Discovery of oxazole-based PDE4 inhibitors with picomolar potency

Article abstract of DOI:10.1016/j.bmcl.2012.01.115

Optimization of oxazole-based PDE4 inhibitors has led to the discovery of a series of quinolyl oxazoles, with 4-benzylcarboxamide and 5-α-aminoethyl groups which exhibit picomolar potency against PDE4. Selectivity profiles and in vivo biological activity are also reported.

Full text of DOI:10.1016/j.bmcl.2012.01.115

Bioorganic & Medicinal Chemistry Letters 22 (2012) 2594–2597
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Discovery of oxazole-based PDE4 inhibitors with picomolar potency
Rongze Kuang a, , Ho-Jane Shue a, Li Xiao b, David J. Blythin a, Neng-Yang Shih a, Xiao Chen a, Danlin Gu a,
⇑
John Schwerdt a, Ling Lin a, Pauline C. Ting a, Jianhua Cao a, Robert Aslanian a, John J. Piwinski a,
Daniel Prelusky c, Ping Wu d, Ji Zhang d, Xiang Zhang d, Chander S. Celly d, Motasim Billah d, Peng Wang d
a Department of Medicinal Chemistry, Merck Research Laboratories, 126 E. Lincoln Avenue, Rahway, NJ 07065, USA
b Department of Computer Modeling and Informatics, Merck Research Laboratories, 126 E. Lincoln Avenue, Rahway, NJ 07065, USA
c Department of DMPK, Merck Research Laboratories, 126 E. Lincoln Avenue, Rahway, NJ 07065, USA
d Department of Pharmacology, Merck Research Laboratories, 2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Optimization of oxazole-based PDE4 inhibitors has led to the discovery of a series of quinolyl oxazoles,
Received 4 December 2011
Revised 26 January 2012
Accepted 30 January 2012
Available online 14 February 2012
with 4-benzylcarboxamide and 5-
Selectivity profiles and in vivo biological activity are also reported.
Ó 2012 Elsevier Ltd. All rights reserved.
a-aminoethyl groups which exhibit picomolar potency against PDE4.
Keywords:
PDE4 inhibitor
Phosphodiesterase 4
Oxazole
Picomolar
Inflammation
Phosphodiesterase 4 (PDE4) is expressed predominantly in
inflammatory and immune cells and is one of the phosphodiester-
ase (PDEs) isozymes that are responsible for the breakdown of the
signaling molecule cAMP.1 Inhibition of PDE4 effectively increases
the intracellular cAMP level, which in turn provides critical nega-
tive regulation of various cellular functions in these cells. The
anti-inflammatory effects of PDE4 inhibitors have been demon-
strated in various animal models of airway diseases as well as in
other biological disorders. Roflumilast, a selective PDE4 inhibitor,
has recently been approved by the FDA for severe COPD,2 while
several PDE4 inhibitors have advanced to various stages of clinical
or preclinical development for COPD, asthma, allergic rhinitis,
Crohn’s disease, psoriasis, atopic dermatitis, and depression.3–10
Despite significant progress in this area, PDE4 inhibitors are often
associated with dose-limiting side-effects such as nausea, emesis,
and vasculopathy, which limit their therapeutic potential.11–13 This
further highlights the need to discover novel pharmacophores for
designing PDE4 inhibitors with an improved therapeutic index.
We have previously reported the discovery of a potent class of
quinolyl oxazole-based PDE4 inhibitors by re-designing the core
structure of SCH 351591.14,15 A novel PDE4 inhibitory pharmaco-
phore revealed in the new core structure consists of an oxazole
motif bearing the 4-carboxamide and 5-aminomethyl groups
(Scheme 1). Weak inhibition of PDE10 and PDE11 has also been
OMe
OMe
OMe
O
N
CF3
N
CF3
N
CF3
convert amide
to oxazole
O
Cl
NH
O
N
O
N
search for potent
Cl
end groups
NH2
H2N
N
O
IC50 = 80 nM
IC50 = 89 nM
SCH 351591
synergistic effects
of polar end groups
OMe
OMe
N
CF3
N
CF3
initial amide
SAR
O
O
N
O
N
H2N
NH
H2N
NH2
O
1b
IC50 = 3.7 nM (PDE4B)
1a
IC50 = 24 nM
⇑
Corresponding author.
E-mail address: rongze.kuang@merck.com (R. Kuang).
Scheme 1. Design and discovery of oxazole-based PDE4 inhibitors.
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2012.01.115

