Potent and orally efficacious benzothiazole amides as TRPV1 antagonists

Article abstract of DOI:10.1016/j.bmcl.2012.08.018

Benzothiazole amides were identified as TRPV1 antagonists from high throughput screening using recombinant human TRPV1 receptor and structure-activity relationships were explored to pinpoint key pharmacophore interactions. By increasing aqueous solubility

Full text of DOI:10.1016/j.bmcl.2012.08.018

Bioorganic & Medicinal Chemistry Letters 22 (2012) 6205–6211
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Potent and orally efficacious benzothiazole amides as TRPV1 antagonists
a
b
a
a
b
b
Yevgeni Besidski , William Brown , Johan Bylund , Michael Dabrowski , Sophie Dautrey , Magali Harter ,
a
a
b
b
b
b
Lucy Horoszok , Yin Hu , Dean Johnson , Shawn Johnstone , Paul Jones , Sandrine Leclerc ,
a
a
b
b
b
a
Karin Kolmodin , Inger Kers , Maryse Labarre , Denis Labrecque , Jennifer Laird , Therese Lundström ,
b
b
b
a
b
a
John Martino , Mickaël Maudet , Alexander Munro , Martin Nylöf , Andrea Penwell , Didier Rotticci ,
Andis Slaitas , Anna Sundgren-Andersson , Mats Svensson , Gitte Terp , Huascar Villanueva ,
Christopher Walpole , Ronald Zemribo , Andrew M. Griffin
AstraZeneca R&D Södertälje, S-151 85 Södertälje, Sweden
a
a
a
a
a
b
a
b,
⇑
a
b
AstraZeneca R&D Montréal, 7171 Frédérick-Banting, St. Laurent, Québec H4S 1Z9, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzothiazole amides were identified as TRPV1 antagonists from high throughput screening using
recombinant human TRPV1 receptor and structure–activity relationships were explored to pinpoint
key pharmacophore interactions. By increasing aqueous solubility, through the attachment of polar
groups to the benzothiazole core, and enhancing metabolic stability, by blocking metabolic sites, the
drug-like properties and pharmokinetic profiles of benzothiazole compounds were sufficiently optimized
such that their therapeutic potential could be verified in rat pharmacological models of pain.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 12 June 2012
Revised 23 July 2012
Accepted 1 August 2012
Available online 14 August 2012
Keywords:
TRPV1 antagonist
Benzothiazoles
Metabolic stability
In vivo pain models
The transient receptor potential cation channel subfamily V
member 1 (TRPV1), also known as VR1, is a member of the tran-
sient receptor potential family of ion channels and is a non-selec-
tive cation channel with high permeability to divalent cations,
of the concomitant pungency and neurotoxicity associated with
the administration of agonists.
9
Benzothiazole 3 (Fig. 2) was identified from a high throughput
screening (HTS) campaign using a fluorescence cell-based assay
1
2+
10a
especially calcium. It is expressed on unmyelinated pain-sensing
that measures Ca influx.
Compound 3 is a potent and compet-
nerve fibres and is activated by noxious stimuli, such as heat
itive antagonist with capsaicin at the TRPV1 receptor, with an IC50
of 27 nM. Liabilities of Compound 3 included low aqueous solubil-
ity (2.2 lM at pH 7.4) and poor metabolic stability in human
2
3
4
(
>43 °C), low pH, capsaicin 1 (the active component of chili pep-
5
pers), capsaicin analogues and endocannabinoids. Additionally,
prostaglandins and bradykinin modulate TRPV1 through PKC-med-
iated receptor phosphorylation. Once activated, the channel opens
microsomes due to oxidation of the tbutyl and benzylic methyl
positions. Our strategy to address these issues included replacing
the lypophilic tbutyl group, reducing lypophilicity through replace-
ment of the right hand aromatic ring with heterocycles, addition of
functional groups at the benzylic methyl position and investigating
changes to the central amide linker (Fig. 2). It is of interest to note
6
to allow the influx of calcium ions, which in turn leads to the exci-
tation of primary sensory neurons that gate nociceptive inputs to
the central nervous system. Studies with TRPV1 knockout mice fur-
ther substantiate the role that TRPV1 plays in the signaling of pain
7
10a
10b–d
stimuli. Thus, TRPV1 is an integrator of pain stimuli, and the mod-
that in addition to our group,
others groups have reported
ulation of TRPV1 therefore represents a potential therapeutic
application for the treatment of pain.
use of the benzothiazole scaffold in their VR1 antagonist programs.
