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Mitochondria-targeted spin-labelled luminescent iridium anticancer complexes.
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Cite this: Chem. Sci., 2017, 8, 8271
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Mitochondria-targeted spin-labelled luminescent
iridium anticancer complexes†
V. Venkatesh, ad Raul Berrocal-Martin, b Christopher J. Wedge, c
Isolda Romero-Canelón, ae Carlos Sanchez-Cano, a Ji-Inn Song, a
James P. C. Coverdale, a Pingyu Zhang, a Guy J. Clarkson, a
Abraha Habtemariam, a Steven W. Magennis, b Robert J. Deeth a
and Peter J. Sadler *a
Mitochondria generate energy but malfunction in many cancer cells, hence targeting mitochondrial
metabolism is a promising approach for cancer therapy. Here we have designed cyclometallated
iridium(III)
complexes,
containing
one
TEMPO
(2,2,6,6-tetramethylpiperidine-1-oxyl)
spin
label
[C43H43N6O2Ir1$PF6]c (Ir-TEMPO1) and two TEMPO spin labels [C52H58N8O4Ir1$PF6]c (Ir-TEMPO2).
Electron paramagnetic resonance (EPR) spectroscopy revealed spin–spin interactions between the
TEMPO units in Ir-TEMPO2. Both Ir-TEMPO1 and Ir-TEMPO2 showed bright luminescence with long
lifetimes (ca. 35–160 ns); while Ir-TEMPO1 displayed monoexponential decay kinetics, the biexponential
decays measured for Ir-TEMPO2 indicated the presence of more than one energetically-accessible
conformation. This observation was further supported by density functional theory (DFT) calculations.
The antiproliferative activity of Ir-TEMPO2 towards a range of cancer cells was much greater than that of
Ir-TEMPO1, and also the antioxidant activity of Ir-TEMPO2 is much higher against A2780 ovarian cancer
Received 23rd July 2017
Accepted 11th October 2017
cells when compared with Ir-TEMPO1. Most notably Ir-TEMPO2 was particularly potent towards PC3
DOI: 10.1039/c7sc03216a
human prostate cancer cells (IC50 ¼ 0.53 mM), being ca. 8 more active than the clinical drug cisplatin,
and ca. 15 more selective towards cancer cells versus normal cells. Confocal microscopy showed that
rsc.li/chemical-science
both Ir-TEMPO1 and Ir-TEMPO2 localise in the mitochondria of cancer cells.
Introduction
The clinical success of platinum complexes has aroused wider
interest in the design of metal-based anticancer drugs. The
major drawbacks of platinum drugs are their systemic toxicities
and development of resistance, now a clinical problem.1 New
metallodrugs that have different mechanisms of action are
needed. Recently we have explored the anticancer activity of
organometallic half-sandwich cyclopentadienyl iridium
complexes.2 They exert anticancer activity by mechanisms
which include DNA binding, induction of reactive oxygen
a
Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK. E-mail: P.J.
Sadler@warwick.ac.uk
b
School of Chemistry, WestCHEM, University of Glasgow, Glasgow G12 8QQ, UK.
E-mail: Steven.Magennis@glasgow.ac.uk
c
Department of Chemical Sciences, University of Hudderseld, Hudderseld HD1 3DH,
UK. E-mail: c.wedge@hud.ac.uk
d
Department of Inorganic and Physical Chemistry, Indian Institute of Science,
Bangalore-560012, India
e
School of Pharmacy, University of Birmingham, Edgbaston B15 2TT, UK
† Electronic supplementary information (ESI) available. CCDC 1522104. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c7sc03216a
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species (ROS), and catalytic oxidation.3–5 In an interesting study,
Wilbuer et al. reported organometallic iridium complexes that
inhibit protein kinase activity,6 and recently Mao's group reported the anticancer activity of cyclometallated iridium
complexes.7,8 They took advantage of the interesting photophysical properties of cyclometallated iridium complexes to
study their sub-cellular targets. Their bright luminescence with
high quantum yields, together with potent anticancer activity,
makes this class of compounds potential theranostic agents.
