Microwave-assisted synthesis of sec/tert-butyl 2-arylbenzimidazoles and their unexpected antiproliferative activity towards ER negative breast cancer cells

Article abstract of DOI:10.3109/14756366.2012.729828

A new series of N-sec/tert-butyl 2-arylbenzimidazole derivatives was synthesised in 85-96% yields within 2-3.5min by condensing ethyl 3-amino-4-butylamino benzoate with various substituted metabisulfite adducts of benzaldehyde under focused microwave irradiation. The benzimidazole analogues were characterised using ¹H NMR, ¹³C NMR, high resolution MS and melting points. Evaluation of antiproliferative activity of the benzimidazole analogues against MCF-7 and MDA-MB-231 revealed several compounds with unexpected selective inhibitions of MDA-MB-231 in micromolar range. All analogues were found inactive towards MCF-7. The most potent inhibition against MDA-MB-231 human breast cancer cell line came from the unsubstituted 2-phenylbenzimidazole 10a.

Full text of DOI:10.3109/14756366.2012.729828

Journal of Enzyme Inhibition and Medicinal Chemistry, 2013; 28(6): 1255–1260
© 2013 Informa UK, Ltd.
ISSN 1475-6366 print/ISSN 1475-6374 online
DOI: 10.3109/14756366.2012.729828
RESEARCH ARTICLE
Microwave-assisted synthesis of sec/tert-butyl
2-arylbenzimidazoles and their unexpected antiproliferative
activity towards ER negative breast cancer cells
Aisyah Saad Abdul Rahim1, Salizawati Muhamad Salhimi1, Natarajan Arumugam1, Lim Chung Pin1,
Ng Shy Yee1, Nithya Niranjini Muttiah1, Wong Boon Keat1, Shafida Abd. Hamid3, Hasnah Osman4,
and Ishak b. Mat4
1School of Pharmaceutical Sciences, Universiti Sains Malaysia, Penang, Malaysia, 2Kulliyyah of Science, International
Islamic University Malaysia, Jalan Istana, Bandar Indera Mahkota, Kuantan, Malaysia, 3School of Chemical Sciences,
Universiti Sains Malaysia, Penang, Malaysia, and 4Advanced Medical and Dental Institute, Universiti Sains Malaysia,
Bandar Putra Bertam, Penang, Malaysia
Abstract
A new series of N-sec/tert-butyl 2-arylbenzimidazole derivatives was synthesised in 85–96% yields within
2–3.5min by condensing ethyl 3-amino-4-butylamino benzoate with various substituted metabisulfite adducts
of benzaldehyde under focused microwave irradiation. The benzimidazole analogues were characterised using
1H NMR, 13C NMR, high resolution MS and melting points. Evaluation of antiproliferative activity of the benzimidazole
analogues against MCF-7 and MDA-MB-231 revealed several compounds with unexpected selective inhibitions of
MDA-MB-231 in micromolar range. All analogues were found inactive towards MCF-7. The most potent inhibition
against MDA-MB-231 human breast cancer cell line came from the unsubstituted 2-phenylbenzimidazole 10a.
Keyword: 2-Arylbenzimidazole, Microwave synthesis, Breast cancer, MTS assay, Antiproliferative activity
Introduction
breast cancer patients, thus they receive more aggressive
Breast cancer ranks as the top cancer in women, amount-
ing to almost one-fifth of all female cancers. It is also the
second leading cause of cancer mortality in women.1–3
Apart from anti-oestrogen and radiation therapies and
surgery, chemotherapy remains the mainstay treatment
for breast cancer.
In oestrogen receptor positive (ER+) breast cancer,
tamoxifen has made substantial reductions in the mortal-
ity rate in both early and advanced breast cancer patients
for almost three decades.4 Long term use of tamoxifen,
however, is of limited success due to resistance in patients.5
In contrast, patients with oestrogen-independent (ER-)
breast cancer are often unresponsive to current treat-
ments with poorer prognosis than hormone dependent
chemotherapy. Despite considerable progress made in the
discovery of novel chemotherapeutic agents for example
imatinib, paclitaxel, ixabepilone; many current anti-
cancer agents are still beseted with clinical problems such
as toxicity, resistance and debilitating adverse effects.6–8
erefore, there is a clear need for new anti-cancer agents
with enhanced selectivity towards breast cancer.
