Synthesis and phosphodiesterase 5 inhibitory activity of novel pyrido[1,2-e]purin-4(3H)-one derivatives

Article abstract of DOI:10.1016/j.bmcl.2005.03.102

Synthesis and primary SAR of a novel series of 2-phenylpyrido[1,2-e]purin- 4(3H)-one derivatives with piperazinyl sulfonamide substituents were described herein. As potential PDE5 inhibitors for erectile dysfunction (ED) treatment, representative compound

Full text of DOI:10.1016/j.bmcl.2005.03.102

Bioorganic & Medicinal Chemistry Letters 15 (2005) 2790–2794
Synthesis and phosphodiesterase 5 inhibitory activity of novel
pyrido[1,2-e]purin-4(3H)-one derivatives
Guangxin Xia,a,c Jianfeng Li,a,c Aiming Peng,b Shunan Lai,b Shujun Zhang,a
Jingshan Shen,a,* Zhonghua Liu,a Xinjian Chena and Ruyun Jia
aShanghai Institute of Materia Medica, SIBS, Chinese Academy of Sciences, 555, Zuchongzhi Road,
Zhangjiang High-Tech Park, Shanghai 201203, China
bTopharman Shanghai Co., Ltd, 1088, Chuansha Road, Shanghai 201209, China
cGraduate School of the Chinese Academy of Sciences, Beijing, China
Received 30 December 2004; revised 25 March 2005; accepted 26 March 2005
Available online 4 May 2005
Abstract—Synthesis and primary SAR of a novel series of 2-phenylpyrido[1,2-e]purin-4(3H)-one derivatives with piperazinyl sulfon-
amide substituents were described herein. As potential PDE5 inhibitors for erectile dysfunction (ED) treatment, representative com-
pounds exhibit improved selectivity versus PDE1 and PDE6. Meanwhile, compound 3e demonstrated functional efficacy on rabbit
corpus cavernosum strip in vitro.
Ó 2005 Elsevier Ltd. All rights reserved.
The phosphodiesterases (PDEs) regulate physiological
processes by the hydrolysis of intracellular second mes-
sengers, cyclic adenosine monophosphate (cAMP), and
cyclic guanosine monophosphate (cGMP, Fig. 1) to
their biologically inactive 50 derivatives.1 In human cor-
pus carvernosum tissue, type 5 PDE (PDE5) is the major
enzyme to degrade cGMP and plays a key role in the
control of penile erection.2 On sexual stimulation, nitric
oxide (NO) is released from nonadrenergic, noncholin-
ergic neurons. The NO activates soluble guanylyl cy-
clase, which cyclizes guanosine triphosphate to
generate cGMP. Increased cGMP levels eventually
cause a decrease in intracellular calcium concentration,
resulting in the relaxation of smooth muscle in the cor-
pus cavernosum. This relaxation leads to an increased
arterial blood flow to the penis and ultimately erection.
PDE5 inhibition slows cGMP breakdown, increases the
half-life of this second messenger, indirectly extending
the activity of NO in penile tissue, and facilitates penile
erection in patients suffering from erectile dysfunction
(ED).3,4
Despite the efficacy and commercial success, ViagraÒ (sil-
denafil, 1a, Fig. 1), as a PDE5 inhibitor for ED treatment,
O
O
N
O
N
N
N
N
NH
O
N
NH
O
N
N
N
N
NH2
O
HO
O2S
O2S
N
N
O
O
P
N
N
HO
O
1a, Sildenafil
Figure 1. cGMP and PDE5 inhibitors with purine-isosteric heterocyclic systems.
cGMP
1b, Vardenafil
Keywords: Pyrido[1,2-e]purin-4(3H)-one; PDE5 Inhibitor; Sildenafil; Erectile dysfunction.
*
Corresponding author. Tel./fax: +86 21 50805891; e-mail: jsshen@mail.shcnc.ac.cn
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.03.102

