Antitumor dinuclear platinum(II) complexes derived from a novel chiral ligand

Article abstract of DOI:10.1016/j.bmcl.2011.08.100

A new chiral ligand, 2-(((1R,2R)-2-aminocyclohexyl)amino)acetic acid (HL), was designed and synthesized to prepare a series of novel dinuclear platinum(II) complexes with dicarboxylates or sulfate as bridges. The evaluation of these metal complexes in vitro cytotoxicity against human HCT-116, MCF-7 and HepG-2 cell lines were made. All compounds showed antitumor activity to HCT-116 and MCF-7. Particularly, compounds M3 and M5 not only exhibited better activity than carboplatin against MCF-7 and HepG-2, but also showed very close activity to oxaliplatin against HCT-116.

Full text of DOI:10.1016/j.bmcl.2011.08.100

Bioorganic & Medicinal Chemistry Letters 21 (2011) 6386–6388
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Bioorganic & Medicinal Chemistry Letters
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Antitumor dinuclear platinum(II) complexes derived from a novel chiral ligand
Chuanzhu Gao a, Gang Xu a,b, Shaohua Gou a,b,
⇑
a Pharmaceutical Research Center and School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China
b Jiangsu Province Hi-Tech Key Laboratory for Bio-medical Research, Southeast University, Nanjing 211189, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new chiral ligand, 2-(((1R,2R)-2-aminocyclohexyl)amino)acetic acid (HL), was designed and synthe-
sized to prepare a series of novel dinuclear platinum(II) complexes with dicarboxylates or sulfate as
bridges. The evaluation of these metal complexes in vitro cytotoxicity against human HCT-116, MCF-7
and HepG-2 cell lines were made. All compounds showed antitumor activity to HCT-116 and MCF-7. Par-
ticularly, compounds M3 and M5 not only exhibited better activity than carboplatin against MCF-7 and
HepG-2, but also showed very close activity to oxaliplatin against HCT-116.
Received 28 May 2011
Revised 15 August 2011
Accepted 24 August 2011
Available online 28 August 2011
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
1R,2R-Diaminocyclohexane derivative
Dinuclear platinum(II) complexes
Cytotoxicity
As molecular structures different from classical mononuclear
platinum-based drugs such as cisplatin and carboplatin, multinu-
clear platinum complexes have attracted much attention since
1990’s, especially rising of BBR3464.1,2 For carrying high positive
charges (+4) and flexible alkyl chains, BBR3464 could form a range
of interstrand GG adducts dictated by the sequence of DNA bases,
while the cytotoxicity of cisplatin is due to the formation of mainly
mono-functional Pt(G) and intrastrand bi-functional Pt(GG) ad-
ducts which have been proved to cause a local distortion of
DNA.3–5 Thus, multinuclear platinum complexes are expected to
be able to circumvent the inherent or acquired cisplatin-resistance
in a panel of human tumor models.6–8 So far a number of various
diamines have been involved as bridges to connect platinum units
in previous researches including BBR3464, however, results for
phase II studies of BBR3464 were not so inspiring9 and severe dose
limiting side-effects such as diarrhea and vomiting limited its
clinic use.10
Since 1R,2R-diaminocyclohexane (DACH) has been believed to
play an important role in the success of oxaliplatin,11,12 we have de-
signed to modify the DACH skeleton by selectively introducing a
functional group to one of its amino moieties so that it can act as
a tridentate ligand to prepare dinuclear platinum(II) complexes
with dicarboxylates as bridges. Upon this design, we have recently
reported a series of dinuclear platinum complexes with 2-(((1R,2R)-
2-aminocyclohexylamino)methyl)phenol as ligand.13 Primary
in vitro tests exhibited that those compounds showed cytotoxicity
towards some selected human cell lines comparable to that of car-
boplatin, but our further work indicated that the steric hindrance
and lipotropy of the aromatic group may reduce their antitumor
activity. So, rather small and hydrophilic carboxylic acids have been
considered as functional groups to combine with one of amino
nitrogen atoms of DACH deliberately. In this paper, we report such
a new ligand, 2-(((1R,2R)-2-aminocyclohexyl)amino)acetic acid
(HL).With HL as carrier group and dicarboxylates/sulfate as bridges,
five novel dinuclear platinum(II) complexes were prepared and
their cytotoxicity was evaluated against three human cell lines.
