New indole amide derivatives as potent CRTH2 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2011.03.085

A new series of indole amide acting as hCRTH2 receptor ligands had been explored and are described herein. Several amide derivatives displaying low nanomolar activity in hCRTH2 binding and whole blood assays were identified. They were found to behave as a full antagonists, exhibiting good selectivity over related prostaglandin receptors. Also, prototypical compounds in this novel series which displayed acceptable CYP profiles and were orally bioavailable in rats were identified.

Full text of DOI:10.1016/j.bmcl.2011.03.085

Bioorganic & Medicinal Chemistry Letters 21 (2011) 3471–3474
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
New indole amide derivatives as potent CRTH2 receptor antagonists
Helmi Zaghdane a, , Michael Boyd a, John Colucci a, Daniel Simard a, Carl Berthelette a, Yves Leblanc a,
⇑
Zhaoyin Wang a, Robert Houle b, Jean François Lévesque b, Carmela Molinaro c, Martine Hamel d,
Rino Stocco d, Nicole Sawyer d, Susan Sillaots d, François Gervais d, Michel Gallant a
a Medicinal Chemistry, Merck Frosst, Center of Therapeutic Research, 16711 Trans Canada Hwy, Kirkland, Quebec, Canada H9H 3L1
b DMPK, Merck Frosst, Center of Therapeutic Research, 16711 Trans Canada Hwy, Kirkland, Quebec, Canada H9H 3L1
c Process Chemistry, Merck Frosst, Center of Therapeutic Research, 16711 Trans Canada Hwy, Kirkland, Quebec, Canada H9H 3L1
d Biochemistry and Molecular Biology, Merck Frosst, Center of Therapeutic Research, 16711 Trans Canada Hwy, Kirkland, Quebec, Canada H9H 3L1
a r t i c l e i n f o
a b s t r a c t
Article history:
A new series of indole amide acting as hCRTH2 receptor ligands had been explored and are described
herein. Several amide derivatives displaying low nanomolar activity in hCRTH2 binding and whole blood
assays were identified. They were found to behave as a full antagonists, exhibiting good selectivity over
related prostaglandin receptors. Also, prototypical compounds in this novel series which displayed
acceptable CYP profiles and were orally bioavailable in rats were identified.
Received 21 February 2011
Revised 17 March 2011
Accepted 22 March 2011
Available online 30 March 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
CRTH2 receptor antagonists
DP2 receptor antagonist
Prostaglandin receptors
MK-7246
Asthma
Prostaglandin D2 (PGD2) is produced from arachidonic acid by
the combined action of cyclooxygenase and synthase.1 It is re-
leased from mast and Th2 cells in response to an immunological
challenge, and has been implicated in different physiological
events such as allergic inflammation.2 PGD2 can activate two dis-
tinct receptors, namely the DP receptor,3 and the chemoattractant
receptor-homologous molecule expressed on T-helper type 2 cells
(i.e., hCRTH2 also referred as the DP2 receptor).4 hCRTH2 is a
G-protein-coupled receptor5 and its activation by PGD2 stimulates
the recruitment of leukocytes, the release of Th2 cytokines,6 and
the degranulation of basophils and eosinophils.7 The combined
pharmacological action of these biological events plays a key role
in late phase allergic inflammation. Such observations represent
a sound biological rational for the development of selective
hCRTH2 antagonists for the treatment of asthma.8
such adverse events are considered as idiosyncratic drug reaction,
we rationalized that the search for an acceptable replacement group
for the sulfonamide moiety present in MK-7246 was a valid starting
point to initiate SAR studies aimed at identifying a back-up series. In
this Letter, we would like to reportthe discoveryof a new class of tet-
CO2
H
F
N
N
O
S
Me
O
1: MK-7246
Figure 1. The structure of MK-7246.
We previously reported the identification of a new class of potent
and selective hCRTH2 antagonists. SAR studies in this class of tetra-
hydropyrido[1,2-a] indoles led to the discovery of the clinical candi-
date MK-7246 (Fig. 1).9 Patients who have been exposed to
sulfonamide containing antibiotics can develop an immune re-
sponse leading to adverse effects such as fever, skin rashes, GI distur-
bance and liver toxicity.10 As a result, they become hypersensitive to
any sulfonamide containing drugs (i.e., sulfa drugs).10,11 Although
CO2H
N
CO2H
N
N
N
F
F
2
3
Me
Me
O
O
⇑
Corresponding author. Tel.: +1 514 582 8299; fax: +1 514 428 4900.
