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Abstract

Smart MR contrast agents exhibit modulation of their relaxivity by specific physiological or
biochemical trigger-events, while targeted MR contrast agents are envisioned to deliver the large
gadolinium chelates into the target tissue. In an effort to develop novel smart and targeted MR
contrast agents, the series of the DO3A based multifunctional chelating agents with the variable
length of the side chain has been synthesized. They serve as valuable multipurpose precursors
for contrast agents based on gadolinium chelates in the design of relaxometric MR probes.
The presence of the amino group in the side chain of the macrocycle allows for conversion into
various functional groups (aminophosponates, aminocarboxylates, etc.) or for conjugation with
different biomolecules, dyes, and polymers. Choice of the functional groups depends on the
further application of the compounds.
A series of gadolinium chelate complexes
based on the compounds 1a-d were developed, in
order to change relaxivity in magnetic resonance
experiments dynamically with Ca2+ concentration.
The potential of Ca2+ MR imaging in
neuroscience is evident in the intensive efforts to
design gadolinium complexes that can act as
calcium-dependent MRI contrast agents. Different
lengths of the phosphonate side chains are
expected to lead to different binding constants of the phosphonate - gadolinium bonds. The latter
property can be exploited for fine-tuning the sensitivity of the agent to calcium ion concentration.
The sensitivity of the contrast agents for changes in Ca2+ concentrations increased with the chain
length of the phosphonate functions, specifically for compound 1d (n=4) the range of Ca2+
concentration is compatible with extracellular physiological conditions.
For the purpose of targeted MRI, two macrocyclic bioconjugates were synthesized. A
conjugate 2 of Gd-DO3A-EA with biotin was synthesized for targeted imaging in an antibodyavidin
system. Mixture of 2 and avidin (4:1) showed 54 relaxivity enhancement for r1
and
311 for r2 relative to the unbound 2.
A macrocyclic conjugate 3 of Gd-DO3A-EA with FITC was designed to track cellular
binding and internalization by both fluorescence detection and MR imaging. The compound did
not show cytotoxicity after treatment up to 50 μM for 24 hrs (NIH-3T3 cells, PI assay).
Fluorescence microscopy of living cells displayed detectable internalisation.