Small molecule inhibitors are the cornerstone of existing
anti-cancer agents, and serve as lead compounds for
many new chemotherapeutic agents.1,2,8 Recently several
structurally related bicyclic privileged scaffolds9,10 were
reported to exhibit strong inhibitory effects on breast cancer
cell lines, for instance benzothiophene 111, benzothiazole
212, benzoxazole13 and benzimidazoles 3, 414,15 (Figure 1).
Address for Correspondence: Aisyah Saad Abdul Rahim, School of Pharmaceutical Sciences, Universiti Sains Malaysia, Penang, 11800
Malaysia. E-mail: aisyah@usm.my; Salizawati Muhammad Salhimi, School of Pharmaceutical Sciences, Universiti Sains Malaysia, Penang,
11800 Malaysia. E-mail: saliza@usm.my
(Received 25 June 2012; revised 06 September 2012; accepted 10 September 2012)
1255

1256 A. S. Abdul Rahim et al.
Figure 1. Some bicyclic privileged structures with cytotoxic activity against breast cell lines.
Of particular interest to us is the benzimidazole core,
which exhibits a wide range of biological and pharmaco-
logical activities16 including against several viruses such
as HIV17, human cytomegalovirus (HCMV)18; Chlamydia
bacteria19; as topoisomerase inhibitors20, angiotensin
inhibitor21, antitumour and anticancer agents.11,22–25 e
proton pump inhibitor Omeprazole, and the antihelmin-
tic Albendazole are examples of benzimidazoles with
established clinical use.16 e remarkable biological sig-
nificance of benzimidazole derivatives together with their
close structural resemblance to benzimidazole analogues
3 and 4 (Figure 1) led us to surmise that this series of
2-arylbenzimidazole derivatives could exert anti-prolifer-
ative effects towards breast cancer cell lines. us in this
paper, we report microwave assisted preparation of a new
series of N-sec/tert-butyl-2-arylbenzimidazole deriva-
tives and the evaluation of their antiproliferative activity
against hormone dependent (MCF-7) and hormone inde-
pendent (MDA-MB-231) breast cancer cell lines.
was irradiated under microwave conditions at 130°C
for 2–3.5 min. e reaction mixture was then diluted
in EtOAc (20 mL) and washed with H2O (20 mL). e
organic layer was collected and dried over Na2SO4. e
solvent was removed under reduced pressure to afford
the crude product.
Ethyl 1-sec-butyl-2-phenyl-1H-benzimidazole-5-car-
boxylate (10a). 1H NMR (CDCl3, 300 MHz), δ (ppm): 0.65
(t, CH3, 3H), 1.44 (t, CH3, 3H), 1.70 (d, CH3, 3H), 1.80–1.93
(m, CH2, 1H), 2.10–2.22 (m, CH2, 1H), 4.42 (q, CH2, 2H),
4.50–4.54 (m, CH, 1H), 7.50–7.58 (m, 3H), 7.61–7.67 (m,
3H), 8.01 (d, J = 9.0 Hz, 1H), 8.56 (s, 1H); 13C NMR (CDCl3,
75 MHz), δ (ppm): 11.22, 14.45, 31.02, 39.04, 55.7, 61.34,
112.32, 123.20, 124.32, 124.85, 128.56, 129.32, 131.26,
137.64, 144.51, 156.24, 167.52; HRMS (ESI/Q-TOF): m/z
calcd for C20H22N2O2 (M+H), 323.1754; found 323.1767.
