Structure-based design of caspase-1 inhibitor containing a diphenyl ether sulfonamide

Article abstract of DOI:10.1016/S0960-894X(01)00573-X

A series of compounds was designed and prepared as inhibitors of interleukin-1β converting enzyme (ICE), also known as caspase-1. These inhibitors, which employ a diphenyl ether sulfonamide, were designed to improve potency by forming favorable interactions between the diphenyl ether rings and the prime side hydrophobic region. An X-ray crystal structure of a representative member of the diphenyl ether sulfonamide series bound to the active site of caspase-1 was obtained.

Full text of DOI:10.1016/S0960-894X(01)00573-X

Bioorganic & Medicinal Chemistry Letters 11 (2001) 2779–2782
Structure-Based Design of Caspase-1 Inhibitor Containing a
Diphenyl Ether Sulfonamide
Aurash B. Shahripour,a,* Mark S. Plummer,a Elizabeth A. Lunney,b Tomi K. Sawyer,a
Charles J. Stankovic,a Michael K. Connolly,a John R. Rubin,b,y Nigel P. C. Walker,c,y
Kenneth D. Brady,c Hamish J. Allen,c Robert V. Talanian,c Winnie W. Wongc
and Christine Humbletb
aDepartment of Chemistry, Pfizer Global Research & Development, 2800 Plymouth Road, Ann Arbor, MI 48105, USA
bDepartment of Discovery Technologies, Pfizer Global Research & Development, 2800 Plymouth Road, Ann Arbor, MI 48105, USA
cDepartment of Biochemistry, BASF Bioresearch Corporation, Worcester, MA 01609, USA
Received 28 May 2001; accepted 10 August 2001
Abstract—A series of compounds was designed and prepared as inhibitors of interleukin-1b converting enzyme (ICE), also known
as caspase-1. These inhibitors, which employ a diphenyl ether sulfonamide, were designed to improve potency by forming favorable
interactions between the diphenyl ether rings and the prime side hydrophobic region. An X-ray crystal structure of a representative
member of the diphenyl ether sulfonamide series bound to the active site of caspase-1 was obtained. # 2001 Elsevier Science Ltd.
All rights reserved.
Interleukin-1 (IL-1) appears to be a central mediator of
the pathogenesis of acute and chronic inflammation.1 Of
the two IL-1 proteins that exist, a and b, it is the latter
that is predominantly released from activated human
monocytes.2 IL-1b is synthesized as a cytosolic inactive
precursor of 33K, which is released only upon cleavage
to the mature 17.5K form. The intracellular protease
(caspase-1), also referred to as interleukin-1b converting
enzyme (ICE), responsible for this processing has been
cloned3 and the crystal structure elucidated.4,5 Caspase-
1 is a homodimer (consisting of two heterodimer sub-
units p10 and p20) cysteine protease that cleaves human
proIL-1b between the Asp116-Ala117. Data from cas-
pase-1 deficient mice show decreased levels of IL-1b
upon lipopolysaccharide (LPS) stimulation, supporting
the concept that caspase-1 inhibition would be beneficial
in treatment of diseases in which IL-1b plays a role.6 IL-
1b has been shown to be important in the pathogenesis
of a number of peripheral inflammatory disorders,
including rheumatoid arthritis and inflammatory bowel
disease. Caspase-1 might also be involved in unsched-
uled programmed cell death (apoptosis).7 In addition,
data in the literature suggest that the inhibition of IL-1b
production by a caspase-1 inhibitor may be useful for
treatment of brain damage in stroke8 and myocardiac
ischemia.9
The structure-based design and X-ray crystallographic
analysis of a new class of active site caspase-1 inhibitors,
which contain a biphenyl sulfonamide-aspartic acid
aldehyde scaffold, have recently been reported from our
laboratory.10 Compound 1, for example, is a reversible
inhibitor of caspase-1. In this structure, a major con-
formational change has been observed in which the
His237 side chain has rotated from a gauche to a trans
orientation, creating a large hydrophobic pocket adja-
cent to the P1 site. Based on our modeling ideas, the
biphenyl series was expanded to include a diphenyl
ether sulfonamide derivative, which was predicted to
bind with His237 in the gauche orientation. The diphe-
nyl ether moiety is predicted to reach toward the pre-
viously recognized prime side hydrophobic pocket.11
The present report describes the structure-based design
and the synthesis of potent compounds, which contain a
diphenyl ether. Based on the initial model, a series of o-
diphenyl ether derivatives was designed with the aim of
maximizing favorable prime side hydrophobic interac-
*Corresponding author. Fax: +1-734-622-3107; e-mail: aurash.
shahipour@pfizer.com
yCorresponding authors regarding X-ray crystallography.
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00573-X

