Discovery of SB-705498: A potent, selective and orally bioavailable TRPV1 antagonist suitable for clinical development

Article abstract of DOI:10.1016/j.bmcl.2006.03.030

Small molecule antagonists of the vanilloid receptor TRPV1 (also known as VR1) are disclosed. Pyrrolidinyl ureas such as 8 and 15 (SB-705498) emerged as lead compounds following optimisation of the previously described urea SB-452533. Pharmacological studies using electrophysiological and FLIPR-Ca²⁺-based assays showed that compounds such as 8 and 15 were potent antagonists versus the multiple chemical and physical modes of TRPV1 activation (namely capsaicin, acid and noxious heat). Furthermore, 15 possessed suitable developability properties to enable progression of this compound into in vivo studies and subsequently clinical development.

Full text of DOI:10.1016/j.bmcl.2006.03.030

Bioorganic & Medicinal Chemistry Letters 16 (2006) 3287–3291
Discovery of SB-705498: A potent, selective and orally bioavailable
TRPV1 antagonist suitable for clinical development
Harshad K. Rami,a,* Mervyn Thompson,a Geoffrey Stemp,a Steve Fell,a Jeffrey C. Jerman,b
Alexander J. Stevens,a Darren Smart,a Becky Sargent,a Dominic Sanderson,c
Andrew D. Randall,a Martin J. Gunthorpea and John B. Davisa
aNeurology and GI CEDD, New Frontiers Science Park, GlaxoSmithKline, Third Avenue, Harlow, Essex CM19 5AW, UK
bDiscovery Research, New Frontiers Science Park, GlaxoSmithKline, Third Avenue, Harlow, Essex CM19 5AW, UK
cPharmaceutical Development, New Frontiers Science Park, GlaxoSmithKline, Third Avenue, Harlow, Essex CM19 5AW, UK
Received 9 February 2006; revised 9 March 2006; accepted 10 March 2006
Available online 31 March 2006
Abstract—Small molecule antagonists of the vanilloid receptor TRPV1 (also known as VR1) are disclosed. Pyrrolidinyl ureas such
as 8 and 15 (SB-705498) emerged as lead compounds following optimisation of the previously described urea SB-452533. Pharma-
cological studies using electrophysiological and FLIPR-Ca2+-based assays showed that compounds such as 8 and 15 were potent
antagonists versus the multiple chemical and physical modes of TRPV1 activation (namely capsaicin, acid and noxious heat).
Furthermore, 15 possessed suitable developability properties to enable progression of this compound into in vivo studies and
subsequently clinical development.
Ó 2006 Elsevier Ltd. All rights reserved.
The vanilloid receptor1 (TRPV1, previously referred to
as VR1) is a non-selective cation channel, predominant-
ly expressed by neurons of the peripheral nervous sys-
tem, which plays a key role in the detection of noxious
painful stimuli such as acid (low pH), noxious heat
and vanilloids such as capsaicin (the pungent compo-
nent of hot chili peppers).
thought to induce analgesia, via the desensitisation or
elimination of TRPV1-bearing nerve terminals. The ini-
tial TRPV1-receptor activation, however, often leads to
pain and extreme discomfort. In addition, such therapy
may also be associated with systemic side-effects. The
delayed onset of action coupled with an appreciation
of these side-effects has hindered the development of sys-
temically bioavailable TRPV1 agonists.
Based on knowledge gleaned from the biological effects
of capsaicin,2 the study of TRPV1 null mice and recent
understanding of the role of TRPV1 receptors in noci-
ceptive pathways and models of disease, it is increasingly
clear that TRPV1 represents an exciting novel target for
the treatment of chronic pain and a range of other
disorders.3
Indeed, more recent efforts have focused on small mole-
cule TRPV1 antagonists since these agents may be
expected to have a faster onset of action and should
avoid the side-effects associated with TRPV1 agonist-
based therapies. Furthermore, because of its different
mode of action, a TRPV1 antagonist may offer an im-
proved safety profile in comparison with current estab-
lished therapy. The appearance of many new TRPV1
antagonists in the literature means that these predictions
may soon be evaluated in the clinic.3
Early efforts at manipulation of the TRPV1 receptor for
the treatment of pain centred on the use of agonists such
as capsaicin and resiniferatoxin. Both of these agents are
Keywords: SB-705498; TRPV1; TRPV1 antagonist; VR1; Pain;
SB-452533; SB-366791; Electrophysiology; Capsaicin; Noxious heat;
FCA.
