α-Methylation at benzylic fragment of N-aryl-N′-benzyl ureas provides TRPV1 antagonists with better pharmacokinetic properties and higher efficacy in inflammatory pain model

Article abstract of DOI:10.1016/j.bmcl.2007.04.105

SAR studies for N-aryl-N′-benzyl urea class of TRPV1 antagonists have been extended to cover α-benzyl alkylation. Alkylated compounds showed weaker in vitro potencies in blocking capsaicin activation of TRPV1 receptor, but possessed improved pharmacokinetic properties. Further structural manipulations that included replacement of isoquinoline core with indazole and isolation of single enantiomer led to TRPV1 antagonists like (R)-16a with superior pharmacokinetic properties and greater potency in animal model of inflammatory pain.

Full text of DOI:10.1016/j.bmcl.2007.04.105

Bioorganic & Medicinal Chemistry Letters 17 (2007) 3894–3899
a-Methylation at benzylic fragment of N-aryl-N0-benzyl
ureas provides TRPV1 antagonists with better pharmacokinetic
properties and higher efficacy in inflammatory pain model
Arthur Gomtsyan,* Erol K. Bayburt, Ryan Keddy, Sean C. Turner,
Tammie K. Jinkerson, Stanley Didomenico, Richard J. Perner, John R. Koenig,
Irene Drizin, Heath A. McDonald, Carol S. Surowy, Prisca Honore, Joe Mikusa,
Kennan C. Marsh, Jill M. Wetter, Connie R. Faltynek and Chih-Hung Lee
Global Pharmaceutical Research and Development, Abbott Laboratories, 100 Abbott Park Road, Abbott Park, IL 60064, USA
Received 26 March 2007; revised 24 April 2007; accepted 30 April 2007
Available online 3 May 2007
Dedicated to Professor Ivars Kalvinsh on the occasion of his 60th birthday.
Abstract—SAR studies for N-aryl-N0-benzyl urea class of TRPV1 antagonists have been extended to cover a-benzyl alkylation.
Alkylated compounds showed weaker in vitro potencies in blocking capsaicin activation of TRPV1 receptor, but possessed
improved pharmacokinetic properties. Further structural manipulations that included replacement of isoquinoline core with inda-
zole and isolation of single enantiomer led to TRPV1 antagonists like (R)-16a with superior pharmacokinetic properties and greater
potency in animal model of inflammatory pain.
Ó 2007 Elsevier Ltd. All rights reserved.
Transient receptor potential vanilloid 1 (TRPV1) is cur-
rently one of the most attractive therapeutic targets in
pain research.1 Nonselective cation channel TRPV1,
originally described as a molecular target for capsaicin,
pungent component of chili pepper, is now considered to
be the principal integrator of noxious information. The
reason for such characterization lies in the fact that
there are multiple mechanisms by which TRPV1 can
be either activated or sensitized. For example, activation
of TRPV1 channel can be achieved by vanilloids such as
capsaicin and resiniferatoxin, noxious heat (>42 °C),
acidic extracellular media (pH < 6), and endogenous
anandamide and arachidonic acid metabolites among
other agents. On the other hand, activation of TRPV1
is potentiated by prostaglandins, bradykinin, and other
pro-inflammatory substances.2 Blocking the painful ef-
fects of activators by TRPV1 antagonists is one of the
major avenues in discovery of novel analgesics.
Recently two groups reported on discovery of N-iso-
quinolin-N0-benzyl urea class of TRPV1 antagonists.3,4
This series produced lead compound 1 (Fig. 1) charac-
terized by good potency at TRPV1,5 modest efficacy in
animal pain models6, and less than desirable pharmaco-
kinetic profile. Here we discuss the effects of alkylation
and arylation at the benzylic carbon atom on in vitro,
in vivo, and pharmacokinetic properties of TRPV1
antagonists.
General synthetic approach to the synthesis of target
TRPV1 antagonists is shown in Scheme 1. Starting aryl-
alkylketones 2 were converted to corresponding oximes
3 which then were reduced to amines 4 and further
O
HN
N
H
CF3
N
Keywords: TRPV1; TRPV1 antagonist; Urea; Pain.
*
1
Corresponding author. Tel.: +1 847 935 4214; fax: +1 847 937
9195; e-mail: arthur.r.gomtsyan@abbott.com
Figure 1. Lead TRPV1 antagonist 1.
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.04.105

