Discovery of highly potent, selective, orally bioavailable, metabotropic glutamate subtype 5 (mGlu5) receptor antagonists devoid of cytochrome P450 1A2 inhibitory activity

Article abstract of DOI:10.1016/j.bmcl.2004.09.018

Structure-activity relationship studies focused on bio-isosteric replacements of 2-pyridyl resulted in mGlu5 receptor antagonists with reduced inhibition of cytochrome P450 1A2. This led to highly potent, selective and orally bioavailable 2-imidazolyl tet

Full text of DOI:10.1016/j.bmcl.2004.09.018

Bioorganic & Medicinal Chemistry Letters 14 (2004) 5481–5484
Discovery of highly potent, selective, orally bioavailable,
metabotropic glutamate subtype 5 (mGlu5) receptor
antagonists devoid of cytochrome P450 1A2inhibitory activity
Nicholas D. Smith,a,* Steve F. Poon,a Dehua Huang,a Mitchell Green,a Christopher King,a
Lida Tehrani,a Jeffrey R. Roppe,a Janice Chung,b Deborah P. Chapman,c
Merryl Cramera and Nicholas D. P. Cosforda
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 12 May 2004; revised 3 September 2004; accepted 7 September 2004
Available online 30 September 2004
Abstract—Structure–activity relationship studies focused on bio-isosteric replacements of 2-pyridyl resulted in mGlu5 receptor
antagonists with reduced inhibition of cytochrome P450 1A2. This led to highly potent, selective and orally bioavailable 2-imidaz-
olyl tetrazoles such as (10) that are devoid of cytochrome P450 inhibitory activity.
Ó 2004 Elsevier Ltd. All rights reserved.
Glutamate is the major excitory neurotransmitter in the
nervous system exerting itÕs effect through both iono-
tropic receptors and the G-protein coupled metabo-
tropic glutamate (mGlu) receptors. The eight mGlu
receptor subtypes identified to date are classified into
three groups based on sequence homology.1 In group
I, mGlu1 and mGlu5 are primarily localized postsynap-
tically and coupled via phospholipase C; activation leads
to phosphoinositide hydrolysis and elevation of intra-
cellular Ca2+ levels.2 Selective antagonists of the mGlu5
receptor may be useful to treat several disease states
including anxiety and depression,3–8 pain,9 drug depend-
ence10 and mental retardation.
We have recently described the discovery of MTEP (1),
a potent and selective mGlu5 receptor antagonist with
anxiolytic properties.11 In a continuing search for alter-
native structural series to diaryl-alkynes derivatives such
as MTEP, we developed a series of heteroaromatic
azoles exemplified by tetrazole 2.12 Compound 2 was
found to be a potent and selective13 mGlu5 receptor
antagonist (mGlu5 Ca2+ flux = 73nM) with good rat
brain receptor occupancy (Table 1). In vitro profiling
of 2 against cytochrome P450 (CYP) isoforms revealed
that although 2 did not significantly inhibit CYP 3A4,
2D6, 2C9 or 2C19 (IC50 > 14lM), it was a potent inhib-
itor of CYP 1A2 (IC50 = 3.8lM). Indeed, this CYP 1A2
inhibition was observed for the tetrazole mGlu5 recep-
tor antagonists as a class—vide supra. At 90% receptor
occupancy, the concentration of 2 in plasma (9.7lM) is
higher than the CYP 1A2 IC50 and this constituted a
potential liability for this series. To eliminate the possi-
bility of drug–drug interactions, we sought to develop
N
S
N
N
N
N
N
N
N
1: MTEP
2
Keywords: Metabotropic glutamate; Antagonist; Tetrazole; Cyto-
chrome P450 1A2.
*
Corresponding author. Tel.: +1 858 202 5248; fax: +1 858 202
5743; e-mail: nicholas_smith@merck.com
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.09.018

5482
N. D. Smith et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5481–5484
Table 1. In vitro data for mGlu5 receptor antagonists
Compound
Structure
mGlu5 Ca2+ flux (nM)a
mGlu5 Ki (nM)b
CYP 1A2 (lM)c
% Occd
90
N
2
73
102
3.8
N
N
N
N
N
N
S
e
—
3
142
183
1.0
N
N
N
N
N
N
S
e
—
4
5
6
7
148
157
1.7
N
N
N
N
N
N
N
e
77
34
>14
—
N
N
N
N
N
N
N
N
e
—
e
—
>2000
>2000
>2000
>2000
N
N
N
N
N
e
—
e
—
N
N
N
N
N
N
N
N
8
3.9
14
2.3
97
N
N
N
N
F
N
N
9
N
47
77
6.7
9.3
19
12
>14
>14
0.7
95
97
90
N
N
N
N
F
O
N
N
N
10
11
N
N
N
N
F
N
N
N
N
N
N
F
N
O
N
N
N
12
13
9.7
5.2
3.2
1.8
5.1
0.5
69
95
N
N
N
F
N
N
N
N
N
N
F
N
N
14
20
3.5
3.5
29
N
N
N
N
N
F
a Using glutamate (10lM) as agonist (n = 2–4, SD < 25%).17
b Displacement by test compounds of [3H]-3-methoxy-5-(pyridin-2-ylethynyl)pyridine from rat cortical membranes (n = 2–4, SD < 25%).18
c Recombinant cytochrome P450 protocols.19
d Measured 1h post-administration of 10mg/kg compound ip.20
e Not tested.

