2-{2-[3-(Pyridin-3-yloxy)phenyl]-2H-tetrazol-5-yl}pyridine: A highly potent, orally active, metabotropic glutamate subtype 5 (mGlu5) receptor antagonist

Article abstract of DOI:10.1016/j.bmcl.2004.09.012

Structure-activity relationship studies on 3-(5-pyridin-2-yl-2H-tetrazol-2- yl)benzonitrile 2 led to the discovery of 2-{2-[3-(pyridin-3-yloxy)phenyl]-2H- tetrazol-5-yl}pyridine (10)-a highly potent and selective mGlu5 receptor antagonist with good brain penetration and in vivo receptor occupancy in rat and cross-species oral bioavailability. Structure-activity relationship studies on 3-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitrile 2 led to the discovery of 2-{2-[3-(pyridin-3-yloxy)phenyl]-2H-tetrazol-5-yl}pyridine (10)-a highly potent and selective mGlu5 receptor antagonist with good brain penetration and in vivo receptor occupancy in rat and cross-species oral bioavailability.

Full text of DOI:10.1016/j.bmcl.2004.09.012

Bioorganic & Medicinal Chemistry Letters 14 (2004) 5473–5476
2-{2-[3-(Pyridin-3-yloxy)phenyl]-2H-tetrazol-5-yl}pyridine:
a highly potent, orally active, metabotropic glutamate
subtype 5 (mGlu5) receptor antagonist
Dehua Huang,a,* Steve F. Poon,a Deborah F. Chapman,c Janice Chung,b Merryl Cramer,a
Thomas S. Reger,a Jeffrey R. Roppe,a Lida Tehrani,a 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 29 September 2004
Abstract—Structure–activity relationship studies on 3-(5-pyridin-2-yl-2H-tetrazol-2-yl)benzonitrile 2 led to the discovery of 2-{2-[3-
(pyridin-3-yloxy)phenyl]-2H-tetrazol-5-yl}pyridine (10)—a highly potent and selective mGlu5 receptor antagonist with good brain
penetration and in vivo receptor occupancy in rat and cross-species oral bioavailability.
Ó 2004 Elsevier Ltd. All rights reserved.
The major excitory neurotransmitter in the central nerv-
ous system, glutamate, activates both ionotropic and
metabotropic glutamate (mGlu) receptors.1 The G pro-
tein-coupled mGlu receptor subtype 5 (mGlu5), which is
predominantly localized post-synaptically couples via
phospolipase C leading to an increase in intracellular
Ca2+ levels.2 A number of reports have indicated that
selective antagonism of mGlu5 receptors may improve
disease states such as anxiety and depression,3–8 pain,9
drug dependence10 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 our search for further structural
classes, we examined the replacement of the alkyne linker
in 1 with a series of heteroaromatic 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, tetraz-
ole 2 showed only moderate in vitro potency and binding
affinity and suffered from relatively poor aqueous solu-
bility.15 Due to these short comings we sought to replace
the nitrile moiety of 2 with groups that would confer im-
proved potency and physicochemical properties. Herein
we describe our efforts towards these goals.
N
S
N
N
N
N
N
N
N
The tetrazole derivatives described herein were synthe-
sized as outlined in Schemes 1–3. A 1,3-dipolar cycload-
dition was employed between a diazonium salt and a
tosyl hydrazone (derived from condensation of 2-pyr-
idylaldehyde with tosyl hydrazide, Scheme 1).16
1: MTEP
2
Keywords: Metabotropic glutamate; Antagonist; Tetrazole.
*
In the case of oxygen and sulfur linked biaryls 3–8, 10,
and 11, the required aniline derivative was prepared
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.2004.09.012