A. Gomtsyan et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3894–3899
3895
O
R1
O
NOMe
NH2
HN
N
R2
H
a
c
b
R1
R1
R1
R2
R2
R2
N
3
2
4
5
Scheme 1. Reagents and conditions: (a) H2NOMeÆHCl, Py, 16 h, rt; (b) H2, 10% Pd/C, MeOH–NH3; (c) 5-isocyanatoisoquinoline, CH2Cl2, rt or
2,2,2-trichloro-N-(isoquinolin-5-yl)acetamide, MeCN, DBU, reflux.
elaborated to desired compounds 5 by reacting with iso-
quinoline isocyanate or its surrogate trichloroaceta-
mide.3 Scheme 2 outlines synthetic detours necessary
to prepare commercially unavailable starting ketones
2. Thus, for the synthesis of cyclopropyl-containing ke-
tone 2p, corresponding Weinreb amide 7 was prepared
from 4-trifluorobenzoyl chloride (6) and then reacted
with cyclopropylmagnesium bromide. Under the same
reaction conditions cyclopentylmagnesium bromide
yielded only traces of the corresponding ketone 2r.
However, reactions between acid chloride 6 and cyclo-
pentyl- and cyclohexylzinc-bromides were more success-
ful providing desired ketones 2r and 2s in 52 and 59%
yields, respectively. 4-Piperidinophenyl-methyl ketone
(2g) was prepared in one step from corresponding
fluoro-substituted ketone 8 by refluxing the mixture of
reactants in pyridine. For the synthesis of compounds
5i–k where phenyl group in the benzyl moiety contains
two substituents one of which is tert-butyl, we
took advantage of known phenomenon of tert-butyl
O
HN
N
O
O
O
c
H
b
a
O
CF3
Cl
N
Me
N
Me
F3C
F3C
F3C
2p
7
6
5p
d
n
O
O
O
HN
N
c
e
H
Cl
CF3
N
n
F3C
F3C
6
2r n=1
2s n=2
5r n=1
5s n=2
O
Me
O
O
HN
N
H
c
f
Me
Me
N
N
N
F
8
2g
5g
O
Me
R
O
R
HN
N
H
c
g
Me
N
R
5i-k
2i-k
9i-k
Scheme 2. Reagents and conditions: (a) NH(OMe)MeÆHCl, Py, THF, 0 °C, rt, 2 h, quantitative; (b) cyclopropylmagnesium bromide, THF, 0 °C, 1 h,
48%; (c) see Scheme 1, a–c; (d) cyclopentylmagnesium bromide, THF, 0 °C, 2 h; (e) cycloalkylzinc bromide, CuCN, LiBr, THF, ꢀ45 °C, rt, 52% for
2r and 59% for 2s; (f) piperidine, Py, reflux, 2 h, 86%; (g) AlCl3, CS2, then MeCOCl, 49–95%.

