Antimalarial activity of HIV-1 protease inhibitor in chromone series

Article abstract of DOI:10.1016/j.bioorg.2014.10.006

Increasing parasite resistance to nearly all available antimalarial drugs becomes a serious problem to human health and necessitates the need to continue the search for new effective drugs. Recent studies have shown that clinically utilized HIV-1 protease (HIV-1 PR) inhibitors can inhibit the in vitro and in vivo growth of Plasmodium falciparum. In this study, a series of chromone derivatives possessing HIV-1 PR inhibitory activity has been tested for antimalarial activity against P. falciparum (K1 multi-drug resistant strain). Chromone 15, the potent HIV-1 PR inhibitor (IC₅₀ = 0.65 μM), was found to be the most potent antimalarial compound with IC₅₀ = 0.95 μM while primaquine and tafenoquine showed IC₅₀ = 2.41 and 1.95 μM, respectively. Molecular docking study of chromone compounds against plasmepsin II, an aspartic protease enzyme important in hemoglobin degradation, revealed that chromone 15 exhibited the higher binding affinity (binding energy = -13.24 kcal/mol) than the known PM II inhibitors. Thus, HIV-1 PR inhibitor in chromone series has the potential to be a new class of antimalarial agent.

Full text of DOI:10.1016/j.bioorg.2014.10.006

Bioorganic Chemistry 57 (2014) 142–147
Contents lists available at ScienceDirect
Bioorganic Chemistry
journal homepage: www.elsevier.com/locate/bioorg
Antimalarial activity of HIV-1 protease inhibitor in chromone series
Pradith Lerdsirisuk a, Chirattikan Maicheen b, Jiraporn Ungwitayatorn b,
⇑
a Chulabhorn Research Institute, Bangkok, Thailand
b Center of Excellence for Innovation in Drug Design and Discovery, Faculty of Pharmacy, Mahidol University, 447 Sri-Ayudhya Road, Bangkok 10400, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2014
Available online 7 November 2014
Increasing parasite resistance to nearly all available antimalarial drugs becomes a serious problem to
human health and necessitates the need to continue the search for new effective drugs. Recent studies
have shown that clinically utilized HIV-1 protease (HIV-1 PR) inhibitors can inhibit the in vitro and
in vivo growth of Plasmodium falciparum. In this study, a series of chromone derivatives possessing
HIV-1 PR inhibitory activity has been tested for antimalarial activity against P. falciparum (K1 multi-drug
Keywords:
Antimalarial activity
Plasmepsin II
Chromone series
Docking
resistant strain). Chromone 15, the potent HIV-1 PR inhibitor (IC50 = 0.65
potent antimalarial compound with IC50 = 0.95 M while primaquine and tafenoquine showed IC50 = 2.41
and 1.95 M, respectively. Molecular docking study of chromone compounds against plasmepsin II, an
lM), was found to be the most
l
l
aspartic protease enzyme important in hemoglobin degradation, revealed that chromone 15 exhibited
the higher binding affinity (binding energy = ꢀ13.24 kcal/mol) than the known PM II inhibitors. Thus,
HIV-1 PR inhibitor in chromone series has the potential to be a new class of antimalarial agent.
Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction
was active against parasites (IC50 0.9–2.1 lM) at concentrations
well below those achieved by ritonavir-boosted lopinavir therapy.
Saquinavir, ritonavir, and lopinavir were evaluated in vivo for anti-
malarial efficacy in mice infected with P. chabaudi AS. Ritonavir
alone and combined with saquinavir or lopinavir significantly
attenuated parasitemia, with the most active regimen being a com-
bination of ritonavir and saquinavir or ritonavir and lopinavir [12].
HIV-1 PR inhibitors also had significant effects on the morphology
of P. falciparum parasites and their hemoglobin digestion. Lopinavir
combined with ritonavir exerted a dose-dependent effect in reduc-
ing liver parasite in mice infected with Plasmodium yoelii [13].
