Synthesis and biological evaluation of novel sulfonyl-naphthalene-1,4-diols as FabH inhibitors

Article abstract of DOI:10.1016/j.bmcl.2008.10.097

A series of analogs of 2-tosylnaphthalene-1,4-diol were prepared and were found to be potent 10-20 nM reversible inhibitors of the Escherichia coli FabH enzyme. The inhibitors were also effective but to a lesser degree (30 nM-5 μM), against the Mycobacter

Full text of DOI:10.1016/j.bmcl.2008.10.097

Bioorganic & Medicinal Chemistry Letters 18 (2008) 6402–6405
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and biological evaluation of novel sulfonyl-naphthalene-1,4-diols
as FabH inhibitors
Mamoun M. Alhamadsheh a, Norman C. Waters b, Sarbjot Sachdeva a, Patricia Lee b, Kevin A. Reynolds a,
*
a Department of Chemistry, Portland State University, Portland, OR 97207, USA
b Division of Experimental Therapeutics, Walter Reed Army Institute of Research, Silver Spring, MD 20910, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of analogs of 2-tosylnaphthalene-1,4-diol were prepared and were found to be potent 10–20 nM
Received 25 August 2008
Revised 17 October 2008
Accepted 17 October 2008
Available online 25 October 2008
reversible inhibitors of the Escherichia coli FabH enzyme. The inhibitors were also effective but to a lesser
degree (30 nM–5 lM), against the Mycobacterium tuberculosis and Plasmodium falciparum FabH enzymes.
Preliminary SAR studies demonstrated that the sulfonyl group and naphthalene-1,4 diol were required
for activity against all enzymes but the toluene portion could be significantly altered and leads to either
modest increases or decreases in activity against the three enzymes. The in vitro activity of the analogs
against E. coli FabH parallel the in vivo activity against E. coli TolC strain and many of the compounds
were also shown to have antimalarial activity against P. falciparum.
Keywords:
FabH
Fatty acid biosynthesis
SAR
Ó 2008 Published by Elsevier Ltd.
Plasmodium falciparum
Inhibition
b-Ketoacyl-ACP-synthase III (FabH) is a key condensing enzyme
in bacterial fatty acid biosynthesis.1 This enzyme is part of the dis-
sociated fatty acid synthase (FAS). FabH is ubiquitous in both
Gram-negative and -positive bacteria, while it bears little homol-
ogy to the mammalian multifunctional type I FAS.2,3 The enzyme
is also present in Plasmodium falciparum4 and Mycobacterium tuber-
culosis5,6 and is considered a target for developing promising new
antibacterial, antiparasitic, and antimycobacterial agents.7,8
2-Tosylnaphthalene-1,4-diol (1) (Fig. 1) was identified as potent
inhibitor of P. falciparum FabH (PfFabH) through a virtual screen
using a pharmacophore developed based on thiolactomycin.9 Here,
we report our effort to further study 1 and develop more potent
sulfonyl-naphthalene-1,4-diols against various FabH enzymes. A
series of 14 compounds were prepared (compounds 1–14, Fig. 2).
Compounds 1–11 were prepared following a general synthetic
strategy as shown in Scheme 1. Nucleophilic addition of alkyl- or
arylsulfinic acids to a variety of 1,4-quinones resulted in the forma-
tion of the corresponding alkyl- or arylsulfonylhydroquinones. The
reaction was conducted using a two-phase dichloromethane-water
system in the presence of trifluoroacetic acid.10 The sulfinic acid
salts 15, 16, and 17 were not commercially available and were pre-
pared as shown in Scheme 2. Sodium 4-propylbenzenesulfinate 15
and sodium naphthalene-2-sulfinate 16 were prepared by reacting
the corresponding sulfonyl chloride with sodium sulfite and so-
dium bicarbonate in water,11 while sodium 2-carboxyethanesulfi-
nate 17 was prepared by oxidation of 3-mercaptopropanoic acid
using meta-chloroperoxybenzoic acid (mCPBA).12
The diaryl sulfone 12 was prepared by palladium-catalyzed
coupling of 2-bromonaphthalene with toluene sulfinic acid as
shown in Scheme 3.13 Oxidation of compound 1 using manganese
dioxide gave the 1,4-quinone 13 (Scheme 3). Nucleophilic addition
of 4-methylbenzenethiol to 1,4-naphthoquinone gave compound
14. Reduction of 14 using zinc dust/acetic acid gave the corre-
sponding 1,4-hydroquinone 18 that converted back to 14 by air
oxidation (Scheme 4). All analogs were purified and characterized
by 1H NMR, melting point, and high-resolution mass spectroscopy
(HRMS).
As shown in Table 1, 1 is an effective inhibitor of all three en-
zymes and is most potent against theEscherichia coli FabH (ecFabH)
(IC50 of 13 nM). It was least effective against the M. tuberculosis
FabH (mtFabH). The analogs 2–14 were then synthesized and eval-
uated against all three enzymes to establish a preliminary SAR.
These efforts focused primarily on modifying either the toluene
OH
O
S
O
HO
1
* Corresponding author. Tel.: +1 503 725 3886; fax: +1 503 725 9525.
E-mail address: reynoldsk@pdx.edu (K.A. Reynolds).
Figure 1. FabH inhibitor 1.
0960-894X/$ - see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2008.10.097

