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Medicinal Chemistry, 2020, 16
RESEARCH ARTICLE
1573-4064/20 $65.00+.00 © 2020 Bentham Science Publishers
Design, Synthesis, Antimalarial Activity and Docking Study of 7-Chloro-4-
(2-(substituted benzylidene)hydrazineyl)quinolines
Jahnabi Kalita1, Dipak Chetia1 and Mithun Rudrapal1,*
1Department of Pharmaceutical Sciences, Dibrugarh University, Dibrugarh-786 004, Assam, India
A R T I C L E H I S T O R Y
Received: March 29, 2019
Revised: July 10, 2019
Accepted: July 22, 2019
DOI:
10.2174/1573406415666190806154722
Abstract: Background: Malaria is a growing infectious disease burden due to the increasing
emergence of resistant strains of Plasmodium falciparum. Because of the limited therapeutic effi-
cacy of available antimalarial drugs, the development of potent antimalarial drug agents is there-
fore an urgent requirement to fight against resistant malaria.
Objective: The objective of this work was to develop novel quinoline-baed antimalarial agents that
would be active against resistant P. falciparum malaria.
Methods: Some 7-chloro-4-(2-(substituted benzylidene)hydrazineyl)quinolines were synthesized
for the evaluation of their potential as possible antimalarial agents, particularly against resistant
malaria. The antimalarial activity of synthesized compounds was evaluated in vitro against blood-
stage parasites of P. falciparum. Further, molecular docking and drug-likeness including ADMET
(Absorption, Distribution, Metabolism, Elimination and Toxicity) studies were also carried out us-
ing in silico tools.
Results: Results reveal the in vitro antimalarial activity of synthesized 7-chloro-4-(2-(substituted
benzylidene)hydrazineyl)quinolines against P. falciparum. The docking study investigates the an-
timalarial effectiveness of synthesized quinolines as novel plasmepsin 2 inhibitors. Drug-likeness
prediction exhibits acceptable drug-likeness and ADMET properties.
Conclusion: Based upon our findings, it is concluded that the molecular scaffold of 7-chloro-4-(2-
(substituted benzylidene)hydrazineyl)quinolines may be used as a lead structure for further modi-
fications in the search of more potent antimalarial drug molecules.
Keywords: Malaria, P. falciparum, drug resistance, 7-Chloro-4-(2-(substituted benzylidene)hydrazineyl)quinolines, antimalar-
ial, plasmepsin 2 inhibitors.
1. INTRODUCTION
Malaria is a growing infectious disease with widespread
occurrence across the globe. According to the World Health
Organization (WHO), about 200-300 million people are af-
flicted by malaria with approximately 430,000 deaths every
year globally [1, 2]. Plasmodium falciparum is the only
causative organism responsible for most of the malaria-
related mortalities in humans. It causes severe infections
such as cerebral malaria in children as well as adults [2]. In
recent days, the increasing emergence of drug-resistant
strains of P. falciparum has become a global concern. The
problem of drug resistance has limited the efficacy of most
of antimalarial drugs (including quinoline-based and artemis-
inin derived drugs) and/or antimalarial therapies [including
artemisinin-based combination therapies (ACTs)] currently
available in clinical practice. The control and prevention of
malaria, has, therefore, become a difficult task.
*Address correspondence to this author at the Department of Pharmaceutical
Sciences, Dibrugarh University, Dibrugarh -786 004, Assam, India;
Mobile: +91-8638724949; E-mail: rsmrpal@gmail.com
In view of the above facts, the development of potent an-
timalarial agents as alternative to existing antimalarial drugs
is an urgent requirement for the treatment of resistant ma-
laria. Traditionally, the development of new antimalarial
drugs is based upon the structure of existing lead molecules/
natural scaffolds on a rational basis of new drug design strat-
egy. This could be accepted as a reliable approach in the
discovery of novel antimalarial drug molecules to address
the above challenging issue. As part of our ongoing research
towards developing potent antimalarial agents, some new 7-
chloro-4-(2-(substituted benzylidene)hydrazineyl)quinolines
were designed, synthesized and evaluated for their antimalar-
ial effectiveness against P. falciparum parasite. Additionally,
docking simulation study and drug-likeness prediction in-
cluding ADMET (Absorption, Distribution, Metabolism,
Elimination and Toxicity) screening were also carried out
using in silico tools with an aim to assess the antimalarial
efficacy and drug-likeness of synthesized compounds. Our
objective was to develop novel antimalarial agents that
would be active against resistant P. falciparum malaria.