E-mail address: Zaghdane.helmi@courrier.uqam.ca (H. Zaghdane).
Figure 2. New series of reverse indole amide (compound 2 (hCRTH2 Ki = 4.0 nM)
and compound 3 (hCRTH2 Ki = 1.7 nM)).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.03.085

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H. Zaghdane et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3471–3474
H
O
a
b
c
d
CO2Et
CO2H
N
N
H
N
H
N
N
CO2Et
CO2tBu
7
4
5
6
8
N3
O
e
O
CO2Me
CO2Me
CO2Me
g
i
h
f
N
N
N
N
O
N
R
R
9
10
I
11
O
S
N
Me
O
II
O
N
N
N
N Me
Me
S
Me
Me
S
O
O
O
O
F
F
F
Scheme 1. Reagents and conditions: (a) (i) LiAlH4, THF, À78 °C, 2 h; (ii) MnO2, CH2Cl2, rt, 3 h, 100% (over two steps); (b) (i) carbethoxymethylene triphenylphosphorane,
CH2Cl2, rt, 12 h, 84%; (ii) tert-butyl bromoacetate, Cs2CO3, DMF, 60 °C, 18 h, 60%; (c) (i) H2, Pd/C, EtOH, 1 atm, 12 h, 90%; (ii) KOtBu, THF, rt, 20 h, 97%; (iii) Silica gel, toluene,
reflux, 2 h, 82%; (d) (i) NaBH4, THF, 0 °C, 1 h, 100%; (ii) methanesulfonyl chloride, Et3N,CH2Cl2, À40 °C, 30 min, 98%; (iii) sodium azide, DMF, 60 °C, 12 h, 75%; (e) (i) H2, Pd/C,
MeOH, rt, 12 h, 96%; (ii) 4-fluorobenzensulfonyl chloride, Et3N, DMAP, CH2Cl2, rt, 4 h, 70%; (iii) MeI, NaH, DMF, 0 °C to rt, 3 h, 96%; (f) (i) oxalyl chloride, CH2Cl2, 0 °C, 1 h; (ii)
MeOH, CH2Cl2, rt, 1 h, 90% (over two steps); (g) (i) NaBH4, THF, 0 °C, 1 h, 100%; (ii) triethylsilane, MeI, CH3CN, 30 min, 95%; (h) (i) Mg, MeOH, 50 °C, 4 h, 54%; (ii) 4-
fluorophenylacetyl chloride, DMAP, Et3N, CH2Cl2, rt, 2 h, 94%; (i) NaOH (2 N), THF/MeOH (2:1), rt, 2 h, 98%.
rahydropyrido[1,2-a] indoles, bearing an amide moiety, (Fig. 2) act-
ing as potent and selective hCRTH2 antagonists derivatives.12
The hCRTH2 antagonists in this novel series were prepared
according to the synthetic route described in Scheme 1. Reduction
of the ester indole 4 followed by an oxidation with manganese
oxide gave the corresponding aldehyde 5. Wittig reaction followed
Table 1
Binding affinity and functional antagonism on hCRTH2 for preliminary aryl amides
CO2 H
N
R
N
by N-alkylation of the indole 5 afforded the a,b-unsaturated ester
Me
O
6, as a mixture (E/Z) of alkene isomers. This intermediate 6 was
then subjected to hydrogenolysis followed by a Dieckman conden-
sation13 with KOtBu. Decarboxylation using silica gel in refluxing
toluene yielded the desired ketone 7. Reduction of the ketone
group to the alcohol followed by activation with methanesulfonyl
chloride and SN2 reaction with sodium azide afforded the desired
azide 8. Compound 9 was generated in three steps from the azide
8. Hydrogenation of the azide gave the corresponding racemic
amine which was N-sulfonylated with 4-fluoro benzenesulfonyl
chloride under basic conditions. The resulting sulfonamide was fi-
nally N-methylated using NaH and MeI. C-3-directed nucleophilic
addition of indole 9 to oxalyl chloride afforded the glyoxylated
derivative 10 following addition of methanol.14 The resulting
keto-ester 10 was reduced with sodium borohydride followed by
the dehydration with triethylsilane and iodomethane to yield the
racemic ester 11.15 Resolution by chiral HPLC allowed the separa-
tion of the two enantiomers. The amide functionality of I was
introduced by radical deprotection of the sulfonamide using mag-
nesium followed by acylation of the resulting amine with an
appropriately substituted acyl chloride. Finally, the ester was
saponified to afford the desired amide II.
