Imidazopyridines: A novel class of hNav1.7 channel blockers

Article abstract of DOI:10.1016/j.bmcl.2008.01.047

A series of imidazopyridines were evaluated as potential sodium channel blockers for the treatment of neuropathic pain. Several members were identified with good hNa_v1.7 potency and excellent rat pharmacokinetic profiles. Compound 4 had good efficacy (52% and 41% reversal of allodynia at 2 and 4 h post-dose, respectively) in the Chung rat spinal nerve ligation (SNL) model of neuropathic pain when dosed orally at 10 mg/kg.

Full text of DOI:10.1016/j.bmcl.2008.01.047

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 1696–1701
Imidazopyridines: A novel class of hNav1.7 channel blockers
Clare London,a,* Scott B. Hoyt,a William H. Parsons,a Brande S. Williams,c
Vivien A. Warren,c Richard Tschirret-Guth,d McHardy M. Smith,c Birgit T. Priest,c
Erin McGowan,b William J. Martin,b Kathryn A. Lyons,a Xiaohua Li,a
Bindhu V. Karanam,d Nina Jochnowitz,b Maria L. Garcia,c John P. Felix,c Brian Dean,d
Catherine Abbadie,b Gregory J. Kaczorowskic and Joseph L. Duffya
aDepartment of Medicinal Chemistry, Merck Research Laboratories, Rahway, NJ 07065, USA
bDepartment of Pharmacology, Merck Research Laboratories, Rahway, NJ 07065, USA
cDepartment of Ion Channels, Merck Research Laboratories, Rahway, NJ 07065, USA
dDepartment of Drug Metabolism, Merck Research Laboratories, Rahway, NJ 07065, USA
Received 3 October 2007; revised 11 January 2008; accepted 14 January 2008
Available online 18 January 2008
Abstract—A series of imidazopyridines were evaluated as potential sodium channel blockers for the treatment of neuropathic pain.
Several members were identified with good hNav1.7 potency and excellent rat pharmacokinetic profiles. Compound 4 had good effi-
cacy (52% and 41% reversal of allodynia at 2 and 4 h post-dose, respectively) in the Chung rat spinal nerve ligation (SNL) model of
neuropathic pain when dosed orally at 10 mg/kg.
Ó 2008 Elsevier Ltd. All rights reserved.
Neuropathic pain is a chronic and debilitating disease
caused by injury or pathological changes to the periph-
eral or central nervous systems. These changes result in
the abnormal processing of external stimuli (e.g., touch,
temperature) by the affected pain sensing neurons (noci-
ceptors).1,2 Voltage-gated sodium channels (Nav1Æx) are
strongly expressed in nociceptors and are known to be
important in nociception.3 Clinically, it has been shown
that voltage-gated sodium channel blockers such as
mexiletine (1) and carbamazepine (2) (Fig. 1) are effica-
cious in the treatment of neuropathic pain. These thera-
pies, however, are restricted by their dose limiting CNS
side effects.4
has also been identified as a possible target for neuro-
pathic pain.7,8
Our research has focused on the development of
hNav1.7 blockers for the treatment of neuropathic pain.
Mexiletine and carbamazepine are only weak blockers
of hNav1.7 (VIPR IC50 = 11 and 22 lM, respectively).9
We reasoned that the identification of more potent
H2N
O
O
N
NH2
Data from recent human genetic studies strongly sup-
port the role of sodium channels in pain signaling.
Individuals with a loss of function mutation in the
gene that encodes hNav1.7 have a complete inability
to sense pain. Conversely, gain of function mutations
in Nav1.7 lead to hyperexcitability in pain signaling
neurons.5,6 In addition to Nav1.7, the Nav1.8 isoform
O
N
O
CF3
O
CF3
N
N
N
H
N
H
NH
O
N
NH
O
O
O
3
4
Keywords: Sodium channel blockers; hNav1.7; Neuropathic pain;
Voltage-gated sodium channels.
*
Figure 1. Sodium channel blockers.
Corresponding author. E-mail: clare_london@merck.com
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.01.047

