Substituted indole-1-acetic acids as potent and selective CRTh2 antagonists-discovery of AZD1981

Article abstract of DOI:10.1016/j.bmcl.2011.08.124

Novel indole-3-thio-, 3-sulfonyl- and 3-oxy-aryl-1-acetic acids are reported which are potent, selective antagonists of the chemoattractant receptor-homologous expressed on Th2 lymphocytes receptor (CRTh2 or DP2). Optimization required maintenance of high

Full text of DOI:10.1016/j.bmcl.2011.08.124

Bioorganic & Medicinal Chemistry Letters 21 (2011) 6288–6292
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Substituted indole-1-acetic acids as potent and selective CRTh2
antagonists—discovery of AZD1981
⇑
,
Tim Luker a, , Roger Bonnert a, , Steve Brough a,à, Anthony R. Cook a, Mark R. Dickinson a, Iain Dougall b,
Chris Logan c, Rukhsana T. Mohammed a,à, Stuart Paine c,à, Hitesh J. Sanganee a, Carol Sargent b,
Jerzy A. Schmidt b, Simon Teague a,à, Stephen Thom a,à
a Medicinal Chemistry, AstraZeneca R&D Charnwood, Loughborough, Leicestershire LE11 5RH, UK
b Bioscience, AstraZeneca R&D Charnwood, Loughborough, Leicestershire LE11 5RH, UK
c Drug Metabolism and Pharmacokinetics, AstraZeneca R&D Charnwood, Loughborough, Leicestershire LE11 5RH, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel indole-3-thio-, 3-sulfonyl- and 3-oxy-aryl-1-acetic acids are reported which are potent, selective
antagonists of the chemoattractant receptor-homologous expressed on Th2 lymphocytes receptor (CRTh2
or DP2). Optimization required maintenance of high CRTh2 potency whilst achieving a concomitant
reduction in rates of metabolism, removal of cyp p450 inhibition and minimization of aldose reductase
and aldehyde reductase activity. High quality compounds suitable for in vivo studies are highlighted, cul-
minating in the discovery of AZD1981 (22).
Received 18 July 2011
Revised 29 August 2011
Accepted 31 August 2011
Available online 5 September 2011
Keywords:
CRTh2
Ó 2011 Elsevier Ltd. All rights reserved.
DP2
Prostaglandin
Inflammation
Asthma
Candidate drug
Optimization
SAR
The chemoattractant receptor homologous expressed on Th2
lymphocytes receptor (CRTh2 or DP2),1 one of the receptors for
prostaglandin D2 (PGD2), is highly expressed in humans on key
cells implicated in the pathology of asthma and other allergic dis-
eases.1,2 Activation of CRTh2 elicits responses including chemo-
taxis1 and mediator release.3 These findings have stimulated
considerable interest in the development of CRTh2 antagonists as
novel treatments for asthma and other allergic diseases, such as
allergic rhinitis.4
A variety of molecular frameworks have emerged as potent
CRTh2 ligands and these have been recently reviewed.5 Indole
acids have emerged as one class of CRTh2 ligands, perhaps inspired
by early reports of surprising CRTh2 activity from compounds such
as Indomethacin2 (1, Fig. 1) a non-selective cyclo-oxygenase inhib-
itor and Ramatroban (2),6 originally developed as a thromboxane
A2 receptor (TP) antagonist. Compounds from the AstraZeneca col-
lection with structural similarity to Indomethacin were screened to
provide both agonists and antagonists of CRTh2, from which a
series of 3-quinolinyl-indole-1-acetic acids (3) were identified.7
Publications from other groups have disclosed alternative indole-
derived CRTh2 series.8
An additional hit molecule
4 intriguingly highlighted the
3-thioaryl indole as a novel CRTh2 core template.
CO2H
CO2H
MeO
N
F
N
O
Cl
N
H
S
O
O
1
2
CO2H
CO2H
Me
N
N
CO2H
N
S
S
Cl
⇑
N
Corresponding author.
E-mail addresses: tim.luker@astrazeneca.com, fxxtim@gmail.com (T. Luker).
Equal first authors.
Former AstraZeneca employee.
