The search for novel TRPV1-antagonists: From carboxamides to benzimidazoles and indazolones

Article abstract of DOI:10.1016/j.bmcl.2006.03.004

Based on a series of diaryl amides the corresponding inverse amides have been found to be potent TRPV1 receptor antagonists. Benzimidazole and indazolone derivatives prepared retained good potency in vitro and indazolone 4a was identified as a novel TRPV1 receptor antagonist suitable for evaluating orally in animal models of analgesia.

Full text of DOI:10.1016/j.bmcl.2006.03.004

Bioorganic & Medicinal Chemistry Letters 16 (2006) 2872–2876
The search for novel TRPV1-antagonists: From carboxamides
to benzimidazoles and indazolones
Stephen Robert Fletcher,* Edward McIver, Stephen Lewis, Frank Burkamp,
Clare Leech, Glenn Mason, Susan Boyce, Denise Morrison,
Gillian Richards, Kathy Sutton and Anthony Brian Jones
Merck Sharp & Dohme, The Neuroscience Research Centre, Terlings Park, Harlow, Essex CM20 2QR, UK
Received 9 February 2006; revised 28 February 2006; accepted 2 March 2006
Available online 24 March 2006
Abstract—Based on a series of diaryl amides the corresponding inverse amides have been found to be potent TRPV1 receptor
antagonists. Benzimidazole and indazolone derivatives prepared retained good potency in vitro and indazolone 4a was identified
as a novel TRPV1 receptor antagonist suitable for evaluating orally in animal models of analgesia.
ꢀ 2006 Elsevier Ltd. All rights reserved.
F3C
The search for transient receptor potential vanilloid 1
(TRPV1) antagonists as potential analgesics has attracted
much attention of late. This has resulted in the disclo-
sure of several structural classes of compounds.1,2 We
now wish to describe novel, potent benzimidazole and
indazolone derivatives.3
F3C
O
H
N
N
H
N
O
N
F3C
2a
F3C
1
Figure 1.
Discovery of a series of biaryl amides, such as 1, has
recently been described.2 Upon investigating the SAR
within that series and in particular to explore the spatial
and orientation requirements of the amide bond, inverse
amide 2a was prepared (Fig. 1). Despite the significant
change in orientation of the carbonyl and NH bonds,
compounds 1 and 2a were found to have comparable
potency at the TRPV1 receptor in vitro (Table 1). This
provided a new avenue for exploration, the results of
which are detailed in this letter.
hepatic portal vein (HPV) and systemic levels detected
following oral dosing at 5 mpk.
In an effort to overcome the pharmacokinetic liability of
the amides, heterocyclisation was explored. N–H hetero-
cyclic derivatives were considered initially and benzimid-
azole derivatives 3 and indazolones 4 were targeted
(Fig. 2).
A versatile synthesis of benzimidazoles 3 enabling struc-
tural variation at both the 2- and 5-positions was em-
ployed as shown in Scheme 1. Cross-coupling of 4-
bromo-2-nitroaniline (5) with a variety of aromatic
derivatives afforded the corresponding nitroanilines
(6). Reduction of the nitro group with iron powder in
acetic acid followed by cyclisation of the resulting
phenylene diamine with a selection of benzaldehydes
by heating at 140 ꢁC in nitrobenzene for 2 h6 gave the
required benzimidazoles 3a–d.
Synthesis of a library of ‘inverse’ amides 2 was
undertaken which rapidly established the in vitro
SAR. Selected data are included in Table 1.
This study showed little tolerance for structural varia-
tion on the distal aromatic rings. Furthermore, evaluation
of pharmacokinetic parameters of 2a in rat (Table 4)
indicated poor oral absorption in the series with low
Benzimidazole 3a showed comparable affinity at the
hTRPV1 receptor to the inverse amide 2a as measured
in a FLIPR-based assay (IC50, 22 nM). Whole-cell patch
Keywords: TRPV-1 antagonists; Indazolone; Benzimidazole.
*
Corresponding author. Tel.: +44 1279 440000; e-mail: stephen_
fletcher@merck.com
0960-894X/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.03.004

S. Robert Fletcher et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2872–2876
2873
the 4-CF3 group with Me, tBu or F, led to loss in potency.
