3-[3-Fluoro-5-(5-pyridin-2-yl-2H-tetrazol-2-yl)phenyl]-4-methylpyridine: A highly potent and orally bioavailable metabotropic glutamate subtype 5 (mGlu5) receptor antagonist

Article abstract of DOI:10.1016/j.bmcl.2004.09.011

Structure-activity relationship studies performed around 3-fluoro-5-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitrile for the purposes of developing novel mGlu5 receptor antagonists are described. Synthesis of a series of four-ring tetrazoles led to the discovery of 3-[3-fluoro-5-(5-pyridin-2-yl- 2H-tetrazol-2-yl)phenyl]-4-methylpyridine (26), a highly potent, brain penetrant, tetrazole-based mGlu5 receptor antagonist. Structure-activity relationship studies performed around 3-fluoro-5-(5-pyridin-2-yl-2H-tetrazol-2- yl)benzonitrile for the purpose of developing novel mGlu5 receptor antagonists are described. Synthesis of a series of four-ring tetrazoles led to the discovery of 3-[3-fluoro-5-(5-pyridin-2-yl-2H-tetrazol-2-yl)phenyl]-4- methylpyridine, a highly potent, brain penetrant, azole-based mGlu5 receptor antagonist.

Full text of DOI:10.1016/j.bmcl.2004.09.011

Bioorganic & Medicinal Chemistry Letters 14 (2004) 5477–5480
3-[3-Fluoro-5-(5-pyridin-2-yl-2H-tetrazol-2-yl)phenyl]-4-
methylpyridine: a highly potent and orally bioavailable
metabotropic glutamate subtype 5 (mGlu5) receptor antagonist
Steve F. Poon,a,* Brian W. Eastman,a Deborah F. Chapman,c Janice Chung,b
Merryl Cramer,a Gregory Holtz,c Nicholas D. P. Cosforda and Nicholas D. Smitha
aDepartment of Medicinal Chemistry, Merck Research Laboratories, MRLSDB2, 3535 General Atomics Court,
San Diego, CA 92121, USA
bDepartment of Molecular Profiling, Merck Research Laboratories, MRLSDB2, 3535 General Atomics Court,
San Diego, CA 92121, USA
cDepartment of Neuropharmacology, Merck Research Laboratories, MRLSDB1, 3535 General Atomics Court,
San Diego, CA 92121, USA
Received 11 May 2004; revised 3 September 2004; accepted 7 September 2004
Available online 28 September 2004
Abstract—Structure–activity relationship studies performed around 3-fluoro-5-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitrile for the
purpose of developing novel mGlu5 receptor antagonists are described. Synthesis of a series of four-ring tetrazoles led to the dis-
covery of 3-[3-fluoro-5-(5-pyridin-2-yl-2H-tetrazol-2-yl)phenyl]-4-methylpyridine, a highly potent, brain penetrant, azole-based
mGlu5 receptor antagonist.
ꢀ 2004 Elsevier Ltd. All rights reserved.
S
Glutamate is the principal excitatory neurotransmitter
in the central nervous system.1 As such, it performs a
critical function in activating modulatory pathways
through Gprotein-coupled metabotropic glutamate
(mGlu) receptors. Group I mGlu receptors, which in-
clude the mGlu1 and mGlu5 subtypes, are coupled to
the stimulation of phospholipase C resulting in the
hydrolysis of inositoltriphosphate and the elevation of
intracellular Ca2+ levels.2,3 Excessive activation of
mGlu5 receptors has been implicated in several neuro-
logical diseases and disorders.4 A growing body of liter-
ature suggests that selective mGlu5 receptor antagonists
may be of therapeutic benefit for the treatment of vari-
ous pain states,5 psychiatric disorders such as anxiety
and depression,6–8 and other neurological impairments
such as drug dependence and mental retardation.9,10
CN
F
N
N
N
N
N
N
N
N
1 MPEP
2 MTEP
3
of the role of mGlu5 receptors in the brain and nervous
system. These compounds are useful tools for in vitro
and in vivo studies and have reinforced the viability of
mGlu5 receptors as a molecular therapeutic target.11,12
In addition to the biaryl alkyne class of antagonists,
we have recently reported the discovery of N-linked
azole derivatives as novel mGlu5 receptor antagonists.13
In particular, the tetrazole derivative 3 was shown to
have good mGlu5 receptor potency, selectivity,14 rat
pharmacokinetics, and receptor occupancy in rat brain.
The discovery of the subtype selective mGlu5 receptor
antagonists 1 and 2 has led to a better understanding
With the goal of further improving the potency and rat
pharmacokinetic properties of this novel class of com-
pounds, we herein describe the structure–activity rela-
tionship studies (SAR) around the phenyl ring of 3.
Specifically, we sought to replace the nitrile moiety with
different aromatic functionalities.
Keywords: Metabotropic glutamate; Antagonist; Tetrazole.
*
Corresponding author. Tel.: +1 858 202 5248; fax: +1 858 202
5743; e-mail: nicholas_smith@merck.com
0960-894X/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.09.011

