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Potent Half-Sandwich Iridium(III) and Ruthenium(II) Anticancer Complexes Containing a P^O-Chelated Ligand
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Potent Half-Sandwich Iridium(III) and Ruthenium(II) Anticancer
Complexes Containing a P^O-Chelated Ligand
Qing Du,† Lihua Guo,*,† Meng Tian,† Xingxing Ge,† Yuliang Yang,† Xiyan Jian,† Zhishan Xu,†,‡
Zhenzhen Tian,† and Zhe Liu*,†
†
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Institute of Anticancer Agents Development and Theranostic Application, The Key Laboratory of Life-Organic Analysis and Key
Laboratory of Pharmaceutical Intermediates and Analysis of Natural Medicine, Department of Chemistry and Chemical
Engineering, Qufu Normal University, Qufu 273165, People’s Republic of China
‡
Department of Chemistry and Chemical Engineering, Shandong Normal University, Jinan 250014, People’s Republic of China
S Supporting Information
*
ABSTRACT: We herein report the synthesis, characterization, and anticancer activity of a series of iridium(III)
and ruthenium(II) half-sandwich complexes of the type
[(Cpx/arene)M(P^O)Cl]PF6 (M = Ir, Cpx = pentamethylcyclopentadienyl (Cp*) or its phenyl (Cpxph = C5Me4C6H5) or
biphenyl (Cpxbiph = C5Me4C6H4C6H5) derivatives; M = Ru,
arene = p-cymene (p-cym); P^O = phosphine phosphonic
amide ligand (PPOA)). The X-ray crystal structures of all
complexes, in which the ligand can form six-membered rings
with the metal center, have been determined. All of the
complexes show remarkable anticancer activities toward HeLa
and A549 cancer cells, activities which are higher than that of
the clinical anticancer drug cisplatin. The incorporation of
phenyl substituents on the Cp* ring for iridium(III) complexes results in little variation in their anticancer activities. These
results can be attributed to the combinatorial action of the metal and PPOA ligand. Hydrolysis and DNA cleavage are not the
major mechanisms of action. These complexes show potent catalytic activity in the transfer hydrogenation of NADH to NAD+.
Additionally, complexes [(η5-C5Me5)Ir(P^O)Cl]PF6 (1) and [(η6-p-cym)Ru(P^O)Cl]PF6 (4) arrest cell cycles at S and G2/M
phase and S phase, respectively. Complexes 1 and 4 both can induce apoptosis of HeLa cancer cells. Reactive oxygen species
(ROS) and mitochondrial membrane potential tests were also performed to explore the mechanism of action. When the
concentration of the complexes is increased, the amount of reactive oxygen species (ROS) increases dramatically and the
mitochondrial membrane potential decreases significantly in HeLa cancer cells. Overall, cell stress including cell cycle
perturbation, apoptosis induction, increase in ROS level, and loss of mitochondrial membrane potential contributes to the
anticancer potency of these complexes. Interestingly, the use of confocal microscopy provides insights into the microscopic
mechanism in which the typical and most active complex 1 can damage lysosomes. This type of complex represents a potent
platform for development of metal anticancer drugs.
