Design, synthesis and in vitro cytotoxicity of novel dinuclear platinum(II) complexes

Article abstract of DOI:10.1016/j.bmcl.2011.01.064

Five dinuclear platinum(II) complexes with a novel chiral ligand, 2-(((1R,2R)-2-aminocyclohexylamino)methyl)phenol (HL), were designed, prepared and spectrally characterized. In vitro cytotoxicity of all the resulting platinum(II) compounds was evaluated

Full text of DOI:10.1016/j.bmcl.2011.01.064

Bioorganic & Medicinal Chemistry Letters 21 (2011) 1763–1766
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis and in vitro cytotoxicity of novel dinuclear
platinum(II) complexes
Chuanzhu Gao b, Shaohua Gou a,b, , Lei Fang a,b, Jian Zhao b
⇑
a Jiangsu Province Hi-Tech Key Laboratory for Bio-medical Research, Southeast University, Nanjing 211189, China
b Pharmaceutical Research Center and School of Chemistry and Chemical Engineering, 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:
Five dinuclear platinum(II) complexes with a novel chiral ligand, 2-(((1R,2R)-2-aminocyclohexylami-
no)methyl)phenol (HL), were designed, prepared and spectrally characterized. In vitro cytotoxicity of
all the resulting platinum(II) compounds was evaluated against human HEPG-2, A549 and HCT-116 cell
lines, respectively. Results indicated that all compounds showed positive biological activity. Particularly,
compound D4 has lower IC50 values than carboplatin toward HEPG-2 and A549, while compound D5
shows better activity than carboplatin against A549.
Received 27 October 2010
Revised 6 January 2011
Accepted 18 January 2011
Available online 22 January 2011
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
1R,2R-Diaminocyclohexane derivative
Dinuclear platinum(II) complexes
Antitumor activity
Cisplatin is one of the most successful and frequently used
drugs in the cancer chemotherapy at present, especially for testic-
ular cancer, for which the overall cure rate exceeds 90%, and is
nearly 100% for early-stage disease.1–3 However, the clinical appli-
cation of cisplatin is greatly limited by its side effects including
nephrotoxicity and neurotoxicity,4,5 narrow range of activity,
intrinsic and/or acquired resistance, and low aqueous
solubility.6–8 So far, much effort has been devoted to designing cis-
platin analogues with reduced toxicity, improved clinical efficacy
and broader anticancer spectrum, which has resulted in successful
developments of several new anticancer platinum drugs. Among
these drugs, carboplatin and oxaliplatin are their representatives.
Breaking through the set of classical structure–activity relation-
ships (SAR) summarized by Cleare and Hoeschele,9 more recently
there have been efforts focused on the design of non-classical
platinum complexes, such as orally active platinum(IV) com-
plexes,10–12 sterically hindered platinum(II) complexes,13–16
trans-platinum complexes,17–19 multinuclear platinum(II) com-
plexes20–23 and sulfur containing platinum(II) complexes,24–26 etc.
Among those non-classical anticancer platinum multinuclear
platinum complexes have attracted much attention. For instance,
BBR3464, shown in Figure 1, is a trinuclear platinum complex own-
ing 1,6-diaminohexyl chains as bridges to connect metal ions.27,28
Much recently, Zhang et al. have reported a number of dinuclear
platinum(II) complexes with iodide anions as the bridges, in which
some compounds exhibited good antitumor activity.29 The corre-
sponding studies indicate that multinuclear platinum complexes,
as compared with conventional mononuclear platinum complexes,
could not only deliver more platinum containing drugs to the
tumor, but also form mutable-binding with tumor cell DNA and
increase the activity to block DNA replication.30–32
In this letter, we report a set of dinuclear platinum(II) com-
plexes with a new tridentate chiral ligand, 2-(((1R,2R)-2-amin-
ocyclohexylamino)methyl)phenols (HL), which is derived from
1R,2R-diaminocyclohexane by several-step synthetic processes.
With Lꢀ as a carrier ligand, and organic dicarboxylates or sulfate
as leaving groups, five novel dinuclear platinum(II) complexes
have been designed and synthesized, whose structures are illus-
trated in Figure 2.
For the synthesis of the ligand (HL), it is difficult to directly get
the monsubstituted derivative due to the equivalent reactivity of
the two amino groups in DACH. Thus, mono-Boc protecting DACH
(1) was used as the starting material which was recently reported.33
Detailed processes are as followed. Reaction of 1 with salicylalde-
hyde offered a mono-Schiff base which was then reduced by NaBH4
to give intermediate 2, then 2 was treated with HCl/EtOAc to remove
the Boc group, leading to 3 of HL hydrochloride, which was finally
neutralized by aqueous Na2CO3 solution to give free HL (Scheme 1).
4+
H3N
NH3
H3N
NH2(CH2)6H2N
Cl
Pt
Pt
Pt
H3N
NH3
NH3
Cl
NH2(CH2)6H2N
BBR3464
⇑
Corresponding author. Tel./fax: +86 25 8327 2381.
E-mail address: sgou@seu.edu.cn (S. Gou).
Figure 1. Structure of BBR3464.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.01.064

