Orally active PDE4 inhibitors with therapeutic potential

Article abstract of DOI:10.1016/j.bmcl.2003.12.018

Based on the successful results in the clinical trial of Ariflo, further optimization of the spatial arrangement of the three pharmacophores (carboxylic acid moiety, nitrile moiety and 3-cyclopentyl-4-methoxyphenyl moiety) in the structure of Ariflo 1 was

Full text of DOI:10.1016/j.bmcl.2003.12.018

Bioorganic & Medicinal Chemistry Letters 14(2004) 1323–1327
Orally active PDE4 inhibitors with therapeutic potential§
Hiroshi Ochiai,a Tazumi Ohtani,a Akiharu Ishida,a Katuya Kishikawa,b
Takaaki Obata,a Hisao Nakaia,* and Masaaki Todaa
aMinase Research Institute, Ono Pharmaceutical Co., Ltd., 3-1-1 Sakurai, Shimamoto, Mishima, Osaka 618-8585, Japan
bDevelopment Planning, Ono Pharmaceutical Co., Ltd., 2-1-5 Doshomachi, Chuo-ku, Osaka 541-8526, Japan
Received 14November 2003; revised 1 December 2003; accepted 3 December 2003
Abstract—Based on the successful results in the clinical trial of ArifloTM, further optimization of the spatial arrangement of the
three pharmacophores (carboxylic acid moiety, nitrile moiety and 3-cyclopentyl-4-methoxyphenyl moiety) in the structure of Ariflo
. .
1 was attempted using a bicyclo[3 3 0]octane template instead of a cyclohexane template. As a result, 2a, 7a and 7b were found to be
orally active and were predicted to have an improved therapeutic potential based on evaluation by cross-species and same-species
comparisons. Structure–activity relationships (SARs) of these compounds are also discussed.
# 2003 Elsevier Ltd. All rights reserved.
The phosphodiesterases (PDEs)1 are involved in intra-
cellular degradation of cyclic adenosine monophosphate
(cAMP) and cyclic guanosine monophosphate (cGMP)
to form their corresponding 50-monophosphates, and
2
phosphodiesterase 4(PDE4) has received considerable
attention as a molecular target for the treatment of
asthma and other inflammatory diseases.3,4 Previous
efforts in this field have mainly been focused on asthma
and chronic obstructive pulmonary disease (COPD).
Many inhibitors have been reported that are under
clinical.5,6 However, the clinical utility of the pioneer
compounds has been limited by side effects such as
nausea, emesis and increased gastric acid secretion. To
obtain efficient PDE4inhibitors with reduced side effect,
two strategies have been tried. The first approach is
based on designing compounds with a reduced affinity
for the so-called high affinity rolipram-binding site
(HPDE4) and high affinity for the catalytic domain
(LPDE4).7,8 Ariflo 1 (Fig. 1) has been reported to be 75-
fold more selective than (R)-rolipram with respect to
LPDE4activity, 9 and has shown efficacy in human with
an improved safety margin. The second strategy is
related to the design of PDE4subtype-selective
inhibitors.10ꢀ12 Ariflo 1 is an orally active second-gen-
. .
Figure 1. Molecular design of bicyclo[3 3 0]octanes.
eration PDE4inhibitor that was also reported to be
PDE4D-selective. Recently, the crystal structures of
PDE4B13 and PDE4D14,15 were determined. These studies
would give us structural information about the rolipram
binding site and the active site of PDE4subtypes.
The importance of the proper spatial arrangement of
the carboxyl moiety relative to the 3-cyclopentyloxy-4-
methoxyphenyl and nitrile moieties on the cyclohexane
ring is illustrated by comparison of Ariflo 1 (cis-isomer)
with its trans-isomer.9 More rigid spatial arrangement
of these three functional groups within their optimized
stereochemistry using another template instead of the
cyclohexane ring was predicted to lead to the discovery
. .
Keywords: PDE4inhibitor; Bicyclo[3 3 0]octane; Three pharmaco-
phores; Orally active.
§Supplementary data associated with this article can be found at doi:
10.1016/j.bmcl.2003.12.018.
* Corresponding author. Tel.: +81-75-961-1151; fax: +81-75-962-
9314; e-mail: hi.nakai@ono.co.jp
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.12.018