More recent study has demonstrated the synergistic interactions
between indinavir and chloroquine against both the chloroquine
sensitive line Plasmodium chabaudi ASS and the chloroquine-
resistant line P. chabaudi ASCQ [14]. These findings suggest that
use of HIV-1 PR inhibitors may offer clinically relevant antimalarial
activity. Although HIV-1 PR inhibitors are less likely to become
first-choice drugs for the treatment of malaria, their antimalarial
activity may lead to the development of a new class of antimalarial
drugs.
Malaria is still one of the major causes of ailment and mortality,
threatening and killing millions of people annually [1,2]. The caus-
ative agents of malaria are four different species of Plasmodium,
i.e., Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae
and Plasmodium ovale. However, almost all deaths are due to inflec-
tion by P. falciparum [3,4]. In recent years, some human cases of
malaria have also occurred with Plasmodium knowlesi, the fifth
species that causes malaria among monkeys and occurs in some
rainforest areas of the South-East Asia [5,6]. Malaria has become
more difficult to treat because of an increase in multi-drug resis-
tant strains [7,8]. This situation underlines the urgent need for
the development of new antimalarial drugs with novel mechanism
of action.
The United States Food and Drug Administration (US FDA)
approved HIV-1 protease (HIV-1 PR) inhibitors, ritonavir and
saquinavir, have been reported to induce CD36 deficiency, result-
ing in decreased CD36-mediated cytoadherence and phagocytosis
of parasite erythrocytes [9]. Recent studies have indicated that
saquinavir, ritonavir, indinavir, nelfinavir, amprenavir, lopinavir
and atazanavir directly inhibited the growth of both drug-sensitive
and drug-resistant P. falciparum in vitro at pharmacologically rele-
vant concentrations [10,11]. The most potent compound, lopinavir,
In the previous study, our research group has designed, synthe-
sized a series of chromone derivatives and evaluated for their
in vitro inhibitory activity against HIV-1 PR (Fig. 1) [15–17]. The
results revealed that the studied chromone compounds exhibited
promising inhibitory effect on the HIV-1 PR activity with IC50
values ranging from 0.34 to 11.50
mone derivatives showing potent HIV-1 PR inhibitory activity
lM) [15]. In this study, the chro-
⇑
Corresponding author. Fax: +66 2 644 8695.
E-mail address: jiraporn.ung@mahidol.ac.th (J. Ungwitayatorn).
http://dx.doi.org/10.1016/j.bioorg.2014.10.006
0045-2068/Ó 2014 Elsevier Inc. All rights reserved.

P. Lerdsirisuk et al. / Bioorganic Chemistry 57 (2014) 142–147
143
(over 70% inhibition) were evaluated for their antimalarial activity
2.1.1. 2,4,5-Trihydroxyacetophenone
solution of 2,4,5-trimethoxyacetophenone
against P. falciparum. In addition, in order to preliminary investi-
gate the potential of chromone derivatives as plasmepsin (PM) II
inhibitor, docking simulation was performed. PM II is one of the
four catalytically active plasmepsins (PM I, PM II, PM IV and
histoaspartyl protease) that has been identified in the food vacuole
of P. falciparum [18]. PM I and PM II initiate the degradative process
by cleaving the native hemoglobin molecule in a highly conserved
hinge region [19–21]. Among several types of plasmepsins, PM II
has received considerable attention as a promising target for anti-
malarial drug design [22–26].
A
(4.00 g,
19.03 mmol) in chlorobenzene (20 mL) was treated with AlCl3
(6.60 g, 49.47 mmol) at room temperature and then refluxed for
12 h. The solvent was evaporated and the residue was hydrolyzed
with cooled 1 M HCl (50 mL) and extracted with ethyl acetate
(3 ꢁ 50 mL). The organic layer was washed with water (2 ꢁ
50 mL), dried over sodium sulfate anhydrous and then filtered.