Ethyl
1-sec-butyl-2-(4-fluorophenyl)-1H-benzimid-
1
azole-5-carboxylate (10b). H NMR (CDCl3, 500 MHz),
δ(ppm): 0.64 (t, CH3, 3H), 1.42 (t, CH3, 3H), 1.70 (d, CH3,
3H), 1.85–1.93 (m, CH2, 1H), 2.13–2.22 (m, CH2, 1H), 4.42
(q, CH2, 2H), 4.45–4.49 (m, CH, 1H), 7.21–7.25 (m, 2H),
7.60–7.64 (m, 3H), 7.99–8.01 (dd, J = 8.5, 1.5 Hz), 8.54 (d, J
= 1.0 Hz, 1H); 13C NMR (CDCl3, 125 MHz), δ (ppm): 10.94,
14.37, 19.89, 28.09, 55.18, 60.80, 111.70, 115.88, 116.05,
122.51, 123.83, 124.78, 126.80, 131.60, 136.68, 143.30,
155.17, 162.68, 164.68, 167.03; HRMS (ESI/Q-TOF): m/z
calcd for C20H21FN2O2 (M+H), 341.1660; found 341.1666.
Materials and methods
Microwave-assisted syntheses were performed using
CEM Discover™ microwave synthesizer. Melting points
were measured on a Stuart SMP10 instrument and are
uncorrected. Compounds 6–8 have been synthesised as
previously described.15,26,27 Preparative thin layer chro-
matography (PLC) was performed using Merck 60 GF254
silica gel coated (1mm) on glass plates (20×20cm). TLC
experiments were performed on alumina-backed silica gel
40 F254 plates (Merck, Darmstadt, Germany). Visualisation
of TLC plates was performed under UV light and aided by
KMnO4 and iodine staining. Routine NMR (1H and 13C)
spectra were recorded on a Bruker 300, 400 and 500 MHz
instruments in CDCl3. Acquisition of high resolution mass
measurements of the compounds was performed on an
Agilent 6520 Quadrupole Time of Flight Mass Spectrometer
(Agilent Technologies, Santa Clara, CA, USA) operating in
the MS mode. All commercially available starting materials
and solvents are used without further purification.
Ethyl
1-sec-butyl-2-(4-bromophenyl)-1H-benzimid-
1
azole-5-carboxylate (10c). H NMR (CDCl3, 500 MHz),
δ(ppm): 0.64 (t, CH3, 3H), 1.43 (t, CH3, 3H), 1.70 (d, CH3,
3H), 1.86–1.94 (m, CH2, 1H), 2.13–2.22 (m, CH2, 1H),
4.43 (q, CH2, 2H), 4.45–4.47 (m, CH, 1H), 7.52 (d, J = 8.0
Hz, 2H), 7.61 (d, J = 8.5 Hz, 1H), 7.69 (d, J = 8.0 Hz, 2H),
8.00–8.02 (dd, J = 8.5, 1.0 Hz, 1H), 8.55 (d, J = 1.0 Hz, 1H);
13C NMR (CDCl3, 125 MHz), δ (ppm): 10.94, 14.38, 19.91,
28.12, 55.25, 60.83, 111.76, 122.60, 123.94, 124.52, 124.87,
129.61, 131.09, 132.04, 136.71, 143.36, 155.01, 167.00;
HRMS (ESI/Q-TOF): m/z calcd for C20H21BrN2O2 (M+H),
401.0859; found 401.0872.
