Synthesis and biological evaluation of new imidazo[2,1-b]thiazole derivatives as anticancer agents

Article abstract of DOI:10.1007/s00044-016-1684-x

A series of arylidenehydrazide compounds (3a–3j) were synthesized from [6-(4-chlorophenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid hydrazide. The newly synthesized compounds 3b and 3h were subjected to the National Cancer Institute’s in vitro disease-oriented antitumor screening to be evaluated for antitumor activity. Compound 3b, the most potent compound examined, displayed broad spectrum antiproliferative activity against all of the tested cell lines with log₁₀GI₅₀ values between ?4.41 and ?6.44. The greatest growth inhibitions were observed against an ovarian cancer cell line(OVCAR-3), a colon cancer cell line (HCT-15), two renal cancer cell lines (CAKI-1 and UO-31) and two leukemia cell lines (CCRF-CEM and SR) with log₁₀GI₁₀ values ?6.44, ?6.33, ?6.11, ?6.30, ?6.13 and ?6.22, respectively.[InlineMediaObject not available: see fulltext.]

Full text of DOI:10.1007/s00044-016-1684-x

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res
DOI 10.1007/s00044-016-1684-x
ORIGINAL RESEARCH
Synthesis and biological evaluation of new imidazo[2,1-b]thiazole
derivatives as anticancer agents
1
Nuray Ulusoy Güzeldemirci1
●
Berin Karaman
Received: 6 March 2016 / Accepted: 6 July 2016
© Springer Science+Business Media New York 2016
Abstract A series of arylidenehydrazide compounds (3a–
3j) were synthesized from [6-(4-chlorophenyl)imidazo[2,1-
b]thiazol-3-yl]acetic acid hydrazide. The newly synthesized
compounds 3b and 3h were subjected to the National
Cancer Institute’s in vitro disease-oriented antitumor
screening to be evaluated for antitumor activity. Compound
3b, the most potent compound examined, displayed broad
spectrum antiproliferative activity against all of the tested
cell lines with log10GI50 values between −4.41 and −6.44.
The greatest growth inhibitions were observed against an
ovarian cancer cell line(OVCAR-3), a colon cancer cell line
(HCT-15), two renal cancer cell lines (CAKI-1 and UO-31)
and two leukemia cell lines (CCRF-CEM and SR) with
log10GI10 values −6.44, −6.33, −6.11, −6.30, −6.13 and
−6.22, respectively.
N
S
Cl
N
O
3b
OVCAR-3;
log10GI50 = -6.44
NH
N
HO
●
●
●
Keywords Anticancer activity Imidazo[2 1-b]thiazole
Arylidenehydrazide
Introduction
Cancer constitutes a broad range of diseases diagnosed with
abnormal cell growth. According to the World Health
Organization, approximately one third of cancer deaths are
related with poor diet and daily habits such as high fat diets,
lack of exercise, low fiber intake, tobacco and alcohol use.
On the other hand, in economically less developed countries
one fifth of cancer deaths are engaged with viral infections
like hepatitis B and C viruses and human papillomavirus (de
Martel et al., 2012). GLOBOCAN reported 14.1 million
new cases and 8.2 million deaths related with cancer in
2012 around the world (Torre et al., 2015). The design and
development of novel molecules to treat cancer has spurred
great interest in many industrial and academic research
laboratories worldwide.
Electronic supplementary material The online version of this article
(doi:10.1007/s00044-016-1684-x) contains supplementary material,
which is available to authorized users.
Present address: Laboratoire de Chimie et Biochimie
Pharmacologiques et Toxicologiques, CNRS UMR8601, Université
Paris Descartes, 45 Rue des Saints-Péres, 75270 Paris Cedex 06,
France.
* Berin Karaman
berin.karaman@parisdescartes.fr
1
Among fused heterocyclic compounds, imidazothiazole
derivatives attracted much attention owing to their
Department of Pharmaceutical Chemistry, Faculty of Pharmacy,
Istanbul University, Beyazıt, 34116 Istanbul, Turkey

Med Chem Res
Fig. 1 The structures of some
imidazo[2,1-b]thiazoles having
antitumor activity
N
S
N
Levamisole
N
R
N
H
HN
N
H3C
N
CH3
N
N
N
R
N
N
S
S
S
II
I
N
N
S
HO
N
N
N
O
N
N
R
NH
NH
NH
NH
O
N
R2
IV
III
R1
O
R
R1
NH
N
N
R
O
N
Br
N
N
S
S
N
R2
V
VI
wide-range of biological properties such as antifungal
(Malik et al., 2013), antitubercular (Samala et al., 2016),
antipsychotic (Cole et al., 2007), antiviral (Wang et al.,
2015) and antitumor activities (Romagnoli et al., 2015).
Over the past years, a large number of imidazo[2,1-b]thia-
zole derivatives (Fig. 1) were recorded to possess anti-
neoplastic activities (Ali et al., 2014 (I); Andreani et al.,
2011 (II); Park et al., 2011 (III); Fidanze et al., 2010 (IV);
Andreani et al., 2008 (V); Gürsoy and Ulusoy
Güzeldemirci, 2007 (VI)).
Besides heterocylic ring systems, hydrazones that carry
an azomethine –NHN=CH– proton constitutes a versitile
class of compounds and are heavily used in drug design and
development campaigns (Tripathi et al., 2011). Compounds
with a hydrazide-hydrazone not only serve as intermediates
in a synthesic pathway, but are in and of themselves also
very effective organic compounds (Raj et al., 2015). For
example, many of the hydrazones were reported to exhibit
significant anticancer activities (Barbosa et al., 2011; Hayat
et al., 2010; Cui et al., 2010).
Levamisole,
2,3,5,6-tetrahydro-6-phenylimidazo[2,1-b]
thiazole has been one of the most extensively studied imi-
dazothiazole derivative in the medicinal chemistry field
(Fascio et al., 2015). Originally used for the treatment of
parasitic infections, Levamisole has spurred great interest
for its immunostimulating activities in cancer (Dillman,
2011). It appears that Levamisole can modulate the immune
system through T-cell activation and proliferation (Chen et
al., 2008), increasing the neutrophil mobility (Earhoudi et
al., 1978) and stimulating the antibody formation. The
combination of Levamisole with 5-fluorouracil was shown
to be beneficial for patients with Dukes stage C colon
cancer (Mutch and Hutson, 1991) and stage III colon cancer
(Sun and Haller, 2005) following the surgical resection.
One of the most promising approaches to treat cancer is
to design hybrid molecules with two different biologically

