Design,: In silico studies, and synthesis of new 1,8-naphthyridine-3-carboxylic acid analogues and evaluation of their H1R antagonism effects

Article abstract of DOI:10.1039/d0ra00746c

New 1,8-naphthyridine-3-carboxylic acid derivatives were designed, synthesized and evaluated for their in vivo antihistaminic activity on guinea pig trachea by using chlorpheniramine as the standard drug. It was found that compound 5a1 displayed a promising bronchorelaxant effect in conscious guinea pigs using the in vivo model. A molecular docking study was performed to understand the molecular interaction and binding mode of the compounds in the active site of the H1 receptor. Furthermore, in silico computational studies were also performed to predict the binding modes and pharmacokinetic parameters of these derivatives. Prior to the start of experimental lab work, PASS software was used to predict the biological activities of these compounds. An in silico PASS, Swiss ADME assisted docking approach was found to be suitable to derive and synthesize effective antihistaminic agents for the present study.

Full text of DOI:10.1039/d0ra00746c

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Design, in silico studies, and synthesis of new 1,8-
naphthyridine-3-carboxylic acid analogues and
evaluation of their H1R antagonism effects†
Cite this: RSC Adv., 2020, 10, 13907
*
Vinod Kumar Gurjar and Dilipkumar Pal
New 1,8-naphthyridine-3-carboxylic acid derivatives were designed, synthesized and evaluated for their in
vivo antihistaminic activity on guinea pig trachea by using chlorpheniramine as the standard drug. It was
found that compound 5a1 displayed a promising bronchorelaxant effect in conscious guinea pigs using
the in vivo model. A molecular docking study was performed to understand the molecular interaction
and binding mode of the compounds in the active site of the H1 receptor. Furthermore, in silico
computational studies were also performed to predict the binding modes and pharmacokinetic
parameters of these derivatives. Prior to the start of experimental lab work, PASS software was used to
predict the biological activities of these compounds. An in silico PASS, Swiss ADME assisted docking
approach was found to be suitable to derive and synthesize effective antihistaminic agents for the
present study.
Received 23rd January 2020
Accepted 11th March 2020
DOI: 10.1039/d0ra00746c
rsc.li/rsc-advances
linked to a terminal amino group via a two or three carbon
chain (chlorpheniramine maleate).21 In contrast, most of the
1 Introduction
non-sedative H1R inhibitors like azelastine, levocetirizine and
fexofenadine22 have a minimal muscarinic effect and show
a better BRA (benet-to-risk ratio). These drugs cannot diffuse
through the blood–brain barrier and thus have a weaker
sedating effect and possess higher receptor selectivity and
affinity,23 and they are marked as ‘non-sedative’ H1R inhibitors.
The ‘non-sedative’ second-generation drugs like terfenadine
and levocetirizine also have many structural features belonging
to old classic antihistamines. The condensed heterocyclic ring
system of the second-generation H1-antihistamines (loratadine,
fexofenadine, and azelastine) does not have the above-described
pharmacophoric groups of H1-antihistamines. This concept
and idea have enabled the discovery of many potent antihista-
minic drugs like temelastine and mangostin. The currently used
antihistaminic compounds are almost completely based on
modifying the structure of some old H1 antagonists (Fig. 1).
The availability of the crystal structure of H(1)R, however,
can enable new ways to study the binding of histamine and its
antagonists and to nd the important receptor–ligand interac-
tions. It is also possible to compare the binding affinity of the
synthesized analogues to predict their possible therapeutic
activity.24,25
Heterocyclic synthesis has become a powerful technique in
organic synthesis for generating new molecules for drug
discovery and development.1–8 Nitrogen atom-containing
heterocyclic compounds provide highly functionalized scaf-
folds on which pharmacophoric features can be arranged to
obtain effective and selective drugs.9–17 Histamine is one of the
most important chemical mediators that inuences immune
regulation via an acute and chronic inammatory response
through 4 different types of G-protein coupled receptors, H1,
H2, H3, and H4. It is involved in the pathophysiology of allergic
disorders like urticaria, rhino conjunctivitis, and asthma.18
Although antihistaminic agents belong to several chemical
classes, such as ethylene-diamines, aminoethyl ethers, propyl-
and propenyl-arnines, phenothiazines, piperidines and pipera-
zines, they show remarkable chemical similarities.19 Presently,
antihistamines are widely prescribed for the management of
various allergic symptoms, but these drugs show complex side
effect proles, which include sedation, light-headedness, motor
incoordination, cardiovascular effects, diminished alertness,
concentration difficulties, fatigue and a tendency to fall
asleep.20 A common structural feature for the classical rst-
generation antihistaminic drugs is an aryl group including
phenyl, substituted phenyl and heteroaryl groups (2-pyridyl)
Azelastine is a phthalazinone derivative and a H1R antago-
nist.26 It is an antiallergic agent that inhibits the release of
histamine and other mediators involved in the allergic
response. It antagonizes histamine and leukotriene-induced
bronchospasm in animal studies and reduces airway respon-
siveness to an inhaled antigen or distilled water and exercise
challenges.27 From a comprehensive literature search of 1,8-
Department of Pharmaceutical Sciences, Guru Ghasidas Vishwavidyalaya (A Central
University), Bilaspur-495 009, CG, India. E-mail: drdilip71@gmail.com; Tel:
+91-7389263761
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra00746c
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Fig. 1 Classical antihistamines.
naphthyridine, it has been found to be a potent antiallergic and
In reference to our continuous interest in establishing
antihistaminic agent.28,29 In addition, its derivatives are re- potential non-sedating, nonclassical histaminergic (H1)
ported to have a broad spectrum of pharmacological activity. blockers with low toxicity proles, we have previously published
Notable amongst these are A1 and A2A adenosine receptor a research paper where the synthesis of some substituted 1,8-
agonist,30 antibacterial,31 analgesic and anti-inammatory32 naphthyridine derivatives with different cyclic amine substitu-
agents, cannabinoid receptor ligands33 and phosphodiesterase ents of 1,8-naphthyridine-3-carboxylic acid35 has been
(PDE 4) inhibitors.34 A review of the literature also supports that described. Consequently, in this work, the key compound 1,8-
azelastine as an asthma prophylactic has three main pharma- naphthyridine-3-carboxylic acid is selected, which is susceptible
cophoric groups: (i) phthalazinone nuclei, (ii) a p-chlorobenzyl to forming amide derivatives. In the present paper, we have
moiety and (iii) a basic amino group, N-methyl azepine. These reported the design, synthesis and in vitro antihistaminic
three parts are essential to be an effective H1R antagonistic activity of 1,8-naphthyridine-3-carboxylic acid derivatives that
compound.
are modied at the terminal 3-carboxylic acid end (Fig. 2). In the
Fig. 2 Design of 1,8-naphthyridine derivatives.
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Scheme 1 Preparation of 1,4-dihydro-4-oxo-1,8-naphthyridine-3-carboxylic acid derivatives. Reagents and conditions: (i) diethyl ethoxy
methylene malonate, 120 ꢀC, 1 h, (ii) diphenyl ether, 250 ꢀC reflux, 4 h, (iii) p-chlorobenzylchloride/benzyl chloride, NaH, DMF, 90 ꢀC reflux, 24 h,
(iv) NaOH, EtOH, 100 ꢀC reflux, 2 h and (v) R–NH R–NH2, heat 25–100 ꢀC in a sealed tube for 24 h.
current drug discovery process, the potential of a novel
compound is frequently studied initially through virtual tools.
The possibility of a compound to exhibit useful therapeutic
2 Materials and methods
2.1 General
activity (sometimes called ‘drug-likeness’) is predicted from its
molecular structure.36,37 Prediction of bioavailability and
bioavailability-related properties, such as solubility and lip-
ophilicity, is signicant before synthesis. This could be the best
method to avoid depletion of chemicals, valuable time and
above all possible environmental problems. Furthermore, we
also performed in silico ADMET prole and PASS analyses of all
compounds.
All chemicals and reagents were purchased from commercial
sources (Himedia, Sigma-Aldrich) having the best analytical-
grade and used without further purication unless otherwise
stated. Melting points were checked using open capillary
methods on a Macro scientic apparatus and were uncorrected.
All reactions were routinely checked by thin-layer chromatog-
raphy (TLC) analysis on silica gel G plates eluted with a mixture
of chloroform : methanol (4 : 1) as a solvent system. The IR
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spectra were recorded using a Shimadzu FTIR spectrophotom- of ethyl ether caused the precipitation of carboxylate 3a and 3b
eter. 1HNMR spectra were recorded using a Bruker (Avance III) as pure solids. The 1,8-naphthyridine-3-carboxylic acid ethyl
400 MHz system spectrometer with tetramethyl silane (TMS) as esters 3a and 3b (4.13 mmol) and a mixture of 10% sodium
the internal standard and DMSO as the solvent. Chemical shis hydroxide (5 mL) and ethyl alcohol (5 mL) were reuxed for 2 h.
were reported as parts per million (ppm). Signals were repre- Aer cooling, the solution was adjusted to pH 4 with aqueous
sented as follows: s: singlet, d: doublet, m: multiplet, and t: 10% hydrochloric acid. The resulting precipitate of 4a and 4b
triplet. High-resolution mass spectra (HRMS) were obtained was collected. A mixture of the 1,8-naphthyridine acids 4a and
using the Electrospray Ionization (ESI) technique on a Bruker 4b (1 mmol) and appropriate amines (2-aminopyridine, 2-
Fourier Transform Ion Cyclotron Resonance Mass chloro aniline, cyclohexane amine, and n-phenyl piperazine)
Spectrometer.
