Azaindoles as potent CRTH2 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2010.11.084

A new class of 7-azaindole analogs of MK-7246 as potent and selective CRTH2 antagonists is reported. The SAR leading to the identification of the optimal azaindole regioisomer as well as the pharmacokinetics and off-target activities of the most potent antagonists are disclosed.

Full text of DOI:10.1016/j.bmcl.2010.11.084

Bioorganic & Medicinal Chemistry Letters 21 (2011) 841–845
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Azaindoles as potent CRTH2 receptor antagonists
⇑
Daniel Simard , Yves Leblanc, Carl Berthelette, M. Helmi Zaghdane, Carmela Molinaro, Zhaoyin Wang,
Michel Gallant, Stephen Lau, Trinh Thao, Martine Hamel, Rino Stocco, Nicole Sawyer, Susan Sillaots,
Francois Gervais, Robert Houle, Jean-François Lévesque
Merck Frosst Centre for Therapeutic Research, 16711 Trans Canada Highway, Kirkland, Québec, 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 class of 7-azaindole analogs of MK-7246 as potent and selective CRTH2 antagonists is reported. The
SAR leading to the identification of the optimal azaindole regioisomer as well as the pharmacokinetics and
off-target activities of the most potent antagonists are disclosed.
Received 28 October 2010
Revised 14 November 2010
Accepted 17 November 2010
Available online 21 November 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
CRTH2
Chemoattractant receptor-homologous
PGD2
Prostanoid
Azaindole
Tetrahydropyridoindolizines
Prostaglandin D2 (PGD2) is known to bind with high affinity to
two G protein-coupled seven-transmembrane receptors DP1 and
DP2. The latter receptor, also known as CRTH2 (chemoattractant
receptor-homologous molecule expressed on Th2 cells), is respon-
sible for the chemoattractant effects of PGD2 on eosinophils, baso-
phils and Th2-cells.1,2 The receptor also induces eosinophils shape
change and degranulation, the production of proinflammatory
cytokines in Th2 cells, and enhances the release of histamine from
basophils. The combined pharmacological action of such biological
events plays a key role in late phase allergic inflammation. CRTH2
is thus pursued as a target for respiratory diseases such as asthma
and COPD.3
We recently reported the identification of a potent and selective
CRTH2 antagonist4 (Fig. 1). As Phase I clinical trials were in progress,
our main focus was to develop a potential backup for MK-7246
which would have an improved off-target activity profile. Our lead
compound had a propensity to undergo moderate covalent binding
on hepatic cells and showed micromolar activity on CYP2C9.
Consequently, efforts in a new series were undertaken.
series and potentially avoiding allergic-reaction-type adverse ef-
fects related to the sulfonamide moiety,6 an amido-azaindole ser-
ies was also explored and is reported.
The original lengthy synthesis of MK-7246 proved to be incom-
patible with azaindoles as starting material. The key Dieckmann
condensation could not be applied to that system and as a result
a new synthetic route was devised.4b
Efforts in other series had led to the development of a new route
taking advantage of the multi-nucleophilic nature of the indole
(Scheme 1).7 The use of an electrophilic aziridine bearing all of
the carbons necessary for the construction of the carbocycle as well
as the stereogenic center required in the final product, dramati-
cally reduced the synthetic efforts needed to pursue SAR in that
series. The number of linear steps fell from 18 to only eight steps.
The synthesis of the aziridine 8 (Scheme 2) started with D-aspar-
tic acid as chiral building block. Methylation of the carboxylic acids
followed by the introduction of the required sulfonamide yields the
diester 9. The diester was then reduced to reveal diol 10 which
One of the preferred approaches was to synthesize close analogs
of the lead compound where the indole core would be replaced by
an azaindole.5 This Letter reports the synthesis of all four possible
regioisomers (Fig. 2) as well as their structure–activity relation-
ship. In view of increasing structural diversity of the newly born
CO2H
F
N
N
S
O
O
⇑
Corresponding author. Tel.: +1 514 428 8508; fax: +1 514 428 4900.
E-mail address: daniel.simard@merck.com (D. Simard).
