A one pot synthesis of novel bioactive tri- substitute-condensed-imidazopyridines that targets snake venom phospholipase A2

Article abstract of DOI:10.1371/journal.pone.0131896

Drugs such as necopidem, saripidem, alpidem, zolpidem, and olprinone contain nitrogencontaining bicyclic, condensed-imidazo[1,2-α]pyridines as bioactive scaffolds. In this work, we report a high-yield one pot synthesis of 1-(2-methyl-8-aryl-substitued-imidazo[1,2-α] pyridin-3-yl)ethan-1-onefor the first-time. Subsequently, we performed in silico mode-ofaction analysis and predicted that the synthesized imidazopyridines targets Phospholipase A₂ (PLA₂). In vitro analysis confirmed the predicted target PLA₂ for the novel imidazopyridine derivative1-(2-Methyl-8-naphthalen-1-yl-imidazo [1,2-α]pyridine-3-yl)-ethanone (compound 3f) showing significant inhibitory activity towards snake venom PLA₂ with an IC₅₀ value of 14.3 μM. Evidently, the molecular docking analysis suggested that imidazopyridine compound was able to bind to the active site of the PLA₂ with strong affinity, whose affinity values are comparable to nimesulide. Furthermore, we estimated the potential for oral bioavailability by Lipinski's Rule of Five. Hence, it is concluded that the compound 3f could be a lead molecule against snake venom PLA₂.

Full text of DOI:10.1371/journal.pone.0131896

RESEARCH ARTICLE
A One Pot Synthesis of Novel Bioactive Tri-
Substitute-Condensed-Imidazopyridines that
Targets Snake Venom Phospholipase A2
Nirvanappa C. Anilkumar1, Mahalingam S. Sundaram2, Chakrabhavi Dhananjaya Mohan3,
Shobith Rangappa4, Krishna C. Bulusu5, Julian E. Fuchs5, Kesturu S. Girish2,6
,
Andreas Bender5, Basappa1*, Kanchugarakoppal S. Rangappa3*
1 Laboratory of Chemical Biology, Department of Chemistry, Bangalore University, Central College campus,
Palace Road, Bangalore-560 001, Karnataka, India, 2 Department of Studies in Biochemistry, University of
Mysore, Mysore-570 006, Karnataka, India, 3 Department of Studies in Chemistry, University of Mysore,
Mysore-570 006, Karnataka, India, 4 Frontier Research Center for Post-genome Science and Technology
Hokkaido University, Sapporo, 060–0808, Japan, 5 Centre for Molecular Science Informatics, Department of
Chemistry, University of Cambridge, Lensfield Road, CB2 1EW, Cambridge, United Kingdom, 6 Department
of Studies and Research in Biochemistry, Tumkur University, Tumkur-572 103, Karnataka, India
* rangappaks@gmail.com (KSR); salundibasappa@gmail.com (B)
OPEN ACCESS
Citation: Anilkumar NC, Sundaram MS, Mohan CD,
Rangappa S, Bulusu KC, Fuchs JE, et al. (2015) A
One Pot Synthesis of Novel Bioactive Tri-Substitute-
Condensed-Imidazopyridines that Targets Snake
Venom Phospholipase A2. PLoS ONE 10(7):
Abstract
Drugs such as necopidem, saripidem, alpidem, zolpidem, and olprinone contain nitrogen-
containing bicyclic, condensed-imidazo[1,2-α]pyridines as bioactive scaffolds. In this work,
we report a high-yield one pot synthesis of 1-(2-methyl-8-aryl-substitued-imidazo[1,2-α]
pyridin-3-yl)ethan-1-onefor the first-time. Subsequently, we performed in silico mode-of-
action analysis and predicted that the synthesized imidazopyridines targets Phospholipase
A2 (PLA2). In vitro analysis confirmed the predicted target PLA2 for the novel imidazopyri-
dine derivative1-(2-Methyl-8-naphthalen-1-yl-imidazo [1,2-α]pyridine-3-yl)-ethanone (com-
pound 3f) showing significant inhibitory activity towards snake venom PLA2 with an IC50
value of 14.3 μM. Evidently, the molecular docking analysis suggested that imidazopyridine
compound was able to bind to the active site of the PLA2 with strong affinity, whose affinity
values are comparable to nimesulide. Furthermore, we estimated the potential for oral bio-
availability by Lipinski's Rule of Five. Hence, it is concluded that the compound 3f could be
a lead molecule against snake venom PLA2.
