Synthesis and structure-activity relationships of 5-substituted pyridine analogues of 3-[2-((S)-pyrrolidinyl)methoxylpyridine, A-84543: A potent nicotinic receptor ligand

Article abstract of DOI:10.1016/S0960-894X(01)00030-0

In an effort to probe the steric influence of C5 substitution of the pyridine ring on CNS binding affinity, analogues of 1 substituted with a bulky moiety - such as phenyl, substituted phenyl, or heteroaryl - were synthesized and tested in vitro for neuro

Full text of DOI:10.1016/S0960-894X(01)00030-0

Bioorganic & Medicinal Chemistry Letters 11 (2001) 631±633
Synthesis and Structure±Activity Relationships of 5-Substituted
Pyridine Analogues of 3-[2-((S)-Pyrrolidinyl)methoxy]pyridine,
A-84543: A Potent Nicotinic Receptor Ligand
Nan-Horng Lin,* Yihong Li, Yun He, Mark W. Holladay, Theresa Kuntzweiler,
David J. Anderson, Je€rey E. Campbell and Stephen P. Arneric
Neurological and Urological Diseases Research, D-47W, Pharmaceutical Products Division, Abbott Laboratories,
100 Abbott ParkRoad, Abbott Park, IL 60064-3500, USA
Received 19 October 2000; accepted 18 December 2000
AbstractÐIn an e€ort to probe the steric in¯uence of C5 substitution of the pyridine ring on CNS binding anity, analogues of 1
substituted with a bulky moietyÐsuch as phenyl, substituted phenyl, or heteroarylÐwere synthesized and tested in vitro for neu-
ronal nicotinic acetylcholine receptor binding anity. The substituted analogues exhibited Ki values ranging from 0.055 to 0.69 nM
compared to a Ki value of 0.15 nM for compound 1. Assessment of functional activity at subtypes of neuronal nicotinic acetyl-
choline receptors led to identify several agonists and antagonists. # 2001 Elsevier Science Ltd. All rights reserved.
Recent evidence indicating the therapeutic potential of
cholinergic channel modulators for the treatment of
CNS disorders as well as the diversity of brain neuronal
nicotinic acetylcholine receptor (nAChR) subtypes have
suggested an opportunity to develop subtype-selective
nAChR ligands for the treatment of speci®c CNS
disorders with reduced side-e€ect liabilities.1À4 We have
recently identi®ed A-84543 (1), a member of a novel
series of 3-pyridyl ether compounds, as a potent
cholinergic channel modulator.5 Several compounds of
this class were found to possess subnanomolar anity
for nAChRs and activate speci®c subtypes of neuronal
nAChRs.5 To explore the structural requirements for
potent interaction with nAChRs, structural modi®ca-
tions of this compound were undertaken. Recently,6 we
demonstrated that subtype selectivity of agonists or
antagonists could be achieved by varying the sub-
stituents on the pyridine ring. For example, compound
2, which possesses a large alkyl group, exhibits a
moderately selective antagonism of the a4b2 receptor
subtype compared with 1. In order to probe the e€ect
of steric volume of substituents on the binding and
functional potency, a large number of 5-substituted
analogues of 1 have been prepared, including those with
large substituents such as phenyl, substituted phenyl,
substituted heteroaryl or heteroaryl groups.6 These
compounds have been screened in assays which mea-
sured binding anity to central nAChRs and in vitro
functional activity (ion ¯ux). With regard to ion ¯ux
assays, a key focus was on functional activity in IMR-32
cells as a model for potential activity at human periph-
eral ganglionic receptors, which appear to at least
partially mediate undesired cardiovascular and gastro-
intestinal e€ects of (S)-nicotine.7,8 In this paper we
describe SAR studies of 5-substituted analogues of 1.
