Identification and optimization of novel 1,3,4-oxadiazole EP1 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2007.06.014

A novel series of oxadiazole EP₁ receptor antagonists was identified by replacing the amide of a known glycine sulfonamide derivative with a 1,3,4-oxadiazole. Optimization of the substitution patterns on the three aromatic rings led to the iden

Full text of DOI:10.1016/j.bmcl.2007.06.014

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4450–4455
Identification and optimization of novel 1,3,4-oxadiazole
EP1 receptor antagonists
Adrian Hall,a,* Susan H. Brown,a Anita Chowdhury,a Gerard M. P. Giblin,a
Mairi Gibson,a Mark P. Healy,a David G. Livermore,a Richard J. McArthur Wilson,b
Alan Naylor,a D. Anthony Rawlings,a Shilina Roman,a
Emma Wardb and Caroline Willaya
aNeurology and Gastrointestinal Centre of Excellence for Drug Discovery, GlaxoSmithKline, New Frontiers Science Park,
Third Avenue, Harlow, Essex CM19 5AW, UK
bDepartment of Screening and Compound Profiling, GlaxoSmithKline, Third Avenue, Harlow, Essex CM19 5AW, UK
Received 2 May 2007; revised 31 May 2007; accepted 2 June 2007
Available online 8 June 2007
Abstract—A novel series of oxadiazole EP1 receptor antagonists was identified by replacing the amide of a known glycine sulfon-
amide derivative with a 1,3,4-oxadiazole. Optimization of the substitution patterns on the three aromatic rings led to the identifi-
cation of high affinity EP1 receptor antagonists. The derivative with highest affinity displayed a binding IC50 of 2.5 nM (pIC50 8.6).
Ó 2007 Elsevier Ltd. All rights reserved.
The EP1 receptor is one of four receptor subtypes that is
activated by prostaglandin E2 (PGE2).1 Preclinical evi-
dence has shown that EP1 receptor antagonism could
provide a potential treatment for several conditions such
as inflammatory pain,2 neuropathic pain,3 postoperative
pain,4 over active bladder5 and ischaemic stroke.6
we wished to replace the amide of this series and estab-
lish whether the metabolically labile alkyl groups on the
A- and B-rings could be replaced by halogens or trifluo-
romethyl groups.
Herein, we describe our work within this series, where
we have replaced the amide in compounds of type 1a–
c with an oxadiazole. This change resulted in a novel ser-
ies of EP1 antagonists which we hoped would have bet-
ter metabolic stability and better developability
characteristics than the amides typified by 1a–c.
We have previously reported some of our efforts to iden-
tify novel EP1 receptor antagonists.2,7–11 To this end, we
recently disclosed a new series of glycine sulfonamides,12
exemplified by 1a (Fig. 1), which was identified as part
of a high throughput screen (HTS). Optimization of this
series gave compounds such as 1b and 1c (Fig. 1).12
The first step in this project was to understand the role
of the amide in compounds such as 1b. Thus, in order
to investigate if the amide was an important pharmaco-
phore we synthesized the ester 1d (Table 1). Based on the
modest decrease in affinity of 1d relative to 1b we
hypothesized that the amide and ester were acting as
hydrogen bond acceptors (HBA). The decreased affinity
of 1d could be explained by the weaker hydrogen bond
acceptor ability of the ester moiety relative to the
amide.13
Compounds from the glycine sulfonamide series gener-
ally suffered from poor metabolic stability (when as-
sessed in rat and human liver microsomes in vitro).
Incubation of compounds from this series with human
liver microsomes allowed qualitative metabolite identifi-
cation by mass spectrometry. The major routes of
metabolism were oxidation of the alkyl groups on the
A- and B-rings and dealkylation of the amide. Thus,
Thus, we sought to replace the amide of 1b (or the ester
of 1d) with a heterocycle with HBA potential such as the
1,3,4-oxadiazole. We felt that the vicinal arrangement of
the lone pairs of electrons on the nitrogen atoms would
Keywords: EP1 antagonist; Amide isostere; Oxadiazole.