e0131896. doi:10.1371/journal.pone.0131896
Editor: Luis Eduardo M Quintas, Universidade
Federal do Rio de Janeiro, BRAZIL
Received: November 28, 2014
Accepted: June 8, 2015
Published: July 21, 2015
Copyright: © 2015 Anilkumar et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any
medium, provided the original author and source are
credited.
Data Availability Statement: All relevant data are
within the paper and its Supporting Information files.
Funding: This research was supported by University
Grants Commission (41-257-2012-SR), Vision Group
Science and Technology, Department of Science and
Technology (NO. SR/FT/LS-142/2012) to Basappa.
KSR thanks DST-JSPS (DST/INT/JAP/P-79/09), DST
Indo-Korea [INT/indo-korea/122/2011-12] and DST-
PURSE for funding. AB thanks Unilever and the
European Research Commission (ERC Starting
Grant 2013) for funding. MSS thanks UGC-BSR for
research fellowship and CDM thanks Department of
Introduction
Imidazole derivatives are the distinct class of heterocyclic compounds which exhibit remark-
able pharmacological activities across a wide range of therapeutic targets [1, 2]. Research in the
previous decade demonstrated that bicyclic condensation of imidazo[1,2-a]pyridines possess
multiple therapeutic properties including anti-cytomegalo-zoster, anti-microbial, anti-cancer,
anti-inflammatory and anti-protozoal activities [3–7]. Imidazo[1,2-a]pyridine ring is a compo-
nent of anxiolytic and sedative drugs such as necopidem, saripidem, alpidem, zolpidem, and
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Novel Imidazopyridines Targets Snake Venom PLA2
Science and Technology for INSPIRE fellowship. The
funders had no role in study design, data collection
and analysis, decision to publish, or preparation of
the manuscript.
olprinone (Fig 1) [8]. These reports suggest the critical role of imidazopyridines in medicinal
chemistry and requirement of easy route for the synthesis of imidazopyridines with improved
efficacy. Initially, copper catalyzed synthesis of imidazopyridines was reported by coupling of
2-aminopyridine with benzaldehyde and propiolic acid as a source of alkyne [9]. Similarly,
NaAuCl4 and Cu(OTf)2 catalyzed synthesis of imidazo[1,2-α]pyridine were reported. The sil-
ver-catalyzed synthesis of substituted-3-methylimidazo[1,2-α]pyridines by cyclo-isomerization
was reported at milder reaction conditions by using N-(prop-2-yn-1-yl)-pyridine-2-amines
[10]. Recently, Dimauro et al reported the Pd(II) and copper iodide catalyzed synthesis of
2-benzylimidazo[1,2-a]pyridines using 2-amino-1-(2-propynyl)pyridinium bromide, aryl
halides and triethylamine [11].
Competing Interests: This study was funded in part
by Unilever. There are no patents, products in
development or marketed products to declare. This
does not alter the authors' adherence to all the PLoS
ONE policies on sharing data and materials.
Additionally, the indium(III) bromide catalyzed multi-component one pot synthesis of imi-
dazo[1,2-a]pyridines by means of 2-aminopyridine, aldehyde and alkyne was reported [6]. In
an another study, Lamblin and colleagues reported theone pot, four-component, microwave
assisted, MgCl2 catalyzed synthesis of imidazopyridines by Ugi-type cyclization of 2-aminopyr-
idine boronic acid pinacol ester, aldehyde and isocyanide, followed by Suzuki coupling with
different aryl halides [12].
