Efficacy switching SAR of mGluR5 allosteric modulators: Highly potent positive and negative modulators from one chemotype

Article abstract of DOI:10.1016/j.bmcl.2011.03.103

A series of metabotropic glutamate 5 receptor (mGluR5) allosteric ligands with positive, negative or no modulatory efficacy is described. The ability of this series to yield both mGluR5 PAMs and NAMs with single-digit nanomolar potency is unusual, and the

Full text of DOI:10.1016/j.bmcl.2011.03.103

Bioorganic & Medicinal Chemistry Letters 21 (2011) 3407–3410
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Efficacy switching SAR of mGluR5 allosteric modulators: Highly potent
positive and negative modulators from one chemotype
Anette Graven Sams a, Gitte Kobberøe Mikkelsen a, Robbin M. Brodbeck b, Xiaosui Pu b, Andreas Ritzén a,
⇑
a Discovery Chemistry and DMPK, H. Lundbeck A/S, Ottiliavej 9, DK-2500 Valby, Denmark
b ST-1 Glutamate Biology, Lundbeck Research USA, Inc., 215 College Road, Paramus, NJ 07652-1431, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of metabotropic glutamate 5 receptor (mGluR5) allosteric ligands with positive, negative or no
modulatory efficacy is described. The ability of this series to yield both mGluR5 PAMs and NAMs with
single-digit nanomolar potency is unusual, and the underlying SAR is detailed.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 14 March 2011
Revised 25 March 2011
Accepted 29 March 2011
Available online 5 April 2011
Keywords:
Glutamate
mGluR5
Allosteric modulator
PAM
NAM
Efficacy switching
Glutamate is the principal excitatory neurotransmitter in the
mammalian central nervous system, where it mediates its effects
through interaction with ionotropic and metabotropic receptors.
The metabotropic glutamate receptors (mGluRs) are class C GPCRs
and eight subtypes are known. They are divided into three groups
based on their preferred G protein coupling and pharmacological
responses to various ligands: group I consisting of mGluR1 and
mGluR5, group II consisting of mGluR2 and mGluR3, and group
III consisting of mGluR4, mGluR6, mGluR7 and mGluR8.1 These
receptors contain two distinct domains; a large extracellular do-
main which binds glutamate at the orthosteric binding site, and
a heptahelical transmembrane domain, which has been found to
bind a variety of ligands at one or more allosteric binding sites.2
Receptor activation by glutamate binding can be positively or neg-
atively modulated or be left unaffected by the binding of ligands to
an allosteric site.
The mGluR5 has been suggested to be involved in a number of
disease states, and both inhibition and activation of this receptor
could have therapeutic benefits. The anxiolytic effect of the mGluR5
negative allosteric modulator (NAM) fenobam 1 (see Fig. 1) has
been demonstrated in a phase II clinical trial,3 and mGluR5 NAMs
have been proposed as treatment of, for example, fragile X mental
retardation,4–6 Parkinson’s disease7–9 and gastroesophageal reflux
disease.10,11 Activation of mGluR5 has been postulated to amelio-
rate both positive symptoms and cognitive deficits in schizophre-
nia.12–18
We previously reported the discovery of a novel class of
mGluR5 positive allosteric modulators (PAMs), exemplified by
compound 2.19 Interestingly, we have now observed the phenom-
enon of efficacy switching in this compound series, from positive to
negative modulation or even allosteric bindning without modula-
tion,20 resulting from small structural variations. We propose the
term neutral allosteric binder (NAB) to denote a compound that
binds to an allosteric site with no effect on receptor signaling. Such
compounds can only be identified by a combination of functional
and binding assays. Four new analogs of 2 were prepared by meth-
ods similar to those reported previously, seeTable 1. Even from this
small sample of compounds, it was observed that efficacy switch-
ing from PAM to NAM and NAB results from variations of either the
R or the left-hand side Ar moieties. Similar efficacy switching has
been observed for other allosteric modulators of mGluR5, both
MPEP-derived21 and with other scaffolds.22–26
Seeking to find more potent PAMs and NAMs, we hypothesized
that locking the amide in the bioactive conformation would reduce
the binding entropy penalty of the ligand thereby increasing its po-
tency. Because the bioactive conformation was not known to us,
we substituted the global energy minimum conformation and
introduced a conformational restraint into scaffold 2 by cycliza-
tion. The SAR at both ends of the scaffold was elaborated, both in
terms of potency and efficacy. To this end, a parallel synthesis
⇑
Corresponding author.