*
Corresponding
adrian.2.hall@gsk.com
author.
Tel.:
+44
1279
643464; e-mail:
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.06.014

A. Hall et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4450–4455
4451
O
O
S
O
O
O
O
O
H
N
H
N
C
H
N
A
A
S
A
S
O
O
N
N
N
C
C
O
O
O
Cl
B
B
B
1a
1b
1c
EP1 binding pIC50 6.6 (0.2)
EP1 binding pIC50 7.6 (0.1)
EP1 binding pIC50 8.1 (0.3)
Figure 1. Profile of HTS hit 1a and optimized derivatives 1b and 1c, with standard deviations in parentheses.
Table 1. Binding data for amide 1b and corresponding ester 1d
Table 2. SAR for substitution of the C-ring
A
A
X
C
O
O
S
O
O
S
L
N
B
N
B
O
N
N
O
C
X
a
a
FLIPR pKia
Compound
L
X
Binding pIC50
Compound
X
Binding pIC50
1b
1d
NH
O
OMe
OMe
7.6 0.1
7.2 0.2
2a
2b
2c
2d
H
Cl
6.0 0.1
6.4 0.1
6.9 0.1
6.9 0.1
6.4b
6.5 0.2c
6.2 0.4
7.4 0.1
OMe
OEt
a Values are means of three experiments.
a Values are means of at least three experiments.
b Single experiment.
c Mean of two experiments.
mimic the lone pairs of electrons on the carbonyl oxygen
atom14 whilst improving metabolic stability.
Before undertaking the synthesis of the oxadiazole
derivatives we carried out molecular overlays with the
minimized structures of 1c and an oxadiazole analogue
(2c), using FLO,15 Figure 2. This indicated a good over-
lap of the three phenyl rings (A, B, and C), including
pendant functionality, and the sulfonamide moieties.
Furthermore, the carbonyl group of the amide was very
close to the nitrogen atoms of the oxadiazole, implying
that a similar HBA interaction could possibly be formed
by the oxadiazole.
Thus, in the first round of synthesis we fixed the tosyl
group (ring A) and the ortho-ethyl aniline (ring B),
found in compounds 1b and 1c, and investigated substi-
tution of the right hand side phenyl ring (ring C), (Table
2) utilising functionality that had proven beneficial for
activity within the glycine sulfonamide series.12 As ex-
pected, the 4-chloro, 4-methoxy and 4-ethoxy deriva-
tives 2b–d showed higher activity than the
unsubstituted derivative 2a (Table 2). Pleasingly, ana-
logue 2c (binding pIC50 6.9, IC50 126 nM) displayed a
5-fold decrease in affinity (binding assay) relative to
the amide 1b (binding pIC50 7.6, IC50 25 nM) and only
a 2-fold decrease in affinity relative to the ester 1d (bind-
ing pIC50 7.2, IC50 63 nM).
Based on these observations we initiated the synthesis of
oxadiazole analogues.
Compound affinities were determined using a [3H]-PGE2
binding assay at the human EP1 receptor recombinantly
expressed in CHO cell membranes.7 Compounds were
also profiled in a functional assay using the recombinant
EP1 receptor in CHO cells to measure their ability to
block the intracellular Ca2+ mobilization driven by
PGE2 (Ca2+ mobilization measured by FLIPR).7
Replacement of the ester/amide by 1,2,4-oxadiazoles
was also briefly investigated. Neither of the regioisomer-
ic 1,2,4-oxadiazole analogues of 2b gave an advantage
over the parent (2b) in terms of binding affinity (results
not shown), hence we focused on the 1,3,4-oxadiazoles.
Encouraged by the results in Table 2, we fixed a 4-alk-
oxy substituent on the C-ring and investigated optimiza-
tion of the substituents on the aniline moiety (B-ring)
(Table 3).
These results demonstrated a marked improvement in
activity, as measured by binding affinity and functional
antagonism, was possible by adding a second substitu-
ent in the 3-position of ring B. A chlorine atom in the
3-position proved optimal for affinity, whereas in the
2-position a methyl group or chlorine atom resulted in
equal activity (Table 3).
