Design, Synthesis and Characterization of Zinc-sensitive or dual-modal Probes for Optical and Magnetic Resonance Imaging

Abstract

Optical imaging (OI) and magnetic resonance imaging (MRI) are powerful molecular imaging techniques widely used in biomedicine and neuroscience. To date, various image-contrast-enhancing molecular sensors have been developed to improve the functional and anatomical visualization of body structures and fluids. Zn(II) plays a fundamental role in many essential biological processes. The noninvasive determination of Zn(II) concentration fluctuations is of paramount importance for understanding its biological role and improve early-stage disease detection. To the best of our knowledge, the OI/MRI combined imaging sensors are much less studied. Aiming to expand research in this field, the efforts towards preparing Ln(III)-based probes sensitive to Zn(II) for MRI/OI dual-modal imaging are described in this thesis. The first part of this thesis begins with an introduction of non-responsive optical probes EuL1-2 consisting of a DO3A-based reporting moiety linked to a Tyr-derived chromophore. Further structural modifications of EuL1-2 include the introduction of a Zn-sensitive DPA moiety, resulting in the Zn-sensitive probes EuL3-4. These novel compounds exhibited strong selectivity to Zn(II) over other studied cations, and demonstrated an up to 7-fold luminescence enhancement. The induced luminescence change enables establishing EuL3-4 as viable Zn-chemosensors for biological applications. Based on the first project on optical probes, the Gd(III) analogues of EuL3-4, GdL3-4, were prepared and evaluated by means of NMR. It showed that both complexes displayed negligible sensitivity to Zn(II) in r1 relaxivity. In order to improve the sensitivity, GdL3-4 were subjected to further modifications. Thus, the phenolic OH group was converted into phenoxyacetic acid providing GdL5-6. These modifications led to a large r1 relaxivity enhancement (~280%) upon the addition of Zn(II) under physiologically relevant conditions. The detailed evaluation of GdL5-6 properties by means of various NMR experiments in HEPES, PBS, HSA and serum accompanied by MRI phantoms, evidenced their outstanding sensitivity to Zn(II). This makes GdL5-6 complexes potential agents for biological applications. In the third part, I focused on combining two different OI and MRI modalities into one entity. Thus, a trimacrocyclic chelator L7 bridged by DA18C6 moiety was designed and synthesized. Its dinuclear Eu(III) complex Eu2L7 displayed a CEST effect, which is typical for EuDOTAM-Gly. Upon metalation of the remaining DA18C6 chelator with Tb(III), the complex converted into a hetero-multinuclear Eu2L7Tb. This mixed lanthanide complex showed interesting emission properties at different excitation wavelengths. Overall, this system paves the way towards the development of bimodal imaging probes with controlled properties.