DNA-Intercalative Platinum Anticancer Complexes Photoactivated by Visible Light

Article abstract of DOI:10.1002/chem.202101168

Photoactivatable agents offer the prospect of highly selective cancer therapy with low side effects and novel mechanisms of action that can combat current drug resistance. 1,8-Naphthalimides with their extended π system can behave as light-harvesting groups, fluorescent probes and DNA intercalators. We conjugated N-(carboxymethyl)-1,8-naphthalimide (gly-R-Nap) with an R substituent on the naphthyl group to photoactive diazido Pt^IV complexes to form t,t,t-[Pt(py)₂(N₃)₂(OH)(gly-R-Nap)], R=H (1), 3-NO₂ (2) or 4-NMe₂ (3). They show enhanced photo-oxidation, cellular accumulation and promising photo-cytotoxicity in human A2780 ovarian, A549 lung and PC3 prostate cancer cells with visible light activation, and low dark cytotoxicity. Complexes 1 and 2 exhibit pre-intercalation into DNA, resulting in enhanced photo-induced DNA crosslinking. Complex 3 has a red-shifted absorption band at 450 nm, allowing photoactivation and photo-cytotoxicity with green light.

Full text of DOI:10.1002/chem.202101168

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doi.org/10.1002/chem.202101168
Chemistry—A European Journal
DNA-Intercalative Platinum Anticancer Complexes
Photoactivated by Visible Light
Huayun Shi,[a] Jana Kasparkova,[b] Clément Soulié,[c] Guy J. Clarkson,[a] Cinzia Imberti,[a]
Olga Novakova,[b] Martin J. Paterson,[c] Viktor Brabec,[b] and Peter J. Sadler*[a]
Abstract: Photoactivatable agents offer the prospect of highly
selective cancer therapy with low side effects and novel
mechanisms of action that can combat current drug resist-
ance. 1,8-Naphthalimides with their extended π system can
behave as light-harvesting groups, fluorescent probes and
DNA intercalators. We conjugated N-(carboxymethyl)-1,8-
naphthalimide (gly-R-Nap) with an R substituent on the
naphthyl group to photoactive diazido PtIV complexes to form
t,t,t-[Pt(py)2(N3)2(OH)(gly-R-Nap)], R=H (1), 3-NO2 (2) or 4-
NMe2 (3). They show enhanced photo-oxidation, cellular
accumulation and promising photo-cytotoxicity in human
A2780 ovarian, A549 lung and PC3 prostate cancer cells with
visible light activation, and low dark cytotoxicity. Complexes
1 and 2 exhibit pre-intercalation into DNA, resulting in
enhanced photo-induced DNA crosslinking. Complex 3 has a
red-shifted absorption band at 450 nm, allowing photoactiva-
tion and photo-cytotoxicity with green light.
Introduction
solubility, and a more intense and red-shifted ligand-to-metal
charge-transfer (LMCT) band compared with their cis isomers.[3b]
Notably, higher photo-cytotoxicity was also detected for the all
trans complexes with UVA irradiation. The replacement of NH3
by pyridine can also red-shift the LMCT absorption and improve
Phototherapy has attracted increasing attention in the clinic,
owing to its cancer selectivity and novel mechanisms of
action.[1] However, the oxygen-dependence of clinically ap-
proved photodynamic therapy (PDT) limits its applications in
hypoxic tumours.[1b] Photoactivated chemotherapy (PACT)
agents, in contrast, exert photo-cytotoxicity independently of
oxygen.[1b] Diazido PtIV complexes are promising PACT agents
that are dark-stable, but decompose upon irradiation to
generate DNA binding PtII species, azidyl radicals and ROS,
showing high photo-cytotoxicity.[2] The first diazido PtIV com-
the
photo-cytotoxicity.[3c]
Complex
trans,trans,trans-[Pt-
(py)2(N3)2(OH)2] (FM-190) shows photo-cytotoxicity with blue
light and is a promising candidate for further modification to
achieve higher photo-cytotoxicity, cancer selectivity, and red-
shifted activation.[2,3d] The conjugation of diazido PtIV complexes