Figure 2. Molecular overlay of amide 1c with oxadiazole 2c.

4452
A. Hall et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4450–4455
Table 5. SAR for replacement of the 4-methyl group on the A-ring
Table 3. Initial exploration of substitution of the aniline moiety,
B-ring
X
A
A
O
O
O
S
N
B
O
S
O
N
N
N
Cl
O
C
N
N
C
2
Y
B
4
X
Cl
Y
3
a
Compound
X
Y
Binding pIC50
FLIPR pKia
a
Compound
X
Y
Binding pIC50 FLIPR pKia
3d
5a
3e
5b
Me
Cl
OMe
OMe
OEt
8.1 0.2
7.7 0.1
8.2 0.1
8.6 0.1
7.4 0.2
7.2 0.1b
7.5 0.2
8.2 0.5
3a
3b
3c
3d
3e
2-Me, 3-CF3 OMe 7.2 0.2
6.4 0.1
7.4 0.2
7.3 0.4
7.4 0.2
7.5 0.2
Me
CF3
2-Me, 3-Cl
2-Me, 3-Cl
2,3-diCl
OMe 8.2 0.2
OEt 8.4 0.3
OMe 8.1 0.2
OEt 8.2 0.1
OEt
a Values are means of at least three experiments.
b Value is a mean of two experiments.
2,3-diCl
a Values are means of at least three experiments.
in 5a, resulted in a moderate decrease in binding affinity
but negligible change in activity in the functional assay.
However, replacement of the methyl group in 3e with a
trifluoromethyl group, compound 5b, led to a marked
improvement in affinity in the binding assay and in-
creased activity in the functional assay (Table 5).
Fixing the 2-methyl-3-chloro aniline as ring B, we ex-
plored a wider range of substituents on various positions
of the C-ring (Table 4). Initial results showed that the
alkoxy groups present in compounds 3a–e could be re-
placed by related electron donating groups such as the
dimethylamino group present in analogue 4a. We also
found that the 4-trifluoromethyl group, as in 4b, was
an effective replacement for the 4-alkoxy group, thereby
demonstrating that electron donating groups were not
required for activity. Based on this result we investigated
a range of halogenated C-ring analogues. Although the
monofluorinated derivatives 4c–e were less active, the 2-
chloro analogue 4f showed promising activity. Addition
of a second chlorine atom retained activity, compounds
4h–i (Table 4).
Finally, in order to ascertain whether the methylene lin-
ker was optimal we investigated various isosteric and
homologated linkers (Table 6). Replacement of the
methylene group by an oxygen atom was poorly toler-
ated (6a) in the binding assay although this compound
still retained activity in the functional assay (FLIPR).
However, the amino replacements 6b–d retained activity
in both assays. Analogue 6d again highlighted the bene-
ficial effect of the 4-trifluoromethyl group on the A-ring.
Finally, the homologated analogues 6e–g also retained
activity. These results correlate well with the activity
of the homologated analogues in the glycine sulfon-
amide series where it was found that both benzyl and
phenethyl amides were active (Table 6).12
Returning to the 2,3-dichloro aniline (B-ring), we sought
to investigate potential replacements for the methyl
group on the sulfonamide A-ring (Table 5). Replace-
ment of the methyl group in 3d by a chlorine atom, as
Table 4. SAR for substitution of the C-ring
Table 6. SAR for derivatives with alternative linker groups
X
A
A
O
O
O
S
2
N
B
O
3
O
S
N
N
L
Y
C
N
B
6
O
N
N
4
C
5
Cl
Y
O
a
Compound
Y
Binding pIC50
FLIPR pKia
Compound
X
Y
L
Binding FLIPR pKia
4a
4b
4c
4d
4e
4f
4-NMe2
4-CF3
4-F
7.9 0.1
7.3 0.1
6.7 0.1
6.3 0.3
6.5 0.2
7.1 0.2
6.8 0.1
7.2 0.2
7.2 0.1
7.7 0.4
6.8 0.2
6.3 0.3
6.2 0.4
6.3 0.2
6.4 0.1
5.7 0.2
6.3 0.3
6.0 0.3
a
pIC50
6a
6b
6c
6d
6e
6f
Me 2-Et
Me 2-Me, 3-Cl NH
O
<6
7.0 0.4
7.5 0.3 7.1 0.2
7.4 0.2 7.3 0.1
8.1 0.0 7.2 0.2
7.6 0.1 6.9 0.1
2-F
3-F
Me 2,3-diCl
CF3 2,3-diCl
Me 2-Et
NH
NH
2-Cl
3-Cl
CH2O
4g
4h
4i
Me 2-Et CH2NH 6.3 0.1 6.1 0.2
Me 2-Me, 3-Cl CH2NH 7.7 0.1 7.1 0.2
2,6-diCl
2,4-diCl
6g
a Values are means of at least three experiments.