Design, Synthesis, Antimalarial Activity of 7-Chloro-4-(2-(substituted benzylidene)hydrazineyl)quinolines Med icinal C hemistry, 2020, Vol. 16, No. 7 929
2. EXPERIMENTAL
2.1. Chemicals and Analysis
Chemicals were procured commercially from Sigma-
Aldrich Corporation (USA), Merck Specialists Pvt. Ltd.
(Germany) or Spectrochem Pvt. Ltd. (India), and were used
without further purification. TLC was performed using the
silica gel-G and the spots were visualized by iodine vapours.
Melting points (MP) were measured in open capillaries on an
electrically heated melting point apparatus. Ultraviolet (UV)-
visible spectra were recorded on Shimadzu UV-1700 UV-
visible spectrophotometer and the wavelengths of maximum
absorption (λmax, nm) are reported. Infrared (IR) spectra were
recorded on a Bruker Alpha Fourier Transform (FT-IR)
spectrometer using KBr disc and are reported in terms of
frequency of absorption (υ, cm-1). 1H & 13C Nuclear Mag-
netic Resonance (NMR) spectra were recorded on Bruker
Avance II 400 FT-NMR spectrometer at 400 and 100 MHz,
respectively using tetramethylsilane (TMS) as an internal
standard (δ 0.00 ppm) and CDCl3 as a solvent. Chemical
shift (δ) values were expressed in parts per million (ppm)
relative to TMS (δ 0.00 ppm). Mass spectra were obtained
on an LC-MS Water 4000 ZQ instrument using electrospray
ionization (ES+). The m/z values of the most intense quasi-
molecular ion [M+H] + peak, with relative intensities in pa-
rentheses are given.
2.2. Synthesis of Compounds
A mixture of 4,7-dichloroquinoline (10 mmol), 1 and hy-
drazine hydrate (200 mmol) was stirred vigorously in ap-
prox. 50 ml of ethanol until a clear solution was achieved.
The solution was then heated under reflux for about 10-12
hours at 100-120 °C. After 12 hours of reaction, the mixture
was kept in the refrigerator overnight. The precipitate of dark
yellow solid was collected by filtration, washed with ice-
cooled water and dried. The crude compound was recrystal-
lized from ethanol to obtain a pure product of yellow-colored
crystalline solid.
To a mixture of aromatic benzaldehyde (13 mmol) and
ethanol (30 ml), the intermediate compound, [(7-Chloro-
quinolin-4-yl)-hydrazine] (1 mmol), 2 was slowly added
followed by the drop-wise addition of conc. sulfuric acid
(0.05 mmol). The reaction mixture was then heated under
reflux for about 8-10 hours. After reflux was over, the reac-
tion mixture was cooled in the refrigerator overnight and the
separated solid was collected by filtration [3, 4]. The crude
product was washed with organic solvents and recrystallized
from ethanol/water mixture to obtain the pure product of
desired quinoline derivatives, 3a-l .
2.2.1. 7-Chloro-4-(2-(2-hydroxybenzylidene)hydrazineyl)
quinoline, 3b
UV (DMSO), λmax (nm): 267.00; IR (KBr), υ (cm
-1):
3321.10 (N-H str.), 3017.82 (C-H str., Ar.), 1608.12,
1578.32, 1542.31, 1489.11 (C=C str., Ar.), 1238.17 (C-O
str., Ar-OH), 1021.12 (Ar. C-Cl); 1H NMR (DMSO-d6, 400
MHz), δ (ppm): 4.34 (s, 1H, =CH-); 7.87 (s, 1H, Ar-OH), 7.66 (d,
J=8.8 Hz, 1H, quinoline-H3), 7.79 (d, J=7.6 Hz, 1H, quinoline-H2),
8.43 (d, J=6.4 Hz, 1H, quinoline-H5), 8.56 (d, J=12.0 Hz, 1H, qui-
noline-H6); 13C NMR (DMSO-d6, 100 MHz), δ (ppm): 46.21
(=CH-), 118.12, 128.12, 135.36, 142.19, 152.10, 158.10,
162.34, 166.10, 172.10 (Ar-C & Quinoline-C); MS (ES+),
m/z (%): 298.14 (100), [M+H]+.
2.2.2. 7-Chloro-4-(2-(4-nitrobenzylidene)hydrazineyl)qui-
noline, 3h
UV (DMSO), λmax (nm): 387.80; IR (KBr), υ (cm
-1):
3362.42 (N-H str.), 3021.45 (C-H str., Ar.), 1556.09,
1389.23 (N-O str., Ar-NO2), 1489.12, 1445.68, 1432.13
(C=C, Ar.), 1024.90 (Ar. C-Cl); 1H NMR (DMSO-d6, 400
MHz), δ (ppm): 4.36 (s, 1H, =CH-); 7.68 (d, J=8.8 Hz, 1H, quino-
line-H3), 7.78 (d, J=7.6 Hz, 1H, quinoline-H2), 8.46 (d, J=6.4 Hz,
1H, quinoline-H5), 8.58 (d, J=12.0 Hz, 1H, quinoline-H6); 13C
NMR (DMSO-d6, 100 MHz), δ (ppm): 48.34 (=CH-), 118.22,
128.78, 134.23, 144.37, 156.10, 159.34, 164.11, 168.39,
170.23 (Ar-C & Quinoline-C); MS (ES+), m/z (%): 327.18
(100), [M+H]+.
