Synthesis and structure-activity relationships of sinenxan A derivatives as multidrug resistance reversal agents

Article abstract of DOI:10.1016/j.bmcl.2010.07.099

Two types of sinenxan A derivatives with different side chains at C-5 were synthesized and evaluated for their in vitro multidrug resistant reversal activities. Several derivatives exhibited better activities than the positive control verapamil. The structure-activity relationships of these derivatives suggested that a carbonyl group at C-13 and the length of side chain at C-5 are important for the activity.

Full text of DOI:10.1016/j.bmcl.2010.07.099

Bioorganic & Medicinal Chemistry Letters 20 (2010) 5418–5421
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and structure–activity relationships of sinenxan A derivatives as
multidrug resistance reversal agents
Meng Huang a, , Xin Zhao b, Meng Zhang a, Jun Gu a, Xiaoguang Chen b, Dali Yin a,
*
a Department of Medicinal Chemistry, Institute of Materia Medica, Chinese Academy of Medical Sciences and Peking Union Medical College, 1 Xian Nong Tan Street,
Beijing 100050, China
b Department of Pharmacology, Institute of Materia Medica, Chinese Academy of Medical Sciences and Peking Union Medical College, 1 Xian Nong Tan Street, Beijing 100050, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two types of sinenxan A derivatives with different side chains at C-5 were synthesized and evaluated for
their in vitro multidrug resistant reversal activities. Several derivatives exhibited better activities than
the positive control verapamil. The structure–activity relationships of these derivatives suggested that
a carbonyl group at C-13 and the length of side chain at C-5 are important for the activity.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 23 March 2010
Revised 23 June 2010
Accepted 26 July 2010
Available online 29 July 2010
Keywords:
Sinenxan A derivatives
Multidrug resistant reversal activity
Anti-cancer
Multidrug resistance (MDR) refers to a phenomenon whereby
cancer cells undergoing chemotherapy simultaneously develop
cross resistance to a number of unrelated anticancer drugs with di-
verse structures and mechanisms of action.1 MDR attributes to a
variety of mechanisms of action which are not fully understood
yet so far. One of the most important mechanisms is due to the
overexpression of a transmembrane protein called P-glycoprotein
(P-gp), which can actively transport anticancer drugs out of the
cancer cells and thus result in a decreased intracellular accumula-
tion of the anticancer drugs.2 Therefore, P-gp has emerged as a
promising target for cancer therapy and great efforts have been fo-
cused on the development of effective reversal agents to overcome
P-gp-mediated MDR.
Some chemically diverse compounds such as verapamil, qui-
nine, and cyclosporin A have been previously reported to directly
bind to P-gp with subsequent inhibition of pump activity and thus
resensitize MDR cells to anticancer drugs.3 It was found recently
that some natural taxoids isolated from the Japanese yew Taxus
cuspidata also enhanced the cytotoxicity of vincristine (VCR) in
MDR human ovarian cancer 2780AD cells and efficiently inhibited
[3H]-azidopine photolabeling of P-gp.4 In addition, surprisingly,
these natural taxoids exhibited weak or no cytotoxicity. The stron-
gest increase of VCR cellular accumulation by taxezopidine G
(Fig. 1) corresponded to be 323% of that by verapamil.5 This has
paved the way to the synthesis and study of potent MDR reversal
agents of the taxoid family. Taxinine (Fig. 1), a major natural tax-
oid, has been chemically modified to yield many compounds show-
ing an increase in activity to reverse MDR and weak cytotoxicity.6
However, taxinine and taxezopidine G can be only isolated from
the seeds of the Chinese yew Taxus chinensis and Japanese yew T.
cuspidata in very low yields of 0.014% and 0.0056% (w/w),
respectively.7
Sinenxan A (Fig. 1), a biosynthetic taxoid consisting of a 6/8/6-
membered ring system, can be obtained in a higher yield of 5.0%
(w/w).8 Nine sinenxan A derivatives with different side chains at
C-5 have been semi-synthesized in our group and showed MDR
reversal activity against VCR-resistant human oral epidermoid car-
cinoma KB/V cells.9 Among these sinenxan A derivatives, three
compounds were selected for further investigation on their
in vitro MDR reversal activities and one compound was evaluated
regarding its in vivo sensitizing activity with VCR-resistant KB/V
tumor xenografts.10 Our preliminary structure–activity relation-
ships (SAR) results9 showed that change of 5-O acyl substituents
has a remarkable impact on the MDR reversal activity.
However, 5-O acyl substituents in our previous study were lim-
ited to side chains containing a phenyl ring. Therefore, side chains
with different lengths containing another similar ring, such as a
heterocycle, a cyclohexane or a naphthalin, could be introduced
at C-5 to investigate the electronic effect and space effect on the
activity. Especially, in view of the poor aqueous solubility of
taxoids, introduction of side chains containing a heterocycle is ex-
pected to improve physicochemical property and subsequent drug
metabolism and pharmacokinetics (DMPK) profile of the whole
* Corresponding author. Tel.: +86 10 63037952; fax: +86 10 63165248.
E-mail address: yindali@imm.ac.cn (D. Yin).
Present address: Department of Pharmacology, School of Medicine, University of
Pennsylvania, 3620 Hamilton Walk, Philadelphia, PA 19104, United States.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.07.099

