Synergy of Key Properties Promotes Dendrimer Conjugates as Prospective Ratiometric Bioresponsive Magnetic Resonance Imaging Probes.

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 T2 / T1 -imaging method that provided a higher contrast-to-noise ratio compared to conventional T1 - or T2 -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 r1 (3.11 to 5.72 mM-1 s-1 ) and a remarkable increase in r2 (7.44 to 34.57 mM-1 s-1 ), resulting in a r2 / r1 ratio increase of the factor 2.52, which is greater than what was previously achieved. These changes in r1 and r2 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 T1 -weighted and ratiometric T2 / T1 -weighted imaging to visualize essential biological processes in a dynamic fashion.