2.2.3. 7-Chloro-4-(2-(3-methoxy-4-hydroxybenzylidene)
hydrazineyl)quinoline, 3i
UV (DMSO), λmax (nm): 360.50; IR (KBr), υ (cm
-1):
3318.52 (N-H str.), 3024.12 (C-H str., Ar.), 2952.12,
2872.10 (C-H, OCH3), 1623.35, 1519.22, 1547.19, 1460.42
(C=C str., Ar.), 1259.21 (C-O str., Ar-OH); 1023.42 (Ar. C-
Cl); 1H NMR (DMSO-d6, 400 MHz), δ (ppm): 1.82 (s, 1H,
OCH3), 4.36 (s, 1H, =CH); 7.88 (s, 1H, Ar-OH), 7.64 (d, J=8.8 Hz,
1H, quinoline-H3), 7.80 (d, J=7.6 Hz, 1H, quinoline-H2), 8.46 (d,
J=6.4 Hz, 1H, quinoline-H5), 8.58 (d, J=12.0 Hz, 1H, quinoline-
H6); 13C NMR (DMSO-d6, 100 MHz), δ (ppm): 46.50 (=CH-),
18.23 (OCH3); 118.27, 129.10, 135.11, 145.26, 152.27,
158.11, 166.22, 166.34, 170.22 (Ar-C & Quinoline-C); MS
(ES+), m/z (%): 328.46 (100), [M+H]+.
2.3. In vitro Antimalarial Screening
The synthesized compounds, 3a-l were evaluated in vitro
for antimalarial activity against CQ-sensitive (3D7) and CQ-
resistant (RKL9) strains of P. falciparum. The in vitro anti-
malarial screening was carried out by Giemsa stained slide
method based on a microculture assay method. The tech-
nique of schizont maturation inhibition assay was used for
the quantitative assessment of parasitemia in the evaluation
of drug sensitivity. The assay was conducted using the blood
parasites of in vitro culture of P. falciparum according to the
previously reported methods [5]. Each test compound was
assayed in duplicate and the number of schizonts was
counted against 200 asexual parasites per replica under light
microscope. To assess the parasite growth, a drug-free nega-
tive control was maintained in duplicate in the microtitre
plate. Chloroquine diphosphate was also maintained in du-
plicate as a positive control to assess the integrity of assay.
Mean number of schizonts for duplicate observations was
calculated at each concentration of test/ standard compound
(chloroquine). Further, the IC50 (concentration at which the
inhibition of parasite growth represents 50%) values in µM
were also calculated using the NonLin v1.1 software. Test
results were compared with the standard results of chloro-
quine.
930 Medicinal Chemistry, 2020, Vol. 16, No. 7 Kalita et al.
2.4. In silico Studies
2.4.1. Molecular Docking
Molecular docking studies were carried out on plasmep-
sin II (PM-II) protein using Biovia Discovery Studio (DS) v
4.5 (2017) software. The x-ray crystal structure of PM-
II/EH-58 (PDB id: 1LF3) was retrieved from the RCSB Pro-
tein Data Bank (http://www.rcsb.org/pdb/). Chain A of the
protein determined at a resolution of 2.9 Å was used in the
study [1]. Flexible molecular docking was performed where
the protein was held rigid, while ligands were allowed to be
flexible during refinement. Prior to docking, energy minimi-
zation of the protein molecule was done following the stan-
dard protocol. A receptor grid was generated around the
binding cavity (active sites) of energy minimized protein
molecule by specifying the key amino acid residues (Phe 16,
Leu 33, Val 78, Ser 79, Ser 215, Gly 216 and Asp 303). For
the receptor grid box, binding site sphere was set with a ra-
dius of 20 Å and x, y, z dimensions of -52.25, -4.46, -19.25,
respectively. Docking was performed using the Dock
Ligands module of LibDock genetic algorithm program. All
docking and consequent scoring parameters used were kept
at their default settings [6, 7]. LibDock scores of docked
ligands were calculated. All docked poses were scored,
ranked and the best pose (having the highest dock score) for
each compound was selected. To study receptor-ligand inter-
action, binding modes of the best pose were also analyzed
for each molecule with the help of a 3D receptor-ligand
complex. Different non-bonding interactions (hydrogen
bonding and hydrophobic) were also analyzed with the help
of 2D diagram of receptor-ligand complexes.
