3-[Substituted]-5-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitriles: Identification of highly potent and selective metabotropic glutamate subtype 5 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2005.07.062

Structure-activity relationship studies on the phenyl ring of 3-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitrile 2 led to the discovery that small, non-hydrogen bond donor substituents at the 3-position led to a substantial increase in in vitro potency. In particular, 3-fluoro-5-(5-pyridin- 2-yl-2H-tetrazol-2-yl)benzonitrile (7) is a highly potent and selective mGlu5 receptor antagonist with good rat pharmacokinetics, brain penetration, and in vivo receptor occupancy.

Full text of DOI:10.1016/j.bmcl.2005.07.062

Bioorganic & Medicinal Chemistry Letters 15 (2005) 5061–5064
3-[Substituted]-5-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitriles:
Identification of highly potent and selective metabotropic
glutamate subtype 5 receptor antagonists
a,
a
a
Lida R. Tehrani,a,* Nicholas D. Smith, Dehua Huang, Steve F. Poon,
*
Jeffrey R. Roppe,a Thomas Jon Seiders,a Deborah F. Chapman,c Janice Chung,b
Merryl Cramera and Nicholas D. P. Cosforda
aDepartment of Medicinal Chemistry, Merck Research Laboratories, MRLSDB2,
3535 General Atomics Court, San Diego, CA 92121-1140, USA
bDepartment of Molecular Profiling, Merck Research Laboratories, MRLSDB2,
3535 General Atomics Court, San Diego, CA 92121-1140, USA
cDepartment of Neuropharmacology, Merck Research Laboratories, MRLSDB1,
3535 General Atomics Court, San Diego, CA 92121-1140, USA
Received 1 June 2005; revised 28 July 2005; accepted 29 July 2005
Available online 23 September 2005
Abstract—Structure–activity relationship studies on the phenyl ring of 3-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitrile 2 led to the
discovery that small, non-hydrogen bond donor substituents at the 3-position led to a substantial increase in in vitro potency. In
particular, 3-fluoro-5-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitrile (7) is a highly potent and selective mGlu5 receptor antagonist
with good rat pharmacokinetics, brain penetration, and in vivo receptor occupancy.
Ó 2005 Elsevier Ltd. All rights reserved.
The excitatory neurotransmitter glutamate activates
both ionotropic receptors and G protein-coupled metab-
otropic glutamate (mGlu) receptors. To date, eight
mGlu receptors have been identified and they are catego-
rized as follows: Group I includes mGlu1 and mGlu5
receptors, Group II comprises mGlu2 and mGlu3 recep-
tors, and Group III encompasses the mGlu4 and
mGlu6–8 subtypes.1 The Group I receptors activate
phospholipase C, which results in the mobilization of
intracellular calcium.2 A number of reports have indicat-
ed that selective antagonism of mGlu5 receptors may im-
prove disease states, such as anxiety and depression,3–8
pain,9 drug dependence,10 and mental retardation.11
Recent publications from this laboratory have described
the discovery of MTEP (1), a potent and selective
mGlu5 receptor antagonist.12 In a continuing search
for alternative structural series to diaryl-alkynes deriva-
tives, such as MTEP, we developed a series of heteroa-
romatic azoles.13 Of the 16 N-linked azoles examined,
tetrazole 2 was found to be the most promising in terms
of potency, selectivity,14 brain penetration, and rat
pharmacokinetics. However, tetrazole 2 showed only
moderate in vitro potency and binding affinity. With
the goal of improving the potency of this novel class
of compounds further, we herein describe the struc-
ture–activity relationship studies (SAR) around the
phenyl ring of 2.
S
N
N
The tetrazole derivatives described herein were con-
structed, as outlined in Scheme 1. Thus, a 1,3-dipolar
cycloaddition was employed between a diazonium salt
(derived from the corresponding 3-aminobenzonitrile)
and a tosyl hydrazone (derived from condensation of
2-pyridylaldehyde with tosyl hydrazide—Scheme 1).15
N
N
N
N
N
N
2
1-MTEP
Keywords: Metabotropic glutamate receptor 5; Antagonist; Tetrazole;
Occupancy.
Those 3-aminobenzonitriles that were not commercially
available were synthesized by installing the nitrile on the
*
Corresponding authors. Tel.: +1 858 202 5248; fax: +1 858 202 5743
(N.D.S.); e-mail: Nicholas_Smith77@yahoo.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.07.062