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INTRODUCTION
The clinical success of platinum-based anticancer agents has
stimulated the exploration for other metal-based diagnostic and
chemotherapeutic drugs, which may be able to reduce side
effects, widen the spectrum of sensitive tumors, and overcome
platinum resistance.1 Among these metallodrugs, organometallic iridium(III) and ruthenium(II) complexes offer rich
versatility for the rational design of anticancer agents.2 In a
search for these complexes as potent chemotherapeutic
compounds, many different fixed ligands such as η5-C5Me5
and η6-arene have been used in combination with monodentate ligands3 and bidentate cationic or neutral C,N-,4
N,N-,5 N,O-,5a,6 P,P-,3c,7 and P,S-ligands.8 In particular,
iridium(III) and ruthenium(II) complexes containing phosphorus ligands have emerged as promising anticancer agents
and even are advancing to clinical trials. For example, complex
© XXXX American Chemical Society
I (Scheme 1) containing a monodentate amphiphilic
phosphorus ligand shows excellent antimetastatic and antiangiogenic behavior in vivo and is able to reduce the growth of
certain primary tumors.3a Our group has reported types of halfsandwich iridium(III) and ruthenium(II) complexes with 2,20bis(diphenylphosphino)-1,10-binaphthyl (BINAP) as a P,Pchelating ligand and showed that the cytotoxicity of the
complexes may be associated with the redox mechanism of
action (II, Scheme 1).7 Broomfield et al. also have highlighted
the potential utility of the bis-phospinoamine scaffold as an
easily tunable auxiliary ligand core in a logical design of
anticancer agents (III, Scheme 1).3c In addition, Ludwig et al.
have developed a series of half-sandwich ruthenium(II)
Received: June 11, 2018
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DOI: 10.1021/acs.organomet.8b00402
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Article
RESULTS AND DISCUSSION
Synthesis and Characterization of Complexes 1−4.
The phosphine phosphonic amide ligand was previously
synthesized and used as a supporting ligand for palladiumcatalyzed organic Suzuki−Miyaura cross-coupling reactions10
and ethylene polymerization.9 However, the iridium(III) and
ruthenium(II) complexes based on this ligand have never been
prepared or studied in bioinorganic chemistry. [(η5-C5Me5)IrCl2]2 (dimer 1), [(η5-C5Me4C6H5)IrCl2]2 (dimer 2), [(η5C5Me4C6H4C6H5)IrCl2]2 (dimer 3), [(η6-p-cym)RuCl2]2
(dmer 4), and phosphine phosphonic amide (PPOA) ligand
were synthesized according to previously reported procedures.9b,10,11 The reactions between the ligand PPOA and the
corresponding dimers were carried out in CH3OH at ambient
temperature and led to complexes 1−4 in 51−75% yields
(Scheme 2). These newly synthesized compounds were all
isolated as PF6− salts and characterized by 1H NMR, 13C
NMR, mass spectroscopy, and elemental analysis.
X-ray Crystal Structures. Crystals of complexes 1−4
suitable for X-ray diffraction were obtained from a slow
diffusion of diethyl ether into dichloromethane solutions of the
complexes at room temperature (Figure 1). X-ray crystallographic data are given in Table S1 (see the Supporting
Information), and selected bond lengths and angles are
summarized in Table S2 (see the Supporting Information).
As expected, these complexes adopt a pseudo-octahedral
“piano-stool” geometry with Cpx or p-cym acting as theseat
and the bidentate phosphine phosphonic amide ligand and a
monodentate chloride acting as the legs. In all cases, the metal
center is part of a six-membered (PO)Ir or (PO)Ru chelate
ring system. Generally, the Ir−P and Ru−P bond distances
(2.325−2.367 Å) are slightly longer than the Ir−O and Ru−O
bond distances (2.097−2.157 Å). The Ir−Cl bond distances
are 2.3925(12), 2.3913(16), and 2.394(2) Å for 1−3,
respectively, and the Ru−Cl bond distance is 2.3878(11) Å
for 4. No intermolecular π−π stacking in the unit cell is
observed in these crystal structures.
In Vitro Cytotoxicity. The cytotoxicities of complexes 1−4
toward HeLa human cervical cancer cells were investigated by
an MTT assay. The IC50 values (concentration at which 50%
of the cell growth is inhibited) after 24 h of exposure to the
compounds are shown in Table 1 and Figure 2. Interestingly,
these new complexes are highly potent toward the human
HeLa cervical cancer cell line and the lung cancer A549 cell
line with IC50 values of 1.2−3.8 and 4.4−7.5 μM, respectively.
The IC50 values of these complexes are much lower than the
values obtained with cisplatin against HeLa cells (1.2−3.8 μM
vs 7.5 μM) and A549 cells (4.4−7.5 μM vs 21.3 μM).