3
4
5
à
Figure 1. Known CRTh2 scaffolds 1–3 and AstraZeneca hits 4–5.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.08.124

T. Luker et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6288–6292
6289
Table 1
Preliminary SAR
This data set showed chloro substituents were more potent at
CRTh2 at the indole 4- and 5-positions than at the 6- or 7-positions
(7, 11–13). Other small substituents were also potent, with 5-fluoro
indole 14 having increased potency compared to the 5-chloro
analogue (7). The larger electron withdrawing methylsulfone
regio-isomers (18–19) had a stronger potency preference for the
4- over the 5-isomer, but interestingly the phenyl substituent was
well tolerated at either position. As no closely related SAR has been
published, it was inferred from these results that a mixture of size,
lipophilicity and electronic factors were affecting CRTh2 potency.
A range of indoles containing polar amide and sulfonamide
groups were prepared (21–25). Given the dramatic reduction of
lipophilicity that these substituents produced, it was somewhat
surprising that compounds 21 and 22 had such exceptional po-
tency. Alkylating the amide lost considerable potency (23 vs 22)
as did reversing the amide link (24 vs 22). Moving the acetamide
group to the indole 5-position resulted in a loss of affinity at CRTh2
(25 vs 22). Finally, derivatives bearing heterocycles at the indole
4-position (26, 29) showed high affinity for CRTh2.
R1
N
R2
R4
R3
S
a
R1
R2
R3
R4
CRTh2 binding IC50 (nM)
5
6
7
8
9
10
CH2CO2H
CH2CO2H
CH2CO2H
CH2CO2H
Me
Me
Me
H
Me
Me
H
Me
Me
Cl
Cl
Cl
20
15
13
89
501
1468
Cl
Cl
Cl
Cl
Cl
CH(Me)CO2H
CH2 (1H-tetraz-5-yl)
Cl
a
Radioligand binding assay (3H-PGD2), mean of n >2 measurements.
Compound 4 was of modest CRTh2 potency (binding7 IC50
794 nM), however by analogy to compounds 1 and 3, the methyl
and acetic acid side chain positions on the indole core were re-
versed and gratifyingly this resulted in a substantial increase in
CRTh2 binding potency (Table 1, 5: IC50 20 nM). Compound 5
was also shown to behave as an antagonist in a functional assay,
blocking PGD2 mediated Ca2+ flux in HEK cells expressing human
CRTh27 (IC50 25 nM). The potency in this functional screen was
similar to that observed from the radioligand binding assay. Given
that the indole and acid components of the molecules are also
present within the pharmacophores known for other related pro-
stanoid targets (and as highlighted by structures 1 and 2) it was
unexpected that 5 was more than 100-fold selective over DP1, TP
and COX-1.
Wider screening across a range of receptors and enzymes re-
vealed rat aldose reductase inhibition to be somewhat of a concern
for 5 (MDS Pharma (now Ricerca), IC50 157 nM). Indomethacin (1)
and other structurally related acetic acids are known to inhibit hu-
man aldose reductase (ALR2).9 ALR2 is a member of the aldo-keto
superfamily of enzymes which includes the closely related alde-
hyde reductase (ALR1). Because both enzymes share a role in the
removal of toxic aldehydes we tested 5 against human recombi-
nant ALR2 and ALR1.10 Gratifyingly the human activity at ALR2
The more lipophilic compounds as a general trend had higher
rates of metabolism (11–12, 27–28) although this SAR was often
compound specific and unpredictable (e.g., 22 and 29 have similar
log D’s but quite different rates of metabolism).