(10 mmol each) was heated in a sealed tube at 120ꢀ for 24 h.
The compounds were synthesized in a sequence of reactions Aer cooling, the reaction mixtures were treated with ethyl
from the starting material 2-aminopyridine (1). Diethyl ethoxy ether to give the title compounds as a pure solid.
methylene malonate (2), p-chloro benzyl chloride/benzyl chlo-
1-(4-Chlorobenzyl)-4-oxo-N-(pyridin-2-yl)-1,4-dihydro-1,8-
ride (3) and 1,8-naphthyridine-3-carboxylic acid (4) were ob- naphthyridine-3-carboxamide (5a1). White crystalline powder
tained using reported methods. The synthetic route to the title (yield 76.6%); mp > 300 ꢀC; (DMSO-d6) d ppm: 5.68 (s, 2H, CH2-
compound is illustrated in the following Scheme 1.35,38
Ph), 8.2 (dd, 1H, aromatic, J ¼ 4.4, Hz), 9.19 (d, 1H, aromatic),
2-Aminopyridine (1 mmol) and diethyl ethoxy methylene 7.36 (d, 4H, Ph-H, J ¼ 8.24 Hz), 7.42 (d, 2H, aromatic, J ¼ 8.4 Hz),
malonate (1 mmol) were heated at 120–130 ꢀC for 2 h producing 7.66 (d, 1H, aromatic, J ¼ 8.8 Hz), 7.76 (d, 2H, aromatic), 8.51 (s,
crude malonate (1) that was puried by recrystallization from 1H, aromatic), IR (KBr, cmꢁ1): 3240 (NH), 3112.7, 3086.0 (C–H
light petroleum ether. The crude ester (1, 0.017 mol) and aromatic), 1686.4 (C]O keto), 1651.1 (C]O amide), 734 (C–Cl);
diphenyl ether in excess were heated at 240–250 ꢀC for one hour, mass (m/z): 390 (M+), chemical formula: C21H15ClN4O2, MW
cooled at room temperature and washed with petroleum ether. 390.82, anal calc.: C, 64.54; H, 3.87; N, 14.34; found: C, 64.44; H,
The resulting white powder of malonate (2) was recrystallized 3.78; N, 14.30%.
from dimethylformamide. Sodium hydride (NaH, 1.81 mmol,
1-(4-Chlorobenzyl)-N-(2-chlorophenyl)-4-oxo-1,4-dihydro-1,8-
50% in mineral oil) and a solution of malonate (2) (1.5 mmol) in naphthyridine-3-carboxamide (5a2). White crystalline powder
10 mL of dry DMF were stirred at room temperature. Aer one (yield 75%); mp 193–195 ꢀC; (DMSO-d6) d, ppm: 8.25 (s, 1H,
hour, p-chlorobenzyl chloride (for 3a) and chlorobenzyl chloride aromatic), 8.52 (dd, 1H, aromatic, J ¼ 4.67 Hz), 7.57 (dd, 1H,
(for 3b) (1 mmol each) were added and the mixture was stirred aromatic,, J ¼ 7.77 Hz), 7.97 (d, 1H, aromatic, J ¼ 7.77 Hz), 7.24
for 24 h. The resultant solution was evaporated and the addition (m, 4H, aromatic), 5.68 (s, 2H, CH2-Ph), 9.19 (d, 1H, NH), 7.24–
Table 1 Antihistaminic and sedative–hypnotic activity of the compounds (5a1–10 and 5b1–10)a
Antihistaminic activity
Compd
Percent CNS depression
1 h
Time of onset of
convulsion (s)
% Protection
2 h
3 h
9.81 ꢂ 1.52**
5a1
5a2
5a3
5a4
5a5
5a6
5a7
5a8
5a9
5a10
5b1
5b2
5b3
5b4
5b5
5b6
371 ꢂ 6.53*
352 ꢂ 2.46*
352 ꢂ 1.82*
341 ꢂ 2.40*
311 ꢂ 2.43*
328 ꢂ 2.86*
305 ꢂ 1.52*
340 ꢂ 4.29*
316 ꢂ 4.56*
311 ꢂ 2.46*
354 ꢂ 5.33*
353 ꢂ 3.87*
366 ꢂ 3.94*
354 ꢂ 4.23*
355 ꢂ 6.42*
304 ꢂ 6.69*
305 ꢂ 6.38*
359 ꢂ 5.41*
301 ꢂ 5.87*
361 ꢂ 6.72*
143 ꢂ 3.29*
411 ꢂ 4.43*
61.45 ꢂ 0.15*
59.38 ꢂ 0.19*
59.37 ꢂ 0.14*
58.06 ꢂ 0.16*
54.01 ꢂ 0.14*
56.40 ꢂ 0.19*
53.11 ꢂ 0.13*
57.94 ꢂ 0.36*
54.74 ꢂ 0.42*
54.01 ꢂ 0.23*
59.60 ꢂ 0.352*
59.49 ꢂ 0.284*
60.92 ꢂ 0.315*
59.60 ꢂ 0.265*
59.71 ꢂ 1.53*
52.96 ꢂ 1.42*
53.11 ꢂ 1.28*
60.16 ꢂ 1.39*
52.49 ꢂ 1.72*
60.38 ꢂ 1.40*
—
13.02 ꢂ 0.22**
9.12 ꢂ 1.02**
10.14 ꢂ 0.56**
7.11 ꢂ 0.88**
8.16 ꢂ 0.73**
7.11 ꢂ 0.88**
7.11 ꢂ 0.88**
6.00 ꢂ 0.58**
8.18 ꢂ 0.74**
9.61 ꢂ 1.42**
8.58 ꢂ 0.09**
9.53 ꢂ 0.13**
6.43 ꢂ 0.26**
8.28 ꢂ 0.10**
8.23 ꢂ 1.23**
9.23 ꢂ 1.72**
11.10 ꢂ 1.52**
12.9 ꢂ 1.72**
6.21 ꢂ 1.94**
8.29 ꢂ 1.93**
6.10 ꢂ 0.49**
38.80 ꢂ 0.32**
13.60 ꢂ 1.26**
9.15 ꢂ 1.02**
11.16 ꢂ 0.77**
8.16 ꢂ 0.73**
8.19 ꢂ 0.76**
7.17 ꢂ 0.87**
9.81 ꢂ 1.52**
7.17 ꢂ 0.87**
10.13 ꢂ 0.55**
10.11 ꢂ 0.56**
11.17 ꢂ 0.10**
11.2 ꢂ 0.09**
8.10 ꢂ 0.09**
9.42 ꢂ 0.17**
12.1 ꢂ 1.51**
13.1 ꢂ 1.60**
14.1 ꢂ 1.62**
15.1 ꢂ 1.83**
12.1 ꢂ 1.87**
10.1 ꢂ 1.04**
4.1 ꢂ 0.59**
6.00 ꢂ 0.58**
8.03 ꢂ 0.91**
5.01 ꢂ 0.43**
6.00 ꢂ 0.58**
5.01 ꢂ 0.43**
6.00 ꢂ 0.58**
5.18 ꢂ 0.4**
6.07 ꢂ 0.59**
7.11 ꢂ 0.88**
5.29 ꢂ 0.11**
5.16 ꢂ 0.08**
4.67 ꢂ 0.13**
5.09 ꢂ 0.10**
6.1 ꢂ 1.62**
9.1 ꢂ 1.54**
7.2 ꢂ 1.93**
9.1 ꢂ 1.7**
5b7
5b8
5b9
5b10
Control
Chlorpheniramine
6.1 ꢂ 1.92**
8.1 ꢂ 1.87**
4 ꢂ 0.91**
65.20 ꢂ 0.33*
34.80 ꢂ 0.72**
29.58 ꢂ 0.72**
a
Each value represents the mean ꢂ SEM (n ¼ 6). Signicance levels *p < 0.001, **p > 0.05.
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7.46 (m, 4H, aromatic), IR (KBr, cmꢁ1): 3112.7, 3086.0 (C–H 13.2 Hz), IR (KBr, cmꢁ1): 3069.1, 2998.7 (C–H), 1714.0 (C]O
aromatic), 1686.4 (C]O amide), 1651.1 (C]O ring), 797.70 (C– keto), 1692.1 (C]O amide), 781.0 (C–Cl); mass (m/z): 395 (M+),
Cl); mass m/z 423 (M+), chemical formula: C22H15Cl2N3O2, MW chemical formula: C22H22ClN3O2, MW 395.87, anal calc.: C,
423.29, anal calc.: C, 62.28; H, 3.56; N, 9.90; found: C, 62.21; H, 66.75; H, 5.60; N, 10.61; found: C, 66.62; H, 5.42; N, 10.52%.
3.72; N, 9.81%.