Br
O
H
H
N
N
N
Me
OMe
N
H
O
Et
*
Corresponding author. Tel.: +44 0 1279 644202; fax: +44 0 1279
875246; e-mail: Harshad.Rami@gsk.com
Cl
SB-366791
SB-452533
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.03.030

3288
H. K. Rami et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3287–3291
Br
Br
H
N
H
N
R
H
N
H
N
N
N
Me
O
( )n
Et
O
SB-452533
1-6
R
NaBH3CN
AcOH, 18h
Br
Br(CH2)2NH2.HBr
R
N
H
Toluene, 100oC, 18h
N
H
NCO
Br
H
N
H
N
N
R
R
N
DCM
O
NH2
Figure 1. Cyclisation of N-ethyl group and generic synthetic scheme.
We recently reported4 that SB-366791, a novel, potent
and selective competitive antagonist of human and rat
TRPV1 represented a useful tool compound for the
study of TRPV1 function. Unfortunately, the poor
pharmacokinetic properties of this compound, like the
prototypical TRPV1 antagonist capsazepine before it,
made it unsuitable for further progression. More recent-
ly we described5 the synthesis and SAR around the urea
SB-452533, a compound identified by high throughput
screening (HTS). SB-452533 showed good potency
against capsaicin, noxious heat and low pH mediated
activation of TRPV1 but it too was deemed unsuitable
for progression due to its high intrinsic clearance deter-
mined in vitro in both rat and human liver microsomes
(intrinsic clearance rates in liver microsomes: rat > 50
and human 41 mL/min/g liver).
ately substituted indole using sodium cyanoborohy-
dride, alkylation with 2-bromoethylamine followed by
addition of 2-bromophenyl isocyanate to afford com-
pounds 1–5 as previously published6 (see Fig. 1). The
parent dihydroindoline compound 1 was 10-fold less po-
tent in comparison to SB-452533. Encouragingly, intro-
duction of either a methyl or fluoro substituent
increased potency with the 4-position showing optimum
potency but still lower when compared with SB-452533.
The tetrahydroquinoline homologue derivative6 6 also
showed good potency, suggesting minimal overall
conformational change and therefore tolerated.
However, despite the encouraging potency achieved
with both the dihydroindole derivatives 1–5 and the
homologue tetrahydroquinoline 6, there was no
improvement in their overall in vitro PK profile (e.g.,
for 5 and 6 rat CLi > 50 mL/min/g liver and human 19
and > 45 mL/min/g liver, respectively).
In this communication, we report our continuing efforts
to identify selective TRPV1 antagonists with improved
pharmacokinetic properties in order to: (i) enable the
assessment of the safety and efficacy of such molecules
in vivo and (ii) identify a molecule suitable for clinical
development.
Our attention next turned to alternative cyclisation of
the N-ethyl group onto the ethylene linker, leading to
the pyrrolidinyl urea derivatives. Preparation of
analogues 7–11 was readily achieved as outlined in generic
Figure 2. Palladium-assisted coupling of an appropriately
substituted bromobenzene with the 3-amino Boc-
protected pyrrolidine derivative afforded the 3-amino
Boc-protected N-arylpyrrolidine. Deprotection under
acidic conditions followed by treatment with 2-brom-
ophenyl isocyanate afforded analogues 7–11 in good
overall yield.7 Preparation of the analogues 12–20 was
readily achieved as outlined in Figure 3 for synthesis of 15.
Since the high clearance associated with SB-452533 ser-
ies could be due to potential dealkylation of the N-ethyl
group, our initial attempt to improve the pharmacoki-
netic properties focused on cyclisation of the N-ethyl
group onto the right-hand side (RHS) phenyl ring
affording 2,3-dihydroindoline derivatives (Fig. 1).
The synthesis of the dihydroindole urea derivatives
(Table 1) was readily achieved by reduction of appropri-
In this pyrrolidine series, the stereochemical require-
ment(s) for activity was initially investigated (Table 2).
With the 3-Me phenyl RHS both the R and S enantio-
mers were prepared. The R enantiomer (8) clearly
showed at least 10-fold greater activity in comparison
with the S enantiomer (7) and demonstrated chiral dis-
crimination at the receptor site. Encouragingly, the
SAR determined from the SB-452533 series could be ap-
plied to the pyrrolidinyl ureas. Thus, the 3-methyl (8)
and 3-fluoro (9) showed equivalent potency to SB-
452533 whilst the disubstituted analogues 10 and 11
exhibited slightly higher potency. Results from rat and
human liver microsomes for analogues 8–11 showed
Table 1. Antagonist activity (FLIPR) versus capsaicin at TRPV1 for
compounds 1–6
Compound
n
R
pKb
1
1
H
6.8
7.4
7.3
7.4
7.1
7.4
7.8
2
1
4-Me
5-Me
4-F
3
1
4
1
5
1
5-F
6-Me
—
6
SB-452533
2
—

H. K. Rami et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3287–3291
3289
Br
Br
H
H
N
R2
H
H
N
N
Remove liability
N
N
N
X
for N-dealkylation
R2
O
X
Et
O
Poor in vitro PK profile
Good in vitro metabolic stability?