Representative compounds were screened against ALR2 and
ALR1 and high selectivity was found across this set of 3-thioaryl
analogues. Gratifyingly, some compounds also had improved selec-
tivity over p450 enzymes (IC50 21: 2C9 10
2D6 all >10 M).
lM; 22: 1A2, 3A4, 2C9,
l
Modifications to the side chain at the 3-position of the indole
were investigated (Table 3). These included changes to the linker
atom between the two aromatic rings as well as variation of the
ring substituents. For example, S-aryl substituents such as 3-Cl
(30) and 2-Cl (31) as well as the 4-methylsulfone (32) and 4-meth-
oxy (33) were highly potent CRTh2 ligands. The high potency of the
alkoxy and methyl sulfone substituents was in contrast to some
other reports.8c,8g For example, SAR now revealed by Novartis8g
showed their 4-methoxyphenyl analogue to lose considerable
activity, highlighting SAR divergence across series of indole/azain-
dole derived CRTh2 antagonists. Unfortunately, 32 was a reason-
ably potent cyp 2C9 inhibitor (IC50 5.2 lM) despite its lower
lipophilicity and the alkoxy substituent in 33 promoted unwanted
(IC50 31.6
lM) was weaker than rat, however the level of human
ALR1 activity.
ALR1 activity (IC50 3.16
l
M) was more concerning but still with
The sulfoxide linker (34) was significantly less potent than the
original sulfide (6) at CRTh2, however the sulfone and oxygen
linked compounds (35–49) had more encouraging levels of CRTh2
activity. Several of the indole and aryl substituents were potent
across the different linkers. For example, the combination of a
5-F-indole core with a 4-Cl-phenyl side chain showed potency in
excess of 10 nM across the S- (14), SO2- (36) and O- (43) linkers.
However this was not always the case and there were a number
of surprises within the SAR across the individual linker types. For
example, with the SO2-linker, a 2-Cl-phenyl was poorly tolerated
when compared to the S-linker (40 vs 31). 2,6-Di-chloro (42) was
also poor as a sulfone link. These observations have subsequently
been published by Novartis in a series of reverse azaindoles with
a sulfone linker.8g This publication also highlighted the superiority
of a 4-Cl over 3-Cl and 2-Cl isomers. However it is difficult in our
data to rationalize why the 3-chloro isomer (41) was of modest
CRTh2 potency in the sulfone series, but highly potent in the
sulfides (30).
ꢀ150-fold separation from CRTh2. Interestingly, we are not aware
of reports from other CRTh2 antagonist series highlighting this po-
tential selectivity concern.
With this encouraging profile, compounds with close structural
changes were prepared to unravel the CRTh2 potency SAR within
this new series (Table 1). A chlorine atom in the 4-position of the
S-aryl gave a small potency increase (5 vs 6), and a methyl group
at the indole 2-position gave a substantial 6-fold potency increase
(8 vs 7). Potent analogues were also shown to be antagonists in the
functional Ca2+ assay (6: IC50 87 nM, 7: IC50 35 nM). Attempts were
also made to modify the structure around the carboxylic acid
group, but unfortunately methyl substitution adjacent to the acid
(9) and replacement of the carboxylate with a tetrazole (10) were
poorly tolerated. Others have also reported poor results with a-Me
acids in their indole derived series.8d Compound 6 was further pro-
filed and had reasonable metabolic stability (rat hepatocyte (rat
hep) CLint 18, human microsomes (hum mic) CLint <3), but was
unfortunately found to be a relatively potent inhibitor of cyto-
chrome p450 (cyp) 2C9 (IC50 800 nM).
O-linked analogues (46, 49) were slightly less potent (ꢀfive-
fold) than the S-linked analogues, but overall retained encouraging
levels of CRTh2 activity. Some changes to the heterocyclic indole
core were also prepared. The indole was successfully replaced with
pyrrolopyridines (50, 51). These were slightly less potent at CRTh2
than their structurally-matched indoles ( 6, 13), but were of
With the goals of increasing potency at CRTh2, minimizing the
rate of metabolism and controlling selectivity over ALR1, ALR2
and cyp 2C9, a wider range of structural changes were investigated.
Substituent changes on the indole core are summarized in Table 2.

