Synthesis and Antitubercular Activity of New Benzo[b]thiophenes

Article abstract of DOI:10.1021/acsmedchemlett.6b00077

In vitro and ex vivo efficacies of four series of benzo[b]thiophene-2-carboxylic acid derivatives were studied against Mycobacterium tuberculosis H37Ra (MTB). Benzo[b]thiophenes were also tested in vitro against multidrug resistant Mycobacterium tuberculosis H37Ra (MDR-MTB), and 7b was found to be highly active against A- and D-MDR-MTB/MTB (MIC ranges 2.73-22.86 μg/mL). The activity of all benzo[b]thiophenes against M. bovis BCG (BCG) was also assessed grown under aerobic and under conditions of oxygen depletion. Compounds 8c and 8g showed significant activity with MICs of 0.60 and 0.61 μg/mL against dormant BCG. The low cytotoxicity and high selectivity index data against human cancer cell lines, HeLa, Panc-1, and THP-1 indicate the potential importance of the development of benzo[b]thiophene-based 1,3-diketones and flavones as lead candidates to treat mycobacterial infections. Molecular docking studies into the active site of DprE1 (Decaprenylphosphoryl-β-d-ribose-2′-epimerase) enzyme revealed a similar binding mode to native ligand in the crystal structure thereby helping to understand the ligand-protein interactions and establish a structural basis for inhibition of MTB. In summary, its good activity in in vitro and ex vivo model, as well as its activity against multidrug-resistant M. tuberculosis H37Ra in a potentially latent state, makes 7b an attractive drug candidate for the therapy of tuberculosis.

Full text of DOI:10.1021/acsmedchemlett.6b00077

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Letter
Synthesis and Anti-tubercular Activity of New Benzo[b]thiophenes
Pravin Sudhakar Mahajan, Mukesh Dattatray Nikam, Laxman U Nawale,
Vijay M. Khedkar, Dhiman Sarkar, and Charansingh Harnamsingh Gill
ACS Med. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acsmedchemlett.6b00077 • Publication Date (Web): 28 Jun 2016
Downloaded from http://pubs.acs.org on June 29, 2016
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ACS Medicinal Chemistry Letters
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Synthesis and Anti-tubercular Activity of New Benzo[b]thiophenes
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Pravin S. Mahajan†, Mukesh D. Nikam†, Laxman U. Nawale‡, Vijay M. Khedkar‡, Dhiman Sarkar‡,
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Charansingh H. Gill*,†
†Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad 431 004, Maharashtra, India
‡CombiꢀChem Resource Centre, CSIRꢀNational Chemical Laboratory, Pune 411008, Maharashtra, India
KEYWORDS. Benzo[b]thiophene, MDRꢀMTB, M. Bovis BCG, Cytotoxicity, Molecular docking.
ABSTRACT: In vitro and ex vivo efficacies of four series of benzo[b]thiopheneꢀ2ꢀcarboxylic acid derivatives were studied against
Mycobacterium tuberculosis H37Ra (MTB). Benzo[b]thiophenes were also tested in vitro against multiꢀdrug resistant Mycobacteꢀ
rium tuberculosis H37Ra (MDRꢀMTB) and 7b was found to be highly active against Aꢀand DꢀMDRꢀMTB / MTB (MIC ranges
2.73ꢀ22.86 µg/ml). The activity of all Benzo[b]thiophenes against M. bovis BCG (BCG) was also assessed grown under aerobic and
under conditions of oxygen depletion. Compounds 8c and 8g showed significant activity with MICs 0.60, and 0.61 ꢁg/mL against
dormant BCG. The low cytotoxicity and high selectivity index data against human cancer cell lines, HeLa, Pancꢀ1, and THPꢀ1 indiꢀ
cate that the potential importance of the development of benzo[b]thiopheneꢀbased 1,3ꢀdiketones and flavones as lead candidates to
treat mycobacterial infections. Molecular docking studies into the active site of DprE1 (DecaprenylphosphorylꢀβꢀDꢀriboseꢀ2'ꢀ
epimerase) enzyme revealed a similar binding mode to native ligand in the crystal structure thereby helping to understand the ligꢀ
andꢀprotein interactions and establish a structural basis for inhibition of MTB. In summary, its good activity in in vitro and ex vivo
model, as well as its activity against multidrugꢀresistant M. tuberculosis H37Ra in a potentially latent state, makes 7b an attractive
drug
candidate
for
the
therapy
of
tuberculosis.
