Discovery and biological evaluation of 5-aryl-2-furfuramides, potent and selective blockers of the Nav1.8 sodium channel with efficacy in models of neuropathic and inflammatory pain

Article abstract of DOI:10.1021/jm070637u

Na_v1.8 (also known as PN3) is a tetrodotoxin-resistant (TTx-r) voltage-gated sodium channel (VGSC) that is highly expressed on small diameter sensory neurons and has been implicated in the pathophysiology of inflammatory and neuropathic pain. Recent studies using an Na_vL8 antisense oligonucleotide in an animal model of chronic pain indicated that selective blockade of Na_v1.8 was analgesic and could provide effective analgesia with a reduction in the adverse events associated with nonselective VGSC blocking therapeutic agents. Herein, we describe the preparation and characterization of a series of 5-substituted 2-furfuramides, which are potent, voltage-dependent blockers (IC₅₀ < 10 nM) of the human Na _v1.8 channel. Selected derivatives, such as 7 and 27, also blocked TTx-r sodium currents in rat dorsal root ganglia (DRG) neurons with comparable potency and displayed > 100-fold selectivity versus human sodium (Na _v1.2, Na_v1.5, Na_v1.7) and human ether-a-go-go (hERG) channels. Following systemic administration, compounds 7 and 27 dose-dependently reduced neuropathic and inflammatory pain in experimental rodent models.

Full text of DOI:10.1021/jm070637u

J. Med. Chem. 2008, 51, 407–416
407
Discovery and Biological Evaluation of 5-Aryl-2-furfuramides, Potent and Selective Blockers of
the Nav1.8 Sodium Channel with Efficacy in Models of Neuropathic and Inflammatory Pain
Michael E. Kort,*,† Irene Drizin,† Robert J. Gregg,† Marc J. C. Scanio,† Lei Shi,† Michael F. Gross,‡ Robert N. Atkinson,‡
Matthew S. Johnson,‡ Gregory J. Pacofsky,‡ James B. Thomas,‡ William A. Carroll,† Michael J. Krambis,‡ Dong Liu,‡
Char-Chang Shieh,† XuFeng Zhang,† Gricelda Hernandez,† Joseph P. Mikusa,† Chengmin Zhong,† Shailen Joshi,†
Prisca Honore,† Rosemarie Roeloffs,‡ Kennan C. Marsh,† Bernard P. Murray,† Jinrong Liu,† Stephen Werness,‡
Connie R. Faltynek,† Douglas S. Krafte,‡ Michael F. Jarvis,† Mark L. Chapman,‡ and Brian E. Marron‡
Global Pharmaceutical Research and DeVelopment, Abbott Laboratories, 100 Abbott Park Road, Abbott Park, Illinois 60064-6100, and Icagen,
Inc., 4222 Emperor BouleVard, Durham, North Carolina 27703
ReceiVed June 4, 2007
Nav1.8 (also known as PN3) is a tetrodotoxin-resistant (TTx-r) voltage-gated sodium channel (VGSC) that
is highly expressed on small diameter sensory neurons and has been implicated in the pathophysiology of
inflammatory and neuropathic pain. Recent studies using an Nav1.8 antisense oligonucleotide in an animal
model of chronic pain indicated that selective blockade of Nav1.8 was analgesic and could provide effective
analgesia with a reduction in the adverse events associated with nonselective VGSC blocking therapeutic
agents. Herein, we describe the preparation and characterization of a series of 5-substituted 2-furfuramides,
which are potent, voltage-dependent blockers (IC50 < 10 nM) of the human Nav1.8 channel. Selected
derivatives, such as 7 and 27, also blocked TTx-r sodium currents in rat dorsal root ganglia (DRG) neurons
with comparable potency and displayed >100-fold selectivity versus human sodium (Nav1.2, Nav1.5, Nav1.7)
and human ether-a-go-go (hERG) channels. Following systemic administration, compounds 7 and 27 dose-
dependently reduced neuropathic and inflammatory pain in experimental rodent models.
Introduction
Voltage-gated sodium channels (VGSCs) are transmembrane
proteins that open in response to changes in membrane potential
to enable selective permeability for sodium ions.1 These
channels belong to a multigene family consisting of nine
members, which can be distinguished pharmacologically on the
basis of their sensitivity to blockade by natural toxins, particu-
larly tetrodotoxin (TTx).2,3 VGSCs contribute to the initiation
and propagation of action potentials in excitable tissues such
as nerve and muscle and participate in many physiological
processes including locomotion, cognition, and nociception.4–6
Considerable data support the hypothesis that hyperexcitabil-
ity and spontaneous action potential firing mediated by VGSCs
in peripheral sensory neurons play a role in the pathophysiology
of chronic pain.7,8 Consistent with this hypothesis, nonselective
blockade of sodium channels contributes to the analgesic activity
of a number of clinically used agents, developed for other
indications, such as mexiletine, lamotrigine, and carbamazepine
(Figure 1). However, all of these drugs penetrate the CNS,
possess off-target activities, and give rise to adverse events in
the clinical setting.9
Nav1.8 (also known as PN3) is a TTx-resistant (TTx-r) VGSC
that is preferentially expressed on sensory neurons10 and carries
a major portion of the TTX-r current in peripheral nerves.11
The highly localized distribution of Nav1.8 in nociceptive small
diameter neurons suggests this channel may be involved in the
generation of action potentials in response to painful stimuli.12
Altered levels of expression of Nav1.8 in chronic pain states,13
gene knockout data,14–16 and analgesic effects of Nav1.8-specific
antisense oligodeoxynucleotides17–19 provide compelling cor-
Figure 1. Clinically used sodium channel blockers.
roborative evidence supporting Nav1.8 as an attractive target
for analgesic drug discovery.20,21
The clinical utility of nonselective VGSC blockers may be a
result of either a voltage-dependent mechanism of inhibition
(preferential binding to the inactivated state of the channel),
frequency-dependent (progressive channel block with repeated
stimulation) biophysical properties, or the combination of both
effects.22,23 The available data indicate that the absence of
subtype selectivity across the VGSC family may be principally
responsible for the limited therapeutic indices associated with
existing therapeutics.12 Consequently, the clinically observed
adverse effects of those agents might be mitigated by selectively
targeting the Nav1.8 subtype. Herein, we report the discovery
of a series of subtype-selective small-molecule Nav1.8 blockers
and their characterization as novel analgesic agents.24
The furfuryl glycinamide derivative 1 (Figure 2) was identi-
fied as a hit from a focused screening strategy using an Nav1.8
isotopic flux assay.25,26 Whole cell voltage clamp electrophysi-
ological characterization of this compound revealed modest
potency in blocking half-maximally inactivated native rat TTX-r
currents in dissociated dorsal root ganglia (DRG) neurons
(estimated IC50 ) 10 µM) and recombinant human Nav1.8
channels (estimated IC50 ≈ 0.9 µM). Furthermore, 1 showed
similar inhibitory activity against other sodium channels (e.g.,
hNav1.2, hNav1.5) and significant activity in a human ether-a-
go-go (hERG) flux assay (IC50 ) 0.8 ( 0.1 µM).
* To whom correspondence should be addressed. Phone: 847-935-4945.
Fax: 847-935-5466. E-mail: michael.e.kort@abbott.com.
†
A preliminary appraisal of 1 and available analogues led to
an iterative truncation strategy whereby the structural determi-
Abbott Laboratories.
Icagen, Inc.
‡
10.1021/jm070637u CCC: $40.75
2008 American Chemical Society
Published on Web 01/05/2008

408 Journal of Medicinal Chemistry, 2008, Vol. 51, No. 3
Kort et al.
Scheme 2a
a Reagents: (a) 4-chlorophenylboronic acid, Pd(PPh3)4, aqueous Na2CO3,
DME, 23 °C; (b) (COCl)2, cat. DMF, CH2Cl2, 23 °C; (c) m-toluidine, Et3N,
CH2Cl2, 23 °C, 51% for three steps; (d) Zn, aqueous NH4OH, 23 °C, 98%;
(e) 4-chlorophenylboronic acid, PdCl2(PPh3)2, Na2CO3, i-PrOH, reflux; (f)
(COCl)2, cat. DMF, CH2Cl2, 23 °C; (g) m-toluidine, Et3N, CH2Cl2, 23 °C,
54% for three steps.
Figure 2. Furan glycinamide screening hit.
