Highly potent PDE4 inhibitors with therapeutic potential

Article abstract of DOI:10.1016/j.bmcl.2003.09.087

Based on the hypothesis that the dose-limiting side effects of PDE4 inhibitors could be mediated via the central nervous system (CNS), design and synthesis of a hydrophilic analogue is considered to be one approach to improving the side-effect profile of

Full text of DOI:10.1016/j.bmcl.2003.09.087

Bioorganic & Medicinal Chemistry Letters 14 (2004) 207–210
Highly potent PDE4 inhibitors with therapeutic potential
Hiroshi Ochiai,a Tazumi Ohtani,a Akiharu Ishida,a Kensuke Kusumi,a Masashi Kato,a
Hiroshi Kohno,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 28August 2003; revised 11 September 2003; accepted 17 September 2003
Abstract—Based on the hypothesis that the dose-limiting side effects of PDE4 inhibitors could be mediated via the central nervous
system (CNS), design and synthesis of a hydrophilic analogue is considered to be one approach to improving the side-effect profile
of ArifloTM 1. Water-soluble piperidine derivatives were found to possess therapeutic potential.
# 2003 Elsevier Ltd. All rights reserved.
Phosphodiesterase 4 (PDE4)1À3 is a cyclic adenosine
monophosphate (cAMP)-specific PDE that is expressed
in inflammatory cells such as eosinophils. Inhibition of
PDE4 results in elevation of the cAMP level in these
cells, which in turn downregulates the inflammatory
response.4 The potential of PDE4 inhibitors as anti-
inflammatory agents for the treatment of asthma and
other inflammatory disorders has therefore received
considerable attention.5 However, no selective PDE4
inhibitor has reached the market yet because the early
compounds (such as rolipram) caused dose-limiting side
effects like nausea and emesis, which severely restricted
their therapeutic utility.
possible approach because compounds that show good
penetration into the CNS have a relatively high lipo-
philicity.16 As shown in Figure 1, focus was placed on
the design and synthesis of a less lipophilic and/or
zwitter-ionic piperidine derivative, which is structurally
related to Ariflo 1 and expected to show the more lim-
ited penetration into the CNS.17 A structure–activity
relationship (SAR) study was started with the expecta-
tion that this approach would provide compounds with
an improved therapeutic index.
As outlined in Scheme 1, test compounds were prepared
from the benzyl cyanide 166 by the following two syn-
thetic pathways. Dialkylation of 16 with cis-2-butenyl-
1,4-dichloride 17 afforded the cyclopentene derivative
18 in 73% yield. Oxidative cleavage of the cyclopentene
moiety of 18 gave a dialdehyde 19, which was used for
the next reaction without isolation. Reductive amina-
tion of the dialdehyde 19 with an appropriate amino
ester in the presence of sodium triacetoxyborohydride
afforded 20a and 20b at moderate yields.18 Deprotection
of the benzylester 20a–b was accomplished by catalytic
The improved therapeutic index of ArifloTM 16 (Fig. 1)
has been attributed to increased selectivity for the abil-
ity to inhibit PDE4 catalytic activity (LPDE4) versus
the ability to compete for high-affinity[3H]rolipram-
binding site (HPDE4).7,8 A second strategy is related to
the design of PDE4 subtype-selective inhibitors.9À13
The dose-limiting side effects of PDE4 inhibitors are
thought to be mediated via the central nervous system
(CNS).14,15 Accordingly, the negatively charged physical
property of Ariflo 1 at a physiological pH might
decrease the occurrence of side effects because of limited
penetration into the CNS. Molecular design of a highly
hydrophilic PDE4 inhibitor was thought to be another
* Corresponding author. Tel.: +81-75-961-1151; fax: +81-75-962-
9314; e-mail: hi.nakai@ono.co.jp
Figure 1. Molecular design of hydrophilic molecules.
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.09.087