The same group utilised cyclometallated iridium complexes as
photosensitisers for pH-dependent singlet oxygen production.9
Nitroxides are stable free radicals, extensively used as spin
labels in electron paramagnetic resonance (EPR) spectroscopy.10
Nitroxides have unique antioxidant properties that mimic
superoxide dismutase (SOD) and catalase.11 The antiproliferative activity of 4-hydroxy/amino-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and their metal complexes is mainly
due to the induction of apoptosis through activation of multiple
caspases and their activity has been demonstrated in various
cancerous and non-cancerous cell lines.12–20 Nitroxides can
inhibit the growth of cancer cells selectively and so are attractive
candidates for chemotherapeutic drug design. Substituted
nitroxides can selectively target specic sub-cellular
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Fig. 1 (a and b) Molecular structures of Ir-TEMPO1 (top) and Ir-TEMPO2 (bottom); (c and d) X-band EPR spectra of Ir-TEMPO1 and Ir-TEMPO2,
as 0.1 mM solutions in deoxygenated DCM. Spectral simulations were performed using EasySpin (see ESI† for details); (e) X-ray crystal structure of
Ir-TEMPO2 showing the close proximity of the two nitroxide radicals (N–N distance 8.63 Å). Hydrogen atoms and counter anion are omitted for
clarity, and thermal ellipsoids are drawn at 50% probability.
compartments, for example mito-TEMPO has a nitroxide
attached to triphenylphosphonium chloride that selectively
localises in mitochondria and traps ROS.21 Mitochondria play
crucial roles in many important biological processes, including
ATP production, calcium homeostasis, and redox signalling.
Mitochondrial dysfunctions are involved in many pathological
diseases such as cancer, cardiovascular and neurodegenerative
diseases. There is growing interest in targeting mitochondria in
cancer cells with therapeutic intervention since they are known
to be defective in their function.22
Here we report the design of novel mono and bis TEMPO
(2,2,6,6-tetramethylpiperidine-N-oxyl)-substituted cyclometallated
iridium complexes (Ir-TEMPO1 and Ir-TEMPO2), shown in Fig. 1a
and b.
We veried the bi-radical structure of Ir-TEMPO2 by X-ray
crystallography. The presence of a spin–spin interaction
between the two TEMPO units in Ir-TEMPO2 was investigated
by EPR spectroscopy, luminescence spectroscopy and DFT
calculations. The anticancer activity of these complexes was
studied in a variety of cancer cell lines. The luminescence of the
complexes allowed us to study their sub-cellular localisation
using confocal microscopy.
Results and discussion
Synthesis and characterisation
The bipyridine ligands with nitroxide substituents (L1 and L2)
were synthesised by using amide-coupling reactions as shown
in Schemes S1 and S2.† Complexes Ir-TEMPO1 and Ir-TEMPO2
were prepared by reacting L1 and L2 with the chloride-bridged
phenylpyridine (N–C) cyclometallated iridium(III) dimer [Ir(N–
C)2(m-Cl)]2. Detailed synthetic procedures are in the ESI
(Schemes S3 and S4†). Ligands and complexes were
8272 | Chem. Sci., 2017, 8, 8271–8278
characterised by 1H NMR, HRMS, and elemental analysis
(details in the ESI†). The presence of free radicals in the ligands
and complexes causes paramagnetic relaxation that leads to
broadening of 1H NMR peaks. The 1H NMR resonances of L2
and Ir-TEMPO2 were much broader due to the presence of two
TEMPO groups (Fig. S1†). Furthermore, the formation of
Ir-TEMPO2 was veried by single-crystal X-ray diffraction
(Fig. 1e). The crystal structure determination showed that the
iridium adopts octahedral geometry in Ir-TEMPO2 with
a distance between the two nitroxyl radicals of 8.63 Å.