a Values are means of at least three experiments.

A. Hall et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4450–4455
4453
The synthesis of analogues was carried out as described
in Schemes 1–4. All final compounds were characterized
by LC/MS and H NMR spectroscopy.
lowed by deprotection gave 13. Amide formation
with glycine sulfonamide derivative 14 under stan-
dard conditions gave 15, which was dehydrated
with Burgess reagent17 under microwave conditions
to give 6a.
1
Reaction of an appropriately substituted aniline, 7, with
a sulfonyl chloride, 8, gave the corresponding sulfona-
mides 9. Alkylation with ethyl bromoacetate gave esters
10. Condensation with hydrazine gave the hydrazides 11
which were converted directly to the 1,3,4-oxadiazoles
2–516 by reaction with substituted phenyl acetic acids
in neat phosphorus oxychloride under microwave condi-
tions at 100 °C (Scheme 1).
The amino-linked derivatives 6b–d were prepared as de-
scribed in Scheme 3. Reaction of an appropriately
substituted phenyl isocyanate derivative, 16, with hydra-
zine gave the amino urea derivative 17. Condensation of
17 with the glycine sulfonamide derivatives 14a in phos-
phorus oxychloride under microwave irradiation at
100 °C furnished the oxadiazoles 6b–d.
The oxygen-linked derivative 6a was prepared as
described in Scheme 2. Reaction of 4-methoxyphe-
nyl chloroformate, 12, with tert-butyl carbazate fol-
The homologated oxy and amino analogues 6e–g were
prepared as described in Scheme 4. Condensation of
X
S
X
S
NH2
SO2Cl
O
Y
O
Y
OEt
b
a
NH
9
N
O
O
Y
+
O
X
8
7
10
c
X
S
X
H
O
O
d
O
S
N
Z
N
N
NH2
O
O
N
N
O
Y
Y
2 - 5
11
Scheme 1. Reagents and condition: (a) DCM, pyridine; (b) NaH, DMF, ethyl bromoacetate; (c) hydrazine hydrate, heat or microwave; (d)
phenylacetic acid derivative, POCl3, l-wave, 100 °C, 30 min.
H
N
Cl
O
b, c
O
a
H2N
O
S
OH
10
+
O
N
O
O
O
O
O
X = Me
Y = 2-Et
13
12
14
d
O
O
O
O
S
H
O
e
N
O
S
N
O
N
N
H
O
N
N
O
O
O
6a
15
Scheme 2. Reagents and conditions: (a) EtOH, 2 M NaOH; (b) BocNHNH2, EtOAc, TEA, 0 °C; (c) TFA then sat. NaHCO3; (d) EDAC, HOBt, N-
ethylmorpholine, DMF; (e) Burgess reagent, THF, l-wave, 130 °C, 15 min.

4454
A. Hall et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4450–4455
X
H
N
H
N
OCN
b
a
H2N
O
Y
S
OH
10
+
N
O
O
O
O
O
17
16
14a
c
X
S
H
N
O
O
Y
N
O
N
N
O
6b - d
Scheme 3. Reagents and conditions: (a) EtOH, 2 M NaOH; (b) DCM, hydrazine hydrate, À60 °C to rt; (c) POCl3, l-wave, 100 °C, 20 min.