Scheme 1. Reported Half-Sandwich Iridium(III) and
Ruthenium(II) Anticancer Complexes I−IV Containing
Phosphorus Ligands and Our Current Work
complexes containing P,S-ligands (IV, Scheme 1). These
complexes displayed high biological potential even against
cisplatin-resistant tumor cell lines.8b On the other hand, P,Ochelating ligands are currently receiving much attention for
their versatile structures and wide applications in the field of
coordination chemistry and catalysis.9 The most distinguishing
feature of this class of electronically nonsymmetric neutral
ligand is the combination of a very strongly σ donating
phosphine moiety and a very weakly σ donating oxygen
moiety. These studies encouraged us to prepare a series of halfsandwich iridium(III) and ruthenium(II) complexes bearing
phosphine phosphonic amide ligands and explore their
anticancer activity and reactivity toward biomolecules.
For this work, the phosphine phosphonic amide ligand was
chosen to prepare new iridium(III) and ruthenium(II) halfsandwich complexes. These complexes have been systematically investigated for their chemical and biological reactivity
and cancer cell toxicity against HeLa and A549 cancer cells.
Flow cytometry experiments, including cell cycle, apoptosis
induction, ROS level, and mitochondrial membrane potential,
were carried out to explore the mechanism of action. In
particular, confocal microscopy was employed to investigate
the lysosome damage.
Scheme 2. Synthesis of Complexes 1−4
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Figure 1. X-ray crystal structures with atom numbering schemes for (a) complex 1, (b) complex 2, (c) complex 3, and (d) complex 4 with the
thermal ellipsoids drawn at the 50% probability level. The hydrogen atoms and PF6− anion have been omitted for clarity. Selected bond lengths
(Å): complex 1, Ir−C(centroid) = 1.814, Ir−O = 2.137(3), Ir−P = 2.3315(12), Ir−Cl = 2.3925(12); complex 2, Ir−C(centroid) = 1.817, Ir−O =
2.157(4), Ir−P = 2.3265(14), Ir−Cl = 2.3913(16); complex 3, Ir−C(centroid) = 1.828, Ir−O = 2.147(5), Ir−P = 2.325(2), Ir−Cl = 2.394(2);
complex 4, Ru−C(centroid) = 1.7018, Ru−O = 2.097(3), Ru−P = 2.3672(12), Ru−Cl = 2.3878(11).
Table 1. IC50 Values of Complexes 1−4 Tested toward Cancer and Normal Cell Lines and Comparison with Cisplatina
IC50 (μM)
complex
HeLa
A549
BEAS-2B
16HBE
[(η5-C5Me5)Ir(P^O)Cl]PF6 (1)
[(η5-C5Me4C6H5)Ir(P^O)Cl]PF6 (2)
[(η5-C5Me3C6H4C6H5)Ir(P^O)Cl]PF6 (3)
[(η6-p-cym)Ru(P^O)Cl]PF6 (4)
PPOA
dimer 1
dimer 2
dimer 3
dimer 4
cisplatin
1.2 ± 0.1
3.8 ± 0.2
1.6 ± 0.3
3.4 ± 0.4
>50
>50
>50
>50
>50
7.5 ± 0.2
4.9 ± 1.2
4.4 ± 0.4
7.5 ± 1.1
5.0 ± 0.1
>50
>50
>50
>50
>50
21.3 ± 1.7
1.3 ± 0.2
3.5 ± 0.1
2.8 ± 0.4
4.9 ± 1.2
b
1.9 ± 0.2
4.5 ± 0.2
3.0 ± 0.1
5.3 ± 0.9
Data are presented as means ± standard deviations. bNot determined.