The pyridine substituted analogues of 1 were synthe-
sized as outlined in Scheme 1. The 5-phenyl substituted
pyridine analogues, as exempli®ed by the preparation of
3, were synthesized employing the Suzuki reaction.9
Thus, 5-bromopyridyl ether was heated with phenyl
boronic
phino)Pd(0) as catalyst to provide the corresponding
acid
employing
tetrakis(triphenylphos-
*Corresponding author. Fax: +1-847-938-5034; e-mail: nanhorng.lin
@abbott.com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00030-0

632
N.-H. Lin et al. / Bioorg. Med. Chem. Lett. 11 (2001) 631±633
5-phenylpyridine compound. The 5-bromopyridine
intermediate was prepared by treatment of 3,5-dibro-
mopyridine with 1-methyl-2(S)-pyrrolidinylmethanol in
the presence of sodium hydride. Employing Stille cou-
pling reaction conditions (Method B), the 5-pyridyl and
pyrimidinyl analogues were synthesized from the corre-
sponding tributylstannanes as depicted in Scheme 1.10
Treatment of 5-bromopyridine with phenylacetylene
in the presence of palladium(II) catalyst furnished the
5-(2-phenyl-1-ethynyl)pyridine analogue 25 (Method
C).11 Employing Heck reaction conditions (method D),12
5-(trans-2-phenyl-1-vinyl)pyridine analogue 26 was pre-
pared from 5-bromopyridine. It should be noted that
only trans product was obtained from the coupling
reaction. Hydrogenation of compound 26 provided the
corresponding 5-(2-phenyl-1-ethyl)pyridine 27 in high
yield.
decrease (Ki=2.4 nM) in binding potency was observed
when tri¯uoromethyl group was introduced at both C3
and C5 positions (compound 15).
To investigate the steric limitation of the C5 position,
an additional ring was introduced to the phenyl moiety.
Thus, introduction of a 2-naphthyl (17) group increased
the Ki value to 0.33 nM (a 3-fold decrease compared to
3), while replacement of phenyl ring with a 1-naphthyl
group (i.e., 16) resulted in a 38-fold decrease in binding
potency. A 3-fold decrease in potency was observed
when phenyl was replaced with 3-quinolyl (18) or
2-indolyl (19) group.
Although 3-pyridyl analogue (20) has a 2-fold decrease
in binding potency when compared to 3, no further
reduction in binding potency was observed upon repla-
cement with a ®ve-membered heteroaryl moiety (cf. 21
and 22). As observed previously, fusion of an additional
phenyl ring to yield the benzofuran analogue 23 had
little e€ect on binding potency. In addition, replace-
ment of the pyridyl moiety with a pyrimidinyl group
(24) resulted in little e€ect on the binding anity.
Table 1. Binding data for pyridine substituted analogues
Scheme 1.
A major subtype of nAChR in the brain is labeled with
high anity by [3H](À)-nicotine and [3H](À)-cytisine
and is composed of a4 and b2 subunits.13 The e€ect of
the substituents on binding anity, as re¯ected by dis-
placement of [3H](À)-cytisine from rat brain mem-
branes, are shown in Table 1. Replacement of the 3-
pyridyl fragment of 1 with a 5-phenylpyridyl group
(compound 3) caused no reduction in binding anity
towards the [3H]-cytisine binding site. However, a 10-
fold reduction in binding anity was observed in ana-
logue 4 (Ki=1.1 nM) having a 2-methylphenylpyridyl
substituent. Replacement of methyl group with formyl
group has a similar e€ect on binding anity. When an
electron-withdrawing group is incorporated into the
phenyl ring to give 3-nitrophenyl analogue 6, a 4-fold
lower anity than phenyl compound 3 is observed.
Introduction of an electron-donating amino group (7) at
the C3 position of the phenyl ring resulted in a 4-fold
decrease in binding anity. In general, compounds
having substituents at C2 or C3 positions of the phenyl
ring reduced receptor binding anity.
