2-Cycloalkyl phenoxyacetic acid CRTh2 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2007.05.019

High throughput screening identified a phenoxyacetic acid scaffold as a novel CRTh2 receptor antagonist chemotype, which could be optimised to furnish a compound with functional potency for inhibition of human eosinophil shape change and oral bioavailability in the rat.

Full text of DOI:10.1016/j.bmcl.2007.05.019

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4347–4350
2-Cycloalkyl phenoxyacetic acid CRTh2 receptor antagonists
David A. Sandham,a,* Clive Aldcroft,a Urs Baettig,a Lucy Barker,b David Beer,b
Gurdip Bhalay,a Zarin Brown,b Gerald Dubois,b David Budd,b Louise Bidlake,a
Emma Campbell,b Brian Cox,a Brian Everatt,a David Harrison,a Catherine J. Leblanc,a
Jodie Manini,b Rachael Profit,b Rowan Stringer,b Katy S. Thompson,a
Katharine L. Turner,a Morris F. Tweed,b Christoph Walker,b
Simon J. Watson,a Steven Whitebread,c Jennifer Willis,b
Gareth Williamsb and Caroline Wilsonb
aGlobal Discovery Chemistry, Novartis Institutes of Biomedical Research, Horsham Research Centre,
Wimblehurst Road, Horsham, West Sussex RH12 5AB, UK
bRespiratory Diseases Area, Novartis Institutes of Biomedical Research, Horsham Research Centre, Wimblehurst Road,
Horsham, West Sussex RH12 5AB, UK
cDiscovery Technologies, Novartis Institutes of Biomedical Research, 250, Massachusetts Ave., Cambridge, MA 02139, USA
Received 30 March 2007; revised 4 May 2007; accepted 8 May 2007
Available online 13 May 2007
Abstract—High throughput screening identified a phenoxyacetic acid scaffold as a novel CRTh2 receptor antagonist chemotype,
which could be optimised to furnish a compound with functional potency for inhibition of human eosinophil shape change and oral
bioavailability in the rat.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Prostaglandin D2 (PGD2) is a product of the arachi-
donic acid cascade and is synthesized primarily by mast
cells, as well as by macrophages and Th2 lymphocytes.
PGD2 has long been associated with the allergic inflam-
matory response, being found at high concentrations in
the lungs of asthmatic patients.1 PGD2 was initially
shown to act via a G-protein coupled receptor known
as DP which shows significant homology with other pro-
stanoid receptors. More recently, a second receptor for
PGD2, known as CRTh2 or DP2, has been identified.2
CRTh2 shows minimal homology with DP and is
expressed on inflammatory cells, in particular showing
selective expression on Th2 over Th1 cells. Using specific
agonists, it has been demonstrated that the PGD2-med-
iated activation and migration of eosinophils, basophils
and Th2 cells in vitro proceeds via CRTh2 rather than
DP receptor activation. Taken together with in vivo
data on pulmonary eosinophil influx in rodents medi-
ated by CRTh2 specific agonists, a CRTh2 antagonist
may be an attractive therapeutic target for allergic dis-
eases such as asthma.2 This premise has been recently
supported by the observation that a CRTh2 antagonist
can inhibit pulmonary tissue eosinophilia in a mouse
model of asthma.3
Ramatroban 1 is a TP receptor antagonist used for treat-
ment of allergic rhinitis which was shown to also exhibit
CRTh2 antagonist activity.4 Recently, close analogues of
1 have been reported which are selective antagonists for
F
H
N
O
S
O
O
HO
O
O
OH
O
N
N
O
Cl
Cl
OH
O
Keywords: CRTh2; Prostaglandin D2; Phenoxyacetic acid; Eosinoph-
ils; Allergic diseases.
1
2
3
CRTh2 ligands
*
Corresponding author. E-mail: david.sandham@novartis.com
0960-894X/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.05.019