M. Huang et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5418–5421
5419
OAc
OAc
OAc
OAc
9
9
AcO
O
13
13
O
O
5
5
O
O
AcO
HO
Taxezopidine G
Taxinine
OAc
14
AcO
5
OAc
AcO
Sinenxan A
Figure 1. Chemical structures of taxezopidine G, taxinine, and sinenxan A.
OAc
OAc
OAc
S
a
b
O
AcO
S
HO
AcO
OAc
AcO
Sinenxan A
AcO
OAc
OAc
2
1
OAc
OAc
OAc
c
d
e
O
1
AcO
AcO
AcO
O
R
OH
OAc
5-17
4
3
Scheme 1. Reagents and conditions: (a) 10 N KOH, CH3OH, 0 °C, 36%; (b) (i) NaH, CS2, THF, reflux, 90%; (ii) CH3I, THF, 45 °C, 90%; (c) Bu3SnH, AIBN, toluene, 80 °C, 90%; (d)
tBuOK, THF, À78 °C, 80%; (e) RCOOH, DCC, DMAP.
taxoid molecule. On the other hand, to our best knowledge, the
information about the effect of 13-oxo on the activity is limited.
It would be necessary to investigate the role of 13-oxo related to
MDR reversal activities.
Herein, we report the most recent progress in this research. Two
series of compounds with different 5-O acylations were synthe-
sized to investigate the structure–activity relationships (SAR) of
5-O acyl substituents as well as that of 13-oxo. The key intermedi-
ates 4 and 19 were synthesized starting from sinenxan A (Schemes
1 and 2)11 followed by 5-O acylation to give targets 5–17 and 20–
32, respectively. The structures of these target compounds were
validated by 1H NMR, FAB-MS or ESI-MS.
OAc
OAc
a, b, c
d
AcO
AcO
OAc
AcO
OAc
3
Sinenxan A
OAc
OAc
OAc
O
f
e
O
O
O
AcO
2
OH
AcO
AcO
O
R
OAc
19
18
20-32
Scheme 2. Reagents and conditions: (a) 10 N KOH, CH3OH, 0 °C, 36%; (b) (i) NaH, CS2, THF, reflux, 90%; (ii) CH3I, THF, 45 °C, 90%; (c) Bu3SnH, AIBN, toluene, 80 °C, 90%; (d) PCC,
NaOAc, celite, benzene, reflux, 60%; (e) tBuOK, THF, À78 °C, 90%; (f) RCOOH, DCC, DMAP or RCOCl.

5420
M. Huang et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5418–5421
Table 1
Effects of sinenxan A derivatives on resensitizing to VCR in KB/V cells
OAc
OAc
R1
R2
O
Compds
IC50 (nM)
Compds
IC50 (nM)
O
O
R1
AcO
O
R2
O
AcO
5
6
706.14
403.58
20
21
1795.99
1203.69
O
7
8
9
1616.67
48.23
22
23
24
191.77
2.80
CH=CH
509.83
55.98
OMe
10
11
370.47
45.36
25
26
71.21
11.05
O
MeO
O
CH=CH
S
O
12
13
14
537.24
763.99
347.62
27
28
29
1184.72
356.51
24.53
O
S
CH=CH
S
O
S
H
N
15
16
17
346.09
52.68
30
31
0.87
H
63.32
N
N
COOH
902.74
32
12.97
18.47
N
Verapamil
The IC50 values of VCR in KB/V cells were determined in the presence of 10 lM of sinenxan A derivatives. Verapamil was used as a positive control.
MDR reversal activity of the derivatives was tested in vitro on
the cellular accumulation of VCR in KB/V cells, which is 100-fold
more resistent to VCR and cross resistance to doxorubicin, paclit-
axel, and colchicine. The results were shown in Table 1. Among
these sinenxan A derivatives, compounds 30 (IC50 = 0.87 nM), 23
(IC50 = 2.80 nM), 26 (IC50 = 11.05 nM), and 32 (IC50 = 12.97 nM),
that were 21.23, 6.60, 1.67, and 1.42 times more potent than verap-
amil, completely reversed the resistance to VCR in KB/V cells over-
expressing P-gp. Compounds 8, 11, 16, 24, 25, 29, and 31 partially
reversed the MDR in KB/V cells. The others had no MDR reversal
activities.
In summary, the in vitro SAR suggested that the presence of a
carbonyl group at C-13 and two or three atoms length of side chain
at C-5 might be important for the activity of reversing MDR. Some
derivatives bearing a cyclohexyl or a heterocycle at C-5 side chain
are favorable for this type of activity.
Acknowledgments
This research work was financially supported by the National
Natural Science Foundation of China (No. 39770872 and No.
30100230). The starting material sinenxan A was supplied by the
Department of Biosynthesis, Institute of Materia Medica, Chinese
Academy of Medical Sciences and Peking Union Medical College.
Most of these sinenxan A derivatives were also evaluated
in vitro on the cytotoxicity against KB cells and exhibited weak
or no cytotoxicity (data not shown).
Most of the derivatives with a heterocyclic acyl at O-5 and a 13-
oxo displayed increased MDR reversal activity in KB/V cells. In
particular, all of the compounds bearing nitrogen-containing het-
erocycles (16, 30, 31, and 32) showed better potency. Compounds
23 possessing a cyclohexyl also showed improved MDR reversal
activity.
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