E-mail address: arit@lundbeck.com (A. Ritzén).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.03.103

3408
A. G. Sams et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3407–3410
Table 1
O
O
O
N
H
R
2: R = cyclopentyl, X = Y = CH
3: R = Me, X = Y = CH
NH
O
N
H
N
4: R= i-Pr, X = Y = CH
N
N
N
H
Cl
N
5: R = cyclopentyl, X = N, Y = CH
6: R = cyclopentyl, X = CH, Y = N
2
Fenobam, 1
X
Y
a
a
a
a
Figure 1. Structures of the mGluR5 NAM fenobam 1 and PAM 2.
Efficacy
IC50 (nM)
Imax (%)
EC50 (nM)
19
Emax (%)
Kib
3.3 1.6
15
7.1 2.2
7.6 1.1
>32
(
l
M)
2
3
4
5
6
PAM
NAB
PAM
NAM
PAM
2
106 10
6
protocol was devised for the preparation of the compounds in
Table 2, see Scheme 1. To prepare the intermediates 14a–d, ethyl
2-chloronicotinate 7 was vinylated in a Stille reaction to yield
8.27 This intermediate was reacted with ammonium chloride and
acetic acid, or with primary amines to directly yield the cyclized
products 9 or 10b–d, while 10a was obtained from methylation
of 9 using NaH and MeI. Next, treatment with m-CPBA gave the
N-oxides 11a–d, which were transformed into the 2-chloro-
dihydronaphthyridinones 12a–d through reaction with POCl3. A
Sonogashira coupling reaction with trimethylsilylacetylene
62 11
>10000
61
9
227 26
90
3
68 14
a
FLIPR functional assay, see Supplementary data for details. Data given as mean
value SEM of four to five experiments. Imax refers to percent inhibition of mGluR5
-Glu EC80 response at the highest tested concentration of the NAM (10 M). Emax
refers to percent stimulation (relative to the response at saturating -Glu concen-
tration in the absence of modulator) of mGluR5 -Glu EC20 response at the highest
L
l
L
L
tested concentration of the PAM (10 lM).
[3H]-MPEP binding assay, see Supplementary data for details. Data given as
b
mean value SEM of four experiments.
Table 2
a
a
a
a
Compound
R
Ar
Efficacy
IC50 (nM)
Imax (%)
EC50 (nM)
Emax (%)
Kib
(lM)
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
Phenyl
o-Tolyl
m-Tolyl
p-Tolyl
2-Chlorophenyl
3-Chlorophenyl
4-Chlorophenyl
3-Methoxyphenyl
2-Pyridyl
3-Pyridyl
2-Thienyl
Phenyl
o-Tolyl
m-Tolyl
p-Tolyl
2-Chlorophenyl
3-Chlorophenyl
4-Chlorophenyl
3-Methoxyphenyl
2-Pyridyl
3-Pyridyl
2-Thienyl
Phenyl
o-Tolyl
NAM
NAM
NAM
NAM
NAM
NAM
NAM
NAM
NAM
NAB
NAM
NAM
NAM
NAM
PAM
NAM
NAM
NAM
NAM
NAM
NAB
NAM
PAM
PAM
PAM
PAM
PAM
PAM
PAM
PAM
NAM
PAM
PAM
PAM
PAM
PAM
NAB
30
776 409
2.9 0.4
1401 409
916 223
22 17
352 161
293 76
184 29
1
58
80
86
68
70
84
73
88
92
3
3
2
5
3
3
5
2
2
0.76 0.09
>32
0.82 0.60
7.45 2.24
7.90 1.27
0.25 0.15
6.15 2.63
3.66 0.88
6.24 2.52
27.38 4.22
2.17 0.98
1.28 0.45
>32
0.51 0.25
20.18 6.99
5.68 0.67
0.10 0.03
27.14 4.46
2.95 0.72
2.74 1.31
19.55 7.39
1.93 0.71
0.89 0.23
18.79 4.34
0.67 0.09
>32
>32
5.23 1.71
>32
>32
1.77 0.63
>32
2.84 0.50
12.43 6.65
>32
3.21 2.52
27.15 4.45
>32
12.02 6.55
>32
23.15 4.88
2.24 0.76
15.61 5.35
0.72 0.24
451 165
706 245
810 171
4.8 1.4
49
60
76
88
6
3
4
1
1012 380
28
7
2517 690
5.7 2.5
4228 808
111 14
61
86
61
88
89
5
2
6
1
2
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
27
4
45
3
78
2
Cyclopentyl
Cyclopentyl
Cyclopentyl
Cyclopentyl
Cyclopentyl
Cyclopentyl
Cyclopentyl
Cyclopentyl
Cyclopentyl
