Identification of prostaglandin D2 receptor antagonists based on a tetrahydropyridoindole scaffold

Article abstract of DOI:10.1016/j.bmcl.2008.03.015

A new series of indole-based antagonists of the PGD₂ receptor subtype 1 (DP1 receptor) was identified and the progress of the structure-activity relationship study to the identification of potent and selective antagonists is presented. Selective DP1 antagonists with high potency and selectivity were prepared. Of particular interest is the DP1 antagonist 26 with a K_i value of 1 nM for the DP1 receptor and an IC₅₀ value of 4.6 nM in a DP1 functional assay for the inhibition of the PGD₂ induced cAMP production in platelet rich plasma (PRP).

Full text of DOI:10.1016/j.bmcl.2008.03.015

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 2696–2700
Identification of prostaglandin D2 receptor antagonists
based on a tetrahydropyridoindole scaffold
Christian Beaulieu,a,* Daniel Guay,a Zhaoyin Wang,a Yves Leblanc,a Patrick Roy,a
Claude Dufresne,a Robert Zamboni,a Carl Berthelette,a Stephen Day,a Nancy Tsou,c
Danielle Denis,b Gillian Greig,b Marie-Claude Mathieub and Gary O’Neillb
aDepartment of Medicinal Chemistry, Merck Frosst Canada & Co., 16711 Trans Canada Hwy, Kirkland, Que., Canada H9H 3L1
bDepartment of Biochemistry, Merck Frosst Canada & Co., 16711 Trans Canada Hwy, Kirkland, Que., Canada H9H 3L1
cDepartment of Medicinal Chemistry, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065, USA
Received 8 January 2008; revised 5 March 2008; accepted 6 March 2008
Available online 10 March 2008
Abstract—A new series of indole-based antagonists of the PGD2 receptor subtype 1 (DP1 receptor) was identified and the progress
of the structure–activity relationship study to the identification of potent and selective antagonists is presented. Selective DP1 antag-
onists with high potency and selectivity were prepared. Of particular interest is the DP1 antagonist 26 with a Ki value of 1 nM for the
DP1 receptor and an IC50 value of 4.6 nM in a DP1 functional assay for the inhibition of the PGD2 induced cAMP production in
platelet rich plasma (PRP).
Ó 2008 Elsevier Ltd. All rights reserved.
Selective antagonists of the prostaglandin D2 receptor
subtype 1 (DP1) are being considered for use in the
treatment of symptoms associated with niacin-induced
flushing1,2 and also for the treatment of allergic airway
responses.3–6 Niacin (nicotinic acid, vitamin B3) has
shown efficacy in reducing cardiovascular events in pa-
tients with dyslipidemia. However, symptoms associated
with niacin-induced vasodilatation (e.g., flushing) have
limited its use. The flushing response is mediated in large
part by PGD2 and recently the DP1 receptor antagonist
laropiprant (MK-0524) has proven to be effective in sup-
pressing both subjective and objective manifestations of
niacin-induced vasodilatation.1,2 In addition, it has been
demonstrated that intranasal instillation of PGD2 re-
sults in upper airway obstruction with 10-fold greater
potency than histamine.7
clinical studies to block the inflammatory effects of
PGD2.8–12
Recently, we reported the identification of the potent
and selective DP1 antagonist MK-0524 (1) (Fig. 1).13
While MK-0524 clinical trials were underway, part of
our efforts were focused on finding additional potential
development candidates. One of our main criteria was
to identify a DP1 antagonist structurally distinct from
our first clinical candidate MK-0524.
Towards this end, we elected to preserve the 6-5-5 or
6-5-6 tricyclic scaffold which was a common motif to
our more potent DP1 antagonists. Thus, one of the
modification considered (Fig. 1) was to invert the indole
The role of the PGD2 receptor (DP1 receptor) in the reg-
ulation of mucous secretion and the efficacy of DP1
receptor blockade in allergic disease models prompted
us to identify a suitable DP1 receptor antagonist for
Keywords: DP1 antagonist; Prostanoid; PGD2 antagonist; PGD2
receptor; Tetrahydropyridoindole; Indole.
Figure 1. Structure of MK-0524 (1) and of proposed novel indole
scaffold (2).
*
Corresponding author. E-mail: christian_beaulieu@merck.com
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.03.015