Ethyl
1-sec-butyl-2-(3-nitrophenyl)-1H-benzimid-
1
azole-5-carboxylate (10d). H NMR (CDCl3, 500 MHz),
δ(ppm): 0.70 (t, CH3, 3H), 1.45 (t, CH3, 3H), 1.77 (d, CH3,
3H), 1.92–2.01 (m, CH2, 1H), 2.19–2.28 (m, CH2, 1H),
4.45 (q, CH2, 2H), 4.47–4.51 (m, CH, 1H), 7.67 (d, J = 8.5
Hz, 1H), 7.76–7.79 (m, 1H), 8.03–8.07 (m, 2H), 8.41–8.43
(m, 1H), 8.53–8.54 (m, 1H), 8.58 (d, J = 1.0 Hz, 1H); 13C
NMR (CDCl3, 125 MHz), δ (ppm): 11.02, 14.38, 19.99,
28.25, 55.65, 60.94, 111.98, 122.89, 124.41, 124.52, 124.67,
General procedure for synthesis of benzimidazoles
10a-e, 11a-e
Ethyl-3-amino-4-(sec-/tert-butylamino) benzoates were
prepared according to previous procedure.27 us, a
solution of ethyl-3-amino-4-(sec-/tert-butylamino) ben-
zoate (200 mg, 0.84 mmol) and sodium bisulfite adduct
of benzaldehyde (353 mg, 1.68 mmol) in DMF (1 ml)
Journal of Enzyme Inhibition and Medicinal Chemistry

2-Arylbenzimidazoles selective towards MDA-MB-231 1257
125.28, 130.05, 132.41, 135.52, 136.72, 143.31, 148.31,
153.36, 166.87; Mass ESI m/z 368.3 (M+1). HRMS (ESI/Q-
TOF): m/z calcd for C20H21N3O4 (M+H), 368.1605; found
368.1620.
(14); HRMS (ESI/Q-TOF): calcd (M+H) 391.0974, found
391.2843; (M-Cl+H) calcd 357.1365, found 357.1362.
Ethyl 1-tert-butyl-2-(2-hydroxyphenyl)-1H-benzimid-
1
azole-5-carboxylate (11e). H NMR (CDCl3, 500 MHz),
Ethyl 1-sec-butyl-2-(2-hydroxyphenyl)-1H-benzimid-
azole-5-carboxylate (10e). H NMR (CDCl3, 400 MHz),
δ(ppm): 1.47 (t, CH3, 3H), 1.70 (s, CH3, 9H), 4.43 (q, CH2,
2H), 6.53 (d, J = 8.5 Hz), 6.87–6.90 (m, 1H), 7.08–7.11 (m,
1H), 7.15–7.16 (dd, J = 7.5, 1.5 Hz 1H), 7.53 (d, J = 8.5 Hz),
7.61–7.64 (m, 2H); 13C NMR (CDCl3, 125 MHz), δ (ppm):
14.52, 30.21, 60.02, 60.63, 114.04, 119.52, 119.70, 120.27,
122.85, 123.58, 124.04, 130.09, 131.16, 136.90, 141.03,
152.91, 155.40, 166.46; HRMS (ESI/Q-TOF): m/z calcd for
C20H22N2O3 (M+H), 339.1703; found 339.1710.
1
δ(ppm): 0.65 (t, CH3, 3H), 1.45 (t, CH3, 3H), 1.77 (d, CH3,
3H), 1.91–2.02 (m, CH2, 1H), 2.13–2.22 (m, CH2, 1H),
4.413 (q, CH2, 2H), 4.79–4.87 (m, CH, 1H), 6.97–7.02 (m,
1H), 7.13–7.15 (dd, J = 8.3, 1.0 Hz, 1H), 7.35–7.39 (m,
1H), 7.40–7.44 (dd, J = 7.8, 1.5 Hz, 1H), 7.63 (d, J = 8.6 Hz,
1H), 7.69–7.99 (m, 1H), 8.44 (d, J = 1.5 Hz, 1H); 13C NMR
(CDCl3, 100 MHz), δ (ppm): 11.21, 14.81, 20.01, 28.54,
56.30, 61.32, 112.59, 114.25, 118.73, 119.57, 121.90, 124.43,
125.56, 128.48, 132.19, 136.65, 142.10, 154.35, 158.40,
167.23; HRMS (ESI/Q-TOF): m/z calcd for C20H22N2O3
(M+H), 339.1703; found 339.1717.
