Structure-activity relationships of N-substituted ligands for the α7 nicotinic acetylcholine receptor

Article abstract of DOI:10.1016/j.bmcl.2009.11.023

A series of α7 neuronal nicotinic acetylcholine receptor ligands were designed based on a structural combination of a potent, but non-selective ligand, epibatidine, with a selective lead structure, 2. Three series of compounds in which aryl moieties were attached via a linker to different positions on the core structure were studied. A potent and functionally efficacious analog, (3aR,6aS)-2-(6-phenylpyridazin-3-yl)-5-(pyridin-3-ylmethyl)octahydropyrrolo[3,4-c]pyrrole (3a), was identified.

Full text of DOI:10.1016/j.bmcl.2009.11.023

Bioorganic & Medicinal Chemistry Letters 20 (2010) 104–107
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Structure–activity relationships of N-substituted ligands for the
nicotinic acetylcholine receptor
a7
*
Kathleen H. Mortell , Michael R. Schrimpf, William H. Bunnelle, David J. Anderson, Jens Halvard Gronlien,
Kirsten Thorin Hagene, Murali Gopalakrishnan
Neuroscience Research, Global Pharmaceutical Research and Development, Abbott Laboratories, 100 Abbott Park Road, Abbott Park, IL 60064, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of a7 neuronal nicotinic acetylcholine receptor ligands were designed based on a structural com-
Received 28 August 2009
Accepted 9 November 2009
Available online 14 November 2009
bination of a potent, but non-selective ligand, epibatidine, with a selective lead structure, 2. Three series
of compounds in which aryl moieties were attached via a linker to different positions on the core struc-
ture were studied. A potent and functionally efficacious analog, (3aR,6aS)-2-(6-phenylpyridazin-3-yl)-5-
(pyridin-3-ylmethyl)octahydropyrrolo[3,4-c]pyrrole (3a), was identified.
Keywords:
Ó 2009 Elsevier Ltd. All rights reserved.
Nicotinic acetylcholine receptors
Neuronal nicotinic receptors
Nicotinic agonist
Nicotine
Epibatidine
Neuroscience
Neuronal nicotinic acetylcholine receptors (nAChRs) are of great
interest as therapeutic targets for the treatment of pain and cogni-
tive disorders, and for smoking cessation.1 Different subtypes in
this receptor family have been shown to mediate different effects,
and compounds that are selective among the subtypes have the po-
tion. Inspection of simple 3D models in which the basic nitrogen of
1 and 2 were overlapped led to the hypothesis that the aromatic
moiety of epibatidine was oriented differently in the binding pock-
et than the biaryl group of 2. We therefore conjectured that addi-
tion of a substituent to compound 2 might offer an additional
binding interaction that would enhance potency. This hypothesis
suggested analogs 3, 4 and 5 with substituents on the pyrrolidine
ring that combined elements of epibatidine 1 with 2 (Fig. 1).
To explore possible substituents, we sought to quickly generate
N-substituted analogs. As shown in Scheme 1, bicyclic diamine
analogs 3a–f were generated from precursor 68 by reaction with al-
kyl or arylmethyl halides. Aminopyrrolidine analogs were synthe-
sized by a coupling of racemic tert-butyl pyrrolidin-3-ylcarbamate
and 3-chloro-6-phenylpyridazine using excess diisopropylethyl-
amine in DMSO. The carbamate nitrogen was reacted with various
alkyl or arylmethyl halides and the Boc group was removed by
treatment with acid. Reductive methylation gave the tertiary
amine analogs 4a–f.
tential to target particular diseases.2 Agonists selective for the
a7
receptor subtype have been shown to have beneficial effects on
cognition,3 offering the potential to treat diseases such as Alzhei-
mer’s disease and schizophrenia.4 Several groups have reported
agonists based on the quinuclidine pharmacophore.3 We sought
to design
product epibatidine (1), isolated from the skin of an Ecuadorian
tree frog, is very potent at the 7 receptor (Ki = 2.7 nM), but also
a7 ligands with a unique core structure. The natural
a
binds to other subtypes including
a4b2 (0.1 nM), a3b4 (0.3 nM)
and ganglionic and neuromuscular nAChRs.5 By combining ele-
ments from epibatidine with structures that were selective for
a7, our research program aimed to produce compounds with im-
proved potency and selectivity for the 7 nAChR.
a
Compound 2 (A-582941) is a nAChR agonist discovered in our
As shown in Table 1, in the bicyclic diamine series (3), the sub-
stituent on the nitrogen had a substantial effect on binding of the
laboratories with activity across a range of cognition models.6 It
binds to the
a
7 receptor with a potency of 11 nM, and it is selective
4b2 Ki >100 M, 3b4 Ki = 4700 nM).