7-20
Br
NHCO2tBu
R2
Cs2CO3, Pd(OAc)2
R2
N
+
tBuO2CNH
BINAP, 1-4-Dioxane
100oC, 18h
N
H
Br
Br
NCO
H
N
H
N
R2
TFA, DCM
40oC, 18h
N
N
H2N
O
DCM
R2
Figure 2. Design of pyrrolidine analogues and generic synthetic scheme for analogues 7–11.
CF3
CF3
K2CO3, DMF
BocNH
NH
+
1000C, 7h, 83%
N
N
BocNH
Cl
N
23
21
22
1) 4M HCl, dioxan (1:1)
800C, 2h
2) basify (pH12), DCM
CF3
CF3
H
N
N
N
(2-Br)PhNCO
Et2O, 84%
N
N
H
H2N
Br
N
O
24
15
Figure 3. Synthesis of compound 15.
Table 2. Antagonist activity (FLIPR) versus capsaicin at TRPV1 for
pyrrolidine urea analogues 7–20
8–11 possess lipophilic left-hand side (LHS) and RHS
and, not surprisingly showed poor aqueous solubility
(<0.1 mg/mL); an improvement in the solubility profile
was desired. The LHS SAR demonstrated by the SB-
452533 series precluded introduction of polar substitu-
ents such as amino groups since this was found to lower
activity and so modification of the properties of the
RHS aryl group was investigated.
Compound
Stereochemistry
X
R2
pKb
7
8
S
R
R
R
R
R
R
R
R
R
R
R
R
R
C
C
C
C
C
N
N
N
N
N
N
N
N
N
3-Me
3-Me
3-F
<6.8
7.7
9
7.8
8.1
10
11
12
13
14
15
16
17
18
19
20
3,4-DiF
3-F-4-Me
H
8.3
<6.6
7.1
3-CF3
4-CF3
5-CF3
6-CF3
5-Br
Evaluation of the parent 2-pyridyl compound 12
(Table 2) indicated poor activity at TRPV1 but intro-
duction of a trifluoromethyl group in the 6-position
(equivalent to 3-Me position when X = CH, e.g., as in
8) showed an encouraging level of activity (analogue
16). Further investigation of the isomeric trifluorometh-
yl analogues showed that the 5-pyridyl position was
optimum (compound 15) as the alternative isomers (13
and 14) showed lower potency. Having established the
5-pyridyl position as optimum for substitution, alterna-
tive substituents were investigated. Neither the 5-bromo
(17) nor the 5-chloro (18) showed an increase in potency
compared to 15 and replacement by 5-cyano (19) and
5-methyl (20) attenuated activity.
6.9
7.5
7.2
7.2
5-Cl
5-CN
5-Me
7.3
6.7
<6.7
that the intrinsic clearance rates were now much lower
than for previous compounds (611 and 64 mL/min/g
liver in rat and human, respectively) and, hence, the
potential for assessing key exemplars of the series in vivo
could be usefully explored. However, all the analogues

3290
H. K. Rami et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3287–3291
Table 3. Antagonist activity (FLIPR) versus capsaicin at rat and
guinea pig TRPV1 receptor and pharmacokinetic profile for 15
starting materials (21 and 22) the pyridyl intermediate
23 was prepared in high yields (K2CO3, DMF, 100 °C,
7 h, 83%). Deprotection of 23 under acidic conditions
followed by basification afforded the primary amine 24
which was used without purification in the next step.
Addition of 24 to an ethereal solution of 2-bromophenyl
isocyanate furnished 15 in an overall yield of 62% with-
out the need for chromatography.
Parameter
Rat
Guinea pig
Dog
pKb
7.5
7.3
4.2
34
nd
0.9
11
CLi (mL/min/g liver)
CLb (mL/min/kg)
Vss (L/kg)
<0.5
4
0.9
3.1
86a
3.6
2.6
39b
1.9
3.4
42a
t1/2 (h)
Fpo (%)
a At 3 mg/kg dose.
b At 1 mg/kg dose.
nd, not determined.
