Cell-permeable lanthanide-platinum(iv) anti-cancer prodrugs

Article abstract of DOI:10.1039/d1dt01688a

Platinum compounds are a vital part of our anti-cancer arsenal, and determining the location and speciation of platinum compounds is crucial. We have synthesised a lanthanide complex bearing a salicylic group (Ln = Gd, Eu) which demonstrates excellent cellular accumulation and minimal cytotoxicity. Derivatisation enabled access to bimetallic lanthanide-platinum(ii) and lanthanide-platinum(iv) complexes. Luminescence from the europium-platinum(iv) system was quenched, and reduction to platinum(ii) with ascorbic acid resulted in a “switch-on” luminescence enhancement. We used diffusion-based¹H NMR spectroscopic methods to quantify cellular accumulation. The gadolinium-platinum(ii) and gadolinium-platinum(iv) complexes demonstrated appreciable cytotoxicity. A longer delay following incubation before cytotoxicity was observed for the gadolinium-platinum(iv) compared to the gadolinium-platinum(ii) complex. Functionalisation with octanoate ligands resulted in enhanced cellular accumulation and an even greater latency in cytotoxicity.

Full text of DOI:10.1039/d1dt01688a

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Cell-permeable lanthanide–platinum(IV)
anti-cancer prodrugs†
Cite this: Dalton Trans., 2021, 50,
8761
Kezi Yao,a Gogulan Karunanithy, ‡a Alison Howarth,§a Philip Holdship,
b
Amber L. Thompson, a Kirsten E. Christensen,a Andrew J. Baldwin,a
Stephen Faulknera and Nicola J. Farrer
*
a
Platinum compounds are a vital part of our anti-cancer arsenal, and determining the location and specia-
tion of platinum compounds is crucial. We have synthesised a lanthanide complex bearing a salicylic
group (Ln = Gd, Eu) which demonstrates excellent cellular accumulation and minimal cytotoxicity.
Derivatisation enabled access to bimetallic lanthanide–platinum(II) and lanthanide–platinum(IV) complexes.
Luminescence from the europium–platinum(IV) system was quenched, and reduction to platinum(II)
with ascorbic acid resulted in a “switch-on” luminescence enhancement. We used diffusion-based 1H
NMR spectroscopic methods to quantify cellular accumulation. The gadolinium–platinum(II) and gadoli-
nium–platinum(IV) complexes demonstrated appreciable cytotoxicity. A longer delay following incubation
before cytotoxicity was observed for the gadolinium–platinum(IV) compared to the gadolinium–platinum(II)
complex. Functionalisation with octanoate ligands resulted in enhanced cellular accumulation and an
even greater latency in cytotoxicity.
Received 29th March 2021,
Accepted 28th May 2021
DOI: 10.1039/d1dt01688a
rsc.li/dalton
uptake for the octanoate complex results in an increased cyto-
toxicity by two orders of magnitude, compared to cisplatin.8
Introduction
Approximately 50% of chemotherapy regimens worldwide
Lanthanide complexes have played a central role in the
include a platinum-based drug1 and platinum(II) complexes development of magnetic resonance imaging (MRI) agents for
such as cisplatin, carboplatin and oxaliplatin are well-estab- over thirty years.9–11 Gadolinium complexes with octadentate
lished, highly effective anti-cancer agents. These complexes are ligands have favourable relaxation properties that have enabled
highly reactive in vivo and the side-effects of treatment caused them to become a key tool in diagnostic imaging.12 While
by off-target reactivity are often debilitating.2,3 Platinum(IV) initially employed for imaging vasculature,13 through careful
complexes can potentially reduce the side-effects of treat- ligand design “smart” systems have been developed that can
ment,4 as they typically require reduction to platinum(II) be targeted to particular tissue types, or that exhibit a response
species before exerting their anti-cancer effect.5 Their cellular modulated by external biochemical stimuli.14,15 To enable us
uptake and rate of reduction inside cells depends strongly on to localise gadolinium compounds, we recently developed a
the coordinated ligands.6 For example, lipophilic ligands such diffusion-based 1H NMR method that can be used to dis-
as octanoate (OA) enable [PtIV(NH3)2(Cl2)(OA)2] to exhibit a tinguish between intra- and extracellular pools of water,
106-fold enhancement in cellular accumulation in ovarian relying on the fact that water diffusion in intracellular fluids is
(A2780) cancer cells in comparison to its synthetic precursor, defined by the boundaries of the cell.16 Since the cell is much
cis,cis,trans-[PtIV(NH3)2(Cl2)(OH)2].7 The additional cellular smaller than the voxel (volume pixel) in an MRI image, this
means that intracellular fluid can be treated as a slow
diffusing pool, whilst extra-cellular fluid is treated as a fast
diffusing pool. The INDIANA (IN cell DIffusion Analysis) meth-
odology allows these two pools and their properties to be
quantitatively described, including relaxation rates.