ligand to the rat a7 receptor. Compound 2, with a methyl substitu-
for 7 over other subtypes (
a
a
l
a
ent, bound potently with a Ki of 11 nM. Larger substituents (Et or
Bn) brought a 30- or 40-fold drop in potency. Among the pyridyl-
methyl substituted analogs, the position of the pyridyl nitrogen
impacted potency. The 2- and 4-pyridylmethyl substituted analogs
were about 50-fold less potent than 2, comparable to the benzyl
analog. In contrast, the 3-pyridylmethyl analog 3e had a Ki of
3.6 nM. The 3-pyridyl isomer in 3e corresponds to the position of
Ligands for nAChRs bind in a highly conserved ‘aromatic box’ of
amino acid sidechains.7 The amine portion of the ligand is held
in the binding site through a hydrogen bond or a cation-pi interac-
* Corresponding author. Tel.: +1 847 936 3417; fax: +1 847 935 5466.
E-mail address: kathleen.mortell@abbott.com (K.H. Mortell).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.11.023

K. H. Mortell et al. / Bioorg. Med. Chem. Lett. 20 (2010) 104–107
105
Cl
N
H
HN
N
N
+
N
N
H
1
2
Ar
H
Ar
H
X
H
Ar
RN
N
N
Ph
N
Ph
N
N
Ph
N
N
N
N
N
N
H
H
H
3
4
5
Figure 1. Proposed analogs.
H
H
RX, KOtBu,
HN
N
RN
N
N
N
DMF, 50-80%
N N
H
H
2, 3b-f
6
DIEA, DMSO
BocHN
BocHN
Cl
Ph
N
Ph
+
NH
N
N
N
N
90 ºC, 18h, 73%
CH2O
R
1. RX, KOtBu, DMF
rt, 18h, 20-50%
NaBH(OAc)3
RHN
N
H3C
N
N
1,2-DCE
rt, 6h, 50-70%
N
7
N
N N
2. 25%TFA/DCM
4a-f
Scheme 1. Preparation of N-substituted derivatives.
Table 1
7 nAChR binding of N-substituted derivatives10
the nitrogen in the 2-chloropyrid-5-yl substituent in epibatidine.
Compound 3g, bearing a chloropyridyl group like epibatidine,
was also superior to the analogs containing other pyridyl isomers,
but it was 28-fold less potent than analog 3e. Compound 6, the NH
a
Compd
R
Series 3
a
7 nAChR
Series 4 a7 nAChR
bindinga Ki (nM)
bindinga,b Ki (nM)
analog of the lead compound 2, bound potently to the
with a Ki of 17.5 nM. But compound 6 was somewhat less selective,
binding the 4b2 nAChR subtype with a Ki of 5.3 M. All other
compounds tested in series 3, 4 or 5 did not bind to the 4b2
receptor at concentrations up to 10
M.9
Compounds containing an aminopyrrolidine group (series 4)
bound less potently to the 7 receptor than the bicyclic diamine
compounds 3. Dimethylaminopyrrolidine 4a had a Ki of 478 nM.
The ethyl analog 4b was about twofold less potent, and larger sub-
stituents, including 3-pyridylmethyl, abolished binding up to
a7 receptor,
2
CH3
Et
11
4a
3b
4b
3c
4c
478
331
476
a
l
746
a
Bn
>104
>104
l
3d
4d
533
3.6
N
a
3e
4e
N
>104
10
l
M. All compounds in the aminopyrrolidine series with a sec-
M.
3f
541
N
ondary amine, that is, 7 had Kis >10
l
4f
>104
Compounds related to 5 were synthesized as shown in Scheme
2. The fused [3.3.0] ring system was accessed by a [3+2] cycloaddi-
tion.11 The azomethine ylide was formed in situ and reacted with
N-benzylmaleimide to give cis isomer 8 and its epimer 9 in approx-
imately a 1:1 ratio.12 The imide was reduced with lithium alumi-
num hydride to amino alcohol 10. Attachment of the pendant
pyridyl ring was achieved by nucleophilic aromatic substitution
to give intermediate 11. Hydrogenation over palladium on carbon
removed the aromatic bromide and the benzyl group. Coupling of
the resulting amine with 3-chloro-6-phenylpyridazine gave com-
pound 12. The relative stereochemistry of 12 was established by
3g
N
102
Cl
4g
6
7
>104
>104
H
CH3
17.5
C-Linked analogs
12
13
NA
NA
320
42
Displacement of [3H]-F-585539 from rat brain.10 Kis reported are the mean of
a
three determinations; SEM versus pKi 6 0.08.
b
Analogs are racemic.