Consequently the pharmacokinetics of 3a was evaluated
prior to further SAR studies to assess whether the benz-
imidazole core would offer any advantage in vivo. The
data obtained (Table 4) showed that whilst the benzimid-
azole led to a modest improvement in absorption
(AUChpv (0–7 h), 1.48 lM h), circulating levels were still
low (AUCsys, 1.25 lM h) following oral dosing at
5 mpk. As a consequence of this finding, attention was
directed towards preparation of the indazolone deriva-
tives 4.
Table 1. Amides
Compound Structure
hIC50 (nM)
FLIPR4 Pclamp5
F3C
F3C
NH
1
13
24
3
33
—
—
—
—
—
—
5
O
O
N
F3C
F3C
2a
HN
N
O
F3C
2b
1500
160
2400
1100
470
640
11
HN
N
N
The approach taken towards a versatile synthesis of the
desired indazolone derivatives was based on the work of
Smalley and co-workers7 who reported that treatment
of o-azidobenzanilides with thionyl chloride leads to the
formation of 2-aryl-3-chloro-2H-indazoles. Consequent-
ly, 2-azido-4-bromobenzoic acid was coupled with 4-
aminobenzotrifluoride and the resulting anilide treated
under the conditions of Smalley et al. with thionyl chlo-
ride at reflux. This led to the smooth formation of the
6-bromo-3-chloro-2H-indazole 11 as a key intermediate.
Selective cross-coupling at the 6-position with4-methoxy-
phenyl boronic acid proved possible and hydrolysis of the
resulting product gave rise to indazolone 4e (Scheme 2).
O
Me
F3C
2c
HN
F3C
O
2d
HN
N
F3C
O
Me
2e
HN
N
F3C
O
2f
HN
N
Me
O
2g
In an alternative use of intermediate 11, hydrolysis of
the 3-chloro substituent was initially carried out using
KOH/MeOH to give bromide 13. Direct cross-coupling
with this derivative proved unsuccessful but the SEM-
protected bromide 14 could be converted to the pinaco-
lato borane derivative for cross-coupling purposes. In
this way, 4a8 and 4c were prepared (Scheme 3).
HN
N
N
F3C
F3C
N
CF3
2h
N
H
R1
H
Azaindalone derivatives 4f and 4g were synthesised via
standard condensation reactions as shown in Schemes
4 and 5.
N
3
X
N
R2
Since the indazolone lacks the inherent symmetry of the
benzimidazole core, two isomeric derivatives 4a and 4b
were prepared and evaluated. Of the two compounds,
4a proved to be more active showing comparable affinity
at the hTRPV1 receptor to the inverse amide 2a and
benzimidazole 3a in the FLIPR-based assay (IC50,
35 nM). This was confirmed by whole-cell patch clamp
technique following stimulation by H+ (Table 3). Some-
what disappointingly a similar SAR was observed with
small functional group changes again leading to signifi-
cant loss in potency. In a final effort to enhance affinity
in vitro, azaindazole 4f was prepared since this change
had proved successful in the amide series, and tetrahydr-
H
N
H
N
R1
R1
N
N
N
N
O
R2
3
4
R2
Figure 2.
clamp experiments were again used to confirm activity
against pH 5.5-dependent activation of the human
receptor (Table 2). As was found with the amides, minor
modifications to the substituents, such as replacement of
NH2
X
N
NO2
H2N
i
iii
R1
N
N
H
R2
R2
Br
5
N
3a-d
6a-d (X=NO2)
7a-d (X=NH2)
ii
Scheme 1. Reagents and conditions: (i) bis(pinacolato)diboron, Pd(dppf)Cl2, KOAc, dioxane, reflux, then Na2CO3(aq), 2-chloro-3-(triflurometh-
yl)pyridine, reflux; (ii) Fe, AcOH, THF–water, (iii) ArCHO, nitrobenzene, 140ꢁ, 2 h.