EPR studies
Continuous-wave EPR spectra of complexes Ir-TEMPO1 and IrTEMPO2 in solution are shown in Fig. 1c and d. In the case of
the Ir-TEMPO1, hyperne coupling to the ligand nitroxide 14N
gives an equal-intensity three line spectrum, with a slight
broadening of the high-eld line, characteristic of a nitroxide
radical undergoing tumbling in the fast motional regime.23,24
Similarly, three-line spectra were observed for the free ligands
L1 and L2 (Fig. S3†). A much more complex spectrum was
observed in the case of Ir-TEMPO2, with an additional six broad
lines resolved. This pattern indicates the presence of an intramolecular electron–electron exchange interaction, J, which
mixes the spin-states of the interacting radicals to generate
singlet and triplet manifolds.25,26
In the limit of strong exchange (J [ A) where A is the
hyperne coupling, the system is well described by a total spin
quantum number S ¼ 1. This means that only transitions in the
triplet manifold are observed and the spectrum is reduced to
a simple 1 : 2 : 3 : 2 : 1 quintet. Similarly for vanishingly small
exchange interaction (J A) the typical three-line spectrum of
a non-interacting nitroxide radical is seen. In the intermediate
exchange region, however, the state mixing induced by the
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hyperne interaction makes S a poor quantum number, and
a complicated spectral pattern emerges from the overlap of
een individual transitions, many of which are only partially
allowed.25 The resonance elds are strongly dependent on the
ratio of the exchange and hyperne interactions, and hence
spectral simulation permits the strength of the exchange
interaction to be determined in the case of a rigid biradical.
The presence of both broad and narrow lines in the spectrum
of Ir-TEMPO2 is an indication that the biradical species is not
rigid.25,26 Three of the transitions in the biradical are found at
eld positions that are independent of the magnitude of the
exchange interaction, giving narrow lines at the eld positions
seen for a mono nitroxide species. As the eld positions of the
remaining transitions are dependent upon the exchange interaction, motional uctuations tend to broaden the observed line
shape. To produce a full spectral simulation is therefore nontrivial, requiring the motional dynamics to be considered. The
interplay of variation in the electron exchange magnitude and
rates of conformational dynamics is such that even in a full
dynamic simulation, it is difficult to obtain a unique solution
when tting a single spectrum.27 Such an attempt was therefore
not made, but as seen in Fig. 1d, reasonable spectral simulation
was nevertheless obtained simply as a weighted sum of just two
components having J ¼ 0 and J ¼ 43 MHz (J/A z 1).26 It is
however not possible to use this information to place rm
bounds on the possible intermolecular approach distances
between the two radicals. Simulation details are in the ESI
(Fig. S5†).
Whereas an appreciable exchange interaction is observed in
the case of Ir-TEMPO2, the typical three-line spectrum seen for
L2 (Fig. S3†) implies no signicant exchange interaction. This is
further supported by the observation of ne structure arising
from partially resolved proton hyperne couplings, showing
that exchange broadening is absent in this system. These
observations imply that metal binding must induce a conformational change in the ligand which permits closer approach of
the nitroxide groups, assuming that the exchange interaction is
dominated by through-space rather than through-bond
coupling.
Photophysical properties
The absorption spectra of Ir-TEMPO1 and Ir-TEMPO2 were
recorded in aerated DMSO, DMF and methanol and were
similar in each solvent (Fig. 2). The strong absorption band
around 260 nm (Fig. 2a and b) is assigned as a spin-allowed
ligand-centred (1LC) transition, the absorbance at 380 nm is
assigned as a 1MLCT transition and the weaker feature near
480 nm is assigned as a spin-forbidden 3MLCT. Both complexes
show similar structured emission from 525 nm to beyond
700 nm in aerated DMSO, DMF and methanol solutions,
following excitation at 480 nm (the extinction coefficient at
480 nm is ca. 1000 M1 cm1 for both samples in all three
solvents). We attribute this broad emission to transitions from
3
MLCT states, which would agree with previous studies of
cyclometallated iridium(III) complexes.28–38 For Ir-TEMPO1, the
emission has a peak around 630 nm and there is a shoulder
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Fig. 2 (a, b) UV-vis absorbance spectra; (c, d) uncorrected emission
spectra of Ir-TEMPO1 and Ir-TEMPO2 in DMSO, DMF and methanol
solutions (insets in c, d show the emission of Ir-TEMPO1 and
Ir-TEMPO2 when excited with a 365 nm hand-held UV lamp in
dichloromethane). The small feature around 560 nm in the emission
spectra is due to Raman scattering from the solvent.
