Substituted biaryl pyrazoles as sodium channel blockers

Article abstract of DOI:10.1016/j.bmcl.2010.07.080

Voltage-gated sodium channels have been shown to play a critical role in neuropathic pain. A series of low molecular weight biaryl substituted pyrazole carboxamides were identified with good in-vitro potency and in-vivo efficacy. Compound 26, a Na_v1.7 blocker has excellent efficacy in the Chung model of neuropathic pain.

Full text of DOI:10.1016/j.bmcl.2010.07.080

Bioorganic & Medicinal Chemistry Letters 20 (2010) 5480–5483
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Substituted biaryl pyrazoles as sodium channel blockers
a
a
a
a
Sriram Tyagarajan a, , Prasun K. Chakravarty , Bishan Zhou , Brett Taylor , Michael H. Fisher ,
Mathew J. Wyvratt a, Kathy Lyons b, Tracy Klatt b, Xiaohua Li b, Sanjeev Kumar b, Brande Williams c,
John Felix c, Birgit T. Priest c, Richard M. Brochu c, Vivien Warren c, McHardy Smith c, Maria Garcia c,
Gregory J. Kaczorowski c, William J. Martin d, Catherine Abbadie d, Erin McGowan d, Nina Jochnowitz d,
William H. Parsons a
*
a Department of Medicinal Chemistry, Merck Research Laboratories, Rahway, NJ 07065, USA
b Department of Drug Metabolism and Pharmacokineticsm, Merck Research Laboratories, Rahway, NJ 07065, USA
c Department of Ion Channels, Merck Research Laboratories, Rahway, NJ 07065, USA
d Department of Pharmacology, Merck Research Laboratories, Rahway, NJ 07065, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Voltage-gated sodium channels have been shown to play a critical role in neuropathic pain. A series of
low molecular weight biaryl substituted pyrazole carboxamides were identified with good in-vitro
potency and in-vivo efficacy. Compound 26, a Nav1.7 blocker has excellent efficacy in the Chung model
of neuropathic pain.
Received 22 February 2010
Revised 16 July 2010
Accepted 20 July 2010
Available online 24 July 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Neuropathic pain
Voltage gated sodium channel
Nav1.7 blockers
Biaryl pyrazoles
Chung model
Voltage-gated sodium channel blockers have been shown to
play a key role in the pathogenesis of neuropathic pain. Neuro-
pathic pain is a complex state that is caused by injury to the
peripheral or central nervous system.1 Certain subtypes of sodium
channels (Nav1.7 and Nav1.8) are over expressed at the site of inju-
ries.2 Neuropathic pain is often treated using existing drugs that
are weak sodium channel antagonists. They provide only partial re-
lief and have adverse effects that limit their use. Our goal is to de-
velop state dependent Nav1.7 blockers, which may afford minimal
CNS and cardiac side effects and improved efficacy for treating
neuropathic pain.3 Recent support for this goal has come from hu-
man specific genetic evidence that has shown a loss of function
mutation in Nav1.7 has been associated with congenital insensitiv-
ity to painful stimuli.4
O
F
S
O
N
N
Cl
N
OCF3
NH2
NH2
O
2
1
Merck biphenyl thiazole
Cocensys / Purdue Pharma
Nav1.7: Ki = 0.013 µM
Nav1.7: Ki = 0.015 µM
R2
O
N
N
NH2
R1
3
Figure 1. Design concept.
Previously, we disclosed a novel class of biphenyl thiazole car-
boxamides, represented by 1 (Fig. 1) as state dependent sodium
channel blockers.5 These compounds displayed good hNav1.7 func-
tional block (Nav1.7 EP Ki = 0.015 lM) and good in-vivo efficacy in
various animal models of neuropathic pain. However, pharmacoki-
underwent extensive hydrolysis of the amide to the acid in dog
hepatocytes. Recently, scientists at Cocensys/Purdue Pharma re-
ported the discovery of aryloxy phenyl pyrazole carboxamide 2
as sodium channel blockers with good hNav1.7 functional block
netic metabolite analysis revealed that this class of compounds
(Nav1.7 EP Ki = 0.013 l
M) in electrophysiology.6 By combining
the two designs, we report a new class of pyrazoles where aryloxy-
phenyl group is replaced by a biphenyl group, yielding pyrazole
carboxamide 3, a novel class of sodium channel blockers.
* Corresponding author. Tel.: +1 732 594 6226; fax: +1 732 594 5350.
E-mail address: sriram_tyagarajan@merck.com (S. Tyagarajan).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.07.080