Cyclopentyl
Cyclopentyl
Cyclohexyl
Cyclohexyl
Cyclohexyl
Cyclohexyl
Cyclohexyl
Cyclohexyl
Cyclohexyl
Cyclohexyl
Cyclohexyl
Cyclohexyl
Cyclohexyl
5.9 2.5
239 55
7.5 3.2
4816 1006
393 65
33 11
109 15
107 13
97 13
97 18
115 13
87
98 14
25
m-Tolyl
p-Tolyl
2-Chlorophenyl
3-Chlorophenyl
4-Chlorophenyl
3-Methoxyphenyl
2-Pyridyl
3-Pyridyl
2-Thienyl
Phenyl
o-Tolyl
4
979 206
462 77
3
199 48
80
2
748 170
68 10
103 13
122 13
62 10
12
40
3
5
466 18
36 16
m-Tolyl
p-Tolyl
54
6
2-Chlorophenyl
3-Chlorophenyl
4-Chlorophenyl
3-Methoxyphenyl
2-Pyridyl
PAM
924 369
171 81
68
47
8
7
PAM
c
NAB
NAM
PAM
PAM
518 128
74
2
3-Pyridyl
2-Thienyl
387 106
73
112 14
7
13
3
a
FLIPR functional assay, see Supplementary data for details. Data given as mean value SEM of four to five experiments. Imax refers to percent inhibition of mGluR5
EC80 response at the highest tested concentration of the NAM (10 M). Emax refers to percent stimulation (relative to the response at saturating -Glu concentration in the
M).
L-Glu
l
L
absence of modulator) of mGluR5 L-Glu EC20 response at the highest tested concentration of the PAM (10 l
b
[3H]-MPEP binding assay, see Supplementary data for details. Data given as mean value SEM of four experiments.
Inactive in functional assays and without affinity for the MPEP binding site.
c

A. G. Sams et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3407–3410
3409
A very clear trend towards NAM efficacy for the lower alkyl R
groups and PAM efficacy for the cycloalkyl R groups was observed.
The efficacy SAR of the Ar group was in most cases dominated by
that of the R group, but the 2-pyridyl and 3-pyridyl groups were
in some cases able to override the influence of the R group. Inter-
estingly, the effects of these two Ar groups were opposite, with
2-pyridyl tending to turn PAMs into NAMs, for example, 37 versus
45 and 48 versus 56, while 3-pyridyl turned NAMs into NABs, see
15 versus 24 and 26 versus 35. Similar but less pronounced was the
influence of 3-methoxyphenyl, which lowered the maximum effi-
cacy of a PAM in one case (37 vs 44) and turned a PAM into a
NAB in another (48 vs 55). The data in Table 1 suggests that this
trend for the Ar group also holds for the original series of analogues
of 2, while the R group does not seem to play the same role there.
Gratifyingly, both NAMs and PAMs with functional IC50 or EC50
below 10 nM were identified, for example, 17, 28, 31, 37 and 39.
For the NAMs, there was little or no difference in potency between
R = Me and R = i-Pr, but the Ar group had a very large influence
with m-tolyl and 3-chlorophenyl yielding the most potent com-
pounds. Among the PAMs, the preferred R group was cyclopentyl
and for the Ar group, Ph, m-tolyl and 2-thienyl gave the most po-
tent compounds, while p-tolyl and 4-chlorophenyl gave the lowest
potency. In particular, for 54 the EC50 was above the detection limit
of the assay, and this compound also did not displace [3H]-MPEP at
O
b
NH
N
9
c
O
CO2Et
Cl
CO2Et
R
a
d
e
N
N
N
N
7
8
10a: R = Me
b: R = i-Pr
c: R = cyclopentyl
d: R = cyclohexyl
O
O
O
R
R
R
f
g
N
N
N
N+
O-
Cl
N
N
X
11a-d
12a-d
13a-d: X = SiMe3
14a-d: X = H
h
O
R
i
N
32
lM, the highest concentration tested.