a
Otherwise, the PPOA ligand and four precursor dimers show
very low cytotoxicity against HeLa cells and A549 cells (>50
μM). In particular, as far as we know, most of the cytotoxic
organometallic ruthenium(II) compounds generally tend to be
less potent than cisplatin.3b,12 However, the ruthenium(II)
complex 4 in this system displays a potency higher than that of
cisplatin, which can be attributed to the combinatorial action
of the metal and PPOA ligand. Further, in contrast to the
reported half-sandwich C,N and N,N-chelating iridium(III)
complexes,5b,13 the presence of the extended phenyl rings in
this system only slightly changes the anticancer activity of the
complexes. Generally, the hydrophobicity and intercalative
ability of extended cyclopentadienyl systems make the major
contribution to the anticancer activity of half-sandwich C,N
and N,N-chelating iridium(III) complexes.4d In this system,
the major role of the chelating ligand PPOA may have offset
Figure 2. Inhibition of the growth of HeLa and A549 cells by
complexes 1-4 and cisplatin.
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Figure 3. UV−vis spectra of the reaction of NADH (87 μM) with complexes 1−4 (0.8 μM) in 5% MeOH/95% H2O (v/v) at 298 K for 8 h: (a)
control: only NADH; (b) complex 1 + NADH; (c) complex 2 + NADH; (d) complex 3 + NADH; (e) complex 4 + NADH. (f) Turnover numbers
(TONs) of complexes 1−4.
complexes. Hence, more structural modification is necessary in
future work to decrease the cytotoxic action toward normal
cells without loss of the selectivity between cancer and normal
cells.
Hydrolysis Studies. Hydrolysis of M−Cl bonds is
considered to be an activation step for half-sandwich
transition-metal analogues.5a,14 M−OH2 aqua complexes are
often more reactive than the corresponding chloride complexes.5a For the investigation of aqueous stability, studies were
conducted by 1H NMR spectroscopy for complexes 1 and 4,
which were dissolved in 85% MeOD-d4/15% D2O (v/v) or in
70% DMSO-d6/30% D2O (v/v). The presence of methanol or
DMSO ensured the solubility of the complexes. The 1H NMR
spectra of 1 and 4 showed no obvious change over 24 h
(Figures S13−S16 in the Supporting Information). It is
possible that the aqua complex was obtained before the 1H
NMR spectra were acquired. Therefore, NaCl (32 mol equiv)
was then added to the solutions to further confirm the
hydrolysis of these complexes. There was also no change in the
the advantages of having the extended arenes on the Cp*.
Considering the aforementioned high anticancer activity of
ruthenium(II) complex 4, this is highly speculative at this
point. Furthermore, the investigated iridium(III) and
ruthenium(II) complexes have been shown to interfere with
mitochondrial activity (see below, in Effect on Mitochondrial
Membrane Potential (MMP)) so that the metabolization of
MTT can reflect an effect of metal complexes on the
mitochondrial metabolism rather than the viability of the
cell. Thus, a cell viability assay was also determined by
colorimetric assays based on neutral red (Table S3 in the
Supporting Information). There is no significant difference
from the results of the MTT assay, indicating that the MTT
test is only slightly affected by changes in the mitochondrial
metabolism.
The cytotoxicities of complexes 1−4 were further evaluated
against the two human bronchial epithelial normal cells BEAS2B and 16HBE (Table 1). Unfortunately, no selectivity was
observed for cancer cells versus normal cells with these
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Figure 4. Cell cycle analysis of HeLa cancer cells after 24 h of exposure to complexes 1 and 4 at 310 K. The concentrations used were 0.25 and 0.5
equipotent concentrations of IC50. Cell staining for flow cytometry was carried out using PI/RNase. (a) FL2 histogram for the negative control
(untreated cells) and complexes 1 and 4 with 0.25 and 0.5 equipotent concentrations of IC50. (b) Cell populations in each cell cycle phase for
control and complexes 1 and 4. Data are quoted as mean ± SD of three replicates.
