Exploration of SAR features by modifications of thiazoleacetic acids as CRTH2 antagonists

Article abstract of DOI:10.1016/j.bmcl.2010.01.092

The SAR features have been further explored for (2-benzhydryl-4-phenyl-thiazol-5-yl)acetic acids as CRTH2 (chemoattractant receptor-homologous molecule expressed on Th2 cells) antagonists. The introduction of a nitrogen or a methyl substituent in the benzhydrylic position offer two alternative drugable scaffolds attractive for unsymmetrically substituted derivatives. An imidazole analogue lacks activity due to formation of a favored coplanar intramolecular hydrogen bond. The pyrimidine derivative 18 represents a potent and selective compound that will be subject to continued investigations.

Full text of DOI:10.1016/j.bmcl.2010.01.092

Bioorganic & Medicinal Chemistry Letters 20 (2010) 1638–1641
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Exploration of SAR features by modifications of thiazoleacetic acids
as CRTH2 antagonists
Marie Grimstrup, Jean-Marie Receveur, Øystein Rist, Thomas M. Frimurer, Peter Aadal Nielsen,
*
Jesper M. Mathiesen, Thomas Högberg
7TM Pharma A/S, Fremtidsvej 3, DK-2970 Hørsholm, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
The SAR features have been further explored for (2-benzhydryl-4-phenyl-thiazol-5-yl)acetic acids as
CRTH2 (chemoattractant receptor-homologous molecule expressed on Th2 cells) antagonists. The intro-
duction of a nitrogen or a methyl substituent in the benzhydrylic position offer two alternative drugable
scaffolds attractive for unsymmetrically substituted derivatives. An imidazole analogue lacks activity due
to formation of a favored coplanar intramolecular hydrogen bond. The pyrimidine derivative 18 repre-
sents a potent and selective compound that will be subject to continued investigations.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 9 December 2009
Revised 10 January 2010
Accepted 12 January 2010
Available online 22 January 2010
Keywords:
CRTH2 antagonists
Chemoattractant receptor-homologous
molecule expressed on Th2 cells
PGD2
Scaffold hopping
Structure–activity relationships
Molecular modelling
Prostaglandin D2 (PGD2) and one of its receptors CRTH2 (che-
moattractant receptor-homologous molecule expressed on Th2
cells) have been implicated in the pathogenesis of various inflam-
matory conditions.1 The CRTH2 receptor is expressed on eosino-
phils, basophils and Th2 cells, mediating their chemotaxis in
response to PGD2 and several other arachidonate metabolites.1–3
The Th2 cells act as central orchestrators of allergic asthma, driving
IgE response and eosinophilia. Hence, CRTH2 induces the produc-
tion of proinflammatory cytokines in Th2 cells,1,4 enhances the re-
lease of histamine from basophils1,5 mediates the respiratory burst
and degranulation of eosinophils,1,6 and inhibits apoptosis of hu-
man Th2 cells induced by cytokine deprivation.7 Accordingly,
CRTH2 antagonists are being under development for the treatment
of asthma, COPD and allergic disease.8–10
We have earlier described the hit-to-lead process of hits ob-
tained from screening an in silico derived library that successfully
delivered some novel chemotypes of CRTH2 antagonists (Fig. 1).11–
14 Initially a pyrazole series 1 was developed12 and in more recent
publications13,14 we described how thiazole hits were optimized
into compounds with western lipophilic moieties (2) and with
eastern benzydrylic motifs (3,4). In this Letter we describe the ef-
forts of making compounds of the latter type with lower lipophil-
icity and further exploration of the structure–activity
relationships.
The introduction of different substituents in the phenyl rings in
the benzyhydryl series leads to a chiral centre. We envisaged one
O
N
Cl
N
O
N
O
COOH
S
HOOC
N
2
1
Br
CH3
F
F
F
N
N
S
S
HOOC
HOOC
3
4
F
F
Figure 1. Representative chemotypes identified after hit-to-lead process of in silico
screening hits. CRTH2 receptor binding IC50: 1 (4 nM),12 2 (3.7 nM),13 3 (1.4 nM)14
and 4 (3.1 nM).14
* Corresponding author. Tel.: +45 3925 7760; fax: +45 3925 7776.
