Discovery of TRPV1 antagonist ABT-116

Article abstract of DOI:10.1016/j.bmcl.2010.04.047

The synthesis and SAR of a series of indazole TRPV1 antagonists leading to the discovery of 21 (ABT-116) is described. Biological studies demonstrated potent in vitro and in vivo activity for 21, as well as suitable physicochemical and pharmacokinetic pro

Full text of DOI:10.1016/j.bmcl.2010.04.047

Bioorganic & Medicinal Chemistry Letters 20 (2010) 3291–3294
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of TRPV1 antagonist ABT-116
*
Brian S. Brown , Ryan Keddy, Richard J. Perner, Stanley DiDomenico, John R. Koenig,
Tammie K. Jinkerson, Steven M. Hannick, Heath A. McDonald, Bruce R. Bianchi, Prisca Honore,
Pamela S. Puttfarcken, Robert B. Moreland , Kennan C. Marsh, Connie R. Faltynek, Chih-Hung Lee
Neuroscience Research, Global Pharmaceutical Research and Development, Abbott Laboratories, Dept R4PM, AP10-207, 100 Abbott Park Road, Abbott Park,
IL 60064-6100, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 February 2010
Revised 7 April 2010
Accepted 12 April 2010
Available online 14 April 2010
The synthesis and SAR of a series of indazole TRPV1 antagonists leading to the discovery of 21 (ABT-116)
is described. Biological studies demonstrated potent in vitro and in vivo activity for 21, as well as suitable
physicochemical and pharmacokinetic properties for advancement to clinical development for pain
management.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
TRPV1
Antagonist
Analgesic
Considerable effort has been recently devoted to the search for
antagonists of the vanilloid receptor TRPV1,1 which is the receptor
for capsaicin, the pungent component of chili peppers. This recep-
tor is also activated in response to noxious heat and low pH, defin-
Accordingly, two series of ureas were examined. In the first ser-
ies, the benzyl group of 1 was replaced by a pyridinylmethyl group,
with the goal of improving aqueous solubility with inclusion of
additional heteroatoms on this moiety. The synthesis of the re-
quired amine fragments was straightforward, employing estab-
lished displacement6 or cross-coupling chemistry7 of nicotinic
nitriles, followed by hydrogenation to the corresponding amines,
as shown in general fashion in Scheme 1. Urea formation pro-
ceeded as previously reported using 3.3a Although the replacement
of the phenyl ring of 1 by the 3-pyridinyl ring of 4 led to a 20-fold
reduction of in vitro activity (a similar reduction was seen for aza-
indans related to 25), the compound was orally active in the rat CFA
pain model. For most other examples in Table 1, potent in vitro
activity was retained, with the 6-CF3 group preferred over 6-Me
and -Ph substituents to varying degrees depending on the 2-substi-
tution (cf. 5 vs 6 or 7, and 8 vs 9). The attempt to further increase
ing it as
a
polymodal receptor of painful stimuli.2 TRPV1
modulators have therefore been broadly examined as potential
therapeutics for pain.
We have previously disclosed our medicinal chemistry studies
around the indazolyl urea TRPV1 antagonists typified by 1, which
showed potent blockade of in vitro activation of hTRPV1 in re-
sponse to capsaicin (Fig. 1).3 Compound 1 had modest activity in
the complete Freund’s adjuvant (CFA) model of inflammatory ther-
mal hyperalgesia4 in rats following intraperitoneal (IP) administra-
tion, but failed to show significant effects upon oral dosing due to
its poor pharmacokinetic properties. Further exploration of this
core structure led to the discovery of indan 2 (ABT-102),5 a more
potent antagonist which demonstrated good oral activity in several
pre-clinical in vivo pain models, and was then advanced to human
clinical studies as a potential analgesic agent. However, the low
aqueous solubility of 2 presented significant challenges in its for-
mulation. Therefore, follow-on efforts were undertaken to find
compounds which retained the potent biological activity of 2, but
possessed improved physical properties.
O
O
HN
N
H
HN
N
H
CF3
N
N
N
H
N
H
1
2
hTRPV1 IC50= 9 1 nM
hTRPV1 IC50= 4 1 nM
* Corresponding author. Tel.: +1 847 937 0750; fax: +1 847 935 5466.
E-mail addresses: brian.s.brown@abbott.com (B.S. Brown), robert.moreland@
us.astellas.com (R.B. Moreland).
CFA ED50 = 60 µmol/kg, ip
(CFA ED50 > 300 µmol/kg, po)
CFA ED50= 10 µmol/kg, po
Aq. Sol. = 0.06 µg/mL (pH 7.2)
Present address: Astellas Pharma Global Development, Inc., 144 113th St,
Pleasant Prairie, WI 53158, United States.
Figure 1. First generation TRPV1 antagonists.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.04.047

