Aryloxyethylamines: Binding at α7 nicotinic acetylcholine receptors

Article abstract of DOI:10.1016/j.bmcl.2006.05.080

Structure-affinity relationships for the binding of 3-[2-(N,N,N-trimethylammonium)ethoxy]pyridine (AXPQ) at α7 nACh receptors were investigated due to its close structural similarity to a known α7 antagonist.

Full text of DOI:10.1016/j.bmcl.2006.05.080

Bioorganic & Medicinal Chemistry Letters 16 (2006) 4283–4286
Aryloxyethylamines: Binding at a7 nicotinic acetylcholine receptors
Hanan M. Ragab,a Jin Sung Kim,a Małgorzata Dukat,a
b
a,
*
´
Hernan Navarro and Richard A. Glennon
aDepartment of Medicinal Chemistry, School of Pharmacy, Virginia Commonwealth University,
Richmond, VA 23298-0540, USA
bCenter for Organic and Medicinal Chemistry, Research Triangle Institute, 3040 Cornwallis Road,
Research Triangle Park, NC 27709-2194, USA
Received 5 April 2006; revised 18 May 2006; accepted 18 May 2006
Available online 9 June 2006
Abstract—Structure–affinity relationships for the binding of 3-[2-(N,N,N-trimethylammonium)ethoxy]pyridine (AXPQ) at a7 nACh
receptors were investigated due to its close structural similarity to a known a7 antagonist.
Ó 2006 Elsevier Ltd. All rights reserved.
Acetylcholine produces many of its effects by interaction
with nicotinic acetylcholine (nACh) receptors, and the
two most abundant receptor types in brain are the
a4b2 and a7 nACh receptors. The medicinal chemistry
of a4b2 receptors has been relatively well studied.1
The latter population, about which much less is known,
is of interest because of its possible role in certain central
disorders such as cognitive and perceptual distur-
bances—especially those associated with schizophrenia.2
There also is evidence that a7 nACh receptors are asso-
ciated with the release of neurotransmitters that are in-
volved in schizophrenia, depression, and anxiety, such
as dopamine, norepinephrine, and serotonin.3,4
yet been examined. Furthermore, very little structure–
affinity data are available for F3 (3). Due to this
structural similarity, and because 3 is reportedly an
a7-selective agent, it was of interest to determine the
affinity of 6-PEN (1) and AXPQ (2) for a7 receptors,
and to examine what common structural features might
account for the affinity of 3-related aryloxyalkylamines.
For example, the presence and position of a pyridyl
nitrogen atom is known to be a major determinant for
high-affinity binding at a4b2 receptors;1 compound 3
lacks this feature. If it could be verified that a pyridyl
nitrogen atom is not required for a7 binding, this would
provide a useful lead for further exploitation. Other is-
sues needing to be addressed include the role of the ether
oxygen atom, the nature of the amine, and the necessity
of the phenylethenyl moiety for a7 binding. The present
investigation was undertaken to address these issues.
( )6-(2-Phenylethyl)nicotine (6-PEN; 1) is a nicotinic
acetylcholine (nACh) receptor antagonist with reason-
able selectivity for a4b2 (Ki = 0.015 lM) over other
nACh receptors,5 and AXPQ (2) is a high-affinity a4b2
agonist (Ki = 0.0005 lM).6 Recently, Gotti et al.,7
reported that choline ether F3 (3) is an a7-selective
antagonist (a7 Ki = 0.057 lM) with little affinity
(Ki = 39 lM) for b2-containing nACh receptors. As
such, 3 might represent a novel template for further
development of a7 ligands.
CH3
O
N
CH3
N
CH3
CH3
N
N
2
1
CH3
N
6-PEN (1), and even more so AXPQ (2), bear structural
similarity to 3 but their binding at a7 receptors has not
CH2
R1
CH3
CH3
O
O
N
R2
CH3
R
X
Keywords: Nicotinic receptors; AXPQ.
*
4 X = N
5 X = CH
3
Corresponding author. Tel.: +1 804 828 8487; fax: +1 804 828
7404; e-mail: glennon@vcu.edu
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.05.080