O
O
O
O
O
O
O
O
O
O
NH
O
NH
NH
NH
Pt
Pt
Pt
Pt
NH2
NH2
NH2
NH2
O
O
O
O
O
O
M1
M2
O
O
O
O
O
O
NH
NH
Pt
Pt
NH2
O
O
O
NH
NH
NH2
Pt
Pt
O
NH2
NH2
O
O
O
O
O
M4
M3
O
O
O
O
O
NH
NH
NH2
Pt
Pt
NH2
O
S
O
O
M5
⇑
Corresponding author. Tel./fax: +86 25 8327 2381.
E-mail address: sgou@seu.edu.cn (S. Gou).
Figure 1. Chemical structures of dinuclear platinum(II) complexes.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.08.100

C. Gao et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6386–6388
6387
Notably, the targeted dinuclear platinum complexes differ from the
previously reported multinuclear platinum complexes which have
employed various diamines as bridges (including BBR3464). Struc-
tures of the dinuclear complexes are shown in Figure 1.
respectively. The IC50 values of the platinum complexes with car-
boplatin and oxaliplatin as positive controls are given in Table 1
As we can see, all dinuclear platinum complexes showed cyto-
toxicity towards HCT-116 cell line with IC50 values varying from
Mono-Boc protecting DACH14 was used as starting material to
prepare the ligand as before, since it is difficult to directly get
the monsubstituted derivative due to the equivalent reactivity of
the two amino groups in DACH. The synthetic process of HL is
shown as following. Reaction of mono-Boc protecting DACH with
ethyl 2-chloroacetate offered intermediate 1, which was then re-
moved the protecting group to give intermediate 2.2 was hydro-
lyzed in the presence of NaOH, leading to the formation of
intermediate 3, which was finally neutralized by aqueous HCl solu-
tion to give a free ligand, HL (Scheme 1).
5.1 to 27.5 lM, the order of the potency is oxalipla-
tin > M5 > M3 > M2 > M1 > M4. Clearly, cytotoxicity increased
along with the extending carbon chain of the carboxylate bridge,
but the cyclobutyl chain in compound M4 had a reverse effect.
Among these dinuclear platinum complexes, compounds M3 and
M5, which have succinate and sulfate as bridges, respectively,
showed very close activity to oxaliplatin.
When tested against MCF-7 cell line, all complexes exhibited
affirmative cytotoxicity, the order of IC50 values is M5 < M3 < car-
boplatin < M2 < M1 < M4. Both compounds M3 and M5 showed
better activity than carboplatin, and the cytotoxicity of compound
M5 was even two times as potent as that of carboplatin. Again,
cytotoxicity increased along with the addition of the carbon chain
in the carboxylate bridge, which was observed in HCT-116 cell line
with these compounds.
Before preparing the targeted dinuclear platinum complexes,
important intermediate [PtLI]15 was firstly prepared, which was
then used to react with different silver salts (dicarboxylates/sul-
fate) to afford compounds M1–M5.16,17
Intermediate [PtLI] and targeted dinuclear platinum complexes
were characterized by IR, 1H NMR, ESI-MS spectra and microanal-
ysis together with TG-thermal analysis. Found values for each com-
pound in the elemental analysis were in good agreement with
calculated values. In the IR spectra, bands of mNH/NH2 and dNH/NH2
in the platinum complexes shifted to lower frequencies than those
of free primary amine and secondary amine. Carboxylate anions
binding with Pt(II) were confirmed by the examination of the
C@O absorptions shifting from free carboxylic acids near
1700 cmÀ1 to bands near 1646–1584 cmÀ1. The complexes showed
[M+H]+ or [M+Na]+ corresponding to their formula weights and rel-
ative fragment peaks in their ESI mass spectra. Typical isotopes of
Pt element: 194Pt (33%), 195Pt (34%), 196Pt (25%), were found with
three protonated ion isotopic peaks. The 1H NMR spectral of all
prepared complexes in Figure 1 were all consistent with their cor-
responding protons both in the chemical shifts and in the number
of protons. TG-thermal analytic data of compounds M2 and M4
showed that they had analogous thermal processes, in which the
weight loss began at about 200–230 °C, and then kept steady until
about 660–700 °C. The final residue, corresponding to 53–56%
weight loss, can be attributed to platinum element.
