Phenylimidazole derivatives as new inhibitors of bacterial enoyl-ACP reductase FabK

Article abstract of DOI:10.1016/j.bmcl.2007.06.040

Novel FabK inhibitors with antibacterial activity against Streptococcus pneumoniae were synthesized and evaluated. Through SAR studies of our initial hit compound 2-(1H-benz[d]imidazol-2-ylthio)-N-(6-methoxycarbonylbenzo[d]thiazol-2-yl)acetamide, a series of novel phenylimidazole derivatives were discovered as potent FabK inhibitors.

Full text of DOI:10.1016/j.bmcl.2007.06.040

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4982–4986
Phenylimidazole derivatives as new inhibitors of bacterial
enoyl-ACP reductase FabK
Hideo Kitagawa,a,* Tomohiro Ozawa,a Sho Takahatab and Maiko Iidaa
aMedicinal Chemistry Research Laboratories, Pharmaceutical Research Center, Meiji Seika Kaisha, Ltd,
760 Morooka-cho, Kohoku-ku, Yokohama 222-8567, Japan
bPharmacology Research Laboratories, Pharmaceutical Research Center, Meiji Seika Kaisha, Ltd, Japan
Received 12 March 2007; revised 26 April 2007; accepted 6 June 2007
Available online 14 June 2007
Abstract—Novel FabK inhibitors with antibacterial activity against Streptococcus pneumoniae were synthesized and evaluated.
Through SAR studies of our initial hit compound 2-(1H-benz[d]imidazol-2-ylthio)-N-(6-methoxycarbonylbenzo[d]thiazol-2-yl)acet-
amide, a series of novel phenylimidazole derivatives were discovered as potent FabK inhibitors.
Ó 2007 Elsevier Ltd. All rights reserved.
Streptococcus pneumoniae is the main causative patho-
gen of community-acquired pneumonia. The infectious
disease caused by S. pneumoniae remains a leading cause
of morbidity and mortality. The increasing prevalence of
penicillin-resistant S. pneumoniae (PRSP) is of concern,
since most of the pathogens have acquired resistance to
many antibacterial agents including macrolide and
quinolone.1–3 Since many conventional therapies are
no longer effective against these drug-resistant patho-
gens, the discovery of new antibiotics is the most impor-
flavoprotein, termed FabK, has been shown to be the sole
enoyl-ACP reductase in S. pneumoniae.23,24 Conse-
quently, a selective FabK inhibitor is expected to be a no-
vel narrow-spectrum antibacterial agent against
S. pneumoniae including PRSP, macrolide and quinolone
resistant S. pneumoniae. There are very few reports of
FabK inhibitorsexceptforasmallnumberofcompounds.
The reported FabK inhibitors 2 and 3 showed weak inhib-
itoryactivity andfailed toshowselectiveinhibition ofbac-
terial FAS.17,25 Atromentin and leucomelone were also
reported as FabK inhibitor,26 but these compounds did
not show any antibacterial activity against S. pneumoniae.
Unlike the reported FabK inhibitors, our developed
phenylimidazole derivatives showed strong FabK-inhibi-
tory activity and potent antibacterial activity against
S. pneumoniae due to the inhibition of FabK. In this
report, we present our investigation of a series of pheny-
limidazole derivatives as novel FabK inhibitors.
tant.
A key strategy to overcome drug-resistant
pathogens is the discovery of antibacterial agents with
novel mechanisms of action.
FabI is an enoyl-acyl carrier protein (ACP) reductase
which catalyzes the final and rate-limiting step of bacte-
rial fatty acid synthase (FAS).4–7 Most bacteria possess
FabI and bacterial FAS shows low overall sequence
homology with mammalian enzymes. Therefore, FabI-
targeting approach to antibacterial drug therapy seems
feasible. Various compounds including isoniazid,8
diazaborines,9,10 triclosan,11–16 indole naphthyridinones
(2,3),17–19 thiopyridine,20 and 4-pyridones (1)21,22 have
been reported (Fig. 1).
To discover novel small-molecule FabK inhibitors, we
screened our compound library for inhibitory activity
toward FabK of S. pneumoniae. Among them, amide
compound 4 exhibited moderate FabK-inhibitory activ-
ity of S. pneumoniae (IC50 = 1.8 lM) and selective inhi-
bition of bacterial FAS. Therefore, compound 4 was
selected as the lead compound27 and we studied for
the structure–activity relationship (SAR).
However, recentstudieshaveshownotherbacterialenoyl-
ACP reductases, in addition to FabI. A triclosan-resistant
Amide-type derivatives were prepared by means of
standard procedures. For example, the synthesis of
compound 9 is illustrated in Scheme 1. Commercially
available 2-aminobenzothiazole 5 was treated with
Keywords: Bacterial enoyl-acyl reductase; FabK; FabI; Fatty acid
biosynthesis; Inhibitors.
*
Corresponding author. Tel.: +81 45 541 2521; fax: +81 45 541
0667.; e-mail: hideo_kitagawa@meiji.co.jp
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.06.040

