Toward the design of novel polynuclear platinum antitumor complexes: A polydentate ligand system based on dipyridylamine and 1,3,5-trimethylenebenzene

Article abstract of DOI:10.1021/ic034604q

A novel hexadentate ligand, N,N,N′,N′,N″,N″ -hexa(2-pyridyl)-1,3,5-tris(aminomethyl)benzene (L), was designed and synthesized. The X-ray structure analysis reveals that the three dipyridylamine (DPA) groups of L are almost perpendicular to the central trimethylenebenzene, and two of them are spacially close to each other while the third one is further apart. The trinuclear Pt(II) complexes [Pt₃LCl₆] (1) and [Pt₃L(CBDCA)₃] (2) (where CBDCA represents cyclobutane dicarboxylic acid) were prepared and fully characterized by IR, NMR, and ESMS spectroscopy. A mononuclear complex, [PtL(CBDCA)] (3), was also prepared and structurally characterized, which suggests that controlled formation of mono-, di-, and trinuclear complexes with L is possible. Spectroscopic data showed that complexes 2 and 3 are able to bind to calf thymus DNA and their CBDCA group can be readily replaced by thiourea.

Full text of DOI:10.1021/ic034604q

Inorg. Chem. 2003, 42, 5795−5797
Toward the Design of Novel Polynuclear Platinum Antitumor
Complexes: A Polydentate Ligand System Based on Dipyridylamine
and 1,3,5-Trimethylenebenzene
Chao Tu, Jun Lin, Ying Shao, and Zijian Guo*
State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute,
Nanjing UniVersity, 210093 Nanjing, P. R. China
Received June 2, 2003
A novel hexadentate ligand, N,N,N′,N′,N′′,N′′-hexa(2-pyridyl)-1,3,5-
tris(aminomethyl)benzene (L), was designed and synthesized. The
X-ray structure analysis reveals that the three dipyridylamine (DPA)
groups of L are almost perpendicular to the central trimethyl-
enebenzene, and two of them are spacially close to each other
while the third one is further apart. The trinuclear Pt(II) complexes
[Pt3LCl6] (1) and [Pt3L(CBDCA)3] (2) (where CBDCA represents
cyclobutane dicarboxylic acid) were prepared and fully characterized
by IR, NMR, and ESMS spectroscopy. A mononuclear complex,
[PtL(CBDCA)] (3), was also prepared and structurally characterized,
which suggests that controlled formation of mono-, di-, and
trinuclear complexes with L is possible. Spectroscopic data showed
that complexes 2 and 3 are able to bind to calf thymus DNA and
their CBDCA group can be readily replaced by thiourea.
while they significantly reduced the rate of the deactivation
by thiol groups.4 2,2′-Dipyridylamine (DPA) is an aromatic
amine that shares certain similarities to 2,2′-dipyridine;
however, the central amine unit introduces more flexibility
to the pyridine rings.5 Although studies on the Pt(II)-DPA
system are rare, several DPA complexes of platinum(II) and
palladium(II) have demonstrated equal or higher antitumor
activity than cisplatin against P388 leukemia cell lines.6
In this work, we designed a novel multidentate ligand L
in which three DPA moieties are linked through a trimeth-
ylenebenzene group, a more rigid backbone compared to
polyamines. Moreover, the ligand offers a certain degree of
flexibility, which provides potential for both intra- and
interstrand DNA cross-linking.
Ligand L is synthesized from the reaction of 1,3,5-tris-
(bromomethyl)benzene (TBB) and DPA in 1:3 molar ratio
in dimethyl sulfoxide (DMSO) solution with the presence
of excessive potassium hydroxide (Scheme 1 and Supporting
Information). The compound is fully characterized by FT-
IR, elemental analysis, ESMS, NMR and X-ray crystal-
lography.
Polyamine-bridged polynuclear platinum(II) complexes
have attracted considerable attention because they represent
a new class of anticancer agents that possess potent and
distinct biological activity from cisplatin.1 A representative
compound of the class, BBR3464, has been in clinical trials
since 1997. The complex is actually composed of two
monofunctional Pt moieties, and the two separated Pt-Cl
bonds maintain the bifunctional binding mode on DNA.2
Encouraged by the success of this compound, many studies
have been conducted for the search for novel polynuclear
complexes with desired chemical and biological properties.3
1
The H NMR spectrum of L in CDCl3 shows six signals
at 8.22d, 7.40t, 7.09s, 6.92d, 6.81t, and 5.34s ppm which
can be assigned, respectively, to the protons of the three
equivalent DPA, benzyl, and methylene groups, suggesting
these groups are free of rotation in solution. As expected,
(3) (a) Komeda, S.; Lutz, M.; Spek, A. L.; Chikuma, M.; Reedijk, J. Inorg.