When the cell line was HepG-2, only compounds M3 and M5
showed significant cytotoxicity that was better than carboplatin.
Based on the structure–activity relationship of these com-
pounds, we have got an interesting finding that sulfate as a bridge
may significantly improve the cytotoxicity of the dinuclear plati-
num complex. It is noted from Figure 2 that the cytotoxicity of
compound M5 is better than carboplatin against HepG-2 and
MCF-7 cell lines and very close to oxaliplatin against HCT-116 cell
line, that is the best performance in all resulting dinuclear plati-
num complexes. When succinate was selected as a bridge, com-
pound M3 also showed interesting activity, in spite of being
lower than compound M5. Other compounds which owned oxa-
late, malonate and cyclobutane-1,1-dicarboxylate as bridges
showed weak antitumor activity.
It is noted that the cytotoxicity of compounds with the bridging
dicarboxylates increased with the linear carbon chain length of
dicarboxylate (M3 > M2 > M1), however, compound M4 with 1,1-
cyclobutyldicarboxylate had the weakest activity among these
complexes. We suppose that the balance between lipotropy and
hydrophily of the platinum-based compounds is closely relative
to their antitumor activity. In fact, aqueous solubility of com-
pounds M3 and M5 are in the middle of these five compounds
(aqueous solubility order: M1 > M2 > M5 > M3 > M4). The
hydrophily of compound M4 has been remarkably reduced with
the introduction of cyclobutyl moiety.
Poor aqueous solubility is a severe problem for some platinum
anticancer drugs in clinic use. Compared with cisplatin (1.0 mg/
ml) and oxaliplatin (7.0 mg/ml), the aqueous solubility of all result-
ing platinum complexes has been greatly improved (30–50 mg/ml
at 25 °C) for introducing hydrophilic carboxyl to DACH as expected.
MTT assay was carried out to evaluate the in vitro cytotoxicity
of the resulting dinuclear platinum complexes as described by
Mosmann et al.,18–21 using HCT-116, MCF-7 and HepG-2 cell lines,
Moreover, we have ever prepared a number of mononuclear
Platinum compounds of the ligand (HL) with several monodentate
carboxylates as leaving group and made in vitro biological tests,
H
H
N
N
Boc
Table 1
Boc
a
In vitro cytotoxicity of complexes M1 to M5
NH
a
NH2
Complex
IC50
(l
M)
COOCH2CH3
HCT-116b
MCF-7c
HepG-2d
1
M1
M2
M3
17.2
13.1
5.8
23.5
16.8
9.9
37.5
31.2
8.2
b
M4
M5
Carboplatin
Oxaliplatin
27.5
5.1
Not tested
4.3
31
7.1
15.3
Not tested
>50
6.8
9.7
Not tested
NH2
NH
NH2
NH
NH2
NH
d
c
2HCl
a
All IC50 values (drug concentration giving 50% survival) calculated based on the
Pt content are means SD (SD <9% of the mean value) from at least three separated
experiments.
COOH
COONa
COOCH2CH3
HL
3
2
b
Human colorectal cancer cell.
Human breast cancer cell.