106
K. H. Mortell et al. / Bioorg. Med. Chem. Lett. 20 (2010) 104–107
a) HCHO,
EtO2C
MeN
O
EtO2C
MeN
O
H
H
PhMe reflux
HN
EtO2C
NBn
+
NBn
b)
O
N
O
H
H
O
O
Bn
8, 45%
9, 34%
N
N
1. H2, Pd/C, 75%
2.
Br
Br
HO
MeN
Br
H
H
LiAlH4
46%
O
O
8
H
H
H
H
N
11
NBn
Cl
Ph
MeN
NBn
MeN
N
N
N
NaH, DMF
20%
45%
N
N
10
12
N
O
H
H
MeN
N
N
9
N
13
Scheme 2. Preparation of diamine analogs.
gesting they were less able to catalyze the conformational change
leading to channel opening.
Table 2
Functional efficacy of lead compoundsa
Previous approaches to designing ligands for nAChRs based on
epibatidine have produced compounds that are selective for the
a
H
N
4b2 receptor subtype over a a7 recep-
7.14 Successful designs for
N
N
N
N
tor ligands have employed specific amine pharmacophores such as
H
quinuclidine3 or other amine groups15 linked to an aromatic or
3e
bisaromatic moiety to give a linear core. In addition, a few
a7 li-
Compd
EC50
(
l
M)
Maximal responsea (%)
gands have been reported that contain a 3-pyridyl group as one
of two aromatic substituents branching from the amine pharmaco-
phore, notably the anabaseine derivative GTS-21.16 The quinucli-
dine pharmacophore has been combined with a 3-pyridylmethyl
2
4.26
9.36
11.77
>100
12.90
52
84
13
—
3e
3g
12
13
a
7 agonists.17 These observations
44
substituent to give very potent
suggested that the 7 binding site could accommodate a branched
a
a
Normalized to the maximal response of ACh (10 mM) in Xenopus oocytes
expressing human
7 receptors.13
ligand displaying two distinct aromatic groups linked to the alkyl-
a
amine headgroup.
In the present study, arylmethyl substituents were attached at
different positions to a diamine-containing core. Consistent with
previous results, we found that a 3-pyridyl group was preferred over
other pyridine isomers or aromatic groups. The most potent analog
discovered was (3aR,6aS)-2-(6-phenylpyridazin-3-yl)-5-(pyridin-
3-ylmethyl)octahydropyrrolo[3,4-c]pyrrole, 3e, inwhicha 3-pyridyl
group was attached by a linker to the terminal nitrogen. This result
was somewhat surprising, given that the substituted nitrogen binds
in a well-defined pocket, and the change from a methyl to ethyl in 3b
caused a 30-fold drop in potency. The nitrogen of the quinuclidine
analogs, by contrast, occupies a bridgehead position that minimizes
steric demand. This suggests the basic amine binding site has the
flexibility to accommodate an appropriate substituent. The binding
energy available from the correctly placed pyridyl group enhanced
thepotencyof 3e, and thesubstituentalso increased itseffectiveness
as an agonist compared to the parent compound, 2.
2D NMR and is consistent with literature.12 Compound 13 was syn-
thesized similarly from compound 9.
Based on previous observations, series 5 compounds were de-
signed with a pendant 3-pyridyl group. The addition of the substi-
tuent in compound 12 gave a 30-fold loss in potency relative to 2
(Ki = 320 nM, Table 1). However, the epimeric compound 13 largely
retained its ability to bind
a7, with a Ki of 42 nM. Although 13 is
slightly less potent than 2, the binding site is able to accommodate
the addition of this substituent.
The stereochemistry and attachment of the pyridyl substituent
also had a dramatic effect on the ability of the ligands to activate
the receptor. The
a7 nAChR is a ligand-gated ion channel. Activa-
tion of the receptor by an agonist opens the channel to allow the
flow of ions across a membrane. Five compounds were tested for
their ability to evoke Ca2+ currents in Xenopus oocytes expressing
recombinant human a
7 nicotinic receptor (Table 2).13 The maximal
References and notes
response was calculated as a percent of the response evoked by
10 mM acetylcholine. Compound 2 is a partial agonist at human
1. (a) D’hoedt, D.; Bertrand, D. Exp. Opin. Ther. Targets 2009, 13, 395; (b) Jensen, A.