2.4.2. Drug-likeness Screening
In silico calculations of molecular properties and drug-
likeness parameters for all the compounds, 3a-l were per-
formed based on theoretical approaches using Biovia DS v
4.5 software (2017). The aim was to identify the molecules
violating the optimum requirements for drug-likeness. Mo-
lecular properties [molecular weight (MW), LogP value,
number of hydrogen bond acceptors (nHBA), number of
hydrogen bond donors (nHBD), total polar surface area
(TPSA)] incorporated in Lipinski’s rule of five [1], and other
physicochemical parameters like aqueous solubility (LogS),
molar refractivity (MR) and molar volume (MV) were also
calculated. The number of rotatable bonds (nRotB) was pre-
dicted using Molsoft Online software
(http://www.molsoft.com/, 2018), and non-violation of drug-
likeness and drug-likeness score were calculated using
Molinspiration online software (http://www.molinspiration.
com/, 2018).
2.4.3. ADMET Prediction
ADME-Toxicity (ADMET) was calculated in silico using
the Biovia DS v 4.5 (2017) software. Six mathematical mod-
els (aqueous solubility, blood-brain barrier penetration, cyto-
chrome P450 2D6 inhibition, hepatotoxicity, human intesti-
nal absorption and plasma protein binding) were used for
quantitative prediction of properties related to ADMET char-
acteristics/ pharmacokinetics (PKs) of molecules [1]. The
study of these properties is important because they influence
oral bioavailability, cell permeation and metabolism of drug
molecules.
3. RESULTS AND DISCUSSION
3.1. Design and Synthesis
A series of quinoline imine derivatives were designed at
the structural moiety of 4-aminoquinoline ring conjugated
with varied aromatic systems having hydrazinoyl-
azomethine (-NH-N=CH-) bridge system (Fig. 1). The 4-
aminoquinoline scaffold was considered as the key structural
requirement for the antimalarial activity, and the substituted
imino component was thought to modulate the activity of
compounds.
NCl
HN
N
R
Fig. (1). General structure of 7-chloro-4-(2-(substituted ben-
zylidene)hydrazineyl)quinolines.
Twelve designed 7-chloro-4-(2-(substituted benzylidene)
hydrazineyl)quinolines were synthesized, characterized and
evaluated in vitro for their antimalarial activities. The stan-
dard reaction procedure involving a synthetic route (Scheme
1) was employed for the preparation of target quinoline imi-
nes. Substitutions with a different substituted aromatic alde-
hyde at the azomethine carbon atom (-N=CH) of 4-
quinolinehydrazinoyl moiety afforded target quinoline imi-
nes as conjugated molecules. The reaction of 4,7-
dichloroquinoline, 1 with hydrazine hydrate first yielded the
intermediate compound, (7-chloro-quinolin-4-yl)-hydrazine,
2 which was later allowed to react with different substituted
NCl
Cl
N
HN
Cl
NH
2
N
Cl
HN
N
R
ab
123
Scheme 1. Scheme of synthesis, reagents and conditions: (a) Hydrazine hydrate/ EtOH, 100-120 °C, 10-12 hr, rt; (b) Ar-CHO, 60-70 oC, 6-8
hr, EtOH/ Conc. H2SO4.
Design, Synthesis, Antimalarial Activity of 7-Chloro-4-(2-(substituted benzylidene)hydrazineyl)quinolines Med icinal C hemistry, 2020, Vol. 16, No. 7 931
aromatic aldehydes in the presence of an acid catalyst, conc.
H2SO4, to obtain desired quinoline imine derivatives, 3a-l.
The physicochemical details of synthesized compounds
are summarized in Table 1. The spectral (UV, IR, 1HNMR,
13CNMR, Mass) data (of representative compounds) depicted
in the experimental section are consistent with structures of
synthesized compounds. In UV spectra (in DMSO), the λmax
values were observed at the range between 267.00 to 387.80
nm. UV data favours the chromophoric quinoline imino
moiety in the basic structure of compounds. IR spectral data
showed peaks of absorption frequencies (υ, cm-1) character-
istic to specific functional groups of the synthesized com-
pounds. The appearance of prominent peaks at their corre-
sponding frequencies is depicted in the experimental sec-
tion. Some characteristic stretching vibrations observed
were C-H (s, aliphatic), C-H (m, aromatic), C=C (s, aro-
matic), C-O (s, aromatic OH), N-O (s, aromatic NO2 and
C-Cl (w, Ar-Cl). 1H NMR spectra exhibited resonance sig-
nals with chemical shift (δ, ppm) values characteristic to
various structural protons. 1H NMR data concorded the an-
ticipated structure of synthesized compounds. A broad sin-
glet for the proton of the aromatic OH/ OCH3 groups and
also characteristic multiplet peaks due to aromatic/ het-
eroaromatic (quinoline-4-yl) structural protons were as-
signed. Depending on the nature of the substituent at imino
carbon of the quinoline imine scaffold, distinct δ values were
also observed with expected splitting patterns (doublets, trip-
lets or multiplets) and coupling constant (J) values. In 13C
NMR spectra, distinguished resonance signals (δ, ppm) due
to structural aromatic/heteroaromatic carbons appeared.
