Discovery of potent and selective DP1 receptor antagonists in the azaindole series

Article abstract of DOI:10.1016/j.bmcl.2009.03.010

Azaindole based structures were evaluated as DP1 receptor antagonists. This work has lead to the discovery of potent, selective and distinct DP1 receptor antagonists.

Full text of DOI:10.1016/j.bmcl.2009.03.010

Bioorganic & Medicinal Chemistry Letters 19 (2009) 2125–2128
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of potent and selective DP1 receptor antagonists
in the azaindole series
a
a
a
a
a
Yves Leblanc a, , Patrick Roy , Claude Dufresne , Nicolas Lachance , Zhaoyin Wang , Gary O’Neill ,
*
Gillian Greig a, Danielle Denis a, Marie-Claude Mathieu a, Deborah Slipetz a, Nicole Sawyer a, Nancy Tsou b
a Merck Frosst Centre for Therapeutic Research, PO Box 1005, Pointe Claire-Dorval, Quebec, Canada H9R 4P8
b Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Azaindole based structures were evaluated as DP1 receptor antagonists. This work has lead to the discov-
ery of potent, selective and distinct DP1 receptor antagonists.
Received 5 February 2009
Revised 2 March 2009
Accepted 4 March 2009
Available online 9 March 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
DP1 receptor antagonists
Azaindoles
Niacin (nicotinic acid) has been clinically demonstrated to im-
prove the lipid profile and reduce cardiovascular events in patients
with cardiovascular disease.1 However, the clinical use of niacin for
treating dyslipidemia has been limited by niacin-induced flushing
(erythema, warmth, pruritis, tingling) of the face, neck and upper
torso.1 Preclinical and clinical studies have shown that blockade
of the prostaglandin D2 (PGD2) G-protein coupled receptor termed
DP1 suppresses the vasodilation and symptoms associated with
niacin administration,2–4 thus significantly improving its tolerabil-
ity. It is noteworthy that PGD2 possesses two distinct receptors;
DP1 and CRTH2.2 The niacin-induced vasodilation adverse effect
is mediated by means of PGD2 acting through the DP1 receptor,
rather than the CRTH2 receptor.2
MK-0524 1 (Fig. 1) an indole derivative has been identified as a
potent (Ki 0.57 nM) and selective DP1 receptor antagonist.5 Follow-
ing the discovery of MK-0524, our group focused on the identifica-
tion of backup compounds in distinct structural classes 2.6
The azaindole group was considered as a potential surrogate for
the indole core, thus providing a distinct series of DP1 receptor
antagonists. Azaindoles are less electron rich as compared to in-
doles and may possess improved metabolic stability. Initially, we
turned our attention to the synthesis of azaindole analogs 3
(Fig. 2) based on the reverse indole series 2 (Fig. 1).6 Therefore,
the development of an efficient method to prepare the azaindole
core structure 4, with substitution at C4 in order to mimic the
Cl
Cl
Me
O S O
R
CO2H
S
N
F
N
CO2H
F
1
2
Figure 1. MK-0524 (1) a potent DP1 receptor antagonist (Ki = 0.57 nM) and reverse
indole series (2).
R
N
R
N
S-Ar
OR3
N
CO2H
n = 1,2
N
H
O
3
4
Figure 2. Core structure of azaindoles.
MK-0524 structure (1), became a key element for the elaboration
of DP antagonists in this series.
One expedient method that allows the preparation of methyl 2-
indole carboxylate in good yields, is the thermolysis of 2-azidophe-
nyl acrylates as described by Hemetsberger et al.7 Only a few
examples were reported in the literature on the application of this
thermolysis for the synthesis of azaindoles.8 In a previous report,
we demonstrated that a large variety of azaindoles can be prepared
from 2-azidopyridine acrylates 5 (Fig. 3) using this approach pro-
vided that appropriate reaction temperatures are chosen.9 Azain-
* Corresponding author. Tel.: +1 514 428 3096; fax: +1 514 428 4900.