S. Tyagarajan et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5480–5483
5481
Previous work in the area of biphenyl thiazolidine-2,4-diones
O
O
NH2
and thiazoles as Nav1.7 blockers has shown that meta arrangement
of the biphenyl (1,3) is more potent than the para biphenyl (1,4).5,7
In case of the biphenyl pyrazoles, the same relationship was main-
tained (Table 1). Exploration of the biphenyl ring system consis-
tently showed that the 2-substituted biphenyl to be the preferred
position. Meta (1,3) biphenyl pyrazole carboxamide 5 exhibited a
significantly more potent hNav1.7 block compared to para (1,4)
biphenyl pyrazole carboxamide 4. However, both of these N-pyra-
zole carboxamides displayed high clearance in rat pharmacokinetic
studies and were rapidly metabolized during incubation in rat or
human liver microsomes. An analysis of the metabolite products
indicated the formation of carboxylic acid due to hydrolysis of
the amide. In an attempt to solve the issue of labile amide, we syn-
thesized a series of various pyrazole carboxamide arrangements.
Biphenyl-5-carboxamide-1H-pyrazole 10 was prepared as out-
lined in Scheme 1. Palladium catalyzed Suzuki cross-coupling of
the commercially available 3-bromoacetophenone 6 with 2-triflu-
oromethoxyphenyl boronic acid afforded the biphenyl acetophe-
a
Br
N
H
+
R1
CO2Et
11
12
R1 = Me, t-butyl
N
N
b
N
Br
N
CO2Et
CO2Et
R1
R1
R
13
14
c
N
N
R1
CONH2
R
15
Scheme 2. Reagents and conditions: (a) AcOH, reflux; (b) ArB(OH)2, Ph3P, Pd(OAc)2,
n-propanol, H2O, 2 M Na2CO3, reflux; (c) ammonia, MeOH.
none 7.8 The biphenyl diketoester
8 was constructed from
alkylation of the acetophenone 7 with diethyl oxalate. Condensa-
tion with hydrazine hydrate afforded the pyrazole ester 9. The car-
boxamide 10 was prepared by aminolysis of the ester 9.
The syntheses of N-aryl pyrazole carboxamides is outlined in
Scheme 2. These analogs were constructed from the 3-bromo pyr-
azole ester 13, a common key intermediate. Condensation of the
commercially available 3-bromophenyl hydrazine 11 and the dike-
toester 12 afforded a regioisomeric mixture of pyrazole esters. The
desired regioisomer 13, the major product, was isolated after chro-
matographic separation. With the pyrazole ester derivative 13 in
hand, the biphenyl pyrazole esters 14 were synthesized via
palladium catalyzed Suzuki cross-coupling with aryl boronic acids.
Aminolysis of the ester 14 afforded the biphenyl pyrazole carbox-
amides 15.
The synthesized compounds were evaluated for their ability to
block voltage gated hNav1.7 using an in-vitro functional membrane
potential FRET based assay.9 Selected compounds were screened
for activity against ancillary targets including hERG (MK-0499
binding assay measures the displacement of 35S-labeled MK-
0499, a known hERG blocker), CYP inhibition and Cav 1.2 (diltia-
zem (DLZ)) binding.10 Compounds selective in these ancillary as-
says were examined in the rat spinal nerve ligation (Chung)
model of neuropathic pain and their pharmacokinetic profiles were
evaluated.11
Table 1
para (1,4) versus meta (1,3) arrangement in pyrazoles
Nav 1.7 EP
Rat Clp (mL/min/kg)
(Ki nM)
(1 mpk, iv)
O
N
NH2
N
As shown in Table 2, the carboxamide regioisomer 10 main-
tained potent hNav1.7 block, but also exhibited a strong affinity to-
wards MK-0499 binding. A series of N-aryl-3-carboxamides were
also investigated. Compound 16 had a very good hNav1.7 block
0.111
0.018
125
123
Cl
4
O
N
N
NH2
Table 2
SAR around the pyrazole ring
Cl
5
OCF3
Entry
R1
Nav1.7 VIPR
(IC50 M)
MK-0499
(% inh @ 10 lM)
,
l
a
b
O
Br
OCF3
7
N
O
NH
6
10
16
0.508
0.065
94%
51%
CONH2
O
c
N
CO2Et
N
N
NH
N
NH2
O
O
OCF3
9
OCF3
8
O
O
EtO
N
17
0.111
44%
NH2
H2N
d
N
NH
O
N
OCF3
N
18
19
0.224
1.368
6%
NH2
O
H2N
10
O
N
N
ND
Scheme 1. Reagents and conditions: (a) ArB(OH)2, Ph3P, Pd(OAc)2, n-propanol, H2O,
2 M Na2CO3, reflux; (b) NaH, diethyl oxalate, THF; (c) hydrazine hydrate, EtOH,
reflux; (d) Ammonia, MeOH.
NH2