In summary, most of the reported synthetic routes of imidazopyridines involve the use of a
catalyst and an alkyne, or the eventual Suzuki-Miyaura cross-coupling reactions. In the present
work, we developed a one pot two-step synthesis of tri-substituted-condensed-imidazopyri-
dines for the first time without using a catalyst for the cyclization, followed by Suzuki coupling
reaction. Further, in silico mode of action analysis predicted phospholipase A2 (PLA2) as a
potential protein target of title compounds, which has subsequently been validated
experimentally.
Materials and Methods
Chemicals/reagents
Vipera russelli (RV) venom was obtained from Hindustan snake park, Kolkata, India. Solvents
and reagents used in this study were of analytical grade and were purchased from Sigma-
Aldrich, St. Louis, USA. 1,2-bis(heptanoylthio)glycerophosphocholine was purchased from
Santa Cruz Biotechnology, Inc. Texas, USA. The VRV-PLA2-VIII was isolated from RV
according to the method of Kasturi and Gowda [13].
General procedure for the synthesis of 1-[(6a-l)-2-methyl-imidazo[1, 2-α]
pyridine-3-yl]ethanone derivatives
The mixture of 3-bromopyridine-2-amine (200 mg, 0.08mmol), 3-bromopentane-2, 4-dione
(142 mg, 0.08 mmol) and 4 mL of tetrahydrofuran (THF) were taken in a sealed tube and
heated at 60°C for 4 h and the reaction was monitored by TLC. After the completion of reac-
tion, boronic acids (0.08 mmol) were added along with Pd(dppf)Cl2 (0.002 mmol) and K2CO3
(0.17 mmol). Finally, 1mL of water was added and the reaction was continued for 4 h at 60°C.
Solvent was evaporated to obtain the crude product and further it was purified by passing
through the column chromatography using hexane and ethyl acetate as solvents.
All IR spectra were obtained in KBr disc on a Shimadzu FT-IR 157 Spectrometer. 1H and
13C NMR spectra were recorded on a Bruker WH-200 (400MZ) spectrometer in CDCl3 or
DMSO-d6 as solvent, using TMS as an internal standard and chemical shifts are expressed
as ppm. Mass spectra were determined using LC-MS. (Shimadzu). The elemental analyses were
carried out using an Elemental Vario Cube CHNS rapid Analyser. The progress of the reaction
was monitored by TLC pre-coated silica gel G plates. Melting points were determined in a
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Novel Imidazopyridines Targets Snake Venom PLA2
Fig 1. Structure of biologically active imidazo[1, 2-α]pyridines.
doi:10.1371/journal.pone.0131896.g001
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Novel Imidazopyridines Targets Snake Venom PLA2
Table 1. Physical data of the tri-substituted-condensed-imidazopyridines and IC50 values towards the binding of PLA2.
Entry Boronic acids
Products
Yield
(%)
Mp
(°C)
IC50 (μM)
VRV-PLA2-VIII
RV
venom
3a
3b
3c
3d
3e
3f
(4-chloro-3-(trifluoromethyl)phenyl)
boronic acid
1-(8-(4-chloro-3-(trifluoromethyl)phenyl)-
2-methylimidazo[1,2-a]pyridin-3-yl)ethanone
70
78
141–
143
110
155
(4-(benzyloxy)-3-fluorophenyl)
boronic acid
1-(8-(4-(benzyloxy)-3-fluorophenyl)-2-methylimidazo
[1,2-a]pyridin-3-yl)ethanone
154–
156
NS
140
Phenylboronic acid
1-(2-methyl-8-phenylimidazo[1,2-a]pyridin-3-yl)ethanone 75
160–
162
194
59.1
46.4
14.3
219
105
123
166
NS
246
(3-chlorophenyl)boronic acid
(3-methoxyphenyl)boronic acid
Naphthalen-1-ylboronic acid
(4-chlorophenyl)boronic acid
1-(8-(3-chlorophenyl)-2-methylimidazo[1,2-a]pyridin-3-yl) 80
ethanone
114–
116
89.1
65.3
23.1
194.8
114.4
134.9
189
1-(8-(3-methoxyphenyl)-2-methylimidazo[1,2-a]pyridin-
3-yl)ethanone
81
116–
