Enzyme-activated nanomaterials for MR imaging and tumor therapy

•By utilizing endogenous enzymes characterized by their remarkable specificity and catalytic activity, enzyme-responsive MRI probes exhibit modified signals in response to abnormal fluctuations in enzyme expression levels. This capability enables the sensitive detection of early-stage diseases.•Enzyme-responsive MRI probes can be employed to understand the dynamic changes of reagents at tumor treatment sites, thereby establishing the most reliable relationship of imaging signals with therapeutic efficacy.•The integration of enzyme-responsive MRI probes with diverse therapeutic strategies, enabling real-time feedback on outcomes and facilitating prompt adjustment of parameters such as treatment dosage, administration timing, and intervals, will significantly enhance the utilization of imaging in therapeutic assessment. Enzymes are key components of the biological microenvironment, possessing outstanding catalytic properties and exceptional biorecognition capabilities. Abnormal expression or activity of the enzyme is associated with many disease-associated micro-environments and aberrant cell processes. Therefore, dynamic imaging for enzyme activity is tremendously meaningful for disease diagnosis and treatment. Magnetic resonance imaging (MRI) is a reliable imaging technology for disease diagnosis with the merits of high resolution, non-invasive, and sufficient penetration depth. To monitor enzyme activity, various enzyme-activated MRI probes were developed, which possess a weak background signal and high-sensitivity enzyme imaging for tumors in vivo. Furthermore, certain probes can realize both diagnosis and treatment of cancer as well as other ailments. This leads to a more efficient and secure therapeutic approach guided by imaging techniques. And the therapeutic methods of enzyme-sensitive MRI probes primarily encompass phototherapy, photodynamic therapy, chemodynamic therapy, chemotherapy, radiotherapy, as well as combination therapy. In this review, we systematically introduced the types and mechanisms of enzyme-activated probes and further summarized their application on disease diagnosis and therapy. Finally, we also prospected the development tendency of enzyme-responsive MRI probes.