Among infectious widespread diseases, tuberculosis
sent agents, there is still an urgent need for the development of
new ideal antiꢀTB agents with low toxicity, and are active to
treat against MDR and XDR bacteria and latent diseases.
Based on these facts, efforts have been continued to
discover new and effective chemotherapeutic agents for the
tuberculosis treatment. We recently reported the antiꢀ
tubercular activity of several new molecules with good miniꢀ
mum inhibitory concentrations (MICs),6ꢀ11 promoted us to
synthesize new compounds.
(TB) remains an active and major health problem worldwide.
TB is deadliest communicable disease, caused by the global
emergence of multi drug resistant (MDR) and extensively drug
resistant (XDR) strains of TB along with acquired, primary
and cross resistance mycobacterium TB (MTB) strains. Despite
the availability of first line and second line drugs regimen to
treat the disease, still unacceptably TB continues to have high
mortality and a global health threat. Resistance to first line
antiꢀTB drugs has been linked to spontaneous mutations in
many genes.1 The probability of resistance arising when riꢀ
fampicin and isoniazid are used in combination is only 1 in
1014, sufficiently low to prevent resistance for either drug.2 As
per WHO, the direct observed therapy strategy (DOTS) is the
most effective means to prevent the emergence of drug reꢀ
sistance.3 With the HIV, several other interꢀrelated factors like
poverty, mobility of people from the countries where TB is
prevalent, the long and complex TB regimen, and poor manꢀ
agement of TB control programs4 posing the resurgence of
TB.
Benzo[b]thiophene moiety is a drugꢀlike scaffold
known to possess potential medicinal value in FDA approved
drugs such as raloxifene, sertaconazole, zileuton and benoꢀ
cyclidine. Therefore, in continuation of our program on the
discovery of antiꢀtubercular compounds, various 1,3ꢀ
diketones, flavones, pyrazoles, and carboxamides were synꢀ
thesized from benzo[b]thiophene carboxylic acid and demonꢀ
strated inhibitory activity against MTB H37Ra and M. bovis
BCG. The cytotoxicity of compounds has been also tested.
In an effort to elucidate the possible mechanism by
which the title compounds can induce antiꢀtubercular activity,
molecular docking studies were performed to visualize the
binding mode of the drug candidate at the molecular level. In
the absence of resources to perform the targetꢀbased assays
experimentally, the inꢀsilico approach of molecular docking
has proved to be a very important tool for identifying the tarꢀ
gets for different ligands and their associated thermodynamic
interꢀmolecular interactions with the target enzyme governing
the inhibition of the pathogen. Molecular docking studies into
Mycobacterium Bovis Bacillus Calmette Guerin
(BCG), an attenuated strain closely related to MTB.5 M. bovis
infections in humans have been reported from many centuries
ago. M. bovis infections have reꢀemerged and are causing TB
in humans, due to mutations during the long in vitro propagaꢀ
tion of this strain and in particularly those who are HIVꢀ
positive. Owing to the appearance of drug resistance surveilꢀ
lance, the significant side effects and drug interactions of preꢀ
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the active site of (DecaprenylphosphorylꢀβꢀDꢀriboseꢀ2'ꢀ Information) results showed that four compounds 7a, 7b, 7d,
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7
8
epimerase) DprE1 enzyme revealed a similar binding mode to
native ligand in the crystal structure thereby helping to underꢀ
stand the ligandꢀprotein interactions and establish a structural
basis for inhibition of MTB.