Compound 15 showed good antagonist potency in
FLIPR assays against both the rat and guinea pig
TRPV1. Furthermore, the low intrinsic clearance trans-
lated to low in vivo clearance in the rat, dog and mod-
erate in the guinea pig with good bioavailability and
an acceptable volume of distribution and half-life across
species (Table 3).
The selectivity of 15 was defined by broad receptor pro-
filing (CEREP) and other in vitro assays where it
showed little or no activity versus a wide range of ion
channels, receptors and enzymes.8 Furthermore, 15
showed improved solubility (0.4 mg/mL in simulated
gastric fluid) and therefore, emerged as the optimized
analogue from the pyrrolidine urea series. Compound
15 (pKb 7.5 0.2), SB-705498, was further investigated
and a full in vitro and in vivo package generated
(Table 3).
Compound 15 was also investigated in electrophysiology
experiments using the whole cell patch clamp method, as
described previously.4,5 The data show that 15 is a rapid,
potent and reversible inhibitor of capsaicin-mediated
activation of human TRPV1 (Fig. 4).
Further aspects of the pharmacological properties
namely effects of pyrrolidinyl ureas on the alternative
modes of activation of TRPV1 were also investigated.
Compound 8 showed full blockade of heat activation
of TRPV1 and this was fully reversible following drug
washout (Fig. 5). Compound 15 also produced full
blockade of heat (97 3% at 1 lM) as well as pH acti-
vation (104 3% at 1 lM, n = 5) of hTRPV1.
The synthesis of compound 15 as detailed above (Fig. 3)
exemplifies a readily accessible route to pyrrolidinyl ure-
as and was applied to preparation of analogues7 high-
lighted in Table 2. From commercially available
Overall, these results show that these pyrrolidinyl ureas
such as 8 and 15 exhibit potent and reversible blockade
against the multiple modes of TRPV1 activation, name-
ly the vanilloid (capsaicin), heat- and acid-mediated acti-
vation of the receptor (full data to be reported
elsewhere).
Having initially demonstrated that 15 showed good
overall in vitro efficacy and oral bioavailability in rat,
the in vivo activity of 15 was investigated in the capsai-
cin-induced secondary hyperalgesia model9 in the rat
(Fig. 6). This model demonstrates the compound’s
antagonist activity in vivo against the specific TRPV1
agonist capsaicin.
Figure 4. Inhibition of the capsaicin-mediated activation of human
TRPV1 by 15. Whole cell currents were recorded from
HEK293 Æ TRPV1 cells using the patch clamp technique. TRPV1-
mediated responses to 1 lM capsaicin (black bar) were completely and
reversibly inhibited by co-application of 1 lM 15 (open bar; 102 1%
inhibition, n = 5).
A
B
Heat (50˚C)
100
80
60
40
20
0
+1 μM
Compound 8
Wash
Control
Control
Wash
+1μM
Compound 8
200 pA
500 ms
Figure 5. Inhibition of human TRPV1 receptors by 8. Whole cell currents were recorded from HEK293 Æ TRPV1 cells using the patch clamp
technique. TRPV1-mediated responses to heat were completely and reversibly inhibited by co-application of 1 lM 8.

H. K. Rami et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3287–3291
3291
icin, heat and low pH). Compound 1510 (SB-705498) is
currently undergoing clinical trials and the outcome,
as well as full in vivo biology details, will be reported
elsewhere.
1
8
6
4
2
0
Vehicle po
Vehicle po ipsilateral
SB705498 3 mg/kg po
SB705498 10 mg/kg
SB705498 30 mg/kg
References and notes
1. (a) Caterina, M. J.; Schumacher, M. A.; Tominaga, M.;
Rosen, T. A.; Levine, J. D.; Julius, D. Nature 1997, 389,
816; (b) Hayes, P.; Meadows, H. J.; Gunthorpe Harries,
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Terrett, J.; Jenkins, O.; Benham, C. D.; Randall, A. D.;
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Gunthorpe, M. J.; Benham, C. D.; Randall, A.; Davis,
J. B. Trends Pharmacol. 2002, 23, 183.
1.4
2
4
6
8
1
1
2
6
Von Frey Hair (g)
Figure 6. Activity of compound 15 (SB-705498) in the capsaicin
induced secondary hyperalgesia model in the rat (3–30 mg/kg po at 2 h
pre-treatment time).
