Furoyl and benzofuroyl pyrroloquinolones as potent and selective PDE5 inhibitors for treatment of erectile dysfunction

Article abstract of DOI:10.1021/jm0202573

Synthesis of furoyl and benzofuroyl pyrroloquinolones as potent and selective PDE5 inhibitors was reported. Their in vitro potencies in inhibiting PDE5 and selectivity in inhibiting other PDE isozymes (PDE1-4 and PDE6) were evaluated. Some of these compounds are more potent than sildenafil with better selectivity toward PDE1 and PDE6. Incorporation of solublizing groups resulted in bioavailable analogues. Selected compounds showed in vivo efficacy in anesthetized dog model for penile erection.

Full text of DOI:10.1021/jm0202573

J . Med. Chem. 2003, 46, 441-444
441
Brief Articles
F u r oyl a n d Ben zofu r oyl P yr r oloqu in olon es a s P oten t a n d Selective P DE5
In h ibitor s for Tr ea tm en t of Er ectile Dysfu n ction
Weiqin J iang,* Zhihua Sui, Mark J . Macielag, Shawn P. Walsh, J ames J . Fiordeliso, J ames C. Lanter,
J ihua Guan, Yuhong Qiu, Patricia Kraft, Sheela Bhattacharjee, Elizabeth Craig, Donna Haynes-J ohnson,
T. Matthew J ohn, and J oanna Clancy
J ohnson & J ohnson Pharmaceutical Research & Development L.L.C., 1000 Route 202 South, P.O. Box 300,
Raritan, New J ersey 08869
Received J une 17, 2002
Synthesis of furoyl and benzofuroyl pyrroloquinolones as potent and selective PDE5 inhibitors
was reported. Their in vitro potencies in inhibiting PDE5 and selectivity in inhibiting other
PDE isozymes (PDE1-4 and PDE6) were evaluated. Some of these compounds are more potent
than sildenafil with better selectivity toward PDE1 and PDE6. Incorporation of solublizing
groups resulted in bioavailable analogues. Selected compounds showed in vivo efficacy in
anesthetized dog model for penile erection.
Sch em e 1. Synthesis of Pyrroloquinolone 16 and 19
In tr od u ction
PDE5 is the major cGMP-hydrolyzing enzyme in
human corpus carvernosal tissue. Upon sexual stimula-
tion, release of nitric oxide from NANC neurons and the
vascular endothelium activates soluble guanylyl cyclase
in the smooth muscle cells, initiating cGMP synthesis.
Inhibition of PDE5 causes further accumulation of
cGMP in the penile tissue. The resulting elevated cGMP
levels cause a decrease in intracellular calcium concen-
tration, leading to relaxation of smooth muscle in the
corpus cavernosum. The relaxation of this tissue permits
increased arterial flow to the penis, which becomes erect
when engorged with blood.1
The large population of people suffering from male
erectile dysfunction (MED)2,3 and the commercial suc-
cess of sildenafil4 have provided a strong stimulus for
the discovery and development of second-generation
PDE5 inhibitors.5,6 One important issue facing such an
agent is selectivity versus other enzymes in the phos-
phodiesterase superfamily. Inhibition of PDE1 and
PDE6 may be associated with some of the adverse side
effects of sildenafil therapy, especially visual distur-
bances (PDE6). Recently, we reported the discovery of
both furoyl â-carbolines7c and pyrimidinyl pyrroloquin-
olones7e as potent and selective PDE5 inhibitors. In an
effort to extend the scope of this discovery, we now
report a series of furoyl and benzofuroyl pyrroloquino-
lones as PDE5 inhibitors with greater potency and
selectivity for PDE5 versus other PDE isozymes.
amine provided â-carbolines 13a and 13b.8 After ben-
zylation with benzyl bromide, compounds 14a and 14b
were rearranged via Winterfeldt oxidation9 to furnish
protected pyrroloquinones 15a and 15b. Hydrogenolysis
of the benzyl group provided pyrroloquinones 16a and
16b in good yield. Alternatively, this sequence can be
reduced to two steps without the use of a protecting
group. The Pictet-Spengler reaction of veratraldehyde
17 provided â-carboline 18, which was directly oxidized
to pyrroloquinolone 19. X-ray analysis on a crystalline
sample of 19 confirmed the structure of this key inter-
mediate (Figure 1).12 This two-step sequence was used
for generating compound 16a in similar yield and
compound 16b in lower yield. Although furoyl pyrrolo-
quinolones were initially made by direct acylation of
pyrroloquinolones 16a and 16b with acid chlorides
(Scheme 2), coupling acids with 16a and 16b using
standard conditions10 was found to be more efficient.
Suzuki coupling of 12a or 12b with aryl- and hetero-
arylboronic acids allowed facile introduction of diversity
elements (Scheme 3).11
Ch em istr y
Pyrroloquinolone core structures 16a and 16b were
synthesized in a four-step sequence (Scheme 1). The
Pictet-Spengler reaction of piperonal (11a ) or 2,3-
dihydrobenzo[b]furan-5-carboxaldehyde (11b) with trypt-
* To whom correspondence should be addressed. Phone: (908) 704-
4351. Fax: (908) 526-6469. E-mail: wjiang1@prdus.jnj.com.
10.1021/jm0202573 CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/28/2002