Ethyl 1-tert-butyl-2-p-tolyl-1H-benzimidazole-5-car-
boxylate (11a). 1H NMR (CDCl3, 300 MHz), δ (ppm):
1.41 (t, CH3, 3H), 1.62 (s, CH3, 9H), 2.42 (s, CH3, 3H), 4.41
(q, CH2, 2H), 7.23 (d, J = 7.8 Hz, 2H), 7.36 (d, J = 8.1 Hz,
2H), 7.73 (d, J = 8.7 Hz, 1H), 7.97–8.00 (dd, J = 8.7, 1.5 Hz,
1H), 8.47 (d, J = 1.5 Hz, 1H); 13C NMR (CDCl3, 75 MHz,),
δ (ppm): 14.78, 21.81, 31.89, 59.80, 61.18, 114.57, 122.53,
122.83, 124.62, 128.89, 129.72, 132.71, 138.62, 139.65,
143.35, 155.95, 167.50; HRMS (ESI/Q-TOF): m/z calcd for
C21H24N2O2 (M+H), 337.1911; found 337.1922.
In vitro cytotoxicity assay
MCF-7 and MDA-MB-231 cell lines were purchased from
ATCC and were grown, respectively, in DMEM and L-15
supplemented with 10% fetal bovine serum. Cytotoxicity
of the compounds was evaluated using Cell Titer 96®
Aqueous Non-Radioactive Cell Proliferation Assay
(Promega), according to manufacturer’s instructions.
Briefly, 90 μL of MCF-7 and MDA-MB-231 at respective
cell number (MCF-7: 2 × 104 cells/well; MDA-MB-231:
2.5 × 104 cells/well) were seeded in triplicates in 96-well
plate and incubated for 24 h. Subsequently, 10 μL of test
compounds at various concentrations were added into
each well and incubated for 48 h. Following incubation,
20 μL of MTS-PMS solution was added into each well
and the plate was incubated 2 h for MCF-7 and 5 h for
MDA-MB-231. Cisplatin and DMSO were used as posi-
tive and negative controls in this assay. e optical den-
sity of each well was determined spectrophotometrically
at 490 nm, with 655 nm as reference wavelength. e
percentage growth inhibition was calculated according
to the following formula:
Ethyl 1-tert-butyl-2-(4-methoxyphenyl)-1H-benzimid-
1
azole-5-carboxylate (11b). H NMR (CDCl3, 300 MHz),
δ(ppm): 1.41 (t, CH3, 3H), 1.62 (s, CH3, 9H), 3.87 (s, OCH3,
3H), 4.40 (q, CH2, 2H), 6.96 (d, J = 9.0 Hz, 1H), 7.39–
7.41(m, 2H), 7.74 (d, J = 9.0 Hz, 1H), 7.96–7.99 (m, 1H),
8.47 (s, 1H); 13C NMR (CDCl3, 75 MHz), δ (ppm): 14.77,
31.87, 55.76, 60.07, 61.21, 114.11, 114.69, 122.39, 123.76,
124.88, 127.25, 131.32, 138.43, 142.67, 155.58, 160.86,
167.37; HRMS (ESI/Q-TOF): m/z calcd for C20H24N2O3
(M+H), 353.1860; found 353.1863.
Percentage of growth inhibition (%) = Absorbance
of negative control – Absorbance of test compound
Absorbance of negative control
Ethyl
1-tert-butyl-2-(2-chlorophenyl)-1H-benzimid-
1
azole-5-carboxylate (11c). H NMR (CDCl3, 500 MHz),
δ(ppm): 1.43 (t, CH3, 3H), 1.66 (s, CH3, 9H), 4.43 (q, CH2,
2H), 7.38–7.40 (m, 1H), 7.45–7.50 (m, 3H), 7.79 (d, J = 8.5
Hz), 8.03–8.05 (dd, J = 8.5, 1.5 Hz, 1H), 8.52 (d, J = 1.5 Hz);
13C NMR (CDCl3, 125 MHz), δ (ppm): 14.39, 30.39, 59.45,
60.80, 114.17, 122.64, 123.56, 124.49, 126.46, 129.32,
130.81, 131.44, 134.49, 135.08, 137.67, 143.17, 151.60,
167.00; HRMS (ESI/Q-TOF): m/z calcd for C20H21ClN2O2
(M+H), 357.1365; found 357.1378.