X
O
L
a
HO
O
O
Y
S
O
N
O
L
O
N
N
18
6e: L = O
6f- g: L = NH
Scheme 4. Reagents and conditions: (a) 11, POCl3, l-wave, 100 °C, 20–40 min.
the required carboxylic acid derivative (18) with hydra-
zide 11 in neat phosphorus oxychloride under micro-
wave irradiation at 100 °C yielded the desired
oxadiazoles.
References and notes
1. (a) Coleman, R. A.; Smith, W. L.; Narumiya, S. Pharma-
col. Rev. 1994, 46(2), 205; (b) Breyer, R. M.; Bagdassarian,
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Toxicol. 2001, 41, 661; (c) Narumiya, S.; Sugimoto, Y.;
Ushikubi, F. Physiol. Rev. 1999, 79, 1193; (d) Coleman, R.
A.; Kennedy, I.; Humphrey, P. P. A.; Bunce, K.; Lumley,
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Oxford, UK, 1990; Vol. 3, pp 643–714.
2. Hall, A.; Atkinson, S.; Brown, S. H.; Chessell, I. P.;
Chowdhury, A.; Clayton, N. M.; Coleman, T.; Giblin, G.
M. P.; Gleave, R. J.; Hammond, B.; Healy, M. P.;
Johnson, M. J.; Michel, A. D.; Naylor, A.; Novelli, R.;
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In conclusion, we have generated a novel series of
1,3,4-oxadiazole EP1 receptor antagonists by replacing
the amide group in a previously described series by a
biosteric oxadiazole. Optimization of this series re-
vealed that a 2,3-disubstituted aniline gave higher affin-
ity than the starting 2-ethyl aniline. The tosyl group
was best replaced by a 4-trifluoromethylbenzenesulfo-
nyl group which led to a significant increase in binding
affinity. The optimal compound in terms of in vitro
activity was derivative 5b, with a binding IC50 of
2.5 nM (pIC50 8.6) and a functional Ki of 6.3 nM
(FLIPR pKi 8.2). Unfortunately the compounds de-
scribed herein did not display sufficiently improved
metabolic stability relative to the glycine sulfonamide
series when assessed in rat and human liver micro-
somes in vitro.18
6. (a) Kawano, T.; Anrather, J.; Zhou, P.; Park, L.; Wang,
G.; Frys, K. A.; Kunz, A.; Cho, S.; Orio, M.; Iadecola, C.
Nat. Med. 2006, 12(2), 225; (b) Ahmad, A. S.; Saleem, S.;
Acknowledgment
We thank Phil East for the metabolite identification.
´
Ahmad, M.; Dore, S. Toxicol. Sci. 2006, 89(1), 265.

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7. Hall, A.; Bit, R. A.; Brown, S. H.; Chaignot, H. M.;
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10. (a) Hall, A.; Brown, S. H.; Chessell, I. P.; Chowdhury, A.;
Clayton, N. M.; Coleman, T.; Giblin, G. M. P.; Ham-
mond, B.; Healy, M. P.; Johnson, M. R.; Metcalf, A.;
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14. The replacement of an ester by a 1,2,4-oxadiazole has been
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15. McMartn, C.; Bohacek, R.S. J. Computer-Aided Molecu-
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release date February 2003.
16. Analytical data for compound 3c: 1H NMR dH (400 MHz,
CDCl3) 1.42 (3H, t), 2.13 (3H, s), 2.45 (3H, s), 3.92–4.15
(4H, m), 4.74–4.98 (2H, m), 6.43 (1H, d), 6.77–6.92 (3H,
m), 7.02–7.17 (2H, m), 7.20–7.40 (3H, m), 7.48–7.60 (2H,
m); m/z [M+H]+ 512 and 514.
17. (a) Burgess, E. M.; Penton, H. R.; Taylor, E. A. J. Org.
Chem. 1973, 38, 26; (b) Burgess, E. M.; Penton, H. R., Jr.;
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Chowdhury, A.; Giblin, G. M. P.; Goldsmith, P.; Healy,
M. P.; Jandu, K. S.; Johnson, M. R.; Michel, A. D.;