near 700 nm. The emission spectra for Ir-TEMPO2 are similar
but the peak is red-shied by ca. 10 nm. This red shi in
emission can be clearly distinguished visually with excitation at
365 nm using a hand held UV lamp (Fig. 2c and d inset). The
emission quantum yields (Table 1) range from 1–5% and vary
depending on the complex and the solvent; they are larger for IrTEMPO1 and the solvent trend is DMSO > DMF > methanol. The
emission detector's sensitivity drops off above 720 nm, causing
an apparent cut off in the long-wavelength region (see ESI† for
details). Therefore, the quantum yields that we report represent
lower limits.
Luminescence lifetimes were determined for the same
solutions using time-correlated single photon counting
(TCSPC). When measured at the peak of the emission spectrum
between 630–650 nm, Ir-TEMPO1 displays a mono-exponential
decay in the three different solvents (DMSO, DMF and methanol), while Ir-TEMPO2 shows a bi-exponential decay in each
solvent (Table 1). Both samples have long-lived excited-states
with lifetimes ranging from 33–155 ns, typical of 3MLCT emission (Table 1). Heterogeneity in the photophysics of organic
molecules is usually attributed to a variation in the molecule's
local environment, conformational change, protonation or
other well-dened structural changes. In contrast, heterogeneous decays and spectra for a range of organometallic
complexes have oen been ascribed to dual emission e.g. from
two localised MLCT states.39 As discussed in a recent comprehensive review,40 dual emission appears to be a feature of the
photophysics of most organometallic complexes, albeit not
always in solution under ambient conditions. Furthermore, the
“dual” emission usually appears to be due to more than two
states or a continuum of states.40 The authors of the review
discuss in detail potential explanations for these ndings,
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Table 1
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Photophysical propertiesa of Ir-TEMPO1 and Ir-TEMPO2 in DMSO, DMF and methanol
Compounds (solvent)
s1 (ms)
A1 (%)
s2 (ms)
A2 (%)
c2
fem
Ir-TEMPO1 (DMSO)
Ir-TEMPO2 (DMSO)
Ir-TEMPO1 (DMF)
Ir-TEMPO2 (DMF)
Ir-TEMPO1 (MeOH)
Ir-TEMPO2 (MeOH)
0.124
0.155
0.096
0.099
0.042
0.047
100
5.45
100
33.4
100
12.0
—
0.067
—
0.062
—
0.033
—
94.6
—
66.6
—
88.0
0.97
0.98
1.02
1.00
1.00
0.96
0.053 0.0023
0.025 0.0013
0.037 0.0010
0.022 0.0005
0.013 0.0004
0.008 0.0004
a
Luminescence lifetimes (si), their amplitudes (Ai) and the c2 goodness of t parameter, and the emission quantum yields (fem) standard
deviation are shown. Excitation wavelengths were 480 nm (lifetime) and 470 nm (quantum yields); decays were recorded at the peak emission
wavelength. Results from repeat decay measurements were in good agreement, and the uncertainties in reported values of lifetimes and
amplitudes are #10%. Quantum yields are likely to be underestimated by ca. 10–20% (see ESI for details).