SAR studies were initiated to identify an amide group that
would be conformationally similar to the sulfonamide moiety pres-
ent in MK-7246 (Table 1). Benzamide (12), naphthamide (13, 14)
and the non-aromatic amide 20, were all found to be unacceptable
alternatives as they all lead to significant loss in potency on the
hCRTH2 receptor. On the other hand, the benzyl amides 15 and
16 were satisfactory replacements to MK-7246’s sulfonamide moi-
ety. Both compounds exhibited low nanomolar potency against
hCRTH2 receptor and behaved as full antagonists as shown by their
relative IC50’s in the functional cAMP assay. Additionally, amides
15 and 16 were selective for hCRTH2 versus the DP and TP recep-
Compd
R
hCRTH2a Ki (nM)
cAMPb IC50 (nM)
3.0
MK-7246
––
2.5
F
12
13
340
120
2500
540
14
15
16
370
15
2100
25
7.4
12
F
17
670
810
18
19
20
110
140
1700
240
3700
630
a
Radioligand competition binding assay using membrane proteins from HEK293
(EBNA) cells stably expressing the receptor CRTH2 in a 10 mM solution of HEPES/
KOH (all values are mean of two or more experiments).16
b
Functional assay: the intracellular concentration of cAMP was determined using
the 125I-cAMP scintillation proximity assay. The assay was performed in Hank’s
balanced salt solution 25 mM HEPES containing 5
lM Forskolin (Ki’s are an average
of at least tow independent titrations).17
atoms or to incorporate additional substituents failed to improve
potency on the hCRTH2 receptor.
Consequently, amide 16 was selected for further profiling. We
performed pharmacokinetic studies in Sprague–Dawley (SD) rats
(Table 4) and found that despite the fact that the clearance, the
volume of distribution and half life were comparable to
MK-7246, the oral bioavailability (14%) needed to be improved.
The in vitro metabolism of 16 in SD rat and human hepatocytes18
was investigated to see if it could shed some light on the origin
of the low oral bioavailability. The extent of metabolism observed
tors (Ki’s >1 lM). Conformationally restricted benzyl amide ana-
logs such as 17, 18, and 19 were also found to be unacceptable
replacements.
With our best amide replacement in hand, we pursued SAR
studies by exploring the substitution pattern on the amide benzyl
group (Table 2). Unfortunately, all attempts to replace the fluorine

H. Zaghdane et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3471–3474
3473
Table 2
human hepatocytes. Neither hydroxylated metabolites, nor amide
hydrolysis was observed. Since metabolism does not seem to be
implicated in the low oral bioavailability of amide 16 in rats, lack
of absorption or active transport in the liver leading to rapid elim-
ination in bile are hypothesized to be involved.
SAR on the benzyl ring substitution
CO2H
N
N
The remaining unexplored segment of the amide moiety was
the substitution at the benzylic position. Several compounds were
prepared and the results are displayed in Table 3. Introduction of a
methyl substituent led to the pair of diastereoisomers 2 and 31.19
As a result, we discovered that the chirality of the newly formed
asymmetric center had a significant impact on potency. Indeed,
the R-isomer 2 was found to be 10-fold more potent then the cor-
responding S-isomer 31 as exemplified by their respective hCRTH2
Ki’s. More importantly, the activity of 2 in the whole blood assay,
measuring eosinophil shape change, was similar to MK-7246.20
The absolute configuration of MK-7246 was determined by X-ray
crystallography.21 Analogs bearing two benzylic substituents (acy-
clic and cyclic; i.e., 32–35) were also found to be potent hCRTH2
antagonists with Ki’s ranging from 1.7 to 4.3 nM and EOS IC50’s
from 2 to 8 nM. To appropriately distinguish these substituted ana-
logs, we performed rat pharmacokinetic studies on all of them.
Compounds 2, 3, and 34 demonstrated superior pharmacokinetic
characteristics, in particular oral bioavailability, and were further
profiled (Table 4).
