Design, synthesis and biological evaluation of new rhodacyanine analogues as potential antitumor agents

Article abstract of DOI:10.1016/j.cclet.2012.01.015

In an attempt to develop potent antitumor agents, new rhodacyanine analogues containing the pyridinium ring (5a-5h), the isoquinolinium ring (6a-6c) and the quinolinium ring (7a-7e) linked to the rhodanine ring via N-N covalent bond were designed, synthesized and evaluated for antitumor activity against human lung cancer cell line (H460) by MTT assay in vitro. Most of the tested compounds showed enhanced antitumor activity with IC₅₀ values ranging from 0.006 to 9.2 μmol/L as compared to the lead compound MKT-077. Among them, the most promising compound 7d (IC₅₀ = 0.006 μmol/L) was 216.7 times more active than MKT-077 (IC₅₀ = 1.3 μmol/L). The preliminary structure-activity relationship of the target compounds was discussed.

Full text of DOI:10.1016/j.cclet.2012.01.015

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 415–418
www.elsevier.com/locate/cclet
Design, synthesis and biological evaluation of new
rhodacyanine analogues as potential antitumor agents
*
Yang Xiong Li, Xin Zhai, Wei Ke Liao, Wu Fu Zhu, Ying He, Ping Gong
Key Laboratory of Structure-based Drug Design and Discovery of Ministry of Education, School of Pharmaceutical Engineering,
Shenyang Pharmaceutical University, Shenyang 110016, China
Received 31 October 2011
Available online 3 March 2012
Abstract
In an attempt to develop potent antitumor agents, new rhodacyanine analogues containing the pyridinium ring (5a–5h), the
isoquinolinium ring (6a–6c) and the quinolinium ring (7a–7e) linked to the rhodanine ring via N–N covalent bond were designed,
synthesized and evaluated for antitumor activity against human lung cancer cell line (H460) by MTT assay in vitro. Most of the
tested compounds showed enhanced antitumor activity with IC50 values ranging from 0.006 to 9.2 mmol/L as compared to the lead
compound MKT-077. Among them, the most promising compound 7d (IC50 = 0.006 mmol/L) was 216.7 times more active than
MKT-077 (IC50 = 1.3 mmol/L). The preliminary structure–activity relationship of the target compounds was discussed.
# 2012 Ping Gong. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Rhodacyanine analogues; Antitumor; Synthesis
The rhodacyanine derivative MKT-077 (Fig. 1), known as 1-ethyl-2-[[3-ethyl-5-(methylbenzothiazolin-2-ylid-
ene)-4-oxothiazolidin-2-ylidene]methyl]pyridinium chloride, displayed remarkable antitumor activity due to its
selective accumulation in the negatively charged mitochondria in carcinoma cells [1,2]. Furthermore, it can bind to
mortalin (a kind of chaperone protein) and activate the tumor suppressor protein p53, thus inducing senescence in
tumor cells [3]. Based on the principles of bioisosterisms, taking MKT-077 as the lead compound and replacing the
C C double bond at 2-position of thiazolidinone ring with C N group, compounds 5a–5h were designed and
synthesized, in which pyridinium group was linked to the rhodanine ring via N–N covalent bond.
Berberine (Fig. 1), an isoquinoline alkaloid, was shown to be a potent telomerase inhibitor [4] as well as MKT-077
[5]. In view of the similar molecular mechanism, we focused on fusing the pharmacophores of the two structures. As a
result, compounds 6a–6c and 7a–7e were designed by introducing isoquinolinium or quinolinium groups into 2-
position of thiazolidinone. The structures and the antitumor activity of the target compounds were listed in Table 1.
The general synthesis of target compounds was described in Scheme 1. Taking benzo[d]thiazole-2-thiol (1a) and
6-chlorobenzo[d]thiazole-2-thiol (1b) as starting materials, 2a and 2b were obtained by alkylation of 1a and 1b
with dimethylsulfate under reflux for 10 h, respectively. Subsequently, reaction of 2a and 2b with the key
intermediate N-substituted rhodanine, which was synthesized according to approach described in the literature [6],
* Corresponding author.