M. M. Alhamadsheh et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6402–6405
6403
OH
OH
OH
OH
OH
OH
O
O
S
O
S
O
S
O
S
H3C
HO
OH
OH
O
O
OH
O
O
1
3
2
4
O
OH
OH
OH
OH
HN
Cl
O
O
O
O
S
S
S
S
O
OH
O
OH
O
OH
OH
O
8
7
6
5
9
O
OH
OH
OH
OH
HN
O
O
S
O
S
O
S
S
O
O
OH
O
12
10
O
11
O
O
O
O
S
S
O
O
13
14
Figure 2. FabH inhibitors 1–14.
or naphthalene portions of the molecule, or changing the oxidation
state of the sulfur and 1,4-diol. The activity of each of these analogs
with each enzyme is shown in Table 1. The toluene portion of 1
was very tolerant to modifications. Replacing the toluene ring with
a methyl (2) or carboxy propyl (3) groups resulted in ꢀ7-fold de-
crease in activity against ecFabH and ꢀ5-fold decrease in activity
against mtFabH. Interestingly, both compounds 2 and 3 were
slightly more active against pfFabH than the parent compounds 1
(2 was ꢀ3 times more active against pfFabH than 1). Replacing
the toluene ring with a phenyl group (4) resulted in a decrease in
activity against the three enzymes. Introducing a hydrophilic acet-
amide group (5) in place of the methyl group of 1 gave comparable
activity against ecFabH and ꢀ2.5 improvement in activity against
pfFabH. A slight improvement in activity against ecFabH was ob-
served when the methyl group of 1 was replaced by more hydro-
phobic groups such as chlolro and propyl (compounds 6 and 7,
respectively). However, these changes did not improve the activity
against pfFabH or mtFabH. Increasing the size of the toluene por-
tion of 1 by an extra phenyl group (8) resulted in no significant
change in activity against ecFabH and a ꢀ5 fold decrease in activity
against pfFabH.
The naphthalene-1,4-diol portion of 1 was important for activ-
ity against all three FabH enzymes. Extending the size of the naph-
thalene-1,4-diol with an extra phenyl group (9) resulted in much
lower activity against the three FabH enzymes. Decreasing the size
of the naphthalene-1,4-diol by replacing it with a hydroquinone
moiety (10 and 11) resulted in a major loss of activity against both
O
OH
O
R
S
O
a
O
O
HO
O
S
S
S
Cl
O
O- Na+
O
O
15
OH
OH
O
S
a
R
O
a
b
O
R
R
O
S
O
S
O
HO
O- Na+
Cl
O- Na+
O
O
16
17
O
S
HOOC
O
S
HOOC
SH
O
O- Na+
HO
Scheme 2. Reagents and condition: (a) Na2SO3, NaHCO3, H2O; (b) mCPBA, CH2Cl2,
À30 °C.
Scheme 1. Reagents: (a) TFA, CH2Cl2, H2O.

