Compartmentalized agents: A powerful strategy for enhancing the detection sensitivity of chemical exchange saturation transfer contrast

Gadolinium-Based Contrast Agents (GBCAs) for MRI: A Benefit-Risk Balance Analysis from a Chemical, Biomedical, and Environmental Point of View

Gadolinium-based contrast agents (GBCAs) have revolutionized medical imaging, enhancing the accuracy and diagnostic value of magnetic resonance imaging (MRI). The increasing use of GBCAs has raised concerns about the release of gadolinium (Gd)(III) into the environment and potential risks for human health. Initially, multiple administrations of GBCAs were associated only with nephrogenic system fibrosis disease in individuals with impaired kidney function. Even if the Gd(III) retention in tissues has not yet been correlated with any specific disease, caution is required for the extensive use of GBCAs. The concerns related to the employment of GBCAs, due to the possible deposition and retention, should be extended also to healthy individuals without renal impairments. To ensure the well-being of patients, there is a need to develop even more stable and better-performing GBCAs, new MRI approaches requiring lower doses of GBCAs and, finally, innovative methods for recovering Gd(III) from both patients' urines and the environment. This can have strong advantages for human health and for environmental sustainability, also considering Gd(III) scarcity, being a rare earth element, and the shared guideline to reduce, as much as possible, the use of rare metals.

Design Chemical Exchange Saturation Transfer Contrast Agents and Nanocarriers for Imaging Proton Exchange in Vivo

Chemical exchange saturation transfer magnetic resonance imaging (CEST MRI) enables the imaging of many endogenous and exogenous compounds with exchangeable protons and protons experiencing dipolar coupling by using a label-free approach. This provides an avenue for following interesting molecular events in vivo by detecting the natural protons of molecules, such as the increase in amide protons of proteins in brain tumors and the concentration of drugs reaching the target site. Neither of these detections require metallic or radioactive labels and thus will not perturb the molecular events happening in vivo. Yet, magnetization transfer processes such as chemical exchange and dipolar coupling of protons are sensitive to the local environment. Hence, the use of nanocarriers could enhance the CEST contrast by providing a relatively high local concentration of contrast agents, considering the portion of the protons available for exchange, optimizing the exchange rate, and utilizing molecular interactions. This review provides an overview of these factors to be considered for designing efficient CEST contrast agents (CAs), and the molecular events that can be imaged using CEST MRI during disease progression and treatment, as well as the nanocarriers for drug delivery and distribution for the evaluation of treatments.

CEST effect of dimethyl sulfoxide at negative offset frequency

Dimethyl sulfoxide (DMSO) has wide biomedical applications such as cryoprotectant and hydrophobic drug carrier. Here, we report for the first time that DMSO can generate a distinctive chemical exchange saturation transfer (CEST) signal at around -2 ppm. Structural analogs of DMSO, including aprotic and protic solvents, also demonstrated CEST signals from -1.4 to -3.8 ppm. When CEST detectable barbituric acid (BA) was dissolved in DMSO solution and was co-loaded to liposome, two obvious peaks at 5 and -2 ppm were observed, indicating that DMSO and related solvent system can be monitored in a label-free manner via CEST, which can be further applied to imaging drug nanocarriers. With reference to previous studies, there could be molecular interactions or magnetization transfer pathways, such as the relayed nuclear Overhauser enhancement (rNOE), that lead to this detectable CEST contrast at negative offset frequencies of the Z-spectrum. Our findings suggest that small molecules of organic solvents could be involved in magnetization transfer processes with water and readily detected by CEST magnetic resonance imaging (MRI), providing a new avenue for detecting solvent-water and solvent-drug interactions.

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.

A Magnetic Resonance Imaging-Chemical Exchange Saturation Transfer (MRI-CEST) Method for the Detection of Water Cycling across Cellular Membranes

Water cycling across the membrane transporters is considered a hallmark of cellular metabolism and it could be of high diagnostic relevance in the characterization of tumors and other diseases. The method relies on the response of intracellular proton exchanging molecules to the presence of extracellular Gd-based contrast agents (GBCAs). Paramagnetic GBCAs enhances the relaxation rate of water molecules in the extracellular compartment and, through membrane exchange, the relaxation enhancement is transferred to intracellular molecules. The effect is detected at the MRI-CEST (Magnetic Resonance Imaging - Chemical Exchange Saturation Transfer) signal of intracellular proton exchanging molecules. The magnitude of the change in the CEST response reports on water cycling across the membrane. The method has been tested on Red Blood Cells and on orthotopic murine models of breast cancer with different degree of malignancy (4T1, TS/A and 168FARN). The distribution of voxels reporting on membrane permeability fits well with the cells' aggressiveness and acts as an early reporter to monitor therapeutic treatments.