1
Plasmid DNA Cleavage. In order to assess the DNA
cleavage ability of these complexes, plasmid pBR322 DNA (10
μM) was incubated with complexes 1 and 4 (0−100 μM) for
24 h and was monitored using agarose gel electrophoresis in a
buffer (40 mM Tris-HCl/1 mM EDTA, pH 8.3). However, no
DNA cleavage was observed for these complexes even at a
concentration of 100 μM (Figure S24 in the Supporting
Information). These results further confirm that DNA is not a
pharmacological target for these complexes.
Reaction with NADH. Coenzymes nicotinamide adenine
dinucleotide NADH and NAD+ play a crucial role in numerous
biocatalyzed processes. Previously, the Sadler group reported
that NADH can donate a hydride to aqua iridium cyclopentadienyl complexes and produce ROS in the form of H2O2,
thus providing a pathway to an oxidant mechanism of action.17
As a result, the catalytic transfer hydrogenation behavior of
complexes 1−4 with coenzyme NADH was also investigated.
To evaluate the real catalytic activity, we incubated 87 μM
NADH in a solution of 5% MeOH/95% H2O (v/v) as a
control. The reaction of complexes 1−4 (ca. 0.8 μM) with
NADH (87 μM) in 5% MeOH/95% H2O (v/v) was
monitored by UV−vis at 298 K after various time intervals.
It is simple to measure the conversion of NADH to NAD+ by
measuring the amount of UV absorption at 339 nm, as NADH
has an absorption at 339 nm while NAD+ does not.17 The
turnover numbers (TONs) of complexes calculated by the
amount of UV absorption at 339 nm (Figure 3) revealed the
following trends in catalytic activity: 1 (10.1), 2 (31.7), 3
(18.2), and 4 (7.7), 2 > 3 > 1 > 4. Despite no clear trend being
observed, it is apparent that the presence of a ruthenium metal
center seems to reduce the catalytic activity. As these
complexes can convert NADH to NAD+, the catalytic
performance may provide a potential pathway to induce
ROS and enhance the killing of cancer cells by an oxidant
mechanism of action.17c
H NMR spectra. These results suggest that the complexes
remained stable under these conditions.
It should be noted that some previously reported halfsandwich metal complexes may undergo Cl−/H2O exchange
more easily in dilute solutions with a higher relative content of
water, as is the case of cell culture.3b,15 As a result, the
hydrolysis behaviors of complexes 1 and 4 in 35% MeOH/65%
H2O (v/v) and complexes 2 and 3 in 10% MeOH/90% H2O
(v/v) were also monitored by UV−vis at 298 K. No
absorbance changes were observed in UV−vis spectra (Figure
S17 in the Supporting Information). Meanwhile, the hydrolysis
behaviors of complexes 1 and 4 in 30% DMSO/70% H2O (v/
v) or 30% DMSO/70% PBS (v/v) were also monitored by
UV−vis at 298 K. No absorbance changes were observed in
UV−vis spectra (Figures S18 and S19 in the Supporting
Information), which indicated that the hydrolysis also did not
occur when a high content of water was employed. As a result,
the stability studies suggest that the compounds have sufficient
stability for the preparation of samples for biological assays.
Interaction with Nucleobases. DNA often represents a
primary target site for many transition-metal anticancer
complexes.16 Thus, the binding of 9-methyladenine (9-MeA)
and 9-ethylguanine (9-EtG) to complexes 1 and 4 was studied.
The addition of 1 mol equiv of 9-MeA or 9-EtG to an
equilibrium solution of 1 and 4 (0.5 mM) in 85% MeOD-d4/
15% D2O (v/v) at 310 K resulted in no additional 1H NMR
peaks over a period of 24 h (Figures S20−S23 in the
Supporting Information), suggesting that no reaction with 9EtG and 9-EtA occurred in this system. In addition, the
formation of nucleobase adducts by these iridium(III) and
ruthenium(II) complexes was not detected by mass spectrometry. These results indicate that DNA may not be the main
target for these iridium(III) and ruthenium(II) P^O anticancer
complexes.