R1
Me
O
R2
Compd
R1
R2
hCRTH2a Ki (nM)
MK-7246
16
21
22
23
24
25
26
27
––
F
F
F
F
F
F
F
––
H
2.5
7.4
184
141
70
22
126
19
31
14
408
1220
2-Cl
2-CF3
2-F
3Cl
3-CF3
3-F
H
Cl
28
29
30
OMe
CF3
NMe2
H
H
H
a
Ki’s are an average of at least tow independent titrations.
Firstly, the binding affinities of 2, 3, and 34 were measured
against other prostaglandin receptors (IP, EP1, EP2, EP3, EP4 and
FP) and in all cases they were found to be selective for the hCRTH2
receptor by at least 500-fold. They showed little or no inhibitory
activities against the CYP3A4 and 2C9, representing a significantly
improvement over MK-7246. Finally, compound 2, 3, and 34 dem-
onstrated improved oral bioavailability compared to the unsubsti-
tuted analog 16.
Table 3
SAR on the amide linker
CO2H
N
W
N
F
Me
O
In summary, we identified a novel series of tetrahydropyri-
do[1,2-a] indole amides acting as potent and selective hCRTH2
antagonists. In particular, analogs 2, 3, and 34 were found to be
equipotent to our clinical candidate MK-7246 in the whole blood
eosinophil shape change assay but more importantly were devoid
of inhibitory activity against CYP2C9. Finally, we successfully dis-
covered a replacement for the sulfonamide moiety present in
MK-7246 by appropriately substituted amide groups.
Compd
W
hCRTH2a Ki (nM)
EOSb,d IC50 (nM)
cAMPc IC50(nM)
MK-7246
16
––
CH2
2.5
7.4
2.2
4.8
3.0
12
2
4.0
0.96
3.0
31
32
33
39
30
3.0
2.0
6.0
4.4
6.2
2.0
References and notes
34
3
1.8
1.7
1.0
1.4
4.0
4.0
1. Haeggström, J. Z.; Rinaldo-Matthis, A.; Wheelock, C. E.; Wetterholm, A. Biochem.
Biophys. Res. Commun. 2010, 21, 135.
2. (a) Sood, A.; Arora, R. J. Clin. Hypertens. 2009, 11, 685; (b) Kostenis, E.; Ulven, T.
Trends Mol. Med. 2006, 12, 148.
3. Walch, L.; Labat, C.; Gascard, J. P.; de Montpreville, V.; Brink, C.; Norel, X. Br. J.
Pharmacol. 1999, 126, 859.
O
35
4.3
2.4
8.0
4. Hirai, H.; Tanaka, K.; Yoshie, O.; Ogawa, K.; Kenmotsu, K.; Takamori, Y.;
Ichimasa, M.; Sugamura, K.; Nakamura, M.; Takano, S.; Nagata, K. J. Exp. Med.
2001, 193, 255.
5. (a) Honda, T.; Tokura, Y.; Miyachi, Y.; Kabashima, K. Curr. Drug Targets 2010, 11,
1605; (b) Jones, R. L.; Giembycz, M. A.; Woodward, D. F. Br. J. Pharmacol. 2009,
158, 104.
a,b,c
d
Ki’s and IC50’s are an average of at least tow independent titrations.
Whole blood eosinophil shape change assay.16
6. Xue, L.; Barrow, A.; Pettipher, R. Clin. Exp. Immunol. 2009, 156, 126.
7. Boyce, J. A.; Broide, D.; Matsumoto, K.; Bochner, B. S. J. Allergy Clin. Immunol.
2009, 123, 569.
was low in rat hepatocytes with 91% of the parent remaining after
1 h of incubation while 60% of the parent was glucuronidated in
Table 4
Pharmacokinetic data in SD rata as well as the CYP inhibition for potent hCRTH2 antagonist compounds
Compd
F (%)
Cl (mL/min/kg)
Vd (L/kg)
t
½
(h)
CYP 3A4 IC50
(lM)
CYP 2C9 IC50 (lM)
MK-7246
2
3
16
34
138
73
74
14
93
3.9
1.1
2.7
0.38
12
1.1
4.1
4.7
2.2
4.6
2.6
34
>50
37
>50
>50
9
0.36
0.47
0.13
2.6
>50
>50
>50
>50
a
po dosing solution: 5 mg/kg (5 mL/kg) in 0.5% methocel (n = 2), IV dosing solution: 1 mg/kg (1 mL/kg) in 60% PEG 200 (n = 2).

