Selective cytotoxic Ru(II) arene Cp* complex salts [R-PhRuCp*]+X- for X = BF4-, PF6-, and BPh4-

Article abstract of DOI:10.1021/ic801159f

A novel series of ionic Ru(II) arene Cp* sandwich complexes has been synthesized and characterized. Screening results for cytotoxicity against a range of human tumor cell lines and normal human cells indicate that the complexes show promising anticancer activity, which varies with changes in the arene ligand and the anionic counterion.

Full text of DOI:10.1021/ic801159f

Inorg. Chem. 2008, 47, 8589-8591
Selective Cytotoxic Ru(II) Arene Cp* Complex Salts [R-PhRuCp*]+X- for
-
-
-
X ) BF4 , PF6 , and BPh4
Bradley T. Loughrey,† Peter C. Healy,† Peter G. Parsons,‡ and Michael L. Williams*,†
Eskitis Institute for Cell and Molecular Therapies, Griffith UniVersity, Brisbane, Australia, and
Drug DiscoVery Group, Queensland Institute of Medical Research, Brisbane, Australia
Received June 24, 2008
A novel series of ionic Ru(II) arene Cp* sandwich complexes has
been synthesized and characterized. Screening results for cyto-
toxicity against a range of human tumor cell lines and normal
human cells indicate that the complexes show promising anticancer
activity, which varies with changes in the arene ligand and the
anionic counterion.
in ViVo has prompted our research group to synthesize and
structurally characterize a series of ionic Ru(II) arene Cp*
(η5-C5(CH3)5) complexes [R-PhRuCp*]+X- in which the
ligands (YZ)(L) of the above systems are replaced by the
Cp* anion. These highly stable, nonlabile complexes have
no readily available leaving groups, potentially indicating
that this series of compounds could display its own unique
mechanism of action.
The aim of this study was to investigate how changes in
arene size and hydrophobicity impact the overall biological
activity of these complexes. Potential applications of this
work include, but are not limited to, complexation of the
RuCp* moiety to known aromatic drugs such as chloroquine
and tamoxifen. These clinical therapeutics have previously
been tethered to ferrocene yielding ferroquine and ferrocifen,
respectively.5,6 Ferroquine is highly active against chloro-
quine-resistant strains of the malaria parasite, while the
ferrocifens were shown to inhibit proliferation of both
hormone-dependent and hormone-independent forms of
breast cancer.5,6
A second aim of the study was to determine if the presence
of differing counterions could effect any noticeable changes
in the biological activity of the compounds, as literature data
obtained on a number of cytotoxic cations indicate that
variation in the chemical properties of the anion can result
in significant changes in biological activity.7-14 To study
this, we prepared each complex as the [BF4]-, [PF6]-, and
[BPh4]- salts. The complexes were prepared through a
combinatorial synthetic approach (Scheme 1), yielding a
small focused library of functional groups incorporated on
the arene ring, including monosubstituted esters (1a-c),
ketones (2a-c), alkyl side chains (3a-c), and amines (4a-c)
(a ) [BF4]-, b ) [PF6]-, c ) [BPh4]-). Synthetic details
and compound characterization (nuclear magnetic resonance,
electrospray ionization mass spectrometry, Fourier transform
infrared spectroscopy, microanalysis, and X-ray crystal-
The significant involvement of metal ions and complexes
in biological processes and systems has, over recent times,
led to the realization that considerable scope exists for the
design of metal-based therapeutics.1 While considerable work
in this area has been in the field of coordination complexes,
the large diversity of structure and unique bonding modes
of organometallic complexes also suggests that these systems
may find use as therapeutic agents.2
Recently, ruthenium(II) organometallic complexes have
been gaining popular interest as potential anticancer agents.3,4
The Ru(II) arene half-sandwich complexes [(R-Ph)Ru(Y-
Z)L] (R-Ph ) substituted arene, Y-Z ) bidentate ligand, and
L ) monodentate anion) have proven to be potent cytotoxic
agents against a range of tumor cell lines while retaining
good stability and aqueous solubility.3 The RAPTA series
of compounds [(R-Ph)Ru(YZ)PTA], which incorporate a
1,3,5-triaza-7-phospha-adamantane (PTA) ligand and two
monodentate ligands (YZ), have proven to be effective
antimetastatic agents with comparable biological effects to
the Ru(III)-based antimetastatic drug NAMI-A.4
The promising biological effects displayed by both of these
classes of Ru(II) organometallic complexes both in Vitro and
* Author to whom correspondence should be addressed. Phone: +61 07
373 57728. E-mail: michael.williams@griffith.edu.au.
†
Griffith University.
Queensland Institute of Medical Research.
‡
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1531. (b) Farrell, N. Bioorganometallics; Jaouen, G., Ed.; Wiley-VCH:
Weinheim, Germany, 2005.
(2) Cohen, S. M. Curr. Opin. Chem. Biol. 2007, 11 (2), 115–120.
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Comm. 2005, 38, 4764–4776. (b) Dougan, S. J.; Sadler, P. J. Chimia
2007, 61 (11), 704–715.
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(b) Dyson, P. J. Chimia 2007, 61 (11), 698–703.
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10.1021/ic801159f CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 19, 2008 8589
Published on Web 09/11/2008