6290
T. Luker et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6288–6292
Table 2
Modulation of indole substituents
CO2H
N
R1
S
Cl
11-29
R1
CRTh2 binding IC50 (nM)
hALR2 IC50 (nM)b
hALR1 IC50 (nM)
Rat hep CLint
Hum mic CLint
Log D7.4
a
c
d
e
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
7-Cl
6-Cl
4-Cl
5-F
60
42
25
1.6
1.3
6.1
178
159
71
11220
1995
5012
>7943
3548
4732
—
—
—
—
>5309
2512
—
—
—
—
—
4467
—
4467
1122
1778
3162
2239
2239
—
—
—
—
1000
794
—
—
—
—
—
1778
—
75
64
40
19
17
6
29
<3
<6
—
6
1
19
<3
8
<1
1
<1
<3
<3
4
4
<3
—
<3
13
<1
32
<3
36
43
<1
16
8
2.0
2.5
2.2
1.8
2.2
1.6
—
0.8
0.5
0.6
0.2
À0.2
1.6
À0.9
0.2
—
4-Me, 5-F
4-CN, 5-Cl
5-Cl, 6-CN
6-SO2Me
5-SO2Me
4-SO2Me
4-MeSO2NH
4-MeC(O)NH
4-MeC(@O)N(Et)
4-MeNHC(@O)
5-MeC(O)NH
4-(2-Thiazolyl)
4-Ph
11
1.8
4.3
224
45
56
3.5
1.0
3.5
0.6
—
25
—
2.6
3.7
0.0
5-Ph
4-(2-Pyrazinyl)
12
a
b
c
Radioligand binding assay (3H-PGD2), n >2 measurements.
Human aldose reductase, details in Ref. 10
Human aldehyde reductase, details in Ref. 10
d
e
Rat hepatocyte intrinsic clearance (
Human liver microsomes intrinsic clearance (ll/min/mg).
l
l/min/1 Â 106 cells).
Table 3
SAR for modulation of side chain and core
CO2H
CO2H
CO2H
N
N
N
N
R2
N
R2
R2
R1
X
X
X
Cl
30-49
50
51
R1
R2
CRTh2 binding IC50 (nM)
hALR2 IC50 (nM)b
hALR1 IC50 (nM)c
Rat hep CLint
Hum mic CLint
LogD7.4
a
d
e
X
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
S
S
S
S
5-Me
5-Me
5-Me
5-Me
5-Me
5-Me
5-F
5-Me
4-MeC(O)NH
4-MeSO2NH
5-Me
5-Me
5-Me
3-Cl
2-Cl
4-SO2Me
4-MeO
4-Cl
4-Cl
4-Cl
3-MeO
4-Cl
4-Cl
2-Cl
3-Cl
2,6-Di-Cl
4-Cl
4-Cl
4-Cl
4-Cl
4-SO2Et
4-Cl
4-Cl
3.0
2.6
1.4
7.1
1000
26
7.1
71
7.1
7.9
178
79
126
5.0
11
3.5
18
3.5
36
4.0
56
3162
11220
>10000
251
—
6310
—
724
4467
>5843
—
—
—
—
—
—
—
1778
5623
7943
282
—
411
—
115
708
63
—
—
—
—
—
—
—
3981
—
—
31
24
7
24
<5
—
<3
14
<1
5
21
—
—
<3
—
8
—
12
<3
<3
<3
3
7
<3
3
<3
<3
<1
—
—
<3
—
<3
—
<7
—
<3
5
—
1.9
1.7
À0.1
1.0
0.1
0.1
À0.2
À0.4
0.4
SO
SO2
SO2
SO2
SO2
SO2
SO2
SO2
SO2
O
O
O
O
O
O
O
S
S
À0.8
—
—
—
5-F
5-CF3
1.3
2.3
À1.0
À1.4
0.3
4-MeSO2NH
4-MeC(O)NH
5-CF3
4-EtSO2NH
4-Ph
>10000
—
—
—
—
6
—
14
5
—
À0.2
—
—
—
4-Cl
4-Cl
—
—
0.4
0.9
32
a
b
c
Radioligand binding assay (3H-PGD2), mean of n >2 measurements.
Human aldose reductase see Ref. 10
Human aldehyde reductase, see Ref. 10
d
e
Rat hepatocyte intrinsic clearance (
Human liver microsomes intrinsic clearance (ll/min/mg).
l
l/min/1 Â 106 cells).