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S. Robert Fletcher et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2872–2876
When indazolone 4a was evaluated in vivo,9 compared
Table 2. Benzimidazoles
with the amide 2a and benzimidazole 3a, a significant-
ly enhanced pharmacokinetic profile was observed
(Table 4). Following oral dosing of 4a at 5 mpk, sig-
Compound Structure
hIC50
(nM)
% inh at
100 nM
FLIPR Pclamp
nificant systemic levels were obtained (AUCsys
,
N
CF3
CF3
CF3
3 lM h). The compound was evaluated further and
found to have low plasma clearance of 8 mL/min/kg
and a half-life of 3.6 h. Thus, indazolone 4a represents
a structurally novel TRPV1 receptor antagonist which
is suitable for evaluating orally in animal models of
analgesia.
F3C
3a
22
224
170
320
95%
—
N
H
N
N
3b
N
H
N
N
Me
Indazolone 4a was evaluated in the carrageenan induced
thermal hyperalgesia Hargreaves model in rat.10 Follow-
ing dosing at 30 mpk (po) a 50% reduction in hyperalge-
sia was obtained. To rationalize the modest activity, 4a
was evaluated at the rat TRPV1 receptor.11
3c
—
N
H
N
N
CF3
F
3d
—
N
H
N
The IC50 measured using whole-cell patch clamp stud-
ies5 was 220 nM when stimulated with 500 nM capsaicin
and 65 nM at pH 5.5 showing a significant drop in
potency compared to the human receptor.
oindazole 4g was evaluated. Both strategies, however,
proved unsuccessful with indazolone 4a remaining the
most active compound in vitro.
H2N
HOOC
N3
N3
iii, iv
68%
i, ii
F3C
NH
O
Br
HOOC
Br
Br
84%
9
8
10
v
85%
vi
86%
Br
N
N
N
F3C
N
F3C
OMe
11Cl
Cl
12
70%
vii
H
N
F3C
N
OMe
O
4e
Scheme 2. Reagents and conditions: (i) NaNO2, 2 N HCl, 5 ꢁC; (ii) NaN3, NaOAc, H2O; (iii) SOCl2, reflux; (iv) 4-aminobenzotrifluoride, DMAP,
CH2Cl2; (v) SOCl2, reflux, 96 h; (vi) 2-methoxyphenylboronic acid, Pddppf, Na2CO3, 160 ꢁC, 900 s; (vii) KOH, MeOH, 150 ꢁC, 10 min.
Cl
N
O
i
F3C
F3C
N
N
11
N
86%
Br
Br
H
13
70%
ii
O
O
X
iii, iv
45%
F3C
N
F3C
N
N
SEM
N
Br
N
SEM
15
v
14
90%
O
X
F3C
N
N
H
N
X
CF3
Me
4a
4c
Scheme 3. Reagents and conditions: (i) KOH, MeOH, 160 ꢁC, 15 min lwave; (ii) SECML, NaH, THF; (iii) bispinacolatodiborane, KOAc, Pddppf,
dioxane, reflux, 18 h; (iv) chloropyridine, K2CO3, Pddppf, dioxane, reflux, 18 h; (v) 5 N HCl, EtOH, reflux, 1 h.

S. Robert Fletcher et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2872–2876
2875
O
NMe2
O
O
i
Me
F3C
N
+
Me
F3C
N
N
H
N
86%
N
NH2
H
N
4f
N
Scheme 4. Reagent and condition: (i) pyridine, reflux.
O
EtOOC
O
EtOOC
O
CF3
CF3
i
ii
86%
F3C
N
NH
N
N
N
H
N
N
4g
Scheme 5. Reagents and conditions: (i) NaH, 2-chloro-3-(trifluoromethyl)pyridine, DMA, 140 ꢁC, 12 h; (ii) CF3-phenylhydrazine, TsOH, xylene,
reflux, 2 h.
Table 3. Indazolones
In conclusion, we have developed a novel series of
potent TRPV1 receptor antagonists. Compound 4a
shows activity in the carrageenan-induced thermal
hyperalgesia model following oral dosing.
Compound
Structure
hIC50 (nM)
FLIPR Pclamp
O
CF3
F3C
N
4a
4b
4c
4d
4e
4f
35
370
180
340
170
270
290
4
N
H
References and notes
N
N
O
1. (a) Xi, N.; Bo, Y.; Doherty, E. M.; Fotsch, C.; Gavva, N.