119
1-(2-methyl-8-(naphthalen-1-yl)imidazo[1,2-a]pyridin-
3-yl)ethanone
86
179–
182
3g
3h
3i
1-(8-(4-chlorophenyl)-2-methylimidazo[1,2-a]pyridin-3-yl) 82
ethanone
111–
114
(3-(cyclopentylcarbamoyl)pentyl)
boronic acid
4-(3-acetyl-2-methylimidazo[1,2-a]pyridin-8-yl)-N-
cyclopentyl-2-ethylbutanamide
87
86
80
76
79
117–
119
(2-fluoro-3-methoxyphenyl)boronic
acid
1-(8-(2-fluoro-3-methoxyphenyl)-2-methylimidazo[1,2-a]
pyridin-3-yl)ethanone
190–
192
3j
o-tolylboronic acid
1-(2-methyl-8-(o-tolyl)imidazo[1,2-a]pyridin-3-yl)
ethanone
157–
159
3k
3l
(4-(trifluoromethyl)phenyl)boronic
acid
1-(2-methyl-8-(4-(trifluoromethyl)phenyl)imidazo[1,2-a]
pyridin-3-yl)ethanone
121–
123
NS
(4-ethylphenyl)boronic acid
1-(8-(4-ethylphenyl)-2-methylimidazo[1,2-a]pyridin-3-yl)
ethanone
169–
172
NS
43.2
NS: Not Significant
doi:10.1371/journal.pone.0131896.t001
melting point apparatus and were uncorrected. The structures of novel imidazopyridine deriv-
atives are presented in Table 1. Spectra (S1 Data) and characterization data is provided as sup-
plementary data (S2 Data).
Cheminformatics based rationalization
Utilizing the increasing amount of available bioactivity data, we were able to rationalize
the mode-of-action for the imidazopyridines using in silico approaches, which is currently of
interest for chemogenomics studies [14]. To obtain the most probable target for imidazopyri-
dines, we applied the Laplacian-modified Naïve Bayesian classifier and predicted the potential
targets as developed by Koutsoukas et al [15, 16]. This classifier was trained on a large dataset
extracted from ChEMBL, comprising approximately 190,000 bioactive compounds covering
477 human protein targets [17]. A score cut-off of 10 was applied to the predictions, meaning
that predictions with a score of 0.2 or greater were considered to be possible protein targets of
the compound.
In vitro PLA2 inhibition assay
a) Indirect haemolytic activity. Indirect haemolytic activity was determined according to
the method described by Boman and Kaletta [18]. Briefly, packed human erythrocytes were
repeatedly washed with phosphate buffered saline (PBS, 10 mM pH 7.4) and the assay stock
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Novel Imidazopyridines Targets Snake Venom PLA2
was prepared by mixing packed human erythrocytes, egg yolk and PBS (1:1:8; v/v/v). The stock
suspension (200 μL) was incubated independently with 1 μg of RV venom in a total volume of
300 μL for 1 h at 37°C. The reaction was terminated by adding 1.7 mL of ice-cold PBS and cen-
trifuged at 160 ×g for 10 min. The amount of haemoglobin released in the supernatant was
measured at 540 nm. Stock suspension (200 μL) with 1.8 mL of ice-cold PBS alone was consid-
ered as 0% lysis. The activity was expressed as percent haemolysis against 100% lysis of cells by
water. For inhibition studies, 1 μg of RV venom was pre-incubated with different concentra-
tions of 3a-l (0–500 μM) for 10 min at 37°C and necessary controls were maintained in the
respective groups. Compounds were dissolved in DMSO and further diluted in PBS and final
concentration of DMSO was less than 0.05% in the reaction mixture.
b) VRV-PLA2-VIII inhibition assay. The assay was carried out according to the method
of Petrovic et al [19] using isolated PLA2 (VRV-PLA2-VIII) and 1,2-bis(heptanoylthio)glycero-
phosphocholineas substrate [18]. Briefly, VRV-PLA2-VIII (5 μg) was pre-incubated with dif-
ferent concentrations of 3a-l (0–100 μM) for 10 min at 37°C in 96 well microtiter plate.