5474
D. Huang et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5473–5476
Table 1. In vitro potencies of tetrazole mGlu5 receptor antagonists
O
N
O
N
Compd
Structure
hmGlu5
Ca2+ flux
IC50 (nM)a
mGlu5 Ki
(nM)b
S
O
N
a
b
N
Y
H
H
N
c
N
N
+
N
Y
N
Y
N
R
Cl
N+
R = H or F
Y = I, OAr, SAr
2
3
73
186
617
N
H2N
N
N
N
N
N
N
R
R
O
Scheme 1. Reagents and conditions: (a) tosyl hydrazide, EtOH, rt; (b)
NaNO2, HCl, H2O, EtOH, 0°C; (c) NaOH, 0°C.
1458
N
N
N
N
O
O
N
4
5
2859
30
633
59
N
O
O
N
N
NH2
Y
N
N
N+
Y
N+
F
Ar
N
N
b
N
a
O
Ar
O
N
N
R
R
R
N
N
O
R = H or F
Y = O or S
6
NAc
1577
737
3.9
NAc
NTd
633
14
N
N
N
N
N
Scheme 2. Reagents and conditions: (a) K2CO3, Ar–OH or ArSNa;
DMF, 80–150°C; (b) H2, Pd/C.
N
O
7
N
N
N
N
N
N
R
Ar
N
N
I
O
N
8
a
N
N
N
N
N
N
N
N
N
N
N
N
N
F
N
F
F
Ar = Pyridyl
R = H or Me
9
N
N
N
N
N
N
N
N
Scheme 3. Reagents and conditions: (a) NaOtBu, biphenyl-2-yl(di-
cyclohexyl)phosphine, Pd2(dba)3, Ar–NH2, dioxane, 110°C.
O
N
N
10
6.7
12
N
via an SNAr reaction of a phenol or thiophenol with an
appropriately substituted nitroarene (Scheme 2).
F
S
N
N
11
13
16
N
N
As shown in Scheme 3, for the N-linked biaryls 13 and
15, the fourth ring was appended using palladium-cata-
lyzed heteroatom cross-coupling chemistry between an
aryl iodide and an appropriately substituted aniline (or
for compound 15, 7-azaindole).17,18
N
N
F
N
N
12
13
17
38
29
N
N
N
N
N
F
N
The data in Table 1 illustrate the effect of replacing the
nitrile moiety in 2 with oxygen-linked aryl groups. Thus
O-phenyl derivative 3 and O-2-pyridyl derivative 4 both
showed a significant loss in potency compared to 2.
However, moving the pyridyl nitrogen to the 3-position
as in 5 gave an increase in the functional Ca2+ flux
(IC50 = 30nM) and binding (Ki = 59nM) assays. Mov-
ing the pyridyl nitrogen to the 4-position as in 6 led to
a loss of potency.19 Finally, if the O-3-pyridyl group
of 5 is placed at the 2- or 4-positions of the phenyl ring
as in 7 and 8, again potency was lost relative to 2 (Table
1).
N
N
124
N
N
N
F
N
N
N
14
15
22
16
30
25
N
N
N
N
N
F
N
N
N
N
N
N
F
a Ca2+ flux assay using glutamate (10lM) as agonist (n = 2–4, SD
< + /À25%).20
Previous research from this laboratory demonstrated the
dramatic increase in potency obtained when benzonitrile
2 is substituted at the 5-position with fluorine to give 9
(Ca2+ flux IC50 = 3.8nM compared to 73nM for 2;
Table 1).13
b Displacement by test compounds of [3H]-3-methoxy-5-(pyridin-2-
ylethynyl)pyridine from rat cortical membranes (n = 2–4, SD
< + /À25%).21
c Not active at 2lM.
d Not tested.
A key question was whether application of this SAR to
3-pyridyl derivative 5 would lead to a similar increase in
potency. Gratifyingly compound 10, containing a 5-flu-
orine substituent, demonstrated an increase in mGlu5

