Synthesis and anti-cancer activity of dinuclear platinum(II) complexes containing bis(thioalkyl)dicarba-closo-dodecaborane(12) ligands

Article abstract of DOI:10.1039/b508340k

The synthesis and anti-cancer activity of dinuclear platinum (II) complexes containing bis(thioalkyl)dicarba-closo-dodecaborane(12) ligands were reported. In vitro anti-cancer screening of the complexes against L1210 leukaemia and its cisplatin-resistant

Full text of DOI:10.1039/b508340k

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C O M M U N I C A T I O N
Synthesis and anti-cancer activity of dinuclear platinum(II)
complexes containing bis(thioalkyl)dicarba-closo-dodecaborane(12)
ligands†
Susan L. Woodhouse,b Erin J. Ziolkowskia and Louis M. Rendina*a
a School of Chemistry, The University of Sydney, Sydney, NSW, 2006, Australia.
E-mail: rendina@chem.usyd.edu.au; Fax: 61 2 9351 3329; Tel: 61 2 9351 4781
b School of Chemistry and Physics, The University of Adelaide, Adelaide, South Australia, 5005,
Australia
Received 14th June 2005, Accepted 12th July 2005
First published as an Advance Article on the web 25th July 2005
A
series of novel, dinuclear (2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine)-
ation of the complexes from DNA in comparison with the
mono-intercalators,2,3 thereby allowing the compound to persist
near the DNA for longer periods and thus maximising cellular
damage by the products of thermal neutron capture involving
the 10B nucleus. Herein we report the synthesis of a series
of novel bis(thioalkyl)carborane ligands and their dinuclear
platinum(II)–trpy complexes, the first examples of potential
bis-intercalator molecules containing a carborane. Preliminary
in vitro anti-cancer screening of the complexes against L1210
leukaemia and its cisplatin-resistant variant (L1210/DDP) is
also reported.
platinum(II) complexes containing bis(thioalkyl)-dicarba-
closo-dodecaborane(12) (carborane) ligands were prepared
and characterised, and their preliminary anti-cancer char-
acteristics have been determined in vitro; the complexes are
the first examples of bis-intercalator complexes containing
a boron-rich carborane cage.
Platinum(II)–trpy (trpy = 2,2ꢀꢀ:6ꢀ,2ꢀꢀ-terpyridine) complexes have
attracted considerable attention in recent years for their interest-
ing biological properties, including anti-cancer activity.1,2 Din-
uclear platinum(II)–trpy complexes linked by simple dithioalkyl
ligands with varying chain lengths have been shown to bi-
functionally intercalate DNA with higher affinities for DNA
over their mononuclear analogues.2,3 Indeed, the linking of
two DNA intercalating moieties to form a ‘bis-intercalator’
frequently increases the binding affinity of the species due to
a decrease in the dissociation rate in comparison with the
mono-intercalator.3–9 Some bis-intercalators are also known to
exhibit potent anti-cancer activity against numerous tumour
cell lines.5,10–14 In many cases, a variation in the rigidity or
length of the linker between the two intercalating moieties
within a bis-intercalator can alter the mode of binding and the
ability of the compound to bis-intercalate.5,8,9,15–17 In the case of
diplatinum(II)–trpy complexes with short linkers, mono- or bis-
intercalation is found to occur, with one base-pair separating
the two intercalating chromophores in the latter. Intermediate-
linked centres bifunctionally intercalate across a short range,
such as two base-pairs. The longest linked metal centres tend
to show mixed mono- and bis-intercalation. In the case of
mono-intercalation, the non-intercalated chromophore may
bind externally to the DNA or aggregate with a chromophore
from an adjacent molecule.2 As found for the corresponding
mononuclear platinum(II)–trpy species, dinuclear complexes
require a GC base pair at the intercalation site and display
growth inhibition of L1210 murine leukaemia cells.2 The mode
of anti-cancer activity is thought to be due to DNA intercalation
of the platinum(II)–trpy moiety and consequent interruption to
cellular processes, action on topoisomerases7,12,18,19 or disruption
of the cell membrane causing cell lysis.2
Recent work in our group20,21 has led to the development
of a series of thioalkylcarborane ligands and their correspond-
ing platinum(II)–trpy complexes for potential application in
Boron Neutron Capture Therapy (BNCT).22,23 These complexes,
e.g. 1, have demonstrated a propensity to intercalate calf-
thymus DNA and display in vitro anti-cancer activity.20,21 It
was anticipated that the use of carboranes containing two
platinum(II)–trpy chromophores would decrease the dissoci-
There exists no precedent for the synthesis of the desired
bridging bidentate bis(thioalkyl)carborane ligands required in
the preparation of the target complexes 2–5. In a related
† Electronic supplementary information (ESI) available: Synthetic
procedures and characterisation data for the bis(thioalkyl)carborane
ligands and complexes 2–5. See http://dx.doi.org/10.1039/b508340k
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 2 8 2 7 – 2 8 2 9
2 8 2 7
©