The synthetic sequences of benzo[b]thiophene derivꢀ
atives are illustrated in Scheme 1. Commercially available 2ꢀ
and 7f in particular exhibit >92% inhibitory activity against
MTB H37Ra and nine compounds 7a-d, 7f, 7g, and 9b-d exꢀ
hibit >97% against M. bovis BCG strain. Compounds 7a-f, 8c
and 8g have been selected for further evaluation to dose reꢀ
sponse by both the IC50 (Supporting information) and MIC90
(minimum concentration bringing 90% inhibition) values preꢀ
sented in Table 1.
fluorobenzaldehyde
1
was reacted with methyl 2ꢀ
mercaptoacetate 2 in N,NꢀDimethyl formamide (DMF) under
basic (K2CO3) condition followed by base (LiOH) hydrolysis
afforded benzo[b]thiopheneꢀ2ꢀcarboxylic acid 4. Further, the
reaction between compound 4 and appropriate oꢀhydroxy aceꢀ
tophenones 5a-g in pyridine and POCl3 provided intermediate
esters 6a-g. In the next step, these intermediate undergoes
BakerꢀVenkataraman rearrangement (BVR) under ultrasoniꢀ
The 1,3ꢀdiketone 7d with no any substitution appear
to provide strong activity against dormant MTB H37Ra (MIC
= 2.05 µg/mL). Compounds 7a and 7b having bromo substituꢀ
ent showed good activity against active as well as dormant
MTB H37Ra with MIC value of 2.87 µg/mL and 2.63 µg/mL
respectively. However, it was observed that the compounds
are more susceptible to M. bovis BCG than MTB H37Ra, proꢀ
vided significantly enhanced biological activities against these
mycobacteria. Compounds 7g, 8g, 8c, 7f, 7d and 7a exhibited
notable inhibitory activity, having MIC values in the 0.56ꢀ1.90
µg/mL range against active M. bovis BCG, and compounds
8g, 8c, 7g, 7a, 7d, 7b and 7c with 0.60ꢀ1.37 µg/mL range
showed excellent activity against dormant M. bovis BCG. We
observed that compound with methyl (7c, 7f and 8c) or both
methyl and chloro groups (7g and 8g) are more effective than
other derivatives. Importantly, the flavone 8g exhibited supeꢀ
rior activity against both active (MIC = 0.56 µg/mL) and
dormant (MIC = 0.60 µg/mL) M. bovis BCG. In other derivaꢀ
tives, the flavone 8c with MIC value of 0.62 µg/mL, and 0.61
µg/mL against active, and dormant M. bovis BCG, respectiveꢀ
ly, and 1,3ꢀdiketone 7g with MIC value of 0.71 µg/mL, and
0.90 µg/mL against active, and dormant M. bovis BCG, reꢀ
spectively also exhibited excellent antiꢀTB activity. Comꢀ
pound with hydrogen (7d), one chloro group (7b) and two
chloro groups (7e) showed significant reduction of M. bovis
BCG growth. Replacement of the chloro group by bromo
group (7a) offer an advantage in the inhibition of M. bovis
BCG growth, resulted in a dramatic enhancement of activity.
The activity of benzo[b]thiopheneꢀ2ꢀcarboxylic acid
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cation,
afforded
1ꢀ(benzo[b]thiophenꢀ2ꢀyl)ꢀ3ꢀ(2ꢀ
hydroxyphenyl)propaneꢀ1,3ꢀdiones 7a-g.
Scheme 1. Synthesis of benzo[b]thiophene derivativesa
derivatives against multidrugꢀresistant H37Ra was determined
by a “microdilution” 96ꢀwell plate assay.13,14 On every microꢀ
titer plate containing one MDR MTB H37Ra strain, a dilution
series of RIF and INH was included as a positive control comꢀ
pounds. The results for all the compounds 7a-f, 8c and 8g at
concentrations of 30ꢀ0.001 µg/mL (one third dilutions) were
scored by using an established XTT Reduction Menadione
assay (XRMA),12 and the results are presented in Table 1. The
results were also cross checked by taking a final spectrophoꢀ
tometric reading of the microtiter plates at 600 nm. The most
active compounds 7a-f, 8c and 8g were highly active against
multidrugꢀresistant M. tuberculosis H37Ra (MICs range, 2.73
aReagents and conditions: (i) K2CO3, DMF. (ii) LiOH,
THF/H2O. (iii) Pyridine, POCl3, RT. (iv) KOH. Pyridine, ))))).