3896
A. Gomtsyan et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3894–3899
O
HN
N
H
a
b
Cl
c
OH
NCO
O
Cl
O
O
N
Cl
Cl
Cl
5t
10
HN
N
H
N
d
e
CF3
NH2
N
F3C
F3C
5u
11
12
Scheme 3. Reagents and conditions: (a) SOCl2, toluene, 80 °C, 2 h, 98%; (b) !(i!) NaN3, H2O, 0 °C, 0.5 h;! (ii!)toluene, 60 °C, 1 h, 96% for two
steps; (c) 5-aminoisoquinoline, THF, 16 h, rt, 38%; (d) methylmagnesium bromide, THF, 0 °C, rt, 16 h; (e) see Scheme 1c.
isomerization/migration that occurs during electrophilic
reactions on tert-butyl-substituted benzenes.7 Thus, Fri-
edel–Crafts acetylation of disubstituted benzene 9i–k in
carbon disulfide in the presence of aluminum chloride
initiated tert-butyl group migration from para- to
meta-position and gave desired ketones 2i–k in good
yields. For the synthesis of gem-dimethyl compounds
5t and 5u two approaches were taken (Scheme 3). Cur-
tius rearrangement strategy starting from the carboxylic
acid 10 led to 5t and reduction of nitrile 11 to amine 12
was the key reaction8 in the preparation of 5u.
Inspection of SAR Table 1 reveals that attachment of
the methyl group to the benzylic carbon atom provides
superior TRPV1 antagonists compared with the ethyl
(5d vs 5l), phenyl (5a vs 5e), cycloalkyl (5a vs 5p–s) or
gem-dimethyl (5a vs 5u) analogs. Although in vitro
activities of the most potent methyl-substituted com-
pounds 5a–d, j, k were still 4- to 5-fold weaker than
for unsubstituted 1, we found that methylation at the
benzylic carbon atom led to TRPV1 antagonists exhibit-
ing longer half-life and larger volume of distribution in
both rat and dog pharmacokinetic studies (Table 2).
Table 1. In vitro functional activity of isoquinoline TRPV1 antagonists 5 in human TRPV1 Ca2+ influx assay
O
R1 R2
HN
N
H
R3
N
5
R1
R2
R3
a
hTRPV1 IC50 (nM)
Compound
1
H
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
Me
4-CF3
4-CF3
4-Br
4
22
3
9
5a
5b
5c
5d
5e
5f
Me
Me
26 10
4-t-Bu
3-F, 4-CF3
4-CF3
24
19
4
5
Me
Ph
147 21
125 30
171 52
600 0b
46 12
Ph
Ph
4-t-Bu
4-Piperidino
H
5g
5h
5i
Ph
Me
2-Me, 4-t-Bu
2-Et, 4-t-Bu
2,4-di-t-Bu
3-F, 4-CF3
H
5j
Me
Me
20
17
5
5
5k
5l
Et
Et
109 11
2580 350
31,000 2200
15,900 1100
149 50
5m
5n
5o
5p
5r
5s
5t
Me2NCH2
NCCH2
Cyclopropyl
Cyclopentyl
Cyclohexyl
Me
H
H
4-CF3
4-CF3
4-CF3
476 180
287 118
143 29
4-Cl
4-CF3
5u
Me
94 14
a All values are means SEM of at least three separate experiments.
b Only one reading.