1764
C. Gao et al. / Bioorg. Med. Chem. Lett. 21 (2011) 1763–1766
O
NH
O
O
O
HN
NH
O
HN
Pt
Pt
Pt
Pt
O
O
O
NH2
H2N
NH2
H2N
O
O
O
O
D1
D2
O
NH
O
NH
O
O
HN
O
HN
Pt
Pt
O
Pt
Pt
O
O
O
NH2
NH2
H2N
NH2
O
O
O
D3
D4
O
O
NH
O
HN
Pt
Pt
O
NH2
H2N
S
O
O
D5
Figure 2. Molecular structures of the resulting dinuclear platinum(II) complexes.
NH2
N
H
a
b
N
HO
N
H
Boc
HO
N
H
Boc
N
H
Boc
2
1
c
N
H
N
H
d
2HCl
HO
HO
NH2
NH2
HL
3
Scheme 1. Preparation of HL. Reagents and conditions: (a) salicylaldehyde, toluene, refluxing and water separating for 2 h; (b) NaBH4; (c) HCl/EtOAc, Et2O; (d) Na2CO3.
Ag2 Z
O
I
NH
HL/NaOH
KI
D1~D5
K2PtCl4
K2PtI4
Pt
NH2
PtLI
O
O
O
O
COO-
COO-
COO-
COO-
COO-
S
;
;
;
Z=
;
COO-
COO-
COO-
Scheme 2. Synthetic scheme for dinuclear platinum(II) complexes D1–D5.