G. Xia et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2790–2794
2791
has shown clinically significant adverse reactions such as
headache, flushing, dyspepsia, and visual disturbances,
which may be linked to insufficient selectivity versus
other PDE isozymes, specifically PDE1 (widely exists in
vasculature) and PDE6 (the sole cGMP PDE in the ret-
ina).5,6 Therefore, both the potency toward PDE5 and
the selectivity against other PDEs are important for the
successful development of new PDE5 inhibitors.4,7
nitrations of 5a–i were performed below 0 °C to afford
the mono-nitro products (6a–i).12 The nitroesters 6a–i
were stirred in aqueous ammonia to give the nitroamides
7a–i in high yields. Reductions of 7a–i to the corre-
sponding aminoamides (8a–i) were achieved by catalytic
hydrogenation with 10% Pd–C.13 Acylations of 8a–i
with 2-ethoxylbenzoyl chloride were carried out in the
presence of a catalytic amount of DMAP to produce
the amides (9a–i) in middle yields. Using KOtBu as base,
dehydrating cyclization of 9a–i in refluxing tBuOH
afforded 10a–i in good yields.
In the wide variety of PDE5 inhibitors reported in the
past few years, compounds with 6/5-fused ring systems,
including 1a and LevitraÒ (vardenafil, 1b), bear more
resemblance to the purine base in the substrate cGMP
(Fig. 1).8,9 The Ôstretched-outÕ purine10 derivatives (2a
and 2b) are more potent and selective ones than 1a,
but the derivatives of 2a did not lead to good result
on the functional model (rabbit corpus cavernosum
strip) or had a poor oral bioavailability, and no efficacy
data of the series of 2b on the functional model was
reported.11
According to the SAR study of pyrazolopyrimidinones
made by Terrett et al., the attachment of 50-polar sub-
stituents on the 20-ethoxyphenyl ring will improve the
PDE5 inhibitory activity distinctly.14 We then synthe-
sized a series of 50-piperazinyl sulfonamide derivatives
of 10. Putatively, these groups may fill a space occupied
by the phosphate of cGMP in the PDE5 active site.9,14
The synthetic method of target compounds 3a–g, i, o
and p was shown in Scheme 3. Chlorosulfonylations of
10a–g or 10i in chlorosulfonic acid at 0 °C gave the 50-
position substituted chlorosulfonyl derivatives, which
were coupled with N-substituted piperazines in CHCl3
at room temperature to produce desired sulfonamides
in high yields.
Other analogs (2c, 2d, and 2e shown in Fig. 2) with 5/6/6
or 6/5/6-fused tricyclic systems, being evaluated in pre-
clinical phase, also have potency and highly selectivity
toward PDE5.8 Herein, we report the design and synthe-
sis of a new template of 2-phenyl pyrido[1,2-e]purin-
4(3H)-one, in which, a ÔpyridineÕ ring was fused on the
imidazole side of the purine structure. Among the target
compounds, the 50-sulfonamide substituted derivative
(3, Fig. 2) shown potent and selective inhibitory activity
toward PDE5.
To get more SAR information of this pyridopurinones,
several additional derivatives were synthesized through
modification of pyridinyl moiety in 10. Compounds
10f and 10g were converted to 10j and 10k, respectively,
by palladium-catalyzed cyanation (Scheme 2). Bromina-
tion of 10e via radical reaction gave 10l, and the latter
was coupled with pyrrolidine or N-methylpiperazine to
afford 10m and 10n, respectively (Scheme 2). We
supposed that the polar cyclic amino group of 10m
and 10n have positive effect on PDE5 inhibitory activity
The synthetic route of 2-(2-ethoxyphenyl)pyrido[1,2-e]-
purin-4(3H)-ones (10a–i) was shown in Scheme 1.
Commercially available aminopyridines (4a–i) were con-
verted into imidazo[1,2-a]pyridines (5a–i) by refluxing in
EtOH with ethyl bromopyruvate. The regiospecific
Figure 2. PDE5 inhibitors under development with Ôstretched-outÕ purine and other tricyclic skeletons.

