5-[(2-Methyl-1,3-thiazol-4-yl)ethynyl]-2,3′-bipyridine: A highly potent, orally active metabotropic glutamate subtype 5 (mGlu5) receptor antagonist with anxiolytic activity

Article abstract of DOI:10.1016/j.bmcl.2004.05.037

Structure-activity relationship studies leading to the discovery of a new, orally active mGlu5 receptor antagonist are described. The title compound, 5-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-2,3′-bipyridine, is highly potent in vitro, has good in vivo recep

Full text of DOI:10.1016/j.bmcl.2004.05.037

Bioorganic & Medicinal Chemistry Letters 14 (2004) 3993–3996
0
5
-[(2-Methyl-1,3-thiazol-4-yl)ethynyl]-2,3 -bipyridine: a highly
potent, orally active metabotropic glutamate
subtype 5 (mGlu5) receptor antagonist with anxiolytic activity
a,
a
a
Jeffrey R. Roppe, Bowei Wang, Dehua Huang, Lida Tehrani, Theodore Kamenecka,
a
a
*
a
Edwin J. Schweiger, Jeffery J. Anderson, Jesse Brodkin, Xiaohui Jiang,
b
b
a
a
c
c
Merryl Cramer, Janice Chung, Grace Reyes-Manalo, Benito Munoz
a
a
and Nicholas D. P. Cosford
a
Department of Medicinal Chemistry, Merck Research Laboratories, San Diego,
MRLSDB2, 3535 General Atomics Court, San Diego, CA 92121, USA
b
Department of Pharmacology, Merck Research Laboratories, San Diego, MRLSDB1,
535 General Atomics Court, San Diego, CA 92121, USA
3
c
Department of Molecular Profiling, Merck Research Laboratories, San Diego,
MRLSDB2, 3535 General Atomics Court, San Diego, CA 92121, USA
Received 2 February 2004; revised 19 May 2004; accepted 20 May 2004
Available online
Abstract—Structure–activity relationship studies leading to the discovery of a new, orally active mGlu5 receptor antagonist are
0
described. The title compound, 5-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-2,3 -bipyridine, is highly potent in vitro, has good in vivo
receptor occupancy, and is efficacious in the rat fear-potentiated startle model of anxiety following oral dosing.
Ó 2004 Elsevier Ltd. All rights reserved.
1
3
Metabotropic glutamate (mGlu) receptors are a family
of G-protein coupled receptors in the mammalian ner-
vous system that are activated by L-glutamate. Eight
tion. The development of potent and selective mGlu5
receptor antagonists as potential therapeutic agents has
therefore been the focus of significant research in these
laboratories.
1;2
mGlu receptor subtypes have been identified to date and
these are organized into three groups (Group I, II, and
III) based on sequence homology. Group I mGlu
receptors (mGlu1 and mGlu5) are primarily localized
postsynaptically and are widely distributed in many
brain regions, including the hippocampus, thalamic
nuclei, and spinal cord. Stimulation of mGlu1 and
mGlu5 leads to phosphoinositide (PI) hydrolysis and
Recently we reported the discovery of the potent and
highly selective mGlu5 receptor antagonists 1a and 1b
(3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine, MTEP),
the latter of which was found to be active in a rodent
14
model of anxiety.
2
þ
2þ
elevation of intracellular Ca levels ([Ca ] ) via G-
protein coupling to phospholipase C. Excessive acti-
vation of mGlu5 receptors has been linked to a number
i
3;4
S
N
5
of CNS disorders including pain, anxiety and depres-
12
6–11;20;21
sion,
drug dependence and mental retarda-
X
1
a X = CH
Keywords: Metabotropic glutamate receptor; mGluR; mGluR5;
MPEP; MTEP.