E-mail address: yves_leblanc@merck.com (Y. Leblanc).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.03.010

2126
Y. Leblanc et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2125–2128
O
H2N
O
1)
CHO
piperidine
CO2Et
OMe
N
R
N
N
R
OEt
N3
OMe
N
2) S8 / Δ
H
5
4
O
13
1) LAH
12
Figure 3. Preparation of azaindoles via thermolysis of 2-azidopyridine acrylates.
10
2) Dess-Martin
Cl
Cl
CO2Me
KOt-Bu
1)
N3
CO2Me
1)
N
N
CHO
NaOMe / MeOH
O
N
N
N
CO2Me
N
2) HCl / 2-propanol
reflux
OMe
H
2) Xylene relux
N
H
7
6
O
14
15
SnBu3
Ph3As / Pd2(dba)3 / DMF / 78oC
1)
Scheme 2. Synthesis of 2-isopropyllpyridine-3-carboxaldehyde.
N
O
N
2) H2 1 atm, Pd(OH)2 / EA
Alternatively, 2-isopropyl-3-pyridinecarboxaldehyde 14 can be
used as starting material for the Hemetsberger–Knittel reaction
(Scheme 2). The ester 13 was obtained in 50% yield from enamine
12 using an adapted protocol.12 After reduction of ester 13 fol-
lowed by oxidation, the aldehyde 14 was obtained in 70% yield.
Methyl azidoacetate was then condensed on aldehyde 14 with sub-
sequent thermolysis to afford azaindole 15 in 50% yield which in
turn was subsequently converted to compound 10 as described
in Scheme 1.
The above approach has been used to prepare several potent
analogs in the 5- and 7-azaindole series.13 A few 6-azaindoles were
also synthesized but they showed no advantage in terms of recep-
tor affinity or selectivity.
8
O
P
O
(MeO)2
OMe
1)
NaH / THF -78oC 0oC
Cl
2) H2 1 atm, Pd(OH)2 / EA
Cl
Cl
S
S
2
N
N
Cl
N
CO2Me
N
CO2Me
SO2Cl2
9
10
DMF / CH2Cl2
The DP1 receptor affinity and selectivity over the TP receptor of
various racemic 7-azaindoles (16–29) are shown in Table 1.14 In
general, their DP1 affinities are comparable to the reverse indole
series 2 but they are less selective. In the 5-azaindole series, the
4-methylsulfone analogs were initially prepared (30–32) and
found to be potent and highly selective as described in Table 2.
These 5-azaindole analogs were found to be more selective than
the corresponding reverse indole compounds 2. However, they
1) Chiracel OD / Column /
2-propanol/hexane
2) NaOH / THF / MeOH
Cl
Cl
Cl
Cl
S
S
N
N
+
N
CO2H
N
CO2H
11a
11b
Table 1
Scheme 1. Synthesis of azaindole DP1 antagonists.
Ki values of 7-azaindoles as PGD2 antagonists14
R
Ar
S
doles 4 were then converted in a straightforward manner to the
desired DP1 antagonists 3.