Initially, strategies to modulate TRPV1 focused on the develop-
ment of agonists, since the administration of agonists, such as cap-
saicin 1, leads to desensitization of the channel and neurite
Cl
O
HO
HO
S
O
N
H
N
N
H
retraction. However, antagonists at TRPV1, such as capsazepine
8
HO
2,
(Fig. 1) may prove more useful, since antagonists are devoid
1
2
⇑
Corresponding author. Tel.: +1 514 832 3200; fax: +1 514 832 3232.
E-mail address: andrew.griffin@astrazeneca.com (A.M. Griffin).
Figure 1. Structures of TRPV1 antagonists 1 and 2.
0
960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.08.018

6
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Y. Besidski et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6205–6211
heteroaromatic
groups
H
N
this work
H
N
S
N
N
O
O
lypophilic
S
group replacements
3
amide
alternatives
Me group
IC50 hVR1 27 nM
replacements
Poor aqueous solubility
Metabolically unstable
Figure 2. Strategy adopted for optimization of HTS hit 3.
Metabolism studies showed that the methyl group of benzimid-
far proved not to be fruitful, such changes being associated with
an important loss in potency. Incorporation of a basic nitrogen
on the benzamide aromatic ring abolished all activity (compounds
14 and 15), whereas ethers were tolerated but showed only mod-
est solubility (compounds 16 and 17). Finally we investigated the
use of heterocycles in place of the right hand aromatic group and
found this change caused a substantial drop in activity – compare
phenyl 9 to pyridyl 18 and pyrrazole 21. Potency could be regained
via addition of a more lypophilic substituent onto the heterocycle
(compounds 19, 20) but the overall profiles in terms of potency,
solubility and metabolic stability did not show significant
improvements compared to aromatic derivatives (see Table 1).
Overall, in most cases potency at VR1 was improved switching
from the benzylic methyl to hydroxyl methyl substitution as well
as demonstrating an improvement in aqueous solubility. These
trends were confirmed when looking at a wider set of compounds
as shown by a matched pairs analysis (see Fig. 3). A clear need for a
lypophilic substituent on the right hand side of the molecule was
also confirmed.
Alternative replacements of the alcohol functionality demon-
strated a large loss in TRPV1 activity (see Table 2). Amine-derived
analogs 22-24 showed at least a hundred fold lose of affinity, with
sulphonamide 23, a typical alcohol bioisostere, the most active
analog. Secondary alcohol 25, a racemic compound, showed a 60
fold loss in potency compared to 5. None of these modifications
were pursued further.
Next our attention turned to examining the role of the central
amide linker. Reverse amides were synthesized according to
Scheme 3. 2-Methyl-1,3-benzothiazol-5-amine was converted to
the corresponding aryl iodide via diazotation, and then trans-
formed into cyanide 26 with zinc cyanide under palladium cataly-
sis. The cyano-intermediate was hydrolyzed to acid 27 followed by
oxidation of the methyl group according to the same 2-step oxida-
tion/reduction sequence used previously, yielding hydroxy acid 28.
Coupling of acid 28 with 4-t-butylaniline furnished 29 while cou-
pling with 4-methyl phenethylamine gave amide 30.
azole 3 was readily oxidized to the corresponding alcohol in vitro.
Given that we were looking for improvements in aqueous solubil-
ity as well as maintaining TRPV1 activity, we started our SAR
exploration by investigating the properties of a set of hydroxym-
ethylbenzimidazoles. Synthesis of hydroxymethylbenzothiazole
derivatives was performed using chemistry shown in Scheme 1.
Commercially available 2-methyl-1,3-benzothiazol-5-amine was
protected as a N-tbutyl carbamate and the methyl group oxidized
with selenium dioxide. The intermediate aldehyde was reduced
with sodium borohydride and the alcohol protected as the corre-
sponding allylcarbonate (Alloc). Removal of the Boc protecting
group gave aniline 4 which was then followed by amide formation
under standard conditions with subsequent removal of the Alloc
protecting group to generate alcohols 5-21.