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H. M. Ragab et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4283–4286
Specifically targeted were compounds 4 (where X = N)
and 5 (where X = CH) where R1 and/or R2 are H or
small alkyl groups, and R is either H, or a 2-phenylethyl
or 2-phenylethenyl substituent as found in 1 and 3,
respectively. Also, examined, to determine the role of
the ether oxygen atom, were a few derivatives where
the ether oxygen was replaced by carbon (i.e., 6), and
an example of ether analog 2 where chain length was in-
creased by one methylene unit (i.e., 7).
O
CH3
O
CH3
N
N
a
CH3
CH3
N
Cl
N
4e
8
b
c
CH3
O
CH3
O
N
N
b
CH3
CH3
CH3
N
N
Certain of the necessary compounds were available from
studies previously conducted in our laboratories.6,8,9
The physicochemical properties of the other target
compounds are shown in Table 1.
4g
4d
b
CH3
O
N
Compound 4e was prepared from halogenated aryl ether
86 by a palladium-catalyzed reaction with potassium
trans-styryltrifluoroborate10 (Scheme 1). Careful quat-
ernization of 4e with MeI afforded 4g.11 Compound 4f
was obtained by catalytic reduction of 4e followed by
quaternization with MeI.11
CH3
CH3
N
4f
Scheme 1. Reagents and conditions: (a) PdCl2(dppf)ÆCH2Cl2, potas-
sium trans-styryltrifluoroborate, Et3N, iPrOH:H2O, 2:1; (b) MeI,
CH2Cl2; (c) H2, 10% Pd/C, EtOH.
Compounds 5a12 and 5d13 were prepared as their HCl
salts according to literature procedures. The remaining
analogs were prepared from 5d in the same manner as
shown in Scheme 1. That is, 5d was quaternized to 5f,
or reduced to 5c, and 5c was quaternized to 5e.
The first issue to be addressed with regard to the aryl-
oxyalkylamines was whether their carbon analogs bind
at a7 receptors; the ethoxy group of several N-substitut-
ed 3-(2-aminoethoxy)pyridines was replaced by an
n-propyl group (i.e., 6a and 6b),8 and by a propenyl
group (6c–e),8 but none of the compounds displayed
significant affinity (i.e., Ki > 10 lM). Initially, these
Although 6-PEN (1) bears some structural semblance to
3, it was found to lack affinity for a7 receptors
(Ki > 10 lM). The mere presence of the 6-position
substituent in this molecule detracts from a7 binding
when compared with (À)-nicotine (Ki = 0.75 lM).
Table 1. Physicochemical properties and a7 nACh receptor binding affinities of target compounds
CH3
O
R1
N
R2
R
X
Compound
R
R1
R2
X
Recrystallization solvent
Mp (°C)
Empirical formulaa
Kib (lM)
4ac
4bc
2c
–H
–H
H
H
N
—
—
—
—
—
—
>10
>10
0.28
>10
>10
1.10
>10
>10
>10
>10
4.32
7.50
1.25
2.17
H
Me
Me
Et
N
–H
–H
Me
H
N
—
—
—
—
—
4cc
5a
5b
4d
5c
4e
5d
4f
N
—
–H
–H
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
CH
CH
N
iPrOH
163–164d
164–165e
164–166
173–175
219–221
238–240f
210–212
208–210
246–248
274–276
—
—
Me
H
abs EtOH
abs EtOH/Et2O
abs EtOH
abs EtOH
abs EtOH
MeOH/abs EtOH
abs EtOH
MeOH/abs EtOH
MeCN
–CH2–CH2–Ph
–CH2–CH2–Ph
–CH@CH–Ph
–CH@CH–Ph
–CH2–CH2–Ph
–CH2–CH2–Ph
–CH@CH–Ph
–CH@CH–Ph
C17H22N2OÆ2C2H2O4
C18H23NOÆHCl
C17H20N2OÆ1.25HCl
—
H
CH
N
H
H
CH
N
Me
Me
Me
Me
C18H25IN2O
C19H26INO
5e
4g
5f
CH
N
C18H23IN2O
C19H24INO
CH
a All compounds were homogeneous on thin-layer chromatography, analyzed within 0.4% of theory for C, H, and N, and 1H NMR spectra were
consistent with assigned structures. C2H2O4 = oxalate salt.
b See Ref. 18 for assay conditions. For comparison, (À)-nicotine was found to bind with Ki = 0.75 lM.
c The synthesis of compounds 4a,8 4b,8 2,6 and 4c8 has been previously reported from this laboratory.
d Lit.,12 mp = 163 °C for HCl salt.
e Lit.,14 mp = 167–169 °C for bromide.
f Lit.,13 mp = 238–240 °C for HCl salt.

