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Project
Luminescent MRI contrast agents for molecular imaging.
Magnetic resonance imaging (MRI) is a very popular technique in medical
diagnostics. Together with the development of contrast agents,a
tremendous interest in its applications has grown. A lot of effort has
been done to investigate the most important parameters in order to
enhance the relaxation efficiency of MRI probes.Taking into account the
requirements for an optimal magnetic performance, different contrast
agents have been synthesized and studied in this PhD thesis. Moreover,
novel bimodal probes have been developed in order to exploit the high
sensitivity and resolution of optical microscopy with the ability of MRI
to image opaque samples. Employing this strategy enables the
simultaneous visualization of the same biological structures at
different resolutions and depths. Throughout this thesis, the aim was to
improve relaxivity especially by increasing the molecular weight and
the rotational tumbling time of the agent, and to find an appropriate
ligand acting as an efficient sensitizer for lanthanide luminescence.
Each synthesized compound has been analyzed by luminescence and
relaxometric measurements in order to evaluate its potential forbimodal
imaging.
DTPA-bisamide derivatives functionalized with p-toluidine,
6-aminocoumarin, 1-naphthalene methylamine and4-ethynylaniline were
synthesized and fully characterized. Ln(III) complexes (Ln = Gd, Eu, Tb,
Y) of the ligands DTPA-bis-p-toluidine-amide (DTPA-BTolA),
DTPA-bis-6-coumarin-amide (DTPA-BCoumA), DTPA-bis-1-naphthylmethyl-amide
(DTPA-BNaphA) and DTPA-bis-4-ethynylphenyl-amide (DTPA-BEthA) were
prepared and studied for theirbimodal MRI/optical properties. Eu(III)
and Tb(III) derivatives in aqueous solutions exhibit characteristic red
and green emission, respectively, with quantum yields of 0.73% for
Eu(III)-DTPA-BNaphA and 2.5% for Tb(III)-DTPA-BEthA. Ligand-centered
photophysical properties of the Gd(III) complexes have been investigated
to get insights into energy transfer processes taking place in these
systems. The Gd(III) complexes were also analyzed by the NMRD technique.
The relaxivity (r1) at 20 MHz and 310 K was in the range from 4.1
s-1mM-1 for Gd-DTPA-BTolA to 6.4 s-1mM-1 for Gd-DTPA-BNaphA which are
higher values compared to 3.8 s-1mM-1 for Gd-DTPA(Magnevist®). The
improved relaxivity is due to a general increase of the rotational
tumbling time τR. Moreover, ina 4% HSA solution, the apparent
relaxivity at 20 MHz increasesto values of 13.9 and 19.1 s-1mM-1 for
Gd-DTPA-BNaphA and Gd-DTPA-BEthA respectively, caused by a further
decrease of molecular motions.
In the second part, the synthesis and characterization of two DTPA bisamide derivatives DTPA-BC12PheA and DTPA-BC14PheAfunctionalized with p-dodecylaniline and p-tetradecylaniline have been described. Mixed micelles consisting of Gd/Eu-DTPA-BC12PheA and DTPA-BC14PheA with a homogeneous size distribution (33 - 40 nm) were prepared bythe assembly of the amphiphilic complexes with phospholipid DPPC and surfactant Tween 80®. Taking into account the sensitivity difference between magnetic resonance and optical imaging techniques, the ratios of Gd and Eu complexes (Gd/Eu) 1:1, 2:1, 3:1, 20:1 and 50:1 were combined in one single micelle and their optical and relaxometric properties have beencharacterized in detail. Upon excitation at 290 nm, the micelles displayed characteristic red emission bands due to the 5D0→7FJ (J = 0-4)transitions of Eu(III). Micelles composed of exclusively europium complexes exhibited quantum yields in the range of 1.0%, decreasing with
the europium concentration when going from 1:1 to 50:1 Gd/Eu contents. The relaxivity r1 per Gd(III) ion at 40 MHz and 310 K reaches a maximum value of 14.2 s-1mM-1 for the Gd-DTPA-BC12PheA assemblies and 16.0 s-1mM-1 for the micellar Gd-DTPA-BC14PheA compared to a value of 3.5 s-1mM-1 for Gd-DTPA (Magnevist®). Theoretical fitting of the 1H NMRD profiles results in large τR values of 4.2 to 6.6 ns. The most optimal concentration ratio of Gd/Eu compounds in the micelles in order to provide the required bimodal performance has been determined to be 20:1. In the search for other bimodal assemblies, this discovery can be used as guideline concerning the load of paramagnetic agents with respect to luminescent probes.
