Potent and selective adenosine A2A receptor antagonists: 1,2,4-Triazolo[1,5-c]pyrimidines

Article abstract of DOI:10.1016/j.bmcl.2008.11.075

Antagonism of the adenosine A_2a receptor offers great promise in the treatment of Parkinson's disease. In the course of exploring pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine A_2A antagonists, which led to clinical candidate SCH 420814, we prepared 1,2,4-triazolo[1,5-c]pyrimidines with potent and selective (vs A₁) A_2a antagonist activity, including oral activity in the rat haloperidol-induced catalepsy model. Structure-activity relationships and plasma levels are described for this series.

Full text of DOI:10.1016/j.bmcl.2008.11.075

Bioorganic & Medicinal Chemistry Letters 19 (2009) 967–971
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Potent and selective adenosine A2A receptor antagonists:
1,2,4-Triazolo[1,5-c]pyrimidines
a
a
a
a
Bernard R. Neustadt a, , Hong Liu , Jinsong Hao , William J. Greenlee , Andrew W. Stamford ,
Carolyn Foster a, Leyla Arik a, Jean Lachowicz a, Hongtao Zhang a, Rosalia Bertorelli b, Silva Fredduzzi b,
Geoffrey Varty a, Mary Cohen-Williams a, Kwokei Ng a
*
a Departments of Chemical Research, CNS Pharmacology, Drug Metabolism and Pharmacokinetics, Schering-Plough Research Institute, 2015 Galloping Hill Road, Kenilworth,
NJ 07033-1310, USA
b Schering-Plough Research Institute, San Raffaele Science Park, 20132 Milan, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Antagonism of the adenosine A2a receptor offers great promise in the treatment of Parkinson’s disease. In
the course of exploring pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine A2A antagonists, which led to
clinical candidate SCH 420814, we prepared 1,2,4-triazolo[1,5-c]pyrimidines with potent and selective
(vs A1) A2a antagonist activity, including oral activity in the rat haloperidol-induced catalepsy model.
Structure–activity relationships and plasma levels are described for this series.
Received 26 September 2008
Revised 19 November 2008
Accepted 20 November 2008
Available online 24 November 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Adenosine A2a
Parkinson’s disease
1,2,4-Triazolo[1,5-c]pyrimidine
Parkinson’s disease (PD) is a very serious neurological disorder,
and current methods of treatment fail to achieve long-term con-
trol. Since adenosine A2A receptor antagonists have been shown
to restore the deficits caused by degeneration of the striatonigral
dopamine system, which is compromised by the loss of striatal
neurons in this disease, A2A antagonism affords a possible treat-
ment for PD.1 The A2A antagonist KW-6002 (istradefylline) was
shown to be effective in animal models of PD, and recent clinical
studies demonstrated efficacy in alleviation of symptoms of the
disease.2
Adenosine A2A receptor antagonists of several structural types
have been described. In our previous report3 we have summarized
these,4 as well as our development of the pyrazolo[4,3-e]-1,2,4-
triazolo[1,5-c]pyrimidine SCH 420814 1a as a potential agent for
the treatment of Parkinson’s disease.5 As part of the exploration of
this series, several structural types of 1,2,4-triazolo[1,5-c]pyrimi-
dines were evaluated. The activity of a set of 7-aryl-1,2,4-triazol-
o[1,5-c]pyrimidines of type 2 has been described.6 In the present
report, we describe a series of 1,2,4-triazolo[1,5-c]pyrimidines 3
bearing a 7-substituent linked through a heteroatom.7 These com-
pounds are high affinity adenosine A2A receptor antagonists with
high selectivity versus the A1 receptor. In addition, oral activity has
been demonstrated in the rat haloperidol-induced catalepsy model.
Other workers have described compounds of type 3 with cyclic
linker groups,8,9 as well as limited efforts with chain linker groups.10
We explored both alkyl and piperazino-alkyl linker groups in depth,
together with efforts to replace the 2-furanyl substituent found in
the majority of reported adenosine A2A receptor antagonists.
We began our studies with a simple alkyl linker. Synthetic
methods were developed to explore a full range of heteroatoms
X in structure 3, as shown in Scheme 1. For O and N heteroatoms,
intermediates 7 and 8 were prepared in a straight-forward manner.
For S as the heteroatom, intermediate 9 was prepared in two steps,
while a keto-ester 6 was employed for the carbon attachment in
10. Formation of hydrazides 11 was unsuccessful under nucleo-
philic displacement conditions and required the seldom employed
acid-catalyzed conditions. Final Dimroth-mediated cyclization to
12, 13, 14, and 15 was achieved by heating 11 with bis-trimethyl-
silylacetamide as previously disclosed.3
Utilizing this methodology11 we prepared the set of targets
12a–e shown in Table 1. A receptor binding assay with human
A2A receptors showed that each of heteroatoms provided a potent
antagonist, but only 12c with the NMe group provided the desired
high selectivity versus the adenosine A1 receptor.12 Compound 12c,
however, showed no detectable plasma level upon oral administra-
tion in rats.13
Additional structure–activity relationships were developed in
relation to chain length (compounds 13a–b), aromatic substitution
(14), and N-substitution (15a–b). While receptor affinity is reduced
with the shorter chain of 13a, affinity is retained with the longer
chain of 13b, but receptor selectivity is reduced. Substitution of
* Corresponding author. Tel.: +1 908 740 3495; fax: +1 908 740 7152.
E-mail address: bernard.neustadt@spcorp.com (B.R. Neustadt).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.11.075