442 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 3
Brief Articles
Sch em e 3. Suzuki Coupling Reaction of Furoyl
Pyrroloquinolonesa
F igu r e 1. X-ray ORTEP picture of compound 19.
Sch em e 2. Synthesis of Furoyl and Benzofuroyl
Pyrroloquinolones via Acylation Reaction
a
(/) All of the compounds were synthesized from 12a except
for 20r and 20w . These were made from 12b.
Ta ble 1. Potencies toward Inhibiting PDE5
compd
Ki (SD),a nM
compd
Ki (SD),a nM
sildenafil
12a
12c
12d
20a
20b
20c
20d
20e
20f
20g
20h
20i
1.91((0.33)
2.07((0.53)
2.49((1.16)
3.32((1.54)
0.98((0.09)
0.61((0.30)
2.09((0.78)
2.64((0.70)
0.47((0.10)
0.61((0.08)
1.30((0.57)
1.01((0.32)
0.99((0.18)
2.59((0.32)
2.71((1.22)
20l
20m
20n
20o
20p
(-)-20q
(()-20q
20r
20s
20t
20u
20v
20w
26a
26b
0.73((0.06)
1.71((0.19)
3.74((0.04)
0.24((0.10)
0.76((0.05)
0.20((0.02)
1.82((0.31)
1.78((0.32)
4.66((0.92)
0.71((0.35)
0.31((0.05)
0.15((0.02)
0.53((0.14)
0.35((0.02)
1.74((0.69)
20j
20k
a
Values are the mean of three experiments. Standard devia-
tions are given in parentheses.
Hydrophilic (20e and 20f), basic (20p and 20q), mod-
erately acidic (20l), and acidic (20h ) groups could all
be introduced to produce compounds with PDE5 inhibi-
tory activities equal to or better than sildenafil. We also
investigated the replacement of the phenyl substituent
in 20c with heterocycles (20s-w ). It was particularly
encouraging to note that moderately basic solublizing
groups (20u and 20v) could be introduced with a
marked increase in potency relative to sildenafil. Fi-
nally, in a bid to further reduce the molecular weight
of the lead series, we prepared benzofurans 26a and 26b
and found their potency comparable to unfused phenol
20l.
Resu lts a n d Discu ssion
Initially, we prepared both aryl (12c) and heteroaryl
(12a and 12d ) derivatives of the pyrroloquinolone core
(Table 1). Although all had potencies comparable to
sildenafil, for purposes of this report, we focused our
attention on the furoyl series. Replacement of the
bromine atom in 12a with a phenyl ring (20c) was
accomplished without loss of potency. The tolerance for
a wide range of substituents on this phenyl ring from
very electron-withdrawing (20b) to very electron-donat-
ing (20n ) indicated that this area was a good place to
adjust the physicochemical properties of the series.
One of the most important issues facing PDE5 inhibi-
tors is their selectivity versus other PDE isozymes, in
particular, PDE1 (found in heart) and PDE6 (primarily
located in retina). All compounds tested in this series
had high selectivity versus PDE1-4 (Table 2). For