particularly in the context of charge transfer states and ion
pairing. It is also clear, however, that there are complexes that
show no or little evidence of dual emission.40 The dual emission
is most common for mixed-ligand complexes with asymmetric
ligands. Both complexes reported here are mixed-ligand species
with two ppy ligands and a bipy with one or two TEMPO units
attached. Interestingly, we nd that Ir-TEMPO1, which has the
asymmetrical L1 ligand displays a single-exponential decay,
while Ir-TEMPO2, where the L2 ligand is symmetrical, has
a clear double exponential decay. To investigate further the
origin of the two lifetimes, we performed a global analysis
where we recorded the decays of Ir-TEMPO2 as a function of
both excitation and emission wavelength. By exciting at 450, 480
and 510 nm and detecting at 580, 640 and 700 nm, we collected
a total of 9 decays (Table S1†). The global analysis involved
tting each decay with the same lifetimes, but allowing the
weights and values of the lifetimes to vary during the tting.
Such an analysis is an excellent method for revealing heterogeneity that is not apparent for an individual decay. In this case,
however, we found that the data can indeed be explained by only
two unique lifetimes. The global ts with two lifetimes are
excellent with a global c2 of 0.98 in contrast a global t with
a single lifetime gave a value of 6.0. In light of these data and the
preceding discussion, we attribute the bi-exponential decay of
Ir-TEMPO2 to the presence of two distinct conformations. The
two conformations are assigned as ground-state species
because the relative weights of the two decay components, at
a particular emission wavelength, changes with excitation
wavelength; these weights should be independent of excitation
wavelength for an excited-state reaction. Furthermore, a rise
time is oen seen at longer emission wavelengths for excited
state reactions, but none was observed here. Finally, we note
that the minor decay component for Ir-TEMPO2 has a lifetime
that is very similar to that of Ir-TEMPO1 in the same solvent,
suggesting that this results from a conformation in which the
TEMPO units in ligand L2 are independent of each other.
Computational study of Ir-TEMPO2
Lifetime measurements and EPR studies of Ir-TEMPO2 suggested the presence of more than one low-energy conformation.
In order to explore the other possible conformations, an ad hoc
ligand eld molecular mechanics (LFMM)41 force eld (FF) was
8274 | Chem. Sci., 2017, 8, 8271–8278
constructed to facilitate large stochastic searches. Favourable
conformations were then subjected to further DFT analysis. The
selection of ‘favourable’ conformations was based on the X-ray
conformation as conformation 1 and then three more conformations chosen from the LFMM results. Conformations were
selected within 7 kcal mol1 of the best LFMM energy with an
N/N distance <6 Å and a noticeably different conformation
compared to the best from LFMM. Table S6† displays the initial
DFT results. The larger, more accurate basis sets suggest that the
X-ray-like conformation leads to the lowest-energy structure
although conformation 4 is within a few kcal mol1. The structures of all four conformations at the BP/ZORA/SVP/D3/COSMO
level are shown in Fig. 3 and the Cartesian coordinates are in
the ESI.†
The presence of H-bonding between one of the TEMPO
groups and the neighbouring amide leads to a signicantly
shorter distance between the two nitroxyl radicals. The N/N
distance drops from ca. 9 Å to 7 Å from conformation 1 to
conformation 4. However, we could not rule out the possibility
that other conformations with low energies and different N/N
distances exist. To partially address this issue, a low-mode
molecular dynamics (MD) conformational search of just an
amido-TEMPO unit was undertaken to assess whether there
might be any other low-energy conformations available to IrTEMPO2. The three lowest-energy conformers are shown in
Fig. 3e and the relative energies were conrmed with DFT
calculations optimised at the B3LYP/SVP/COSMO (CH2Cl2) level
with TZVP single-point energies. Both molecular mechanics
(MM) and DFT suggest that rotation of the TEMPO group relative to the amide moiety might generate an energeticallyaccessible conformation for the Ir-TEMPO2. Inspection of
Fig. 3 indicates that an approximately 180 rotation of the
‘terminal’ TEMPO unit would generate a shorter N/N contact
only for conformation 4 (Fig. 3d).