4348
D. A. Sandham et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4347–4350
CRTh2 over TP.5 The COX-1/COX-2 inhibitor indo-
methacin 2 has also been shown to function as a CRTh2
agonist and the indole substituents have been modified to
deliver CRTh2 selective antagonists devoid of COX
activity.6 In this paper, we describe the discovery and
exploration of an alternative phenoxyacetic acid chemo-
type7 which functions as a CRTh2 antagonist.
TP prostanoid receptors and was also tested in a panel
of 45 additional receptors and 25 kinases, where no inhi-
bition >50% at 10 lM concentration was found.
Encouraged by the profile of 3, we initiated an optimisa-
tion programme to explore the CRTh2 structure–activ-
ity
relationship
(SAR)
of
the
2-cycloalkyl
phenoxyacetic acid scaffold. Representative synthetic
methods employed are depicted in Scheme 1. Commer-
cially available 2-cyclohexyl-4-chlorophenol could be
functionalised by standard methods. The cyclopentyl
and cycloheptyl analogues were prepared by AlCl3-med-
iated Friedel-Crafts alkylation of 4-chlorophenol using
the appropriate cycloalkene and the quaternary
analogue was prepared by treatment of the phenol with
1-methylcyclohexanol and H3PO4. In the case of 4-triflu-
oromethyl analogue, the Friedel-Crafts methodology
was unsuccessful and an alternative Claisen rearrange-
ment was employed. Using the 4-bromo analogue as a
handle, further functionality could be introduced by
transition metal-mediated coupling reactions, to furnish
either the 4-cyano or the 4-(4-fluorophenyl) derivatives.
Following a high throughput screen (HTS) of the Nov-
artis compound archive, the known phenoxyacetic acid
38 was identified as a potent ligand for the CRTh2 recep-
tor in a [3H]-PGD2 radioligand binding assay using
CHO cells stably transfected with the human CRTh2
receptor. Functional activity was assessed using an
eosinophil shape change assay.9 Leukocyte shape
change responses are mediated through rearrangements
of the cellular cytoskeleton and are essential to the pro-
cess of migration from the microcirculation to sites of
inflammation. Follow-up profiling (Table 1) confirmed
3 as an antagonist of the CRTh2 receptor, exhibiting po-
tent inhibition of shape change in isolated human
eosinophils stimulated by the CRTh2 specific agonist10
13,14-dihydro-15-ketoprostaglandin D2 (DK-PGD2).
In addition, 3 showed selectivity over the DP, EP2 and
We initially focused on the acetic acid moiety (Table 2,
compounds 4a–4h) confirming the acid itself was re-
quired and could not be replaced by either tetrazole or
acylsulfonamide isosteres 4c or 4d. The acetic acid itself
was shown to provide the optimal length of linker to the
aromatic core, with extended analogues 4e and 4f being
less potent. Substitution a- to the acid gave a racemic
compound of similar potency to 3, which on resolution
by preparative chiral HPLC11 showed 30-fold potency
difference between the enantiomers 4g and 4h.
Table 1. Initial profiling of HTS hit 3
CRTh2
binding
Ki (lM)
Eosinophil
shape change
IC50 (lM)
DP
binding
Ki (lM)
EP2
binding
Ki (lM)
TP binding
Ki (lM)
0.154
0.093
7.6
3.0
>10
Values represent means of at least two experiments.
O
N
O
N
HO
O
NH
N
Cl
Cl
4d
4e
vi), vii)
iv), v)
OH
HO
H
N
O
i), ii)
S
O
iii)
O
O
O
O
Cl
Cl
Cl
3
4c
HO
OH
O
OH
O
O
viii)
ix)
i), ii), x)
CF3
CF3
CF3
CF3
5d
Scheme 1. Representative syntheses. Reagents and conditions: (i) ethyl bromoacetate, Cs2CO3, DMF, rt (90%); (ii) aq NaOH, MeOH, rt (85%); (iii)
˚
MeSO2NH2, water soluble carbodiimide, DMAP, 4 A sieves, CH2Cl2 (27%); (iv) chloroacetonitrile, K2CO3, acetone, reflux (81%); (v) NaN3, AcOH,
1-butanol, reflux (76%); (vi) Triton-B, acrylonitrile, reflux (37%); (vii) concd HCl, AcOH, rt (65%); (viii) 3-bromocyclohexene, K2CO3, acetone, rt
(80%); (ix) 160 ꢁC, neat (crude); (x) H2, Pd/C, EtOH (96%).