Complexes which incorporate lanthanides such as euro-
pium also offer attractive luminescence properties. The
general concept of generating “switch-on” luminescence fol-
lowing reduction of a platinum(IV) prodrug to platinum(II) is a
highly promising strategy for tracking (sub)cellular distri-
bution and reduction of platinum(IV) prodrugs. Previously
aChemistry Research Laboratory, University of Oxford, Mansfield Road, OX1 3TA,
UK. E-mail: Nicola.Farrer@chem.ox.ac.uk
bDepartment of Earth Sciences, University of Oxford, OX1 3AN, UK
†Electronic supplementary information (ESI) available: Experimental details,
material characterisation data and crystallographic details. CCDC 1960280. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1dt01688a
‡Current address: Institute of Structural and Molecular Biology, Division of
Biosciences, University College London, London, WC1E 6BT, UK.
§Current address: Centre for Medicines Discovery, Nuffield Department of
Medicine, University of Oxford, Oxford OX3 7DQ, UK.
This journal is © The Royal Society of Chemistry 2021
Dalton Trans., 2021, 50, 8761–8767 | 8761

View Article Online
Paper
Dalton Transactions
reported strategies include axial platinum(IV) coordination of
quenched fluorophores such as fluorescein17 and selective
fluorescence reporting from organic probes following reaction
with specific platinum(II) reduction fragments.18 Lanthanides
provide a significant advantage over purely organic fluoro-
phores, by producing a highly distinctive emission profile
when excited, from which background autofluorescence can
also be effectively removed through time-gating.19
In order to develop redox-active platinum anti-cancer
agents, whilst simultaneously providing a means to accurately
track their distribution in the body, two gadolinium(III)–Pt(IV)
complexes derived from carboplatin and cisplatin have been
recently developed.20 Moreover, a series of platinum(IV) pro-
drugs conjugated to Gd-texaphyrin have shown greater stability
towards hydrolysis and nucleophilic attack compared to their
Fig. 1 Structures of the complexes 1–4 reported, where Ln = Gd, Eu.
platinum(II) analogues, while the Gd-texaphyrin fragment pre- a ligand to a second metal centre – such as platinum – to give
sented the ability to activate the platinum(IV) prodrug species 2. The synthesis of the ligands and complexes are summarized
through redox cycling.21
in Scheme 1. Triester (L1)27 was reacted with methyl 5-bromo-
Microscopy on luminescent analogues can provide insights acetyl-2-hydroxybenzoate (B), to yield the protected pro-ligand,
in isolated cells or tissue slices,22,23 but does not translate L2. Deprotection was accomplished by treating L2 with TFA to
readily to whole body imaging. Similarly, MRI resolution does give the unmasked pro-ligand, L3, which was reacted with the
not approach the diameter of a single cell, meaning that it is appropriate lanthanide trifluoromethane sulfonate to produce
difficult to assess distribution of a contrast agent between the desired Ln(III)–Pt(II) complex 1·Ln (where Ln = Gd, Eu).
intra- and extra-cellular fluid. In this manuscript, we address Reaction of 1·Ln with cis-[Pt(NH3)2(OH2)2]2+ yielded the hetero-
the issue of how MR methods can be used to define cellular metallic complex 2·Ln.28 Oxidation of 2·Ln with H2O2 pro-
uptake by exploiting the constraints on diffusion imposed by duced the Ln(III)–Pt(IV) di-hydroxido complex 3·Ln.29 Reaction
intracellular localization of a bimetallic lanthanide–platinum of 3·Ln with octanoyl chloride afforded the Ln(III)–Pt(IV) di-
complex.
Drawing these concepts together, we have synthesized and by LCMS, with characterisation data consistent with the pro-
octanoic acid conjugate, 4·Ln.8 All compounds were purified
characterized
a
dual-purpose lanthanide(III)–platinum(IV) posed structures (see ESI†) and in accordance with previously
system which includes a sensitizing salicylic group. This can reported Ln(III)–Pt(II) complexes.30 The complexes were also
enable both in cellulo tracking in preclinical development (Ln characterized by ICP-MS to confirm the Ln : Pt ratio and
= Eu, Gd), and in vivo tracking (Ln = Gd), through simple ensure that complexation had occurred (Table S1†).
modulation of the lanthanide, whilst retaining very similar
Additionally, complex 2·Eu was crystallised from aqueous
pharmacological properties regardless of the choice of lantha- solution at 4 °C and characterised by single crystal X-ray diffr-
nide. This first reported example of a Eu(III)–Pt(IV) complex action. The structure was solved by charge-flipping using
that has the potential to provide highly diagnostic real-time.
