← Back
Highly Charged, Cytotoxic, Cyclometalated Iridium(III) Complexes as Cancer Stem Cell Mitochondriotropics.
DOI: 10.1002/chem.201803521
Communication
& Antitumor Agents
Highly Charged, Cytotoxic, Cyclometalated Iridium(III) Complexes
as Cancer Stem Cell Mitochondriotropics
Kristine Laws,[a] Arvin Eskandari,[a] Chunxin Lu,[a, b] and Kogularamanan Suntharalingam*[a]
drial DNA content, metabolic phenotype, intracellular ATP, reactive oxygen species (ROS) level, and mitochondrial membrane potential. Furthermore, independent studies showed
that CSCs have a higher mitochondrial mass than bulk cancer
cells, highlighting the significance of mitochondrial function to
CSC regulation.[6] Therefore, mitochondrial targeting could
offer a viable approach to remove CSCs. We recently reported
a metallopeptide containing dichloro(1,10-phenanthroline)
copper(II), a ROS-generating CSC-potent complex, affixed to an
established mitochondrial-penetrating peptides (MPPs), capable of selectively killing breast CSCs over bulk breast cancer
cells through mitochondrial dysfunction.[7] Although this metallopeptide exhibited promising in vitro CSC potency, potential
in vivo application is limited by the high reactivity of the copper(II)-phenanthroline warhead, and akin to other peptidebased chemotherapeutics, challenges relating to pharmacokinetics and clearance.[8] Targeting CSC mitochondria using
chemically inert small molecules could be a more effective and
translatable strategy.
Several iridium complexes possess impressive anticancer
properties.[9] Of note, organometallic iridium(III) complexes
bearing electron-rich pentamethylcyclopentadienyl ligands display promising potency against various cancer cell types (up
to sub-micromolar).[10] Organometallic iridium(III) complexes
with pyridocarbazole exhibit antiangiogenicity in vivo (zebrafish models) and photocytotoxicity against bulk cancer cells.[11]
Cyclometalated iridium(III) complexes with polypridyl ligands
have attracted a lot of attention as therapeutic and diagnostic
tools over the last decade, due to their high stability (low spin,
d6) and rich phosphorescent properties.[12] Certain cyclometalated iridium(III) complexes also act as effective photoinduced
singlet oxygen producers in bulk cancer cells.[13] According to
the large body of work already published on the anticancer
properties of iridium(III) complexes, the cell toxicity and associated mechanism of action is highly dependent on the coordinating ligands.[14] The coordinating ligands also dictate intracellular localization, with bulky, lipophilic ligands promoting mitochondrial accumulation.[15] Mitochondrial targeting is generally
achieved by lipophilic (log P > 1.7) and cationic (Z > 0) species.[16] Here, we have sought to exploit these determinants to
develop a series of intrinsically inert, lipophilic, and cationic cyclometalated iridium(III) complexes that can target and disrupt
mitochondrial function in breast CSCs. Specifically, we present
cyclometalated iridium(III) complexes with two charged 1methyl-2-(2-pyridyl)pyridinium ligands and a lipophilic polypyridine ligand, with an overall + 3 charge. As certain CSCs have a
higher mitochondrial load than bulk cancer cells and normal
Abstract: The cancer stem cell (CSC) toxicity and mechanism of action of a series of iridium(III) complexes bearing
polypridyl and charged 1-methyl-2-(2-pyridyl)pyridinium ligands, 1–4 is reported. The most effective complex (containing 1,10-phenanthroline), 3, kills CSCs and bulk cancer
cells with equal potency (in the micromolar range), indicating that it could potentially remove heterogenous
tumour populations with a single dose. Encouragingly, 3
also inhibits mammopshere formation to a similar extent
as salinomycin, a well-established anti-CSC agent. This
complex induces CSC apoptosis by mitochondrial membrane depolarization, inhibition of mitochondrial metabolism, and intracellular reactive oxygen species (ROS) generation. To the best of our knowledge, this is the first study
to investigate the anti-CSC properties of iridium complexes.