Alternative functional groups were incorporated at the benzylic
position and compound 3 served as the starting material for the
syntheses of amino-derived analogs 22–24 (see Scheme 2). Com-
pound 3 was converted into amine 22 through a three step se-
quence of oxidation to the corresponding benzylic aldehyde,
followed by oxime formation and finally oxime reduction. The ami-
no-derivative 22 was then sulfonylated and acetylated to yield
derivatives 23 and 24, respectively. Synthesis of secondary alcohol
5 was realized by oxidation of 3 to the corresponding aldehyde,
followed by addition of methyl magnesium bromide.
Human VR1 antagonism data, as well as aqueous solubility and
human microsomal stability data (where available) for alcohols 5-
1 is shown in Table 1. Alcohol 5, formed as a metabolite of 3,
maintained excellent potency at VR1 with reasonable metabolic
stability. The tbutyl benzamide group present in 3 and 5 was met-
abolically labile and efforts were made to examine alternative aro-
matic amides with various substitution patterns. Replacing the
tbutyl group by a halogen atom usually caused a 10-20 fold loss
of potency, although the chloro 6 showed improved metabolic sta-
bility. Alternative lypophilic groups such as a trifluoromethyl 9 and
alkyl ethers 11 and 12 also showed reasonable activity with mod-
erate solubility, with the para position being preferred for activity
2
2
Sulfonamide 31 was prepared by reacting the corresponding
sulfonyl chloride with aminobenzothiazole intermediate 4, fol-
lowed by hydrolysis of the allylcarbonate protecting group under
(
compare para Otbutyl ether 11 to meta Otbutyl ether 13). Efforts
to add polar groups in place of the lypophilic groups described so
NH2
a
HO
N
S
NHBoc
b
N
S
AllocO
N
S
NH2
4
c,d
H
N
HO
N
S
R
O
5-21
Scheme 1. Reagents and conditions:(a) i) Boc
2
3
O, Et N, MeOH, 56%; ii) SeO
2
, dioxane then NaBH , MeOH, 78%; (b) i) allylchloroformate, DMAP, dichloromethane, ii) TFA,
4
2
dichloromethane, 71%; (c) RCO H, EDC, DMAP, dichloromethane d) 1 N NaOH, THF, (50–62% for two steps, compounds 5–21)

Y. Besidski et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6205–6211
6207
O O
S
H
N
d
H
N
N
S
O
O
H
a, b, c
H
N
H N
2
N
S
N
23
N
S
O
O
e
H
H
N
N
N
O
22
S
2
3
4
H
N
H
N
f
HO
N
S
H
N
a
O
N
S
N
S
O
O
H
O
2
5
3
Scheme 2. Reagents and conditions:(a) SeO
2
, Dioxane, 83%; (b) NH
2 3
OH.HCl, KOAc, EtOH; (c) Zn, KOAc, AcOH, 48% from steps b–c; (d) MsCl, Et N, dichloromethane, 20%; (e)
3
AcCl, Et N, dichloromethane, 75%; (f) MeMgBr, THF, 67%
Table 1
Potency of hydroxylated benzothiazole amides at human recombinant TRPV1
H
N
R
HO
N
S
O
Compound
R
hIC50
Sol
Human CLint
L/ min/mg)
Compound
R
hIC50
(nM)
Sol
M) b
Human CLint
(lL/min/ mg)
a
M)b
a
(
nM)
(
l
(
l
(
l
N
5
3.7
6
57
14
14
>10,000
>10,000
5161
Cl
Br
N
6
7
68.2
26.5
3
5
15
16
419
37
13
24
O
OEt
OtBu
489.5
8
9
175.4
11.9
25
17
18
52.2
11
61
CF3
CF3
N
CF3
417.2
13
N
N
N
O
1
0
1
>10,000
77
64
19
20
85.6
41.0
4
9
OtBu
OiPr
CF3
1
9.8
19
12
CF3
1
2
3
33.4
38
21
N
157.0
14
9
N
1
1072
181
OtBu
a
Test compound induced inhibition of the increase in intracellular calcium in response to capsaicin addition in whole cells quantified in a Molecular Devices FLIPR IITM
instrument imaging assay.
b
Solubility of test compound in 0.1 M sodium phosphate buffer.