E-mail address: th@7tm.com (T. Högberg).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.01.092

M. Grimstrup et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1638–1641
1639
R3
R2
R2
R2
I
b
a
R3
R3
+
HN
H2N
N
NH2
S
II
III
Br
O
R1
R1
N
S
R2
R3
OH
N
O
c
I
IV
Figure 2. Superimpositions of A: thiazole
nitrogen analogue (X = N) and B: thiazole 3 and the corresponding pyrimidine 18.
3
(X = CH) and the corresponding
OH
O
Scheme 1. Reagents and conditions: (a) (DPPF)PdCl2.CH2Cl2, DPPF, sodium
t-butoxide, THF, 100 °C or Pd(OAc)2, X-Phos, Cs2CO3, toluene, t-BuOH, microwave
reactor, 150 °C, 5 min; 80–95%; (b) (i) FmocNCS, CH2Cl2; (ii) piperidine, MeOH; 40–
60%; (c) MeCN or DMF, microwave reactor 100–120 °C; 10–20%.
functional activity somewhat. Such reduction in activity caused
by the introduction of eastern para substituents was not seen in
the benzhydryl series,14 which might be attributed to the different
positioning of the para substituents imposed by the different
hybridization stage for carbon and nitrogen as reflected in the
superimpositions (cf. Fig. 2A). The more polar monosubstituted
derivatives, that is, methylsulfone 10, nitrile 11 and 3-pyridyl 12,
display a further reduction in activity.
We also explored the introduction of a methyl group in the ben-
zhydrylic position to see if activity was retained to potentially be
able to block an epimerisation of chiral compounds. We assessed
this possibility by making the simple compound 13 by conversion
of the commercial 2,2-diphenylpropionitrile to thioamide and con-
densation with the p-fluorophenyl butyric acid derivative V as
shown in Scheme 2. This compound possessed a binding affinity
of 8 nM and a full functional antagonistic activity comparable to
the unmethylated compound 14.14 Thus, the methyl group can
be assumed to be without interaction with the receptor binding
site.
way to circumvent chirality, and simultaneously add another
acceptor site, was replacement of the benzydryl carbon with nitro-
gen. Diarylamines II, required for the synthesis of the target mole-
cules IV, were prepared by palladium assisted cross-coupling.
Subsequent conversion of II into the corresponding thioureas III,
followed by microwave assisted condensation with 3-bromo-4-
oxo-butyric acid derivatives I led to the desired 2-aminothiazoles
IV (Scheme 1).
The compounds were characterized with respect to binding
affinity and functional antagonistic activity using a biolumines-
cence resonance energy transfer (BRET) assay in HEK385-7 cells
13, 15
(Table 1).12,
Superimposition of carbon and nitrogen ana-
logues reveals a slightly different orientation of the eastern phenyl
rings (Fig. 2A). However, the compounds 5, 6 and 7, lacking eastern
substituents, display activities that are comparable to the carbon
analogues described before having binding affinities of 14, 9 (com-
pound 14) and 3 nM, respectively.14 Introduction of para fluoro or
methoxy substituents in the eastern rings (8 and 9) lowers the
In the pyrazole series exemplified by compound 1 the carboxyl
function is connected with a two atom linker to the phenyl ring so
we wanted to explore this possibility also in the thiazole series by
making 15 and 16 (Table 2). The thiazolepropionate 15 was made
by reaction of 4-bromo-5-(4-fluorophenyl)-5-oxopentanoic acid
and diphenylthioacetamide analogous to the synthesis of the ace-
tate (cf. Scheme 2).
Table 1
Binding affinity and functional antagonism on hCRTH2 of 4-(p-chlorophenyl)thiaz-
The oxymethylene-linked compound 16 was made in a modest
yield according to Scheme 3 by alkylation with bromoacetate of
the hydroxythiazole VI followed by hydrolysis. The intermediate
VI was conveniently obtained from the thioamide VII, which was
made by treatment or the corresponding amide with Lawesson’s
reagent. Both compounds 15 and 16 having the acidic side chain
extended with one atom showed a considerable drop in potency
and functional activity compared to 3.
oleacetic acids
R1
R2
N
X
S
R3
COOH
To increase the polarity in the core nucleus the thiazole was re-
placed with an imidazole ring according to the synthesis in Scheme
4. The nitrile was converted to 2,2-diphenylacetamidine that was
reacted with the p-fluorophenyl butyrate ester V to produce the
imidazoleacetic acid ester that was hydrolyzed to give 17 in a poor
yield. The imidazole analogue is virtually devoid of CRTH2 affinity
and lacks functional activity. This observation is likely to be ex-
a
b
No.