5482
S. Tyagarajan et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5480–5483
and reduced affinity towards MK-0499. Introduction of amino
group at C-5 position of the pyrazole, 17, resulted in loss of
Nav1.7 activity.
Replacing the amino group of 17 with a methyl group at C-5
afforded pyrazole 18, which was a potent hNav1.7 blocker with
dramatically reduced affinity for MK-0499. Introduction of a bulky
t-butyl group at C-5 of the pyrazole, 18, led to 19 with a dramatic
loss in hNav1.7 potency. Based on these results, a series of N-aryl-
3-methyl-5-carboxamides pyrazoles were selected for further
investigation.
Table 3 shows a series of novel N-aryl pyrazole carboxamide so-
dium channel blockers. Consistent with our prior work, we found
lipophilic substituents were preferred at the R2 position.5,7 Lipho-
philic substituents such as the trifluoromethyl or the trifluoro-
methoxy were found to be optimal at the 2-position of the
biphenyl group. Most of these pyrazoles displayed good hNav1.7
block. Compounds with similar hNav1.7 potency were distin-
guished on basis of MK-0499 binding data and/or SNL (Chung)
model in-vivo efficacy data (at 10 mg/kg).11 Compounds with sig-
nificant reversal of allodynia at 2 and 4 h in the SNL model were
further examined for their pharmacokinetic profile.
Pyrazole 21 with a trifluoromethyl substituent is a more potent
Nav1.7 blocker than the 2-chloro pyrazole analog 20. However, 21
displayed poor efficacy in the in-vivo Chung model. With a trifluo-
romethyl group installed at the R2 position of the biphenyl pyra-
zole, a series of di-substituted analogs, 22–28 were explored. The
inclusion of a second substituent (a fluoro or trifluoromethyl
group) at R6, pyrazole 25 and 28 resulted in excellent hNav1.7
block but little or no effect in the in-vivo Chung pain model. Com-
paratively, a second trifluoromethyl group installed at R4 or R5 po-
sition of the distal phenyl ring, compounds 26 and 27, imparted
modest hNav1.7 block but improved efficacy significantly at 2
and 4 h in the Chung model.
efficacious in the Chung model. Substitution of the middle ring
with a fluoro or trifluoromethoxy group at R6 or R7 position of
the biphenyl, as in compounds 33 and 34, increased the affinity
for MK-0499 binding while maintaining the same level of hNav1.7
block. Lack of a R2 substituent, 34 or introduction of a bulky group
at R2, 35, resulted in dramatic loss of hNav1.7 potency.
From these results, 26 and 27 were identified for additional
characterization based on the efficacy in the SNL (Chung) model
and favorable ion channel profile. As illustrated in Table 4, pyrazole
26 had a lower affinity for cytochrome P450 enzymes and a low po-
tential for induction as measured by an hPXR induction assay,
which indicated that 26 may have lower liability for drug–drug
interactions.
Compound 26 was evaluated for pharmacokinetic properties in
rat and dog (Table 5). In rat, it showed moderate clearance (11 mL/
min/kg), good oral AUC (1.21 lM h kg/mg), good plasma half-life
(2 h) and desirable bioavailability (55%). However, in dog, the PK
profile showed slightly higher clearance (16 mL/min/kg) and lower
plasma half-life (1.4 h).
This potent Nav channel blocker was efficacious in various pre-
clinical pain models. In the rat spinal nerve ligation model (SNL) of
neuropathic pain, 26 attenuates mechanical allodynia to the same
Table 4
P450 inhibition and hPXR induction data for compound 26 and 27
Entry
CYP3A4
IC50 M)
CYP2C9
CYP2D6
IC50 M)
PXR
IC50
(
l
IC50
(l
M)
(
l
(lM)
26
27
>100
51
9
2.6
>100
33
>25
ND
Table 5
Pharmacokinetic profile of 26
Compound 18, possessing a trifluoromethoxy group at the R2
position, afforded moderate hNav1.7 block and reduced activity
in MK-0499 binding. The pyrazole 18 exhibited moderate in-vivo
efficacy in the Chung model. R2 substitution with a more liphophil-
ic trifluoroethoxy (29) or pentafluroethoxy (30) group maintained
the potency of hNav 1.7 block but these compounds were even less
t1/2 (h)
Vd
(L/kg)
Clp
(mL/mg/kg)
nAUC (po)
(lM h kg/mg)
F (%)
Rat
Dog
2.0
1.4
3.10
1.43
11
16
1.2
1.2
55
50
Table 3
Biphenyl-3-carboxamide-5-methyl pyrazoles
R8
R7
R2
R5
R3
O
N
N
NH2
R4
R6
Entry
R2
R3
R4
R5
R6
R7
R8
Nav1.7 VIPR (IC50
,
l
M
MK-0499 (% inh
SNL % Reversala
SNL % Reversala
or % inh @ 10
l
M)
@ 10
l
M)
(t = 2 h)
(t = 4 h)
20
21
22
23
24
25
26
27
28
18
29
30
31
32
33
34
35
Cl
CF3
CF3
CF3
CF3
CF3
CF3
CF3
H
H
F
H
H
H
Cl
H
H
CF3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
F
H
H
CF3
H
H
H
H
H
H
H
CF3
H
H
H
H
H
H
F
H
H
CF3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
F
H
H
H
H
H
H
H
H
H
H
H
H
H
F
0.738
0.163
0.781
0.243
0.412
0.071
0.426
0.272
0.041
0.225
0.187
0.128
0.162
0.222
0.373
17%
8%
ND
14
ND
8
ND
19
ND
0
ND
2
32
35
6
11
4
9
ND
8
ND
ND
ND
68%
73%
45%
41%
74%
53%
51%
67%
6%
43%
49%
66%
62%
77%
ND
H
H
H
H
H
H
H
H
H
H
H
H
CF3
H
ND
12
35
39
16
33
18
15
ND
17
ND
ND
ND
CF3
OCF3
OCH2CF3
OCH2CF2CF3
OCF3
OCF3
OCF3
H
H
OCF3
H
H
H
H
PhO
H
0.875
13%
a
Reversal of rat spinal nerve ligation induced allodynia (Chung model – Von Frey) versus pre-surgery sensitivity.

