The discovery and synthesis of novel adenosine receptor (A2A) antagonists

Article abstract of DOI:10.1016/j.bmcl.2005.01.019

In high throughput screening of our file compounds, a novel structure 1 was identified as a potent A_2A receptor antagonist with no selectivity over the A₁ adenosine receptor. The structure-activity relationship investigation using 1 as a template lead to identification of a novel class of compounds as potent and selective antagonists of A_2A adenosine receptor. Compound 26 was identified to be the most potent A_2A receptor antagonist (K_i = 0.8 nM) with 100-fold selectivity over the A₁ adenosine receptor.

Full text of DOI:10.1016/j.bmcl.2005.01.019

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1333–1336
The discovery and synthesis of novel adenosine receptor
(A2A) antagonists
Julius J. Matasi, John P. Caldwell, Jinsong Hao, Bernard Neustadt, Leyla Arik,
Carolyn J. Foster, Jean Lachowicz and Deen B. Tulshian*
Department of Medicinal Chemistry—CV & CNS, Schering–Plough Research Institute, 2015 Galloping Hill Road,
Kenilworth, NJ 07033, USA
Received 13 December 2004; revised 10 January 2005; accepted 11 January 2005
Abstract—In high throughput screening of our file compounds, a novel structure 1 was identified as a potent A2A receptor antago-
nist with no selectivity over the A1 adenosine receptor. The structure–activity relationship investigation using 1 as a template lead to
identification of a novel class of compounds as potent and selective antagonists of A2A adenosine receptor. Compound 26 was iden-
tified to be the most potent A2A receptor antagonist (Ki = 0.8 nM) with 100-fold selectivity over the A1 adenosine receptor.
Ó 2005 Elsevier Ltd. All rights reserved.
The purine nucleoside, adenosine, is known to act via
four major receptor subtypes, A1, A2A, A2B, and A3,
which have been characterized according to their pri-
mary sequences.1 Adenosine A2A receptors are abun-
dant in the caudate putamen, nucleus accumbens, and
olfactory tubercle in several species.2 In the caudate
putamen, adenosine A2A receptors are localized in
several neurons and have been shown to modulate the
neurotransmission of c-aminobutyric acid (GABA),
acetylcholine, and glutamate.3 These actions of the
A2A adenosine receptor could contribute to motor
behavior.4 For instance, A2A agonists have been shown
to inhibit locomotor activity and induce catalepsy in
rodents.5 In contrast, adenosine A2A antagonists
prevent the motor disturbances of dopamine D2
receptor null mice.6
discover A2A receptor antagonists, screening of our file
compounds identified a novel nonpurine structure 1 as
a potent A2A antagonist (Ki = 1.7 nM). However, it
had no selectivity over the A1 adenosine receptor. We
began the SAR using Compound 1 as a starting point
to improve selectivity of this novel class of compounds
over the A1 adenosine receptor. Our strategy was to
achieve the selectivity via modifications of rings A and
B as well as the amino moiety. We also planned to ad-
dress the role of ketone functionality of lead 1. It was
quickly realized that the amino functionality was essen-
tial for A2A receptor affinity, since either alkylation or
acylation produced compounds with significantly re-
duced affinity for the A2A adenosine receptor.
NH2
N
O
N
OMe
OMe
N
N
N
Recently, evaluation of A2A receptor antagonist, KW-
6002, exhibited antiparkinsonian activity in the parkin-
sonian monkey without producing hyperactivity and
provoking dyskinesia,7 suggesting that A2A receptor
antagonists have potential to be a new class of antisymp-
tomatic drug for ParkinsonÕs disease. A number of A2A
selective adenosine receptor antagonists8 have been
developed using purine as a template. In our quest to
N
O
A
B
O
KW-6002
1
It was also established that reduction of the ketone to a
methylene as in 6b or its replacement with the ether link-
age to compound 6c produced compounds with A2A
affinity similar to 1 without any improvement in the
selectivity over A1 adenosine receptor (see Table 1).
Based on these results, the SAR was focused only in
the methylene series represented by 6b. Hence, both
Keywords: Adenosine receptor; Antagonist; Arylindenopyrimidines.
*
Corresponding author. Tel.: +1 908 740 3505; fax: +1 908 740
7152; e-mail: deen.tulshian@spcorp.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.01.019