X
R1
R2
R3
IC50 Bind (nM) IC50 BRET
(lM)
5
6
7
8
N
N
N
N
N
N
N
N
4-Cl
4-F
3-F
4-F
3-F
4-F
4-F
4-F
H
H
H
4-F
4-OMe
4-MeSO2
4-CN
3-pye
H
H
H
H
4-F
18
15
4.4
23
0.072
0.098
0.079
0.24
9
4-OMe 15
0.26
10
11
12
13
H
H
H
H
H
190
84
43
2.0d
0.46c
1.5d
CMe 4-F
4-F
7.9
8.7
0.085
0.077
F
14f CH
H
a
[3H]PGD2 equilibrium competition binding in HEK385-7 cells.
Antagonistic activity as inhibition of b-arrestin translocation measured in a
1) a
N
b
NC
CH3
CH3
F
S
bioluminescence resonance energy transfer (BRET) assay in HEK385-7 cells. All
compounds displayed efficacy above 80% unless noted. All values are single or mean
of double determinations.
Br
O
COOH
2) b
OMe
c
O
Compound having 55–65% antagonistic efficacy.
Compound having about 30% antagonistic efficacy.
13
V
d
e
One phenyl ring replaced with 3-pyridyl system.
Scheme 2. Reagents and conditions: (a) H2S in pyridine/Et3N, rt, 17%; (b) (i) DMF,
microwave reactor, 100 °C, 10 min; (ii) aq LiOH, THF; 63%.
f
Previously described.14

1640
M. Grimstrup et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1638–1641
Table 2
Binding affinity and functional antagonism on hCRTH2 of modified benzhydryl substituted heterocyclic acids
R2
R1
Ar
X
A
COOH
R2
No.
Ar
X
A
R1
R2
F
IC50 Binda (nM)
1.4
IC50 BRETb
0.018
(lM)
N
3c
CH
CH2
3-F
3-F
3-F
S
N
15
16
CH
CH
CH2CH2
OCH2
F
F
75
83
1.0
S
N
0.20
S
N
17
CH
CH2
4-F
F
9140
>100
N
H
N
N
N
N
18
19
20
21
22c
CH
CH
N
CH2
3-F
3-F
3-F
3-F
4-F
F
1.9
0.012
>100
>100
>100
0.031
N
N
N
N
CH2CH2
CH2
F
2070
1260
2790
3.9
F
N
CH2
OMe
OMe
N
S
CH
CH2
a
c
b
and as in Table 1.
Previously described.14
F
F
F
F
F
F
F
F
F
H
1) a
H2N
HN
O
N
N
a
b
NC
O
OMe
N
H
HO
2) b
F
S
O
OMe
VII
O
HO
NH2
F
F
F
17
c
F
F
F
F
F
Scheme 4. Reagents and conditions: (a) NH4Cl, Me3Al, toluene, 80 °C; 78%; (b) (i)
bromo ester V, KHCO3, THF, 80 °C; (ii) aq LiOH, THF, 5%.
d
N
N
S
S
Thus, our interest of making a somewhat more polar core moi-
ety turned to a central pyrimidine ring. Compared to the nitrogen
analogues discussed above (Fig. 2A), superimpositions of thiazole 3
and pyrimidine 18 showed almost identical spatial arrangements
including the eastern para substituents (Fig. 2B). This indicates that
SAR generated for the thiazole series is to a large extent transferra-
ble to the pyrimidine series. The pyrimidines were synthesized by
reacting an amidine VIII with the bromo enamine IX as exempli-
fied in Scheme 5 for 18 and 20. The N,N-diphenylguanidines (VIII,
X = N) required for synthesis of the 2-aminopyrimidines 20 and 21
were obtained by treatment of anilines II with cyanamide and tri-
methylsilyl chloride in acetonitrile during microwave conditions at
180 °C.
O
HO
HOOC
F
VI
16
Scheme 3. Reagents and conditions: (a) isobutyl chloroformate, N-methylmorpho-
line; (b) (i) microwave reactor, 100 °C, 30 min; (ii) Lawesson’s reagent, toluene,
80 °C; (c) (i) aq LiOH, THF; (ii) TFA, 80 °C; 13% overall yield from bis-(4-
fluorophenyl)acetic acid; (d) (i) ethyl bromoacetate, K2CO3, acetone; (ii) aq LiOH,
THF; 17%.
plained by the formation of a favored coplanar intramolecular
hydrogen bond, indicated in Scheme 4, making the receptor bound
acetic acid conformation energetically inaccessible (vide infra).