N
N
CO2H
For example (Scheme 1) methyl 4-chloroazaindole carboxylate
610 was condensed on methyl acrylate with potassium tert-butox-
ide followed by decarboxylation with HCl/iPrOH at 100 °C to pro-
vide ketone 7 in 69% yield. The isopropyl moiety was introduced
by stannylation using 2-methylvinyltributyltin in the presence of
Pd2(dba)3 followed by hydrogenation at 1 atm with Pd(OH)2 to
provide almost quantitatively ketone 8. Subsequently, the ketone
was converted to the conjugated ester via the Horner–Wads-
worth–Edmmons reaction followed by hydrogenation to afford es-
ter 9. The thioaryl substituent was then introduced by treatment of
9 with bis(3,4-dichlorophenylsulfide) and SO2Cl2 in DMF/CH2Cl2 to
give compound 10 in 62% yield. The ester was then hydrolyzed un-
der standard conditions to provide racemic acid 11. Resolution was
achieved on ester 10 by chiral HPLC using OD column (10% isopro-
panol in hexane) followed by hydrolysis to provide acid 11a from
more mobile ester and 11b from less mobile ester. The absolute
stereochemistry was determined by X-ray crystallographic analy-
sis of resolved ester 10.11
R
Ar
DPa Ki (nM)
TPa Ki (nM)
16
17
18
19
20
21
22
23
24
25
26
27
28
29
SO2Me
SO2Me
SO2Me
SO2Me
SO2Me
SO2Me
SO2Me
SO2Me
SO2Me
SO2Me
SMe
4-Methylphenyl
Naphthyl
6.0
1.6
1.9
4.0
1.3
3.3
3.7
3.7
171.6
41.5
17.9
39.4
23.1
35.7
64.8
24.5
3.8
646.4
78.9
339.8
13.9
7.0
2,3-Dichlorophenyl
4-Chlorophenyl
3,4-Dichlorophenyl
4-Bromophenyl
4-Trifluoromethylphenyl
2-Chloro-4-fluorophenyl
2,4-Dichlorophenyl
Phenyl
4-Chlorophenyl
4-Chlorophenyl
3,4-Dichlorophenyl
4-Chlorophenyl
1.8
16.3
12.4
107.4
2.3
OMe
Isopropyl
Isopropyl
0.88
a
Radioligand competition binding assays using recombinant human prosta-
glandin D2 (DP1) and recombinant human thromboxane receptor (TP).Values are
means from at least three experiments.

Y. Leblanc et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2125–2128
2127
Table 4
Table 2
Human DP1 and TP receptor functional activity of compounds 11a and 11b15
Ki values of 5-azaindoles as PGD2 antagonists14
a
IC50 (nM)
DP PRPc
R
Ar
DPWPb
TP PRPd
—
S
N
11a
11b
0.22 0.10 (n = 5)
0.14 0.04 (n = 8)
4.99 1.63 (n = 5)
2.15 0.57 (n = 7)
N
CO2H
4040 1330 (n = 11)
a
n
Values are means from at least three experiments. IC50 = concentration at which
process is inhibited by 50%.
b
DP WP assay involved inhibition of the accumulation of cAMP in washed
n
R
Ar
DPa Ki
(nM)
TPa Ki
(nM)
platelets challenged with PGD2.
c
DP PRP assay involved inhibition of the accumulation of cAMP in platelet-rich
30
31
32
33
34
35
36
37
38
39
2
1
1
1
1
2
1
2
1
2
SO2Me
SO2Me
SO2Me
H
Phenyl
Ethyl
Ethyl
Cyclopropyl
Cyclopropyl
1-Methyl-
cyclopropyl
1-Methyl-
cyclopropyl
n-Propyl
Isopropyl
Isopropyl
Isopropyl
Isopropyl
Isopropyl
Isopropyl
3,4-Dichlorophenyl
3,4-Dichlorophenyl
4-Chlorophenyl
3,4-Dichlorophenyl
3,4-Dichlorophenyl
3,4-Dichlorophenyl
3,4-Dichlorophenyl
3,4-Dichlorophenyl
3,4-Dichlorophenyl
3,4-Dichlorophenyl
1.3
1.4
2.1
13.6
1.0
1.0
0.91
2.5
0.8
1.0
995
plasma challenged with PGD2.
6887
12183
2917
d
TP PRP assay involved the inhibition of U44619-induced platelet aggregation in
platelet-rich plasma.