‘Superflip’,31 and refined by full-matrix least squares on F2
We established the minimal cytotoxicity of the gadolinium using CRYSTALS suite (Fig. 2).32–34 The structure demonstrated
salicylic acid precursor, and the contrasting cytotoxicity of the a monocapped square antiprism (SAP) configuration of the Eu
Gd(III)–Pt systems, and correlated accumulation with the effect centre.
that oxidation state and choice of axial ligand of the platinum
To the best of our knowledge, this is the first reported euro-
group has on the cytotoxicity of these complexes. Information pium–platinum X-ray single crystal structure with a kinetically
on the reductive activation of a Pt(IV) prodrug. Our established stable macrocyclic lanthanide complex; bond lengths and
1
diffusion-based H NMR method enabled us to determine the angles are essentially consistent with other previously reported
extent to which the new complexes 1–4 (Fig. 1) accumulated Ln(III)–Pt(II) complexes.35,36
within cells.
Complexes 1–4 were purified by HPLC with different reten-
tion times (see Fig. S13† for the HPLC traces of (1–4)·Gd com-
plexes). The complexes were also characterised by mass spec-
troscopy and NMR spectroscopy. 195Pt NMR spectroscopy of
2·Lu revealed a single resonance at −1608 ppm (D2O, Fig. S8†),
Results and discussion
We have previously used phenacyl–DO3A derivatives to good while 3·Lu gave a resonance at 1691 ppm (D2O, Fig. S9†),
effect, as kinetically stable building blocks for more compli- which are consistent with Pt(II) and Pt(IV) oxidation states,
cated architectures.24–26 The lanthanide complex (1) (Fig. 1) respectively.37 Whilst the Pt(II) species 2·Lu was observed as a
(where Ln = Gd, Eu) builds on this work, and was conceived as singlet, the Pt(IV) complex 3·Lu was observed as a quintet, as a
a system in which the lanthanide is bound in an octadentate result of coupling to the two equivalent quadrupolar (I = 1) 14
N
1
binding site, while the pendent salicylic acid group can act as nuclei of the NH3 ligands. The magnitude of the J (195Pt, 14N)
8762 | Dalton Trans., 2021, 50, 8761–8767
This journal is © The Royal Society of Chemistry 2021

View Article Online
Dalton Transactions
Paper
Scheme 1 Synthetic route to the complexes (Ln = Gd, Eu). Reagents and conditions: (a) K2CO3, MeCN, room temperature, 16 h; (b) TFA, CH2Cl2,
24 h; (c) Ln(OTf)3, MeOH, 60 °C, 48 h; (d) Pt(NH3)2I2, AgNO3, H2O2, pH 10, room temperature, 24 h; (e) H2O2, H2O, room temperature, 16 h; (f) octa-
noyl chloride, acetone, pyridine, room temperature, 16 h.
Fig. 2 Molecular structure of 2·Eu from single crystal X-ray diffraction
studies; thermal ellipsoids are drawn at 20% probability and water in the
solvent sphere is omitted for clarity.
Fig. 3 Variation of 1/T1 with concentration of gadolinium for complexes
coupling constant (213 Hz) is consistent with other 1J (14N,
(1–4)·Gd at different concentrations (at 25 °C, in DMEM, 14.1 T).