R. Kuang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2594–2597
2595
Table 1
SAR of 4-benzylcarboxamide groupa
OMe
N
CF3
O
N
O
R1
H2N
N
R2
Compd
–N R1R2
IC50 (nM)
PDE4B
3.7
PDE10
PDE11
4800
NH
NH
NH
NH
NH
1b
1c
1d
1e
1f
1200
4.1
3.4
26
1300
2100
ND
2700
2700
ND
F
Cl
Figure 1. Modeling of 2n binding to PDE4 with (S)-methyl group being highlighted.
CN
OMe
N
OMe
N
O
CF3
OMe
N
CF3
37
ND
ND
SO2Me
N
CF3
a
F
b, c
NH
NH
1g
1h
1i
1.8
1.5
0.9
1.2
0.5
810
770
500
610
320
3900
2100
3600
2300
870
MeS
O
NH
OMe
O
OH
OMe
Cl
O
A
C
D
F
NH
NH
OMe
OMe
N
CF3
N
CF3
Cl
f, g
1j
d, e
O
N
O
N
O
MeO
NH
R3
N
BocHN
OH
1k
H2N
R4
O
F
2
E
NH
NH
1l
0.46
0.1
240
410
2500
1300
Scheme 2. Reagents and conditions: (a) SOCl2, toluene, reflux, then glycine-
OEtÁHCl, TEA, 100%; (b) Lawesson’s reagent, THF, 70 °C, 1 h, 87%; (c) Me3OÁBF4,
F
F
CH2Cl2, 0 °C, 3 h, 95%; (d) Boc- -Ala-F, KHMDS, THF, À78 to À50 °C, 1 h, 50–70%; (e)
L
LiOH, H2O/THF, 100%; (f) HNR3R4, HATU, DMF/CH2Cl2, rt; (g) 4 N HCl, dioxane, rt
100%.
1m
Cl
F
zation of this series of compounds which led to the discovery of
highly selective PDE4 inhibitors with picomolar potency.
NH
NH
NH
NH
1n
1o
1p
1q
0.6
2.7
4.3
2.6
1000
2200
1300
1300
2200
2700
1100
2200
F
A series of oxazoles with substituted N-benzylcarboxamides as
the 4-carboxamide moiety was first prepared according to the pre-
viously reported route.15 These compounds were screened in vitro
against PDE4B, PDE10 and PDE11.16 Subtype B of PDE4 was chosen
because it is expressed predominantly in inflammatory cells, and
our goal is to develop PDE4 inhibitors as anti-inflammatory agents.
The IC50 data is summarized in Table 1.
We first examined the effects of substitution on the phenyl ring
of the N-benzyl group. Para-substitution with small halogens (F
and Cl in compounds 1c and 1d, respectively) shows little effect
on potency, while larger groups such as –CN, and –SO2Me (com-
pounds 1e–1f) result in 5- to 10-fold decrease in PDE4 potency.
In contrast, meta- and especially ortho-substitution with –F, –Cl,
and –OMe (compounds 1g–1k) improve the IC50 for PDE4. The
importance of the ortho-F substitution was further confirmed with
the disubstituted benzylamide analogs, in which the most potent
compounds (compounds 1l–1n) are those with two ortho-substitu-
tions (–F or –Cl), and a small para-substitution (–F or –H). This
F
Cl
Cl
F
Cl
F
ND = not determined.
a
Values of IC50 are the means of at least two experiments.
observed for compounds derived from this scaffold. Among the
potent carboxamides identified from our initial screening, the
N-benzylcarboxamide (1b) was found to exhibit good selectivity
for PDE4 over PDE10 and PDE11. Herein, we report on our optimi-

