Fused tricyclic mGluR1 antagonists for the treatment of neuropathic pain

Article abstract of DOI:10.1016/j.bmcl.2011.12.131

A series of fused tricyclic mGluR1 antagonists containing a pyridone ring were synthesized. In vitro, these antagonists were potent against both human and rat isozymes, as well as selective for inhibiting mGluR1 over mGluR5. When dosed orally, several examples were active in vivo in a rat SNL test.

Full text of DOI:10.1016/j.bmcl.2011.12.131

Bioorganic & Medicinal Chemistry Letters 22 (2012) 1575–1578
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Bioorganic & Medicinal Chemistry Letters
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Fused tricyclic mGluR1 antagonists for the treatment of neuropathic pain
Chad E. Bennett a, , Duane A. Burnett a, William J. Greenlee a, Chad E. Knutson a, Peter Korakas a, Cheng Li a,
⇑
Deen Tulshian a, Wen-Lian Wu a, Rosalia Bertorelli b, Silva Fredduzzi b, Mariagrazia Grilli b,
Gianluca Lozza b, Angelo Reggiani b, Alessio Veltri b
a Merck Research Laboratories, 2015 Galloping Hill Road, MS 2800, Kenilworth, NJ 07033-0539, USA
b Schering-Plough Research Institute, San Raffaele Science Park, Via Olgettina, 58, 20132 Milan, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of fused tricyclic mGluR1 antagonists containing a pyridone ring were synthesized. In vitro, these
antagonists were potent against both human and rat isozymes, as well as selective for inhibiting mGluR1
over mGluR5. When dosed orally, several examples were active in vivo in a rat SNL test.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 28 October 2011
Accepted 29 December 2011
Available online 3 January 2012
Keywords:
mGluR1 (metabotropic glutamate receptor 1)
Neuropathic pain
Spinal nerve ligation (SNL)
Fused tricyclic pyridones
GPCR
Metabotropic glutamate receptors (mGluRs) are G-protein cou-
pled receptors (GPCRs) located primarily in the nervous system.1
These receptors possess a highly conserved, extracellular gluta-
mate binding region as well as a transmembrane domain contain-
ing a binding site for noncompetitive, allosteric modulators.2,3
Eight of these receptors have been identified and classified into
three subgroups based on sequence homology and function, with
mGluR1 and R5 comprising group I. Several studies have demon-
strated that mGluR1 knockout animals exhibit reduced sensitivity
to pain, thus identifying mGluR1 as a potential target for treating
neuropathic pain.4,5
mGluR1 (see Fig. 1).6 Furthermore, testing of compound 1 in the
Chung rat spinal nerve ligation (SNL) assay produced an ED50 of
1.2 mg/kg.7 Based on these results, a series of analogs that
combined a left side pyridine or pyrimidine ring with a right side
pyridone (2) were proposed and synthesized.8
R
R
NH2
CO2Me
X
c or d
a or b
CO2Me
S
S
N
N
3
4
5, R = NMe2, X = Br
6, R = OMe, X = I
, R = NMe2
, R = OMe
R
R
CHO
N X
e, f, org
CO2Me
N
N
R
N
N
O
S
N
S
N
OMe
N X
O
N
Z
7, R = NMe2
8, R = OMe
9, R = NMe2
10, R = OMe
O
S
S
1
2
, Z = CH or N
h-mGluR1 IC50 = 9.5 nM
Figure 2. (a) 3, CuBr2, aq 48% HBr, NaNO2, 39%; (b) 4, I2, t-BuONO, CH3CN, 59%; (c)
(i) 5, (Z)-EtOCHCHSnBu3, i-Pr2NEt, (Ph3P)4Pd, toluene, microwave, 180 °C, 20 min,
75%; (ii) aq 1 M HCl, THF, reflux, 75%; (d) (i) 6, (Z)-EtOCHCHSnBu3, i-Pr2NEt,
(Ph3P)2PdCl2, toluene, 110 °C; (ii) aq 1 M HCl, THF, reflux, 36% over two steps; (e) 7,
cyclohexylamine, HOAc, toluene, 110 °C, 85%; (f) 7, amine, 3 Å mol sieves, THF,
reflux, 2 h, then NaH, 20–76%; (g) 8, amine, Me3Al, toluene, 110 °C, 10–77%.
r-mGluR1 Ki = 7.9 nM
Figure 1. Commercially available lead (1) and proposed analogs.
