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

Imaging of proteases using activity-based probes

Proteases (proteolytic enzymes) are proteins that catalyze one of the most important biochemical reactions, namely the hydrolysis of the peptide bond in peptide and protein substrates. Therefore these molecular biocatalysts participate in virtually all living processes. The proper balance between intact and processed protease substrates enables to maintenance of homeostasis from a single-cell level to the whole living system. However, when the proteolytic activity is altered, this delicate balance is disturbed, which might lead to the development of a plethora of diseases. Given this, monitoring proteolytic activity is indispensable to understanding how proteases operate in disease lesions and how their altered catalytic activity might be harnessed for a better diagnosis and treatment. In this manuscript, we provide a critical review of the recent development of protease chemical probes which are small molecules that detect proteolytic activity by interacting with protease active site, individual proteases as well as complex proteolytic networks.

19F Magnetic Resonance Imaging and Spectroscopy in Neuroscience

¹H magnetic resonance imaging (MRI) has established itself as a key diagnostic technique, affording the visualization of brain anatomy, blood flow, activity and connectivity. The detection of other atoms (e.g. ¹⁹F, ²³Na, ³¹P), so called hetero-nuclear MRI and spectroscopy (MRS), provides investigative avenues that complement and extend the richness of information that can be gained from ¹H MRI. Especially ¹⁹F MRI is increasingly emerging as a multi-nuclear (¹H/¹⁹F) technique that can be exploited to visualize cell migration and trafficking. The lack of a ¹⁹F background signal in the brain affords an unequivocal detection suitable for quantification. Fluorine-based contrast material can be engineered as nanoemulsions, nanocapsules, or nanoparticles to label cells in vitro or in vivo. Fluorinated blood substitutes, typically nanoemulsions, can also carry oxygen and serve as a theranostic in poorly perfused brain regions. Brain tissue concentrations of fluorinated pharmaceuticals, including inhalation anesthetics (e.g. isoflurane) and anti-depressants (e.g. fluoxetine), can also be measured using MRS. However, the low signal from these compounds provides a challenge for imaging. Further methodological advances that accelerate signal acquisition (e.g. compressed sensing, cryogenic coils) are required to expand the applications of ¹⁹F MR imaging to, for instance, determine the regional pharmacokinetics of novel fluorine-based drugs. Improvements in ¹⁹F signal detection and localization, combined with the development of novel sensitive probes, will increase the utility of these multi-nuclear studies. These advances will provide new insights into cellular and molecular processes involved in neurodegenerative disease, as well as the mode of action of pharmaceutical compounds.

Perfluorocarbons-Based 19F Magnetic Resonance Imaging in Biomedicine

Fluorine-19 (¹⁹F) magnetic resonance (MR) molecular imaging is a promising noninvasive and quantitative molecular imaging approach with intensive research due to the high sensitivity and low endogenous background signal of the ¹⁹F atom in vivo. Perfluorocarbons (PFCs) have been used as blood substitutes since 1970s. More recently, a variety of PFC nanoparticles have been designed for the detection and imaging of physiological and pathological changes. These molecular imaging probes have been developed to label cells, target specific epitopes in tumors, monitor the prognosis and therapy efficacy and quantitate characterization of tumors and changes in tumor microenvironment noninvasively, therefore, significantly improving the prognosis and therapy efficacy. Herein, we discuss the recent development and applications of ¹⁹F MR techniques with PFC nanoparticles in biomedicine, with particular emphasis on ligand-targeted and quantitative ¹⁹F MR imaging approaches for tumor detection, oxygenation measurement, smart stimulus response and therapy efficacy monitoring, et al.