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Figure 5. Apoptosis analysis of HeLa cells after 24 h of exposure to complexes 1 and 4 at 310 K determined by flow cytometry using annexin VFITC vs PI staining. (a) HeLa cells left untreated (control) or treated with different concentrations of complexes 1 and 4 for 24 h. (b) Histogram
showing populations for HeLa cells in four stages treated by complexes 1 and 4. Data are quoted as mean ± SD of three replicates.
Figure 6. Analysis of ROS level by flow cytometry after HeLa cells were treated with complexes 1 and 4 at 0.25 and 0.5 equipotent concentrations
of IC50 for 24 h and stained with H2DCFDA. Data are quoted as mean ± SD of three replicates.
Cellular-Metal Accumulation. As iridium and ruthenium
are exogenous elements, the cellular uptake levels of Ir(III) and
Ru(II) can be quantitatively determined by inductively
coupled plasma mass spectrometry (ICP-MS). HeLa cells
were incubated with 5 μM complexes 1−4 for 6 h. The
intracellular metal content induced by incubation of these
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Figure 7. Changes in mitochondrial membrane potential of HeLa cancer cells induced by complexes 1 and 4. (a) Flow cytometry histograms of the
changes induced by the complexes 1 and 4 at concentrations of 0.25 × IC50, 0.5 × IC50, 1 × IC50, and 2 × IC50. (b) Populations of cells that exhibit
a reduction in the mitochondrial membrane potential. Data are quoted as mean ± SD of three replicates.
complexes is in the range of 78−90 ppb per 1 × 105 cells
(Figure S25 in the Supporting Information). In comparison to
complex 1, complex 3 showed a higher level of intracellular
iridium content (90 ppb per 1 × 105 cells vs 78 ppb per 1 ×
105 cells). This result is most likely due to the extended phenyl
rings in complex 3, which increase its lipophilicity and thus
enhance the cell membrane penetration. However, the
differences in the levels of cellular-metal accumulation are
small. This can be also attributed to the major role of the
PPOA ligand in this system.
Cell Cycle Arrest. Cell cycle arrest analysis for complexes 1
and 4 toward HeLa cells was also performed by flow cytometry
to confirm whether the induced cell growth inhibition was the
result of cell cycle arrest. The treatment of HeLa cells with
complex 1 at 0.5 × IC50 concentration led to mainly S and G2/
M phase arrest, where the percentages of cells increased from
25.5% and 10.0% to 31.6% and 14.8%, respectively, in
comparison to untreated cells. On the other hand, complex 4
blocked the cell cycle at S phase, where the percentages of cells
showed an increase from 17.0% to 25.6% under the same test
conditions (Figure 4 and Tables S4 and S5 in the Supporting
Information). These results suggested that the anticancer
mechanism of 1 and 4 on HeLa cells is dominantly S and G2/
M phase arrest in a concentration-dependent manner.
Apoptosis Assay. Apoptosis is a programmed cell death
process, and it has been reported to inhibit cell growth by
activating apoptosis for a large number of transition-metalbased anticancer drugs.18 Complexes 1 and 4 were selected for
further biological investigation on their mechanism of action.
In order to investigate whether the reduction in cell viability
observed in the MTT assay is based on apoptosis, HeLa cells
were treated with complexes 1 and 4 at 0.5, 1, 2, and 3
equipotent concentrations of IC50 for 24 h and then stained
with annexin V/propidium iodide and analyzed by flow
cytometry. This allowed determination of cell populations as
viable (unstained, only self-fluorescence), early apoptosis
(stained by annexin V only, green fluorescence), late apoptosis
(stained by annexin V and PI, green and red fluorescence), and
nonviable (stained by PI only, red fluorescence). As shown in
Figure 5 and Tables S6 and S7 (see the Supporting
Information), when HeLa cells were incubated with complexes
1 and 4 at 3 × IC50 concentration, totals of 71.9% and 95.4%
(early apoptotic + late apoptotic) of the cells were undergoing
apoptosis, respectively. Notably, the most apoptotic cells were
in late apoptosis stage. In addition, because active caspase 3 is a
common effector in several apoptotic pathways, it is a good
marker to detect apoptotic cells by flow cytometry.19 At a time
of 24 h after the treatment of complex 4, 11.3% of active
caspase 3-positive cells was detected, which was significantly
higher than the percentage (1.2%) in untreated HeLa cells
(Figure S26 in the Supporting Information). This result further
confirmed that these complexes can induce cell death mainly
through the apoptotic pathway.