5478
S. F. Poon et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5477–5480
Synthesis of the tetrazole derivatives described below
began with the preparation of tri-substituted anilines,
which were synthesized using transition metal-catalyzed
couplings starting with 1,3,5-dibromofluoroaniline
(Scheme 1). The amine functionality was installed with
a Cu-catalyzed amidation using acetamide followed by
saponification with NaOH, yielding the aniline 4 in an
economical fashion. The remaining bromide in 4 was
further derivatized via Pd-catalyzed coupling with
Zn(CN)2 or bis(pinacolato)diboron to afford the nitrile
5 and boronate ester 6, respectively.15,16
reasonable (50–70%) and the purity excellent (>90%
by HPLC).
The in vitro functional activity of potential mGlu5
receptor antagonists was determined using a cell-based,
high throughput assay that measured the changes
in cytosolic Ca2+ concentrations by fluorescence detec-
tion.18 Compounds were also tested in a binding assay,
which measured the displacement of [H3]-3-methoxy-5-
(pyridin-2-ylethynyl)pyridine from rat cortical mem-
branes.19 The mGlu5 receptor potency data for the
initial set of tetrazole derivatives synthesized is displayed
in Table 1.
The 1,3-disubstituted tetrazole core was synthesized in a
combined three step process from the starting aniline
(Scheme 2). The 2-pyridylcarboxaldehyde was con-
densed with tosyl hydrazide to afford the hydrazone 7,
and each of the anilines was converted to the corre-
sponding diazonium salt under acidic conditions. The
two intermediates were combined under aqueous basic
conditions to yield the respective tetrazole product in
50–75% yield.
Phenyl derivative 13 was determined to have moderate
in vitro activity (IC50 = 247nM, Ki = 53nM). Subse-
quent structure–activity relationship studies on substi-
tuted phenyl analogues showed that compounds
containing an ortho-substituted phenyl ring (14, 17, 19)
were particularly potent in both the functional and bind-
ing assays. Furthermore, the 2-pyridyl (20) and the 3-
pyridyl (21) derivatives also maintained mGlu5 receptor
potency. Notably, the 2-benzonitrile and the 2-aniline
derivatives, compounds 17 (IC50 = 6nM, Ki = 4nM)
and 19 (IC50 = 8nM, Ki = 7nM), were comparable in
potency with compound 3 (IC50 = 4nM, Ki = 12nM)
in both the functional and binding assays.
Tetrazole derivatives in which the nitrile moiety of 3 is
replaced with aryl rings were prepared by a Suzuki cou-
pling reaction. With compounds 11 and 12, both aryl
boronic acids and aryl halides could be attached to the
appropriate tetrazole coupling partner to give the final
compounds (Scheme 3). By making use of the basicity
of the products, these were readily purified with the
use of an acidic ion-exchange resin.17 The yields were
In an attempt to further enhance potency, a second iter-
ation of compounds was synthesized to test whether the
Br
F
Ar
F
g
N
N
H2N
CN
N
N
N
N
N
N
N
b
N
H2N
Br
Br
Br
a
F
11
5
c
O
B
O
B
F
F
H2N
Ar
O
O
h
4
N
N
N
N
N
N
N
F
6
N
N
N
F
F
12
Scheme 1. Reagents and conditions: (a) i. acetamide, CuI, trans-1,2-
diaminocyclohexane, K2CO3, toluene, 130ꢁC, 18h (76%), ii. NaOH
(99%); (b) Zn(CN)2, Pd2dba3, dppf, DMF, 90ꢁC, 18h (85%); (c)
bis(pinacolato)diboron, Pd(dppf), KOAc, DMSO, 85ꢁC, 2h (82%).
Scheme 3. Reagents and conditions: (g) ArB(OH)2, Pd(PPh3)4,
K2CO3, 5:1 DME/H2O, 90ꢁC; (h) ArBr, Pd(PPh3)4, K2CO3, 5:1
DME/H2O, 90ꢁC.
N2+Cl-
R
NH2
R
d
R
N
F
F
N
N
4 R=Br
8 R=Br
N
f
N
5 R=CN
9 R=CN
F
6 R=boronate
10 R=boronate
3
R=CN
11 R=Br
12 R=
O2
e
O
O
N
S
B
N
N
H
H
N
O
7
Scheme 2. Reagents and conditions: (d) NaNO2, HCl, 1:1 H2O EtOH, 0ꢁC; (e) tosyl hydrazide, EtOH; (f) NaOH, 0ꢁC.