C. London et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1696–1701
1697
hNav1.7 blockers could afford new therapies that show
significant analgesic effects as well as improved thera-
peutic indices over existing treatments.
Once synthesized, compounds were tested for their abil-
ity to block the hNav1.7 channel. This was done using
hNav1.7 channels that were stably expressed in a
HEK-293 cell line. The degree of channel block was
measured using a functional, membrane potential-based
assay, the protocols for which have been previously de-
scribed.9 Using the same assay protocol, hNav1.8 chan-
nel block was also measured for selected compounds. In
addition to this primary hNav1.7 functional assay, com-
pounds were also screened in an IKr binding assay
(MK499) and a cytochrome 3A4 inhibition assay. Since
block of hERG K+ channels can result in fatal ventric-
ular arrythmias and inhibition of CYP 3A4 can lead
to drug–drug interactions, compounds that did not have
these activities would be more promising clinical
candidates.13
We have recently reported the discovery of benzazepi-
none hNav1.7 blockers (e.g., 3).10,11 Compounds in this
class suffered from modest oral exposures and high
clearance rates (3: Clp = 50 ml/min/kg, AUCN (po) =
0.22 lM h kg/mg). Incubation of these compounds with
rat liver microsomes indicated that the major sites of
oxidative metabolism lay in the benzolactam core and
the N-Boc group. In an effort to reduce the clearance
rates of these compounds, we investigated alternatives
to the benzazepinone moiety. These studies resulted
in the discovery of a series of imidazopyridines (e.g.,
4) that show improved pharmacokinetic (PK) profiles
as well as excellent efficacy in the Chung rat spinal
nerve ligation (SNL) model of neuropathic pain.
The effect of the R1 substituent on potency and PK was
investigated. As shown in Table 1, to maintain hNav1.7
potency the imidazole R1 substituent must be no smaller
than a tert-butyl group (17: hNav1.7 IC50 = 185 nM).
Compounds bearing larger alkyl groups such as 18–21
showed good hNav1.7 potency, but were compromised
by their poor PK profiles. These compounds such as
18 suffered from high clearance and low oral exposure.14
In an effort to reduce clearance by blocking potential N-
dealkylation, branched alkyl analogs 15 and 16 were
synthesized. These analogs, however, showed reduced
hNav1.7 potency.
The synthesis of these compounds, outlined in Scheme
1, commenced from commercially available 2-chloro-
3-nitropyridine 5. Treatment of a DMF solution of
5 with an alkylamine in the presence of sodium car-
bonate afforded the 2-aminopyridine product 6. The
nitro group of 6 was reduced with Raney nickel to af-
ford diaminopyridine 7; subsequent amide coupling
with N-Boc glycine yielded compound 8. Deprotection
of 8 with TFA and coupling to the appropriate amino
acid afforded the acyclic precursor 9, which was ther-
mally cyclized in the presence of acetic acid to the
aza-benzimidazole 10. Amide derivatives 11 were
readily prepared by removal of the Boc group with
TFA, followed by coupling with a carboxylic acid.
Compounds were synthesized from commercially
available starting materials except in cases where the
phenylalanine derivatives were unavailable. These
amino acids were synthesized using the Schollkopf
method.12
Incorporation of a benzyl substituent at R1 led to signif-
icant improvements in rat pharmacokinetics. Compound
4 showed a much improved rat PK profile, displaying
both modest clearance and higher exposure (Clp =
15 ml/min/kg, F = 41%, AUCN (po) = 0.86 lM h kg/
mg). Incubation of 4 with rat liver microsomes revealed
that the primary site of oxidative metabolism was the
benzyl substituent. Assuming that oxidation of the
benzylic methylene was the metabolic liability, we syn-
thesized a-methyl analogs 22 and 23. Unfortunately,
these compounds afforded no improvement in PK. The
cyclopropyl analog 24, however, did show a reduced
clearance rate (3 ml/min/kg) and fivefold higher oral
exposure (AUCN (po) = 4.63 lM h kg/mg).
NHR1
NO2
N
NHR1
NH2
N
c
N
N
Cl
b
a
NO2
5
6
7
As shown in Table 1, this series of compounds are po-
tent inhibitors of cytochrome 3A4 (e.g., 4:
IC50 = 980 nM). Block of hERG does not appear to be
an issue, as compounds in this series typically displayed
<50% inhibition of MK499 at 10 lM.
N
NHR1
NH
NHR1
NH
d, e
f
NHBoc
Ar
H
N
NHBoc
O
O
O
With the imidazole substituent optimized for pharmaco-
kinetics, we focused our efforts on modifying the N-Boc
group. Previous work in the related benzazepinone ser-
ies (3) had shown that a secondary amide or carbamate
was required at this position to maintain potency. As
shown in Table 2, tert-butyl (25) and neopentyl amides
(26) were acceptable surrogates for the Boc group and
resulted in analogs with similar hNav1.7 potency. While
the simple phenyl amide 27 afforded a 7-fold loss in po-
tency, incorporation of a 2-substituent increased po-
tency (28 and 29). The 2-trifluoromethoxy analog, 28,
had comparable potency to the parent (4). Changing
9
8
R1
R1
N
O
O
N
g, h
N
N
N
Ar
N
H
Ar
NHBoc
H
N
NH
R2
N
O
10
11
Scheme 1. Reagents and conditions: (a) R1NH2, Na2CO3, DMF,
90 °C (10–85%); (b) Raney-Nickel, H2 (1 atm), MeOH, rt (50–76%); (c)
EDC, HOBt, N-Boc glycine, THF, rt (60–74%); (d) 30% TFA/DCM,
rt; (e) EDC, HOBt, i-Pr2NEt, amino acid, THF, rt; (f) AcOH, 100 °C;
(g) 30% TFA/DCM, rt; (h) EDC, HOBt, i-Pr2NEt, R2CO2H, THF, rt.

