Bridging the gap in molecular imaging
Proteases constitute a class of enzymes that are often up-regulated during diseased states. Their inherent ability to cleave proteins combined with their abundant expression and catalytic activity could be exploited for diagnostic imaging purposes. Developing specific probes that could become activated in the presence of proteases was the key objective of the EU-funded 'Light-based functional in vivo monitoring of diseases related enzymes' (LIVIMODE) project. Such selective smart imaging probes were envisaged for visualisation and investigation of disease-related molecular processes. Apart from diagnostic imaging, these probes could find application in monitoring disease progression and assessing efficacy of therapeutic agents. More than 60 probes for a total of 11 proteases were generated, including cathepsins and matrix metalloproteases (MMPs). These probes were conjugated to various polymers (polyglutamates and dendritic polyglycerols) or lipids. This should improve homing and avoid rapid clearance from the site of interest due to increased protease activity. The design was mainly based on the principles of fluorescence resonance energy transfer. The fluorescent dye that was used for each probe was carefully selected based on the intended application. One of the main achievements of the study entailed the construction and use of a probe that detected MMP-13, a collagenase associated with osteoarthritis. When used in a mouse model of the disease, this probe provided proof that such probes could be used to monitor the therapeutic efficacy of an MMP-13 inhibitor in vivo. In a similar way, novel probes developed for proteases, such as cathepsins and FAP, exhibited very promising results in diagnostic imaging of non-invasive cancer. Dual probes that could combine optical imaging and magnetic resonance imaging were also developed, opening up new avenues for improved diagnostic strategies. Given the limitations of existing imaging technologies, the LIVIMODE probes constitute an innovative means of non-invasive monitoring of molecular processes in vivo.
