1. • All the PEG complexes were basically noncytotoxic in the dark (IC50 values > 300 μM).
However, they became very cytotoxic upon irradiation (ex = 365 nm).
Photodynamic indices (ratio of the dark and light IC50 values) ranging from 12.9 to
88.2 μM have been determined for the PEG complexes.
• PEG-free complexes (the PEG pendant was replaced by an ethyl group) were also
prepared for comparison studies.
Structure of the Iridium(III) PEG Complexes
(PF6)
Ir
N
C
C
N
N
N
CH3
O
NH
O
O
CH3
n
MWPEG = 5 kDa
PDI = 1.08
N
C
C
N
N
N
Ir
CH3
H
N
O
O
O
CH3
n
+
N
C
C
N
N
N
Ir
CH3
H
N
O
O
O
CH3
n
+
S
S
N
C
C
N
N
N
Ir
CH3
H
N
O
O
O
CH3
n
+
OHC
OHC
N
C
C
N
N
N
Ir
CH3
H
N
O
O
O
CH3
+
Ph
Ph
Mitochondria-targeting Phosphorescent Cyclometalated
Iridium(III) Poly(ethylene glycol) (PEG) Complexes with
Tunable Photodynamic Activity
Steve Po-Yam Li, Chris Tsan-Shing Lau, Man-Wai Louie, Yun-Wah Lam, Shuk Han Cheng,
and Kenneth Kam-Wing Lo*
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue,
Kowloon, Hong Kong, P. R. China
Photodynamic therapy (PDT) is a well-established therapeutic modality for cancer
treatment that involves photochemical reactions mediated by the interaction of a
photosensitizer, light, and oxygen. Upon irradiation, the photosensitizer
generates highly reactive singlet oxygen (1O2) and other reactive oxygen species
(ROS) that cause cytotoxic effects to targeted cells.
We believe that the attachment of a poly(ethylene glycol) (PEG) pendant to
cyclometalated iridium(III) polypyridine complexes will minimize their unfavorable
interactions with intracellular biomolecules and organelles, whereas it is still
possible to induce cytotoxic activity upon irradiation. Also, we envisage that the
photo-induced cytotoxicity can be readily tuned using different cyclometalating
and diimine ligands.
Introduction
It has been well established that the excited states of
many phosphorescent transition metal complexes are
effectively quenched by molecular oxygen to
generate different ROS through electron transfer
and/or energy transfer. Although many inorganic and
organometallic complexes exhibit interesting
photoactivated biological activity, most of these
complexes still display high cytotoxicity in the dark.
• The  values were determined by steady-state irradiation (ex = 365 nm) of a
solution of the complex and 1,3-diphenylisobenzofuran (DPBF). The change of
absorbance at 418 nm was monitored over time.
• The  values increased in the order: ppy < pppy < pq < pba < bsn, which is in
accordance with their increasing excited-state lifetimes.
• The  values of the PEG complexes were slightly lower than those of the PEG-free
complexes. It is likely that the PEG pendant prevented (1) molecular oxygen from
getting close to the excited complexes and (2) the generated 1O2 from leaving the
PEG cage to react with DPBF.
Quantum Yields for 1O2 Production ()
Complex em [nm] [a] [s]
Ir-ppy-PEG 613 0.17
Ir-ppy-Et 611 0.16
Ir-pppy-PEG 603 0.24
Ir-pppy-Et 603 0.21
Ir-pq-PEG 570, 598 sh 0.31
Ir-pq-Et 567, 600 sh 0.29
Ir-pba-PEG 544, 576 sh 0.54
Ir-pba-Et 542, 577 sh 0.54
Ir-bsn-PEG 599 (max), 650, 709 sh 0.80
Ir-bsn-Et 598 (max), 650, 709 sh 0.78
[a] The lifetimes were measured at the emission maxima.
[b] Methylene blue ( = 0.52 in aerated DMSO) was used as a reference.
0.24
0.38
0.38
0.50
0.51
0.58
0.58
0.79
0.69
0.83

0 10 20 30 40 50 60
-0.16
-0.14
-0.12
-0.10
-0.08
-0.06
-0.04
-0.02
0.00
DPBF only
Ir-ppy-PEG
Ir-pppy-PEG
Ir-pq-PEG
Ir-bsn-PEG
Ir-pba-PEG
400 450 500 550 600 650
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Irradiation time / s
Absorbance(A.U.)
AA
Wavelength / nm
O
O
O
• The cells remained viable after they were incubated with the complex at 100 μM for
24 h.
• The high Pearson’s co-localization coefficients (> 90%) obtained from the co-staining
experiments indicated the mitochondria-targeting properties of the complexes.
Mitochondria-targeting Properties
2 h 5 h 10 h 24 h
5 M
50 M
100 M
time
conc.
Co-staining Studies
MitoTracker
Deep Red
Ir-pq-PEG
Overlaid
N
C
C
N
N
N
Ir
CH3
H
N
O
O
O
CH3
n
+
Ir-ppy-PEG
Ir-pppy-PEG Ir-pq-PEG Ir-bsn-PEG
Ir-pba-PEG
Photo-induced Cytotoxicity ( dark; light)
N
C
C
N
N
N
Ir
N
C
C
N
N
N
Ir
Ph
Ph
N
C
C
N
N
N
Ir
N
C
C
N
N
N
Ir
S
S
Pt
Cl NH3
Cl NH3
0
20
40
60
80
100
300150753819952
0
20
40
60
80
100
300150753819952
0
20
40
60
80
100
5 30015075381992
0
20
40
60
80
100
300150753819952
0
20
40
60
80
100
5 30015075381992
0
20
40
60
80
100
Cisplatin
30015075381995
PercentageofSurvivingcells/%
[complex] / M
IC50 = 23.2 M IC50 = 11.9 M
IC50 = 5.4 M IC50 = 3.4 M
IC50 > 300 M IC50 = 16.7 M
a b c
Detection of Oxidative Stress
d e
CellROX Deep Red
em = 660  20 nm
Brightfield
Irradiation
Ir-pq-PEG
H2O2
In vivo Visualization
DPBF 1,2-dibenzoyl
benzene
[complex] / M
We thank The Hong Kong Research Grants Council (Project Nos. CityU 102212 and
102311) for financial support.
Acknowledgements
Immediately after -injection: the intravascular loaded complex moved from the point of
injection (white arrow) to notochord, spinal cord (yellow arrow), and brain ventricle (red
arrow) via blood vessels.
24 h after -injection: the loaded complex gradually accumulated at the space around the yolk
sac and cardiac cavity (yellow) and the head space (red arrow) 24 h after loading.
Immediately
after -injection
(48 hpf)
24 h
after -injection
(72 hpf)
• Increased
intracellular
oxidative stress
has been
observed,
indicating that
the complexes
can act as
photosensitizers
for ROS.
N
C
C
N
N
N
Ir
OHC
OHC
1O2
e
Energy Transfer
(Type II)
Electron Transfer
(Type I)
3O2
Radicals,
OH