1b X = N
*
Corresponding author. Tel.: +1-858-202-5454; fax: +1-858-202-5752;
e-mail: jeffrey_roppe@merck.com
In this communication structure–activity relationship
(SAR) studies around the aryl ring appended to the
0
960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.05.037

3994
J. R. Roppe et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3993–3996
a
Table 1. In vitro potency data for mGlu5 receptor antagonists
S
S
S
S
N
R
N
N
N
R
R
Cmpds 1a, 2-7
Cmpds 8, 1b, 9
Cmpds 10-13
N
Cmpds 14-17
N
R
2
þ
b
c
d
Compound
a
R
hmGlu5 Ca flux IC50 (nM)
mGlu5 K
i
(nM)
log D
1
2
3
4
5
6
7
8
1
9
1
1
1
1
1
1
1
1
H
4 (0.04; 2)
15 (0.19; 5)
2 (0.04; 3)
9 (0.18; 3)
51 (0.02; 2)
10 (0.12; 2)
638 (0.08; 2)
145 (0.09; 3)
16 (0.05; 3)
43 (0.13; 4)
54 (0.13; 3)
12 (0.10; 2)
218 (0.07; 3)
40 (0.15; 2)
28 (0.13; 3)
1 (0.36; 4)
3.3
3.8
3.9
3.9
––
2-Me
3-Me
4-Me
2-Ph
3-Ph
4-Ph
2-Py
3-Py
4-Py
Ph
845 (0.22; 3)
423 (0.20; 6)
41 (0.15; 3)
22 (0.25; 11)
53 (0.04; 4)
5 (0.4; 3)
4.1
5.0
1.9
2.1
2.1
4.1
2.8
2.4
2.6
3.9
3.7
3.1
2.9
b
127 (0.14; 4)
2 (0.09; 2)
0
1
2
3
4
5
6
7
2-Py
3-Py
4-Py
Ph
22 (0.39; 12)
3 (0.16; 12)
18 (0.09; 4)
0.5 (0.33; 4)
4 (0.24; 5)
15 (0.18; 4)
2 (0.38; 2)
2-Py
3-Py
4-Py
16 (0.17; 4)
2 (0.18; 3)
21 (0.17; 2)
2 (0.05; 3)
29 (0.26; 3)
a
b
c
Data are presented as the geometric mean followed in parentheses by the standard deviation and the number of replicates.
15–17
2þ
Ca flux assay using glutamate (10 lM) as agonist.
3
18
Displacement by test compounds of [ H]-3-methoxy-5-(pyridin-2-ylethynyl)pyridine from rat cortical membranes.
See Ref. 14.
d
thiazole alkyne motif in 1a and pyridyl group in 1b will
be described. These efforts have led to the discovery of
new derivatives of 1b that display improved in vitro
potency, good rodent pharmacokinetics, and potent
anxiolytic activity in vivo. The functional potencies of
these compounds in vitro was assessed using an auto-
mated assay employing Ltk cells that stably express
human recombinant mGlu5 receptors (Table 1). This
The general scheme for the synthesis of the alkyne
14;18
derivatives in Table 1 has been described previously.
Briefly, utilizing Sonogashira cross-coupling methodol-
ogy, compounds 1a,b and 2–9 could be readily prepared
from 4-(trimethylsilylethynyl)-2-methyl-1,3-thiazole 18
and the appropriate commercially available pyridyl, aryl
or biaryl halide (Scheme 1).
2þ
cell based assay measures changes in cytosolic Ca
concentrations ([Ca ] ) by fluorescence detection using
Compounds 10–13 were prepared in two steps beginning
with the Sonogashira cross-coupling of 18 with 3,5-
dibromopyridine (Scheme 2). The resulting common
intermediate 19 was used as a Suzuki cross-coupling
partner with the appropriate aryl or pyridylboronic acid
to obtain 10, 12, and 13. Compound 11 was obtained via
Negishi cross-coupling of 19 with 2-pyridylzinc bro-
mide.
2þ
i
2
þ
15–17
the Ca -sensitive dye fura-2.
Selectivity over the
2þ
mGlu1receptor was also determined using the Ca flux
assay, testing compounds as agonists and antagonists.