1-(4-Chlorobenzyl)-3-(piperidine-1-carbonyl)-1,8-naphthyr-
1-(4-Chlorobenzyl)-N-(3-chlorophenyl)-4-oxo-1,4-dihydro-1,8- idin-4(1H)-one (5a8). Pale yellow crystalline powder (yield
naphthyridine-3-carboxamide (5a3). Off white crystalline 76.7%); mp > 300 ꢀC; (DMSO-d6) d, ppm: 9.29 (s, 1H, aromatic),
powder (yield 67%); mp > 300 ꢀC; (DMSO-d6) d ppm: 9.19 (s, 1H, 8.52 (dd, 1H, aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H, aromatic, J ¼ 7.8
aromatic), 8.53 (dd, 1H, aromatic, J ¼ 4.67 Hz), 7.57 (dd, 1H, Hz), 7.97 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.24–7.46 (m, 4H,
aromatic,, J ¼ 7.77 Hz), 7.97 (d, 1H, aromatic, J ¼ 7.77 Hz), 7.24 aromatic), 5.68 (s, 2H, CH2-Ph), 8.71 (s, 1H, aromatic), 8.71 (s,
(m, 4H, aromatic), 5.68 (s, 2H, CH2-Ph), 9.91 (d, 1H, NH), 7.24– 1H, aromatic), 1.5–1.8 (m, 6H, piperidine), 1.9–2.2 (t, 4H,
7.46 (m, 4H, aromatic), IR (KBr, cmꢁ1): 3112.7, 3086.0 (C–H piperidine), IR (KBr, cmꢁ1): 3112.7, 3086.0 (C–H aromatic),
aromatic), 1686.4 (C]O keto), 1651.1 (C]O amide), 780 (C–Cl); 1686.4 (C]O keto), 1651.1 (C]O), 780.10 (C–Cl); mass (m/z):
mass (m/z): 423 (M+), chemical formula: C22H15Cl2N3O2, MW 381 (M+), chemical formula: C21H20ClN3O2, MW 381.87, anal
424.28, anal calc.: C, 62.28; H, 3.56; N, 9.90; found: C, 62.21; H, calc.: C, 66.05; H, 5.28; N, 11.00; found: C, 66.01; H, 5.22; N,
3.72; N, 9.81%.
10.87%.
1-(4-Chlorobenzyl)-3-(piperazine-1-carbonyl)-1,8-naphthyr-
1-(4-Chlorobenzyl)-N-(4-chlorophenyl)-4-oxo-1,4-dihydro-1,8-
naphthyridine-3-carboxamide (5a4). Off white crystalline idin-4(1H)-one (5a9). Pale yellow crystalline powder (yield
powder (yield 66%); mp 193–195 ꢀC; (DMSO-d6) d, ppm: 9.19 (d, 76.1%); mp 193–195 ꢀC; (DMSO-d6) d, ppm: 9.29 (s, 1H,
1H, NH), 9.29 (s, 1H, H2), 8.52 (dd, 1H, aromatic, J ¼ 4.66 Hz), aromatic), 8.52 (dd, 1H, aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H,
7.56 (d, 1H, aromatic, J ¼ 7.77 Hz), 7.96 (d, 1H, aromatic, J ¼ aromatic, J ¼ 7.8 Hz), 7.97 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.24–
7.77 Hz), 8.71 (s, 1H, aromatic), 7.24–7.46 (m, 4H, aromatic), 7.46 (m, 4H, aromatic), 5.68 (s, 2H, CH2-Ph), 8.71 (s, 1H,
5.68 (s, 2H, CH2-Ph), 7.32–7.47 (m, 4H, aromatic), IR aromatic), 7.22–734 (m, 4H, aromatic), 1.5–1.7 (m, 4H, pipera-
(KBr, cmꢁ1): 3112.7, 3086.0 (C–H aromatic), 1686.4 (C]O keto), zine), 1.9–2.1 (m, 4H, piperazine), IR (KBr, cmꢁ1): 3112.7, 3086.0
1651.1 (C]O amide), 797.7 (C–Cl); mass m/z 423 (M+), chemical (C–H aromatic), 1686.4(C]O amide), 1651.1 (C]O ring), 797.8
formula: C22H15Cl2N3O2, MW 424.28, anal calc.: C, 62.28; H, (C–Cl); mass (m/z): 382 (M+), chemical formula: C20H19ClN4O2,
3.56; N, 9.90; found: C, 62.28; H, 16.71; N, 9.90%.
MW 382.85, anal calc.: C, 62.74; H, 5.00; N, 14.63; found: C,
1-(4-Chlorobenzyl)-3-(pyrrolidine-1-carbonyl)-1,8-naphthyr- 62.65; H, 5.01; N, 14.54%.
idin-4(1H)-one (5a5). Off white crystalline powder (yield 56%);
1-(4-Chlorobenzyl)-3-(4-methylpiperazine-1-carbonyl)-1,8-
mp 181–183 ꢀC; (DMSO-d6) d, ppm: 9.19 (s, 1H, aromatic), 8.52 naphthyridin-4(1H)-one (5a10). Pale yellow crystalline powder
ꢀ
(dd, 1H, aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H, aromatic, J ¼ 7.8 Hz), (yield 55.10%); mp 181–183 C; (DMSO-d6) d, ppm: 9.29 (s, 1H,
7.97 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.24–7.48 (m, 4H, aromatic), aromatic), 8.52 (dd, 1H, aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H,
5.68 (s, 2H, CH2-Ph), 8.71 (s, 1H, aromatic), 3.37 (t, 4H, pyrro- aromatic, J ¼ 7.8 Hz), 7.97 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.24–
lidine, J ¼ 13.2 Hz), 1.67 (t, 4H, pyrrolidine, J ¼ 15.2 Hz), IR 7.46 (m, 4H, aromatic), 5.68 (s, 2H, CH2-Ph), 8.71 (s, 1H,
(KBr, cmꢁ1): 3069.1, 2998.7 (C–H), 1714.0 (C]O keto), 1692.1 aromatic), 1.5–1.7 (m, 4H, piperazine), 1.9–2.1 (m, 4H, pipera-
(C]O amide), 781 (C–Cl); mass (m/z): 367 (M+), chemical zine), 2.88 (s, 3H, CH3), IR (KBr, cmꢁ1): 3069.1, 2998.7 (C–H),
formula: C20H18ClN3O2, MW 367.84, anal calc.: C, 65.31; H, 1714.0 (C]O keto), 1692.1 (C]O), 781.0 (C–Cl); mass (m/z): 396
4.93; N, 11.42; found: C, 65.22; H, 4.84; N, 11.41%.
1-(4-Chlorobenzyl)-3-(morpholine-4-carbonyl)-1,8-naphthyr- C, 63.55; H, 5.33; N, 14.12; found: C, 63.48; H, 5.24; N, 14.09%.
(M+), chemical formula: C21H21ClN4O2, MW 396.87, anal calc.:
idin-4(1H)-one (5a6). Off white crystalline powder (yield 76.6%);
1-Benzyl-4-oxo-N-(pyridin-2-yl)-1,4-dihydro-1,8-naphthyri-
ꢀ
mp 147–149 C; (DMSO-d6) d ppm: 9.19 (s, 1H, aromatic), 8.52 dine-3-carboxamide (5b1). Pale yellow crystalline powder (yield
(dd, 1H, aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H, aromatic, J ¼ 7.8 Hz), 76%); mp > 300 ꢀC; (DMSO-d6) d, ppm: 8.51 (dd, 1H, aromatic, J
7.97 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.24–7.46 (m, 4H, aromatic), ¼ 4.8 Hz), 8.24 (dd, 1H, aromatic, J ¼ 4.7 Hz), 5.68 (s, 2H, CH2-
5.68 (s, 2H, CH2-Ph), 8.71 (s, 1H, aromatic), 1.91 (d, 1H, NH), Ph), 9.19 (dd, 1H, aromatic, J ¼ 7.8 Hz), 7.36 (d, 4H, Ph-H, J ¼
2.1–2.3 (m, 4H, morpholine, J ¼ 15.2 Hz), 2.6–2.8 (m, 4H, 8.24 Hz), 7.31–6.96 (m, 5H, aromatic), 7.40 (m, 2H, aromatic),
morpholine, J ¼ 15.2 Hz), IR (KBr, cmꢁ1): 3069.1, 2998.7 (C–H), 7.77 (dd, 1H aromatic, J ¼ 4.7 Hz), 6.59–8.08 (m, 4H, pyridine),
1714.0 (C]O keto), 1692.1 (C]O amide), 781.00 (C–Cl); mass IR (KBr, cmꢁ1): 3112.7, 3086.0 (C–H aromatic), 1686.4 (C]O
(m/z): 383 (M+); chemical formula: C20H18ClN3O3, MW 383.84, keto), 1651.1 (C]O ring), 780.10 (C–Cl); mass (m/z): 356 (M+),
anal calc.: C, 62.58; H, 4.73; N, 10.95; found: C, 62.51; H, 4.70; N, chemical formula: C21H16N4O2, MW 356.38, anal calc.: C, 70.77;
10.86%.
1-(4-Chlorobenzyl)-N-cyclohexyl-4-oxo-1,4-dihydro-1,8-naph-
H, 4.53; N, 15.72; found: C, 70.70; H, 4.46; N, 15.65%.
