Pyrimidinylpyrroloquinolones as highly potent and selective PDE5 inhibitors for treatment of erectile dysfunction

Article abstract of DOI:10.1021/jm025545d

A series of N-pyrimidinylpyrroloquinolones were discovered as extremely potent and selective PDE5 inhibitors. Representative compounds demonstrated in vivo efficacy in dog erectile dysfunction models and are orally bioavailable.

Full text of DOI:10.1021/jm025545d

4094
J . Med. Chem. 2002, 45, 4094-4096
Letters
Sch em e 1
P yr im id in ylp yr r oloqu in olon es a s High ly
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
Zhihua Sui,* J ihua Guan, Mark J . Macielag,
Weiqin J iang, Suying Zhang, Yuhong Qiu,
Patricia Kraft, Sheela Bhattacharjee,
T. Matthew J ohn, Donna Haynes-J ohnson, and
J oanna Clancy
J ohnson & J ohnson Pharmaceutical Research &
Development, Drug Discovery, 1000 Route 202,
Raritan, New J ersey 08869
Ch a r t 1
Received May 31, 2002
Abstr a ct: A series of N-pyrimidinylpyrroloquinolones were
discovered as extremely potent and selective PDE5 inhibitors.
Representative compounds demonstrated in vivo efficacy in
dog erectile dysfunction models and are orally bioavailable.
In tr od u ction . Cyclic nucleotides (adenosine cyclic
3′,5′-phosphate (cAMP) and cyclic guanosine 5′-phos-
phate (cGMP)) are important secondary messengers
that control many physiological processes. The levels of
intracellular cyclic nucleotides are determined by the
activities of cyclases that synthesize them and phos-
phodiesterases (PDEs) that degrade them. Upon extra-
cellular stimulation, the levels of cyclic nucleotides
change rapidly, producing the physiological responses.
PDEs play critical roles in modulating the levels of cyclic
nucleotide, their duration of action, and thus, the
physiological outcome. To date, 21 mammalian PDE
genes have been cloned and they are classified into 11
families according to the sequence homology and bio-
proton correlated with potency against PDE5. Consist-
ent with this finding, we now report a series of ex-
tremely potent PDE5 inhibitors, pyrroloquinolones,
which we discovered during our work on the â-carbo-
lines.
Resu lts a n d Discu ssion . In the SAR studies of the
â-carboline series (1), we wanted to determine the effect
of alkylation of the indole nitrogen atom on PDE5
potency (Scheme 1). Although the N-alkylation reaction
was usually high-yielding, we occasionally observed a
very minor byproduct. Upon further examination, we
realized that the minor product (2) was the result of
Winterfeldt17 oxidation of compound 1, due to the
presence of a trace amount of oxygen in the reaction
mixture. Quinolones 2 are more acidic (pKa ∼ 9) than
â-carbolines such as 3 (pKa ∼ 14), which according to
the SAR in our previous report might provide more
potent compounds against PDE5 probably due to the
increased ability as a hydrogen-bonding donor (Chart
1).
SAR studies of the pyrroloquinolone series demon-
strated that substituents on the phenyl ring in the
pyrimidine portion of the molecule did not significantly
influence the inhibitory activity against PDE5 (Table
1). In particular, the monomethoxy and dimethoxy
analogues showed identical inhibition (2a vs 2b). Re-
placing methoxy with hydroxy decreased the Ki value
only 4-fold (2c vs 2d ). Introduction of large, basic
substituents (2e,f) or electron-withdrawing groups such
as 4-chloro and 4-nitro (not shown) also did not have
significant effects on PDE5 potency. In fact, eliminating
the entire phenyl ring resulted in only a moderate
decrease in potency (2g,h in Chart 2), thereby present-
ing an opportunity to modify the physical properties of
the series. Further studies revealed that the pyrimidine
moiety can be replaced with other heterocycles or acyl
groups, which will be reported in separate publications.
chemical properties.1-3 These families are PDE1, Ca2+
/
calmodulin-dependent; PDE2, cGMP-stimulated; PDE3,
cGMP-inhibited; PDE4, cAMP specific and rolipram
sensitive; PDE5, cGMP specific; PDE6, photoreceptor
cGMP specific; PDE7, cAMP specific and rolipram
insensitive; PDE8, cAMP specific and IBMX insensitive;
PDE9, cGMP specific; and PDE10 and PDE11, hydro-
lyzing both cAMP and cGMP. Among the 11 families,
PDE5 is the first recognized cGMP-binding PDE dis-
covered initially in lung tissues. Subsequent studies
revealed that it is the major isozyme of PDEs in corpus
cavernosum tissue of the penis, where it plays an
important role in penile erection.4,5 Many potent PDE5
inhibitors have been reported in the literature as
vasodilators.6 The introduction of sildenafil, a PDE5
inhibitor, as an agent treating male erectile dysfunction
(MED) in 1998 has raised the public awareness of MED
and generated significant interest in the discovery of
more selective PDE5 inhibitors.7-11 As part of our
ongoing effort to identify novel PDE5 inhibitors that
have improved selectivity vs PDE1 and PDE6,12-16 we
are interested in compounds that are structurally
distinct from sildenafil. During the structure-activity
relationship (SAR) studies of the pyrimidinyl-â-carbo-
lines,16 we discovered that the acidity of the indole
10.1021/jm025545d CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/09/2002