H. Kitagawa et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4982–4986
4983
Figure 1. Inhibitors of FabI.
to afford the corresponding amide adduct 9 in 73%
yield.
Although several of the amide derivatives showed
good FabK-inhibitory activity, they showed no anti-
bacterial activity against S. pneumoniae in the presence
of blood. Examination of the stability of the com-
pounds under the MIC measurement conditions by
means of liquid chromatography–mass spectrometry
indicated that 4 readily decomposed to compounds
10–12 (Fig. 2), which lacked FabK-inhibitory activity
(Table 1). For improving the chemical stability of
the amide bond, we prepared carbamate type 15 and
ureido type 17.
Scheme 1. Synthesis of compound 9. Reagents and conditions: (a)
toluene, 60 °C (70%); (b) NaOMe (1.5 equiv), DMF, rt (73%).
The synthesis of compound 15 is illustrated in Scheme 2.
The activated amide 13 was prepared from 2-amin-
obenzothiazole 5 with 1,10-carbonyldiimidazole (CDI),
and used without purification. Reaction of commercially
chloroacetyl chloride 6 in toluene at 60°C to afford
compound 7 in 70% yield. The reaction of 7 with 2-
mercaptobenzimidazole 8 was carried out in DMF at
room temperature in the presence of sodium methoxide
Figure 2. Expected compounds that were converted from compound 4.
Table 1. Antibacterial activities for FabK inhibitors: Expected compounds that were converted from compound 4
Compound
Structure
FabK IC50a(lM)S. pneumoniae
MIC(lg/mL)S. pneumoniae
N
S
HN
O
N
CO2H
10
>32
>32
S
S
N
H
OH
N
11
12
>32
>32
ntb
ntb
O
N
H
N
H2N
S
CO2H
a S. pneumoniae KU197.
b nt, not tested.