(v) PEGꢀ400, Cat. H2SO4, ))))). (vi) NH2NH2.H2O, ))))). (vii)
EDC.HCl, HOBt, DMF, ))))).
The intramolecular cyclization of 1,3ꢀdiketones 7a-g
yielded novel 2ꢀ(benzo[b]thiophenꢀ2ꢀyl)ꢀ4Hꢀchromenꢀ4ꢀones
8a-g in good yields. The formation of the chromenꢀ4ꢀones was
accomplished through a different protocol, PEGꢀ400 as a
green solvent with a nonꢀconventional approach, ultrasound
irradiation was successfully utilized. Moreover, 2ꢀ(5ꢀ
(benzo[b]thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrazolꢀ3ꢀyl)phenols 9a-g synꢀ
thesis proceeds via a condensation reaction between substitutꢀ
to 22.86
µg/ml against active and dormant stages). Comꢀ
pounds 7b having one bromo and one chloro substituent was
highly active against multidrugꢀresistant MTB H37Ra (MIC =
2.73 µg/ml against dormant stage). In the same experiment,
ed
1ꢀ(benzo[b]thiophenꢀ2ꢀyl)ꢀ3ꢀ(2ꢀhydroxyphenyl)propaneꢀ
the activity of compounds was compared to those of clinically
available compounds RIF, INH and were found to be slightly
better than that of RIF and INH.
These synthetic compounds 7a-f, 8c and 8g were
further tested to determine their MIC against mycobacteria
within THPꢀ1 host macrophages (Table 1). Ex vivo studies
against MTB revealed the strong antiꢀTB activity of benꢀ
zo[b]thiopheneꢀ2ꢀcarboxylic acid derivatives (7a-f, 8c and 8g).
1,3ꢀdiones 7a-g and hydrazine hydrate in ethanol under ultraꢀ
sound irradiation. Further, the condensation reaction between
compound 4 and anilines 10a-i using EDC.HCl and HOBt as
coupling reagents under ultrasound irradiation within 1ꢀ2 hrs
afforded carboxamides 11a-i. The structures of the compounds
were chemically characterized by Thin Layer Chromatography
(TLC), spectroscopic data and elemental analyses.
All 31 synthesized compounds were tested for their
in vitro antiꢀTB activity.9,12 The primary screen (Supporting
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Table 1. Anti-tubercular Activity of Compounds 7a-b, 7d, 7f, 8c and 8g
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7
8
Ex vivo
In vitro
Drug Resistant
MTB H37Ra
H37Ra
MTB H37Ra
M. bovis BCG
Compd
cLogPa
Active Dormant
Active
MIC90
8.04
5.3
Dormant
MIC90
7.11
2.73
2.87
9.42
7.22
9.1
Active
MIC90
2.87
Dormant Active Dormant
MIC90
4.94
2.83
2.96
25.84
8.39
7.67
0.5
MIC90
3.91
0.91
1.82
2.61
2.76
2.18
0.8
MIC90
2.63
2.79
MIC90
1.16
MIC90
1.77
7a
7b
5.12
4.97
4.13
4.62
5.23
5.94
ꢀ
3.29
2.22
2.7
9
7d
12.41
8.97
22.86
9.58
>10
25.17
5.45
28.87
25.36
0.048
0.074
2.05
2.34
1.36
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7f
8c
4.41
1.68
1.61
29.98
24.34
0.043
0.075
0.62
0.61
8g
0.56
0.60
RP
INH
>10
>10
0.004
0.075
0.0042
0.0078
0.05
0.075
>10
ꢀ
acLogP calculated using ChemDraw Ultra 12.0 software by Cambridge Soft.
Amongst derivatives, 7b was found to be highly effective and
inhibited both active and dormant mycobacteria with MIC
ranged from 0.91ꢀ2.83 µg/ml, corroborating its antimicrobial
nature. However, the activity is not as profound as that of RIF
and INH. The pattern was similar in ex vivo THPꢀ1 infection
model assay with MDR MTB H37Ra (in vitro) with comꢀ
pounds having highest efficiency to inhibit MTB.
