Protease Sensitive Particles for Molecular Imaging of Cancer by CT

X-ray CT instruments are among the most available, efficient and cost-effective imaging modalities in hospitals. The field of CT molecular imaging is emerging which relies mainly on the detection of gold and bismuth nanoparticles, iodine and gadolinium-labeled compounds directed to tagging a variety of abundant biomolecules. Here, for the first time we attempted to detect enzymatic activity, while the low sensitivity of CT scanners to contrast reagents made this a challenging task.

We invented a new class of nano-sized cathepsin-targeted activity-based probes (ABPs) for functional CT imaging of cancer. ABPs are small molecules designed to covalently modify enzyme targets in an activity-dependent manner. Using a CT instrument, these novel probes enable detection of the elevated cathepsin activity within cancerous tissue, thus creating a direct link between biological processes and imaging signals.

Here we present the generation and biochemical evaluation of a library of ABPs tagged with different sized gold nano-particles (GNPs), with various ratios of cathepsin-targeting moiety and a combination of different polyethylene glycol (PEG) protective layers. The most potent and stable GNP-ABPs were applied for non-invasive cancer imaging in mice. Detection of CT contrast from the tumor depended on GNP size and the amount of targeting moiety. Interestingly, TEM images of tumor sections show intercellular lysosomal sub-cellular localization of the GNP-ABPs. In conclusion, we demonstrate the utility of GNP-ABPs as a promising tool for functional CT imaging.