N
In order to detect NABs, and to establish that the compounds
Ar
bind to the same site on the receptor as the prototypical NAM
MPEP, all compounds were also tested in a radioligand displace-
ment binding assay. In general, binding Ki values were much higher
than the functional potency (IC50 or EC50), and not perfectly corre-
lated. This discrepancy has been observed in the literature28 but it
is to the best of our knowledge not completely understood. Our
working hypothesis is that the discrepancy is based, in part, on par-
tially overlapping but not identical binding sites between our test
compounds and [3H]-MPEP. The use of an alternative radioligand
closer to the structure of the test compounds would likely clarify
15-58
(See table 2)
Scheme 1. Reagents and conditions: (a) vinyl–SnBu3, Pd(PPh3)2Cl2, 2,6-di-t-Bu-4-
Me-phenol, DMF, 55 °C, 90%; (b) NH4Cl, AcOH, H2O, reflux, 32%; (c) NaH, MeI, DMF,
91%; (d) RNH2, 50 °C, 68–74%; (e) m-CPBA, CHCl3, 5–20 °C, 88–99%; (f) POCl3,
120 °C, 12–27%; (g) TMS-acetylene, Pd(dppf)Cl2, CuI, DMF, NEt3, 80–98%; (h) NaOH,
MeOH, H2O, 64–85%; (i) ArI, (PPh3)2PdCl2, CuI, NEt3 or i-Pr2NEt, THF, 60 °C.
afforded the intermediates 13a–d, which were transformed into
14a–d by desilylation under basic conditions. These building
blocks were coupled with eleven different aryl halides under
Sonogashira conditions in a library format to yield the desired
target compounds. Functional and binding data are summarized
in Table 2, and functional data is presented graphically in Figure 2.
this debate. At the highest concentration tested (32 lM) many,
but not all, compounds completely displaced [3H]-MPEP. Based
on Hill slopes near unity and the lack of evidence to the contrary,
we believe that all compounds would achieve full displacement if
high enough concentrations were to be tested.
Figure 2. Functional response of compounds 15–58. NAMs are shown in red; PAMs in blue. For explanation of Imax and Emax, see legend of Table 1.

3410
A. G. Sams et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3407–3410
9. Rylander, D.; Iderberg, H.; Li, Q.; Dekundy, A.; Zhang, J.; Li, H.; Baishen, R.;
The ability of a chemical series to yield both mGluR5 PAMs and
Danysz, W.; Bezard, E.; Cenci, M. A. Neurobiol. Dis. 2010, 39, 352.
10. Jensen, J.; Lehmann, A.; Uvebrant, A.; Carlsson, A.; Jerndal, G.; Nilsson, K.;
Frisby, C.; Blackshaw, L. A.; Mattsson, J. P. Eur. J. Pharmacol. 2005, 519, 154.
11. Gasparini, F.; Bilbe, G.; Gomez-Mancilla, B.; Spooren, W. Curr. Opin. Drug Disc.
2008, 11, 655.
12. Kinney, G. G.; Burno, M.; Campbell, U. C.; Hernandez, L. M.; Rodriguez, D.;
Bristow, L. J.; Conn, P. J. J. Pharmacol. Exp. Ther. 2003, 306, 116.
13. Homayoun, H.; Stefani, M. R.; Adams, B. W.; Tamagan, G. D.; Moghaddam, B.
Neuropsychopharmacology 2004, 29, 1259.
14. Chavez-Noriega, L. E.; Marino, M. J.; Schaffhauser, H.; Campbell, U. C.; Conn, P. J.
Curr. Neuropharmacol. 2005, 3, 9.
15. Kinney, G. G.; O’Brien, J. A.; Lemaire, W.; Burno, M.; Bickel, D. J.; Clements, M.
K.; Chen, T. B.; Wisnoski, D. D.; Lindsley, C. W.; Tiller, P. R.; Smith, S.; Jacobson,
M. A.; Sur, C.; Duggan, M. E.; Pettibone, D. J.; Conn, P. J.; Williams, D. L., Jr. J.
Pharmacol. Exp. Ther. 2005, 313, 199.
NAMs of such high potency is unusual although not unprece-
dented.25 Most reported cases of efficacy switching within one
chemotype have provided high potency NAMs with a few exam-
ples of moderately potent PAMs.22–24 In one case, optimization
from a NAM screening hit to a moderately potent PAM was possi-
ble.26 In general, there is a scarcity of high potency mGluR5 PAMs
in the literature, and the two most potent such compounds in the
present report, 37 (EC50 = 5.9 nM) and 39 (EC50 = 7.5 nM), are
among the most potent mGluR5 PAMs reported to date.29
The clear and predictable efficacy SAR trend of the present ser-
ies dominated by the R group appears to be unusual for allosteric
modulators of the mGluR5 receptor. In contrast, literature prece-
dence has often highlighted the unpredictable nature of efficacy
switching as a function of small structural changes.25,26
16. Lecourtier, L.; Homayoun, H.; Tamagnan, G.; Moghaddam, B. Biol. Psychiatry
2007, 62, 739.