3474
H. Zaghdane et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3471–3474
8. (a) Schuligoi, R.; Sturm, E.; Luschnig, P.; Konya, V.; Philipose, S.; Sedej, M.;
Waldhoer, M.; Peskar, B. A.; Heinemann, A. Pharmacology 2010, 85, 372–382;
(b) Pettipher, R.; Hansel, T. T. Drug News Perspect. 2008, 21, 317.
9. (a) Indole derivatives as CRTH2 receptor antagonists, Wang, Z. WO2007019675
and WO2010031182.; (b) Gervais, F. G.; Sawyer, N.; Stocco, R.; Hamel, M.;
Krawczyk, C.; Sillaots, S.; Denis, D.; Wong, E.; Wang, Z.; Gallant, M.; Abraham,
W. M.; Slipetz, D.; Crackower, M. A.; O’Neill, G. P. Mol. Phamacol. 2011, 79, 1.
10. Slatore, C. G.; Tilles, S. A. Immunol. Allergy. Clin. N. Am. 2004, 24, 477.
11. Castrejon, J. L.; Berry, N.; El-Ghaiesh, S.; Gerber, B.; Pichler, W. J.; Park, B. K.;
Naisbitt, D. J. J. Allergy Clin. Immunol. 2010, 125, 411.
M.; Dufresne, C.; Gareau, Y.; Ruel, R.; Juteau, H.; Labelle, M.; Ouimet, N.;
Metters, K. M. Biochim. Biophys. Acta 2000, 1483, 285.
17. Sawyer, N.; Cauchon, E.; Chateauneuf, A.; Cruz, R. P.; Nicholson, D. W.; Metters,
K. M.; O’Neill, G. P.; Gervais, F. G. Br. J. Pharmacol. 2002, 137, 1163.
18. The compounds were incubated at 10 lM in the presence of fresh SD rat or
fresh human hepatocytes (1e6 cells/mL, Krebs–Henseleit buffer + 3 g/L Hepes,
pH = 7.4) for 2 h at 37 °C. Incubations were stopped by adding an equal volume
of acetonitrile + 1% formic acid, centrifuged and supernatants were analyzed by
UPLC-PDA-Qtof.
19. The absolute configuration of
2 and 31 was determined using Evans’
12. Indole derivatives as CRTH2 receptor antagonists, Colucci, J.; Boyd, M.;
Zaghdane, M. H. WO2010031183.
13. Christie, B. D.; Rapoport, H. J. Org. Chem. 1985, 50, 1239.
14. Liu, Y.; Wu, J.; Weissig, H.; Betancort, J. M.; Gai, W. Z.; Leventhal, P. S.; Patricelli,
M. P.; Samii, B.; Szardenings, A. K.; Shreder, K. R.; Kozarich, J. W. Bioorg. Med.
Chem. Lett. 2008, 18, 5955.
15. Collot, V.; Schmitt, M.; Marwah, P.; Bourguignon, J.-J. Heterocycles 1999, 51,
2823.
oxazolidinone chiral auxiliaries (data not included).; Evans, D. A.; Bartroli, J.;
Shih, T. L. J. Am. Chem. Soc. 1981, 103, 2127; Bull, S. D.; Davies, S. G.; Garner, A.
C.; Kruchinin, D.; Key, M.-S.; Roberts, P. M.; Savory, E. D.; Smith, A. D.;
Thomson, J. E. Org. Biomol. Chem. 2006, 4, 2945.
20. (a) Gervais, F. G.; Cruz, R. P.; Chateauneuf, A.; Gale, S.; Sawyer, N.; Nantel, F.;
Metters, K. M.; O’Neill, G. P. J. Allergy Clin. Immunol. 2001, 108, 982; (b)
Monneret, G.; Gravel, S.; Diamond, M.; Rokach, J.; Powell, W. S. Blood 2001, 98,
1942.
16. Abramovitz, M.; Adam, M.; Boie, Y.; Carrière, M.-C.; Denis, D.; Godbout, C.;
Lamontagne, S.; Rochette, C.; Sawyer, N.; Tremblay, N. M.; Belley, M.; Gallant,
21. The absolute configuration of MK-7246 was determined by X-ray
crystallography (Unpublished data).