Med Chem Res
O
O
O
Br
N
H3C
CH3
+
Cl
CH3
H2N
S
O
O
O
Cl
Br-
N+
Br-
N+
O
O
CH3
CH3
H2N
NH2 . H2O
abs.EtOH
Cl
O
N
H
S
S
H2N
1
O
O
H
R
NH2
N
NH
NH
O
R
N
N
Cl
Cl
H
N
S
N
S
2
3a-3j
CH
3
OH
NO
2
O
;
;
;
;
3d
3a
3b
3c
R=
;
3e
HO
H C
O
3
Cl
O
CH
3
CH
3
3
;
3f
;
3j
N
;
;
;
3g
3h
3i
Cl
OH
NO2
O
CH
O
H C
3
Scheme 1 Synthesis of [6-(4-chlorophenyl)imidazo[2,1-b]thiazol-3-yl] acetic acid arylidenehydrazide derivatives (3a–3j)
active components that are built into one single molecule.
In light of the above statement, we hypothesize that
including both an imidazo[2,1-b]thiazole scaffold and a
hyrazide-hydrazon core in our compounds should in
return enhance the antitumoral properties of the two dis-
tinct drug entities. We previously demonstrated a suc-
cessful application of such hybrid approach and showed
that [6-(4-bromophenyl)imidazo[2,1-b]thiazol-3-yl]acetic
acid hydrazide derivatives exhibit growth inhibitory
effects on a broad range of human cancer cell lines
(Gürsoy and Ulusoy Güzeldemirci, 2007). In continuation
of our previous studies on the biological properties of
imidazo[2,1-b]thiazole containing derivatives (Çapan et
al., 1999; Ulusoy, 2002; Ulusoy et al., 2002; Ulusoy
Güzeldemirci and Küçükbasmacı, 2010; Ulusoy
Güzeldemirci et al., 2013) we have designed and syn-
thesized a number of novel arylidenehyrazides bearing
imidazo[2,1-b]thiazole moiety compounds as potential
anticancer agents.
Results and discussion
Chemistry
In this study, we synthesized novel imidazo[2,1-b]thiazole
derivatives bearing hydrazone moieties and investigated
their anticancer activities. The target compounds were
synthesized in four steps as shown in Scheme 1. The
structures of the compounds were confirmed by analytical
and spectral (UV, IR, 1H NMR, 13C NMR (APT), 13C NMR
(DEPT), HSQC-2D and ESI-MS/MS) data.
Firstly, ethyl 2-aminothiazole-4-acetate treated with
4-chloro-2′-bromoacetophenone in acetone to obtain ethyl
6-(4-chlorophenyl)imidazo[2,1-b]thiazole-3-acetate hydro-
bromide (1) The key intermediate 2-[6-(4-chlorophenyl)
imidazo[2,1-b]thiazol-3-yl]acetohydrazide (2) was prepared
by the reaction of hydrazine hydrate with 1. Finally, con-
densation of 2 with appropriate aromatic aldehydes furn-
ished the target hydrazones (3a–3j). The structures of the

Med Chem Res
1
compounds (3a–3j) were confirmed by UV, IR, H NMR,
cell viability and growth, a 48-h continuous drug exposure
protocol was followed and a sulphorhodamine B (SRB)
protein assay was conducted. Bar graphs containing means
were constructed for each effect and each bar was depicted
with standard deviations of individual tumor cell lines. The
center points on the mean graphs represent the mean of all
growth inhibition percentages over all cell lines. Bars that
point to the right are cell lines where the inhibition was
greater than the average, while bars that point to the left are
cell lines where the inhibition was less than the average.
Preliminary results revealed that the lowest growth per-
centages of R-2,5-dimethoxyphenyl substituted compound
3h were determined against two leukemia cell lines SR and
K-562 and a prostate cancer cell line DU-145 with 32.07,
51.07 and 55.11 % growth, respectively (Fig. 3). On the
other hand, R-2-hydroxyphenyl substituted 3b showed a
better antiproliferative activity profile than the compound
3h. While compound 3b exhibited a promising inhibitory
activity over most of the cancer cell lines, the most
remarkable effects were observed against leukemia cell
lines SR and HL-60 (TB) and the prostate cancer cell line
DU-145 with −11.51, −3.50 and −8.65 % growth, respec-
tively (Fig. 2).
Compound 3b was selected further for a five-dose assay
and evaluated against 60 cell lines of nine tumor subpanels
in 5 × 10-fold dilutions with the top dose being 10−4 M. The
in vitro test results retrieved from five-dose assay for
compound 3b are summarized in Supplementary Table S1
(see Supporting Information). From the dose response
curves illustrated in Supplementary Fig. S29 (see Support-
ing Information) three response parameters were calculated
for each cell line. The GI50 value was defined as the
negative log10 concentration that was required to inhibit
50 % of cell growth relative to untreated cells, while the
TGI value was defined as the negative log10 minimum
concentration resulting in total growth inhibition (TGI), and
lastly the LC50 value was the negative log10 concentration
required to kill 50 % of the total cell population. The larger
negative values indicated the more sensitive cell lines.
Therefore, compounds with log10GI50 values −4 and greater
were considered to be active. As can be seen from Table 1,
the log10GI50 values of compound 3b was less than −4 for
all tested cancer cell lines and thus, exhibited a broad
spectrum of antitumor properties.
13C-NMR (APT) (3a, 3d, 3h and 3i), 13C-NMR (DEPT) (3a
and 3d), HSQC-2D (3a, 3d, 3e and 3i), ESI-MS spectral
data and elemental analyses. Spectral data of compounds
3a, 3b and 3h are presented in Supplementary Fig. S1–S28
(see Supporting Information). UV spectral data of com-
pounds were consistent with literature values (Küçükgüzel
et al., 2003). In the IR spectra exhibited the NH, C=O and
C=N bands in the 3212–3135, 1687–1660 and 1622–1561
cm−1 regions, respectively. The absence of the NH2
1
absorptions of hydrazide 2 (δ 4.33 ppm) in the H NMR
spectra of hydrazide hydrazones (3a–j) and the presence of
new resonances for N=CH proton of 3a–j confirmed the
hydrazone formation. Furthermore, the H NMR spectra of
1
3a–j indicated the presence of two geometrical isomers in a
ratio of 3:7 in DMSO-d6 as deduced from the NH, N=CH
and CH2 protons resonating as double singlets at about δ
12.04–11.35/12.12–11.44, 8.39–7.91/8.57–8.07 and 3.97–
3.85/4.39–4.27 ppm, respectively. The N=CH double bond
restricted rotation is thought to give rise to the formation of
E and Z isomers (Gürsoy et al., 1997; Gürsoy and Ulusoy
Güzeldemirci, 2007; Çapan et al., 1999; Ayhan-Kılcıgil et
al., 2012). The formation of the aimed hydrazide-hydrazone
structure was approved by the 13C NMR of 3a, 3d, 3e, 3h
and 3i which were selected as prototypes (Palagiano et al.,
1995; Santos et al., 1998). Molecular weights of the
selected compounds were confirmed using electrospray
ionization tandem mass spectra (ESI-MS/MS) The cleavage
of CH2–CO, CO–NH and NH–N bonds of the hydrazide
moiety was the major fragmentation pattern observed
(Kingston et al., 1970; Ulusoy et al., 2001; Gürsoy and
Ilhan, 1995).
Anticancer activity
Structures of the newly synthesized imidazo[2,1-b]thiazole
derivatives (3a-j) were submitted to the National Cancer
Institute (NCI), Bethesda, Maryland, USA (Alley et al.,
1988; Grever et al., 1992; Boyd and Paull, 1995;
Shoemaker, 2006) and compounds 3b and 3h were selected
for the evaluation of their anticancer activity. The cell lines
used in the NCI screen were leukemia, non-small cell lung
cancer (NSCL), colon cancer, central nervous system (CNS)
cancer, melanoma, ovarian cancer, renal cancer, prostate
cancer, and breast cancer cell lines.
The selected compounds 3b and 3h were initially tested
at a single dose of 10−5 M concentration in the full NCI
60 cell panel and the percentages of the growth inhibition
(GI) over the tested cell lines were determined. The results
retrieved from the single dose screen of compounds 3b and
3h were reported as a mean graph in Figs. 2 and 3,
respectively. The growth percentage was measured using a
spectrophotometer and compared to controls. To estimate
In our previous studies we presented that [6-(4-bromo-
phenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid arylidenehy-
drazides show pronounced effects against the full panel of
60 human tumor cell lines in the five-dose assay (Gürsoy
and Ulusoy Güzeldemirci, 2007). Among the tested com-
pounds, 4-hydroxybenzylidenehydrazide derivative was the
most active member and the most remarkable antitumor
effect of this compound was against the prostate cancer cell
line PC-3 with a log10GI50 value of o−8.00. Moreover,