C. Beaulieu et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2696–2700
2697
central core to form a new structure (2) in which the
MK-0524’s indole nitrogen atom and the carbon on
3-position of indole were exchanged. Based on the sim-
ilarities between both indole structures, we expected
some features of the structure–activity relationship
(SAR) to be common to both series. In this letter, we de-
scribe the SAR of this new series of DP1 receptor antag-
onists based on the above proposal.
such as methyl ketone and secondary alcohols (com-
pounds 5–7) resulted in potent compounds with im-
proved selectivity profiles. However, compounds 5–7
were only modestly potent in a DP1 functional assay
(DP1 PRP) in inhibiting the PGD2 induced production
of cAMP in platelet rich plasma (PRP)16 with IC50 val-
ues of 101, 56 and 509 nM, respectively. Gratifyingly,
potency in this DP1 PRP assay was improved by the
introduction of a methoxyether (8) or a methylthioether
(9) with IC50 values of 6.7 and 4.1 nM, respectively.
Therefore, the DP1 selectivity profile of 8 was far supe-
rior to 9 (180-fold vs 17-fold selectivity). Interestingly,
replacement of the methoxyether by a methylsulfone
(11) resulted in an antagonist as potent and selective
as 8. The bulkier ethylsulfone 10 was 3-fold less potent
than 11 in the DP1-PRP functional assay. Further
SAR with alkyl chains and/or carbocycles as for com-
pounds 12–14 resulted in less potent and/or less selective
derivatives. Metabolic profiles of 8 and 11 were evalu-
ated (data not shown) and revealed higher rate of
metabolism in human microsomes and hepatocytes for
8 compared to 11. The overall profile of 11 being supe-
rior, the methylsulfone group was identified as optimal
at the 4-position position of the indole.
Previous SAR studies from the MK-0524 (1) series
demonstrated the advantage of fluorine substitution at
the 5-position of the indole ring which corresponds to
the 6-position of our new indole series.13 The presence
of an acetic acid side chain was also found to be essential
for activity. To initiate SAR work in this new series, we
kept the same substitution pattern and initially focused
our attention on identifying an optimal substituent for
the 4-position of the indole ring. The template used
for this SAR was a dihydropyrroloindole scaffold substi-
tuted with a 4-chlorophenylthioether at the 3-position of
the indole, chosen for its ease of synthesis. In general,
compounds were screened for affinity against all nine
human prostanoid receptors and were synthesized as
racemates. However, our attention was mainly directed
at the thromboxane A2 receptor (TP) which was the
most significant ‘off-target’ activity for the compounds
in this series. In addition, antagonism of compounds
was characterized by a DP1 CRE-SEAP assay.14 As
shown in Table 1, the unsubstituted compound 3 main-
tains a modest binding affinity15 for the DP1 receptor
(Ki = 132 nM) and is actually 8-fold more potent on
the TP receptor (Ki = 17 nM). Substitution of the 4-po-
sition with a bromide atom (4) resulted in a gain of po-
tency on the DP1 receptor and led to a modestly DP1
selective antagonist. Introduction of more polar groups
The effect of the substituent at the 3-position of the
indole was also studied and is presented in Table 2.
Modification of the substitution pattern on the 4-chloro-
phenylthioether such as for compound 15 did not
improve the potency profile of the series. Antagonists,
for which the chloride atom was replaced, were found
to be less active as observed with 16 which is 3-fold less
active than 11 in the DP1 PRP functional assay. The
replacement of the phenyl ring with a naphthyl ring
(17) had the same effect. The benzyl analog 18 exhibited
a better affinity for the DP1 receptor but the selectivity
over the TP receptor was significantly reduced (23-fold
selectivity only). The addition of an additional methy-
lene to 18 to increase the distance between the phenyl
and the indole ring (19) diminished the potency by 38-
fold. A similar loss of activity was also observed for
the a-methyl benzyl analog 20. Surprisingly, the replace-
ment of the phenylthioether with a 3-biphenyl group
was well tolerated (21). However, this biphenyl analogs
were not as potent as 11. Of particular interest, the ben-
zoyl analog 22 was somewhat less potent on DP1 than
the corresponding phenylthioether but displayed an
excellent selectivity versus TP (1000-fold). The SAR
around the 3-position of indole did not result in signifi-
cant gain in potency and the thioether analogs appeared
as the most attractive for further studies. Nonetheless,
due to the high selectivity profile of benzoyl 22, we
decided to evaluate the metabolic profile of both of these
DP1 antagonists. Unfortunately for compound 11 and
close analogs, high levels of covalent protein labeling
were identified as a major issue with these antagonists.
Indeed, all compounds tested from the thioether series
exhibited in vitro covalent protein binding levels higher
than 50 pmol equiv/mg of protein, which is considered
as the upper limit for covalent binding modification.17
Since high propensity of a compound for covalent pro-
tein labeling is potentially associated with liabilities
in vivo such as organ toxicity and unwanted immune
Table 1. SAR of the indole 4-position
Compound
R
DP1a
TPa
DP1 PRPb
Ki (nM) Ki (nM) IC50 (nM)
3
4
H
Br
132
7.7
3.6
2.5
7.9
1.1
17
17
—
—
5
COMe
674
273
484
200
27.5
250
226
46
101
56
6
CH(OH)Me
CH(OH)i-Pr
CH(OMe)Et
7
509
6.7
4.1
34
8
9
CH(SMe)Me 1.6
10
11
12
13
14
SO2Et
SO2Me
i-Pr
2.3
1.3
2.4
8.1
5.7
10.4
8.6
21.6
—
CH(Et)2
Cyclopropyl
363
136
a Radioligand competition binding assays using recombinant human
prostaglandin D2 (DP1) and recombinant human thromboxane
receptor (TP).
b Inhibition of the accumulation of cAMP in platelet rich plasma
(PRP) challenged with PGD2.