Scheme 1a
Scheme 3a
a Reagents: (a) (COCl)2, cat. DMF, CH2Cl2, 23 °C; (b) m-toluidine, Et3N,
CH2Cl2, 23 °C; (c) arylboronic acid, PdCl2(PPh3)2, aqueous Na2CO3, i-PrOH,
reflux; (d) m-toluidine, BOP reagent, Et3N, CH2Cl2, 23 °C.
a Reagents: (a) m-toluidine hydrochloride, AlMe3, PhCH3, 23 f110 °C,
67%; (b) Lawesson’s reagent, PhCH3, 110 °C, 99%; (c) MeI, CH3CN, 23
°C; (d) NCNH2, Et3N, EtOH, 80 °C, 34% for two steps.
nants conferring sodium channel activity and selectivity could
be distilled from this relatively large (MW ) 600), conforma-
tionally mobile starting point. Initial SAR studies revealed that
the entire region comprising the cyclopropyl ketone, aryloxy,
and alkylpiperazine moieties of 1 could be eliminated (1 f 2)
without appreciable loss of Nav1.8 activity. Similarly, the
D-glycine residue linking the furfuroic acid and piperazine
fragments could be excised if appropriate substitution was
reintroduced in the form of an N-substituted furan carboxamide
(2 f 3). On the other hand, deviation from sp2-hybridizaton at
the C-2 substituent on the furan core had a deleterious effect
on activity. Likewise, removal of the C-5 aryl ring gave inactive
compounds. Introduction of a basic nitrogen (ionizable at
physiological pH) imparted favorable physicochemical proper-
ties albeit with erosion of selectivity versus sodium channel
subtypes. These findings set the stage for a more detailed
investigation of 5-aryl-2-furfuramide derivatives, the syntheses
of which are detailed in Schemes 1–3.
dibromo-2-furfuroic acid (14) with zinc metal.30 The preparation
of several isosteric replacements for the carbonyl residue in 3
is outlined in Scheme 3. Addition of 3-aminotoluene to
2-cyanofuran 1731 in the presence of trimethylaluminum
provided amidine 18, which could be conveniently isolated as
the hydrochloride salt. Exposure of 3 to Lawesson’s reagent32
generated 19, which was further elaborated to the corresponding
cyanoamidine 20 by S-methylation and reaction with cyanamide.
Biological Evaluation
Compounds were evaluated for their ability to block the
recombinant mouse Nav1.8 sodium channel stably expressed in
HEK293 cells using a modified version of an isotopic efflux
assay.25 Subsequently, the activity of a subset of potent blockers
was confirmed by measuring their inhibition of TTx-r currents
in dissociated rat DRG neurons and sodium currents in human
Nav1.8 expressing HEK293 cells using conventional voltage-
clamp electrophysiology.24 Selected analogues were also further
examined for their selectivity versus other human sodium
channel subtypes stably expressed in HEK293 cells. Since the
biophysical properties vary among sodium channel subunits,2,3
electrophysiological protocols were designed to set the mem-
brane potential to the midpoint of voltage-dependent steady-
state inactivation (i.e., the voltage at which 50% of channels
are inactivated) or to a voltage that sets all channels in a resting
state to allow a direct comparison of compound effects across
channel subtypes (see Table 4 for details). Finally, compounds
were tested against the hERG channel33 and a series of other
channels and receptors expressed in peripheral sensory neurons
and, in some cases, against a broad (n ) 70) receptor screening
panel.24
Chemistry
The influence of the furyl C-5 substituent on Nav1.8 activity
was evaluated via a series of 5-aryl derivatives (3, 6-11) that
were prepared (Scheme 1) from the commercially available
5-bromo-2-furfuroic acid (4). Reaction of the acid chloride 4
with m-toluidine and subsequent Suzuki coupling27 provided a
convenient route to the desired compounds. Alternatively, the
sequence of carbon-carbon bond generation and amide forma-
tion could be reversed without a deleterious effect on overall
yield. Coupling with BOP reagent28 proved particularly suitable
for the synthesis of certain amide derivatives. The regioisomeric
3,5- and 2,4-variants of 3 (13 and 16, respectively) were made
by a similar series of transformations (Scheme 2) involving
precursors 12 and 14, respectively. In particular, access to 1629
was accomplished via the intermediacy of 15, which was
delivered in high yield by selective dehalogenation of 4,5-
On the basis of their pharmacokinetic profiles, compounds
were further evaluated for analgesic efficacy using in vivo

5-Aryl-2-furfuramides as Blockers
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 3 409
Table 1. In Vitro Nav1.8 Activity of Substituted Furan Derivativesa
Table 2. In Vitro Nav1.8 Activity of Selected 5-(4-Chlorophenyl)furan
Carboxamidesa
compd
isomer
X
R
Nav1.8 IC50 (µM)
3
6
7
8
2,5-
2,5-
2,5-
2,5-
2,5-
2,5-
2,5-
3,5-
2,4-
2,5-
2,5-
O
O
O
O
O
O
O
O
O
4-Cl
0.48 ( 0.32
0.43 ( 0.07
0.48 ( 0.023
0.29 ( 0.04
>30b
4-OEt
4-CN
4-OCF3
4-SO2Me
3-CN
3-CF3
4-Cl
4-Cl
4-Cl
4-Cl
9
10
11
13
16
18
20
30 ( 9
7.5 ( 0.7
20 ( 9
5.3 ( 1.1
1.9 ( 0.1
0.03 ( 0.01
NH
NCN
a IC50 values were determined by least squares fitting of a logistic
equation to data from full eight-point, half-log concentration response curves
using an Nav1.8 isotopic efflux assay as described in Experimental Section.
Data shown with standard error ((SEM) represent the mean of two to five
separate determinations. b An IC50 value could not be calculated because
of low potency.
models of inflammatory and neuropathic pain.34 These assays
included the complete Freund’s adjuvant (CFA) induced hype-
ralgesia model of chronic inflammatory pain, as well as the L5/
L6 spinal nerve tight ligation (Chung) and sciatic nerve loose
ligation (Bennett) models of neuropathic pain.18,34,35
Results and Discussion
Our investigation of the furfuramides began by probing the
requirements of the C-5 furan substituent (Table 1). It quickly
became evident that full or partial saturation, deletion, or one-
carbon homologation of the C-5 aryl ring resident in 3 led to a
significant drop in Nav1.8 blocking activity (data not shown).
Our focus was thus directed to aromatic substitution at the
5-position of the furan. Within this selection of derivatives,
substitution at the 4-position of the appended phenyl ring with
relatively small (one to three atoms) substituents as in 3 and
6-8 proved optimal. Although functionalization at the 3-position
of the aryl ring was tolerated (e.g., 10 and 11), it generally led
to a half-log or greater decrease in potency in the mouse Nav1.8
flux assay (compare 7 versus 10)., The corresponding ortho-
substituted aryl subunits typically displayed very weak (IC50
>
30 µM, data not shown) inhibition of Nav1.8 channels. The 3,5-
and 2,4-regioisomers 13 and 16, respectively, proved to be
significantly less potent Nav1.8 blockers than the 2,5-isomer 3.
The limited aqueous solubility of these initial anilide deriva-
tives (e.g., <0.1 µg/mL for 3) was an impediment to definitively
assessing their pharmacokinetic properties in vivo.36 Subsequent
exploration of structure–activity relationships was therefore
driven by the need for changes that could improve physico-
chemical properties. Modification of the carboxamide linker was
one approach pursued. Unsubstituted amidine 18 was less potent
relative to 3 but offered the advantage of an ionizable group,
which imparted enhanced water solubility (250 µg/mL as the
hydrochloride salt). Furthermore, 18 possessed modest in vitro
intrinsic clearance (220 µL min-1 mg-1) and excellent oral
bioavailability (F ) 67% at 10 mg/kg) in rat. Conversely,
increased potency was realized with the corresponding cyano-
amidine 20, wherein the amidine nitrogen is completely non-
basic. Analogues derived from replacement of either the amide
carbonyl or nitrogen with a methylene unit or from inversion
of the amide connectivity to the furan ring were inactive (IC50
> 30 µM) against Nav1.8 channels.
a IC50 values were determined by least-squares fitting of a logistic
equation to data from full eight-point, half-log concentration–response curves
using an Nav1.8 isotopic efflux assay as described in Experimental Section.
Data shown with standard error ((SEM) represent the mean of two to five
separate determinations. b An IC50 value could not be calculated because
of low potency.
nucleus, an extensive survey of amide substitution was under-
taken. In addition to defining the structural requirements for
the pharmacophore, these efforts were focused on improving
the aforementioned solubility limitations, as well as the rapid
With the establishment of the preferred relative orientation
and substitution pattern of appendages on the furfuramide

410 Journal of Medicinal Chemistry, 2008, Vol. 51, No. 3
Kort et al.