basic conditions (Scheme 4). Synthesis of reverse sulfonamide
the corresponding sulfonyl chloride with sodium sulfite and
analog 32 required diazotization of aniline 4 and conversion to
copper sulfite in hydrochloric acid. Analog 33 was synthesized in

6
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Y. Besidski et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6205–6211
Table 2
Potency of derivatives with hydroxy replacements at human recombinant TRPV1
H
N
N
R
S
O
Compound
R
hIC50 (nM)
Compound
R
hIC50 (nM)
5
–CH
–CH
2
OH
NH
3.7
987
708
24
25
–CH
–CH(Me)OH
2
NHAc
2415
223
22
3
2
2
2
2
CH NHSO
2
Me
Figure 3. Matched pairs analysis for methyl to hydroxyl methyl transform on VR1 potency and aqueous solubility.
O
NH2
b
N
S
a
CN
N
S
N
S
OH
26
27
O
c
O
d
HO
N
S
OH
HO
N
S
N
H
28
29
Scheme 3. Reagents and conditions:(a) i) NaNO
2 3 4 2
, HCl, KI, acetone, water, ii) Zn(CN)2, Pd(PPh ) , dimethylformamide (b) HCl, 32% yield from (a) to (b); (c) i) SeO , dioxane, ii)
NaBH , MeOH; (d) 4-t-butylaniline, EDC, DMAP, CH
4
2
Cl , 22% from (c) to (d)
2
a similar fashion. Amine 34 was furnished by reductive amination
of aniline 4 with 4-tert-butylbenzaldehyde followed by deprotec-
tion of the allylcarbonate protecting group.
Representative data is shown in Table 3 and demonstrates that
replacement of the central amide with reverse amide, sulfonamide
or reverse sulfomanide groups resulted in a dramatic decrease in

Y. Besidski et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6205–6211
6209
Br
Br
H
H
N
AllocO
N
S
NH2
a
AllocO
N
S
N
b
HO
N
S
S
O
S
O
O O
4
31
e
c
F
F
F
O
O
H
N
SO Cl
2
HO
N
S
HO
N
S
d
HO
N
S
S
N
H
32
34
Scheme 4. Reagents and conditions:(a) ArSO
2
Cl, NaHCO
3
, THF; (b) NaOH, THF, 78% yield from (a) to (b); (c) i) HCl, NaNO
2
, water, ii) Na
2
SO
3
, CuSO
3
, HCl, water; (d) 4-
trifluoromethylaniline, NaHCO , THF, 13% yield from (c) to (d). (e) i) 4-tert-butylbenzaldehyde, acetic acid:water 1:1, NaBH(OAc) , ii) NaOH, THF, 15%.
3
3
activity. By comparing amide 7 with sulfonamide 31, amide 9 with
reverse sulfonamide 32, amide 5 with reverse amide 29 and amide
We postulated that steric hinderence ortho to the amide group
would decrease amide hydrolysis and hence decrease the clearance
in vitro in rat hepatocytes, resulting in a favourable in vivo pharm-
okinetic profile. An ortho-methyl group on the benzamide showed
a successful reduction in the measured clearance in rat hepato-
cytes as shown in Table 4. Compare the pairs of compounds 6
and 35, 12 and 36 and 7 and 37 Furthermore, potency at the target
was generally unaffected as was aqueous solubility. The resulting
improvements in compound stability yielded compounds 35, 36
and 37 with pharmacokinetic profiles suitable for exploration of
compound effects in rodent inflammatory pain models. Of these
three compounds, analog 37 was evaluated in two rodent models
of pain.
5
compared to aniline 34 it is clear that the original amide group is
preferred for TRPV1 activity, possibly due to the defined direction
of the hydrogen and carbonyl, that is, optimal for activity at the ion
channel. Exploration of extended lypophilic groups, as highlighted
by benzylic substituted 33 and phenethyl substituted amide 30 did
show some indication of activity but physiochemical properties
were suboptimal and it was felt that these modifications would
not lead to productive areas for further exploration.