Figure 1. Lead compound MK-7246.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.11.084

842
D. Simard et al. / Bioorg. Med. Chem. Lett. 21 (2011) 841–845
CO2H
CO2H
R
R
(a)
N
N
F
F
N
N
N
N
N
H
N
Cl
Cl
OTBS
NHTsF
12
11
N
N
S
O
S
O
O
O
1
2
(b)
N
CO2H
CO2H
R
R
(c)
F
F
N
N
N
N
N
N
N
Cl
Cl
OH
NHTsF
O
N
N
S
O
S
NHTsF
O
O
14
13
R = CO2Me, CH2OTHP
O
3
4
Figure 2. Azaindole analogs.
Scheme 3. General synthesis of condensation precursor from aziridine 8. Reagents
and conditions: (a) NaH, 8, DMF, 38–85%; (b) TBAF, THF, 86–100%; (c) DMP, DCM,
45–95%.
underwent selective intramolecular Mitsunobu reaction with the
vicinal sulfonamide. The resulting alcohol bearing aziridine could
then be protected with a TBS group and used for rapid construction
of our inhibitors.
The synthesis began with the addition of the aziridine 8 onto
the azaindole core 11 with sodium hydride in DMF. Simple syn-
thetic transformations then allowed the formation of an electro-
philic aldehyde 14 (Scheme 3). Although, this aldehyde was
known to cyclize in high yield to generate the corresponding tricy-
clic compound with the indole core, the same reactivity was not
observed with all azaindole regioisomers.
Indeed, most regioisomers cyclized with low yields except
7-azaindoles 18 and 19 (Scheme 4). The 4-azaindole 15 failed to
condense under normal reaction condition (PPTS, toluene, reflux).
Only when the solvent was switched to NMP, a low yield cyclization
was observed. 4-Azaindole and 6-azaindole cyclizations failed in all
conditions attempted. Cyclization was observed in low yield only
when a chlorine substituent was incorporated onto the azaindole
core (i.e., 16 and 17).
Next, the reduction of the condensation double bond and the
carbon–chlorine bonds was investigated (for condensation prod-
ucts of compounds 16 and 17). Condensation product of compound
19 was singularly hydrogenated using Monsanto catalyst11 to
avoid dehalogenation of chlorine.
CO2Me
CO2H
(a),(f),(i)
(b),(f),(g),(i)
(c),(f),(i)
N
N
O
N
N
N
N
N
15
Cl
1
2
N TsF
CO2H
HN TsF
OTHP
CO2R
N
N
N
CO2H
CO2R
O
N
3
N
Cl
Cl
2
N
N
16
N
H
N TsF
CO2H
HN TsF
CO2Me
N
7
N
1
OTBS
N
6
ArO2S
N
NH
ArO2S
ArO2S
N
5
O
8
N
17
3
4
Scheme 1. Retrosynthetic analysis of key step.
N TsF
CO2H
HN TsF
CO2Me
(d),(f),(i)
NH2
(a),(b)
(d),(e)
TsF
O
HN
MeO2C
N
N
N
CO2H
N
HO2C
CO2Me
(c)
18
N TsF
CO2H
HN TsF
CO2Me
D-aspartic acid
9
TsF
N
(e),(h),(i)
TsF
HN
HO
O
OTBS
N
Cl
N
OH
Cl
N
8
O O
S
10
19
20
N TsF
HN TsF
TsF =
Scheme 4. Condensation to tricylic azaindoles. Reagents and conditions: (a) PPTS,
NMP, 180 °C, 31%; (b) PPTS, toluene, 110 °C, 27%; (c) PPTS, 4:1 toluene/NMP, 150 °C,
6%; (d) PPTS, toluene, 110 °C, 86%; (e) PPTS, toluene, 110 °C, 80%; (f) Pd/C, H2, MeOH,
50–95%; (g) DMP, DCM, 99% then NaClO2, H3PO4, 2-methyl-2-butene, 72%; (h)
½RhðCODÞðR; RÞ-DIPAMPꢀþBF4ꢁ, 40 psi H2, MeOH, 95%. (i) NaH, MeI, THF, 70–95%
then aq NaOH, MeOH, THF, 90–99%.
F
Scheme 2. Synthesis of chiral aziridine 8. Reagents and conditions: (a) SOCl2,
MeOH, 95%; (b) 4-fluorobenzene sulfonyl chloride, TEA, THF, 95%; (c) NaBH4, EtOH,
70%; (d) PBu3, 1,10-(azodicarbonyl)dipiperidine (ADDP), THF, 85%; (e) TBSCl,
imidazole, THF, 80%.