S. F. Poon et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5477–5480
Table 2. In vitro data of 4-ring tetrazoles
5479
Table 1. In vitro data of 4-ring tetrazoles
X
X
N
N
N
N
N
N
N
N
N
N
F
F
Cpd
X
mGlu5 Ca2+ flux
IC50 (nM)a
mGlu5
Ki (nM)b
Compound
X
mGlu5 Ca2+ flux
IC50 (nM)a
mGlu5
Ki (nM)b
N
3
CN
Ph
4
14
53
23
24
31
63
24
13
14
15
16
17
18
19
20
21
22
247
120
1467
>3000
6
H3C
H3C
o-Tolyl
m-Tolyl
p-Tolyl
23
NTc
NTc
4
N
33
45
2
2-Benzonitrile
3-Benzonitrile
2-Aniline
2-Pyridyl
3-Pyridyl
4-Pyridyl
253
8
12
25
26
41
5
7
95
N
39
12
H3C
H3C
37
575
N
956
a Ca2+ flux assay using glutamate (10lM) as agonist (n = 2–4,
SD ꢀ 25%).20
N
N
b Displacement by test compounds of [3H]-3-methoxy-5-(pyridin-2-
ylethynyl)pyridine from rat cortical membranes (n = 2–4,
27
28
57
36
CH3
SD ꢀ 25%).17
c Not tested.
499
272
CH3
N
potency enhancing effects of the aryl substitution and
the pyridyl moiety were additive. Thus, a series of sub-
stituted halopyridines were coupled to 12 using a micro-
wave assisted Suzuki method, affording good yields of
the biaryl tetrazole products (75–85%).21 The substi-
tuted pyridyl analogues are listed in Table 2.
29
30
108
19
20
31
H2N
N
H2N
a Ca2+ flux assay using glutamate (10lM) as agonist (n = 2–4,
SD ꢀ 25%).15
As shown in Table 2, when the methyl group was fixed
at the ortho position and the nitrogen of the pyridyl ring
was systematically moved around the ring (23, 24, 25,
26), reasonable levels of mGlu5 receptor potency was
maintained in each case. In particular, compound 26,
which combines the ortho-tolyl of 14 with the 3-pyridyl
of compound 21, had significant potency in the
Ca2+ flux assay (IC50 = 5nM) and in the binding assay
(Ki = 2nM). In turn, when the 3-pyridyl functionality
was fixed and the methyl substituent moved around
the ring as in 26–28, the potency levels decreased,
respectively. Interestingly, the 2-aminopyridine deriva-
tive 29, showed diminished potencies relative to the cor-
responding methyl derivative 26, and the original aniline
derivative 19. As 26 was the most potent compound syn-
thesized in this series, it was selected for in vivo
profiling.
b Displacement by test compounds of [3H]-3-methoxy-5-(pyridin-2-
ylethynyl)pyridine from rat cortical membranes (n = 2–4,
SD ꢀ 25%).17
Table 3. Rat pharmacokinetic data comparison
PK parameter
3
26
a
Occ ED50
%Fb
97%
26
95%
12
Cmax (lM)b
2.5
2.9
17
0.9
1.6
39
t1/2 (h)b
Clp (mL/min/kg)b
a Following 10mg/kg dose of 50% PEG400/H 20 (ip) at 1h.
b iv dosing at 2mg/kg of 50% PEG400/H 20, po dosing at 10mg/kg.
A comparison of the rat receptor occupancy and phar-
macokinetics for 3 and 26 are presented in Table 3. As
with 3, 26 possessed significant occupancy levels at
95% in the rat receptor occupancy assay when dosed
at 10mg/kg intraperitoneally (measuring at 1h). At this
time point, the hippocampal and plasma levels were
determined to be 1.0lM and 0.6lM, respectively, indi-
cating good rat brain penetration. In comparing the
pharmacokinetic parameters, both 3 and 26 are bio-
available, however, 3 has a longer half-life (t1/2 = 2.9h
vs 1.6h) and a lower clearance than 26 (Cl = 15mL/
min vs 35mL/min).
In conclusion, SAR studies on the phenyl ring of tetraz-
ole 3 have shown that the nitrile moiety maybe replaced
with a variety of ortho-substituted aryl groups. In parti-
cular, 26 was highly potent and selective against the
mGlu5 receptor. It also had excellent rat brain receptor
occupancy, brain penetration, and acceptable pharma-
cokinetic characteristics. Compound 26 should prove
to be a valuable tool for future in vitro and in vivo
studies.

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S. F. Poon et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5477–5480
E. C.; Hess, S. D.; Rao, S. P.; Sacaan, A. I.; Santori, E.
Acknowledgements
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1493–1503.
The authors thank Jeffrey Anderson, Merryl Cramer,
Janice Chung, and BillBray for expert technical assistance.
12. Cosford, N. D. P.; Tehrani, L.; Roppe, J.; Schweiger, E.;
Smith, N. D. J. Med. Chem. 2003, 46, 204–206.
13. Roppe, J.; Smith, N. D.; Huang, D.; Tehrani, L.; Wang,
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14. The selectivity of the prototypical tetrazole, 3-(5-pyridin-
2-yl-2H-tetrazol-2-yl)benzonitrile, was determined by
extensive profiling against a battery of in vitro assays
(MDS Pharma Services screen).
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