(Supporting Information).According to drug susceptibility
study of TB, compounds that exhibited SI values >10 in all
three cell lines were considered nontoxic.20 Simultaneous deꢀ
tection of active and dormant stage inhibitors against tubercuꢀ
lar bacilli, XRMA assay protocol was used, which follows
about similar principle of hypoxia model of dormancy.21
When comparing the toxicity exhibited by these poꢀ
tent antiꢀtubercular 1,3ꢀdiketones and flavones, one compound
8g exhibited comparatively higher selectivity index (SI >30) at
active and dormant state of THPꢀ1 cell line of both MTB
H37Ra and M. bovis BCG. Compounds 7f (>13), and 8g (>35)
against both active and dormant HeLa cell line of M. bovis
BCG possesses superior selectivity index. Importantly, 1,3ꢀ
diketone derivative 8c possessed a more favorable selectivity
index (SI >20) against all active and dormant three cell lines
of M. bovis BCG. In addition, compound 7a (>10) against
active and dormant Pancꢀ1 cell line, compounds 7c and 7e
(>10) against dormant Pancꢀ1 cell line, compounds 7g and 8c
(>22) against active and dormant state of THPꢀ1 cell line exꢀ
hibited better selectivity index for M. bovis BCG. These reꢀ
sults demonstrated that compounds 8c and 8g have been found
to be potential against MTB H37Ra and M. bovis BCG.
Mycobacteria can induce ROS (reactive oxygen speꢀ
The cytotoxicity is important to find out potent antiꢀ
biotics, and to get insight into potential toxicity of identified
inhibitor molecules. The cytotoxicity of most potent antiꢀTB
agents 7a-g, 8c, and 8g was further assessed against a panel of
three human cancer cell lines HeLa (human epithelial cervical
cancer), Pancꢀ1 (pancreas carcinoma) and THPꢀ1 (acute monꢀ
ocytic leukemia) using modified MTT cell viability assay.15ꢀ17
The cytotoxic effect of these compounds was checked to deꢀ
termine the Growth Inhibition (GI), GI50 (Supporting Inforꢀ
mation) and GI90, and paclitaxel was used as positive control
(Table 2).
Table 2. Cytotoxicity of Compounds 7a-g, 8c, and 8g
HeLa
GI50 GI90
6.75 20.28 18.33 >30
Pancꢀ1
GI50 GI90
THPꢀ1
Compd
GI50
>100
7.48
GI90
>100
>30
7a
7b
7c
7d
7e
7f
7g
8c
8g
cies) production by activating phagocytes.22 Although, these
are an important part of the host defence against mycobacteria,
enhanced ROS generation may promote tissue injury and inꢀ
flammation, which may further contributes to immunosupꢀ
pression,23 particularly, in HIV infected patients.24 Therefore,
the antioxidant activity of the antiꢀtubercular compounds was
assessed by DPPH (1,1ꢀdiphenylꢀ2ꢀpicrylꢀhydrazil) radical
scavenging method25,26 (Supporting Information). The interacꢀ
tion of all tested compounds with the stable free radical DPPH
indicates their radical scavenging activity by EC50 (Effective
Concentration). These results indicate that the antioxidant
activity of the 1,3ꢀdiketone and flavone derivatives could not
simply be attributed to their antiꢀtubercular activity. The 1,3ꢀ
diketone 7f (EC50 = 20.00 µg/mL) was the best synthetic antiꢀ
oxidant as compared to standard antioxidant drug BHT.
Promising antiꢀtubercular activities demonstrated by
4.42
7.12
7.86
8.24
>30 15.83 >30
>30 7.77 >30
8.44 26.03
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
5.81
>100 >100 26.22 >30
23.11 >30 9.24 >30
>100 >100 20.36 >30
15.55 27.8 12.87 29.75
21.22 >30
>100 >100
aPaclitaxel 0.004 0.075 0.127 5.715 0.133
aStandard anticancer drug and also positive control.