1-Benzyl-N-(2-chlorophenyl)-4-oxo-1,4-dihydro-1,8-naphthyr-
thyridine-3-carboxamide (5a7). Off white crystalline powder idine-3-carboxamide (5b2). Pale yellow powder (yield 75%); mp
ꢀ
(yield 65.1%); mp 182–184 C; (DMSO-d6) d, ppm: 9.29 (s, 1H, 193–195 ꢀC; (DMSO-d6) d, ppm: 8.25 (s, 1H, aromatic), 8.52 (dd,
aromatic), 8.52 (dd, 1H, aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H, 1H, aromatic, J ¼ 4.67 Hz), 7.57 (dd, 1H, aromatic, J ¼ 7.77 Hz),
aromatic, J ¼ 7.8 Hz), 7.97 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.24– 7.97 (d, 1H, aromatic, J ¼ 7.77 Hz), 7.24–7.33 (m, 5H, aromatic),
7.46 (m, 4H, aromatic), 5.68 (s, 2H, CH2-Ph), 8.71 (s, 1H, 5.68 (s, 2H, CH2-Ph), 9.19 (d, 1H, NH), 7.14–8.06 (m, 5H,
aromatic), 3.85 (m, 1H, CH), 2.17–1.59 (m, 10H, cyclohexyl, J ¼ aromatic), IR (KBr, cmꢁ1): 3112.7, 3086.0 (C–H aromatic),
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1686.4 (C]O amide), 1651.1 (C]O amide), 797.8 (C–Cl); mass 361.44, anal calc.: C, 73.11; H, 6.41; N, 11.63; found: C, 73.03; H,
(m/z): 389 (M+), chemical formula: C22H16ClN3O2, MW 389.84, 6.32; N, 11.53%.
anal calc.: C, 67.78; H, 4.14; N, 10.78; found: C, 67.71; H, 4.05; N,
10.70%.
1-Benzyl-3-(piperidine-1-carbonyl)-1,8-naphthyridin-4(1H)-
one (5b8). Yellow crystalline powder yield (%): 76.6; mp >300 ꢀC;
1-Benzyl-N-(3-chlorophenyl)-4-oxo-1,4-dihydro-1,8-naphthyri- (DMSO-d6) d, ppm: 9.19 (s, 1H, aromatic), 8.52 (dd, 1H,
dine-3-carboxamide (5b3). Pale yellow crystalline powder (yield aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.97 (d,
76.2%); mp > 300 ꢀC; (DMSO-d6) d, ppm: 8.25 (s, 1H, aromatic), 1H, aromatic, J ¼ 7.8 Hz), 7.24–7.46 (m, 5H, aromatic), 5.68 (s,
8.52 (dd, 1H, aromatic, J ¼ 4.67 Hz), 7.57 (dd, 1H, aromatic, J ¼ 2H, CH2-Ph), 8.71 (s, 1H, aromatic), 8.71 (s, 1H, aromatic), 1.5–
7.77 Hz), 7.97 (d, 1H, aromatic, J ¼ 7.77 Hz), 7.24–7.33 (m, 5H, 1.8 (m, 6H, piperidine), 1.9–2.2 (t, 4H, piperidine), IR
aromatic), 5.68 (s, 2H, CH2-Ph), 9.19 (d, 1H, NH), 7.14–7.99 (m, (KBr, cmꢁ1): 3112.7, 3086.0 (C–H aromatic), 1686.4 (C]O keto),
4H, aromatic), IR (KBr cmꢁ1): 3112.7, 3086.0 (C–H aromatic), 1651.1 (C]O); mass (m/z): 347 (M+), chemical formula:
1686.4 (C]O keto), 1651.1 (C]O amide), 780 (C–Cl); mass (m/
C21H21N3O2, MW 347.41, anal calc.: C, 72.60; H, 6.09; N, 12.10;
z): 389 (M+), chemical formula: C22H16ClN3O2, MW 389.83, anal found: C, 72.54; H, 6.01; N, 12.04%.
calc.: C, 67.78; H, 4.14; N, 10.78; found: C, 67.71; H, 4.05; N,
10.70%.
1-Benzyl-3-(piperazine-1-carbonyl)-1,8-naphthyridin-4(1H)-
one (5b9). Yellow crystalline powder yield: 75%; mp 193–195 ꢀC;
1-Benzyl-N-(4-chlorophenyl)-4-oxo-1,4-dihydro-1,8-naphthyri- (DMSO-d6) d, ppm: 9.19 (s, 1H, aromatic), 8.52 (dd, 1H,
dine-3-carboxamide (5b4). Pale yellow crystalline powder (yield aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.97 (d,
71%); mp 193–195 ꢀC; (DMSO-d6) d, ppm: 8.25 (s, 1H, aromatic), 1H, aromatic, J ¼ 7.8 Hz), 7.24–7.46 (m, 4H, aromatic), 5.68 (s,
8.52 (dd, 1H, aromatic, J ¼ 4.67 Hz), 7.57 (dd, 1H, aromatic, J ¼ 2H, CH2-Ph), 8.71 (s, 1H, aromatic), 7.22–734 (m, 5H, aromatic),
7.77 Hz), 7.97 (d, 1H, aromatic, J ¼ 7.77 Hz), 7.24–7.33 (m, 5H, 1.5–1.7 (m, 4H, piperazine), 1.9–2.1 (m, 4H, piperazine), IR
aromatic), 5.68 (s, 2H, CH2-Ph), 9.19 (d, 1H, NH), 7.14–7.99 (m, (KBr, cmꢁ1): 3112.7, 3086.0 (C–H aromatic), 1686.4 (C]O keto),
5H, aromatic), IR (KBr, cmꢁ1): 3112.7, 3086.0 (C–H aromatic), 1651.2 (C]O); mass (m/z): 348 (M+), chemical formula:
1686.4 (C]O keto), 1651.1 (C]O amide), 797.71 (C–Cl); mass
C20H20N4O2, MW 348.41, anal calc.: C, 68.95; H, 5.79; N, 16.08;
(m/z): 389 (M+), chemical formula: C22H16ClN3O2, MW 389.83, found: C, 68.86; H, 5.69; N, 16.02%.
anal calc.: C, 67.78; H, 4.14; N, 10.78; found: C, 67.71; H, 4.05; N,
10.70%.
1-Benzyl-3-(4-methylpiperazine-1-carbonyl)-1,8-naphthyr-
idin-4(1H)-one (5b10). Off white crystalline powder (yield
1-Benzyl-3-(pyrrolidine-1-carbonyl)-1,8-naphthyridin-4(1H)- 69); mp 181–183 ꢀC; (DMSO-d6) d, ppm: 9.19 (s, 1H, aromatic),
one (5b5). Pale yellow crystalline powder (yield 59%); mp 181– 8.52 (dd, 1H, aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H, aromatic, J ¼ 7.8
183 ꢀC; (DMSO-d6) d, ppm: 9.19 (s, 1H, aromatic), 8.52 (dd, 1H, Hz), 7.97 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.24–7.46 (m, 5H,
aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.97 (d, aromatic), 5.68 (s, 2H, CH2-Ph), 8.71 (s, 1H, aromatic), 1.5–1.7 (m,
1H, aromatic, J ¼ 7.8 Hz), 7.24–7.48 (m, 5H, aromatic), 5.68 (s, 4H, piperazine), 1.9–2.1 (m, 4H, piperazine), 2.88 (s, 3H, CH3), IR
2H, CH2-Ph), 8.71 (s, 1H, aromatic), 3.37 (t, 4H, pyrrolidine, J ¼ (KBr, cmꢁ1): 3069.1, 2998.7 (C–H), 1714.0 (C]O keto), 1692.1 (C]
13.2 Hz), 1.67 (t, 4H, pyrrolidine, J ¼ 15.2 Hz), IR (KBr, cmꢁ1): O); mass (m/z): 362 (M+), chemical formula: C21H22N4O2, MW
3069.1, 2998.7 (C–H), 1714.0 (C]O keto), 1692.1 (C]O) cmꢁ1
.
362.43, anal calc.: C, 69.59; H, 6.12; N, 15.46; found: C, 69.51; H,
Mass (m/z): 333 (M+), chemical formula: C20H19N3O2, MW 6.03; N, 15.37%.
333.39, anal calc.: C, 72.05; H, 5.74; N, 12.60; found: C, 72.06; H,
5.75; N, 12.61%.