S. Tyagarajan et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5480–5483
5483
+
120%
Na channel blocker
Other MOA
100%
Duration of effect (hr)
Acute dose (mg/kg)
80%
0.25
n.a.
2
Lidocaine (5-iv)
Mexiletine (30)
(100)
*
Rat
n.s.
60%
40%
20%
0%
Dog
*
Monkey
Human
>1
1
Lamotrigine (100)
*
Carbamazepine (100)
*
n.a.
2
Amitriptyline (10)
n.s.
0
10
20
30
40
50
60
70
Gabapentin (30)
Pyrazole 26 (30)
*
Time (min)
4
*
Figure 4. Metabolic stability of compound 26 in liver microsomes.
0
20
40
60
80
100
Maximal Possible Effect (%)
dog, human, and monkey but significantly more pronounced with
rat liver microsomes.
Figure 2. Comparison of pyrazole 26 with standards: SNL model of neuropathic
pain.
In summary, a new class of sodium channel blockers based on a
biphenyl pyrazole core with a primary carboxamide at C-3 position
has been identified. Many of the analogs in this series exhibit good
in-vitro potency and in-vivo efficacy. Compound 26 has excellent
efficacy in the Chung model of neuropathic pain. It has a clean pro-
file in human P450 inhibition and PXR induction assays, thus reduc-
ing the potential for drug–drug interactions. One of the concerns in
this monoamide pyrazole carboxamide design is the hydrolysis of
the amide to the acid. This issue is being addressed and is the sub-
ject of a forthcoming publication.
extent as and for a longer duration (4 h) than several drugs used for
treatment of neuropathic pain (Fig. 2). As can be seen from Table 6,
the compound reverses mechanical allodynia at 2 and 4 h in a dose
dependent manner. When dosed orally at 3 mg/kg bid for 2 days,
26 was very efficacious producing 90% (p <0.01) and 55%
(p <0.001) reversal of mechanical allodynia at 2 and 4 h, respec-
tively. Compound 26 produced 90% protection from tonic seizures
at 10 mg/kg po in the mouse MES (maximal electroshock induced
seizures to measure anticonvulsant activity) model (4 h post-
dose).12
Acknowledgements
In the rat rotarod model of motor coordination, no significant
change was observed at 30 mg/kg for 26.13 In contrast, mexiletine
and Gabapentin at subefficacious dose of 100 mg/kg, po signifi-
cantly reduced latency on the rotarod (Fig. 3).
In incubation of 26 with liver microsomes, significant amount of
a carboxylic acid metabolite was detected which was formed via
hydrolysis of the carboxamide. As illustrated in Figure 4, this con-
version was modest after 60 min in microsomal preparations from
The authors thank Dr. Joseph Duffy and Dr. Tesfaye Biftu for
proof-reading the manuscript.
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a
In-vivo SNL (Chung) activity of compound 26 (po)
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% Reversal of allodynia (2 h)
% Reversal of allodynia (4 h)
10 mpk
30 mpk
3 mg/kg bid
35%
69%
80%
32%
55%
87%
a
Treatment with compound 21 days after surgery.
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Figure 3. Efficacy of compound 26 in SNL model versus motor coordination.