A. Gomtsyan et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3894–3899
3897
Table 2. Pharmacokinetic properties of N-benzyl-N0-isoquinolinyl urea 1 and its methylated analog 5aa
Compound
hTRPV1 IC50 (nM)
iv T1/2 (h)
iv Vb (L/kg)
iv Clp (L/h kg)
po F (%)
O
HN
N
H
4
0.6 (rat)
1.0 (dog)
0.6 (rat)
0.5 (dog)
0.6 (rat)
0.4 (dog)
46 (rat)
32 (dog)
CF3
N
1
O
Me
HN
N
H
22
1.1 (rat)
1.6 (dog)
1.1 (rat)
1.1 (dog)
0.7 (rat)
0.5 (dog)
47 (rat)
98 (dog)
CF3
N
5a
a Pharmacokinetic parameters determined in rats and dogs following administration of 10 lmol/kg.
In an effort to further improve pharmacokinetic proper-
ties of target molecules, we replaced the isoquinoline
with an indazole moiety that was previously used in
the unsubstituted series.9 Thus, 4-nitroindazole (13)10
was acylated by methylchloroformate followed by cata-
lytic hydrogenation over Pd/C to provide 4-aminoindaz-
ole derivative 14 which was converted to isocyanate
intermediate 159 (Scheme 4). The latter reacted with sev-
eral a-methyl-benzylamines to afford corresponding ur-
eas, which after indazole N-deprotection by NaOH in
MeOH gave target compounds 16. In order to resolve
individual enantiomers of 16a, several approaches were
taken such as (1) asymmetric synthesis of the chiral
amines where the key reaction was chiral oxazaboroli-
dine catalyzed reduction of acetophenones to secondary
alcohols11,12 followed by elaboration to amines; (2) chi-
ral resolution of racemic amines either by chiral acids or
diastereomer formation; (3) HPLC separation of final
racemic products on chiral columns. The most useful
and reproducible results were obtained by chiral resolu-
tion of racemic amine 4a through diastereomer forma-
tion (Scheme 5). Thus, acetophenone 2a was converted
to racemic amine 4a in two steps consisting of oxime for-
mation followed by reduction. Then, the diastereomeric
mixture of amides 17, prepared from 4 and (R)-O-acet-
ylmandelic acid, was separated on silica gel to give dia-
stereomerically pure amides (R,R)-17 and (S,R)-17 in 42
and 39% yields, respectively. Finally, the cleavage of the
chiral auxiliary by treating the amides with aq HBr
afforded desired chiral amines (R)-4a and (S)-4a in good
yields. Absolute stereochemistry was assigned based on
the sign of reported optical rotation.13 NMR and HPLC
analysis of Mosher amides of synthesized amines con-
firmed enantiomeric ratios of 96:4 and 97:3 for (R)-4a
and (S)-4a, respectively. Synthesis of target chiral com-
pounds (R)-16a and (S)-16a was completed in two steps
from amines (R)-4a and (S)-4a by their acylation with
isocyanate 15 followed by deprotection of methoxycar-
bonyl group. Commercially available chiral amines were
used for the synthesis of single enantiomers of 16b,c.
Appraisal of enantiomeric pairs of TRPV1 antagonists
16 in indazole series indicated clear effect of chirality
on the functional potency of TRPV1 antagonists in
the Ca2+ influx assay (Table 3). (R)-Enantiomers of
16a–c were ꢁ5- to 30-fold more potent than their (S)-
counterparts.
The most potent compound from these series, (R)-16a,
was tested in rat model of inflammatory pain and the re-
sult was compared with the activity of compound 1 in
the same model. Compound (R)-16a demonstrated
greater potency in vivo than 1 in relieving CFA (Com-
plete Freund’s Adjuvant)-induced thermal hyperalge-
sia14 after oral administration (Table 4), despite the
fact that it was 10-fold weaker than 1 in Ca2+ influx as-
say. The lack of correlation between in vitro and in vivo
activities for 1 and (R)-16a can be explained by better
pharmacokinetic properties of (R)-16a compared with
1. The most notable differences were larger volume of
distribution (2.9 vs 0.6 L/kg) and longer half-life (2.1
vs 0.6 h) for (R)-16a.
O
Me
NO2
HN
N
H
R
NH2
NCO
a
c
b
N
N
N
N
N
N
N
N
H
H
MeO2C
MeO2C
16
14
15
13
Scheme 4. Reagents and conditions; (a)! (i!) mix NaH (60% in mineral oil) and DMF, add 13 at 0 °C, then methylchloroformate, 0 °C, rt, 51%;!
(ii!) H2, 10% Pd/C, MeOH, 60 psi, 60 °C, 1 h, used crude; (b) 20% COCl2 in toluene, reflux, 3 h, used crude; (c)! (i!) benzylamines, Et2O, Et3N,
16 h, rt; !(ii!) 5 M NaOH in MeOH, 2 h, rt.