The DFT-optimised structure is shown in Fig. 3f and the
Cartesian coordinates are in the ESI.† Not only does conformation 5 have a signicantly shorter nitroxyl–nitroxyl N/N
distance of only 5.7 Å, but it is also 2.1 kcal mol1 more stable
than conformation 1 at the BP/ZORA/TZVP/D3/COSMO
(CH2Cl2) level. The computational results indicate that there
is more than one accessible conformation for Ir-TEMPO2 and
that these conformers are likely to have signicantly different
N/N separations. The EPR signal could thus comprise
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(a–d) Structures of selected conformations of Ir-TEMPO2 optimised at the BP/ZORA/SVP/D3/COSMO level showing the nitroxyl–nitroxyl
N/N distance; (e) three lowest-energy conformers from low-mode MD conformational search for TEMPO-amide. Non-polar hydrogen atoms
are omitted for clarity; (f) DFT-optimised structure for lowest-energy Ir-TEMPO2 complex. The energies were calculated for these structures
using a larger, more accurate, basis set (TZVP).
Fig. 3
a component from a conformer with a large N/N separation
which yields a ‘normal’ three-line nitroxyl signal together with
one or perhaps more components where the N/N distance is
signicantly shorter leading to coupling between the two
nitroxyl units.
In vitro antiproliferative activity
The antiproliferative activity of Ir-TEMPO1, Ir-TEMPO2, and
cisplatin (CDDP) against A2780 human ovarian, cisplatinresistant A2780Cis human ovarian, A549 human lung and PC3
human prostate cancer cells was determined. Their selectivity
was also investigated by comparing their toxicity in cancer cells
with that towards the non-cancerous human lung broblast cell
line (MRC5). The IC50 values of Ir-TEMPO2 are ca. 2–30 lower
than for Ir-TEMPO1 in all the cell lines tested, suggesting
a signicant role for the TEMPO radical unit in anticancer
activity. Ir-TEMPO2 is 5 more active than cisplatin against
cisplatin-resistant A2780Cis ovarian cancer cells, showing no
cross-resistance with the platinum drugs and suggesting
a different mechanism of action. For comparison we also
determined the antiproliferative activity of the precursors [Ir(N–
C)2(m-Cl)]2, and [Ir(ppy)2(bpy)]+, as analogues without the
TEMPO units, and mono- and bis-TEMPO ligands L1 and L2.
The dimer and [Ir(ppy)2(bpy)]+ were inactive up to the concentrations tested (100 mM), and the ligands were only weakly active
(IC50 values 74–84 mM, Table S7†). Importantly, the complexes
Ir-TEMPO1 and Ir-TEMPO2 are more potent in all cell lines than
related free ligands. Furthermore, both complexes were less
toxic towards MRC5 normal lung broblasts (Table 2). IrTEMPO2 is highly active towards the PC3 prostate cancer cell
line with an IC50 value of 0.53 mM, 8 more potent than the
clinical drug cisplatin (CDDP), and importantly ca. 15-fold
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selective for this cancer cell line compared with non-cancerous
MRC5 cell line (Tables 2 and S7†).
Antioxidant activity
The antioxidant activity of complexes Ir-TEMPO1 and IrTEMPO2 was determined in A2780 ovarian cancer cells using
20 ,70 -dichlorouorescein diacetate (DCFH-DA) and ROS induction by hydrogen peroxide as well as the organic hydroperoxide
TBHP (tert-butyl hydroperoxide), as shown in Fig. 4. The uorescence based-experiment conrmed that, as expected, IrTEMPO1 and Ir-TEMPO2 do not induce intracellular ROS per se,
as there is no increased DCFH-DA uorescence upon drug
exposure. Furthermore, it also shows a reduction in uorescence when cells have been pre-treated with Ir-TEMPO1 and IrTEMPO2 for 24 h prior to induction of ROS by hydrogen
peroxide or TBHP. This reduction of uorescence is
concentration-dependent, as the values for cells treated with
equipotent IC50 concentrations of the TEMPO-appended
IC50 values for Ir-TEMPO1, Ir-TEMPO2, and cisplatin (CDDP)
against A2780 human ovarian, A2780Cis cisplatin-resistant human
ovarian, A549 human lung, PC3 human prostate cancer cell lines, and
normal MRC5 human lung fibroblast cell line. Corresponding values for
precursors can be found in the ESI (Table S7)
Table 2
IC50 (mM)
Cell lines
Ir-TEMPO1
Ir-TEMPO2
CDDP
A2780
A2780Cis
A549
PC3
MRC5
14.5 0.5
8.6 0.1
13.8 0.5
16.21 0.08
32.7 0.5
3.0 0.2
2.57 0.08
7.5 0.6
0.53 0.02
8.2 0.7
1.2 0.2
13.4 0.3
3.2 0.1
4.1 0.5
12.8 0.4
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Fig. 4 Antioxidant activity of Ir-TEMPO1 and Ir-TEMPO2 in A2780
ovarian cancer cells. (a) Comparison between the fluorescence of
20 ,70 -dichlorofluorescein diacetate (DCFH-DA) on its own or in the
presence of equipotent concentrations of Ir-TEMPO1 or Ir-TEMPO2.