Med Chem Res
Fig. 2 One dose mean graph of 3b
it was shown that 2-hydroxybenzylidenehydrazide derivative
of 4-bromophenyl substituted imidazo[2,1-b]thiazole also
displayed favorable in vitro growth inhibitory effects on all
nine tumor subpanels with log10GI50 values between −4.00
and −7.43. In the case of the newly synthesized compund
3b, the only difference between the two compounds is the
substitution of imidazo[2,1-b]thiazole moiety with
4-chlorophenyl moiety instead of 4-bromophenyl. The
comparison of the tumor growth inhibiton effects of these
two compounds revealed that the replacement of the bromo
ion with chloro did not improve the overall cytotoxic effects
of imidazo[2,1-b]thiazoles. Nevertheless, the chloro

Med Chem Res
Fig. 3 One dose mean graph of 3h
substitution improved the anticancer effect on particular cell
lines such as CCRF-CEM (log10GI50 value −6.13) and
MOLT-4 (log10GI50 value −5.66) from the leukemia sub-
panel, A549/ATCC (log10GI50 value −5.29), EKVX
(log10GI50 value −5.49), NCI-H322M (log10GI50 value
−5.81) and NCI-H460 (log10GI50 value −5.86) from the
NSCL panel, COLO 205 (log10GI50 value −5.53), HCC-
2998 (log10GI50 value −5.62), HCT-15 (log10GI50 value
−6.33), KM-12 (log10GI50 value −5.55) from the colon
cancer subpanel, SF-268 (log10GI50 value −5.90), SNB-75
(log10GI50 value −5.36), and U251 (log10GI50 value −5.75)
from the CNS cancer subpanel, LOX IMVI (log10GI50

Med Chem Res
Table 1 Comparison of in vitro tumor cell GI of compound 3b, Cisplatin and Sorafenib
Panel/cell line
3b
Cisplatin
Sorafenib
Log10GI50 Log10TGI Log10LC50 Log10GI50 Log10TGI Log10LC50 Log10GI50 Log10TGI Log10LC50
Leukemia
CCRF-CEM
HL-60(TB)
K-562
−6.13
−5.78
−5.47
−5.66
−5.47
−6.22
4−4.00
−5.07
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
−5.257
−5.18
−4.215
−4.047
−4.062
−4.186
−4.073
−4.266
−4.06
−4
−5.665
−5.793
−5.547
−5.538
−5.837
−5.515
−5.037
−4.595
−4
−4
−4
−4
−4
−4
−4
4−4.00
4−4.00
4−4.00
4−4.00
−4.577
−4.877
−4.774
−5.113
−4
MOLT-4
RPMI-8226
SR
−4
−4
−4
−5.319
−4.885
−4
Non-small cell lung cancer
A549/ATCC
EKVX
−5.29
4−4.00
4−4.00
4−4.00
−4.49
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
−4.664
−4.452
−5.063
−4.706
−4.594
−5.412
−4.507
−5.627
−4.152
−4.097
−4.349
−4.156
−4.235
−4.413
−4.103
−4.493
−4
−5.533
−5.602
−5.719
−5.796
−5.721
−5.748
−5.552
−5.634
−4.867
−4.91
−4.068
−4.062
−4.995
−4.303
−4.442
−4.197
−4.314
−4.521
−5.49
−5.33
−5.47
−5.33
−5.27
−5.81
−5.86
−4
HOP-62
−4.02
−4.013
−4.025
−4
−5.372
−5.242
−5.159
−5.122
−5.082
−5.189
HOP-92
NCI-H226
NCI-H23
4−4.00
4−4.00
−4.23
NCI-H322M
NCI-H460
Colon cancer
COLO 205
HCC-2998
HCT-15
−4
4−4.00
−4.025
−5.53
−5.62
−6.33
−5.55
−5.17
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
−4.162
−4.746
−4.406
−4.405
−4.715
−4.061
−4.265
−4.113
−4.064
−4.013
−4.03
−4.081
−4.026
−4.022
−4
−5.661
−5.519
−5.607
−5.803
−5.56
−5.273
−5.003
−5.036
−5.229
−5.059
−4.451
−4.367
−4.348
−4.573
−4.474
KM12
SW-620
CNS cancer
SF-268
−5.90
−5.28
−5.10
−4.91
−5.36
−5.75
−4.26
4−4.00
4−4.00
4−4.00
4−4.00
−4.45
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
−5.4
−4.531
−4.232
−4.618
−4.077
−4.341
−4.31
−4
−5.598
−5.786
−5.787
−5.465
−5.529
−5.677
−5.13
−4.573
−4.667
−4.733
−4.322
−4.52
SF-295
−5.188
−5.29
−4.019
−4.194
−4
−5.354
−5.292
−4.923
−5.018
−5.236
SF-539
SNB-19
−4.755
−5.057
−4.947
SNB-75
−4.02
−4.127
U251
−4.713
Melanoma
LOX IMVI
MALME-3M
MDA-MB-435
SK-MEL-28
SK-MEL-5
UACC-257
UACC-62
Ovarian cancer
IGROV1
−5.71
−5.26
−5.21
−5.26
−5.53
−5.32
−5.47
−4.44
−4.03
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
−5.178
−4.908
−4.609
−4.658
−4.957
−4.597
−4.889
−4.413
−4.429
−4.245
−4.158
−4.391
−4.196
−4.353
−4.178
−4.208
−4.05
−4
−5.784
−5.668
−5.751
−5.579
−5.816
−5.668
−5.773
−5.351
−4.86
−4.865
−4
4−4.00
4−4.00
4−4.00
4−4.00
−4.41
−5.326
−5.127
−5.495
−5.239
−5.35
−4.63
−4.621
−5.174
−4.632
−4.868
−4.163
−4.009
−4.12
−5.52
−6.44
−5.82
−4.41
−5.52
−5.58
−5.30
4−4.00
−4.08
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
4−4.00
−5.032
−4.99
−4.383
−4.506
−4.259
−4.116
−4.112
−4.083
−4.113
−4.152
−4
−5.584
−5.532
−5.455
−5.531
−5.528
−5.596
−5.64
−5.063
−5.014
−4.635
−4.811
−4.675
−4.766
−5.214
−4.216
−4.535
−4
OVCAR-3
OVCAR-4
OVCAR-5
OVCAR-8
NCI/ADR-RES
SK-OV-3
4−4.00
4−4.00
−4.10
−4.934
−4.799
−4.553
−4.696
−4.632
−4
−4
−4.058
−4
−4.042
−4
−4.09
−4
4−4.00
−4
−4.608