Results and discussion
Microwave-assisted synthesis of 2-arylbenzimidazoles
Synthesis of our target benzimidazoles 10 and 11 began
with esterification of benzoic acid 5 in the presence of
catalytic sulfuric acid in ethanol by refluxing for 8 h to
afford ethyl ester 6 in 75% yield (Scheme 1). e benzo-
ate 6 was then treated with sec- or tert-butylamine and
N,N-diisopropylethylamine (DIPEA) in dichlorometh-
ane overnight to afford compound 7 and 8. Reduction of
the aryl nitro 7 using 1.5 equivalent of ammonium for-
mate in 10% Pd/C in ethanol gave a 30% yield; repeating
it at 3.5 equivalent doubled the yield to 65%. When this
reaction was performed under microwave conditions
at 100oC for 2 min, the yields of the diamine intermedi-
ate improved significantly to 85%.27 e pleasant smell
compound was found to decompose by turning to dark
red if kept any longer. Hence, the diamino compound
was used immediately in the next step without further
purification.
Ethyl
1-tert-butyl-2-(2,4-dichlorophenyl)-1H-benz-
imidazole-5-carboxylate (11d). 1H NMR (CDCl3, 500
MHz), δ (ppm): 1.42 (t, CH3, 3H), 1.65 (s, CH3, 9H), 4.42
(q, CH2, 2H), 7.37–7.39 (dd, J = 8.5, 2.0 Hz, 1H), 7.43 (d,
J = 8.5 Hz, 1H), 7.51 (d, J = 2.0 Hz, 1H), 7.77 (d, J = 9.0
Hz, 1H), 8.03–8.05 (dd, J = 9.0, 1.5 Hz, 1H), 8.50 (d, J =
1.0 Hz); 13C NMR (CDCl3, 125 MHz), δ (ppm): 14.37,
30.45, 59.57, 60.85, 114.23, 122.63, 123.77, 124.70, 126.95,
129.28, 132.17, 133.60, 135.33, 136.32, 137.61, 143.03,
150.44, 166.87; EIMS m/z calcd for C20H20Cl2N2O2 (M+H),
391.1; found 391.1 (100), 392.1 (19), 393.0 (59), 394.1
© 2013 Informa UK, Ltd.

1258 A. S. Abdul Rahim et al.
Scheme 1. Synthesis of N-sec/tert-butyl 2-arylbenzimidazoles 10a-e and 11a-e.
Table 2 Cytotoxicity of sec-/tert-butyl 2-arylbenzimidazoles
against human breast cancer cell lines.
Table 1. Cyclocondensation reaction of aldehydes 9a-j under
microwave conditions and analytical data of the benzimidazole
products 10a-e and 11a-e.
IC50 (µM)
Aldehydes Aromatic
Time
Compound
R1 = sec-butyl
R2
MDA-MB-231a,c MCF-7b,d
9
a
b
c
d
e
f
substitution (min)a Products Yieldb (%) Mp oC
H
2
3
10a
10b
10c
10d
10e
11a
11b
11c
11d
11e
92
94
96
89
95
91
92
85
87
93
121
160
122
109
178
165
156
155
148
140
10a
10b
10c
10d
10e
R1 = tert-butyl
H
29.7
>200
81.5
36.8
>200
>200
>200
>200
>200
>200
4-F
4-F
4-Br
2
4-Br
3-NO2
2-OH
3-NO2
2-OH
4-CH3
4-OCH3
2-Cl
3
2.5
2
g
h
i
2
11a
11b
11c
11d
4-CH3
4-OCH3
2-Cl
>200
88.6
>200
47.6
>200
>200
>200
>200
>200
>200
3.5
3
2,4-Cl
2-OH
j
2
2,4-Cl
2-OH
aReaction time under optimised microwave conditions. bIsolated
yield.
11e
ae concentration required to inhibit the proliferation of the cells
by 50% expressed as IC50 (µM). Values given represent the mean
obtained from three consecutive experiments. bIC50 values could
not be determined since the concentrations exceeded the range
tested. cHormone independent human breast cancer cell line.
dHormone dependent human breast cancer cell line.