After evaporation, the crude product was purified by column chro-
matography (methanol/dichloromethane [0.5:9.5]) to provide
2,4,5-trihydroxyacetophenone as the pale yellow solid (2.42 g,
75.87%); m.p. 206–207 °C; FTIR (KBr) (cmꢀ1): 3405, 3238 (OAH
st.), 1634 (C@O st.) 1589, 1536 (C@C st.), 1300, 1211, 1135 (CAO
st.); 1H NMR 300 MHz (CD3OD): d 2.46 (s, 3H, CH3), 6.27 (s, 1H,
H3), 7.14 (s, 1H, H6); HRMS (ESI) m/z calculated for C8H8O4,
168.0428 [M]+, 167.0350 [MꢀH]+; found 167.0356 [MꢀH]+.
2. Experimental
2.1. Synthesis
Chromones 1–20 were synthesized via one-pot cyclization reac-
tion as shown in pathway a in Fig. 2. More details of the synthesis
procedures and spectroscopic data were reported in Ref. [15].
Chromone 21 was prepared via pathway b as follow:
2.1.2. 4,5-Bis(benzyloxy)-2-hydroxyacetophenone
To
a
solution of 2,4,5-trihydroxyacetophenone (1.50 g,
8.93 mmol) in acetone (24 mL) was added anhydrous potassium
carbonate (2.22 g, 16.07 mmol). The mixture was stirred at room
temperature for 10 min. Then benzyl bromide (2.75 g, 16.08 mmol)
was added and the reaction mixture was refluxed for 5 h. After
cooling to room temperature, the solvent was evaporated and
water was added to the residue. The aqueous mixture was
extracted with ethyl acetate (3 ꢁ 40 mL). The combined organic
layer was washed with water (2 ꢁ 40 mL) and dried over sodium
sulfate anhydrous, then filtered. After removing the solvent, the
crude product was purified by column chromatography (ethyl ace-
tate/hexane [1:4]) to provide 4,5-bis(benzyloxy)-2-hydroxyaceto-
phenone as the white solid (2.18 g, 70.19%); m.p. 99–100 °C; FTIR
(KBr) (cmꢀ1): 3065, 3037 (aromatic CAH st.), 2867 (aliphatic
CAH st.), 1633 (C@O st.), 1510, 1455 (C@C st.), 1370 (CAH bend-
ing), 1263, 1210, 1166 (CAO st.); 1H NMR 300 MHz (CDCl3): d
2" 3"
O
O
C
5
A
8
4"
4
D
3
2
6
7
C
5"
6"
2'
1'
6'
3'
O
1
B
4'
5'
chromone nucleus
Fig. 1. General structure of synthesized chromone derivatives.
Pathway a
R5
O
O
R5
O
O
O
O
O
R5
, DBU, pyridine
R2
reflux 8 hrs.
R6
R7
R6
R7
R6
R7
Cl
R
1)
2)
CH3
OH
+
10%NaOH, CH OH, dioxane
3
R2
R2
R8
R8
R
= phenyl, substituted phenyl
R8
2
chromones 1-20
R , R , R , R = H, OH
5
6
7
8
Pathway b
O
O
O
BnO
H3CO
HO
BnBr, K CO3
CH3
OH
2
AlCl3, PhCl
CH3
CH3
reflux 5 hrs.
reflux 12 hrs.
BnO
H3CO
OCH3
HO
OH
O
OCH3
Cl
Baker-Venkataraman
rearrangement
K CO
2
3
reflux 8 hrs.
O
O
O
HO
HO
conc. H2SO4, AcOH
BnO
BnO
OCH3
o
120 C, 3 hrs.
OCH3
O
OH
chromone 21
Fig. 2. Synthesis pathways for chromone derivatives 1–21.

