SAR development of polycyclic guanine derivatives targeted to the discovery of a selective PDE5 inhibitor for treatment of erectile dysfunction

Article abstract of DOI:10.1016/j.bmcl.2003.12.027

Development of structure-activity relationship of cyclic guanines I lead us to discovery of a potent and selective series of phosphodiesterase 5 inhibitors 52-59 (IC₅₀=1.3-11.0 nM, PDE6/5=116-600).

Full text of DOI:10.1016/j.bmcl.2003.12.027

Bioorganic & Medicinal Chemistry Letters 14 (2004) 1291–1294
SAR development of polycyclic guanine derivatives targeted
to the discovery of a selective PDE5 inhibitor for treatment
of erectile dysfunction
Dmitri A. Pissarnitski,* Theodros Asberom, Craig D. Boyle, Samuel Chackalamannil,
Madhu Chintala, John W. Clader, William J. Greenlee, Yueqing Hu, Stanley Kurowski,
Joyce Myers, Jairam Palamanda, Andrew W. Stamford, Subbarao Vemulapalli,
Yuguang Wang, Peng Wang, Ping Wu and Ruo Xu
Schering-Plough Research Institute, 2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
Received 27 September 2003; revised 3 December 2003; accepted 6 December 2003
Abstract—Development of structure–activity relationship of cyclic guanines I lead us to discovery of a potent and selective series of
phosphodiesterase 5 inhibitors 52–59 (IC50=1.3–11.0 nM, PDE6/5=116–600).
# 2003 Elsevier Ltd. All rights reserved.
Introduction of sildenafil (ViagraTM) for the treatment
of erectile dysfunction in 1998 has led to a fundamental
shift in this therapeutic area. This effective, orally active
drug has virtually displaced previously developed
options, such as intracavernosal or intraurethral injec-
tions of vasoactive substances or penile implants as
mechanical aids.
Despite its commercial success, sildenafil has shown
clinically significant side effects,2 some of which may be
linked to insufficient selectivity versus other PDEs. Spe-
cifically, reported incidences of visual disturbances may
be linked to inhibition of PDE6.
Several approaches to an improved PDE5 inhibitor
were reported in the literature during the last several
years,3 with the most challenging aspect being achieving
In males, erection is triggered by the release of nitric
oxide from non-adrenergic, non-cholinergic (NANC)
neurons located in the corpus cavernosum of the penis,
which takes place upon sexual stimulation. Nitric oxide
initiates cGMP synthesis via activation of soluble gua-
nylyl cyclase in the smooth muscle cells. cGMP med-
iates intracellular signal transduction, leading to a
decrease in calcium concentration, followed by relax-
ation of smooth muscle in corpus cavernosum, increase
of arterial flow to this tissue and engorgement of penis
with blood. The mechanism of action of sildenafil stems
from the inhibition of phosphodiesterase type 5 (PDE5),
which is the major cGMP hydrolyzing enzyme in corpus
cavernosum. As the breakdown of cGMP is slowed
down, increased levels of this mediator lead to
enhancement of erection.1
* Corresponding author. Tel.: +1-908-740-3844; fax: +1-908-740-
7152; e-mail: dmitri.pissarnitski@spcorp.com
Figure 1. Structure II as initial lead.
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.12.027