Cancer stem cells (CSCs) are a radiotherapy and chemotherapy
resistant subset of tumour cells found in many solid cancers
and leukaemias.[1] CSCs are believed to be partly responsible
for cancer relapse due to their inherent ability to initiate tumours and promote metastasis.[2] Current cancer therapies
work by removing bulk cancer cells, however, they are unable
to remove CSCs, which remain untouched and can seed secondary tumour growth.[3] Given the clinical implications of
CSCs it is imperative that future cancer therapies have the ability to remove bulk cancer cells and CSCs, so the possibility of
cancer reoccurrence is diminished. Certain CSC traits have
been recognised as potential therapeutic targets such as overexpressed cell surface proteins, deregulated cell signalling
pathways, and components within the CSC microenvironment;
nevertheless, a clinically effective anti-CSC agent remains elusive.[4] Recent studies on lung and breast cancer, and leukaemia populations revealed that some mitochondrial features
were distinctly different in CSCs compared to the bulk tumour
populations.[5] These include, but are not limited to, mitochon[a] K. Laws, A. Eskandari, Dr. C. Lu, Dr. K. Suntharalingam
Department of Chemistry, King’s College London, London SE1 1DB (UK)
E-mail: kogularamanan.suntharalingam@kcl.ac.uk
[b] Dr. C. Lu
College of Biological, Chemical Sciences and Engineering
Jiaxing University, Jiaxing 314001 (China)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.201803521.
Chem. Eur. J. 2018, 24, 1 – 7
These are not the final page numbers! ÞÞ
1
2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Communication
cells, compounds that can target mitochondria, such as the
cationic iridium(III) complexes presented here, have the potential to kill mitochondria-rich CSCs effectively.
The cyclometalated iridium(III) complexes, 1–4 (Figure 1 A)
used in this study were synthesised by refluxing [{Ir(m-Cl)(1methyl-2-(2-pyridyl)pyridinium)2}2][PF6]4 with > 3 equiv of the
porting Information). Selected bond lengths and bond angle
data are presented in Table S2. The complex exhibits a distorted octahedral geometry with the iridium centre coordinated to
two 1-methyl-2-(2-pyridyl)pyridinium ligands and one 4,4’-dimethyl-2,2’-bipyridine ligand, each forming a five-membered
chelated ring. The average Ir C (2.011 ) bond length is consistent with bond parameters for related octahedral iridium(III)
complexes.[17] As expected, the Ir N (2.136 ) bonds trans to
the cyclometalated bonds are longer than the Ir N bonds
(2.056 ) in the cis position.
The photophysical properties of 1–4 were studied in acetonitrile, dichloromethane, water, and PBS and are summarised
in Table 1 and Tables S3 and S4 (also see Figures S9–S16 in the
Supporting Information). The iridium(III) complexes, 1–4 displayed intense absorption bands between 250 and 300 nm,
Table 1. Absorbance, emission (lex = 450 nm), lifetime and quantum yield
data for 1–4 (50 mm) in acetonitrile.
Compound
lmax[nm] (e [m 1 cm 1])
lem [nm]
t [ms]
F [%]
1
223 (42 777),
255 (17 218),
298 (16 803),
312 (14 083),
356 (4912),
443 (1223)
527, 553
1.79
11.5
2
234 (45 490),
257 (32 653),
298 (31 838),
311 (27 762),
356 (8755),
446 (1951)
530, 558
1.86
13.9
3
226 (85 122),
270 (69 144),
290 (43 333),
354 (11 000),
442 (2624)
528, 549
2.15
17.3
4
223 (84 811),
288 (56 900),
378 (10 700),
445 (2110)
528, 553
2.08
22.5
Figure 1. (A) Chemical structures of the cyclometalated iridium(III) complexes, 1–4 investigated in this study. The charged iridium(III) complexes
were all isolated as hexafluorophosphate salts. (B) X-ray structure of 2. Ellipsoids are shown at 50 % probability, C in grey, N in light blue, and Ir in dark
blue. H atoms and hexafluorophosphate counter anions have been omitted
for clarity.
