Luminescent Lanthanide Doped Nanoparticles as Bimodal Contrast Agents for MRI and Optical Imaging.

This manuscript covers the development of five different multimodal contrast agents for MRI and optical imaging, which are all unique in their design and properties.

The first approach involves linking multiple paramagnetic gadolinium(III)-chelates based on 2-[4,7,10-tris(carboxymethyl)-1,4,7,10-tetrazacyclododec-1-yl]acetate (DOTA) ligand to the surface of NaGdF4:Yb3+,Tm3+ upconverting nanoparticles with an average particle size of 20 nm. This resulted in the assembly that has favorable properties for bimodal Magnetic Resonance Imaging (MRI) and Optical Imaging (OI). An improved synthetic pathway was used to couple the paramagnetic precursor to the nanoparticles. The nanoparticles were rendered water dispersible via citrate capping, leaving one acid group free for amide coupling with the mono-amino precursor of the DOTA ligand. Luminescence spectroscopy measurements have shown that the excitation of the nanoconstruct at 980 nm resulted in intense upconverted emission of thulium(III) at 800 nm. The assembly of several paramagnetic centers on the nanoparticles scaffold reduces the overall tumbling rate resulting in enhanced longitudinal relaxation times and improved relaxivity. The proton NMRD profiles show a characteristic hump at higher frequencies, which is caused by the slow rotation of the nanoconstruct, resulting in r1 values of 25 mM-1s-1 per gadolinium(III)-ion at 60 MHz and 310 K. This is a significant improvement compared to the Gd-DO3A-ethylamine precursor for which a value of r1 of 3.23 mM-1s-1 was observed under the same conditions. Theoretical fitting by two different approaches showed an increase of τR from 57.3 ps for the Gd-DO3A-ethylamine precursor to 392.0 ps for the nanoconstruct, which is responsible for the overall substantial increase in relaxivity.

The second design is a terbium(III) ion coordinated to four novel 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) amphiphilic bisamide ligands. The complexes were assembled into monodisperse micellar nano-aggregates and for the first time TbIII has been evaluated as a single lanthanide negative bimodal contrast agent for MRI/OI. The complexes show characteristic TbIII emission with quantum yields reaching 7.3% and transverse relaxivity r2 per TbIII micelle at 500 MHz and 310 K reaches maximum values up to 60 s-1 micelle-1. The efficient T2 relaxation at high magnetic field strengths is sustained by the increased rotational correlation time of the nanoaggregates and high magnetic moment of the terbium(III) ion.

In the third approach upconverting nanoparticles are further extended for use, now for optical imaging and as negative contrast agents for MRI. Making core-shell particles, where the core exist of upconverting NaGdF4:Yb3+,Er3+ or NaGdF4:Yb3+,Tm3+ NP and by doping the shell with dysprosium(III) or holmium(III). The doping amount was varied from 0.1 mol% to 1 mol% to 10 mol%. The photophysical properties were not affected much by the doping in the shell, and gadolinium(III) emission was observed at 310 nm while a weak dysprosium(III) emission at 575 nm due to energy migrated upconversion. Relaxometric studies showed increase of T2 especially for higher magnetic field strengths, but no real correlation could be drawn between the degree of doping of the paramagnetic lanthanide(III) ion and the increase or r2.

In the fourth approach, two structurally similar nanoparticles were designed for multimodal imaging and possible radiotherapy. The assembly consists of Ultrasmall Superparamagnetic Iron Oxide nanoparticles that act as contrast agent for MRI, with a luminescent rhenium complex, for optical imaging, attached to the surface. Rhenium has the advantage of being luminescent and carries two radio-isotopes 186Re and 188Re making it possible to act as a contrast agent for SPECT(γ) and to be used for radiotherapy(β). The iron oxide nanoparticles were treated with a silane and further functionalized with picolyl. This picolyl was used to capture rhenium(I)(CO)3-1,10-phenanthroline (ReL1) or rhenium(I)(CO)3-2,2’-bipyridine (ReL2) and form the final product Fe3O4-picolyl-rhenium(I)(CO)3-1,10-phenanthroline (IO-ReL1) or Fe3O4-picolyl-rhenium(I)(CO)3-2,2’-bipyridine (IO-ReL2) respectively. All products were characterized properly (TEM, XRD, NMR, IR and TXRF) and a full investigation of the relaxometric and optical properties was conducted. Although iron oxide nanoparticles suffer from strong Rayleigh scattering, an efficient sensitized luminescence was observed and the orange emission wavelength was found at 585 nm for IO-ReL1 and at 592 nm for IO-ReL2 after irradiation at 395 nm. The relaxometric study of these ultrasmall nanoparticles showed very promising results. The r2 values measured at a magnetic field strength of 60 MHz of the nanoparticles being 92.9 mM-1s-1 and 97.5 mM-1s-1 for IO-ReL1 and IO-ReL2 respectively were at least 1.5 times larger than Sinerem®.

The last and fifth multimodal contrast agent consists of linking multiple europium(III)-chelates based on 2-[4,7,10-tris(carboxymethyl)-1,4,7,10-tetrazacyclododec-1-yl]acetate (DOTA) ligand to the surface of iron oxide nanoparticles. A silane derivate was made of DOTA and this was firmly attached to the surface of iron oxide nanoparticles and afterwards coordinated with europium(III) ions. Europium(III) luminescence was not affected by the Rayleigh scattering since the emission was observed in the 560 – 720 nm region of the electromagnetic spectrum. The relaxometric properties of the particles still have to be studied, but since they are based on the same batch as the IO-Re particles, similar results are expected.