H. M. Ragab et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4283–4286
4285
compounds were of particular interest because of their
known lack of affinity for a4b2 receptors8 and, had they
shown affinity, might have been selective for a7
receptors.
bind with about three times the affinity of their phenyl-
ethyl counterparts 4f and 5e, respectively. Furthermore,
although the presence of the pyridyl nitrogen atom was
found unnecessary for a7 binding, the pyridyl series
seems to bind with 2- to 3-fold higher affinity than the
phenyl series (compare 2 with 5b, 4f with 5e, and 4g with
5f). Interestingly, neither the phenylethyl nor the phenyl-
ethenyl analogs of AXPQ (2), 4f and 4g, respectively, dis-
played higher affinity than AXPQ itself.
R
R
N
N
CH3
CH3
N
NH-CH3
N
N
6c R = H
6d R = CH3
6a R = H
6b R = CH3
6e
Compound 5f differs in structure from choline ether F3
(3) only in that the latter possesses a methyl group a to
the terminal amine, yet 5f binds with >35-fold lower
affinity than that reported for 3. The affinity of 5f at a7
receptors was re-determined and found (Ki = 2.03
0.2 lM) to be comparable to the result shown in Table
1. Evidently, the a-methyl group of 3 might play a major
role in its higher affinity for a7 receptors. Consistent with
this concept, Gotti et al.7 have shown that moving this
methyl group to the adjacent chain position is detrimen-
tal to binding. Future studies might wish to retain this
substituent. In addition, it is rather interesting that
AXPQ (2) binds with several-fold higher affinity at a7
receptors than either its phenylethyl or phenylethenyl
counterparts 4f and 4g, respectively; because AXPQ
binds with such high affinity at a4b2 receptors
(Ki = 0.0005 lM; 560-fold a4b2 vs a7 selectivity), the
phenylethenyl group of 3 might contribute to its selectiv-
ity for a7 receptors. Finally, although the 3-pyridyl
substituted analogs tended to display several-fold higher
affinity than their phenyl counterparts for a7 receptors,
the presence of the pyridyl nitrogen atom is known to
be important for a4b2 binding; hence, the absence of this
feature in F3 (3) might additionally contribute to its
selectivity for a7 versus a4b2 receptors, and it might be
profitable to continue targeting phenyl rather than pyr-
idyl analogs to achieve enhanced a7 selectivity.
The simple secondary and tertiary (i.e., N-methyl and
N,N-dimethyl) amine derivatives of AXPQ (2) also
failed to bind at a7 receptors (4a and 4b; Ki > 10 lM;
Table 1). Extension of the N-methyl group of 4b to an
N-ethyl group likewise resulted in a compound with
low affinity (i.e., 4c; Ki > 10 lM). However, AXPQ
itself (2; Ki = 0.28 lM) displayed twice the affinity of
(À)-nicotine (Ki = 0.75 lM). Similar findings were ob-
tained with phenyl counterparts 5a and 5b; that is, the
N,N-dimethyl tertiary amine 5a (Ki > 10 lM) failed to
bind, whereas the quaternary amine 5b displayed en-
hanced affinity (Ki = 1.10 lM). Evidently, higher affinity
is associated with a quaternary amine in both series.
Nevertheless, there must also be a chain-length require-
ment because the n-propoxy counterpart of 2 lacked
appreciable affinity (7;6 Ki > 10 lM).
CH3
O
N CH3
CH3
N
7
It might be noted that while our work was in progress
Gundisch et al.14 reported that 5b binds at a7 receptors
¨
with Ki ꢀ 0.2 lM. Possible explanations for the nearly
6-fold difference in affinity are that the published study
employed [3H]MLA rather than [125I]iodo-MLA as
radioligand, and used rat forebrain rather than rat
cerebral cortical homogenates.
Since this work was initiated, several investigative
groups have identified novel a7 nACh receptor ligands
that bind in the nanomolar range.15–17 Nevertheless,
we have extended the findings of Gotti et al.7 on the
binding of aryloxyalkylamines to a7 nACh receptors
by providing additional structure–affinity data, and
have shown that 6-PEN (1) and AXPQ (2) lack signifi-
cant affinity for a7 nACh receptors relative to their
affinity for a4b2 receptors.
Introduction of a 6-(2-phenylethyl) group had no appar-
ent effect on the affinity of 4b or 5a (4d and 5c, respec-
tively; Ki > 10 lM). Likewise, their phenylethenyl
counterparts 4e and 5d (Ki > 10 lM) lacked affinity for
a7 receptors. Again, affinity seems to be associated only
with the quaternary amines. The N,N,N-trimethyl
quaternary amine counterpart of 4d (i.e., 4f; Ki =
4.32 lM) and 5b (i.e., 5e; Ki = 7.50 lM) showed en-
hanced affinity (Table 1). The corresponding phenyl-
ethenyl quaternary analogs 4g (Ki = 1.25 lM) and 5f
(Ki = 2.17 lM) displayed about 3-fold enhanced affinity
both in the pyridyl and phenyl series.
Acknowledgments
This work was supported in part by DA 05274. H.M.R.
was supported by the Channel Program through the
Egyptian Cultural Bureau.
References and notes
The results of this structure–affinity investigation indi-
cate that the quaternary amine nature of both the pyridyl
(i.e., 4) and phenyl (i.e., 5) series is a major determinant
of affinity; N-methyl secondary amines and N,N-dimeth-
yl tertiary amines bind with reduced affinity. The results
also show that the phenylethenyl compounds 4g and 5f
1. Glennon, R. A. Prog. Med. Chem. 2004, 42, 55.
2. Martin, L. F.; Kem, W. R.; Freedman, R. Psychophar-
macology 2004, 174, 54.
3. Summers, K. L.; Kem, W. R.; Giacobini, E. Jpn. J.
Pharmacol. 1997, 74, 139.

