Design, synthesis and biological evaluation of novel thiazole derivatives as potent FabH inhibitors

Article abstract of DOI:10.1016/j.bmcl.2009.09.111

Fatty acid biosynthesis is essential for bacterial survival. Components of this biosynthetic pathway have been identified as attractive targets for the development of new antibacterial agents. FabH, β-ketoacyl-acyl carrier protein (ACP) synthase III, is a

Full text of DOI:10.1016/j.bmcl.2009.09.111

Bioorganic & Medicinal Chemistry Letters 19 (2009) 6750–6754
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis and biological evaluation of novel thiazole derivatives
as potent FabH inhibitors
*
Peng-Cheng Lv, Kai-Rui Wang, Ying Yang, Wen-Jun Mao, Jin Chen, Jing Xiong, Hai-Liang Zhu
State Key Laboratory of Pharmaceutical Biotechnology, Nanjing University, Nanjing 210093, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Fatty acid biosynthesis is essential for bacterial survival. Components of this biosynthetic pathway have
been identified as attractive targets for the development of new antibacterial agents. FabH, b-ketoacyl-
acyl carrier protein (ACP) synthase III, is a particularly attractive target, since it is central to the initiation
of fatty acid biosynthesis and is highly conserved among Gram positive and negative bacteria. Three ser-
ies of Schiff bases containing thiazole template were synthesized and developed as potent inhibitors of
FabH. This inhibitor class demonstrates strong antibacterial activity. Escherichia coli FabH inhibitory assay
and docking simulation indicated that the compounds 11 and 18 were potent inhibitors of E. coli FabH.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 17 August 2009
Revised 23 September 2009
Accepted 26 September 2009
Available online 2 October 2009
Keywords:
Design
Synthesis
Thiazole derivatives
FabH inhibitors
The emergence of bacterial resistance to most of all antibiotics
poses a threat to health care, and novel therapeutics are needed.
Recently, the research has been focused toward development of
new antibacterial agents with novel target. A promising target is
the fatty acid synthase (FAS) pathway in bacteria. Fatty acid bio-
synthesis (FAB) is an essential metabolic process for prokaryotic
organisms and is required for cell viability and growth.1 b-Ketoa-
cyl-acyl carrier protein (ACP) synthase III, also known as FabH or
KAS III, plays an essential and regulatory role in bacterial FAB.2,3
The enzyme initiates the fatty acid elongation cycles4,5 and is in-
volved in the feedback regulation of the biosynthetic pathway via
product inhibition.6 Therefore, it represents a promising target
for the design of novel antimicrobial drugs. Because of this, various
kinds of compounds were screened by enzymatic assays to gener-
ate leads that were co-crystallized with various pathogenic FabH
proteins and subsequently optimized using structure guided drug
design methods.7–11 For example, Kim and co-workers reported
the YKAs3003, a Schiff base condensed by 4-hydroxy salicylalde-
hyde and cyclohexanamine as a potent inhibitor of Escherichia coli
(E. coli) FabH with antimicrobial activity.12 Further optimization of
this compound is required to improve its antimicrobial activity.
Thiazolesand their derivativeshave attracted continuing interest
over the years because of their varied biological activities,13,14 re-
cently found application in drug development for the treatment of
allergies,15 hypertension,16 inflammation,17 schizophrenia,18 bacte-
rial,19 HIV infections,20 hypnotics,21 and more recently for the treat-
ment of pain.22 Besides, Kitagawa et al. find that thiazole derivatives
show strong FabI and FabK inhibitory activities with potent antibac-
terial activity.23
In view of the above mentioned findings, we report in the pres-
ent work the synthesis of three series of Schiff bases containing thi-
azole template. This combination was suggested in an attempt to
investigate the inhibitory activity against E. coli FabH. Biological
evaluation indicated that compounds 11 and 18 displayed not only
significant antibacterial activity against E. coli ATCC 35218, but also
favorable activity against other five bacterial strains, indicating
that they possessing broad-spectrum antibacterial activity. Be-
sides, further E. coli FabH inhibitory assay was undertaken and
the results suggested that compounds 11 and 18 were potent
E. coli FabH inhibitors. In addition, docking simulation were per-
formed to position compounds 11 and 18 into the E. coli FabH ac-
tive site to determine the probable binding conformation and the
results confirmed that the two compounds were potential inhibi-
tors of E. coli FabH.
The synthesis of compounds 3–29 followed the general path-
way outlined in Scheme 1. Nine aryl aldehydes reacted directly
with thiosemicarbazide, and the obtained thiosemicarbazones sub-
sequently reacted with suitable 2-halogenoketones to yield the 4-
substituted thiazole ring derivatives. In the synthesis of all of the
compounds, isopropyl alcohol proved to be the best solvent for
our purpose. As a matter of fact, the reaction products precipitate
upon cooling, and they can be filtered and purified by crystalliza-
tion from ethanol or ethanol/isopropanol. All of the synthetic com-
pounds gave satisfactory analytical and spectroscopic data, which
were in full accordance with their depicted structures.
All the compounds prepared were evaluated for their antibacte-
rial activities against three Gram positive bacterial strains (Bacillus
* Corresponding author. Tel./fax: +86 25 8359 2672.
E-mail address: zhuhl@nju.edu.cn (H.-L. Zhu).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.09.111

