Molecular imaging of enzyme activity in vivo using activatable probes

Precise measurement of enzyme activity in living systems with molecular imaging probes is becoming an important technique to unravel the functional roles of different enzymes in biological processes. Recent progress has been made in the development of a myriad of molecular imaging probes featuring different imaging modalities, including optical imaging, magnetic resonance imaging, nuclear imaging, and photoacoustic imaging, allowing for non-invasive detection of various enzyme activities in vivo with high sensitivity and high spatial resolution. Among these imaging probes, activatable or “smart” probes, whose imaging signal can be specifically switched from the “off” to “on” state upon interaction with a target enzyme, are particularly attractive due to their improved sensitivity and specificity. Here, recent advances in the development of activatable probes capable of imaging different enzyme activities in vivo are summarized based on different imaging modalities, and current challenges and future perspectives are discussed.