In all instances, the compounds in Table 1 were inactive
at mGlu1 receptors up to 10 lM test concentra-
tion. Binding to native mGlu5 receptors in vitro was
determined by measuring the displacement by test
3
compounds of [ H]-3-methoxy-5-(pyridin-2-ylethynyl)-
pyridine from rat cortical membranes.
Similarly, compounds 14–17 were prepared via Sono-
gashira cross-coupling of 18 with 2-chloro-5-iodopyr-
18
1
a Ar = Phenyl
2
Ar = 2-Tolyl
S
3 Ar = 3-Tolyl
S
4
5
Ar = 4-Tolyl
Ar = 2-Biphenyl
N
a
N
6 Ar = 3-Biphenyl
7 Ar = 4-Biphenyl
+
Ar-Br
Si
18
Ar
8 Ar = 2-Pyridyl
1b Ar = 3-Pyridyl
9 Ar = 4-Pyridyl
3 4 3
Scheme 1. Reagents and conditions: (a) Pd(PPh ) , CuI, TBAF, Et N, DMF, 70 °C, 4–18 h (41–87%).

J. R. Roppe et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3993–3996
3995
S
N
S
N
Br
Br
18
+
a
b
Ar
Br
N
19
N
N
1
0 Ar = Ph
1
1
1
1 Ar = 2-pyridyl
2 Ar = 3-pyridyl
3 Ar = 4-pyridyl
Scheme 2. Reagents and conditions: (a) Pd(PPh
CO , DME/H
idylboronic acid, PdCl
0 °C, 18 h (47%).
3
)
4
, CuI, TBAF, TEA, DMF, 60 °C, 2.5 h (88%); (b) for Ar ¼ Ph; phenylboronic acid, PdCl
2 3 2
(PPh ) ,
K
2
3
2
O, 80 °C, 5 h (87%); for Ar ¼ 2-pyridyl; 2-pyridylzinc bromide, Pd(PPh
3
)
4
, THF, 65 °C, 18 h (38%); for Ar ¼ 3-pyridyl; 3-pyr-
2 3 2 2 3 2
2
(PPh
3
)
2
, K
2
CO
3
, DME/H
2
O, 80 °C, 4 h (69%); for Ar ¼ 4-pyridyl; 4-pyridylboronic acid, PdCl
(PPh ) , K CO , DME/H O,
8
S
N
S
N
I
18
+
a
b
N
Cl
20
N
Cl
N
Ar
1
1
1
1
4 Ar = Ph
5 Ar = 2-pyridyl
6 Ar = 3-pyridyl
7 Ar = 4-pyridyl
Scheme 3. Reagents and conditions: (a) Pd(PPh
CO , DME/H
idylboronic acid, PdCl
0 °C, 18 h (42%).
3
)
4
, CuI, TBAF, TEA, DMF, 70 °C, 5 h (72%); (b) for Ar ¼ Ph; phenylboronic acid, PdCl
, THF, 65 °C, 30 h (47%); for Ar ¼ 3-pyridyl; 3-pyr-
O, 80 °C, 6 h (61%); for Ar ¼ 4-pyridyl; 4-pyridylboronic acid, PdCl (PPh , K CO , DME/H O,
2 3 2
(PPh ) ,
K
2
3
2
O, 80 °C, 4 h (84%); for Ar ¼ 2-pyridyl; 2-pyridylzinc bromide, Pd(PPh
3 4
)
2
(PPh , K CO , DME/H
3
)
2
2
3
2
2
3
)
2
2
3
2
8
idine to provide intermediate 20 (Scheme 3). Compound
was then subjected to Suzuki cross-coupling
conditions with the appropriate aryl or pyridylboronic
acid to obtain 14, 16, and 17. Finally, Negishi cross-
coupling of intermediate 20 with 2-pyridylzinc bromide
gave 15.
