Cell Labeling with Responsive MRI Contrast Agents is Enabled through Solid-Phase Synthesis

Bioresponsive or smart contrast agents (SCAs) for magnetic resonance imaging (MRI) can facilitate functional molecular imaging of numerous biological processes. These are MRI probes that alter the MRI signal along with the concentration changes of different biomarkers in their microenvironment, thus enabling the assessment of tissue physiology with high spatiotemporal resolution. One of the common shortcomings of SCA is their structural and functional insufficiency for accumulation in the targeted region, i.e., most frequently internalization into the cells to study the intracellular processes. Here, we report a strategy to prepare a multifunctional SCA that can be successfully incorporated into the cell membrane and internalized. We used the solid-phase synthesis methodology to obtain a trimeric SCA responsive to calcium ions, which bears a hydrophobic tetradecanoyl group to facilitate interaction with primary rat astrocytes. The developed MRI probe maintained high activity, exhibiting high calcium-triggered longitudinal and transverse relaxivity changes. Concurrently, it showed the ability to label the cell membranes and internalize into the astroglial cells while not causing cytotoxicity or affecting the electrophysiology of the cells.