2.2 Antihistaminic activity study
1-Benzyl-3-(morpholine-4-carbonyl)-1,8-naphthyridin-4(1H)-
one (5b6). Yellow crystalline powder (yield 76.2); mp 147– The antihistaminic screening of the synthesized compounds
149 ꢀC; (DMSO-d6) d, ppm: 9.19 (s, 1H, aromatic), 8.52 (dd, 1H, was carried out through the process of protection from
aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.97 (d, histamine-induced constriction on guinea pig trachea. The
1H, aromatic, J ¼ 7.8 Hz), 7.24–7.48 (m, 5H, aromatic), 5.68 (s, animals were maintained in a fully controlled animal facility at
2H, CH2-Ph), 8.71 (s, 1H, aromatic), 1.91 (d, 1H, NH), 2.1–2.3 23 ꢂ 2 ꢀC, a relative humidity (RH) of 45–55%, and under a 12 h
(m, 5H, morpholine, J ¼ 15.2 Hz), 2.6–2.8 (m, 4H, morpholine, J light–dark cycle; and they had sufficient standard nutritional
¼ 15.2 Hz), IR (KBr, cmꢁ1): 3069.1, 2998.7 (C–H), 1714.0 (C]O animal feed and puried water. All the animals were acclima-
keto), 1692.1 (C]O); mass (m/z): 349 (M+); chemical formula: tized to their environment for one week before carrying out the
C
20H19N3O3, MW 349.37, anal calc.: C, 68.75; H, 5.48; N, 12.03; experiments. The Institutional Animal Ethics Committee
found: C, 68.62; H, 5.41; N, 12.00%. approved the protocol adopted for the experimentation on
1-Benzyl-N-cyclohexyl-4-oxo-1,4-dihydro-1,8-naphthyridine- animals. All animal procedures were performed in accordance
3-carboxamide (5b7). Yellow crystalline powder (yield 60%); mp with the Guidelines for Care and Use of Laboratory Animals of
181–183 ꢀC; (DMSO-d6) d, ppm: 9.19 (s, 1H, aromatic), 8.52 (dd, Pinnacle Biomedical Research Institute (PBRI), Bhopal, India
1H, aromatic, J ¼ 4.7 Hz), 7.56 (d, 1H, aromatic, J ¼ 7.8 Hz), 7.97 and approved by the Animal Ethics Committee of the Institute
(d, 1H, aromatic, J ¼ 7.8 Hz), 7.24–7.46 (m, 5H, aromatic), 5.68 (Approval reference number PBRI/IAEC/PN-17082). In all
(s, 2H, CH2-Ph), 8.71 (s, 1H, aromatic), 3.85 (m, 1H, CH), 2.17– experimental procedures, efforts were made to minimize pain
1.59 (m, 10H, cyclohexyl, J ¼ 13.2 Hz), IR (KBr, cmꢁ1): 3069.1, and suffering.
2998.7 (C–H), 1714.0 (C]O keto), 1692.1 (C]O amide), 781 (C–
A modied Van Arman technique was adopted to determine
Cl). Mass (m/z): 361 (M+), chemical formula: C22H23N3O2, MW the in vivo antihistaminic potential of the synthesized
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derivatives.39 Male Dunkin Hartley Guinea pigs of either sex (ANOVA) followed by Dunnett's test (manually). In all studies,
(250–300 g) were fasted prior to the experiment for 24 h. Three the signicance level of the means of each test group was
animals were chosen per individual group. The test compounds calculated and compared with the control group. A level of p <
and reference drug (chlorpheniramine maleate) were given by 0.05 was considered signicant in all cases.
oral administration at the dose level of 10 mg kgꢁ1 of body
weight suspended in 1% CMC and combined with histamine
hydrochloride aerosol (aqueous solution 0.2%) in a vaponephr-
ine Pocket Nebulizer and drizzled into a closed transparent
2.5 Molecular docking study
Molecular docking was performed using Auto Dock Tools (ADT)
(version 1.5.4) and Auto Dock Vina (version 1_1_2 docking
programs) (interactive molecular graphics programs) to under-
stand the drug molecule interaction with the protein, the
potential binding mode, and energy. The structures relating to
the human histamine H1 receptor in complex with doxepin
(PDB ID: 3RZE) retrieved from protein data bank (PDB).
chamber for 15 seconds. Proconvulsive (time for onset of
convulsions) time in seconds, during the aerosol experiment,
was measured. The delitescence of convulsion and tumble
numbers for each animal were noted within a 6 min interval of
exposure. The test animals remaining balanced (behaving nor-
mally) for over 6 min were treated as guarded against histamine-
induced bronchoconstriction. Chlorpheniramine maleate was
given intraperitoneal (Ip) at a dose level of 25 mg kgꢁ1 of body
weight for the restoration of the experimental animals. Bron-
chial challenge by histamine forced a bronchospastic response
in all sensitive animals. The delay in the rise of the broncho-
spastic response was measured as a bronchoprotective effect
and expressed in terms of percent protection compared to the
control group.
The ligands were drawn with Chem Draw Ultra version 12.0
(Cambridge So) followed by subsequent molecular mechanics
(MM2) energy minimization of ligands using ChemBio3D Ultra
version 12.0 with GAMESS Interface by assuring connection
error in the bonds. These energy-minimized ligands (structures)
were employed for molecular docking study. The torsions of the
ligands were set by detecting the roots in Auto Dock Vina 1.1.2.
Ligand preparation was done by adding Gasteiger charges.
Redundant chains, non-essential ions, water molecules, and
ligands were discarded. The only exception was made for one
structural water molecule that mediates a salt-bridge for the co-
crystal ligand in both crystal structures. The PDBQT formats of
ligands and protein were prepared by the AutoDock Tools 1.5.4
package (http://mgltools.scripps.edu).47 Protein and Grid prep-
aration was done using Auto dock tools, and Auto dock Vina
1.1.2 (ref. 48) was used to perform molecular docking. The
search grid of the histamine H1 receptor was identied as
centre: 15.386, centre: 32.27 and centre: 20.835 with dimensions
of size: 30, size: 28, and size: 28. The active site was dened to
Percent protection ¼ [1 ꢁ (T1/T2)] ꢃ 100
where T1 is the pre-convulsive breathing time (s) of the control
group and T2 is the pre-convulsive breathing time (s) of the test
group. The potency of the synthesized admixtures was corre-
lated with the reference antihistaminic drug chlorpheniramine.
2.3 Sedative-hypnotic exercise
The sedative potential of the compounds was determined by
measuring the decrease in locomotor activity using an astropho-
tometer (INCO, Ambala, India).40–46 Swiss albino mice of either sex
(20–25 g) were chosen as test animals where six animals were kept
per individual group in polypropylene cages in a controlled envi-
ronment, with free access to food and water and maintained on
a 12 h/12 h day/night cycle. The basal activity score for all animals
was noted. Then, test compounds and the standard drug at the
dose of 5 mg kgꢁ1 of body weight were given by the oral route in
a 1% CMC suspension. The activity score was noted at 1, 2 and 3 h
aer drug application. Sedation was determined by a substantial
decline in locomotor activity. The percentage fall in the count was
taken as the measure of sedative activity. The percent reduced
locomotor activity was noted and expressed in terms of percentage
fall in locomotor exercise determined by the formula:
ꢀ
include all atoms within 6.0 A radius of the native ligand. The
rst ten top-ranked docking poses were saved for each docking
run. To validate the molecular docking protocol, the respective
Percent fall in locomotor activity ¼ [(A ꢁ B)/A] ꢃ 100
where A is the basal score and B is the score aer drug
administration.
The percentage decrease in photoactometer reading is re-
ported as the degree of sedation produced.
2.4 Statistical analysis
Statistical evaluation of the biological activities of the test
Fig. 3 The docking poses of 5a1 in the binding site of the human
admixtures on various animals was performed by one-way histamine H1 receptor in the complex (PDB ID: 3RZE).
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Fig. 4 (a) The docking poses of 5a1–5a4 in the binding site of the human histamine H1 receptor in the complex (PDB ID: 3RZE). (b) The docking
poses of 5a5–5a8 in the binding site of the human histamine H1 receptor in the complex (PDB ID: 3RZE). (c) The docking poses of 5a9 and 5a10 in
the binding site of the human histamine H1 receptor in the complex (PDB ID: 3RZE). (d) The docking poses of 5b1–5b4 in the binding site of the
human histamine H1 receptor in the complex (PDB ID: 3RZE). (e) The docking poses of 5b5 and 5b6 in the binding site of the human histamine H1
receptor in the complex (PDB ID: 3RZE). (f) The docking poses of 5b7–5b10 in the binding site of the human histamine H1 receptor in the
complex (PDB ID: 3RZE).
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reference ligands were initially docked into the crystal structure
of the enzymes.