208
H. Ochiai et al. / Bioorg. Med. Chem. Lett. 14 (2004) 207–210
Table 1. Activity profile of piperidine derivatives
Compd
X
LPDE4a
IC50 (nM)
Inhibition of TNF-ab
ID50 (mg/kg, po)
2a
2b
66Æ39
(74%)c
0.04
0.080Æ0.050
3a (Y=OH)
3b (Y=NHOH)
>300
0.91Æ1.9
NTe
(26%)d
4a (Y=OH)
4b (Y=NHOH)
>300
5.9Æ7.9
NTe
(49%)d
5a (Y=OH)
5b (Y=NHOH)
>1000
34Æ11
NTe
NTe
6a (Y=OH)
6b (Y=NHOH)
>1000
4.6Æ1.4
NTe
2.0
Scheme 1. (a) 17, LiHMDS, THF, À78 ꢀC; (b) O3, CH2Cl2, then PPh3,
À78 ꢀC; (c) NaBH(OAc)3, AcOH, DCE, NH2CR3R4COR5; (d) H2,
10% Pd/C, MeOH; (e) EDC, HOBt, DMF, NH2OC(CH3)2(OCH3),
then 2 N HClaq, MeOH; (f) 21, LiHMDS, THF, À78 ꢀC; (g) TFA,
a Inhibition of PDE4 prepared from U937 cells (a cell line derived
from human monocytes). IC50 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.
C6H5SCH3, CH2Cl2, then
4
N
HCl/EtOAc; (h) K2CO3,
BrCH2COOEt, DMF; (i) 2 N NaOHaq, EtOH; (j) methyl-S-(À)-lac-
d Inhibition % at 1 mg/kg, po.
tate or methyl-R-(+)-lactate, Tf2O, CH2Cl2, 2,6-lutidine, Et3N.
e Not tested.
hydrogenation to give 5a and 6a, which were converted
to the corresponding hydroxamic acid analogues 5b and
6b, respectively, by condensation with (1-methoxy-1-
methyethyl)oxyamine followed by acidic deprotection.
Compounds 2a, 7a–11a, and 13a–15a were also pre-
pared from 16 as follows. Dialkylation of 16 with
dimesylate 21 gave the piperidine derivative 22, acidic
deprotection of which produced an amine derivative 23.
N-Alkylation of 23 with ethyl 2-bromoacetate afforded
24, alkaline hydrolysis of which gave the carboxylic acid
analogues 2a, 7a–11a, and 13a–15a. Condensation of
these carboxylic acid analogues with (1-methoxy-1-
methyethyl)oxyamine, followed by acidic deprotection,
produced the hydroxamic acid analogues 2b, 7b–11b,
and 13b–15b, respectively.
causing 74% inhibition of LPS-induced TNF-a pro-
duction in rats. Based on reported information,11 the
corresponding hydroxamic acid analogue 2b was extre-
mely potent in the in vitro LPDE4 assay and showed an
ID50 value of 0.04 mg/kg, orally in the in vivo TNF-a
production assay. To improve the predicted rapid
metabolic hydrolysis of 2b, the design and synthesis of
a-substituted and a,a-disubstituted analogues 3b–6b
was carried out as illustrated in Table 1. Compounds
Table 2. Activity profile of piperidine derivatives
The series of piperidine derivatives listed in the Tables
1–3 were synthesized and biologically evaluated for the
ability to inhibit PDE4 prepared from U937 cells19
(derived from human monocytes). The results of the
assays were expressed as IC50 values, that is the test
compound concentration that caused 50% inhibition.
Test compounds were also evaluated for the ability to
inhibit lipopolysaccharide (LPS)-induced production of
tumor necrosis factor-a (TNF-a) in rats.20 The results
were expressed as ID50 values, that is the dose that resul-
ted in 50% inhibition relative to the effect of the vehicle.
Compd
R
LPDE4a
IC50 (nM)
Inhibition of TNF-aa
ID50 (mg/kg, po)
7a (X=OH)
7b (X=NHOH)
8a (X=OH)
8b (X=NHOH)
9a (X=OH)
9b (X=NHOH)
10a (X=OH)
10b (X=NHOH)
11a (X=OH)
11b (X=NHOH)
12 (X=OH)
Me-
Et-
>100
4.7Æ1.1
>100
NTd
0.23
(0%)b
0.04
0.52Æ0.12
>100
0.29Æ0.03
>100
(12%)b
0.03
i-Propyl—
c-Butyl
NTd
1.0Æ0.80.09
>100
0.18Æ0.02
67Æ19
NTd
0.03
(29%)c
In Table 1, the biological activities of these piperidine
derivatives are summarized. Compound 2a was
designed to reduce the hydrophobicity of Ariflo 1 and to
remove the configurational isomerism; it exhibited
potent LPDE4 activity, with an oral dose of 3 mg/kg
a See corresponding footnotes from Table 1.
b Inhibition % at 1 mg/kg, po.
c Inhibition % at 3 mg/kg, po.
d Not tested.