Brief Articles
J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 3 443
Ta ble 2. Selectivities of PDE5 over PDE1-4 and -6a
F igu r e 2. Results of cell-based functional assays, showing
fold increase in cGMP levels above 10 µM SNP-stimulated
baseline in RFL6 cells.
a
(/) Values are the mean of three experiments for Ki of PDE5
and the mean of at least two experiments for Ki of PDE1-4 and
PDE6 (retina cone and rod).
PDE2, -3, and -4, these compounds showed selectivity
comparable to that of sildenafil, and for PDE1, they
showed selectivity a minimum of 100-fold better than
sildenafil. Since our initial analogues such as 20r had
selectivity for PDE6 similar to that of sildenafil, we
focused our efforts on improving the PDE6/PDE5 ratio.
Replacement of methylenedioxyphenyl group (20q and
20u ) with dihydrobenzofuran (20r and 20v, respec-
tively) improved the PDE6/PDE5 selectivity 6- and
3-fold, respectively, without compromising the potency
and the selectivity versus other isozymes. Selectivity for
PDE5 over PDE6 increased even more (20-fold) when
this modification was applied to benzofuroyl pyrrolo-
quinolone (26a versus 26b). This result was consistent
with previous findings in the pyrimidinyl pyrroloqui-
nolone series.7e We also synthesized the enantiomer (-)-
20q, which likely has the (R)-configuration,7e by using
chiral starting material (R)-(-)-13a .8b Biological testing
confirmed that (-)-20q is also more potent and selective
than its racemic form, (()-20q.
F igu r e 3. In vivo efficacy of 20o in anesthetized dogs, showing
ICP increase after intravenous administration.
In Vivo Effica cy. Selected analogues from this series
were studied in vivo in an anesthetized dog model of
erection. In these studies, drugs were administered via
the intravenous route using sildenafil citrate as a posi-
tive control. Although there is no dose response at lower
doses, compound 20o significantly potentiated ICP
increase at 300 µg/kg. This suggested that compound
20o is efficacious in this model. As shown in Figure 3,
the ED50 for compound 20o is around 284 µg/kg.
Con clu sion s
In this paper, we have reported the discovery of highly
selective and potent PDE5 inhibitors, furoyl and ben-
zofuroyl pyrroloquinolones. Some of these compounds
are more potent than sildenafil against the isolated
PDE5 enzyme. Substituents on the phenyl group can
be introduced to reach reasonable oral bioavailibility.
Representative compounds showed improvement in
their selectivity for PDE1 and -6 when compared to
sildenafil. The SAR results in this work indicate that
PDE6/PDE5 selectivity can be increased moderately
when the methylenedioxyphenyl group adjacent to the
chiral center was replaced with dihydrobenzofuran
group. Finally, we have discovered a compound that is
efficacious in an anesthetized dog model.
Cell-Ba sed F u n ction a l Assa ys. Further evaluation
of the compounds was carried out by testing their ability
to increase cGMP levels in RFL-6 cells. In comparison
with sildenafil, these compounds possess similar poten-
cies in the cell-based assays. A representative example
is shown in Figure 2, where data for 20q and (-)-20q
are almost superimposable on that of sildenafil.
P r elim in a r y P h a r m a cok in etics Stu d ies. Next, we
addressed the issue of oral bioavailability. Eight com-
pounds bearing a range of acidic and basic functional
groups were tested in a preclinical pharmacokinetics
study in male rats. The sodium salt of 20l and 26b and
the hydrochloride of 20w showed no oral bioavailability.
The hydrochloride salts of 20p , 20u and the sodium salt
of 20h showed only a trace of oral bioavailablity, i.e.,
4%, 3%, and 0.4%, respectively. The sodium salt of 26a
showed modest bioavailabitity (17%), while the hydro-
chloride salt of the methylpiperazine derivative 20q
showed the best bioavailabitity (31%).
Ack n ow led gm en t. The authors thank Drs. William
V. Murray, Do Won Hahn, and Scott G. Lundeen for
discussions and support, and Dr. Bill Hageman, Mary
Evangelisto, and Vernon Alford for technical assistance.
Su p p or tin g In for m a tion Ava ila ble: Biological assay,
characterization of compounds reported (12a -d , 13a ,b, 14a ,b,
15a ,b, 16a ,b, 18, 19, 20a -w , 26a ,b). This material is available

444 J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 3
Brief Articles
â-Carbolines as PDE5 Inhibitors for Treatment of Male Erectile
Dysfunction. Abstracts of Papers, 224th National Meeting of the
American Chemical Society, Boston, MA, August 18-22, 2002;
MEDI-278. (d) J iang, W.; Sui, Z.; Guan, J .; Macielag, M. J .;
Walsh, S. P.; Lanter, J . C.; Fiordeliso, J . J .; Qiu, Y.; Kraft, P.;
Bhattacharjee, S.; Haynes-J ohnson, D.; Lombardi, E.; J ohn, T.
M.; Clancy, J . Furoyl Pyrroloquinolones as Selective PDE5
Inhibitors for Treatment of Male Erectile Dysfunction. Abstracts
of Papers, 224th National Meeting of the American Chemical
Society, Boston, MA, August 18-22, 2002; MEDI-280. (e) Sui,
Z.; Guan, J .; Macielag, M. J .; J iang, W.; Zhang, S.; Qiu, Y.; Kraft,
P.; Bhattacharjee, S.; J ohn, T. M.; Haynes-J ohnson, D.; Clancy,
J . Pyrimidineyl Pyrroloquinolones as Highly Potent and Selec-
tive PDE5 Inhibitors for Treatment of Erectile Dysfunction. J .
Med. Chem. 2002, 45, 4094-4096.
free of charge via the Internet at http://pubs.acs.org. Crystal-
lographic data for 19 have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication
CCDC 194662. Copies of the data can be obtained, free of
charge, on application by e-mail to deposit@ccdc.cam.ac.uk.
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