T. Luker et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6288–6292
6291
CO2Et
CO2H
H
N
H
a or b or c
A
N
e
N
N
d
R1
R1
R1
R1
A = NH2
or H
Cl
S
S
S
S
I
II
Cl
Cl
Cl
e
R1
4-NO2
=
R1 = 4-NHSO2Me
or 4-NHC(O)Me
O
f, g or h
N
CO2H
CO2Et
CO2H
i
N
N
R2
R1
R1
S
S
X
Me
HS
HN
Cl
R2
X = SO2 or X = C(O)
Scheme 1. Reagents and conditions: (a) A=NH2, AcOH, MeCN, H2O; (b) A=H, SO2Cl2, proton sponge, Et3N, CH2Cl2; (c) A=H, t-BuOCl, CH2Cl2, Et3N, 17% for product R1 = 4-NO2;
(d) BrCH2CO2Et, NaHMDS or NaH, THF or K2CO3, DMF, 80% for R1 = 4-NO2; (e) NaOH, H2O, THF or THF/EtOH or MeOH, 44% for 21, 55% for 22; (f) Pt/C, EtOH, H2, 83%; (g) AcCl,
Et3N, CH2Cl2, 94%; (h) MeSO2Cl, Et3N, CH2Cl2, 73%; (i) I2, DMF, 55% for 30.
significantly lower log D and so provided interesting additional
diversity.
desired acidic CRTh2 antagonists. A complimentary route (step i)
allowed late stage introduction of substituted S-aryl side chains.
SO2-linked compounds could be prepared by S-oxidation chemis-
try (not shown).
Overall, there were no discernable trend of CRTh2 affinity with
log D, however compounds that combined high metabolic stability
and low cyp 2C9 activity tended to have log D <0.5 (Figs. 2a and b).
Unfortunately, for a given (low) log D value, a wide variety of
compound performance was attained. For example at a log D of
ꢀ0.3 there is at least a 10-fold variation in both the cyp 2C9 po-
A selection of additional compounds was screened against ALR2
and ALR1. Some of the more promising SO2-linked compounds
showed surprisingly reduced selectivity against ALR1 (35, 37, 39
each <20-fold selective). One can speculate that these CRTh2
antagonists might mimic parts of the pharmacophore present
within some known ALR2/1 inhibitors (e.g., Indomethacin,
Zopolrestat), although other parts of these ligands appear quite
different.9b,9c From the sulfones screened, only 38 had 100-fold
selectivity over ALR1. It was also devoid of significant cyp
liabilities. The high ALR1 selectivity of O-linked 47 was unexpected
given that aryloxy side chains had diminished selectivity in other
compounds (37, 33).
Synthetic methods for the preparation of the CRTh2 antagonists
are summarized in Scheme 1. Additional details have been
reported elsewhere.11 The 3-thioaryl indoles (I) were synthesized
from 2-thioaryl-ketones using either Fisher (step a) or Gass-
mann12a (step b/c) methodology. The corresponding 3-oxyaryl in-
doles (not shown) were made from the analogous 2-oxyaryl
ketones using a modified Fisher indole synthesis catalyzed by
PCl3.12b Additional transformations (steps d–h) afforded the
Figure 2b. Rat hepatocyte CLint versus log D. Less than values are marked where
appropriate.
Table 4
Rata pharmacokinetic properties of key analogues
Rat Cl (ml/min/kg)b
Rat Vss (L/kg)b
Rat t½ (h)b
Rat F%c
21
22
38
45
11
4
4
1.1
0.5
0.2
1.7
1.7
1.9
0.9
1.4
42
63
<10
>46
20
a
b
c
Male sprague dawley.
IV dose 1 mg/kg.
Oral dose 4 mg/kg (21, 22) or 3 mg/kg (38, 45).
Figure 2a. cyp 2C9 pIC50 versus log D. Less than values are marked where
appropriate.

6292
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S.; Jorand-Lebrun, C.; Page, P.; Campbell, G.; Colovray, V.; Missotten, M.;
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A. Med. Chem. Lett. 2011. ASAP.
tency and rate of metabolism (rat hep CLint), rendering it challeng-
ing to predict the profile of individual compounds before synthesis.