1698
C. London et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1696–1701
Table 1. The effect of imidazole substituent on hNav1.7 binding, off-target activities, PK and in vivo efficacy
R1
N
O
CF3
N
N
H
N
NH
O
O
Compound
R1
hNav1.7 IC50 (nM)
MK-499
(% inhibition at
CYP 3A4
IC50 (lM)
Clp
(mL/min/kg)
AUC (norm. po,
lM h kg/mg)
F (%)
the rate of 10 lM)
12
13
14
H
>10,000
>10,000
>10,000
13
0
Me
iPr
0
15
16
1000
54
19
0.86
>1000
F3C
17
18
19
20
21
4
t-Bu
185
81
32
0
36
113
87
0.28
0.02
0.07
0.07
0.08
0.86
31
7
CF3CH2
CF3CF2CH2
c-PrCH2
t-BuCH2
PhCH2
123
243
83
19
20
19
43
10
25
17
41
2.43
0.98
114
64
80
15
22
23
24
255
191
186
1
65
50
21
22
3
0.50
0.82
4.63
36
57
44
Ph
Ph
Ph
0.82
the imidazole substituent from a benzyl to a cyclopro-
pylbenzyl consistently resulted in a decrease in hNav1.7
potency (e.g., cf. 28 with 33). Trifluoromethylcyclopro-
pyl and 2-trifluoromethoxyphenyl amides 32 and 33
had comparable potency to the parent (24), and had
no MK-499 liability. These amides, which were poten-
tially more stable to oxidative metabolism, were incor-
porated into subsequent compounds. However, CYP
3A4 inhibition in this series of imidazopyridines re-
mained an issue (e.g., compounds 32 and 33). As shown
in Table 2 these compounds are more potent blockers of
hNav1.7 than hNav 1.8.
(cf. 35 with 41 or 34 with 38). This SAR study afforded
compounds with clean off-target profiles, for example,
35, 36, and 40 (<50% at the rate of 10 lM MK-499 bind-
ing and >10 lM inhibitor of CYP 3A4). Substitution at
the para position affords a large decrease in potency (cf.
compound 24 with 47). Otherwise, substitution on the
phenyl substituent R3 is widely tolerated. Several analogs
emerged with greater potency than the parent (34) with
the 2,3,6-trifluoro analog affording a 3-fold increase (43)
(hNav1.7 IC50 = 63 nM). Analogs such as 2-tolyl or 2-
methoxyphenyl were not synthesized due to their poten-
tial PK liabilities. Compounds in Table 3 continue to
greater potency for hNav1.7 over hNav1.8, with selectivity
ranging from 2- to 7-fold.
With optimized R1 and R2 substituents in hand, we ex-
plored SAR at the R3 position. Earlier work in the related
benzazepinone series (3) had shown that a lipophilic aro-
matic group was necessary at R3 to maintain hNav1.7 po-
tency. The effect of substitution at R3 was investigated and
it was discovered that CYP 3A4 inhibition could be atten-
uated with the correct substitution pattern (e.g., 36 and
40). As shown in Table 3, analogs with a 2-CF3 substitu-
ent are all potent inhibitors of CYP3A4. Adding a substi-
tuent at the 3-position also increases CYP3A4 potency
With these systematic SAR studies complete, com-
pounds were synthesized that combined all of the opti-
mized substituents. As shown in Table 4, several
analogs with hNav1.7 potency, clean off-target profiles
and good rat PK were identified (e.g., 49 and 50).
These optimized compounds were tested in the Chung
rat spinal nerve ligation model of neuropathic pain.15