D. Simard et al. / Bioorg. Med. Chem. Lett. 21 (2011) 841–845
843
CO2Me
N TsF
CO2H
CO2Me
CO2H
(a),(b),(c)
(a),(b)
F
N
N
N
N
Cl
N
N
N
N
R
F
21
22
N
S
N
N TsF
23
24
O
O
O
Scheme 5. Diversification at position-6 of the 7-azaindole core. Reagents and
conditions: (a) 4-fluorophenyl boronic acid, NaHCO3, Pd(dppf)2(CH2Cl2)2, 40%;
(b) aq NaOH, MeOH, THF, 85%.
Scheme 6. Synthesis of azaindole amides. Reagents and conditions: (a) Mg, MeOH,
80%; (b) RCO2H, HATU, DIPEA, DMF, 65–96%; (c) aq. NaOH, THF, MeOH, 85–99%.
Compound 16 was generated using a literature procedure12 and
required two-step Dess–Martin-periodinane/Pinnick oxidation
prior to the methylation of the sulfonamide. Compound 22
(Scheme 5) was obtained from the Suzuki coupling of the methyl
ester intermediate 21 (en route to compound 20). Hydrolysis of
all methyl ester intermediates with sodium hydroxide yielded
compounds 1–4, 20 and 22 which were assayed for binding on pro-
stanoid receptors (Table 1).
From the results shown in Table 1, only the 7-azaindoles 4, 20
and 22 demonstrated low nanomolar activity in the hCRTH2
binding assay. Similar results in the cAMP functional assay clearly
indicated that this class of compounds behaves as a full antago-
nist. Additionally, compounds 4, 20 and 22 showed superior
selectivity for hCRTH2 against DP and TP prostanoid receptors
in comparison to compound MK-7246. Conversely, the 4- and
5-azaindole regioisomers 1 and 2 showed very low affinity on
all prostanoid receptors tested, and although the 6-azaindole 3
displayed some affinity for hCRTH2 (139 nM), we focused our ef-
forts on the more promising 7-azaindole series. Finally, com-
pound 4 was found not to be a CYP2C9 inhibitor (IC50 >50
lM)
as compared to the lead MK-7246 (IC50 = 9.4 M) but failed to ad-
l
dress the in vivo covalent binding issue associated with MK-7246
(vide infra).
Functionalization at the position-6 of the 7-azaindole with chlo-
rine is well tolerated. In the EOS whole blood assay, the azaindole
20 exhibited an activity similar compared to MK-7246. Since the
presence of a 6-chloro substituent creates an electrophilic site on
the azaindole core, we considered introducing a metabolically
more robust substituent that would maintain the gain in potency
while reducing the possibility of a nucleophilic addition mediated
metabolism. With the introduction of an aromatic ring at position-
6, as in compound 22 (Scheme 5), the binding affinity for hCRTH2
was conserved, but an eight fold shift in the whole blood EOS assay
was observed.
Although the overall profile of 4 was encouraging, we pursued
its optimization based on SAR studies imported from a novel class
of compounds in which the sulfonamide moiety of MK-7246 was
replaced by an amide group.13 Potential liabilities related to the
Table 1
Activities of azaindole sulfonamides
CO2H
4
5
F
N
R6
N
7
N
S
O
O
Compd
Na
R
hCRTH2b Ki (nM)
cAMPc IC50 (nM)
EOSd IC50 (nM)
DPb Ki (nM)
TPb Ki (nM)
MK-7246
1
2
3
4
20
22
—
4
5
6
7
7
7
H
H
H
H
H
Cl
4F-Ph
2.5
3.0
—
—
351
3.4
3.2
3.6
2.2
—
—
373
>1300
>3400
>12000
>47000
>18000
>3800
3804
>7000
>6800
>22000
>22000
>27000
>1000
7189
6725
139
3.3
1.8
1.9
—
7.0
3.3
15.8
a
Azaindole nitrogen position.
b
Radioligand competition binding assay using membrane proteins from HEK293 (EBNA) cells stably expressing the receptor hCRTH2 in a 10 mM solution of HEPES/KOH
(all values are mean of two or more experiments).8
c
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 l
M Forskolin (all values are mean of two or more experiments).9
d
Human whole blood eosinophil shape change assay (all values are mean of two or more experiments).10
Table 2
Covalent binding and CYP profiles of selected antagonistsa
Compd
In vivo CB (pmol equiv/mg)
CYP2C9 IC50
(lM)
CYP3A4 IC50
(l
M)
3A4 TDI Kobs (minꢁ1) at 50 mM
6 h
24 h
MK-7246
4
24a
61
52
8
30
50
6
9.4
>50
>50
34
>50
>50
0.047
—
<0.004
a
IC50 are averages of at least two experiments.