Human hepatoma cell.
c
Scheme 1. Reagents: (a) ethyl 2-chloroacetate, K2CO3, acetonitrile; (b) HCl/EtOAc;
(c) NaOH, EtOH/H2O; (d) HCl/H2O.
d

6388
C. Gao et al. / Bioorg. Med. Chem. Lett. 21 (2011) 6386–6388
10. Wheate, N. J.; Walker, S.; Craig, G.; Oun, R. Dalton Trans. 2010, 39, 8113.
11. Tyagi, P.; Gahlot, P.; Kakkar, R. Polyhedron 2008, 18, 3567.
12. Sanofi-Synthélabo, Lilly Drugs Future 2000, 25, 644.
13. Gao, C. Z.; Gou, S. H.; Fang, L.; Zhao, J. Bioorg. Med. Chem. Lett. 2011, 6, 1763.
14. Lee, D. W.; Ha, H. J.; Lee, W. K. Synth. Commun. 2007, 4–6, 737.
15. Synthesis of [PtLI]: To a stirring aqueous solution of KI (80 mmol), K2PtCl4
(12 mmol) in water (40 ml) was added. The blending solution was stirred at
25 °C for 30 min under a nitrogen atmosphere to get a black solution of K2PtI4.
Then an aqueous solution (40 ml) of HL (12 mmol) and NaOH (12 mmol) was
added dropwise under stirring in the dark at 25 °C. After 24 h, the dark yellow
precipitate was filtered, washed sequentially with water, ethanol and ether,
and then dried in vacuum.
Data for [PtLI]: Yield: 60%, dark yellow solid. IR (m
, cmÀ1): 3546s (br), 3176s,
2934m, 2858m, 1642vs, 1586m, 1445m, 1334s, 1291m, 1076w, 1010m, 911w,
648w, 459m; 1H NMR (D2O/TMS): d 1.10–1.50 (m, 6H, 3CH2 of DACH), 1.80–
2.07 (m, 2H, CH2 of DACH), 2.30–2.81 (m, 2H, 2CH of DACH), 3.35–3.85 (m, 2H,
NHCH2COOÀ). ESI-MS: m/z [M+Na]+ = 516 (25%), [M+K]+ = 532 (60%). Anal.
(C8H15IN2O2Pt) C, H, N.
Figure 2. Cytotoxicity of compounds M3 and M5 with carboplatin or oxaliplatin as
positive controls.
16. Vollano, J. F.; Al-Baker, S.; Dabrowiak, J. C.; Schurig, J. E. J. Med. Chem. 1987, 4,
716.
but the IC50 values of those mononuclear compounds against the
same tumor cells were disappointing.
17. General preparation of the complexes: M1, M2, M3, M4 and M5: A suspension
of the corresponding silver dicarboxylate or Ag2SO4 (5 mmol) and [PtLI]
(10 mmol) in 100 ml water was stirred at 60 °C under a nitrogen atmosphere in
the dark for 24 h, the resulting yellow deposit was filtered off and washed with
water for two times. The filtrate was then evaporated to nearly dryness and
light yellow solids precipitated, which were washed with small icy water for
two times, and dried in vacuum.
In conclusion, a new N-monosubstituted chiral DACH ligand, 2-
(((1R,2R)-2-aminocyclohexyl)amino)acetic acid, has been designed
and synthesized to prepare a series of novel dinuclear platinum(II)
complexes which own dicarboxylates/sulfate as bridges, and that
differ from the previously reported multinuclear platinum com-
plexes which have employed various diamines as bridges. In vitro
cytotoxicity tests indicated that all resulting dinuclear platinum(II)
complexes exhibited antitumor activity to HCT-116 and MCF-7 cell
lines. Compounds M3 and M5 not only showed very close activity
to oxaliplatin against HCT-116 but also showed better cytotoxicity
than carboplatin against MCF-7 and HepG-2 cell lines. Further-
more, all compounds showed much better aqueous solubility than
oxaliplatin. Consequently, compounds M3 and M5 may be de-
served for further investigation as leading compounds.