A.; Frolund, B.; Liljefors, T.; Krogsgaard-Larsen, P. J. Med. Chem. 2005, 48, 4705.
2. Broad, L. M.; Sher, E.; Astles, P. C.; Zwart, R.; O’Neill, M. J. Drugs Future 2007, 32,
161.
3. (a) Mullen, G.; Napier, J.; Balestra, M.; DeCory, T.; Hale, G.; Macor, J.; Mack, R.;
Loch, J.; Wu, E.; Kover, A.; Verhoest, P.; Sampognaro, A.; Phillips, E.; Zhu, Y.;
Murray, R.; Griffith, R.; Blosser, J.; Gurley, D.; Machulskis, A.; Zongrone, J.;
Rosen, A.; Gordon, J. J. Med. Chem. 2000, 43, 4045; (b) Bodnar, A. L.; Cortes-
a7 with an EC50 of 4.3 lM. N-Substituted compound 3e is about
twofold less potent, but closer in efficacy to ACh. Compound 13
is a partial agonist, threefold less potent than 2. But compound
12 shows no efficacy up to 100 lM. While compounds bearing a
non-optimal substituent might lose binding potency, ligands 3g,
12 and 13 suffered a loss in functional potency and efficacy, sug-

K. H. Mortell et al. / Bioorg. Med. Chem. Lett. 20 (2010) 104–107
Burgos, L. A.; Cook, K. K.; Dinh, D. M.; Groppi, V. E.; Hajos, M.; Higdon, N. R.;
9. Binding of ligands to the
107
a
4b2 receptor was measured via displacement of
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; (c) Wishka, D. G.; Walker, D. P.; Yates,
K. M.; Reitz, S. C.; Jia, S.; Myers, J. K.; Olson, K. L.; Jacobsen, E. J.; Wolfe, M. L.;
Groppi, V. E.; Hanchar, A. J.; Thornburgh, B. A.; Cortes-Burgos, L. A.; Wong, E. H.
F.; Staton, B. A.; Raub, T. J.; Higdon, N. R.; Wall, T. M.; Hurst, R. S.; Walters, R. R.;
Hoffmann, W. E.; Hajos, M.; Franklin, S.; Carey, G.; Gold, L. H.; Cook, K. K.;
Sands, S. B.; Zhao, S. X.; Soglia, J. R.; Kalgutkar, A. S.; Arneric, S. P.; Rogers, B. N. J.
Med. Chem. 2006, 49, 4425; (d) Tatsumi, R.; Fujio, M.; Takanashi, S.-I.; Numata,
A.; Katayama, J.; Satoh, H.; Shiigi, Y.; Maeda, J.-I.; Kuriyama, M.; Horikawa, T.;
Murozono, T.; Hashimoto, K.; Tanaka, H. J. Med. Chem. 2006, 49, 4374.
4. Hu, M.; Waring, J. F.; Gopalakrishnan, M.; Li, J. J. Neurochem. 2008, 106, 1371.
5. Sullivan, J. P.; Briggs, C. A.; Donnelly-Roberts, D.; Brioni, J. D.; Radek, R. J.;
McKenna, D. G.; Campbell, J. E.; Arneric, S. P.; Decker, M. W.; Bannon, A. W.
Med. Chem. Res. 1994, 4, 502.
6. (a) Tietje, K. R.; Anderson, D. J.; Bitner, R. S.; Blomme, E. A.; Brackemeyer, P. J.;
Briggs, C. A.; Browman, K. E.; Bury, D.; Curzon, P.; Drescher, K. U.; Frost, J. M.;
Fryer, R. M.; Fox, G. B.; Gronlien, J. H.; Hakerud, M.; Gubbins, E. J.; Halm, S.;
Harris, R.; Helfrich, R. J.; Kohlhaas, K. L.; Law, D.; Malysz, J.; Marsh, K. C.; Martin,
R. L.; Meyer, M. D.; Molesky, A. L.; Nikkel, A. L.; Otte, S.; Pan, L.; Puttfarcken, P.
S.; Radek, R. J.; Robb, H. M.; Spies, E.; Thorin-Hagene, K.; Waring, J. F.; Ween, H.;
Xu, H.; Gopalakrishnan, M.; Bunnelle, W. H. CNS Neurosci. Ther. 2008, 14, 65; (b)
Bitner, R. S.; Bunnelle, W. H.; Anderson, D. J.; Briggs, C. A.; Buccafusco, J.;
Curzon, P.; Decker, M. W.; Frost, J. M.; Gronlien, J. H.; Gubbins, E.; Li, J.; Malysz,
J.; Markosyan, S.; Marsh, K.; Meyer, M. D.; Nikkel, A. L.; Radek, R. J.; Robb, H. M.;
Timmermann, D.; Sullivan, J. P.; Gopalakrishnan, M. J. Neurosci. 2007, 27,
10578; (c) Bunnelle, W. H.; Tietje, K. R.; Frost, J. M.; Peters, D.; Ji, J.; Li, T.;
Scanio, M. J. C.; Shi, L.; Anderson, D. J.; Dyhring, T.; Grønlien, J. H.; Ween, H.;
Thorin-Hagene, K.; Meyer, M. D. J. Med. Chem., in press.