Table 3. Activity of Selected Furyl Carboxamides in Blocking Native TTx-r Currents in Rat Dorsal Root Ganglia and in HEK-293 Cells Expressing
Human Nav1.8a
rat DRG TTx-r
hNav1.8
% inhib
% inhib
b
c
b
c
compd
concn (µM)
V1/2
V0
concn (µM)
V1/2
V0
3
0.3
0.1
0.3
0.1
0.3
0.1
1
0.3
0.03
0.003
0.3
0.1
0.03
0.003
10
60 ( 6
37 ( 4
74 ( 4
41 ( 5
63 ( 2
40 ( 1
60 ( 1
45 ( 3
71 ( 6
51 ( 1
71 ( 2
42 ( 1
79 ( 1
44 ( 7
88 ( 2
36 ( 1
18 ( 1
3 ( 2
0.1
77 ( 1
68 ( 1
51 ( 3
33 ( 2
73 ( 1
40 ( 9
75 ( 1
46 ( 4
99 ( 2
76 ( 1
59 ( 2
30 ( 4
70 ( 3
56 ( 7
81 ( 4
42 ( 6
67 ( 6
61 ( 1
29 ( 2
14 ( 3
33 ( 3
7 ( 3
0.03
0.03
0.01
0.003
0.001
1
7
52 ( 2
38 ( 1
30 ( 7
25 ( 5
10 ( 3
2 ( 1
45 ( 4
22 ( 3
48 ( 1
14 ( 4
28 ( 9
31 ( 3
42 ( 6
9 ( 3
8
18
20
27
30
33
5 ( 2
0.1
1 ( 1
0.03
0.001
0.01
0.003
0.3
0.03
3
1
94 ( 1
46 ( 2
19 ( 2
5 ( 3
66 ( 1
27 ( 3
83 ( 2
58 ( 6
1
a Data shown with standard error ((SEM) represent the mean of two to six separate determinations. b Inactivated state protocol: the prepulse voltage at
which 50% of channels are inactivated (V1/2 ) -40 mV). c Resting state protocol: the prepulse voltage at which 100% of channels are available to be
activated (V0 ) -100 mV).24
Table 4. Sodium Channel Selectivity of Selected Furylcarboxamide Nav1.8 Blockers Measured at Half-Maximal Inactivation for Each Channel Subtypea
% inhib
compd
concn (µM)
% inhib hNav1.8a
concn (µM)
hNav1.2
hNav1.3
hNav1.5
hNav1.7
hERG IC50 (µM)c
27 ( 3
3
0.1
77 ( 1
68 ( 1
51 ( 3
33 ( 2
73 ( 1
40 ( 9
75 ( 1
46 ( 4
99 ( 2
76 ( 1
59 ( 2
30 ( 4
70 ( 3
55 ( 5
81 ( 4
42 ( 6
3
1
10
3
1
0.1
1
54 ( 4
33 ( 3
57 ( 4
32 ( 10
84 ( 4
47 ( 1
52 ( 1
61 ( 7
17 ( 5
58 ( 7
47 ( 3
NDb
51 ( 6
41 ( 1
0.03
0.03
0.01
0.003
0.001
1
NDb
NDb
7
74 ( 5
61 ( 7
87 ( 6
52 ( 9
56 ( 3
54 ( 3
26 ( 6
NDb
5.8 ( 0.8
>30d
8
18
20
27
30
33
36 ( 3
NDb
2.1 ( 0.3
>30
0.1
0.03
0.001
0.01
0.003
0.3
0.03
3
1
0.3
0.03
10
3
94 ( 2
91 ( 4
53 ( 3
41 ( 5
38 ( 6
24 ( 5
NDb
70 ( 4
52 ( 3
37 ( 6
69 ( 5
32 ( 3
49 ( 2
44 ( 2
71 ( 1
70 ( 3
NDb
54 ( 4
42 ( 3
44 ( 1
>30
3
5.3 ( 0.9
>30
1
12 ( 1
5 ( 5
24 ( 7
NDb
a Data shown with standard error ((SEM) represent the mean of two to six separate determinations. Data were collected using an inactivated state
protocol (the prepulse voltage at which 50% of channels are inactivated). V1/2 ) -60 mV for hNav1.2, hNav1.3, hNav1.7; V1/2 ) -90 mV for hNav1.5.24
b Not determined (compound was not tested). c IC50 values were determined by least-squares fitting of a logistic equation to data from full eight-point,
half-log concentration–response curves. Data shown with standard error ((SEM) represent the mean of two to five separate determinations using a Nav1.8
isotopic efflux assay as described in the Experimental Section. d An IC50 value could not be calculated because of low potency.
in vitro microsomal turnover generally observed with more
lipophilic derirvatives (e.g., 850 µL min-1 mg-1 for 3). As
summarized in Table 2, relatively conservative alterations, such
as one- or two-carbon elongation of the amide-aryl spacer (22
and 23, respectively), were tolerated but did not provide superior
potency or microsomal stability. Placement of substituents at
the 3-position (24-27) of the aryl amide, or at both the 3- and
4-positions as a fused heterocycle (28-30) or fused heteroaro-
matic (30, 31), yielded active Nav1.8 blockers with varying
levels of potency. The inclusion of functionality bearing a
heteroatom at these positions enhanced physicochemical proper-
ties, although weaker activity generally was noted with bicyclic
systems, such as quinoline 31. The activities of 33 and 34, both
pyridine analogues of 3, were sensitive to regiochemistry, with
33 affording potency comparable to the most active disubstituted
derivative (27). As evidenced with 35, N-methylation of the
amide residue resulted in an appreciable loss of Nav1.8 activity
relative to 3. Saturated functional groups on the amide, including
those bearing a heteroatom (e.g., 36, 37), were typically weaker
Nav1.8 blockers.
Active compounds, as assessed by preliminary Nav1.8 flux
data, were studied further using voltage-clamp electrophysi-
ological recordings. As shown in Table 3 significant block of
TTx-r currents in dissociated DRG neurons and in human Nav1.8
expressing HEK293 cells was observed at both resting and half-
maximal inactivated states; a consistent trend toward greater
potency relative to the flux assay was noted. A number of these
analogues (8, 20, 27) possessed estimated IC50 values in the
single-digit nanomolar range. For comparison, mexiletine and
lamotrigine produced <50% Nav1.8 block at 30 µM under these
assay conditions. In addition, the furfuramides generally ex-
hibited greater potency following more depolarized prepulse
potentials (-40 mV), possibly reflecting preferential affinity for
the inactivated (-40 mV) versus resting (-100 mV) states of
the Nav1.8 channel. This voltage-dependent/state-dependent
behavior is similar to that observed with other sodium channel
blockers such as local anesthetics and other clinically used
agents, although the absolute differences in potency between
states are not as great.12,37 A trend toward decreased potency
was noted for blockade of native TTx-r currents in rat DRG

5-Aryl-2-furfuramides as Blockers
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 3 411
Table 5. In Vivo Activitya and Pharmacokinetic Profileb of Nav1.8
Blockers 7 and 27
mg/kg (ip), 27 did not significantly impair either motor function
or motor coordination and balance.24
7
27
human Nav1.8 IC50 (µM)c
0.079
0.240
38 (86 ( 9)
0.008
0.125
47 (70 ( 12)
Conclusion
native rat Nav1.8 IC50 (µM)c
Chung ED50, ip, mg/kg (% effect)
We have identified a novel series of furan-based derivatives
that are potent, selective blockers of the Nav1.8 sodium channel.
To our knowledge, this disclosure is the first demonstration of
a small-molecule blocker of sodium channels showing both high
potency and subtype selectivity among the sodium channel
family.38–40 SAR studies established the preferred C-5 aryl and
amide substituents for Nav1.8 activity in the more potent 2,5-
furan regioisomers. This activity was confirmed with human
Nav1.8 channels and in isolated rat DRG neurons using
electrophysiological recordings. In general, significant structural
variation was not well tolerated on the C-5 phenyl group or in
the 2-furfuramide region. Incorporation of certain heterocycles
or amidine isosteres of the amide imparted a more favorable
balance of Nav1.8 and physiochemical properties, but typically
the improvements came at the expense of subtype selectivity.
The limited physicochemical and pharmacokinetic properties
of many lead compounds complicated evaluation of their
analgesic profiles in vivo. However, consistent with their in vitro
profiles, the systemic administration of both 7 and 27 produced
dose-dependent antinociceptive effects in three rodent pain
models. In summary, these data demonstrate that acute and
selective pharmacological blockade of Nav1.8 sodium channels
produces significant antinociception in animal models of neu-
ropathic and inflammatory pain.
Bennett ED50, ip, mg/kg (% effect) >100 (21 ( 8) 85 (56 ( 10)
CFA ED50, ip, mg/kg (% effect)
CLint ((µL/min)/mg)d
F, po (%)
F, ip (%)
Clp (L/(h ·kg))
T1/2 (h)
Cmax, ip (µg/mL)
Vss (L/kg)
[brain]/[plasma]
plasma protein binding, rat (%)
Cerep (at 10 µM)e
98 (55 ( 11)
260
1 ( 0.3
36 ( 4
2.4 ( 0.5
1.9 ( 0.5
0.75
1.9 ( 0.5
0.35
41 (64 ( 6)
140
13 ( 2
26 ( 4
1.5 ( 0.2
7.3 ( 1.2
0.35
6.4 ( 0.9
1.1
98.7 ( 0.4
98.3 ( 0.4
BZD peripheral BZD peripheral,
CCKA, D1,
ML1, 5-HT2A
a Values shown with standard error ((SEM) represent the mean for
experiments in rats, n ) 6 per dose group. b Determined in rats (n ) 6)
following administration of a 5 mg/kg iv or 10 mg/kg ip dose. c Measured
at V1/2 (-40 mV). IC50 values were determined by least-squares fit of the
Hill equation and were based on two to six independent determinations at
four or more concentrations. d Intrinsic clearance based on incubation with
rat microsomes (1 h, 37 °C). e Displayed >60% inhibition of control specific
binding at indicated receptor.
neurons versus recombinant human Nav1.8 channels. This
apparent difference in activity most likely reflects a difference
in native versus recombinant channels rather than a species
difference between rat and human Nav1.8 channels.24 Nonethe-
less, it was still possible to identify highly potent analogues
(e.g., 3, 30) in the native rat system.
Experimental Section
The potent Nav1.8 blockers identified in Table 3 were
evaluated for their activity at other sodium channel subtypes
and liability targets. Table 4 shows the activity of these
compounds at Nav1.2, 1.3, 1.5, and 1.7 (3 µM), as well as at
the hERG channel (IC50). The majority of compounds in Table
4 were approximately 100- to 1000-fold more potent in blocking
Nav1.8 compared to their activity at other sodium channels (e.g.,
27). For example, 3, 7, and 30 displayed Nav1.8 selectivity
comparable to 27 for all subtypes except Nav1.5, where the
potency versus Nav1.8 was reduced to approximately 10- to 100-
fold. Insertion of additional nitrogen atoms anywhere in the
molecule was usually detrimental to sodium channel selectivity
(e.g., 20, 33) and hERG selectivity (e.g., 7, 30) or both (e.g.,
18). In contrast, more lipophilic derivatives did not show any
significant activity at the hERG channel. Compounds 7 and 27
were also tested against other channels and receptors expressed
in peripheral sensory neurons including TRPV1, P2X2/3, Cav2.2
calcium channels, and KCNQ2/3 potassium channels; neither
had significant activity at these channels (IC50 > 10 µM). In
addition, both were evaluated in a broad screening panel (n )
70) of cell-surface receptors, ion channels, and enzymes
(CEREP, Poitiers, France) and showed no or weak (IC50 > 2
µM) activity (see Table 5).