195Pt) constants that we have previously observed, for example
in the platinum(IV) species, cis,cis,cis-[Pt(N3)2(OH)2(NH3)2] (167
Hz).38 The 1H NMR spectra of the europium complexes
(Fig. S10†) are consistent with those of other phenacyl–DO3A
Table 1 Relaxivities of complexes 1·Gd–4·Gd
analogues, and show structures dominated by the square anti-
prismatic diastereoisomers of the complexes, in agreement
with the molecular structure of 2·Eu determined using single
crystal X-ray diffraction. Measurement of the T1 relaxation
enhancements obtained with complexes (1–4)·Gd offers
further insights into their solution state structure. The relaxiv-
ity of the complexes was determined by observing the variation
of 1/T1 with complex concentration in media (DMEM) as
Compound
Relaxivity (s−1 mM−1
)
( )
1·Gd
2·Gd
3·Gd
4·Gd
2.21
3.96
3.21
2.89
0.04
0.12
0.1
0.11
This is corroborated by the molecular structure of the euro-
shown in Fig. 3. In all cases, the resulting plots showed a pium analogue, 2·Eu, determined using single crystal X-ray
linear response to concentration across a wide range, indicat- diffraction. This shows a single bound water molecule in the
ing that all complexes were essentially stable in solution, and inner coordination sphere of the lanthanide (Fig. 2). 1·Eu dis-
that their properties are not changed by their local environ- plays luminescence properties in line with other phenacyl–
ment (since a change in speciation would be expected to result DO3A derivatives, exhibiting intense lanthanide-centred emis-
in curvature of the plots). All the complexes (1–4)·Gd exhibit sion. The luminescence lifetimes (τH O = 0.66 ms and, τD O
=
2
2
relaxivities consistent with a single water molecule in their 2.13 ms, Table S3†) were used to calculate the number of inner
inner coordination sphere (Table 1).
sphere solvent molecules using established methods.39 The
This journal is © The Royal Society of Chemistry 2021
Dalton Trans., 2021, 50, 8761–8767 | 8763

View Article Online
Paper
Dalton Transactions
value obtained (q = 1.0) is consistent with expectations for a cellular processing it is anticipated that the lanthanide–sal-
europium ion complexed by an octadentate ligand. 2·Eu dis- icylic acid ligand will dissociate from the platinum(II) complex
plays very similar luminescence properties: the observed which is likely to further affect the lanthanide luminescence,
luminescence lifetimes (τH O = 0.61 ms, τD O = 1.87 ms) are further study is needed to investigate this modulation in
2
2
also consistent with q = 1.0 and with the structure of 2·Eu detail.
determined from X-ray diffraction studies.
As shown previously, using the INDIANA model,16 the
Furthermore, the steady state spectra of the two complexes diffusive behaviour of water within cellular systems can be
show strong similarities, suggesting that the coordination robustly described using a two-pool model with the water
environments at the europium ion are similar. In comparison, residing in either an intra- or extra-cellular pool. This results
3·Eu and 4·Eu exhibited only very weak luminescence, and from the intrinsic diffusion of water being slower in the
poor signal-to-noise ratios were obtained (Fig. S15†). However, viscous intra-cellular compartment and its maximum displace-
when the platinum(IV) centre in 3·Eu was treated with ascorbic ment being restricted by the presence of the cell membrane.
acid (AA), a dramatic enhancement in luminescence intensity The model also explicitly accounts for exchange between the
was observed – resulting in a 37-fold enhancement of signal two environments. This methodology builds on previous
on regenerating 2·Eu (Fig. 4). The excited-state manifold of the experiments to characterise restricted diffusion in cellular
platinum(IV) complex acts to quench the europium emission, systems.44–47
meaning that switching on of signal can potentially be used to
When fitting variable diffusion delay data from 1H NMR
observe reduction of the complex in situ. The bis-octanoic acid spectroscopic experiments, it is possible to derive a number of
platinum(IV) complex 4·Eu also showed a “switch-on” in system properties including intra- and extra-cellular diffusion
luminescence when reduced with ascorbic acid (Fig. S16†).
coefficients, populations of water in the two environments,
This is consistent with other Pt(IV)/Pt(II) couples that exchange rate of water over the membrane, average cell radius
display reductive “switch on” fluorescence; the quenching in and, crucially for this study, intra- and extra-cellular longitudi-
the Pt(IV) state being thought to arise from the presence of nal relaxation rates of water. Due to the relaxation enhance-
metal-based d-orbitals energetically positioned between the ment caused by MRI imaging reagents, by comparing the
HOMO and LUMO of the fluorophore.40 Quenching of the difference in relaxation rates in the presence and absence of
Eu(III) emission by the Pt(IV) centre is therefore suggested to these agents in cellular systems we can quantitatively assess
occur through energy transfer to low-lying electronic states in the degree to which they are localised within cells. As the relax-
the platinum(IV) complex. Luminescence in the Pt(IV) oxidation ivities of the four complexes are different, this must be con-
state is more likely to be observed if one or more ligands are sidered when using an increased intra-cellular relaxation rate
cyclometallated.41,42 Although the mechanism of reduction as a proxy for cellular uptake of imaging agent. To address
in cellulo is likely to be complicated and involve more than one this, we used the relaxivity of the complexes measured in
reducing species,43 ascorbate acid-mediated reduction gives an
initial insight into the potential for monitoring the reduction
of these compounds through fluorescence microscopy. During
Fig. 5 Calculated partitioning of complexes 1–4·Gd between extra-
and intra-cellular environments following 1.5 h incubation with KNS42
Fig. 4 Luminescence spectrum of 3·Eu; λex = 330 nm, emission slit =
1 nm. (a) before addition, (b) 24 h after addition of excess (20-fold)
ascorbic acid (AA) and (c) 72 h after addition of AA, (d) 120 h after
addition of AA, (e) 216 h after addition of AA. The red arrow shows that
as time increases the intensity of emission increases.