From work described so far, the hydroxyl methyl benzamide ser-
ies was the most promising series for evaluation in rodent models of
pain. However, despite advances in improving the solubility and
metabolic stability of the series when evaluated in human and rat
microsomes, rat in vivo clearance of these analogs was generally
higher than expected as well as the measured clearance in vitro in
rat hepatocytes (data not shown). Improvements in rat hepatic sta-
bility were required to generate compounds with an appropriate
pharmokinetic profile to test in rodent models of pain. Biotransfor-
mation studies indicated that in vitro metabolism in rat hepato-
cytes (as well as human hepatocytes) occurred through amide
Compound 37 has an affinty of 1 lM at rat VR1 when stimu-
lated by capsaicin or protons, a bioavailability of 53% in rat and
i.v. half life of 2.4 h. Evaluation in a rat capsaicin model of inflam-
1
2
matory pain at a single dose experiment of 90 lmol/kg p.o. of 37,
gave a robust 72% block of paw withdrawal latency (Fig. 4). Like-
wise, evaluation in a rat carrageenan model of inflammatory pain12
at a single dose experiment of 100 lmol/kg p.o. of 37, gave a mod-
erate but significant 35% reversal of the paw withdrawal latency.
By comparison, the COX-2 inhibitor celecoxib in the same model
also produced 35% reversal at the equivalent dose (data not
shown). Interestingly, at the same dose no effect on core body tem-
perature was observed. Previous reports have documented a rise in
core body temperature during rodent experiments as well as in
1
1
hydrolysis, with the liberated aniline acetylated (Scheme 5). Fur-
ther experimental details can be found in ref. 11 and structures of
the metabolites were confirmed by comparison with chemically
synthesized standards. We suspected that the amide hydrolysis
was in part responsible for the high clearance observed in rats
and that a strategy was required to reduce amide hydrolysis.
1
3
human clinical trials and although we cannot rule out body
Table 3
Potency of amide linker replacements at human recombinant TRPV1
HO
N
S
R
Compound
R
hIC50 (nM)
>10,000
Compound
R
hIC50 (nM)
5575
CF3
O
O
O
O
S
S
2
9
32
N
N
H
H
O
O
CF
3
3
3
0
1
1605
33
34
N
H
2871
833
N
H
Br
H
N
>10,000
H
N
S
O
O

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Y. Besidski et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6205–6211
R'
H
N
NH2
HO
H
N
HO
N
S
N
S
HO
N
S
O
O
Scheme 5. Amide hydrolysis on incubation in vitro with rat hepatocytes.
Table 4
Effect of introducing a methyl group ortho to the amide
H
HO
N
S
N
R
O
Compound
R
hIC50 (nM)
68.2
Rat hepatocytes (
66
l
L/min/106 cells)
Sol (
lM)
Cl
Cl
6
3
3
1
3
7
3
5
2
6
85.9
33.4
46.7
26.5
52.3
17
57
12
54
24
7
38
9
O
O
Br
Br
5
7
10
Figure 4. Effect of compound 37 in rat capsaicin model of inflammatory pain, effect in reversing thermal hyperalgesia in the carrageenan thermal hyperalgesia model and
effect on core body temperature
temperature effects with 37 at higher doses, we do not observe
body temperature effects at an efficacious dose.
pain with no observed body temperature effects seen at an effica-
cious dose.
Conclusion
References and notes
Starting from a benzothiazole HTS hit we have systematically
probed the key requirements for potent TRPV1 activity, while aim-
ing to improve metabolic stability and aqueous solubility of the
series. Although improvements in metabolic stability have been
realized, improvements in aqueous solubility have been modest,
highlighting a preference for TRPV1 antagonists to be reasonably
lypophilic. From our studies, the hydroxymethylbenzimidazole
series demonstrated the best overall profile with steric blocking
of the amide group essential to reduce rat in vivo clearance. Com-
pound 37 was active in a rat carrageenan model of inflammatory
1
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.
Walpole, C. S. J.; Bevan, S.; Bovermann, G.; Boelsterli, J. J.; Breckenridge, R.;
Davies, J. W.; Hughes, G. A.; James, I.; Oberer, L.; Winter, J.; Wrigglesworth, R. J.
Med. Chem. 1942, 1994, 37; (b) Walker, K. M.; Urban, L.; Medhurst, S. J.; Patel, S.;
Panesar, M.; Fox, A. J.; McIntyre, P. J. Pharmacol. Exp. Ther. 2003, 304, 56.