E-mail address: gongpinggp@126.com (P. Gong).
1001-8417/$ – see front matter # 2012 Ping Gong. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2012.01.015

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Y.X. Li et al. / Chinese Chemical Letters 23 (2012) 415–418
O
O
CH3
N
N
S
C2H5
Cl
S
N
N
H3CO
O
C2H5
Cl
OCH3
MKT-077
Berberine
R
CH
N
CH
N
3
3
CH
3
N
N
S
S
N
N
S
N
N
N
I
S
S
R
1
N
R
1
N
S
I
I
R
N
1
O
O
R
R
2
2
O
R
2
__
__
7a
7e
6a
__
6c
5a 5h
Fig. 1. The structures of MKT-077, Berberine, 5a–5h, 6a–6c and 7a–7e.
Table 1
Structure and antitumor activity against H460 cells in vitro of target compounds 5a–5h, 6a–6c and 7a–7e.
Compound
R1
R2
R3
IC50 (mmol)
5a
ÀH
ÀCH3
ÀC2H5
9.2
N
N
N
5b
5c
5d
5e
5f
ÀH
ÀH
ÀCl
ÀCl
ÀCl
8.6
1.4
1.9
2.0
1.1
CH
CH
3
3
ÀCH3
ÀC2H5
N
N
CH
CH
3
3
N
CH3
CH3
5g
ÀCl
ÀCH3
1.98
N
N
CH3
5h
ÀCl
ÀC2H5
2.3
6a
6b
6c
7a
ÀH
ÀCH3
ÀCH3
ÀC2H5
0.3
0.3
0.9
0.05
N
N
N
ÀCl
ÀCl
ÀCl
N
7b
ÀH
ÀH
ÀH
ÀCl
ÀCH3
0.35
0.09
0.006
0.05
1.3
N
N
N
N
7c
7d
ÀC2H5
ÀC2H5
7e
MKT-077

Y.X. Li et al. / Chinese Chemical Letters 23 (2012) 415–418
417
CH
N
3
CH
N
CH
N
3
3
N
S
a
b
S
c
S
S
SH
SC H
SCH
2
5
3
S
R
R
S
N
S
1
1
R
R
N
1
1
BF
O
4
R
2
O
R
2
1a, 1b
2a, 2b
3a-3f
4a-4f
1a: R = H
1b: R =Cl
2a: R = H
2b: R = Cl
3a, 4a: R =H, R =CH
3b, 4b: R =H, R =C H
1
1
1
1
1
1
2
2
3
d, e, f
2
5
3c, 4c: R =H, R =
1
2
CH
3
N
3d, 4d: R =Cl, R =CH
1
2
3
5
R
S
3
3e, 4e: R =Cl, R =C H
N
1
2
2
S
R
N
1
I
3f, 4f: R =Cl, R =
1
2
O
R
2
5a-5g, 6a-6c, 7a-7e
Scheme 1. Regents and conditions: (a) (CH3O)2SO2, reflux, 10 h, yield 50–60%; (b) CH3CN, Et3N, N-substituted rhodanine, 0 8C, 5 h, yield 30–
50%; (c) 1, 4-dioxane, BF3ÁEt2O, HC(OC2H5)3, 80 8C, 3 h, yield 70–80%; (d) CH3CN, Et3N, substituted N-aminopyridinium iodide, 0 8C, 4 h, yield
20–40%; (e) CH3CN, Et3N, N-aminoisoquinolinium iodide, 0 8C, 4 h, yield 20–40%; (f) CH3CN, Et3N, N-aminoquinolinium iodide, 0 8C, 4 h, yield
20–40%.
