Discovery of orally active prostaglandin D2 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2004.07.039

Discovery process of a new PGD₂ receptor antagonists 3i is reported. A series of N-(p-alkoxy)benzoyl-2-methylindole-4-acetic acids were synthesized and evaluated for prostaglandin D₂ (DP) receptor affinity and antagonist activity. So

Full text of DOI:10.1016/j.bmcl.2004.07.039

Bioorganic & Medicinal Chemistry Letters 14 (2004) 4891–4895
Discovery of orally active prostaglandin D2 receptor antagonists
a
a
a
Kazuhiko Torisu,a, Kaoru Kobayashi, Maki Iwahashi, Yoshihiko Nakai,
*
Takahiro Onoda,b Toshihiko Nagase,a Isamu Sugimoto,a Yutaka Okada,a
Ryoji Matsumoto,a Fumio Nanbu,a Shuichi Ohuchida,b Hisao Nakaia and Masaaki Todaa
aMinase Research Institute, Ono Pharmaceutical Co., Ltd, Shimamoto, Osaka, Mishima 618-8585, Japan
bFukui Research Institute, Ono Pharmaceutical Co., Ltd, Technoport, Yamagishi, Mikuni, Fukui, Sakai 913-8538, Japan
Received 14 June 2004; revised 15 July 2004; accepted 17 July 2004
Available online 6 August 2004
Abstract—A series of N-(p-alkoxy)benzoyl-2-methylindole-4-acetic acids were synthesized and evaluated for prostaglandin D2 (DP)
receptor affinity and antagonist activity. Some of them exhibited strong receptor binding and were potent in the cAMP formation
assays. These antagonists also suppressed allergic inflammatory responses such as the PGD2-induced increase of microvascular per-
meability. Structure–activity relationship (SAR) data are presented.
Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction
Here we report on the identification of an orally active
hDP receptor antagonist 3i, which exhibited efficacy in
animal models, starting from chemical modification of
2d (Fig. 1).
Prostaglandin D2 (PGD2) is considered to play an
important role in various allergic diseases such as aller-
gic rhinitis,1 atopic asthma,2 allergic conjunctivitis,3 and
atopic dermatitis.4 However, there have been very few
reports about the efficacy of PGD2 receptor (DP) antag-
onists in animal models of allergy or patients with aller-
gic diseases.5 A DP receptor selective antagonist was
considered to possess potential therapeutic value for
various allergic disorders.
2. Chemistry
A series of N-benzoyl-2-methylindole-4-acetic acids 1,
2a–d, and 3a–i listed in Tables 1 and 2 were synthesized
as outlined in Scheme 1. Efficient synthetic method of 2-
methylindole-4-acetic acid has been unknown. In the
preceding paper,6 we reported on the synthesis of 2-
methylindole-4-acetic acid using palladium-catalyzed
carbon monoxide insertion reaction starting from 2-
methyl-4-hydroxyindole 4a. Here we report on the more
efficient synthesis consisting of palladium-catalyzed C2
homologation followed by the hydroboration.
In the preceding paper,6 we reported on the discovery of
a new chemical lead 1 for DP receptor selective antago-
nists starting from the chemical modification of indo-
methacin analogs. Further optimization of the new
chemical lead 1 was carried out. Modification of the
chemical lead 1 was concentrated on the terminal alkyl
moiety, which was still not optimized. The p-phenyloxy-
ethyloxy derivative 2d showed the most potent mDP re-
ceptor antagonist activity among the compounds listed
in Table 1. However, the human (h)DP receptor antag-
onist activity of 2d was found to be relatively lower than
the mDP receptor antagonist activity as described in
Table 1. Further optimization of 2d was continued to in-
crease its hDP receptor antagonist activity.
Replacement of the trifluoromethane sulfonate function
of 4c with trimethylsilylacetylene moiety was success-
fully carried out to afford 5 by the palladium catalyzed
reaction in the presence of cuprous iodide.7 Hydrobora-
tion of 5 followed by an alkaline deprotection of indole
nitrogen and the appropriate esterification afforded 6a
and 6b, which were converted to 1, 2a–d, and 3a–i by
the following sequential procedures: (1) N-acylation
with p-acetyloxybenzoyl chloride; (2) aminolysis with
pyrollidine; (3) Mitsunobu reaction with an appropriate
alcohol; (4) deprotection. The alcohol 11 for 3i were
prepared from 7 as outlined in Scheme 2. N-Tosylation
Keywords: Prostaglandin; DP receptor; Antagonist.
*
Corresponding author. Tel.: +81-75-961-1151; fax: +81-75-962-
9314; e-mail: torisu@ono.co.jp
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.07.039