P.-C. Lv et al. / Bioorg. Med. Chem. Lett. 19 (2009) 6750–6754
6751
R2
R2
R3
R4
R1
R3
R4
R1
S
i
N
H
N
H
NH2
2
O
1
R5
R6
R2
ii
R3
R4
R1
N
N
S
N
H
3-29
Scheme 1. General synthesis of compounds 3–29. Reagents and conditions: (i) thiosemicarbazide, 2-propanol, acetic acid; (ii) 2-haloacetophenone, 2-propanol, rt.
subtilis ATCC 6633, Staphylococcus aureus ATCC 6538 and Strepto-
coccus faecalis ATCC 9790) and three Gram negative bacterial
strains (E. coli ATCC 35218, Pseudomonas aeruginosa ATCC 13525
and Enterobacter cloacae ATCC 13047) activities by MTT method,
and the results are shown in Table 1. Also included was the activity
of reference compounds Penicillin G and Kanamycin B. The results
revealed that most of the synthesized compounds showed signifi-
cant antibacterial activity.
As shown in Table 1, Compounds 10 (MIC = 1.56
(MIC = 0.78 g/mL) and 18 (MIC = 1.56 g/mL) exhibited potent
activity against E. coli ATCC 35218, which were superior to the po-
lg/mL), 11
l
l
sitive control kanamycin B (MIC = 3.125
lg/mL). Compounds 17
Table 1
MICs (minimum inhibitory concentrations) (
l
g/mL) of the synthetic compounds
Gram positive
Compounds
Microorganisms
Gram negative
B. subtilis ATCC 6633
S. aureus ATCC 6538
S. faecalis ATCC 9790
P. aeruginosa ATCC 13525
E. coli ATCC 35218
E. cloacae ATCC 13047
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
6.25
12.5
6.25
>50
25
>50
12.5
12.5
3.125
12.5
>50
12.5
6.25
12.5
6.25
25
>50
>50
12.5
25
>50
>50
25
0.78
1.562
3.125
25
25
12.5
25
6.25
6.25
6.25
12.5
12.5
25
6.25
25
25
25
25
25
25
12.5
12.5
25
12.5
12.5
25
12.5
6.25
6.25
1.56
0.78
50
>50
12.5
6.25
>50
12.5
12.5
3.125
25
>50
12.5
6.25
>50
12.5
6.25
>50
6.25
>50
25
>50
>50
12.5
>50
12.5
6.25
12.5
>50
25
12.5
>50
25
12.5
12.5
>50
>50
25
1.562
>50
>50
25
12.5
25
25
1.562
3.125
25
12.5
12.5
6.25
3.125
25
6.25
>50
25
25
25
12.5
12.5
3.125
1.56
6.25
6.25
25
12.5
>50
>50
25
12.5
12.5
25
25
6.25
3.125
25
25
12.5
6.25
>50
25
6.25
25
12.5
25
>50
>50
25
12.5
6.25
6.25
25
12.5
25
12.5
25
>50
12.5
25
25
>50
>50
>50
25
12.5
25
>50
12.5
25
12.5
1.562
0.39
27
28
29
>50
>50
1.562
1.562
>50
25
6.25
3.125
Penicillin G
Kanamycin B
3.125
1.562