D. A. Sandham et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4347–4350
4349
Table 2. CRTh2 and DP binding activities
Table 3. Functional potency of selected compounds
5b 5d 5e 6a
0.448 0.172 0.052 1.013 0.248 0.451
OH
6b
6c
R
OH
O
O
O
Eosinophil shape
R
O
O
change IC50(lM)
IC50 values represent means of at least two experiments.
Cl
R
Cl
gested an additional steric contribution. The trifluorom-
ethyl analogue 5d gave the optimal combination of
binding activity with improved selectivity over DP. In
contrast, electron rich substituents such as 5g gave re-
duced binding potency. Interestingly, incorporation of
an aromatic group maintained binding potency in 5h.
We next explored the ortho cycloalkyl group (6a–6e),
taking the 4-chlorophenyl moiety of 3 as the optimal
substituent. Ring contraction to 6a resulted in a reduc-
tion in binding potency, whereas the novel cycloheptyl
analogue 6b12 was somewhat more potent than the ori-
ginal hit, albeit with a slightly reduced selectivity over
the DP receptor. Notably, the 2-allyl derivative 6d
showed dual, albeit weak, activity at CRTh2 and DP.
4a - 4i
5a - 5h
6a - 6e
R
CRTh2 Ki
3.28
DP Ki
8.9
O
4a
4b
4c
*
O
O
OH
>10
>10
>10
*
*
H
N
>10
S
O
O
N
*
N
4d
>10
>10
N
H
N
O
4e
4f
0.799
1.99
>10
>10
OH
OH
*
*
With CRTh2 binding SAR established we next profiled
selected compounds for human eosinophil shape change
functional activity, as indicated in Table 3. All com-
pounds were found to be antagonists, with some varia-
tion in the binding-functional potency correlation. For
compound 6b, the eosinophil assay was also run using
the CCR-3 receptor agonist eotaxin13 as a shape change
stimulus. No inhibition was found up to 10 lM under
these conditions, supporting the concept of a CRTh2
specific effect for the compound.
O
CH3
4ga
4ha
0.075
2.25
4.49
>10
OH
OH
*
*
O
CH3
O
5a
5b
5c
5d
5e
5f
–H
–Br
0.311
0.083
0.446
0.020
0.230
0.353
0.782
>10
5.9
Compound 6b14 was selected for further profiling, and
showed IC50 > 10 lM in a panel of cytochrome P450
(CYP) enzyme isoforms 3A4, 2D6 1A2, 2C19 and
2C9. Table 4 shows the results of in vitro and in vivo
ADME profiling. Moderate to high microsomal clear-
ance and good permeability were observed, along with
a high level of plasma protein binding, consistent with
the carboxylic acid functionality. In the rat, good expo-
sure was observed after oral dosing of the compound.
–F
>10
>10
>10
>10
>10
–CF3
–CN
–CH3
5g
–OCH3
*
5h
0.099
6.4
F
In conclusion, HTS identified a novel phenoxyacetic
acid chemotype for selective CRTh2 antagonists. The
SAR of the scaffold was explored, resulting in identifica-
tion of compound 6b which is a functional antagonist of
the human CRTh2 receptor and shows oral bioavailabil-
ity in the rat.
6a
6b
0.150
0.059
4.8
*
1.88
*
6c
0.090
1.21
*
CH3
6d
6e
5.13
>10
4.74
>10
Table 4. ADME profile of compound 6b
*
Human microsomes, Clint
Rat microsomes Clint
Caco-2 permeability, A–B
Caco-2 permeability, B–A
Human plasma protein binding
T1/2 (iv, 0.5 mg/kg)
Cl
109 ll/min/mg
205 ll/min/mg
8.4 · 10À6 cm/s
21.4 · 10À6 cm/s
>99.9%
CH3
*
Ki values are reported in lM and represent the mean of at least two
experiments.
a Absolute configurations arbitrarily assigned.
71 min
Having established the requirement for the phenoxy ace-
tic acid moiety, we next investigated variation of the
para substituent (Table 2, compounds 5a–5h). Elec-
tron-deficient substituents favoured binding activity,
although the low activity of fluoro derivative 4c sug-
7.3 ml/min/kg
0.52 L/kg
58%
Vss
Bioavailability (po, 2 mg/kg)
AUCall
Cmax
694.9 lM min
4.7 lM

4350
D. A. Sandham et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4347–4350
J. PCT Int. Appl. WO2006056752, 2006; Chem Abstr.
2006, 145, 28015.
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