(a) Walpole, C. S. J.; Wrigglesworth, R.; Bevan, S.; Campbell, E. A.; Dray, A.;
James, I. F.; Perkins, M. N.; Reid, D. J.; Winter, J. J. Med. Chem. 1993, 36, 2362; (b)
Walpole, C. S. J.; Wrigglesworth, R.; Bevan, S.; Campbell, E. A.; Dray, A.; James, I.
F.; Masdin, K. J.; Perkins, M. N.; Reid, D. J.; Winter, J. J. Med. Chem. 1993, 36,
induced by the single administration of 100 ll of 1% carrageenan solution,
Sigma) injected into the subcutaneous space of the plantar aspect of the left
hind paw, in the center of the pads. All experiments were conducted 3 h after
carrageenan administration.
9
In order to assess the degree of heat hyperalgesia, the rats were placed
individually in Plexiglas boxes on the glass surface (maintained at 30 °C) of the
paw thermal stimulator system (IITC Life Science, Woodland Hills, USA, Model
390 Series 8), and allowed to acclimate for 30 min. A thermal stimulus, in the
form of a radiant heat beam was focused onto the plantar surface of the
affected paw. In each test session, rats were tested twice at approximately
5 min apart. Paw withdrawal latencies (PWLs) were calculated as the mean of
the two values. An assay cut off was set at 20 s. Induction and assessment of the
rat capsaicin model of inflammatory pain: Rats were gently placed in light
restrainers with the left hind paw protruding. The left hind paw was
immobilized and the needle was inserted intradermally (superficial layer)
and laterally on the plantar aspect of the paw proximal to the heel. Capsaicin
2
373; (c) Walpole, C. S. J.; Wrigglesworth, R.; Bevan, S.; Campbell, E. A.; Dray,
A.; James, I. F.; Masdin, K. J.; Perkins, M. N.; Reid, D. J.; Winter, J. J. Med. Chem.
993, 36, 2381; (d) Wrigglesworth, R.; Walpole, C. S. J.; Bevan, S.; Campbell, E.
A.; Dray, A.; Hughes, G. A.; James, I.; Masdin, K. J.; Winter, J. J. Med. Chem. 1996,
9, 4942; (e) Walpole, C. S. J.; Bevan, S.; Bloomfield, G.; Breckenridge, R.; James,
I. F.; Ritchie, T.; Szallasi, A.; Winter, J.; Wrigglesworth, R. J. Med. Chem. 1996, 39,
939; (f) Dray, A.; Bettaney, J.; Rueff, A.; Walpole, C.; Wrigglesworth, R. Eur. J.
Pharmacol. 1990, 181, 289.
1
3
2
1
0. (a) Besidski, Y.; Brown, W.; Johnstone, S.; Labrecque, D.; Munro, A.; Rotticci, D.;
Walpole, C.; Zemribo, R. WO 2004096784.; (b) Westaway, S. M.; Thompson, M.;
Rami, H. K.; Stemp, G.; Trouw, L. S.; Mitchell, D. J.; Seal, J. T.; Medhurst, S. J.;
Lappin, S. C.; Biggs, J.; Wright, J.; Arpino, S.; Jerman, J. C.; Cryan, J. E.; Holland,
V.; Winborn, K. Y.; Coleman, T.; Stevens, A. J.; Davis, J. B.; Gunthorpe, M. J.
Bioorg. Med. Chem. Lett. 2008, 18, 5609; (c) Duncton, M. A. J.; Ayala, L.; Kaub, C.;
Janagani, S.; Edwards, W. T.; Orike, N.; Ramamoorthy, K.; Kincaid, J.; Kelly, M.
was injected at a dose 10 lg/5 lL. Rats were placed individually in Plexiglas
boxes on the glass surface of the paw thermal stimulator system and tested
30 min for heat hyperalgesia as described above.
13. (a) Gavva, N. R. Trends in Pharm. Sci. 2008, 29, 550; (b) Tamayo, N.; Liao, H.;
Stec, M. M.; Wang, X.; Chakrabarti, P.; Retz, D.; Doherty, E. M.; Surapaneni, S.;
Tamir, R.; Bannon, A. W.; Gavva, N. R.; Norman, M. H. J. Med. Chem. 2008, 51,
2744.