Synthesis and biological activity of trisubstituted adenines as A 2A adenosine receptor antagonists

Article abstract of DOI:10.1080/15257770701542264

The discovery of new drugs for the treatment of neurodegenerative disorders, such as Parkinson's disease, has become an attractive field of research. Due to the regulation of D2 receptor activity by A2 A adenosine receptor, potent and selective ligands of

Full text of DOI:10.1080/15257770701542264

Nucleosides, Nucleotides, and Nucleic Acids, 26:1443–1446, 2007
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1525-7770 print / 1532-2335 online
DOI: 10.1080/15257770701542264
SYNTHESIS AND BIOLOGICAL ACTIVITY OF TRISUBSTITUTED
ADENINES AS A2A ADENOSINE RECEPTOR ANTAGONISTS
Catia Lambertucci, Sauro Vittori, Ram Chandra Mishra, and Diego Dal Ben
Dipartimento di Scienze Chimiche, University of Camerino, Camerino (MC), Italy
2
Karl-Norbert Klotz
Institut fu¨r Pharmakologie und Toxicologie, Universita¨t of
2
Wu¨rzburg, Wu¨rzburg, Germany
Rosaria Volpini and Gloria Cristalli
Dipartimento di Scienze Chimiche, University
2
of Camerino, Camerino (MC), Italy
The discovery of new drugs for the treatment of neurodegenerative disorders, such as Parkinson’s
2
disease, has become an attractive field of research. Due to the regulation of D2 receptor activity by
A2A adenosine receptor, potent and selective ligands of A2A subtype could be useful tools to study
neurodegenerative disorders. A series of 2,8-disubstituted-9-ethyladenine derivatives was synthesized
and tested in binding affinity assay at human adenosine receptors. New compounds showed good
affinity and selectivity at A2A receptor versus the other subtypes. The introduction of a bromine atom
in 8-position increased the affinity of these compounds, leading to ligands with Ki in the nanomolar
range.
Keywords Adenosine receptor ligands; adenosine receptor antagonits; A2A antago-
nists; 9-ethylpurine derivatives; substituted adenines
INTRODUCTION
Adenosine (Ado) is an endogenous modulator of a variety of physiolog-
ical and pathophysiological processes that acts through the interaction with
specific membrane receptors termed A1, A2A, A2B, and A3.[1]
In particular, adenosine is deeply involved in the control of motor
behaviour and substantial evidences indicate that adenosine A2A receptor
antagonists improve motor deficits in animal models of Parkinson’s disease.
For this reason development of potent and selective A2A adenosine receptor
antagonists has become an attractive field for the discovery of new drugs
This work was supported by Fondo di Ricerca di Ateneo (University of Camerino) and by grants
from the Italian Ministry of Research: FIRB 2001, FIRB 2003, and PRIN 2005.
Address correspondence to Catia Lambertucci, Dipartimento di Scienze Chimiche, Universita´ di
Camerino, I-62032 Camerino (MC), Italy. E-mail: catia.lambertucci@unicam.it
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FIGURE 1 Structure of 9-ethyl and 8-bromo-9-ethyladenines.
for the treatment of neurodegenerative disorders, such as Parkinson’s
disease.[2]
Ado receptor (AR) agonists are analogues of the natural ligand, whereas
antagonists are characterized by a wide range of different structures. In par-
ticular, replacement of the ribose moiety of Ado with alkyl chains led to
compounds that maintain affinity, but are not able to activate the receptors
so behaving as antagonists.[3]
In a previous article we have reported the synthesis of a number of 9-
ethylpurines bearing various substituents in 2-, 6-, or 8-position.[3] While
9-ethyladenine (1, Figure 1) showed micromolar affinity at the human A1
and A2A subtypes, the introduction of a bromine atom in 8-position led to
an increase of binding affinity at all AR subtypes (see data for 9-ethyladenine
(1) versus 8-bromo-9-ethyladenine (2) in Table 1). Furthermore, the substi-
tution in the 2-position of 1 with a phenylethylamino (4) or a phenethoxy
substituent (5) resulted in compounds endowed with increased A2A affinity
compared to 1 (Table 1).[3]
These observations prompted us to synthesize 9-ethyladenine derivatives
substituted in 2-position with phenylalkylamino and phenylalkoxy groups
and bearing a bromine atom in 8-position.
SYNTHESIS
The synthesis of the 2-substituted 9-ethyladenines was carried out start-
ing from the 2-chloro-9-ethyladenine (3; Figure 2), which was obtained from
the 2,6-dichloropurine in two steps.[3]
FIGURE 2 Synthesis of 2,8-disubstituted-9-ethyladenines.

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Treatment of the 2-chloro-9-ethyladenine (3) with the phenethylamine
or phenethyl alcohol gave the corresponding 2-substituted derivatives 4, 5.
The reactions were performed under high temperature and, in the second
case, with addition of NaOH. 2-Substituted-9-ethyladenines were reacted
with N-bromosuccinimide to obtain the 8-bromoderivatives 6 and 7, (62%
and 67% yield, respectively; Figure 2). Synthetic procedure and characteri-
zation of these compounds will be reported elsewhere.
BIOLOGICAL DATA
The new compounds were evaluated at the human recombinant ARs,
stably transfected into Chinese hamster ovary (CHO) cells, utilizing radioli-
gand binding studies (A1, A2A, A3) or adenylyl cyclase activity assay (A2B).
Receptor binding affinity was determined using [3H]CCPA as the radioli-
gand for A1 receptors, whereas [3H]NECA was used for the A2A and A3 sub-
types. In the case of A2B receptors Ki-values were calculated from IC50 values
determined by inhibition of NECA-stimulated adenylyl cyclase activity.[4]
Binding data showed that the new compounds 6 and 7 are endowed with
good affinity for ARs and are slightly A2A selective (Table 1). In fact, intro-
duction of a bromine atom in 8-position improved affinity at all adenosine
receptors, leading to compounds which showed affinity at A2A receptor in
the low nanomolar range and good selectivity for the A2A versus A3 subtype
(6: Ki A2A = 19 nM, A3/A2A = 163; 7: Ki A2A = 1.7 nM, A3/A2A = 640), the
2-phenethoxy derivative being the most active compound.
CONCLUSION
The newly synthesized trisubstituted adenines 6 and 7 are endowed with
good affinity for the human A2A adenosine receptor subtype; the 8-bromo-
9-ethyl-2-phenethyloxy-9H-purine-6-ylamine (7), showing the highest A2A
affinity and selectivity, could be a starting point for searching new A2A AR
antagonists.
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