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H. Ochiai et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1323–1327
Scheme 2. Synthesis of compounds 6a–b–10a–b: (a) LiHMDS,
PhNTf2, THF, ꢀ78 ꢁC rt; (b) Pd(OAc)2, Et3N, PPh3, CO, MeOH,
DMF; (c) H2, Pd/C, MeOH; (d) RX, K2CO3, DMF or ROH, ADDP,
PPh3, CH2Cl2; (e) 1N NaOHaq, MeOH, THF; (f) EDC, HOBt, Et3N,
NH2OC(CH3)2OCH3, DMF then HClaq, MeOH.
Scheme 1. Synthesis of compounds 2a, 2b, 3, 4 and 5: (a) NaHMDS,
THF, ꢀ78ꢁC; (b) O3, CH2Cl2 then PPh3, ꢀ78 ꢁC; (c) NaClO2,
NaH2PO4, t-BuOH, H2O, 2-methyl-2-butene; (d) CH2N2, Et2O, 0ꢁC;
(e) NaH, DME, reflux; (f) NaCl, DMSO, H2O, 165ꢁC; (g) 2-tri-
methylsilyl-1, 3-dithiane, n-BuLi, THF, ꢀ78 ꢁC; (h) TFA, H2O2aq,
CH3CN, H2O, then 2N NaOHaq; (i) CH2N2, Et2O; (j) 2N KOHaq,
THF, MeOH; (k) NH2OC(CH3)2OCH3, EDC, HOBt, DMF then
MeOH, 1N HCl aq.
isomer 20. O-Alkylation of 20 by the conventional
method gave 21a–e, alkaline hydrolysis of which pro-
duced 6a–10a, respectively. Condensation of 6a–10a
with a protected hydroxylamine, followed by acidic
deprotection, led to the corresponding hydroxamic
acids 6b–10b, respectively.
of a new inhibitor with improved therapeutic potential.
. .
Design and synthesis of a series of bicyclo[3 3 0]octane
derivatives was carried out with the expectation of
obtaining an improved therapeutic potential because the
. .
A series of bicyclo[3 3 0]octane derivatives were synthe-
. .
bicyclo[3 3 0]octane template provides more stereo-
chemical diversity (four stereoisomers) and it was
sized and evaluated for their ability to inhibit PDE4
enzyme prepared from U937 cells16 (a cell line derived
from human monocytes). Results are expressed as IC50
values, that is, the test compound concentration that
gave 50% inhibition relative to the effect of the vehicle.
These compounds were also evaluated for their ability
to inhibit lipopolysaccharide (LPS)-induced tumor
necrosis factor (TNF)-a production in rats.17 Results are
expressed as ID50 values, that is, the dose that caused
50% inhibition relative to the effect of the vehicle.
. .
expected that derivatives based on the bicyclo[3 3 0]-
octane template might show greater potential with
respect to PDE4inhibition, LPDE4selectivity, and
subtype selectivity compared with the corresponding
cyclohexane derivatives (two stereoisomers).
As shown in Figure 1, this paper describes the config-
urational requirements of the above-mentioned three
. .
pharmacophores on a new template, the bicyclo[3 3 0]-
octane ring, which result in increased beneficial activity
and an improved side effect profile.
All the four possible isomers 2–5 were synthesized and
evaluated as demonstrated in Table 1. Compounds 2a–b
and 3, in which the aromatic moiety was located outside
. .
As outlined in Scheme 1, analogues were prepared from
a benzylcyanide 119 by the following synthetic pathway.
Dialkylation of 11 with cis-4,5-bis(bromomethy)-
cyclohex-1-ene provided 12 at a good yield. Ozonolysis
of 12, followed by oxidation and then esterification,
gave the diester 13. Dieckmann condensation of 13,
followed by demethoxycarbonylation, resulted in two
separable diastereoisomers 14a and 14b, which were con-
verted to ketenedithioacetals 15a and 15b, respectively.
Deprotection of 15a gave two separable isomers 16a
and 16b, which were converted to the carboxylic acid
analogues 2a and 3, respectively. Condensation of 2a
with O-protected hydroxyamine followed by acidic
deprotection affored 2b. According to the procedure
described above, 15b was converted to 16c and 16d,
which were transformed to 4 and 5, respectively.
the concave bicyclo[3 3 0]octane framework, showed
potent inhibitory activity against PDE4. The LPDE4
inhibitory activity of 2a–b was equipotent to that of
Ariflo 1, while that of 3 was 15-fold weaker than that
of 1. Compounds 4 and 5, with the aromatic moiety
located inside the concave framework, exhibited
no LPDE4inhibitory activity at a concentration of
300 nM.
The carboxylic acid group was preferably oriented the
syn-direction of the nitrile function and located inside
the concave molecule, as illustrated by the greater
potency of 2a than 3. Thus, these results showed that
the PDE4enzyme might recognize the more accesssible
aromatic moiety first, while favoring more hindered
carboxylic acid and nitrile groups.
The synthesis of 6–10 is outlined in Scheme 2. The enol
triflate 18 was prepared from 17, which was derived
from 3-benzyloxy-4-methoxybenzyl cyanide according
to the procedure described for the preparation of 14a
from 11. Palladium-catalyzed insertion of carbon mon-
oxide into 18 in the presence of methanol resulted in 19,
after which catalytic hydrogenation afforded a single
Inhibition of TNF-a production in rats was also eval-
uated using 2a–b and 3. The in vivo activity of 2a was 4-
fold more potent than that of 1, while 2b and 3 were less
potent than 1. Compound 2a was nearly 10-fold more
potent than 3 (based on ID50 values) for inhibition of
LPS-induced TNF-a production in rats, as predicted
from their in vitro SAR.