The potency, selectivity, and pharmacokinetics (Table 5) of
several 5-aryl-2-furfuramide analogues made them useful tools for
evaluation of the analgesic effects of Nav1.8 blockade. Intraperi-
toneal (ip) administration of either 7 or 27 resulted in dose-
dependent attenuation of mechanical allodynia in the L5/L6 spinal
nerve injury (Chung) model of neuropathic pain (ED50 ) 56 and
47 mg/kg, respectively). These two analogues also dose-depen-
dently reduced mechanical allodynia in the chronic constriction
injury of the sciatic nerve (Bennett) model of neuropathic pain and
thermal hyperalgesia in a CFA-induced inflammatory pain model.
Both 7 and 27 penetrated the CNS; however, at doses up to 300
General Procedures. Nuclear magnetic resonance spectra were
obtained on a General Electric QE 300 or QZ 400 MHz instrument
with chemical shifts (δ) reported relative to tetramethylsilane as
internal standard. Mass spectra determinations were obtained using
an electrospray (ESI) technique or by direct chemical ionization
(DCI) methods employing ammonia. Melting points were deter-
mined with capillary apparatus and are uncorrected. Elemental
analyses were performed by Robertson Microlit Laboratories, Inc.,
Madison, NJ. Analytical thin layer chromatography was done on 2
cm × 6 cm Kieselgel 60 F-254 plates precoated with 0.25 mm
thick silica gel distributed by E. Merck. LC-MS analyses were
performed on ThermoQuest Navigator systems using 10–100%
acetonitrile/10 mM ammonium acetate gradient with MS data
obtained using atmospheric pressure chemical ionization (APCI)
positive ionization over the range of m/z from 170 to 1200. Unless
otherwise specified, column chromatography was performed on
silica gel (230–400 mesh). The term in vacuo refers to solvent
removal using a rotary evaporator at 30 mmHg. With the exception
of amines, solvents and reagents were purchased from Aldrich
Chemical Co. and were used without further purification unless
otherwise specified. All amines were dried over molecular sieves
(4 Å) for at least 24 h prior to use.
High-Throughput Mouse Nav1.8 and hERG Isotopic Flux
Assays. HEK293 cells stably expressing mouse Nav1.8 Na channels
or CHO cells stably expressing hERG K channels were loaded
overnight with an appropriate radiotracer, followed by stimulation
using variations on protocols previously described.25,26 The ra-
diotracer efflux was measured at a single time point that had been
previously established to be on the linear portion of the efflux curve.
Percentage inhibition of efflux was calculated as
% inhibition )
efflux(test compound) - efflux(ref blocker)
1 -
× 100
efflux(control) - efflux(ref blocker)
Reference blockers for Nav1.8 and hERG assays were tetracaine
(30 µM) and terfenadine (30 µM), respectively. Concentration-

412 Journal of Medicinal Chemistry, 2008, Vol. 51, No. 3
Kort et al.
dependent activity was established via eight-point concentration-
–response curves, and assays were performed in duplicate.
In Vivo Evaluation. Male Sprague-Dawley rats (Charles River,
Wilmington, MA) weighing 200–300 g were utilized. All animals
were group housed in AAALAC approved facilities at Abbott
Laboratories in a temperature-regulated environment with lights on
between 0700 and 2000 h. Food and water were available ad libitum
except during testing. All animal handling and experimental
protocols were approved by an institutional animal care and use
committee (IACUC). All experiments were performed during the
light cycle. Unless otherwise noted, all experimental and control
groups contained at least six animals per group and data are
expressed as mean ( SEM Data analysis was conducted using
analysis of variance and appropriate posthoc comparisons (P <
0.05) as previously described.35 ED50 values were estimated using
least-squares linear regression.
Spinal Nerve (L5/L6) Ligation Model of Neuropathic Pain.
As previously described in detail by Kim and Chung,43 a 1.5 cm
incision was made dorsal to the lumbosacral plexus in anesthetized
rats. The paraspinal muscles (left side) were separated from the
spinous processes, and the L5 and L6 spinal nerves were isolated
and tightly ligated with 3–0 silk threads. Following hemostasis,
the wound was sutured and coated with antibiotic ointment. The
rats were allowed to recover and then placed in a cage with soft
bedding for 14 days before behavioral testing for mechanical
allodynia.
Sciatic Nerve Ligation Model of Neuropathic Pain. As
previously described in detail by Bennett and Xie,44 a 1.5 cm
incision was made 0.5 cm below the pelvis in anesthetized rats,
and the biceps femoris and the gluteous superficialis (right side)
were separated. The sciatic nerve was exposed and isolated, and
four loose ligatures (5–0 chromic catgut) with 1 mm spacing were
placed around it. The rats were allowed to recover and then placed
in a cage with soft bedding for 14 days before behavioral testing
for mechanical allodynia as described above. Animals were also
tested for mechanical allodynia using calibrated von Frey filaments
(Stoelting, Wood Dale, IL). Briefly, rats were placed into individual
Plexiglas containers and allowed to acclimate for 15–20 min before
testing. Paw withdrawal threshold was determined by increasing
and decreasing stimulus intensity and estimated using a Dixon
nonparametric test. Only rats with threshold scores of e4.5 g were
considered allodynic and utilized in compound testing experiments.
Complete Freund’s Adjuvant (CFA) Induced Thermal
Hyperalgesia. Unilateral inflammation was induced by injecting
150 µL of a 50% solution of complete Freund’s adjuvant (CFA)
(Sigma Chemical Co., St. Louis, MO) in physiological saline into
the plantar surface of the right hind paw of the rat. CFA was injected
2 days before behavioral testing for thermal hyperalgesia.
Representative Procedure for Conversion of Carboxylic
Acid Derivatives to Amides via Acid Chlorides (Method A).
5-Bromo-N-(3-methylphenyl)furan-2-carboxamide (4). A solu-
tion of 5-bromo-2-furoic acid (1.00 g, 5.00 mmol) in dichloro-
methane (50 mL) was treated with oxalyl chloride (0.650 mL, 7.50
mmol) and a catalytic amount of N,N-dimethylformamide (100 µL).
The reaction mixture was stirred at ambient temperature for 2 h,
and then solvent and excess oxalyl chloride were removed under
reduced pressure to yield the intermediate acid chloride, which was
used without further purification.
Electrophysiology.24,41 Rat Dorsal Root Ganglion Neu-
rons.42 Whole-cell patch clamp recordings were performed on
dissociated rat small diameter DRG neurons (18–25 µm) from the
L4 and L5 lumbar region at room temperature. For voltage clamp
recordings, pipet solution contained (mM) the following: CsF 135,
NaCl 5, CsCl 10, EGTA 5, HEPES 10, pH 7.2 (osmolarity, 285).
The external solution contained the following (mM): NaCl 22,
choline-Cl 110, CaCl2 1.8, MgCl2 0.8, HEPES 10, glucose 5, TTx
0.0005, CdCl2 0.1, pH 7.4 (osmolarity, 310).
Recombinant Human Sodium Channels. Human embryonic
kidney (HEK-293) cells expressing recombinant sodium channels
were grown in DMEM/high glucose Dulbecco’s media, 10% fetal
bovine serum, 2 mM sodium pyruvate, G418. For whole-cell voltage
clamp recordings, patch pipettes were pulled from borosilicate glass
on a Flaming-Brown micropipette puller (Sutter Instruments, Inc.).
Pipettes had a tip resistance of 0.8-2.5 MΩ using the internal
solutions (mM): 135 CsF, 10 CsCl, 5 EGTA, 5 NaCl, 10 HEPES-
free acid, pH to 7.3 with 5 M CsOH. Voltage offset was zeroed
prior to seal formation. The external buffer consisted of the
following (mM): 132 NaCl, 5.4 KCl, 0.8 MgCl2, 1.8 CaCl2, 5
glucose, 10 HEPES-free acid, pH to 7.3 with 6 N NaOH. After
establishment of a whole-cell recording, cellular capacitance was
minimized using the analogue compensation available on the
recording amplifier (Axopatch 200B). Series resistance was less
than 5 MΩ and was compensated by >85% in all experiments,
resulting in a final series resistance no greater than 0.75 MΩ. Signals
were low-pass-filtered at 5–10 kHz, digitized at 20–50 kHz, and
stored on a computer for later analysis. Voltage protocols were
generated, and data acquisition and analysis were performed using
pCLAMP software (version 8.1, Axon Instruments, Inc.). All
experiments were performed at room temperature. Liquid junction
potentials were <10 mV and were not corrected.
