Investigation of glycine α-ketoamide HCV NS3 protease inhibitors: Effect of carboxylic acid isosteres

Article abstract of DOI:10.1016/j.bmcl.2005.06.003

The design and synthesis of tetrapeptide-based α-ketoamides containing prime side acid isosteres HCV NS3 protease inhibitors are described. Tetrazole, sulfonic acid, and N-sulfonylcarboxamids were demonstrated to be efficient carboxylic acid replacements.

Full text of DOI:10.1016/j.bmcl.2005.06.003

Bioorganic & Medicinal Chemistry Letters 15 (2005) 3487–3490
Investigation of glycine a-ketoamide HCV NS3 protease
inhibitors: Effect of carboxylic acid isosteres
Wei Han,* Xiangjun Jiang, Zilun Hu, Zelda R. Wasserman and Carl P. Decicco
Discovery Chemistry, Pharmaceutical Research Institute, Bristol-Myers Squibb Company, P.O. Box 5400, Princeton, NJ 08543, USA
Received 28 April 2005; revised 27 May 2005; accepted 2 June 2005
Abstract—The design and synthesis of tetrapeptide-based a-ketoamides containing prime side acid isosteres HCV NS3 protease
inhibitors are described. Tetrazole, sulfonic acid, and N-sulfonylcarboxamids were demonstrated to be efficient carboxylic acid
replacements. Further optimization yielded a series of potent HCV NS3 protease inhibitors with IC50 of 0.020–0.060 lM.
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
tigate this lead compound, we investigated replacing the
acid with functional groups that can maintain a similar
hydrogen bonding network on the prime side (structure
2). In this communication, we wish to report results
based on prime side investigations.
The hepatitis C virus (HCV) is the major etiologic agent of
transfusion-associated and community-acquired non-A,
non-B hepatitis. About 3% of the world population are
infected by HCV.1 In most cases, HCV establishes a per-
sistent infection resulting in chronic hepatitis and cirrho-
sis, which can lead to hepatocellular carcinoma. Current
therapies used include treatment with interferon-a
(INF-a) alone and in combination with ribavirin.2 These
therapies have limited efficacy and are frequently accom-
panied by side effects. Consequently, there is a clear need
to develop more effective therapeutics for the treatment
of HCV-associated hepatitis. Among the multiple
approaches, HCV NS3 protease has been studied exten-
sively and well characterized.3 Kolykhalov et al. demon-
strated that NS3 protease was required for HCV
infection in Chimpanzee, hence provided preclinical vali-
dation of this target.4 Furthermore, the availability of
crystal structures of NS3 protease makes it an attractive
target for small-molecule computer-assisted inhibitor
design.5
2. Results and discussion
Simple sulfonic acid (2a, Table 1) gave comparable
potency (IC50 = 0.22 lM)7 to 1. Tetrazole, a well known
carboxylic acid isosteres, was also effective, resulting in
only a twofold loss of activity (2b). The N-(methylsulfo-
nyl)glycinamide analog 2c was two orders of magnitude
less potent than the acid 1, however, the triflouro-
methylsulfonyl analog 2d restored most of the potency.
Compared to the methyl analog 2c, phenyl- and ben-
zylsulfonyl glycinamides gave only modest potency
(8.9 and 12 lM, respectively). Naphthyl further im-
proved potency to 1.2 lM, indicating that further SAR
work in this area was warranted.
Early studies from our laboratory showed that an a-
ketoamide of glycine (1) was an effective inhibitor of
HCV NS3 protease.6 Docking of 1 in the active site of
the protease indicated that the carboxylic acid moiety
can engage in hydrogen bonding with Arg 109 and
Lys 136 of the enzyme.6a In an attempt to further inves-
Keyword: HCV protease inhibitors.
*
Corresponding author. Tel.: +6098185288; fax: +6098183331; e-mail:
wei.han1@bms.com
Molecular modeling of the N-phenylsulfonylglycin-
amide analog 2e in the active site of NS3 protease
revealed that the sulfonamide group appear to interact
Present address: Incyte Pharmaceuticals, Wilminton, DE 19880,
USA.
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.06.003