Compound
R
Method [3H]Cytisine binding
Ki (nM)a
1
3
4
5
6
7
8
9
H
Ph
2-Me-Ph
2-CHO-Ph
3-NO2-Ph
3-NH2-Ph
4-OMe-Ph
4-CF3-Ph
4-F-Ph
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
B
0.15Æ0.05
0.11Æ0.07
1.1Æ0.1
0.56Æ0.13
0.46Æ0.21
0.43Æ0.05
0.32Æ0.21
0.15Æ0.03
0.11Æ0.008
0.13Æ0.02
0.082Æ0.022
0.16Æ0.03
0.055Æ0.007
2.4Æ0.7
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
4-Me-Ph
4-Cl-Ph
2-Cl-4-Cl-Ph
3-Cl-4-F-Ph
3-CF3-5-CF3-Ph
1-Naphthyl
2-Naphthyl
3-Quinolyl
N-Me-2-indolyl
3-Pyridyl
4.2Æ1.9
0.33Æ0.11
0.39Æ0.15
0.32Æ0.07
0.27Æ0.08
0.19Æ0.01
0.31Æ0.05
0.15Æ0.06
0.37Æ0.17
0.16Æ0.02
0.24Æ0.06
0.22Æ0.06
0.31Æ0.13
0.12Æ0.03
0.064Æ0.012
2-Furyl
2-Thienyl
2-Benzofuryl
5-Pyrimidinyl
1-Ph-CCÀ
trans-1-Ph-CHˆCHÀ
Ph-CH2CH2À
4-Me-Ph-CCÀ
4-Py-CHˆCHÀ
4-Py-CH2-CH2À
To examine whether substituents at the C4 position of
the phenyl ring have a similar e€ect on the nicotinic
receptors as observed at the C2 or C3 positions, the
corresponding 4-methyl analogue was evaluated for the
nicotinic binding anity. Table 1 reveals that C4 methyl
(11) analogue was as potent as compound 3. The OMe,
CF3, F and Cl moieties (compounds 8, 9, 10, and 12)
were all well tolerated at this position. Introduction of
an additional chlorine atom at either the C2 or C3
positions had only minor e€ect on the binding anity
(cf. 12 and 13; 10 and 14). In contrast, a 22-fold
C
D
b
C
D
c
aThe ability of compound to displace [3H](À)-cytisine binding to whole
rat brain membranes was performed as described.14 Values are the
meansÆSEM; n=3±4. In all cases, the Hill coecient was close to
unity indicative of an interaction with a single class of binding sites.
bSynthesized by hydrogenation of compound 25.
cSynthesized by hydrogenation of compound 29.

N.-H. Lin et al. / Bioorg. Med. Chem. Lett. 11 (2001) 631±633
633
Table 2. Antagonist properties for pyridine substituted analoguea
analogues tested stimulated cation e‚ux with very low
potency (EC50>1000 mM) and ecacy (<20%) at
human a3bx receptor subtypes (data not shown). In
general, those 5-substituted phenyl analogues exhibited
very low potency and ecacy at a3bx subtypes regard-
less of the position and functionality of substitution
(Table 2).
Calcium dynamics in IMR32 Cells15
Compound
R
IC50 (mM)b
3
4
5
6
7
8
9
Ph
2-Me-Ph
2-CHO-Ph
3-NO2-Ph
3-NH2-Ph
4-OMe-Ph
4-CF3-Ph
4-F-Ph
4-Me-Ph
4-Cl-Ph
2-Cl-4-Cl-Ph
3-Cl-4-F-Ph
3-CF3-5-CF3-Ph
1-Naphthalyl
2-Naphthalyl
3-Quinolyl
N-Me-2-indolyl
3-Pyridyl
2-Furyl
5.1Æ0.9
1.25Æ0.32
6.31Æ1.45
2.33Æ0.94
6.9Æ0.2
Since most of the analogues displayed very low ecacy
(<20%) at this subtype, they were then tested for their
antagonist properties at a3bx receptor subtype. At the
a3bx subtype, all of the analogues except compound 24,
which possesses EC50 value of 32 mM, antagonized
cation e‚ux mediated by 100 mM (S)-nicotine with an
IC50 value of 0.7±25 mM.
6.97Æ1.95
5.71Æ1.65
6.48Æ1.57
0.8Æ0.2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
1.99Æ0.28
0.89Æ0.34
7.3Æ0.4
2.35Æ0.87
0.69Æ0.34
3.4Æ2.3
In summary, we have shown that varying the sub-
stituent pattern at the C5 position of the pyridyl moiety
of A-84543 alters binding and particularly functional
properties of compounds. The binding data in Table 1
indicated that large substituents are well tolerated at the
C5 position. As demonstrated previously,6 this study
has also shown that preparation of agonists or antago-
nists could be achieved by varying the substituents on
the pyridine ring. The 5-pyrimidinyl analogue 24 was
identi®ed as a a3bx partial agonist and several analogues
(compounds 11, 13, 16, and 25) were identi®ed as potent
a3bx antagonist.
5.1Æ0.7
6.1Æ0.5
24.7Æ4.6
7.9Æ0.9
2-Thienyl
1.7Æ0.6
2-Benzofuryl
5-Pyrimidinyl
1-Ph-CCÀ
2.9Æ1.3
c
0.79Æ0.14
4.8Æ0.9
1.1Æ0.6
1.5Æ0.3
7.6Æ3.2
6.3Æ0.8
trans-1-Ph-CHˆCHÀ
Ph-CH2CH2À
4-Me-Ph-CCÀ
4-Py-CHˆCHÀ
4-Py-CH2-CH2À
aCompounds were tested against 100 mM of (À)-nicotine.
bValues represent meanÆSEM; n=3±5.
cEC50: 32.4Æ8.1 mM; % Max: 38.9Æ14.7. % Max represents the
maximal ecacy of the compounds relative to 100 mM nicotine.