Electrophysiological Recordings. Coverslips were mounted in
a small flow-through chamber on the stage of an inverted
microscope and were continuously perfused with bath external
solution (see below). Cells were voltage-clamped via the whole
cell configuration of the patch clamp with an Axopatch-200B
amplifier (Molecular Devices/Axon Instruments, Foster City, CA)
using standard techniques. Micropipettes were pulled from thin-
walled borosilicate glass capillaries (TW-150F, World Precision
Instruments, Saratoga, FL) with a Flaming Brown micropipette
puller (P97, Sutter Instrument, Novato, CA) and polished on a
microforge (Narishige, Tokyo) to obtain electrode resistances
ranging from 1.0 to 2.5 MΩ. The pipet solution contained the
following (in mM): 135 CsF, 5 NaCl, 10 HEPES, 10 CsCl, and 5
EGTA, pH 7.4 with CsOH (295 mOSM). Capacity transients were
canceled, and series resistance was compensated (>80%) using the
facilities of the amplifier. The bath solution contained the following
(in mM): 110 choline-Cl, 22 NaCl, 5.4 KCl, 1.8 CaCl2, 0.8 MgCl2,
10 HEPES, and 5 glucose, plus 500 nM tetrodotoxin (TTx) and 50
µM CdCl2, pH 7.4 with NaOH (300 mOSM). The pipet potential
was zeroed before seal formation. Calculated liquid junction
potentials were <5 mV and were not corrected. Whole cell currents
were filtered at 5 kHz and acquired at 20 kHz using Clampex 8.2
software (Molecular Devices/Axon Instruments) and analyzed using
Clampfit 8.2 (Molecular Devices/Axon Instruments). All experi-
ments were performed at room temperature (21–25 °C). Compounds
were dissolved in DMSO (10 mM) and added to extracellular
solution (final DMSO concentration of <0.1%) immediately prior
to use. Compounds were applied directly to the voltage-clamped
cells at a flow rate of 1–2 mL/min via a custom-made perfusion
manifold connected to an array of gravity-fed reservoirs. Sodium
currents were evoked from a holding potential of -100 mV by a
test pulse of 0 mV for 20 ms and pulsed every 15 s until stabilized.
To measure drug effects, sodium currents were evoked with 20 ms
steps to 0 mV following 8 s prepulses to either -100 or -40 mV.
The prepulse was followed by a brief (20 ms) repolarization to
-100 mV to relieve fast inactivation.
The crude acid chloride was redissolved in dichloromethane (10
mL), treated with m-toluidine (530 mg, 5.00 mmol) and triethyl-
amine (1.50 mL, 11.0 mmol), and stirred at ambient temperature
for 2 h. The mixture was diluted with additional dichloromethane
(5 mL) and washed with 0.5 M HCl (10 mL) and brine (10 mL).
The organic portion was dried (MgSO4), filtered, and concentrated
in vacuo. The residue was purified by flash chromatography on
silica gel (elution with 5% EtOAc in CH2Cl2), and the isolated
product was crystallized from 25% EtOAc/hexanes to provide
1.15 g (4.12 mmol, 82%) of the desired product as off-white
1
needles: mp 98–100 °C; H NMR (400 MHz, DMSO-d6) δ 10.09
(s, 1H), 7.55 (s, 1H), 7.52 (d, J ) 8.0 Hz, 1H), 7.36 (d, J ) 3.4
Hz, 1H), 7.22 (t, J ) 7.7 Hz, 1H), 6.93 (d, J ) 7.4 Hz, 1H), 6.82
(s, 1H), 2.30 (s, 3H); MS (DCI/NH3) m/z 280, 282 (M + H)+.
Anal. (C12H10BrNO2) C, H, N.

5-Aryl-2-furfuramides as Blockers
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 3 413
Representative Procedure for Suzuki Reaction of Brom-
ofurans and Arylboronic Acids (Method B). .5-(4-Chlorophe-
nyl)-N-(3-methylphenyl)furan-2-carboxamide (3). To a suspen-
sion of 4 (375 mg, 1.34 mmol) in isopropanol (15 mL) and water
(5 mL) was added 3-chlorophenylboronic acid (262 mg, 1.67
mmol). PdCl2(PPh3)2 (29 mg, 0.039 mmol) and sodium carbonate
(708 mg, 6.68 mmol) were added, the reaction vessel was equipped
with a reflux condenser, and the mixture was heated at 80 °C for
5 h. The reaction mixture was diluted with EtOAc (30 mL) and
additional water (10 mL) and was then partitioned. The organic
portion was washed with 2 N HCl (5 mL) followed by brine (10
mL) and was then dried (Na2SO4), filtered, and concentrated in
vacuo. The oily residue obtained was purified by flash chromatog-
raphy on silica gel (elution with 25% EtOAc/hexanes) to provide
330 mg (1.06 mmol, 79%) of the desired product as a crystalline
nylboronic acid according to method B: mp 149–152 °C; 1H NMR
(400 MHz, DMSO-d6) δ 10.23 (s, 1H), 8.42 (d, J ) 8.6 Hz, 2H),
8.09 (d, J ) 8.6 Hz, 2H), 7.65–7.59 (m, 2H), 7.41 (s, 2H), 7.26
(dd, J ) 8.5, 7.5 Hz, 1H), 6.97 (d, J ) 7.6 Hz, 1H), 3.42 (s, 3H),
2.33 (s, 3H); MS (DCI/NH3) m/z 356 (M + H)+. Anal.
(C19H14N2O2) C, H, N.
5-(3-Cyanophenyl)-N-(3-methylphenyl)furan-2-carboxam-
ide (10). 10 was prepared from 4 and 3-cyanophenylboronic acid
according to method B: mp 142–143 °C; 1H NMR (400 MHz,
DMSO-d6) δ 10.15 (s, 1H), 8.30 (d, J ) 7.8 Hz, 1H), 7.87 (d, J )
7.8 Hz, 1H), 7.71 (t, J ) 8.5 Hz, 1H), 7.61–7.55 (m, 2H), 7.40 (d,
J ) 3.7 Hz, 1H), 7.38 (d, J ) 3.7 Hz, 1H), 7.26 (t, J ) 7.5 Hz,
1H), 6.97 (d, J ) 6.4 Hz, 1H), 2.37 (s, 3H); MS (ESI) m/z 303 (M
+ H)+. Anal. (C19H14N2O2 ·0.25H2O) C, H, N.
5-(3-Trifluoromethylphenyl)-N-(3-methylphenyl)furan-2-car-
boxamide (11). 11 was prepared from 4 and (3-trifluorometh-
yl)phenylboronic acid according to method B: mp 199–200 °C; 1H
NMR (400 MHz, DMSO-d6) δ 10.15 (s, 1H), 8.30 (m, 1H,) 8.28
(t, J ) 4.4 Hz, 1H), 7.77–7.73 (m, 2H), 7.61–7.55 (m, 2H), 7.42
(d, J ) 3.7 Hz, 1H), 7.38 (d, J ) 3.7 Hz, 1H), 7.26 (t, J ) 7.8 Hz,
1H), 6.97 (d, J ) 6.5 Hz, 1H), 2.32 (s, 3H); MS (APCI) m/z 346
(M + H)+. Anal. (C19H14F3NO2) C, H, N.
1
white solid: mp 139–140 °C; H NMR (400 MHz, DMSO-d6) δ
10.11 (s, 1H), 8.01 (d, J ) 8.8 Hz, 2H), 7.61–7.54 (m, 4H), 7.39
(d, J ) 3.7 Hz, 1H), 7.29–7.22 (m, 1H), 7.23 (d, J ) 3.7 Hz, 1H),
6.95 (d, J ) 7.5 Hz, 1H), 2.32 (s, 3H); MS (DCI/NH3) m/z 312 (M
+ H)+. Anal. (C18H14ClNO2) C, H, N.
5-(4-Ethoxyphenyl)-N-[(3-methyl)phenyl)]furan-2-carboxam-
ide (6). 6 was prepared from 5-(4-ethoxyphenyl)furan-2-carboxylic
acid45 and m-toluidine according to method A: mp 128–130 °C;
1H NMR (400 MHz, DMSO-d6) δ 10.02 (s, 1H), 7.9 (d, J ) 8.8
Hz, 2H), 7.60–7.53 (m, 2H), 7.35 (d, J ) 3.4 Hz, 1H), 7.24 (dd, J
) 8.6, 7.6 Hz, 1H), 7.04 (d, J ) 8.8 Hz, 1H), 7.01 (d, J ) 3.7 Hz,
1H), 6.93 (d, J ) 7.8 Hz, 1H), 4.1 (q, J ) 7.1 Hz, 2H), 1.36 (t, J
) 7.1 Hz, 3H), 2.32 (s, 3H); MS (ESI) m/z 322 (M + H)+. Anal.
(C20H19NO3 ·H2O) C, H, N.
Representative Procedure for Suzuki Reaction and Amide
Formation Using Bromofuroic Acids (Method C). 5-(4-Cy-
anophenyl)-N-(3-methylphenyl)furan-2-carboxamide (7). To a
suspension of 5-bromo-2-furoic acid (300 mg, 1.57 mmol) in 3:1
isopropanol-water (10 mL) was added 4-cyanophenylboronic acid
(240 mg, 1.64 mmol) and sodium carbonate (306 mg, 2.88 mmol).
Argon was bubbled through the stirred mixture for 2 min, and then
PdCl2(PPh3)2 (32 mg, 0.046 mmol) was added. The reaction vessel
was equipped with a reflux condenser and heated at reflux for 3 h.