4286
H. M. Ragab et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4283–4286
4. Aznar, S.; Kostova, V.; Christiansen, S. H.; Knudsen, G.
M. Synapse 2005, 55, 196.
12. Schinckaneder, H.; Loesen, R.; Grill, H. German Patent
DE 3239610, 1984.
5. Ramunno, A.; Dukat, M.; Lee, M.; Young, R.; El-Zahabi,
M.; Damaj, M. I.; Martin, B.; Glennon, R. A. Bioorg.
Med. Chem. Lett. 2005, 15, 3237.
13. Massarani, A. Il Farmaco 1957, 12, 380.
14. Gundisch, D.; Andra¨, M.; Munoz, L.; Tilotta, M. C.
¨
Bioorg. Med. Chem. 2004, 12, 4953.
6. Simsek, R.; Chang-Fong, J.; Lee, M.; Damaj, M. I.;
Martin, B. R.; Glennon, R. A. Bioorg. Med. Chem. Lett.
2003, 13, 2917.
7. (a) Gotti, C.; Carbonnelle, E.; Moretti, M.; Zwart, R.;
Clementi, F. Behav. Brain Res. 2000, 113, 183; (b) Gotti,
C.; Balestra, B.; Moretti, M.; Rovati, G. E.; Maggi, L.;
Rossoni, G.; Berti, F.; Villa, L.; Pallavicini, M.; Clementi,
F. Br. J. Pharmacol. 1998, 124, 1197.
15. Bodnar, A. L.; Cortes-Burgos, L. A.; Cook, K. K.; Dinh,
D. M.; Groppi, V. E.; Hajos, M.; Higdon, N. R.;
Hoffmann, W. E.; Hurst, R. S.; Myers, J. K.; Rogers, B.
N.; Wall, T. M.; Wolfe, M. L.; Wong, E. J. Med. Chem.
2005, 48, 905.
16. Tatsumi, R.; Fujio, M.; Satoh, H.; Katayama, J.; Takan-
ashi, S.; Hashimoto, K.; Tanaka, H. J. Med. Chem. 2005,
48, 2678.
8. Cheng, Y.; Dukat, M.; Dowd, M.; Fiedler, W.; Martin, B.;
Damaj, M. I.; Glennon, R. A. Eur. J. Med. Chem. 1999,
34, 177.
9. Glennon, R. A.; Dukat, M. In Neuronal Nicotinic Recep-
tors; Arneric, S. P., Brioni, J. D., Eds.; Wiley-Liss: New
York, 1999; pp 271–284.
17. Mazurov, A.; Klucik, J.; Miao, L.; Phillips, T. Y.;
Seamans, A.; Schmitt, J. D.; Hauser, T. A.; Johnson, R.
T., Jr.; Miller, C. Bioorg. Med. Chem. Lett. 2005, 15, 2073.
18. Radioligand binding assays were conducted and the data
analyzed as previously reported.19 Rat cerebral cortex
homogenates were used and final [125I]iodo-MLA (sa
2175 mCi/mmol) concentration was 180 pM. The IC50
values determined from competition binding experiments
were used to calculate the Ki using the formula Ki = IC50/
(1 + [L]/Kd) where [L] is the concentration of radioligand
and Kd its apparent dissociation constant.20 A Kd of
1.98 nM was used. In cases where the Ki value was
repeated the SD was under 10%.
10. Molander, G. A.; Bernardi, C. R. J. Org. Chem. 2002, 67,
8424.
11. Alkylation with MeI can conceivably result in either one,
or both, of two quaternized products. The methylation
reaction was conducted at 0 °C and resulted in homoge-
neous products after purification. Proton NMR signals for
the methyl groups of 4g (d 3.16; s, 9H) and 4f (d 3.18; s,
9H) appeared as singlets using DMSO-d6 as solvent
indicative of a common environment. Furthermore, the
signals were nearly identical to that found for 5f (d 3.19; s,
9H) which can only undergo methylation at one position.
19. Navarro, H. A.; Zhong, D.; Abraham, P.; Xu, H.; Carroll,
F. I. J. Med. Chem. 2000, 43, 142.
20. Cheng, Y.; Prusoff, W. H. Biochem. Pharmacol. 1973, 22,
3099.