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W. Han et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3487–3490
Table 1. SAR of analogs with acid isostere as prime groups
shown in Table 2, chloro substitution at the meta-posi-
tion of the phenyl ring (2h) improved the potency by
35-fold over the un-substituted counterpart (2e), to give
a 0.25 lM inhibitor. Nitro and phenyl groups (2i and 2j)
at the meta-position also improved the potency, both
under 1 lM. The para-congeners of the nitro and phenyl
groups (2k and 2l) were also prepared and tested. How-
ever, they are less potent than their meta-congeners.
Introduction of a second substituent to the remaining
meta-position of the phenyl group proved beneficial to
potency. Despite the difference in electronic and steric
properties, both (benzamido)sulfonyl and chloro substi-
tutions gave comparable potency, and both improved
the IC50 value to about 0.1 lM. Additionally, the 2,5-di-
substituted phenylsulfonylglycinamide derivative 2o
(0.11 lM) provided equal potency as compared to 2m
Compound
P0
IC50 (lM)
1
0.23
2a
2b
2c
2d
2e
2f
0.22
0.46
25
Table 2. SAR of analogs with N-sulfonylglycinamide as prime groups
0.47
8.9
Compound
P0
IC50 (lM)
2e
8.9
12
2h
2i
0.25
0.41
0.64
0.92
2.4
2g
1.2
with the polar side chain of Lys 136 near the S10 site of
the enzyme (Fig. 1). The phenyl group appears to point
to an open space, which explains the SAR observed for
compounds 2e–g. To further explore the binding site of
this region, a series of substituted N-phenylsulfonylgly-
cinamide analogs were designed and evaluated. As
2j
2k
2l
2m
2n
0.12
0.13
2o
2p
0.11
0.25
Figure 1. Sectional view of a computer-generated model of 2e (in stick
presentation) in the active site of HCV NS3 protease (in surface
presentation).

W. Han et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3487–3490
3489
and 2n. Besides N-phenylsulfonylglycinamide, the elec-
tron rich N-thiadiazolesulfonylglycinamide was also
well tolerated and provided an inhibitor 2p with IC50
of 0.25 lM. The enhanced potency for the substituted
analogs can be explained by the hydrophobic interac-
tions gained from the substitution groups on the phenyl.
Figure 1 also reviewed that the para-position of the
phenyl is closer to the enzyme surface compared to the
meta-position, which explains the better tolerance of a
meta-substitution.
Previously, we reported that when the ethyl P1 group of
compound 1 was replaced with 2,2-difluoroethyl group,
potency was enhanced.8 Therefore, the 2,2-difluoroethyl
P1 group was incorporated to tetrazole and sulfonylgly-
cinamide series. As shown in Table 3, the tetrazole deriv-
ative 3a was a 40 nM inhibitor, about 10-fold more
potent than the ethyl counterpart (2b). The difluoroethyl
P1 also enhanced the potency of sulfonylglycinamide
series (3b–3f), all gave IC50 less than 100 nM. Among
them, the two thiadiazolesulfonyl analogs were most
potent, with IC50 value of 20 nM. Compared to the
2,2-difluoroethyl P1 optimized carboxylic acid inhibitor
3g, the acid isosteres analogs (3a–3f) provided equal po-
tent or slightly better HCV NS3 protease inhibitors.
Scheme 1. Reagents and conditions: (a) LiOH, THF, H2O, 0 °C, 2 h
(81–87%); (b) NH2CH2X, BOP, DIEA, DMF, 0–25 °C, 2 h (61–74%);
(c)Dess–Martin oxidation, CH2Cl2, 25 °C, 4 h (65–84%).
3. Synthesis
The synthesis of tetrazole and sulfonic acid analogs is
outlined in Scheme 1. Starting from intermediate 4,
which was prepared from cyclohexylalanine as described
previously,8a saponification followed by coupling with
5-(aminomethyl)tetrazole or aminomethanesulfonic acid
provided the corresponding a-hydroxy amides. Dess–
Table 3. SAR of analogs with CH2CHF2 as P1 group
Compound
P0
IC50 (lM)
3a
0.040
3b
3c
0.060
0.060
3d
3e
0.050
0.020
3f
0.020
0.060
Scheme 2. Reagents and conditions: (a) LiOH, THF, H2O, 0 °C, 2 h
(87–91%); (b) HATU, HOBt, TMP, H-Gly-OtBu, 0 °C to rt, 2 h (65–
87%); (c) Dess–Martin oxidation, CH2Cl2, rt, 8 h (50–83%); (d) TFA,
CH2Cl2, rt, 1 h (74–91%); (e) NH2SO2R0, EDCI, DMAP, DMF, 0 °C
to rt, 2 h (56–87%).
3g