Med Chem Res
Table 1 continued
Panel/cell line
3b
Cisplatin
Sorafenib
Log10GI50 Log10TGI Log10LC50 Log10GI50 Log10TGI Log10LC50 Log10GI50 Log10TGI Log10LC50
Renal cancer
A498
−5.08
−5.75
−6.11
−5.57
−4.96
−6.30
4−4.00
4−4.00
−4.80
4−4.00
4−4.00
−4.80
−4.284
−5.14
−4.146
−4.489
−4.316
−4.247
−4.082
−4.28
−4.054
−4.015
−4
−5.645
−5.553
−5.544
−5.472
−5.628
−5.59
−5.175
−4.808
−5.058
−5.012
−5.12
−4.654
−4.029
−4.261
−4.328
−4.555
−4.337
ACHN
CAKI-1
RXF 393
SN 12C
UO-31
−5.351
−4.691
−4.627
−4.794
−4.40
−4.40
−4.027
−4
4−4.00
−4.36
4−4.00
−4.36
−4.036
−5.208
Prostate cancer
PC-3
−5.46
−5.57
4−4.00
4−4.00
4−4.00
4−4.00
−4.608
−5.165
−4.124
−4.324
−4
−4
−5.69
−5.104
−4.899
−4
DU-145
Breast cancer
MCF7
−5.476
−4.353
−5.48
4−4.00
4−4.00
4−4.00
4−4.00
−5.31
4−4.00
4−4.00
4−4.00
4−4.00
−5.31
−4.908
−4.124
−4.634
−4.301
−5.196
−4
−4
−5.553
−5.899
−5.582
−5.758
−5.696
−4.999
−5.379
−4.708
−5.158
−5.117
−4.104
−4.743
−4
MDA-MB-231/ATCC −4.88
−4
−4
HS 578T
−5.00
−5.38
−5.80
−4.058
−4
−4
T-47D
−4
−4.092
−4.369
MDA-MB-468
−4.715
−4.194
value −5.71), and SK-MEL-28 (log10GI50 value −5.26) from
the melanoma subpanel, OVCAR-3 (log10GI50 value −6.44)
and SK-OV-3 (log10GI50 value −5.30) from the ovarian
cancer subpanel, RXF 393 (log10GI50 value −5.57) and UO-
31 (log10GI50 value −6.30) from the renal cancer subpanel,
PC3 (log10GI50 value −5.46) and DU-145 (log10GI50 value
−5.57) from the prostate cancer subpanel, and finally MCF7
(log10GI50 value −5.48), HS 578T (log10GI50 value −5.00)
and T-47D (log10GI50 value −5.38) from the breast cancer
subpanel. On the same cancer cell lines, the log10GI50 values
of the bromo substituted compound were −5.74, −5.63,
−5.21, −5.41, −5.67, −4.80, −5.29, −5.46, −6.02, −5.44,
4−4.00, −4.89, −5.44, −5.46, −4.92, −5.49, −4.89, −4.42,
−5.15, −5.37, −5.54, −4.88, −4.87, and –5.28.
In the next step, we compared the cytotoxic effect of
compound 3b with the two well-known anticancer drugs
Cisplatin (CAS no. 15663-27-1) and Sorafenib (CAS no.
475207-59), which differ in their molecular mechanism of
action, but show remarkable cytotoxic activities on a wide
range of human cancer cell lines such as compound 3b.
While Cisplatin is an alkylating agent that covalently binds
to DNA and induces cellular apoptosis (Dasari and
Tchounwou, 2014), Sorafenib targets several protein kina-
ses such as vascular endothelial growth factor (VEGF),
platelet-derived growth factor (PDGF) receptor families,
and serine/threonine kinase Raf (Adnane et al., 2006).
Cisplatin was first approved for the treatment of ovarian and
testicular cancer by the US Food and Drug Administration
(FDA) in 1978 (Kelland, 2007). Since that time,
Cisplatin has been used in clinical trials alone or in com-
bination with other drugs for the treatment of a variety of
cancer types, including head and neck, bladder, cervical,
lung, ovarian, and testicular cancers (Dasari and Tchoun-
wou, 2014). In comparison to Cisplatin, Sorafenib is a much
newer chemotherapeutic agent used for cancer treatment.
Sorafenib received its first FDA approval in 2005 for the
treatment of renal cell carcinoma (RCC) (Fishman et al.,
2015), which was followed by two more FDA approvals in
2007 and 2013 for the treatment of hepatocellular and
thyroid carcinomas, respectively (Cervello et al., 2012;
Wang et al., 2013; White and Cohen, 2015).
Biological data of Cisplatin (NSC-119875) and
Sorafenib (NSC-747971) was taken from publicly available
NCI Anti-cancer Agent Mechanism Database (Weinstein et
al., 1997; van Osdol et al., 1994) for comparison of three
response parameters, GI50, TGI and LC50 values, with that
of compound 3b. Compound 3b demonstrated higher
potencies against all human cancer cell lines except NCI-
H23 (a NSCL cancer cell line), SF-539 (a CNS cancer cell
line), and OVCAR-5 (an ovarian cancer cell line) than that
of the reference compound, Cisplatin (Table 1). Never-
theless, compound 3b showed only a higher cytotoxic effect
on CCRF-CEM, MOLT-4, and SR out of 9 leukemia cell
lines than that of Sorafenib. In addition, Sorafenib demon-
strated a greater inhibitory effect on all melanoma cancer
cell lines in comparison to compound 3b. A similar trend
could be seen for the prostate and breast cancer cell lines.
Sorafenib inhibited the cell growth more effectively than

Med Chem Res
compound 3b except for the prostate cancer cell line; PC-3
and the breast cancer cell line; MDA-MB-468. Among
seven ovarian cancer cell lines, compound 3b exhibited a
higher anitumoral activity on OVCAR-3 and OVCAR-4
cell lines than Sorafenib did. Most surprisingly, compound
3b showed a better inhibitory effect on tumor cell growth
for all renal cancer cell lines except A498 and SN 12C than
the well-known anti-RCC agent, Sorafenib.
1112 elemental analyzer. IR spectra were recorded (in KBr)
using a Perkin Elmer Spectrum 100 FT-IR spectrometer and
Shimadzu IRAffinity-1 FTIR spectrophotometer. 1H and
UNITY
13C-NMR spectra were obtained on Varian
INOVA
500 MHz spectrometer using DMSO-d6 as an internal
standard. All chemical shifts were reported as δ (ppm)
values and spin–spin couplings (J) were exposed in Hz.
MS (ESI-) were determined on a Finnigan LCQ Advantage
Max mass spectrometer.
Conclusion
General procedure for synthesis of compounds
In this work, we present a synthesis and primary cyto-
toxicity evaluation of a new series of imidazo[2,1-b]thia-
zoles. Among these compounds, R-2-hydroxyphenyl
substituted compound 3b showed significant anti-
proliferative activity towards the 60 human tumor cell
lines in nine cancer types. The most pronounced effect of
this compound was seen on the ovarian cancer cell line
OVCAR-3 (log10GI50 value −6.44). On the same cancer
cell line, the log10GI50 values of the two well-known
anticancer drugs, Cisplatin and Sorafenib, were −4.99 and
−5.53, respectively. In addition, compound 3b also
exhibited significant antiproliferation activity on CCRF-
CEM and SR, two leukemia cell lines, NCI-H322M and
NCI-H460, two NSCL cancer cell lines, HCT-15, a colon
cancer cell line, SF-268, a CNS cancer cell line, CAKI-1
and UO-31, two renal cancer cell lines and MDA-MB-
468, a breast cancer cell line with log10GI50 values of
−6.13, −6.22, −5.81, −5.86, −6.33, −5.90, −6.11, −6.30,
and −5.80, respectively. Furthermore, compound 3b
inhibited the in vitro growth of ACHN (log10GI50 value
−5.75), CAKI-1 (log10GI50 value −6.11), RXF-393
(log10GI50 value −5.57) and UO-31 (log10GI50 value
−6.30) renal cancer cell lines more effectively than Sor-
afenib. Collectively, these findings suggest that compound
3b could be a promising starting point to develop antic-
ancer agents, especially for the treatment of ovarian and
renal cell carcinomas. Currently, further synthesis and
biological investigations for similar novel analogs are
underway in our laboratory.
Ethyl 6-(4-chlorophenyl)imidazo[2,1-b]thiazole-3-acetate
hydrobromide (1)
These compounds were obtained according to the procedure
described by Robert et al. (1975).
2-[6-(4-Chlorophenyl)imidazo[2,1-b]thiazol-3-yl]
acetohydrazide (2)
These compounds were prepared according to the procedure
described by Kühmstedt et al. (1983).
General synthesis of [6-(4-chlorophenyl)imidazo[2,1-b]
thiazol-3-yl]acetic acid arylidenehydrazide derivatives
3a–3j
A solution of 0.005 mol of compound 2 and 0.005 mol of an
appropriate aromatic aldehyde in 100 ml ethanol was heated
under reflux for 5 h. The precipitate obtained was purified
either by recrystallization from ethanol or by washing with
hot ethanol.
[6-(4-Chlorophenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid
benzylidenehydrazide 3a
Yield 66 %, mp 265–266 °C, UV λmax (nm): 226 (ɛ 22053),
277 (ɛ 40470). IR νmax (cm−1): 3151 (N–H stretching), 1685
(amide I C=O stretching), 1613 (hydrazone C=N stretch-
1
ing). H NMR (500MHz, δ, ppm, DMSO-d6): 3.93, 4.36
(2s, 2H, CH2CO), 7.14, 7.15 (2s, 1H, imidazothiazole
C2-H), 7.43–7.46 (m, 5H, arylidene C3,4,5-H and Cl–Ph
Experimental
Chemistry
C
C
3,5-H), 7.69, 7.73 (2d, 2H, J = 7.81, 7.81 Hz, arylidene
2,6-H), 7.81, 7.84 (2d, 2H, J = 8.30, 8.29 Hz, Cl–Ph C2,6
-
H), 8.07, 8.25 (2s, 1H, N=CH), 8.32, 8.35 (2s, 1H, imi-
dazothiazole C5-H), 11.66, 11.77 (2s, 1H, CONH). 13C
NMR (APT) (125 MHz, δ, ppm, DMSO-d6): 32.52, 34.04
(CH2), 109.75, 110.03 (imidazothiazole C5), 111.29, 111.44
(imidazothiazole C2), 127.01, 127.06 (Cl–Ph C2,6), 127.12,
127.54 (imidazothiazole C3), 127.62, 127.81 (arylidene
All chemicals were purchased from Merck (Darmstadt,
Germany) or Sigma-Aldrich (St. Louis, MO, USA) chemi-
cal companies. Using a Büchi B-540 melting point appa-
ratus (Flawil, Switzerland) with open capillaries, melting
points were determined and are uncorrected. Elemental
analyses were performed on a Thermo Finnigan Flash EA
C2,6), 129.36, 129.38 (Cl–Ph C3,5), 129.52 (arylidene C3,5),
130.66, 130.84 (arylidene C4), 132.09, 132.14 (arylidene C1),