6404
M. M. Alhamadsheh et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6402–6405
a
O
O
+
S
S
Br
O- Na+
O
12
O
O
OH
b
O
S
O
S
O
O
OH
13
1
Scheme 3. Reagents and condition: (a) Pd2(dba)3, Xantphos, Cs2CO3, nBu4NCl, Toluene, 80 °C; (b) MnO2, acetonitrile.
Table 2
O
O
O
In vitro activity of 1 and selected analogs against ecFabH and E. coli tolC mutant.
Compound
ecFabHIC50 (nM)
E. coli tolC mutant (MIC99) (lg/ml)
a
1
2
5
10
11
12
13. 6 2.3
87.3 6.4
11.8 1.3
14.1
9.7
12.3
>90
>90
>90
+
S
SH
O
14
>10
>10
>10
lM
lM
lM
Air Oxidation
b
MIC99, minimum concentration at which no visible growth was detected.
OH
changing the toluene part of 1 on the activity against mtFabH.
The naphthalene-1,4-diol portion of 1 is important for activity
and cannot be modified, as increasing its size reduced the activity,
while decreasing its size resulted in major loss of activity. Chang-
ing the oxidation state of the naphthalene-1,4-diol moiety resulted
also in major loss of activity. Interestingly, the major loss of activity
observed by changes in the naphthalene-1,4-diol was universal
among the three FabH enzymes. Finally, the loss of activity ob-
served by changing the sulfone group (compound 14) suggests a
role of this group in critical interaction with the FabH enzymes.
All FabH enzymes catalyze a Claisen-type condensation reaction
between malonyl-ACP (MACP) and an acyl–enzyme intermediate
formed by an initial transacylation from a short-chain acyl-CoA
primer with a conserved active site cysteine.1 We have recently
shown that compounds that covalently bind to these active site
cysteines are potent inhibitors of FabH enzymes.14,15 A reversibility
study, as previously reported,16 was carried out to determine the
mode of binding of 1 to FabH. Compound 1 and ecFabH were incu-
bated under conditions that resulted in greater than 95% enzyme
inhibition. Dialysis of the inhibited enzyme for 16 h in phosphate
buffer (at 4 °C) resulted in a ꢀ90% restoration of its activity. These
data demonstrate that compound 1 is a reversible inhibitor of ec-
FabH and that there is likely no covalent interaction with the ec-
FabH active site cysteine (Cys112). We have shown previously
that no restoration of activity was observed when these experi-
ments were carried out with irreversible inhibitors that form cova-
lent adducts with the active site cysteine.14,15
S
OH
18
Scheme 4. Reagents and conditions: (a) CH2Cl2; (b) Zinc dust, acetic acid.
ecFabH and pfFabH and ꢀ50-fold decrease in activity against
mtFabH. The 1,4-diol of 1 was crucial for activity. Replacing the
naphthalene-1,4-diol with a simple naphthalene group (12) re-
sulted in a drastic loss of activity against the three FabH enzymes.
The quinone 13 (generated through oxidation of 1) was also much
less active against FabH, as compared to 1. The sulfone moiety is
also crucial for activity as compound 14 did not have significant
activity against FabH.
These SAR data shows that toluene portion of 1 is tolerant to
modifications. A hydrophobic group with a limited size is impor-
tant for activity against ecFabH. On the other hand, pfFabH has is
slightly more sensitive to analogs of 1 in which the toluene is re-
placed with a smaller (compound 2) or more hydrophilic (com-
pounds 3 and 5) groups. There was no significant effect in
Table 1
In vitro activity of 1–14 against three FabH enzymes.
Compound
ecFabH IC50 (nM)
pfFabH IC50 (nM)
mtFabH IC50 (nM)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
13.6 2.3
87.3 6.4
96.5 5.6
21.2 1.9
11.8 1.3
8.8 2.6
9.7 1.0
15.4 1.32
112.4 7.6
NA
73.6
27.45
46.5
278.7
29.6
128.7
72.6
393
0.61 0.16
3.53 0.12
3.53 0.11
2.84 0.17
3.76 0.68
3.61 0.94
3.09 0.66
1.49 0.12
8.47 1.63
34.4 2.85
31.4 2.7
The activity of 1 and some of its analogs against E. coli tolC mu-
tant was evaluated (Table 2). Interestingly, there is a clear correla-
Table 3
IC50 of 1 and selected analogs against P. falciparum D6.
Compound
P. falciparum D6 IC50 (lg/ml)
341.1
NA*
1
5
6
10
11
12
7.69
4.98
3.23
31.35
3.96
8.74
NA
NA
187.2 9.3
NA
NA*
NA*
138.1 6.4
1.99 0.21
112 4.3
1425
NA*
NA, not active below 10 l lM.
M; NA*, not active below 100

M. M. Alhamadsheh et al. / Bioorg. Med. Chem. Lett. 18 (2008) 6402–6405
6405
tion between the growth inhibition of this strain and the observed
References and notes
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get of these compounds in E. coli. However, an E. coli strain,
carrying an ecFabH expression plasmid, displayed a similar MIC va-
lue for 1 compared to the same E. coli strain lacking the ecFabH
plasmid (data not shown). The absence of increase in resistance
to 1 when the ecFabH was overexpressed in E. coli suggests that ec-
FabH might not be the sole target for this class of inhibitors.
Most compounds showed in vitro antimalarial activity against
the chloroquine sensitive D6 strain of P. falciparum (IC50 between
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2008.10.097.