M. Grimstrup et al. / Bioorg. Med. Chem. Lett. 20 (2010) 1638–1641
1641
F
In conclusion, we have explored the SAR features further for
the (2-benzhydryl-4-phenyl-thiazol-5-yl)acetic acids series and
extended our understanding regarding the carboxyl side chain
conformation and other interaction requirements for binding to
the CRTH2 receptor. The introduction of a nitrogen or a methyl
substituent in the benzhydrylic position offer two alternative
drugable scaffolds that could be attractive for unsymmetrically
substituted derivatives. The pyrimidine derivative 18 represents
a potent and selective compound that will be subject to continued
investigations.
F
F
F
F
Br
N
O
X=CH: a
X=N: b
H2N
HN
OEt
N
+
X
X
N
HO
O
F
X = CH 18
X = N 20
VIII
IX
Scheme 5. Reagents and conditions: (a) (i) sodium t-pentoxide, tBuOH, microwave
reactor, 135 °C, 1 h; (ii) aq LiOH, THF; 20% or (i) sodium t-butoxide, EtOH, 50 °C,
overnight; (ii) aq LiOH, THF; 17%; (b) (i) EtOH, reflux, overnight; (ii) aq LiOH, THF;
19%.
Acknowledgements
The authors thank Joan Gredal, Kirsten Nymann Petersen, Stina
Hansen, Ann Christensen and Helle Zancho Andresen, for excellent
technical assistance.
References and notes
1. Reviews with relevant references therein: (a) Pettipher, R. Br. J. Pharmacol.
2008, 153, S191; (b) Pettipher, R.; Hansel, T. T.; Armer, R. Nat. Rev. Drug Disc.
2007, 6, 313; (c) Kostenis, E.; Ulven, T. Trends Mol. Med. 2006, 12, 148; (d)
Herlong, J. L.; Scott, T. R. Immunol. Lett. 2006, 102, 121; (e) Moore, M. L.; Peebles,
R. S., Jr. J. Allergy Clin. Immunol. 2006, 117, 1036.
2. (a) Nagata, K.; Hirai, H.; Tanaka, K.; Ogawa, K.; Aso, T.; Sugamura, K.;
Nakamura, M.; Takano, S. FEBS Lett. 1999, 459, 195; (b) 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.
3. Bohm, E.; Sturm, G. J.; Weiglhofer, I.; Sandig, H.; Shichijo, M.; McNamee, A.;
Pease, J. E.; Kollroser, M.; Peskar, B. A.; Heinemann, A. J. Biol. Chem. 2004, 279,
7663.
4. Xue, L.; Gyles, S. L.; Wettey, F. R.; Gazi, L.; Townsend, E.; Hunter, M. G.;
Pettipher, R. J. Immunol. 2005, 175, 6531.
5. Yoshimura-Uchiyama, C.; Iikura, M.; Yamaguchi, M.; Nagase, H.; Ishii, A.;
Matsushima, K.; Yamamoto, K.; Shichijo, M.; Bacon, K. B.; Hirai, K. Clin. Exp.
Allergy 2004, 34, 1283.
6. 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.
7. Xue, L.; Barrow, A.; Pettipher, R. J. Immunol. 2009, 182, 7580.
8. Reviews: (a) Medina, J. C.; Liu, J. Ann. Rep. Med. Chem. 2006, 41, 221; (b) Ulven,
T.; Kostenis, E. Curr. Top. Med. Chem. 2006, 6, 1427; (c) Ly, T. W.; Bacon, K. B.
Exp. Opin. Invest. Drugs 2005, 14, 769.
Figure 3. Docking of pyrimidine derivative 18 in the hCRTH2 receptor.
9. (a) Stearns, B. A.; Baccei, C.; Bain, G.; Broadhead, A.; Clark, R. C.; Coate, H.;
Evans, J. F.; Fagan, P.; Hutchinson, J. H.; King, C.; Lee, C.; Lorrain, D. S.; Prasit, P.;
Prodanovich, P.; Santini, A.; Scott, J. M.; Stock, N. S.; Truong, Y. P. Bioorg. Med.