3292
B. S. Brown et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3291–3294
Cl
Cl
NR2
N
OPh
iPr
a,b
c,b
c,b
e,b
NC
NC
N
H2N
H2N
H2N
H2N
R'
R'
R'
R'
OR
N
R'
R
R'
R
d,b
d,b
H2N
N
H2N
R'
R'
O
O
N
O
R
X
HN
O
O
HN
N
H
N
N
MeO2C
R
R'
N
3
H2N
X
N
H
f, g
R'
(X = N or CH)
4-20 (X = N or CH)
Scheme 1. Preparation of urea antagonists. Reagents and conditions: (a) HNR2, EtOH, 83–98%; (b) Ra Ni, H2, NH3, MeOH, 61–100%; (c) ROH, Na or NaH, 18–86%; (d) HCCR,
Pd2dba3 CHCl3, Cul, DavePhos, Et3N, 64–94%; (e) iPr2Zn, PdCl2(dppf), THF, 62%; (f) iP2NEt, DMF; (g) NaOH, MeOH, 8–82% for two steps.
Table 1
attributable to a loss of CNS exposure, related to an increase in
the polar surface area of these compounds.8 For example, 8 and
SAR of pyridinylmethyl urea TRPV1 antagonists
O
R1
13 each have [brain]:[plasma] ratios of only 0.02. TRPV1 receptors
are found in both the peripheral and central nervous systems, with
expression in primary afferents, dorsal root ganglion (DRG) neu-
rons, and various brain regions, and the influence of receptors in
different compartments on different modalities of pain has been
demonstrated previously.9a,b Since CNS exposure is required for
broad-spectrum analgesic activity, we changed our strategy to find
less polar, more CNS-penetrant compounds which could be formu-
lated using lipid vehicles. We re-examined the benzyl urea series,
maintaining the preferred 4-CF3 substituent from the pyridinylm-
ethyl amines, and varying the lipophilic groups at R1 (Table 2).
The benzyl compounds showed potent in vitro antagonism, with
some offering improved in vivo activities relative to the corre-
sponding pyridinyl analogs. The 2-(tert-butyl)ethyl substitution
imbued 20 with the best in vitro activity of the analogs, as well
as potent in vivo activity, with a sevenfold improvement over the
pyridinyl 14 in the rat CFA model. Methyl ether 15 was not exam-
ined in vivo due to its low CNS penetration, equivalent to that of 8.
Given these promising initial results, additional characteriza-
tion of 20 was undertaken. The synthetic route was straightfor-
ward and efficient (77% over four steps), providing ready access
to material for advanced studies. In further in vivo studies in rats,
20 demonstrated good activity in the carageenan-induced thermal
hyperalgesia model and modest potency in an osteoarthritis pain
model4b (Table 3). Compound 20 also displayed a favorable phar-
HN
N
H
N
R2
N
N
H
R1
H
R2
IC50 (nM)
184
CFA ED50 (lmol/kg, po)
a
Compd
4
CF3
CF3
5
81
30
5
6
7
8
N
N
N
3.5 0.4
Me
Ph
16
14
12
3
4
1
53
37
33
O
N
CF3
9
10
11
O
N
Me
CF3
CF3
125 14
—
PhO–
9
2
1
30
—
O
18
MeO
MeO
12
13
CF3
CF3
12
6
1
1
>30
O
>100
14
CF3
5
1
100
macokinetic profile, increased CNS exposure relative to
2
a
All values are the mean SEM of at least two independent experiments run in
([brain]:[plasma] = 0.45 vs 0.32 for 2), and a sevenfold increase in
aqueous solubility, despite the solubility remaining quite low.
Increasing the lipophilicity of the series to improve CNS exposure
also led to good solubility in lipid vehicles, as shown below for
PEG-400. Generally favorable safety findings were also observed
for 20, which displayed a >80-fold therapeutic index for cardiovas-
cular safety (based on its CFA efficacious plasma level). Also, a
binding screen showed little cross-reactivity against a panel of
79 neurotransmitter receptors and ion channels (Cerep, Paris),
and no genetic toxicity was observed in the Ames test of mutage-
nicity or the micronucleus test of clastogenicity. However, 20 was
triplicate.
hydrophilicity by replacing the piperidine group with a morpho-
line led to reduced in vitro antagonist potency (5 vs 8, 6 vs 9).
Several 2-alkoxyl groups also offered potent antagonism, and the
corresponding 2-alkyl groups showed similar, or marginally better,
in vitro activity (e.g., 11 vs 12, and 13 vs 14). A further preference
for large, lipophilic groups tethered to the 2-position was observed
(12 vs 14).
Overall, however, the in vivo activities of the pyridinyl series
were weaker than the lead compound 2. This effect is partially
found to be
a potent, time-dependent inhibitor of CYP3A4