Further, PLA2 substrate (2 mM) containing 1 mM DTNB was added to each well to a final
reaction volume of 100 μL with assay buffer (50 mM Tris-HCl, pH 7.5 containing 150 mM KCl
and 10 mM CaCl2) and incubated for 60 min at room temperature. The resulting absorbances
were measured at 415 nm and 600 nm.
Molecular docking studies
In silico molecular docking was performed based on the X-ray structure of Russell's viper PLA2
in complex with nimesulide with a resolution of 1.1Å (PDB: 1ZWP) [20, 21].The structures
were prepared by removing the sulphate ions and bound methanol to subject it to docking in
MOE [21].The synthesized molecules were docked to the active site of PLA2using the co-crys-
tallized nimesulide as starting point. To enforce reasonable docking poses, we introduced a
pharmacophore filter to discard poses not showing a hydrogen bond acceptor feature at the
position of the nitro group of nimesulide. We applied the standard flexible docking work-flow
implemented in MOE including placement with Triangle Matcher, primary scoring with Lon-
don dG and subsequent refinement using the mmff94x forcefield [22]. The highest scoring
pose for each compound according to GBVI/WSA dG was finally considered. Resulting poses
were visualized in Pymol [23].
Structure-activity relationships
We calculated molecular descriptors for all twelve newly synthesized compounds using MOE
aiming to identify correlations between physico-chemical properties and biological activities.
Molecular were treated in their predicted biologically active conformation from docking. Cor-
relations were calculated as non-parametric Spearman rank correlation coefficient. Further-
more, we estimated the potential for oral bioavailability by Lipinski's Rule of Five. Descriptors
and correlations are provided in Table 2.
Statistical analysis
Results were expressed as mean SEM of three independent experiments.IC50values of indi-
vidual imidazopyridine derivatives on VRV-PLA2-VIII and indirect haemolytic activity were
obtained from dose response curve for each derivative.
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Novel Imidazopyridines Targets Snake Venom PLA2
Table 2. Structure-activity relationships of newly synthesized tri-substituted-condensed-imidazopyridines.
Compound
Weight(g/
mol)
Hydrogen Bond
Acceptors
Net
Charge
Hydrogen Bond
Donors
SlogP Surface Area
Lipinski
Violations
(Ų)
3a
3b
3c
3d
3e
3f
352.7
374.4
250.3
251.3
280.3
300.4
284.7
355.5
298.3
264.3
318.3
278.4
0.31
3
0
0
0
0
0
0
0
0
0
0
0
0
0
5.11
5.11
3.13
2.52
3.14
4.28
3.78
4.13
3.28
3.44
4.46
3.69
0.43
323.6
386.5
280.6
273.4
306.0
319.7
294.7
415.3
310.4
293.1
309.7
317.1
1
1
0
0
0
0
0
0
0
0
0
0
4
0
3
0
4
0
4
0
3
0
3g
3h
3i
3
0
5
1
4
0
3j
3
0
3k
3l
3
0
3
0
rspearman
(IC50)
-0.19
-0.18
0.24
NS: Not Significant
doi:10.1371/journal.pone.0131896.t002
Results
One pot synthesis and characterization of imidazopyridine derivatives
We previously reported the newer routes for the synthesis of biologically active heterocycles
[24–31] and in continuation we herein report the one pot synthesis of imidazopyridine deriva-
tives. Initially, 3-bromopentane-2,4-dione required for the first step was prepared by treating
acetyl acetone with N-Bromosuccinimide (NBS) in chloroform, instead of hazardous molecular
bromine (Fig 2). The filtrate of the brominated compound was used directly in the first step
without purification, which renders it practically close to the one-step synthesis of the title
compounds. The 1H NMR spectrum of bromo-derivative of imidazopyridines showed a CH
proton at 10 ppm confirming the formation of imidazopyridine ring. In conclusion, we have
developed a successful one pot two-step synthesis of Suzuki coupled imidazopyridine deriva-
tives in THF solvent.