H. Ochiai et al. / Bioorg. Med. Chem. Lett. 14 (2004) 207–210
Table 3. Activity profile of piperidine derivatives
209
which had slightly less in vitro activity and retained its
in vivo potency. Interestingly, indan-2-yloxy analogue
1221 demonstrated equipotent LPDE4 activity com-
pared with 2a, while it exhibited less in vivo potency
than 2a. The corresponding carboxylic acids 7a–11a
demonstrated markedly reduced LPDE4 inhibitory
activity compared with 2a.
Compd
R
LPDE4a
IC50 (nM)
Inhibition of TNF-aa
ID50 (mg/kg, po)
As shown in Table 3, further optimization of the meth-
oxy moiety of 2a–b was carried out. Replacement of the
methoxy moiety of 2b with an ethyloxy moiety gave 13b,
which had nearly 100-fold less LPDE4 inhibitory activ-
ity and nearly 10-fold less in vivo activity. Replacement
of the methoxy moiety of 2b with an isopropyloxy moi-
ety resulted in 15b, also with markedly reduced LPDE4
inhibitory activity. Interestingly, replacement of the
methoxy moiety of 2b with a difluoromethoxy22 moiety
produced 14b, which had slightly increased LPDE4
activity and demonstrated 2-fold greater in vivo
potency. The corresponding carboxylic acid derivatives
13a and 15a showed much lower potency both in vitro
and in vivo, although 14a had nearly the same potency
as 2a in both the LPDE4 assay and the LPS-induced
TNF-a production assay. As described above, chemical
modification of the methoxy moiety of 2a–b was found
to be much more limited than that of the cyclopentyl
moiety, while the difluoromethoxy moiety was identified
as another optimized moiety that could be replaced by
methoxy. Among the several possible changes in the
physical properties of a molecule that occur when a
hydrogen atom is replaced by a fluorine atom, an
increase of at least three factors can be expected, which
are steric bulkiness, lipophilicity, and metabolic stabi-
lity. More details will be discussed using the Log D
values and the molal volume calculated by
13a (X=OH)
13b (X=NHOH)
14a (X=OH)
14b (X=NHOH)
15a (X=OH)
15b (X=NHOH)
Et-
2500Æ980
6.2Æ1.1
65Æ11
NTb
0.7
CHF2-
1.1
0.051Æ0.007
0.02
NTb
NTb
i-Propyl—
>1000
>30
a See corresponding footnotes from Table 1.
b Not tested.
3a–6a were also synthesized and evaluated for compar-
ison with the corresponding hydroxamic acid analogues.
All of the corresponding carboxylic acid analogues 3a–
6a exhibited much less potency in the LPDE4 assay
than their corresponding hydroxamic acid analogues
3b–6b, respectively. Compounds 3a–4a and 5a–6a
exhibited less than 50% inhibition at 0.3 and 1 mM,
respectively. The above chemical modification was
found to cause marked reduction of the activity of the
new chemical lead 2b. More details will be discussed in
the subsequent full report.
As shown in Table 2, further attempts at optimization
of the cyclopentyl moiety of 2a–b were made. Replace-
ment of the cyclopentyl moiety of 2b with n-alkyl chains
such as methyl and ethyl moieties resulted in 7b and 8b,
respectively. Compound 7b showed reduced potency in
both in vitro and in vivo tests, while 8b showed slightly
reduced in vitro potency and retained its in vivo
potency. Replacement of the cyclopentyl moiety of 2b
with a branched alkyl moiety, such as an isopropyl
moiety, gave 10b, which showed nearly 10-fold reduc-
tion of in vitro potency and 2-fold reduction of in vivo
potency. Replacement of the cyclopentyl moiety of 2b
with a cyclopropylmethyl moiety afforded 9b, which
also had slightly reduced in vitro potency and retained
its in vivo potency. Ring contraction of the cyclopentyl
moiety of 2b produced a cyclobutyloxy analogue 11b,
QMPRPLUSTM 23
in the subsequent full report.
,
Further biological evaluation of compounds 2b, 8b, 9b
and 14a, which were selected based on the in vitro
LPDE4 assay and in vivo TNF-a production, was car-
ried out as shown in Table 4. These compounds were
evaluated for the inhibition of slow reacting substance
of anaphylaxis (SRS-A)-mediated bronchoconstric-
tion24,25 for their beneficial effect and gastric acid emp-
tying in rats26 for their side effect. These compounds
were also evaluated for inhibition of LPS-induced TNF-
Table 4. Biological profile of 1, 2b, 8b, 9b and 14a
Compd
SRS-A-mediated
bronchoconstrictiona
ID50 (mg/kg, po)
Inhibition of TNF-a
productionb
ID50 (mg/kg, po)
Inhibition of gastric
emptyingc
ID50 (mg/kg, po)
Inhibition of TNF-a
productiond
IC50 (mM) in HWB
1 (Ariflo)
2b
8b
9b
4.5
0.3
0.04
0.3
4.2e
1.7
5.7
0.3
0.7
0.3
4.7
18
0.04
0.04
0.03
1.1
0.0089
0.0050
0.0021
0.84
14a
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 See corresponding footnotes from Table 1.
c Inhibition of gastric emptying in rats (n=5).
d Inhibition of LPS-induced TNF-a production in human whole blood. IC50 represent a mean of n=3.
e Inhibition of SRS-A-mediated bronchoconstriction and airway microvascular leakage in actively sensitized guinea pigs (n=3–6); OVA challenge
0.5 mg/kg 1 h after oral dosing of a test compound.

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H. Ochiai et al. / Bioorg. Med. Chem. Lett. 14 (2004) 207–210
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and 9b were much less potent in guinea pigs than
expected from their effects on TNF-a production, while
8b was equipotent in each assay. To assess one of the
safety problems, inhibition of gastric emptying by 2b,
8b, 9b and 14a was evaluated in rat. The ID50 values at
which these compounds affected gastric emptying were
found to be higher than the values for TNF-a produc-
tion, which is one of their beneficial effects.
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