1334
J. J. Matasi et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1333–1336
Table 1. Modification of the five-membered ring (indenyl ring)
reactions. Those that were not commercially available
were prepared as shown in Schemes 3 and 4. Using the
Doebner modification of the Knoevenagel condensation
between substituted aldehydes 9 and malonic acid in
pyridine in presence of piperidine afforded cinnamic acid
derivatives 10. After catalytic hydrogenation with 10%
palladium on carbon, the carboxylic acids were then
converted to the acyl chlorides. Cyclization of the acyl
chlorides using aluminum trichloride provided the re-
quired indanones 11.
NH2
N
N
X
Compound
R
A2A Ki (nM)
A1/A2A
1.4
1
–(C@O)–
–(CHOH)–
–CH2–
1.7
168.3
11.2
40.5
9.0
6a
6b
8
1.2
9.7
2
–(NH)–
–O–
–CH2CH2–
6c
6d
Indanone derivative 14 was prepared from 6-bromo-1-
indanone 12 using Scheme 4. Sodium borohydride
reduction of 12 in methanol followed by protection of
the resulting alcohol as silyl ether, and a standard palla-
dium-catalyzed amination gave 13 in 78% yield.
172
<1
rings A and B of 6b were modified. Compounds shown
in Table 1 were prepared according to the procedures
described in Schemes 1 and 2. An aldol condensation
at room temperature between ketone derivatives 2 or 4
with benzaldehyde 5 in the presence of a catalytic
amount of sodium hydroxide produced the benzylidene
adduct, which was not isolated.
Compound 13 was deprotected and then treated under
Swern oxidation conditions to give 14. This ketone
was then condensed with various aldehydes following
the method in Scheme 1 to provide compounds pre-
sented in Table 3. All compounds gave satisfactory ana-
lytical results.10
This adduct was heated to reflux in ethanol with a solu-
tion of salt free guanidine hydrochloride under basic
condition to produce 1 and 6. Sodium borohydride
reduction of 1 produced compound 6a in quantitative
yield. Compound 7, prepared as described in the litera-
ture,9 was converted to compound 8 under the reaction
conditions shown in Scheme 2.
The results of A2A adenosine receptor binding assay11
are expressed as inhibition constants (Ki nM). The A1/
A2A describes their selectivity over A1 adenosine recep-
tor. Results in Table 1 show that the modification of
the indenylene ring does not result in improvement of
selectivity over the A1 adenosine receptor. No significant
change in affinity for A2A adenosine receptor was ob-
served when ketone 1 was converted to either methylene
6b or ether derivative 6c. Reduction of ketone 1 to com-
pound 6a or the ring expanded derivative 6d produced a
significant drop in affinity for the A2A adenosine recep-
tor where as amino derivative 8 showed a moderate de-
crease in affinity.
Some of the starting indanones and derivatives were
purchased from commercial sources and used in these
NH2
O
O
O
N
N
a
b
O
Initial SAR had established that substitution at the 8-
position of these derivatives was optimal for A2A affinity
O
2
3
1
c
O
O
NH2
N
CO2H
R
a
b, c, d
N
H
O
O
b
R
+
R
H
X
X
9
10
11
R = Ph, tBu, iPr, Pr, Cl, Br
4
5
6
-
-
-
X = -CH2 , -O , -CH2CH2 , -CH(OH)-
Scheme 3. Reagents and conditions: (a) HO2CCH2CO2H, piperidine,
pyridine; (b) H2, 10% Pd/C, MeOH; (c) (COCl)2, CH2Cl2; (d) AlCl3,
CH2Cl2.
Scheme 1. Reagents and conditions: (a) benzaldehyde, NaOH, EtOH;
(b) guanidine, HCl, NaOH, EtOH; (c) NaBH4, MeOH.
NH2
N
O
OTBS
O
Br
N
N
a, b, c
d, e
O
O
N
a
+
H
N
N
H
O
12
13
14
7
5
8
Scheme 4. Reagents and conditions: (a) NaBH4, MeOH; (b) TBSCl,
imidazole, DMF; (c) pyrrolidine, Pd(OAc)2, P(tBu)3, NaOtBu, toluene;
(d) TBAF, THF; (e) DMSO, (COCl)2, Et3N, CH2Cl2.
Scheme 2. Reagents and conditions: (a) guanidine, HCl, NaOH,
EtOH.