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A. B. Shahripour et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2779–2782
tions. Specifically, it was hoped that appended hydro-
phobic groups from the diphenyl ether sulfonamide
would serve to further isolate the oxyanion from the
solvent. As a first step the un-substituted diphenyl ether
sulfonamide 2 was synthesized and was found to be
equipotent (IC50=24 mM) with 1.
between the SO2NH group and the enzyme. The X-ray
crystal structure shows that His237 is in the standard
gauche orientation rather than trans as is observed in
the caspase-1 structure bound with compound 1. The
isopropyl phenyl group is positioned in the hydrophobic
region formed by Ile176, Pro177, and Ile239.
Initially, simple alkyl substituents on the remote phenyl
ring were evaluated to probe the prime side hydro-
phobic pocket. As can be seen in Table 1, addition of
these simple alkyl groups did provide improved potency
when compared to compound 2. Activity of the 3, 4, 5-
trimethyl substituted compound 7 suggests that attach-
ing branched alkyl groups to either the meta or para
position may provide additional potency.
Examination of the X-ray structure of 9 reveals that
incorporation of a methyl group adjacent to the iso-
propyl group, as in 10, would result in improved
potency. Indeed, compound 10 is 5-fold more potent
than 9. This increase in potency was attributed to the
methyl group, which causes the isopropyl group to twist
relative to aromatic ring, thereby allowing better com-
plementarity of the isopropyl group with the enzyme
hydrophobic surface. Replacement of the isopropyl
group in 9 with a 2-butyl group to give 11 produced a 2-
fold increase in IC50. Addition of a methyl group to 11
enhanced potency by 2-fold providing 12, which is
equipotent to 10. Compound 13 was synthesized in
order to avoid the issue of the isobutyl stereochemistry.
Molecular modeling12 suggested caspase-1 could
Specifically, appending a branched alkyl group like
meta-isopropyl on the remote phenyl ring provided a
more potent inhibitor 8 (IC50=3.70 mM), with a 2-fold
increase in potency relative to compound 7 (Table 2). A
para-isopropyl substitution on the remote phenyl ring
provides analogue, 9, and is nearly as potent as 8
(IC50=5.30 mM).
Table 2. Evaluation of inhibitors against caspase-1
The crystal structure of 9 in the caspase-1 active site was
determined to further guide our efforts exploring the
hydrophobic prime side (Fig. 1). The aspartic acid
aldehyde in 9 forms the standard covalent bond with
catalytic Cys285 and orients the acidic side chain in the
electropositive pocket formed by Arg179, Gln283, and
Arg341. Additional polar interactions are formed
Compd
R
IC50 (mM)
Ki (mM)
8
3.7
0.41
Table 1. Evaluation of inhibitors against caspase-1
9
5.3
1.0
2.2
0.62
0.11
0.49
10
11
Compd
R
IC50 (mM)
Ki (mM)
1
20
1.6
2
3
4
5
24
14
11
9
2.4
3
12
13
14
15
16
17
1.0
3.5
4.8
4.3
4.4
58
0.10
0.44
0.88
0.90
0.29
4.2
0.43
1.1
6
7
13
1.0
7.8
0.45

A. B. Shahripour et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2779–2782
2781
accommodate a 3-pentyl substituent found in 13. Fur-
thermore, replacement of the isopropyl group in 9 with
cyclic nonpolar groups resulted in 14 and 15, which had
binding affinities equipotent to 9.
binding in the prime side hydrophobic pocket. Although
this substitution did not improve activity, this phenyl
ring served as a template for the addition of polar
groups which may interact favorably with the His248
side chain or Glu250 (NH). Unfortunately, compound
17 with a para carboxyl group exhibited poor binding
affinity, suggesting a favorable interaction with His248
or Glu250 (NH) was not obtained (Table 2).
Replacing the cyclohexyl group in compound 15 with
the phenyl group as in compound 16 provides a com-
parison between an aromatic and cyclic alkyl group
The synthetic route to compounds of type 2 is shown in
Scheme 1. The phenol was treated with potassium t-
butoxide in dry THF for 1 h. The o-fluoro-nitrobenzene
was added to afford the diphenyl ether 18. The nitro
group was subsequently reduced by catalytic hydro-
genation using Raney nickel to give aniline 19. The
resulting aniline was treated with sodium nitrite/HCl or
isoamyl nitrite/boron trifluoride etherate to form the
diazonium salt which was treated with a mixture of sul-
fur dioxide/HOAc/CuCl2 to produce the sulfonyl chlo-
ride 20.13 The resulting sulfonyl chloride was used to
acylate Asp–N(OCH3)CH3/pyridine/CH2Cl2, giving
the diphenyl ether sulfonamide 21. The Weinreb
amide was reduced with lithium aluminum hydride in
Et2O at ꢀ65 ꢁC yielded the aldehyde 22. The tert-
butyl ester protecting group was then removed with
15% TFA in CH2Cl2 to provide the acid 23 as a final
product.
Figure 1. Crystal structure of 9 in complex with caspase-1. The cata-
lytic residue (Cys285) forms the standard covalent bond with the
aspartic acid aldehyde and orients the acidic side chain in the electro-
positive pocket formed by residues (Arg179, Gln283, Arg341) and
inhibitor 9 is shown with thicker bonds. Protein and inhibitor atoms
are shown with atoms in green (carbon), blue (nitrogen), red (oxygen),
and yellow (sulfur).
In conclusion, the diphenyl ether sulfonamides descri-
bed above provide a novel lead template for the devel-
opment of caspase-1 inhibitors. This series differs from
previously described biphenyl sulfonamide small mole-
cule inhibitors of caspase-1, in that His237 binds in a
Scheme 1. Synthesis of diphenyl ether sulfonamide 2.

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gauche orientation and the distal substituted phenyl ring
binds in the prime side hydrophobic pocket. The 20-fold
increase in binding affinity over biphenyl sulfonamide
series10 observed in this diphenyl ether series is
encouraging, suggesting that caspase-1 inhibitors built
on this novel structure have the potential for future
drug development.
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