provided 3a-3f which were purified by recrystallization with methanol in 30–50% yields. 4a–4f were accomplished
by treating 3a–3f with boron trifluoride etherate and triethyl orthoformate in 1,4-dioxane at 80 8C for 3 h. Reaction of
the intermediates 4a–4f with substituted N-aminopyridinium iodide, N-aminoisoquinolinium iodide and N-
aminoquinolinium iodide in the presence of triethylamine at 0 8C for 4 h afforded the corresponding compound 5a–
5h, 6a–6c and 7a–7e, respectively. The crude product of the target compound precipitated from the mixture and was
collected by vacuum filtration. Then the residue was suspended in acetone, and the mixture was stirred under reflux
for 30 min. After being cooled to room temperature, the precipitate was collected and washed with acetone to give the
1
pure product. Structures of the target compounds were confirmed by H NMR data and ESI–MS [7].
The target compounds were evaluated for antitumor activity on human lung cancer cell line H460 in vitro by MTT
assay, using MKT-077 as the positive control. As shown in Table 1, compounds 5a–5h showed moderate activity with
IC50 values ranging from 1.1 to 9.2 mmol/L. In contrast, compounds 6a–6c displayed good antitumor activity, which
was a slight improvement compared with the positive control, while compounds 7a–7e exhibited excellent antitumor
activity. In particular, the most promising compound 7d (IC50 = 0.006 mmol/L) was 216.7 times more active than
MKT-077 (IC50 = 1.3 mmol/L).
Based on the pharmacological data, the following conclusions could be drawn: (1) The antitumor activity was
significantly altered by changing the heteroaromatic ring linked to the rhodanine ring via N–N covalent bond, and
activity increased in the following order: quinolinium > isoquinolinium > pyridinium. (2) Changing the N-substituent
(R2) on the rhodanine ring from methyl to cyclopropyl group had little influence on their antitumor activity. (3) R1
group altering with hydrogen or chloride atom had no significant effect on the pharmacological activity. The study will
provide valuable information for further research on the antitumor rhodacyanine analogues.
References
[1] K. Takasu, H. Inoue, H.S. Kim, et al. J. Med. Chem. 45 (2002) 995.
[2] J.S. Modica-Napolitano, K. Koya, E. Weisberg, et al. Cancer Res. 56 (1996) 544.
[3] R. Wadhwa, T. Sugihara, A. Yoshida, et al. Cancer Res. 60 (2000) 6818.
[4] J. Li, Y. Chen, J. Sperry, et al. Med. Chem. Commun. 2 (2011) 229.
[5] I. Naasani, H. Seimiya, T. Yamori, et al. Cancer Res. 59 (1999) 4004.
[6] C.E. Redemann, R.N. Icke, G.A. Alles, et al. Org. Synth. 3 (1955) 763.
