Application of High-spin Species Designed by the Studies on Molecule-based on Magnet to MRI ImagingAgents

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The molecule for MRI imaging agent requires the high solubility in water and the high relaxivities, r_1 and r_2, of the water proton. One of the factors affecting to the relaxation time of the water proton is the rotational correlation time (τ_R) of the molecule for MRI imaging agent and the relaxivities increase with increasing the τR value. Because the τ_R value is affected by the size of the molecule; τR is proportional to r^3, where r is a radius of the molecule spherical in shape, the molecule with large molecular weight is expected to give high relaxivities. Based on this idea, we performed two approaches, DNA and dendrimer systems. In DNA system, the DNA monomer carrying a stable radical, its oligomer (15mer), and its duplex with a complimentary DNA oligomer were prepared and their relaxivities were investigated by a pulse NMR spectrometry. Obtained values of r_1 and r_2 increased with increasing the molecular weight, as expected. The values of r_1 for a monomer, an oligomer, and a duplex derivatives are 0.25, 0.44, and 0.57 sec^<-1>mM^<-1>, respectively. In water-soluble dendrimer system, sixteen stable radical units could be introduced to the third generation in the amine dendrimer carrying the carboxylic acid moieties by an ion exchange reaction. The value of r_1= 2.28 sec^<-1>mM^<-1> per molecule was obtained. This system is improved by using a hyper-branched polymer (HBP) and the value higher than the one for the Magnevist which is a gadolinium complex and is used in clinical field, is obtained at present stage.