appropriate polypyridine ligand (2,2’-bipyridine, 4,4’-dimethyl2,2’-bipyridine, 1,10-phenanthroline, or 4,7-diphenyl-1,10-phenanthroline) and > 2 equiv of AgPF6 in 2-methoxyethanol/
water (2:1) for 3–5 days under nitrogen. Upon work up by
Celite filtration, evaporation and anion exchange using NBu4Cl
or NaCl, the resultant solids were purified by Sephadex column
using 0.012 mm NaCl acetone/water (1:1) solution and subsequently converted to the corresponding hexafluorophosphate
salt. The iridium(III) complexes, 1–4 isolated as yellow solids,
were characterised by 1H NMR, 13C NMR, high-resolution ESI
mass spectrometry, IR spectroscopy, elemental analysis and Xray crystallography (for 2) (see the Supporting Information, Figures S1–S4, CCDC 1836118).[25] Distinctive molecular ion peaks
corresponding to 1–4 with the appropriate isotopic pattern
were observed in the ESI mass spectra (m/z = 979.1384 a.m.u,
[1-PF6] + ; 1007.1726 a.m.u, [2-PF6] + ; 237.7271 a.m.u, [3-3PF6]3 + ;
1155.2108 a.m.u, [4-PF6] + ) (Figures S5–S8, Supporting Information), confirming formation of the desired products. The purity
of 1–4 was established by elemental analysis. The structure of
2 was further confirmed by X-ray diffraction studies and is depicted in Figure 1 B. Crystals of 2 suitable for an X-ray diffraction analysis were obtained by slow diffusion of diethyl ether
into a concentrated acetonitrile solution of 2 (Table S1, Sup&
&
Chem. Eur. J. 2018, 24, 1 – 7
www.chemeurj.org
which are tentatively assigned to p–p* and high energy metalto-ligand charge-transfer (MLCT) transitions involving both the
1-methyl-2-(2-pyridyl)pyridinium and corresponding polypyridine ligands. Weaker bands between 350 and 450 nm are assigned largely to MLCT (d–p*) transitions involving the 1methyl-2-(2-pyridyl)pyridinium ligand. The iridium(III) complexes, 1–4 exhibited green emission comprising of two broad
signals, upon excitation at 450 nm. The bands at 524–532 nm
and 548–560 nm correspond to 3LC (associated to the 1methyl-2-(2-pyridyl)pyridinium ligand) and 3MLCT transitions,
respectively. Solvent polarity (water/PBS > acetonitrile > dichloromethane) has a minor effect on emission wavelength,
but significantly influences emission intensity. All of the complexes have emission lifetimes in the microsecond region,
ranging from 1.79–2.08 ms (in acetonitrile) with mono-exponen2
2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Communication
tial decay kinetics (Figure S17), suggesting that emission originates from a triplet excited state. The quantum yield, F (in
acetonitrile) of the iridium(III) complexes increases in the following order; 1 (11.5 %) < 2 (13.9 %) < 3 (17.3 %) < 4 (22.5 %),
and is consistent with those reported for similar complexes
(Figure S18).[17b]
The lipophilicity of 1–4 was determined by measuring the
extent to which it partitioned between octanol and water, P.
The experimentally determined Log P values of 1–4 varied
from 0.86 to 1.42 (Table S5). The Log P values of 1–4 are
consistent with other mitochondriotropics (Log P > 1.7), suggesting that the iridium(III) complexes should be readily taken
up by cells and enter the mitochondrial matrix.[16, 18] UV/Vis and
high-resolution ESI mass spectroscopy studies were carried out
to assess the stability of 3 and 4, taken as representative members of the iridium(III) series, in biologically relevant solutions.
The UV/Vis p–p* and MLCT absorption bands of 3 and 4
(50 mm) in PBS:DMSO (200:1) remained unchanged over 24 h
at 37 8C (Figures S19 and S20). In PBS:DMSO (200:1) containing
5 equiv of ascorbic acid or glutathione, the UV/Vis absorption
of 3 and 4 (50 mm) remained unaltered over 24 h at 37 8C (Figures S21–S24). Under these conditions, peaks corresponding to
the molecular ion of 3 and 4 with the expected isotopic distribution (m/z = 1003.1290 a.m.u, [3-PF6] + ; 1155.2111 a.m.u, [4PF6] + ) were observed in the positive mode of the ESI mass
spectrum (Figures S25 and S26). Collectively, this suggests that
the iridium complexes are stable and remain intact under biologically reducing conditions. Before carrying out cellular studies, the stability of 1–4 in mammary epithelial cell growth
medium (MEGM) was investigated (Figures S27–S30). The UV/
Vis trace of 1–4 (50 mm) in MEGM/DMSO (200:1) displayed no
changes after incubation at 37 8C for 24 hours. Therefore 1–4
were deemed stable for cellular studies.