[7] Data for new compounds. 5a: yield 25.4%. mp: 301–303 8C; ESI–MS: m/z 355.2 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 3.38 (s,
3H), 3.77 (s, 3H), 7.36–7.27 (m, 1H), 7.47 (m, 1H), 7.56 (d, 1H, J = 8.2 Hz), 7.88 (d, 1H, J = 7.3 Hz), 8.29 (m, 2H), 8.73–8.59 (m, 1H), 9.12 (m,
2H). 5b: yield 36.5%. mp 294–296 8C; ESI–MS: m/z 383.3 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 1.33 (t, 3H), 2.68 (s, 3H), 3.77
(s, 3H), 3.97 (q, 2H), 7.32 (dd, 1H), 7.48 (dd, 1H), 7.56 (d, 1H, J = 8.2 Hz), 7.88 (d, 1H, J = 7.6 Hz), 8.12 (d, 2H, J = 6.5 Hz), 9.01 (d, 2H,

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Y.X. Li et al. / Chinese Chemical Letters 23 (2012) 415–418
J = 6.7 Hz). 5c: yield 31.7%. mp 277–279 8C; ESI–MS: m/z 395.4 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 1.24–1.04 (m, 4H),
2.68 (s, 3H), 2.96–2.84 (m, 1H), 3.74 (s, 3H), 7.28 (dd, 1H), 7.45 (dd, 1H), 7.53 (d, 1H, J = 8.2 Hz), 7.86 (d, 1H, J = 7.8 Hz), 8.12 (d, 2H,
J = 6.6 Hz), 8.97 (d, 2H, J = 6.6 Hz). 5d: yield 28.9%. mp 290–293 8C; ESI–MS: m/z 403.3 [M]+, 126.9 [M]À; 1H NMR(300 MHz, DMSOÀd6):
d 2.68 (s, 3H), 3.73 (s, 3H), 3.37 (s, 3H), 7.58–7.45 (m, 2H), 8.03 (s, 1H), 8.13 (d, 2H, J = 6.4 Hz), 8.95 (d, 2H, J = 6.5 Hz). 5e: yield 32.8%. mp
274–276 8C; ESI–MS: m/z 417.4 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 1.33 (t, 3H, J = 7.1 Hz), 2.68 (s, 3H), 3.74 (s, 3H), 3.97
(q, 2H, J = 6.9 Hz), 7.52 (dt, 2H, J = 8.8, 5.4 Hz), 8.04 (d, 1H, J = 1.9 Hz), 8.12 (d, 2H, J = 6.6 Hz), 9.01 (d, 2H, J = 6.6 Hz). 5f: yield 37.3%. mp
273–275 8C; ESI–MS: m/z 429.4 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 1.20–1.01 (m, 4H), 2.68 (s, 3H), 2.96–2.84 (m, 1H),
3.71 (s, 3H), 7.57–7.44 (m, 2H), 8.01 (d, 1H, J = 1.6 Hz), 8.12 (d, 2H, J = 6.5 Hz), 8.97 (d, 2H, J = 6.6 Hz). 5g: yield 26.7%. mp 299–302 8C;
ESI–MS: m/z 403.3 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 2.55 (s, 3H), 3.38 (s, 3H), 3.74 (s, 3H), 7.58–7.46 (m, 2H), 8.03 (d,
1H, J = 1.8 Hz), 8.19 (dd, 1H), 8.51 (d, 1H, J = 8.0 Hz), 8.96 (d, 1H, J = 6.2 Hz), 9.05 (s, 1H). 5h: yield 33.8%. mp 291–293 8C; ESI–MS: m/z
417.4 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 1.34 (t, 3H, J = 6.9 Hz), 2.55 (s, 3H), 3.75(s, 3H), 3.98 (q, 2H, J = 7.1 Hz), 7.33 (m,
1H), 7.53 (m, 1H), 8.01 (d, 1H, J = 9.4 Hz), 8.24–8.15 (m, 1H), 8.49 (t, 1H, J = 9.9 Hz), 9.00 (d, 1H, J = 6.2 Hz), 9.09 (s, 1H). 6a: yield 35.6%.
mp 284–286 8C; ESI–MS: m/z 405.1 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 3.44 (s, 3H), 3.71 (s, 3H), 7.32 (t, 1H), 7.47 (t, 1H),