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C. Beaulieu et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2696–2700
Table 2. SAR of the indole 3-position
Table 3. SAR for the size of the carbocycle ring fused to the indole
ring
Compound
R
DP1 Ki TP Ki
(nM)
DP1 PRP
IC50 (nM)
(nM)
Compound
n
DP1
Ki (nM)
TP
Ki (nM)
DP1 PRP
IC50 (nM)
22
23
24
1
2
3
6.9
3
6972
513
—
11
1.3
226
10.4
32.1
4.6
5.2
1.4
675
activity and resulted in a compound more shifted than
the cyclohexyl analog 23 in the PRP functional assay.
At this point, 23 was identified as the optimal DP1
antagonist based on its excellent in vitro profile and its
good pharmacokinetics profile in rats (100% bioavail-
ability, 4.4 h half-life). Since indole 23 was a mixture
of enantiomers, it was resolved by chiral HPLC separa-
tion into its individual isomers, namely 25 and 26. As
illustrated in Table 4, the orientation of the acetic acid
side chain was important to maintain the activity on
the DP1 receptor, 26 being 36-fold more potent than
25 in the DP1 binding assay. Not surprisingly, the abso-
lute stereochemistry of the acetic acid side chain of 26
was found to be the same as in 1.18 The antagonist 26
exhibits a similar activity on the DP receptor to 1,13
and is at least 100-fold selective for the DP receptor over
all the other prostanoids receptors.
15
16
17
3.5
1.8
1.2
198
286
583
29.2
40
18
19
20
0.5
18.9
14.7
11.7
4.7
219
—
The general synthetic route used to prepare compounds
of the dihydropyrroloindole, the tetrahydropyridoindole
and the tetrahydroazepinoindole series is outlined in
Scheme 1. The indole ring formation involved the ther-
molysis of azidocinnamate19 28 which affords the methyl
indole-2-carboxylate 29. Indole 29 is then alkylated with
a halo alkyl alkanoate to afford a 5, 6 or 7 membered
ring carbocycle fused to the indole20 and the resulting
ketoester is decarboxylated under acidic conditions to
afford ketone 30. Introduction of the acetic acid side
chain is then accomplished by a Reformatsky type addi-
tion on ketone 30 followed by deoxygenation21 of hy-
droxyl ester 31. The reaction proceeds using in situ
generated TMSI to provide the important intermediate
32 which was used as a common building block for
the thioester series as well as for the benzoyl series of
DP1 antagonist. Thus, the sulfenylation22 of 32 allowed
the preparation of thioester 33 while Friedel–Crafts
acylation permitted the preparation of the benzoyl ana-
log 34. The presence of a bromide atom on 4-position of
indoles 33 and 34 allowed the introduction of a variety
of substituents on the indole ring such as methylsulfone,
introduced by Ullmann type coupling23 to afford the de-
sired DP1 antagonists 35 and 36 after ester hydrolysis.24
3190
—
21
22
1.7
6.9
310
31.6
6972
—
response,17 we decided to focus the SAR studies around
the benzoyl series which had acceptable levels of cova-
lent protein labeling (<50 pmol equiv/mg of protein).
We next examined the importance of the size of the car-
bocycle fused to the indole. The expansion of the carbo-
cycle from a cyclopentyl (22) to a cyclohexyl (23)
improved the potency of the DP1 antagonist by 2-fold
in the binding assay as shown in Table 3. Moreover,
23 was found to be highly potent in the DP1 PRP assay
(4.6 nM) and also exhibited a satisfactory selectivity ver-
sus TP (>100-fold). Further expansion of the carbocycle
to a cycloheptyl ring (24) had a minimal effect on the
In conclusion, a new series of potent and selective DP1
receptor antagonists was identified by modification of
the tetrahydrocarbazole core of MK-0524 (1). Tetrahyd-
ropyridoindole 26 displayed the best overall profile
among the compounds studied and fulfills the criteria of