J. J. Matasi et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1333–1336
1335
Table 3. Optimization of substituents at position 8 (ring B)
and moderate selectivity. With this knowledge, we direc-
ted our efforts around the 8-methyl derivative.
NH2
N
N
The SAR of the modifications of ring A is summarized
in Table 2. These results showed that the replacement
of phenyl with furan or substituted furans is well toler-
ated by the A2A adenosine receptor. There is a moderate
to significant improvement in selectivity for these deriv-
atives. Among these, the 2-furanmethanol derivative,
compound 19 was identified to be the most potent A2A
adenosine receptor antagonist with almost 100-fold
selectivity over the A1 receptor. The 2- or 4-pyridyl
derivatives (20 and 21) exhibited a significant drop in
A2A receptor affinity. These results suggested that the
furan was a favorable substitute for the phenyl. We fo-
cused on the 5-bromo-2-furnyl derivative 18 for further
SAR since A2A receptor affinities were similar for all furan
derivatives.
O
Br
R
Compound
R
A2A Ki (nM)
A1/A2A
22
23
–C6H5
–C(CH3)3
10.9
8.1
68
26
24
3.9
32
N
25
26
27
28
29
–Cl
1.0
0.8
4.2
1.6
2.6
70
103
49
–CH(CH3)2
–CH3
–OCH3
21
42
–CH2CH3
A2A receptor antagonists. Compared to our initial non-
selective lead 1, several compounds in this series with
moderate to high selectivity were identified. In particu-
lar, compounds 19 and 26 show very high affinity
(Ki = 2.6, 0.8 nM, respectively) with approximately
100-fold selectivity over A1 receptor.
The A2A adenosine receptor affinities for derivatives of
the modified ring B are presented in Table 3. These re-
sults showed that the introduction of substituents at
the 8-position is well tolerated by the A2A adenosine
receptor and can be substituted with a variety of susbti-
tuents. There is no significant increase in selectivity over
A1 receptor for these derivatives when compared with
compound 18. Compound 26 was found to have higher
affinity for A2A adenosine receptor and slightly im-
proved selectivity. The selectivity against A2B and A3
adenosine receptors was not determined. This current
class of compounds does not contain purine as a tem-
plate, which makes them structurally different than
known class14 of A2A adenosine receptor antagonists
in the literature namely SCH-58261 and ZM-241385.
Acknowledgements
We would also like to thank Dr. William Greenlee for
his support for this work. Finally, we would like to
thank Zheng Tan for her help in the preparation of this
manuscript.
References and notes
1. Fredholm, B. B.; Abbracchio, M. P.; Burnstock, G.; Daly,
J. W.; Harden, T. K.; Jacobson, K. A.; Legg, P.; Williams,
M. Pharmacol. Rev. 1994, 46, 143.
In summary, 4-aryl-5H-indeno[1,2-d]pyrimidin-2-yl-
amine derivatives were identified as a novel class of
2. (a) Svenningsson, P.; Le Moine, C.; Fisone, G.; Fredholm,
B. B. Prog. Neurobiol. 1999, 59, 355; (b) Schiffmann, S. N.;
Libert, F.; Vassart, G.; Dumont, J. E.; Vanderhaeghen,
J. J. Brain Res. 1990, 519, 333.
3. (a) Ochi, M.; Koga, K.; Kurokawa, M.; Kase, H.;
Nakamura, J.; Kuwana, Y. Neuroscience 2000, 100, 53;
(b) Mori, A.; Shindou, T.; Ichimura, M.; Nonka, H.;
Kase, H. J. Neurosci. 1996, 16, 605.
4. Kuwana, Y.; Shiozaki, S.; Kanda, T.; Kurokawa, M.;
Koga, K.; Ochi, M.; Ikeda, K.; Kase, H.; Jackson, M. J.;
Smith, L. A.; Pearce, R. K. B.; Jenner, P. Adv. Neurol.
1999, 80, 121.
5. (a) Shiozaki, S.; Ichikawa, S.; Nakamura, J.; Kitamura, S.;
Yamada, K.; Kuwana, Y. Psychopharmacology 1999, 147,
90; (b) Shiozaki, S.; Ichikawa, S.; Nakamura, J.; Kuwana,
Y. In Effects of Adenosine Receptors, in Adenosine Recep-
tors and Parkinson’s Disease; Kase, H., Richardson, P. J.,
Jenner, P., Eds.; Academic, 1999; 193–210.
6. Aoyama, S.; Kase, H.; Borreli, E. J. Neurosci. 2000, 20,
5848.
7. Kanda, T.; Jackson, M. J.; Smith, L. A.; Pearce, R. K. B.;
Nakamura, J.; Kase, H.; Kuwana, Y.; Jenner, P. Exp.
Neurol. 2000, 162, 321.
8. (a) Baraldi, P. G.; Cacciari, B.; Romagnoli, R.; Spalluto,
G.; Monopoli, A.; Ongini, E.; Varani, K.; Borea, P. A.
J. Med. Chem. 2002, 45, 101; (b) Baraldi, P. G.; Cacciari,
B.; Spalluto, G.; Bergonzoni, M.; Dionosotti, S.; Ongini,
E.; Varani, K.; Borea, P. A. J. Med. Chem. 1998, 41, 2126.
Table 2. Optimization of substituents at position 4 (ring A)
NH2
N
N
R
Compound
R
A2A Ki (nM)
A1/A2A
9
15
11
O
O
16
17
18
19
20
21
6.7
7.1
29.4
25.7
48.8
94.1
5.5
O
O
4.2
Br
2.6
OH
154.2
56.9
N
9.0
N