The mixture was cooled to ambient temperature and filtered to
remove solids. Isopropanol was removed under reduced pressure,
and the resulting aqueous solution was adjusted to pH 2 with 6 N
HCl and was then extracted with EtOAc (2 × 10 mL). The
combined organic extracts were dried (Na2SO4), filtered, concen-
trated, and filtered through a plug of silica gel (∼2 g, 5% ethanol/
EtOAc rinse) to provide 247 mg of the product 5-(4-cyanophenyl-
)furan-2-carboxylic acid as a white solid, which was used without
further purification. MS (DCI/NH3) m/z 214 (M + H)+.
The crude 5-(4-cyanophenyl)furan-2-carboxylic acid was dissolved
in THF (9 mL) and treated sequentially with triethylamine (0.430 mL,
3.30 mmol), m-toluidine (210 mg, 2.02 mmol), and BOP reagent (667
mg, 1.50 mmol). The mixture was stirred at ambient temperature for
16 h and then transferred to a separatory funnel and washed with water
(6 mL). The organic layer was then dried (Na2SO4), and the solvent
was concentrated in vacuo. The residue was purified by flash
chromatography on silica gel (elution with 30% EtOAc/hexanes) to
provide 285 mg (0.942 mmol, 60%) of the desired product as a white
crystalline solid: mp 134–135 °C; 1H NMR (400 MHz, DMSO-d6)
δ 10.19 (s, 1H), 8.18 (d, J ) 8.5 Hz, 2H), 7.98 (d, J ) 8.5 Hz,
2H), 7.61–7.54 (m, 2H), 7.43 (s, 2H) 7.26 (dd, J ) 8.6, 7.6 Hz,
1H), 6.96 (d, J ) 7.5 Hz, 1H), 2.33 (s, 3H); MS (ESI) m/z 302 (M
+ H)+. Anal. (C19H14N2O2) C, H, N.
5-[(4-Trifluoromethoxy)phenyl]-N-(3-methylphenyl)furan-2-
carboxamide (8). 8 was prepared from 5-bromo-2-furoic acid, (4-
trifluoromethoxy)phenylboronic acid, and m-toluidine according to
method C: mp 149–150 °C; 1H NMR (400 MHz, DMSO-d6) δ
10.15 (s, 1H), 8.11 (d, J ) 8.8 Hz, 2H), 7.63–7.57 (m, 2H), 7.50
(d, J ) 8.1 Hz, 2H), 7.42 (d, J ) 3.7 Hz, 1H), 7.29–7.22 (m, 1H),
7.23 (d, J ) 3.7 Hz, 1H), 6.95 (d, J ) 7.5 Hz, 1H), 2.33 (s, 3H);
MS (DCI/NH3) m/z 362 (M + H)+. Anal. (C19H14F3NO3) C, H, N.
5-[(4-Methylsulfonyl)phenyl]-N-(3-methylphenyl)furan-2-car-
boxamide (9). 9 was prepared from 4 and (4-methylsulfonyl)phe-
5-(4-Chlorophenyl)-N-(3-methylphenyl)furan-3-carboxam-
ide (13). 5-Bromofuran-3-carboxylic acid (12)29 (1.00 g, 5.20
mmol), 4-chlorophenylboronic acid (860 mg, 5.20 mmol), and
tetrakis(triphenylphosphine)palladium(0) (300 mg, 0.260 mmol)
were combined in DME (13 mL) and 2.0 M aqueous Na2CO3 (6.5
mL) and heated at 95 °C for 4 h. The reaction mixture was allowed
to cool to ambient temperature and diluted with EtOAc (40 mL).
The mixture was washed with 2.0 N HCl (2 × 25 mL), dried
(Na2SO4), filtered through a pad of Celite, and evaporated under
reduced pressure. The crude product was triturated from Et2O to
provide 688 mg of 5-(4-chlorophenyl)furan-3-carboxylic acid as a
1
white powder. H NMR (400 MHz, DMSO-d6) δ 12.80 (s, 1H),
8.37 (d, J ) 0.7 Hz, 1H), 7.78 (d, J ) 8.8 Hz, 2H), 7.51 (d, J )
8.8 Hz, 2H), 7.28 (d, J ) 0.9 Hz, 1H); MS (ESI-) m/z 221 (M -
H)+.
The crude carboxylic acid was then treated with m-toluidine
according to method A to provide 823 mg (2.65 mmol, 51% for
three steps) of the desired product, a white semicrystalline solid:
1
mp 177–179 °C; H NMR (400 MHz, DMSO-d6) δ 9.92 (s, 1H),
8.46 (d, J ) 0.7 Hz, 1H), 7.77 (d, J ) 8.8 Hz, 2H), 7.57–7.48 (m,
4H), 7.46 (d, J ) 0.7 Hz, 1H), 7.29–7.19 (m, 1H), 6.92 (d, J ) 7.8
Hz, 1H), 2.31 (s, 3H); MS (ESI) m/z 310 (M-H)+. Anal.
(C18H14ClNO2 ·0.5H2O) C, H, N.
4-Bromofuran-2-carboxylic Acid (15).30 4,5-Dibromofuran-2-
carboxylic acid (12) (5.50 g, 20.3 mmol) was suspended in water
(63 mL) and NH4OH (18 mL) with vigorous stirring at ambient
temperature. Powdered zinc metal (1.30 g, 20.3 mmol) was added,
and the mixture was allowed to stir at ambient temperature for 3 h.
The reaction mixture was filtered through a pad of Celite and
acidified (pH 2) with 2 N HCl. The filtrate was extracted with
EtOAc (4 × 50 mL), dried (Na2SO4), and concentrated to dryness
under reduced pressure to provide 3.80 g (19.9 mmol, 98%) of the
1
desired product as a white powder. H NMR (400 MHz, DMSO-
d6) δ 13.42 (br s, 1H), 8.17 (d, J ) 1.0 Hz, 1H), 7.39 (d, J ) 1.0
Hz, 1H); MS (ESI-) m/z 189 (M - H)+.
4-(4-Chlorophenyl)-N-(3-methylphenyl)furan-2-carboxam-
ide (16). To a suspension of 4-bromo-2-furoic acid (15) (900 mg,
4.71 mmol) in 3:1 isopropanol-water (30 mL) was added 4-chloro-
boronic acid (769 mg, 4.92 mmol) and sodium carbonate (918 mg,
8.64 mmol). Argon was bubbled through the stirred mixture for 3
min, and then PdCl2(PPh3)2 (96 mg, 0.14 mmol) was added. The
reaction vessel was equipped with a reflux condenser and heated
at reflux for 4 h. The mixture was then cooled to ambient
temperature and filtered to remove solids. Isopropanol was removed
under reduced pressure, and the resulting aqueous solution was
adjusted to pH 2 with 6 N HCl and was then extracted with EtOAc
(3 × 10 mL). The combined organic extracts were dried (Na2SO4),
filtered, concentrated, and filtered through a plug of silica gel (∼5
g, 5% ethanol/EtOAc rinse) to provide 787 mg of the product 4-(4-

414 Journal of Medicinal Chemistry, 2008, Vol. 51, No. 3
Kort et al.
chlorophenyl)furan-2-carboxylic acid as a white solid, which was
used without further purification. 1H NMR (400 MHz, DMSO-d6)
δ 8.41 (d, J ) 1.0 Hz, 1H), 7.72 (d, J ) 8.5 Hz, 2H), 7.65 (s, 1H),
7.46 (d, J ) 8.5 Hz, 2H); MS (DCI/NH3) m/z 240 (M + NH4)+.
The (4-chlorophenyl)furan-2-carboxylic acid obtained in the
previous step was reacted with m-toluidine according to method A
to yield 793 mg (2.54 mmol, 54% for three steps) of the desired
provide the final compound. For 31 and 33–34, a modification of
method A was employed using pyridine as the base and acetonitrile
as the solvent. Compounds 26 and 32 were synthesized using the
BOP coupling protocol (see method C).
5-(4-Chlorophenyl)-N-(2-methylbenzyl)furan-2-carboxam-
ide (22). Mp 144–146 °C; 1H NMR (400 MHz, DMSO-d6) δ 8.97
(t, 1H), 7.95 (d, J ) 8.5 Hz, 2H), 7.53 (d, J ) 8.5 Hz, 2H),
7.27–7.22 (m, 1H), 7.22 (d, J ) 3.7 Hz, 1H), 7.19–7.10 (m, 4H),
4.47 (d, J ) 5.8 Hz, 2H), 2.33 (s, 3H); MS (DCI/NH3) m/z 326 (M
+ H)+. Anal. (C19H16ClNO2) C, H, N.
1
product as a white solid: mp 147–148 °C; H NMR (400 MHz,
DMSO-d6) δ 10.16 (s, 1H), 8.53–8.48 (m, 1H), 7.82–7.78 (m, 1H),
7.74 (d, J ) 8.8 Hz, 2H), 7.62–7.59 (m, 1H), 7.57 (d, J ) 8.1 Hz,
1H), 7.51 (d, J ) 8.5 Hz, 2H), 7.23 (t, J ) 7.5 Hz, 1H), 6.95 (d,
J ) 7.5 Hz, 1H), 2.31 (s, 3H); MS (ESI) m/z 310 (M - H)+. Anal.
(C18H14ClNO2) C, H, N.