Table 2 Docking interaction of active compounds (5a1–a10 and 5b1–
b10) with human histamine H1 receptor in complex
No. of H
bonds
Docking
score kcal molꢁ1
H-bond with amino acid
2.6 ADME analysis
Comd
The success of a drug candidate is determined not only by its
good potential but also by a satisfactory ADME prole. Because
a wide variety of experimental media and high throughput in
vitro ADME screens are available, it has the capacity to predict
some important properties in silico and is valuable for analysis
of the good qualities of the molecules. Nowadays, it is recom-
mended that the employment of computational ADME, in
combination with in vivo and in vitro predictions, should be
done as early as possible in the drug discovery process so as to
reduce the number of safety issues.49 Lipinski's rule of ve is
helpful in describing molecular properties of a drug candidate,
which is necessary to evaluate the important pharmacokinetic
parameters such as ADME. The rule is helpful in drug design
and development of a potential drug molecule.50,51 The ADME
study was carried out using the SWISS ADME predictor for the
present investigation. This is a free web tool to evaluate the
ADMET properties, like aqueous solubility (log S), skin perme-
ability (log Kp), synthetic accessibility score (SA), percentage
absorption, pharmacokinetics, drug-likeness, and medicinal
chemistry friendliness properties of small molecules.52 The
criteria of molecular weight #500, #5 hydrogen bond donors
(HBDs), 10 hydrogen bond acceptors (HBAs), and #10 rotatable
bonds (RBs) were selected for the present study.53
5a1
5a2
5a3
5a4
5a5
5a6
5a7
5a8
5a9
7
5
5
5
8
7
3
7
5
ꢁ7.9
ꢁ9.5
ꢁ7.1
ꢁ7.5
ꢁ8.2
ꢁ7.2
ꢁ7.4
ꢁ7.4
ꢁ7.6
A:Asp-107, Ser111, Tyr431
A:Asp-107, Tyr431, Tyr108
A:Asp-107, Tyr431, Tyr108
A:Asp-107, Tyr431, Ser111
A:Asp-107, Tyr431, Tyr108
A:Asp-107, Tyr431, Tyr108
A:Asp-107, Ser111, Asb198
A:Asp-107, Tyr431, Tyr108
A:Asp-107, Tyr431, Tyr108,
Ser111
A:Asp-107, Tyr431, Tyr108,
Ser111
A:Asp-107, Tyr431, Ser111
A:Asp-107, Tyr431, Tyr108,
Ser111
A:Asp-107, Tyr431, Ser111
A:Asp-107, Tyr431, Ser111
A:Asp-107, Tyr431, Tyr108,
Ser111
A:Asp-107, Tyr431, Tyr108,
Ser111
A:Asp-107, Tyr431, Tyr108
A:Asp-107, Tyr431, Tyr108
A:Asp-107, Tyr431, Tyr108
A:Asp-107, Tyr431, Tyr108,
Tyr458
5a10
6
ꢁ7.1
5b1
5b2
7
5
ꢁ8.0
ꢁ9.8
5b3
5b4
5b5
7
4
6
ꢁ7.4
ꢁ7.7
ꢁ8.4
5b6
6
ꢁ7.2
5b7
5b8
5b9
5b10
5
6
7
8
ꢁ8.2
ꢁ8.6
ꢁ7.4
ꢁ8.2
Azelastine 3
ꢁ10.6
A:Asp-107, Tyr431, Ser111
The search engine further gave a compiled result on lip-
ophilicity and hydrophilicity of these molecules by integrating
Table 3 Physicochemical descriptors and ADME parametersa
R-
H- H-
W log P Lipinski PAINS Synthetic
ESOL GI
BBB
log Kp
Compd
MW bonds
A
D
TPSA MR
(lipophilicity) log S absorption permeant (cm sꢁ1
) violations alerts accessibility
5a1
5a2
5a3
5a4
5a5
5a6
5a7
5a8
5a9
5a10
5b1
5b2
5b3
5b4
5b5
5b6
5b7
5b8
5b9
5b10
CLPH
390.8
424.3
424.3
424.3
367.8
383.8
395.9
381.9
382.8
396.9
356.4
389.8
389.8
389.8
333.4
349.4
361.4
347.4
348.4
362.4
274.8
5
5
5
5
4
4
5
4
4
4
5
5
5
5
4
4
5
4
4
4
5
3
4
3
3
3
3
4
3
3
4
4
4
3
3
3
3
4
3
3
4
4
2
3
1
1
1
1
0
0
1
0
1
0
1
1
1
1
0
0
1
0
1
0
0
0
76.88 108.77 3.55
63.99 115.99 4.81
63.99 115.99 4.81
63.99 115.99 4.81
55.2 106.07 2.95
64.43 107.16 2.19
63.99 111.68 4.16
55.2 110.88 3.34
67.23 112.79 1.38
58.44 117.69 1.72
76.88 103.76 2.9
63.99 110.98 4.16
63.99 110.98 4.16
63.99 110.98 4.16
55.2 101.06 2.3
64.43 102.15 1.54
63.99 106.67 3.51
55.2 105.87 2.69
67.23 107.78 0.73
58.44 112.68 1.07
16.13 108.77 3.82
38.13 115.99 3.92
ꢁ4.65 High
ꢁ5.69 High
ꢁ5.69 High
ꢁ5.69 High
ꢁ4.35 High
ꢁ3.89 High
ꢁ5.15 High
ꢁ4.65 High
ꢁ3.71 High
ꢁ4.07 High
ꢁ4.06 High
ꢁ5.11 High
ꢁ5.11 High
ꢁ5.11 High
ꢁ3.76 High
ꢁ3.3 High
ꢁ4.55 High
ꢁ4.06 High
ꢁ3.12 High
ꢁ3.48 High
ꢁ3.82 High
ꢁ5.20 High
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
ꢁ6.28
ꢁ5.52
ꢁ5.52
ꢁ5.52
ꢁ6.24
ꢁ6.95
ꢁ5.61
ꢁ6.07
ꢁ7.14
ꢁ6.9
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2.67
2.59
2.58
2.56
2.46
2.58
2.69
2.57
2.61
2.72
2.65
2.56
2.55
2.53
2.41
2.54
2.65
2.52
2.57
2.68
2.7
ꢁ6.51
ꢁ5.75
ꢁ5.75
ꢁ5.75
ꢁ6.48
ꢁ7.19
ꢁ5.84
ꢁ6.31
ꢁ7.38
ꢁ7.13
ꢁ5.57
ꢁ5.53
Yes
Yes
Yes
Azelastine 381.9
3.62
a
R bond ¼ rotatable bond, H-A ¼ hydrogen bond acceptor, H-D ¼ hydrogen bond donor, TPSA ¼ topological polar surface area, log P ¼
lipophilicity, log S ¼ water solubility, log Kp ¼ permeability coefficient, PAINS ¼ pan-assay interference structure, CLPH ¼ chlorpheniramine,
AZT ¼ azelastine.
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results obtained from various log P and S prediction programs biological effects55 like antihistaminic and other related activi-
called ILOGP, XLOGP3, WLOGP, ESOL, and SILICOS-IT. log P, ties, i.e., antiallergic, anti-asthmatic, histamine release inhibi-
a measure of lipophilicity of a molecule, is the logarithm of the tion, rhinitis treatment, IgE antagonism, bronchodilation,
ratio of the concentration of a drug substance in two solvents in immunomodulation, IL antagonism, PDE inhibition, 5HT3
a unionized form. The lower the log P value, the stronger the lip- antagonism, anti-inammatory activities, etc. This soware
ophilicity of the chemical substance. The aqueous solubility of program designed a tool for evaluating the general biological
a compound signicantly affects its absorption and distribution potential of an organic drug-like candidate. PASS provides
characteristics.54 On the other hand, low water solubility oen simultaneous predictions of a wide range of biological activity
leads to bad absorption, and, therefore, the general aim is to avoid based on the structure of organic compounds. Thus, PASS can
poorly soluble compounds. log S is a unit expressing solubility, be used to estimate the biological activity proles for virtual
and it is the 10-based logarithm of the solubility measured molecules, prior to their chemical synthesis and biological
in mol Lꢁ1. The distribution of log S in traded drugs reveals a value testing. It predicts the required pharmacological effect, as well
somewhere between ꢁ1 and ꢁ4 and will be optimized for better as molecular mechanisms of action and generation of unnec-
absorption and distribution of drugs in the body.
essary side effects such as mutagenicity, teratogenicity, carci-
nogenicity, and embryotoxicity. This tool provided quantitative
structure–activity relationships based on the decomposition of
chemical structures using 2D and/or 3D descriptors, followed by
the generation of models obtained from bioactive ligands.56 The
activity was estimated in terms of Pa (probable activity) and Pi
2.7 Prediction of activity spectra for substances (PASS)
We used PASS (Prediction of Activity Spectra for Substances,
available
at
http://www.pharmaexpert.ru/PASSonline/
predict.php) for computational screening of possible
Fig. 5 (a) Bioavailability radar graph of 1,8-naphthyridine derivatives 5a1–a9 (pink area reflects the allowed values of drug likeness properties of
the molecule). (b) Bioavailability radar graph of 1,8-naphthyridine derivatives 5a10–5b8 (pink area reflects the allowed values of drug-likeness
properties of the molecule). (c) Bioavailability radar graph of 1,8-naphthyridine derivatives 5b9–5b10 (pink area reflects the allowed values of
drug-likeness properties of the molecule).
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(probable inactivity). Structures with Pa greater than Pi were the 4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylic acids 4a and
only compounds considered for a particular pharmacological 4b. The coupling step was achieved in 24 h by heating the cor-
activity.57
responding acids with the appropriate amines (2-amino-
pyridine, chloroaniline, morpholine, pyrrolidine, piperazine,
etc.) in a dry DMF solution in a sealed tube. The target
compounds 5a1–10 and 5b1–10 were puried by recrystalliza-
tion from appropriate solvents.
3 Results and discussion
3.1 Synthetic chemistry
The synthesized compounds were puried via column
A series of cyclic amine substituted 1,8-naphthyridine-3-
carboxylic acid analogues was efficiently synthesized based on
the methods we had developed in our previous schemes.35
The key compounds, 1-(4-chlorobenzyl)-4-oxo-1,4-dihydro-
1,8-naphthyridine-3-carboxylic acid 4a and 1-benzyl-4-oxo-1,4-
dihydro-1,8-naphthyridine-3-carboxylic acid 4b, were prepared
as outlined via a four-step methodology in excellent yield.