Characteristic downfield signals for aliphatic carbons/ other
similar groups were also observed for CH3, CH groups etc.
The mass spectra of compounds exhibited prominent mo-
lecular ion peaks, [M+H]+ corresponding to their respective
molecular mass.
Table 2. Antimalarial activity data.
Compounds
IC50 in µM*
-
P f 3D7
P f RKL9
3b
0.0008
0.0008
3e
0.0014
0.0014
3f
0.0010
0.0010
3h
0.0009
0.0009
3i
0.0009
0.0009
CQ
0.0007
0.0008
*Calculated using NonLin v1.1 software.
#Standard drug, Chloroquine.
3.2. Antimalarial Activity
Results of antimalarial activity presented in Table 2 re-
veal that among all the synthesized compounds, the activity
of compounds, 3b, 3e, 3f, 3h, and 3i was found to be better
than rest of the compounds. These compounds showed activ-
ity against both CQ-sensitive (3D7) and CQ-resistant
(RKL9) strains of P. falciparum. The IC50 values range from
0.0008 to >0.014 µM against both sensitive and resistant
strains of P. falciparum. Out of five compounds, two com-
pounds, 3e and 3f showed poor activity with IC50 values of
>0.001 µM against both the strains. Compounds, 3b, 3h and
3i exhibited moderate activity with IC50 values of in the
range between 0.0008 and 0.0009 µM against both sensitive
and resistant strains. Compound 3b showed better activity
against resistant strain than the sensitive one compared to
compound 3h and compound 3i. Compound 3b was found to
be comparatively more potent than compound 3h and com-
pound 3i. Results are comparable with that of the standard
drug, CQ. The IC50 values of CQ were found to be 0.0007
Table 1. Physicochemical data.
Compounds
R
Color (State)
% Yield
MP (oC)
Rf*
3a
H
Yellow (Solid)
80
156-158
0.69
3b
2-OH
Pale yellow (Solid)
77
-
0. 70
3c
3-OH
Off-white (Solid)
60
-
0.72
3d
4-OH
Brown (Solid)
88
189-191
0.73
3e
3-OCH3
Yellow (Solid)
72
159-161
0.72
3f
4-OCH3
Dark yellow (Solid)
74
160-162
0.73
3g
4-Cl
Yellow (Solid)
83
170-173
0.76
3h
4-NO2
Pale yellow (Solid)
74
-
0.71
3i
4-OH, 3-OCH3
Yellow (Solid)
84
180-183
0.74
3j
3,4-(OCH3)2
Yellow (Solid)
80
168-170
0.77
3k
4-CH3
Brown (Solid)
85
240-244
0.78
3l
4-N(CH3)2
Brown (Solid)
77
292-296
0.80
*n-Hexane: EtOAc: Glacial acetic acid (3:2:0.5).
932 Medicinal Chemistry, 2020, Vol. 16, No. 7 Kalita et al.
Fig. (2). (a): Optimized co-crystal structure of chain A of plasmepsin 2 (PM-II)/EH-58, (b): Receptor grid. (A higher resolution / colour ver-
sion of this figure is available in the electronic copy of th e article).
Fig. (3). (a): Redocked conformer of co-crystal ligand, EH-58 in the active site of PM-II (left) and 2D representation of the binding interac-
tion (right); (b): Binding mode (left) and 2D receptor-ligand interaction diagram (right) of compound 3b at the binding pocket of PM-II pro-
tein. (A higher resolution / colour version of this figure is available in the electronic copy of the article).
Design, Synthesis, Antimalarial Activity of 7-Chloro-4-(2-(substituted benzylidene)hydrazineyl)quinolines Med icinal C hemistry, 2020, Vol. 16, No. 7 933
and 0.0008 µM against sensitive and resistant strains of P.
falciparum, respectively.
3.3. Docking Simulation
The protein model of PM-II/EH-58 co-crystal structure
was validated and used for the docking simulation study.