Commercially available, fused tricycle 1 was found to be a po-
tent, in vitro antagonist against both human and rat forms of
Starting from known aminoester 3, the amino group was con-
verted to a bromide via a diazonium intermediate and CuBr2 to
provide aryl bromide 5. Stille coupling of 5 with 2-ethoxyvinyl-
stannane, followed by hydrolysis of the resulting enol ether
⇑
Corresponding author.
E-mail address: chadbennett3@yahoo.com (C.E. Bennett).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.12.131

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C. E. Bennett et al. / Bioorg. Med. Chem. Lett. 22 (2012) 1575–1578
Table 1
R
R
N
In vitro mGluR1 activity of N,N-dimethylaminopyridyl derivatives 9 and selectivity
N X
O
c
N X
O
versus mGluR59
S
S
N
N
10, R = OMe
11, R = Cl
R = NR1R2
a or b
N X
O
S
N
Figure 3. (a) (i) 33% HBr in HOAc, 100 °C, 85–89%; (ii) POCl3, 115 °C, 50–87%; (b)
X = 4-MeO-Ph, 3-F,4-MeO-Ph, or 2-F,4-MeO-Ph; (i) pyridine–HCl, CHCl3, 65 °C, 74–
97%; (ii) TsCl, iPr2NEt, dioxane, 65 °C, 4 h, then LiCl, Et4NCl, 65 °C, 16 h, 40–87%; (c)
amine, DMSO, 65 °C, 26–89%.
9
Compd
X
h-mGluR1 IC50
h-mGluR5 IC50
(nM)
r-mGluR1 Ki
(nM)
(nM)
9a
9b
9c
9d
9e
9f
9g
9h
9i
4-MeO-Ph
4-Me-Ph
4-F-Ph
6.3
4.7
5.2
0.4
5.7
10
4624
1201
17
1.9
29
1.9
0.4
56
18
11
13
nation of ortho-methyl and para-methoxy substituents (10e) affor-
ded good human and rat potency. From this set of data (10a–e), it
is clear that only small groups are tolerated at the meta- and ortho-
positions. The para-position of the appended phenyl ring also only
tolerated small groups. While the para-methoxyphenyl group of
10a provided a potent inhibitor, the para-ethyl group of 10f and
the para-cyclopropyl group of 10g resulted in 5-fold and 12-fold
drops in IC50, respectively. Replacing the phenyl ring substituent
with a cyclohexane ring (10h, IC50 = 5.4 nM) did not cause a loss
in potency. Reducing the size of the alkyl group to an ethyl (10i),
however did result in a >150-fold drop in IC50, relative to 10a
and 10h. Gratifyingly, the mGluR1 human IC50’s and rat Ki’s mir-
rored each other for each substrate. The data in Tables 1 and 2
clearly indicate that the right hand side substituent binds in a
space in which phenyl rings substituted with small groups fit well,
but groups significantly larger (10f) or smaller (10i) are not
tolerated.
>1000
>10,000
7844
>1000
515
4-Cl-Ph
4-Br-Ph
4-CN-Ph
4-Et-Ph
Cyclohexyl
3-F,4-MeO- 3.9
8.2
8.1
>10,000
>1000
Ph
9j
2-F,4-MeO- 14
Ph
>1000
19
N
9k
9l
5.0
1321
11
12
S
S
3.1
>10,000
N
Table 2
In vitro mGluR1 activity of 4-methoxypyridyl derivatives 109
With the size requirements of the right side substituent estab-
lished, we examined next the effects of substituent changes at the
4-position of the left side pyridine. For these studies, a series of
4-methoxypyridine derivatives (10) were converted to their corre-
sponding 4-chloropyridine analogs (11), as shown in Figure 3.
