currently available.

NIH Support

Studies reported in this chapter were supported by National Institutes of Health grants DA020777 and MH079017.

References

1.

dos Santos Coura R., Granon S. Prefrontal neuromodulation by nicotinic receptors for cognitive processes. Psychopharmacology (Berl). 2012;221(1):1–18. [PubMed: 22249358]

2.

Albuquerque E.X., Pereira E.F., Alkondon M., Rogers S.W. Mammalian nicotinic acetylcholine receptors: from structure to function. Physiol Rev. 2009;89(1):73–120. [PMC free article: PMC2713585] [PubMed: 19126755]

3.

Kawamata J., Suzuki S., Shimohama S. alpha7 nicotinic acetylcholine receptor mediated neuroprotection in Parkinson's disease. Curr Drug Targets. 2012;13(5):623–30. [PubMed: 22300030]

4.

Hernandez C.M., Dineley K.T. alpha7 nicotinic acetylcholine receptors in Alzheimer's disease: neuroprotective, neurotrophic or both? Curr Drug Targets. 2012;13(5):613–22. [PubMed: 22300028]

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Schuller H.M. Regulatory role of the alpha7nAChR in cancer. Curr Drug Targets. 2012;13(5):680–7. [PubMed: 22300035]

6.

Radek R.J., Kohlhaas K.L., Rueter L.E., Mohler E.G. Treating the cognitive deficits of schizophrenia with alpha4beta2 neuronal nicotinic receptor agonists. Curr Pharm Des. 2010;16(3):309–22. [PubMed: 20109141]

7.

Horti A.G., Ravert H.T., Gao Y., Holt D.P., Bunnelle W.H., Schrimpf M.R., Li T., Ji J., Valentine H., Scheffel U., Kuwabara H., Wong D.F., Dannals R.F. Synthesis and evaluation of new radioligands [(11)C]A-833834 and [(11)C]A-752274 for positron-emission tomography of alpha7-nicotinic acetylcholine receptors. Nucl Med Biol. 2013;40(3):395–402. [PMC free article: PMC3596482] [PubMed: 23294899]

8.

Horti A.G., Wong D.F. Clinical Perspective and Recent Development of PET Radioligands for Imaging Cerebral Nicotinic Acetylcholine Receptors. PET Clin. 2009;4(1):89–100. [PMC free article: PMC2761691] [PubMed: 20046884]

9.

Chopra, A., 2-(6-[11C]methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-7-(6-methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-9H-fluoren-9-one. Molecular Imaging and Contrast agent Database (MICAD) [database online]. National Library of Medicine, NCBI, Bethesda, MD, USA. Available from www​.micad.nih.gov, 2004 -to current.