Except 7a, 7b and 8c, all compounds showed GI90
values >30 ꢁg/mL in HeLa cell line. In case of Pancꢀ1 cell
line, except 7b and 8c, all other compounds showed GI90 valꢀ
ues >30 ꢁg/mL. Notably, all compounds showed cytotoxic
GI90 >30 ꢁg/mL on THPꢀ1 cell line. Overall, all the tested
compounds had low cytotoxic effect on these three human cell
lines. The ratios between IC50 for human cancer cell lines (cyꢀ
totoxicity) and MIC (antimycobacterial activity) in vitro
against both active and dormant MTB H37Ra and M. bovis
BCG.18,19 enabled the determination of selectivity index (SI)
benzo[b]thiopheneꢀ2ꢀcarboxylic acid derivatives motivated us
to perform molecular docking studies to identify a potential
target for them, and thereby gaining an insight into the key
molecular mechanisms in exerting inhibitory activity against
MTB.
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After scanning through the crystal structures available for myꢀ rivatives snuggly fitted into the binding pocket making close
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cobacterium targets in the Protein Data Bank (PDB), we obꢀ
tained a fairly good agreement between experimental antiꢀ
tubercular data and the docking results against DprE1 enzyme.
contacts with the surrounding residues.
Their docking score varied from ꢀ9.198 for the most
active analogue 7a to ꢀ6.995 for the least active 7e while the
docking score for the native ligand was found to be ꢀ7.953.
This trend of docking scores corroborated well with the obꢀ
served antiꢀtubercular activity where the active compounds
showed higher scores while those with relatively low inhibiꢀ
tion were also predicted to have a lower docking score. Quanꢀ
titative measures of the docking scores along with binding
energy and nonꢀcovalent interactions observed for these derivꢀ
atives are summarized in the Table 4 (Supporting Inforꢀ
mation). A detailed perꢀresidue interaction analysis between
the protein and the most active analogue 7a only is elucidated
in the next section for the sake of brevity through which we
can speculate regarding the binding patterns in the cavity.
The binding mode of 7a is presented in Figure 2.
It is
a key enzyme involved in the biosynthesis of
decaprenylphosphorylꢀDꢀarabinose (DPA) which is the only
known donor of Dꢀarabinofuranosyl residues for the synthesis
of arabinogalactan, a basic precursor for the mycobacterial cell
wall core.27,28 Therefore, DprE1 is also essential for the cell
growth and survival making it a potential target for antimycoꢀ
bacterial drug design strategy.29ꢀ32 Molecular docking study
was performed using the standard protocol implemented in the
GLIDE (Gridꢀbased Ligand Docking with Energetics) module
of the Schrodinger Molecular modeling package (Schrodinger,
LLC, New York, NY, 2015).33, 34
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Though it showed multiple interactions with the residues in
the active site, however for visibility and clarity only selected
interacting residue are exhibited. Analysis of the docked comꢀ
plex showed very strong binding affinity (ꢀ50.095 kcal/mol)
with the receptor wherein the contribution of the van der
Waals interactions (ꢀ45.951kcal/mol) was found to be more
than the coulombic interactions (ꢀ4.144kcal/mole). The comꢀ
pound was found to be stabilized within the active site through
an extensive chain of favorable van der Waals interactions
observed with Cys387(ꢀ3.463), Asn385(ꢀ1.651), Phe369(ꢀ
1.374), Lys367(ꢀ2.704), Phe366(ꢀ1.268), Val365(ꢀ4.669),
Leu363(ꢀ2.049), Gln336(ꢀ1.763), Gln334(ꢀ1.44), Arg325(ꢀ
10.823), Asn324(ꢀ1.039), Phe320(ꢀ2.084), Leu317(ꢀ2.551),
Trp230(ꢀ1.012), His132(ꢀ3.631), Ile131(ꢀ1.393), Gly117(ꢀ
1.577), Pro116(ꢀ2.175), Tyr60(ꢀ1.545). Analysis of polar conꢀ
tacts such as electrostatic and piꢀpi stacking interactions disꢀ
played that 7a formed multiple closed interactions with
Lys418(ꢀ2.185), Tyr415(ꢀ1.565), Arg413(ꢀ9.938), Lys367(ꢀ
1.418), Lys134(ꢀ1.73), Arg119(ꢀ1.872), Gly117(ꢀ1.463),
Pro116(ꢀ1.49), Arg58(ꢀ1.037) residues of DprE1. Strong bindꢀ
Figure 1. Overlay of the crystallographically observed binding
mode of the native ligand on its best docked conformation.