ROS Determination. Reactive oxygen species (ROS) play
important roles in regulating cell proliferation, death, and
signaling, even in the mechanism of action of anticancer
agents.13 The levels of reactive oxygen species in HeLa cancer
cells induced by complexes 1 and 4 were determined by flow
cytometry analysis (Figure 6). When HeLa cancer cells were
exposed to complexes 1 and 4 for 24 h, increased and
concentration-dependent ROS total levels (combined levels of
H2O2, peroxy, hydroxyl radicals, peroxynitrite, and NO) in the
cells were observed. It should be noted that similar halfsandwich C,N-chelating iridium anticancer complexes generating the ROS H2O2 by catalytic hydride transfer from the
coenzyme NADH to oxygen has been reported previously.17c
In addition, as mentioned above, the conversion of NADH to
NAD+ by these complexes was also observed in this system. As
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a result, H2O2 production was further determined. We
observed an increase in the H2O2 levels in cells treated with
complex 1 or complex 4 in comparison to untreated cells
(Figure S27 in the Supporting Information). Moreover, in this
system, the main type of ROS was H2O2, which accounted for
ca. 60% of the total ROS levels. The increased ROS level for
these complexes played an important role in their anticancer
activity and thus was also deemed as the mechanism of action
in this system.
Effect on Mitochondrial Membrane Potential (MMP).
The mitochondrion is the bioenergetic center of the cell, and
more importantly, it is also an essential component of the
intrinsic apoptotic signaling pathway. Once the membrane
integrity of mitochondria is destroyed, the pro-death factors
are released from mitochondria and initiate the death signaling
cascade. In addition, mitochondrial dysfunction can induce cell
death. Mitochondrial dysfunction can be assessed by
measuring changes in mitochondrial membrane potential
(ΔΨm).20 Analysis of the ΔΨm values in HeLa cancer cells
after exposure to complexes 1 and 4 (at concentrations of 0.25,
0.5, 1, and 2 × IC50) was carried out by observing the
fluorescence of JC-1 (an ideal fluorescent probe widely used to
detect mitochondrial membrane potential) using flow
cytometry (Figure 7). When the concentration of complexes
1 and 4 was increased, the mitochondrial membrane potential
decreased significantly in HeLa cancer cells. For example, when
the concentration of the complex 1 was increased from 0.25 ×
IC50 to 2 × IC50, the percentage of cells with mitochondrial
membrane depolarization increased from 23.7% to 92.8%
(Tables S8 and S9 in the Supporting Information). The ζ
potentials of complex 1 (8.9 ± 0.6) and complex 4 (6.3 ± 0.5)
were positively charged, which could contribute to binding of
negatively charged mitochondria after entering the cytosol
(Figure S28 in the Supporting Information). As a result, these
complexes can induce cancer cell death through the
dysfunction of the mitochondrial membrane potential.
