Molecular imaging that provides a non invasive means to visu
Molecular imaging that provides a non-invasive means to visualize and measure a biological process of interest at the molecular and cellular level in living subjects has emerged as a promising approach for in vivo detection of enzyme activity in recent decades . In enzyme substrate to anatomical imaging, which merely provides morphologic information, molecular imaging can detect abnormal enzyme activity before morphologic changes in disease tissues, allowing for a better understanding of enzyme function and early diagnosis of disease . Molecular imaging requires an imaging probe that can produce an analytical signal in response to a specific enzyme. As such, it is pivotal to develop effective molecular imaging probes for the successful detection of enzyme activity in vivo. A number of molecular imaging probes with different modalities, including optical imaging, magnetic resonance imaging (MRI), nuclear imaging, and photoacoustic imaging, have been developed for various enzymes to obtain imaging results with high sensitivity and high spatial resolution in vivo . These imaging probes can be classified into two main categories: “always on” probes and activatable probes (Fig. 1). When using “always on” probes to image enzyme activity, the continuous signal, regardless of proximity or interaction with the enzyme target, produces a considerable background signal that often interferes detection at the target site. A time delay is required to clear the nontargeted probes to generate a sufficient target-to-background ratio (TBR) for imaging (Fig. 1a). In contrast, activatable probes, where the imaging signal can be switched “on” from an “off” state in response to an enzyme, have shown improved TBR for enzyme detection (Fig. 1b). The background signal is inherently low due to the initial “off” signal of the probes; however, enzymatic catalysis could trigger continuous activation of probes, allowing for signal amplification and/or specific retention at the site of interaction to produce enhanced imaging contrast. Therefore, activatable probes are generally characterized by improved sensitivity and specificity, which is advantageous for the rapid and accurate detection of enzyme activity in vivo by molecular imaging. In lymphatic system review, we summarize recent advances in the development of activatable probes capable of non-invasive imaging of the activity of different enzyme in vivo using different modalities. The review is organized based on enzyme-activatable optical imaging probes, MRI probes, and photoacoustic probes.
Activatable optical imaging probes Optical fluorescence imaging, which directly detects photons emitted from fluorescent probes, has become as a powerful analytical method to study biological processes both in vitro and in vivo. It is characterized by high sensitivity, is free of radioactive irradiation, and has the capacity for real-time imaging to obtain quantitative information about molecular targets . Activatable fluorescence imaging probes that amplify the fluorescent signal upon interaction with a certain enzyme have several advantages for in vivo applications, including low background signal and high specificity to detect enzyme activity . Moreover, activatable fluorescence imaging probes can also detect and monitor enzyme activity in real time. There are several principles to consider when designing activatable fluorescence probes for in vivo imaging. First, the probes should be designed to have fluorescence emission in the near-infrared (NIR) range (700–900nm) to guarantee high tissue-penetration capacity and low autofluorescence from tissues. Second, the probes should have high specificity to a target enzyme, that is, the fluorescence signal emission should only occur when the probe interacts with the enzyme of interest to ensure accurate detection. Third, the probe should exhibit minimal inhibition of enzyme activity upon recognition, allowing a single target enzyme to activate many molecular probes, thus amplifying the fluorescence emission and generating high SBR at the target site. Furthermore, upon activation, the probes should prolong accumulation at the target site with a continuous fluorescence signal, enabling long-term monitoring of enzyme activity.