Development of high-relaxivity Gd-based and responsive CEST contrast agents

MRI-based contrast agents are often required in order to improve the diagnostic content of the tomographic image. Currently used small molecular weigth Gd-complexes, optimized for the clinical scenario, have been used in pre-clinical settings to report on vessel architecture and on tumor vessel permeability. Such Gd-based complexes have, unfortunately, sub-optimal contrast properties when used with low-field (1 Tesla) pre-clinical MRI scanners in addition to sub-optimal binding properties to mouse serum albumin.
Despite Gd-based agents are an indispensable part of MRI examinations, in the past decades, two major clinical studies questioned their safety profiles and alternatives to Gd-based agents are not readily available. Our lab discovered that Iodinated contrast media, playing a key-role in oncological imaging can provide contrast enhancement in tumors (by exploiting the MRI-CEST approach) that is correlated with corresponding CT enhanced images. In addition, we have demosntrated that most of them can be exploited as pH-responsive contrast agents.
We are interested in developing novel blood-pool Gd-based and CEST-based contrast agents with a focus for preclinical imaging applications. In addition, we are exploring radiographic contrast media as novel MRI contrast agents for assessing tumor perfusion and acidosis.

New Gd-based blood-pool agent


Gd-AAZTA-MADEC, an improved blood pool agent for DCE-MRI studies on mice

In the past years several blood pool Gd-complexes, optimized for the clinical scenario, have been used in pre-clinical settings to report on vessel architecture and on tumor vessel permeability. Such Gd-based complexes have, unfortunately, sub-optimal contrast properties when used with the recently developed low-field (1 Tesla) pre-clinical MRI scanners in addition to sub-optimal binding properties to mouse serum albumin. We have developed a new blood pool Gd(III)-based CA [Longo et al. 2016], comparing its remarkable MRI contrast properties in comparison to the clinically approved MS-325 (Vasovist or Ablavar) and to B22956/1.

Gd-AAZTA-MADEC showed stronger binding affinity to HSA, higher relaxivity both with human and with mouse serum albumin (at 40 MHz and 37°C), as well as more prolonged blood circulation time with respect to the other CAs. These properties turned out into brightest and prolonged MIP angiographic images.

Transforming CT to MRI-CEST contrast agents

Nonionic Iodinated Radiographic Molecules as MRI-CEST Tumor Perfusion Agents

Gd-based agents are an indispensable part of MRI examinations to characterize tumor physiology and delineate tumor margins. Iodinated contrast media (CM) are commonly used x-ray contrast agents, playing a key-role in oncological imaging. Recently we have shown [Longo et al. 2016] that iodinated CM can provide contrast enhancement in tumors (by exploiting the MRI-CEST approach) that is correlated with corresponding CT enhanced images. In addition, both iodinated and Gd-based CM have similar molecular weights, likely possessing similar tumor perfusion properties. We demonstrated that iodinated CM can provide MR enhanced-tumor images and perfusion values comparable to that provided by a Gd-based contrast agent [Anemone et al. 2017] .

This work is the first comparison of tumor contrast enhancements and tumor perfusion values using iodinated CM and a Gd-based agent in a preclinical breast tumor model. The obtained enhancement and perfusion maps by both MRI-based approaches proved that the spatial information was similar, suggesting that iodinated CM, within the MRI-CEST approach, might be able to replace Gd-based agents.

Novel pH-responsive CEST contrast agents

pH responsive agents owing a single exchanging proton pool


Responsive agents capable of reporting physicochemical properties of diagnostic interest of the microenvironment in which they distribute (such as pH, temperature, metabolites,..) require multiple magnetically non-equivalent protons in order to apply a ratiometric approach to rule out the concentration term. We have proposed a novel ratiometric pH MRI method based on the analysis of CEST effects under different radio frequency irradiation power levels to a CEST molecule possessing a single proton pool. We demonstrated that the proposed RF power-based ratiometric pH MRI method extends conventional ratiometric pH MRI to CEST agents with at least one exchangeable site, with enhanced pH sensitivity [Longo et al. 2014].

All these ratiometric methods have been demonstrated by using a continuous wave saturation scheme that limits its translation to clinical scanners. We have developed a new ratiometric CEST-MRI pH-mapping approach based on a pulsed CEST saturation scheme for a radiographic contrast agent (iodixanol) possessing a single chemical exchange site. This approach is based on the ratio of the CEST contrast effects at two different flip angles combinations (180/360 and 180/720), keeping constant the mean irradiation RF power (Bavg power). The proposed ratiometric approach index is concentration independent and it showed good pH sensitivity and accuracy in the physiological range between 6.0 and 7.4 [Arena et al. 2018]. .