Prior to behavioral evaluation, the rat pharmacokinetic
profiles of 12 and 16 were assessed. MTEP (1b) is also
included for reference (Table 2).
2
0
Because of its superior oral bioavailability and high
plasma levels, the hydrochloride salt of compound 16
was taken forward to determine in vivo receptor occu-
pancy. Thus at time zero rats were dosed between 0 and
In an effort to probe receptor space around the phenyl
ring in 1a, the effect of methyl and aryl substitution at
the ortho, meta, and para positions was determined. It
was found that ortho (2) and meta (3) substituted methyl
derivatives were preferred, while para-substitution as in
3
31.25 mg/kg orally and at 59 min [ H]-3-methoxy-5-
(pyridin-2-ylethynyl)pyridine was administered via tail
19
vein injection. One minute later, the animals were
sacrificed and brain binding was measured. With this
paradigm, a dose–response relationship was determined
for binding of 16 to mGlu5 receptors in vivo, resulting in
an oral Occ50 value of 0.52 mg/kg (Figure 1).
4 led to a greater than 200-fold reduction in potency.
Phenyl substitution, on the other hand, revealed a
preference for the meta (6) and para (7) biphenyl
14
derivatives over the ortho (5) analog. Previously we
had shown that the 3-pyridyl moiety (as in 1b) could be
used to replace the phenyl group in 1a, resulting in a
lower log D while still maintaining high potency in vitro.
With this in mind, a 3-pyridyl ring was incorporated
into the most potent biphenyl isomers 6 and 7, providing
The potential use of mGlu5 receptor antagonists for the
treatment of mood disorders such as anxiety and
6–11;20;21
depression has been documented recently.
Of
2
þ
Table 2. Selected rat pharmacokinetic data for 1b, 12, and 16
1
0 and the highly potent 14 (IC50 ¼ 0:5 nM in the Ca
assay, K
i
¼ 2 nM). In an effort to further decrease the
PK parameter
Compound
a
Compound
b
12
Compound
a
1
b
16
100
6.5
0.8
15
log D and thereby improve aqueous solubility, a second
basic nitrogen was introduced into the phenyl ring of 10
and 14. This led to compounds 11–13 (meta-substituted)
and 15–17 (para-substituted), and of these bipyridyl
derivatives, 12 and 16 were revealed as the most potent
compounds in the series and were therefore selected for
further profiling in vivo.
%F
16
2.0
8.3
Clp (mL/min/kg) 28
26
C
max (po, lM)
0.8
1.6
29
t
1=2 (iv, h)
a
iv dosing at 2 mg/kg, po dosing at 10 mg/kg.
iv dosing at 1 mg/kg, po dosing at 2 mg/kg.
b

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J. R. Roppe et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3993–3996
References and notes
100
Occ50= 0.52 mg/kg
1
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0.25
1.25
6.25
31.25
Compound 16: (mg/kg) p.o.
6
. Brodkin, J.; Busse, C.; Sukoff, S. J.; Varney, M. A.
Pharmacol. Biochem. Behav. 2002, 73, 359.
Figure 1. In vivo receptor occupancy dose–response for compound 16.
7
. Spooren, W. P. J. M.; Vassout, A.; Neijt, H. C.; Kuhn, R.;
Gasparini, F., et al. J. Pharmacol. Exp. Ther. 2000, 295,
particular interest to our research is the ability of these
compounds to block fear-conditioning in rats as deter-
mined by the fear-potentiated startle (FPS) model of
anxiety. Previously 1b (MTEP) was reported to be five
times more potent than the classical mGlu5 receptor
antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP)
when dosed ip in the rat FPS model of anxiety
1
267.
8. Spooren, W. P. J. M.; Schoeffter, P.; Gasparini, F.; Kuhn,
R.; Gentsch, C. Eur. J. Pharmacol. 2002, 435, 161.
9. Tatarczynska, E.; Klodzinska, A.; Chojnacka-Wojcik, E.;
Palucha, A.; Gasparini, F., et al. Br. J. Pharmacol. 2001,
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32, 1423.