The cytotoxicity of 1–4 against bulk bone (U2OS), liver
(HepG2), and breast (HMLER) cancer cells, and breast CSC-enriched cells (HMLER-shEcad) was determined using the MTT
assay. The IC50 values were determined from dose–response
curves (Figures S31–S34) and are summarised in Table 2. The
1,10-phenanthroline- and 4,7-diphenyl-1,10-phenanthrolinebearing complexes, 3 and 4 displayed micromolar potency towards all of the cell lines tested, whereas 1 and 2 were relatively non-toxic. The potency of 3 and 4 towards CSC-enriched
HMLER-shEcad cells was comparable to salinomycin, an established breast CSC-potent agent.[19] Notably, 3 indiscriminately
killed HMLER-shEcad and HMLER cells, and exhibited 3.6- and
12.6-fold greater potency (p < 0.05, n = 18) for HMLER-shEcad
cells over U2OS and HepG2 cells, respectively. As a measure of
therapeutic potential, we determined the cytotoxicity of the
most potent complex, 3 towards normal skin fibroblast
GM07575 cells and mitochondria-rich human embryonic
kidney HEK 293T cells. The complex, 3 was less potent towards
GM07575 (IC50 value = 64.1 0.4 mm, Figure S35) and HEK 293T
cells (IC50 value = 22.1 1.6 mm, Figure S36) than HMLER and
HMLER-shEcad cells, indicating selective toxicity for breast bulk
cancer cells and breast CSCs over non-tumorigenic cells. Markedly, 3 displayed 12-fold higher potency for HMLER-shEcad
cells than GM07575 cells. Taken together, this suggests that 3
can potentially remove breast cancer cell populations in their
entirety (bulk cancer cells and CSCs) with a single dose, with
reduced toxicity towards bulk cancer and normal cells derived
from other tissues.
The iridium(III) complexes remain intact in MEGM over the
course of 24 h at 37 8C (Figures S27–S30), and thus cell toxicity
is unlikely to result from the free ligands. Nevertheless, the cytotoxicity values of all of the ligands (2,2’-bipyridine, 4,4’-dimethyl-2,2’-bipyridine, 1,10-phenanthroline, 4,7-diphenyl-1,10phenanthroline, and 1-methyl-2-(2-pyridyl)pyridinium hexafluorophosphate) were determined against HMLER-shEcad cells
(72 h incubation, Figure S37, Table S6). The neutral ligands displayed micromolar potency against HMLER-shEcad cells, increasing in the following order; 2,2’-bipyridine < 4,4’-dimethyl2,2’-bipyridine < 4,7-diphenyl-1,10-phenanthroline < 1,10-phenanthroline. Interestingly this trend is similar to that observed
for the corresponding iridium(III) complexes, 1–4 against
HMLER-shEcad cells. The positively charged ligand, 1-methyl-2(2-pyridyl)pyridinium hexafluorophosphate was non-toxic towards HMLER-shEcad cells (IC50 < 100 mm).
The capability of 1–4 to inhibit spheroid formation from
single-cell suspensions of CSC-enriched HMLER-shEcad cells
was investigated using the mammospheres formation assay.[21]
This method provides a reliable measure of CSC potency and
in vivo potential. The treatment of 3 (at a non-lethal dose, IC20
value for 5 days) to single-cell suspensions of HMLER-shEcad
cells dramatically reduced the number and size of mammospheres formed, comparable to salinomycin (Figure 2 and Figure S38). Dosage with 1, 2, and 4 (at a non-lethal dose, IC20
Table 2. IC50 values of 1–4, against U2OS, HepG2, HMLER, and HMLERshEcad cells, and HMLER-shEcad mammospheres.
Cmpnd
U2OS
IC50
[mm]
HepG2
IC50
[mm]
HMLER
IC50
[mm]
HMLERshEcad
IC50 [mm]
Mammosphere
IC50 [mm]
1
> 100
> 100
> 100
> 100
> 133
2
> 100
> 100
52.5 3.7 64.4 0.1 > 133
3
18.5 3.0 65.4 4.8 5.4 0.3
5.2 0.1
21.0 0.2
4
37.5 0.4 34.4 0.3 9.1 0.3
14.3 1.8 > 133
salinomycin n.d.
n.d.
11.4 0.4[a] 4.2 0.4[a] 18.5 1.5[a]
Figure 2. Representative bright-field images ( 10) of the mammospheres in
the absence and presence of 1–4 and salinomycin at their respective IC20
values.
[a] Taken from reference [20]. n.d. = not determined.
Chem. Eur. J. 2018, 24, 1 – 7
www.chemeurj.org
These are not the final page numbers! ÞÞ
3
2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Communication
value for 5 days) did not significantly affect the number of
mammospheres formed; however, the size of mammospheres
formed was markedly reduced (Figure 2 and Figure S38). To investigate the effect of 1–4 on mammosphere viability, the colorimetric resazurin-based reagent, TOX8 was used. Only the
1,10-phenanthroline-bearing complex 3 exhibited mammosphere potency, with an IC50 value (21.0 0.2 mm) similar to salinomycin (18.5 1.5 mm) under identical conditions (Figure S39). Given the cytotoxicity data in HMLER-shEcad monolayer systems (Table 2), it was unsurprising that 1, 2, and 4 displayed low mammosphere potency (IC50 > 133 mm) (Figure S39).