to up-conversion nanoparticles (UCNPs) allows photoactivation
with near infrared light, but the longest excitation wavelength
for reported simple diazido PtIV complexes to exert photo-
cytotoxicity is 465 nm until now.[2,4] Longer wavelength excita-
tion allows deeper tissue penetration, and therefore potentially
more efficient photo-cytotoxicity in tissues.[5]
plexes
cis,trans-[Pt(en)(N3)2(OH)2]
and
cis,cis,trans-[Pt-
(NH3)2(N3)2(OH)2] reported in 2003 exhibited photo-cytotoxicity
upon irradiation with UVA light.[3a] Interestingly, PtIV complexes
with two azide ligands in trans positions have higher aqueous
1,8-Naphthalimides with their extended π-conjugated sys-
tem are well-known versatile fluorescent molecules that have
been widely used in anticancer treatment.[6] Substitution and
aromatic ring extension can influence the photochemical and
photophysical properties of 1,8-naphthalimides effectively.[6,7]
The highly potent naphthalimides Mitonafide and Amonafide,
have entered clinical trials for treatment of various tumours.[8]
The conjugation of 1,8-naphthalimides with anticancer com-
plexes enhances their cytotoxicity and enables cellular
imaging.[7,9] Pt complexes containing 1,8-naphthalimides exhibit
dual DNA binding modes and enhanced cytotoxicity.[10] 1,8-
Naphthalimides can also be photosensitisers which generate
ROS,[6c] thus improving photo-cytotoxicity when appended to
photoactive complexes.[11]
[a] Dr. H. Shi, Dr. G. J. Clarkson, Dr. C. Imberti, Prof. P. J. Sadler
Department of Chemistry,
University of Warwick
Coventry CV4 7AL (UK)
E-mail: p.j.sadler@warwick.ac.uk
[b] Prof. J. Kasparkova, Dr. O. Novakova, Prof. V. Brabec
Institute of Biophysics,
Czech Academy of Sciences
Kralovopolska 135, 61265 Brno (Czech Republic)
[c] C. Soulié, Prof. M. J. Paterson
Institute of Chemical Sciences
School of Engineering & Physical Sciences
Heriot-Watt University
Edinburgh EH14 4AS (UK)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202101168
In this work, we have synthesised and characterised three
photoactive diazido PtIV complexes with axial fluorescent 1,8-
naphthalimide ligands, trans,trans,trans-[Pt(py)2(N3)2(OH)(gly-R-
Nap)], R=H (complex 1), 3-NO2 (2) or 4-NMe2 (3; Figure 1). Since
the optical properties of 1,8-naphthalimides are sensitive to
© 2021 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided the
original work is properly cited.
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at around 340 nm is observed for 1 and 2, and an intense band
at 450 nm for 3. In contrast, no absorbance at 300 nm is
observed for ligands L1–L3 (Figure S15c, Table S1). However,
peaks at around 340 nm for 1 and 2, and 450 nm for 3, were
detected. Upon excitation at λ=355 nm, complex 1 exhibited
purple emission (λmax =395 nm) with a quantum yield of 0.002;
complex 3 exhibited green emission (λmax =546 nm) upon
excitation at λ=450 nm, and a quantum yield of 0.006 (Fig-
ure S15b, Table S1). Similar emission was found for their
corresponding ligands (L1 and L3) with a higher quantum yield
(Figure S15d, Table S1). However, the emission from complex 2
and ligand L2 was too weak to detect.
X-ray crystallography
Crystals suitable for X-ray diffraction studies of complexes 1–3
were obtained through the diffusion of ethers into solutions in
DMSO/DMF for 1 and DCM/MeOH for 2 and 3. In the crystal
structures of 1–3, the Pt centres with slightly distorted
octahedral geometry resemble those of reported photoactive
Figure 1. Trans,trans,trans-[Pt(py)2(N3)2(OH)(gly-R-Nap)], R=H (1), 3-NO2 (2) or
4-NMe2 (3), complexes containing conjugated 1,8-naphthalimide ligands
studied in this work and the parent complex FM-190.