Prior to docking, the receptor grid model was generated and
optimized in terms of binding site sphere for predictive in-
teraction between a receptor molecule and ligands (Fig. 2).
The co-crystallized ligand, EH-58 (a selective PM-II inhibi-
tor) was re-docked using flexible docking simulations into
the original structure of the receptor molecule using all dock-
ing parameters to the software’s default values. The refer-
ence ligand, EH-58 was successfully re-docked to the pre-
dicted active site of PM-II molecule with an acceptable
RMSD value of 1.190 Å. This study was performed in order
to reproduce the results of experimentally observed ligand
binding modes in protein-ligand docking. Results confirmed
experimental binding modes/conformations of EH-58 in the
binding pocket of receptor molecule with well defined pro-
tein-ligand interactions (Fig. 3).
A docking simulation study was performed to predict the
efficacy of newly designed 7-chloro-4-(2-(substituted ben-
zylidene)hydrazineyl)quinolines as possible novel PM-II
inhibitors. Molecular docking is a virtual tool intended to
find the best binding orientation of small molecules bound to
their target protein molecules. It is used to predict the bind-
ing affinity as well as biological efficacy of small molecules
[1, 8-10]. Thus, docking thus plays an important role in the
identification of bioactive molecules based on the target pro-
tein structure in rational drug discovery program. In protein-
ligand docking, LibDock program successfully docked all
the compounds into the binding pocket of PM-II molecule.
Compounds could bind well with the pre-defined active site
residues of the receptor sphere. High binding affinity was
observed with LibDock scores ranging from 90.287 to
108.085. The LibDock scores of compounds, 3a-3l are
summarized in Table 3. Five compounds which showed
good in vitro activity ranked top in docking with LibDock
scores ranging from 108.085 to 112.34. Results indicate that
docking scores of synthesized compounds are consistent
with the in vitro antimalarial activity. Docking study may,
therefore, serve as an important tool for assessing the anti-
malarial efficacy of newer quinoline derivatives as P. falci-
parum PM-II inhibitors.
The three-dimensional poses of bound ligand molecules
reveal the best molecular orientation relative to the structure/
orientation of receptor molecule. Analysis of 2D diagram
indicates that polar hydrogen-bonding interactions were pri-
marily involved between receptor and ligand molecule. Mo-
lecular interaction reveals well-defined binding between
binding site residues of PM-II molecule and complementary
moieties/atoms of ligands. The higher the number of hydro-
gen bonds, the higher the binding affinity. Other non-bonded
interactions like hydrophobic bonding were also observed,
but to a lesser extent. Table 4 shows details of hydrogen
bonding interactions for the three best compounds (3b, 3h,
3i). The following active site residues, Glu 36, Gly 40, Cys
42, Gly 83, Ile 85, Ser 149, Asp 173, His 174 and Asp 234
were involved in hydrogen bonding interactions. The 3D
binding modes and 2D interaction diagrams of the most po-
tent compound, compound 3b are displayed in Fig. (3).
Analysis of 2D interaction diagram for compound 3b reveals
that the compound formed four strong hydrogen bonds with
residues like Cys 42 (O··H··S), 174 (O··H··N), Ser 149
(N··H··O) and Asn 173 (O··H··O) with bonding distances of
3.41, 3.39, 3.68 and 2.43 Å, respectively. Further analysis of
3D diagram reveals that the quinoline scaffold was oriented
in the binding cavity (active site residues) of PM-II mole-
cule. In 2D diagram, the quinoline moiety could occupy the
binding sites of PM-II mainly through some hydrophobic
interactions like Van der Waals interaction. Such interactions
afforded good stability of the complex formed between PM-
Table 3. Dock scores and no. of H-bonds.
Compounds
Libdock Score
No. of H-Bond(s)
3a
90.28
3
3b
112.34
4
3c
94.27
3
3d
90.87
2
3e
95.62
2
3f
95.74
3
3g
94.99
4
3h
95.65
4
3i
97.65
3
3j
95.88
5
3k
90.98
3
3l
105.96
6
934 Medicinal Chemistry, 2020, Vol. 16, No. 7 Kalita et al.
II and ligand molecule, compound 3b. Quinoline imine moi-
ety interacted with multiple amino acid residues of receptor
molecule through hydrogen bonding as depicted above.
Strong H-bonding interactions between CH-C=N group of
the imine component and amino acid residues were also ob-
served. The OH of the substituent (2-hydroxyphenyl) was
also involved in H-bonding interactions with several residues
of receptor molecule. In further analysis of docking interac-
tions, it is assumed that quinoline ring plays a crucial role in
protein-ligand binding. The presence of substituents in-
creases binding strength by forming additional H-bonds that
facilitate a stronger interaction of ligands with the receptor
molecule. The imino component present as a bridging moi-
ety also contributes significantly to the docking interaction
of quinoline derivatives.
