Development of Off-On Switching 19F MRI Probes for Cathepsin K Activity Detection

Cathepsin K is a protease expressed in osteoclasts that degrades bone tissue, such as type I collagen fibers. Overexpression of cathepsin K is involved in osteoporosis, rheumatoid arthritis, and bone metastasis. Therefore, detecting cathepsin K activity is important for understanding the mechanism of these diseases and developing new drugs. However, current chemical probes cannot be employed for the detection of cathepsin K activity in animal deep-tissue. In this study, we developed novel 19F magnetic resonance imaging (MRI) probes (FLAME-(Gd-X), X = Acp, Deg, Deg2) to detect cathepsin K. In FLAME-(Gd-X), the Gd3+ complex was modified on the surface of perfluorocarbon-encapsulated silica nanoparticles through cathepsin K substrate and three different hydrophobic/hydrophilic linkers. The 19F NMR signal intensities of these probes were suppressed by the paramagnetic relaxation enhancement (PRE) effect of the Gd3+ complexes. The 19F MRI signal intensities of FLAME-Gd-Acp and FLAME-Gd-Deg specifically increased with the substrate cleavage by cathepsin K. The 19F MRI probes based on the PRE effect can be applied to the in vivo detection of cathepsin K activity. We developed 19F MRI probes to detect cathepsin K activity. These probes consist of perfluorocarbon-encapsulated silica nanoparticles, Gd3+ complexes, cathepsin K substrates, and linkers. The 19F NMR signals of these probes were suppressed by the paramagnetic relaxation enhancement effect of the Gd3+ complex. The 19F MRI signal intensities significantly increased after cleaving the substrates with Gd3+ complexes by cathepsin K.