Synthesis and anticancer evaluation of 3-aryl-6-phenylimidazo[2,1-b]thiazoles

Article abstract of DOI:10.1016/j.bmcl.2014.10.030

A series of new 3,6-diphenylimidazo[2,1-b]thiazole derivatives (4a-l) are synthesized and evaluated for their anticancer activity. Some of the synthesized compounds have shown potent anti-proliferative activity against HeLa, MDA-MB-231, A549 and THP1 huma

Full text of DOI:10.1016/j.bmcl.2014.10.030

Accepted Manuscript
Synthesis and anticancer evaluation of 3-aryl-6-phenylimidazo[2,1-b]thiazoles
Satish Koppireddi, Deepika Raj Kumari Chilaka, Sreenivas Avula, Jayaram
Reddy Komsani, Srigiridhar Kotamraju, Rambabu Yadla
PII:
S0960-894X(14)01090-7
DOI:
http://dx.doi.org/10.1016/j.bmcl.2014.10.030
BMCL 22085
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
23 June 2014
20 August 2014
9 October 2014
Please cite this article as: Koppireddi, S., Chilaka, D.R.K., Avula, S., Komsani, J.R., Kotamraju, S., Yadla, R.,
Synthesis and anticancer evaluation of 3-aryl-6-phenylimidazo[2,1-b]thiazoles, Bioorganic & Medicinal Chemistry
Letters (2014), doi: http://dx.doi.org/10.1016/j.bmcl.2014.10.030
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Synthesis and anticancer evaluation of 3-aryl-6-phenylimidazo[2,1-b]thiazoles
Satish Koppireddia, Deepika Raj KumariChilakab, Sreenivas Avulaa, Jayaram Reddy Komsania, Srigiridhar Kotamrajub,* and
Rambabu Yadlaa,*
Antiproliferative activity of
compound 4j
IC50 = 6.5 µM (HeLa)
IC50 = 8.9 µM (A549)
IC50 = 10.9 µM (MDA-MB-231)
IC50 = 17.4 µM (THP1)

1
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.els evier.com
Synthesis and anticancer evaluation of 3-aryl-6-phenylimidazo[2,1-b]thiazoles
Satish Koppireddia, Deepika Raj Kumari Chilakab, Sreenivas Avulaa, Jayaram Reddy Komsania, Srigiridhar
Kotamrajub,* and Rambabu Yadlaa,∗
aFluoroorganic division, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad-500007, India.
bCentre for Chemical Biology, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad-500007, India.
ARTICLE INFO
ABSTRACT
Article history:
Received
A
series of new 3,6-diphenylimidazo[2,1-b]thiazole derivatives (4a-l) are
synthesized and evaluated for their anticancer activity. Some of the synthesized
compounds have shown potent anti-proliferative activity against HeLa, MDA-MB-
231, A549 and THP1 human cancer cell lines. Among the active compounds, 3-(3-
trifluoromethylphenyl)-6-phenylimidazo[2,1-b]thiazole (4j) has caused significant
cytotoxicity in HeLa cells, with IC50 as low as 6.5 µM. Compound 4j has induced
caspase-3 and caspase-8 activation, leading to an apoptotic cell death. FACS
analysis has revealed that compound 4j arrests cells in G0/G1 phase. The presence
of 3-(3-trifluoromethylphenyl)- or 3-(3-chlorophenyl)- substituent, in that order, on
the 6-phenylimidazo[2,1-b]thiazole impacts more positively than other aryl-
substituents, on the anti-proliferative properties of these compounds.
Revised
Accepted
Available online
Key words:
Imidazo[2,1-b]thiazoles
Thiazole
Anticancer activity
Cell cycle effects
Caspase assay
Thiazole scaffold is found in many natural and synthetic
compounds with numerous applications in medicinal chemistry.
Many thiazole derivatives have been reported in the drug
development for the treatment of allergies,1 inflammation,2
hypertension,3 bacterial infection,4 schizophrenia,5 hypnotics,6
HIV infections7 and thrombosis.8 Sulfathiazol (antimicrobial
drug), bleomycine and tiazofurin (antineoplastic drug) are some
of the thiazole containing drugss. It has been found that 2-amino-
4-phenylthiazole derivatives have shown anesthetic activity9.
lines.15,16 The known immunomodulator and antihelminthic agent
levamisole (1) and the antitumor agent YM-201627 (2, Fig 1) are
some of the orally active imidazo[2,1-b]thiazole derivatives.17
Human SIRT1 is an NAD+-dependent deacetylase protein that
plays a role in cell death/survival, senescence and endocrine
signaling.18 N-(2-(3-(3-hydroxycyclopentylmethyl)imidazo[2,1-
b]thiazol-6-yl)phenyl)-2-naphthamide (3, Fig 1) has been
reported as
a
SIRT1 activator.19 Recently some thiazole
compounds and imidazo[2,1-b]benzothiazole derivatives have
been reported to exhibit anticancer activity.20,21 These facts have
inspired us to synthesize new 3,6-diphenylimidazo[2,1-b]thiazole
derivatives for evaluating their anticancer potential.
Some thiazole analogs have also exhibited antimicrobial10,11
,
antitumor,12,13 analgesic,10,14 anti-inflammatory and antipyretic10
properties.
O
S
i
N
75-90 %
yield
R
H2N
NH2
R
NH2
S
5a-l
6a-l
7
O
Br
ii
N
S
R
+
NH2
N
R
N
S
5a-l
8
4a-l
Reagents and reaction conditions: i). I2, 90 oC, 12 h;
ii). Ethanol, MW, 180 W, 5-6 min;
Scheme 1. Synthesis of 3,6-diarylimidazo[2,1-b]thiazoles (4a-l).
Fig 1. Representative structures of biologically active imidazo[2,1-b]thiazole
derivatives.
Imidazo[2,1-b]thiazole derivatives are reported to possess
anticancer activity against different types of human cancer cell
———
∗ Corresponding author. Tel.: +91-40-27193171; fax: +91-40-27193185; e-mail: ryadla@yahoo.com, srigiridhark@gmail.com