D. Huang et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5473–5476
Table 3. Pharmacokinetic data for compound 10
5475
receptor functional (IC50 = 6.7nM) and binding
(Ki = 12nM) potency compared to 5 (IC50 = 30nM
and Ki = 59nM).
Rata
Dogb
Monkeyb
Clp (mL/min/kg)
Vd (L/kg)
t1/2 (h)
21
22
14
7.7
0.7
2.5
1.5
2.3
54
Encouraged by this result, we decided to also investigate
the effect of the atom linker between the phenyl and 3-
pyridyl rings. As seen from the data in Table 1, sulfur
linked derivative 11 and carbon linked derivative 12
both maintain good potency in the Ca2+ flux assay at
IC50 = 13 and 17nM, respectively, while nitrogen linked
derivative 13 lost functional potency (IC50 = 124nM).
Interestingly, functional potency can be regained with
a nitrogen linkage by methylation of the secondary ani-
line as in 14 (IC50 = 22nM) or by using the conforma-
2.7
100
11
%F
Cmax (lM)
27
0.3
1.4
a Iv dosing at 2mg/kg, p.o. dosing at 10mg/kg. In solution of PEG 400/
water (V:V = 1:1).
b Iv dosing at 1mg/kg, p.o. dosing at 1mg/kg. In solution of PEG 400/
water (V:V = 1:1).
brain penetration and cross-species pharmacokinetics
and demonstrates good rat receptor occupancy when
dosed orally.
tionally restricted
(IC50 = 16nM).
7-azaindole
derivative
15
Having identified compounds with good potency at the
mGlu5 receptor, brain penetration and receptor occu-
pancy was evaluated using receptor occupancy assay in
rats (Table 2).22 As represented by 10, 11, 12, and 14,
this class of compounds has good rat brain penetration
with levels of 1.5–9.7lM at 1h following a dose 10mg/
kg ip. In particular, compound 10 showed brain levels
of 6.7lM, which translated into excellent rat receptor
occupancy of 90%.
Acknowledgements
We would like to thank Bill Bray for HRMS character-
ization of compounds. Gregory Holtz, and Darlene
Giracello for expert technical assistance.
References and notes
Since derivative 10 exhibited excellent mGlu5 receptor
potency and in vivo receptor occupancy and the corre-
sponding hydrochloride salt showed improved water
solubility15 (compared to hydrochloride salts of 2 and
9). It was selected for further in vivo profiling.
1. Siegel, G. J.; Aganoff, B. W.; Albers, R. W.; Fisher, S. K.;
Uhler, M. D. Basic Neurochemistry: Molecular, Cellular
and Medicinal Aspects, 6th ed.; Lippincott, Williams and
Wilkins: Philadelphia, PA, 1999.
2. Pin, J.-P.; Acher, F. Curr. Drug Targets: CNS Neurol.
Disord. 2002, 1, 297–317.
3. Brodkin, J.; Busse, C.; Sukoff, S. J.; Varney, M. A.
Pharmacol., Biochem. Behav. 2002, 73, 359–366.
4. Spooren, W. P. J. M.; Vassout, A.; Neijt, H. C.; Kuhn, R.;
Gasparini, F.; Roux, S.; Porsolt, R. D.; Gentsch, C. J.
Pharmacol. Exp. Ther. 2000, 295, 1267–1275.
5. Schulz, B.; Fendt, M.; Gasparini, F.; Lingenho¨hl, K.;
Kuhn, R.; Koch, M. Neuropharmacology 2001, 41, 1–7.
6. Klodzinska, A.; Tatarczynska, E.; Chojnacka-Wojcik, E.;
Pilc, A. Pol. J. Pharmacol. 2000, 52, 463–466.
The pharmacokinetic data for 10 in rat, dog and mon-
key are summarized in Table 3. In all three species good
to excellent bioavailability was observed (27–100%),
with a similar half-life across species (t1/2 = 2.3–2.7h).
A rat receptor occupancy–dose titration for 10 was also
carried out with the occupancy ED50 determined to be
10.7mg/kg, p.o.
In conclusion, we have demonstrated that the 3-nitrile
moiety of 2 and 9 may be replaced with an oxygen
linked 3-pyridyl group leading to compounds 5 and
10, respectively. Tetrazole 10 has excellent potency and
selectivity against the mGlu5 receptor, has good rat
7. Spooren, W. P. J. M.; Schoeffter, P.; Gasparini, F.; Kuhn,
R.; Gentsch, C. Eur. J. Pharmacol. 2002, 435, 161–
170.
8. Tatarczynska, E.; Klodzinska, A.; Chojnacka-Wojcik, E.;
Palucha, A.; Gasparini, F.; Kuhn, R.; Pilc, A. Br. J.
Pharmacol. 2001, 132, 1423–1430.
9. Varney, M. A.; Gereau, R. W. I. Curr. Drug Targets: CNS
Neurol. Disord. 2002, 1, 283–296.
Table 2. Rat receptor occupancy, brain levels and plasma for selected
compoundsa,b
10. Chiamulera, C.; Epping-Jordan, M. P.; Zocchi, A.;
Marcon, C.; Cottiny, C.; Tacconi, S.; Corsi, M.; Orzi,
F.; Conquet, F. Nat. Neurosci. 2001, 4, 873–874.
11. Huber, K. M.; Gallagher, S. M.; Warren, S. T.; Bear, M.
F. Proc. Nat. Acad. Sci. 2002, 99, 7746.
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,
B.; Anderson, J.; Brodkin, J.; Chung, J.; Jiang, X.; King,
C.; King, C.; Munoz, B.; Varney, M. A.; Prasit, P.;
Cosford, N. D. P. J. Med. Chem. 2004, 47, 4645–4648.
14. The selectivity of the prototypical tetrazole 2 was deter-
mined by extensive profiling against a battery of in vitro
assays (MDS Pharma Services screen). In addition, 2 is
highly selective for mGlu5 over mGlu1 (mGlu1 Ca2+ flux
IC50 > 10lM).
Compd
Recep. occ (%)
Hippocampus
levels (lM)
Plasma levels
(lM)
2
96
60
97
90
83
43
58
12
13
5
NDc
9.7
6.7
1.5
2.7
4.9
NDc
8.7
7.4
3.4
0.9
3.5
9
10
11
12
14
a Measured at 1h following 10mg/kg dose ip.19
b All compounds were administered in solution, except 2, 9 in the form
of suspension, of PEG 400/water (V:V = 1:1).
c Not determined.