Lysosomal Damage. Acridine orange (AO) is an effective
probe used to measure the functional state of the acidic
organelles, due to its characteristic of emitting red fluorescence
at high concentrations in lysosomes and green fluorescence at
low concentrations in the cytosol and the nucleus.21 Therefore,
the lysosomal integrity of HeLa cells was observed by AO
staining. As shown in Figure 8, HeLa cells treated with acridine
orange (AO) alone displayed distinct red fluorescence in
lysosomes. However, the red fluorescence of AO significantly
decreased with an increase in drug concentration, suggesting
that lysosomal integrity was damaged under the treatment of
complex 1. It seems reasonable that, in accordance with the
behavior of some previously reported potent anticancer
agents,22 the introduction of nitrogen-containing groups
(−N(i-Pr)2) to these complexes may increase the total basicity
of the molecule and further result in the accumulation and
damage in acidic lysosomes. Therefore, we conclude that
complex 1 can induce apoptosis through lysosomal damage.
Figure 8. Observation of lysosomal disruption in HeLa cells loaded
with complex 1 for 12 h at 37 °C and then stained with acridine
orange (AO) (5 μM) at 37 °C for 15 min. Emission was collected at
510 ± 20 nm (green) and 625 ± 20 nm (red) upon excitation at 488
nm. Scale bar: 20 μm. The HeLa cells were treated with acridine
orange (AO), acridine orange (AO) and complex 1 (1× IC50),
acridine orange (AO) and complex 1 (2 × IC50), and acridine orange
(AO) and complex 1 (3 × IC50), respectively.
drug cisplatin. The presence of the extended phenyl rings in
this system only slightly changed the anticancer activity of the
complexes. These results can be attributed to the combinatorial action of metal and PPOA ligand. These types of
iridium(III) and ruthenium(II) complexes could be promising
candidates to build potential anticancer agents for future
cancer therapy. Several conclusions can be drawn from
chemical reactivity and biological activity studies for these
complexes.
(1) Hydrolysis and DNA cleavage were not the major
mechanisms of action. The 1H NMR spectra and UV−vis
spectra of these complexes showed that no hydrolysis occurs
and that the complexes remain stable under these conditions.
No nucleobase binding or DNA cleavage was observed for
these types of complexes under the conditions mentioned in
the experiment, suggesting that DNA is not a possible target.
(2) The mechanism of action (MoA) of these complexes is
attributed to cell stress including cell cycle perturbation,
apoptosis induction, increase of ROS level, and loss of
mitochondrial membrane potential. For example, complexes
1 and 4 arrested the cell cycle at S and G2/M phase, induced
obvious cell apoptosis and high increase in the level of ROS in
HeLa cancer cells, and aroused a decrease in mitochondrial
membrane potential.
(3) The generation of the ROS H2O2 in this system arises
from catalytic hydride transfer from the coenzyme NADH to
oxygen. In addition, these complexes may induce apoptosis
through lysosomal damage.
■
CONCLUSIONS
A series of PPOA-based half-sandwich iridium(III) and
ruthenium(II) anticancer complexes have been prepared and
fully characterized. The X-ray crystal structures of all
complexes, in which the ligand can form six-membered rings
with the metal center, have been determined. All the
complexes display higher potency toward HeLa and A549
human cancer cells in comparison to the clinical anticancer
H
DOI: 10.1021/acs.organomet.8b00402
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Article
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ASSOCIATED CONTENT
S Supporting Information
*
TThe Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organomet.8b00402.
Experimental section and Figures S1−S28 and Tables
S1−S9 as described in the text (PDF)
Accession Codes
CCDC 1565352, 1565354−1565355, and 1815607 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail for L.G.: guolihua@qfnu.edu.cn.
*E-mail for Z.L.: liuzheqd@163.com.
ORCID
Lihua Guo: 0000-0002-0842-9958
Zhe Liu: 0000-0001-5796-4335
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank the Shandong Provincial Natural Science Foundation (ZR2018MB023), the National Natural Science Foundation of China (Grant Nos. 21671118 and 21706147), the
Taishan Scholars Program, the Key Laboratory of Polymeric
Composite & Functional Materials of the Ministry of
Education (PCFM-2017-01), and the excellent experiment
project of Qufu Normal University (jp201705) for support.
We thank Juanjuan Li for stimulating discussions.
■
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