2
Representative structures of biologically active imidazo[2,1-
b]thiazole derivatives along with the target molecules (4) are
shown in Fig 1. The 3,6-diphenylimidazo[2,1-b]thiazole
derivatives (4a-l) are synthesized by the condensation of the
individual 4-aryl-1,3-thiazol-2-amine (5a-l) and 2-bromo-1-
phenylethanone (8) in refluxing ethanol for 12 h under N2
atmosphere. Microwave irradiation of an ethanolic solution of 4-
aryl-1,3-thiazol-2-amine (5a-l) and 2-bromo-1-phenylethanone
(8) in a pressure tube at 100 oC has also resulted in condensation-
cyclization reaction leading to the formation of respective 3,6-
diphenylimidazo[2,1-b]thiazole (4a-l) as a solid product in
reasonable yield, albeit in just 5-6 min. The 4-aryl-1,3-thiazol-2-
amines 5a-l are obtained in 75-90 % yield, as per the procedure
reported for compounds 5a-i by us earlier.22 These
transformations are depicted in Scheme 1. The imidazo[2,1-
b]thiazole derivatives 4a and 4c-l reported here are synthesized
for the first time. 3-(4-Chlorophenyl)-6-phenylimidazo[2,1-
b]thiazole (4b) has been reported earlier.23 However, no spectral
data is available for this compound.
trifluoromethylphenyl)- (4k) and 3-(4-ethylphenyl)- (4l)
substituent have shown significant antiproliferative activity
against HeLa and moderate activity against A549 human cancer
Table 1
IC50 values for compounds (4a-l) against different human cancer
cell lines
IC50 (µM)
Comp
R
MDA-
MB-231c
19.7 0.71
HeLaa
A549b
THP1d
4a
4b
4c
4-F
20.2 3.06
25.9 2.47
28.3 1.43
31.6 3.47
14.0 3.06
NA
51 3.67
15.9 0.4
15.5 0.11
48.6 4.62
15.0 3.1
NA
27.7 0.3
25 0.8
NA
4-Cl
4-Br
2-Cl
3-Cl
2-F
21 0.76
21.1 0.48
27.4 0.21
12.8 1.5
39.1 0.3
25.8 0.23
4d
4e
55.5 0.3
21.7 1.2
78.9 0.2
55.5 0.3
25 2.4
50 1.03
4f
4g
4h
4i
2-Me
3-Me
2-OMe
3-CF3
4-CF3
4-Et
-
36.9 1.69
NA
13.7 1.02 40.2 4.03 NA
The imidazo[2,1-b]thiazole derivatives (4a-l) synthesized in
1
this study are characterized by H NMR, 13C NMR, ESI-MS,
9.5 1.09
6.5 0.56
8.5 2.43
11.3 1.53
6.0 0.7
39.0 2.9
8.9 0.46
21.5 0.2
44.8 1.21
3.4 0.9
25.3 0.18
ESI-HRMS and IR spectroscopic data. In the IR spectra, the
cyclic imine (C=N) stretching absorption band is seen around
4j
10.9 0.44 17.4 1.34
1
1460-1478 cm-1. The H NMR signal due to the lone hydrogen
4k
NA
71.4 0.8
68.2 0.5
3.5 0.8
4l
NA
atom present on thiazole ring has appeared as a singlet between δ
6.64-6.90 ppm, while the singlet signal due to the lone hydrogen
on the imidazole moiety is seen in the range of δ 7.57-7.94 ppm,
overlapping with multiplet signals of the aromatic hydrogens.
Similarly, the proton decoupled 13C NMR spectra of imidazo[2,1-
b]thiazole compouds 4a-l contained two singlet signals due to the
two CH-carbons of imidazo[2,1-b]thiazole skeleton appearing at
δ 106.7-108.5 (C-2) and 107.6-111.0 (C-5) ppm, respectively.
The signals due to the remaining three quaternary carbons of
imidazo[2,1-b]thiazole skeleton are seen at δ 128.7-130.7 (C-3),
144.9-148.8 (C-6), and 147.7-150.3 (C-8) ppm. In the case of 3-
(2/4-fluorophenyl)-6-phenylimidazo[2,1-b]thiazoles (4a and 4f),
the signal due to the aromatic carbon attached to fluorine
appeared as a doublet at δ163.3 (1J(C-F) = 250.2 Hz) and 159.6
(1J(C-F) = 251.4 Hz) ppm respectively. The spectral data is