Fig 2. Synthesis of tri-substituted-condensed-imidazopyridines.
doi:10.1371/journal.pone.0131896.g002
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Novel Imidazopyridines Targets Snake Venom PLA2
Cheminformatics based rationalization of putative human targets for
imidazopyridines
The newly synthesized imidazopyridines were subjected to in silico target prediction protocols,
which are able to anticipate the most likely protein targets of a small molecule, based on molec-
ular structure. Using the well-established Laplacian-modified Naïve Bayes classifier, in silico
target prediction of bioactive molecules was carried out based on 155,000 ligand-protein pairs
covering 894 human protein targets from ChEMBL.
Among the predicted human targets, B2 bradykinin receptor, cGMP-dependent 3',5'-cyclic
phosphodiesterase, Type-2 angiotensin II receptor, Phospholipase A2 and TGF-beta receptor
type-1 were found to have likelihood scores of 11.54, 9.32, 8.8, 8.14 and 8.05 respectively,
which were all empirically classified as being significant. Evidently, imidazopyridines were
reported as potent inhibitors for leukotriene A4 hydrolase, a pro-inflammatory mediator impli-
cated in the pathogenesis of a number of diseases including inflammatory bowel disease and
arthritis [32].Therefore, we considered the Phospholipase A2 as a predictive target for the novel
imidazopyridines.
Effect of imidazopyridines towards the inhibitory activity of PLA2
The in silico analysis predicted PLA2 as a target for the newly synthesized imidazopyridines
and we hence tested the imidazopyridines against RV venom PLA2 as the target enzyme[20,
33].The series of tri-substituted-condensed-imidazopyridines 3a-l were assessed for PLA2 inhi-
bition by indirect haemolytic activity and the results are tabulated in Table 1. All the tested
compounds from tri-substituted-condensed-imidazopyridine series displayed venom-PLA2
inhibition in the dose dependent manner. Among the tested compounds, 3f showed maximum
inhibitory efficacy against PLA2 with an IC50 value of 14.3 μM (Fig 3). None of the compounds
induce haemolysis up to the tested concentrations which served as negative control. Addition-
ally, we have tested the effect of imidazopyridines against purified VRV-PLA2-VIII, using
1,2-bis(heptanoylthio)glycerophosphocholine as the substrate. The results are summarized in
Table 1. The analysis of the results indicated that the imidazopyridines inhibited the catalytic
Fig 3. IC50 values of imidazopyridine derivatives on Vipera russelli (RV) venom induced indirect
haemolytic activity. RV venom (1 μg) was pre-incubated with different concentrations of imidazopyridine
derivatives for 10 min at 37°C. Assay was performed as described in methods section and IC50 values for
individual imidazopyridine derivatives obtained from dose response curve is presented.
doi:10.1371/journal.pone.0131896.g003
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Novel Imidazopyridines Targets Snake Venom PLA2
activity of VRV-PLA2-VIII, whose IC50 values are comparable to the results of indirect haemo-
lytic assay. These results further confirms that imidazopyridines catalytically inhibits
VRV-PLA2-VIII effectively.
Structure-based in silico docking analysis of imidazopyridine small
molecule that targets PLA2
To structurally understand the molecular mechanism of inhibition by imidazopyridines, dock-
ing studies of imidazopyridines and PLA2 were performed. We chose an X-ray structure of
Russell's viper PLA2 in complex with nimesulide (PDB: 1ZWP) as basis for our docking studies
[20]. We docked all 12imidazopyridines (3a-l) using MOE to the active site of PLA2, thereby
replacing the co-crystallized ligand nimesulide.
All the docked compounds occupy a similar region in the PLA2 binding site, thereby replac-
ing the nitro group of nimesulide with either a ketone or an ether functionality. The position of
the phenyl ring of nimesulide is occupied by the imidazopyridine for most predicted poses;
thereby showing pronounced π-π stacking interactions with Trp-31 (Fig 4). The most active
compound, imidazopyridine 3f, ranks second amongst the twelve docked compounds. The
naphthyl system of 3f forms additional stacking interactions with Trp-31 and extends towards
Gly-32 potentially adding further amide-pi stacking contributions. Therefore, molecular dock-
ing studies were found helpful in rationalization of PLA2 in vitro binding.