Med Chem Res
133.54 (Cl–Ph C1), 134.76, 134.79 (Cl–Ph C4), 144.43,
147.80 (N=CH), 144.89, 145.16 (imidazothiazole C6),
149.38, 149.49 (imidazothiazole C7a), 163.98, 169.70
(CONH). 13C NMR (DEPT) (125 MHz, δ, ppm, DMSO-
d6): 32.51, 34.01 (CH2), 109.81, 110.10 (imidazothiazole
C5), 111.45, 111.57 (imidazothiazole C2), 127.03, 127.08
(Cl–Ph C2,6), 127.62, 127.81 (arylidene C2,6), 129.38 (Cl–
Ph C3,5), 129.53 (arylidene C3,5), 130.66, 130.84 (arylidene
C4), 144.42, 147.78 (N=CH). 13C NMR (HSQC) (125
MHz, δ, ppm, DMSO-d6): 32.52, 34.04 (CH2), 109.75,
110.03 (imidazothiazole C5), 111.29, 111.45 (imidazothia-
zole C2), 127.01, 127.07 (Cl–Ph C2,6), 127.12, 127.54
(imidazothiazole C3), 127.62, 127.81 (arylidene C2,6),
129.36, 129.38 (Cl–Ph C3,5), 129.50, 129.52 (arylidene
C3,5-H), 7.01, 7.07 (2s, 1H, imidazothiazole C2-H), 7.40–
7.45 (m, 2H, Cl–Ph C3,5-H), 7.51–7.55 (m, 2H, arylidene
C2,6-H), 7.80–7.85 (m, 2H, Cl–Ph C2,6-H), 7.95, 8.12 (2s,
1H, N=CH), 8.25, 8.27 (2s, 1H, imidazothiazole C5-H),
9.88, 9.89 (2s, 1H, arylidene C4-OH), 11.43, 11.52 (2s, 1H,
CONH). MS (ESI-) m/z (rel intensity %): 411 ([M–H+2]−,
16), 409 ([M–H]−, 100). MS2 (ESI-) m/z (rel intensity %):
409 ([M–H]−, 17), 289 (52), 288 (100), 274 (1), 273 (3),
247 (10). Anal. for C20H15ClN4O2S, Calcd. C, 58.46 H,
3.68 N, 13.64 Found C, 58.05 H, 3.78 N, 13.74.
[6-(4-Chlorophenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid
4-methoxybenzylidenehydrazide 3d
C
3,5), 130.66, 130.84 (arylidene C4), 132.10, 132.15 (ary-
Yield 88 %, mp 239–240 °C, UV λmax (nm): 228 (ɛ 36033),
290 (ɛ 43979). IR νmax (cm−1): 3212, 3138 (N–H stretching),
1660 (amide I C=O stretching), 1608 (hydrazone C=N
lidene C1), 133.53, 133.55 (Cl–Ph C1), 134.76, 134.79 (Cl–
Ph C4), 144.43, 147.80 (N=CH), 144.88, 145.15 (imida-
zothiazole C6), 149.38, 149.48 (imidazothiazole C7a),
163.97, 169.70 (CONH). MS (ESI-) m/z (rel intensity %):
395 ([M–H+2]−, 31), 393 ([M–H]−, 100). MS2 (ESI-) m/z
(rel intensity %): 393 ([M–H]−, 30), 289 (100), 273 (1), 247
(10). Anal. for C20H15ClN4OS.H2O, Calcd. C, 58.18 H,
4.15 N, 13.57 Found C, 57.93 H, 3.74 N, 13.29.
1
stretching). H NMR (500 MHz, δ, ppm, DMSO-d6): 3.78,
3.79 (2s, 3H, arylidene C4-OCH3), 3.88, 4.30 (2s, 2H,
CH2CO), 6.97–7.00 (m, 2H, arylidene C3,5-H), 7.08 (s, 1H,
imidazothiazole C2-H), 7.40–7.44 (m, 2H, Cl–Ph C3,5-H),
7.63–7.67 (m, 2H, arylidene C2,6-H), 7.81–7.85 (m, 2H, Cl–
Ph C2,6-H), 8.00, 8.17 (2s, 1H, N=CH), 8.26, 8.28 (2s 1H,
imidazothiazole C5-H), 11.51, 11.60 (2s, 1H, CONH). 13C
NMR (APT) (125 MHz, δ, ppm, DMSO-d6): 32.56, 34.10
(CH2), 56.00 (arylidene OCH3), 109.61, 109.89 (imida-
zothiazole C5), 110.84, 111.09 (imidazothiazole C2), 115.02
(arylidene C3,5), 126.97, 127.03 (Cl–Ph C2,6), 127.14, 127.50
(imidazothiazole C3), 127.33, 127.37 (arylidene C1), 129.21,
129.42 (arylidene C2,6), 129.29, 129.34 (Cl–Ph C3,5), 131.91,
132.01 (Cl–Ph C1), 133.89, 133.98 (Cl–Ph C4), 144.30,
147.72 (N=CH), 145.37, 145.52 (imidazothiazole C6), 149.46,
149.55 (imidazothiazole C7a), 161.43, 161.61 (arylidene C4),
163.69, 169.44 (CONH). 13C NMR (DEPT) (125 MHz, δ,
ppm, DMSO-d6): 32.55, 34.08 (CH2), 56.00 (arylidene
OCH3), 109.92, 109.89 (imidazothiazole C5), 110.85 (imida-
zothiazole C2), 115.02 (arylidene C3,5), 126.96, 127.02 (Cl–Ph
C2,6), 129.21, 129.42 (arylidene C2,6), 129.30, 129.35 (Cl–Ph
C3,5), 144.27, 147.67 (N=CH). 13C NMR (HSQC) (125 MHz,
δ, ppm, DMSO-d6): 32.55, 34.10 (CH2), 55.99 (arylidene
OCH3), 109.61, 109.90 (imidazothiazole C5), 110.84, 111.09
(imidazothiazole C2), 115.02 (arylidene C3,5), 126.97, 127.03
(Cl–Ph C2,6), 127.13, 127.50 (imidazothiazole C3), 127.33,
127.37 (arylidene C1), 129.21, 129.42 (arylidene C2,6), 129.29,
129.34 (Cl–Ph C3,5), 131.90, 132.01 (Cl–Ph C1), 133.89,
133.99 (Cl–Ph C4), 144.29, 147.71 (N=CH), 145.37, 145.52
(imidazothiazole C6), 149.45, 149.55 (imidazothiazole C7a),
161.43, 161.60 (arylidene C4), 163.69, 169.44 (CONH). MS
(ESI-) m/z (rel intensity %): 425 ([M–H+2]−, 32); 423 ([M–
H]−, 100). MS2 (ESI-) m/z (rel intensity %): 423 ([M–H]−, 8),
290 (20), 289 (97), 247 (47), 208 (100). Anal. for
C21H17ClN4O2S, Calcd. C, 59.36 H, 4.03 N, 13.19 Found C,
59.25 H, 3.94 N, 13.18.
[6-(4-Chlorophenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid
2-hydroxybenzylidenehydrazide 3b
Yield 85 %, mp 257–258 °C, UV λmax (nm): 228 (ɛ 23374),
279 (ɛ 31437), 291 (ɛ 26785), 323 (ɛ 9613). IR νmax (cm−1):
3138 (N-H stretching), 1685 (amide I C=O stretching),
1
1622 (hydrazone C=N stretching). H NMR (500 MHz, δ,
ppm, DMSO-d6): 3.93, 4.31 (2s, 2H, CH2CO), 6.83-6.91
(m, 2H, arylidene C3,5-H), 7.09, 7.12 (2s, 1H, imida-
zothiazole C2-H), 7.23–7.29 (m, 1H, arylidene C4-H), 7.41–
7.45 (m, 2H, Cl–Ph C3,5-H), 7.54, 7.74 (2d, 1H,
J = 7.81, 7.32 Hz, arylidene C6-H), 7.81, 7.84 (2d, 2H,
J = 7.81, 8.30 Hz, Cl–Ph C2,6-H), 8.28, 8.29 (2s, 1H, imi-
dazothiazole C5-H), 8.36, 8.45 (2s, 1H, N=CH), 10.05,
10.97 (2s, 1H, arylidene C2-OH), 11.58, 11.95 (2s, 1H,
CONH). MS (ESI-) m/z (rel intensity %): 411 ([M–H+2]−,
32), 409 ([M–H]−, 100), 273 (3). MS2 (ESI-) m/z (rel
intensity %): 409 ([M–H]−, 51), 290 (1), 273 (100). Anal.
for C20H15ClN4O2S, Calcd. C, 58.46 H, 3.68 N, 13.64
Found C, 58.30 H, 3.82 N, 13.44.
[6-(4-Chlorophenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid
4-hydroxybenzylidenehydrazide 3c
Yield 75 %, mp 278–278 °C, UV λmax (nm): 228 (ɛ 37801),
284 (ɛ 38253). IR νmax (cm−1): 3149 (N-H stretching), 1667
(amide I C=O stretching), 1607 (hydrazone C=N stretch-
ing). H NMR (500 MHz, δ, ppm, DMSO-d6): 3.86, 4.28
(2s, 2H, CH2CO), 6.81 (d, 2H, J = 8,30 Hz, arylidene
1

