Highly stable and soluble bis-aqua Gd, Nd, Yb complexes as potential bimodal MRI/NIR imaging agents

Article abstract of DOI:10.1039/c0dt00994f

A tripodal ligand based on the 8-hydroxyquinolinate binding unit yields a soluble and highly stable bis-hydrated Gd³⁺ complex in water (pGd = 19.2(3)) with relaxivity change in the pH range 4.5-7.4 and Nd³⁺, Yb³⁺ analogues with sizeable NIR emission upon excitation at 370 nm providing a new architecture for the development of bimodal agents.

Full text of DOI:10.1039/c0dt00994f

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Highly stable and soluble bis-aqua Gd, Nd, Yb complexes as potential
bimodal MRI/NIR imaging agents†
Gaylord Tallec, Daniel Imbert, Pascal H. Fries and Marinella Mazzanti*
Received 5th May 2010, Accepted 12th August 2010
DOI: 10.1039/c0dt00994f
A tripodal ligand based on the 8-hydroxyquinolinate binding
unit yields a soluble and highly stable bis-hydrated Gd3+
complex in water (pGd = 19.2(3)) with relaxivity change in
the pH range 4.5–7.4 and Nd3+, Yb3+ analogues with sizeable
NIR emission upon excitation at 370 nm providing a new
architecture for the development of bimodal agents.
Scheme 1 Structure of the lanthanide complexes [Ln(thqN-SO3)(H2O)2].
The recent development of high-field instruments for magnetic
◦
5.16 mM-1 s-1 at 200 MHz, pH = 7.4 and 25 C, while the Nd3+
resonance imaging (MRI) has refocused the search for more
efficient contrast agents (CAs) towards paramagnetic Gd3+ com-
plexes with a high number of coordinated water molecules.1,2
The enhancement of the water proton relaxation rate per mmol
L-1 concentration of dissolved MRI contrast agents is named
relaxivity and gauges the contrast efficiency of these agents.
The inner sphere relaxivity, which is directly proportional to
the number q of water molecules coordinated to Gd3+, can be
increased at any magnetic field by reducing the denticity of
the chelating ligand. However, ligands of reduced denticity can
lead to Gd3+ chelates of lower thermodynamic stability, likely to
release this toxic ion in physiologic medium. In spite of numerous
attempts, there are only very few examples of multi-hydrated
Gd3+ chelates showing high stability and solubility in physio-
logical conditions.2–5 In the search for new structural motifs, the
preparation of ligand scaffolds suitable to simultaneously sensitize
lanthanide Ln3+ emission and yield stable multi-hydrated Gd3+
complexes is of primary importance because it is a route towards
bimodal agents for both optical and MR imaging.6–11 Only two
bimodal MRI/NIR luminescence have been reported recently.10,12
Obtaining sizeable luminescence emission from NIR emitting
lanthanide complexes and high inner sphere relaxivity of the
gadolinium complex using the same ligand is challenging because
of the unfavourable effect of O–H oscillators.13 Recent studies have
suggested that 8-hydroxyquinolinate based lanthanide complexes
are good candidates for the design of near infra-red (NIR) emitting
luminescent probes for biomedical application due to their good
stability, low cytotoxicity, sizeable quantum yields in water, long
excitation wavelength, ability to interact with proteins.14–19 In spite
of these attractive properties, the hydroxyquinolinate groups have
never been envisaged for the design of Gd3+ CAs. Here, we report
on the new tripodal heptadentate ligand H3thqN-SO3 containing
three 8-hydroxiquinolinate groups connected to a nitrogen anchor
(Scheme 1). This ligand yields highly stable (pGd = 19.2(3)) bis-
aqua Ln3+ complexes. The Gd3+ complex shows a relaxivity of
and Yb3+ analogues present sizeable NIR-emission.