3898
A. Gomtsyan et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3894–3899
O
O
O
NH2
Me
HN
HN
a
b
OAc
Me
OAc
Me
Me
F3C
F3C
F3C
F3C
(R,R)-17
(S,R)-17
2a
4a
c
c
NH2
Me
NH2
Me
F3C
F3C
(S)-4a
(R)-4a
d
d
O
Me
O
Me
HN
N
HN
N
H
H
CF3
CF3
N
N
N
N
H
H
(S)-16a
(R)-16a
Scheme 5. Reagents and conditions: (a) see Scheme 1, a and b; (b) !(i!) (R)-O-acetylmandelic acid, DCC, DMAP, CH2Cl2, rt, 16 h; !(ii!) silica
gel chromatography, 42% for (R,R)-17 and 39% for (S,R)-17; (c) 48% HBr (aq), H2O, reflux, 75% for (R)-4a and 72% for (S)-4a; (d) see Scheme 4, c.
Table 3. In vitro functional activity of indazole TRPV1 antagonists 16 in human TRPV1 Ca2+ influx assay
O
Me
HN
N
R
H
N
N
H
16
a
hTRPV1 IC50 (nM)
Compound
R
(rac)-16a
(S)-16a
(R)-16a
(S)-16b
(R)-16b
(S)-16c
(R)-16c
4-CF3-phenyl
4-CF3-phenyl
4-CF3-phenyl
4-Me-phenyl
4-Me-phenyl
naphthyl
47 11
248 62
41 10
749 115
73
3750 1900
123
5
naphthyl
7
a All values are means SEM of at least three separate experiments.
Table 4. Comparison of in vitro, in vivo, and pharmacokinetic properties of compounds 1 and (R)-16a
Compound
hTRPV1 IC50 (nM)
CFA ED50 (lmol/kg) poa
iv T1/2 (h)b
iv Vb (L/kg)b
po Fb (%)
1
(R)-16a
4
41
40
13
0.6
2.1
0.6
2.9
46
53
a The data represent means of n = 6 per dose group.
b Pharmacokinetic parameters determined in rats following administration of 10 lmol/kg.

A. Gomtsyan et al. / Bioorg. Med. Chem. Lett. 17 (2007) 3894–3899
3899
5. El Kouhen, R.; Surowy, C. S.; Bianchi, B. R.;
Neelands, T. R.; Mcdonald, H. A.; Niforatos, W.;
Gomtsyan, A.; Lee, C.-H.; Honore, P.; Sullivan, J. P.;
Jarvis, M. F.; Faltynek, C. R. J. Pharmacol. Exp. Ther.
2005, 314, 400.
6. Honore, P.; Wismer, C. T.; Mikusa, J.; Zhu, C. Z.;
Zhong, C.; Gauvin, D. M.; Gomtsyan, A.; El Kouhen,
R.; Lee, C.-H.; Marsh, K. C.; Sullivan, J. P.;
Faltynek, C. R.; Jarvis, M. F. J. Pharmacol. Exp.
Ther. 2005, 314, 410.
7. Burgstahler, A. W.; Chien, P.-L.; Abdel-Rahman, M. O.
J. Am. Chem. Soc. 1964, 86, 5281.
8. Neuvonen, K.; Pihlaja, K. Chem. Soc., Perkin Trans. 1988,
2, 461.
9. Drizin, I.; Gomtsyan, A.; Bayburt, E. K.; Schmidt, R. G.;
Zheng, G. Z.; Perner, R. J.; Didomenico, S.; Koenig, J. R.;
Turner, S. C.; Jinkerson, T. K.; Brown, B. S.; Keddy, R.
G.; McDonald, H. A.; Honore, P.; Wismer, C. T.; Marsh,
K. C.; Wetter, J. M.; Polakowski, J. S.; Segreti, J. A.;
Jarvis, M. F.; Faltynek, C. R.; Lee, C.-H. Bioorgan. Med.
Chem. 2006, 14, 4740.
In conclusion, several modifications to the early pharma-
cological tool compound 1 were made with the goal of
improving both pharmacokinetic properties and in vivo
activity. These modifications included methylation at
benzylic carbon atom, replacement of isoquinoline core
with indazole, and finally resolving individual enantio-
mers. While this process resulted in compounds showing
weaker activity at TRPV1, their improved pharmacoki-
netic properties resulted in greater potency in an animal
pain model. The current data also demonstrated that
introduction of a stereogenic center may offer additional
opportunities to improve pharmacological and pharma-
cokinetic properties of TRPV1 compounds by virtue of
one of the enantiomers being superior to the other.
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