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K. Torisu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 4891–4895
Table 1. Effect of the p-alkoxy substituent of the N-benzoyl moiety on the activity profiles
CO2H
4
3
2
Me
N
1
R
O
O
Compound
R
Binding Ki (lM)
IC50 (lM)
mDP
mEP1
>10
mEP2
2.0
mEP3
mEP4
>10
mDP
0.010
hDP
NTa
1
3.3
1.2
0.30
1.3
CH3
2a
1.2
0.33
1.3
>10
1.9
0.068
NTa
NTa
2b
0.40
2.2
0.0018
0.12
2c
2d
7.7
5.6
NTa
0.54
3.3
2.1
0.17
NTa
NTa
0.25
O
>10
>10
0.017
0.053
Using membrane fractions of CHO cells expressing the prostanoid receptors, the mouse (m) EP receptor or (m) DP receptor, Ki values were
determined by competitive binding assay, which was performed according to the method of Kiriyama et al. with some modifications.7 With regard to
the mDP and human (h) DP receptor antagonist activity, IC50 values were determined based on the effects of the test compounds on the increase in
intracellular cAMP formation in the presence of bovine serum albumin (BSA) (0.1%) evoked by PGD2 in mDP or hDP receptor expressing CHO
cells.
a NT: not tested.
stably expressing each prostanoid receptor and for inhi-
bition of cAMP formation evoked by PGD2 in CHO
cells8 in the presence of BSA (0.1%). Test compounds
were also evaluated for binding to all subtypes of the
mouse PGE2 receptor (mEP1, mEP2, mEP3, and
mEP4).9
CO2H
Me
N
O
R
O
In the preceding paper,6 we reported on the discovery of
1 as a new chemical lead for DP receptor antagonists
(Fig. 1). We then conducted further optimization of N-
benzoyl-2-methylindole-4-acetic acid as a subtype-selec-
tive DP receptor antagonist.
(mDP :Ki = 0.010µM)
1 : R =
CH3
O
2d : R =
O
F
As shown in Table 1, the effect of the p-alkoxy substitu-
ent of the N-benzoyl moiety of 2-methylindole-4-acetic
acid was investigated. Replacement of the p-butyloxy
moiety of 1 with benzyloxy, phenylethyloxy, and phen-
ylpropyloxy moieties provided 2a–c, respectively.
N-(p-Benzyloxy)benzoyl analog 2a demonstrated nearly
7-fold lower mDP receptor binding affinity, while the N-
(p-phenylethyloxy)benzoyl and N-(p-phenylpropyl-
oxy)benzoyl analogs 2b–c demonstrated nearly 5-fold
stronger and 17-fold weaker mDP receptor binding
affinity, respectively. Regarding mDP receptor antago-
nist activity, 2a showed nearly 4-fold lower potency rel-
ative to 1, while 2b showed nearly 2-fold greater
potency. Replacement of the p-butyloxy moiety of 1
with a p-phenyloxyethyloxy moiety, which was designed
for further optimization of the chain length, expected
formation of another hydrogen bond and others,
provided 2d with 2-fold weaker mDP receptor binding
affinity and nearly 6-fold more potent mDP receptor
antagonist activity.
3i : R =
N
Me
Figure 1. Discovery of new DP receptor antagonists.
of 7 with tosyl chloride gave 8. N-Alkylation of 8 with
(S)-(À)-glycidyl triphenylmethyl ether provided 9. Cycli-
zation reaction of 9, followed by removal of the N-tosyl
moiety, produced 10. N-Methylation of 10 followed by
acidic deprotection produced 11.
3. Results and discussion
The compounds listed in Tables 1 and 2 were evaluated
for inhibition of the specific binding of a radioligand,
[3H]PGD2, to membrane fractions prepared from cells