1336
J. J. Matasi et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1333–1336
9. Sugasawa, T.; Adachi, M.; Sasakura, K.; Kitagawa, A. J.
GF/B filters. Filteres were washed seven times with 1 mL
cold (4 °C) distilled water, air dried, and radioactivity
retained on filters were counted in PackardÕs TopCount
NXT microplate scintillation counter. Compounds were
tested at 10 different concentrations ranging from 0.1 nM
to 3 lM. Nonspecific binding for adenosine A2A and A1
receptors were determined in the presence of 10 lM CGS-
15943 and 10 lM NECA, respectively. Assays were
performed in duplicates and compounds were tested
twice. Data were fitted in one site competition binding
model for IC50 determination using the program GRAPH-
PAD PRISM (GraphPad Software, Inc., San Diego, CA)
and Ki values were calculated using Cheng and PrusoffÕs
formula.13
Org. Chem. 1979, 44, 578.
10. Compound 19: 1H NMR (DMSO-d6, 400 MHz): d 7.66 (s,
1H), 7.55 (d, J = 8.0 Hz, 1H), 7.32 (d, J = 8.0 Hz, 1H),
7.14 (d, J = 3.4 Hz, 1H), 6.60 (br s, 2H), 6.53 (d,
J = 3.4 Hz, 1H), 5.39 (t, J = 5.8 Hz, 1H), 4.51 (d,
J = 5.8 Hz, 2H), 3.98 (s, 2H), 2.41 (s, 3H). Compound
1
26: H NMR (CDCl3, 400 MHz): d 7.90 (s, 1H), 7.55 (d,
1H), 7.39 (d, 1H), 7.19 (d, 1H), 6.52 (d, 1H), 5.25 (s, 2H),
4.02 (s, 2H), 3.04 (m, 1H), 1.32 (d, 6H).
11. Adenosine A2A and A1 binding assays: [3H]SCH-58261
and [3H]DPCPX binding assays for adenosine A2A and A1
receptors, respectively, were performed as described
before.12 Briefly, 5 lg HEK cell membranes expressing
human adenosine A2A receptors were incubated with
different concentrations of compounds and 1 nM
[3H]SCH-58261 in 200 lL assay buffer containing
2.7 mM KCl, 1.1 mM KH2PO4, 137 mM NaCl, 7.6 mM
Na2HPO4, 10 mM MgCl2, 0.04% methyl cellulose, 20 lg/
mL adenosine deaminase, and 4% dimethyl sulfoxide.
Adenosine A1 binding assays were performed on 10 lg
CHO cell membranes expressing human adenosine A1
receptors and 1 nM [3H]DPCPX in 200 lL assay buffer.
Reactions were carried out for 60 min at room tempera-
ture (23 °C) and were terminated by rapid filtration over
12. (a) Dionisotti, S.; Ongini, E.; Zocchi, C.; Kull, B.; Arslan,
G.; Fredholm, B. B. Br. J. Pharmacol. 1997, 121, 353; (b)
Klotz, K.-N.; Hessling, J.; Hegler, J.; Owman, C.; Kull,
B.; Fredholm, B. B.; Lohse, M. J. Arch. Pharmacol. 1998,
357, 1.
13. Cheng, Y.; Prusoff, W. H. Biochem. Pharmacol. 1973, 22,
3099.
14. (a) Zocchi, C.; Ongini, E.; Conti, A.; Monopoli, A.;
Negretti, A.; Baraldi, P. G.; Dionisotti, S. J. Pharmacol.
Exp. Ther. 1996, 276, 398; (b) Jones, G.; James, R.;
Hargreaves, R. B. U.S. Patent 5,380,714, 1995.