Design and one-pot synthesis of new 7-acyl camptothecin derivatives as potent cytotoxic agents

Article abstract of DOI:10.1016/j.bmcl.2012.10.002

New 7-acyl camptothecin derivatives were designed and synthesized from camptothecin in a one-pot reaction through a Minisci type-reaction and were evaluated for cytotoxicity against four tumor cell lines, A-549, DU-145, KB, and KB-vin. All of the new comp

Full text of DOI:10.1016/j.bmcl.2012.10.002

Bioorganic & Medicinal Chemistry Letters 22 (2012) 7659–7661
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design and one-pot synthesis of new 7-acyl camptothecin derivatives as
potent cytotoxic agents
Ying-Qian Liu a,c, , Wei Dai a, Chih-Ya Wang b, Susan L. Morris-Natschke b, Xing-Wen Zhou a, Liu Yang d,
⇑
⇑
,
Xiao-Ming Yang b, Wen-Qun Li a, Kuo-Hsiung Lee b,c,
⇑
a School of Pharmacy, Lanzhou University, Lanzhou 730000, PR China
b Natural Products Research Laboratories, UNC Eshelman School of Pharmacy, University of North Carolina, Chapel Hill, NC 27599, USA
c Chinese Medicine Research and Development Center, China Medical University and Hospital, Taichung, Taiwan
d Environmental and Municipal Engineering School, Lanzhou Jiaotong University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2012
Revised 24 September 2012
Accepted 1 October 2012
Available online 11 October 2012
New 7-acyl camptothecin derivatives were designed and synthesized from camptothecin in a one-pot
reaction through a Minisci type-reaction and were evaluated for cytotoxicity against four tumor cell lines,
A-549, DU-145, KB, and KB-vin. All of the new compounds showed significant inhibition of human tumor
cell growth, with IC50 values ranging from 0.01538 to 13.342 lM. Most of the derivatives were more cyto-
toxic than irinotecan, and the (7a) and 7-propionyl (7b) analogs exhibited the highest cytotoxic activity
against the tumor cell lines tested. This compound class merits further development as anticancer clinical
trial candidates.
Keywords:
Camptothecin
Synthesis
Ó 2012 Elsevier Ltd. All rights reserved.
Cytotoxic activity
Camptothecin (CPT, 1, Fig. 1), a cytotoxic natural alkaloid iso-
lated from Camptotheca acuminata, acts as a selective poison of
the nuclear enzyme topoisomerase I by forming a ternary complex
with topoisomerase I and DNA.1–4 Elucidation of the mechanism
and extensive structural modifications led to the successful identi-
fication and development of the antitumor agents topotecan (2)
and irinotecan (3), as well as other CPT analogs that are currently
in clinical trials.5,6
From structure-activity relationship (SAR) investigations, mod-
ifications at the 7- or 10-position or both appear to be the most
efficient approach to increase the antitumor potency.7 Many stud-
ies8–11 have shown that appropriate substitutions at either or both
of these positions can decrease or abolish the high-affinity binding
of the inactive ‘carboxylate form’ of 1’s E-ring to human serum
albumin (HSA), and thus, shift the hydrolysis equilibrium in human
blood to favor the active ‘lactone-closed form’. Furthermore, a
binding model of 1 with biological macromolecules indicated that
the 7-position can accommodate various substituents without ste-
ric problems.12–14 Therefore, most structural modifications of 1
have focused on positions 7 and 10. In particular, various substitu-
ents, such as ethyl, alkylsilyl, oxyiminoalkyl, and alkylsilylalkyl,
were introduced at the 7-position of 1 to provide potent antitumor
agents. Indeed, most of the second-generation 1-derivatives that
have currently reached preclinical or clinical development studies
are 7-substituted compounds, for example, gimatecan (4)15, CKD-
602 (5),16 and BNP-1350 (6),17 which all bear highly lipophilic sub-
stituents intended to increase lactone stability and antitumor
activity. These successful examples imply that C-7 substitution
plays an important role in the activity profiles of 1-analogs and
that optimization of this compound class through rational C-7
modification is quite feasible. On this basis, we designed a series
of new 1-derivatives with different substituents at position-7.
Herein, we report an efficient one-pot synthesis of the new 7-acyl
1-derivatives as potent cytotoxic agents.
As shown in Scheme 1, the new 7-acyl camptothecin derivatives
(7a–l) were obtained by Minisci free radical acylation of 1 with the
appropriate aldehydes in sulfuric acid in the presence of t-BuOOH
and ferrous sulfate. The structures of the target molecules were
characterized through 1H NMR, 13C NMR, IR and HR-MS analyses.
Target compounds 7a–l were evaluated for in vitro cytotoxic
activity against four different tumor cell lines, KB (nasopharyn-
geal), A549 (lung), DU-145 (prostate), and KBvin (an MDR KB sub-
line), using a sulforhodamine B colorimetric assay,18 in triplicate
experiments, with 1 and 3 used as positive controls. The screening
results are shown in Table 1. All of the new compounds exhibited
significant in vitro cytotoxic activity against the four tested tumor
cell lines, with IC50 values ranging from 0.01538 to 13.342 lM, and
were as or more potent than 3. Among the new derivatives, 7a and
7b, with 7-acetyl and 7-propionyl groups, respectively, exhibited
⇑
Corresponding authors. Tel.: +1 919 962 0066; fax: +1 919 966 3893.
E-mail addresses: yqliu@lzu.edu.cn (Y.-Q. Liu), khlee@email.unc.edu (K.-H. Lee).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.10.002