B. S. Brown et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3291–3294
3293
Table 4
Table 2
In vitro and in vivo characterization of 21
SAR of benzyl urea TRPV1 antagonists
PKc
O
R1
Assay
hTRPV1 IC50 (3
l
M NADA, nM) 15 3a
Vss
CLp
Cmax
3.4 L/kg
0.8 L/kg h
HN
N
H
hTRPV1 IC50 (pH 5.5, nM)
rTRPV1 IC50 (cap, nM)
Carageenan ED50
OA ED50
7
1a
277 106
0.14
3.1 h
58%b
lg/mL
CF3
N
25
66
l
l
mol/kg, po t1/2
mol/kg, po F
N
H
Solubility, aq
<0.010
lg/mL [brain]:[plasma] 0.32
Solubility, lipidb
>67 g/mL
l
Compd
R1
IC50 (nM)
CFA ED50 (lmol/kg, po)
a
a
All values are the mean SEM of at least two independent experiments run in
triplicate in transiently transfected HEK-293 cells.
15
16
8
8
1
3
—
MeO
Ph
b
Caprol MPGO:Capmul MCM:Acconon CC-6 (10:10:80) vehicle.
30
c
10 lmol/kg, po.
17
18
PhO–
iPr
9
9
1
1
22
14
F
19
20
6
1
58
15
midazolam oxidation at 10 lM, which no longer occurred in a
time-dependent manner. The metabolism of 21 was examined,
since the formation of 20 by in vivo demethylation would restore
the potential for drug–drug interactions. In a PK study in rats,
the primary site of metabolism was found to be the tert-butyl
group, predominantly forming the corresponding primary alcohol,
as well as the carboxylic acid oxidation products. Based on AUC,
the ratio of 20:21 was only ꢀ1:8, indicating relatively little metab-
olism by demethylation. Moreover, the rate of demethylation of 21
by human microsomes was only ꢀ30% that of rat microsomes,
which further reduced concerns of drug–drug interactions in
humans.
The favorable in vitro activity and CYP inhibition profile of 21
prompted the development of an optimized synthesis12 to provide
material in sufficient quantities for characterization in pre-clinical
efficacy and pharmacokinetic studies. Various biological studies in
rats showed 21 to possess similar activities in inflammatory and
arthritic pain models as 20, with similar CNS exposures (Table 4).
Additionally, 21 showed greater effects following chronic treat-
ment in the OA model relative to a single dose (67% effect after
4.3 0.4
a
All values are the mean SEM of at least two independent experiments run in
triplicate.
Table 3
In vitro and in vivo characterization of 20
Assay
PK
Carageenan ED50
OA ED50
7
l
l
mol/kg, po
mol/kg, po
g/mL
g/mL
Vb (iv)
CLp (iv)
t1/2 (po)
F (po)
5.4 L/kg
1.2 L/kg h
4.3 h
35%
0.45
71
0.42
>100
Solubility, aq
Solubility, PEG-400
l
l
[brain]:[plasma]
metabolism10 (86% inhibition of midazolam oxidation @ 10
lM),
which precluded any further development due to the likelihood
of drug–drug interactions.
12 days, 10
sponse). The compound also reduced pain after a single dose in a
post-operative pain model in rats (45% effect @ 100 mol/kg,
lmol/kg, po BID dosing vs 20% acute 10 lmol/kg, po re-
Previously, interactions of unsubstituted indazoles have been
described with CYP enzymes showing binding to N2, and hydrogen
bonding to the N1 hydrogen.11 Since studies of earlier indazole
TRPV1 antagonists found that methylation of N1 only slightly re-
duced antagonist activity,3b indazole alkylation was pursued to
disrupt this CYP interaction while retaining potency as a TRPV1
antagonist. Alkylation of 20 was accomplished by standard chem-
istry, providing 21, 22, and 23, as well as varying amounts of the
corresponding N2 alkylation products. The SAR of 21–23 showed
a correlation of TRPV1 antagonist activity with reduced steric bulk,
with the methyl-substituted 21 being 10-fold more potent than the
isopropyl analog 23 (Scheme 2). Most importantly, while 21 was
nearly as potent a TRPV1 antagonist as 20, the CYP3A4 inhibition
was almost completely abrogated, showing only 10% inhibition of
l
po).4b The ꢀ40-fold lower potency of 21 versus capsaicin at rat
TRPV1 than at hTRPV1 is similar to shifts observed with other
antagonists in our assays, and appears to be an effect of cell line
differences. The reversal of antagonism at hTRPV1 to agonism at
rTRPV1 has been previously reported for some compounds,13 and
could potentially produce similar in vivo pain reduction via ago-
nist-induced TRPV1 desensitization. However, no agonism was ob-
served for 21 in either species indicating antinociception via TRPV1
antagonism. Furthermore, in rat telemetry studies 21 produced
hyperthermia characteristic of TRPV1 antagonists (0.9 °C increase
in body temperature 1 h after a 100 lmol/kg oral dose). Compound
O
HN
N
H
a
CF3
20
N
N
R
21-23
R =
hTRPV1 IC50 (cap, nM)a
Me
21
Et
22
53 17
iPr
23
69 15
7
1
aAll values are the mean SEM of at least two independent experiments run in triplicate.
Scheme 2. Synthesis and activities of alkylindazoles. Reagents and conditions: (a) For 21, NaH, (MeO)2SO2, DMF, 50%; for 22, NaH, EtI, DMF, 66%; for 23, NaH, (iPrO)2SO2,
DMF, 41%.

