6-Acylamino-penam derivatives: Synthesis and inhibition of cathepsins B, L, K, and S

Article abstract of DOI:10.1016/S0960-894X(02)00766-7

The synthesis of a new series of 6-acylamino penam derivatives and their inhibition of cysteine proteases cathepsins B, L, K, and S is described. The 6-acylamino-penam sulfone compounds showed excellent cathepsin L, K, and S inhibition activity with IC

Full text of DOI:10.1016/S0960-894X(02)00766-7

Bioorganic & Medicinal Chemistry Letters 12 (2002) 3417–3419
6-Acylamino-penam Derivatives: Synthesis and Inhibition
of Cathepsins B, L, K, and S
Nian E. Zhou,a Jadwiga Kaleta,a Enrico Purisima,b Robert Menard,b
Ronald G. Miceticha and Rajeshwar Singha,*
aSynPhar Laboratories, currently NAEJA Pharmaceutical Inc., 4290-91A Street, Edmonton, Alberta, Canada T6E 5V2
bNational Research Council Canada, BRI, 6100 Royalmount Ave, Montreal, Quebec, Canada H4P 2R2
Received 5 July 2002; accepted 26 August 2002
Abstract—The synthesis of a new series of 6-acylamino penam derivatives and their inhibition of cysteine proteases cathepsins B, L,
K, and S is described. The 6-acylamino-penam sulfone compounds showed excellent cathepsin L, K, and S inhibition activity with
IC50 values in the nanomolar and subnanomolar range.
# 2002 Elsevier Science Ltd. All rights reserved.
The cysteine proteases cathepsin B, L, K, and S may be
involved in diseases such as osteoporosis, cancer meta-
stasis, rheumatoid arthritis, and infectious diseases.1
These proteases are implicated as an important targets
for the development of inhibitors as potential thera-
peutic agents.2 Several types of chemical functionalities
served as the central pharmacophore for cysteine pro-
teases inhibitors, such as aldehydes, nitriles, a-keto-
heptan-7-one skeleton is reported to show cytotoxic
activity.6 The alternative structure related to 4-oxa-1-aza-
bicyclo[3,2,0]heptan-7-one skeleton is the penam which is
a well known pharmacophore for antibiotics and is widely
used in clinic.7 Therefore, a new series of 6-acylamino-
penam derivatives was synthesized and their inhibitions
with cathepsins B, L, K, and S were evaluated. The
synthesis of 6-acylamino-penam compounds is outlined in
Scheme 1. The intermediate 4-acetoxy-3-acylamino-azet-
din-2-one 2 was prepared according to our previous
report8 from 6-aminopenicillanic acid. Substitution of the
acetate 2 with mercaptoethanol in the presence of NaOH
gave 4-hydroxyethylthio-azetidin-2-one. Treatment of the
hydroxy compound with toluenesulfonyl chloride under
standard conditions gave toluenesulfonyloxyethylthio-
azetidin-2-one, which was converted to 4-bromoethylthio-
azetidin-2-one, 3, with lithium bromide. Without further
purification, cyclization of 4-bromoethylthio-azetidin-2-
one 3 with base gave two optically pure isomers of the
penam derivatives 4 and 5 in high yield after silica gel
column chromatography. A careful and controlled oxida-
tion of compound 4 with hydrogen peroxide in acetic
acid9 gave a mixture of 4b- and 4a-penam sulfoxides 6 in
ratio of 3:1.10 Complete oxidation of 4 or 5 with KMnO4
resulted in penam sulfones 7 or 8, respectively. For com-
parison, two cephem sulfone derivatives 9 and 10 were
also prepared by a similar method as described above.
Using a similar method, compounds 11–15 were prepared
as well, where phenylalanine was replaced by cyclohexyl-
alanine, 2-naphthylalanine, 2-thiophenalanine, 3-fluoro-
phenylalanine, or 3-pyridylalanine, respectively.
carbonyl
compounds,
halomethyl
ketones,
acyloxymethyl ketone, epoxide, vinyl sulfones, cyclo-
propenone and cyclohexanone.3,4
We have reported on the rational design and synthesis of
a series of 6-substituted-4-oxa-1-azabicyclo[3,2,0]heptan-
7-one derivatives, 1, as cysteine protease inhibitors.5
Although potent inhibition of cathepsin L and K was
achieved with IC50 values in the nanomolar range (for
compound 1; IC50 is 4 and 5 nM for cathepsin L and K,
respectively), one concern with such inhibitors was their
potential toxicity, since the 4-oxa-1-azabicyclo[3,2,0]
*Corresponding author. Fax:+1-780-461-0196; e-mail: rsingh@naeja.
com
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00766-7

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N. E. Zhou et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3417–3419
Scheme 1. Reagents and conditions: (a) HS(CH2)2OH, 1 equivNaOH, THF/H 2O, rt 1 h; (b) TsCl, pyridine, 5 ꢀC, 6 h; (c) LiBr, HMPA, 60 ꢀC, 2 h;
(3) 45% for three step reaction; (d) K2CO3, DMSO, rt overnight; (4) 65%; (5) 25%; (e) 1.5 equiv H2O2, 4 equivAcOH, DCM, rt 2 days; (6) 50%;
(f) KMnO4, AcOH, H2O, rt, 1 h; (6) 65%; (7) 45%.
The inhibitory activities of these compounds with
cathepsin B, L, K, and S were determined according to
the procedure described in the literature11 by using Cbz-
Phe-Arg-AMC for cathepsin B, L, and K and Cbz-Val-
Val-Arg-AMC for cathepsin S. The inhibitory activities
of the compounds are shown in Table 1. The 6-acyl-
amino-penam sulfone derivatives are most potent
inhibitors of cathepsin L, K, and S with IC50 in sub-
nanomolar or low nanomolar range. It appears that
activity correlates strongly with an oxidation state of
the penam ring (compare the activity of compound 4
with compounds 6 and 7). The sulfones are more active
than sulfoxide and sulphide, may be due to the
increased chemical reactivity of azetidin-2-one skelton.
The 5S stereo isomer compound 7 is preferred for better
cysteine proteases inhibitory activity over the 5R stereo
isomer compound 8 as in our previous finding.5 In con-
trast, six-membered ring cepham sulfones do not exhibit
any inhibitory activity. Different aromatic and hetero-
cyclic substitutions at R, mimicking presumably P2
portion revealed lack of selectivity (11, 13, 14, and 15),
with exception of a naphthyl substituent which is not
tolerated well with cathepsin K, but better with other
cathepsins (12). Penam sulfoxides and sulfides are fully
reversible inhibitors, while sulfones are slow reversible
and time dependent inhibitors.
Table 1. In vitro inhibitory activity of azetidin-2-one derivatives with
cysteine proteases
Compd
IC50 (mM)
Cath B
Cath L
CathK
Cath S
4
6
7
8
>50
16.4
0.35
6.01
>50
>50
0.43
0.39
0.43
1
6.68
0.068
0.0014
0.0096
10.6
>2.5
0.035
0.0005
0.0075
>2.5
>2.5
0.0076
0.4
n.d.a
0.07
0.01
0.21
>2.5
>2.5
0.0007
0.003
0.0046
0.008
0.058
9
10
11
12
13
14
15
>50
References and Notes
0.0007
0.003
0.003
0.003
0.0017
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0.006
0.0025
1.3
an.d., not determined.

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