CSC uptake was determined by dosing HMLER-shEcad cells
with 1–4 (10 mm for 16 h at 37 8C) and measuring the iridium
content by ICP-MS (Figure S40). The iridium(III) complexes, 1–4
were readily taken up by HMLER-shEcad cells, with whole cell
uptake ranging from 44.5 0.1 ppb of Ir/million cells for 2 to
163.6 1.3 ppb of Ir/million cells for 4. To determine if CSC
uptake was temperature dependent (and thereby active or
passive), identical studies were conducted at 4 8C. HMLERshEcad cells incubated with 1–4 (10 mm for 16 h at 4 8C)
showed a significant decrease in uptake for 1 and 4 but not 2
and 3 (Figure S40). This indicates that the mechanism of CSC
uptake is active for 1 and 4, and passive for 2 and 3. It should
be noted that the iridium(III) complexes contain different polypyridyl ligands (2,2’-bipyridine, 4,4’-dimethyl-2,2’-bipyridine,
1,10-phenanthroline, or 4,7-diphenyl-1,10-phenanthroline).
Thus, although the structural differences between 1–4 is
subtle, their biological properties (such as cellular uptake pathway) can be drastically different. The varying modes of CSC
uptake for 1–4 could be partly responsible for the lack of correlation between cellular uptake and lipophilicity or CSC cytotoxicity. To determine the CSC localization of 3 (the most effective complex within the iridium(III) series) fluorescence microscopy studies were performed. Co-treatment of HMLER-shEcad
cells with the green-emitting iridium(III) complex 3 (10 mm for
24 h) and MitoTracker Deep Red FM (100 nm for 30 min), revealed a high level of overlap (Figure S41) suggesting that 3
localises in CSC mitochondria. The quality of the microscopy
images was somewhat compromised by the reduced lifetime
(0.80 ms) and quantum yield, F (4.7 %) of 3 in phenol-free
MEGM cell media (Figures S42 and S43). The imaging studies
suggests that 3 can potentially cause mitochondrial dysfunction in CSCs.
Mitochondrial damage can lead to detrimental effects on mitochondrial metabolism and mitochondrial membrane potential.[22] The ability of 3 to inhibit cytochrome c oxidase, an
enzyme responsible for reducing molecular oxygen to water in
the respiratory electron transport chain, was determined.
HMLER-shEcad cells dosed with 3 (10 mm for 24 h) displayed a
significant decrease (45 %, p < 0.01) in cytochrome c oxidase
activity compared to control cells (Figure 3 A), signifying mitochondrial respiration inhibition. Control studies with potassium
cyanide (5–10 mm for 10 min), an established cytochrome c oxidase inhibitor, also showed considerable inhibition (Figure 3 A). The integrity of the mitochondrial membrane potential of HMLER-shEcad cells upon treatment with 3 was probed
&
&
Chem. Eur. J. 2018, 24, 1 – 7
www.chemeurj.org
Figure 3. (A) Normalised cytochrome c oxidase activity in HMLER-shEcad
cells treated with 3 (10 mm for 24 h) and KCN (5–10 mm for 10 min). Error
bars = SD and Student t-test, ** = p < 0.01. (B) Representative 2D plots displaying the fluorescence emitted by JC-1 aggregates (red) and JC-1 monomers (green) by untreated HMLER-shEcad cells (top left), and HMLER-shEcad
cells treated with 3 (10 mm for 24 h, top right), 3 (20 mm for 24 h, bottom
left) and carbonyl cyanide m-chlorophenyl hydrazone (CCCP) (1 mm for 24 h,
bottom right). (C) Normalised ROS activity in untreated HMLER-shEcad cells
(control) and HMLER-shEcad cells treated with 3 (10 mm for 3–48 h). Error
bars represent standard deviations and Student t-test, * = p < 0.05,
** = p < 0.01.
using the JC-1 assay. HMLER-shEcad cells incubated with 3
(10–20 mm for 24 h) showed up to 30 % increase in the population of cells with depolarised mitochondrial membrane com4
2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Communication
pared to untreated control cells (Figure 3 B). A similar, but
more pronounced effect was observed for HMLER-shEcad cells
dosed with carbonyl cyanide m-chlorophenyl hydrazone
(CCCP) (1 mm for 24 h), a well-known mitochondrial membrane
depolarizer (Figure 3 B).