provided as part of the supplementary information.
Doxoe
2.0 1.0
aHeLa
=
Human cervical carcinoma cells; bA549
=
Human lung
adenocarcinoma epithelial cells; cMDA-MB-231 = Human breast carcinoma
cells; dTHP1 = Human leukemic cells; NA = denotes activity >100 µM;
eDoxorubicin is used as a positive control; The IC50 values represent the
required concentration (µM) of test compound to inhibit tumor cell
proliferation by 50%; The data presented here is obtained from three
independent experiments and standard deviation(S.D.) values are derived.
cell lines. The 3-(3-substituted-phenyl)-6-phenylimidazo[2,1-
b]thiazoles (4e, 4h and 4j) have exhibited better activity than their
analogs containing 3-(4-or 2-substituted-phenyl)- moiety. The 3-
(4-halophenyl)- (4a-c) or 3-(2-(halo/methyl/methoxy)phenyl)-
(4d, 4f, 4g and 4i) substituted 6-phenylimidazo[2,1-b]thiazoles
have possessed moderate activity. The 3-(3-trifluorophenyl)-6-
phenylimidazo[2,1-b]thiazole (4j) has shown the best
antiproliferative activity at a concentration range of 6-17.5 µM in
different cell lines tested. As compound 4j has caused significant
cytotoxicity at lower concentration (6.5 0.56 µM) which
coincided with the doxorubicin-induced cytotoxicity (6.0 0.7
µM) in HeLa cells, we have carried out further experiments in
these cells. To understand the cell cycle phase specific alterations
induced by compound 4j, fluorescence activated cell sorting
(FACS) analysis is also performed.24
The 3-aryl-6-phenylimidazo[2,1-b]thiazoles 4a-l are
evaluated for their anti-proliferative activity against a panel of
four different human cancer cell lines namely, cervical (HeLa),
breast (MDA-MB-231), lung (A549) and leukemic cancer cell
lines (THP1) using the MTT assay.24 The IC50 for each individual
compound with respect to the four human cancer cell lines are
calculated and the results are summarized in Table 1. These
values represent the concentration at which 50% decrease in cell
growth is observed after 48 h incubation in presence of the drug.
They are compared with control cells treated with DMSO and
positive control doxorubicin under similar conditions. Some of
the synthesized compounds (4a-l) have exhibited promising anti-
proliferative activity against HeLa (cervical), MDA-MB-231
(breast), A549 (lung), and THP1 (leukemia) human cancer cell
lines. The substituent effect on their antiproliferative activity has
been observed. The 3-aryl-6-phenylimidazo[2,1-b]thiazole
derivatives containing 3-(3-chlorophenyl)- (4e) and the lipophilic
3-trifluoromethyl- (4j) moiety in the 3-aryl-substituent have
shown significant antiproliferative activity against all the four
human cancer cell lines tested (Table 1).
Table 2
Cell cycle distribution of compound 4j
Cell cycle distribution (%)
Concn
(µM)
Compound
Sub G1
G0/G1
S
G2/M
Control
0
1.38
17.28
19.0
35.73
51.54
54.78
18.18
12.75
8.33
44.71
18.43
17.89
4j
4j
6.5
10
It is interesting to note that 6-phenylimidazo[2,1-b]thiazoles
having lipophilic group in the form of 3-(4-tolyl)- (4h), 3-(2-
Cell cycle analysis has been performed to explore the basis for
observed antiproliferative properties of 3-(3-
trifluoromethylphenyl)-6-phenylimidazo[2,1-b]thiazole (4j) at 6.5
anisyl)-
(4i),
3-(3-trifluoromethylphenyl)-
(4j),
3-(4-