The H3thqN-SO3 ligand was synthesised in 3 steps‡ with
an overall yield of 34% by coupling three 2-functionalized 8-
hydroxyquinoline followed by a regiospecific sulfonation by using
oleum. The pKa s of the protonated form of the ligand H7thqN-
SO3 were determined (the three sulfonates remain completely
deprotonated under the used experimental conditions) as well as
the stability constant of the resulting chelate with Gd3+. Analysis
of the potentiometric curves allowed the determination of the
’
+
acidity constants, pKa1+2 = 6.36, pKa3 = 4.08, pKa4 = 7.66, pKa5+6
=
17.36, pKa7 = 9.16 corresponding to the deprotonation of the
pyridinium groups, the central nitrogen and the hydroxyl oxygen
atoms, respectively. For pKa1 and pKa2, as often described for
acidity constants in the same pH range, only the sum could be
calculated and a similar situation prevails for pKa5 and pKa6. This
assignment compares well with that of the previously described
ligands TsoxMe and H3thqtcn-SO3.17,18
The anionic lanthanide complexes [Ln(thqN-SO3)] (Ln = Nd,
Gd, Yb) were prepared in situ by reacting the protonated
ligand with the appropriate lanthanide triflate salt followed by
adjustment of the pH. The lanthanide complexes of thqN-
3-
SO3 are highly soluble in water (> 35 mM). To evaluate their
potential as imaging probes, we investigated the thermodynamic
stability of the gadolinium complex at physiological pH (7.4)
by spectrophotometric competition batch titration versus H5dtpa
(diethylenetriamine-pentaacetic acid).20,21 The titration yielded
a pGd value (pGd = -log [Gdaq] for a total concentration
of [Gd]tot = 10-6 M and [L]tot = 10-5 M) of 19.2(3) at pH =
7.4 and of 12.7(3) at pH = 4.5. At pH = 7.4 the major
solution species is the 1 : 1 complex of the deprotonated ligand
[Gd(thqN-SO3)]. A model with one complexation constant
(b110(LMH)) was used to fit the titration data with the Hyss2
software22 yielding a value of logb110 = 23.05(5). The stability
of the complex at physiological pH is very high compared to
other bis-aqua complexes of acyclic ligands.23,24 A strong stability
enhancement is also found for these complexes with respect to
the previously reported 8-hydroxyquinolinate based podates15,17 in
spite of the decreased ligand denticity. The obtained pGd value is
comparable to that of the mono-aqua complex [Gd(dtpa)(H2O)],
a clinically approved MRI contrast agent.25 Moreover, the high
Laboratoire de Reconnaissance Ionique et Chimie de Coordination, SCIB,
(UMR-E 3 CEA-UJF), Institut Nanosciences et Cryoge´nie, INAC, CEA-
Grenoble, 17 rue des Martyrs, 38054, Grenoble Cedex 09, France
† Electronic supplementary information (ESI) available: Full experimental
procedures, spectroscopic data. See DOI: 10.1039/c0dt00994f
9490 | Dalton Trans., 2010, 39, 9490–9492
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L
Table 1 Metal ion centered lifetimes t, absolute quantum yields QLn
,
and number q of water molecules bound in the inner coordination sphere
for Nd(4F3/2), Yb(2F5/2) in the 1 : 1 [Ln(thqN-SO3)] complexes at pH = 7.4
(TRIS buffer) in H2O/D2O solutions
t/ms
H2O
QLnL/%
H2O
t/ms
D2O
Compound
q
[Nd(thqN-SO3)]
[Yb(thqN-SO3)]
0.0521(60)
0.329(5)
0.00068(4)
0.00122(8)
0.350(3)
1.86(7)
1.7(2)
2.3(2)
ligand acidity renders it more resistant than polyaminocarboxylate
ligands to protonation and a reasonable stability is maintained
after partial ligand protonation (pGd = 12.7(3) at pH = 4.5).
The high value of the pGd suggests that decoordination does
not occur after protonation. Moreover, solid state and solution
studies of the two previously reported lanthanide complexes of
partially protonated hydroxyquinoline based nonadentate26 and
tetradentate27 ligands showed that these ligands can efficiently
coordinate lanthanide also in their protonated form. 1H NMR of
the diamagnetic La3+ and Lu3+ at 298 K and pH = 7.4 show 5
signals (see ESI†). In the case of the Lu3+ complex, the 1H NMR
at 278 K shows two different signals for the diastereotopic protons
of the methylene group, which coalesce at room temperature. This
suggests the presence of a slow exchange between the two possible
helical (K and D) conformations of the Lu3+ complex in which
the chelating arms remain coordinated to the metal ion. A faster
exchange is observed for the La3+ and Nd3+ ions. The number q
of water molecules bound to the Ln3+ ion in water was calculated
to be 1.7 0.2 for Nd3+ and 2.3 0.2 for Yb3+ from the measured
luminescence lifetimes of the Nd(4F3/2) and Yb(2F5/2) excited states
which re-emit the near UV light (370 nm) absorbed by the ligand
levels (Table 1). A similar number of coordinated molecules is
expected for the Gd3+ ion since its ionic radius is intermediate
between those of Nd3+ and of Yb3+. These results indicate that
the Gd3+ complex of thqN-SO3 is a rare example of highly stable
bis-aqua complex with potential as MRI contrast agent.
Fig. 1 Relaxivity values of Gd(thqNSO3) at different pH, in water, 25 ◦C,
200 MHz.
associated to cancer and iskemic or kidney diseases,29,30 low pH
sensors are potentially attractive to monitor the pH within cells
(pH = 5.5–6 in endosomes, 4–5 in lysosomes).31–34 Further studies
will be directed to investigate the origins of the observed pH
dependency and the possibility of tuning the pH range.