Disruption of regular mitochondrial processes can lead to an
increase in intracellular ROS levels.[23] The ROS levels in untreated HMLER-shEcad cells and 3-dosed (10 mm) HMLER-shEcad
cells was measured using 6-carboxy-2’,7’-dichlorodihydrofluorescein diacetate, a ROS indicator, over the course of 48 h. As
depicted in Figure 3 C, 3 steadily increases ROS levels from 3 h
(33 % increase) to 48 h (141 % increase). Similar time-dependent ROS generation has been reported for other mitochondrial-targeting agents. Control studies with HMLER-shEcad cells
dosed with H2O2 (100 mm for 3–48 h) also showed an increase
in ROS levels (up to 2.3-fold) (Figure S44). Cell viability studies
in the presence of NAC (2.5 mm, 72 h), a ROS scavenger,
showed that the potency of 3 towards HMLER-shEcad cells decreased significantly (IC50 value increased from 5.2 0.1 mm to
15.1 0.8 mm, p < 0.05) (Figure S45). This indicates that 3-mediated CSC death is related to intracellular ROS generation. Mitochondrial dysfunction and ROS elevation can trigger intrinsic
apoptosis, and thus an increase in the expression of active executioner caspases.[24] Immunoblotting studies showed that
HMLER-shEcad cells treated with 3 (2.5, 5, and 10 mm for 72 h)
expressed higher levels of cleaved caspase-3 and -7 than control cells, indicative of caspase-dependent apoptosis (Figure S46). Cytotoxicity studies in the presence of z-VAD-FMK
(5 mm), a potent peptide-based apoptosis inhibitor, showed
that the potency of 3 towards HMLER-shEcad cells decreased
significantly (IC50 value increased from 5.2 0.1 mm to 8.2
0.2 mm, p < 0.05) (Figure S47). This confirms that 3 induces caspase-dependent CSC death.
In summary, we report a series of CSC-potent, iridium(III)
complexes with high overall charge and mitochondrial targeting capabilities. The 1,10-phenanthroline-bearing complex 3
exhibited equal potency (in the micromolar range) towards
bulk breast cancer cells and breast CSCs, implying that it could
potentially remove heterogeneous breast cancer populations
with a single dose. Mechanistic studies show that 3 enters
breast CSCs, inhibits cytochrome c oxidase activity, promotes
mitochondrial membrane depolarisation, elevates intracellular
ROS, and triggers caspase-dependent apoptosis. This study,
highlights the expanding potential of mitochondrial-targeting
agents as anti-CSC agents and opens the door for the development of other iridium(III) complexes as CSC-active mitochondriotropics. Furthermore, in light of the findings reported in
this manuscript, the anti-CSC potential of mitochondrial targeting iridium(III) complexes previously reported in the literature
should be determined as they could provide promising antiCSC leads.
thanks the Natural Science Foundation of China (Grant
No.21401078) for financial support. We are grateful to Prof.
Robert Weinberg (Whitehead Institute, MIT) for providing the
cell lines used in this study. We thank Prof. Fiona Watt for insightful discussions on the biological studies.
Conflict of interest
The authors declare no conflict of interest.
Keywords: bioinorganic chemistry · cancer · cyclometallation ·
iridium · mitochondria
[1] a) L. V. Nguyen, R. Vanner, P. Dirks, C. J. Eaves, Nat. Rev. Cancer 2012, 12,
133 – 143; b) M. Dean, T. Fojo, S. Bates, Nat. Rev. Cancer 2005, 5, 275 –
284.
[2] a) J. Marx, Science 2007, 317, 1029 – 1031; b) D. R. Pattabiraman, R. A.
Weinberg, Nat. Rev. Drug Discovery 2014, 13, 497 – 512; c) Y. Yu, G.
Ramena, R. C. Elble, Front. Biosci. 2012, E4, 1528 – 1541.
[3] a) L. N. Abdullah, E. K. Chow, Clin. Transl. Med. 2013, 2, 3; b) J. Kaiser, Science 2015, 347, 226 – 229; c) K. Rycaj, D. G. Tang, Int. J. Radiat. Biol.
2014, 90, 615 – 621.
[4] a) K. Chen, Y.-h. Huang, J.-l. Chen, Acta. Pharmacol. Sin. 2013, 34, 732 –
740; b) X. Ning, J. Shu, Y. Du, Q. Ben, Z. Li, Cancer Biol. Ther. 2013, 14,
295 – 303.
