Molecualr Imaging with MRI
q-Modulated MR Probes: EGad
Bioactivated q-modulated contrast agents alter the coordination environment of the Gd(III) center as a means to modulate contrast. These probes allow for real-time in vivo imaging of dynamic cellular processes such as gene expression and ion fluxes.
The Meade lab pioneered the development of q-modulated Gd(III) contrast agents with Egad. In the absence of the reporter protein β-galactosidas (β-gal), the pendant sugar blocks water access to the Gd(III) center. Hydrolysis of the glycosidic bond by β-gal results in a change in the coordination environment (q) resulting in enhanced image contrast. Several generations of enzyme-activated agents have been developed with increased changes in contrast and improved kinetics of activation.
In response to the biological event of interest, q-modulated contrast agents transform from a low-relaxivity q=0 state to a high relaxivity q=1 state.Lanthanide probes for bioresponsive imaging. Chem Rev 2014, 114, 4496.
q-Modulated MR Probes: BAPTA-based Ca(II)-sensitive MR agents
We have developed q-modulated Gd(III) contrast agents responding selectively and specifically to Ca(II). Calcium is the most important intracellular secondary messenger in the body. Spatial and temporal detection of Ca(II) fluxes provides insight into many biological mechanisms. The Meade lab developed the first ion sensing q-modulated contrast agent. The Ca(II) agent is comprised of a aminopolycarboxylic acid Ca(II)-binding ligand known as BAPTA. In the absence of Ca(II), the acetate groups of BAPTA are believed to bind to the Gd(III) center, restricting water access. Ca(II) concentrations in the millimolar range are bound by the BAPTA domain resulting in an increase in MR signal.
Calcium binding the BAPTA domain of the Gd(III)-DOPTA agent. This mechanism mechanism of activation is sensitive to millimolar changes in Ca(II) concentration.
Enzyme-Activated MR Shift Probes
Imaging enzyme activity is a key challenge in MR molecular imaging because unknown concentrations of on- and off-states can confound T1 measurements in vivo. MR chemical shift probes enable specific detection of activation using the NMR signal of a reporter group. When placed near a paramagnetic Ln(III), the chemical shift is sensitive to the Ln(III) coordination environment, giving unique signals pre- and post-activation.
T1e Modulated Contrast Agents
The electronic relaxation time (T1e) of Gd(III) is the only parameter that influences all water relaxivity spheres. By shortening the T1e through magnetic coupling to a fast relaxing paramagnetic species, we can improve the sensitivity of our probe by reducing the background in the off-state and activate it to the on-state through a transition metal redox switch.
Caspase-Activated Multimodal FL-MR Contrast Agent
Effective cancer therapy largely depends on inducing apoptosis in cancer cells via chemotherapy and/or radiation. Monitoring apoptosis in real-time provides invaluable information for evaluating cancer therapy response and screening preclinical anticancer drugs. We have developed caspase probe 1 (CP1), a multimodal fluorescence-magnetic resonance (FL-MR) probe that exhibits simultaneous FL-MR turn-on response to caspase-3/7. Both caspases exist as inactive zymogens in normal cells but are activated during apoptosis and are unique biomarkers for this process. CP1 has three distinct components: a DOTA-Gd(III) chelate that provides the MR signal enhancement, tetraphenylethylene as the aggregation induced emission luminogen (AIEgen), and DEVD peptide which is a substrate for caspase-3/7. In response to caspase-3/7, the water-soluble peptide DEVD is cleaved and the remaining Gd(III)-AIEgen (Gad-AIE) conjugate aggregates leading to increased FL-MR signals.
Contrast Agent Nanoconstructs
We utilize the multimodal capacity of AuNPs to create targeted, bimodal MRI agents.