Benzimidazoles are typically prepared by condensing
o-phenylenediamine with acid chloride, aldehydes and
carboxylic acids.28,29 To effect the condensation of phen-
ylenediamines with carboxylic acids generally require
harsh, dehydrating conditions; in contrast, access via alde-
hydes employs oxidative reagents for example, copper(II)
acetate, 1,4-benzoquinone, I2/KI, sodium metabisulfite
and, interestingly, air.30–32 us, we prefer using bisulfite
adducts of aldehyde as this method has been reported to
be more efficient and environmental friendly affording
good to excellent yield of benzimidazoles when compared
to other synthetic methods.15,31 Initial condensation reac-
tions of the unstable substituted o-phenylenediamines
with sodium bisulfite adduct of some substituted benzal-
dehydes, by heating in DMF for 2–4h, afforded moderate
yields of benzimidazoles. e cyclocondensation reaction
was further optimised under microwave conditions and
then performed on commercially available aldehydes 9a-j
to assess the generality of the optimised reaction condi-
tions. We found that this microwave-irradiated cyclocon-
densation reactions proceeded in 2–3.5min and afforded
excellent yields (85–96%) of 2-arylbenzimidazoles as
shown in Table 1.
Pharmacology
e newly synthesised compounds 10a-e and 11a-e were
evaluated in in vitro antiproliferative (MTS assay) studies
against two human breast cancer cell lines MDA-MB-231
and MCF-7. e inhibitory activities are summarised in
Table 2. Several compounds were found to exhibit mod-
erate antiproliferative activity towards MDA-MB-231 cell
lines. e unsubstituted 2-phenylbenzimidazole 10a
emerged as the compound with most potent inhibition
against MDA-MB-231 cell line (IC50 value of 29.7 µM) fol-
lowed by 3-nitrophenyl 10d and 2,4-dichlorophenyl 11d
with IC50 of 36.8 µM and 47.6 µM, respectively.
In the sec-butyl series, the replacement of the hydrogen
on the para-position of 10a with an isosteric fluoro in 10b
Journal of Enzyme Inhibition and Medicinal Chemistry

2-Arylbenzimidazoles selective towards MDA-MB-231 1259
seems detrimental to its activity. Unlike the fluoro 10b, the
4-bromophenyl derivative 10c (IC50 of 81.5 µM) showed a
2.7-fold reduced activity compared to the unsubstituted
10a. In terms of substitution on the phenyl ring, the activ-
ity seen in 2,4-dichloro 11d however, was abolished in its
ortho-substituted analogue 11c. Another ortho-substituted
phenylbenzimidazoles 10e and 11e also showed no antip-
roliferative effects against both breast cancer cell lines. is
suggests the unlikely role of the 2-substitution in the phenyl
ring towards activity. All compounds showed no antiprolif-
erative activity on MCF-7 cell lines (IC50 > 200 µM).
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Various substituted benzimidazoles were previously
reported to show potent antiproliferative activity on
MCF-7 and other breast cancer cell lines; however with
little or no selectivity.11,23,33–38 In this study, even though
these benzimidazole analogues showed moderate antip-
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stituted benzimidazole derivatives was accomplished
in excellent yields (85–96%) and short reaction time
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e results of the antiproliferative studies showed that
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Acknowledgements
is research was funded by Universiti Sains Malaysia
(USM)ResearchUniversityGrant1001/PFARMASI/815026,
Ministry of Higher Education (MoHE), under the FRGS
grants: 203/PFARMASI/671156 and 671159, and Ministry
of Science and Technology and Innovation (MOSTI)
under the R&D initiative Grant No: 09-05-lfn-meb-004.
We are grateful to Dr Mohd Nazri Ismail and Dr Michael
Harvey at USM Doping Control Centre for helpful tech-
nical discussions and assistance in providing the HRMS
service. We thank Ms Nurasyikin Hamzah for NMR and
MS sample preparations. N.A. thanks USM for the award
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