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G. Xia et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2790–2794
Scheme 1. Reagents and conditions: (a) ethyl bromopyruvate, EtOH, reflux, 6 h; (b) HNO3, H2SO4, ꢀ5–0 °C, 2 h; (c) NH4OH, H2O, THF, rt
overnight; (d) H2, 10% Pt–C, 50 °C; (e) 2-ethoxybenzoyl chloride, pyridine, DMAP, CH2Cl2; (f) KOt-Bu, t-BuOH, reflux.
Scheme 2. Reagents and conditions: (a) NaCN, CuI, Pd(PPh3)4, DMF, reflux, 24 h; (b) NBS, CCl4, hm, 2 h; (c) pyrrolidine, K2CO3, acetone, rt, 20 h;
(d) N-methylpiperazine, K2CO3, acetone, rt, 20 h.
(10b–e) led to more potent activity than 10a
(IC50 >10 lM), and the order of potencies was 9-methyl
(10e, IC50 = 0.116 lM) >8-methyl (10d, IC50 = 0.381
lM) >7-methyl (10c, IC50 = 0.849 lM) >6-methyl (10b,
IC50 = 1.05 lM). Introduction of electron-withdrawing
substitutents (Br, CN) at 8-position and 6-position re-
sulted in the diminishing of activity. IC50 of 10f (8-Br
analog of 10d) was only 2.16 lM. Both 8-Br and 6,8-di
Br derivatives of 10e decrease potency (10g, IC50
=
0.710 lM, 10h, IC50 = 2.10 lM). The replacement of
Br in 10f and 10g with cyano group caused further loss
of activities (10j, IC50 = 3.34 lM, 10k, IC50 = 4.87 lM).
Unexpectedly, introduction of tertiary amine on 9-
methyl group resulted in a marked loss of PDE5 inhib-
Scheme 3. Reagents and conditions: (a) (i) chlorosulfonic acid, 0–5 °C,
3 h; (ii) N-substituted piperazine, Et3N, CHCl3, rt, overnight.
like the 50-piperazinyl sulfonamide substituents men-
tioned above.
itory activities (10m, IC50 = 1.33 lM and 10n, IC50
1.57 lM).
=
Using [3H]-cGMP SPA kit, the pyridopurinones (10a–k,
m, n) and its sulfonamide derivatives (3a–g, i, o, p) were
evaluated for the inhibitory activities against PDE5 iso-
lated from human platelet.15 Table 1 summarizes the
structure and PDE5 inhibitory potency of 10a–k, m, n.
Introduction of methyl group in the pyridinyl ring
As shown in Table 2, the inhibitory potencies toward
PDE5 of all the piperazinyl sulfonamide derivatives
(3a–g, i, o, p) were correspondingly higher than the par-
ent ones (10a–g, i). The position of methyl group on
pyridinyl ring affects the activity. The potency of