5-(4-Chlorophenyl)-N-[2-(2-chlorophenyl)ethyl]furan-2-carboxa-
1
mide (23). H NMR (400 MHz, DMSO-d6) δ 8.74 (t, J ) 5.9 Hz,
1H), 7.93 (d, J ) 8.5 Hz, 2H), 7.55 (d, J ) 8.8 Hz, 2H), 7.48 (d, J )
0.7 Hz, 1H), 7.45 (s, 1H), 7.29 (dd, J ) 8.5, 7.5 Hz, 1H), 7.14 (s,
2H), 3.48 (q, J ) 6.9 Hz, 2H), 3.18 (t, J ) 7.3 Hz, 2H); MS (ESI
APCI) m/z 361 (M + H)+. Anal. (C19H15Cl2NO2) C, H, N.
5-(4-Chlorophenyl)-N-(3-methylphenyl)furan-2-carboxami-
dine Hydrochloride (18). m-Toluidine hydrochloride (300 mg, 2.09
mmol) was suspended in toluene (10 mL) at ambient temperature,
and AlMe3 (1.20 mL of a 2.0 M solution in toluene, 2.28 mmol)
was added. The mixture was stirred at ambient temperature for 5
min. Then 5-(4-chlorophenyl)furan-2-carbonitrile (17)46 (387 mg,
1.89 mmol) was added, and the mixture was heated at reflux for
1 h. The mixture was allowed to cool to ambient temperature, and
saturated aqueous NaHCO3 (75 mL) was added. The mixture was
extracted with EtOAc (2 × 75 mL), and the combined extracts
were dried (Na2SO4), filtered, and absorbed on silica gel. The
mixture was chromatographed on silica gel (elution with EtOAc)
and the purified product converted to the HCl salt with ethanolic
HCl to provide the 440 mg (1.40 mmol, 67%) of the desired product
as a white solid: mp >250 °C; 1H NMR (400 MHz, DMSO-d6) δ
11.55 (s, 1H), 9.84 (s, 1H), 8.88 (s, 1H), 8.13 (d, J ) 8.5 Hz, 2H),
8.00 (d, J ) 3.7 H, 1H), 7.62 (d, J ) 8.5 Hz, 2H), 7.49 (t, J ) 8.0
Hz, 1H), 7.44 (d, J ) 3.7 Hz, 1H), 7.34–7.25 (m, 3H), 2.40 (s,
5-(4-Chlorophenyl)-N-[(3-morpholin-4-yl)phenyl]furan-2-car-
boxamide Hydrochloride (24). 24 was prepared from 20 and
3-morpholin-4-yl-phenylamine (see Supporting Information). The
product was converted to the HCl salt by treatment of the ethanolic
solution with HCl in ether: mp 186–187 °C; 1H NMR (400 MHz,
DMSO-d6) δ 10.05 (s, 1H), 8.01 (d, J ) 8.8 Hz, 2H), 7.57 (d, J )
8.5 Hz, 2H), 7.38 (d, J ) 3.4 Hz, 1H), 7.35 (t, J ) 2.0 Hz, 1H),
7.31–7.25 (m, 1H), 7.25–7.15 (m, 2H), 6.74 (ddd, J ) 8.1, 2.4, 1.0
Hz, 1H), 3.81–3.71 (m, 4H), 3.16–3.05 (m, 4H); MS (DCI/NH3)
m/z 383 (M + H)+. Anal. (C21H19ClN2O3 ·HCl) C, H, N.
5-(4-Chlorophenyl)-N-[3-(4,5-dihydrooxazol-2-yl)phenyl]fu-
ran-2-carboxamide Hydrochloride (25). 25 was prepared from
21 and [3-(4,5-dihydro-oxazol-2-yl)phenyl]amine.31 The product
was converted to the HCl salt by treatment of the ethanolic solution
with HCl in ether: mp 225–227 °C (dec); 1H NMR (400 MHz,
DMSO-d6) δ 10.34 (s, 1H), 8.81 (t, J ) 1.7 Hz, 1H), 8.02 (d, J )
8.8 Hz, 2H), 8.00–7.97 (m, 1H), 7.62 (dt, J ) 8.1, 1.2 Hz, 1H),
7.58 (d, J ) 8.8 Hz, 2H), 7.47 (t, J ) 8.0 Hz, 1H), 7.44 (d, J ) 3.4
Hz, 1H), 7.24 (d, J ) 3.7 Hz, 1H), 4.43 (t, J ) 9.7 Hz, 2H), 3.98
(t, J ) 9.5 Hz, 2H); MS (DCI/NH3) m/z 367 (M + H)+. Anal.
(C20H15ClN2O3 ·HCl·0.75H2O) C, H, N.
3H); MS (ESI +) m/z 311 (M
(C18H14ClN2O·HCl·0.75H2O) C, H, N.
- HCl +
H)+. Anal.
5-(4-Chlorophenyl)-N-(3-methylphenyl)furan-2-carbothioam-
ide (19). Furfuramide 3 (1.79 g, 5.74 mmol) and Lawesson’s reagent
(1.28 g, 3.16 mmol) were suspended in toluene (75 mL) and heated
at reflux. After 30 min, the reaction became homogeneous, and
heating was continued for 4 h. The mixture was allowed to cool to
ambient temperature and was absorbed on silica gel. Purification
by silica gel chromatography (elution with 20% EtOAc/hexanes)
provided 1.86 g (3.12 mmol, 99%) of the desired product as a pale-
5-(4-Chlorophenyl)-N-[(3-cyanomethyl)phenyl]furan-2-car-
boxamide (26). Mp 161–162 °C; 1H NMR (400 MHz, DMSO-d6)
δ 10.29 (s, 1H), 8.01 (d, J ) 8.8 Hz, 2H), 7.80 (s, 1H), 7.73 (d, J
) 8.5 Hz, 1H), 7.60 (d, J ) 8.8 Hz, 2H), 7.42 (d, J ) 3.4 Hz, 1H),
7.37 (d, J ) 8.1 Hz, 1H), 7.24 (d, J ) 3.7 Hz, 1H), 7.10 (d, J )
8.5 Hz, 1H), 4.28 (s, 2H); MS (DCI/NH3) m/z 354 (M + NH4)+.
Anal. (C19H13ClN2O2) C, H, N.
1
yellow powder. H NMR (400 MHz, DMSO-d6) δ 11.54 (s, 1H),
8.10 (d, J ) 8.5 Hz, 2H), 7.57 (d, J ) 8.5 Hz, 2H), 7.53–7.42 (m,
3H), 7.34 (t, J ) 7.8 Hz, 1H), 7.23 (d, J ) 3.7 Hz, 1H), 7.13 (d,
J ) 7.5 Hz, 1H), 2.36 (s, 3H); MS (ESI +) m/z 328 (M + H)+.
(E)-5-(4-Chlorophenyl)-N-cyano-N′-(3-methylphenyl)furan-
2-carboxamidine (20). Thioamide 19 (186 mg, 0.567 mmol) and
iodomethane (299 mg, 2.10 mmol) were dissolved in acetonitrile
(3 mL), and the mixture was stirred at ambient temperature for
16 h in a sealed vessel. The mixture was diluted with Et2O (10
mL) and the product was collected by filtration and dried under
vacuum to provide the thioimidate salt (184 mg) as a bright-yellow
powder, which was used without further purification. MS (ESI+)
m/z 342 (M - HI + H)+.
5-(4-Chlorophenyl)-N-(3,5-dimethoxyphenyl)furan-2-carbox-
1
amide (27). Mp 138–139 °C; H NMR (400 MHz, DMSO-d6) δ
10.10 (s, 1H), 7.98–8.03 (m, 2H), 7.55–7.60 (m, 2H), 7.40 (d, J )
3.7 Hz, 1H), 7.23 (d, J ) 3.7 Hz, 1H), 7.05 (d, J ) 2.4 Hz, 1H),
6.29 (t, J ) 2.4 Hz, 1H), 3.75 (s, 6H); MS (DCI/NH3) m/z 358 (M
+ H)+. Anal. (C19H16ClNO4) C, H, N.
5-(4-Chlorophenyl)-N-(1-methyl-2,3-dihydro-1H-indol-6-yl)-
furan-2-carboxamide Hydrochloride (28). A solution of the 5-(4-
chlorophenyl)-N-(2,3-dihydro-1H-indol-6-yl)furan carboxamide (100
mg, 0.200 mmol) (see Supporting Information) in methanol (5 mL)
at ambient temperature was treated with a 35% aqueous solution
of formaldehyde (0.100 mL, 1.20 mmol) followed by sodium
cyanoborohydride (100 mg, 1.58 mmol). Acetic acid (0.10 mL)
was added, and the mixture was stirred for 1 h. The mixture was
concentrated, and the residue was purified by flash chromatography
on silica gel (elution with 30% EtOAc/hexanes). The resulting
material was treated with 2 N HCl in ethanol, which generated a
white precipitate that was filtered and rinsed with diethyl ether to
provide 54 mg (0.15 mmol, 77%) of the desired compound: mp
225–227 °C (dec); 1H NMR (400 MHz, DMSO-d6) δ 10.15 (s,
1H), 8.20 (d, J ) 8.8 Hz, 2H), 7.57 (d, J ) 8.8 Hz, 2H), 7.35 (d,
J ) 3.7 Hz, 1H), 7.22 (d, J ) 3.7 Hz, 1H), 7.06–7.00 (m, 2H),
6.93–6.88 (m, 1H), 3.34–3.26 (m, 2H), 2.88–2.79 (m, 2H), 2.70
The imidate from the previous step (100 mg, 0.392 mmol),
cyanamide (17 mg, 0.392 mmol), and triethylamine (0.111 mL,
0.784 mmol) were combined in EtOH (4 mL) and heated at 80 °C
in a sealed vessel for 16 h. The mixture was concentrated in vacuo
and triturated with 50% EtOAc/hexanes to provide 65 mg (0.193
mmol, 34% for two steps) of the desired product as a white powder:
mp 221–222 °C; 1H NMR (400 MHz, DMSO-d6) δ 10.62 (s, 1H),
8.09 (d, J ) 8.8 Hz, 2H), 7.88 (d, J ) 4.1 Hz, 1H), 7.61 (d, J )
8.8 Hz, 2H), 7.45–7.38 (m, 3H), 7.33 (t, J ) 8.0 Hz, 1H), 7.09 (d,
J ) 7.5 Hz, 1H), 2.35 (s, 3H); MS (ESI+) m/z 336 (M + H)+.