The condensation reaction of 2-aminopyridine with ethoxy
methylene malonate by Gould–Jacobs reaction yielded diethyl 2-
((pyridine-2-ylamino)methylene) malonate 2 that was cyclized
during reuxing with phenoxy ether to give ethyl 4-oxo-1,4-
dihydro-1,8-naphthyridine-3-carboxylate 3.58 N-alkylation of 3
in anhydrous DMF with appropriate alkyl chloride in the pres-
ence of sodium hydride gave the 1-alkyl-4-oxo-1,4-dihydro-1,8-
chromatography using
a
methanol : chloroform (10 : 40)
mixture as the eluent. Characterization of the synthesized
molecules was performed using FTIR spectroscopy, 1HNMR
spectroscopy, elemental analysis and mass spectrometry, which
conrmed the formation of the structure. Further, the struc-
tures were established by mass spectral data in accordance with
their molecular formula. Compound 5a, for example, exhibited
a molecular ion peak at m/z 390.2 (M+) in its mass spectrum. Its
1H NMR spectrum included the characteristic signals at 8.02 (d,
1H, Ar–H), 9.11 (d, 1H, Ar–H), 7.66 (d, 1H, Ar–H), and 7.76 (d, 2H
Ar–H), corresponding to the naphthyridine ring structure.
3.2 Antihistaminic activity study
naphthyridine-3-carboxylic acid ethyl esters. Hydrolysis in A series of compounds containing the 1,8-naphthyridine-3-
10% aqueous NaOH solution yielded the corresponding 1-alkyl- carboxylic acid scaffold was evaluated for their in vivo
Fig. 6 ADME properties of compounds 5a1–a10 and 5b1–b10 and azelastine by graphical representation (boiled-egg) (predict gastrointestinal
absorption and brain penetration of small molecules).
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antihistaminic activity. All the test compounds were found to
exhibit comparable antihistaminic activity (Table 1). The
percentage protection result showed that all the test
compounds of the series exhibited comparable protection in the
range of 52–61%. The pharmacological studies of the test
compounds indicated that different substituents present at the
third position of 1,8-naphthyridine-3-carboxylic acid exert
varied pharmacological activity. Replacement of the hydroxyl
group of the acid with alicyclic amine (pyrrolidinyl and piper-
idinyl compounds 5a5, 5a8, 5b5, and 5b8, respectively) showed
good antihistaminic activity. Replacement of the heterocyclic
amine (morpholinyl, piperazinyl and N-methyl piperazinyl
compounds 5a6, 5a9, 5a10, 5b6, 5b9 and 5a10, respectively)
showed comparable antihistaminic activity. Furthermore,
replacement with aromatic amine (amino pyridinyl and cyclo-
hexylamine compounds 5a1, 5a7, 5b1 and 5b7, respectively) led
to a further increase in activity. Amongst the series, compound
5a1 (pyridinyl derivative) was the most potent with the
percentage protection of 61.45 that is comparable to that of the
standard drug (65.20%).
3.3 Sedative-hypnotic exercise
Sedation is the most common side effect associated with more
than 20% of antihistamines used by human beings. Hence, in
vivo sedative–hypnotic activity was also evaluated for the test
compounds. The percentage fall in photoactometer count was
measured as the degree of sedation produced. The results of
this study shown in Table 1 demonstrate that most of the tested
compounds exhibited insignicant sedation (less than 10%).
Chlorpheniramine maleate (rst-generation H1R inhibitor) and
Cetirizine (second-generation H1R inhibitor) were used as
reference drugs for this study. Chlorpheniramine maleate
showed approximately 30% sedation and cetirizine showed
around 10–12% sedation. All the test compounds showed lower
sedative potentials; while compared to the reference drug, the
test compounds exhibited equipotent activity. Thus, this series
can be developed as a clinically useful novel class of non-
sedative antihistamines.
3.4 Molecular docking study
Molecular docking is a valuable technique in computational
chemistry to deeply analyse ligand recognition and it has led to
important breakthroughs in drug discovery and design in the
eld of medicinal chemistry.59 Molecular docking methodology
explores the binding mode and affinity of a small molecule
within the binding site of the receptor target protein. The
docked ligands were ranked according to their binding affinity
in ligand–receptor complexes (Fig. 3 and 4a–f).
Molecular docking was performed on
a set of test
compounds, viz. 5a1–10 and 5b1–10, against the H1 receptor in
order to identify the critical ligand–protein interactions. The
scores due to docking for the tested ligands determined that all
the prepared admixtures possessed potential for interaction
with one or more amino acids in the active site (binding pocket)
of the receptor. Compound 5a1 was the most potent inhibitor of
the receptor. In the docking, the binding of 5a1 was supported
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by three hydrophilic interactions between residues Asp107,
Ser111, and Tyr431 and the ligand at bond distances of 2.84 and
ꢀ
3.08 and 3.18 A. According to the dockings, compounds 5a2–10
and 5b1–10 shared a similar binding prole with 5a and also
showed two hydrophilic interactions with residue Asp107. The
docked ligands were found to have a similar binding capacity to the
co-crystallized ligands. Theoretically, the entire synthesized
compounds showed moderate to good docking scores and binding
energy to the selected protein target ranged from ꢁ7.1 to
ꢁ10.6 kcal molꢁ1, which is in good agreement with the observed
antihistaminic activity. Most of the test ligands showed compa-
rable docking scores in comparison to the reference drug azelas-
tine, which was used as a reference for interpretations of biological
study and analysis. Amongst these compounds, the complex of 5b2
(o-chloroaniline derivative) showed high binding affinities during
the docking study in the active sites of the H1R receptor (PDB ID:
3RZE). The results are shown in Table 2.
3.5 ADME analysis
The results obtained from in silico studies clearly indicate that
the compounds had drug-like candidate properties with no
violation of any of the drug-likeness rules discussed above. It
was interesting to note that the results of the SWISS ADME
predictor values of log P, molar refractivity and the total polar
surface area in these molecules were in excellent agreement
with the most important rules of drug-likeness.
Though these compounds exhibited a good hydrophilic–lipo-
philic balance and the same predicted bioavailability, the halogen
derivative with high lipophilicity was expected to show decent GI
absorption. In addition, we calculated the total polar surface area
(TPSA) since it is another key property that is related to drug
bioavailability. Thus, passively absorbed molecules with TPSA >140
are thought to have low oral bioavailability. The results obtained
from the Swiss ADME search engine are listed in Table 3.
The analysis indicates that the derivatives fell within the
permissible range of standard drugs, as is evident from the
boiled-egg diagram (Fig. 5a–c and 6).
It is also clear that compounds 5a1–5a10 and 5b1–5b10
cannot be affected by the P-glycoprotein of the CNS system.
Compounds present in the yellow zone except 5a9 and 5b9 can
permeate through the blood–brain barrier (BBB). All
compounds present in the white area can be absorbed very
easily by the gastrointestinal tract. In the present study, the
synthesized ligand and its complexes were found to be in good
agreement with the given criteria and can be said to possess
good oral bioavailability.
3.6 Biological activity spectrum PASS analysis
The biological activity spectra of the synthesized compounds
were determined by using an online version of PASS soware.
The results obtained (Tables 4 and 5) were interpreted and used
in a exible manner. The antihistaminic activities of the
derivatives were only slightly changed by amidation at position
3. However, an additional chloro-substituent increased the
activity remarkably, as was observed for compound 5a3.
Compound 5a3 showed the highest Pa for H1R inhibitory
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activity (0.359), whereas compound 5b1 showed the highest Pa
for antiallergic and antiasthmatic activity (0.575 and 0.520,
respectively), compound 5a2 showed the highest Pa for hista-
mine release inhibitor (0.373), compound 5a9 showed the
highest Pa for rhinitis treatment (0.479), and compound 5b6
showed the highest Pa for IL antagonist activity (0.300).
Acknowledgements
The authors acknowledge the SLT Institute of Pharmaceutical
Sciences, Guru Ghasidas University, Bilaspur Chhattisgarh and
Punjab University Chandigarh for performing IR, NMR and
Mass spectroscopy and PBRI, Bhopal, M. P. for the evaluation of
the biological activities of the compounds.
4 Conclusions
The synthesis, characterization, in silico ADME and in vivo
antihistaminic activity of a new series of 1,8-naphthyridine-3-
carboxylic acid derivatives have been described in this manu-
script. A combination of cyclic amines with a naphthyridine
ring and possession of sufficient hydrogen donor–acceptor sites
resulted in good antihistaminic activity.
References
1 D. Pal and S. Saha, J. Adv. Pharm. Technol. Res., 2012, 4, 98–
104.
2 P. Rani, D. Pal, R. R. Hegde and S. R. Hashim, Adv. Anticancer
Agents Med. Chem., 2016, 16, 898–906.
These compounds showed signicant H1R inhibitory (anti-
histaminic) activity through histamine-induced bronchocon-
striction on conscious guinea pigs by the in vivo model. Out of
the synthesized compounds, the pyridyl derivative 5a1 showed
maximum antihistaminic activity with 61.45% protection.