PtIV complexes with the OÀ PtÀ O angles less than 180 (1
°
°
°
°
substitution in the aromatic ring, unsubstituted 1,8-naphthali-
mide (1) and those with electron-withdrawing À NO2 (2) or
electron-donating -NMe2 (3) substituents are compared. The
crystal structures of the three complexes have been determined
by X-ray diffraction. Theoretical absorption spectra are com-
pared with the observed spectra to assign the bands. The dark
stability, photo-decomposition, photo-reactions with NADH and
5’-GMP, DNA intercalation and crosslinking, photo-cytotoxicity
and cellular accumulation, and ROS generation upon irradiation
have been investigated. The introduction of the 1,8-naphthali-
mides can allow DNA-targeting, red-shifted excitation wave-
lengths, and improve the photo-cytotoxicity of diazido PtIV
complexes.
(177.17(7) ),
2
(175.53(10) ), and
3
(170.01(9) ), Figure 2,
Tables S2 and S3). The naphthalimide rings in 1 form infinite π-
stacks along the a axis of the cell with the distance between
closest antiparallel naphthalimide rings being 3.516 Å. The large
planar structure of the naphthalimide ring indicates the
possibility of DNA intercalation. Due to the π stacking and H-
bonds (O1À H1···N3), an infinite chain is formed with all adjacent
molecules being antiparallel to each other (Table S4). The
substituents cause slight distortions of the naphthalimide ring
in 2 and 3, which reduces their ability to π-stack. Hydrogen
bonds between the axial OÀ H and O in the carbonyl group
close to Pt in an adjacent molecule (O100À H100···O15) in
complex 2, and between an axial OÀ H and terminal azide N of
an adjacent molecule (O1À H1···N3) in complex 3 are observed.
Results and Discussion
Synthesis and characterisation of complexes 1–3
Complexes 1–3 containing conjugated 1,8-naphthalimide li-
gands were synthesised by coupling trans,trans,trans-[Pt-
(py)2(N3)2(OH)(CO2CH2NH2)] (Pt-gly-NH2), containing an axial
glycine, with the corresponding anhydride at 353 K. They have
high HPLC purity (>95%, Figure S1 in the Supporting Informa-
tion), and were characterised by ESI-HRMS and NMR spectro-
scopy (Figures S2–S7). In 1H NMR spectra, Pt-coordinated
pyridine has characteristic doublets with 195Pt satellites at
around 8.8 ppm, and triplets at around 8.1 and 7.7 ppm, and 13
C
NMR resonances at ca. 149, 142 and 126 ppm. 1,8-Naphthali-
1
mide ligands give H NMR peaks in the range 8.6 to 7.1 ppm
and 13C NMR peaks from 135 to 113 ppm. 1,8-Naphthalimide
ligands L1-L3 were prepared for comparison using modified
literature methods (Figures S8–S14).[12] All complexes display a
high-energy band at around 260 nm, and an absorption
maximum at around 300 nm (Figure S15a, Table S1). A shoulder
Figure 2. X-ray crystal structures of octahedral PtIV complexes a) 1, b) 2, and
c) 3, with thermal ellipsoids drawn at the 50% probability level.
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DFT calculations of absorption and fluorescence spectra
The crystal structure-based DFT-calculated UV/vis spectra exhib-
it absorption bands comparable with experimental spectra
(Figure S16). Two main types of absorbing states are observed
in the predicted absorption spectra. The transitions responsible
for the band at approximately 300 nm correspond to ligand-to-
PtIV charge-transfer (LMCT) states, transferring electronic density
from the azide π system to an antibonding orbital centred on
PtIV (302 and 308 nm for 1, 301, 303, 306 and 309 nm for 2, and
301 and 308 nm for 3). The antibonding characters of these
transitions are either directed towards the azide ligand, or
against the hydroxide and naphthalimide ligands, which
resembles the calculation for FM-190, and are responsible for
the photo-release of azide, hydroxido and naphthalimide
ligands due to the antibonding character of lowest unoccupied
natural transition orbitals.[3d] The transitions of 1 located at
347 nm correspond to the naphthalimide π-to-π* excitation.