We also investigated the viability of the Nd3+ and Yb3+ com-
plexes of H3thqN-SO3 to act as NIR-emitting luminescent probes
in biomedical applications. Ligand excitation at 370 nm results in
sizeable NIR luminescence of the complexed Nd3+ and Yb3+ ions
in water (Fig. 2). The measured absolute quantum yields, which
amount to 5 ¥ 10-4 % for Nd3+ and 1.2 ¥ 10-3 % for Yb3+, show
that the H3thqN-SO3 tripod is an efficient sensitizer of the NIR
emission of these ions in spite of the presence of two coordinated
water molecules. These yields are in the range of values reported
for non-hydrated lanthanide complexes.15–19,35 They are remarkable
for a bis-hydrate lanthanide complex and comparable values have
only been reported once recently for a bis-aqua Nd3+ complex of
a pyridine based aminocarboxylate ligand (9.7 ¥ 10-4 %) upon
excitation at 266 nm.36 In conclusion, for the first time, the 8-
hydroxyquinolinate chelating unit has been used to prepare very
stable Ln3+ complexes with potential bimodal imaging properties:
a bis-aqua Gd3+ complex with a relaxivity of 5.16 mM-1 s-1 at
200 MHz and its Nd3+ or Yb3+ analogues showing sizeable NIR
emission and long excitation wavelength. These properties indicate
that 8-hydroxyquinolinate is a promising complexing motif which
can be the starting point of a new class of multimodal reporters.
Future work will be directed to the rational optimization of their
relaxivity after proper characterization at the molecular level.37–40
The measured relaxivity r1 of the Gd3+ complex of thqN-
◦
SO3 at physiological pH (7.4) and 25 C is 5.16 mM-1 s-1 at
200 MHz and 5.73 mM-1 s-1 at 20 MHz. This value is higher than
those found for the mono-aqua complexes [Gd(dtpa)(H2O)]2- or
[Gd(dota)(H2O)]- (4.3 mM-1 s-1 at 25 ◦C), but somewhat low for
a bis-aqua complex.1 It remains unchanged when the relaxivity
is measured in anaerobic conditions excluding the possibility of
partial water displacement by carbonate.4 It still keeps unaltered
even after addition of 200 equivalents of carbonate. Note that the
relaxivity can be limited in this complex by a rather slow exchange
rate of coordinated water and preliminary 17O NMR1,25 results
support this interpretation. Besides, the relaxivity value is higher
in physiological serum than in water (r1 = 7.01 mM-1 s-1 instead
of 5.16 mM-1 s-1 at 200 MHz). The origin of this increase will
be the object of future studies. Finally, as shown in Fig. 1, the
relaxivity values increase markedly, e.g., from 5.16 to 7.38 mM-1
s-1 at 200 MHz, when the pH drops from 7.4 to 4.5. Such increase
parallels the protonation of the 8-hydroxyquinolinate groups
followed by a relaxivity decrease observed after pH 7.5 probably
associated to the formation of lanthanide hydroxo complexes.9,28
[Gd(thqN-SO3)] is a new example of a pH sensitive complex. While
most attention has been directed to the development of sensors in
the pH range 6–8 because they can serve to detect pH decreases
Fig. 2 Normalized emission spectra (lex = 370 nm) of water solutions
(1 mM) of [Nd(thqN-SO3)]3- and [Yb(thqN-SO3)]3-, Nd(4F3/2), Yb(2F5/2).
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Acknowledgements
This research was carried out in the frame of the EC COST Action
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Notes and references
‡ Synthesis and characterisation of H3thqN-SO3, 2,2¢,2¢¢-nitrilo-
tris(methylene)tris(8-hydroxyquinoline-5-sulfonic
acid):
2,2¢,2¢¢-
nitrilotris(methylene)triquinolin-8-ol (130 mg, 0.26 mmol) (see ESI†) was
dissolved in the minimum amount of oleum (5 mL) and stirred for one
night at room temperature. The mixture was then poured on crushed iced.
The resulting yellow solid was collected, washed with cold water (2 ¥
10 mL), cold ethanol (2 ¥ 10 mL), cold diethylether (2 ¥ 10 mL) and then
dried to give a brown solid (Yield = 99%). 1H NMR (D2O) d (ppm): 9.23
(d, 1H, H4, 8.9 Hz), 8.05 (d, 1H, H6, 8.2 Hz), 7.79 (d, 1H, H3, 8.9 Hz),
7.20 (d, 1H, H7, 8.2 Hz), 4.98 (s, 2H, CH2). Elemental Anal. Calcd. for
H7thqN-SO3·5.5H2O·1.5H2SO4; (%) C30H38N4O23.5S4.5 (974.93): C 36.96,
H 3.93, N 5.75; found: C 36.99, H 3.93, N 4.56.
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9492 | Dalton Trans., 2010, 39, 9490–9492
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©