1292
D. A. Pissarnitski et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1291–1294
significant (e.g., several hundred-fold) selectivity over
the closely related PDE6 isoform. In the light of this
fact we wish to report our studies which culminated in
the discovery of a novel, potent and selective series of
PDE5 inhibitors.4
PDE1 and PDE5. A representative of this class with
N-5 ethyl substitution (II) was reported5 to have modest
levels of PDE5 inhibition, and was chosen as the lead
molecule for the present study (Fig. 1).
Molecule II allowed easy chemical modification at the
N-3 atom by known N-arylation methodology6 as well
as by alkylation or benzylation in DMF in the presence
of potassium carbonate (Scheme 1).7 As the N-1 center
Cyclic guanines with framework of type I were dis-
covered in these labs in the course of antihypertensive
drug search, and were identified as dual inhibitors of
represents
diminished so that a complete N-3 selectivity is achieved.
a
vinylogous amide, its reactivity is
While simple alkyl- and aryl-substituted analogues of II
lacked PDE5 activity, N-3 benzylation was tolerated. In
order to systematize the investigation of benzyl sub-
stitution, a modified Topliss tree8 approach was under-
taken (Table 1).
Introduction of an electron-donating methyl group at
the para-position of the phenyl ring (2) resulted in >3
fold improvement of PDE5 potency as compared to the
unsubstituted case (1). meta- And ortho-substitution
was not tolerated (3, 4). Introduction of electron-with-
drawing para-substituent (Cl, NO2) resulted in
diminishing (5) or loss (6) of PDE5 activity. As was
expected, a substituent with stronger electron-donating
properties, such as methoxy- and hydroxy-, allowed for
further improvement in IC50 as in 7 and 8, the latter
compound having potency and selectivity on a par with
sildenafil. Despite the well-documented electron-donat-
ing properties of a p-amino group, it resulted in a drastic
loss of activity (9). Di-substituted derivatives 14–16 as
well as aryl substituted compound 10 were less active.
Scheme 1. Synthesis of compounds of Table 1. (a) ArB(OH)2,
Cu(OAc)2, Pyr, DCM, air; (b) Alk-I, DMF, K2CO3; (c) Br–CH2–Ar,
DMF, K2CO3; (d) LDA, THF, I2; (e) phenylacetylene, PdCl2(PPh3)2,
CuI, TEA; (f) BCl3, DCM.
Table 1. Discovery of Y=p-MeO and p-OH as optimized substituents
at N-3 center
In line with the previous observations,9 installation of
an alkynyl substituent at C-2 provided excellent PDE5
inhibitors 17–20. A combination of the optimized
p-hydroxybenzyl substituent at N-3 with a phenyl-
acetylene at C-2 resulted in a sub-nanomolar inhibitor
(20).
Despite these promising results, we were concerned that
conjugation of the acetylene in 17–20 with the electron-
Entry
X
Y
PDE5 IC50, nM PDE6/5
1
2
3
4
5
6
7
8
9
–CH2–Ph
–CH2–Ph
–CH2–Ph
–CH2–Ph
–CH2–Ph
–CH2–Ph
–CH2–Ph
–CH2–Ph
–CH2–Ph
H
130.0
40.0
2.6
1.5
p-Me
m-Me
o-Me
p-Cl
p-NO2
p-MeO
p-OH
p-NH2
26%a
18%a
160.0
8%a
1.6
20.0
3.9
550.00
1.1
7.4
0.4
10
–CH2–Ph
32%a
—
11
12
13
14
15
16
17
18
19
20
–CH2–Ph
–CH2–Ph
–CH2–Ph
–CH2–Ph
–CH2–Ph
m-Cl
m-Br
m-MeO
p-Me, m-Cl
p-MeO, m-Cl
26%a
33%a
14%a
27%a
58.0
46.0
8.0
—
—
—
—
6.7
0.7
2.6
3.4
0.5
10.3
–CH2–Ph 3,4-methylenedioxy
Scheme 2. Synthesis of compounds of Tables 2 and 3. (a) LDA; (b)
ClCOOMe; (c) TsCN; (d) TsN3; (e) PhSSPh; (f) BrF2C–CF2Br or I2;
(g) Hal=Br: Me2NH, THF, 100 ꢀC, sealed tube; (h) Hal=I: ‘CuCF3’
(lit.10), DMF, HMPA; (i) Hal=I: RONa; (j) Hal=Br: EtSNa; (k)
BCl3, DCM; (l) AlCl3, EtSH:DCM=1:1; (m) PPh3, THF, H2O; (n)
NH3, MeOH; (o) NaBO3, AcOH, 50 ꢀC.
–CC–Ph
–CC–Ph
–CC–Ph
–CC–Ph
H
p-Cl
p-MeO
p-OH
5.0
3.1
0.3
a % Inhibition at 100 nM.