C. Gao et al. / Bioorg. Med. Chem. Lett. 21 (2011) 1763–1766
1765
Table 2
When preparing the targeted platinum complexes, we first pre-
In vitro cytotoxicity against selected human tumor cell lines of complexes D1–D5
pared the important intermediate [PtLI], which was then used to
react with related silver dicarboxylates and silver sulfate, respec-
tively, to afford compounds D1–D5(Scheme 2).
a
Complex
IC50 (lM)
HEPG-2b
A549c
HCT-116d
All compounds were spectrally characterized by IR, 1H NMR and
ESI-MS spectra together with microanalyses.34,35 Elemental analy-
sis for each compound was in good agreement with the empirical
formula proposed. In the IR spectra, the amino group participation
in binding with Pt(II) was confirmed by the examination of
D1
D2
D3
D4
26.3
37.2
48.5
15.1
24.2
22.2
0.318
5.3
18.6
29.2
>50
4.2
5.1
9.9
48.2
36.6
28.2
9.6
D5
7.8
Carboplatin
Cisplatin
Oxaliplatin
Not tested
Not tested
4.3
m
NH2/mNH and dNH2/dNH frequencies, which were shifted to lower
1.1
2.3
frequencies comparing with the free amino group, due to
Pt(II)-NH2/Pt(II)-NH coordinations. The shifts of the C@O absorp-
tion from free carboxylic acids near 1700 cmꢀ1 to a band near
1640–1593 cmꢀ1 proved that the carboxylate anion was coordi-
nated to Pt2+ in each case.36 All dinuclear compounds showed
peaks of [MꢀZ2ꢀ+CH3Oꢀ]+ in their positive ESI mass spectra, sev-
eral compounds gave [M+Na]+ or [M+K]+ peaks, which are in agree-
ment with their molecular formula weights. For three isotopes of
Pt element: 194Pt(33%), 195Pt(34%), 196Pt(25%), all the mass spectra
of platinum compounds were found with three isotopic peaks. The
molecular structures of all compounds showed in Figure 1 were
also proved by their related 1H NMR spectral data.
To explore the stability of our compounds, compound D4 in D2O
at room temperature was studied by 1H NMR. 1H NMR spectra
were subsequently recorded at 0, 3, 12 and 24 h. All resulting spec-
tra were compared each other and found that there were not any
obvious differences, indicating that compound D4 kept stable in
water at room temperature.
Some platinum anticancer drugs, such as cisplatin, are limited
due to their poor aqueous solubility. Thus, the aqueous solubility
of all compounds was measured at 25 °C. Compared with cisplatin
whose aqueous solubility is only 1 mg/mL, our platinum com-
plexes have been improved, with aqueous solubility ranging from
4.1 to 7.5 mg/mL. Due to the introduction of a chiral ligand, specific
rotations of these dinuclear platinum complexes were tested by an
automatic digital polarimeter. All compounds turned out to be
optical with values of specific rotations from +32.0° to +74.2°, sim-
ilar to oxaliplatin(+76.1°). The corresponding values of aqueous
solubility and specific rotations for the complexes are showed in
Table 1.
In vitro cytotoxicity of the dinuclear platinum complexes was
tested by MTT assay37,38 using A549, HEPG-2 and HCT-116 cell
lines. The IC50 values of these compounds as well as positive con-
trols, cisplatin, carboplatin and oxaliplatin, are given in Table 2.
As we can see, most dinuclear platinum complexes showed po-
sitive cytotoxicity against selected cell lines. Compared with carbo-
platin, the target compounds showed potent antitumor activity
against HEPG-2 cell line with IC50 values varying from 15.1 to
a
All IC50 values (drug concentration giving 50% survival) calculated based on the
Pt content are means SD (SD <12% of the mean value) from at least three sepa-
rated experiments.
b
Human hepatocellular carcinoma cell.
Human lung cancer cell.
Human colorectal cancer cell.
c
d
> D5 > carboplatin > D1 > D2 > D3. When the cell line is HCT-116,
D4 and D5 also showed comparable activities to oxaliplatin.
Based on the above results, it can be found that the selection of
different bridges has a clear impact on the antitumor activities.
When HEPG-2 and A549 cell lines were employed, the longer the
dicarboxylate bridge is the lower the antitumor activity is. Inter-
estingly, when a cyclobutyl moiety was introduced to the dicar-
boxylate bridge, the antitumor activity significantly increased,
indicating the cyclobutyl fragment had a positive impact on the
activity. Employing inorganic sulfate as a bridge instead of the or-
ganic dicarboxylate chain also achieved a good result, as the cyto-
toxicity of D5 was obviously higher than that of D1, D2 and D3. In
particular, when HCT-116 cell line was tested, sulfate-bridged
dinuclear complex showed better activity than all organic dicar-
boxylates. Notably, the length of the bridge showed a different
influence on the cytotoxicity against HCT-116 cell line: D3 had a
longer bridge chain, but its cytotoxicity was higher than that of
D1 which contained a short bridge chain, suggesting antitumor
activities may increase when carbon chains is prolonged against
this cell line. One of important reasons that our prepared dinuclear
complexes showed positive cytotoxicity against selected cell lines
may be attributed to their stereo-structures similar to oxaliplatin,
because the ligand, HL, was synthetically derived from 1R,2R-
diaminocyclohexane without changing its absolute configuration.
Apart from the above, steric effect in the compound could also
play a significant role. For example, complex D4 with 1,1-cyclobu-
tyl dicarboxylate as the bridge, had the minimum specific rotation
of +32.0° but showed the relatively good cytotoxicity. We specu-
lated steric effect may produce between the cyclobutyl moiety
and the aromatic species. When entering the tumor cell, the steric
effect could make complex D4 more easier than other dinuclear
complexes lose the bridge and form positive ions to bind with
DNA.
In conclusion, with a monosubstituted chiral DACH derivative
(HL) as a carrier group, we designed and synthesized five novel
dinuclear platinum complexes which have some organic dicarbox-
ylates/sulfate as bridges. All of the target compounds showed bet-
ter aqueous solubility than cisplatin. In vitro cytotoxicity tests
showed that most prepared dinuclear complexes gave positive
antitumor activity against selected cell lines. Compound D4 which
takes 1,1-cyclobutyl dicarboxylate as a bridge not only showed
better antitumor activity against HEPG-2 and A549 than carbo-
platin, but also showed a comparable activity against HCT-116 to
oxaliplatin. Besides, compound D5 which takes sulfate as a bridge
also exhibited better activity than carboplatin against A549. Conse-
quently, the obtained dinuclear platinum complexes, especially
compound D4, may be deserved for further investigation.
48.5 lM, particularly, compound D4 somehow showed a better
activity than carboplatin. Compounds D1 and D5 also showed com-
parable activity to carboplatin, the order of cytotoxicity is cis-
platin > oxaliplain > D4 > carboplatin > D5 > D1 > D2 > D3. As for
A549 cell line, compounds D4 and D5 gave lower IC50 values than
carboplatin, the order of cytotoxicity is cisplatin > oxaliplain > D4
Table 1
Aqueous solubility and specific rotations of complexes
D1–D5
½ ꢁ (c 1, MeOH)
a 1D5
Complex
Aqueous solubility
(mg/mL, 25°C)
D1
D2
D3
D4
D5
5.6
6.2
5.0
4.1
7.5
+74.2
+46.2
+85.4
+32.0
+59.5