1
1
1
0. Klodzinska, A.; Tatarczynska, E.; Chojnacka-Wojcik, E.;
Pilc, A. Pol. J. Pharmacol. 2000, 52, 463.
1. Schulz, B.; Fendt, M.; Gasparini, F.; Lingenhohl, K.;
Kuhn, R., et al. Neuropharmacology 2001, 41, 1.
2. Chiamulera, C.; Epping-Jordan, M. P.; Zocchi, A.; Mar-
con, C.; Cottiny, C., et al. Nat. Neurosci. 2001, 4, 873.
14
(
ED50 ¼ 1 mg/kg ip vs 5 mg/kg ip for MPEP). How-
ever, following oral dosing in this model, MTEP showed
reduced potency (ED50 ¼ 7 mg/kg po). On the other
hand, as a result of superior oral bioavailability in rats
(
Table 2), compound 16 at 1 h post-dose exhibited an
13. Huber, K. M.; Gallagher, S. M.; Warren, S. T.; Bear,
M. F. Proc. Nat. Acad. Sci. 2002, 99, 7746.
14. Cosford, N. D. P.; Tehrani, L.; Roppe, J.; Schweiger, E.;
Smith, N. D., et al. J. Med. Chem. 2003, 46, 204.
ED50 of 1 mg/kg (po) in the FPS model. Based on this
result in conjunction with its in vivo receptor occupancy
profile, compound 16 was determined to be the most
orally active mGlu5 antagonist described to date.
1
1
1
1
5. Varney, M. A.; Cosford, N. D. P.; Jachec, C.; Rao, S. P.;
Sacaan, A., et al. J. Pharmacol. Exp. Ther. 1999, 290, 170.
6. Varney, M. A.; Suto, C. M. Drug Discovery Today: HTS
Suppl. 2000, 1, 20.
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S. D., et al. Neuropharmacology 1995, 34, 871.
8. Cosford, N. D. P.; Roppe, J.; Tehrani, L.; Seiders, T. J.;
Schweiger, E. J., et al. Bioorg. Med. Chem. Lett. 2003, 13,
351.
In conclusion, a detailed exploration of receptor space
around the phenyl ring of 1a led to the discovery of the
highly potent mGlu5 antagonist 16. As a result of
excellent rodent oral pharmacokinetics, 16 was profiled,
following po dosing, in both an in vivo receptor occu-
pancy assay and the fear-potentiated startle model of
anxiety. As compared to the prototypical mGlu5
antagonists MPEP and MTEP, 16 should be consid-
ered a significant improvement due to an excellent oral
pharmacokinetic profile. Further details of the SAR and
pharmacological profile of 16 and related analogues will
be reported in due course.
19. Anderson, J. J.; Bradbury, M. J.; Giracello, D. R.;
Chapman, D. F.; Holtz, G.; Roppe, J., et al. Eur. J.
Parmacol. 2003, 473, 35.
14
2
0. Busse, C. S.; Brodkin, J.; Tattersall, D.; Anderson, J. J.;
Warren, N. A.; Tehrani, L.; Bristow, L. J.; Varney, M. A.;
Cosford, N. D. P. The behavioral profile of the potent and
selective mGlu5 receptor antagonist 3-[(2-methyl-1,3-thia-
zol-4-yl)ethynyl]pyridine (MTEP) in rodent models of
anxiety, Neuropsychopharmacology, 2004, in press.
1. Klodzinska, A.; Tatarczynska, E.; Chojnacka-Wojcik, E.;
Nowak, G.; Cosford, N. D. P.; Pilc, A. Anxiolytic-like
effects of MTEP, a potent and selective mGlu5 receptor
agonist does not involve GABAA signaling, Neurophar-
macology, 2004, in press.
2
Acknowledgements
The authors thank Greg Holtz, Darlene Giracello, and
Bill Bray for expert technical assistance.