COMMUNICATION
Scheme 1. Synthesis of the Ru(II) Arene Cp* Sandwich Complexes
lography for 2c) are supplied within the Supporting Informa-
tion of this communication. An ORTEP plot of complex 2c
is presented in Figure 1.
The cytotoxicities of 1-4 (Table 1) were investigated
using a sulphorhodamine B colorimetric assay of cell number
following drug treatment in microlite wells for 6 days.15 The
cell lines chosen for study were MCF7 (hormone-dependent
breast cancer), MDA-MB-231 (hormone-independent breast
cancer), MM96L (human melanoma), and normal human
cells (NFF, neonatal foreskin fibroblasts). Each of these cell
lines is susceptible to a variety of applied chemotherapeutics
and also displays different mechanisms of cross-resistance
to such chemotherapies.
Our results indicate that each of the complexes, 1-4, is
biologically active with respect to all three tumor cell lines,
while displaying a moderate level of selectivity toward NFF
(Table 1, Figures 2 and 3). The ammonium salts of the
counterions were also screened for toxicity. Each anion
obtained IC50 values >1000 µM against all four cell lines
and was considered nontoxic when not in the presence of
the organometallic cation.
Selectivity of the complexes was found to be dependent
on the nature of the monosubstituted arene functional group
with the counterion prompting little to no difference (Figure
3). The most selective functional groups were the nonpolar
monosubstituted methyl complex 3 and the monosubstituted
amine complex 4. In relation to the MM96L human
melanoma cell line, this series of complexes achieved
selectivity ratios of 34.9 (3a), 35.1 (3b), 40.4 (3c), 30.0 (4a),
20.5 (4b), and 28.1 (4c), respectively. These results can be
compared to cisplatin, which achieved a selectivity ratio of
only 1.94 against the same tumor cell line.
Figure 1. Representative view of the Ru(II) propiophenone Cp* tetraphe-
nylborate complex (2c), anion omitted for clarity.
The results show that the complexes incorporating the
tetraphenylborate (TPB) anion (1c-4c) are significantly more
toxic than those with the tetrafluoroborate (1a-4a) and
hexafluorophosphate (1b-4b) anions. The TPB organome-
-
tallic salts are nearly 3 times more active than their BF4
and PF6- counterparts against all three of the tumor cell lines
investigated and yielded IC50 values comparable in mag-
nitude to that of cisplatin (Table 1).
It is postulated that hydrophobic interactions may occur
between the arene ligand of the organometallic cation and
the aromatic hydrocarbons of TPB. These interactions could
prompt strong ion pairing, allowing the TPB to potentiate
transport of the organometallic cation across the cell
membrane and into various organelles within the cell.
Ion-pair formation of this kind has been proposed to
account for a number of physiochemical phenomena in which
lipophilic anions modulate the lipid solubility of cationic
species and vice versa.7-14,16-26
Studies carried out on a range of structurally diverse
organic hydrophobic amines such as 1-methyl 4-phenyl
1,2,3,6-tetrahydropyridine (MPTP),8-12 4,4′-diethylamino-
ethoxyhexestrol,13 and 2-[3-chloro-8-(4-chlorophenyl)-1,7-
diazabicyclonona-2,4,6,8-tetraen-9-yl]-N,N-dipropyl-aceta-
mide (Alpidem)14 concluded that TPB was capable of
modulating both drug uptake and toxicity through hydro-
phobic ion-pairing interactions.8-14 These studies also found
Toxicity of the organometallic complexes is shown to
change not only with variation of the monosubstituted arene
ligand but also with variation of the counterion (Figure 2).
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J. Neurochem. 1990, 54, 743–750.
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Nahamura, Y. Neurochem. Int. 1988, 12, 525–531.
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(21) Boroukerdi, M. Drug DeV. Ind. Pharm. 1987, 13, 181–191.
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U. Pharmazie 1987, 42, 393–394.
(13) Berson, A.; De Beco, V.; Letteron, P.; Robin, M. A.; Moreau, C.; El
Kahwaji, J.; Verthier, N.; Feldmann, G.; Fromenty, B.; Pessayre, D.
Gastroenterology 1998, 114, 764–774.
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205–215.
(25) Graefe, U.; Stengel, C.; Moellmann, U.; Heinisch, L. Pharmazie 1994,
49, 343–346.
(14) Berson, A.; Descatoire, V.; Sutton, A.; Fau, D.; Maulny, B.; Vadrot,
N.; Feldmann, G.; Berthon, B.; Tordimann, T.; Pessayre, D. Phar-
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8590 Inorganic Chemistry, Vol. 47, No. 19, 2008