H. Ochiai et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1323–1327
1325
. .
Table 1. Activity profile of bicyclo[3 3 0]octane derivatives
Compd
R1
R2
R3
R4
Inhibition of LPDE4a
IC50 (nM)
Inhibition of TNF-ab
ID50 (mg/kg, po)
1 (Ariflo)
10
1.7
2a
2b
CN
CN
H
H
COOH
CONHOH
9.0
9.7
0.4
(52%)c
3
4
5
CN
COOH
COOH
H
H
H
150
>300
>300
3.8
CN
CN
NTd
COOH
NTd
a Inhibition of PDE4prepared from U937 cells (a cell line derived from human monocytes). IC 50 represent a mean of n=2.
b ID50 for inhibition of LPS-induced TNF-a production in rats (n=7) 0.5 h after oral dosing of a test compound.
c Inhibition% at 3 mg/kg, po.
d Not tested.
. .
Table 2. Activity profile of bicyclo[3 3 0]octane derivatives
Compd
R
Inhibition of LPDE4a
IC50 (nM)
Inhibition of TNF-aa
ID50 (mg/kg, po)
2a (X=COOH)
2b (X=CONHOH)
9.0
9.7
0.4
(52%)c
6a (X=COOH)
6b (X=CONHOH)
Me—
Et—
160
62
NTd
(55%)c
7a (X=COOH)
7b (X=CONHOH)
26
(56%)b
2.9
1.8
8a (X=COOH)
8b (X=CONHOH)
i-Pr—
62
12
(57%)c
(47%)c
9a (X=COOH)
9b (X=CONHOH)
1.3
0.85
1.7
(17%)c
10a (X=COOH)
10b (X=CONHOH)
100
29
NTd
3.0
a See corresponding footnotes from Table 1.
b Inhibition% at 10 nM.
c Inhibition% at 3 mg/kg, po.
d Not tested.
PDE4is reported to be inhibited by various hydroxamic
acid derivatives.18,19 This information led us to synthesize
a series of hydroxamic acid analogues which were expected
to possess strong metal-mediated interactions. Conversion
of the carboxylic acid function of 2a to the hydroxamic
acid function of 2b maintained PDE4inhibitory activ-
ity, but the inhibition of TNF-a production in rats was
markedly decreased with 2b relative to 2a, probably
because of problems such as instability of the hydro-
xamic acid function and/or poor oral availability.
As illustrated in Table 2, further structural optimization
of the cyclopentyl moiety of 2a, which exhibited the
most potent activity among the compounds listed in