D. A. Pissarnitski et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1291–1294
Table 2. Optimization of substituent X at C-2 site Table 3. Effect of substituent Z
1293
Entry
X
Y
PDE5 IC50 (nM)
PDE6/5
Entry
X
Y
Z
PDE5 IC50 (nM)
PDE6/5
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
SO2Et
NMe2
H
CF3
OEt
SEt
COOMe
CN
CONH2
NH2
CF3
MeO
MeO
MeO
MeO
MeO
MeO
MeO
MeO
MeO
OH
31%a
27%a
55.0
48.0
18.0
13.0
6.8
6.3
4.0
22.0
7.0
—
—
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
H
H
H
MeO
MeO
OH
MeO
OH
OH
MeO
OH
MeO
MeO
OH
Br
Cl
Br
Br
Br
Cl
Cl
Cl
Br
Cl
Cl
Br
Cl
Br
Cl
6.5
8.3
6.3
1.5
0.9
1.1
2.3
1.3
5.4
6.5
2.1
2.8
5.0
1.6
2.0
6
4
0.6
1.7
1.4
1.0
0.8
1.2
3
13
23
28
25
24
9.3
50
82
6.5
32
11
8
57
32
10
194
100
CONH2
CONH2
CONH2
CONHMe
CONHMe
OEt
OEt
OEt
OMe
OMe
OH
OH
OH
OH
CONH2
SEt
OEt
2.1
1.8
0.9
0.61
MeO
MeO
OH
21
67
N3
OH
OMe
OMe
OH
a The value represents % inhibition at 100 nM of inhibitor.
withdrawing moiety would set the former for Michael
reaction with biological nucleophiles. Based on a
hypothesis that the both electron-withdrawing p-system
and sterically unencumbered nature of acetylene is
important for PDE5 binding, we prepared a series of
surrogates 21–35, all bearing a p-methoxy- or p-hydro-
xybenzyl group on N-3 and either electron-negative or
heteroatomic substituent X at C-2 (Table 2, Scheme 2).
the highest selectivity of 194-fold was achieved for Z¼
Br (49).
We were interested in determining if the developed SAR
would be maintained in a series with a modified C8–C7
region. Structure IX is a direct analogue of II featuring
a benzyl substituent at C-7, which resulted in an
improved PDE1 selectivity (Fig. 2). Gratifyingly, com-
bination of the two SARs allowed us to further improve
the profile of inhibitors (Table 4). Compounds 52–59
displayed 116–600-fold selectivity over PDE6 and ꢁ400
to several thousand-fold selectivity over PDE1-4.
Dimethylamino and alkylsulfone groups at C-2 were
not tolerated probably because of their size. SAR of
other
C-2
substituents
followed
the
trend:
H<CF3<alkoxyꢁthioakyl ꢁakoxycarbonylꢁcyano
ꢁcarboxamide.
Subsequently, we became aware that other heterocyclic
systems with similarly substituted benzyl groups were
also developed as PDE5 inhibitors.11 The notion
suggests that this moiety is an important enzyme
recognition element.
Transformation of the N-3 substituent from p-methoxy
to p-hydroxybenzyl resulted in a 2–20-fold increase in
potency, with the effect most pronounced for C-2
alkoxy substituent. No useful PDE6 selectivity yet was
observed, with the highest value of 67-fold for azide 35,
which could not be pursued as a drug due to potential
toxicity.
In the next round of SAR development, we kept the
optimized alkoxy and carboxamido substituents at C-2,
p-methoxy or p-hydroxybenzyl at N3, and introduced
an additional substituent Z=Cl or Br at the phenyl ring
(Table 3).
For all the C-2 substituents studied, a single-digit
nanomolar PDE5 potency was achieved. As before, free
phenols (Y¼OH) were superior in both the potency and
selectivity. For C-2 carboxamides, Z¼ Cl was preferred
over Z¼ Br (41 versus 40). For C-2 alkoxides, however,
Figure 2. Overlap of structure X on determined SAR.

