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isolated and used as the first antimalarial drug in 1820 (Meshnick
and Dobson, 2001; Smith, 1976). The treatment for this disease
was accelerated after the discovery of its causative pathogen,
Plasmodium and its vector (Guillemin, 2002). The journey of
chemotherapy for malaria started in 1891 with the drug, methylene
blue (Guttmann and Ehrlich, 1891) and the optimistic result
prompted researchers to search for novel antimalarial drugs. In this
endeavor, pamaquine, mepacrine and chloroquine were discovered.
Among these, chloroquine is the most effective and significant drug
with well-defined pharmaceutical properties and served as a
worldwide first line drug to treat malaria for about five decades
(Salas et al., 2013; WHO, 2010). The extended efforts to find
out chemical entities similar to chloroquine led to the discovery
of amodiaquine, piperaquine, pyronaridine, mefloquine and
tafenoquine etc., (Salas et al., 2013). The other important class of
antimalarial agents is artemisinin and its semi-synthetic derivatives.
Presently, these are the recommended drugs for the treatment of
malaria caused by P. falciparum. Artemether, arteether, artesunate
and dihydroartemisinin are the important compounds of this class.
In addition to these, the significant antifolate combinations such as
pyrimethamine and sulphadoxine, atovaquone and proguanil are
in clinical use to treat malaria (Newton and White, 1999). In the
antibiotics class, fosmidomycin, clindamycin, tetracycline and
doxycycline are noteworthy candidates (Rathore et al., 2005). In
addition to antimalarial drugs, dichlorodiphenyltrichloroethane
(DDT), the vector control pesticide was also used to control
mosquito breeding. Due to DDT and antimalarial drugs, malaria
was seemed to be eradicated in most parts of the world (Carter
and Mendis, 2002).
The success of chemotherapy did not cherish for a long time pri-
marily due to the emergence of drug-resistant strains of the Plas-
modium (White, 2004). The synergy of malaria with HIV/AIDS
pandemic (Abu-Raddad et al., 2006) and ban on DDT prolonged
the eradication of malaria. Among the 1400 drugs registered
worldwide during 1975–1999, it was only four drugs registered
for treating malaria (Trouiller et al., 2002). This clearly indicates
the high attrition rates of efforts to treat this disease. In addition
to these hurdles, climatic changes, social, financial and political
factors are constantly contributing to evolve malaria as the biggest
threat to the world health since 1970s (Shiff, 2002). Despite the
high activity, the hope on artemisinin and artemisinin-combina-
tion therapies (ACTs) is diminishing as Plasmodium is developing
resistance to these drugs in recent times (Enserink, 2010). If ACTs
treatment fails, the world health will further deteriorate as all
the last generation drugs gained resistance and no immediate cur-
rent generation drug is ready to use. Hence, there is an urgent need
to limit the spread of ACTs resistance strains and to discover new
chemical entities which can exert high efficacy in short time. In
an attempt to identify the new chemical entities to treat malaria,
herein we report the synthesis of the 2-hydrazinyl thiazole deriv-
atives and their in vitro evaluations using a blood stage assay
against P. falciparum, NF54. Using computational methods, docking
studies have also been conducted to identify the possible mode of
action of these compounds with enoyl acyl carrier protein reduc-
tase (PfENR) of P. falciparum.
transform infra red (FT-IR) spectra were recorded on a Thermo
Nicolet 6700 FT-IR spectrometer and only major peaks are re-
ported. Elemental analysis of all the compounds was performed
on Elementar Vario EL-II CHNS analyzer. Mass spectra (MS) were
recorded on a Thermo Scientific High Resolution Magnetic Sector
MS DFS mass spectrometer by either chemical ionization (CI) or
negative ion electro spray ionization (ESI) method. The single crys-
tals were analyzed using Oxford diffractometer and data collection,
cell refinement and data reduction were carried out using CrysAlis
PRO, Oxford Diffraction Ltd., Version 1.171.34.44 (release 25-10-
2010 CrysAlis171.NET) software. Programs used to solve and refine
the structures are olex2 (compiled Oct 25 2010, 18:11:34) and
SHELXL respectively. All the IUPAC names for the synthesized com-
pounds were deduced using Marvin Sketch software version
5.11.5.
2.1. General procedure for the synthesis of thiosemicarbazone
derivatives
The synthesis of thiosemicarbazones was carried out according
to the reported procedures (Alahari et al., 2007; Coxon et al., 2013;
Mei Hsiu et al., 2007; Yi et al., 2011) and the brief procedure is gi-
ven here. The appropriate thiosemicarbazide (1.0 mmol) and alde-
hyde or ketone (1.1 mmol) were dissolved in ethanol (10 ml). To
this, catalytic amount of acetic acid was added. The reaction mix-
ture was refluxed for 5–6 h and then cooled to room temperature.
The resulting precipitate was filtered, washed with ether and
recrystallized from ethanol to obtain the corresponding thiosemi-
carbazones. All the compounds were characterized by spectral
techniques and were found to be identical with the reported data
in the literature (Alahari et al., 2007; Coxon et al., 2013; Mei Hsiu
et al., 2007; Yi et al., 2011). The compounds 1h, 1s and 1t are newly
synthesized compounds and the spectral data for these compounds
are given in the supplementary information.
2.2. General procedures for the synthesis of 2-(2-hydrazinyl)thiazole
derivatives
The procedure for the synthesis of 2-(2-hydrazinyl)thiazole
derivatives was already reported in our previous publication
(Makam et al., 2013), but, the brief procedure is given here. The
appropriate thiosemicarbazone (1.0 mmol) and aliphatic
a-halo
ketone (1.1 mmol) were dissolved in ethanol (10 ml). The reaction
mixture was refluxed, monitored by TLC and at the end of the reaction
it was cooled to room temperature. The resulting precipitate was
filtered, washed with cold ethanol and recrystallized from ethanol
to obtain the corresponding 2-(2-hydrazinyl)thiazole derivatives.
All the compounds were characterized by spectral techniques
and the data confirm the proposed structures. The spectral
data of the synthesized 2-(2-hydrazinyl)thiazole derivatives have
been published recently (Makam et al., 2013) and the data are
included in the Supplementary information.
2.3. In vitro assay for evaluation of antimalarial assay
To determine the potency of a compound against P. falciparum,
384 well SyBR Green I IC50 determination assay was used. First,
2. Materials and methods
1
lL of test compound was dispensed in a 10 point concentration
response format using a BiomekFx liquid dispenser. To this, 50
l
L
Thiosemicarbazide and carbonyl compounds were purchased
from Sigma–Aldrich, USA or Himedia Biosciences, Mumbai, India
or Spectrochem Pvt. Ltd., Mumbai, India. a-Haloketones were pur-
chased from Himedia Biosciences, Mumbai, India. All the chemicals
were used without further purification. 1H and 13C nuclear mag-
netic resonance (NMR) spectra were recorded using 400 MHz and
101 MHz Bruker Avance-II NMR instrument respectively. Fourier
of synchronous P. falciparum ring stage culture at 2% parasitemia
and 0.3% hematocrit was added using a multidrop dispenser. The
plates were then incubated at 37 °C for 72 h in the dual gas mixture
incubator. After incubation, the percent parasitemia in one of the
untreated culture control wells was determined by light micros-
copy. In general, the percent parasitemia at the end of the assay
was ranged from 8% to 12% and the cultures progressed to the tro-
Makam, Parameshwar