5476
D. Huang et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5473–5476
15. Solubility of HCl salt of 2 is 1.6mg/mL in PEG 400/water
(V/V = 1:1). While the HCl salt of 9 is 36mg/mL in PEG/
400 water (V/V = 1:1).
16. Ito, S.; Tanaka, Y.; Kakehi, K. Chem. Lett. 1973, 10,
1071–1072.
17. Old, D. W.; Harris, M. C.; Buchwald, S. L. Org. Lett.
2000, 2, 1403–1406.
18. Ali, M. A.; Buchwald, S. L. J. Org. Chem. 2001, 66, 2560–
2565.
19. On a related system, when a methyl group was scanned at
the ortho, meta and para positions of the terminal phenyl
of 3, the compounds exhibited activities >2mM in the
Ca2+ flux assay.
20. Daggett, L. P.; Sacaan, A. I.; Akong, M.; Rao, S. P.; Hess,
S. D.; Liaw, C.; Urrutia, A.; Jachec, C.; Ellis, S. B.;
Dreeseen, J.; Knopfel, T.; Testa, C. M.; Landwehrmeyer,
G. B.; Young, A. B.; Varney, M.; Johnson, E. C.;
Velicelebni, G. Neuropharmacology 1995, 34, 871–
886.
21. Cosford, N. D. P.; Roppe, J.; Tehrani, L.; Seiders, T. J.;
Schweiger, E. J.; Chaudary, S.; Rao, S.; Varney, M. A.
Bioorg. Med. Chem. Lett. 2003, 13, 351–354.
22. Anderson, J. J.; Bradbury, M. J.; Giracello, D. R.;
Chapman, D. F.; Holtz, G.; Roppe, J.; King, C.; Cosford,
N. D. P.; Varney, M. A. Eur. J. Pharmacol. 2003, 473, 35–
40.