Med Chem Res
1
[6-(4-Chlorophenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid
(hydrazone C=N stretching). H NMR (500 MHz, δ, ppm,
4-nitrobenzylidenehydrazide 3e
DMSO-d6): 3.78, 3.79 (2s, 3H, arylidene C3-OCH3), 3.87,
4.31 (2s, 2H, CH2CO), 6.82 (d, 1H, J = 8.30 Hz, arylidene
C5-H), 7.07, 7.08 (2s, 1H, imidazothiazole C2-H), 7.10,
7.11 (2d, 1H, J = 8.05, 1.71 Hz, arylidene C6-H), 7.26, 7.29
(2d, 1H, J = 1.96, 1.95 Hz, arylidene C2-H), 7.41, 7.43 (2d,
2H, J = 8.79, 8.78 Hz, Cl–Ph C3,5-H), 7.80, 7.84 (2d, 2H,
J = 8.78, 8.78 Hz, Cl–Ph C2,6-H), 7.94, 8.11 (2s, 1H,
N=CH), 8.25, 8.27 (2s, 1H, imidazothiazole C5-H), 9.48,
9.51 (2s, 1H, arylidene C4-OH), 11.47, 11.55 (2s, 1H,
CONH). MS (ESI-) m/z (rel intensity %): 441 ([M–H+2]−,
40), 439 ([M–H]−, 100). MS2 (ESI-) m/z (rel intensity %):
439 ([M–H]−, 19), 289 (43), 288 (100), 273 (2), 208 (92).
Anal. for C21H17ClN4O3S, calcd. C, 57.21 H, 3.89 N, 12.71
found C, 56.98 H, 4.27 N, 12.83.
Yield 86 %, mp 260–261 °C, UV λmax (nm): 233 (ɛ 32431),
258 (ɛ 35406), 321 (ɛ 25013). IR νmax (cm−1): 3206, 3146 (N-
H stretching), 1682 (amide I C=O stretching), 1608 (hydra-
1
zone C=N stretching). H NMR (500 MHz, δ, ppm, DMSO-
d6): 3.96, 4.39 (2s, 2H, CH2CO) 7.11 (s, 1H, imidazothiazole
C2-H), 7.41–7.45 (m, 2H, Cl–Ph C3,5-H), 7.82, 7.85 (2d, 2H,
J = 8.79, 8.78 Hz, Cl–Ph C2,6-H), 7.96, 8.00 (2d, 2H, J = 8.79,
8.79 Hz, arylidene C2,6-H), 8.16, 8.34 (2s, 1H, N=CH), 8.26–
8.29 (m, 3H, arylidene C3,5-H and imidazothiazole C5-H),
11.94, 12.04 (2s, 1H, CONH). 13C NMR (HSQC) (125 MHz,
δ, ppm, DMSO-d6): 32.58, 34.09 (CH2), 109.64, 109.89
(imidazothiazole C5), 111.14, 111.35 (imidazothiazole C2),
124.70 (arylidene C3,5), 126.79, 127.17 (imidazothiazole C3),
126.97, 127.03 (Cl–Ph C2,6), 128.56, 128.75 (arylidene C2,6),
129.31, 129.34 (Cl–Ph C3,5), 131.93, 132.02 (Cl–Ph C1),
133.87, 133.96 (Cl–Ph C4), 141.07, 141.14 (arylidene C1),
142.01, 145.34 (N=CH), 145.42, 145.55 (imidazothiazole C6),
148.49, 148.64 (arylidene C4), 149.47, 149.54 (imidazothia-
zole C7a), 164.49, 170.17 (CONH). MS (ESI-) m/z (rel
intensity %): 440 ([M–H+2]−, 30), 438 ([M–H]−, 100). MS2
(ESI-) m/z (rel intensity %): 438 ([M–H]−, 49), 290 (2), 273
(100). Anal. for C20H14ClN5O3S, Calcd. C, 54.61 H, 3.21 N,
15.92 Found C, 54.98 H, 3.39 N, 15.89.
[6-(4-Chlorophenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid
2,5-dimethoxybenzylidenehydrazide 3h
Yield 90 %, mp 240–242 °C, UV λmax (nm): 229 (ɛ 18554),
270 (ɛ 30284), 345 (ɛ 9322). IR νmax (cm−1): 3172, 3142
(N–H stretching), 1671 (amide I C=O stretching), 1599
1
(hydrazone C=N stretching). H NMR (500 MHz, δ, ppm,
DMSO-d6): 3.71 (s, 3H, arylidene C5-OCH3), 3.79, 3.80 (2s,
3H, arylidene C2-OCH3), 3.88, 4.33 (2s, 2H, CH2CO), 6.99
(dd, 1H, J = 9.27, 2.92 Hz, arylidene C4-H), 7.03–7.09 (m,
2H, imidazothiazole C2-H and arylidene C3-H), 7.28–7.44
(m, 3H, arylidene C6-H and Cl–Ph C3,5-H), 7.81, 7.84 (2d,
2H, J = 8.30, 7.80 Hz, Cl–Ph C2,6-H), 8.26, 8.27 (2s, 1H,
imidazothiazole C5-H), 8.36, 8.54 (2s, 1H, N=CH), 11.60,
11.75 (2s, 1H, CONH). 13C NMR (APT) (125 MHz, δ, ppm,
DMSO-d6): 32.64, 34.12 (CH2), 56.15, 56.17 (arylidene
5-OCH3), 56.94, 56.99 (arylidene 2-OCH3), 109.63, 109.85
(imidazothiazole C5), 110.47, 110,87 (imidazothiazole C2),
114.03 (arylidene C3), 114.17 (arylidene C6), 117.78, 118.47
(arylidene C4), 123.29, 123.45 (arylidene C1), 126.96,
127.02 (Cl–Ph C2,6), 127.00, 127.51 (imidazothiazole C3),
129.28, 129.34 (Cl–Ph C3,5), 131.90, 132.01 (Cl–Ph C1),
133.88, 133.98 (Cl–Ph C4), 139.88, 143.16 (N=CH),
145.40, 145.53 (imidazothiazole C6), 149.47, 149.55 (imi-
dazothiazole C7a), 152.88, 153.01 (arylidene C5), 153.97,
154.00 (arylidene C2), 163.79, 169.66 (CONH). MS (ESI-)
m/z (rel intensity %): 455 ([M–H+2]−, 21), 453 ([M–H]−,
100). MS2 (ESI-) m/z (rel intensity %): 455 ([M–H+2]−, 3),
453 ([M–H]−, 30), 290 (10), 289 (100), 273 (18), 247 (7),
208 (97). Anal. for C22H19ClN4O3S, calcd. C, 58.08 H, 4.21
N, 12.32 found C, 57.72 H, 4.21 N, 12.31.
[6-(4-Chlorophenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid
4-dimethylaminobenzylidenehydrazide 3f
Yield 88 %, mp 257–259 °C, UV λmax (nm): 255 (ɛ 23401),
346 (ɛ 27780). IR νmax (cm−1): 3149 (N–H stretching), 1674
(amide I C=O stretching), 1613 (hydrazone C=N stretching).
1H NMR (500 MHz, δ, ppm, DMSO-d6): 2.95 (s, 6H, aryli-
dene C4-N(CH3)2), 3.85, 4.27 (2s, 2H, CH2CO), 6.70, 6.72
(2d, 2H, J = 8.78, 8.79 Hz, arylidene C3,5-H), 7.07 (s, 1H,
imidazothiazole C2-H), 7.40, 7.43 (2d, 2H, J = 8.78, 8.78 Hz,
Cl–Ph C3,5-H), 7.49, 7.51 (2d, 2H, J = 7.32, 8.79 Hz, aryli-
dene C2,6-H), 7.81, 7.84 (2d, 2H, J = 8.29, 8.78 Hz, Cl–Ph
C2,6-H), 7.91, 8.07 (2s, 1H, N=CH), 8.26, 8.28 (2s, 1H,
imidazothiazole C5-H), 11.35, 11.44 (2s, 1H, CONH). MS
(ESI-) m/z (rel intensity %): 438 ([M–H+2]−, 41), 436 ([M–
H]−, 100). MS2 (ESI-) m/z (rel intensity %): 436 ([M–H]−, 31),
288 (100), 247 (66), 208 (88). Anal. for C22H20ClN5OS,
calcd. C, 60.34 H, 4.60 N, 15.99 found C, 60.35 H, 4.46 N,
15.75.
[6-(4-Chlorophenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid
3-methoxy-4-hydroxybenzylidenehydrazide 3g
[6-(4-Chlorophenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid
2,4-dichlorobenzylidenehydrazide 3i
Yield 82 %, mp 256–257 °C, UV λmax (nm): 230 (ɛ 25735),
282 (ɛ 29635), 317 (ɛ 23148). IR νmax (cm−1): 3187 (N–H
Yield 91 %, mp 254–255 °C, UV λmax (nm): 231 (ɛ 16051),
286 (ɛ 30100), 319 (ɛ 11367). IR νmax (cm−1): 3135
stretching), 1683 (amide
I C=O stretching), 1601