C. Beaulieu et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2696–2700
Table 4. DP1 and prostanoids binding affinities for enantiomers 25 and 26
2699
Compound
Ki (nM)
IC50 (nM) DP1 PRP
DP1
CRTH2
TP
EP1
EP2
EP3
EP4
FP
IP
25
26
36
1
—
19,200
7154
168
2174
142
>29,188
3404
>21,360
5063
>16,061
>16,500
>24,826
21,057
>22,498
>21,200
—
4.6
Scheme 1. General synthesis of compounds 11 and 22–24. Reagents and conditions: (a) NBS, BzOOH, CCl4, reflux; (b) N-methylmorpholine N-
oxide, dioxane, 70 °C; (c) N3CH2CO2Me, MeONa, MeOH, 70 °C; (d) xylene, reflux; (e) n = 1: methyl acrylate, KOtBu, THF, reflux; n = 2: i—
Br(CH2)3CO2Et, NaH, Bu4NI, DMF, 0 °C; ii—KOtBu, THF, 0 °C; n = 3: i—Br(CH2)4CO2Et, NaH, Bu4NI, DMF, 0 °C; ii—KOtBu, THF, 0 °C; (f)
HCl concd, EtOH, reflux; (g) BrCH2CO2Me, Zn–Cu couple, HMPA, THF; (h)TMSCl, NaI, CH3CN, Et2O; (i) bis(4-chlorophenyl)disulphide,
SO2Cl2, 1,2-DCE, DMF; (j) 4-ClPhCOCl, AlCl3, 1,2-DCE; (k) CuI, NaSO2Me, DMSO, 100 °C; (l) LiOH, H2O, MeOH, THF.
4. Naclerio, R. M.; Proud, D.; Togias, A. G.; Adkinson, N.
F., Jr.; Meyers, D. A.; Kagey-Sobotka, A.; Plaut, M.;
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313, 65.
potency and selectivity deemed suitable for further
development.
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16. The DP1 functional assays were performed on blood
collected from normal human volunteers. Platelet rich
plasma (PRP) was prepared by centrifugation at 150g for
15 min. A portion of the PRP fraction was used to prepare
the washed platelet fraction by isolating the platelets by
centrifugation (10 min at 800g) and resuspension of the
platelet cell pellet in buffer (25 mM Hepes, pH 7.4, HBSS
without Ca2+, Mg2+). The WP and PRP assays were
conducted as follows: isobutylmethylxanthine (IBMX;
2 mM final concentration) was added to prevent degrada-
tion of cAMP. Samples (100 lL) of either human WP or
PRP were then preincubated (10 min at 37 °C) with
increasing concentrations of test compound in DMSO.