These 4-chloropyridine intermediates were used to introduce var-
ious amines into the 4-position of the pyridine ring. Typically, this
transformation was achieved by treating a 4-methoxypyridine
with HBr in acetic acid, and then converting the resulting 4-
hydroxypyridine into a 4-chloropyridine with POCl3. When the
right side substituent X was a methoxy substituted phenyl ring,
milder conditions had to be used. In these cases, the 4-methoxy-
pyridine was demethylated with excess pyridine–HCl in
chloroform at 65 °C to provide a 4-hydroxypyridine. This resulting
hydroxyl group was sulfonylated with p-toluenesulfonyl chloride,
and then the tosylate was displaced with chloride ion to provide
OMe
N
X
O
S
10
N
Compd
X
h-mGluR1 IC50 (nM)
r-mGluR1 Ki (nM)
10a
10b
10c
10d
10e
10f
10g
10h
10i
4-MeO-Ph
3-MeO-Ph
2-MeO-Ph
2-Me-Ph
2-Me,4-MeO-Ph
4-Et-Ph
4-Cyclopropyl-Ph
Cyclohexyl
Ethyl
6.3
6.5
118
>1000
85
54
35
79
5.4
>1000
566
>1000
204
32
42
119
14
778
afforded aldehyde 7. This intermediate was then treated with var-
ious amines to give fused tricycles 9. As an initial comparison to
the original lead 1, the N,N-dimethylamino derivatives 9 listed in
Table 1 were synthesized. Happily, most derivatives were very po-
tent antagonists of both the human and rat versions of mGluR1
(Table 1). Additionally, these molecules were very selective for
h-mGluR1 over h-mGluR5, with selectivities typically greater than
100 to 1. From these analogs, it is readily apparent that para-
substituted phenyl rings are well tolerated (9a–g), as well as a sat-
urated cyclohexyl ring (9h). Additional, small substitutions such as
fluorines (9i–j) or benzthiazoles (9k–l) also afforded potent
inhibitors.
To further examine the SAR on the right side of this pyridone
series, methoxypyridine derivatives 10 (Table 2) were synthesized
in a manner analogous to that used for 9 (Fig. 2). These derivatives
quickly established the patterns and size of substitutions tolerated
by mGluR1. Not surprisingly, the para-methoxyphenyl analog 10a
was quite potent. Moving the methoxy group to the meta- and then
ortho-positions (10b–c), however, resulted in a ꢀ20-fold and >150-
fold drop in IC50, respectively. Replacing the ortho-methoxy group
of 10c with a methyl substituent provided compound 10d with an
IC50 of 85 nM, albeit with slightly reduced rat potency. The combi-
Table 3
In vitro mGluR1 activity of 4-methoxyphenyl derivatives 129
R
N
OMe
O
S
N
12
Compd
R
h-mGluR1 IC50 (nM)
r-mGluR1 Ki (nM)
9a
Me2N
MeO
HO
MeNH
EtNH
n-PrNH
Cyclopropylamino
HOCH2CH2NH
6.3
6.3
>1000
27
15
55
12
124
192
17
6.5
>1000
18
8.4
24
5.2
136
108
10a
12a
12b
12c
12d
12e
12f
12g
MeOCH2CH2NH
HO
NH
12h
12i
133
17
86
63
HO
N
Me

C. E. Bennett et al. / Bioorg. Med. Chem. Lett. 22 (2012) 1575–1578
1577
OMe
NH2
OMe
N
Table 4
In vitro mGluR1 activity of N methylaminopyridyl derivatives 139
a, b, c
d or e
N
X
N
N
N
CO2Me
O
Me
HN
S
S
N
16
f
N X
O
15
g
R
N
OH
S
N
13
N
O
X
N
X
h
N
O
S
S
N
Compd
R
h-mGluR1 IC50 (nM)
r-mGluR1 Ki (nM)
17
18
13a
13b
13c
13d
13e
13f
4-Me-Ph
4-F-Ph
4-Cl-Ph
4-Br-Ph
4-CN-Ph
3-F,4-MeO-Ph
2-F,4-MeO-Ph
3.4
68
2.0
0.8
203
75
16
270
13
6.1
309
40
Figure 4. (a) I2, t-BuONO, CH3CN, 51%; (b) (Z)-EtOCHCHSnBu3, Pd2(dba)3, DMF,
80 °C, 77%; (c) aq 1 M HCl, THF, reflux, 40%; (d) aniline, 3 Å mol sieves, THF, reflux,
2 h, then NaH, 13–68%; (e) aniline, Me3Al, toluene, 110 °C, 70%; (f) X = Me, amine,
DMSO, 50 °C, 23–88%; (g) X = OMe, pyridine–HCl, CHCl3, 65 °C, 52%; (h) 2,4,6-tri-
iso-propylbenzenesulfonyl chloride, DMAP, iPr2NEt, CHCl3, 1 h, then amine, 5–31%.