NO2 and NMe2 substituents, respectively, blue- and red-shift
naphthalimide absorption from 347 nm in 1 to 334 nm in 2,
and 436 nm in 3. In addition, two other transitions with partial
LMCT character were observed for 3 at 256 and 257 nm,
corresponding to π-donation from the naphthalimide ligand to
antibonding orbitals between PtIV and pyridines, hydroxido and
naphthalimide ligands. The computed emission wavelengths of
Figure 3. UV/vis spectral changes for 3 in RPMI-1640 cell culture medium
upon irradiation with a) indigo 420 nm, b) blue 463 nm, and c) green
517 nm light. d) Fluorescence change of 3 in PBS upon irradiation with blue
light (463 nm).
Figure S21b). In contrast, FM-190 exhibited negligible decom-
position after similar irradiation with green light (Figure S22).
the free protonated ligand released from 1 and 3 are λmax
=
387 nm and λmax =497 nm, respectively. They are both blue-
shifted compared to experimental measurements, but support
emission from the photo-released ligands.
Photo-reaction with 5’-GMP (guanosine 5’-monophosphate)
The interaction between photoactivated 3 and 5’-GMP as a
model nucleotide was investigated using LC–MS. An aqueous
solution of 3 and 5’-GMP was irradiated with indigo (420 nm) or
green (517 nm) light at 298 K, then analysed by LC–MS
(Figure S23). Major PtII-GMP adducts were detected as {PtII-
Photoactivation and dark stability
Complexes 1–3 are stable in DMSO for 2 h in the dark
(Figure S17). Complex 3 also shows high dark stability in RPMI-
1640, and in the presence of GSH (2.4 mM, Figures S17b and
S18). The photo-decomposition of 1–3 was monitored by UV/vis
and fluorescence spectroscopy after various time intervals and
irradiation with indigo (420 nm), blue (463 nm) or green
(517 nm, for 3 only) light at 298 K (Figures 3, S19 and S20). A
rapid decrease in the LMCT (N3!Pt) bands of 1–3 at around
300 nm, was observed upon irradiation, indicating the release
of azide ligands. The absorbance of 3 at 450 nm decreased and
blue-shifted after irradiation; this suggests the release of the
naphthalimide ligand. The emission of 1 and 3 increases
gradually with irradiation (463 nm), indicating the release of the
axial naphthalimide ligand (Figures 3d and S20). As neither
complex 2 nor the free nitro-naphthalimide ligand L2 shows
any apparent fluorescence, no change in emission was detected
for 2.
Upon irradiation of 3 with indigo light (420 nm), the LC-MS
peak for 3 decreased gradually and disappeared within 30 min,
accompanied by the formation of PtIV species {PtIV-
(py)2(N3)(OH)(gly-4-NMe2-Nap)}+ (708.81 m/z, b), PtII species {PtII-
(py)2(CH3CN)(N3)}+ (436.00 m/z, a) and released axial ligand gly-
4-NMe2-Nap (+H, 298.48 m/z, c; Figure S21). About 66% of 3
decomposed after 60 min irradiation with green light (517 nm,
(py)2(CH3CN)(GMPÀ H)}+
(755.73 m/z,
G1)
and
{PtII-
(py)2(N3)(GMP)}+ (757.74 m/z, G2). Indigo light induced the
formation of more adducts than green light after the same
irradiation time.
DNA binding calculations and experimental studies in
cell-free media
Electrostatic maps (ESMs) of iso-density surface (Figure S24)
show that all complexes exhibit a cavity-like region of negative
electrostatic potential, produced by the spatial proximity of the
N3 ligand, the carbonyl of the gly carboxylate and the nearby
naphthalimide carbonyls. The minimum of electrostatic poten-
tial (ESP) for all complexes is in the cavity, with minima of
À 0.100, À 0.090, and À 0.106 au, for complexes 1, 2, and 3,
respectively. This nucleophilic region could be responsible for
the initial step of the intercalation process in these complexes.