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P.-C. Lv et al. / Bioorg. Med. Chem. Lett. 19 (2009) 6750–6754
showed significant activity with MIC values of 3.125
l
g/mL against
out to identify a plausible malonyl-binding mode.24 They found that
in one of the binding modes appeared in the lower scored conforma-
tions, the malonyl carboxylate formed hydrogen bonds to the back-
bone nitrogen of Phe304. Enlightened by this report, Kim and co-
workers designed pharmacophore maps considering the interaction
with Phe304 and performed receptor-oriented pharmacophore
based in silico screening of E. coli FabH.12 YKAs3003, a Schiff base
condensed by 4-hydroxy salicylaldehyde and cyclohexanamine,
was hit by pharmacophore map I consisting of three features (two
hydrogen bond donors that involving the backbone oxygen of
Phe304 and Gly209, respectively, and one hydrophobic interaction
with Ile156, Phe157 and Met207). The results of in silico screening
indicated that YKAs3003 is a potent inhibitor of E. coli FabH with
antibacterial activity.
Based on these facts, in this present work, compound 11 and 18
with the most potent antibacterial activity were hit by pharmaco-
phore map I mentioned above. The binding model of compounds
11, 18 and E. coli FabH are depicted in Figures 1 and 2, respectively.
In the binding model of compound 11 and E. coli FabH, amino
hydrogen of Asn247 forms hydrogen bond with phenolic hydroxyl
of compound 11. However, in the binding model of compound 18
and E. coli FabH, amino hydrogen of GLY306 forms hydrogen bond
with the sulfur atom of the thiazole ring of compound 18. These
results, along with the data of E. coli FabH inhibitory activity assay
E. coli ATCC 35218, which was comparable to the positive control
kanamycin B. Studies were performed by modification of the par-
ent compounds to determine how the substituents of the subunits
affected the antibacterial activities. The parent compounds can be
divided into two rings: A ring and B ring (Scheme 1 and Table 3).
Compounds 3–11 have two chlorine atoms on the 3-position
and 4-position of A ring. Among them, compounds 10 and 11 with
two halogen atoms on 3-position and 5-position of A ring dis-
played better antibacterial activity against E. coli ATCC 35218 than
that of compounds 3–9. In addition, compounds 12–20 have a
chlorine atom on the 4-position of B ring, among them, compounds
17 and 18 with a halogen atom on 5-position of A ring exhibited
better antibacterial activity against E. coli ATCC 35218 than that
of compounds 12–16, 19 and 20.
Among compounds 3–11, compound 3 displayed potent activity
with MIC value of 0.78
was superior to the positive control penicillin G. Compound 4
exhibited significant activity with MIC value of 1.562 g/mL
lg/mL against B. subtilis ATCC 6633, which
l
against B. subtilis ATCC 6633, which was comparable to the positive
control penicillin G. A comparison of the substitution on A ring
demonstrated that 4-position-halogen-substituted derivatives
(compounds 3–5) have more potent antibacterial activity against
B. subtilis ATCC 6633 than others. It can be concluded that
electron-withdrawing halogen groups on the 4-position of A ring
were conductive to the antibacterial activity. Meanwhile, the
derivatives (compounds 6 and 7) which have electron-donating
substituents (such as CH3, OCH3) on 4-position of A ring exhibited
less potent activity against B. subtilis ATCC 6633, and their MICs
Table 3
Chemical structures of compounds 3–29
R5
values were 25 lg/mL. In general, compounds 11 and 18 displayed
not only potent antibacterial activity against E. coli ATCC 35218,
but also favorable activity against other five bacterial strains, indi-
cating that they possessing broad-spectrum antibacterial activity.
The E. coli FabH inhibitory potency of the selected compounds 8–
11, 17–19 and 20 was examined and the results are summarized in
Table 2. As shown in Table 2, among the tested compounds, com-
pounds 11 and 18 showed potent inhibitory activity with IC50 of
R6
R2
R3
R1
N
3.6 and 6.8
moderate inhibitory activity with IC50 ranging from 11.7
78.4 M. It also can be seen from Table 2 that the selected com-
l
M, respectively. Other tested compounds displayed
N
R4
S
l
M to
N
H
l
pounds displayed low hemolytic activity. In addition, molecular
docking of compound 11, 18 and E. coli FabH were performed on
the binding model based on the E. coli FabH-CoA complex structure
(1HNJÁpdb).24 The FabH active site generally contains a catalytic
triad tunnel consisting of Cys-His-Asn, which is conserved in various
bacteria. This catalytic triad plays an important role in the regulation
of chain elongation and substrate binding. Since the alkyl chain of
CoA is broken by Cys of the catalytic triad of FabH, interactions
between Cys and substrate appear to play an important role in sub-
strate binding. Qiu et al. have refined three-dimensional structure of
E. coli FabH in the presence and absence of malonyl-CoA by X-ray
spectroscopy. Since malonyl moiety is degraded by E. coli FabH,
molecular docking studies for FabH and malonyl-CoA was carried
Compounds
R1
R2
R3
R4
R5
R6
3
4
5
6
7
H
H
H
H
H
H
H
H
H
H
H
Cl
Br
H
H
H
H
H
H
H
Cl
Br
H
H
H
H
H
H
H
Cl
Br
F
Cl
Br
CH3
OCH3
H
H
H
H
F
H
H
H
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
H
H
H
H
H
H
H
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
8
9
OH
OH
OH
OH
H
H
H
H
H
OH
OH
OH
OH
H
H
H
H
H
Cl
Br
Cl
Br
H
H
H
H
H
Cl
Br
Cl
Br
H
H
H
H
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Cl
Br
CH3
OCH3
H
H
H
H
F
Cl
Br
CH3
OCH3
H
H
H
Table 2
E. coli FabH inhibitory activity of the selected compounds
a
Compounds
E. coli FabH IC50
(lM)
Hemolysis LC30 (mg/mL)
8
9
39.6
43.4
16.5
3.6
11.7
6.8
>10
>10
>10
>10
>10
>10
>10
>10
10
11
17
18
19
20
OH
OH
OH
OH
Cl
Br
Cl
Br
57.6
78.4
H
a
Lytic concentration 30%.