968
B. R. Neustadt et al. / Bioorg. Med. Chem. Lett. 19 (2009) 967–971
NH2
O
F
OMe
1a Ar =
N
N
O
N
N
1b Ar =
N
N
N
N
Ar
F
NH2
NH2
N
N
N
N
N
O
O
N
N
N
linker
Ar
X
Ar
2
3
Ar (CH2)n Cl
R'-NH2
Ar (CH2)n XH
NaH for X=O
Et3N for X=NR'
NH2
NH2
NH2
N
H2NHN
O
O
N
N
O
N
N
N
O
NH
forX =S
cat. HCl, EtOH
Ar (CH2)n
X
N
H
Ar (CH2)n
X
X=O
Cl
Cl
Cl
1. Na2S
11
5
7
2.
Cl
8 X=NR'
9 X=S
Ar
N-TMS
O-TMS
Me
NH2
1. guanidine carbonate
2. POCl3
10
X=CH2
120oC
N
O
N
N
Ar
COOEt
N
(CH2)n
X
Ar
6
O
12 13 14 15
,
,
,
Scheme 1. Synthetic methods for preparation of compounds 12, 13, 14, and 15.
Table 1
Structure–activity relationships for aralkyl compounds
of these analogs typically approached unity. Compounds 16d and
16g were of particular interest because of their in vivo activity
(vide infra). We further explored elaboration of the N-methyl moi-
ety and demonstrated that functionalized alkyl groups were very
well tolerated (16i–n).
NH2
N
N
O
N
N
R
(CH2)n X
Encouraged by these results, we employed the aryl-piperazine
moieties present in 16a–n to explore SAR for the 2-furanyl group
(Table 3), which is a common structural feature of reported A2A
antagonists. A 5-chloro substituent on the furan ring resulted in
a significant loss in receptor affinity (17a vs 16b), as did replace-
ment with an unsubstituted phenyl group (17b vs 16h). However,
appropriate substitution on the phenyl group, such as 3-CN, re-
stored affinity and receptor selectivity (17d), in addition to provid-
ing good plasma levels in the rat following oral administration.
Certain aza-heterocycles were well tolerated as replacements for
the furan (17e and 17f). In general, the active compounds did not
possess oral anti-cataleptic activity superior to that of furan deriv-
ative 16g (Table 4).
a
Compound
X
n
R
A2A Kia (nM)
A2A/A1
12a
12b
12c
12d
12e
13a
13b
14
O
NH
NMe
S
CH2
NMe
NMe
NMe
2
2
2
2
2
1
3
2
2
2
OMe
OMe
OMe
OMe
OMe
OMe
OMe
F
2.8
5.8
1.8
1.6
11.5
46
5.7
2.0
3.0
1.0
58
20
473
58
31
40
76
248
525
566
15a
15b
NEt
NCH2CH2OMe
F
F
a
Average of duplicate determinations, human receptors.
We were particularly desirous of demonstrating in vivo activ-
ity in the rat haloperidol-induced catalepsy model.15 Compounds
1a and 1b have shown potent and robust activity in this assay.3
Activity of compounds from the current series is shown in Table
4. After oral administration (3 mg/kg) compounds 16d and 16g
showed substantial activity at the 1-h time point, but for 16g
this activity was attenuated at four hours. The N-cyclopropylm-
ethyl derivatives 16m and 16n were active at 3 mg/kg, but
16m showed attenuated activity at four hours. The non-furan
compounds 17c and 17d, in spite of high potency and good plas-
ma and brain levels, failed to show pronounced in vivo activity.
Reasons for the lack of in vivo activity of 17c and 17d are not
understood; possibilities include unfavorable striatal distribution
or high non-specific binding to brain tissue. Finally, comparison
with 1a and 1b shows that the most potent compounds of this
fluoro for methoxy (14) retained both attributes, while larger
N-substitution was tolerated, as seen in 15a and 15b. Nevertheless,
following oral administration of selected analogs, for example, 15b,
to rats at the dose of 3 mg/kg, plasma levels of the compounds
were below the limit of detection.14
To improve bioavailability, we investigated aryl-piperazine
derivatives related to 1a and 1b.3 These compounds were prepared
by the methods shown in Scheme 2.10 Compounds 16e and 16g
have been described in a previous paper.10 Assessment of com-
pounds 16a–h resulted in identification of significant plasma levels
(16b, 16d, and 16h) as shown in Table 2. Plasma levels of the
4-(methoxyethoxy)phenyl compounds were uniformly superior
to those of the 2,4-difluorophenyl analogs, and brain/plasma ratios

