Accelerated ¹⁹ F·MRI Detection of Matrix Metalloproteinase-2/-9 through Responsive Deactivation of Paramagnetic Relaxation Enhancement

Paramagnetic gadolinium ions (Gd ^III ), complexed within DOTA-based chelates, have become useful tools to increase the magnetic resonance imaging (MRI) contrast in tissues of interest. Recently, "on/off" probes serving as ¹⁹ F·MRI biosensors for target enzymes have emerged that utilize the increase in transverse (T2∗ or T ₂ ) relaxation times upon cleavage of the paramagnetic Gd ^III centre. Molecular ¹⁹ F·MRI has the advantage of high specificity due to the lack of background signal but suffers from low signal intensity that leads to low spatial resolution and long recording times. In this work, an "on/off" probe concept is introduced that utilizes responsive deactivation of paramagnetic relaxation enhancement (PRE) to generate ¹⁹ F longitudinal (T ₁ ) relaxation contrast for accelerated molecular MRI. The probe concept is applied to matrix metalloproteinases (MMPs), a class of enzymes linked with many inflammatory diseases and cancer that modify bioactive extracellular substrates. The presence of these biomarkers in extracellular space makes MMPs an accessible target for responsive PRE deactivation probes. Responsive PRE deactivation in a ¹⁹ F biosensor probe, selective for MMP-2 and MMP-9, is shown to enable molecular MRI contrast at significantly reduced experimental times compared to previous methods. PRE deactivation was caused by MMP through cleavage of a protease substrate that served as a linker between the fluorine-containing moiety and a paramagnetic Gd ^III -bound DOTA complex. Ultrashort echo time (UTE) MRI and, alternatively, short echo times in standard gradient echo (GE) MRI were employed to cope with the fast ¹⁹ F transverse relaxation of the PRE active probe in its "on-state." Upon responsive PRE deactivation, the ¹⁹ F·MRI signal from the "off-state" probe diminished, thereby indicating the presence of the target enzyme through the associated negative MRI contrast. Null point ¹ H·MRI, obtainable within a short time course, was employed to identify false-positive ¹⁹ F·MRI responses caused by dilution of the contrast agent.