844
D. Simard et al. / Bioorg. Med. Chem. Lett. 21 (2011) 841–845
Table 3
Activities of azaindole amides
CO2H
N
N
W
O
F
N
Compd
W
hCRTH2a Ki (nM)
cAMPb IC50 (nM)
5.7
EOSc IC50 (nM)
1.2
DPa Ki (nM)
TPa Ki (nM)
24a
3.4
>38000
>71000
24b
24c
24d
21.1
4.7
73.8
3.5
—
>12000
>10000
>3700
>22000
>21000
>6800
2.3
3.1
3.6
7.5
24ed
24f
5.1
3.4
3.9
4.4
4.7
4.5
2.4
1.2
3.4
>4000
>4000
>4000
>7200
>7200
>7200
24g
O
24h
11.5
7.0
-
>4000
>7200
a–c
See Table 1.
Diastereomeric mixture.
d
presence of a sulfonamides group (i.e., allergic reaction to ‘sulfa’
drugs) as well as the potential for in vivo covalent binding found
for compound MK-7246 and 4 (Table 2) were the main drivers
for exploring this new chemical space.
Following the sequence shown in Scheme 6, we converted the
previously obtained sulfonamide intermediate 23 into amides via
a magnesium radical reduction of the sulfonamide to the second-
ary amine. The resulting amine was diversified using a series of dif-
ferent carboxylic acids using HATU coupling conditions followed
by hydrolysis under standard conditions to obtain antagonists
24a–h.
results are based on only two animals. The improved CYP profile
(i.e., lower CYP2C9 inhibition) observed with 4 translated equally
well in the amide series as shown by the similar results obtained
for compound 24a (Table 2). In addition, 24a was found to display
a lower activation constant (Kobs), a hallmark measurement of
CYP3A4 time dependant inhibition (TDI)14 and a significantly low-
er in vivo covalent binding as compared to MK-7246 (Table 2). A
reduced TDI of CYP3A4 for compound 24a lowers the risks of po-
tential drug–drug interaction caused by gradual accumulation of
irreversible or quasi irreversible binders to the cytochrome active
site.
Most amides obtained through this process showed low nano-
molar activity on both the binding and the whole blood assays
(Table 3). They also showed excellent selectivity for the hCRTH2
receptor over the other prostanoid receptors. Pharmacokinetic
studies in rats on this class of compounds revealed that they were
orally bioavailable and exhibited a wide range of clearance rates
and half-lives (Table 4).
In conclusion, the indole core found in MK-7246 was effectively
replaced by a 7-azaindole core while retaining affinity for the
hCRTH2 receptor and activity in the eosinophil shape change
whole blood assay. The selectivity over other prostanoid receptors,
mainly DP and TP, was improved as well as the CYP profile. The
introduction of an amide group in lieu of the sulfonamide moiety
reduced the shift in the EOS whole blood assay but more impor-
tantly solved the covalent binding issue that was observed with
our lead compound. As a result of the SAR studies described in this
manuscript, we identified the azaindole 24a as a potential backup
compound that addressed the potential liabilities associated with
MK-7246.
From these studies, two compounds (4, 24a) stood out for their
low clearance and relatively high bioavailability, although, these
Table 4
Sprague–Dawley (SD) rat pharmacokinetic data for selected CRTH2 antagonistsa
Compd
F (%)
Cl (ml/min/kg)
Vd (L/kg)
T (h)
½
Acknowledgments
MK-7246
4
24a
24c
24d
24f
138b
50
3.9
0.4
3.6
21
12
28
1.15
0.23
0.67
1.54
2.20
4.58
1.72
4.1
7.5
4
1.9
3.1
2.1
The authors would like to thank Nicolas Lachance, Scott
Peterson and Michel Gallant, for proofreading this Letter.
84
156b
63
20
38
References and notes
24g
18
11.6
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