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W. Han et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3487–3490
2. (a) Bartenschlager, R. Antivir. Chem. Chemother. 1997, 8,
281; (b) Brass, V.; Blum, H. E.; Moradpour, D. Expert
Opin. Ther. Targets 2004, 8, 295; (c) De Francesco, R.;
Rice, C. M. Clin. Liver Dis. 2003, 7, 211.
3. (a) Bartenschlager, R. J. Viral Hepat. 1999, 6, 165;
(b) LaPlante, S. R.; Cameron, D. R.; Aubry, N.;
Lefebvre, S.; Kukolj, G.; Maurice, R.; Thibeault, D.;
Martin oxidation provided the corresponding a-keto-
amides 5a and 5b in good yields.
The synthesis of the N-sulfonylglycinamide analogs is
illustrated in Scheme 2. Hydrolysis of 4 followed by cou-
pling with glycine t-butyester gave intermediate 6. Dess–
Martin oxidation yielded the corresponding a-keto-
amides, which was hydrolyzed under acidic conditions
to yield acid 7. Treatment of 7 with various sulfona-
mides in the presence of EDCI and DMAP provided
the desired a-keto sulfonylglycinamides 8.
`
Lamarre, D.; Llinas-Brunet, M. J. Biol. Chem. 1999,
274, 18618; (c) DiMarco, S.; Rizzi, M.; Volpari, C.;
Walsh, M. A.; Narjes, F.; Colarusso, S.; De France-
sco, R.; Matassa, V. G.; Sollazzo, M. J. Biol. Chem.
2000, 275, 7152.
4. (a) Kolykhalov, A. A.; Agapov, E. V.; Blight, K. J.;
Mihalik, K.; Feinstone, S. M.; Rice, C. M. Science 1997,
277, 570; (b) Kolykhalov, A. A.; Mihalik, K.; Feinstone, S.
M.; Rice, C. M. J. Virol. 2000, 74, 2046.
5. (a) Love, R. A.; Parge, H. E.; Wickersham, J. A.;
Hostomsky, Z.; Habuka, N.; Moomaw, E. W.; Adachi,
T.; Hostomska, Z. Cell 1996, 87, 331; (b) Kim, J. L.;
Morgenstern, K. A.; Lin, C.; Fox, T.; Dwyer, M. D.;
Landro, J. A.; Chambers, S. P.; Markland, W.; Lepre, C.
A.; OÕMalley, E. T.; Harbeson, J. A. Cell 1996, 87, 343;
(c) Yao, N.; Reichert, P.; Taremi, S. S.; Prosise, W. W.;
Weber, P. C. Structure 1999, 7, 1353.
In summary, acid isosteres such as sulfonic acid, tetrazole,
and N-sulfonylglycinamide were investigated as carbox-
ylic acid replacement using a tetrapeptide-based a-keto-
amide template. All three types of acid isosteres were
efficient carboxylic acid replacements. Incorporation of
the optimized difluoroethyl P1 group yielded a series of
potent HCV NS3 protease inhibitors with IC50 of 20–
60 nM.
6. (a) Han, W.; Hu, Z.-L.; Jiang, X.-J.; Decicco, C. P. Bioorg.
Med. Chem. Lett. 2000, 10, 711, and references therein; (b)
Han, W.; Jiang, X.-J.; Hu, Z.-L.; Wasserman, Z. R.;
Decicco, C. P. Abstracts of presentation, National Meeting
of the American Chemical Society, San Diego, Apr 2001;
MEDI-119 and MEDI-121.
7. For IC50 determination conditions, see reference 6a.
8. (a) Han, W.; Hu, Z.-L.; Jiang, X.-J.; Wasserman, Z. R.;
Decicco, C. P. Bioorg. Med. Chem. Lett. 2003, 13, 1111; (b)
The 2,2-difluoroethyl P1 was first described by Narjes et al.
Narjes, F.; Koehler, K. F.; Koch, U.; Gerlach, B.;
Acknowledgments
We thank Drs. Bruce D. Korant and Marina G. Bukhti-
yarova for providing HCV protease, Dr. James L. Meek
and Ms. Lorraine Gorey-Feret for determination of
IC50 values.
References and notes
Colarusso, S.; Steinkuhler, C.; Brunetti, M.; Altamura, S.;
¨
De Francesco, R.; Matassa, V. G. Bioorg. Med. Chem. Lett.
2002, 12, 701.
1. (a) Houghton, M. In Fields Virology, 3rd ed.; Raven Press:
New York, 1996, pp P1035–P1058; (b) Narjes, F.; Koch, U.;
Steinkuhler, C. Expert Opin. Investig. New Drugs 2003, 12, 153.