1294
D. A. Pissarnitski et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1291–1294
Table 4. Combination of structure IX with previously established SAR
Entry
X
Y
Z
PDE5 IC50 (nM)
PDE1/5
PDE2/5
PDE3/5
PDE4/5
PDE6/5
51
52
53
54
55
56
57
58
59
CONH2
CONH2
OBn
OMe
OMe
OMe
OMe
OMe
OMe
MeO
OH
OH
MeO
MeO
MeO
OH
Cl
Cl
Cl
CN
Br
Cl
CN
Br
Cl
2.8
3.0
2.1
11.0
2.2
1.6
43.0
1.3
1.9
—
—
—
—
61
600
162
445
173
206
116
562
416
2600
4762
536
4091
4375
—
3333
4762
909
4545
6250
—
3333
4762
909
4545
6250
—
1733
2168
464
418
394
—
3769
3105
OH
OH
7692
5263
7692
5263
7692
5263
References and notes
Neustadt, B.; Puchalski, C.; Pula, K.; Silverman, L.;
Smith, E.; Stamford, A.; Tedesco, R. P.; Tsai, H.;
Tulshian, D.; Vaccaro, H.; Watkins, R. W.; Weng, X.;
Witkowski, J. T.; Xia, Y.; Zhang, H. J. Med. Chem. 1997,
40, 2196.
1. Corbin, J. D.; Francis, S. H. J. Biol. Chem. 1999, 274,
13729.
2. Sildenafil is contraindicated for patients taking NO donors
or organic nitrates because of a synergistic effect on blood
pressure. However, there was no deducible evidence of an
increase in the mortality rate among sildenafil users.
Gresser, U.; Gleiter, C. H. Eur. J. Med. Res. 2002, 7, 435.
3. Rotella, D. P. Nature Reviews Drug Discovery 2002, 1,
674.
4. Human PDEs were isolated and assayed as described
previously: Wang, P.; Wu, P.; Myers, J. G.; Stamford, A.;
Egan, R. W.; Billah, M. M. Life Sci. 2001, 68, 1997.
Recombinant human PDE1A3 was assayed at the final
concentration of cAMP of 10 mM. Recombinant human
PDE1A3 was assayed at the final concentration of cAMP
of 10 mM. Recombinant human PDE4B2 was assayed at
the final concentration of cAMP of 1 mM. Human PDE6
isolated from retina was assayed at the final concentration
of cGMP of 0.5 mM. All assays were performed in the
presence of 2% DMSO and 0.1% BSA..
6. Lam, P. Y. S.; Clark, C. G.; Saubern, S.; Adams, J.;
Winters, M. P.; Chan, D. M. T.; Combs, A. Tetrahedron
Lett. 1998, 39, 2941.
7. All new compounds were characterized by LCMS and
NMR.
8. Toplis, J. G. J. Med. Chem. 1972, 15, 1006.
9. Ho, G. D.; Silverman, L.; Bercovici, A.; Puchalski, C.;
Tulshian, D.; Xia, Y.; Czarniecki, M.; Green, M.; Cleven,
R.; Zhang, H.; Fawzi, A. Bioorg. Med. Chem. Lett. 1999,
9, 7.
10. Mawson, S. D.; Weavers, R. T. Tetrahedron Lett. 1993,
34, 3139.
11. (a) Bunnage, M. E.; Mathias, J. P.; Street, S. D. A.;
Wood, A. WO 9849166, Chem. Abstr. 1998, 129, 330741.
(b) Rotella, D. P.; Sun, Z.; Zhu, Y.; Krupinski, J.; Pon-
grac, R.; Seliger, L.; Normandin, D.; Macor, J. E. J. Med.
Chem. 2000, 43, 1257. (c) Rotella, D. P.; Sun, Z.; Zhu, Y.;
Krupinski, J.; Pongrac, R.; Seliger, L.; Normandin, D.;
Macor, J. E. J. Med. Chem. 2000, 43, 5037. (d) Bi, Y.;
Stoy, P.; Adam, L.; He, B.; Krupinski, J.; Normandin, D.;
Pongrac, R.; Seliger, L.; Watson, A.; Macor, J. E. Bioorg.
Med. Chem. Lett. 2001, 11, 2461.
5. Ahn, H.-S.; Bercovici, A.; Boykow, G.; Bronnenkant, A.;
Chackalamannil, S.; Chow, J.; Cleven, R.; Cook, J.;
Czarniecki, M.; Domalski, C.; Fawzi, A.; Green, M.;
Guendes, A.; Ho, G.; Laudicina, M.; Lindo, N.; Ma, K.;
Manna, M.; McKittrick, B.; Mirzai, B.; Nechuta, T.;