References
1. Holladay, M. W.; Lebold, S. A.; Lin, N.-H. Drug Dev. Res.
1995, 35, 191.
2. Arneric, S. P.; Sullivan, J. P.; Williams, M. In Psycho-
pharmacology: Fourth Generation of Progress; Bloom, E.,
Kuper, D. J., Eds., Raven: New York, 1995; pp 95±110.
3. Meyer, E. M.; de Fiebre, C. M.; Hunter, B. E.; Simpkins,
C. E.; Frauworth, N.; de Fiebre, N. E. C. Drug Dev. Res. 1994,
31, 127.
4. Garvey, D. S.; Carrera, G. M.; Arneric, S. P.; Shue, Y.-K.;
Lin, N.-H.; He, E.; Lee, E.; Lebold, S. A. U.S. Patent 5 409
946, 1995.
5. Abreo, M. A.; Lin, N.-H.; Garvey, D. S.; Gunn, D. E.;
Hettinger, A.-M.; Wasicak, J. T.; Pavlik, P. A.; Martin, Y. C.;
Donnelly-Roberts, D. L.; Anderson, D. J.; Sullivan, J. P.;
Williams, M.; Arneric, S. P.; Holladay, M. W. J. Med. Chem.
1996, 39, 817.
6. Lin, N.-H.; Gunn, D. E.; Li, Y.; He, Y.; Bai, H.; Ryther,
K. B.; Kuntzweiler, T.; Donnelly-Roberts, D. L.; Anderson,
D. J.; Campbell, J. E.; Sullivan, J.; Arneric, S. P.; Holladay,
M. W. Bioorg. Med. Chem. Lett. 1998, 8, 249.
7. McBride, P. E. Med. Clin. N. Am. 1992, 76, 333.
8. Carlson, G. M.; Ruddon, R. W.; Hug, C. C.; Schmiege,
S. K., Jr.; Bass, P. J. Pharmacol. Exp. Ther. 1970, 173, 377.
9. Suzuki, A. Acc. Chem. Res. 1982, 15, 178.
The e€ect of phenyl substituents with longer aliphatic
chains on the binding anities was also examined.
Changing the phenyl moiety to a phenylethynyl moiety
(25) gave a compound possessing the same binding
potency as 3, while replacement of the phenyl moiety
with a phenylethyl or phenylvinyl group (i.e., 27 or 26)
resulted in only little di€erence in binding potency. In
general, introduction of a two-carbon chain between the
phenyl and the C5 position of pyridine ring caused only
a minor e€ect on binding potency regardless of the
¯exibility of the two-carbon moiety. Introduction of a
nitrogen atom at the C4 position of the phenyl ring (29)
resulted in a 2-fold decrease in binding potency com-
pared with 26. Interestingly, replacement of the phenyl
group of 27 with a 4-pyridyl functionality caused a 3.5-
fold decrease in binding potency (27 vs 30).
We conclude that the steric volume of substitutions at
the C5 position of pyridine has little e€ect on nAChR
binding anity. Hence, the phenyl analogue 3 possesses
similar binding activity as that of compound 1. Intro-
duction of an additional ring or aliphatic chain, such as
2-naphthyl (17) and phenylethyl analogue (27), resulted
in only a 2- or 3-fold reduction in binding potency.
10. Milstein, D.; Stille, K. J. Am. Chem. Soc. 1979, 101, 4992.
11. Castro, C. E.; Stephens, R. D. J. Org. Chem. 1963, 28,
2163.
12. Cabri, W.; Candiani, I. Acc. Chem. Res. 1995, 28, 2.
13. Flores, C. M.; Rogers, S. W.; Pabreza, L. A.; Wolfe, B. B.;
Kellar, K. J. Mol. Pharmacol. 1991, 41, 31.
The ability of these analogues to activate nAChRs was
investigated using a cell line that express the human
ganglionic-like (a3bx) nAChR (IMR 32 cells). With
the exception of compound 24 (5-pyrimidinyl), all of
14. Pabreza, L. A.; Dhawan, S.; Kellar, K. J. Mol. Pharmacol.
1990, 39, 9.
15. Kuntzweiler, T. A.; Arneric, S. P.; Donnelly-Roberts,
D. L. Drug Dev. Res. 1998, 44, 14.