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Table 1 IC50 (lM) values (n = 2) for 1–5. IC50 values were determined
procedure to that reported for the monodentate thioalkylcarbo-
rane ligands,23 treatment of the benzylthiolate anion with 1,12-
bis(bromopropyl)-1,12-carborane, for example, afforded the S-
benzyl protected carborane 6 in 96% yield (Scheme 1). This
method was also successfully applied to the synthesis of the cor-
responding 1,2- (90%) and 1,7-carborane isomers (95%). The S-
benzyl groups in 6 were readily◦cleaved by treatment with freshly
sublimed AlCl3 in C6H6 at 50 C to afford the bis(thiopropyl)-
1,12-carborane ligand 7 in reasonable yield (50%) (Scheme 1).23
Similar reaction conditions afforded the 1,2- (54%) and 1,7-
carborane isomers (37%). Small quantities (<5%)ofunidentified
using a Coulter Counting (CC) assay
Cell line
Cisplatin
1
2
3
4
5
L1210
L1210/DDP
0.5
6.9
1.6
0.9
24.5
26.5
5.3
7.0
7.4
10
0.9
0.8
Addition of the labile precursor [Pt(trpy)(MeCN)](OTf)2 to 7
in DMF solution resulted in an immediate and dramatic colour
change from pale-yellow to deep purple, indicating substitution
of the S-donor ligand for the MeCN ligand had readily occurred
to afford 3 in 61% yield (Scheme 1). Similar reaction conditions
were successfully applied to the synthesis of 2 (32%, Scheme 2),
4 (87%) and 5 (84%). The identities of 2–5 were confirmed
by microanalysis and multinuclear (1H, 13C, 11B and 195Pt)
NMR spectroscopy. 2D-NMR (COSY, HMBC and HMQC)
experiments were performed to allow complete assignment of all
1
impurities were detected by H NMR spectroscopy and may
be the result of thiol oxidation but the products could not
be successfully purified by recrystallisation, distillation, subli-
mation or chromatography methods. Characterisation of the
bis(thiopropyl)carborane ligands was confirmed, however, by
means of ESI-MS, with molecular ion peaks detected at m/z
292. The bis(thiomethyl)-1,12-carborane ligand 9 was prepared
from the corresponding dithioester derivative 8 by reduction
with BH3·SMe2 (Scheme 2). The preparation of 8 followed a
related procedure to that reported by Nachman et al.24 for
the preparation of the corresponding thiomethyl-1,12-carborane
derivative from 1,12-carborane using two equivalents of nBuLi,
MeI and CS2 in the presence of CuBr and LiBr.
1
1H and 13C{ H} NMR signals for the complexes. In particular,
coordination of the thiol ligand to the platinum(II) centre in 3, for
1
example, was confirmed by a shift in the 13C{ H} NMR spectrum
from d 23.8 (CH2SH) to d 30.2 (CH2SPt) and a characteristic
1
peak in the 195Pt{ H} NMR spectrum at d −3206 which is
characteristic of a PtN3S core.22,23,25 Despite repeated attempts
no molecular ion peaks were detected for 2–5 in the ESI-MS;
only peaks resulting from fragmented ions were detected, e.g.
[PtS(trpy)]+.
The dinuclear complexes 2–5 were screened against the L1210
murine leukaemia cell line and its cisplatin-resistant variant
L1210/DDP.26 Cisplatin was used as the control compound,
and the mononuclear platinum-carborane complex 1 was also
examined in order to ascertain any differences in cytotoxicity be-
tween related mono- and di-nuclear species. The concentrations
of complexes required to achieve 50% inhibition of cell growth
(IC50) are presented in Table 1.
The relative cytotoxicities of 1–5 are similar in both the
cisplatin-sensitive L1210 and cisplatin-resistant L1210/DDP
cell lines. This clearly indicates that the mechanism of cytotoxic-