[3H]-cytisine from rat brain homogenate: Pabreza, L. M.; Dhawan, S.; Kellar, K.
J. Mol. Pharmacol. 1990, 39, 9.
10. Anderson, D. J.; Bunnelle, W. H.; Surber, B.; Du, J.; Surowy, C.; Tribollet, E.;
Marguerat, A.; Bertrand, D.; Gopalakrishnan, M. J. Pharmacol. Exp. Ther. 2008,
324, 179.
11. (a) Tsuge, O.; Kanemasa, S.; Ohe, M.; Yorozu, K.; Takenaka, S.; Ueno, K. Bull.
Chem. Soc. Jpn. 1987, 60, 4067; (b) Lubineau, A.; Bouchain, G.; Queneau, Y. J.
Chem. Soc., PT1 1995, 19, 2433.
12. The assignment was made by conversion to 12. The cis relative
stereochemistry of 12 was established by 2D NMR and is consistent with
literature.
13. Briggs, C. A.; Schrimpf, M. R.; Anderson, D. J.; Gubbins, E. J.; Gronlien, J. H.;
Hakerud, M.; Ween, H.; Thorin-Hagene, K.; Malysz, J.; Li, J.; Bunnelle, W. H.;
Gopalakrishnan, M.; Meyer, M. D. Br. J. Pharmacol. 2008, 153, 1054.
14. (a) Carroll, F. I. Bioorg. Med. Chem. Lett. 2004, 14, 1889; (b) Barlocco, D.;
Cignarella, G.; Tondi, D.; Vianello, P.; Villa, S.; Bartolini, A.; Ghelardini, C.;
Nicoletta, G.; Anderson, D. J.; Kuntzweiler, T. A.; Colombo, D.; Toma, L. J. Med.
Chem. 1998, 41, 674; (c) Wei, Z.-L.; Xiao, Y.; Kellar, K. J.; Kozikowski, A. P.
Bioorg. Med. Chem. Lett. 2004, 14, 1855.
15. (a) Macor, J. E.; Gurley, D.; Lanthorn, G.; Loch, J.; Mack, R. A.; Mullen, G.; Tran,
O.; Wright, N.; Gordon, J. C. Bioorg. Med. Chem. Lett. 2001, 11; (b) Biton, B.;
Bergis, O. E.; Galli, F.; Nedelec, A.; Lochead, A. S.; Jegham, S.; Godet, D.; Lanneau,
C.; Santamaria, R.; Chesney, F.; Leonardon, J.; Granger, P.; Debono, M. W.;
Bohme, G. A.; Sgard, F.; Besnard, F.; Graham, D.; Coste, A.; Oblin, A.; Curet, O.;
Vige, X.; Voltz, C.; Rouquier, L.; Souilhac, J.; Santucci, V.; Gueudet, C.; Francon,
D.; Steinberg, R.; Griebel, G.; Oury-Donat, F.; George, P.; Avenet, P.; Scatton, B.
Neuropsychopharmacology 2007, 32, 1.
16. Papke, R. L.; Meyer, E. M.; Lavieri, S.; Bollampally, S. R.; Papke, T. A. S.;
Horenstein, N. A.; Itoh, Y.; Porter Papke, J. K. Neuropharmacology 2004, 46,
1023.
7. Cashin, A. L.; Petersson, E. J.; Lester, H. A.; Dougherty, D. A. J. Am. Chem. Soc.
2005, 127, 350.
8. Basha, A.; Bunnelle, W. H.; Dart, M. J.; Gallagher, M. E.; Ji, J.; Li, T.; Pace, J. M.;
Ryther, K. B.; Tietje, K. R. U.S. 7,399765 B2.
17. Mazurov, A.; Klucik, J.; Miao, L.; Phillips, T. Y.; Seamans, A.; Schmitt, J. D.;
Hauser, T. A.; Johnson, R. T.; Miller, C. Bioorg. Med. Chem. Lett. 2005, 15, 2073.