13g
88
32
N
13h
47
71
S
para-methyl-, -chloro-, and -bromo-phenyl substitutions (13a,
13c, and 13d) were by far the most potent against both the human
and rat isozymes (Table 4). These same results were also seen
when the left side pyrindine was substituted at the 4-position with
either ethylamine or cycloproplyamine. Only the data for the most
potent derivatives with para-methyl-, -chloro-, and -bromo-phenyl
substitutions are listed in Table 5. As shown, these examples were
consistently potent against both the human and rat isozymes.
Starting from known aminoester 15 (Fig. 4), tricycles 16 with a
left side pyrimidine ring were synthesized in the same manner as
tricycles 10 (Fig. 2). For derivatives 17f and 17g, treatment of the
corresponding intermediate 16 (X = Me) with either methylamine
or ethylamine provided the desired products. For all other cases,
the methyl ether of the pyrimidyl ring was cleaved to afford
hydroxypyrimidine 18. This hydroxyl group was then activated
as a sulfonate ester and displaced with amines to provide analogs
17a–e,h.
Table 5
In vitro mGluR1 activity of 4-ethylamino- and 4-cyclopropyl aminopyridyl derivatives
149
R
N
X
O
S
N
14
Compd
R
X
h-mGluR1 IC50 (nM) r-mGluR1 Ki (nM)
14a
14b
14c
14d
14e
14f
EtNH
Me 7.7
9.6
12
9.4
7.3
7.3
3.3
Cyclopropylamino Me 9.8
EtNH
Cyclopropylamino Cl
EtNH Br
Cyclopropylamino Br
Cl
3.0
4.4
2.0
2.3
From the results in Table 6, it can be seen that N,N-dimethyl-
amino- and cyclopropylamino-derivatives provided the greatest
potency at both the human and rat isozymes. Surprisingly, the
methylamino derivatives 17b and 17f were the least potent, with
the ethylamino analogs in between the two groupings.
A number of the molecules presented here were tested in the
rat spinal nerve ligation (SNL) assay.7 Two of the best results were
obtained for compounds 9a and 12e (Fig. 5). Both molecules
4-chloropyridine analogs (11). Treatment of these 4-chloropyri-
dine analogs (11) with various amines in DMSO afforded the
desired 4-aminopyridine compounds.
The initial amine scan of the 4-position of the pyridine ring was
conducted with para-methoxyphenyl substituted tricycle 12. For
comparison, previously described derivatives 9a and 10a are listed
in Table 3, with 9a used as the point of reference. In the course of
generating these analogs, 4-methoxypyridine 10a was demethylat-
ed to provide hydroxypyridine 12a. The net result of this demeth-
ylation was a >150-fold decrease in mGluR1 antagonism compared
to 10a. Methylamino- and ethylamino-derivatives 12b and 12c
exhibited only slight increases in h-mGluR1 IC50, while remaining
equipotent or more potent at the rat isozyme, compared to 9a.
Increasing the length of the alkyl chain to n-propyl (12d), resulted
in a 9-fold decrease in h-mGluR1 potency. Interestingly, cyclopro-
pylamino-derviative 12e was very potent against both human and
rat isozymes (IC50 = 12 nM and Ki = 5.2 nM, respectively). Replacing
the terminal methyl group of 12d with hydroxyl and methoxy
groups provided derivatives 12f and 12g, which exhibited
20- and 30-fold loss in h-mGluR1 activity, respectively. While sec-
ondary alcohol analog 12h showed no improved potency over 12f,
secondary amino-derivative 12i did show improved potency
against the human isozyme (IC50 = 17 nM). From this data, it seems
clear that only amines substituted with small alkyl groups are well
tolerated at the 4-pyridyl position of 12. As a result, we limited
substitution at this position to methylamino-, N,N-dimethyl-
amino-, ethylamino-, or cyclopropylamino-groups during our
future SAR development.
Table 6
In vitro mGluR1 activity of A-ring pyrimidyl, C-ring pyridonyl derivatives 179
R
N
X
N
O
S
N
17
Compd
R
X
h-mGluR1 IC50
(nM)
r-mGluR1 Ki
(nM)
17a
17b
17c
17d
17e
17f
Me2N
MeNH
EtNH
MeO 6.4
MeO 88
MeO 34
19
77
32
13
28
97
52
22
Cyclopropylamino MeO 15
Me2N
MeNH
EtNH
Me
Me
Me
6.7
31
15
12
17g
17h
Cyclopropylamino Me
provided
a 50% reversal of the allodynic response at their
Next, we decided to combine the aforementioned small alkyl
amino substituents with the best right side aryls groups from
Tables 1 and 2. From the data in Table 4, it can be seen that
corresponding Cmax’s when dosed at 10 mpk (po). From rat PK
experiments, significant demethylation occurred with 9a. Indeed,
a M-14 metabolite was present in a 6-fold excess over parent.