Chem. 2000, 39, 4230-4236. (b) Komeda, S.; Lutz, M.; Spek, A. L.;
Yamanaka, Y.; Sato, T.; Chikuma, M.; Reedijk, J. J. Am. Chem. Soc.
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Kozelka, J.; Viossat, B. Inorg. Chem. 2000, 39, 6131-6133. (d)
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Sato, T.; Chikuma, M.; Reedijk, J. J. Med. Chem. 2003, 46, 1210-
1219.
It has been shown recently that the introduction of bulky
planar ligands such as pyridine and substituted pyridines
maintained the potent cytotoxicity of the Pt(II) complexes
* To whom correspondence should be addressed. E-mail: zguo@
netra.nju.edu.cn.
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J. Am. Chem. Soc. 2001, 123, 1316-1326. (b) Davies, M. S.; Thomas,
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10.1021/ic034604q CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/27/2003
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COMMUNICATION
Figure 2. Molecular structure and labeling scheme of complex 3.
Figure 1. Molecular structure and labeling scheme of L.
Scheme 1. Synthetic Pathway of Ligand L
L in 1:1 molar ratio in a refluxed methanol solution was
conducted, which gave the formation of [PtL(CBDCA)] (3).
In the H NMR spectrum of 3, two sets of signals with an
1
integral ratio of ca. 1:2 were observed, suggesting that the
coordination of Pt(II) to one of the DPA groups resulted in
their inequivalency (Supporting Information).
The ellipsoid plot with the numbering scheme of 3 is
shown in Figure 2. The Pt(II)-bound DPA group in 3 is
situated above the central trimethylenebenzene plane, while
the other two DPA groups remain almost the same as
observed in L. The Pt(II) center is coordinated in a square
planar geometry, and the CBDCA moiety displays a similar
conformation to that described in the literature.8,9 The two
six-membered chelate rings which DPA and CBDCA formed
with the Pt(II) atom adopt a boat conformation. The
cyclobutane ring, which is nearly perpendicular to the Pt(II)
coordination sphere, is distorted significantly from the best
plane of C42-C43-C44-C45 with the dihedral angle
between C42-C43-C44 and C42-C45-C44 being 16.1°.
In order to study the potential DNA binding ability of 1,
2, and 3, their reactions with calf thymus (CT) DNA were
followed by the UV method. Upon addition of an increasing
amount of DNA (10-5-10-4 M) to the complexes (10-5 M),
40% and 9% hyperchromisms were observed for 2 and 3 at
306 and 305 nm, respectively, which suggested possible
interactions between complexes 2 and 3 with DNA. How-
ever, no obvious change was found for complex 1. A similar
hyperchromism was previously observed for copper com-
plexes,10 which was attributed to the dissociation of complex
aggregates and the breakage of intermolecular hydrogen
bonds when bound to DNA. A time-dependent UV experi-
ment for the reaction of complex 3 with DNA (Figure 3)
showed that, upon addition of a fixed amount of DNA (1.3
× 10-4 M) to the solution of complex 3 (6.7 × 10-5 M), a
very slight hyperchromism was observed at 305 nm initially,
followed by the significant decrease in intensity. After 24
eight signals were observed at 156.7, 147.8, 139.2, 137.4,
123.8, 117.1, 114.7, and 51.4 ppm in the 13C NMR spectrum.
Figure 1 shows an ellipsoid plot of the crystal structure
of L with the numbering scheme. It can be noted that the
three DPA groups are almost perpendicular to the central
trimethylenebenzene, and two of them are spacially close to
each other while the third one is further apart. The dihedral
angles between the two pyridyl rings in each DPA group
are 131.1°, 127.0°, and 128.2°, respectively.