cells. The concentration of complexes in the two environments is found
by comparing longitudinal relaxation rates in the presence and absence
of the compounds (ΔR1) and then dividing by the relaxivity. The error
bars are calculated by propagating the error in ΔR1 for three indepen-
dent samples and taking the standard deviation.
8764 | Dalton Trans., 2021, 50, 8761–8767
This journal is © The Royal Society of Chemistry 2021

View Article Online
Dalton Transactions
Paper
1
Table 2 Cytotoxicity (IC50, µM) values of compounds (1–4)·Gd in KNS42 cell line. Cells were exposed to 2 log serial dilutions of compounds for 1.5,
24, 48 and 72 h. IC50 values were determined using a 3 parameter log inhibitor vs. concentration curve fit. Two independent sets of three (6 in total)
were performed for all compounds. Mean IC50 values and 95% confidence interval (CI) are shown below; P values are given in the ESI (Table S4†)
1·Gd
2·Gd
3·Gd
4·Gd
Complex
Time/h
IC50
95% CI
IC50
95% CI
IC50
95% CI
IC50
95% CI
1.5
24
48
72
>1000
>1000
>1000
>1000
—
—
—
—
637.9
316.7
75.3
446.1–934.3
226.6–445.9
65.79–86.25
64.68–88.82
227.6
96.5
91.4
53.2
183.2–283.7
80.13–116.4
73.43–114.0
43.34–65.50
>1000
>1000
747.5
142.8
—
—
644.4–871.2
111.2–84.5
75.7
DMEM (Fig. 3). To estimate the partitioning of the four com-
A paediatric glioma cell line was specifically chosen for
plexes between extra- and intra-cellular environments we investigation, due to our interest in developing novel anti-
assumed that the relaxivity of the compounds inside cells could cancer compounds for these malignancies.54 Complex 1·Gd
be approximated by their relaxivity in media. Using the was well-tolerated by KNS42 (paediatric glioblastoma) cells
INDIANA method described above we found the extra- and intra- (viability <90%) across all concentrations and time-points
cellular longitudinal relaxation and compared this to the values tested. This indicated minimal cytotoxicity for 1·Gd, with IC50
in the absence of any complex to give ΔR1 for each compound values exceeding 1000 µM (Table 2, Table S5, Fig. S18†). The
in the two environments. This value was then divided by the Gd(III)–Pt(II) complex 2·Gd demonstrated increasing cyto-
relaxivity to show how the compound was partitioned. The toxicity after 1.5 h exposure (IC50 = 637.9 µM at 1.5 h incu-
results of our analysis are shown in Fig. 5. The 1H diffusion bation, Fig. S18†). The Gd(III)–Pt(IV) complex 3·Gd was signifi-
measurements were used to quantify the accumulation of cantly more cytotoxic than complex 2·Gd after 1.5 h (P <
the compounds within cells. Complex 1·Gd accumulated rela- 0.0001) and 24 h (P = 0.0031) exposure. However, no significant
tively well in the cells, whilst there was a small drop reduction difference in cytotoxic effect between 2·Gd and 3·Gd was
in accumulation for 2·Gd which incorporated the Pt(II) observed at longer (48 h and 72 h) exposure times (P values
group. Gadolinium complexes enter cells predominantly by 0.0584 and 0.1482 respectively) (Fig. S18†). Complex 4·Gd was
micropinocytosis,48,49 whereas platinum(II) complexes enter cells less cytotoxic than 2·Gd and 3·Gd at all exposure times, with
through a combination of copper transporters, anion/cation IC50 > 1000 µM at 1.5 h and 24 h, with increasing cytotoxicity
transporters and passive diffusion.50 Significantly lower cellular after 48 h exposure (P = 0.0027 and <0.0001) (Fig. S18†). Whilst
accumulation was observed for the platinum(IV) compound, complex 1·Gd showed no toxicity towards cells up to a concen-
3·Gd, consistent with the oxidation state and ligand sphere of tration of 1000 µM, the platinum-containing bimetallic com-
the Pt modulating the ability of the complex to enter cells. The plexes 2–4·Gd displayed varying degrees of cytotoxicity. As
copper transporter mechanism is less viable for platinum(IV) kinetically-inert 5d6 complexes, platinum(IV) complexes are
complexes, since there are no vacant positions available in the suggested to reduce to platinum(II) intracellularly before