C. London et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1696–1701
Table 2. Effect of the amide substituent on potency and off-target activities
1699
R1
N
O
CF3
N
N
H
N
NH
R2
O
Compound
R1
R2
hNav1.7 IC50
(nM)
hNav1.8
IC50 (nM)
MK-499 % inhibition at
the rate of 10 lM
CYP 3A4 IC50
(lM)
25
26
27
28
29
Bn
Bn
Bn
Bn
Bn
tBu
261
86
1458
361
58
63
19
0
CHt2Bu
Ph
552
129
232
2-OCF3Ph
2-CF3Ph
230
366
0
30
31
32
33
Bn
476
1195
261
45
6
Ph
2975
1782
825
Ph
Ph
27
36
1.9
1.8
F3C
2-OCF3Ph
181
Table 3. Effect of the phenylalanine substitution on hNav1.7 and off-target activities
Ph
O
N
N
N
N
H
R3
NH
O
O
Compound R3
hNav1.7 IC50 (nM) hNav1.8 IC50 (nM) MK-499 % inhibition at the rate of 10 lM CYP 3A4 IC50 (lM)
34
35
36
24
37
38
39
40
41
42
43
44
45
46
47
48
Ph
2-F-Ph
2-Cl-Ph
394
238
132
186
148
221
1053
148
275
171
63
19
40
12
50
35
33
48
5
6
880
500
793
985
>10
>10
0.82
1.3
2.4
2-CF3-Ph
2-OCF3Ph
3-Cl-Ph
4-Cl-Ph
2,5-diF-Ph
2,3-diF-Ph
2,6-diF-Ph
2,3,6-triF-Ph
2-Cl-6-F-Ph
2-Cl-5-F-Ph
2-CF3-3-F-Ph
2-CF3-4-F-Ph
2-CF3-6-F-Ph
>20
4.6
16
30
5
685
446
6.1
3.9
147
102
158
744
153
4
5.3
2.0
295
806
13
41
54
18
0.81
1242
3.4
In this model, neuropathic pain is induced in rats by sur-
gically ligating and transecting the rat’s L5 spinal nerve.
Sensitivity to mechanical stimulation is measured before
and seven days after surgery using calibrated Von Frey
filaments. Rats that exhibit significant allodynia are
dosed orally with test compound, and the reversal of
allodynia is measured. Despite good oral exposure and
hNav1.7 potency, compounds 35 and 49–51 lacked effi-
cacy in this model. Oral dosing (10 mg/kg) of compound
24 afforded efficacy at 2 h post-dose similar to that of
mexilitene but no efficacy at the 4-h time point (% rever-
sal at 2 and 4 h = 45% and 12%, respectively). Com-

Table 4. The effect of imidazole substituent on hNav1.7 binding, off-target activities, PK and in vivo efficacy
R1
O
N
N
N
H
R3
N
NH
R2
O
Compound
R1
R2
R3
hNav1.7
IC50 (nM)
hNav
1.8 IC50
(nM)
MK-499 %
inhibition
at the rate
of 10 lM
CYP 3A4
IC50 (lM)
Clp (mL/min/kg)
AUC norm.,
po (lM h kg/mg)
F
(%)
% Reversal
of allodynia
(10 mg/kg
po) in the
SNL model
at
2 h
52
4 h
41
4
Bn
OtBu
2-CF3-Ph
2-CF3-Ph
2-Cl-Ph
80
186
329
540
270
793
43
50
19
1
1.0
0.8
15
0.86
4.6
2.5
4.0
41
44
78
54
Ph
24
49
50
OtBu
3.0
9.3
4.2
45
20
15
12
13
5
Ph
Ph
F3C
>20
>20
F3C
F3C
2,6-di-F-Ph
1869
446
Ph
Ph
51
2,3,6-tri-F
2-F-Ph
182
158
3
6.4
11
6.6
7.5
3.7
2.0
85
47
32
4
35
OtBu
880
40
25
48
24
23
Mexiletine

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allodynia when dosed orally at 10 mg/kg (% reversal at
2 and 4 h = 52% and 41%, respectively). As shown in
Table 4, this efficacy compares favorably to the efficacy
we obtained with mexiletine, a compound used clinically
to treat neuropathic pain.
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tory pain. In this model, the rat’s hind paw is subjected
to intradermal injection of complete Freund’s adjuvant
(CFA) to induce inflammation.16 Hyperalgesia to
mechanical pressure is measured using a Randal–Sellito
apparatus. Compound 4 exhibited significant reversal of
hyperalgesia in this model even at a low oral dose of
3 mg/kg (% reversal at 2 and 4 h = 44% and 44%, respec-
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with indomethacin, a compound that is used clinically to
treat inflammatory pain (3 mg/kg po: % reversal at 2 and
4 h = 43% and 56%, respectively).
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dazopyridine hNav1.7 blockers. A benchmark com-
pound from this class displayed robust rat
pharmacokinetics, and was orally efficacious in rat mod-
els of neuropathic and inflammatory pain.
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