K. Torisu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 4891–4895
Table 2. Further optimization of the p-alkoxy substituent of the N-benzoyl moiety
4893
CO2H
Me
N
O
R
O
Compound
R
Binding Ki (lM)
mEP3 mEP4
IC50 (lM)
mEP1
5.6
mEP2
0.54
mDP
0.017
hDP
0.29
hDP
O
O
2d
3a
>10
>10
>10
0.25
7.8
0.45
0.98
0.022
0.029
0.036
O
O
3b
3c
0.78
3.8
0.25
0.75
>10
0.0094
0.061
0.039
0.052
0.056
0.030
O
0.092
>10
3.1
3d
3e
>10
2.0
0.28
>10
>10
3.5
0.18
0.39
NTa
O
O
R
S
0.064
>10
0.016
0.018
0.031
O
O
3f
2.3
6.2
1.3
>10
1.3
0.12
0.13
0.27
O
O
S
R
3g
0.10
1.7
>10
0.016
0.0035
0.0012
N
Me
O
3h
>10
>10
0.41
2.1
>10
>10
>10
0.23
0.17
0.019
N
Me
O
F
S
3i
1.7
0.023
0.0053
0.00081
N
Me
a NT: not tested.
The three compounds 2a–b and 2d also demonstrated
weak to moderate affinity for other EP receptors. In par-
ticular, 2b showed moderate affinity for all of the EP re-
ceptors despite its increased mDP receptor binding
affinity and antagonist activity, while the p-phenyloxy-
ethyloxy analog 2d showed relatively higher subtype
selectivity with more potent mDP receptor antagonist
activity relative to 1.
attention on improvement of the hDP receptor antago-
nist activity of 2d (IC50 = 0.25lM). Further optimiza-
tion of 2d was continued to increase its hDP receptor
antagonist activity. A breakthrough was achieved by
synthesis and evaluation of the series of compounds
listed in Table 2. Replacement of the p-phenyloxyethyl-
oxy moiety of 2d with a dihydrobenzofuranyl-2-methyl-
oxy moiety and a 1,3-dioxaindanyl-2-methyloxy moiety
afforded 3a–b, respectively, which showed increased
hDP receptor binding affinity and antagonist activity
relative to 2d and nearly equivalent mDP receptor bind-
ing affinity. Replacement of the phenyloxyethyloxy
moiety of 2d with a benzopyranyl-2-methyloxy moi-
ety or benzopyranyl-3-methyloxy moiety gave 3c–d,
Compound 2d was also evaluated for hDP10 receptor
antagonist activity. Unexpectedly, 2d showed decreased
hDP receptor antagonist activity despite its high mDP
receptor binding affinity. Since our final goal was to
identify a potent hDP receptor antagonist, we focused

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K. Torisu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 4891–4895
TMS
CO2 R2
CO2H
d
e
Me
Me
Me
N
N
N
H
R3
Boc
5
O
6a : R2 = allyl (38%)
O
6b : R2 = Bn (35%)
1, 2a-d and 3a-i (20-80%)
c
(quant.)
OX
Me
N
R1
4a : X = H, R1 = H
a
b
4b : X = Tf, R1 = H (quant.)
4c : X = Tf, R1 = Boc (quant.)
Scheme 1. Synthesis of N-benzoyl-2-methylindole-4-acetic acid analogs 1, 2a–d, and 3a–i. Reagents and conditions: (a) Tf2O, 2,6-lutidine, CH2Cl2,
À78°C; (b) Boc2O, DMAP (cat.), CH3CN; (c) PdCl2(PPh3)2, trimethylsilylacetylene, CuI, TBAI, TEA, DMF, 70°C; (d) (1) dicyclohexylborane; (2)
NaOH aq, H2O2, THF; (3) NaOH aq, MeOH, 1,4-dioxane; (4) allyl bromide or benzyl bromide, K2CO3, DMF; (e) (1) NaOH (powdered), p-
acetyloxybenzoyl chloride, TBACl, CH2Cl2; (2) pyrollidine, CH2Cl2; (3) alcohol, DEAD, PPh3; (4) morpholine, Pd(PPh3)4, THF or H2, Pd–C,
i-PrOH, EtOAc.
F
F
F
F
F
F
b
a
(quant.)
NHTs
N
OTr
(quant.)
NH2
Ts OH
9
8
7
O
F
O
F
c, d
OH
OTr
e, f
(42%)
N
Me
11
N
H
(69%)
10
Scheme 2. Synthesis of 11. Reagents and conditions: (a) TsCl, pyridine, CH2Cl2; (b) (S)-(À)-glycidyl triphenylmethyl ether, K2CO3, BnEt3NCl, 1,4-
t
dioxane; (c) BuOK, THF; (d) Na, naphthalene, DME; (e) MeI, K2CO3, acetone; (f) AcOH, THF, H2O.
respectively. Compound 3c showed weaker mDP recep-
tor binding affinity relative to 2d, while it showed stron-
ger hDP receptor binding affinity and antagonist
activity. Its regioisomer 3d showed nearly equivalent in
hDP receptor binding affinity relative to 2d. Benzo-1,4-
dioxan- yl-2-methyloxy derivatives 3e–f were also syn-
thesized as optically active forms and evaluated. Com-
pound 3e (2R-form) showed nearly equal mDP
receptor binding affinity relative to 2d, while it showed
stronger hDP receptor binding affinity and more potent
hDP receptor antagonist activity. On the other hand, 3f
(2S-form) demonstrated nearly 10-fold weaker mDP re-
ceptor binding affinity and nearly equal hDP receptor
antagonist activity relative to 2d. The 4N-methylbenzo-
morpholinyl-2-methyloxy analogs 3g–i were also synthe-
sized as their optically active forms and evaluated.
increase of hDP receptor binding affinity and antagonist
activity.
The enantiomer 3h (2R-form) was more than 10-fold
weaker in the three assays, that is, mDP receptor binding
affinity, hDP receptor binding affinity and hDP receptor
antagonist activity. Introduction of a 7-fluoro moiety
into the N-methylbenzomorpholin moiety of the more
active enantiomer 3g afforded 3i with slightly less potent
mDP receptor binding affinity and hDP receptor binding
affinity although 3i showed nearly 1.5-fold more potent
hDP receptor antagonist activity. Regarding subtype
selectivity, 3a–b and 3g showed moderate to weak affinity
for other mEP receptors, such as mEP1, mEP2, and
mEP3, while 3c and 3f showed moderate to weak affinity
for the mEP1, mEP2, and mEP4 receptors. Compound
3d showed moderate to weak affinity for the mEP2 and
mEP4 receptors. The less potent enantiomer 3h showed
moderate mEP2 receptor affinity.
Compound 3g (2S-form) showed equal mDP receptor
binding affinity relative to 2d, while it showed a marked