Med Chem Res
(N–H stretching), 1687 (amide I C=O stretching), 1594
(hydrazone C=N stretching). H NMR (500 MHz, δ, ppm,
In vitro evaluation of anticancer activity
1
DMSO-d6): 3.93, 4.35 (2s, 2H, CH2CO), 7.09, 7.10 (2s, 1H,
imidazothiazole C2-H), 7.41–7.44 (m, 2H, Cl–Ph C3,5-H),
7.48 (dd, 1H, J = 8.78, 1.95 Hz, arylidene C5-H), 7.71, 7.72
(2d, 1H, J = 1.95, 1.95 Hz, arylidene C3-H), 7.80–7.86 (m,
2H, Cl–Ph C2,6-H), 7.93, 8.04 (2d, 1H, J = 8.79, 8.30 Hz,
arylidene C6-H), 8.27, 8.28 (2s, 1H, imidazothiazole C5-H),
8.39, 8.57 (2s, 1H, N=CH), 11.87, 12.02 (2s, 1H, CONH).
13C NMR (APT) (125 MHz, δ, ppm, DMSO-d6): 32.55,
34.07 (CH2), 109.91 (imidazothiazole C5), 111.09 (imida-
zothiazole C2), 126.77, 127.21 (imidazothiazole C3),
126.96, 127.03 (Cl–Ph C2,6), 128.65 (arylidene C5), 128.79,
128.86 (arylidene C6), 129.31, 129.34 (Cl–Ph C3,5), 130.10
(arylidene C3), 131.13, 131.18 (arylidene C1), 131.92,
132.01 (Cl–Ph C1), 133.88, 133.97 (arylidene C2), 134.38,
134.54 (Cl–Ph C4), 135.68, 135.90 (arylidene C4), 139.37,
142.60 (N=CH), 145.39, 145.54 (imidazothiazole C6),
149.45 (imidazothiazole C7a), 164.25, 169.96 (CONH).
13C NMR (HSQC) (125 MHz, δ, ppm, DMSO-d6): 32.55,
34.07 (CH2), 109.64, 109.89 (imidazothiazole C5), 111.09,
111.39 (imidazothiazole C2), 126.77, 127.20 (imidazothia-
zole C3), 126.96, 127.03 (Cl–Ph C2,6), 128.64, 128.76
(arylidene C5), 128.79, 128.85 (arylidene C6), 129.30,
129.33 (Cl–Ph C3,5), 130.09 (arylidene C3), 131.13, 131.17
(arylidene C1), 131.93, 132.01 (Cl–Ph C1), 133.87, 133.96
(arylidene C2), 134.38, 134.54 (Cl–Ph C4), 135.67, 135.90
(arylidene C4), 139.38, 142.61 (N=CH), 145.39, 145.54
(imidazothiazole C6), 149.46, 149.53 (imidazothiazole C7a),
164.26, 169.96 (CONH). MS (ESI-) m/z (rel intensity %):
465 ([M–H+4]−, 28), 463 ([M–H+2]−, 100), 461 ([M–H]−
81). MS2 (ESI-) m/z (rel intensity %): 463 ([M–H+2]−, 70),
462 (6), 290 (5), 289 (11), 275 (45), 273 (100), 247 (7), 247
(1), 208 (10). Anal. for C20H13Cl3N4OS, calcd. C, 51.80 H,
2.83 N, 12.08 found C, 51.69 H, 3.25 N, 12.31.
All in vitro cell line screenings were conducted at the NCI
in Bethesda, Maryland, USA (Alley et al., 1988; Grever et
al., 1992; Boyd and Paull, 1995; Shoemaker, 2006). RPMI
1640 medium containing 5 % fetal bovine serum and 2 mM
L-glutamine was used for growth of the human tumor cell
lines of the cancer screening panel. A typical screening
experiment consisted of cells that were inoculated in
96-well microtiter plates, each well contained a 100 µL, at
plating densities ranging from 5000 to 40,000 cells per well.
This range depended on the growth and doubling time of
individual cell lines. The microtiter plates were incubated at
37 °C, 5 % CO2, 95 % air and 100 % relative humidity
(RH), after cell inoculation, for 24 h prior to addition of
experimental drugs.
Two plates of each cell line were then fixed in situ with
TCA, which represented a direct measure of cell popula-
tions for each cell line at the time of drug addition (Tz).
Experimental drugs were frozen and stored until use, but
first were solubilized in dimethyl sulfoxide at 400-fold the
desired final maximum test concentration. Before adding
the drug, an aliquot of frozen concentrate was thawed and
diluted to twice the desired final maximum test concentra-
tion. The complete medium contained 50 µg/ml gentamicin.
There were a total of five drug concentrations plus a control.
The concentrations were 10-fold or ½ log serial dilutions
and aliquots of 100 µl of these different drug dilutions were
added to the appropriate microtiter wells containing 100 µl
of medium. Finally, each well contained the desired final
drug concentrations.
The plates were then incubated for an additional 48 h at
37 °C, 5 % CO2, 95 % air, and 100 % RH. Cold TCA was
added to terminate the assay for adherent cells. Cells were
fixed in situ by the careful and slow addition of 50 µl cold
50 % (w/v) TCA (10 % TCA, final concentration) that was
then incubated for 60 min at 4 °C. First, the supernatant was
discarded, and then the plates were washed five times with
tap water, followed by air drying. The plates were incubated
for 10 min at room temperature after adding SRB solution
(100 µl) at 0.4 % (w/v) in 1 % acetic acid to each well.
Unbound dye was removed after staining by washing five
times with 1 % acetic acid. Then the plates were air dried
once again. A 10 mM trizma base was used to solubilize
any bound stain, and the absorbance was read using an
automated plate reader set for maximum absorption at
515 nm. The same methods were used for suspension cells,
except that the assay was terminated by fixing settled cells
at the bottom of the wells with the addition of 50 µl of 80 %
TCA (final concentration, 16 % TCA). The percentage of
growth was calculated at each drug concentration by using
seven absorbance measurements time zero, (Tz), control
growth, (C), and amount of growth in the presence of the
[6-(4-Chlorophenyl)imidazo[2,1-b]thiazol-3-yl]acetic acid
5-nitro-2-furfurylidenehydrazide 3j
Yield 41 %, mp 254–256 °C, UV λmax (nm): 233 (ɛ 24501),
260 (ɛ 39632), 349 (ɛ 16893). IR νmax (cm−1): 3203, 3147
(N–H stretching), 1670 (amide I C=O stretching), 1561
1
(hydrazone C=N stretching). H NMR (500 MHz, δ, ppm,
DMSO-d6): 3.97, 4.33 (2s, 2H, CH2CO), 7.11 (s, 1H,
imidazothiazole C2-H), 7.24, 7.30 (2d, 1H, J = 3.90, 3.90
Hz, furan C3-H), 7.41–7.44 (m, 2H, Cl–Ph C3,5-H), 7.77–
7.85 (m, 3H, Cl–Ph C2,6-H and furan C4-H), 8.01, 8.21 (2s,
1H, N=CH), 8.24, 8.27 (2s, 1H, imidazothiazole C5-H),
12.04, 12.12 (2s, 1H, CONH). MS (ESI-) m/z (rel intensity
%): 430 ([M–H+2]−, 31); 428 ([M–H]−, 100). MS2 (ESI-)
m/z (rel intensity %): 428 ([M–H]−, 18), 290 (1), 274 (8),
273 (100), 208 (5). Anal. for C18H12ClN5O4S, calcd. C,
50.30 H, 2.81 N, 16.29 found C, 50.09 H, 3.13 N, 16.54.