3.4. Drug-likeness
Results of calculated molecular properties, predicted Lip-
inski’s parameters and other drug-likeness properties are
presented in Table 5. Results reveal that all the synthesized
compounds possess good drug-like properties based on Lip-
inski’s rule of five with additional parameters of drug-
likeness. All compounds obey ‘Lipinski’s rule of five’ and
‘Veber rule’. According to Lipinski’s rule, compounds are
more likely to be drug-like and orally bioavailable, if they
Table 4. Details of hydrogen bonding interaction.
H-binding Ligand
H-binding Receptor
Compounds
H-bond(s)
Element
Type
Residue
Element
Type
H-Bond
Distance (Å)
O
A
Cys 42
S
D
3.41
O
A
His 174
N
D
3.39
3b
4
N
H
D
D
Ser 149
Asn 173
C
O
A
A
3.68
2.81
O
A
His 174
H
D
2.92
H
D
Gly 83
N
A
2.67
3h
4
N
H
D
D
Ser 149
Gly 40
C
O
A
A
3.12
2.44
N
A
Cys 42
S
D
2.63
O
A
Ile 85
N
D
2.84
H
D
Gly 83
O
A
2.22
3i
4
H
D
Asp 234
O
A
3.00
Table 5. Drug-likeness parameters.
Lipinski’s Parameters
Compounds
MW
LogP
nHBA
nHBD
TPSA
(A2)
nViolations
MS
MV
(A3)
nRotB
DL Score
3a
281.74
4.299
3
1
37.28
0
-6.07
252.27
3
-0.17
3b
297.74
4.005
4
2
57.51
0
-5.52
262.80
3
-0.09
3c
297.74
4.057
4
2
57.51
0
-5.58
262.89
3
0.14
3d
297.74
4.057
4
2
46.51
0
-5.62
262.82
3
-0.01
3e
311.77
4.283
4
1
46.51
0
-6.21
284.19
4
0.21
3f
311.77
4.283
4
1
37.28
0
-6.23
284.12
4
0.12
3g
316.19
4.963
3
1
83.10
0
-6.85
269.46
3
-0.15
3h
326.74
4.193
5
1
66.74
0
-6.46
290.45
4
0.20
3i
327.77
4.041
5
2
55.74
0
-5.69
295.54
4
0.33
3j
341.80
4.266
5
1
37.28
0
-6.31
315.54
5
0.30
3k
295.77
4.785
3
1
37.28
0
-6.60
273.21
3
-0.16
3l
324.81
4.659
3
1
40.52
0
-7.03
293.58
4
-0.19
MW- Molecular weight, LogP- Log of octanol/water partition coefficient, nHBA- No. of hydrogen bond accepror(s), nHBD- No. of hydrogen bond donor(s), TPSA- Total polar
surface area, nViolations- No. of rule of five violations, MS- Molar aqueous solubility, MR- Molar refractivity, MV- Molar volume, nRotB- No. of rotable bonds, DL- Drug-likeness.
Design, Synthesis, Antimalarial Activity of 7-Chloro-4-(2-(substituted benzylidene)hydrazineyl)quinolines Med icinal C hemistry, 2020, Vol. 16, No. 7 935
meet the following criteria: LogPo/w (octanol/water partition
coefficient) ≤ 5, MW ≤ 500, nHBAs ≤ 10 and nHBDs ≤5. To
further substantiate drug-likeness, Veber et al. stated that
compounds with ≤ 10 RotB and TPSA of ≤ 140 A2 are more
likely to show optimum membrane permeability and good
bioavailability. ‘Lipinski rule of five’ is considered signifi-
cant for the prediction of oral bioavailability; however, 16%
of oral drugs violate at least one of the criteria and 6% fail in
two or more [11, 12]. In our study, compounds did not vio-
late ‘Lipinski rule of five’ parameters. Poor absorption or
permeation of a ligand is more likely if a drug-like molecule
has more than one of five rule violations (nViolations). Val-
ues of LogP, MW, and TPSA indicate that all the compounds
possess good membrane permeability and oral bioavailabil-
ity, whereas, Rotb bonds suggest that compounds have good
intestinal availability. MS data indicate good oral bioavail-
ability of compounds if given by oral route. MV and MR
values are also in permissible range, which in turn indicate
good oral bioavailability for all the compounds. Hydropho-
bicity, membrane permeability and bioavailability are de-
pendent on molecule’s MW, LogP, MS, HBA and HBD.