2. (a) Szallasi, A.; Blumberg, P. M. Pharmacol. Rev. 1999, 51,
159; (b) Caterina, M. J.; Leffler, A.; Malmberg, A. B.;
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Davey, P. T.; Overend, P.; Harries, M. H.; Latcham, J.;
Clapham, C.; Atkinson, K.; Hughes, S. A.; Rance, K.;
Grau, E.; Harper, A. J.; Pugh, P. L.; Rogers, D. C.;
Bingham, S.; Randall, A.; Sheardown, S. A. Nature 2000,
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Br
H
N
H
N
SB-452533 pKb 7.5
Me
N
Et
O
Remove liability for N-dealkylation
Br
H
H
N
N
N
3. Rami, H. K.; Gunthorpe, M. J. Drug Discovery Today:
Therapeutic Strategies 2004, 1, 197.
8
pKb 7.7
O
Me
4. Gunthorpe, M. J.; Rami, H. K.; Jerman, J. C.; Smart, D.;
Gill, C. H.; Soffin, E. M.; Luis Hannan, S.; Lappin, S. C.;
Egerton, J.; Smith, G.; Worby, A.; Howett, L.; Owen, D.;
Nasir, S.; Davies, C. H.; Thompson, M.; Wyman, P. A.;
Randall, A. D.; Davis, J. B. Neuropharmacology 2004, 46,
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5. 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.
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Improve solubility and PK
Br
H
N
H
N
CF3
N
15 pKb 7.5
O
N
Figure 7. Design of pyrrolidine analogues.
6. Rami, H. K.; Thompson, M. W.O. Patent 03053945, 2003.;
7. (a) Rami, H. K.; Thompson, M.; Wyman, P. A. W.O.
Patent 0290326, 2002; (b) Rami H. K.; Thompson M.;
Wyman, P. A. W.O. Patent 03022809, 2003.
As an early indicator of potential pharmacodynamic
activity (consistent with activity at the target receptor),
15 showed excellent activity at 10 and 30 mg/kg po with
good reversal of allodynia. Compound 8 also showed a
similar level of activity at the equivalent dose (data not
presented). Furthermore, compound 15 was also shown
to give 80% reversal of allodynia in the guinea pig FCA
model at 10 mg/kg po (full data to be reported else-
where). This latter result suggests the potential for a
TRPV1 antagonist in the treatment of inflammatory
pain.
8. Gunthorpe, M. J.; Hannan, S. L.; Jerman, J. C.; Egerton,
J.; Smart, D.; Rami, H. K.; Thompson, M.; Randall, A.
D.; Davis, J. B. SB-705498: A novel potent and selective
TRPV1 antagonist which inhibits the capsaicin-, acid- and
heat-mediated activation of the receptor, Programe No.
36.7, 2005 Abstract Viewer/Itinerary Planner, Washing-
ton, DC: Society for Neurosciences, 2005, online.
9. Bingham, S.; Beswick, P. J.; Bountra, C.; Brown, T.;
Campbell, I. B.; Chessell, I. P.; Clayton, N.; Collins, S. D.;
Davey, P. T.; Goodland, H.; Gray, N.; Haslam, C.;
Hatcher, J. P.; Hunter, A. J.; Lucas, F.; Murkitt, G.;
Naylor, A.; Pickup, E.; Sargent, B.; Summerfield, S. G.;
Stevens, A.; Stratton, S. C.; Wiseman, J. J. Pharmacol.
Exp. Ther. 2005, 312, 1161.
In conclusion, we have shown that modification of the
HTS hit SB-452533 led to 8 that showed improved in vi-
tro metabolic stability. Several cycles of lead optimisa-
tion chemistry, as summarized in Figure 7, detail the
process of identification of our cyclised pyrrolidine ure-
as. Further SAR work identified 15 that exhibited an
excellent in vitro and in vivo PK profile together with
highly encouraging activity in preclinical models of pain
(Fig. 6) and activities across different modalities (capsa-
10. Characterisation data for 15 (SB-705498): 1H NMR
(400 MHz, DMSO-d6) d (ppm): 8.41 (1H, s), 8.12 (1H. dd,
J 8.3, 1.4 Hz), 7.76–7.79 (2H, m), 7.55 (1H, dd, J 8.0,
1.4 Hz), 7.47 (1H, d, J 6.7 Hz), 7.28 (1H, t, J 8.5 Hz), 6.89
(1H, t, J 7.8 Hz), 6.62 (1H, d, J 9.0 Hz), 4.34–4.36 (1H, m),
3.67–3.71 (1H, m), 3.56 (2H, br s), 3.31–3.41 (1H, m), 2.19–
2.28 (1H, m) and 1.94–1.98 (1H, m). MH+ = 429, 431.