In order to rationalize the observed antiꢀtubercular results and
to get more insight into the inhibition pattern, interactions of
benzo[b]thiopheneꢀ2ꢀcarboxylic acid derivatives with the
binding pocket of mycobacterial (DprE1) were analyzed and
depicted using molecular docking studies.28 Visual inspection
of the minimum energy docked poses revealed that these deꢀ
ing
of
7a
with
active
site
of
DprE1
is also contributed by its position in the piꢀinteraction in space
of His 132 and through a significant hydrogen bonding interꢀ
actions with Lys418 and Gly117. Although all benꢀ
zo[b]thiopheneꢀ2ꢀcarboxylic acid derivatives were observed to
show similar binding pattern, the presence of relatively
stronger nonꢀcovalent interactions in terms of van der Waals,
electrostatic and piꢀpi interactions and the higher number of
intermolecular hydrogen bonds augments the stronger binding
of the compound 7a to the active site of DprE1.
In summary, our work demonstrates benꢀ
zo[b]thiophenes as a new family of inhibitors with potent anꢀ
timycobacterial properties. The most promising compounds in
the present series, 8c and 8g showed excellent activity and
inhibited the growth of both active and dormant M. bovis BCG
with MIC90 of 0.56ꢀ0.62 µg/mL range. Molecular docking
study of these benzo[b]thiopheneꢀ2ꢀcarboxylic acid derivaꢀ
tives have shown a high binding affinity towards the active
site of DprE1 enzyme which provides a strong platform for
development of the lead molecules for this series forming poꢀ
tent antiꢀtubercular agents. Overall, our ex vivo infection modꢀ
el indicate that there is significant potential for 7b for the
treatment of tuberculosis. Meanwhile, diverse structural modiꢀ
Figure 2. Binding mode of the most active compound 7a into
active site of DprE1.
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BCG for inhibitors against dormant and active tubercle bacilꢀ
li. J. Micro. Methods 2008, 73, 62ꢀ68.
fications are currently being investigated through iterative
synthesis in conjugation with computer modeling and the reꢀ
sults will be given in due course.
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ASSOCIATED CONTENT
Supporting Information
Experimental procedures for the synthesis of 4, 7a-g, 8a-g, 9a-g,
1
and 11a-i, representative H NMR, 13C NMR, FTꢀIR and HRMS
spectra for final compounds, and details of in vitro, and ex vivo
assay. The Supporting Information is available free of charge on
the ACS Publications website.
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AUTHOR INFORMATION
Corresponding Author
*Tel No: (+91) 240 2403311. Fax: (+91) 240 2400491. Eꢀmail:
chgill16@gmail.com (C. H. Gill).
(14) Ashtekar, D. R.; CostaꢀPerira, R.; Nagrajan, K.; Vishvanaꢀ
than, N.; Bhatt, A. D.; Rittel, W. In vitro and in vivo activiꢀ
ties of the nitroimidazole CGI 17341 against Mycobacterium
tuberculosis. Antimicrob. Agents Chemother. 1993, 37, 183ꢀ
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and survival: application to proliferation and cytotoxicity asꢀ
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evaluation of cell/biomaterial interaction by MTT assay. Biꢀ
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ri, B.; Ahmad, A.; Khan, M. I. J. Microbiol. Biotechnol.
2009, 19, 1342ꢀ1347.
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Reynolds, R. Search for New Drugs for Treatment of Tuberꢀ
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S.; Back, D. J.; Khoo, S. H. Differential drug susceptibility
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Author Contributions
P.S.M. performed the chemical syntheses. VMK performed the
molecular docking study. L.U.N., and D.S. performed in vitro,
and ex vivo biological screenings. C.H.G. participated in the deꢀ
sign and execution of this study.
Funding Sources
This work was not supported by any funding.
ACKNOWLEDGMENT
PSM thanks UGC, and MDN thanks CSIR for a fellowship. We
are also grateful to the National Chemical Laboratory, Pune for
providing antiꢀtubercular activity.
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Graphical Table of Contents
Synthesis and Anti-tubercular Activity of New Benzo[b]thiophenes
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