Highly Sensitive Detection of Caspase-3/7 Activity in Living Mice Using Enzyme-Responsive 19F MRI Nanoprobes.

Highly sensitive imaging of enzymatic activities in the deep tissues of living mammals provides useful information about their biological functions and for developing new drugs; however, such imaging is challenging. 19F magnetic resonance imaging (MRI) is suitable for noninvasive visualization of enzymatic activities without endogenous background signals. Although various enzyme-responsive 19F MRI probes have been developed, most cannot be used for in vivo imaging because of their low sensitivity. Recently, we developed unique nanoparticles, called FLAMEs, that are composed of a liquid perfluorocarbon core and a robust silica shell, and demonstrated their outstanding sensitivity in vivo. Here, we report a highly functionalized nanoprobe, FLAME-DEVD 2, with an OFF/ON 19F MRI switch for detecting caspase-3/7 activity based on the paramagnetic relaxation enhancement effect. To improve the cleavage efficiency of peptides by caspase-3, we designed a novel Gd3+ complex-conjugated peptide, DEVD X ( X = 1, 2), which is a substrate peptide sequence tandemly repeated X times, and demonstrated that DEVD 2 showed faster cleavage kinetics than DEVD 1. By incorporating this novel concept into a signal activation strategy, FLAME-DEVD 2 showed a high 19F MRI signal enhancement rate in response to caspase-3 activity. After intravenous injection of FLAME-DEVD 2 and an apoptosis-inducing reagent, caspase-3/7 activity in the spleen of a living mouse was successfully imaged by 19F MRI. This imaging platform shows great potential for highly sensitive detection of enzymatic activities in vivo.