>20
687
lM
1687
990
415
587
Table 5
Pharmacokinetic data of 11b
Rat
Beagle dog
Cyno monkey
Squirrel monkey
F (%)
Terminal t1/2 (h)
Cmax (lM)
CLp (ml/min/kg)
53
23
4
43
40
1
3,4-Dichlorophenyl
0.74
3428
8.3 (10)
28 (30)
1.6 (10)
0.47 (10)
8.6 (10)
9.0 (10)
4.0 (2)
1.2 (2)
13.8 (10)
0.06 (4)
21 (2)
1.2 (2)
1.4 (10)
8.5 (10)
7.3 (5)
0.78 (5)
41
42
43
11
44
45
46
2
1
2
1
1
1
1
3,4-Dichlorophenyl
4-Chlorophenyl
1.14
0.71
0.88
0.53
1.2
626
2813
468
2081
>20uM
387
Vdss (L/kg)
3,4-Dichlorophenyl
3,4-Dichlorophenyl
2,6-Dichlorophenyl
2,4-Dichlorophenyl
4-
Doses (mg/kg) are indicated in parenthesis.
0.39
0.67
6144
4). Compound 11b also demonstrates high levels of functional
selectivity versus TP (IC50 4040 nM) as predicted from the receptor
binding assay. Excellent pharmacokinetics were observed with
compound 11b (Table 5) in rat, dog, cynomolgus monkey and
squirrel monkey.
Trifluoromethylphenyl
2,4,5-Trichlorophenyl
47
1
Isopropyl
0.39
186
a
Radioligand competition binding assays using recombinant human prosta-
glandin D2 (DP1) and recombinant human thromboxane receptor (TP).Values are
means from at least three experiments.
In summary, potent and selective DP1 antagonists were identi-
fied in both the 5- and 7-azaindoles series. 5-Azaindoles in partic-
ular, possess excellent pharmacokinetics and selectivity profile for
further development.
exhibited short half-lives and low exposure in rats. For example
compound 32, in rat, has a half-life of 2.2 h (IV 5 mg/kg) and rat
bile cannulation study (IV 5 mg/kg) indicated rapid excretion in
bile (56% of parent compound over 5 h, (Cmax 2 mM, Tmax 0.5 h).
Replacement of the methylsulfone functional group by an alkyl
substituent was shown to provide highly potent, selective com-
pounds that possess favorable pharmacokinetics. The racemic iso-
propyl analog 11 exhibited the best overall profile. The in vitro
profiles of both enantiomers 11a and 11b are described in Table
3. It is interesting to note that both enantiomer 11a and 11b are al-
most equipotent, whereas the enantiomer of MK-0524 is 1000-fold
less active than MK-0524.
References and notes
1. Capuzzi, D. M.; Morgan, J. M.; Brusco, O. A.; Intenzo, C. M. Curr. Atheroscler Rep.
2000, 2, 64.
2. Cheng, K.; Wu, T. J.; Wu, K. K.; Sturino, C.; Metters, K.; Gottesdiener, K.; Wright,
S. D.; Wang, Z.; O’Neill, G.; Lai, E.; Waters, M. G. Proc. Natl. Acad. Sci. U.S.A. 2006,
103, 6682.
3. Lai, E.; Wenning, L. A.; Crumley, T. M.; De Lepeleire, I.; Liu, F.; de Hoon, J. N.; Van
Hecken, A.; Depré, M.; Hilliard, D.; Greenberg, H.; O’Neill, G.; Metters, K.;
Gottesdiener, K. G.; Wagner, J. A. Clin. Pharmacol. Ther. 2008, 83, 840.
4. Paolini, J. F.; Bays, H. E.; Ballantyne, C. M.; Davidson, M.; Pasternak, R.;
Maccubbin, D.; Norquist, J. M.; Lai, E.; Waters, M. G.; Kuznetsova, O.; Sisk, C. M.;
Mitchel, Y. B. Cardiol. Clin. 2008, 26, 547.
From Table 3 data it can be concluded that both enantiomers
have comparable overall profile in term of DP1 binding affinity
and selectivity. However, antagonist 11b displays superior potency
in the DP1 functional assay15 where it inhibits the PGD2 induced
production of cAMP in platelet rich plasma with an IC50 value of
2.2 nM compared to 5.0 nM in the case of enantiomer 11a (Table
5. Sturino, C. F.; O’Neill, G.; Lachance, N.; Boyd, M.; Berthelette, C.; Labelle, M.; Li,
L.; Roy, B.; Scheigetz, J.; Tsou, N.; Aubin, Y.; Bateman, K. P.; Chauret, N.; Day, S.