3
Fig 2. The effect of compound 4j on cell cycle of HeLa (FACS analysis).@
@ HeLa cells are treated with compound 4j (at 6.5 and 10 µM concentration) for a period of 24 h before FACS analysis is performed. Control = untreated cells.
Fig 3. Effect of compound 4j on caspase-3 and caspase-8 activation in HeLa cells.§
§ HeLa cells are treated with compound 4j at indicated concentrations for a period of 24 h and caspase-3/-8 activities are measured as described in the
supplementary information, *, p < 0.05 compared to no treatment.
µM (IC50) and 10 µM concentrations in HeLa cells treated for a
period of 24 h. It is observed that 51.54 % and 54.78% of cells are
accumulated in G0/G1 phase with 6.5 µM and 10 µM of
compound 4j respectively, as compared to 35.73 % in control
cells. Thereby, suggesting that compound 4j arrests cell cycle
events in G0/G1 phase (Table 2 & Fig 2). At the same time, it is
observed that compound 4j has caused increase in the cell
population in sub-G1 phase (apoptosis) from 1.38 % in control
cells to 17.28 % and 19 % at 6.5 µM and 10 µM concentration,
respectively (Table 2 & Fig 2). This result shows that compound
4j induces an apoptotic cell death in HeLa cells.
Apoptosis is an important process of cell death during which
programmed cell extinction occurs to maintain harmony in
multicellular organisms. To find out whether the 3-(3-
trifluoromethylphenyl)-6-phenylimidazo[2,1-b]thiazole
(4j)
induces cell death in HeLa cells by apoptosis, we have measured
caspase-3 and caspase-8 like activities in cells treated at
concentrations of 6.5 and 10 µM for a period of 24 h.25 It is found
that 4j has significantly increased caspase-3 and caspase-8
activities (Fig 3), suggesting that it induces HeLa cell death by
apoptosis.
Thus, a series of new 3,6-diphenylimidazo[2,1-b]thiazole
derivatives synthesized in this study are evaluated for their
cytotoxic activity with reasonable success. Majority of these
compounds have shown significant cytotoxic potential against the
cervical cancer (HeLa) and lung cancer (A549) cell lines, while
they are moderately active against human breast cancer (MDA-
MB-231) and leukemic (THP1) cells. The effect of the presence
of various substituents on 3-phenyl-ring on the antiproliferative

4
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activity of 3,6-diphenylimidazo[2,1-b]thiazole derivatives is
observed. The FACS analysis and measurement of caspase-3 and
caspase-8 activities to study the basis of antiproliferative activity
of the most active compound in HeLa cells has indicated that
compound 4j causes accumulation of cells in G0/G1 phase,
thereby inducing apoptotic cell death in human cervical
carcinoma cell line at 6.5 µM concentration.
Acknowledgments
The authors sincerely thank Koteswara Rao Kanugula
(Centre for Chemical Biology, CSIR-IICT) for providing biology
data. The authors are grateful to the Council of Scientific and
Industrial Research, New Delhi for the financial assistance under
the 12th Five Year plan projects, “Small Molecules in Lead
Exploration (SMiLE)” and “Development of Innovative
Technologies for Strategic Fluorochemicals (DITSF)”.
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M.; Narasimha Rao, M.P.; Ananthamurthy, N.; Ramakrishna,
K.V.S.; Anthony, A.; Kotamraju, S. Chem. Med. Chem., 2013,
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