Water Diffusion Modulates the CEST Effect on Tb(III)-Mesoporous Silica Probes

Co(II) Macrocyclic Complexes with Amide-Glycinate Pendants as ParaCEST and Liposomal CEST Agents

Macrocyclic Co(II) complexes with appended amide-glycinate groups were prepared to develop paramagnetic Co(II) chemical exchange saturation transfer (CEST) agents of reduced overall charge. Complexes with reduced charge and lowered osmolarity are important for their loading into liposomes and to provide complexes that are highly water soluble and well tolerated in animals. Co(L1) has two non-coordinating benzyl groups and two amide-glycinate pendants, whereas Co(L2) has two unsubstituted amide pendants and two amide-glycinate pendants on cyclam (1,4,8,11-tetraazacyclododecane). The 1H NMR spectrum of Co(L1) is consistent with a single cis-pendant isomer with both amide protons in the trans-configuration, as supported by an X-ray crystal structure. Co(L2) has a mixture of different isomers in solution, including the trans-1,4 and 1,8 pendant isomers. The Z-spectrum of Co(L1) shows one highly-shifted CEST peak, whereas Co(L2) exhibits six CEST peaks. Encapsulation of 40 mM Co(L1) in a liposome with osmotically-induced shrinking at 300 mOsm/L produces a liposomal CEST agent with saturation frequency offset of 3 ppm. Addition of the amphiphilic 1,4,7-triazacyclononane-based complex Co(L5) to the liposomal bilayer at 18 mM with Co(L1) encapsulated in the liposome at 50 mM changes the sign and increases the magnitude of the saturation frequency offset to -7.5 ppm at 300 mOsm/L.

Prospects and limitations of paramagnetic chemical exchange saturation transfer agents serving as biological reporters in vivo

The concept of using paramagnetic metal ion complexes as chemical exchange saturation transfer agents (paraCEST) for molecular imaging of various biological processes first appeared in the literature about 20 years ago. The first paraCEST agent was based on a highly shifted, inner-sphere, slowly exchanging water molecule that could be activated at a frequency far away from bulk water, a substantial advantage for selective activation of the agent alone. Many other paraCEST agent designs followed that were based on activation of exchanging -NH or -OH proton on the chelate itself. Both types of paraCEST designs are attractive for molecular imaging because the rates of water molecule or ligand proton exchange can be designed to be sensitive to a biological or physiological property such as pH, enzyme activity, or redox. Hence, the intensity or frequency of the resulting CEST signal provides a direct readout of that property. Many molecular designs have appeared in the literature over the past 20 years, mostly reported as proof-of-concept designs but, unfortunately, only a few reports have explored the limitations of paraCEST agents for imaging a biological process in vivo. As a community, we now know that the sensitivity of paraCEST agents is lower than one might anticipate based upon simple chemical exchange principles and, in general, it appears the sensitivity of paraCEST agents is even lower in vivo than in vitro. In this short review, we address some of the factors that contribute to the limited sensitivity of paraCEST agents in vivo, offer some thoughts on approaches that could lead to dramatically improved paraCEST sensitivity, and challenge the scientific community to perform more in vivo experiments designed to test these ideas.

Redox-Responsive MRI Probes Based on First-Row Transition-Metal Complexes

The presence of multiple oxidation and spin states of first-row transition-metal complexes facilitates the development of switchable MRI probes. Redox-responsive probes capitalize on a change in the magnetic properties of the different oxidation states of the paramagnetic metal ion center upon exposure to biological oxidants and reductants. Transition-metal complexes that are useful for MRI can be categorized according to whether they accelerate water proton relaxation (T₁ or T₂ agents), induce paramagnetic shifts of ¹H or ¹⁹F resonances (paraSHIFT agents), or are chemical exchange saturation transfer (CEST) agents. The various oxidation state couples and their properties as MRI probes are summarized with a focus on Co(II)/Co(III) or Fe(II)/Fe(III) complexes as small molecules or as liposomal agents. Solution studies of these MRI probes are reviewed with an emphasis on redox changes upon treatment with oxidants or with enzymes that are physiologically important in inflammation and disease. Finally, we outline the challenges of developing these probes further for in vivo MRI applications.