The six DTPA bisamide derivatives reported in
the previous paragraphs were coordinated to dysprosium(III) and the
magnetic and optical properties of the corresponding complexes and
micelles were examined in detail. Upon excitation into the ligand
levels, the complexes displayed characteristic Dy(III) emission with
quantum yields of 0.3 - 0.5% despite the presence of one water molecule
in the first coordination sphere. Since the luminescence quenching
effect is decreased by the intervention of non-ionic surfactant, quantum
yields up to 1% are obtainedfor the micelles. The transverse
relaxivity r2 per Dy(III) ionat 500 MHz and 310 K reaches a maximum
value of 27.4 s-1mM-1 for Dy-DTPA-BEthA and 36.0 s-1mM-1 for the
Dy-DTPA-BC12PheA assemblies compared to a value of 0.8 s-1mM-1 for the
parent Dy(III)-DTPA. The efficient T2 relaxation, especially at high
magnetic field strengths, is sustained by the high magnetic moment of
the dysprosium ion, the coordination of water molecules with slow water
exchange kinetics and long rotational correlation times. These findings
open the way to the further development of bimodal optical and magnetic
resonance imaging probes starting from singlelanthanide compounds.
In a final and conceptually different approach, a novel synthetic strategy
towards a heteropolymetallic lanthanide complex with selectively
incorporated gadolinium and europium ions is outlined. For this means, a
ditopic ligand able to coordinate to two different lanthanide ions has
been synthesized. A DTPA-based moiety taking care of gadolinium(III)
chelation is linked via an amide bond to a pyridine-2,6-dicarboxylate
derivative ensuring self-assembly around europium(III). Due to the easy
accessibility of water to the three paramagnetic components per
molecule, efficient relaxation enhancement of 31 mM-1s-1 is achieved.On
the other hand, since water is excluded from the first coordination
sphere of Eu(III), a bright emissive compound exhibiting a quantum yield
up to 10% is obtained. Stability studies revealed highbinding
constants, allowing in vivo studies on the newly developed contrast
agent offering interesting applications in bimodal magnetic resonance
and optical imaging.
diagnostics. Together with the development of contrast agents,a
tremendous interest in its applications has grown. A lot of effort has
been done to investigate the most important parameters in order to
enhance the relaxation efficiency of MRI probes.Taking into account the
requirements for an optimal magnetic performance, different contrast
agents have been synthesized and studied in this PhD thesis. Moreover,
novel bimodal probes have been developed in order to exploit the high
sensitivity and resolution of optical microscopy with the ability of MRI
to image opaque samples. Employing this strategy enables the
simultaneous visualization of the same biological structures at
different resolutions and depths. Throughout this thesis, the aim was to
improve relaxivity especially by increasing the molecular weight and
the rotational tumbling time of the agent, and to find an appropriate
ligand acting as an efficient sensitizer for lanthanide luminescence.
Each synthesized compound has been analyzed by luminescence and
relaxometric measurements in order to evaluate its potential forbimodal
imaging.
DTPA-bisamide derivatives functionalized with p-toluidine,
6-aminocoumarin, 1-naphthalene methylamine and4-ethynylaniline were
synthesized and fully characterized. Ln(III) complexes (Ln = Gd, Eu, Tb,
Y) of the ligands DTPA-bis-p-toluidine-amide (DTPA-BTolA),
DTPA-bis-6-coumarin-amide (DTPA-BCoumA), DTPA-bis-1-naphthylmethyl-amide
(DTPA-BNaphA) and DTPA-bis-4-ethynylphenyl-amide (DTPA-BEthA) were
prepared and studied for theirbimodal MRI/optical properties. Eu(III)
and Tb(III) derivatives in aqueous solutions exhibit characteristic red
and green emission, respectively, with quantum yields of 0.73% for
Eu(III)-DTPA-BNaphA and 2.5% for Tb(III)-DTPA-BEthA. Ligand-centered
photophysical properties of the Gd(III) complexes have been investigated
to get insights into energy transfer processes taking place in these
systems. The Gd(III) complexes were also analyzed by the NMRD technique.
The relaxivity (r1) at 20 MHz and 310 K was in the range from 4.1
s-1mM-1 for Gd-DTPA-BTolA to 6.4 s-1mM-1 for Gd-DTPA-BNaphA which are
higher values compared to 3.8 s-1mM-1 for Gd-DTPA(Magnevist®). The
improved relaxivity is due to a general increase of the rotational
tumbling time τR. Moreover, ina 4% HSA solution, the apparent
relaxivity at 20 MHz increasesto values of 13.9 and 19.1 s-1mM-1 for
Gd-DTPA-BNaphA and Gd-DTPA-BEthA respectively, caused by a further
decrease of molecular motions.