Med Chem Res
Shoemaker RH (2011) Imidazo[2,1-b]thiazole guanylhydrazones
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five drug concentration levels (Ti). Percent GI was calcu-
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Andreani A, Burnelli S, Granaiola M, Leoni A, Locatelli A, Morigi R,
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ꢀ
ꢁ
Ti ꢀTz
C ꢀTz
´ 100 for concentrations when Ti>= ¼ Tz
´ 100 for concentrations for when Ti < Tz:
ꢀ
ꢁ
Ti ꢀ Tz
Tz
For each experimental agent three dose response para-
meters were calculated. The GI50 defined as 50 % GI, was
calculated from the following equation [(Ti - Tz)/(C - Tz)] ×
100 = 50. The parameters for the equation were obtained
from the drug concentration resulting in a 50 % reduction in
control cell net protein increase (as measured by SRB
staining) during the drug incubation. The drug concentra-
tion resulting in TGI was determined by calculating Ti = Tz.
The LC50, which is defined as the concentration of drug
resulting in a decline of 50 % protein at the end of the drug
treatment relative to the beginning of the treatment. This
indicated a net loss of cells after treatment and was calcu-
lated using the following equation [(Ti - Tz)/Tz] × 100 =
−50. Only if the level of activity was reached were the
values calculated for each of these three parameters. How-
ever, the value for that particular parameter was expressed
as either greater or less than the maximum or minimum
concentration tested, if the effect was not reached or was
exceeded.
chloroimidazo[2,1-b][1,3]thiazole-5-sulfonyl)tryptamine as
a
potent, selective, and orally active 5-HT(6) receptor agonist.
J Med Chem 50(23):5535–5538
Acknowledgments We thank the Drug Research and Development,
Division of Cancer Research, National Cancer Institute, Bethesda,
Maryland, USA for the anticancer activity screening of the com-
pounds. This work was supported by the Research Fund of Istanbul
University (Project Number: T-3731, 40810 and BYP-27363).
Cui Z, Li Y, Ling Y, Huang J, Cui J, Wang R, Yang X (2010) New
class of potent antitumor acylhydrazone derivatives containing
furan. Eur J Med Chem 45:5576–5584
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[2,1-b]thiazole derivatives: synthesis and antifungal activity.
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cular mechanisms of action. Eur J Pharmacol 740:364–378
de Martel C, Ferlay J, Franceschi S, Vignat J, Bray F, Forman D,
Plummer M (2012) Global burden of cancers attributable to
infections in 2008: a review and synthetic analysis. Lancet Oncol
13:607–615
Compliance with ethical standards
Conflict of interest The authors declare that they have no com-
peting interests.
Dillman RO (2011) Cancer immunotherapy. Cancer Biother Radio-
pharm 26(1):1–64
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