17. Darrah, J. M.; Stefani, M. R.; Moghaddam, B. Behav. Pharmacol. 2008, 19, 225.
18. Liu, F.; Grauer, S.; Kelley, C.; Navarra, R.; Graf, R.; Zhang, G.; Atkinson, P. J.;
Popiolek, M.; Wantuch, C.; Khawaja, X.; Smith, D.; Olsen, M.; Kouranova, E.; Lai,
M.; Pruthi, F.; Pulicicchio, C.; Day, M.; Gilbert, A.; Pausch, M. H.; Brandon, N. J.;
Beyer, C. E.; Comery, T. A.; Logue, S.; Rosenzweig-Lipson, S.; Marquis, K. L. J.
Pharmacol. Exp. Ther. 2008, 327, 827.
In conclusion, we have discovered an improved series of alloste-
ric mGluR5 modulators that provided both PAMs and NAMs of
excellent potency. The efficacy and potency SAR has been
outlined.30
19. Ritzén, A.; Sindet, R.; Hentzer, M.; Svendsen, N.; Brodbeck, R. M.; Bundgaard, C.
Bioorg. Med. Chem. Lett. 2009, 19, 3275.
20. Rodriguez, A. L.; Nong, Y.; Sekaran, N. K.; Alagille, D.; Tamagnan, G. D.; Conn, P.
J. Mol. Pharmacol. 2005, 68, 1793.
Acknowledgment
21. Sharma, S.; Rodriguez, A. L.; Conn, P. J.; Lindsley, C. W. Bioorg. Med. Chem. Lett.
2008, 18, 4098.
22. Rodriguez, A. L.; Williams, R.; Zhou, Y.; Lindsley, S. R.; Le, U.; Grier, M. D.; vid
Weaver, C.; Jeffrey Conn, P.; Lindsley, C. W. Bioorg. Med. Chem. Lett. 2009, 19,
3209.
The authors thank Mr. Jacob Bøgesvang for technical assistance.
Supplementary data
23. Zhou, Y.; Rodriguez, A. L.; Williams, R.; Weaver, C. D.; Conn, P. J.; Lindsley, C. W.
Bioorg. Med. Chem. Lett. 2009, 19, 6502.
24. Felts, A. S.; Saleh, S. A.; Le, U.; Rodriguez, A. L.; Weaver, C. D.; Conn, P. J.;
Lindsley, C. W.; Emmitte, K. A. Bioorg. Med. Chem. Lett. 2009, 19, 6623.
25. Sharma, S.; Kedrowski, J.; Rook, J. M.; Smith, R. L.; Jones, C. K.; Rodriguez, A. L.;
Conn, P. J.; Lindsley, C. W. J. Med. Chem. 2009, 52, 4103.
26. Zhou, Y.; Manka, J. T.; Rodriguez, A. L.; Weaver, C. D.; Days, E. L.; Vinson, P. N.;
Jadhav, S.; Hermann, E. J.; Jones, C. K.; Conn, P. J.; Lindsley, C. W.; Stauffer, S. R.
ACS Med. Chem. Lett. 2010, 1, 433.
27. Showalter, H. D. H. J. Heterocycl. Chem. 2006, 43, 1311.
28. Chen, Y.; Nong, Y.; Goudet, C.; Hemstapat, K.; de Paulis, T.; Pin, J. P.; Conn, P. J.
Mol. Pharmacol. 2007, 71, 1389.
29. For another example of highly potent mGluR5 PAMs, see: Rodriguez, A. L.;
Grier, M. D.; Jones, C. K.; Herman, E. J.; Kane, A. S.; Smith, R. L.; Williams, R.;
Zhou, Y.; Marlo, J. E.; Days, E. L.; Blatt, T. N.; Jadhav, S.; Menon, U. N.; Vinson, P.
N.; Rook, J. M.; Stauffer, S. R.; Niswender, C. M.; Lindsley, C. W.; Weaver, C. D.;
Conn, P. J. Mol. Pharmacol. 2010, 78, 1105.
30. During the preparation of this manuscript, a paper describing a very similar
series of mGluR5 allosteric modulators was published, see: Williams, R.;
Mankaa, J. T.; Rodriguez, A. L.; Vinson, P. N.; Niswender, C. M.; Weaver, C. D.;
Jones, C. K.; Conn, P. J.; Lindsley, C. W.; Stauffer, S. R. Bioorg. Med. Chem. Lett.
2011, 21, 1350.
Supplementary data (these data include experimental details
for the synthesis of compounds 3–58 and descriptions of functional
and binding assays) associated with this article can be found, in the
online version, at doi:10.1016/j.bmcl.2011.03.103.
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