In the second part, the synthesis and characterization of two DTPA bisamide derivatives DTPA-BC12PheA and DTPA-BC14PheAfunctionalized with p-dodecylaniline and p-tetradecylaniline have been described. Mixed micelles consisting of Gd/Eu-DTPA-BC12PheA and DTPA-BC14PheA with a homogeneous size distribution (33 - 40 nm) were prepared bythe assembly of the amphiphilic complexes with phospholipid DPPC and surfactant Tween 80®. Taking into account the sensitivity difference between magnetic resonance and optical imaging techniques, the ratios of Gd and Eu complexes (Gd/Eu) 1:1, 2:1, 3:1, 20:1 and 50:1 were combined in one single micelle and their optical and relaxometric properties have beencharacterized in detail. Upon excitation at 290 nm, the micelles displayed characteristic red emission bands due to the 5D0→7FJ (J = 0-4)transitions of Eu(III). Micelles composed of exclusively europium complexes exhibited quantum yields in the range of 1.0%, decreasing with
the europium concentration when going from 1:1 to 50:1 Gd/Eu contents. The relaxivity r1 per Gd(III) ion at 40 MHz and 310 K reaches a maximum value of 14.2 s-1mM-1 for the Gd-DTPA-BC12PheA assemblies and 16.0 s-1mM-1 for the micellar Gd-DTPA-BC14PheA compared to a value of 3.5 s-1mM-1 for Gd-DTPA (Magnevist®). Theoretical fitting of the 1H NMRD profiles results in large τR values of 4.2 to 6.6 ns. The most optimal concentration ratio of Gd/Eu compounds in the micelles in order to provide the required bimodal performance has been determined to be 20:1. In the search for other bimodal assemblies, this discovery can be used as guideline concerning the load of paramagnetic agents with respect to luminescent probes.
The six DTPA bisamide derivatives reported in
the previous paragraphs were coordinated to dysprosium(III) and the
magnetic and optical properties of the corresponding complexes and
micelles were examined in detail. Upon excitation into the ligand
levels, the complexes displayed characteristic Dy(III) emission with
quantum yields of 0.3 - 0.5% despite the presence of one water molecule
in the first coordination sphere. Since the luminescence quenching
effect is decreased by the intervention of non-ionic surfactant, quantum
yields up to 1% are obtainedfor the micelles. The transverse
relaxivity r2 per Dy(III) ionat 500 MHz and 310 K reaches a maximum
value of 27.4 s-1mM-1 for Dy-DTPA-BEthA and 36.0 s-1mM-1 for the
Dy-DTPA-BC12PheA assemblies compared to a value of 0.8 s-1mM-1 for the
parent Dy(III)-DTPA. The efficient T2 relaxation, especially at high
magnetic field strengths, is sustained by the high magnetic moment of
the dysprosium ion, the coordination of water molecules with slow water
exchange kinetics and long rotational correlation times. These findings
open the way to the further development of bimodal optical and magnetic
resonance imaging probes starting from singlelanthanide compounds.
In a final and conceptually different approach, a novel synthetic strategy
towards a heteropolymetallic lanthanide complex with selectively
incorporated gadolinium and europium ions is outlined. For this means, a
ditopic ligand able to coordinate to two different lanthanide ions has
been synthesized. A DTPA-based moiety taking care of gadolinium(III)
chelation is linked via an amide bond to a pyridine-2,6-dicarboxylate
derivative ensuring self-assembly around europium(III). Due to the easy
accessibility of water to the three paramagnetic components per
molecule, efficient relaxation enhancement of 31 mM-1s-1 is achieved.On
the other hand, since water is excluded from the first coordination
sphere of Eu(III), a bright emissive compound exhibiting a quantum yield
up to 10% is obtained. Stability studies revealed highbinding
constants, allowing in vivo studies on the newly developed contrast
agent offering interesting applications in bimodal magnetic resonance
and optical imaging.
Date:13 Oct 2009 → 30 Sep 2013
Keywords:Supramolecular complexes, Lanthanides, Optical imaging, Contrast agents, Magnetic resonance imaging
Disciplines:Diagnostics, Laboratory medicine, Medicinal products, Condensed matter physics and nanophysics, Inorganic chemistry
Project type:PhD project