The reaction of L with K2[PtCl4] or [Pt(dmso)2CBDCA]7
in 1:3 molar ratio gives readily formation of [Pt3LCl6] (1)
or [Pt3L(CBDCA)3] (2) (See Supporting Information for
details) in quantitative yield. The 1H NMR spectroscopy of
1 in DMSO-d6 shows that the three DPA moieties maintain
their equivalency upon coordination to platinum; the chemi-
cal shifts are 8.83d, 8.03s, 7.71t, 7.30d, 7.25t, and 5.34s ppm,
respectively. The aryl proton signals are shifted to lower field
compared to L which suggested the coordination of DPA
groups to platinum. The ESMS spectrum of 1 contains three
major peaks at m/z values of 1389.9, 1448.7, and 1463.7
which can be assigned to [Pt3LCl5]+, [Pt3LCl6+Na]+, and
[Pt3LCl6+K]+, respectively. Similarly, the 1H NMR spectrum
of 2 suggested the presence of three equivalent Pt(II)-bound
DPA groups, and its ESMS spectrum contains one major
peak with an m/z value of 1662.1, assignable to [Pt3L-
(CBDCA)3 + Na]+ (Supporting Information).
The UV spectra of complexes 1 and 2 show the absorption
bands at 304 and 306 nm, respectively, which can be
attributed to the metal to ligand charge transfer absorption
(MLCT).
In order to investigate the possibility of the stepwise
coordination of L, the reaction of [Pt(dmso)2CBDCA] with
(8) (a) Guo, Z.; Habtemariam, A.; Sadler, P. J.; Palmer, R. B.; Potter, S.
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115, 6699-6704. (b) Liu, J.; Zhang, T. X.; Lu, T. B.; Qu, L. H.; Zhou,
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COMMUNICATION
chloride. Sulfur-containing molecules such as L-methionine
have been suspected to be involved in the activation of
carboplatin.12 Transfer of Pt onto DNA via sulfur-containing
peptides or proteins may also be possible.13 Therefore, the
reaction of 3 and thiourea was conducted in order to
investigate the potential replacement of CBDCA. As shown
1
by H NMR spectroscopy (Supporting Information), the
signals for free CBDCA were observed after 30 min and
increased in intensity with time, suggesting that CBDCA can
be readily substituted by thiourea. No ring-opened mono-
dentate intermediate was observed under the NMR conditions
used here, which indicates the fast replacement of CBDCA
by the second thiourea.
In conclusion, the novel polydentate ligand designed in
this work provides the possibility for the synthesis of mono-
and polynuclear platinum complexes which could bind
strongly to DNA and thereby be antitumor active. The
rigidity of the backbone offers the possibility for designed
length of DNA cross-linking, while the flexibility of the DPA
groups provides the possibility for DNA intercalation.
Moreover, the uncoordinated DPA moieties furnish the
potential binding sites for metal ions such as Cu(II) and Ru-
(II), giving rise to formation of heteromultinuclear complexes
that possess antitumor potential. Further work on the
biological properties of this ligand and its metal compounds
is currently ongoing in our group.
Figure 3. Time dependent absorption spectra of 3 (6.7 × 10-5 mol‚L-1
)
in the absence (- - -) and presence (s, represent 0, 4, 8, 12, 16, 20, and 24
h, respectively) of a fixed amount of CT DNA (1.3 × 10-4 mol‚L-1) at 37
°C. Inset shows decrease in absorbance at 305 nm (ln A) with time for the
reaction of 3 with DNA.
h, a 21% hypochromism was observed which is much larger
than that observed for potential DNA intercalators,11 which
suggests that complex 3 is able to bind to DNA strongly.
The platinum(II) in complex 3 could bind covalently to the
nucleobases such as guanine in DNA upon leaving the
CBDCA group, and the four uncoordinated pyridine groups
could form hydrogen bonding with or intercalate into DNA.
As shown in the crystal data, the distances between Pt1-
N4 and Pt1-N7 are 9.043 and 7.089 Å, respectively,
providing the possibility of both binding modes simulta-
neously.
The fluorescent experiments showed that, upon titrating
the solution of 2 or 3 (20 µL per scan) into the samples
containing 4.9 × 10-5 M of DNA and 4.9 × 10-5 M of
ethidium bromide (EB), the emission band at 600 nm of the
DNA-EB system decreased in intensity with increasing the
concentration of the Pt(II) complexes, which indicates that
the complexes can replace EB from the DNA-EB system.
Such a characteristic change is often observed in the
intercalative DNA interaction.10c The data further suggested
that intercalation may also be an alternative binding mode
for the complexes.
Acknowledgment. We acknowledge the financial support
from the National Natural Science Foundation of China
(29925102, 20231010, and 20228102).
Supporting Information Available: Crystallographic data of
L and complex 3. Experimental procedures for the preparation of
L and 1-3 and their ESMS, NMR, UV-vis, and fluorescent
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC034604Q
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shown to be less susceptible to hydrolysis compared to
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