144
P. Lerdsirisuk et al. / Bioorganic Chemistry 57 (2014) 142–147
2.48 (s, 3H, CH3), 5.10 (s, 2H, CH2-Ph), 5.21 (s, 2H, CH2-Ph), 6.54 (s,
1H, H3), 7.19 (s, 1H, H6), 7.34–7.48 (m, 10H, H20, H30, H40, H50, H60,
H200, H300, H400, H500, H600); LRMS (ESI) m/z [M+Na]+ 371.29 (46.0),
280.20 (100.0), 189.29 (10.0).
OCH3), 6.85 (s, 1H, H3), 7.03 (s, 1H, H8), 7.12 (dd, J = 8.00,
2.35 Hz, 1H, H40), 7.28 (s, 1H, H5), 7.45 (t, J = 8.00 Hz, 1H, H50),
7.52 (s, 1H. H20), 7.59 (d, J = 8.00 Hz, 1H, H60); HRMS (ESI) m/z cal-
culated for C16H12O5, 284.0681 [M]+, 285.0759 [M+H]+; found
285.0755 [M+H]+.
2.1.3. 6,7-Dihydroxy-2-(30-methoxyphenyl) chromone, 21
The Baker-Venkataraman rearrangement was performed by
adding potassium carbonate anhydrous (1.90 g, 13.79 mmol) to a
solution of 4,5-bis(benzyloxy)-2-hydroxyacetophenone (1.20 g,
3.45 mmol) in acetone (25 mL). The mixture was stirred at room
temperature for 20 min then 3-methoxybenzoyl chloride
(0.56 mL, 4.14 mmol) was added dropwise. The reaction mixture
was refluxed for 24 h. After the reaction mixture was allowed to
cool to room temperature, the solvent was evaporated and water
was added to the residue. The aqueous mixture was extracted with
ethyl acetate (3 ꢁ 40 mL). The organic layer was washed with
water (2 ꢁ 40 mL), dried over anhydrous sodium sulfate and
filtered. After removing the solvent, the yellow residue of 1,3-dike-
tone was obtained.
To a mixture of 1,3-diketone in glacial acetic acid (20 mL) was
added concentrated sulfuric acid (0.28 mL) and refluxed at 120 °C
for 4 h. After cooling to room temperature, the reaction mixture
was poured into cool water and extracted with ethyl acetate
(3 ꢁ 40 mL). The combined organic layer was washed with water
(2 ꢁ 40 mL), dried over sodium sulfate anhydrous, filtered and sol-
vent was evaporated. The crude product was purified by column
chromatography (ethyl acetate/hexane [3:2]) to provide chromone
21 as the pale yellow solid (547.4 mg, 55.89%); m.p. 246–247 °C;
FTIR (KBr) (cmꢀ1): 3495 (OAH st.), 3092 (aromatic CAH st.), 1630
(C@O st.), 1602, 1590, 1471 (C@C st.), 1346 (CAH bending), 1293,
1145 (CAO st.); 1H NMR 300 MHz (DMSO-d6): d 3.84 (s, 3H,
2.2. In vitro antimalarial activity assay
In vitro cultivation of P. falciparum (K1, multi-drug resistant
strain) was performed according to the method previously
described by Trager and Jensen [27]. The parasites were cultivated
in RPMI 1640 medium containing 25 mM HEPES (N-2-hydroethyl-
piperazine-N0-2-ethanesulfonic acid), 25 mM NaHCO3, 10% heat-
activated human serum and 3% erythrocytes. The culture was
incubated at 37 °C in a humidified incubator with 3% CO2-enriched
atmosphere (3% CO2, 17% O2 and 80% N2). Daily passaged to fresh
medium containing erythrocyte in order to maintain parasite
growth was performed. Before the assay, the parasite at an early
ring-stage growth was collected and prepared to a parasite mixture
of 1% parasitemia in 1.5% erythrocytes.