(b, c) The relative fluorescence normalised to the controls of A2780
cancer cells stained with DCFH-DA when ROS has been induced with
hydrogen peroxide (575 mM, b) or tert-butyl hydroperoxide (TBHP)
(250 mM, c). In all cases: the concentrations of the complexes used
were equipotent with 1- or 2-fold of their IC50 concentrations in this
cancer cell line, drug exposure to the iridium complexes was 24 h prior
to staining, followed by 4 h of ROS induction. Fluorescence
measurements were carried out using excitation at 485 nm and
emission at 530 nm for DCFH-DA. Each measurement was carried out
in duplicates of triplicates and their statistical significance was determined using an independent two-sample t-tests with unequal variances, Welch's tests, (p < 0.01 for **, and p < 0.05 for *).
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Confocal microscope images of Ir-TEMPO1 and Ir-TEMPO2 in
PC3 prostate adenocarcinoma cells. (a) Co-localisation with MitoTracker Green FM (400 nM; 30 min at 37 C; lex ¼ 490 nm/lem ¼ 516
nm). (b) Co-localisation with LysoTracker Green DND-26 (50 nM;
30 min at 37 C; lex ¼ 504 nm/lem ¼ 511 nm).
Fig. 5
LysoTracker Green and MitoTracker Green conrmed that the
emission is observed only in the presence of complexes (Fig. S6†).
Mitochondrial membrane potentials
complexes are higher than those treated with 2-fold the IC50
values (Fig. 4). Consistent with the antiproliferative activity
results, complex Ir-TEMPO2 shows higher antioxidant activity
than Ir-TEMPO1, showing again the inuence of the TEMPO
radical in these complexes.
Aer conrming by optical imaging that complexes Ir-TEMPO1
and Ir-TEMPO2 localise highly in mitochondria, we investigated
their effects on the mitochondrial membrane potential. For
this, we carried out ow cytometry analysis of PC3 cells exposed
to equipotent concentrations of the complexes stained with
JC-10. This dye accumulates selectively in cellular mitochondria
Cell imaging studies
The strong luminescence of Ir-TEMPO1 and Ir-TEMPO2 allowed
us to study their sub-cellular targets in PC3 prostate adenocarcinoma cells by optical imaging. The cells were seeded in 8-well
microscopy chambers and le to attach for 48 h, and then treated
for another 4 h with equipotent IC50 concentrations of Ir-TEMPO1
(16 mM) and Ir-TEMPO2 (0.5 mM). Cells were then washed with
phosphate buffer saline (PBS) and fresh phenol-red-free medium
to remove the excess complexes that were not taken up by cells.