B. R. Neustadt et al. / Bioorg. Med. Chem. Lett. 19 (2009) 967–971
969
N-TMS
H2NHN
NH2
N
NH2
NH2
N
Me
120oC
O
O-TMS
O
N
N
O
O
N
N
N
O
N
K2CO3, EtOH
100oC
NH
X
Cl
Cl
Cl
N
N
H
Cl
5
18
19
NH2
N
DMF, K2CO3
H
X
130oC
N
Ar
Br
N
N
N
N
N
R
O
NH
N
N
for 21
N
N
N
N
DMF, DIPEA, RT Ar
Ar
RNH2, EtOH,
100oC
Ar
N
R
23
21 X=Cl
22
16g-n
20
X=OH
from 22: NaH,THF
NH2
N
NH2
N
H2NHN
NH2
N
from 23 (R=H): NaH, THF
O
Ar
Ar
N
O
O
from 21,Na2S, DMF
N
N
O
N
N
NH
cat. HCl, EtOH
Cl
Cl
X
Cl
X
N
H
5
27
24 X=O
N-TMS
O-TMS
25 X=NH
Me
26
X=S
120 oC
O
NH2
Ar
N
N
N
N
N
N
X
16a-f
Scheme 2. Synthetic methods for preparation of compounds 16.
Table 2 (continued)
Compound X
a
Ar
A2A Kia
A
2A/A1
Rat plasma
Table 2
(nM)
AUC at 3 mg/kg,
Structure–activity relationships for aryl-piperazine compounds
ng h/mlb
NH2
Ar
N
N
F
N
N
N
16g
NMe
1.0
1580
53
N
O
F
X
a
Compound
X
Ar
A2A Kia
(nM)
A
2A/A1
Rat plasma
AUC at 3 mg/kg,
ng h/mlb
O
16h
16i
16j
NMe
NEt
2.5
0.9
1.5
694
312
102
521
50
OMe
F
16a
16b
O
O
6.0
2.8
275
405
187
943
F
F
F
F
O
O
OMe
OMe
NEt
230
OMe
OMe
F
16c
16d
S
S
1.4
1.5
365
965
229
910
F
16k
16l
NCH2CH2OMe
0.5
0.7
1059
338
<LOQ
355
F
F
O
O
NCH2-
CH2OMe
F
16e
16f
NH
NH
1.5
9.5
262
145
<LOQ
103
F
16m
NCH2-c-Pr
NCH2-c-Pr
0.3
0.8
831
234
17
F
O
O
16n
245
OMe
OMe
a
Average of duplicate determinations, human receptors.
LOQ at each time point 10 ng/ml.
b