Based on the optimal in vitro properties the best 4 compounds
were progressed to in vivo profiling (Table 4). Highly polar com-
pounds such as these are at theoretical risk of low levels of perme-
ability and absorption.13 For example, passive permeability drops
considerably as log D falls below 1–2. The four compounds tested
had low to moderate clearance as expected from the low in vitro
rates of metabolism in rat hepatocytes, and low volumes of distri-
bution in common with many strong acids. As hoped, this drove
encouraging half-lives. Although compound 38 had very low oral
bioavailability, interestingly, given their low lipophilicity, the other
compounds (21, 22, 45) had much more promising oral PK profiles.
Compound 22 was particularly noteworthy having the longest ter-
minal half life coupled with high bioavailability.
Compound 22 successfully balanced a number of the desirable
in vitro and in vivo properties. Additional profiling showed no dis-
cernable activity at related receptors and enzymes such as DP1 and
arachidonic acid-mediated platelet aggregation (a marker of cyclo-
oxygenase 1 and TP activity) at concentrations of 1 lM or above.
Importantly, 22 was potent antagonist in a disease relevant cell
system, inhibiting DK-PGD2-induced CD11b expression in human
eosinophils (IC50 10 nM).
In summary, this lead optimization program generated a range
of high quality compounds which met the aspirations of high
CRTh2 potency, functional antagonism, low rates of metabolism,
no significant cyp 2C9 inhibition and high selectivity against re-
lated receptors. It was challenging to balance these important driv-
ers of quality given compounds of similar bulk properties often had
very different metabolic and selectivity profiles and the finding
that promising compounds often had low lipophilicity which sug-
gested that in vivo DMPK properties could be compromised.
In conclusion, a novel series of CRTh2 antagonists was opti-
mized from an initial hit via a variety of changes to the indole core,
linker atom and substituents on the pendant aryl. Compound 22
was progressed into development by AstraZeneca as AZD1981
and is currently in clinical studies.
9. (a) Chaudhry, P. S.; Cabrera, J.; Juliani, H. R.; Varma, S. D. Biochem. Pharmacol.
1983, 32, 1995; (b) Ratliff, D. M.; Martinez, F. J.; Vander Jagt, T. J.; Schimandle,
C. M.; Robinson, B.; Hunsaker, L. A.; Vander Jagt, D. L. Adv. Exp. Med. Biol. 1999,
463, 493; (c) Alexiou, P.; Pegklidou, K.; Chatzopoulou, M.; Nicolaou, I.;
Demopoulos, V. J. Curr. Med. Chem. 2009, 16, 734; (d) Mylari, B. L.; Larson, E.
R.; Beyer, T. A.; Zembrowski, W. J.; Aldinger, C. E.; Dee, M. F.; Siegel, T. W.;
Singleton, D. H. J. Med. Chem. 1991, 34, 108.
10. Purified human recombinant aldehyde reductase (ALR1) and aldose reductase
(ALR2) were kindly provided by Dr K. Bohren, Dept. of Pediatrics, Baylor College
of Medicine, Texas Children’s Hospital, Houston, Texas, USA. ALR1 and ALR2
activity was determined spectrophotometrically by monitoring the change in
absorbance at 340 nM due to the oxidation of the co-factor NADPH. Assays
were performed at 22 °C in U.V. clear polystyrene 96 well plates (Costar) and
Acknowledgments
the change in absorbance monitored using
a Spectramax 250 (Molecular
We acknowledge Elizabeth Akam, Fiona Bell, Angela Dymond,
Simon Barber, Tom McInally, Gwen McNicol, Tim Phillips, Wendy
Tomlinson, Carol Weyman-Jones and Sara Yendell for additional
data provision and Rachel Lowry for helpful advice during the
preparation of this manuscript.
Devices) spectrophotometer. For both assays, all reagents apart from NADPH
were incubated for 15 min. at 22 °C and the reaction started by the addition of
the co-factor. Absorbance readings were taken and the initial (linear) rate of
reaction recorded. For calculation of enzyme inhibition IC50 values, the initial
rates of reactions were corrected for non-enzymatic NADPH oxidation and the
percent inhibition at any given compound concentration was calculated
relative to the no-compound control. IC50 values are the mean of at least 2
separate determinations and were estimated from concentration effect curves
fitted to a 4 parameter logistic function.
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