In vitro antimalarial activity of chromone derivatives against P.
falciparum was assessed using microculture radioisotope method
described by Desjardins et al. [28]. The assay was performed in
duplicate wells in 96-well plate. In each well, 200
mixture (1% parasitemia and 1.5% erythrocytes) was pre-exposed
with 25 L of the medium containing a test sample dissolved in
1% DMSO (0.1% final concentration) for 24 h. Twenty-five L of
medium containing 0.5
Ci [3H]-hypoxanthine (Perkin Elmer,
lL of parasite
l
l
l
USA) was added to each well. The plates were incubated for an
additional 24 h. Levels of incorporated radioactive labeled hypo-
xanthine indicating parasite growth were determined using the
Table 1
Structures, antimalarial activity and AutoDock binding energy against PM II of chromone derivatives.
O
R5
R6
R7
R3
R2
O
R8
Compd
R2
R3
R5
R6
R7
R8
Activitya
Antimalarial IC50
(l
M)
AutoDock Binding energy (kcal/mol)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Benzyl
Phenyl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
OH
H
H
H
H
H
H
H
H
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
H
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
H
H
H
H
H
H
H
H
H
H
OH
OH
OH
H
H
H
Active
Active
Active
Active
Inactive
Inactive
Active
Active
Active
Inactive
Inactive
Active
Inactive
Inactive
Active
Active
Active
Active
Active
Active
Active
9.43
19.66
11.41
11.07
–
ꢀ8.86
ꢀ8.16
ꢀ8.37
ꢀ8.60
ꢀ8.32
ꢀ8.54
ꢀ8.87
ꢀ8.27
ꢀ8.89
ꢀ8.70
ꢀ8.93
ꢀ7.95
ꢀ10.53
ꢀ10.83
ꢀ13.24
ꢀ10.56
ꢀ11.84
ꢀ11.79
ꢀ12.21
ꢀ13.03
ꢀ8.89
40-(t-butyl)-Phenyl
30-(CF3)-Phenyl
40-(F)-Phenyl
OH
OH
OH
OH
OH
OH
OH
OH
H
H
H
H
H
30,40-(diF)-Phenyl
40-(t-butyl)-Phenyl
30-(Cl)-Phenyl
–
9.15
13.83
11.25
–
30,40-(diCl)-Phenyl
40-(OCH3)-Phenyl
30-(OCH3)-Phenyl
30-(OCH3)-Phenyl
30-(CF3)-Phenyl
40-(F)-Phenyl
–
13.23
–
–
300-(CF3)-Benzoyl
400-(F)-Benzoyl
400-(NO2)-Benzoyl
300,400-(diF)-Benzoyl
300-(CF3)-Benzoyl
400-(NO2)-Benzoyl
400-(t-butyl)-Benzoyl
400-(NO2)-Benzoyl
H
40-(NO2)-Phenyl
30,40-(diF)-Phenyl
30-(CF3)-Phenyl
40-(NO2)-Phenyl
40-(t-butyl)-Phenyl
40-(NO2)-Phenyl
30-(OCH3)-Phenyl
0.95
12.40
4.87
9.85
5.46
5.91
13.94
2.02 nM
30.1 nM
2.41
1.95
0.42
17
18
19
20
H
H
H
OH
H
H
H
H
21
DHA
Mefloquine
Primaquine
Tafenoquine
Chloroquine
a
Less than 50% inhibition of parasite growth = Inactive. More than 50% inhibition of parasite growth = Active.

P. Lerdsirisuk et al. / Bioorganic Chemistry 57 (2014) 142–147
145
TopCount NXT Microplate Scintillation and Luminescence Counters
2.3. Docking study
(Perkin Elmer, USA). The percentage of parasite growth was calcu-
lated from the signal count per minute of treated (CPMT) and
untreated samples (CPMU) using the following equation:
The 3-dimensional (3-D) structures of chromone compounds
were constructed using the standard parameters of the molecular
modeling software package SYBYL 8.0 (Tripos Associates, Saint
Louis, MO, USA). Geometrical optimization was performed using
Powell method with a root-mean-squared (RMS) energy gradient
of 0.05 kcal/mol Å. Tripos force field with Gasteiger-Hückel charges
was employed during the energy minimization. These conforma-
tions of chromones were then docked into active sites of PM II
using AutoDock version 4.2 (The Scripps Research Institute, Molec-
ular Graphics Laboratory, Department of Molecular biology, CA,
USA).