Molecules violating more than one of these rules fail to ex-
hibit optimum bioavailability. Sufficient water solubility is
also important for optimal bioavailability of drugs. RotB is
important for molecular conformational studies (i.e., stereo-
selectivity of drug molecules) for binding/ interaction with
the receptor molecule. Reduced molecular flexibility, as
measured by the NRotB, and TPSA and total hydrogen bond
count (sum of donors and acceptors) are some important
predictors of good oral bioavailability, independent of mo-
lecular weight. The drug score combines all drug-likeness
properties (i.e., lipophilicity, solubility, molecular weight,
the risk of toxicity) into a single numerical value that can be
used to predict a global value for a new drug molecule [1].
Drug-likeness scores were obtained in the range of -0.19 and
+0.33. Scores were in an acceptable range indicating the
suitability of compounds to be transformed into potential
drug candidates. The overall analysis of drug-likeness stud-
ies strongly suggests that quinoline imine derivatives possess
good drug-likeness behavior favorable for optimal mem-
brane permeability, transport and bioavailability, and even-
tual binding/ interaction with the receptor molecule.
3.5. ADMET
The predicted ADMET values of compounds (Table 6)
were found in an acceptable range. All the compounds were
predicted to have good intestinal absorption and non-
inhibitors of cytochrome P450 2D6 (CYP 2D6), with me-
dium to moderate blood-brain barrier (BBB) penetration.
Penetration through BBB is mandatory for a drug molecule
to be effective in the treatment of cerebral malaria. The CYP
2D6 enzyme is one of the important metabolic enzymes in-
volved in drug metabolism [13]. The aqueous solubility (de-
fined in water at 25 oC) indicates that most of the compounds
were soluble in water. The predictive hepatotoxicity was not
observed for the compounds. Some of the compounds were
found to be highly protein bound, while some were poorly
protein-bound [14-16].
CONCLUSION
Newly designed 7-chloro-4-(2-(substituted ben-
zylidene)hydrazineyl)quinolines possess antimalarial poten-
tial in P. falciparum malaria. These compounds have in vitro
Table 6. Predicted ADMET properties.
Compounds
Aqueous
Solubility
BBB Penetration
CYP P450 2D6
Inhibition
Hepatotoxicity
Intestinal
Absorption
PP Binding
3a
2
1
FALSE
FALSE
0
TRUE
3b
2
1
FALSE
FALSE
0
FALSE
3c
2
1
FALSE
FALSE
0
TRUE
3d
2
1
FALSE
FALSE
0
FALSE
3e
2
1
FALSE
FALSE
0
TRUE
3f
2
1
FALSE
FALSE
0
TRUE
3g
1
0
FALSE
FALSE
0
FALSE
3h
2
2
FALSE
FALSE
0
TRUE
3i
2
1
FALSE
FALSE
0
TRUE
3j
2
1
FALSE
FALSE
0
FALSE
3k
2
0
FALSE
FALSE
0
FALSE
3l
2
0
FALSE
FALSE
0
FALSE
Aqueous solubility: 3-Good, 2-Low; BBB (Blood brain barrier) penetration: 3-Lo w, 2-Medium, 1-Mod erate; Cytoc hrome (CYP) P450 2D6 inhibitio n: T rue-Inhibit or, False-Non-
inhibitor; Hepatotoxicity: True-Toxic, False-Non-toxic; Intestinal absorption: 0-Good; Plasma protein (PP) binding: True-Highly bounded, False-Poorly bounded
936 Medicinal Chemistry, 2020, Vol. 16, No. 7 Kalita et al.
antimalarial effectiveness against both CQ-sensitive and CQ-
resistant strains of P. falciparum parasite. In silico studies
further, substantiate the biological potential of quinoline de-
rivatives with desired drug-like and ADMET properties. Our
study also reports the antimalarial efficacy of quinoline imi-
nes as novel plasmepsin 2 (PM-II) inhibitors. It is finally
concluded that the molecular scaffold of quinoline imine
may be used as a lead structure for further modifications in
search for more potent antimalarial drug molecules. Further,
apart from their antimalarial activity, the quinoline deriva-
tives reported in our study may also have potential in other
areas of medicine as biologically important heteroaromatic
compounds.
ETHICS APPROVAL AND CONSENT TO PARTICI-
PATE
Not applicable.
HUMAN AND ANIMAL RIGHTS
No Animals/Humans were used for studies that are base
of this research.
CONSENT FOR PUBLICATION
Not applicable.
AVAILABILITY OF DATA AND MATERIALS
Not applicable.
FUNDING
None.
CONFLICT OF INTEREST
The authors declare no conflict of interest, financial or
otherwise.
ACKNOWLEDGEMENTS
Authors express their sincere thanks to the Head, SAIF,
NEHU, Shillong for providing spectral data of synthesized
compounds.
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