Accordingly, the ability to attract DNA bases through electro-
static interactions toward the naphthalimide ligand in these
complexes can be ranked as 3>1>2. The naphthalimide ligand
can then stabilise the intercalation between bases due to π-
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stacking. However, complex 3 may not give rise to proper π-
stacking because of its bulky NMe2 ligand at the 4 position.
To investigate the binding of complexes 1–3 to double-
stranded calf thymus DNA in the dark, LD spectra of DNA in the
absence or presence of Pt complexes were recorded in 10 mM
Tris-Cl, pH 7.4 containing 0.77–2.3 v/v DMSO. Complex 1 and, to
a lesser extent 2, induced a negative LD signal in the 300–
500 nm region, where the complexes exhibit weak absorption
(Figure S25). Thus, the existence of LD signals in this region
gives evidence that 1 and 2 interact with double-stranded ct-
DNA and become oriented in flow. Moreover, the interaction of
1 and 2 with DNA gives rise to LD signals of the same sign as
DNA itself (i.e., negative). This result shows that the chromo-
phores of 1 and 2 are oriented parallel with DNA base pairs,
which is indicative of intercalative binding. In contrast, 3 did
not show any induced LD bands, despite its higher absorbance
in the visible region. This suggests that 3 does not bind to DNA,
or its binding affinity is very low. However, all three complexes
may interfere with the DNA band in the 260 nm region due to
their own absorptivity. The presence of DMSO can also
contribute slightly to DNA structural changes (Figure S25).
Hence the changes in the LD band around 260 nm cannot be
interpreted reliably.
does not lengthen the DNA helix, as no increase in the viscosity
was observed. These data correspond to LD results and confirm
intercalative DNA binding of 1 and, to a lesser extent, 2.
However, either no, or extremely weak DNA binding was
confirmed for complex 3, ascribed to its bulky 4-NMe2
substituent.
To verify that complexes 1–3 can bind strongly to DNA
under irradiation, we analysed linearised plasmid DNA modified
by 1–3 by gel electrophoresis under denaturing conditions
(Figure 4b). With 1 h exposure (420 nm), a significant fraction of
slowly migrating DNA was observed, which can be attributed to
interstrand crosslinking by irradiated Pt compounds, but no
crosslinked fraction was observed for these samples when kept
in the dark. The intensity of the interstrand crosslink-containing
fraction was concentration-dependent (Figure S26). Notably,
complexes 1 and 2 displayed ca. 2x higher interstrand cross-
linking fractions compared with 3, which may result from their
pre-intercalation in the dark, shortening the distance between
Pt and DNA bases thus facilitating the formation of strong
crosslinks (Figure S26d).
Photo-cytotoxicity and cellular accumulation studies
To characterise the DNA binding mode in the dark further,
viscometry measurements were performed (Figure 4a). Com-
plexes 1 and 2 increase the length of DNA, resulting in an
increased viscosity, 2 being less effective than 1. In contrast, 3
Due to the poor aqueous solubility of 1, the dark-photo-
cytotoxicity of only complexes 2 and 3 was determined in
human A2780 ovarian, A549 lung and PC3 prostate cancer cells,
and MRC5 lung normal cells using the sulforhodamine B (SRB)
colorimetric assay (Table 1). Satisfactory dark stability with IC50
values >50 μM was observed for 2, and >100 μM for 3 in cells.
However, 2 and 3 exhibited significant photo-cytotoxicity upon
irradiation (465 nm, 4.8 mWcmÀ 2, 1 h) with IC50 values of 1.2–
13.5 μM. Pre-intercalation of the NO2-Nap naphthalimide ligand
of 2 might contribute to the enhanced photo-cytotoxicity of 2.
Notably, photo-cytotoxicity with green light (520 nm,
Table 1. IC50 values and photo-cytotoxicity indices (PI) for complexes 2
and 3 obtained after 1 h of incubation, 1 h of irradiation (465/520 nm) and
24 h of recovery for human A549 lung, PC3 prostate, A2780 ovarian cancer
cells, and MRC5 normal fibroblasts. Data for complex FM-190 and ligands
L2 and L3 are listed for comparison.