Letters
J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 19 4095
Ta ble 1. SAR of the Phenylpyrimidine Region
Ch a r t 4
compds
strain
R
Ki, nM (SD)a
2a
2b
2c
2d
2e
2f
A
A
B
B
B
B
3,4-OMe
4-OMe
4-OMe
4-OH
4-O(CH2)2morpholino
4-O(CH2)2N(C2H5)2
0.31 ((0.05)
0.31 ((0.05)
0.40 ((0.05)
1.75 ((0.05)
0.29 ((0.06)
0.53 ((0.14)
a
Means of three determinations.
Ch a r t 2. Ki Values of Pyrimidine Analogues Against
PDE5
substitution reaction of the chloropyrimidine to give the
R-enantiomer in >97% ee. No racemization was ob-
served at any step, as demonstrated by chiral high-
performance liquid chromatography.
The effect of the aromatic substituent attached to the
chiral center was also studied as shown in Chart 4.
Interestingly, replacing the methylenedioxyphenyl with
other bicyclic structures, such as dihydrobenzofuran (2j),
benzodioxane (2i), and indane (2l) did not significantly
decrease the potency, while replacing the bicyclic struc-
ture with dimethoxy (2k ) had a dramatic effect. This
indicates that a bicyclic structure is preferred in this
region for the binding to PDE5.
Ch a r t 3. Influence of the Chiral Center
One of the most important issues for the clinical
utility of PDE5 inhibitors is their selectivity vs other
PDE isozymes, especially PDE1, PDE3, and PDE6. All
compounds tested in this series have high selectivity vs
PDE1-PDE4. However, some of our initial analogues
such as 2i,l have a similar selectivity vs PDE6 as
sildenafil. To eliminate this potential liability to visual
function, we focused our efforts on improving the PDE6/
PDE5 ratio. To this end, we were able to identify
compounds with high selectivity vs PDE6 by modifying
the aromatic substituent attached to the chiral center.
Replacement of methylenedioxyphenyl (e.g., 2b,g) with
dihydrobenzofuran (e.g., 2c,h ) improved the PDE6
selectivity without compromising the potency and the
selectivity vs other isozymes. Table 2 summarizes the
selectivity of representative compounds.
Sch em e 2
To further evaluate the compounds, we have tested
them in RFL-6 cells for their ability to increase cGMP
levels. In comparison with sildenafil, these compounds
possess better efficacy in the cell-based assays. As
shown in Figure 1, both 2c and R-2a (Ki ) 0.21 nM)
induced a 3- and 5-fold increase in intracellular cGMP
concentration at lower compound concentrations than
sildenafil.
Select analogues from this series were studied in vivo
in the dog model of erectile dysfunction. In these studies,
drugs were dosed by the intravenous route using
sildenafil citrate as a positive control. As shown in
Figure 2, compound R-2a demonstrated good efficacy
in this model. Similarly, R-2c had an ED50 of 9.6 µg/kg.
The chiral center has a significant impact on PDE5
inhibition as shown in Chart 3. The R-enantiomer (R-
2c) is the predominant active form with Ki of 0.21 nM,
as compared with the racemate with Ki of 0.31 nM and
the S-enantiomer with Ki in the micromolar range. The
pure enantiomers could be obtained by chiral column
chromatography or chemical resolution. Alternatively,
they can be synthesized from the enantiomerically pure
â-carboline precursor prepared by literature proce-
dures.18 The synthesis of the R-enantiomers is exempli-
fied with R-2c (Scheme 2). Thus, the benzyl-protected
â-carboline 5 was subjected to Winterfeldt oxidation
with NaH as base in dimethylformamide to give 6,
followed by debenzylation and nucleophilic aromatic

4096 J ournal of Medicinal Chemistry, 2002, Vol. 45, No. 19
Letters
Ta ble 2. Selectivity of Selected Compounds Against PDE
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails of compounds reported, PDE assay protocol, and in vivo
efficacy protocol. This material is available free of charge via
the Internet at http://pubs.acs.org.
Isozymesa
PDE5,
Ki (nM)
PDE1/
PDE5
PDE2/
PDE5
PDE3/ PDE4/ PDE6/
compd
R-2a
2b
2c
R-2c
2g
2h
2i
PDE5
PDE5 PDE5
0.21
0.31
0.40
0.23
1.20
1.08
0.52
1.14
1.87
34 000
>50 000
>50 000
4320
9.690
790
0.59
18.5
55.9
27
Refer en ces
>50 000 >50 000 19 380
>50 000 >50 000 20 000
22 000
>50 000 >50 000 1070
>50 000 >50 000 6270
>50 000 >50 000 1050
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Preliminary pharmacokinetics studies have also been
conducted with select analogues. In particular, com-
pound 2c had an oral bioavailability of 20% in rats, as
compared with 11% for sildenafil in our system. This
result demonstrated the potential of these compounds
as oral agents for male erectile dysfunction and female
sexual disorders.
In summary, we have discovered a series of novel
pyrroloquinolone analogues as extremely potent and
selective PDE5 inhibitors. Representative compounds
demonstrated in vivo efficacy in a dog model of erectile
dysfunction. Representative compounds are also orally
bioavailable in preliminary pharmacokinetics studies.
Ack n ow led gm en t. We thank Drs. Do Won Hahn
and William V. Murray for helpful discussions and
support and Dr. William Hageman and Mary Evange-
listo for technical support.
J M025545D