octahedral platinum(IV) coordination sphere. Cellular uptake of ligand exchange and binding to cellular targets (e.g. DNA, pro-
platinum(IV) complexes is therefore thought to occur through a teins) can occur.55 Complexes 3–4·Gd all demonstrated
combination of passive diffusion and active pathways.51
increased cytotoxicity towards the cells with prolonged contact
Finally, 4·Gd, which incorporates OA ligands, showed a time. However, complex 4·Gd showed a more pronounced
similar uptake to 1·Gd, demonstrating the notable effect of the “latent” period of initial lower cytotoxicity, compared with
modified coordination sphere of the platinum(IV) on the complexes 2·Gd and 3·Gd, consistent with an outer-sphere
degree of cellular accumulation. If diffusion is the predomi- mechanism of reduction for 4·Gd which is likely to be slower
nant cellular uptake of the mechanism for 3·Gd, increasing than an inner-sphere mechanism of reduction for 3·Gd. For
the lipophilicity through inclusion of octanoic acid groups as 3·Gd, the reduction mechanism could involve bridging to one
in 4·Gd would be anticipated to significantly enhance uptake, or both axial hydroxido groups,5 which is not possible for
and the data are consistent with this, although without dis- 4·Gd. An investigation into the sub-cellular location and time-
counting the possibility of uptake through micropinocytosis. resolved fluorescence reporting of complexes 1·Eu–4·Eu using
The dominant route(s) of cellular uptake for other lanthanide– microscopy is planned.
platinum conjugates are unknown, since only a handful of
examples have been reported, with either labile,52,53 or kineti-
cally inert lanthanide chelation.19,20,27 From these data, we
suggest that the route(s) vary significantly depending on the
Conclusions
coordination sphere platinum. These diffusion results are par- Complexes (1–4)·Gd all show cellular accumulation in paedia-
ticularly enlightening in combination with the cytotoxicity tric glioblastoma (KNS42) cells, as demonstrated by diffusion
results to give an overall picture of the cellular behaviour of 1H NMR spectroscopic experiments. At equimolar concen-
the complexes.
trations, the platinum(IV) complex 3·Gd showed significantly
This journal is © The Royal Society of Chemistry 2021
Dalton Trans., 2021, 50, 8761–8767 | 8765

View Article Online
Paper
Dalton Transactions
lower cellular accumulation than the platinum(II) complex
2·Gd after 1.5 h. Complex 3·Gd was derivatised through
inclusion of octanoate axial ligands to produce 4·Gd which
enhanced the cellular accumulation of the complex in com-
parison to 3·Gd. The lack of cellular toxicity of 1·Gd suggests
that lanthanide complexes based on 1 have significant poten-
tial for development as cell-permeant probes. The cellular
accumulation, cytotoxicity and “switch-on” luminescence fol-
lowing reduction of prodrugs 3·Ln and 4·Ln makes them ideal
6 Z. Xu, Z. Wang, Z. Deng and G. Zhu, Coord. Chem. Rev.,
2021, 442, 213991.
7 M. Alessio, I. Zanellato, I. Bonarrigo, E. Gabano, M. Ravera
and D. Osella, J. Inorg. Biochem., 2013, 129, 52.
8 V. Novohradsky, I. Zanellato, C. Marzano, J. Pracharova,
J. Kasparkova, D. Gibson, V. Gandin, D. Osella and
V. Brabec, Sci. Rep., 2017, 7, 1.
9 M. C. Heffern, L. M. Matosziuk and T. J. Meade, Chem.
Rev., 2014, 114, 4496.
candidates for studying the real-time accumulation and 10 J. Wahsner, E. M. Gale, A. Rodríguez-Rodríguez and
reduction of platinum(IV) prodrugs in cellulo, as well as making P. Caravan, Chem. Rev., 2019, 119, 957.
them candidates for further development as anti-cancer pro- 11 G. Ferrauto, E. Di Gregorio, S. Lanzardo, L. Ciolli, M. Iezzi
drugs. 1H NMR diffusion-based models have considerable
and S. Aime, Sci. Rep., 2018, 8, 1.
potential in establishing compound localisation and internal- 12 J. Lohrke, T. Frenzel, J. Endrikat, F. C. Alves, T. M. Grist,
ization and open up a new approach for using magnetic reso-
nance imaging modalities to achieve this goal in vivo.
