A Multimodal Ca(II) Responsive Near IR-MR Contrast Agent Exhibiting High Cellular Uptake

Ca­(II) ions are critical for the proper function of neurons by contributing to synaptic signaling and regulating neuronal plasticity. Dysregulation of Ca­(II) is associated with a number of pathologies that cause neurodegeneration; therefore the ability to monitor Ca­(II) intracellularly is an important target for molecular imaging. Contrast-enhanced MR imaging is a promising modality for imaging changes in Ca­(II) concentrations. However, the majority of Ca­(II) responsive MR agents are limited to the extracellular space or hindered by poor cellular uptake. Here, we describe a new class of multimodal, bioresponsive Ca­(II) magnetic resonance agents that are coupled to the NIR probe IR-783. This new design is based on previous generations of our Ca­(II) MR agents but overcomes two significant challenges: (1) the presence of the NIR probe dramatically increases cellular uptake of the agent and (2) provides histological validation of the MR signal using NIR fluorescence imaging. IR-783 targets organic anion transporter polypeptides, and we demonstrate that the agents are not toxic in HT-22 or U-87 MG cells up to 20 μM. The cellular uptake of complex 1 was measured to be greater than 16 femtomoles per cell (where ∼1 femtomole/cell is detectable in acquired MR images). Complex 1 is simultaneously detectable by both MR and NIR fluorescence imaging in vitro and is activated (turned on) by intracellular Ca­(II) at concentrations between 1 and 10 μM.