P.-C. Lv et al. / Bioorg. Med. Chem. Lett. 19 (2009) 6750–6754
6753
Figure 1. Binding model of compound 11 and E. coli FabH. H-bonds are displayed as dashed lines. Amino hydrogen of Asn247 forms hydrogen bond with phenolic hydroxyl of
compound 11.
Figure 2. Binding model of compound 18 and E. coli FabH. H-bonds are displayed as dashed lines. Amino hydrogen of GLY306 forms hydrogen bond with the sulfur atom of
the thiazole ring of compound 18.
indicated that compounds 11 and 18 would be potential inhibitors
of E. coli FabH with potent antibacterial activity.
ginosa ATCC 13525, and E. cloacae ATCC 13047) activities by MTT
method. Data obtained indicated that compounds 11 and 18 dis-
played potent antibacterial activity against E. coli ATCC 35218. Fur-
ther E. coli FabH inhibitory activity assay suggested that they are
potent E. coli FabH inhibitors. Besides, docking simulation were
performed to position compound 11 and 18 into the E. coli FabH ac-
tive site to determine the probable binding conformation and the
In conclusion, three series of Schiff bases containing thiazole
template were synthesized and evaluated for their antibacterial
activity against three Gram positive bacterial strains (B. subtilis
ATCC 6633, S. aureus ATCC 6538 and S. faecalis ATCC 9790) and
three Gram negative bacterial strains (E. coli ATCC 35218, P. aeru-

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P.-C. Lv et al. / Bioorg. Med. Chem. Lett. 19 (2009) 6750–6754
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Acknowledgment
The work was financed by a Grant (Project 30772627) from
National Natural Science Foundation of China.
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