%parasite growth ¼ CPMT=CPMU ꢁ 100
If % parasite growth was >50%, the antimalarial activity was
reported as active and 650%, reported as inactive.
The IC50 values (the concentration of sample required to inhibit
50% parasite growth) were calculated by linear interpolation
[29–31] as follow:
The 3-D structures of PM II complexed with inhibitor (pdb code
1SME and 1ME6) were retrieved from the Brookhaven Protein Data
Bank (http://www.rcsb.org/pdb). The protein templates were pre-
pared for docking study by removing all the native ligand struc-
tures and all water molecules from the complex structures. The
polar hydrogen atoms were added and Gasteiger charges were
assigned to protein atoms. The optimized run parameters for dock-
ing study were as follow: the maximum number of energy evalua-
tions was increased to 2,500,000 per run and the number of GA run
was 100. All other parameters were maintained at their default set-
ting. One hundred independent docking runs were carried out for
each ligand. The docked poses were clustered using a tolerance
of 2.0 Å root-mean-square deviations (RMSD). The docking results,
i.e. the docked pose, binding mode, and binding free energy were
analyzed to evaluate the interaction between the ligand and the
amino acid residues of PM II.
hn ꢀ ꢁ ꢀ
ꢁo
i
C
1
2
G
G
ꢀ50
1
2
2
log
ꢁ
þlogðC2Þ
C
ꢀG
IC50 ¼ 10
where
C1 = concentration of sample with parasite growth <50%.
C2 = concentration of sample with parasite growth P50%.
G1 = percentage of parasite growth at C1.
G2 = percentage of parasite growth at C2.
Dihydroartemisinin (DHA) and mefloquine were used as the
positive controls. The negative control was 0.1% DMSO. All of
chromone compounds were dissolved in 100% of DMSO as the
stock solution at concentration 10 mg/mL. The test concentrations
of chromone compounds were 10, 1 and 0.1 lg/mL for evaluating
the % parasite growth. All assays were performed by Bioassay
laboratory of the National Center of Genetic Engineering and
Biotechnology (BIOTEC).
The PM II template validation was performed by re-docking and
cross-docking experiments. Each ligand was docked into the native
O
H
O
O
H
N
HO
N
N
H
N
H
N
H
O
OH
O
OH
O
Pepstatine A: Ki = 0.006 nM [22]
binding enrgy = -8.39 kcal/mol
H
N
N
H
N
N
O
NH
N
S
N
O
N
N
N
N
HN
N
Compound b: IC50 = 7.00 µM [34]
binding enrgy = -8.06 kcal/mol
Compound a:
binding enrgy = -8.75 kcal/mol
= 4.62 µM [34]
IC50
OCH3
H
N
O
H
N
H
N
OCH3
H3CO
O
N
OCH3
Compound d: IC50 > 250 µM [36]
Compound c: IC50 > 100 µM [35]
binding enrgy = -8.54 kcal/mol
binding enrgy = -7.46 kcal/mol
Fig. 3. Structures and binding energies of the reported PM II inhibitors.

146
P. Lerdsirisuk et al. / Bioorganic Chemistry 57 (2014) 142–147
protein in order to determine the ability of AutoDock program to
reproduce the orientation and position of the ligand observed in
the crystal structure. Then, cross-docking of each ligand into the
non-native protein was performed. The RMSD values were
obtained from the best cluster conformation of the re-docking
and cross-docking studies. The PM II template, 1SME, with the
lowest RMSD values was used for further docking study.
growth was determined in vitro using microculture radioisotope
technique [28]. The amount of [3H]-hypoxanthine taken up by
malaria parasite for purine salvage pathway and DNA synthesis
was an indicator of parasites growth and their multiplication.
