br Activatable MRI probes MRI is commonly used in
Activatable MRI probes MRI is commonly used in clinics around the world and has the advantages of outstanding tissue-penetration depth and extremely high spatial resolution for in vivo imaging . MRI has relatively poor sensitivity and usually requires notch signaling agents to enhance the imaging contrast by reducing the local T1 and T2 relaxation times of water protons in pathological tissues. However, most of the current clinical contrast agents are nonspecific and are not capable of imaging biochemical activity; therefore, recent efforts have been devoted to the design of activatable probes for molecular MRI 34., 35., 36., 37.. These activatable probes, whose MR properties (relaxivity) are modulated by a specific molecular target from either receptor binding or molecular activation, are particularly attractive because the resulting signal amplification can greatly improve the detection sensitivity and specificity. Several effective strategies have been developed to design activatable MR probes according to the Solomon, Bloembergen and Morgan (SBM) theory, including modulation of the number of inner-sphere water molecules (q), the rotational tumbling time (τ) and the residence lifetime of inner-sphere water molecules (τm) 35., 38.. Modulation of the accessibility of hydration molecules (q) to the paramagnetic metal ion e.g., Gd3+) in the contrast agents has been demonstrated as an efficient approach, in which a “shielding” group is initially placed around the paramagnetic metal core and the subsequent removal of the “shielding” group by the target enzyme allows water molecules to coordinate with the paramagnetic ion, resulting in a significant increase in MR relaxivity . Another type of activatable MRI probe involves receptor-induced magnetization enhancement (RIME), where the specific binding of contrast agents with the macromolecules of proteins (receptors or enzymes) prolongs the τR of the probe, leading to higher MR relaxivity 40., 41.. In addition, the alteration of MR contrast based on chemical exchange saturation transfer (CEST) has been demonstrated as a new class of activatable probe for MRI . Weissleder and co-workers  reported an intriguing MR signal amplification strategy based on oxidoreductase-mediated polymerization of Gd monomers into oligomers with higher r1 relaxivity, which is amenable to the design of activatable probes for MR imaging of myeloperoxidase (MPO) activity in inflammation tissue. MPO has been recognized as an important biomarker of various inflammatory diseases, and accurate measurement of its activity is highly useful for both the early diagnosis of diseases and the monitoring of treatment efficacy. They designed the MPO-activatable MRI probes by conjugation of 5-hydroxytryptamide and Gd chelate (Fig. 3a). Upon interaction with MPO, these probes are catalyzed to form highly reactive radicals that can either proceed through rapid condensation into paramagnetic oligomers or react with other phenolic residues on the protein surface to form cross-linked structures with the proteins, resulting in a 70 %–100 % increase in r1 relaxivity due to a prolonged τR. MR imaging of MPO activity in vivo was first performed using an artificial Matrigel model implanted in mice, which showed striking differences both in MR contrast and pharmacokinetics between the MPO-implanted site and control site. They later applied the probes to in vivo MRI of lipopolysaccharide (LPS)-induced endogenous mouse MPO secretion in a myositis model, showing prolonged MR contrast enhancement relative to MPO-nonactivatable probes (Fig. 3b). Further application to in vivo imaging of MPO activity in the myocardium of mice was also demonstrated, which accurately detected MPO activity and had good sensitivity and dynamic range to monitor treatment effects . Encouraged by this, the strategy of enzyme-mediated polymerization of Gd monomers into oligomers was extended by Liang and co-workers  to design an activatable MRI probe for highresolution visualization of furin levels in a human breast MDA-MB-468 tumor xenograft mouse model. Additionally, recent efforts have been made to develop new activatable MRI probes to detect other classes of enzymes, such as protein disulfide isomerase, which will allow for the detection of acute thrombosis .