N. D. Smith et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5481–5484
5483
O
N
O
to 3.8lM for 2). In turn, moving the nitrogen around
the ring as in 3-imidazolyl derivative 6 and 2-pyrazolyl
derivative 7 led to a dramatic loss of mGlu5 receptor po-
tency (Ca2+ flux >2000nM).
HET
N
HET
S
O
a
b
H
H
R
HET
N
c
N
+
N
N
R
Cl
R
Having demonstrated the improvement in CYP 1A2
inhibition upon replacing the 2-pyridyl moiety of 2 with
a 2-imidazolyl moiety (as in 3), we applied this finding
to the lead compounds derived from SAR studies on
the phenyl ring of tetrazole 2.12,15,16 Thus tetrazole 8
is highly potent against the mGlu5 receptor (Ca2+
flux = 3.9nM, Ki = 14nM) and showed high rat receptor
occupancy (97% at 10mg/kg ip) but inhibited CYP 1A2
with an IC50 of 2.3lM. Gratifyingly, replacement of the
2-pyridyl ring in 8 with a 2-imidazolyl ring (9) greatly re-
duced CYP 1A2 inhibition (IC50 > 14lM) while mGlu5
receptor binding potency (Ki = 9.3nM) and rat receptor
occupancy (95%) were maintained. Similarly, 4-methyl-
2-imidazolyl derivative 10 maintained potency in the
binding assay (Ki = 19nM) and was a poor inhibitor
of CYP 1A2 (IC50 > 14lM). Interestingly, both 9
(Ca2+ flux = 47nM) and 10 (Ca2+ flux = 77nM) were
somewhat less active in the Ca2+ flux functional assay
compared to 8 (Ca2+ flux = 3.9nM). However when
tested in a phosphatidylinositol hydrolysis assay,18 func-
tional potency for 10 was maintained (IC50 = 23nM)
compared to 8 (IC50 = 41nM).
N+
H2N
N
Scheme 1. Reagents and conditions: (a) tosyl hydrazide, EtOH, rt; (b)
NaNO2, HCl, H2O, EtOH, 0°C; (c) NaOH, 0°C.
tetrazole derivatives that did not inhibit the major CYP
isoforms and in particular CYP 1A2. Our efforts to-
wards this goal are described herein.
Briefly, the N-linked tetrazoles derivatives were pre-
pared by reaction of a tosyl hydrazone (derived from
the condensation of a heterocyclic aldehyde with tosyl
hydrazide) with a diazonium salt under basic conditions
(Scheme 1).12,14 In turn the diazonium salt was derived
from an appropriately substituted aniline.
Recent disclosures from this laboratory have described
structure–activity relationship (SAR) studies around
the phenyl ring of tetrazole 2 and demonstrated that sig-
nificant increases in potency at the mGlu5 receptor
maybe achieved by employing the appropriate substitu-
tion pattern.12,15,16 However, in each case these
improvements in potency also led to an increase in
CYP 1A2 inhibition (Table 1, compounds 2, 8, 11, 13).
Concerned that the pyridyl moiety in 2 might be respon-
sible for CYP 1A2 inhibition, we carried out a survey of
pyridyl ring replacements while keeping constant the 3-
cyanophenyl ring. Selected examples from these SAR
studies are shown in Table 1.
The trend of reducing CYP 1A2 inhibition while main-
taining mGlu5 receptor potency and selectivity was ob-
served with other lead compounds also. Thus in
comparing aryl ethers 11 and 12, 2-imidazolyl derivative
12 has reduced CYP 1A2 inhibition while maintaining
mGlu5 receptor potency in both the functional
(Ca2+ flux = 9.7nM) and binding (Ki = 3.2nM) assays.
Similarly, in comparing biaryls 13 and 14, mGlu5 recep-
tor potency is maintained while CYP 1A2 inhibition is
reduced. However, in both 12 and 14 there is a reduction
of mGlu5 receptor occupancy in vivo.
Compared with 2, 2-thiazolyl derivative 3 (Ca2+
flux = 142nM) and 5-thiazolyl derivative
4
(Ca2+
flux = 148nM) maintained potency against the mGlu5
receptor but in both cases showed an undesired increase
in CYP 1A2 inhibition. On the other hand, 2-imidazolyl
derivative 5 also maintained mGlu5 receptor potency in
the functional (Ca2+ flux = 77nM) and binding
(Ki = 34nM) assays but more importantly CYP 1A2
inhibition was greatly reduced (IC50 > 14lM: compared
A full profile of CYP inhibition and rat pharmacokinet-
ics comparing 8 and 10 is shown in Tables 2 and 3.
Both 8 (Ki = 14nM) and 10 (Ki = 19nM) are potent and
selective mGlu5 receptor antagonists. When screened
Table 2. CYP P450a profile of 8 and 10
Compound
mGlu5 Ki (nM)
1A2 (lM)
3A4 (lM)
2D6 (lM)
2C9 (lM)
2C19 (lM)
8
10
14
19
2.3
70
>14
>14
>14
>14
>14
>14
>14
>14
a Recombinant cytochrome P450 Gentest based protocols.19
Table 3. Rat occupancya and PKb profile of 8 and 10
Compound
Rec Occb (%)
Brain (lM)c
%F
t1/2 (h)
Vd (L/kg)
Cl (mL/min/kg)
AUC (lMh)
8
10
97
97
9.7
8.1
24
22
2.9
0.3
1.1
0.6
15
40
9.6
3.4
a Rat brain receptor occupancy measured 1h post-administration of 10mg/kg ip (PEG 400).20
b 2mg/kg dosed iv (solution in PEG 400/water (1/1)), 10mg/kg dosed po (Sprague–Dawley rats).
c Measured at 1h post-administration of 10mg/kg ip (PEG 400).

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Acknowledgements
We would like to thank Bill Bray and Darlene Giracello
for expert technical assistance.
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