Further, the average sedative properties of the synthesised
compounds were found to be very small (<16%) when compared
to the standard drug chlorpheniramine maleate (38%). The
molecules showed promising in silico results, as indicated by
their signicant scoring functions and high protein–ligand
interaction energy, which simultaneously predicted the activity
of the test compounds. In conjugation with the in silico results,
the compounds showed very promising antihistaminic activi-
ties. The in silico ADME proling, toxicity, drug-likeness, drug-
scoring results, PASS analysis and in vitro antihistaminic activ-
ities suggested that the compounds are promising leads for the
development of selective, safe and potent antihistamines. The
noteworthy conclusion of the study is the development of 1,8-
napthyridine derivatives with pyrazolines, morpholine, piperi-
dine and piperazine like cyclic amines as remarkable anti-
histaminic agents in general. Further study is in progress with
these compounds and related heterocyclic scaffolds to get
information about the chemical structure that efficiently
modulates histamine release to discover new small molecule(s)
endowed with noteworthy antihistaminic activities.
3 P. Rani, D. Pal, R. R. Hegde and S. R. Hashim, J. Chemother.,
2016, 28, 255–265.
4 P. E. Silva Junior, L. C. D. Rezende, J. P. Gimenes,
V. G. Maltarollo, J. Dale, G. H. G. Trossini, F. S. Emery and
A. Ganesand, RSC Adv., 2016, 6, 22777–22780.
5 D. Pal, R. Tripathy, D. D. Pandey and P. Mishra, J. Adv. Pharm.
Technol. Res., 2014, 5, 196–201.
6 S. Saha, D. Pal and S. Kumar, Trop. J. Pharm. Res., 2016, 15,
1401–1411.
7 D. Pal and S. Saha, RSC Adv., 2019, 9, 28061–28077.
8 S. B. Nimse and D. Pal, RSC Adv., 2015, 5, 27986–28006.
9 S. Haider, Journal of Phytochemistry and Biochemistry, 2017, 1,
e101.
10 S. Saha, D. Pal and S. Kumar, Inven. Impact Med. Chem., 2017,
1, 1–8.
11 S. Saha, D. Pal and S. Kumar, Indian J. Exp. Biol., 2017, 55,
831–837.
12 S. Saha, D. Pal and S. Kumar, Int. J. Pharmacol. Pharm. Sci.,
2017, 9, 247–251.
13 B. Eekhari–Sis, M. Zirak and A. Akbari, Chem. Rev., 2013,
113, 2958–3043.
14 Y. Ju and R. S. Varma, J. Org. Chem., 2006, 71, 135–141.
15 E. M. Gordon, R. W. Barrett, W. J. Dower, S. P. A. Fodor and
M. A. Gallop, J. Med. Chem., 1994, 37, 1385–1401.
16 D. Pal and S. Saha, J. Adv. Pharm. Technol. Res., 2012, 3, 92–
99.
17 P. Rani, D. Pal, R. R. Hegde and S. R. Hashim, BioMed Res.
Int., 2014, 386473, 1–9.
18 S. Zhou and G. Huang, RSC Adv., 2020, 10, 5874–5885.
19 P. Kuna, D. Jurkiewicz, M. M. Czarnecka-Operacz,
Conflicts of interest
The authors declare that there are no conicts of interest in this
manuscript.
´
R. Pawliczak, J. Woron, M. Moniuszko and A. Emeryk,
Alergol, 2016, 33, 397–410.
20 L. M. Du Buske, J. Allergy Clin. Immunol., 1996, 98, S307–
S318.
21 M. Gobinath, N. Subramanian and V. Alagarsamy, J. Saudi
Chem. Soc., 2015, 19, 282–286.
22 N. Chairungsrilerd, K. Furukawa, T. Ohta, S. Nozoe and
Y. Ohizumi, Eur. J. Pharmacol., 1996, 314, 351–356.
23 R. J. Hopp, A. Bewtra, N. M. Nair and R. G. Townley, J. Allergy
Clin. Immunol., 1985, 76, 609–613.
Abbreviations
RPMC Rat peritoneal mast cells
PDE 4 Phosphodiesterase 4
ADMET Absorption distribution, metabolism, execration,
toxicity
PASS
TPSA
Prediction of activity spectra for substances
Total polar surface area
´
24 R. Kiss, Z. Kovari and G. Keseru, Eur. J. Med. Chem., 2004, 39,
959–967.
13920 | RSC Adv., 2020, 10, 13907–13921
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
25 S. Thangapandian, N. Krishnamoorthy, J. Shalini, 41 D. Pal, S. Sannigrahi and U. K. Mazumder, Indian J. Exp.
S. Sakkiah, P. Lazar, Y. Lee and K. Woo Lee, Bull. Korean
Chem. Soc., 2010, 31, 52–58.
26 K. Tasaka and M. Akagi, Arzneimittelforschung, 1979, 29, 488–
493.
Biol., 2009, 47, 743–747.
42 D. Pal, S. K. Pahari and A. K. Pathak, Asian J. Chem., 2007, 19,
4452–4458.
43 D. Pal, S. B. Nimse, S. Khatun and P. Bandyopadhyaya, Nat.
Prod. Sci., 2006, 12, 44–49.
27 D. McTavish and E. M. Sorkin, Drugs, 1989, 38, 778–800.
28 M. H. Sherlock, J. J. Kaminski, W. C. Tom, J. F. Lee, 44 D. Pal, M. Sahoo and A. Mishra, European Bulletin of Drug
S. F. Wong, W. Kreutner, R. W. Bryant and A. T. McPhail, J.
Med. Chem., 1988, 31, 2108–2121.
29 Y. Nishikawa, T. Shind, K. Ishi, H. Nakamura, T. Kon, H. Uno
Research, 2005, 13, 91–97.
45 D. Pal, C. Panda, S. Sinhababu, A. Dutta and S. Bhattacharya,
Acta Pol. Pharm., 2003, 60, 481–486.
and J. Matsumoto, Chem. Pharm. Bull., 1989, 37, 1256–1259. 46 M. F. Sanner, J. Mol. Graphics Modell., 1999, 17, 57–61.
30 C. Manera, L. Betti, T. Cavallini, G. Giannaccini, 47 O. Trott and A. J. Olson, J. Comput. Chem., 2010, 31, 455–461.
A. Martinelli, G. Ortore, G. Saccomanni, L. Trincavelli, 48 M. T. Khan, Curr. Drug Metab., 2010, 11, 285–295.
T. Tuccinardia and P. L. Ferrarini, Bioorg. Med. Chem. Lett., 49 C. A. Lipinski, F. Lombardo, B. W. Dominy and P. J. Feeney,
2005, 15, 4604–4610.
Adv. Drug Delivery Rev., 1997, 23, 3–25.
31 J. D. Gohil, H. B. Patel and M. P Patel, RSC Adv., 2016, 6, 50 D. F. Veber, S. R. Johnson, H. Y. Cheng, B. R. Smith,
74726–74733.
K. W. Ward and K. D. Kopple, J. Med. Chem., 2002, 45,
2615–2623.
32 M. Di Braccio, G. Grossi, S. Alfei, V. Ballabeni, M. Tognolini,
L. Flammini, C. Giorgio, S. Bertoni and E. Barocelli, Eur. J. 51 A. Daina, O. Michielin and V. Zoete, Sci. Rep., 2017, 7, 42717,
Med. Chem., 2014, 86, 394–405. DOI: 10.1038/srep42717.
33 P. L. Ferrarini, V. Calderone, T. Cavallini, C. Manera, 52 B. C. Doak, B. Over, F. Giordanetto and J. Kihlberg, Chem.
G. Saccomanni, L. Pani, S. Ruiu and G. L. Gessa, Bioorg.
Med. Chem., 2004, 15, 1921–1933.
34 K. Takayama, M. Iwata, H. Hisamichi, Y. Okamoto, M. Aoki
and A. Niwa, Chem. Pharm. Bull., 2002, 50, 1050–1059.
35 V. K. Gurjar and D. Pal, Int. J. Pharm. Biol Arch., 2018, 9, 265–
273.
36 W. P. Walters and M. A. Murcko, Adv. Drug Delivery Rev.,
2002, 54, 255–271.
37 M. S. Lajiness, M. Vieth and J. Erickson, Curr. Opin. Drug
Discovery Dev., 2004, 7, 470–477.
Biol., 2014, 21, 1115–1142.
53 G. Moroy, V. Y. Martiny, P. Vayer, B. O. Villoutreix and
M. A. Miteva, Drug Discovery Today, 2012, 17, 44–55.
54 A. Lagunin, A. Stepanchikova, D. Filimonov and V. Poroikov,
Bioinformatics, 2000, 16, 747–748.
55 M. N. Drwal and R. Griffith, Drug Discovery Today: Technol.,
2013, 10, e395–e401.
56 P. G. Jamkhande, S. K. Pathan and S. J. Wadher, Int. J.
Mycobact., 2016, 5, 417–425.
57 G. Y. Leshere, E. J. Froelich, M. D. Gruett, J. H. Bailey and
R. P. Brundage, J. Med. Chem., 1962, 5, 1063–1065.
58 S. Vilar, E. Sobarzo-Sanchez, L. Santana and E. Uriarte, Curr.
Med. Chem., 2017, 24, 4340–4359.
38 C. G. Van Arman, L. M. Miller and M. P. O'Malley, J.
Pharmacol. Exp. Ther., 1961, 133, 90–97.
39 P. B. Dews, Br. J. Pharmacol., 1953, 8, 46–48.
40 D. Pal, S. Dutta and A. Sarkar, Acta Pol. Pharm., 2009, 66, 535– 59 N. S. Pagadala, K. Syed and J. Tuszynski, Biophys. Rev., 2017,
541.
9, 91–102.
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