Synthesis and structure-activity relationships of guanine analogues as phosphodiesterase 7 (PDE7) inhibitors

Article abstract of DOI:10.1016/S0960-894X(01)00125-1

The synthesis of a novel series of guanine analogues is reported. The compounds have been assessed in vitro and some analogues have been found to be inhibitors of phosphodiesterase type 7 (PDE7).

Full text of DOI:10.1016/S0960-894X(01)00125-1

Bioorganic & Medicinal Chemistry Letters 11 (2001) 1081–1083
Synthesis and Structure–Activity Relationships of Guanine
Analogues as Phosphodiesterase 7 (PDE7) Inhibitors
Matthew J. Barnes, Nicola Cooper, Richard J. Davenport,* Hazel J. Dyke,
Fiona P. Galleway, Frances C. A. Galvin, Lewis Gowers, Alan F. Haughan,
Christopher Lowe, Johannes W. G. Meissner, John G. Montana, Trevor Morgan,
C. Louise Picken and Robert J. Watson
Celltech R & D, Granta Park, Great Abington, Cambridge, CB1 6GS, UK
Received 18 December 2000; revised 26 January 2001; accepted 23 February 2001
Abstract—The synthesis of a novel series of guanine analogues is reported. The compounds have been assessed in vitro and some
analogues have been found to be inhibitors of phosphodiesterase type 7 (PDE7). # 2001 Elsevier Science Ltd. All rights reserved.
Introduction
and PDE3 isozymes. As inhibition of the other PDE
isoenzymes may result in some side effects such as
emesis and cardiotoxicity, selectivity for PDE7 over the
other members of the phosphodiesterase family of
enzymes is important. In this communication, we
exemplify a series of guanine analogues which possesses
PDE7 inhibitory activity in vitro and demonstrates
some evidence of selectivity over the PDE4 isoenzyme.
The secondary messengers cAMP and cGMP are
responsible for the regulation of numerous intracellular
processes. They are regulated by phosphodiesterases
(PDEs) which hydrolyze them to the corresponding
inactive 50-monophosphate nucleotides. Eleven PDE
gene families have been identified to date, varying in
substrate specificity, inhibitor sensitivity and regulatory
characteristics.1 PDE7 is a low Km (0.2 mM) cAMP spe-
cific enzyme which is insensitive to the standard PDE4
inhibitor, rolipram. Two splice variants, PDE7A1 and
PDE7A2 were originally identified (>90% homology).2
A further PDE7 gene has recently been identified,3
which has been designated PDE7B as it possesses 70%
homology with PDE7A in the catalytic domain. PDE7
mRNA has been found to be widely distributed,
although active protein has been identified pre-
dominantly in T-cells.4 The proposed role of PDE7 in
T-cell activation5 implies that selective inhibitors could
have benefits in T-cell mediated diseases. Additionally, the
presence of PDE7 in airway epithelial cells6 suggests that
inhibitors could be beneficial in diseases of the airway.
To find a lead, screening of internal and external data-
bases was performed, and an initial guanine based hit
(1) was identified.
Chemistry
To date only two series of synthetic PDE7 inhibitors
have been described,7 generally having low micromolar
IC50 values and no significant selectivity over the PDE4
Synthesis of these guanine analogues was initially
attempted by alkylation of either 2-amino-6-chloro-
purine or 6-chloropurine (2). Intermediates (3) were
generated using potassium carbonate as the base and a
suitable alkylating agent. Displacement of the chlorine
was then achieved using sodium hydroxide to yield the
*Corresponding author. Tel.:+44-1-223-896-491; fax: +44-1-223-896-
400; e-mail: richarddavenport@celltechgroup.com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00125-1

1082
M. J. Barnes et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1081–1083
guanines (4).8 Bromination9 of the 8-position gave the
desired analogues (5) (Scheme 1).
was then brominated15 with bromine in a methanol/
acetate buffer to yield the bromides (12). In cases where
access to the desired alcohol was not possible it was
found that the substituent could be introduced via
alkylation of 8-bromoadenine (13) using the same alky-
lation conditions which had been employed for the
guanine series of analogues.
It was found that this route was not generally applic-
able, and so the synthesis of the guanine ring was
necessary in several cases (Scheme 2).
Starting from commercially available 2-amino-4,6-
dichloropyrimidine (6), saponification with sodium
hydroxide10 followed by nitration11 yielded the 6-
hydroxy pyrimidine (7). Displacement of the chlorine
with a suitable amine followed by sodium dithionite
reduction12 afforded the diamines (8). Ring closure
using triethylorthoformate was unsuccessful, due to the
acid sensitivity of the diamine, however the use of for-
mamide13 was successful. Subsequent bromination9
yielded the desired guanine analogues (9).
Structure–Activity Relationships
Initial results for the inhibiton of PDE7 using our gua-
nine analogues (Table 1) showed removal of the bro-
mine reduced activity (compare 1 and 5a) as did the
removal of the amino group (5a and 5b).
Several analogues were synthesised starting with the
replacement of the saturated six membered ring with a
five membered (5c) and a seven membered ring (9a)
(Table 2). Neither of these alterations was found to
enhance either the activity or the selectivity. However
when the tetralin ring was substituted with bromine
(9b), methoxy (9d) or a nitro group (9c) the results were
far more encouraging, and bromine substitution (9b)
was found to provide the most potent compound. The
most active analogues were also screened against the
other PDE isoenzymes present in T-cells, PDE3 and
PDE4, and found to have good selectivity for PDE7
over these enzymes.
Synthesis of adenine analogues was performed by
reacting adenine (10) (Scheme 3) with a series of commer-
cially available alcohols under Mitsunobu14 conditions
producing the alkylated compounds (11). The 8-position
The natural ligand for PDE7 is cAMP, which contains
an adenine base. By replacing the guanine moeity with
an adenine in our compounds, a better overlay with
cAMP would be predicted. Potentially this would
improve inhibitor potency. This hypothesis was investi-
gated by the synthesis of a series of analogues (12) based
around the initial guanine lead (1).
The results for the inhibition of PDE7 using the adenine
analogues were surprisingly poor. None of the analogues
Scheme 1. (a) K2CO3, RX, (b) aq NaOH, (c) Br2, H2O.
Scheme 2. (a) aq NaOH, (b) H2SO4, HNO3, (c) Et3N, RNH2, DCM,
(d) Na2S2O4, DMF, H2O, (e) HCONH2, (f) Br2, H2O.
Scheme 3. (a) PPh3, DEAD, ROH, (b) Br2, MeOH, NaOAc/AcOH,
(c) K2CO3, RX.

