Bioresponsive or smart contrast agents (SCAs) sensitive to Ca2+ are of extreme interest in the development of functional magnetic resonance imaging (MRI) techniques as they can aid in tracking neural activity in vivo. To this end, the design of macromolecular systems based on nanoscaffolds such as dendrimers functionalized with multiple MRI contrast agents has been used to conveniently increase the local concentration of paramagnetic MR reporters and slow the diffusion time of the probe, which are favorable in vivo characteristics. Moreover, previous studies with Ca-sensitive dendrimeric MRI probes revealed favorable properties crucial in the development of a ratiometric T 2/T 1-imaging method that provided a higher contrast-to-noise ratio compared to conventional T 1- or T 2-weighted imaging protocols. We therefore developed a series of novel dendrimeric MRI probes (DCAs) with differing structural properties and charge distributions. We thoroughly studied their features such as the relaxometric behavior and size change and examined their electrostatic behaviors prior to and after the addition of Ca2+. The most active DCA displayed a common increase in r 1 (3.11 to 5.72 mM–1 s–1) and a remarkable increase in r 2 (7.44 to 34.57 mM–1 s–1), resulting in a r 2/r 1 ratio increase of the factor 2.52, which is greater than what was previously achieved. These changes in r 1 and r 2 were followed with a hydrodynamic diameter increase from 7.1 ± 1.2 to 8.5 ± 0.7 nm upon the addition of Ca2+, along with a decrease in the negative surface charge of the nanoparticle. Overall, our findings indicate that highly responsive DCAs can be developed only through a combination of properties such as changes in hydration and size of the molecule, which are a consequence of intramolecular structural and electrostatic changes in the particle. In turn, they provide a model for future preparations of responsive DCAs that can be utilized for both T 1-weighted and ratiometric T 2/T 1-weighted imaging to visualize essential biological processes in a dynamic fashion.

The relevance of MRI as a diagnostic methodology has been expanding significantly with the development of molecular imaging. Partially, the credit for this advancement is due to the increasing potential and performance of targeted MRI contrast agents, which are able to specifically bind distinct receptors or biomarkers. Consequently, these allow for the identification of tissues undergoing a disease, resulting in the over- or underexpression of the particular molecular targets. Here we report a multimeric molecular probe, which combines the established targeting properties of the Arg-Gly-Asp (RGD) peptide sequence toward the integrins with three calcium-responsive, Gd-based paramagnetic moieties. The bifunctional probe showed excellent 1 H MRI contrast enhancement upon Ca 2+ coordination and demonstrated a longer retention time in the tissue due to the presence of the RGD moiety. The obtained results testify to the potential of combining bioresponsive contrast agents with targeting vectors to develop novel functional molecular imaging methods.

We review the use of the solid phase synthesis methodology for the preparation of diverse and potent MRI probes.

The development of bifunctional imaging probes can often be challenging with difficult and time-consuming solution phase chemistry protocols and purification techniques. A solid phase synthetic protocol was therefore utilized to produce a functionalized derivative of a potent bismacrocyclic calcium-responsive contrast agent for magnetic resonance imaging. Through a convenient building block approach, the applicability of this methodology in the preparation and simple future development of multifunctional imaging probes was demonstrated.