Anal. (C19H14ClN3O) C, H, N.
General Procedure for the Synthesis of Compounds 22–37.
Compounds 22-37 were prepared from commercially available
5-(4-chlorophenyl)furan-2-carboxylic acid (21) and commercially
available amines unless indicated otherwise. Method A was used
for the synthesis of 22-25, 27-30, and 35-37; in the case of 28,
N-methylation was carried out subsequent to amine coupling to
(s, 3H); MS (DCI/NH3) m/z 353 (M
(C20H17ClN2O2 ·HCl) C, H, N.
+
H)+. Anal.
5-(4-Chlorophenyl)-N-(4-methyl-3,4-dihydro-2H-benzo[1,4]oxazin-
7-yl)furan-2-carboxamide (29). 29 was prepared from 24 and 3,4-

5-Aryl-2-furfuramides as Blockers
Journal of Medicinal Chemistry, 2008, Vol. 51, No. 3 415
dihydro-4-methyl-7-nitro-1,4-benzoxazine47 using method A: mp
180–181 °C; 1H NMR (400 MHz, DMSO-d6) δ 9.92 (s, 1H), 7.99 (d,
J ) 8.8 Hz, 2H), 7.20 (d, J ) 3.4 Hz, 1H), 7.19–7.11 (m, 2H), 6.70
(d, J ) 8.5 Hz, 1H), 4.27–4.21 (m, 2H), 3.24–3.15 (m, 2H), 2.81 (s,
3H); MS (DCI/NH3) m/z 369 (M + H)+. Anal. (C20H17ClN2O3) C,
H, N.
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5-(4-Chlorophenyl)-N-[(1-methyl-1,2,3,4-tetrahydroquinolin)-
6-yl]furan-2-carboxamide Hydrochloride (30). 30 was prepared
from 24 and [(1-methyl-1,2,3,4-tetrahydroquinolin)-6-yl]amine (see
Supporting Information) using method A. The product was con-
verted to the HCl salt by treatment of the ethanolic solution with
1
HCl in ether: mp 232–233 °C; H NMR (400 MHz, DMSO-d6) δ
9.95 (s, 1H), 8.00 (d, J ) 8.8 Hz, 2H), 7.56 (d, J ) 8.8 Hz, 2H),
7.42 (d, J ) 8.1 Hz, 1H), 7.37–7.31 (m, 1H), 7.31 (d, J ) 3.7 Hz,
1H), 7.20 (d, J ) 3.7 Hz, 1H), 6.71 (s, 1H), 3.21 (t, J ) 5.4 Hz,
2H), 2.87 (s, 3H), 2.73 (t, J ) 6.3 Hz, 2H), 1.94 (quintet, J ) 5.8
Hz, 2H); MS (DCI/NH3) m/z 367 (M
(C21H19ClN2O2 ·HCl) C, H, N.
+
H)+. Anal.
5-(4-Chlorophenyl)-N-(quinolin-6-yl)furan-2-carboxamide (31).
The product was converted to the HCl salt by treatment of the ethanolic
solution with HCl in ether. 1H NMR (400 MHz, DMSO-d6) δ 10.50
(s, 1H), 8.86–8.81 (m, 1H), 8.50–8.43 (m, 1H), 8.35 (d, J ) 6.4 Hz,
1H), 8.12–8.00 (m, 4H), 7.60 (d, J ) 8.5 Hz, 2H), 7.57–7.51 (m, 1H),
7.47 (d, J ) 3.7 Hz, 1H), 7.27 (d, J ) 3.7 Hz, 1H); MS (DCI/NH3)
m/z 358 (M + H)+. Anal. (C20H13ClN2O2 ·HCl·1.1H2O) C, H, N.
5-(4-Chlorophenyl)-N-(1H-indol-6-yl)furan-2-carboxamide (32).
1
Mp 214–215 °C; H NMR (400 MHz, DMSO-d6) δ 11.09 (s, 1H),
10.15 (s, 1H), 8.02 (d, J ) 8.5 Hz, 2H), 8.01–7.98 (m, 1H), 7.57 (d,
J ) 8.5 Hz, 2H), 7.51 (d, J ) 8.5 Hz, 1H), 7.39 (d, J ) 3.7 Hz, 1H),
7.31 (t, J ) 2.7 Hz, 1H), 7.27 (dd, J ) 8.5, 1.7 Hz, 1H), 7.21 (d, J )
3.7 Hz, 1H), 6.40 (t, J ) 2.4 Hz, 1H); MS (DCI/NH3) m/z 337 (M +
H)+. Anal. (C19H13ClN2O2) C, H, N.
5-(4-Chlorophenyl)-N-[(5-methyl)pyridin-3-yl]furan-2-car-
boxamide (33). The product was converted to the HCl salt by
treatment of the ethanolic solution with HCl in ether. 1H NMR
(400 MHz, DMSO-d6) δ 10.98 (s, 1H), 8.28 (d, J ) 5.1 Hz, 1H),
8.04 (d, J ) 8.8 Hz, 2H), 8.01–7.98 (m, 1H), 7.59 (d, J ) 3.7 Hz,
1H), 7.57 (d, J ) 8.8 Hz, 2H), 7.25 (d, J ) 3.7 Hz, 1H), 7.09 (dd,
J ) 5.4, 1.0 Hz, 1H), 2.39 (s, 3H); MS (DCI/NH3) m/z 313 (M +
H)+. Anal. (C17H13ClN2O2 ·HCl) C, H, N.
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5-(4-Chlorophenyl)-N-(2-methyl-pyridin-4-yl)furan-2-carbox-
amide Hydrochloride (34). 34 was prepared from 21 and (2-
methyl-pyridin-4-yl)amine. The product was converted to HCl salt
by treatment of the ethanolic solution with HCl in ether: mp >250
°C; 1H NMR (400 MHz, DMSO-d6) δ 11.29 (s, 1H), 8.62 (d, J )
6.8 Hz, 1H), 8.17–8.08 (m, 2H), 8.01 (d, J ) 8.5 Hz, 2H), 7.70 (d,
J ) 3.7 Hz, 1H), 7.61 (d, J ) 8.5 Hz, 2H), 7.34 (d, J ) 3.7 Hz,
1H), 2.67 (s, 3H); MS (DCI/NH3) m/z 313 (M + H)+. Anal.
(C17H13ClN2O2 ·HCl) C, H, N.
5-(4-Chlorophenyl)-N-methyl-N-(3-methylphenyl)furan-2-car-
1
boxamide (35). Mp 104–106 °C; H NMR (400 MHz, CDCl3) δ
7.32–7.16 (m, 4H), 7.09–7.02 (m, 2H), 6.55 (d, J ) 3.4 Hz, 2H),
6.48 (d, J ) 3.4 Hz, 2H), 3.44 (s, 3H), 2.38 (s, 3H); MS (DCI/
NH3) m/z 326 (M + H)+. Anal. (C19H16ClNO2) C, H, N.
[5-(4-Chlorophenyl)furan-2-yl]-[4-(dimethylamino)piperidin-
1-yl]methanone (36). Mp 172–174 °C; 1H NMR (400 MHz,
DMSO-d6) δ 7.88 (d, J ) 7.2 Hz, 2H), 7.55 (d, J ) 7.3 Hz, 2H),
7.15 (d, J ) 3.4 Hz, 1H), 7.08 (d, J ) 3.3 Hz, 1H), 4.38 (br s, 2H),
4.33 (br s, 2H), 2.29–2.25 (m, 1H), 2.21 (br s, 6H), 1.87 (br s,
2H), 1.84 (br s, 2H); MS (ESI) m/z 333 (M + H)+. Anal.
(C18H21ClN2O2) C, H, N.
[5-(4-Chlorophenyl)furan-2-yl](morpholino)methanone (37).
1
Mp 179–180 °C; H NMR (400 MHz, DMSO-d6) δ 7.80 (d, J )
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8.9 Hz, 2H), 7.53 (d, J ) 8.8 Hz, 2H), 7.17 (d, J ) 3.7 Hz, 1H),
7.14 (d, J ) 3.7 Hz, 1H), 3.82–3.71 (m, 4H), 3.68–3.61 (m, 4H);
MS (DCI/NH3) m/z 292 (M + H)+. Anal. (C15H14ClNO3) C, H, N.
Supporting Information Available: Synthetic procedures and
characterization data for precursors to 24, 28, and 30; elemental
analysis data for the reported compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.

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(39) (a) A number of approaches to selective sodium channel blockers have
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Garcia, M. L.; Kaczorowski, G. J.; Li, C.; Lyons, K. A.; Martin, W. J.;
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G. J.; Lyons, K. A.; Meinke, P. T.; Priest, B. T.; Schmalhofer, W. A.;
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