7.53 (d, 1H, J = 8.2 Hz), 7.89 (d, 1H, J = 7.4 Hz), 8.13 (t, 1H), 8.31 (t, 1H), 8.45 (d, 1H, J = 8.2 Hz), 8.53 (d, 1H, J = 8.2 Hz), 8.76 (d, 1H,
J = 6.8 Hz), 8.85 (d, 1H, J = 6.7 Hz), 10.19 (s, 1H). 6b: yield 29.1%. mp 280–282 8C; ESI–MS: m/z 439.3 [M]+, 126.9 [M]À; 1H
NMR(300 MHz, DMSOÀd6): d 3.44 (s, 3H), 3.68 (s, 3H), 7.51 (m, 2H), 8.03 (d, 1H, J = 2.1 Hz), 8.17–8.08 (m, 1H), 8.30 (t, 1H), 8.45
(d, 1H, J = 8.3 Hz), 8.53 (d, 1H, J = 8.2 Hz), 8.76 (d, 1H, J = 7.0 Hz), 8.85 (d, 1H, J = 6.9 Hz), 10.18 (s, 1H). 6c: yield 33.5%. mp 302–304 8C;
ESI–MS: m/z 453.1 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 1.44 (t, 3H), 3.75 (s, 3H), 4.44 (q, 2H), 6.09 (s, 1H), 7.48 (dd, 1H,
J = 8.8, 2.1 Hz), 7.55 (d, 1H, J = 8.8 Hz), 7.99 (dd, 1H, J = 5.3, 1.5 Hz), 8.06–8.00 (m, 1H), 8.17–8.07 (m, 1H), 8.33 (dd, 2H), 8.73 (d, 1H,
J = 8.1 Hz), 9.22 (d, 1H, J = 7.1 Hz). 7a: yield 28.6%. mp 287–289 8C; ESI–MS: m/z 465.4 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6):
d 1.22 (m, 2H), 1.59 (m, 2H), 3.48–3.37 (m, 1H), 3.78 (s, 3H), 6.37 (s, 1H), 7.48 (dd, 1H, J = 8.8, 2.1 Hz), 7.59 (d, 1H, J = 8.8 Hz), 7.97 (t, 1H,
J = 4.1 Hz), 8.03 (d, 1H, J = 7.9 Hz), 8.24 (t, 1H), 8.40 (d, 1H, J = 7.8 Hz), 8.62 (d, 1H, J = 9.0 Hz), 8.83 (t, 2H). 7b: yield 37.3%. mp 298–
300 8C; ESI–MS: m/z 405.0 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 3.55 (s, 3H), 3.65 (s, 3H), 7.29 (t, 1H), 7.44 (t, 1H), 7.50 (d,
1H, J = 7.9 Hz), 7.87 (d, 1H, J = 7.6 Hz), 8.09 (dd, 1H), 8.23 (dd, 1H), 8.31 (dd, 1H), 8.59 (d, 2H, J = 8.7 Hz), 9.36 (d, 1H, J = 8.3 Hz), 9.63 (d,
1H, J = 5.0 Hz). 7c: yield 26.8%. mp 270–272 8C; ESI–MS: m/z 431.4 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 1.21 (m, 4H), 3.09
(m, 1H), 3.64 (s, 3H), 7.27 (t, 1H), 7.51–7.38 (m, 2H), 7.85 (d, 1H, J = 7.7 Hz), 8.10 (t, 1H, J = 7.5 Hz), 8.36–8.19 (m, 2H), 8.51 (d, 1H,
J = 8.7 Hz), 8.58 (d, 1H, J = 8.1 Hz), 9.36 (d, 1H, J = 8.2 Hz), 9.67 (d, 1H, J = 5.7 Hz). 7d: yield 30.2%. mp 280–282 8C; ESI–MS: m/z 419.4
[M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6): d 1.46 (t, 3H), 3.64 (s, 3H), 4.15 (q, 2H), 7.27 (dd, 1H), 7.41 (dd, 1H), 7.50 (d, 1H,
J = 8.2 Hz), 7.86 (d, 1H, J = 7.8 Hz), 8.10 (t, 1H, J = 7.6 Hz), 8.29 (m, 2H), 8.45 (d, 1H, J = 8.8 Hz), 8.61 (d, 1H, J = 8.2 Hz), 9.39 (d, 1H,
J = 8.3 Hz), 9.73 (d, 1H, J = 5.9 Hz). 7e: yield 27.3%. mp 290–292 8C; ESI–MS: m/z 453.1 [M]+, 126.9 [M]À; 1H NMR (300 MHz, DMSOÀd6):
d 1.50 (t, 3H), 3.79 (s, 3H), 4.55 (q, 2H), 6.15 (s, 1H), 7.59–7.41 (m, 2H), 8.02–7.91 (m, 2H), 8.25–8.13 (m, 1H), 8.38 (d, 1H, J = 8.0 Hz), 8.61 (d,
1H, J = 9.1 Hz), and 8.83 (dd, 2H, J = 8.8, 4.4 Hz).