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L. R. Tehrani et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5061–5064
Table 1. In vitro data for mGlu5 receptor antagonists
Compound Structure
mGlu5 Ca2+ mGlu5
flux IC50
(nM)a
Ki (nM)b
N
N
Ν
2
73
190
N
N
Scheme 1. Reagents and conditions: (a) Tosyl hydrazide, EtOH, rt. (b)
NaNO2, HCl, H2O, EtOH, 0 °C. (c) NaOH, 0 °C.
N
N
N
N
N
3
334
>600
26
N
corresponding bromide using palladium catalyzed cya-
nation with Zn(CN)2. An example is shown in Scheme
2 for the preparation of the key intermediate 3-amino-
5-fluorobenzonitrile.
N
N
Cl
N
4
7
N
N
N
N
Those derivatives with an alkoxy substituent were pre-
pared by nucleophilic displacement of the fluoro substi-
tuent of 7 with an alcohol in the presence of cesium
carbonate at an elevated temperature (Scheme 3).
N
Cl
N
5
722
1270
4
>600
>600
14
N
Cl
N
N
The in vitro functional activity of mGlu5 receptor
antagonists was determined using a cell-based, high-
throughput assay that measured the changes in cytosolic
Ca2+ concentrations by fluorescence detection.16 Com-
pounds were also tested in a binding assay that mea-
sured the displacement of [H3]3-methoxy-5-(pyridin-2-
ylethynyl)pyridine from rat cortical membranes.17 The
mGlu5 receptor potency data for the initial set of tetra-
zole derivatives synthesized are shown in Table 1.
N
N
Cl
Cl
N
6
N
N
N
N
N
7
N
N
N
N
N
F
We initially examined the effect of chloride substitution
at different positions around the phenyl ring of 2, while
maintaining the 5-nitrile substituent. Thus, derivatives 3,
5, and 6 all lost potency, relative to the parent 2; howev-
er, 4 with a 3-chloro-5-nitrile orientation showed a sub-
stantial improvement in potency (mGlu5 Ki = 26 nM).
Encouraged by this, we decided to focus on the 3,5-ori-
entation and to investigate the effect of other substitu-
ents at the 3-position, while fixing the nitrile group at
the 5-position.
N
N
8
9
34
42
18
42
N
N
N
N
N
N
N
N
N
N
N
N+
O
O
N
Br
Br
N
N
N
Br
a
b
10
11
12
13
6
7
6
N
N
N
N
F
F
F
N
N
Scheme 2. Reagents and conditions: (a)—(i) acetamide, CuI, trans-1,2-
diaminocyclohexane, K2CO3, toluene, 130 °C, 18 h (76%); (ii) NaOH
(99%). (b) Zn(CN)2, Pd2dba3, dppf, DMF, 90 °C, 18 h (85%).
N
N
10
N
N
N
O
N
N
N
a
N
N
N
N
N
N
308
562
95
N
N
N
N
N
N
N
N
N
OR
F
N
7
R =
H, Me, CH2(c-propyl),
CH2Ph, 3-pyridyl
N
318
N
N
Scheme 3. Reagents and conditions: (a) ROH, Cs2CO3, DMF, 140 °C,
16 h.
N
N
N
OH