Incorporation of [3H]-hypoxanthine was quantified with a liquid
b-scintillation counter as the signal count per minute (CPM). The
percentage of inhibition of parasite growth and IC50 were
calculated as described in experimental section and results were
summarized in Table 1. As shown in Table 1, most of the test
compounds exhibited promising inhibitory activity against
P. falciparum. The four most potent compounds were chromones
3. Results and discussion
3.1. Chemical synthesis
15, 17, 19 and 20 with IC50 = 0.95, 4.87, 5.46 and 5.91 lM, respec-
tively. Although the most active, chromone 15, showed the lower
potency than the positive controls (DHA and mefloquine), it
showed the higher potency than the antimalarial drugs currently
used in patients, i.e., primaquine and tafenoquine (IC50 = 2.41 and
Chromones 1–20 were prepared by the one pot cyclization reac-
tion modified from the method of Riva et al. (pathway a, Fig. 2)
[32]. More details in synthesis procedures were described in Ref.
[15]. Chromone 21 was synthesized according to pathway b
(Fig. 2) which required only simple and inexpensive reagents.
The Baker-Venkataraman rearrangement and subsequent intramo-
lecular cyclization with a catalytic amount of strong acid were
used to provide the desired chromone structure with satisfactory
yield higher than 55%.
1.95 lM, respectively) [33].
3.3. Molecular modeling
As a preliminary study toward the potential of chromone series
as PM II inhibitor, the docking simulation study was performed
using AutoDock 4.2. The docking results were reported as binding
energy (Table 1), the lower the binding energy the higher the
binding affinity. All chromone compounds bearing substituents at
C-2 and C-3 as well as 7-OH group displayed strong binding ener-
gies (ꢀ10.53 to ꢀ13.24 kcal/mol). Interestingly, the results from
docking indicated that chromone 15, the experimentally observed
most potent antimalarial, showed the highest binding affinity
against PM II (binding energy = ꢀ13.24 kcal/mol. Chromones 17,
3.2. Evaluation of antimalarial activity
In this study, chromone derivatives which exhibited potent
HIV-1 PR inhibitory activity (higher than 70% inhibition) from
the previously study [15] were evaluated for their antimalarial
activity against P. falciparum (K1 multi-drug resistant strain). The
ability of the chromone derivatives to inhibit the malaria parasite
NO2
4"
3"
5"
2"
6"
O
5
4
3
6
O
2'
2
3'
4'
7
HO
O
8
1
OH
6'
NO2
5'
(a)
(b)
Fig. 4. (a) Binding interaction of chromone 15 against PM II. (b) Schematic view of the binding conformation of chromone 15 in the enzyme active sites.

P. Lerdsirisuk et al. / Bioorganic Chemistry 57 (2014) 142–147
147
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A. Thomas, D.J. Conway, Lancet 363 (2004) 1017–1024.
19 and 20 also showed strong binding energies, ꢀ11.84, ꢀ12.21
and ꢀ13.03 kcal/mol, respectively.
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energy = ꢀ8.75 and ꢀ8.06 kcal/mol, respectively while the poor
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The binding interaction of chromone 15 against PM II is
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bonding interaction with the carbonyl oxygen of Asn76 in S10
subsite.
4. Conclusions
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In this study chromone 15, one of the most potent HIV-1 PR
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P. falciparum. From docking result, chromone 15 also exhibited the
strongest binding affinity against PM II. Though the mechanism
underlying this activity remains to be fully investigated, the results
from this study raise the prospect of chromone series as new anti-
malarial drug.
Acknowledgments
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This research is supported by the Office of the Higher Education
Commission and Mahidol University under the National Research
Universities Initiative. The authors thank the High Performance
Computer Center (HPCC), National Electronic and Computer
Technology Center (NECTEC) of Thailand for providing SYBYL facil-
ities and the Bioassay laboratory (BIOTEC) of the National Center of
Genetic Engineering and Biotechnology (BIOTEC) for antimalarial
activity assay.
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