Complex
Irradiation IC50 [μM][a]/PI
A549
PC3
A2780
MRC5
2
dark
>50
>50
465 nm
PI blue
dark
11.8�0.1
6.4�0.7
>4.2
>7.5
3
>100
>100
>100
>100
465 nm
520 nm
PI blue
PI green
dark
13.5�1.3
92.8�13.5
>7.4
>1.0
>100
10.1�0.2
32.7�0.5
>9.8
>3.0
>100
1.2�0.1
35.3�2.2
>82.6
>2.8
FM-190[13]
>100
>100
Figure 4. a) The relative specific viscosity of calf thymus DNA in the presence
of complexes 1, 2 or 3 in the dark. Data are presented as (η/η0)1/3 vs. r([Pt]/
[DNA]), where η is the viscosity of DNA in the presence of Pt complex and η0
is the viscosity of DNA alone in the buffer containing the same percentage
of DMSO as the respective sample. b) Agarose gel electrophoresis for
determination of DNA interstrand crosslinking. The interstrand crosslinks
(IEC) were formed by 1, 2, or 3 under irradiation conditions (420 nm light) in
linear pSP73 plasmid DNA. Lanes 1, 2, 5, and 6: control, unmodified DNA;
lanes 3 and 4: DNA modified by 1 at r=0.0009; lanes 7 and 8: DNA modified
by 2 at r=0.0012; lanes 9 and 10: DNA modified by 3 at r=0.0012.
465 nm
520 nm
PI blue
dark
465 nm
dark
51.9�2.5
55.6�0.9
7.1�0.4
>100
>100
>100
>1.9
>100
>100
>100
>1.7
>100
>100
>100
>14.0
L2
L3
465 nm
>100
>100
[a] All data are from two independent experiments.
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11.7 mWcmÀ 2, 1 h) was detected for complex 3 (IC50 =32.7–
92.8 μM), which can be attributed to its photoactivation with
green light. No cytotoxicity of L2 and L3 ligands alone was
observed regardless of the presence of irradiation due to the
short incubation time, indicating the important role of PtIV
fragments in killing cancer cells.
Pt accumulation from complexes 2 and 3 (10 μM dose) in
A2780 ovarian, A549 lung and PC3 prostate cancer cells in the
dark was at least 15x higher than from FM-190 (Table S5).
Notably, approximately 3x enhanced Pt accumulation for
complex 2 over 3 was observed; this is consistent with its
higher photo-cytotoxicity. The high lipophilicity of 2 and 3
probably contributes to their high cellular accumulation.
Photo-oxidation of NADH
The coenzyme nicotinamide adenine dinucleotide (NADH) plays
an important role in maintaining the redox balance in cancer
cells and is a potential target of photooxidation.[14] A solution of
NADH (2 mM) was irradiated with indigo light (420 nm) in the
presence of complex 3, then analysed by LC-MS (Figure S27).
The decrease in intensity of the peak for NADH (665.61 m/z,
NADH+H+) and the appearance of a new peak for NAD+
(663.62 m/z) were observed after 1 h irradiation. The amount of
NAD+ produced by 3 is about 1.9x as much as that produced
by FM-190. Little NAD+ was detected after irradiation of NADH
alone. The results indicate the ability of these PtIV complexes to
induce photo-oxidation of biomolecules, giving rise to a
combined mechanism of action of DNA binding and ROS
generation.
Figure 5. Confocal fluorescence microscopy images of ROS generation in
A549 cells treated with complex 3 (IC50 concentration, 2 h in the dark, or 1 h
in the dark and 1 h of irradiation, 465 nm) then probed by DCFH-DA (20 μM,
λex =488 nm, λem =507–611 nm) in the absence and presence of antioxidant
N-acetyl-l-cysteine (NAC, 10 mM). Scale bars: 50 μm.