L. R. Tehrani et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5061–5064
5063
Table 1 (continued)
Table 3. Rat occupancy and brain penetration for 2 and 7
Plasma Brain Brain/ Occ ED50 Occ ED50
levels (lM) levels (lM) plasma (%) (mg/kg ip) (mg/kg po)
Compound Structure
mGlu5 Ca2+ mGlu5
flux IC50
(nM)a
Ki (nM)b
2
7
10a
2.7d
15a
2.4d
150
90
3.0b
1.3b
3.0c
3.6e
N
a Measured at 1 h following 10 mg/kg dose ip.
N
N
N
b Measured 1 h post-administration (n = 5–6 Sprague–Dawley rats/
group).
14
975
271
N
N
c Measured 30 min post-administration (n = 6–7 Sprague–Dawley rats/
O
group).
N
d Measured at 1 h following 3 mg/kg dose ip.
e Measured 2 h post-administration (n = 5–7 Sprague–Dawley rats/
group).
N
15
>1000
>600
N
N
N
N
OCH2Ph
profile these compounds in terms of rat pharmacokinet-
ics (Table 2).
N
O
N
16
15
16
N
N
Although not as impressive as parent tetrazole 2, fluoro-
derivative 7 still exhibits promising pharmacokinetics
with good bioavailability and half-life in rats
(F = 26%; t1/2 = 2.9 h). Methyl derivative 10 is also bio-
available in rats (F = 15%); however, it suffers from high
clearance and a short half-life.
N
N
N
a Using glutamate (10 lM) as agonist.
b Displacement by test compounds of [3H]3-methoxy-5-(pyridin-2-
ylethynyl)pyridine from rat cortical membranes.
With its excellent in vitro potency against the mGlu5
receptor (Ca2+ flux = 4 nM; Ki = 14 nM) and encourag-
ing rat pharmacokinetics, we next profiled 7 in terms of
rat brain penetration and in vivo receptor occupancy
(Table 3).19,20
Thus, it was found that small, non-hydrogen bond do-
nor groups at the 3-position, such as fluorine (7), ni-
trile (8), methyl (10), and methoxy (11), all had good
levels of mGlu5 receptor antagonist activity (mGlu5
Ki = 6–18 nM). Further, electronic nature of the substi-
tuent was not important, as an electron-donating
methoxy substituent 11 (Ki = 6 nM) and an electron-
withdrawing nitrile 8 (Ki = 18 nM) are both potent.
However, introduction of a hydrogen bond donor
group at the 3-substituent led to a loss of potency
against the mGlu5 receptor. Thus, aniline 12
(Ki = 95 nM) or phenol 13 (Ki = 318 nM) both lost sig-
nificant potency compared to 4. Similarly, increasing
the steric bulk of the alkoxy substituent of 11
(Ki = 6 nM) to methylenecyclopropyl 14 (Ki = 271 nM)
or benzyl 15 (Ki = >600 nM) both resulted in a loss of
potency. One interesting exception to the loss of
potency with increasing steric bulk was the 3-pyridyl-
oxy derivative 16 (Ki = 16 nM). The unexpected activ-
ity of this compound probably reflects the fact that
the 3-pyridyloxy group is a potent nitrile replacement
in this series of mGlu5 receptor antagonists, as dis-
closed previously.18
Similar to the parent tetrazole 2, fluoro-derivative 7 has
good rat brain penetration (90% for 7) with measured
drug levels in the brain of 2.4 lM, following a 3 mg/kg
ip dose. Together with the high in vitro potency against
the mGlu5 receptor, this leads to an excellent occupancy
ED50 of 1.3 mg/kg ip for 7 (for 2, ED50 = 3.0 mg/kg ip).
When examining the po dosing route, reflecting its oral
bioavailability of 26%, 7 has an ED50 of 3.6 mg/kg po,
while the parent tetrazole 2 with 100% bioavailability
has an ED50 of 3.0 mg/kg po.
Having shown the beneficial effect of substituting 2 with
a 3-fluoro substituent on in vitro potency, we next
attempted to apply this SAR to related series of mGlu5
receptor antagonists (Table 4).
Thus, substitution of imidazole 17 (Ki = 34 nM),21 pyr-
role 19 (Ki = >600 nM),13 and bi-aryl 21 (Ki = 69 nM),22
with a 3-fluoro substituent on the phenyl ring to give 18
(Ki = 9.3 nM), 20 (Ki = 5 nM), and 22 (Ki = 37 nM),
respectively, led in each case to an increase in mGlu5
receptor potency.
Having shown that a significant increase in in vitro
potency against the mGlu5 receptor may be achieved
with derivatives, such as 7 and 10, we next sought to
In conclusion, SAR studies on the phenyl ring of 2 have
shown that small, non-hydrogen bond donor groups at
the 3-position increase in vitro potency against the
mGlu5 receptor. Specifically, the 3-fluoro derivative 7
shows excellent in vitro potency, good rat pharmacoki-
netics, and excellent in vivo rat receptor occupancy
and brain penetration. Subsequent studies have shown
that installation of a 3-fluoro substituent in other scaf-
folds also leads to an improvement in in vitro potency.
Table 2. Rat pharmacokinetic data for 2, 7, and 10
Clp (mL/min/kg)a Vd (L/kg)a t1/2 (h)a F%b Cmax (lM)b
2
7
33
17
50
5.0
1.1
1.4
6.9
2.9
0.4
100 5.8
26 2.5
15 0.8
10
a 2 mg/kg dosed iv (n = 2 Sprague–Dawley rats/group).
b 10 mg/kg dosed po (n = 3 Sprague–Dawley rats/group).

5064
L. R. Tehrani et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5061–5064
Table 4. In vitro data for mGlu5 receptor antagonists
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Compound Structure
mGlu5 Ca2+
flux IC50 (nM)a Ki (nM)b
mGlu5
N
N
N
17
18
77
47
34
N
N
N
N
N
N
N
N
N
9.3
N
N
F
N
19
20
190
>600
N
N
N
N
F
3
5
N
N
N
21
22
63
12
69
37
N
N
N
N
N
N
N
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N
N
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N
F
a Using glutamate (10 lM) as agonist.
b Displacement by test compounds of [3H]3-methoxy-5-(pyridin-2-yl
ethynyl)pyridine from rat cortical membranes.
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Acknowledgment
The authors thank Grace Reyes-Manalo for expert tech-
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