[5] I. S. Song, J. Y. Jeong, S. H. Jeong, H. K. Kim, K. S. Ko, B. D. Rhee, N. Kim,
J. Han, World J. Stem Cells 2015, 7, 418 – 427.
[6] R. Lamb, G. Bonuccelli, B. Ozsvari, M. Peiris-Pages, M. Fiorillo, D. L.
Smith, G. Bevilacqua, C. M. Mazzanti, L. A. McDonnell, A. G. Naccarato,
M. Chiu, L. Wynne, U. E. Martinez-Outschoorn, F. Sotgia, M. P. Lisanti, Oncotarget 2015, 6, 30453 – 30471.
[7] K. Laws, G. Bineva-Todd, A. Eskandari, C. Lu, N. O’Reilly, K. Suntharalingam, Angew. Chem. Int. Ed. 2018, 57, 287 – 291; Angew. Chem. 2018, 130,
293 – 297.
[8] C. Rathi, B. Meibohm, Pharmacokinetics of Peptides and Proteins in Reviews in Cell Biology and Molecular Medicine, 2015.
[9] Z. Liu, P. J. Sadler, Acc. Chem. Res. 2014, 47, 1174 – 1185.
[10] a) J. M. Hearn, I. Romero-Canelon, B. Qamar, Z. Liu, I. Hands-Portman,
P. J. Sadler, ACS Chem. Biol. 2013, 8, 1335 – 1343; b) Z. Liu, A. Habtemariam, A. M. Pizarro, S. A. Fletcher, A. Kisova, O. Vrana, L. Salassa, P. C.
Bruijnincx, G. J. Clarkson, V. Brabec, P. J. Sadler, J. Med. Chem. 2011, 54,
3011 – 3026; c) Z. Liu, I. Romero-Canelon, B. Qamar, J. M. Hearn, A. Habtemariam, N. P. Barry, A. M. Pizarro, G. J. Clarkson, P. J. Sadler, Angew.
Chem. Int. Ed. 2014, 53, 3941 – 3946; Angew. Chem. 2014, 126, 4022 –
4027; d) S. J. Lucas, R. M. Lord, R. L. Wilson, R. M. Phillips, V. Sridharan,
P. C. McGowan, Dalton Trans. 2012, 41, 13800 – 13802; e) S. Wirth, C. J.
Rohbogner, M. Cieslak, J. Kazmierczak-Baranska, S. Donevski, B. Nawrot,
I. P. Lorenz, J. Biol. Inorg. Chem. 2010, 15, 429 – 440; f) J. M. Hearn, G. M.
Hughes, I. Romero-Canelon, A. F. Munro, B. Rubio-Ruiz, Z. Liu, N. O. Carragher, P. J. Sadler, Metallomics 2018, 10, 93 – 107; g) M. Gras, B. Therrien,
G. Sss-Fink, A. Casini, F. Edafe, P. J. Dyson, J. Organomet. Chem. 2010,
695, 1119 – 1125.
[11] a) A. Wilbuer, D. H. Vlecken, D. J. Schmitz, K. Kraling, K. Harms, C. P. Bagowski, E. Meggers, Angew. Chem. Int. Ed. 2010, 49, 3839 – 3842; Angew.
Chem. 2010, 122, 3928 – 3932; b) A. Kastl, A. Wilbuer, A. L. Merkel, L.
Feng, P. Di Fazio, M. Ocker, E. Meggers, Chem. Commun. 2012, 48,
1863 – 1865.
[12] a) D. L. Ma, S. Lin, W. Wang, C. Yang, C. H. Leung, Chem. Sci. 2017, 8,
878 – 889; b) K. K.-W. Lo, K. Y. Zhang, RSC Adv. 2012, 2, 12069 – 12083.
[13] a) R. Gao, D. G. Ho, B. Hernandez, M. Selke, D. Murphy, P. I. Djurovich,
M. E. Thompson, J. Am. Chem. Soc. 2002, 124, 14828 – 14829; b) S. Moromizato, Y. Hisamatsu, T. Suzuki, Y. Matsuo, R. Abe, S. Aoki, Inorg. Chem.
2012, 51, 12697 – 12706.
[14] R. Cao, J. Jia, X. Ma, M. Zhou, H. Fei, J. Med. Chem. 2013, 56, 3636 –
3644.
[15] a) V. Venkatesh, R. Berrocal-Martin, C. J. Wedge, I. Romero-Canelon, C.
Sanchez-Cano, J.-I. Song, J. P. C. Coverdale, P. Zhang, G. J. Clarkson, A.
Acknowledgements
K.L. and A.E. are supported by a KCL Ph.D. scholarship. K.S.
thanks the Leverhulme Trust for funding (ECF-2014-178). C.L.