M. Law, J. M. Lee, T. Leiner, K. C. Li, K. Nikolaou,
M. R. Prince, H. H. Schild, J. C. Weinreb, K. Yoshikawa and
H. Pietsch, Adv. Ther., 2016, 33, 1.
13 K. Van Dijke, N. Van Bruggen, J. Mann and S. Ostrowitzki.
14 H. Li and T. J. Meade, J. Am. Chem. Soc., 2019, 141, 17025.
15 C. T. Yang and K. H. Chuang, MedChemComm, 2012, 3,
552.
16 G. Karunanithy, R. J. Wheeler, L. R. Tear, N. J. Farrer,
S. Faulkner and A. J. Baldwin, J. Magn. Reson., 2019,
302, 1.
17 Y. Song, K. Suntharalingam, J. S. Yeung, M. Royzen and
S. J. Lippard, Bioconjugate Chem., 2013, 24, 1733.
18 J. X. Ong, C. S. Q. Lim, H. Van Le and W. H. Ang, Angew.
Chem., Int. Ed., 2019, 58, 164.
Author contributions
SF and NF and AB were responsible for conceptualization,
formal analysis, funding acquisition, methodology, project
administration, resources, supervision, visualization and
writing. KY carried out synthesis and characterisation, GK and
KY carried out diffusion NMR experiments. AH carried out the
biological experiments with assistance from KY. PH conducted
ICP-MS experiments. ALT and KEC solved the X-ray crystallo-
graphic structure.
19 S. Faulkner, S. J. A. Pope and B. P. Burton-Pye, Appl.
Spectrosc. Rev., 2005, 40, 1.
Conflicts of interest
20 C. J. Adams and T. J. Meade, Chem. Sci., 2020, 11, 2524.
21 G. Thiabaud, G. He, S. Sen, K. A. Shelton, W. B. Baze,
L. Segura, J. Alaniz, R. M. MacIas, G. Lyness, A. B. Watts,
H. M. Kim, H. Lee, M. Y. Cho, K. S. Hong, R. Finch,
Z. H. Siddik, J. F. Arambula and J. L. Sessler, Proc. Natl.
Acad. Sci. U. S. A., 2020, 117, 7021.
There are no conflicts to declare.
Acknowledgements
22 M. Delbianco, V. Sadovnikova, E. Bourrier, G. Mathis,
L. Lamarque, J. M. Zwier and D. Parker, Angew. Chem., Int.
Ed., 2014, 53, 10718.
23 C. P. Montgomery, B. S. Murray, E. J. New, R. Pal and
D. Parker, Acc. Chem. Res., 2009, 42, 925.
This work was supported by the Wellcome Trust (201406/Z/16/
Z), Cancer Research UK (CR-UK) grant number C5255/A18085,
through the Cancer Research UK Oxford Centre, L’Oréal, the
John Fell Fund and the EPSRC (EP/S019901/1).
24 J. D. Routledge, X. Zhang, M. Connolly, M. Tropiano,
O. A. Blackburn, A. M. Kenwright, P. D. Beer, S. Aldridge
and S. Faulkner, Angew. Chem., Int. Ed., 2017, 56, 7783.
Notes and references
1 N. J. Wheate, S. Walker, G. E. Craig and R. Oun, Dalton 25 M. Tropiano and S. Faulkner, Chem. Commun., 2014, 50,
Trans., 2010, 39, 8113. 4696.
2 S. R. McWhinney, R. M. Goldberg and H. L. McLeod, Mol. 26 J. Lehr, M. Tropiano, P. D. Beer, S. Faulkner and J. J. Davis,
Cancer Ther., 2009, 8, 10. Chem. Commun., 2015, 51, 15944.
3 R. J. Browning, P. J. T. Reardon, M. Parhizkar, R. B. Pedley, 27 A. Dadabhoy, S. Faulkner and P. G. Sammes, J. Chem. Soc.,
M. Edirisinghe, J. C. Knowles and E. Stride, ACS Nano,
2017, 11, 8560.
Perkin Trans. 2, 2002, 348.
28 F. J. Dijt, G. W. Canters, J. H. J. den Hartog,
A. T. M. Marcelis and J. Reedijk, J. Am. Chem. Soc., 1984,
106, 3644.
29 N. J. Farrer, J. A. Woods, L. Salassa, Y. Zhao,
K. S. Robinson, G. Clarkson, F. S. Mackay and P. J. Sadler,
Angew. Chem., Int. Ed., 2010, 49, 8905.