Confocal microscopy showed that Ir-TEMPO1 and Ir-TEMPO2
were taken up by PC3 prostate adenocarcinoma cells (lex ¼ 405
nm/lem ¼ 600 nm). Co-localisation experiments of Ir-TEMPO1 and
Ir-TEMPO2 using LysoTracker Green DND-26 and MitoTracker
Green FM revealed that both complexes were highly localised in
mitochondria, with only very minimal localisation in lysosomes
(Fig. 5). Control experiments with and without complexes,
8276 | Chem. Sci., 2017, 8, 8271–8278
Fig. 6 Ratio of cellular populations in Q3/Q2 of a flow cytometry dot
plot for PC3 cells exposed for 24 h to Ir-TEMPO1 and Ir-TEMPO2 and
treated with JC10 mitochondrial staining.
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as red uorescent aggregates. Upon mitochondrial membrane
potential changes, it leaks as a monomeric green uorescent
form. This experiment included cells treated with carbonyl
cyanide m-chlorophenyl hydrazone (CCCP) as positive controls
and untreated cells as negative controls, both of which were also
used for compensation purposes. In both cases, Ir-TEMPO1 and
Ir-TEMPO2 induced depolarisation of the mitochondrial
membrane. Flow cytometry dot plots establish four different
cellular populations: Q1, high red uorescence, Q2 high red and
green uorescence, Q3 high green uorescence and Q4 low
uorescence (Fig. S7†). The ratio between the cellular populations in Q3 and Q2 indicates cellular populations in which
the JC-10 dye has leaked from the mitochondria and has
generated high green uorescence as a response to changes in
the membrane potential. Ir-TEMPO1 induces similar changes to
the positive control CCCP (ratios of 10.4 and 10.5 respectively)
while Ir-TEMPO2 is capable of achieving a Q3/Q2 ratio of 37.2
(Fig. 6).
Conclusions
We have designed octahedral cyclometallated iridium(III)
complexes containing one and two TEMPO nitroxide spin
labels. They are highly luminescent with long emission lifetimes (33–155 ns). In the complex Ir-TEMPO2, unlike in the free
bisTEMPO ligand (L2), there is a communication between the
two nitroxide radicals. The EPR spectra (Fig. 1d) not only
showed evidence for strong spin–spin exchange interaction
between the nitroxide radicals, but also for their conformational exibility. Fluorescence lifetime measurements also
showed the existence of more than one energetically accessible
conformation for Ir-TEMPO2. A second conformation (in addition to that seen in the X-ray structure) with 2.1 kcal mol1 lower
energy was revealed by DFT calculations and involved in
hydrogen bonding between the amide of one TEMPO unit and
that of a neighbouring TEMPO (Fig. 3).
The anticancer activity of the bis-nitroxide complex IrTEMPO2 is 2 (A549 lung cancer cells) to 15 (PC3 prostate
cancer cells) higher compared to the one TEMPO containing
complex Ir-TEMPO1 (Table 2) showing an important role for the
TEMPO radical in the antiproliferative activity. Moreover, these
complexes are not cross-resistant with cisplatin in ovarian
cancer cells, and Ir-TEMPO2 is 7 more active than CDDP
against PC3 prostate cancer cells and 15 more selective versus
normal cells (MRC5 broblast cells). The antioxidant activity of
Ir-TEMPO1 and Ir-TEMPO2 was demonstrated in ovarian cancer
cells. Luminescence images showed that the complexes localise
in mitochondria of PC3 prostate cancer cells and ow cytometry
studies have conrmed that both iridium(III) complexes induce
changes in the mitochondria membrane potential of cancer
cells. Mitochondria are attractive as target sites for anticancer
drugs because they are known to malfunction in cancer cells.42
Conflicts of interest
There are no conicts to declare.
This journal is © The Royal Society of Chemistry 2017
Chemical Science
Acknowledgements
We thank the EPSRC and CRUK (grants EP/F034210/1, EP/
N007875/1 and C53561/A19933) the Royal Society (Newton
International Fellowship for VV), Chemical Computing Group
(provision of MOE for RJD), and WCPRS and Bruker Daltonics
(Studentship for J.P.C.C.) for support. We thank Dr Lijiang Song
and Philip Aston for their assistance with mass spectrometry,
and Matthew J. Te for help with ow cytometry.
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