br Experimental br Results and discussion br Conclusion

Experimental
Results and discussion
Conclusion
Acknowledgements This work was funded by the National Natural Science Foundation of China (81572080, 81873972and81873980), the National Science and Technology Major Project of the Ministry of Science and Technology of China (2018ZX10732202), the Natural Science Foundation Project of Chongqing (cstc2015shmszx120086) and the Training Program for Advanced Young Medical Personnel of Chongqing (2017HBRC003).
Introduction Precise detection of enzyme activity has been of constant interest due to the essential roles of enzymes in various biological and physiological processes. Abnormal enzyme activity is closely related to the pathology of many human diseases [1]. For instance, up-regulation of matrix metalloproteinase (MMP) activity has been implicated in tumor invasiveness, metastasis, and angiogenesis 2., 3.. A high level of β-galactosidase has been demonstrated as an important biomarker for cell senescence and primary ovarian cancers 4., 5.. Hence, direct monitoring of the activity in vivo in real time is crucial to unravel their functional roles, which could greatly aid in the early detection of diseases and the rapid assessment of disease progression during treatment. However, the complex and dynamic physiological environment in vivo presents a significant challenge to determine enzyme activity within the context of the natural biological environment in living subjects. 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 [6]. In Berberine Sulfate sale 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 [7]. 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 [8]. 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 this 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.