7660
Y.-Q. Liu et al. / Bioorg. Med. Chem. Lett. 22 (2012) 7659–7661
N
O
N
O
HO
N
O
N
O
N
N
N
N
O
O
N
O
O
O
O
O
OH
OH
OH
topotecan (2)
Camptothecin (1)
irinotecan (3)
O
N
Si
N
H
O
O
N
O
N
N
N
N
N
O
O
O
O
O
O
OH
OH
OH
Gimatecan(4)
CKD-602 (5)
BNP-1350(6)
Figure 1. Structures of camptothecin (1), topotecan (2), irinotecan (3), gimatecan (4), CKD-602 (5), and BNP-1350 (6).
R
O
O
O
RCHO/Ac2O-H2O
N
N
FeSO / -BuOOH/H SO
4 t
2
4
N
N
O
O
O
O
OH
OH
7a-l
1
7a
7b
OCH3
OCH3
CH3
CH2CH3
7h
7i
7k
H3CO
7e
Cl
OH
CH3
7f
7c
7d
7l
OCH3
OCH3
OCH3
O
CH3
OH
O
7g
7j
O
OCH3
Scheme 1. Synthesis of target compounds 7a–l.
the highest potency. However, except for 7a and 7b, the new 7-acyl
derivatives showed substantially reduced cytotoxic potency
relative to 1, which exhibited remarkable cytotoxic effects against
the four tumor cell lines. The 7-carbonyl would provide an addi-
tional hydrogen bond receptor and affect the conjugation in the
molecule. Either of these effects could influence the binding with
the enzyme and might result in reduced activity. An alternate
explanation for the reduced activity is steric hindrance resulting
from the more rigid and larger aromatic systems in 7c–7l.
To assist in further identification of more efficient 7-acyl
1-derivatives, preliminary SAR correlations were formulated. As
the length of the 7-acyl group increased from acetyl to propionyl
with 7a and 7b, the corresponding cytotoxic activity was generally
reduced. For example, the IC50 value of compound 7a was
compound 7b was 0.0898 lM. In addition, the 7-alkylcarbonyl
substituted derivatives (compounds 7a and 7b) were more potent
than the 7-arylcarbonyl substituted compounds (compounds 7c–
7l) against all tested tumor cell lines. This result indicated that
an aryl group in the 7-acyl moiety is much less favorable than a
short aliphatic group. The conjugation of the aromatic group with
the carbonyl group might disturb the electronic properties of ring B
to a greater extent than alkyl groups. For those analogs bearing a 7-
benzoyl group (compounds 7d–7l), substituents on the aromatic
ring resulted in substantially increased cytotoxic activity com-
pared to the unsubstituted derivative 7d. This effect was generally
independent of the number, electronic, and steric characteristics of
the groups, particularly against the A-549 cancer cell line. Com-
pounds 7e, 7f, and 7h–7l, with 4-chloro, 4-methoxy, 3,4-methyl-
enedioxy, 3,4-dimethyl, 3-hydroxy-4-methoxy, 2,5-dimethoxy,
0.0402 lM against the DU-145 cancer cell line, while that of