4984
H. Kitagawa et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4982–4986
available (1H-benz[d]imidazol-2-yl)methanol 14 with 13
in the presence of sodium methoxide gave compound 15.
group at 6-position of benzothiazole showed moderate
FabK-inhibitory activity. In particular, compound 27
having methylsulfonyl group exhibited a good FabK-
inhibitory activity. Compounds 29 and 30 having meth-
oxy group at 4- or 5-position of benzothiazole showed
decreased FabK-inhibitory activities compared with
28. These results indicate that substitutions at 6-position
improve FabK-inhibitory activity.
The ureido compound 17 was prepared as illustrated in
Scheme 3. Reaction of commercially available (1H-
benz[d]imidazol-2-yl)methylamine dihydrochloride salt
16 with 13 in THF at room temperature in the presence
of N,N-diisopropylethylamine gave compound 17.
FabK-inhibitory activity and antibacterial activity
against S. pneumoniae of compounds 15 and 17 are
shown in Table 2. Compound 15 having a carbamate
group lacked FabK-inhibitory and antibacterial activi-
ties, while compound 17 having an ureido group showed
weak FabK-inhibitory activity. Moreover, compound
17 showed antibacterial activity against S. pneumoniae
due to the improved chemical stability in the standard
MIC assay medium including 2% lysed horse blood.
We therefore selected an ureido group as a basic
structure.
Introduction of other heterocyclic rings (compounds 31
and 32) instead of benzimidazole ring did not improve
both activities, but the phenylimidazole derivative (com-
pound 25) showed strong FabK-inhibitory activity
(0.14 lM) and antibacterial activity (0.5 lg/mL). Since
compounds 17 and 25–30 did not show FabI-inhibitory
activity (Escherichia coli FabI IC50 > 32 lM), these
compounds appear to be selective FabK inhibitors. In
addition, compound 25 did not show any antibacterial
activity against S. aureus (>32 lg/mL) without FabK
and elevated MIC value (>8-fold) was observed against
S. pneumoniae mutant possessing an amino acid substi-
tution in FabK.27 These results indicate the antibacterial
activity of compound 25 is due to the inhibition of
FabK.
SAR of (benzothiazol-2-yl)ureido derivatives is shown
in Table 3. Compounds 26–28 having various functional
Compound 25 was prepared as illustrated in Scheme
4. Reaction of commercially available benzyl cyanom-
ethylcarbamate 18 with sodium methoxide followed by
ammonium chloride treatment gave compound 19 in
97% yield. Compound 19 was treated with 2-phen-
acylbromide 20 to give phenylimidazole derivative
21, and cleavage of the benzyloxycarbonyl moiety
afforded compound 22. Compound 25 was prepared
by similar procedures described for compounds 15
and 17.
Scheme 2. Synthesis of compound 15. Reagents and conditions: (a)
CDI (2 equiv), THF, rt; (b) NaOMe (0.1 equiv), THF, 40 °C (26%).
We discovered the phenylimidazole derivative 25 as a
novel inhibitor of bacterial enoyl-ACP reductase
(FabK). Compound 25 selectively exhibited strong
FabK-inhibitory and potent antibacterial activities
against S. pneumoniae in the standard MIC assay med-
ium including blood. The present results support the
idea that FabK is a valid antibacterial target, and that
small-molecule FabK inhibitors are drugs for the treat-
ment of bacterial infections, and further suggest that the
Scheme 3. Synthesis of compound 17. Reagents and condition: (a) i-
Pr2NEt (2 equiv), THF, rt (78%).
Table 2. Antibacterial activities for FabK inhibitors: Effect of combination units
Compound
Structure
FabK IC50 (lM) S. pneumoniae
MIC (lg/mL) S. pneumoniaea
N
S
HN
O
N
9
>32
>32
S
N
H
N
S
HN
O
O
N
15
>32
28
>32
32
N
H
N
S
HN
O
HN
N
17
N
H
a S. pneumoniae KU197.

H. Kitagawa et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4982–4986
Table 3. SAR of (benzothiazol-2-yl)ureido derivatives
4985
H
H
R1
N
N
R2
O
MIC (lg/mL) S. pneumoniaeb
a
Compound
Structure
R2
FabK IC50 (lM) S. pneumoniae
R1
N
N
17
26
27
28
29
30
25
31
28
32
8
N
H
N
S
N
4.7
N
H
N
S
N
CN
0.76
7.6
4
N
H
N
S
N
S
SO2Me
16
ntc
ntc
0.5
>32
N
H
N
OMe
OMe
N
S
>32
>32
0.14
24
N
H
N
OMe
N
N
H
S
N
N
S
N
SO2Me
SO2Me
N
H
N
S
N
S
S
32
24
>32
SO2Me
a Average of two experiments.
b S. pneumoniae KU197.
c nt, not tested.
phenylimidazole derivatives represent a good lead struc-
ture. Further structural development studies are in
progress.
Acknowledgments
The authors are especially thankful to Dr. Kunio
Atsumi and Dr. Eiki Shitara Meiji Seika Kaisha
Ltd, for their stimulating discussion and strong
encouragement of this work. The authors also wish
to thank Miss Shigeko Miki, Mrs. Takako Miyara,
and Mrs. Kazue Sasaki Meiji Seika Kaisha, Ltd,
for their kind help with mass spectrometry
analysis.
Supplementary data
Scheme 4. Synthesis of compound 25. Reagents and conditions: (a)
NaOMe (0.1 equiv), MeOH, rt; (b) NH4Cl (1 equiv), rt (97%); (c)
K2CO3 (1 equiv), DMF, rt (29%); (d) 10% Pd/C (30 w/w%), H2,
MeOH, HClaq (76%); (e) CDI (2 equiv), THF, rt; (f) i-Pr2NEt (2.2
equiv), THF, rt, (33%).
Spectrum data of the new compounds and biological
method. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.bmcl.2007.06.040.

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H. Kitagawa et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4982–4986
N.; Wallis, N. G.; Huffman, W. F. J. Med. Chem. 2003,
46, 1627.
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