G. Xia et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2790–2794
2793
Table 1. Structure and PDE5 inhibitory potency of 2-(2-ethoxyphenyl)
pyrido[1,2-e]purin-4(3H)-ones (10a–k, 10m and 10n)a
80 nM; 3c, IC50 = 112 nM), respectively. Meanwhile, it
seems that one carbon elongation of 9-position alkyl
chain did not improve the activity (the 9-ethyl derivative
3i were less potent than 3e). Almost equivalent potency
was shown by the activity data of 3e, 3o, and 3p, indicat-
ing that the N-alkyl substituents on the piperazine moi-
ety play a limited role in the PDE5 inhibitory activity. It
was consistent with the results reported by J. D. Corbin
recently.16
Compd
R6
R7
R8
R9
IC50 (lM)b
10a
10b
10c
10d
10e
10f
H
H
H
H
>10
Besides PDE5 inhibitory activity, the piperazinyl sulfon-
amide derivatives (3a–g, i, o, p) were also investigated
for other inhibitory activity against two isoforms of
PDEs isolated from bovine heart (PDE1) and bovine
retina (PDE6).17 All the compounds displayed an im-
proved selectivity over PDE1, while most of them over
PDE6 (Table 2). Compared with 1a, compound 3o
was 2.7-fold higher in its selectivity over PDE6 (3o,
IC50 ratio of PDE6/PDE5 = 22.6; 1a, IC50 ratio of
PDE6/PDE5 = 8.3).
CH3
H
H
H
H
1.05
CH3
H
H
H
0.849
0.381
0.116
2.16
H
CH3
H
H
H
H
CH3
H
H
H
Br
Br
Br
H
10g
10h
10i
H
H
CH3
CH3
C2H5
H
0.710
2.10
Br
H
H
H
0.634
3.34
4.87
10j
H
H
CN
CN
H
10k
10m
10n
H
H
CH3
CH2-N-Pr
CH2-N-Pi
H
H
1.33
1.57
H
H
H
Moreover, representative compound was evaluated in
vitro for efficacy on rabbit corpus cavernosum
strip.17b,18 To show activity in this function model, mole-
cules must efficiently diffuse into the smooth muscle cell
of corpus cavernosum. Because molecular weight (MW)
influences cell penetration ability of molecules,11a 3e
(MW = 482, IC50 ratio of PDE6/PDE5 = 14.2, PDE1/
PDE5 >7690), with similar MW to 1a (MW = 474)
and higher isozyme selectivity than 1a (IC50 ratio of
PDE6/PDE5 = 8.3, PDE1/PDE5 = 187), was chosen as
the first compound in this test. As shown in Figure 3,
compound 3e significantly enhanced the electrical fre-
quency stimulation (EFS)-induced relaxation of rabbit
corpus cavernosum. The slight difference in efficacy of
3e and 1a reflects in part the potency difference of the
two compounds.
a Assays carried out as described in Ref. 15b. PDE5 was isolated from
human platelets.
b The IC50 values were determined from the logarithmic concentra-
tion–inhibition curve (at least five points). The value is given as the
mean of at least two duplicate experiments.
Table 2. Structure and PDE inhibitory potency of 50-sulfonamide
substituted derivatives of 2-(2-ethoxyphenyl)pyrido[1,2-e]purin-4(3H)-
ones (3a–g, i, o, p)a
In summary, novel pyrido[1,2-e]purin-4(3H)-one deriva-
tives were discovered as a new class of potent and selec-
tive PDE5 inhibitors. The improved selectivities of
representative compounds may translate into decreasing
of side-effect in vivo. Furthermore, compound 3e dem-
onstrated functional efficacy on rabbit corpus caverno-
sum strip in vitro. Encouraged by these primary
results, we are continuing to optimize the potency of this
Compd Starting material
R
IC50 (lM)b
PDE5 PDE1 PDE6
3a
3b
3c
3d
3e
3f
10a
10b
10c
10d
10e
10f
10g
10i
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
C2H5
0.878
0.080
0.112
—
—
—
5.13
0.472
1.05
0.048 >100 0.693
0.013 >100 0.184
0.146
—
0.860
3g
3i
0.033 >100 0.289
0.038 >100 0.455
0.009 >100 0.204
3o
3p
1a
10e
10e
CH(CH3)2 0.015 >100 0.275
0.003 0.561 0.025
a Assays carried out as described in Ref. 15b. enzyme sources: PDE5:
human platelets; PDE1: bovine heart; PDE6: bovine retina.
b The IC50 values were determined from the logarithmic concentra-
tion–inhibition curve (at least five points). The value is given as the
mean of at least two duplicate experiments.
9-methyl derivative (3e, IC50 = 13 nM) is 5-fold, 8-fold,
and 11-fold higher than that of 8-methyl, 6-methyl,
Figure 3. Electrical frequency stimulated (EFS)-induced relaxation of
rabbit corpus cavernosum. (*p <0.05 vs vehicle, **p <0.01 vs vehicle).
and 7-methyl derivatives (3d, IC50 = 48 nM; 3b, IC50
=

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tissues were then incubated with vehicle, or test compound
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electrical stimulation was repeated. The relaxation
responses were expressed as the percentage of total
relaxation (loss in tone) induced by the addition of
0.1 mM papaverine HCl to the chambers at the end of
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