R.; Han, N.; Hungate, R. W.; Klionsky, L.; Liu, Q.;
Tamir, R.; Xu, S.; Treanor, J. J. S.; Norman, M. H.
Bioorg. Med. Chem. Lett. 2005, 15, 5211; (b) Shao, B.;
Huang, J.; Sun, Q.; Valenzano, K. J.; Schmid, L.; Nolan,
S. Bioorg. Med. Chem. Lett. 2005, 15, 719–723; (c)
Swanson, D. M.; Dubin, A. E.; Shah, C.; Nasser, N.;
Chang, L.; Dax, S. L.; Jetter, M.; Breitenbucher, J. G.;
Liu, C.; Mazur, C.; Lord, B.; Gonzales, L.; Hoey, K.;
Rizzolio, M.; Bogenstaetter, M.; Codd, E. E.; Lee, D. H.;
Zhang, S.-P.; Chaplan, S. R.; Carruthers, N. I. J. Med.
Chem. 2005, 48, 1857; (d) Rami, H. K.; Thompson, M.;
Wyman, P.; Jerman, J. C.; Egerton, J.; Brough, S.;
Stevens, A. J.; Randall, A. D.; Smart, D.; Gunthorpe,
M. J.; Davis, J. B. Bioorg. Med. Chem. Lett. 2004, 14,
3631.
—
—
—
—
—
—
F3C
F3C
N
CF3
Me
N
H
O
N
N
H
N
O
F3C
F3C
N
N
H
O
OMe
Me
N
N
H
2. Park, H.-g.; Choi, J.-y.; Kim, M.-h.; Choi, S.-h.; Park,
M.-k.; Lee, J.; Suh, Y. G.; Cho, H.; Oh, U.; Kim, H.-D.; Joo,
Y. H.; Shin, S. S.; Kim, J. K.; Jeong, Y.-S.; Koh, H.-J.; Park,
Y.-H.; Jew, S.-s. Bioorg. Med. Chem. Lett. 2005, 15, 631.
3. Burkamp, F.; Fletcher, S. R. WO 2005049601.
4. Inhibition of capsaicin evoked increases in intracellular
[Ca2+] was measured in CHO cells, stably expressing
recombinant human VR1 receptors using 1uM Fluo-3-
AM dye in a Molecular Devices FLIPR. Increases in
intracellular [Ca2+] after addition of test compound, prior
to addition of capsaicin, allowed determination of intrinsic
agonist or partial agonist activity, if present. All com-
pounds tested behaved as antagonists. For further details,
see Ref. 3.
O
F3C
F3C
N
N
H
N
N
N
O
CF3
N
4g
N
N
H
Table 4. Pharmacokinetic parameters8
5. Whole-cell patch clamp electrophysiology was used to
measure the block of currents stimulated by acidic pH
(5.5) at human TRPV1 stably expressed in CHO cells.
For experimental protocol, see: Sutton, K. G.; Gar-
rett, E. M.; Rutter, A. R.; Bonnert, T. P.; Jarolimek,
W.; Seabrook, G. R. Br. J. Pharmacol. 2005, 146,
702.
6. Yadagiri, B.; Lown, J. W. Synth. Commun. 1990, 20, 955.
7. Ardakani, M.; Smalley, R. K.; Smith, R. H. Synthesis
1979, 308.
2a
3a
4a
podose (mpk)
AUChpv (lM h)
AUCsys (lM h)
ivdose (mpk)
Cl (mL/min/kg)
T1/2 (h)
5
5
5
0.08
0.04
1.48
1.25
6.2
3.0
1
8
3.6
2.0
Vd (L/kg)

2876
S. Robert Fletcher et al. / Bioorg. Med. Chem. Lett. 16 (2006) 2872–2876
8. Preparation of 4a: 2-Azido-4-bromo-N-(4-(trifluoro meth-
no)ferrocenyl palladium(II) dichloride (20 mg) and
saturated Na2CO3 solution (1 mL) were added, and the
mixture heated at reflux for a further 18 h. Purification by
column chromatography gave 0.2 g of product as an oil,
MS: (ES (M+1)) 554. This material was dissolved in concd
HCl (2 mL) and EtOH (3 mL) and heated at reflux for 1 h.