4 H. Anderson, J. Wagstaff, D. Crowther, R. Swindell,
M. J. Lind, J. McGregor, M. S. Timms, D. Brown and
P. Palmer, Eur. J. Cancer Clin. Oncol., 1988, 24, 1471.
5 E. Wexselblatt and D. Gibson, J. Inorg. Biochem., 2012, 117,
220.
8766 | Dalton Trans., 2021, 50, 8761–8767
This journal is © The Royal Society of Chemistry 2021

View Article Online
Dalton Transactions
Paper
30 O. J. Stacey, A. J. Amoroso, J. A. Platts, P. N. Horton, 41 J. E. Expósito, M. Álvarez-Paíno, G. Aullón, J. A. Miguel and
S. J. Coles, D. Lloyd, C. F. Williams, A. J. Hayes, P. Espinet, Dalton Trans., 2015, 44, 16164.
J. J. Dunsford and S. J. A. Pope, Chem. Commun., 2015, 51, 42 J. C. López-López, D. Bautista and P. González-Herrero,
12305. Chem. – Eur. J., 2020, 26, 11307.
31 L. Palatinus and G. Chapuis, J. Appl. Crystallogr., 2007, 40, 43 E. Wexselblatt and D. Gibson, J. Inorg. Biochem., 2012, 117,
786. 220.
32 P. W. Betteridge, J. R. Carruthers, R. I. Cooper, K. Prout 44 A. Szafer, J. Zhong and J. C. Gore, Magn. Reson. Med., 1995,
and D. J. Watkin, J. Appl. Crystallogr., 2003, 36, 1487. 33, 697.
33 P. Parois, R. I. Cooper and A. L. Thompson, Chem. Cent. J., 45 C. Meier, W. Dreher and D. Leibfritz, Magn. Reson. Med.,
2015, 9, 1. 2003, 50, 500.
34 R. I. Cooper, A. L. Thompson and D. J. Watkin, J. Appl. 46 E. Fieremans, D. S. Novikov, J. H. Jensen and J. A. Helpern,
Crystallogr., 2010, 43, 1100. NMR Biomed., 2010, 23, 711.
35 N. M. Shavaleev, L. P. Moorcraft, S. J. A. Pope, Z. R. Bell, 47 I. Åslund, A. Nowacka, M. Nilsson and D. Topgaard,
S. Faulkner and M. D. Ward, Chem. Commun., 2003,
1134.
36 T. K. Ronson, T. Lazarides, H. Adams, S. J. A. Pope,
J. Magn. Reson., 2009, 200, 291.
48 E. Mathieu, A. Sipos, E. Demeyere, D. Phipps, D. Sakaveli
and K. E. Borbas, Chem. Commun., 2018, 54, 10021.
D. Sykes, S. Faulkner, S. J. Coles, M. B. Hursthouse, 49 E. J. New and D. Parker, Org. Biomol. Chem., 2009, 7, 851.
W. Clegg, R. W. Harrington and M. D. Ward, Chem. – Eur. 50 D. P. Gately and S. B. Howell, Br. J. Cancer, 1993, 67, 1171.
J., 2006, 12, 9299.
51 S. B. Howell, R. Safaei, C. A. Larson and M. J. Sailor, Mol.
Pharmacol., 2010, 77, 887.
52 I. M. Etchells, M. C. Pfrunder, J. A. G. Williams and
E. G. Moore, Dalton Trans., 2019, 48, 2142.
53 Z. Zhu, X. Wang, T. Li, S. Aime, P. J. Sadler and Z. Guo,
Angew. Chem., Int. Ed., 2014, 53, 13225.
37 B. M. Still, P. G. A. Kumar, J. R. Aldrich-Wright and
W. S. Price, Chem. Soc. Rev., 2007, 36, 665.
38 N. J. Farrer, P. Gierth and P. J. Sadler, Chem. – Eur. J., 2011,
17, 12059.
39 A. Beeby, I. M. Clarkson, R. S. Dickins, S. Faulkner,
D. Parker, L. Royle, A. S. de Sousa, J. A. G. Williams and 54 M. H. C. Boulet, L. K. Marsh, A. Howarth, A. Woolman and
M. Woods, J. Chem. Soc., Perkin Trans. 2, 1999, 2, 493. N. J. Farrer, Dalton Trans., 2020, 5703.
40 J. J. Wilson and S. J. Lippard, Inorg. Chim. Acta, 2012, 55 N. Graf and S. J. Lippard, Adv. Drug Delivery Rev., 2012, 64,
389, 77.
993.
This journal is © The Royal Society of Chemistry 2021
Dalton Trans., 2021, 50, 8761–8767 | 8767