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C. E. Bennett et al. / Bioorg. Med. Chem. Lett. 22 (2012) 1575–1578
The best examples were capable of providing a 50% reversal in
the rat SNL test.
N
N
HN
N
N
OMe
N
OMe
O
O
S
S
References and notes
9a
12e
1. Kew, J. N. C.; Kemp, J. Psychopharmacology 2005, 179, 4.
Rat SNL (10 mpk, p.o.):
Rat SNL(10 mpk, p.o.):
2. Gasparini, F.; Kuhn, R.; Pin, J.-P. Curr. Opin. Pharm. 2002, 2, 43.
3. Lavreysen, H.; Janssen, C.; Bischoff, F.; Langlois, X.; Leysen, J. E.; Lesage, A. S. Mol.
Pharmacol. 2003, 63, 1082.
4. Scheryantz, J. M.; Kingston, A. E.; Johnson, M. P. J. Med. Chem. 2007, 50, 2563.
5. Neugebauer, V. Pain 2002, 98, 1.
50% reversal @ 2h
50% reversal @ 3 h
Rat PK (10 mpk, p.o.):
Parent 9a:
Rat PK (10 mpk, p.o.):
AUC = 8866 ng-h/mL
B/P @ 6 h = 0.2
AUC = 873 ng-h/mL
B/P @ 6 h = 0.3
6. For related series of mGluR1 antagonists, see: (a) Zheng, G. Z.; Bhatia, P.;
Daanen, J.; Kolasa, T.; Patel, M.; Latshaw, S.; El Kouhen, O. F.; Chang, R.; Uchic, M.
E.; Miller, L.; Nakane, M.; Lehto, S. G.; Honore, M. P.; Moreland, R. B.; Brioni, J. D.;
Stewart, A. O. J. Med. Chem. 2005, 48, 7374; (b) Zheng, G. Z.; Bhatia, P.; Kolasa, T.;
Patel, M.; El Kouhen, O. F.; Chang, R.; Uchic, M. E.; Miller, L.; Baker, S.; Lehto, S.
G.; Honore, P.; Wetter, J. M.; Marsh, K. C.; Moreland, R. B.; Brioni, J. D.; Stewart,
A. O. Bioorg. Med. Chem. Lett. 2006, 16, 4936; (c) Wu, W.-L.; Burnett, D. A.;
Domalski, M.; Greenlee, W. J.; Li, C.; Bertorelli, R.; Fredduzzi, S.; Lozza, G.; Veltri,
A.; Reggiani, A. J. Med. Chem. 2007, 50, 5550; (d) Sasikumar, T. K.; Qiang, L.;
Burnett, D. A.; Greenlee, W. J.; Li, C.; Heimark, L.; Pramanik, B.; Grilli, M.;
Bertorelli, S.; Lozza, G.; Reggiani, A. Bioorg. Med. Chem. Lett. 2009, 19, 3199; (e)
Sasikumar, T. K.; Qiang, L.; Burnett, D. A.; Greenlee, W. J.; Li, C.; Grilli, M.;
Bertorelli, S.; Lozza, G.; Reggiani, A. Bioorg. Med. Chem. Lett. 2010, 20, 2474.
7. Kim, S. H.; Chung, J. M. Pain 1992, 50, 355.
M-14 Metabolite:
AUC = 5475 ng-h/mL
B/P @ 6 h = 0.12
Figure 5. In vivo data for compounds 9a and 12e.
Demethylation was not observed with 12e, suggesting that 9a was
being demethylated at nitrogen.
In summary, a series of fused tricyclic mGluR1 antagonists con-
taining a pyridone ring were synthesized. In vitro, many of these
compounds proved to be potent against both human and rat iso-
zymes, as well as selective for inhibiting mGluR1 over mGluR5.
Several of these molecules were active in vivo when dosed orally.
8. Bennett, C. E.; Wu, W.-L.; Burnett, D. A. WO 2007/024593 A1.
9. IC50 values are means of three experiments, with standard deviations being
620% of the mean and typically 610%. Ki values are means of three experiments,
with confidence values being typically 620%.