The mixture was concentrated and the residue crystallized
from EtOH/water to give 70 mg of 4a as a colourless solid.
1H NMR (360 MHz, CDCl3): 7.33 (1H, s), 7.39 (1H, d,
J = 8.0 Hz), 7.43 (1H, s), 7.5–7.55 (1H, m), 7.73 (2H, d,
J = 8.0 Hz), 7.99 (1H, d, J = 8.0 Hz), 8.09 (2H, d,
J = 8.0 Hz), 8.14 (1H, d, J = 8.0 Hz), 8.9 (1H, d,
J = 8.0 Hz).
yl)phenyl)benzamide 10 (3 g, 0.0078 mol) in thionyl chlo-
ride (15 mL) was heated at reflux for 24 h. The mixture was
concentrated and the residue partitioned between ether (3·
20 mL) and aq Na2CO3 (10 mL). The organic phase was
separated, dried (MgSO4) and concentrated to give 2.5 g
1
(85% yield) of 11 as a tan powder. H NMR (360 MHz,
CDCl3): 7.25 (1H, d, J = 8.9 Hz), 7.37 (1H, d, J 8.9 Hz),
7.80–7.95 (5H, m). A solution of 11 (2.4 g, 0.0064 mol) in
1 M KOH/MeOH (16 mL) was heated at 150 ꢁC in a sealed
tube via microwave irradiation for 15 min. The mixture was
acidified with 1 N HCl and the precipitate collected by
filtration to give 1.0 g (44% yield) of 13. 1H NMR
(360 MHz, CDCl3): 7.37 (1H, dd, J = 1.4 and 8.5 Hz),
7.66 (1H, d, J = 1.4 Hz), 7.73 (1H, d, J = 8.5 Hz), 7.89 (2H,
d, J = 8.5 Hz), 8.13 (2H, d, J = 8.5 Hz), 11.0 (1H, s). A
suspension of NaH in mineral oil (60%, 0.17 g, 0.0041 mol)
was added to a solution of 13 (1.3 g, 0.0035 mol) in dry THF
(50 mL) at room temperature under nitrogen. After 20 min,
SEM chloride (0.68 g, 0.0041 mol) in THF (15 mL) was
added and the resulting slurry stirred overnight. The
mixture was quenched with brine (10 mL) and extracted
into ether (3· 20 mL). The organic phase was washed with
1 N HCl (10 mL), 2 N NaOH (10 mL) and brine (10 mL),
dried (MgSO4) and concentrated to give crude 14 as an oil.
14 (1.3 g, 2.6 mmol) was dissolved in dioxane (10 mL) under
N2 and bis(pinacolato) diboron (680 mg, 2.6 mmol),
bis(diphenylphosphino)ferrocenyl palladium(II) dichloride
(60 mg) and KOAc (380 mg, 3.9 mmol) were added and the
mixture stirred at 100 ꢁC overnight. 2-Chloro-3-(trifluoro-
methyl)pyridine (470 mg, 2.6 mmol), bis(diphenylphosphi-
9. For experimental protocol, see: Scott-Stevens, P.; Atack,
J. R.; Sohal, B.; Worboys, P. Biopharm. Drug Dispos.
2005, 26, 13.
10. A beam of light is focussed on the underside of the foot
until the animal responds by removing the foot from the
light source; three recordings are made on each hind limb
with stimulation separated by at least 5 min. Animals were
injected intra-plantar with either carrageenan (positive
control and drug tested animals) or saline (0.1 mL in the
right hind limb). Two hours later, animals were dosed (po)
with the 4a at 30 mpk. After 1 h, animals were retested
three times on each hind limb. The change in response
following a saline injection is assumed to be an uninflamed
state with 0% hyperalgesia, while carrageenan injection is
assumed to produce a hyperalgesic response of 100%.
11. Grant, S. K.; Bansal, A.; Mitra, A. W.; Dai, G.;
Kaczorowski, G. J.; Middleton, R. E. Anal. Biochem.
2001, 294, 27.