Structure-activity relationships
We found only modest correlations between physico-chemical descriptors and experimental
bioactivity. Interestingly, we observe weak correlations in our compound set indicating that
high molecular weight and high lipophilicity reduce PLA2 binding. Only two of twelve com-
pounds (3a, 3b) show a single violation of Lipinski's Rule of Five due to a calculated logP of
5.11. Therefore, the compounds are predicted to be orally bioavailable.
Discussion
The imidazopyridine scaffold has been extensively incorporated in many drugs because of its
medicinal properties over the other heterocyclic cores. In the present work, we developed a
new method to prepare imidazopyridine-based compounds without the use of catalyst for
cyclization. Additionally, replacement of halogen atom with desired moiety in the title com-
pounds provides a platform for the derivatization and diversification of imidazopyridines. On
the other hand, secretory PLA2s are ubiquitous in mammalian tissues as well as animal venom.
PLA2s are the lipolytic enzymes with the ability to catalyze the hydrolysis of sn-2 ester bonds in
a variety of glycerophospholipid molecules releasing fatty acids and lysophospholipids [34, 35].
The catabolic products of glycerophospholipids are known to be the mediators of various
inflammatory diseases [36]. In this study, snake venom PLA2 was used as a model enzyme to
study the inhibitory efficacy of newly synthesized tri-substituted-condensed-imidazopyridines.
Conclusion
In conclusion, we herein report a simple, efficient, catalyst free and one pot synthetic route
to prepare tri-substituted-condensed-imidazopyridines and our in silico target prediction
presented PLA2 as a likely target for the newly synthesized compounds. The prediction was
experimentally validated using VRV-PLA2-VIII and indirect haemolytic assay. Of the new
compounds synthesized, 1-(2-Methyl-8-naphthalen-1-yl-imidazo [1,2-α]pyridine-3-yl)-etha-
none was identified as the lead compound with an IC50 value of 14.3 μM. Molecular docking
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Novel Imidazopyridines Targets Snake Venom PLA2
Fig 4. Predicted interactions of imidazopyridine 3f with PLA2: PLA2 is shown as green cartoon with highlighted Trp-31 and peptide bond to Gly-32.
Co-crystallized ligand nimesulide is shown in stick representation with cyan carbon atoms. The highest scoring docking pose of compound 3f is shown in
purple sticks. In silico docking predicts π-π stacking interactions with Trp-31 and additional amide-π interactions with the backbone of Gly-32.
doi:10.1371/journal.pone.0131896.g004
analysis displayed that the imidazopyridine compounds could make a favourable π-π stacking
interactions with Trp-31. Exploration of PLA2 inhibitory activity of imidazopyridine deriva-
tives contributes to the development of the title compounds as therapeutic agents to block the
PLA2 associated inflammatory diseases. Thus, synthesis of more imidazopyridine derivatives
and optimization of their biological activity according to the identified structure-activity rela-
tionship is envisaged.
Supporting Information
S1 Data. Scanned spectral images of novel imidazopyridine derivatives.
(DOCX)
S2 Data. Structural analysis of novel imidazopyridine derivative.
(DOCX)
Acknowledgments
This research was supported by University Grants Commission (41-257-2012-SR), Vision
Group Science and Technology, Department of Science and Technology (NO. SR/FT/LS-142/
2012) to Basappa. KSR thanks DST-JSPS (DST/INT/JAP/P-79/09), DST Indo-Korea [INT/
indo-korea/122/2011-12] and DST-PURSE for funding. A B thanks Unilever and the European
Research Commission (ERC Starting Grant 2013) for funding. MSS thanks UGC-BSR for
research fellowship and CDM thanks Department of Science and Technology for INSPIRE
fellowship.
Author Contributions
Conceived and designed the experiments: KSG AB B KSR. Performed the experiments: NCA
MSS B JEF. Analyzed the data: B CDM SR KSG KCB AB KSR. Contributed reagents/materials/
analysis tools: B KSG KSR AB. Wrote the paper: MSS CDM SR B KSG KCB AB KSR.
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