970
B. R. Neustadt et al. / Bioorg. Med. Chem. Lett. 19 (2009) 967–971
Table 3
Activity of compounds with furan moiety modified or replaced
NH2
N
Ar
N
N
N
N
R
N
X
a
Compound
X
O
Ar
R
A2A Kia (nM)
A2A/A1
Rat plasma AUC at 3 mg/kg, ng h/ml
6
O
17a
40
40
Cl
O
OMe
OMe
O
17b
17c
17d
17e
NMe
NMe
NMe
NMe
NMe
25
56
620
F
1.8
0.7
0.3
0.3
1295
546
CN
F
O
1071
7000
7368
CN
OMe
N
F
O
F
F
Me
N
F
17f
N
a
Average of duplicate determinations, human receptors.
Table 4
Activity of compounds in the rat haloperidol-induced catalepsy model
series were nevertheless several-fold less potent than 1a and 1b
in vivo.
Compound
Hetero-link
Dose, mg/kg
Rat catalepsy, % inhibitiona
In summary, we have shown potent and selective adenosine
A2A receptor antagonist activity for 1,2,4-triazolo[1,5-c]pyrimi-
dines bearing either an aralkyl or arylpiperazino-alkyl chain in
the 7-position. Compounds of the latter series achieved signifi-
cant rat plasma levels. Successful replacements for the widely-
employed furanyl moiety were demonstrated. In vivo activity
was observed for a number of compounds, although the level
of potency seen in tricyclic compounds 1a and 1b was not
achieved.
1 h
4 h
16a
16b
16c
O
O
S
3.0
1.0
3.0
1.0
3.0
1.0
3.0
1.0
1.0
3.0
3.0
1.0
3.0
1.0
3.0
1.0
3.0
3.0
1.0
0.3
0.1
1.0
0.3
25
45
50
25
65
50
75
50
15
56
57
29
65
40
65
7
35
29
77
60
25
75
75
0
25
45
5
65
15
35
30
0
16d
16g
S
NMe
16h
16k
16l
NMe
NCH2CH2OMe
NCH2CH2OMe
57
37
6
References and notes
16m
16n
NCH2-c-Pr
NCH2-c-Pr
41
34
77
32
14
31
70
60
25
80
NT
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NMe
NMe
—
1b
—
a
Average for n = 3. Maximum reduction attainable is 60–80%.

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5. Sch 420814 has recently received the designation of preladanant.
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Compounds 16 and 18 of that report correspond to 16e and 16g of this report.
Utilization of our intermediate 19 is described.
11. This synthetic methodology differs significantly from that reported in Refs. 8
and 9.
12. A detailed description of the adenosine receptor binding assays is provided in
Bioorg. Med Chem. Lett. 2005, 15, 1333, Refs. 11. For Ki values cited here, SEM
was <15% of the derived Ki.
13. The antagonist nature of the receptor binding is concluded from functional
assay of 16g, Kb = 0.5 nM, the similarity to the series of antagonist 1a, and the
anti-cataleptic activity shown by these compounds.
14. Plasma levels were determined in the rat over a 6h period employing cassette
dosing as described in Korfmacher, W. A.; Cox, K. A.; Ng, K. J.; Veals, J.; Hsieh,
Y.; Wainhaus, S.; Broske, L.; Prelusky, D.; Nomeir, A.; White, R. E. Rapid
Commun. Mass Spectrom. 2001, 15, 335.
15. Assay methodology described in Ongini, E.; Dionosotti, S.; Gessi, S.; Irenius, E.;
Fredholm, B. B. Naunyn Schmiederbergs Arch. Pharmacol. 1999, 359, 7.