ity by 1–5 is not affected by the cisplatin resistance mechanism(s)
operating in the L1210/DDP cell line. In comparison with
cisplatin, 1 and 5 are considerably more potent in the cisplatin-
resistant L1210/DDP line. In contrast, 2–4 are not as active. It
appears that the inclusion of 1,7- and 1,12-carborane moieties
in 2–4 adversely affects the ability of these complexes to exert
a cytotoxic effect. Although subtle electronic effects associated
with the nature of the carborane cage may explain the observed
differences in biological activity, these results are more likely
to be consistent with the poor aqueous solubility of 2–4 which
is largely attributed to the low polarity of these complexes. It
appears that the significantly higher cytotoxicity of 5, compared
to 3 and 4, results from its somewhat higher solubility in aqueous
solution. Indeed, while all the complexes prepared in this work
are freely soluble in DMF, 1 and 5 are also soluble in MeOH
and, unlike 2–4, do not precipitate out of solution upon dilution
with water. Any precipitation of the complexes from solution
during the 48 h incubation period with the tumour cell lines
would prevent the full cytotoxic effect from being exerted, thus
resulting in the observation of low cytotoxicity.
Scheme 1 Reagents: (i) NaOEt, BnSH; (ii) AlCl3; (iii) [Pt(MeCN)-
(trpy)](OTf)2.
The exact basis of the cytotoxicity exhibited by the complexes
cannot be established with any certainty until their exact
mechanism(s) of action is determined, but both DNA- and
protein-binding are possible as has been observed with related
complexes.2,3,27,28 It is also feasible that the complexes embed
themselves within the cell membrane, possibly by a membrane-
spanning mechanism for the larger bis(thiopropyl) derivatives
3–5, which may ultimately lead to cell death due to lysis.2
In conclusion, the first examples of dinuclear platinum(II)–
trpy complexes containing 1,2-, 1,7- and 1,12-carboranes were
prepared and fully characterised by microanalysis and multin-
uclear (1H, 13C, 11B and 195Pt) 1D- and 2D-NMR spectroscopy.
Scheme 2 Reagents: (i) nBuLi, CuBr/LiBr, CS2, MeI; (ii) BH3·SMe2,
conc. HCl; (iii) [Pt(MeCN)(trpy)](OTf)2.
2 8 2 8
D a l t o n T r a n s . , 2 0 0 5 , 2 8 2 7 – 2 8 2 9

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All complexes were screened against the L1210 leukaemia cell
line and its cisplatin-resistant variant, thus allowing a direct
comparison of their IC50 values with those of cisplatin and the
mononuclear species 1. We are currently exploring methods for
improving the aqueous solubility of the complexes and we are
also determining their DNA-binding characteristics, cell uptake
and intra-cellular distribution. The results of this work will be
reported in due course.
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Acknowledgements
We are grateful to Dr C. Cullinane (Peter MacCallum Cancer
Institute, Melbourne, Australia) for the cytotoxicity data. We
thank Dr I. Luck (The University of Sydney) for assistance with
1
1
the recording of 11B{ H} and 195Pt{ H} NMR spectra, and Dr
K. Fisher (The University of Sydney) for assistance with the
collection of MS data. We are grateful to Johnson-Matthey for
the generous loan of K2[PtCl4]. Finally, we acknowledge funding
from the Australian Research Council (ARC) and Anti-Cancer
Foundation of South Australia.
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D a l t o n T r a n s . , 2 0 0 5 , 2 8 2 7 – 2 8 2 9
2 8 2 9