2596
R. Kuang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2594–2597
series of analogs are generally very selective for PDE4 over PDE11.
Weak inhibition of PDE10 is present in this series of compounds.
Substitution on the benzyl group appears to have parallel effects
on PDE4 and PDE10, but these effects are much more profound
on PDE4 than on PDE10. Therefore, the most potent PDE4 com-
pounds (compounds 1m–1o) are also the most selective com-
pounds. However, subtype selectivity between PDE4B and PDE4D
has generally not been found in the series. For example, the IC50
of compound 1n is 0.6 and 0.7 nM for PDE4B and PDE4D,
respectively.
With the PDE4 inhibitory potency attaining the sub-nanomolar
level through the optimization of the benzylamide group,
computer modeling studies indicated that optimization of the 5-
aminomethyl moiety may provide another opportunity for further
improvement of the PDE4 potency. Docking studies revealed that
the –NH2 of the 5-aminomethyl group chelates with Zn2+ and
makes a hydrogen bond with the His-234 side chain, which are
ocated at the bottom of the active site. A small hydrophobic pocket
exists between the methylene group of the 5-aminomethyl moiety
and the enzyme. Substitution on the methylene group with small
groups such as an (S)-methyl or an (S)-ethyl should occupy this
hydrophobic space and may further increase PDE4 inhibitory po-
tency (Fig. 1).
fluoride18 in the presence of 2 equiv of KHMDS at À78 to À50 °C.
The
of chiral
ies of compounds with different
a-substituents can be introduced directly from the side-chains
a
-amino acids, and no racemization was observed. A ser-
a-substituents has been prepared
and their IC50 data for PDE4 are tabulated in Table 2.
For each of the three different benzylamide moieties used for
comparison in Table 2, the (S)-methyl analogs (compounds 2d,
2n, and 2r) are consistently 10-fold more potent than the corre-
sponding unsubstituted analogs (compounds 1d, 1n, and 1r) and
also more potent than the (S)-ethyl (3d) and (S)-isopropyl (5n)
analogs. The hydrophilic (S)-hydroxymethyl group (4d) decreases
the PDE4 inhibitory potency by more than three-fold. As predicted
from modeling studies, the (S)-isomer of the
a-substitution is
preferred and is 80-fold more potent than the corresponding
(R)-isomer (comparing compounds 2r and 6r).
With the (S)-methyl group clearly positioned as the optimal
a-substituent, a series of analogs were synthesized to explore the
potential synergistic effects of combining the (S)-methyl
Table 3
SAR of 4-benzylcarboxamide groupa
OMe
To explore the effects of
a-substitution on PDE4 inhibition, a
N
CF3
novel synthesis of the oxazole core has been developed (Scheme
2), where the oxazole ring formation step involves the reaction of
a thioimidate intermediate (D)17 with an N-Boc amino acid
O
N
O
R1
H2N
N
R2
Table 2
SAR of
a
-substitution on the 5-aminomethyl group
Compd
–N R3R4
IC50 (nM)
PDE10
Me
O
PDE4B
0.35
PDE11
1100
N
CF3
NH
2d
2g
650
160
Cl
F
O
N
O
R1
NH
0.3
0.3
1400
1100
H2N
N
R2
R3
F
Compd
–R3
H
–N R1R2
PDE4B IC50 (nM)
3.4
NH
NH
2i
2l
80
NH
1d
F
Cl
Cl
Cl
Cl
NH
NH
NH
0.05
0.03
125
1400
1600
2d
3d
4d
(S)-Me
0.35
0.81
F
F
(S)-Et
NH
2m
120
(S)-CH2OH
12.7
Cl
F
F
NH
NH
NH
1n
2n
5n
H
0.6
NH
NH
NH
NH
NH
F
F
F
2n
2o
2p
2q
0.06
0.7
950
700
2300
1100
1800
2000
F
F
F
F
(S)-Me
(S)-iPr
0.06
0.36
Cl
Cl
F
Cl
0.7
1700
1400
NH
F
1r
2r
6r
H
1.6
HO
HO
HO
0.6
NH
NH
(S)-Me
(R)-Me
0.11
8.4
F
F
F
2s
0.04
500
3400
a
aValues of IC50 are the means of at least two experiments.
Values of IC50 are the means of at least two experiments.

R. Kuang et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2594–2597
2597
substitution with the ortho-F benzylamide on both PDE4 inhibitory
Acknowledgment
potency and selectivity (Table 3).
After introducing the (S)-methyl group to the 5-aminomethyl
moiety, all compounds in the benzylamide series in Table 3 exhibit
PDE4 inhibitory potency at the subnanomolar level. Compounds
2l, 2m, 2n, and 2s, containing the ortho-F benzylamide moiety, ex-
hibit IC50 values in the range of 30–60 pM and represent the most
potent PDE4 inhibitors reported to date. In comparing the corre-
sponding compounds from Tables 1 and 3, the (S)-methyl group
has a much greater effect on PDE4 than on PDE10 and PDE11
potency, which results in further improvement of the selectivity
for PDE4 over PDE10 and PDE11. Compound 2n is the most potent
PDE4 inhibitor with the highest relative selectivity for PDE4/PDE10
(15800-fold). This compound was chosen for profiling in our
in vivo studies.
We thank Dr. Jesse Wong, Dr. Mark Liang, and Mr. Yan Jin for
scale-up of intermediates.
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IC50 of 0.6 nM. In a rat PK study, compound 2n produces a
Cmax = 960 nM with 24% bioavailability at a 10 mg/kg oral dose. This
plasma level is quite sufficient for in vivo activity, and in the in vivo
rat LPS(lipopolysaccharide)-induced pulmonary inflammation mod-
el,19 compound 2n is highly efficacious with an ED50 = 0.1 mg/kg PO.
In addition, compound 2n exhibits no P450 liver enzyme issues with
IC50 for 3A4, 2D6, 2C9, and 2C19 > 8 lM. Further development of this
and related compounds will be reported in the future.
In summary, a novel class of PDE4 inhibitors based on the qui-
nolyl oxazole scaffold has been discovered. Optimization of both
the 4-carboxamide and the 5-aminomethyl groups has led to the
discovery of the most potent PDE4 inhibitors reported to date
(IC50 = 30–60 pM), which are also highly selective and orally active.
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The oxazole motif, bearing the 4-carboxamide and 5-a-aminoethyl
groups, is a highly potent and unprecedented pharmacophore for
phosphodiesterase inhibition. It may also provide a new working
model for the rational design of inhibitors for other PDE enzymes.