H.; Levesque, J. F.; Seto, C.; Silva, J. H.; Trimble, L. A.; Carriere, M. C.; Denis, D.;
Greig, G.; Kargman, S.; Lamontagne, S.; Mathieu, M.-C.; Sawyer, N.; Slipetz, D.;
Abraham, W. M.; Jones, T.; McAuliffe, M.; Piechuta, H.; Nicoll-Griffith, D. A.;
Wang, Z.; Zamboni, R.; Young, R. N.; Metters, K. M. J. Med. Chem. 2007, 50, 794.
6. Beaulieu, C.; Guay, D.; Wang, Z.; Leblanc, Y.; Roy, P.; Dufresne, C.; Zamboni, R.;
Berthelette, C.; Day, S.; Tsou, N.; Denis, D.; Greig, G.; Mathieu, M.-C.; O’Neill, G.
Bioorg. Med. Chem. Lett. 2008, 18, 2696.
7. Hemtsberger, H.; Knittel, D.; Wiedmawn, H. Monatsh. Chem. 1970, 101, 161.
8. Fresneda, P. M.; Molina, P.; Delgado, S.; Bleda, J. A. Tetrahedron Lett. 2000, 41,
4777.
9. Roy, P. J.; Dufresne, C.; Lachance, N.; Leclerc, J.-P.; Boisvert, M.; Wang, Z.;
Leblanc, Y. Synthesis 2005, 16, 2751.
10. Roy, P. J.; Boisvert, M.; Leblanc, Y. Org. Synth. 2007, 84, 262.
11. X-ray coordinates of resolved ester 10 (less mobile on OD column 10%
isopropanol in hexane) were deposited with the Cambridge Crystallographic
Centre Data for small molecules (deposit number 721867).
12. Kao, B. C.; Doshi, H.; Rejes-Rivera, H.; Titus, D. D.; Yin, M.; Dalton, D. R. J.
Heterocycl. Chem. 1991, 28, 1315.
13. Leblanc, Y.; Dufresne, C.; Roy, P. WO2004039807, 2004 May 13.
14. Abramovitz, M.; Adam, M.; Boie, Y.; Carriere, M.-C.; Denis, D.; Godbout, C.;
Lamontagne, S.; Rochette, C.; Sawyer, N.; Tremblay, N. M.; Belley, M.; Gallant,
M.; Dufresne, C.; Gareau, Y.; Ruel, R.; Juteau, H.; Labelle, M.; Ouimet, N.;
Metters, K. M. Biochim. Biophys. Acta 2000, 1483, 285.
Table 3
Receptor binding profiles of enantiomers 11a and 11b
Ki (nM)
11a
11b
DP1
TP
0.64 0.12
5039 1426
28,500
7323 3181
710 55
6549 5412
25,700
>25,700
0.32 0.13
935 220
28,500
3975 2768
851 320
>11,300
>25,700
>20,500
4789 2440
EP1
EP2
EP3
EP4
FP
IP
CRTH2
233 59

2128
Y. Leblanc et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2125–2128
15. 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
were then preincubated (10 min at 37 °C) with increasing concentrations of
test compound in DMSO. Samples were then challenged with PGD2 (300 nM
final) added in DMSO and incubated for an additional 2 min at 37 °C. The
reaction was then terminated by the addition of 200
the cells and extract the cAMP. The samples were mixed thoroughly and
centrifuged at 1400g for 10 min at 4 °C. Supernatant aliquots (100 L) were
removed and the ethanol removed by evaporation. cAMP was measured by
125I] cAMP scintillation proximity assay (SPA) (RPA556), Amersham).
lL of ethanol to disrupt
l
to prevent degradation of cAMP. Samples (100
lL) of either human WP or PRP
[