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H. Ochiai et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1323–1327
Table 3. Biological profile of 1, 2a, 7a and 7b
Compd
Inhibition of
bronchoconstricitona
ID50 (mg/kg, po)
Inhibition of
TNF-a productionc
ID50 (mg/kg, po)
Inhibition of
gastric emptyingd
ID50 (mg/kg, po)
Inhibition of
TNF-a production of HWBf
ID50 (mM)
1 (Ariflo)
4.5
9.6
NTb
5.0
1.7
5.7
e
18
21
5.8
0.80
2a
7a
7b
0.4(73%)
2.9
1.8
17
12
a Inhibition of SRS-A-mediated bronchoconstriction and airway microvascular leakage in actively sensitized guinea pigs. (n=3–6); OVA challenge
0.15 mg/kg 1 h after oral dosing of a test compound.
b Not tested.
c See corresponding footnotes from Table 1.
d Inhibition of gastric emptying in rats (n=5).
e Inhibition% at 10 mg/kg, po.
f Inhibition of LPS induced TNF-a production in human whole blood. IC50 represent a mean of n=3.
Table 1, was carried out. Replacement of the cyclo-
pentyl moiety of 2a with a methyl, ethyl and isopropyl
groups led to 6a, 7a, and 8a, respectively, which showed
reduced LPDE4inhibitory activity. Replacement of the
cyclopentyl moiety of 2a with an isoindanyl group
afforded 9a, which had 7-fold more potent LPDE4
inhibitory activity, but was nearly 4-fold less potent in
the inhibition of LPS-induced TNF-a production
presumably because of pharmacodynamic problems.
Replacement of the cyclopentyl moiety of 2a with a
cyclopropylmethyl group produced 10a, which showed
11-fold less potent LPDE4inhibitory activity. When the
corresponding hydroxamic acid analogues 6b–10b were
synthesized and evaluated, increased LPDE4inhibitory
activity was shown by all of these compounds.
However, the in vivo potency of 9b, which exhibited the
strongest LPDE4inhibitory activity, was not increased
as expected from its in vitro potency and this was also
true for the in vivo potency of 8b.
To assess safety within a single species, inhibition of
gastric emptying by 2a, 7a and 7b was evaluated. Com-
pound 2a caused more than 50% inhibition of gastric
emptying at 10 mg/kg, po, while 7a and 7b had ID50
values of 17 and 12 mg/kg, po, respectively. The dose of
7b that inhibited gastric emptying was found to be
higher than that causing a beneficial effect. Based on its
greater potency than that of Ariflo 1 for blocking the
LPS-induced TNF-a production in HWB, 7b was esti-
mated to have an improved therapeutic potential as well
as an improved side effect profile.
Based on the hypothesis that more rigid spatial fixing of
the three pharmacophores (the carboxylic acid, nitrile,
and aromatic moieties) of Ariflo within their optimized
stereochemistry using another template could lead to
the discovery of a new inhibitor, the design, synthesis,
. .
and evaluation of bicyclo[3 3 0]octane derivatives that
showed more stereochemical diversity than the cyclo-
hexane derivatives was carried out. Among the com-
pounds tested, 2a, 7a, and 7b potentially had improved
therapeutic potential with less side effects based on bio-
logical data obtained by both cross-species comparison
and same-species comparison. Full details will be
reported in due course.
Further evaluation of compounds 2a, 7a and 7b, which
were selected based on their potency in the in vivo TNF-
a production assay, was carried out as shown in Table
3. These compounds were evaluated for their abilities to
inhibit slow reacting substance of anaphylaxis (SRS-A)-
mediated bronchoconstriction20,21 (a beneficial effect)
and the inhibition of gastric emptying11 (a side effect) in
rats. The results were expressed as ID50 values, that is,
the dose that caused 50% inhibition relative to the
vehicle. These compounds were also evaluated for inhi-
bition of LPS-induced TNF-a production in human
whole blood (HWB)22 to predict their clinical potential.
Results were expressed as IC50 values, that is, the test
compound concentration that caused 50% inhibition
relative to the vehicle.
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