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C. Gao et al. / Bioorg. Med. Chem. Lett. 21 (2011) 1763–1766
32. Elmroth, S. K. C.; Lippard, S. J. J. Am. Chem. Soc. 1994, 116, 3673.
33. Lee, D. W.; Ha, H. J.; Lee, W. K. Synth. Commun. 2007, 37, 737.
34. Synthesis of complex [PtLI]: To stirring aqueous solution (40 ml) of KI
Acknowledgements
a
This work is supported by the National Natural Science Founda-
tion of China (Project 20971022) and the Six Top Talents Funding
of Jiangsu Province (Project 2008046) as well as the New Drug Cre-
ation Project of the National Science and Technology Major Foun-
dation of China (Project 2010ZX09401-401) to S.G.
(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
6 h, the dark yellow precipitate was filtered, washed sequentially with water,
ethanol and ether, then dried in vacuum.
Data for [PtLI]: yield: 95%, dark yellow solid. IR(m
, cmꢀ1): 3418m(br), 3219m,
3180m, 2933m, 2859m, 1594m, 1562m, 1480s, 1448s, 1289m, 1266m, 1038w,
876w, 748m, 591w, 457m. 1H NMR (DMSO/TMS): d 6.72–7.09 (m, 4H, 4H of
C6H4), 5.83–6.54 (m, 3H, 3H of NH2 and NH), 3.88–4.09 (m, 2H of CH2C6H5),
1.06–2.74 (m, 10H of DACH). ESI-MS: m/z [M+H]+ = 541 (60%), [M+Na]+ = 564
(100%). Anal. (C13H19IN2OPt) C, H, N.
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Data for D1: Yield: 32%, white solid. IR(m
, cmꢀ1): 3403s(br), 3185s, 2935s,
2860s, 1640vs, 1594vs, 1480s, 1448s, 1385m, 1292vs, 1148m, 1115m, 1038m,
877m, 763s, 516m, 464m. 1H NMR (DMSO/TMS): d 6.51–6.90 (m, 8H of 2C6H4),
5.10–6.36 (m, 6H of 2NH2 and 2NH), 3.73–4.18 (m, 4H of 2CH2C6H5), 0.85–2.73
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[M+K]+ = 955(40%). Anal. (C28H38N4O6Pt2) C, H, N.
Data for D2 yield: 32%, white solid. IR(m
, cmꢀ1): 3184s(br), 2934s, 2859s,
1593vs, 1480s, 1448s, 1357s, 1296s, 1267s, 1114m, 1039m, 963m, 877w,
752m, 622w, 466m. 1H NMR (DMSO/TMS): d 6.50–6.96 (m, 8H of 2C6H4), 5.02–
6.31 (m, 6H of 2NH2 and 2NH), 3.87–4.08 (m, 4H of 2CH2C6H4), 2.68–2.70 (m,
2H of COOCH2COO), 1.08–2.63 (m, 20H of 2DACH). ESI-MS: m/z
[MꢀC3O4H 2ꢀ+CH3Oꢀ]+ = 859 (100%). Anal. (C29H40N4O6Pt2) C, H, N.
Data for D32yield: 21%, white solid. IR(
m
, cmꢀ1): 3391s, 3184s(br), 2936s, 2860s,
1594vs, 1560s, 1480s, 1449s, 1385s, 1294s, 1268s, 1151m, 1039m, 877m,
753m, 570m, 467m. 1H NMR (DMSO/TMS): d 6.32–7.12 (m, 8H of 2C6H4), 4.98–
6.31 (m, 6H of 2NH2 and 2NH), 3.70–4.12 (m, 4H of 2CH2C6H5), 0.90–2.45 (m,
2ꢀ
24H, 20H of 2DACH and 4H of ꢀC4O4H42ꢀ). ESI-MS: m/z [MꢀC4O4H4
+
CH3Oꢀ]+ = 859 (100%), [M+H]+ = 945(45%), [M+Na]+ = 967(13%). Anal.
(C30H42N4O6Pt2) C, H, N.
Data for D4 yield: 45%, white solid. IR(m
, cmꢀ1): 3434s, 3185s(br), 2938s, 2861s,
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