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B. S. Brown et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3291–3294
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. Bioorg. Med. Chem. 2006, 14, 4740.
21 also potently inhibited hTRPV1 versus the putative endogenous
activators N-arachidonoyldopamine (NADA) and low pH, which
may have more relevance to native pain states. While aqueous sol-
ubility was greatly reduced relative to 20, 21 maintained very good
solubility in lipid mixtures, affording good oral bioavailability in
the vehicle described below.
Although failing to improve upon the aqueous solubility of 2, 21
did meet the overarching goal of enhanced ease of formulation
with the use of lipid-based vehicles. Additionally, 21 provided an
improved PK profile relative to 2, with predictions of reduced hu-
man clearance and extended half-life, and was therefore advanced
for clinical development as ABT-116.
4. The CFA and carageenan models evaluate inflammatory thermal hyperalgesia
following hindpaw inflammation, and the OA model measures grip strength
following monoiodoacetate-induced knee pain. For the CFA protocol, see: (a)
Honore, P.; Chandran, P.; Hernandez, G.; Gauvin, D. M.; Mikusa, J. P.; Zhong, C.;
Joshi, S. K.; Ghilardi, J. R.; Sevcik, M. A.; Fryer, J. M.; Segreti, J. A.; Banfor, P. N.;
Marsh, K.; Neelands, T.; Erol Bayburt, E.; Daanen, J. F.; Gomtsyan, A.; Lee, C.-H.;
Kort, M. E.; Reilly, R. M.; Surowy, C. S.; Kym, P. R.; Mantyh, P. W.; Sullivan, J. P.;
Jarvis, M. F.; Faltynek, C. R. Pain 2009, 142, 27; For carageenan and OA
protocols, see: (b) Hsieh, G. C.; Chandran, P.; Salyers, A. K.; Pai, M.; Zhu, C. Z.;
Wensink, E. J.; Witte, D. G.; Miller, T. R.; Mikusa, J. P.; Baker, S. J.; Wetter, J. M.;
Marsh, K. C.; Hancock, A. A.; Cowart, M. D.; Esbenshade, T. A.; Brioni, J. D.;
Honore, P. Pharmacol. Biochem. Behav. 2010, 95, 41.
5. Gomtsyan, A.; Bayburt, E. K.; Schmidt, R. G.; Surowy, C. S.; Honore, P.; Marsh, K.
C.; Hannick, S. M.; McDonald, H. A.; Wetter, J. M.; Sullivan, J. P.; Jarvis, M. F.;
Faltynek, C. R.; Lee, C.-H. J. Med. Chem. 2008, 51, 392.
6. Carroll, W. A.; Perez-Medrano, A.; Florjancic, A. S.; Nelson, D. W.; Peddi, S.;
Bunnelle, E. M.; Hirst, G. PCT application WO2005111003, 2005.
7. Lee, C.-H.; Brown, B. S.; Perner, R. J., Darbyshire, J. PCT application
WO2008024945, 2008.
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