DFT calculations assigned the LMCT transitions with antibond-
ing character, favourable for the photo-release of azide,
hydroxide and naphthalimide ligands. Complexes 1–3 undergo
photo-decomposition upon irradiation with visible light (420
and 463 nm). Photoactivated complexes can form DNA cross-
links by binding to guanine bases. Complexes 1 and 2 pre-
intercalate into DNA in the dark, facilitating the formation of
covalent crosslinks, and thus displayed approximately twice as
many interstrand crosslinks as 3. In addition, complex 2
displayed approximately three times as much cellular accumu-
lation as that of 3, contributing to enhanced photo-cytotoxicity.
Photo-induced cellular ROS production contributes to their
photo-cytotoxicity. Notably, a 4-NMe2 substituent induced red-
shifted absorption of the naphthalimide ring to 450 nm
allowing photo-decomposition of 3 with green light (517 nm),
giving rise to its green-light photo-cytotoxicity. The introduc-
tion of versatile naphthalimide ligands into photoactive diazido
PtIV is a promising strategy to improve photo-cytotoxicity,
capable of providing pre-irradiation intercalative DNA recogni-
tion and red-shifting activation wavelengths, which might
facilitate their preclinical development.
ROS generation
Photo-induced singlet oxygen was detected for complex 3 in
acetonitrile directly by infrared fluorescence spectroscopy (Fig-
ure S28). The cellular ROS generation of 3 was determined by
the DCFH-DA assay (Figure 5). A549 lung cancer cells were
treated with 3 for 1 h at 310 K in the dark at 13.5 μM (photo IC50
concentration). One dish was exposed to 1 h irradiation
(465 nm), while the other was kept in the dark. Cells were
stained with DCFH-DA (20 μM) then analysed using confocal
microscopy.
Clear green fluorescence (λex =488 nm, λem =507–611 nm)
in irradiated cells indicates the generation of ROS (Figure 5). In
contrast, cells kept in the dark or treated with antioxidant N-
acetyl cysteine (NAC, 10 mM) exhibited no apparent
fluorescence under the same conditions.
Acknowledgements
Conclusions
We acknowledge financial support from the EPSRC (EP/
G006792, EP/F034210/1, P.J.S.; EP/P001459/1, EP/T021675/1,
M.J.P.), Anglo-American Platinum (post-doctoral fellowship for
H.S.), and Chancellor’s International Scholarship from the
University of Warwick (H.S.), and Czech Science Foundation (21-
27514S, J.K., O.N., and V.B.). This research was funded in part by
We have synthesised and characterised three novel photoactive
diazido PtIV-naphthalimide complexes 1–3 that are dark-stable,
even in the presence of bio-reductant GSH. Their X-ray crystal
structures highlight the large π surface of the naphthalimide.
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G. M. Tocci, L. J. Gillespie, D. O. Frimannsson, J. M. Kelly, T. Gunnlaugs-
son, Chem. Soc. Rev. 2013, 42, 1601–1618.
the Wellcome Trust [Grant number 209173/Z/17/Z for CI]. For
the purpose of open access, the author has applied a CC BY
public copyright licence to any Author Accepted Manuscript
version arising from this submission.
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: anticancer activity
· DFT calculations · DNA
intercalation · interstrand crosslinking · photoactive platinum
complexes
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Manuscript received: March 31, 2021
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Chem. Eur. J. 2021, 27, 1–7
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These are not the final page numbers!

FULL PAPER
Dr. H. Shi, Prof. J. Kasparkova, C. Soulié,
Dr. G. J. Clarkson, Dr. C. Imberti, Dr. O.
Novakova, Prof. M. J. Paterson, Prof. V.
Brabec, Prof. P. J. Sadler*
1 – 7
DNA-Intercalative Platinum Anti-
cancer Complexes Photoactivated by
Visible Light
Green to go: Photoactive all-trans
diazido PtIV complexes with an axial
1,8-naphthalimide ligand that can
pre-intercalate into DNA have been
synthesised. They can be photoacti-
vated with visible light to generate PtII
DNA interstrand crosslinks and
cytotoxic ROS, and are thus promising
photo-chemotherapeutic agents.
Notably, a 4-NMe2 substituent red-
shifted absorption of the naphthali-
mide ring, giving rise to its green-light
photo-cytotoxicity.