Chem. Lett. 2009, 19, 4647; (b) Sandham, D. A.; Adcock, C.; Bala, K.; Barker, L.;
Brown, Z.; Dubois, G.; Budd, D.; Cox, B.; Fairhurst, R. A.; Furegati, M.; Leblanc,
C.; Manini, J.; Profit, R.; Reilly, J.; Stringer, R.; Schmidt, A.; Turner, K. L.; Watson,
S. J.; Willis, J.; Williams, G.; Wilson, C. Bioorg. Med. Chem. Lett. 2009, 19, 4794;
(c) Liu, J.; Wang, Y.; Sun, Y.; Marshall, D.; Miao, S.; Tonn, G.; Anders, P.; Tocker,
J.; Tang, H. L.; Medina, J. Bioorg. Med. Chem. Lett. 2009, 19, 6840; (d) Royer, J. F.;
Schratl, P.; Carrillo, J. J.; Jupp, R.; Barker, J.; Weyman-Jones, C.; Beri, R.; Sargent,
C.; Schmidt, J. A.; Lang-Loidolt, D.; Heinemann, A. Eur. J. Clin. Invest. 2008, 38,
663; (e) Crosignani, S.; Page, P.; Missotten, M.; Colovray, V.; Cleva, C.; Arrighi, J.-
F.; Atherall, J.; Macritchie, J.; Martin, T.; Humbert, Y.; Gaudet, M.; Pupowicz, D.;
Maio, M.; Pittet, P.-A.; Golzio, L.; Giachetti, C.; Rocha, C.; Bernardinelli, G.;
Filinchuk, Y.; Scheer, A.; Schwarz, M. K.; Chollet, A. J. Med. Chem. 2008, 51, 2227.
10. Clinical studiers: (a) AZD1981 from AstraZeneca in asthma and COPD; http://
www.clinicaltrials.gov; (b) OC000459 from Oxagen in asthma and allergic
rhinoconjunctivitis; http://www.clinicaltrials.gov; (c) ADC3680 reported from
Argenta; (d) AM211 reported from Amira Pharmaceuticals.
11. Frimurer, T. M.; Ulven, T.; Elling, C. E.; Gerlach, L.-O.; Kostenis, E.; Högberg, T.
Bioorg. Med. Chem. Lett. 2005, 15, 3707.
12. Ulven, T.; Receveur, J.-M.; Grimstrup, M.; Rist, Ø.; Frimurer, T. M.; Gerlach, L.-
O.; Mathiesen, J. M.; Kostenis, E.; Uller, L.; Högberg, T. J. Med. Chem. 2006, 49,
6638.
13. Rist, Ø.; Grimstrup, M.; Receveur, J.-M.; Frimurer, T. M.; Ulven, T.; Kostenis, E.;
Högberg, T. Bioorg. Med. Chem. Lett. 2010, 20, 1177.
14. Grimstrup, M.; Rist, Ø.; Receveur, J.-M.; Frimurer, T. M.; Ulven, T.; Mathiesen, J.
M.; Kostenis, E.; Högberg, T. Bioorg. Med. Chem. Lett. 2010, 20, 1181.
15. Vrecl, M.; Jørgensen, R.; Pogacnik, A.; Heding, A. J. Biomol. Screen. 2004, 9, 322.
The pyrimidine 18 is a very potent CRTH2 receptor antagonist
(Table 2) also showing appreciable activity in other species suit-
able for PD studies, that is rat 7.5 nM, mouse 3.4 nM, and 29 nM
guinea pig, in contrast to the other PGD2 receptor DP (human
8.5
lM, guinea pig >100
lM, mouse 5.9 lM), and thromboxane
A2 (31
l
M). Molecular modelling of 18 in the CRTH2 receptor bind-
ing site shows a tight fit in the hydrophobic subpocket containing
several aromatic residues, a hydrogen bond to one of the pyrimi-
dine nitrogens with 266Ser and a crucial interaction with 210Lys to
the carboxyl function that is out of the pyrimidine plane (Fig. 3).
This latter side chain conformation is not achieved for the poorly
binding imidazole 17 having the carboxyl group oriented in the
imidazole ring plane by an intramolecular hydrogen bond. The
chain elongated compound 19 shows a pronounced drop in po-
tency also in line with poor adherence to proper interaction with
the lysine residue. In comparison to the thiazole series, the eastern
para substituted nitrogen analogues 20 and 21 display a much
more drastic reduction in activity, that is, being 100-fold less active
than the benzhydryl derivatives, which could be due to conforma-
tional or electronic reasons that we do not fully understand.