Y.-Q. Liu et al. / Bioorg. Med. Chem. Lett. 22 (2012) 7659–7661
7661
Table 1
In vitro cytotoxicity results for 7a–7l against four cancer cell lines
Cmpd
IC50(lM)
A-549
DU-145
KB
KB-vin
7a
7b
7c
7d
7e
7f
7g
7h
7i
7j
7k
7l
1
3
0.0154 0.0002
0.0317 0.0146
2.02 0.2037
11.0 0.952
0.949 0.0366
0.911 0.0502
1.13 0.0054
2.51 0.268
1.00 0.0616
1.02 0.0295
1.01 0.0707
1.00 0.042
0.0158 0.0005
9.48 0.106
0.0402 0.0056
0.0898 0.0049
2.01 0.146
11.0 0.0422
1.68 0.0786
1.61 0.0302
2.60 0.192
0.0233 0.0038
0.0592 0.0059
2.73 0.145
13.3 0.563
3.07 0.391
4.78 0.318
11.8 0.530
12.5 0.141
2.49 0.0275
1.73 0.0214
2.92 0.200
3.14 0.0715
0.121 0.0091
9.83 0.481
0.131 0.156
0.0287 0.0032
1.80 0.181
11.9 0.527
1.12 0.0345
3.14 0.278
1.77 0.150
1.79 0.0192
4.620 0.268
1.14 0.106
2.25 0.257
1.17 0.0183
0.0371 0.0031
>20
2.99 0.399
1.70 0.0618
1.543 0.0644
2.061 0.155
2.11 0.202
0.0287 0.0025
9.30 0.613
and 3,4,5-trimethoxy substitution, showed similar cytotoxicity
against A-549 cells. Compound 7h, with 4-hydroxy substitution,
was somewhat less potent than most of the 7-substituted benzoyl
derivative, except against the KB-vin cell line. These results indi-
cated that the electronegativity of the substituted groups and their
positions on the phenyl ring affected the cytotoxicity only slightly.
Consequently, we plan to synthesize additional 7-acyl derivatives
of 1 based on our preliminary results with the current 7-acyl com-
pounds in order to produce compounds with greater potency.
In summary, 12 new 7-acyl camptothecin derivatives (7a–l)
were designed, synthesized, and evaluated for cytotoxicity against
four tumor cell lines (A-549, DU-145, KB, and KB-vin) in a sulforho-
damine B colorimetric assay. Several derivatives, particularly com-
pounds 7a and 7b, showed impressive cytotoxicity compared with
irinotecan. Certain variations in the acyl groups at the 7-position of
1 markedly affected the activity profiles of this compound class.
Interestingly, with the two most potent compounds, 7a showed a
five-fold decrease in potency against KB-vin (drug-resistant) com-
pared with the KB (normal) cell line, while 7b showed a two-fold
increase in potency against the same two cell lines. Overall, impor-
tant SAR information has been achieved that will assist the design
and development of new 1-derivatives as cytotoxic agents.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.10.002.
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This work was supported financially by the National Natural
Science Foundation of China (30800720), the Post-Doctor Research
Foundation (20090450142), the Fundamental Research Funds for
the Central Universities (860120), and the Young Scholars Science
Foundation of Lanzhou Jiaotong University (2011011). This study
was also supported in part by the Cancer Research Center of Excel-
lence, Taiwan (DOH-100-TD-C-111-005)