K. Torisu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 4891–4895
4895
Table 3. Inhibitory effect of 3i on PGD2 (0.01%, 20lL/eye) induced
vascular permeability in guinea pig conjunctiva (n = 8)
Full details (including more detailed chemistry) will be
reported in due course.
Compound
Dose (mg/kg, po)
0.3
%Inhibition
60**
3i
References and notes
** < 0.01 versus control with DunnettÕs test. Inhibition of increase in
conjunctival vascular permeability caused by topical application of
PGD2 (0.01%, 20lL/eye) in guinea pig. 3i was administered po 1h
before the challenge.
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Compound 3i exhibited better subtype selectivity with
more optimized hDP receptor antagonist activity rela-
tive to 3g mainly because of its decreased mEP2 receptor
affinity with the retained potent antagonist activity.
Overall, the compounds listed in Table 2 demonstrated
stronger hDP receptor binding affinity and their hDP re-
ceptor antagonist activity (IC50 value) was relatively
close to their receptor affinity (Ki value). One of the rea-
sons for the increased hDP receptor binding and antag-
onist activity of these analogs was estimated to be
ascribed to the restricted conformation of the terminal
phenyl moiety in addition to the interaction of these
analogs with hDP receptor by the newly introduced
heteroatoms such as ether oxygen and/or N-methyl
functions.
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These compounds were also evaluated for their TP re-
ceptor affinity, because PGD2 has been known to be a
TP agonist. All of the compounds listed in Tables 1
and 2 showed less than 1000-fold potent affinity to TP
receptor.
To assess potential DP receptor antagonist activity in
vivo, the inhibitory effect of 3i on the PGD2 (0.01%) in-
duced increase of vascular permeability in guinea pig
conjunctiva was evaluated (n = 8).11 Compound 3i
showed 60% inhibition of the PGD2-induced increase
of vascular permeability at an oral dose of 0.3mg/kg
(Table 3).
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In summary, a series of N-(p-alkoxy)benzoyl-2-methyl-
indole-4-acetic acid analogs were synthesized and eval-
uated as a new class of DP receptor antagonists.
Among the compounds tested, 3i demonstrated the
strongest hDP receptor antagonist activity and good
subtype selectivity. In addition, 3i showed in vivo DP re-
ceptor antagonist activity. Thus, a highly potent orally
active DP receptor antagonist 3i was discovered by
starting from chemical modification of Indomethacin
analogs. This could be a potential orally active drug.
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