Highly sensitive MRI chemical exchange saturation transfer agents using liposomes.

Chemical exchange saturation transfer (CEST) agents represent an emerging class of magnetic resonance imaging (MRI) contrast media that has huge potential. They act as negative agents by reducing the signal intensity of the water protons through a saturation transfer mediated by chemical exchange. Saturation transfer occurs through selective irradiation at the absorption frequency of the exchanging protons of the agent. The great potential of CEST agents lies in the possibility of switching the contrast “on” and “off” at will, since each response is uniquely related to the specific absorption frequency of their mobile protons, thus making it possible to detect several agents in the same sample. Basically, the saturation transfer efficiency is proportional to the exchange rate (kex) and to the number of mobile protons in the CESTagent. However, the value of kex must be smaller than the frequency separation (Dw) between the resonances of bulk water and the exchanging protons of the agent if the mobile protons are to be selectively irradiated. For this reason, it might be possible to find efficient CEST agents in the class of paramagnetic metal complexes (the socalled PARACEST agents), in which the presence of unpaired electrons induce large Dw values. However, the sensitivity of a PARACEST agent is still below that of the conventional relaxing agents based on the Gd ion, even when kex and Dw are optimized. It has been suggested that an improved sensitivity may be obtained by increasing the number of exchangeable protons. Van Zijl and co-workers suggested the use of polymers, such as polyamino acids, polyamidoamine-based (PAMAM-based) dendrimers, and single-stranded RNA, containing a large number of labile amide protons. 5] We have further extended this approach by designing supramolecular adducts between a paramagnetic shift reagent and a substrate containing many, fast-exchanging protons. The latter approach appears promising, but it bears the drawback of showing a severe broadening of the absorption corresponding to the exchangeable protons that, in turn, results in the need to apply a much more intense B1 field to maximize their saturation. To design a system with a well-defined paramagnetically shifted resonance that corresponds to a high number of exchangeable protons, we considered the use of liposomes containing a paramagnetic shift reagent for water protons in the aqueous inner cavities. In such systems, which we call LIPOCESTagents, the resonance of the water protons inside the liposomes is shifted from that of the external water to an extent that depends upon the structure and the concentration of the shift reagent entrapped within the liposomes. Thus, the H NMR spectrum of such a suspension will display two signals for water that correspond to water confined in the liposome cavity and to the external solvent. The exchange of water molecules across the liposome membrane is usually slow and can be controlled, to some extent, by varying the composition of the lipidic bilayer. Suitable candidates for the shift reagent should be found among paramagnetic metal complexes containing at least one highly shifted and fastexchanging water molecule. In principle, several macrocyclic lanthanide(iii) chelates derived from 1,4,7,10-tetraaza1,4,7,10-tetrakis(carboxymethyl)cyclododecane (DOTA), a,a’,a’’,a’’’-tetramethyl-1,4,7,10-tetraacetic acid (DOTMA), and related ligands display an axially coordinated water that nicely fulfills the latter requirement. Herein we report the results obtained with the [Tm(dotma)] complex, which has recently been proposed as a non-invasive MR probe for in vivo thermometry.