Data for M1: Yield: 29%, pale yellow solid. Formula: C18H30N4O8Pt2, Mol.Wt:
820. C 26.41(26.34), H 3.81(3.66), N 6.73(6.83), Pt 47.38(47.56). IR (m
, cmÀ1):
3421s(br), 3179s, 2935s, 2860s, 1634vs, 1448m, 1392s, 1309s, 1073w, 1013w,
916w, 809m, 777s, 649m, 562m, 518m, 460m. 1H NMR(D2O/TMS): d 1.09–
1.42(m, 8H, 4CH2 of 2DACH), 1.59–1.60 (m, 4H, 2CH2 of 2DACH), 2.02–2.83 (m,
8H, 2CH2 of 2DACH and 4CH of 2DACH), 3.37–3.82 (m, 4H, 2NHCH2COOÀ). ESI-
MS m/z: [M+H]+ = 821 (32%).
Data for M2 Yield: 22%, pale yellow solid. Formula: C19H32N4O8Pt2, Mol.Wt:
834. C 27.40(27.34), H 3.89(3.84), N 6.66(6.71), Pt 46.62(46.76). IR (m
, cmÀ1):
3410s(br), 3183s, 2936s, 2861m, 1591vs, 1352vs, 1254m, 1175m, 1073m,
1017m, 928m, 701s, 594s(br). 1H NMR (D2O/TMS): d 1.09–1.61 (m, 12H, 6CH2
of 2DACH), 2.02–2.78 (m, 8H, 2CH2 of 2DACH and 4CH of 2DACH), 3.21–3.96
(m, 6H, 2NHCH2COOÀ and CH2(COOÀ)2). ESI-MS m/z: [M+H]+ = 835 (25%).
Data for M3 Yield: 31%, pale yellow solid. Formula: C20H34N4O8Pt2, Mol.Wt:
848. C 28.40(28.30), H 3.96(4.01), N 6.68(6.60), Pt 45.81(45.99). IR (m
, cmÀ1):
Acknowledgments
3404s(br), 3197s, 2934s, 2859m, 1632vs, 1570vs, 1394vs, 1290s, 1173m,
1074w, 1011w, 878w, 650m(br), 458m(br). 1H NMR (D2O/TMS): d 1.09–1.36
(m, 8H, 4CH2 of 2DACH), 1.55–1.57 (m, 4H, 2CH2 of 2DACH), 1.75–2.69 (m, 12H,
2CH2 of 2DACH, 4CH of 2DACH, 4H of (CH2COOÀ)2), 3.09–3.67 (m, 4H,
2NHCH2COOÀ). ESI-MS m/z: [M+H]+ = 849 (30%), [M+Na]+ = 871 (70%).
Data for M4 Yield: 36%, pale yellow solid. Formula: C22H36N4O8Pt2, Mol.Wt:
We are grateful to the National Natural Science Foundation of
China (Project No.20971022) and the Six Top Talents Funding of
Jiangsu Province (Project No. 2008046)) for the financial aids to
this work.
874. C 30.08(30.20), H 4.18(4.12), N 6.52(6.41), Pt 44.48(44.62). IR (m
, cmÀ1):
3419vs(br), 3198vs, 2941s, 2863s, 1584vs, 1337vs, 1251m, 1158m, 1119m,
1021w, 911m, 888m, 762m, 707m, 615m, 571m. 1H NMR (D2O/TMS): d 1.16–
1.93 (m, 14H, 6CH2 of 2DACH and CH2 of cyclobutyl group), 2.03–2.95 (m, 12H,
2CH2 of 2DACH, 4CH of 2DACH and 2CH2 of cyclobutyl group), 3.21–3.77 (m,
4H, 2NHCH2COOÀ). ESI-MS m/z: [M+H]+ = 875 (25%), [M+Na]+ = 897 (30%).
Data for M5 Yield: 28%, pale yellow solid. Formula: C16H30N4O8SPt2, Mol.Wt:
References and notes
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, cmÀ1):
3433m(br), 3184m, 2935m, 2853m, 1646s, 1455m, 1394m, 1338m, 1293m,
1184s, 1110vs, 1012m, 990m, 927w, 619vs. 1H NMR (D2O/TMS): d 1.09–1.37
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2CH2 of 2DACH), 2.44–2.70 (m, 4H, 4CH of 2DACH), 3.48–3.75 (m, 4H,
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