M. J. Barnes et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1081–1083
1083
Table 1. In vitro data for guanine analoguesa
synthesised were found to offer any advantage over the
guanine series. The adenine amino group was also
replaced with both a methoxy group and a chloro sub-
stituent, but in each case no improvement of activity or
selectivity was observed.
In conclusion a series of analogues based around our
original hit has been successfully synthesised. Our ori-
ginal guanine lead (1) was found to have low micro-
molar activity against PDE7 with good selectivity over
both PDE4 and PDE3. This lead has been enhanced to
improve the activity, whilst retaining its selectivity over
the other key PDE isoenzymes. The 8-bromo-9-sub-
stituted series are the most potent and selective com-
pounds published to date. Efforts are ongoing in the
PDE7 area of research at Celltech R & D and the results
of further studies will be published in due course.
Compd
R
X
Y
PDE716
4.88
PDE417
15%
1
NH2
Br
5a
5b
NH2
H
H
H
4%
IAb
7%
IAb
aCompound results are IC50 (mM) or when quoted as a ‘%’ refer to
percentage inhibition at 10 mM.
References and Notes
bCompound results denoted by ‘IA’ were considered inactive at
10 mM.
1. Beavo, J. A. Biochemistry of PDE Isoenzymes and their
Regulation. Presented at William Harvey research conference,
Nice 12/99.
Table 2. In vitro data for tetralin replacementsa
2. Bloom, T. J.; Beavo, J. A. Proc. Natl. Acad. Sci. U.S.A.
1996, 93, 14188.
3. Hetman, J. M.; Soderling, S. H.; Glavas, N. A.; Beavo, J. A.
Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 472.
4. Hoffmann, R.; Abdel’al, S.; Engels, P. Cell Biochem. Bio-
phys. 1998, 28, 103.
5. Li, L.; Yee, C.; Beavo, J. A. Science 1999, 283, 848.
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Barnes, P. J.; Hippenstiel, S.; Kraemer, H. J.; Suttorp, N. Am.
J. Respir. Cell Mol. Biol. 1999, 20, 292.
Compd
R
PDE716
4.88
PDE417
15%
PDE318
30%
1
7. Martinez, I.; Castro, A.; Gil, C.; Miralpeix, M.; Segarra,
V.; Domenech, T.; Beleta, J.; Palacious, J. M.; Ryder, H.;
Miro, X.; Bonet, C.; Casacuberta, F. A.; Benjami, P.; Puig-
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Med. Chem. 1984, 27, 1486.
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Chem. 1985, 22, 1035.
12. Nair, M. G.; Saunders, C.; Chen, S.-Y.; Kisliuk, R. L.;
Gaumont, Y. J. Med. Chem. 1980, 23, 59.
13. Sen, D.; Dasgupta, A.; Sengupta, P. Indian J. Chem. 1985,
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14. Yu, K.-L.; Bronson, J. J.; Yang, H.; Patick, A.; Branko-
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Chem. 1993, 36, 2726.
15. Gani, D.; Johnson, A. W.; Lappert, M. F. J. Chem. Soc.,
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5c
9a
9b
9c
9d
22%
10%
1.31
2.31
4.22
27%
13%
10%
42%
20%
NTb
NTb
14%
19%
NTb
16. PDE7 was isolated from HUT-78 cells as previously
described2 with some minor modifications to suit larger-scale
preparation.
17. Thompson, W. J.; Terasaki, W. L.; Epstein, P. M.; Strada,
S. J. Adv. Cyc. Nuc. Res. 1979, 10, 69.
18. Simpson, A. W.; Reeves, M. L.; Rink, T. J. Biochem.
Pharmacol. 1988, 37, 2315.
aCompound results are IC50 (mM) or when quoted as a ‘%’ refer to
percentage inhibition at 10 mM.
bCompound results denoted by ‘NT’ were not tested in the listed
assay.