COMMUNICATION
Table 1. Inhibitory Concentration That Limits Proliferation by 50% (IC50) and Drug Selectivity Ratios of Ru(II) Arene Cp* Sandwich Complexes
against a Range of Cancer Cell Lines and a Normal Human Cell Line
IC50 values (µM)a
selectivity ratiosb
complex
#
MDA-MB-
IC50 (NFF)/
IC50 (MCF7)
IC50 (NFF)/IC50
(MDA-MB-231)
IC50 (NFF)/
IC50 (MM96L)
MCF7
231
MM96L
NFF
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
5 -
cisplatin
12.0
14.3
2.33
11.3
16.8
3.00
8.55
13.6
4.99
13.1
10.9
4.73
1.80
16.9
17.8
3.36
32.0
29.5
9.14
13.7
20.8
5.20
24.9
30.9
12.0
N/A
7.60
6.15
2.54
6.76
7.18
2.85
4.62
4.53
2.28
6.10
9.30
3.50
1.70
48.4
55.6
10.6
134
136
32.7
161
159
92.2
183
191
98.2
3.30
4.03
3.89
4.55
11.9
8.10
10.9
18.8
11.7
18.5
14.0
17.5
20.8
1.83
2.86
3.12
3.15
4.19
4.61
3.58
11.8
7.64
17.7
7.35
6.18
8.18
N/A
6.37
9.04
4.17
19.8
18.9
11.5
34.9
35.1
40.4
30.0
20.5
28.1
1.94
a Errors within the range of (5-10% of the reported value. Results are the average of three separate experiments. b Selectivity ratios are the direct
comparison of drug cytotoxicity between the normal human cell line (NFF) and the annotated tumor cell line.
as increased the overall level of mitochondrial inhibi-
tion.10-12 This enhanced activity was considered to be a
consequence of TPB ion pairing with MPP+ to facilitate
penetration of the cytotoxic material into the mitochondrial
organelle as well as allow MPP+ access to hydrophobic
inhibition sites on NADH hydrogenase.12 Experiments car-
ried out in ViVo indicated that, although anion exchange
reactions would be expected, TPB successfully modulated
the dopaminergic neurotoxic effects of MPTP within male
SWISS-webster mice.9
The present results obtained on the prepared focused
library of ruthenium(II)-based organometallic complexes
indicate that TPB’s ability to potentiate the biological effects
of cytotoxic compounds is not limited to just organic
hydrophobic amines.
Figure 2. Complex IC50 values for the MM96L cell line. Comparison of
the effect of the arene ligand and counterion choice on cytotoxicity for the
BF4 (a), PF6 (b), and B(C6H5)4 (c) derivatives of the complexes (1-4).
In summary, this manuscript details the first recorded
biological study of Ru(II) arene Cp* ionic complexes. The
results show that these complexes exhibit potent antiprolif-
erative effects against all screened tumorigenic cell lines
while retaining moderate to good selectivity toward the
normal human cell line NFF. Cytotoxicity exhibited by the
complexes was found to be dependent on both the nature of
the arene ligand and the anionic counterion. In particular,
the large hydrophobic counterion tetraphenylborate was
shown to significantly increase the toxic effects exhibited
by the complexes in Vitro, with the TPB ionic salts achieving
IC50 values comparable to that of cisplatin. Of significance
is the potential that these organometallic complexes may hold
for optimization, both through the attachment of the RuCp*
moiety to an established aromatic therapeutic and also
through the incorporation of counterions that aid in both
complex solubility and transport.
Figure 3. Complex selectivity ratios witnessed against the MM96L cell
line. Comparison of the effect of arene ligand and counterion choice on
tumor specificity for the BF4 (a), PF6 (b), and B(C6H5)4 (c) derivatives of
the complexes (1-4).
that the toxicity of these hydrophobic amines arose from their
capacity to inhibit cellular mitochondria, and that TPB alone
exhibited little to no effect on the health of intact mitochon-
dria.7-14 Coadministration of TPB with each hydrophobic
amine, however, significantly increased both mitochondrial
drug uptake and subsequent inhibition of the organelle.8-14
The series of in Vitro and in ViVo studies carried out on
TPB and MPTP found that TPB highly potentiated the rate
of ATP depletion of neuroblastoma X glioma hybrid NG
108-15 cells induced by MPTP.8,9 TPB also accelerated the
onset of respiratory inhibition on intact mitochondria as well
Supporting Information Available: Experimental details and
spectroscopic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
IC801159F
Inorganic Chemistry, Vol. 47, No. 19, 2008 8591