Chem. Eur. J. 2018, 24, 1 – 7
www.chemeurj.org
These are not the final page numbers! ÞÞ
5
2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
Communication
[20] a) C. Lu, K. Laws, A. Eskandari, K. Suntharalingam, Dalton Trans. 2017,
46, 12785 – 12789; b) J. N. Boodram, I. J. McGregor, P. M. Bruno, P. B.
Cressey, M. T. Hemann, K. Suntharalingam, Angew. Chem. Int. Ed. 2016,
55, 2845 – 2850; Angew. Chem. 2016, 128, 2895 – 2900.
[21] G. Dontu, W. M. Abdallah, J. M. Foley, K. W. Jackson, M. F. Clarke, M. J.
Kawamura, M. S. Wicha, Genes Dev. 2003, 17, 1253 – 1270.
[22] X. Wang, Genes Dev. 2001, 15, 2922 – 2933.
[23] a) M. P. Murphy, Biochem. J. 2009, 417, 1 – 13; b) C. H. Wang, S. B. Wu,
Y. T. Wu, Y. H. Wei, Exp. Biol. Med. 2013, 238, 450 – 460.
[24] S. W. G. Tait, D. R. Green, Nat. Rev. Mol. Cell Biol. 2010, 11, 621 – 632.
[25] CCDC 1836118 (2) contains the supplementary crystallographic data for
this paper. These data are provided free of charge by The Cambridge
Crystallographic Data Centre.
Habtemariam, S. W. Magennis, R. J. Deeth, P. J. Sadler, Chem. Sci. 2017,
8, 8271 – 8278; b) K. Qiu, Y. Liu, H. Huang, C. Liu, H. Zhu, Y. Chen, L. Ji, H.
Chao, Dalton Trans. 2016, 45, 16144 – 16147; c) K. Xiong, Y. Chen, C.
Ouyang, R.-L. Guan, L.-N. Ji, H. Chao, Biochimie 2016, 125, 186 – 194;
d) L. He, K.-N. Wang, Y. Zheng, J.-J. Cao, M.-F. Zhang, C.-P. Tan, L.-N. Ji,
Z.-W. Mao, Dalton Trans. 2018, 47, 6942 – 6953; e) C. Caporale, C. A.
Bader, A. Sorvina, K. D. M. MaGee, B. W. Skelton, T. A. Gillam, P. J. Wright,
P. Raiteri, S. Stagni, J. L. Morrison, S. E. Plush, D. A. Brooks, M. Massi,
Chem. Eur. J. 2017, 23, 15666 – 15679.
[16] R. W. Horobin, S. Trapp, V. Weissig, J. Controlled Release 2007, 121, 125 –
136.
[17] a) B. J. Coe, M. Helliwell, J. Raftery, S. Sanchez, M. K. Peers, N. S. Scrutton,
Dalton Trans. 2015, 44, 20392 – 20405; b) B. J. Coe, M. Helliwell, S. Sanchez, M. K. Peers, N. S. Scrutton, Dalton Trans. 2015, 44, 15420 – 15423.
[18] R. A. J. Smith, C. M. Porteous, A. M. Gane, M. P. Murphy, Proc. Natl. Acad.
Sci. USA 2003, 100, 5407 – 5412.
[19] P. B. Gupta, T. T. Onder, G. Jiang, K. Tao, C. Kuperwasser, R. A. Weinberg,
E. S. Lander, Cell 2009, 138, 645 – 659.
&
&
Chem. Eur. J. 2018, 24, 1 – 7
www.chemeurj.org
Manuscript received: July 10, 2018
Revised manuscript received: July 26, 2018
Accepted manuscript online: July 27, 2018
Version of record online: && &&, 0000
6
2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Communication
COMMUNICATION
& Antitumor Agents
K. Laws, A. Eskandari, C. Lu,
K. Suntharalingam*
&& – &&
Iridium mitochondriotropics: A series
of iridium(III) complexes bearing polypridyl and charged 1-methyl-2-(2-pyridyl)pyridinium ligands, is reported and
their anti-cancer stem cell properties are
Chem. Eur. J. 2018, 24, 1 – 7
described in detail. The highly charged,
lipophilic compounds are shown to kill
breast cancer stem cells by disrupting
mitochondrial membrane potential and
mitochondrial respiration.
www.chemeurj.org
These are not the final page numbers! ÞÞ
7
Highly Charged, Cytotoxic,
Cyclometalated Iridium(III) Complexes
as Cancer Stem Cell
Mitochondriotropics
2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&